Pb < 2.0743) isotopes reflect the progressive drying of central Asia triggered by the westward re... more Pb < 2.0743) isotopes reflect the progressive drying of central Asia triggered by the westward retreat of the paleo-Tethys. Comparisons between the changes with time in the isotopically well-defined dust flux and Nd and Pb isotopic compositions of Pacific deep water allow one to draw two major conclusions: (1) dust-bound Nd became a resolvable contribution to Pacific seawater only after the one order of magnitude increase in dust flux starting at $3.5 Ma. Therefore eolian Nd was unimportant for Pacific seawater Nd prior to 3.5 Ma. (2) The lack of a response of Pacific deep water Pb to this huge flux increase suggests that dust-bound Pb has never been important. Instead, mobile Pb associated with island arc volcanic exhalatives probably consists of a significant contribution to Pacific deep water Pb and possibly to seawater elsewhere far away from landmasses.
The global supply of Mo and much of Cu and Au comes from porphyry-type ore deposits associated wi... more The global supply of Mo and much of Cu and Au comes from porphyry-type ore deposits associated with hydrous magmas of broadly calc-alkaline composition, thought to be generated by contemporaneous subduction zone processes. Molybdenum is generally considered to be derived from the continental crust while Cu and Au are sourced in the mantle wedge above subducting slabs. Here we show that neither contemporaneous subduction nor derivation of Mo from crustal sources is required to explain the genesis of porphyry-Cu-Mo-Au deposits on Proterozoic lithosphere in the eastern Rocky Mountains. Uniform Pb isotope ratios measured by LA-MC-ICP-MS in individual fluid inclusions from distinct Cu-Au and later Mo ore-forming stages at Bingham Canyon, USA, demonstrate a common metal source. Uranogenic Pb isotope ratios are particularly non-radiogenic (17.494 b 206 Pb/ 204 Pb b 17.534; 15.553 b 207 Pb/ 204 Pb b 15.588) and plot to the left of the geochron and above the mantle Pb evolution line. In 207 Pb/ 206 Pb vs. 208 Pb/ 206 Pb space, the fluid Pb isotope data cluster at the non-radiogenic end of a mixing line described by N 80 feldspar data from igneous rocks intimately associated with magmatic-hydrothermal ore formation, which extends to modern depleted mantle or upper crust. Forward Monte Carlo simulations require three events for the U-Th-Pb isotope evolution of the fluid: (1) Late Archean formation of enriched crust is followed by (2) preferential extraction of Pb from this aged crust into a subduction fluid characterized by drastically reduced U/Pb that metasomatized lithospheric mantle at ∼ 1.8 Ga. This mantle reservoir then evolved to produce the retarded uranogenic Pb isotope signatures of the Bingham Canyon Cu-Mo-Au deposit in the Cenozoic (3). Similarly retarded uranogenic Pb isotope data characterize the giant porphyry-Mo and Climax-type Mo deposits of Henderson, Questa, Butte, and SE Arizona that occur in Proterozoic sutures of the central and eastern Rocky Mountains. We propose that Cenozoic melting of subcontinental lithospheric mantle metasomatized by subduction fluids during early Proterozoic amalgamation of terranes to the Wyoming Craton provides the metal endowment and subduction flavour to the giant magmatic-hydrothermal Cu-Mo-Au ore deposits in western North America, which together constitute the world's major molybdenum ore province.
Of the isotopically distinctive mantle domains, the so-called HIMU ("high-μ"; μ = 238 U/ 204 Pb) ... more Of the isotopically distinctive mantle domains, the so-called HIMU ("high-μ"; μ = 238 U/ 204 Pb) source is the most extreme, and its genesis continues to be debated. We report very strong U enrichment at unchanged Th concentrations in Cretaceous oceanic serpentinites with exceptionally high 206 Pb/ 204 Pb (reaching 56) but unchanged 208 Pb/ 204 Pb. Similar, but less extreme, features are found in 1.9 billion years old altered oceanic crust (AOC). Forward modelling demonstrates that mantle, if metasomatised by supercritical liquids derived from AOC and serpentinites, evolves to the HIMU Pb isotope signatures, while satisfying experimental and empirical constraints on subduction zone element processing. By contrast, no model solutions for the conventional proposal of the HIMU source representing residual igneous altered oceanic crust can be reconciled with 208 Pb/ 204 Pb, strengthening the need for a paradigm shift regarding HIMU OIB genesis. Over time, the net U addition to the convecting mantle via deeply subducted serpentinite has expressed itself as the so-called second terrestrial Pb isotope paradox, or kappa conundrum.
Oxide minerals contained in ultramafic rocks are useful tools to assess the redox conditions of t... more Oxide minerals contained in ultramafic rocks are useful tools to assess the redox conditions of the rock and fluids liberated upon progressive serpentinite dehydration during subduction, as these minerals contain a relevant redox-sensitive element, iron. Previous studies have revealed that magnetite predominates across the antigorite-out reaction. However, the fate of magnetite and other oxides at higher pressure and temperature conditions has remained underexplored. We present a comprehensive petrological and geochemical study of oxide-sulfide-silicate mineral assemblages in metaperidotites beyond antigorite-and chlorite-out reactions (T = 650-850 °C and P = 1-3 GPa). Several ultramafic lenses, covering different bulk rock compositions and extents of oxidation upon oceanic serpentinization, were investigated from the Central Alps, Switzerland. Results point to two endmember scenarios: (i) Most frequently, metaperidotites have olivine with a Mg# of 89-91 (defined as molar Mg/(Mg + Fe tot) × 100) and contain low oxide modes (0.06-1.41 vol.%), hematite is absent, and redox conditions are weakly oxidized and buffered by orthopyroxene-olivine-magnetite. (ii) Rare occurrence, high olivine Mg# > 94.5 metaperidotites display coexisting hematite and magnetite, high oxide modes (up to 4 vol.%), and redox conditions are hematite-magnetite (HM) buffered (Δlog 10 fO 2 , QFM of + 3 to + 4). Spinel displays evolving compositions from magnetite over chromite to Al-Cr-spinel, roughly correlating with increasing temperature. Most of the samples buffered by the olivineorthopyroxene-magnetite assemblage contain coexisting pentlandite ± pyrrhotite, thus identifying stable sulfides beyond antigorite dehydration for these weakly oxidized samples (Δlog 10 fO 2 , QFM < 2.5). No sulfides were recognized in the highly oxidized sample. The transition of magnetite to chromite at around 700 °C goes along with a shift in fO 2 to lower values. At the prevailing oxygen fugacity in the weakly oxidized metaperidotites sulfur in a coexisting fluid is always present in its reduced form. However, oxidized sulfur can be stable in the dehydration fluids released from highly oxidized serpentinites.
Magnetite stability in ultramafic systems undergoing subduction plays a major role in controlling... more Magnetite stability in ultramafic systems undergoing subduction plays a major role in controlling redox states of the fluids liberated upon dehydration reactions, as well as of residual rocks. Despite their relevance for the evaluation of the redox conditions, the systematics and geochemistry of oxide minerals have remained poorly constrained in subducted ultramafic rocks. Here, we present a detailed petrological and geochemical study of magnetite in hydrous ultramafic rocks from Cerro del Almirez (Spain). Our results indicate that prograde to peak magnetite, ilmenite-hematite solid solution minerals, and sulfides coexist in both antigorite-serpentinite and chlorite-harzburgite at c. 670 C and 1Á6 GPa, displaying successive crystallization stages, each characterized by specific mineral compositions. In antigorite-serpentinite, magnetite inherited from seafloor hydration and recrystallized during subduction has moderate Cr (Cr 2 O 3 < 10 wt%) and low Al and V concentrations. In chlorite-harzburgite, polygonal magnetite is in textural equilibrium with olivine, orthopyroxene, chlorite, pentlandite, and ilmenite-hematite solid solution minerals. The Cr 2 O 3 contents of this magnetite are up to 19 wt%, higher than any magnetite data obtained for antigoriteserpentinite, along with higher Al and V, derived from antigorite breakdown, and lower Mn concentrations. This polygonal magnetite displays conspicuous core to rim zoning as recognized on elemental maps. Cr-V-Al-Fe 3þ mass-balance calculations, assuming conservative behavior of total Fe 3þ and Al, were employed to model magnetite compositions and modes in the partially dehydrated product chlorite-harzburgite starting from antigorite-serpentinite, as well as in the serpentinite protolith starting from the chlorite-harzburgite. The model results disagree with measured Cr and V compositions in magnetite from antigorite-serpentinites and chlorite-harzburgites. This indicates that these two rock types had different initial bulk compositions and thus cannot be directly compared. Our mass-balance analysis also reveals that new magnetite formation is required across the antigorite-breakdown reaction to account for the mass conservation of fluid-immobile elements such as Cr-V-Al-Fe 3þ. Complete recrystallization and formation of new magnetite in equilibrium with peak olivine (Mg# 89-91), chlorite (Mg# $95), orthopyroxene (Mg# 90-91), and pentlandite buffer the released fluid to redox conditions of $1 log unit above the quartz-fayalite-magnetite buffer. This is consistent with the observation that the Fe-Ti solid solution minerals (hemo-ilmenite and ilmeno-hematite) crystallized as homogeneous phases and exsolved upon exhumation and cooling. We conclude that antigorite-dehydration reaction fluids carry only a moderate redox budget and therefore may not be the only reason why the magmas are comparatively oxidized.
Serpentinites are central to water (re)cycling in subduction zones and thus effect fluid-mediated... more Serpentinites are central to water (re)cycling in subduction zones and thus effect fluid-mediated element transfer between the hydrosphere and the Earth's mantle and back into continental crust via calcalkaline magmatism. Diverse and often controversial models exist on the relevance of various source contributions to the budget of fluid mobile elements of hydrous peridotites and how these evolve during the subduction cycle. This work offers novel constraints on ongoing debates. We present a comprehensive bulk rock and silicate mineral major to trace element study covering the antigorite dehydration reaction based on Cerro del Almirez antigorite-serpentinites and chlorite-harzburgites, including the systematics of As, Sb, B, W, Li, In, Tl, Bi, Cd, and Sn-so far unavailable for Almirez, and there exist only few such data for orogenic serpentinites in general. We integrate these with reviewed literature data and develop a general model for the geochemical systematics of subducting hydrous slab mantle covering magmatic peridotite conditioning, element enrichment upon oceanic hydration, compositional evolution with progressive subduction to peak temperature antigorite dehydration, and retrograde metasomatism upon exhumation. Pre-hydration magmatic processes produce strong compositional variations on centimetre to metre to kilometre scales. Serpentinisation via seawater and sediment-equilibrated pore fluids produces highly variable fluidmobile element (FME) bulk rock enrichments in B, As, Sb, W, Cs, ±Li, ±Bi, ±Pb, ±U exceeding primitive mantle concentrations. Hydration enrichment numbers represent a novel concept introduced in this work to refine the extent of hydration-mediated FME enrichment. They represent the measured ratio of fluid-mobile element over a fluid-immobile element of closely comparable magmatic compatibility normalised to its corresponding primitive mantle abundance ratio. Hydration enrichment numbers are highest for Sb and As (up to 650) and lowest for Ba and Rb (down to 0.06) for Almirez data, quantifying fractions of minimal enrichment (values >1) and minimal prograde subduction loss (values <1). FME enrichment occurred primarily in ocean floor to trench to shallow forearc settings where sediment-equilibrated pore fluids are relevant, while addition from deeper sediment metamorphic dehydration fluids with progressive subduction is subordinate at best. Prominent fractions of As,
... Recharge altitude estimation of thermal springs using stable isotopes in MoroccoAnne Winckel... more ... Recharge altitude estimation of thermal springs using stable isotopes in MoroccoAnne Winckel, Christelle Marlin, Laurent Dever, Jean-Luc Morel ... 12, 2002 Fluid circulation at Stromboli volcano (Aeolian Islands, Italy) from self-potential and CO 2 surveysAnthony Finizola ...
The processes of carbon mobilisation at convergent plate boundaries are hotly debated. Recent fin... more The processes of carbon mobilisation at convergent plate boundaries are hotly debated. Recent findings suggest that carbon release along subduction geothermal gradients may well be more relevant than previously thought; however, it has remained difficult to achieve steady state atmospheric CO 2 conditions over geological time scales based on current volatile cycle models. Here, we report on meta-ophicarbonate rocks in the contact metamorphic aureole of the Bergell intrusion, Val Malenco, European Alps, that reached T-P conditions of at least 650°C at 0.35 GPa. We demonstrate by combined field evidence, geochemistry, and closed-system thermodynamic modelling that over 50% of the rock carbonate has been mobilised locally in response to reactive fluid flow upon progressive isobaric heating from 350 to >650°C. Despite complete mineral transformation at the antigorite + calcite devolatilisation reaction, the resulting tremolite-ophicarbonate preserves the original ophicarbonate texture by olivine-chlorite clasts, representing the former antigorite-serpentinite fraction, embedded in a monomineralic tremolite matrix formed from the calcite fraction that often exceeded 50 vol%. Closed system thermodynamic modelling based on an ophicarbonate composition of 80 wt% serpentinite +20 wt% calcite reveals that rock-buffered fluid X CO2 values evolve from <0.09 to as high as $0.16, and the total H 2 O-CO 2 fluid fraction released may be as high as 14 wt%; values that are highly sensitive to bulk ophicarbonate composition. Major to trace element geochemistry reveals a history of peridotite melt depletion followed by ocean floor hydration and ophicarbonate formation, consistent with the ocean continent transition (OCT) setting of the mantle rocks in the Mesozoic. This is demonstrated by positive anomalies in B, U, As, Sb, Bi, and W, and by bulk rock and notably carbonate primitive mantle normalised REE patterns that are basically identical to that of calcite precipitated from Jurassic seawater except for its negative Eu anomaly. Prograde metamorphic tremolite and diopside possess enrichments notably in Sr and REE inherited from reactant calcite, as is also supported by a good match between measured and modelled silicate mineral compositions. Specific geochemical characteristics of the prograde meta-ophicarbonate rocks imply fluid mediated, open system processes. Antigorite dehydration fluid ingress, likely produced in neighbouring antigorite-serpentinites, may have shifted bulk tremolite ophicarbonate rock compositions towards higher SiO 2 /MgO ratios along with an increase in B contents and depletion in fluid mobile elements such as As, Sb, and Sr. The massive, contact metamorphic CO 2 mobilisation documented here occurred at P-T conditions that are similar to those that can be achieved in collisional orogenic settings, whose clockwise P-T-t paths are often characterised by further heating upon initial decompression. They thus evolve at large angle to devolatilisation reactions and silicate-carbonate rock-buffered fluid X CO2 isopleths, thus fostering carbon mobilisation towards metamorphic peak temperatures at moderate to low pressures (in the region of between >700°C-1.0 GPa and >600°C-0.3 GPa). Barrovian metamorphism, too, can reach T-P conditions of 800°C-1 GPa. Amagmatic CO 2 mobilised this way will eventually reach the atmosphere via dispersion in the groundwater
Eclogites, metagabbros, and paragneisses from the Variscan Münchberg Massif record a complex succ... more Eclogites, metagabbros, and paragneisses from the Variscan Münchberg Massif record a complex succession of igneous, hydrothermal and metamorphic processes. The geodynamic setting related to the protolith formation and the impact of different types of fluid-rock interactions have been uncertain up to now. We use major and trace element chemistry as well as oxygen isotopes to disentangle the geochemical signatures related to the different stages of the rocks' history. In the Münchberg Massif, dark eclogites (kyanite-free; Fe-Ti-MORB signature) are distinguished from light eclogites (kyanite-bearing; higher Mg#, Al 2 O 3 , and Cr; lower incompatible element contents; positive Eu anomalies; MORB to arc basalt signature). The δ 18 O values for both types (+5.0 to +10.8‰) are equal to, or higher than those of MORB. Amphibolite facies metagabbros have a more enriched, almost OIB-like trace element signature and high δ 18 O values (+9.4 to +10.3‰). Good linear correlations between fluid-immobile elements throughout the eclogite types confirm their derivation from a common, N-MORB to E-MORB-like parental magma. We interpret the light eclogites as former plagioclase-rich cumulates and the dark eclogites as their complementary differentiates. This relationship is partly obscured by variable degrees of magma contamination by sediments, which also affected the metagabbros. However, the metagabbros originated from a more enriched mantle source than the eclogites. Following intrusion, the eclogites were subjected to hydrothermal alteration under the influence of seawater, as indicated by positive correlations between Li, B, Sb, and δ 18 O. Metamorphic fluid-rock interactions appear to be mostly of limited extent, probably due to the lack of lawsonite dehydration as a fluid source. Nevertheless, the contents at least of some fluid-mobile elements, such as LILE, Li, and Pb, were probably modified during the subductionexhumation cycle of the eclogites. The crustal contamination of the protolith magmas argues against derivation of the eclogites and metagabbros from typical oceanic crust. Instead, a rift-drift transition setting related to the opening of the Rheic or Saxothuringian Ocean seems most likely. The eclogites and metagabbros, alongside with similar rocks in the Mariánské Lázně complex and other resembling high-pressure massifs, may record different stages of this rift-drift transition.
The large-diameter piston core LL44-GPC3 from the central North Pacific Ocean records continuous ... more The large-diameter piston core LL44-GPC3 from the central North Pacific Ocean records continuous sedimentation of eolian dust since the Late Cretaceous. Two intervals resolved by Nd and Pb isotopic data relate to dust coming from America (prior to ~40 Ma) and dust coming from Asia (since ~40 Ma). The Intertropical Convergence Zone (ITCZ) separates these depositional regimes today and may have been at a paleolatitude of ~23°N prior to 40 Ma. Such a northerly location of the ITCZ is consistent with sluggish atmospheric circulation and warm climate for the Northern Hemisphere of the early to middle Eocene. Since ~40 Ma, correlations between Nd (~7.55 > epsilon-Nd(t) > ~10.81) and Pb (18.625 < 206/4Pb < 18.879; 15.624 < 207/4Pb < 15.666; 38.611 < 208/4Pb < 38.960; 0.8294 < 207/6Pb < 0.8389; 2.0539 < 208/6Pb < 2.0743) isotopes reflect the progressive drying of central Asia triggered by the westward retreat of the paleo-Tethys. Comparisons between the c...
15 Anisotropy of magnetic susceptibility (AMS) is often used as a proxy for mineral fabric in 16 ... more 15 Anisotropy of magnetic susceptibility (AMS) is often used as a proxy for mineral fabric in 16 deformed rocks. In order to do so quantitatively, it is necessary to quantify the intrinsic 17 magnetic anisotropy of single crystals of rock-forming minerals. Amphiboles are common in 18 mafic igneous and metamorphic rocks and often define rock texture due to their general 19 prismatic crystal habits. Amphiboles may dominate the magnetic anisotropy in intermediate to 20 felsic igneous rocks and in some metamorphic rock types, because they have a high Fe 21 concentration and they can develop a strong crystallographic preferred orientation. In this 22 study, the AMS is characterized in 28 single crystals and one crystal aggregate of 23 compositionally diverse clinoand ortho-amphiboles. High-field methods were used to isolate 24 the paramagnetic component of the anisotropy, which is unaffected by ferromagnetic 25 inclusions that often occur in amphibole crystals. Laue imaging, laser ablati...
Selected Conclusions 2 Combining the results with halogen [2] and noble gas data [3] suggests tha... more Selected Conclusions 2 Combining the results with halogen [2] and noble gas data [3] suggests that serpentinisation of oceanic lithospheric mantle occurred along bend faults or as detached slices in the shallow forearc by fluids equilibrated within the accretionary prism 2 Initial heterogeneities will govern the spatial extent of serpentinisation and FME enrichment, ultimately amplifying heterogeneities 2 FME enrichments from serpentinisation are largely retained and reincorporated into the convecting mantle, providing potential fractionation mechanisms for U-Th systematics (HIMU) and enriched mantle (EM) sources 2 FMEs such as As, Sb, W and Bi, which are rarely quantified, can provide essential information to further discriminate between serpentinisation environments
Vielleicht haben Sie gerade Licht eingeschaltet, um diesen Artikel zu lesen. Vielleicht geistern ... more Vielleicht haben Sie gerade Licht eingeschaltet, um diesen Artikel zu lesen. Vielleicht geistern die letzten Nachrichten betreffend des steigenden Ölpreises noch in ihrem Hinterkopf umher. Nicht unerschöpflich ist die Quelle fossiler Brennstoffe; erneuerbar ist nur die Wasserkraft, predigt die hiesige Energielobby. Seit dem Altertum basieren verblüffend viele menschliche Errungenschaften auf metallischen Rohstoffen: Eine Aufgabe der Geologen ist es, die zukünftige Versorgung unserer Gesellschaft mit essenziellen Metallen zu sichern.
An efficient, clean procedure for bulk rock major to trace element analysis by 193 nm Excimer LA-... more An efficient, clean procedure for bulk rock major to trace element analysis by 193 nm Excimer LA-ICP-MS analysis of nanoparticulate pressed powder pellets (PPPs) employing a binder is presented. Sample powders are milled in water suspension in a planetary ball mill, reducing average grain size by about one order of magnitude compared to common dry milling protocols. Microcrystalline cellulose (MCC) is employed as a binder, improving the mechanical strength of the PPP and the ablation behaviour, because MCC absorbs 193 nm laser light well. Use of MCC binder allows for producing cohesive pellets of materials that cannot be pelletized in their pure forms, such as quartz powder. Rigorous blank quantification was performed on synthetic quartz treated like rock samples, demonstrating that procedural blanks are irrelevant except for a few elements at the 10 ng g−1 concentration level. The LA-ICP-MS PPP analytical procedure was optimised and evaluated using six different SRM powders (JP-1, ...
Serpentinites are central to carry water and fluid-mobile elements down subduction zones. The bre... more Serpentinites are central to carry water and fluid-mobile elements down subduction zones. The breakdown of antigorite represents the most prominent aqueous fluid release, boosting fluid-mediated element cycling from the slab to the mantle wedge. At Cerro del Almirez, Spain, an antigorite dehydration reaction front is preserved in subducted serpentinites. Bulk rock and mineral major formed from the reaction of brucite with antigorite (Ol-1a) or from the reaction of diopside and antigorite (Ol-1b; Fig. 2). Atg-1 forms the variably foliated rock matrix, and larger antigorite blades are less well oriented and may grow over inclusion-rich olivine or clinopyroxene but not tremolite. Dusty Cpx-1 (Fig. 3a) is the only relic mantle mineral, showing occasional magnetite lamellae. Prograde Cpx-2 forms clear rims on Cpx-1 or isolated clear neoblasts. Euhedral Tr-1 may overgrow clinopyroxene (Fig. 3b) or is rarely present as up to 700 µm sized rhombohedral, unoriented crystals that may grow over Ol-1 and the Atg-1 matrix. Chlorite occurs either as a fine-grained felt associated with magnetite (Chl-1, <0.5 vol%) or as inclusions in recrystallized relic clinopyroxene (Chl-2). Ti-chu-1 forms small grains dispersed throughout the matrix. Opaque phases include Cr-rich magnetite (up to 500 µm), subordinate Fe-Ni sulphide, and a Fe-Ti-oxide. Orthopyroxene crystals (Padrón-Navarta et al., 2011) were not observed in our Atg-serpentinite samples. Chlorite serpentinite and Atg-Chl-Opx-Ol rock: These lithologies characterise the cm to m thick, irregular reaction front between Atg-serpentinites and Chl-harzburgites and document the progressive replacement of antigorite from Atg-serpentinite by antigorite + chlorite, followed by growth of orthopyroxene and olivine until complete antigorite exhaustion. Contacts to the neighbouring rocks are gradational at cm to dm scale and oblique to the regional foliation. Chl-serpentinite represents the recrystallisation product of foliated Atg-1 from Atg-serpentinite into coarser grained and randomly oriented Atg-2 and Chl-3. The most conspicuous features are (i) euhedral Chl-3 flakes of up to 1 mm, topotactically overgrown by Atg-2 (Fig. 3c and d), and (ii) the absence of clinopyroxene. Tr-1 occurs either as euhedral crystals containing microscopic inclusions of olivine, or interstitially between Chl-3-Atg-2 intergrowths. Ol-2 forms mm-sized aggregates of individual, clear crystals. Atg-2 grain boundaries against talc are irregular. Atg-Chl-Opx-Ol rock (i) represents the appearance of orthopyroxene (Opx-1), (ii) has abundant flaky Chl-3 not overgrown by antigorite, (iii) is characterised by reduction in Atg-content to below 10 vol% along with disappearance of fine-grained Atg-1, and (iv) often shows the development of a granular texture similar to that of the granular Chl-harzburgite (see below). Opx-1 is either granular-anhedral and associated with granular-anhedral olivine (Ol-2/Ol-3, Fig. 3e), or Opx-1 forms radial aggregates of thin laths. Olivine is granular, colourless (Ol-2), and may have subordinate brown rims (Ol-3). Rarely, brown olivine grains (Ol-3) can also be observed, and colourless rims (Ol-4) on brown Ol-3 may also 8 occur. Unoriented Chl-3 flakes not overgrown by antigorite are distributed throughout the rock, while the Chl-3-Atg-2 intergrowths have a higher chlorite to antigorite ratio compared to those of the Chlserpentinite. Few, randomly oriented antigorite blades (<1.25 mm) grow over chlorite, olivine and orthopyroxene. Atg-2 and Opx-1 grain boundaries against talc are irregular. Chlorite harzburgite: Chl-harzburgite occurs as a granular or a spinifex-textured rock (Schönbächler, 1999; Trommsdorff et al., 1998; Padrón-Navarta et al., 2011). Both share an identical mineralogy with only slightly variable modal abundances (Table 1). Their spatial occurrence relative to the contact with Atg-serpentinite is not systematic except that granular rocks are more abundant near the contact to Atg-serpentinites. In fact, granular Chl-harzburgite is texturally very similar to the transitional Atg-Chl-Opx-Ol rock. Transitions between granular and spinifex-textured Chl-harzburgites can be as short as a few centimetres. Rock-forming minerals are randomly oriented. Brown elongated olivine (up to 10 cm long) predominates in the spinifex-textured Chl-harzburgite, while granular olivine in the granular Chl-harzburgite is exclusively colourless. Opx-1, Chl-3, and magnetite fill the interstices between these olivine crystals. Olivine in variable modal abundances occurs in three growth stages. Clear Ol-2 sometimes form cores of brown Ol-3 (Fig. 3f) that itself may be overgrown by a clear Ol-4 rim. Clear small isometric Ol-4 crystals showing equilibrium texture may also occur in the matrix (Fig. 3f). Clinopyroxene is absent, and Ti-clinohumite is very rare in our samples. Rare tremolite displays equilibrium textures with Chl-3 (Fig. 3h) or overgrows brown Ol-3 or Opx-1. Inclusions of orthopyroxene and olivine in euhedral tremolite were confirmed by RAMAN spectroscopy. Retrograde talc variably replaces orthopyroxene (e.g. Fig. 3g). Magnetite occurrence and contents: Magnetite modes are higher for Atg-serpentinites (2.8 vol%) than for Chl-harzburgites (1.4 vol%); however, variations within a given rock type are prominent (Atgserpentinites: 2-4 vol%; Chl-harzburgites: 0.4-2 vol%). Modes of opaque minerals determined by point counting (Table 1) thereby reflect mostly magnetite for Atg-serpentinites, while the fraction of sulphides in the opaque part can amount to some 30% in Chl-harzburgites. Texturally, magnetite is overgrown by Atg-1 and Ol-1 in Atg-serpentinites (Fig. 4a), testifying to its crystallisation upon ocean floor hydration (Fig. 2). Granular magnetite in Chl-harzburgite (Fig. 4b) commonly displays sharp and even grain boundaries and is also included in Chl-3 and Ol-3 (and Opx-1; not illustrated) resulting from the antigorite breakdown reaction. This equilibrium texture suggests that ocean floor magnetite 9 may have partially equilibrated upon antigorite breakdown, possibly in association with Fe-sulphide crystallisation. Traces of other oxides (not further investigated) were also observed (compare Debret et al., 2015). Variations in magnetite modes between Atg-serpentinites and Chl-harzburgites at Almirez were also recognized by other studies (Debret et al., 2015; Marchesi et al., 2013; Padrón-Navarta et al., 2011). 3.2. Bulk rock major element concentrations Major element data acquired for 15 bulk rock samples that cover all lithotypes across the antigoriteout reaction are reported in Supplementary Table 1, employing the methods explained in the Supplementary Material 1. Measured loss on ignition (LOI) values determine bulk rock water contents (subordinate carbonate cannot be excluded), which decrease across the antigorite-out reaction from 11.3 wt% H 2 O in Atg-serpentinites typical for complete hydration down to 4.1 wt% H 2 O in Chlharzburgites. Bulk rock major element systematics reveal the combined effects of (i) magmatic mantle history, (ii) oceanic metasomatism, and (iii) partial metamorphic dehydration, of which the magmatic mantle history exerts a dominant control. The mantle rocks are variably refractory and narrowly range in Mg# from 87.7 to 90.9 (all Fe as FeO), typical for upper mantle rocks. In a plot of Al 2 O 3 /SiO 2 vs. MgO/SiO 2 (Fig. 5a), the samples follow the trend of residual mantle rocks following melt extraction. Some of the samples possess variably low MgO/SiO 2 for their given Al 2 O 3 /SiO 2 (compare Marchesi et al., 2013) that is typical for abyssal serpentinized peridotites, indicating either addition of SiO 2 or removal of Mg (and Fe) that may have balanced antigorite growth at the expense of chrysotile (e.g., Kodolányi and Pettke, 2011). Alternatively, this may reflect MgO loss during low temperature seafloor alteration (e.g., Niu, 2004). Al 2 O 3 /SiO 2 ratios are considered to remain unmodified during serpentinization (e.g., Niu, 2004). Alumina concentrations tend to be low, resulting in Al 2 O 3 /SiO 2 ratios that are typical for abyssal peridotites (0.01 < Al 2 O 3 /SiO 2 < 0.07; Niu, 2004) but higher than most mantle wedge serpentinites (Al 2 O 3 /SiO 2 < 0.03; Deschamps et al., 2013). Despite significant amounts of retrograde talc present in partial to complete antigorite breakdown product rocks, bulk rock SiO 2 concentrations do not show an absolute enrichment relative to the peridotite melting residue trend. Al 2 O 3 and SiO 2 show a positive trend across the lithotypes from Atg-serpentinites to Chl-harzburgites, both Al 2 O 3 and SiO 2 being higher in Chl-harzburgites compared to all other lithologies. FeO tot
Pb < 2.0743) isotopes reflect the progressive drying of central Asia triggered by the westward re... more Pb < 2.0743) isotopes reflect the progressive drying of central Asia triggered by the westward retreat of the paleo-Tethys. Comparisons between the changes with time in the isotopically well-defined dust flux and Nd and Pb isotopic compositions of Pacific deep water allow one to draw two major conclusions: (1) dust-bound Nd became a resolvable contribution to Pacific seawater only after the one order of magnitude increase in dust flux starting at $3.5 Ma. Therefore eolian Nd was unimportant for Pacific seawater Nd prior to 3.5 Ma. (2) The lack of a response of Pacific deep water Pb to this huge flux increase suggests that dust-bound Pb has never been important. Instead, mobile Pb associated with island arc volcanic exhalatives probably consists of a significant contribution to Pacific deep water Pb and possibly to seawater elsewhere far away from landmasses.
The global supply of Mo and much of Cu and Au comes from porphyry-type ore deposits associated wi... more The global supply of Mo and much of Cu and Au comes from porphyry-type ore deposits associated with hydrous magmas of broadly calc-alkaline composition, thought to be generated by contemporaneous subduction zone processes. Molybdenum is generally considered to be derived from the continental crust while Cu and Au are sourced in the mantle wedge above subducting slabs. Here we show that neither contemporaneous subduction nor derivation of Mo from crustal sources is required to explain the genesis of porphyry-Cu-Mo-Au deposits on Proterozoic lithosphere in the eastern Rocky Mountains. Uniform Pb isotope ratios measured by LA-MC-ICP-MS in individual fluid inclusions from distinct Cu-Au and later Mo ore-forming stages at Bingham Canyon, USA, demonstrate a common metal source. Uranogenic Pb isotope ratios are particularly non-radiogenic (17.494 b 206 Pb/ 204 Pb b 17.534; 15.553 b 207 Pb/ 204 Pb b 15.588) and plot to the left of the geochron and above the mantle Pb evolution line. In 207 Pb/ 206 Pb vs. 208 Pb/ 206 Pb space, the fluid Pb isotope data cluster at the non-radiogenic end of a mixing line described by N 80 feldspar data from igneous rocks intimately associated with magmatic-hydrothermal ore formation, which extends to modern depleted mantle or upper crust. Forward Monte Carlo simulations require three events for the U-Th-Pb isotope evolution of the fluid: (1) Late Archean formation of enriched crust is followed by (2) preferential extraction of Pb from this aged crust into a subduction fluid characterized by drastically reduced U/Pb that metasomatized lithospheric mantle at ∼ 1.8 Ga. This mantle reservoir then evolved to produce the retarded uranogenic Pb isotope signatures of the Bingham Canyon Cu-Mo-Au deposit in the Cenozoic (3). Similarly retarded uranogenic Pb isotope data characterize the giant porphyry-Mo and Climax-type Mo deposits of Henderson, Questa, Butte, and SE Arizona that occur in Proterozoic sutures of the central and eastern Rocky Mountains. We propose that Cenozoic melting of subcontinental lithospheric mantle metasomatized by subduction fluids during early Proterozoic amalgamation of terranes to the Wyoming Craton provides the metal endowment and subduction flavour to the giant magmatic-hydrothermal Cu-Mo-Au ore deposits in western North America, which together constitute the world's major molybdenum ore province.
Of the isotopically distinctive mantle domains, the so-called HIMU ("high-μ"; μ = 238 U/ 204 Pb) ... more Of the isotopically distinctive mantle domains, the so-called HIMU ("high-μ"; μ = 238 U/ 204 Pb) source is the most extreme, and its genesis continues to be debated. We report very strong U enrichment at unchanged Th concentrations in Cretaceous oceanic serpentinites with exceptionally high 206 Pb/ 204 Pb (reaching 56) but unchanged 208 Pb/ 204 Pb. Similar, but less extreme, features are found in 1.9 billion years old altered oceanic crust (AOC). Forward modelling demonstrates that mantle, if metasomatised by supercritical liquids derived from AOC and serpentinites, evolves to the HIMU Pb isotope signatures, while satisfying experimental and empirical constraints on subduction zone element processing. By contrast, no model solutions for the conventional proposal of the HIMU source representing residual igneous altered oceanic crust can be reconciled with 208 Pb/ 204 Pb, strengthening the need for a paradigm shift regarding HIMU OIB genesis. Over time, the net U addition to the convecting mantle via deeply subducted serpentinite has expressed itself as the so-called second terrestrial Pb isotope paradox, or kappa conundrum.
Oxide minerals contained in ultramafic rocks are useful tools to assess the redox conditions of t... more Oxide minerals contained in ultramafic rocks are useful tools to assess the redox conditions of the rock and fluids liberated upon progressive serpentinite dehydration during subduction, as these minerals contain a relevant redox-sensitive element, iron. Previous studies have revealed that magnetite predominates across the antigorite-out reaction. However, the fate of magnetite and other oxides at higher pressure and temperature conditions has remained underexplored. We present a comprehensive petrological and geochemical study of oxide-sulfide-silicate mineral assemblages in metaperidotites beyond antigorite-and chlorite-out reactions (T = 650-850 °C and P = 1-3 GPa). Several ultramafic lenses, covering different bulk rock compositions and extents of oxidation upon oceanic serpentinization, were investigated from the Central Alps, Switzerland. Results point to two endmember scenarios: (i) Most frequently, metaperidotites have olivine with a Mg# of 89-91 (defined as molar Mg/(Mg + Fe tot) × 100) and contain low oxide modes (0.06-1.41 vol.%), hematite is absent, and redox conditions are weakly oxidized and buffered by orthopyroxene-olivine-magnetite. (ii) Rare occurrence, high olivine Mg# > 94.5 metaperidotites display coexisting hematite and magnetite, high oxide modes (up to 4 vol.%), and redox conditions are hematite-magnetite (HM) buffered (Δlog 10 fO 2 , QFM of + 3 to + 4). Spinel displays evolving compositions from magnetite over chromite to Al-Cr-spinel, roughly correlating with increasing temperature. Most of the samples buffered by the olivineorthopyroxene-magnetite assemblage contain coexisting pentlandite ± pyrrhotite, thus identifying stable sulfides beyond antigorite dehydration for these weakly oxidized samples (Δlog 10 fO 2 , QFM < 2.5). No sulfides were recognized in the highly oxidized sample. The transition of magnetite to chromite at around 700 °C goes along with a shift in fO 2 to lower values. At the prevailing oxygen fugacity in the weakly oxidized metaperidotites sulfur in a coexisting fluid is always present in its reduced form. However, oxidized sulfur can be stable in the dehydration fluids released from highly oxidized serpentinites.
Magnetite stability in ultramafic systems undergoing subduction plays a major role in controlling... more Magnetite stability in ultramafic systems undergoing subduction plays a major role in controlling redox states of the fluids liberated upon dehydration reactions, as well as of residual rocks. Despite their relevance for the evaluation of the redox conditions, the systematics and geochemistry of oxide minerals have remained poorly constrained in subducted ultramafic rocks. Here, we present a detailed petrological and geochemical study of magnetite in hydrous ultramafic rocks from Cerro del Almirez (Spain). Our results indicate that prograde to peak magnetite, ilmenite-hematite solid solution minerals, and sulfides coexist in both antigorite-serpentinite and chlorite-harzburgite at c. 670 C and 1Á6 GPa, displaying successive crystallization stages, each characterized by specific mineral compositions. In antigorite-serpentinite, magnetite inherited from seafloor hydration and recrystallized during subduction has moderate Cr (Cr 2 O 3 < 10 wt%) and low Al and V concentrations. In chlorite-harzburgite, polygonal magnetite is in textural equilibrium with olivine, orthopyroxene, chlorite, pentlandite, and ilmenite-hematite solid solution minerals. The Cr 2 O 3 contents of this magnetite are up to 19 wt%, higher than any magnetite data obtained for antigoriteserpentinite, along with higher Al and V, derived from antigorite breakdown, and lower Mn concentrations. This polygonal magnetite displays conspicuous core to rim zoning as recognized on elemental maps. Cr-V-Al-Fe 3þ mass-balance calculations, assuming conservative behavior of total Fe 3þ and Al, were employed to model magnetite compositions and modes in the partially dehydrated product chlorite-harzburgite starting from antigorite-serpentinite, as well as in the serpentinite protolith starting from the chlorite-harzburgite. The model results disagree with measured Cr and V compositions in magnetite from antigorite-serpentinites and chlorite-harzburgites. This indicates that these two rock types had different initial bulk compositions and thus cannot be directly compared. Our mass-balance analysis also reveals that new magnetite formation is required across the antigorite-breakdown reaction to account for the mass conservation of fluid-immobile elements such as Cr-V-Al-Fe 3þ. Complete recrystallization and formation of new magnetite in equilibrium with peak olivine (Mg# 89-91), chlorite (Mg# $95), orthopyroxene (Mg# 90-91), and pentlandite buffer the released fluid to redox conditions of $1 log unit above the quartz-fayalite-magnetite buffer. This is consistent with the observation that the Fe-Ti solid solution minerals (hemo-ilmenite and ilmeno-hematite) crystallized as homogeneous phases and exsolved upon exhumation and cooling. We conclude that antigorite-dehydration reaction fluids carry only a moderate redox budget and therefore may not be the only reason why the magmas are comparatively oxidized.
Serpentinites are central to water (re)cycling in subduction zones and thus effect fluid-mediated... more Serpentinites are central to water (re)cycling in subduction zones and thus effect fluid-mediated element transfer between the hydrosphere and the Earth's mantle and back into continental crust via calcalkaline magmatism. Diverse and often controversial models exist on the relevance of various source contributions to the budget of fluid mobile elements of hydrous peridotites and how these evolve during the subduction cycle. This work offers novel constraints on ongoing debates. We present a comprehensive bulk rock and silicate mineral major to trace element study covering the antigorite dehydration reaction based on Cerro del Almirez antigorite-serpentinites and chlorite-harzburgites, including the systematics of As, Sb, B, W, Li, In, Tl, Bi, Cd, and Sn-so far unavailable for Almirez, and there exist only few such data for orogenic serpentinites in general. We integrate these with reviewed literature data and develop a general model for the geochemical systematics of subducting hydrous slab mantle covering magmatic peridotite conditioning, element enrichment upon oceanic hydration, compositional evolution with progressive subduction to peak temperature antigorite dehydration, and retrograde metasomatism upon exhumation. Pre-hydration magmatic processes produce strong compositional variations on centimetre to metre to kilometre scales. Serpentinisation via seawater and sediment-equilibrated pore fluids produces highly variable fluidmobile element (FME) bulk rock enrichments in B, As, Sb, W, Cs, ±Li, ±Bi, ±Pb, ±U exceeding primitive mantle concentrations. Hydration enrichment numbers represent a novel concept introduced in this work to refine the extent of hydration-mediated FME enrichment. They represent the measured ratio of fluid-mobile element over a fluid-immobile element of closely comparable magmatic compatibility normalised to its corresponding primitive mantle abundance ratio. Hydration enrichment numbers are highest for Sb and As (up to 650) and lowest for Ba and Rb (down to 0.06) for Almirez data, quantifying fractions of minimal enrichment (values >1) and minimal prograde subduction loss (values <1). FME enrichment occurred primarily in ocean floor to trench to shallow forearc settings where sediment-equilibrated pore fluids are relevant, while addition from deeper sediment metamorphic dehydration fluids with progressive subduction is subordinate at best. Prominent fractions of As,
... Recharge altitude estimation of thermal springs using stable isotopes in MoroccoAnne Winckel... more ... Recharge altitude estimation of thermal springs using stable isotopes in MoroccoAnne Winckel, Christelle Marlin, Laurent Dever, Jean-Luc Morel ... 12, 2002 Fluid circulation at Stromboli volcano (Aeolian Islands, Italy) from self-potential and CO 2 surveysAnthony Finizola ...
The processes of carbon mobilisation at convergent plate boundaries are hotly debated. Recent fin... more The processes of carbon mobilisation at convergent plate boundaries are hotly debated. Recent findings suggest that carbon release along subduction geothermal gradients may well be more relevant than previously thought; however, it has remained difficult to achieve steady state atmospheric CO 2 conditions over geological time scales based on current volatile cycle models. Here, we report on meta-ophicarbonate rocks in the contact metamorphic aureole of the Bergell intrusion, Val Malenco, European Alps, that reached T-P conditions of at least 650°C at 0.35 GPa. We demonstrate by combined field evidence, geochemistry, and closed-system thermodynamic modelling that over 50% of the rock carbonate has been mobilised locally in response to reactive fluid flow upon progressive isobaric heating from 350 to >650°C. Despite complete mineral transformation at the antigorite + calcite devolatilisation reaction, the resulting tremolite-ophicarbonate preserves the original ophicarbonate texture by olivine-chlorite clasts, representing the former antigorite-serpentinite fraction, embedded in a monomineralic tremolite matrix formed from the calcite fraction that often exceeded 50 vol%. Closed system thermodynamic modelling based on an ophicarbonate composition of 80 wt% serpentinite +20 wt% calcite reveals that rock-buffered fluid X CO2 values evolve from <0.09 to as high as $0.16, and the total H 2 O-CO 2 fluid fraction released may be as high as 14 wt%; values that are highly sensitive to bulk ophicarbonate composition. Major to trace element geochemistry reveals a history of peridotite melt depletion followed by ocean floor hydration and ophicarbonate formation, consistent with the ocean continent transition (OCT) setting of the mantle rocks in the Mesozoic. This is demonstrated by positive anomalies in B, U, As, Sb, Bi, and W, and by bulk rock and notably carbonate primitive mantle normalised REE patterns that are basically identical to that of calcite precipitated from Jurassic seawater except for its negative Eu anomaly. Prograde metamorphic tremolite and diopside possess enrichments notably in Sr and REE inherited from reactant calcite, as is also supported by a good match between measured and modelled silicate mineral compositions. Specific geochemical characteristics of the prograde meta-ophicarbonate rocks imply fluid mediated, open system processes. Antigorite dehydration fluid ingress, likely produced in neighbouring antigorite-serpentinites, may have shifted bulk tremolite ophicarbonate rock compositions towards higher SiO 2 /MgO ratios along with an increase in B contents and depletion in fluid mobile elements such as As, Sb, and Sr. The massive, contact metamorphic CO 2 mobilisation documented here occurred at P-T conditions that are similar to those that can be achieved in collisional orogenic settings, whose clockwise P-T-t paths are often characterised by further heating upon initial decompression. They thus evolve at large angle to devolatilisation reactions and silicate-carbonate rock-buffered fluid X CO2 isopleths, thus fostering carbon mobilisation towards metamorphic peak temperatures at moderate to low pressures (in the region of between >700°C-1.0 GPa and >600°C-0.3 GPa). Barrovian metamorphism, too, can reach T-P conditions of 800°C-1 GPa. Amagmatic CO 2 mobilised this way will eventually reach the atmosphere via dispersion in the groundwater
Eclogites, metagabbros, and paragneisses from the Variscan Münchberg Massif record a complex succ... more Eclogites, metagabbros, and paragneisses from the Variscan Münchberg Massif record a complex succession of igneous, hydrothermal and metamorphic processes. The geodynamic setting related to the protolith formation and the impact of different types of fluid-rock interactions have been uncertain up to now. We use major and trace element chemistry as well as oxygen isotopes to disentangle the geochemical signatures related to the different stages of the rocks' history. In the Münchberg Massif, dark eclogites (kyanite-free; Fe-Ti-MORB signature) are distinguished from light eclogites (kyanite-bearing; higher Mg#, Al 2 O 3 , and Cr; lower incompatible element contents; positive Eu anomalies; MORB to arc basalt signature). The δ 18 O values for both types (+5.0 to +10.8‰) are equal to, or higher than those of MORB. Amphibolite facies metagabbros have a more enriched, almost OIB-like trace element signature and high δ 18 O values (+9.4 to +10.3‰). Good linear correlations between fluid-immobile elements throughout the eclogite types confirm their derivation from a common, N-MORB to E-MORB-like parental magma. We interpret the light eclogites as former plagioclase-rich cumulates and the dark eclogites as their complementary differentiates. This relationship is partly obscured by variable degrees of magma contamination by sediments, which also affected the metagabbros. However, the metagabbros originated from a more enriched mantle source than the eclogites. Following intrusion, the eclogites were subjected to hydrothermal alteration under the influence of seawater, as indicated by positive correlations between Li, B, Sb, and δ 18 O. Metamorphic fluid-rock interactions appear to be mostly of limited extent, probably due to the lack of lawsonite dehydration as a fluid source. Nevertheless, the contents at least of some fluid-mobile elements, such as LILE, Li, and Pb, were probably modified during the subductionexhumation cycle of the eclogites. The crustal contamination of the protolith magmas argues against derivation of the eclogites and metagabbros from typical oceanic crust. Instead, a rift-drift transition setting related to the opening of the Rheic or Saxothuringian Ocean seems most likely. The eclogites and metagabbros, alongside with similar rocks in the Mariánské Lázně complex and other resembling high-pressure massifs, may record different stages of this rift-drift transition.
The large-diameter piston core LL44-GPC3 from the central North Pacific Ocean records continuous ... more The large-diameter piston core LL44-GPC3 from the central North Pacific Ocean records continuous sedimentation of eolian dust since the Late Cretaceous. Two intervals resolved by Nd and Pb isotopic data relate to dust coming from America (prior to ~40 Ma) and dust coming from Asia (since ~40 Ma). The Intertropical Convergence Zone (ITCZ) separates these depositional regimes today and may have been at a paleolatitude of ~23°N prior to 40 Ma. Such a northerly location of the ITCZ is consistent with sluggish atmospheric circulation and warm climate for the Northern Hemisphere of the early to middle Eocene. Since ~40 Ma, correlations between Nd (~7.55 > epsilon-Nd(t) > ~10.81) and Pb (18.625 < 206/4Pb < 18.879; 15.624 < 207/4Pb < 15.666; 38.611 < 208/4Pb < 38.960; 0.8294 < 207/6Pb < 0.8389; 2.0539 < 208/6Pb < 2.0743) isotopes reflect the progressive drying of central Asia triggered by the westward retreat of the paleo-Tethys. Comparisons between the c...
15 Anisotropy of magnetic susceptibility (AMS) is often used as a proxy for mineral fabric in 16 ... more 15 Anisotropy of magnetic susceptibility (AMS) is often used as a proxy for mineral fabric in 16 deformed rocks. In order to do so quantitatively, it is necessary to quantify the intrinsic 17 magnetic anisotropy of single crystals of rock-forming minerals. Amphiboles are common in 18 mafic igneous and metamorphic rocks and often define rock texture due to their general 19 prismatic crystal habits. Amphiboles may dominate the magnetic anisotropy in intermediate to 20 felsic igneous rocks and in some metamorphic rock types, because they have a high Fe 21 concentration and they can develop a strong crystallographic preferred orientation. In this 22 study, the AMS is characterized in 28 single crystals and one crystal aggregate of 23 compositionally diverse clinoand ortho-amphiboles. High-field methods were used to isolate 24 the paramagnetic component of the anisotropy, which is unaffected by ferromagnetic 25 inclusions that often occur in amphibole crystals. Laue imaging, laser ablati...
Selected Conclusions 2 Combining the results with halogen [2] and noble gas data [3] suggests tha... more Selected Conclusions 2 Combining the results with halogen [2] and noble gas data [3] suggests that serpentinisation of oceanic lithospheric mantle occurred along bend faults or as detached slices in the shallow forearc by fluids equilibrated within the accretionary prism 2 Initial heterogeneities will govern the spatial extent of serpentinisation and FME enrichment, ultimately amplifying heterogeneities 2 FME enrichments from serpentinisation are largely retained and reincorporated into the convecting mantle, providing potential fractionation mechanisms for U-Th systematics (HIMU) and enriched mantle (EM) sources 2 FMEs such as As, Sb, W and Bi, which are rarely quantified, can provide essential information to further discriminate between serpentinisation environments
Vielleicht haben Sie gerade Licht eingeschaltet, um diesen Artikel zu lesen. Vielleicht geistern ... more Vielleicht haben Sie gerade Licht eingeschaltet, um diesen Artikel zu lesen. Vielleicht geistern die letzten Nachrichten betreffend des steigenden Ölpreises noch in ihrem Hinterkopf umher. Nicht unerschöpflich ist die Quelle fossiler Brennstoffe; erneuerbar ist nur die Wasserkraft, predigt die hiesige Energielobby. Seit dem Altertum basieren verblüffend viele menschliche Errungenschaften auf metallischen Rohstoffen: Eine Aufgabe der Geologen ist es, die zukünftige Versorgung unserer Gesellschaft mit essenziellen Metallen zu sichern.
An efficient, clean procedure for bulk rock major to trace element analysis by 193 nm Excimer LA-... more An efficient, clean procedure for bulk rock major to trace element analysis by 193 nm Excimer LA-ICP-MS analysis of nanoparticulate pressed powder pellets (PPPs) employing a binder is presented. Sample powders are milled in water suspension in a planetary ball mill, reducing average grain size by about one order of magnitude compared to common dry milling protocols. Microcrystalline cellulose (MCC) is employed as a binder, improving the mechanical strength of the PPP and the ablation behaviour, because MCC absorbs 193 nm laser light well. Use of MCC binder allows for producing cohesive pellets of materials that cannot be pelletized in their pure forms, such as quartz powder. Rigorous blank quantification was performed on synthetic quartz treated like rock samples, demonstrating that procedural blanks are irrelevant except for a few elements at the 10 ng g−1 concentration level. The LA-ICP-MS PPP analytical procedure was optimised and evaluated using six different SRM powders (JP-1, ...
Serpentinites are central to carry water and fluid-mobile elements down subduction zones. The bre... more Serpentinites are central to carry water and fluid-mobile elements down subduction zones. The breakdown of antigorite represents the most prominent aqueous fluid release, boosting fluid-mediated element cycling from the slab to the mantle wedge. At Cerro del Almirez, Spain, an antigorite dehydration reaction front is preserved in subducted serpentinites. Bulk rock and mineral major formed from the reaction of brucite with antigorite (Ol-1a) or from the reaction of diopside and antigorite (Ol-1b; Fig. 2). Atg-1 forms the variably foliated rock matrix, and larger antigorite blades are less well oriented and may grow over inclusion-rich olivine or clinopyroxene but not tremolite. Dusty Cpx-1 (Fig. 3a) is the only relic mantle mineral, showing occasional magnetite lamellae. Prograde Cpx-2 forms clear rims on Cpx-1 or isolated clear neoblasts. Euhedral Tr-1 may overgrow clinopyroxene (Fig. 3b) or is rarely present as up to 700 µm sized rhombohedral, unoriented crystals that may grow over Ol-1 and the Atg-1 matrix. Chlorite occurs either as a fine-grained felt associated with magnetite (Chl-1, <0.5 vol%) or as inclusions in recrystallized relic clinopyroxene (Chl-2). Ti-chu-1 forms small grains dispersed throughout the matrix. Opaque phases include Cr-rich magnetite (up to 500 µm), subordinate Fe-Ni sulphide, and a Fe-Ti-oxide. Orthopyroxene crystals (Padrón-Navarta et al., 2011) were not observed in our Atg-serpentinite samples. Chlorite serpentinite and Atg-Chl-Opx-Ol rock: These lithologies characterise the cm to m thick, irregular reaction front between Atg-serpentinites and Chl-harzburgites and document the progressive replacement of antigorite from Atg-serpentinite by antigorite + chlorite, followed by growth of orthopyroxene and olivine until complete antigorite exhaustion. Contacts to the neighbouring rocks are gradational at cm to dm scale and oblique to the regional foliation. Chl-serpentinite represents the recrystallisation product of foliated Atg-1 from Atg-serpentinite into coarser grained and randomly oriented Atg-2 and Chl-3. The most conspicuous features are (i) euhedral Chl-3 flakes of up to 1 mm, topotactically overgrown by Atg-2 (Fig. 3c and d), and (ii) the absence of clinopyroxene. Tr-1 occurs either as euhedral crystals containing microscopic inclusions of olivine, or interstitially between Chl-3-Atg-2 intergrowths. Ol-2 forms mm-sized aggregates of individual, clear crystals. Atg-2 grain boundaries against talc are irregular. Atg-Chl-Opx-Ol rock (i) represents the appearance of orthopyroxene (Opx-1), (ii) has abundant flaky Chl-3 not overgrown by antigorite, (iii) is characterised by reduction in Atg-content to below 10 vol% along with disappearance of fine-grained Atg-1, and (iv) often shows the development of a granular texture similar to that of the granular Chl-harzburgite (see below). Opx-1 is either granular-anhedral and associated with granular-anhedral olivine (Ol-2/Ol-3, Fig. 3e), or Opx-1 forms radial aggregates of thin laths. Olivine is granular, colourless (Ol-2), and may have subordinate brown rims (Ol-3). Rarely, brown olivine grains (Ol-3) can also be observed, and colourless rims (Ol-4) on brown Ol-3 may also 8 occur. Unoriented Chl-3 flakes not overgrown by antigorite are distributed throughout the rock, while the Chl-3-Atg-2 intergrowths have a higher chlorite to antigorite ratio compared to those of the Chlserpentinite. Few, randomly oriented antigorite blades (<1.25 mm) grow over chlorite, olivine and orthopyroxene. Atg-2 and Opx-1 grain boundaries against talc are irregular. Chlorite harzburgite: Chl-harzburgite occurs as a granular or a spinifex-textured rock (Schönbächler, 1999; Trommsdorff et al., 1998; Padrón-Navarta et al., 2011). Both share an identical mineralogy with only slightly variable modal abundances (Table 1). Their spatial occurrence relative to the contact with Atg-serpentinite is not systematic except that granular rocks are more abundant near the contact to Atg-serpentinites. In fact, granular Chl-harzburgite is texturally very similar to the transitional Atg-Chl-Opx-Ol rock. Transitions between granular and spinifex-textured Chl-harzburgites can be as short as a few centimetres. Rock-forming minerals are randomly oriented. Brown elongated olivine (up to 10 cm long) predominates in the spinifex-textured Chl-harzburgite, while granular olivine in the granular Chl-harzburgite is exclusively colourless. Opx-1, Chl-3, and magnetite fill the interstices between these olivine crystals. Olivine in variable modal abundances occurs in three growth stages. Clear Ol-2 sometimes form cores of brown Ol-3 (Fig. 3f) that itself may be overgrown by a clear Ol-4 rim. Clear small isometric Ol-4 crystals showing equilibrium texture may also occur in the matrix (Fig. 3f). Clinopyroxene is absent, and Ti-clinohumite is very rare in our samples. Rare tremolite displays equilibrium textures with Chl-3 (Fig. 3h) or overgrows brown Ol-3 or Opx-1. Inclusions of orthopyroxene and olivine in euhedral tremolite were confirmed by RAMAN spectroscopy. Retrograde talc variably replaces orthopyroxene (e.g. Fig. 3g). Magnetite occurrence and contents: Magnetite modes are higher for Atg-serpentinites (2.8 vol%) than for Chl-harzburgites (1.4 vol%); however, variations within a given rock type are prominent (Atgserpentinites: 2-4 vol%; Chl-harzburgites: 0.4-2 vol%). Modes of opaque minerals determined by point counting (Table 1) thereby reflect mostly magnetite for Atg-serpentinites, while the fraction of sulphides in the opaque part can amount to some 30% in Chl-harzburgites. Texturally, magnetite is overgrown by Atg-1 and Ol-1 in Atg-serpentinites (Fig. 4a), testifying to its crystallisation upon ocean floor hydration (Fig. 2). Granular magnetite in Chl-harzburgite (Fig. 4b) commonly displays sharp and even grain boundaries and is also included in Chl-3 and Ol-3 (and Opx-1; not illustrated) resulting from the antigorite breakdown reaction. This equilibrium texture suggests that ocean floor magnetite 9 may have partially equilibrated upon antigorite breakdown, possibly in association with Fe-sulphide crystallisation. Traces of other oxides (not further investigated) were also observed (compare Debret et al., 2015). Variations in magnetite modes between Atg-serpentinites and Chl-harzburgites at Almirez were also recognized by other studies (Debret et al., 2015; Marchesi et al., 2013; Padrón-Navarta et al., 2011). 3.2. Bulk rock major element concentrations Major element data acquired for 15 bulk rock samples that cover all lithotypes across the antigoriteout reaction are reported in Supplementary Table 1, employing the methods explained in the Supplementary Material 1. Measured loss on ignition (LOI) values determine bulk rock water contents (subordinate carbonate cannot be excluded), which decrease across the antigorite-out reaction from 11.3 wt% H 2 O in Atg-serpentinites typical for complete hydration down to 4.1 wt% H 2 O in Chlharzburgites. Bulk rock major element systematics reveal the combined effects of (i) magmatic mantle history, (ii) oceanic metasomatism, and (iii) partial metamorphic dehydration, of which the magmatic mantle history exerts a dominant control. The mantle rocks are variably refractory and narrowly range in Mg# from 87.7 to 90.9 (all Fe as FeO), typical for upper mantle rocks. In a plot of Al 2 O 3 /SiO 2 vs. MgO/SiO 2 (Fig. 5a), the samples follow the trend of residual mantle rocks following melt extraction. Some of the samples possess variably low MgO/SiO 2 for their given Al 2 O 3 /SiO 2 (compare Marchesi et al., 2013) that is typical for abyssal serpentinized peridotites, indicating either addition of SiO 2 or removal of Mg (and Fe) that may have balanced antigorite growth at the expense of chrysotile (e.g., Kodolányi and Pettke, 2011). Alternatively, this may reflect MgO loss during low temperature seafloor alteration (e.g., Niu, 2004). Al 2 O 3 /SiO 2 ratios are considered to remain unmodified during serpentinization (e.g., Niu, 2004). Alumina concentrations tend to be low, resulting in Al 2 O 3 /SiO 2 ratios that are typical for abyssal peridotites (0.01 < Al 2 O 3 /SiO 2 < 0.07; Niu, 2004) but higher than most mantle wedge serpentinites (Al 2 O 3 /SiO 2 < 0.03; Deschamps et al., 2013). Despite significant amounts of retrograde talc present in partial to complete antigorite breakdown product rocks, bulk rock SiO 2 concentrations do not show an absolute enrichment relative to the peridotite melting residue trend. Al 2 O 3 and SiO 2 show a positive trend across the lithotypes from Atg-serpentinites to Chl-harzburgites, both Al 2 O 3 and SiO 2 being higher in Chl-harzburgites compared to all other lithologies. FeO tot
Uploads
Papers by Thomas Pettke