Located in the heart of the Lesser Caucasus mountains, where the Arabian and Eurasian tectonic pl... more Located in the heart of the Lesser Caucasus mountains, where the Arabian and Eurasian tectonic plates collide, Armenia occupies an exceptional geological position shaped through millions of years of subduction and collision. It is a unique place on the Earth recording extensive intrusive and volcanic activity related to the long-standing continental convergence. The volcanoes of Armenia provide a rare opportunity to study the sources and processes involved in this unusual type of magmatism. More than 500 Quaternary volcanoes have been mapped in Armenia, most of them formed from single eruptive episodes. Among several large composite volcanoes, the mighty Aragats stands out as the largest volcano in Armenia and the region altogether. Volcanic deposits testify to the range of eruptive styles—from the ignimbrites formed in eruptions as explosive and voluminous as any seen globally in the modern era to the enormous fissure-fed lava flows that form the Southern Caucasus flood basalt province, the smallest and youngest Large Igneous Province in the world. Several pre-historical and historical eruptions have been documented, highlighting the potential for future volcanic activity in the region. In recent years, research has focused on the volcanic hazards associated with the Armenian Nuclear Power Plant, located in the foothills of Aragats volcano. This article highlights some of the extraordinary volcanic and intrusive features observed in Armenia and summarizes aspects of recent volcanological and petrological research.
Alkaline igneous rocks are relatively rare in settings of tectonic convergence and little is know... more Alkaline igneous rocks are relatively rare in settings of tectonic convergence and little is known about their pet-rogenesis in these settings. This study aims to contribute to a better understanding of the formation of alkaline igneous rocks by an investigation of the Tezhsar volcano-intrusive alkaline ring complex (TAC) in the Armenian Lesser Caucasus, which is located between the converging Eurasian and Arabian plates. We present new petro-logical, geochemical and Sr\ \Nd isotope data for the TAC to constrain magma genesis and magma source characteristics. Moreover, we provide a new 40 Ar/ 39 Ar age of 41.0 ± 0.5 Ma on amphibole from a nepheline syenite that is integrated into the regional context of ongoing regional convergence and widespread magmatism. The TAC is spatially concentric and measures~10 km in diameter representing the relatively shallow plumbing system of a major stratovolcano juxtaposed by ring faulting with its extrusive products. The plutonic units comprise syenites and nepheline syenites, whereas the extrusive units are dominated by trachytic-phonolitic rocks. The characteristic feature of the TAC is the development of pseudomorphs after leucite in all types of the volcanic, subvolcanic and intrusive alkaline rocks. Whole-rock major element data show a metaluminous (Alkalinity Index = 0-0.1), alkalic and silica-undersatu-rated (Feldspathoid Silica-Saturation Index b0) character of the TAC. The general trace element enrichment and strong fractionation of REEs (La N /Yb N up to 70) indicate a relatively enriched magma source and small degrees of partial melting. All TAC rocks show a negative Nb\ \Ta anomalies typical of subduction zone settings. The initial 87 Sr/ 86 Sr ratios (0.704-0.705) and positive εNd values (+3 to +5) indicate an isotopically depleted upper mantle and lack of significant crustal influence, which in turn suggests the TAC magma has formed via differentiation from lithospheric mantle melts. Regionally, the age of~41 Ma places the TAC amid a Lesser Caucasian Eocene period of dominantly calc-alkaline magmatism. The TAC's arc-like geochemical signatures are interpreted to result from prior subduction of the Te-thyan slab beneath the Eurasian continental margin. The alkaline character, distinct from regional trends, is attributed to Neotethyan slab rollback causing extension and inducing small degrees of decompression melting of metasomatised lithospheric mantle.
Fluid-mediated mineral dissolution and reprecipitation processes are the most common mineral reac... more Fluid-mediated mineral dissolution and reprecipitation processes are the most common mineral reaction mechanism in the solid Earth and are fundamental for the Earth's internal dynamics. Element exchange during such mineral reactions is commonly thought to occur via aqueous solutions with the mineral solubility in the coexisting fluid being a rate limiting factor. Here we show in high-pressure/low temperature rocks that element transfer during mineral dissolution and reprecipitation can occur in an alkali-Al-Si-rich amorphous material that forms directly by depolymerization of the crystal lattice and is thermodynamically decoupled from aqueous solutions. Depolymerization starts along grain boundaries and crystal lattice defects that serve as element exchange pathways and are sites of porosity formation. The resulting amorphous material occupies large volumes in an interconnected porosity network. Precipitation of product minerals occurs directly by repolymerization of the amorphous material at the product surface. This mechanism allows for significantly higher element transport and mineral reaction rates than aqueous solutions with major implications for the role of mineral reactions in the dynamic Earth.
This study presents boron (B) concentration and isotope data for white mica from (ultra)high-pres... more This study presents boron (B) concentration and isotope data for white mica from (ultra)high-pressure (UHP), subduction-related metamorphic rocks from Lago di Cignana (Western Alps, Italy). These rocks are of specific geological interest, because they comprise the most deeply subducted rocks of oceanic origin worldwide. Boron geochemistry can track fluid-rock interaction during their metamorphic evolution and provide important insights into mass transfer processes in subduction zones. The highest B contents (up to 345 μg/g B) occur in peak metamorphic phengite from a garnet-phengite quartzite. The B isotopic composition is variable (δ 11 B = − 10.3 to − 3.6%) and correlates positively with B concentrations. Based on similar textures and major element mica composition, neither textural differences, prograde growth zoning, diffusion nor a retrograde overprint can explain this correlation. Modelling shows that B devolatilization during metamorphism can explain the general trend, but fails to account for the wide compositional and isotopic variability in a single, well-equilibrated sample. We, therefore, argue that this trend represents fluid-rock interaction during peak metamorphic conditions. This interpretation is supported by fluid-rock interaction modelling of boron leaching and boron addition that can successfully reproduce the observed spread in δ 11 B and [B]. Taking into account the local availability of serpentinites as potential source rocks of the fluids, the temperatures reached during peak metamorphism that allow for serpentine dehydration, and the high positive δ 11 B values (δ 11 B = 20 ± 5) modelled for the fluids, an influx of serpentinite-derived fluid appears likely. Paragonite in lawsonite pseudomorphs in an eclogite and phengite from a retrogressed metabasite have B contents between 12 and 68 μg/g and δ 11 B values that cluster around 0% (δ 11 B = − 5.0 to + 3.5). White mica in both samples is related to distinct stages of retrograde metamorphism during exhumation of the rocks. The variable B geochemistry can be successfully modelled as fluid-rock interaction with low-to-moderate (< 3) fluid/rock ratios, where mica equilibrates with a fluid into which B preferentially partitions, causing leaching of B from the rock. The metamorphic rocks from Lago di Cignana show variable retention of B in white mica during subduction-related metamorphism and exhumation. The variability in the B geochemical signature in white mica is significant and enhances our understanding of metamorphic processes and their role in element transfer in subduction zones.
Understanding the Earth's geological nitrogen (N) and carbon (C) cycles is fundamental for assess... more Understanding the Earth's geological nitrogen (N) and carbon (C) cycles is fundamental for assessing the distribution of these volatiles between solid Earth (core, mantle and crust), oceans and atmosphere. This Special Communication about the Earth's N and C cycles contains material that is relevant for researchers who are interested in the Topical Collection on planetary evolution "Reading Terrestrial Planet Evolution in Isotopes and Element Mea-surements". Variations in the fluxes of N and C between these major reservoirs through geological time influenced the evolution and determined the unique composition of the Earth's atmosphere. Here we review several key geological aspects of the N and C cycles of which our understanding has significantly advanced during the last decade through field-based, experimental and theoretical studies. Subduction zones are the most important pathway of both N and C from the Earth's surface into the deep Earth. A key question in the flux quantifica-tion is how much of the volatile elements is stored in the downgoing slab and introduced into the mantle and how much is returned back to the surface and the atmosphere through arc magmatism. For N, the retention of N as NH + 4 in minerals has a major influence on fluxes between reservoirs. The temperature-dependent stability of NH + 4-bearing minerals determines whether N is predominantly retained in the slab to mantle depths (in subduction zones with a low geothermal gradient) or devolatilized (in subduction zones with a high geothermal gradient). Several lines of evidence suggest that the mantle is regassing with respect to N due to a net influx of subducted N over time, but this issue is highly debated and evidence to the contrary also exists. Nevertheless, there is consensus that the majority of the planetary N budget is stored in the Earth's mantle, with the continental crust also constituting a significant N reservoir. For C, release from the subducting slab occurs through decarbona-tion reactions, dissolution and formation of carbonatitic liquids, but reprecipitation of C in the slab or the forearc mantle wedge may limit the effectiveness of direct return of C into the atmosphere. Carbon release through regional metamorphism in collision zone orogens also has potentially profound effects on C release into the atmosphere and consensus has emerged that such orogens are sources rather than sinks of atmospheric CO2. On shorter timescales, contact metamorphism through interaction of mantle-derived magmas with C-bearing country rocks, and the resulting release of large quantities of CH4 and/or CO2 , has been linked to global warming events.
We present the first boron abundance and δ 11 B data for young (1.5-0 Ma) volcanic rocks formed i... more We present the first boron abundance and δ 11 B data for young (1.5-0 Ma) volcanic rocks formed in an active continent-continent collision zone. The δ 11 B of post-collisional volcanic rocks (−5 to +2) from the Armenian sector of the Arabia-Eurasia collision zone are heavier than mid-ocean ridge basalts (MORB), confirming trace element and isotope evidence for their derivation from a subduction-modified mantle source. Based on the low B/Nb (0.03-0.25 vs 0.2-90 in arc magmas), as well as low Ba/Th and Pb/Ce, this source records a subduction signature which is presently fluid-mobile element depleted relative to most arc settings. The heavier than MORB δ 11 B of post-collision volcanic rocks argues against derivation of their subduction signature from a stalled slab, which would be expected to produce a component with a lighter than MORB δ 11 B, due to previous fluid depletion. Instead, the similarity of δ 11 B in Plio-Pleistocene post-collision to 41 Ma alkaline igneous rocks also from Armenia (and also presented in this study), suggests that the subduction signature is inherited from Mesozoic-Paleogene subduction of Neotethys oceanic slabs. The slab component is then stored in the mantle lithosphere in amphibole, which is consistent with the low [B] in both Armenian volcanic rocks and metasomatic amphibole in mantle xenoliths. Based on trace element and radiogenic isotope systematics, this slab component is thought to be dominated by sediment melts (or supercritical fluids). Previously published δ 11 B of metasediments suggests a sediment-derived metasomatic agent could produce the B isotope composition observed in Armenian volcanic rocks. The lack of evidence for aqueous fluids preserved over the 40 Myr since initial collision supports observations that this latter component is transitory, while the lifetime of sediment melts/supercritical fluids can be extended to >40 Myr.
We present the first boron abundance and δ 11 B data for young (1.5-0 Ma) volcanic rocks formed i... more We present the first boron abundance and δ 11 B data for young (1.5-0 Ma) volcanic rocks formed in an active continent-continent collision zone. The δ 11 B of post-collisional volcanic rocks (−5 to +2) from the Armenian sector of the Arabia-Eurasia collision zone are heavier than mid-ocean ridge basalts (MORB), confirming trace element and isotope evidence for their derivation from a subduction-modified mantle source. Based on the low B/Nb (0.03-0.25 vs 0.2-90 in arc magmas), as well as low Ba/Th and Pb/Ce, this source records a subduction signature which is presently fluid-mobile element depleted relative to most arc settings. The heavier than MORB δ 11 B of post-collision volcanic rocks argues against derivation of their subduction signature from a stalled slab, which would be expected to produce a component with a lighter than MORB δ 11 B, due to previous fluid depletion. Instead, the similarity of δ 11 B in Plio-Pleistocene post-collision to 41 Ma alkaline igneous rocks also from Armenia (and also presented in this study), suggests that the subduction signature is inherited from Mesozoic-Paleogene subduction of Neotethys oceanic slabs. The slab component is then stored in the mantle lithosphere in amphibole, which is consistent with the low [B] in both Armenian volcanic rocks and metasomatic amphibole in mantle xenoliths. Based on trace element and radiogenic isotope systematics, this slab component is thought to be dominated by sediment melts (or supercritical fluids). Previously published δ 11 B of metasediments suggests a sediment-derived metasomatic agent could produce the B isotope composition observed in Armenian volcanic rocks. The lack of evidence for aqueous fluids preserved over the 40 Myr since initial collision supports observations that this latter component is transitory, while the lifetime of sediment melts/supercritical fluids can be extended to >40 Myr.
The Sesia zone in the Italian Western Alps is a piece of continental crust that has been subducte... more The Sesia zone in the Italian Western Alps is a piece of continental crust that has been subducted to eclogite-facies conditions and records a complex metamorphic history. The exact timing of events and the significance of geochronological information are debated due to the interplay of tectonic, metamorphic, and metasomatic processes. Here we present new geochronological data using Rb-Sr internal mineral isochrons and in situ 40Ar/39Ar laser ablation data to provide constraints on the relative importance of fluid-mediated mineral replacement reactions and diffusion for the interpretation of radiogenic isotope signatures, and on the use of these isotopic systems for dating metamorphic and variably deformed rocks. Our study focuses on the shear zone at the contact between two major lithological units of the Sesia zone, the eclogitic micaschists and the gneiss minuti. Metasedimentary rocks of the eclogitic micaschists unit contain phengite with step-like zoning in major element chemistry as evidence for petrologic disequilibrium. Distinct 40Ar/39Ar spot ages of relict phengite cores and overprinted rims demonstrate the preservation of individual age domains in the crystals. The eclogitic micaschists also show systematic Sr isotope disequilibria among different phengite populations, so that minimum ages of relict assemblage crystallization can be differentiated from the timing of late increments of deformation. The preservation of these disequilibrium features shows the lack of diffusive re-equilibration and underpins that fluid-assisted dissolution and recrystallization reactions are the main factors controlling the isotope record in these subduction-related metamorphic rocks. Blueschist-facies mylonites record deformation along the major shear zone that separates the eclogitic micaschists from the gneiss minuti. Two Rb-Sr isochrones that comprise several white mica fractions and glaucophane constrain the timing of this deformation and accompanying near-complete blueschist-facies re-equilibration of the Rb-Sr system to 60.1 ± 0.9 Ma and 60.9 ± 2.1 Ma, respectively. Overlapping ages in eclogitic micaschists of 60.1 ± 1.1 (Rb-Sr isochron of sheared matrix assemblage), 58.6 ± 0.8, and 60.9 ± 0.4 Ma (white mica 40Ar/39Ar inverse isochron ages) support the significance of this age and show that fluid-rock interaction and partial re-equilibration occurred as much as several kilometers away from the shear zone. An earlier equilibration during high-pressure conditions in the eclogitic micaschists is recorded in minimum Rb-Sr ages for relict assemblages (77.2 ± 0.8 and 72.4 ± 1.1 Ma) and an 40Ar/39Ar inverse isochron age of 75.4 ± 0.8 Ma for white mica cores, again demonstrating that the two isotope systems provide mutually supporting geochronological information. Local reactivation and recrystallization along the shear zone lasted >15 m.y., as late increments of deformation are recorded in a greenschist-facies mylonite by a Rb-Sr isochron age of 46.5 ± 0.7 Ma.
Interaction between magma and crustal carbonate at active arc volcanoes has recently been propose... more Interaction between magma and crustal carbonate at active arc volcanoes has recently been proposed as a source of atmospheric CO2 , in addition to CO2 released from the mantle and subducted oceanic crust. However, quantitative constraints on efficiency and timing of these processes are poorly established. Here, we present the first in situ carbon and oxygen isotope data of texturally distinct calcite in calc-silicate xenoliths from arc volcanics in a case study from Merapi volcano (Indonesia). Textures and C-O isotopic data provide unique evidence for decarbonation, magma-fluid interaction, and the generation of carbonate melts. We report extremely light δ13CPDB values down to −29.3‰ which are among the lowest reported in magmatic systems so far. Combined with the general paucity of relict calcite, these extremely low values demonstrate highly efficient remobilisation of crustal CO2 over geologically short timescales of thousands of years or less. this rapid release of large volumes of crustal CO2 may impact global carbon cycling. Crustal magma-carbonate interaction has been suggested as a process that may dominate the CO2 output in several volcanic arcs 1,2 and a possible source of magmatic carbonate melts 3-5. Direct evidence for this process often remains elusive, but the occurrence of calc-silicate (skarn) xenoliths in the eruptive products of some active volcanoes 6-9 provide a unique opportunity to study high temperature magma-carbonate interaction, and the subsequent effects on the host magmatic system. Recent work on such xenoliths has additionally linked magma-carbonate interaction to influencing eruptive dynamics via volatile exsolution 7,10-12 , and driving mag-matic differentiation trends towards highly desilicated potassic compositions 13,14. Carbon and oxygen isotopes are powerful tracers of fluid-rock interaction processes during contact meta-morphism of carbonates, where skarn rocks form via reactions between magmatic fluids and carbonate country rocks 15-18. Sedimentary carbonates are isotopically distinct from mantle-derived igneous rocks 15 , allowing for quantification of chemical exchange during magma-carbonate interaction. Decarbonation reactions release CO2 that is enriched in 13C and 18O into the magmatic system 19 , depleting the carbonate protolith in these isotopes, following batch or Rayleigh fractionation trends 15. However, some skarns show additional isotopic modifications that cannot result from equilibrium decarbonation reactions alone, but instead require mixing with magmatic fluids to produce values in near-exchange equilibrium with the adjacent magmatic system 16,20-22. The degree of decarbonation and CO2 released into the magmatic system and ultimately the atmosphere, and the extent of fluid modification of the isotopic signature, can be quantified utilising coupled carbon and oxygen isotopes. In addition , interaction with meteoric water and secondary alteration can usually be distinguished using this approach 16. Merapi is Indonesia's most active volcano, characterised by periods of lava dome growth punctuated by dome failure producing pyroclastic density currents, and intermittent explosive events 23. Compositionally the eruptive products are restricted to high-K basalt to basaltic andesite 23. The upper crust underlying Merapi consists of a 8-11 km thick unit of Cretaceous to Cenozoic limestones, marls and volcaniclastic deposits 24 ,
The mid-Proterozoic Isortoq dike swarm in the Gardar Province, South Greenland, comprises a varie... more The mid-Proterozoic Isortoq dike swarm in the Gardar Province, South Greenland, comprises a variety of alkaline rocks ranging from gabbroic to syenitic in composition. Major magmatic mineral phases are olivine, clinopyroxene, Fe -Ti oxides, amphibole, plagioclase and alkali feldspar. Quartz occurs in some samples as a late magmatic phase. Liquidus temperatures of olivine-bearing samples range between 1120 and 1145 jC and solidus temperatures are 850 -930 jC. Calculated silica activities are highly variable between 0.53 and unity. Oxygen fugacities vary from À 3 to + 1 log units relative to the fayalite -magnetite -quartz buffer.
In order to determine the effects of fluid–rock interaction on nitrogen elemental and isotopic
sy... more In order to determine the effects of fluid–rock interaction on nitrogen elemental and isotopic systematics in high-pressure metamorphic rocks, we investigated three different profiles representing three distinct scenarios of metasomatic overprinting. A profile from the Chinese Tianshan (ultra)high-pressure–low-temperature metamorphic belt represents a prograde, fluid-induced blueschist–eclogite transformation. This profile shows a systematic decrease in N concentrations from the host blueschist (~26 μg/g) via a blueschist–eclogite transition zone (19–23 μg/g) and an eclogitic selvage (12–16 μg/g) towards the former fluid pathway. Eclogites and blueschists show only a small variation in δ15Nair (+2.1 ± 0.3‰), but the systematic trend with distance is consistent with a batch devolatilization process. A second profile from the Tianshan represents a retrograde eclogite–blueschist transition. It shows increasing, but more scattered, N concentrations from the eclogite towards the blueschist and an unsystematic variation in δ15N values (δ15N = + 1.0 to +5.4‰). A third profile from the high-P/T metamorphic basement complex of the Southern Armorican Massif (Vendée, France) comprises a sequence from an eclogite lens via retrogressed eclogite and amphibolite into metasedimentary country rock gneisses. Metasedimentary gneisses have high N contents (14–52 μg/g) and positive δ15N values (+2.9 to +5.8‰), and N concentrations become lower away from the contact with 11–24 μg/g for the amphibolites, 10–14 μg/g for the retrogressed eclogite, and 2.1–3.6 μg/g for the pristine eclogite, which also has the lightest N isotopic compositions (δ15N = + 2.1 to +3.6‰). Overall, geochemical correlations demonstrate that phengitic white mica is the major host of N in metamorphosed mafic rocks. During fluid-induced metamorphic overprint, both abundances and isotopic composition of N are controlled by the stability and presence of white mica. Phengite breakdown in high-P/T metamorphic rocks can liberate significant amounts of N into the fluid. Due to the sensitivity of the N isotope system to a sedimentary signature, it can be used to trace the extent of N transport during metasomatic processes. The Vendée profile demonstrates that this process occurs over several tens of metres and affects both N concentrations and N isotopic compositions.
Trace and major element compositions of mid-Proterozoic (1.20-1.16 Ga) basaltic lava flows and di... more Trace and major element compositions of mid-Proterozoic (1.20-1.16 Ga) basaltic lava flows and dikes from the Gardar Province (South Greenland) provide evidence for two geochemically distinct magma sources. Based on distinct features of incompatible trace element ratios, such as Th/Ta, Th/Tb, or Th/Hf, they differ by the composition of their mantle source and by their partial melting trends. One mantle source is compositionally transitional between midocean ridge basalt (MORB)-type and ocean-island basalt (OIB)-type sources with relatively low Ta/Hf ratios (-0.2), moderate enrichment in light rare-earth elements (LREE), and slightly positive initial ε Nd values (+2). It can be attributed to either a lithospheric mantle source or a depleted astenospheric mantle plume component that has been enriched shortly prior to eruption. The other mantle source is characterized by high Ta/Hf ratios (-0.6), a more pronounced LREE enrichment, and initial ε Nd values around 0. Elevated Ce N /Yb N (7.0-9.8) and Tb N /Yb N ratios (1.6-1.8) of the rocks derived from this source indicate the presence of garnet during melting, suggesting melt generation at depths > 70 km. This mantle source has the geochemical characteristics of an OIB-type source and is interpreted as originating from a mantle plume. Samples from the slightly younger (1.14 Ga) Abitibi dike swarm (Superior Province, Canada), spatially connected to the Gardar Province, show very similar trace element characteristics and the same two distinct magma sources can be identified. The geochemical similarities between the magma sources in South Greenland and Canada support the idea of a genetic link between the two magmatic provinces. This link strengthens the idea that the system was a long-lived major intracontinental rift zone.
In situ UV laser spot 40 Ar/ 39 Ar analyses of distinct phengite types in eclogite-facies rocks f... more In situ UV laser spot 40 Ar/ 39 Ar analyses of distinct phengite types in eclogite-facies rocks from the Sesia-Lanzo Zone (Western Alps, Italy) were combined with SIMS boron isotope analyses as well as boron (B) and lithium (Li) concentration data to link geochronological information with constraints on fluid-rock interaction. In weakly deformed samples, apparent 40 Ar/ 39 Ar ages of phengite cores span a range of $20 Ma, but inverse isochrons define two distinct main high-pressure (HP) phengite core crystallization periods of 88-82 and 77-74 Ma, respectively. The younger cores have on average lower B contents ($36 lg/g) than the older ones ($43-48 lg/g), suggesting that loss of B and resetting of the Ar isotopic system were related. Phengite cores have variable d 11 B values (À18& to À10&), indicating the lack of km scale B homogenization during HP crystallization.
Abstract Harzburgitic xenoliths cut by pyroxenite veins from Avachinsky volcano, Kamchatka, are d... more Abstract Harzburgitic xenoliths cut by pyroxenite veins from Avachinsky volcano, Kamchatka, are derived from the sub-arc mantle and record element transfer from the slab to the arc. Olivine and orthopyroxene in the harzburgites have Li isotopic compositions (�� 7 Li=+ 2.8 to+ 5.6) comparable to estimates of the upper mantle (�� 7 Li~+ 4��2).
We apply new International Atomic Energy Agency guidelines to the assessment of volcanic hazards ... more We apply new International Atomic Energy Agency guidelines to the assessment of volcanic hazards at the proposed site of a nuclear power plant in Armenia (ANPP). Potential volcanic hazards at the ANPP site arise from its location near the base of the 70-km-diameter Aragats shield volcano, composed of high-K series trachy-basalt to trachy-dacite lavas, and low aspect ratio dacitic ignimbrites of Pliocene to mid-Pleistocene age. In addition, the site is located in the vicinity of a mid-Pleistocene monogenetic volcano cluster and in the region of potentially active composite volcanoes and calderas . Hazard analysis is based upon extensive geological, geophysical and volcanological investigations, including preparation of maps at 1:10000 and 1:100000 scales, drilling, and new geochemical, petrological and radiometric age (Ar-Ar) determinations. Probabilistic hazard assessments are done for numerous potential volcanic phenomena, including: lava flow inundation, tephra fallout, pyroclastic flows and probability of opening of new vents. Quantitative assessment shows the site may be inundated by lava flows from the monogenetic volcano cluster, and from pyroclastic eruptions of Aragats, should these long-dormant volcanic systems reactivate. Given the cessation of activity 490 ka ago, there is a 95% probability that the recurrence rate of volcanic activity is < 4e-6 per year, and the probability of inundation of the site by lavas is 2e-6 per year, or by pyroclastic flows is <1e-6 per year. Should these levels of hazard be judged acceptable for site development, a monitoring programme should be implemented and operational plan developed for ANPP response to potential volcanic hazards.
In situ 40Ar/39Ar dating of zoned and recrystallized phengite in metasomatized high-pressure rocks
In situ 40Ar/39Ar dating provides spatial resolution that is critical to understand the temporal ... more In situ 40Ar/39Ar dating provides spatial resolution that is critical to understand the temporal information in 40Ar/39Ar dates of metasomatized and partially recrystallized rocks as these dates can be combined with petrologic and geochemical information about the rock's history. Here, we evaluate the effects of deformation and fluid flux on 40Ar/39Ar dates by investigating a structural profile with strain- and recrystallization gradient across a major crustal shear zone that separates two tectonometamorphic units in the Sesia-Lanzo Zone (Western Alps). Weakly deformed, eclogite-facies samples contain phengites that show significant major element compositional differences between pristine cores and overprinted rims. In contrast, samples from the blueschist-facies shear zone contain fine-grained, mylonitic phengite. To test the hypothesis that increasing deformation and fluid flux promote Ar transmissivity, we compare in situ 40Ar/39Ar data and geochemical tracers of fluid flow (...
Tracing fluid-induced metasomatism across a blueschist-facies shear zone (Sesia-Lanzo zone, Western Alps) using boron isotopes
Boron (B) is a useful tracer for slab-mantle-arc fluid transport and recycling processes in subdu... more Boron (B) is a useful tracer for slab-mantle-arc fluid transport and recycling processes in subduction zones due to its high mobility in hydrous fluids and its significant isotope fractionation by fluid-rock interaction. Although B isotope geochemistry provides key evidence for slab to arc transfer by aqueous fluids, B isotope data on natural high-pressure metamorphic rocks are still scarce and the effects and extent of fluid-rock interaction remain poorly studied. Secondary ion mass spectrometry (SIMS) analyses generally confirm that slab dehydration significantly lowers δ11B of subducted oceanic crust and sediments, but the lack of a systematic relationship with peak metamorphic conditions suggests additional processes have affected the exhumed rocks. Here, we evaluate the effects of deformation and fluid flux on the B isotopic composition of phengitic mica along a structural profile with strain- and recrystallization gradient across a major crustal shear zone that separates two t...
Combined thermodynamic–geochemical modeling in metamorphic geology: Boron as tracer of fluid–rock interaction
Lithos, 2014
ABSTRACT Quantitative geochemical modeling is today applied in a variety of geological environmen... more ABSTRACT Quantitative geochemical modeling is today applied in a variety of geological environments from the petrogenesis of igneous rocks to radioactive waste disposal. In addition, the development of thermodynamic databases and computer programs to calculate equilibrium phase diagrams has greatly advanced our ability to model geodynamic processes. Combined with experimental data on elemental partitioning and isotopic fractionation, thermodynamic forward modeling unfolds enormous capacities that are far from exhausted.
Located in the heart of the Lesser Caucasus mountains, where the Arabian and Eurasian tectonic pl... more Located in the heart of the Lesser Caucasus mountains, where the Arabian and Eurasian tectonic plates collide, Armenia occupies an exceptional geological position shaped through millions of years of subduction and collision. It is a unique place on the Earth recording extensive intrusive and volcanic activity related to the long-standing continental convergence. The volcanoes of Armenia provide a rare opportunity to study the sources and processes involved in this unusual type of magmatism. More than 500 Quaternary volcanoes have been mapped in Armenia, most of them formed from single eruptive episodes. Among several large composite volcanoes, the mighty Aragats stands out as the largest volcano in Armenia and the region altogether. Volcanic deposits testify to the range of eruptive styles—from the ignimbrites formed in eruptions as explosive and voluminous as any seen globally in the modern era to the enormous fissure-fed lava flows that form the Southern Caucasus flood basalt province, the smallest and youngest Large Igneous Province in the world. Several pre-historical and historical eruptions have been documented, highlighting the potential for future volcanic activity in the region. In recent years, research has focused on the volcanic hazards associated with the Armenian Nuclear Power Plant, located in the foothills of Aragats volcano. This article highlights some of the extraordinary volcanic and intrusive features observed in Armenia and summarizes aspects of recent volcanological and petrological research.
Alkaline igneous rocks are relatively rare in settings of tectonic convergence and little is know... more Alkaline igneous rocks are relatively rare in settings of tectonic convergence and little is known about their pet-rogenesis in these settings. This study aims to contribute to a better understanding of the formation of alkaline igneous rocks by an investigation of the Tezhsar volcano-intrusive alkaline ring complex (TAC) in the Armenian Lesser Caucasus, which is located between the converging Eurasian and Arabian plates. We present new petro-logical, geochemical and Sr\ \Nd isotope data for the TAC to constrain magma genesis and magma source characteristics. Moreover, we provide a new 40 Ar/ 39 Ar age of 41.0 ± 0.5 Ma on amphibole from a nepheline syenite that is integrated into the regional context of ongoing regional convergence and widespread magmatism. The TAC is spatially concentric and measures~10 km in diameter representing the relatively shallow plumbing system of a major stratovolcano juxtaposed by ring faulting with its extrusive products. The plutonic units comprise syenites and nepheline syenites, whereas the extrusive units are dominated by trachytic-phonolitic rocks. The characteristic feature of the TAC is the development of pseudomorphs after leucite in all types of the volcanic, subvolcanic and intrusive alkaline rocks. Whole-rock major element data show a metaluminous (Alkalinity Index = 0-0.1), alkalic and silica-undersatu-rated (Feldspathoid Silica-Saturation Index b0) character of the TAC. The general trace element enrichment and strong fractionation of REEs (La N /Yb N up to 70) indicate a relatively enriched magma source and small degrees of partial melting. All TAC rocks show a negative Nb\ \Ta anomalies typical of subduction zone settings. The initial 87 Sr/ 86 Sr ratios (0.704-0.705) and positive εNd values (+3 to +5) indicate an isotopically depleted upper mantle and lack of significant crustal influence, which in turn suggests the TAC magma has formed via differentiation from lithospheric mantle melts. Regionally, the age of~41 Ma places the TAC amid a Lesser Caucasian Eocene period of dominantly calc-alkaline magmatism. The TAC's arc-like geochemical signatures are interpreted to result from prior subduction of the Te-thyan slab beneath the Eurasian continental margin. The alkaline character, distinct from regional trends, is attributed to Neotethyan slab rollback causing extension and inducing small degrees of decompression melting of metasomatised lithospheric mantle.
Fluid-mediated mineral dissolution and reprecipitation processes are the most common mineral reac... more Fluid-mediated mineral dissolution and reprecipitation processes are the most common mineral reaction mechanism in the solid Earth and are fundamental for the Earth's internal dynamics. Element exchange during such mineral reactions is commonly thought to occur via aqueous solutions with the mineral solubility in the coexisting fluid being a rate limiting factor. Here we show in high-pressure/low temperature rocks that element transfer during mineral dissolution and reprecipitation can occur in an alkali-Al-Si-rich amorphous material that forms directly by depolymerization of the crystal lattice and is thermodynamically decoupled from aqueous solutions. Depolymerization starts along grain boundaries and crystal lattice defects that serve as element exchange pathways and are sites of porosity formation. The resulting amorphous material occupies large volumes in an interconnected porosity network. Precipitation of product minerals occurs directly by repolymerization of the amorphous material at the product surface. This mechanism allows for significantly higher element transport and mineral reaction rates than aqueous solutions with major implications for the role of mineral reactions in the dynamic Earth.
This study presents boron (B) concentration and isotope data for white mica from (ultra)high-pres... more This study presents boron (B) concentration and isotope data for white mica from (ultra)high-pressure (UHP), subduction-related metamorphic rocks from Lago di Cignana (Western Alps, Italy). These rocks are of specific geological interest, because they comprise the most deeply subducted rocks of oceanic origin worldwide. Boron geochemistry can track fluid-rock interaction during their metamorphic evolution and provide important insights into mass transfer processes in subduction zones. The highest B contents (up to 345 μg/g B) occur in peak metamorphic phengite from a garnet-phengite quartzite. The B isotopic composition is variable (δ 11 B = − 10.3 to − 3.6%) and correlates positively with B concentrations. Based on similar textures and major element mica composition, neither textural differences, prograde growth zoning, diffusion nor a retrograde overprint can explain this correlation. Modelling shows that B devolatilization during metamorphism can explain the general trend, but fails to account for the wide compositional and isotopic variability in a single, well-equilibrated sample. We, therefore, argue that this trend represents fluid-rock interaction during peak metamorphic conditions. This interpretation is supported by fluid-rock interaction modelling of boron leaching and boron addition that can successfully reproduce the observed spread in δ 11 B and [B]. Taking into account the local availability of serpentinites as potential source rocks of the fluids, the temperatures reached during peak metamorphism that allow for serpentine dehydration, and the high positive δ 11 B values (δ 11 B = 20 ± 5) modelled for the fluids, an influx of serpentinite-derived fluid appears likely. Paragonite in lawsonite pseudomorphs in an eclogite and phengite from a retrogressed metabasite have B contents between 12 and 68 μg/g and δ 11 B values that cluster around 0% (δ 11 B = − 5.0 to + 3.5). White mica in both samples is related to distinct stages of retrograde metamorphism during exhumation of the rocks. The variable B geochemistry can be successfully modelled as fluid-rock interaction with low-to-moderate (< 3) fluid/rock ratios, where mica equilibrates with a fluid into which B preferentially partitions, causing leaching of B from the rock. The metamorphic rocks from Lago di Cignana show variable retention of B in white mica during subduction-related metamorphism and exhumation. The variability in the B geochemical signature in white mica is significant and enhances our understanding of metamorphic processes and their role in element transfer in subduction zones.
Understanding the Earth's geological nitrogen (N) and carbon (C) cycles is fundamental for assess... more Understanding the Earth's geological nitrogen (N) and carbon (C) cycles is fundamental for assessing the distribution of these volatiles between solid Earth (core, mantle and crust), oceans and atmosphere. This Special Communication about the Earth's N and C cycles contains material that is relevant for researchers who are interested in the Topical Collection on planetary evolution "Reading Terrestrial Planet Evolution in Isotopes and Element Mea-surements". Variations in the fluxes of N and C between these major reservoirs through geological time influenced the evolution and determined the unique composition of the Earth's atmosphere. Here we review several key geological aspects of the N and C cycles of which our understanding has significantly advanced during the last decade through field-based, experimental and theoretical studies. Subduction zones are the most important pathway of both N and C from the Earth's surface into the deep Earth. A key question in the flux quantifica-tion is how much of the volatile elements is stored in the downgoing slab and introduced into the mantle and how much is returned back to the surface and the atmosphere through arc magmatism. For N, the retention of N as NH + 4 in minerals has a major influence on fluxes between reservoirs. The temperature-dependent stability of NH + 4-bearing minerals determines whether N is predominantly retained in the slab to mantle depths (in subduction zones with a low geothermal gradient) or devolatilized (in subduction zones with a high geothermal gradient). Several lines of evidence suggest that the mantle is regassing with respect to N due to a net influx of subducted N over time, but this issue is highly debated and evidence to the contrary also exists. Nevertheless, there is consensus that the majority of the planetary N budget is stored in the Earth's mantle, with the continental crust also constituting a significant N reservoir. For C, release from the subducting slab occurs through decarbona-tion reactions, dissolution and formation of carbonatitic liquids, but reprecipitation of C in the slab or the forearc mantle wedge may limit the effectiveness of direct return of C into the atmosphere. Carbon release through regional metamorphism in collision zone orogens also has potentially profound effects on C release into the atmosphere and consensus has emerged that such orogens are sources rather than sinks of atmospheric CO2. On shorter timescales, contact metamorphism through interaction of mantle-derived magmas with C-bearing country rocks, and the resulting release of large quantities of CH4 and/or CO2 , has been linked to global warming events.
We present the first boron abundance and δ 11 B data for young (1.5-0 Ma) volcanic rocks formed i... more We present the first boron abundance and δ 11 B data for young (1.5-0 Ma) volcanic rocks formed in an active continent-continent collision zone. The δ 11 B of post-collisional volcanic rocks (−5 to +2) from the Armenian sector of the Arabia-Eurasia collision zone are heavier than mid-ocean ridge basalts (MORB), confirming trace element and isotope evidence for their derivation from a subduction-modified mantle source. Based on the low B/Nb (0.03-0.25 vs 0.2-90 in arc magmas), as well as low Ba/Th and Pb/Ce, this source records a subduction signature which is presently fluid-mobile element depleted relative to most arc settings. The heavier than MORB δ 11 B of post-collision volcanic rocks argues against derivation of their subduction signature from a stalled slab, which would be expected to produce a component with a lighter than MORB δ 11 B, due to previous fluid depletion. Instead, the similarity of δ 11 B in Plio-Pleistocene post-collision to 41 Ma alkaline igneous rocks also from Armenia (and also presented in this study), suggests that the subduction signature is inherited from Mesozoic-Paleogene subduction of Neotethys oceanic slabs. The slab component is then stored in the mantle lithosphere in amphibole, which is consistent with the low [B] in both Armenian volcanic rocks and metasomatic amphibole in mantle xenoliths. Based on trace element and radiogenic isotope systematics, this slab component is thought to be dominated by sediment melts (or supercritical fluids). Previously published δ 11 B of metasediments suggests a sediment-derived metasomatic agent could produce the B isotope composition observed in Armenian volcanic rocks. The lack of evidence for aqueous fluids preserved over the 40 Myr since initial collision supports observations that this latter component is transitory, while the lifetime of sediment melts/supercritical fluids can be extended to >40 Myr.
We present the first boron abundance and δ 11 B data for young (1.5-0 Ma) volcanic rocks formed i... more We present the first boron abundance and δ 11 B data for young (1.5-0 Ma) volcanic rocks formed in an active continent-continent collision zone. The δ 11 B of post-collisional volcanic rocks (−5 to +2) from the Armenian sector of the Arabia-Eurasia collision zone are heavier than mid-ocean ridge basalts (MORB), confirming trace element and isotope evidence for their derivation from a subduction-modified mantle source. Based on the low B/Nb (0.03-0.25 vs 0.2-90 in arc magmas), as well as low Ba/Th and Pb/Ce, this source records a subduction signature which is presently fluid-mobile element depleted relative to most arc settings. The heavier than MORB δ 11 B of post-collision volcanic rocks argues against derivation of their subduction signature from a stalled slab, which would be expected to produce a component with a lighter than MORB δ 11 B, due to previous fluid depletion. Instead, the similarity of δ 11 B in Plio-Pleistocene post-collision to 41 Ma alkaline igneous rocks also from Armenia (and also presented in this study), suggests that the subduction signature is inherited from Mesozoic-Paleogene subduction of Neotethys oceanic slabs. The slab component is then stored in the mantle lithosphere in amphibole, which is consistent with the low [B] in both Armenian volcanic rocks and metasomatic amphibole in mantle xenoliths. Based on trace element and radiogenic isotope systematics, this slab component is thought to be dominated by sediment melts (or supercritical fluids). Previously published δ 11 B of metasediments suggests a sediment-derived metasomatic agent could produce the B isotope composition observed in Armenian volcanic rocks. The lack of evidence for aqueous fluids preserved over the 40 Myr since initial collision supports observations that this latter component is transitory, while the lifetime of sediment melts/supercritical fluids can be extended to >40 Myr.
The Sesia zone in the Italian Western Alps is a piece of continental crust that has been subducte... more The Sesia zone in the Italian Western Alps is a piece of continental crust that has been subducted to eclogite-facies conditions and records a complex metamorphic history. The exact timing of events and the significance of geochronological information are debated due to the interplay of tectonic, metamorphic, and metasomatic processes. Here we present new geochronological data using Rb-Sr internal mineral isochrons and in situ 40Ar/39Ar laser ablation data to provide constraints on the relative importance of fluid-mediated mineral replacement reactions and diffusion for the interpretation of radiogenic isotope signatures, and on the use of these isotopic systems for dating metamorphic and variably deformed rocks. Our study focuses on the shear zone at the contact between two major lithological units of the Sesia zone, the eclogitic micaschists and the gneiss minuti. Metasedimentary rocks of the eclogitic micaschists unit contain phengite with step-like zoning in major element chemistry as evidence for petrologic disequilibrium. Distinct 40Ar/39Ar spot ages of relict phengite cores and overprinted rims demonstrate the preservation of individual age domains in the crystals. The eclogitic micaschists also show systematic Sr isotope disequilibria among different phengite populations, so that minimum ages of relict assemblage crystallization can be differentiated from the timing of late increments of deformation. The preservation of these disequilibrium features shows the lack of diffusive re-equilibration and underpins that fluid-assisted dissolution and recrystallization reactions are the main factors controlling the isotope record in these subduction-related metamorphic rocks. Blueschist-facies mylonites record deformation along the major shear zone that separates the eclogitic micaschists from the gneiss minuti. Two Rb-Sr isochrones that comprise several white mica fractions and glaucophane constrain the timing of this deformation and accompanying near-complete blueschist-facies re-equilibration of the Rb-Sr system to 60.1 ± 0.9 Ma and 60.9 ± 2.1 Ma, respectively. Overlapping ages in eclogitic micaschists of 60.1 ± 1.1 (Rb-Sr isochron of sheared matrix assemblage), 58.6 ± 0.8, and 60.9 ± 0.4 Ma (white mica 40Ar/39Ar inverse isochron ages) support the significance of this age and show that fluid-rock interaction and partial re-equilibration occurred as much as several kilometers away from the shear zone. An earlier equilibration during high-pressure conditions in the eclogitic micaschists is recorded in minimum Rb-Sr ages for relict assemblages (77.2 ± 0.8 and 72.4 ± 1.1 Ma) and an 40Ar/39Ar inverse isochron age of 75.4 ± 0.8 Ma for white mica cores, again demonstrating that the two isotope systems provide mutually supporting geochronological information. Local reactivation and recrystallization along the shear zone lasted >15 m.y., as late increments of deformation are recorded in a greenschist-facies mylonite by a Rb-Sr isochron age of 46.5 ± 0.7 Ma.
Interaction between magma and crustal carbonate at active arc volcanoes has recently been propose... more Interaction between magma and crustal carbonate at active arc volcanoes has recently been proposed as a source of atmospheric CO2 , in addition to CO2 released from the mantle and subducted oceanic crust. However, quantitative constraints on efficiency and timing of these processes are poorly established. Here, we present the first in situ carbon and oxygen isotope data of texturally distinct calcite in calc-silicate xenoliths from arc volcanics in a case study from Merapi volcano (Indonesia). Textures and C-O isotopic data provide unique evidence for decarbonation, magma-fluid interaction, and the generation of carbonate melts. We report extremely light δ13CPDB values down to −29.3‰ which are among the lowest reported in magmatic systems so far. Combined with the general paucity of relict calcite, these extremely low values demonstrate highly efficient remobilisation of crustal CO2 over geologically short timescales of thousands of years or less. this rapid release of large volumes of crustal CO2 may impact global carbon cycling. Crustal magma-carbonate interaction has been suggested as a process that may dominate the CO2 output in several volcanic arcs 1,2 and a possible source of magmatic carbonate melts 3-5. Direct evidence for this process often remains elusive, but the occurrence of calc-silicate (skarn) xenoliths in the eruptive products of some active volcanoes 6-9 provide a unique opportunity to study high temperature magma-carbonate interaction, and the subsequent effects on the host magmatic system. Recent work on such xenoliths has additionally linked magma-carbonate interaction to influencing eruptive dynamics via volatile exsolution 7,10-12 , and driving mag-matic differentiation trends towards highly desilicated potassic compositions 13,14. Carbon and oxygen isotopes are powerful tracers of fluid-rock interaction processes during contact meta-morphism of carbonates, where skarn rocks form via reactions between magmatic fluids and carbonate country rocks 15-18. Sedimentary carbonates are isotopically distinct from mantle-derived igneous rocks 15 , allowing for quantification of chemical exchange during magma-carbonate interaction. Decarbonation reactions release CO2 that is enriched in 13C and 18O into the magmatic system 19 , depleting the carbonate protolith in these isotopes, following batch or Rayleigh fractionation trends 15. However, some skarns show additional isotopic modifications that cannot result from equilibrium decarbonation reactions alone, but instead require mixing with magmatic fluids to produce values in near-exchange equilibrium with the adjacent magmatic system 16,20-22. The degree of decarbonation and CO2 released into the magmatic system and ultimately the atmosphere, and the extent of fluid modification of the isotopic signature, can be quantified utilising coupled carbon and oxygen isotopes. In addition , interaction with meteoric water and secondary alteration can usually be distinguished using this approach 16. Merapi is Indonesia's most active volcano, characterised by periods of lava dome growth punctuated by dome failure producing pyroclastic density currents, and intermittent explosive events 23. Compositionally the eruptive products are restricted to high-K basalt to basaltic andesite 23. The upper crust underlying Merapi consists of a 8-11 km thick unit of Cretaceous to Cenozoic limestones, marls and volcaniclastic deposits 24 ,
The mid-Proterozoic Isortoq dike swarm in the Gardar Province, South Greenland, comprises a varie... more The mid-Proterozoic Isortoq dike swarm in the Gardar Province, South Greenland, comprises a variety of alkaline rocks ranging from gabbroic to syenitic in composition. Major magmatic mineral phases are olivine, clinopyroxene, Fe -Ti oxides, amphibole, plagioclase and alkali feldspar. Quartz occurs in some samples as a late magmatic phase. Liquidus temperatures of olivine-bearing samples range between 1120 and 1145 jC and solidus temperatures are 850 -930 jC. Calculated silica activities are highly variable between 0.53 and unity. Oxygen fugacities vary from À 3 to + 1 log units relative to the fayalite -magnetite -quartz buffer.
In order to determine the effects of fluid–rock interaction on nitrogen elemental and isotopic
sy... more In order to determine the effects of fluid–rock interaction on nitrogen elemental and isotopic systematics in high-pressure metamorphic rocks, we investigated three different profiles representing three distinct scenarios of metasomatic overprinting. A profile from the Chinese Tianshan (ultra)high-pressure–low-temperature metamorphic belt represents a prograde, fluid-induced blueschist–eclogite transformation. This profile shows a systematic decrease in N concentrations from the host blueschist (~26 μg/g) via a blueschist–eclogite transition zone (19–23 μg/g) and an eclogitic selvage (12–16 μg/g) towards the former fluid pathway. Eclogites and blueschists show only a small variation in δ15Nair (+2.1 ± 0.3‰), but the systematic trend with distance is consistent with a batch devolatilization process. A second profile from the Tianshan represents a retrograde eclogite–blueschist transition. It shows increasing, but more scattered, N concentrations from the eclogite towards the blueschist and an unsystematic variation in δ15N values (δ15N = + 1.0 to +5.4‰). A third profile from the high-P/T metamorphic basement complex of the Southern Armorican Massif (Vendée, France) comprises a sequence from an eclogite lens via retrogressed eclogite and amphibolite into metasedimentary country rock gneisses. Metasedimentary gneisses have high N contents (14–52 μg/g) and positive δ15N values (+2.9 to +5.8‰), and N concentrations become lower away from the contact with 11–24 μg/g for the amphibolites, 10–14 μg/g for the retrogressed eclogite, and 2.1–3.6 μg/g for the pristine eclogite, which also has the lightest N isotopic compositions (δ15N = + 2.1 to +3.6‰). Overall, geochemical correlations demonstrate that phengitic white mica is the major host of N in metamorphosed mafic rocks. During fluid-induced metamorphic overprint, both abundances and isotopic composition of N are controlled by the stability and presence of white mica. Phengite breakdown in high-P/T metamorphic rocks can liberate significant amounts of N into the fluid. Due to the sensitivity of the N isotope system to a sedimentary signature, it can be used to trace the extent of N transport during metasomatic processes. The Vendée profile demonstrates that this process occurs over several tens of metres and affects both N concentrations and N isotopic compositions.
Trace and major element compositions of mid-Proterozoic (1.20-1.16 Ga) basaltic lava flows and di... more Trace and major element compositions of mid-Proterozoic (1.20-1.16 Ga) basaltic lava flows and dikes from the Gardar Province (South Greenland) provide evidence for two geochemically distinct magma sources. Based on distinct features of incompatible trace element ratios, such as Th/Ta, Th/Tb, or Th/Hf, they differ by the composition of their mantle source and by their partial melting trends. One mantle source is compositionally transitional between midocean ridge basalt (MORB)-type and ocean-island basalt (OIB)-type sources with relatively low Ta/Hf ratios (-0.2), moderate enrichment in light rare-earth elements (LREE), and slightly positive initial ε Nd values (+2). It can be attributed to either a lithospheric mantle source or a depleted astenospheric mantle plume component that has been enriched shortly prior to eruption. The other mantle source is characterized by high Ta/Hf ratios (-0.6), a more pronounced LREE enrichment, and initial ε Nd values around 0. Elevated Ce N /Yb N (7.0-9.8) and Tb N /Yb N ratios (1.6-1.8) of the rocks derived from this source indicate the presence of garnet during melting, suggesting melt generation at depths > 70 km. This mantle source has the geochemical characteristics of an OIB-type source and is interpreted as originating from a mantle plume. Samples from the slightly younger (1.14 Ga) Abitibi dike swarm (Superior Province, Canada), spatially connected to the Gardar Province, show very similar trace element characteristics and the same two distinct magma sources can be identified. The geochemical similarities between the magma sources in South Greenland and Canada support the idea of a genetic link between the two magmatic provinces. This link strengthens the idea that the system was a long-lived major intracontinental rift zone.
In situ UV laser spot 40 Ar/ 39 Ar analyses of distinct phengite types in eclogite-facies rocks f... more In situ UV laser spot 40 Ar/ 39 Ar analyses of distinct phengite types in eclogite-facies rocks from the Sesia-Lanzo Zone (Western Alps, Italy) were combined with SIMS boron isotope analyses as well as boron (B) and lithium (Li) concentration data to link geochronological information with constraints on fluid-rock interaction. In weakly deformed samples, apparent 40 Ar/ 39 Ar ages of phengite cores span a range of $20 Ma, but inverse isochrons define two distinct main high-pressure (HP) phengite core crystallization periods of 88-82 and 77-74 Ma, respectively. The younger cores have on average lower B contents ($36 lg/g) than the older ones ($43-48 lg/g), suggesting that loss of B and resetting of the Ar isotopic system were related. Phengite cores have variable d 11 B values (À18& to À10&), indicating the lack of km scale B homogenization during HP crystallization.
Abstract Harzburgitic xenoliths cut by pyroxenite veins from Avachinsky volcano, Kamchatka, are d... more Abstract Harzburgitic xenoliths cut by pyroxenite veins from Avachinsky volcano, Kamchatka, are derived from the sub-arc mantle and record element transfer from the slab to the arc. Olivine and orthopyroxene in the harzburgites have Li isotopic compositions (�� 7 Li=+ 2.8 to+ 5.6) comparable to estimates of the upper mantle (�� 7 Li~+ 4��2).
We apply new International Atomic Energy Agency guidelines to the assessment of volcanic hazards ... more We apply new International Atomic Energy Agency guidelines to the assessment of volcanic hazards at the proposed site of a nuclear power plant in Armenia (ANPP). Potential volcanic hazards at the ANPP site arise from its location near the base of the 70-km-diameter Aragats shield volcano, composed of high-K series trachy-basalt to trachy-dacite lavas, and low aspect ratio dacitic ignimbrites of Pliocene to mid-Pleistocene age. In addition, the site is located in the vicinity of a mid-Pleistocene monogenetic volcano cluster and in the region of potentially active composite volcanoes and calderas . Hazard analysis is based upon extensive geological, geophysical and volcanological investigations, including preparation of maps at 1:10000 and 1:100000 scales, drilling, and new geochemical, petrological and radiometric age (Ar-Ar) determinations. Probabilistic hazard assessments are done for numerous potential volcanic phenomena, including: lava flow inundation, tephra fallout, pyroclastic flows and probability of opening of new vents. Quantitative assessment shows the site may be inundated by lava flows from the monogenetic volcano cluster, and from pyroclastic eruptions of Aragats, should these long-dormant volcanic systems reactivate. Given the cessation of activity 490 ka ago, there is a 95% probability that the recurrence rate of volcanic activity is < 4e-6 per year, and the probability of inundation of the site by lavas is 2e-6 per year, or by pyroclastic flows is <1e-6 per year. Should these levels of hazard be judged acceptable for site development, a monitoring programme should be implemented and operational plan developed for ANPP response to potential volcanic hazards.
In situ 40Ar/39Ar dating of zoned and recrystallized phengite in metasomatized high-pressure rocks
In situ 40Ar/39Ar dating provides spatial resolution that is critical to understand the temporal ... more In situ 40Ar/39Ar dating provides spatial resolution that is critical to understand the temporal information in 40Ar/39Ar dates of metasomatized and partially recrystallized rocks as these dates can be combined with petrologic and geochemical information about the rock's history. Here, we evaluate the effects of deformation and fluid flux on 40Ar/39Ar dates by investigating a structural profile with strain- and recrystallization gradient across a major crustal shear zone that separates two tectonometamorphic units in the Sesia-Lanzo Zone (Western Alps). Weakly deformed, eclogite-facies samples contain phengites that show significant major element compositional differences between pristine cores and overprinted rims. In contrast, samples from the blueschist-facies shear zone contain fine-grained, mylonitic phengite. To test the hypothesis that increasing deformation and fluid flux promote Ar transmissivity, we compare in situ 40Ar/39Ar data and geochemical tracers of fluid flow (...
Tracing fluid-induced metasomatism across a blueschist-facies shear zone (Sesia-Lanzo zone, Western Alps) using boron isotopes
Boron (B) is a useful tracer for slab-mantle-arc fluid transport and recycling processes in subdu... more Boron (B) is a useful tracer for slab-mantle-arc fluid transport and recycling processes in subduction zones due to its high mobility in hydrous fluids and its significant isotope fractionation by fluid-rock interaction. Although B isotope geochemistry provides key evidence for slab to arc transfer by aqueous fluids, B isotope data on natural high-pressure metamorphic rocks are still scarce and the effects and extent of fluid-rock interaction remain poorly studied. Secondary ion mass spectrometry (SIMS) analyses generally confirm that slab dehydration significantly lowers δ11B of subducted oceanic crust and sediments, but the lack of a systematic relationship with peak metamorphic conditions suggests additional processes have affected the exhumed rocks. Here, we evaluate the effects of deformation and fluid flux on the B isotopic composition of phengitic mica along a structural profile with strain- and recrystallization gradient across a major crustal shear zone that separates two t...
Combined thermodynamic–geochemical modeling in metamorphic geology: Boron as tracer of fluid–rock interaction
Lithos, 2014
ABSTRACT Quantitative geochemical modeling is today applied in a variety of geological environmen... more ABSTRACT Quantitative geochemical modeling is today applied in a variety of geological environments from the petrogenesis of igneous rocks to radioactive waste disposal. In addition, the development of thermodynamic databases and computer programs to calculate equilibrium phase diagrams has greatly advanced our ability to model geodynamic processes. Combined with experimental data on elemental partitioning and isotopic fractionation, thermodynamic forward modeling unfolds enormous capacities that are far from exhausted.
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Papers by Ralf Halama
Metasedimentary rocks of the eclogitic micaschists unit contain phengite with step-like zoning in major element chemistry as evidence for petrologic disequilibrium. Distinct 40Ar/39Ar spot ages of relict phengite cores and overprinted rims demonstrate the preservation of individual age domains in the crystals. The eclogitic micaschists also show systematic Sr isotope disequilibria among different phengite populations, so that minimum ages of relict assemblage crystallization can be differentiated from the timing of late increments of deformation. The preservation of these disequilibrium features shows the lack of diffusive re-equilibration and underpins that fluid-assisted dissolution and recrystallization reactions are the main factors controlling the isotope record in these subduction-related metamorphic rocks.
Blueschist-facies mylonites record deformation along the major shear zone that separates the eclogitic micaschists from the gneiss minuti. Two Rb-Sr isochrones that comprise several white mica fractions and glaucophane constrain the timing of this deformation and accompanying near-complete blueschist-facies re-equilibration of the Rb-Sr system to 60.1 ± 0.9 Ma and 60.9 ± 2.1 Ma, respectively. Overlapping ages in eclogitic micaschists of 60.1 ± 1.1
(Rb-Sr isochron of sheared matrix assemblage), 58.6 ± 0.8, and 60.9 ± 0.4 Ma (white mica 40Ar/39Ar inverse isochron ages) support the significance of this age and show that fluid-rock interaction and partial re-equilibration occurred as much as several kilometers away from the shear zone. An earlier equilibration during high-pressure conditions in the eclogitic micaschists is recorded in minimum Rb-Sr ages for relict assemblages (77.2 ± 0.8 and 72.4 ± 1.1 Ma) and
an 40Ar/39Ar inverse isochron age of 75.4 ± 0.8 Ma for white mica cores, again demonstrating that the two isotope systems provide mutually supporting geochronological information. Local reactivation and recrystallization along the shear zone lasted >15 m.y., as late increments of deformation are recorded in a greenschist-facies mylonite by a Rb-Sr isochron age of 46.5 ± 0.7 Ma.
systematics in high-pressure metamorphic rocks, we investigated three different profiles representing
three distinct scenarios of metasomatic overprinting. A profile from the Chinese Tianshan
(ultra)high-pressure–low-temperature metamorphic belt represents a prograde, fluid-induced
blueschist–eclogite transformation. This profile shows a systematic decrease in N concentrations
from the host blueschist (~26 μg/g) via a blueschist–eclogite transition zone (19–23 μg/g) and an
eclogitic selvage (12–16 μg/g) towards the former fluid pathway. Eclogites and blueschists show
only a small variation in δ15Nair (+2.1 ± 0.3‰), but the systematic trend with distance is consistent
with a batch devolatilization process. A second profile from the Tianshan represents a retrograde
eclogite–blueschist transition. It shows increasing, but more scattered, N concentrations from the
eclogite towards the blueschist and an unsystematic variation in δ15N values (δ15N = + 1.0 to
+5.4‰). A third profile from the high-P/T metamorphic basement complex of the Southern
Armorican Massif (Vendée, France) comprises a sequence from an eclogite lens via retrogressed
eclogite and amphibolite into metasedimentary country rock gneisses. Metasedimentary gneisses
have high N contents (14–52 μg/g) and positive δ15N values (+2.9 to +5.8‰), and N concentrations
become lower away from the contact with 11–24 μg/g for the amphibolites, 10–14 μg/g for
the retrogressed eclogite, and 2.1–3.6 μg/g for the pristine eclogite, which also has the lightest N
isotopic compositions (δ15N = + 2.1 to +3.6‰).
Overall, geochemical correlations demonstrate that phengitic white mica is the major host of N
in metamorphosed mafic rocks. During fluid-induced metamorphic overprint, both abundances
and isotopic composition of N are controlled by the stability and presence of white mica. Phengite
breakdown in high-P/T metamorphic rocks can liberate significant amounts of N into the fluid.
Due to the sensitivity of the N isotope system to a sedimentary signature, it can be used to trace
the extent of N transport during metasomatic processes. The Vendée profile demonstrates that
this process occurs over several tens of metres and affects both N concentrations and N isotopic
compositions.
Metasedimentary rocks of the eclogitic micaschists unit contain phengite with step-like zoning in major element chemistry as evidence for petrologic disequilibrium. Distinct 40Ar/39Ar spot ages of relict phengite cores and overprinted rims demonstrate the preservation of individual age domains in the crystals. The eclogitic micaschists also show systematic Sr isotope disequilibria among different phengite populations, so that minimum ages of relict assemblage crystallization can be differentiated from the timing of late increments of deformation. The preservation of these disequilibrium features shows the lack of diffusive re-equilibration and underpins that fluid-assisted dissolution and recrystallization reactions are the main factors controlling the isotope record in these subduction-related metamorphic rocks.
Blueschist-facies mylonites record deformation along the major shear zone that separates the eclogitic micaschists from the gneiss minuti. Two Rb-Sr isochrones that comprise several white mica fractions and glaucophane constrain the timing of this deformation and accompanying near-complete blueschist-facies re-equilibration of the Rb-Sr system to 60.1 ± 0.9 Ma and 60.9 ± 2.1 Ma, respectively. Overlapping ages in eclogitic micaschists of 60.1 ± 1.1
(Rb-Sr isochron of sheared matrix assemblage), 58.6 ± 0.8, and 60.9 ± 0.4 Ma (white mica 40Ar/39Ar inverse isochron ages) support the significance of this age and show that fluid-rock interaction and partial re-equilibration occurred as much as several kilometers away from the shear zone. An earlier equilibration during high-pressure conditions in the eclogitic micaschists is recorded in minimum Rb-Sr ages for relict assemblages (77.2 ± 0.8 and 72.4 ± 1.1 Ma) and
an 40Ar/39Ar inverse isochron age of 75.4 ± 0.8 Ma for white mica cores, again demonstrating that the two isotope systems provide mutually supporting geochronological information. Local reactivation and recrystallization along the shear zone lasted >15 m.y., as late increments of deformation are recorded in a greenschist-facies mylonite by a Rb-Sr isochron age of 46.5 ± 0.7 Ma.
systematics in high-pressure metamorphic rocks, we investigated three different profiles representing
three distinct scenarios of metasomatic overprinting. A profile from the Chinese Tianshan
(ultra)high-pressure–low-temperature metamorphic belt represents a prograde, fluid-induced
blueschist–eclogite transformation. This profile shows a systematic decrease in N concentrations
from the host blueschist (~26 μg/g) via a blueschist–eclogite transition zone (19–23 μg/g) and an
eclogitic selvage (12–16 μg/g) towards the former fluid pathway. Eclogites and blueschists show
only a small variation in δ15Nair (+2.1 ± 0.3‰), but the systematic trend with distance is consistent
with a batch devolatilization process. A second profile from the Tianshan represents a retrograde
eclogite–blueschist transition. It shows increasing, but more scattered, N concentrations from the
eclogite towards the blueschist and an unsystematic variation in δ15N values (δ15N = + 1.0 to
+5.4‰). A third profile from the high-P/T metamorphic basement complex of the Southern
Armorican Massif (Vendée, France) comprises a sequence from an eclogite lens via retrogressed
eclogite and amphibolite into metasedimentary country rock gneisses. Metasedimentary gneisses
have high N contents (14–52 μg/g) and positive δ15N values (+2.9 to +5.8‰), and N concentrations
become lower away from the contact with 11–24 μg/g for the amphibolites, 10–14 μg/g for
the retrogressed eclogite, and 2.1–3.6 μg/g for the pristine eclogite, which also has the lightest N
isotopic compositions (δ15N = + 2.1 to +3.6‰).
Overall, geochemical correlations demonstrate that phengitic white mica is the major host of N
in metamorphosed mafic rocks. During fluid-induced metamorphic overprint, both abundances
and isotopic composition of N are controlled by the stability and presence of white mica. Phengite
breakdown in high-P/T metamorphic rocks can liberate significant amounts of N into the fluid.
Due to the sensitivity of the N isotope system to a sedimentary signature, it can be used to trace
the extent of N transport during metasomatic processes. The Vendée profile demonstrates that
this process occurs over several tens of metres and affects both N concentrations and N isotopic
compositions.