Analysis of trace elements in fossil foraminiferal tests is widely used by palaeoceanographers to... more Analysis of trace elements in fossil foraminiferal tests is widely used by palaeoceanographers to reconstruct past environmental conditions in the oceans. However, there remain a number of problems associated with the interpretation of these trace element data. Firstly, there is a lack of understanding as to the controls on the uptake of trace elements into foraminiferal tests. Secondly, fossil test chemistry may be influenced by post-depositional diagenesis, and thirdly, the impact of chemical cleaning techniques, used to remove contaminant detrital and authigenic mineral phases from the test surface, on test chemistry is uncertain. In order to address these problems, information is required as to the variability of trace elements within individual foram tests. To this end, we have developed a technique for the analysis of trace elements in foraminiferal calcite by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). This technique is reproducible (generally <12 % (1σ) for the elements studied), and has low detection limits. The accuracy of the technique is difficult to assess, but Mg/Ca, Sr/Ca, Ba/Ca and Li/Ca ratios for foraminiferal calcite are within the range reported elsewhere in the literature when normalised to a calcite standard. Analysis of different parts of a Gr. tumida test shows that Li/Ca, Sr/Ca and Ba/Ca of calcite pustules surrounding its aperture are distinctly different from the chamber and keel calcite. Successive analyses of the same Orbulina universa test throughout a chemical cleaning procedure show that the Mg/Ca ratio of the test falls significantly after removal of ferromanganese oxide phases, but is unchanged after oxidative cleaning. Rastering of relatively flat parts of G. sacculifer has enabled us to investigate changes in chemistry through the test wall. High Li/Ca on the outermost part of the test can be attributed to contaminant phases, but Li/Ca is relatively constant at >3 μm depth. B, Na, Mg, Mn, Zn, and Ba are also present in this contaminant phase.
The exceptional exposure of the Oman ophiolite allows to study in detail the mantle processes occ... more The exceptional exposure of the Oman ophiolite allows to study in detail the mantle processes occurring beneath a spreading centre. We have carried out a Re-Os pilot study focussed on the Maqsad diapir in the western part of the Sumail massif. The Maqsad diapir is roughly constituted of 3 petrographic units: 1, the main harzburgitic domain (MHD); 2, the diapir harzburgite (DH) showing plunging lineation; 3, a thick dunitic, mantle-crust transition zone (MTZ) topping the DH section. The MHD samples are characterised by Os content about 4 ppb and 187Os/188Os down to 0.1210. These features are in agreement with a >15% melt extraction. These unradiogenic Os compositions yield a model Re-depletion age ca. 1 Ga, similar to other Re-Os model age obtained for other oceanic related mantle samples worldwide. Samples from the DH domain despite similar range in Al2O3 content show more radiogenic Os composition (0.1240<187Os/188Os <0.1298). The MTZ dunites show even more radiogenic Os composition (0.1352<187Os/188Os <0.1399). The MTZ dunite are further characterised by low Os content (<1.5 ppb) not in agreement with the well know high compatible nature of Os. These observations are inconsistent with a residual origin after melting for these dunites. In contrast, Os systematic indicate than the MTZ dunites as well as the diapir harzburgites are not a mere product of melt depletion but rather result from extensive melt-rock reaction as previously demonstrated (e.g., Kelemen et al., 1995; Godard et al., 2000). Those authors have postulated that dunites are the product of melt-rock reaction leading to the crystallisation of olivine and Fe-enrichment (leading to low Mg-number) similar but less extensive process has been envisaged for the diapir harzburgite. 187Os/188Os is inversely correlated with the Mg-number of the MTZ and DH samples. Suggesting that indeed the petrographic and geochemical characteristics of both domains are due to the same process occurring at various extend. The processes leading to the overprinting of the Os isotopic signature in the DH and MTZ domain is currently under investigation but could be related to the dissolution and precipitation of sulphides and to the S-saturation and under-saturation of the percolating melt. These preliminary results suggest that the Re-Os system could be a very efficient tracer of melt-rock reaction occurring at large melt-rock ratios and indicate that a fair amount of melt-rock interaction occurred. Such processes may have important bearings for the Os-systematic of MORB. Kelemen et al., Nature, 375, 747-753, 1995; Godard et al, Earth Planet. Sci. Lett., 180, 133-148, 2000.
The decay of 187Re to 188Os provides an exceptional tracer of recycled crustal materials in the E... more The decay of 187Re to 188Os provides an exceptional tracer of recycled crustal materials in the Earth's convective mantle, and radiogenic Os isotope compositions of many mid-ocean ridge basalts (MORB) (with187Os/188Os isotope ratios ranging from 0.127 to 0.164) have been taken by some to indicate the presence of recycled oceanic crust in the upper mantle [1]. Others, however, have attributed the data to seawater contamination [2] or an age effect (due to the decay of 187Re following igneous crystallisation) [3]. Despite the controversy surrounding the MORB glass data, no attempt has been made to reproduce these measurements. This study presents new U-Th-Ra and ultra low blank Re-Os isotope data for MORB glass from the Atlantic, Pacific and Indian Oceans (including 18 samples previously measured [1]). Coupled Th and Ra excesses rule out an age effect, but surprisingly the new Os isotope measurements do not reproduce the radiogenic compositions previously observed. These data were most likely due to an analytical artefact. The range of 187Os/188Os is significantly smaller than that observed in previous work varying from 0.126 to 0.144 with an average value of 0.133 that is indistinguishable from estimates for the modern primitive upper mantle, and within the range of abyssal peridotites. Os isotope ratios are not correlated with Os abundances or with 187Re/188Os and hence no evidence for binary mixing can be inferred. The small but resolvable variations that do exist can not be easily explained by age or seawater contamination. Rather co-variations in Os concentration with rare earth elements, Cl and B, are most readily attributed to fractional crystallisation. While subtile but systematic covariations with Sr and Pb isotopes, and distinctive signatures between the Atlantic-Pacific and Indian Oceans, are consistent with some degree of chemical heterogeneity in the MORB mantle. [1] P. Schiano, J.-L. Birck, C.J. Allegre, EPSL 150, 363-379 (1997). [2] S.B. Shirey, R.J. Walker, Ann. Rev. EPS, 26, 423-500 (1998). [3] E.H. Hauri, Phil. Trans. Roy. Soc. Lon. 360, 2371-2382 (2002)
The bulk composition of highly refractory peridotites is characterised by a strong depletion in t... more The bulk composition of highly refractory peridotites is characterised by a strong depletion in trace elements together with relative enrichments in the most incompatible elements, such as Rb, Ba, Sr and U (considered as fluid-mobile), but also in Pb and LREE, Th, Nb and Ta (less/not affected by fluid-rock interactions). The origin of these trace element variations in the mantle exhumed on the seafloor has been little discussed: they may result either from mantle processes and/or serpentinization, but the later process is often considered as dominant in the highly altered abyssal peridotites. In order to better characterize the distribution of highly incompatible elements between primary and alteration minerals in these rock samples, we carried out an in situ multi-elemental study of a series of serpentinized harzburgites sampled on the Mid-Atlantic Ridge at ODP Site 920 (23°N, MARK) and at ODP Site 1274 drilled to the north of the Fifteen-Twenty Fracture Zone using ICP-MS and HR-ICP-MS coupled to laser ablation. Site 1274 peridotites preserve abundant primary minerals (up to 40%-50%). Site 920 peridotites are almost entirely serpentinized and few relics of primary minerals are observed (0%-<10%). At both sites, clinopyroxene (cpx) has interstitial or/and reactional textures, which suggest they are secondary. In spite of their highly variable degree of alteration, the peridotites sampled at Sites 920 and 1274 have the depleted yet spiked trace element patterns (mostly < 0.1 x PM) that is typical of refractory mantle peridotites, except for a few Site 920 serpentinites which are distinguished by high U and Pb contents (up to 7 x PM). Pyroxene and olivine have depleted compositions, although Site 920 pyroxenes are slightly enriched relative to that of Site 1274 (Yb: cpx = 1.6-3 x PM (Site 920) and 0.5-1 x PM (Site 1274), opx (orthopyroxene) ~ 0.5 x PM (Site 920) and 0.2-0.4 x PM (Site 1274), olivine ~ 0.02 x PM). Pyroxenes and olivine are characterized by selective enrichments in highly incompatible elements relative to REE (e.g., ~0.1 x PM up to ~1 x PM for U and Pb and Ce ~ 0.001 x PM in opx), similar to what observed in the bulk rock except for Sr. The bulk rock trace element content is mostly controlled the opx trace element budget. In detail, cpx is not in equilibrium with opx. These compositions cannot be solely explained by mantle partial melting and indicates melt rock interactions in the upwelling mantle, which is also marked by the late precipitation of cpx. Our results suggest that the bulk rock geochemistry of the serpentinized samples preserves the peridotite primary signature for most trace elements. Nevertheless, in situ analyses of alteration minerals indicate extensive chemical exchanges at the thin section scale (Andreani et al, this session). These results are discussed in terms of trace element re-distribution between phases and at the mineral boundaries during magmatic processes and consequences on the composition and type of alteration minerals during serpentinization.
The highly siderophile element (Platinum Group elements (PGE)+ Au+Re) form a coherent group which... more The highly siderophile element (Platinum Group elements (PGE)+ Au+Re) form a coherent group which behave very differently to lithophile elements and thus provide a different perspective on the formation and evolution of the lithospheric mantle. In addition, the Re-Os isotopic system may provide reliable melt depletion ages. However, the distribution and behaviour of the HSE in the mantle is poorly understood. Although base metal sulfides are thought to be the main host for HSE, little attention has been paid to sulfide abundance, distribution, mineralogy and microstructural sites in mantle. Further understanding of HSE behaviour in the mantle will depend on the studies of the relationships between HSE distribution, ReOs isotope systematic, and sulfide petrographic characteristics. In-situ techniques as LA-ICPMS or LA-MC-ICPMS allow to investigate the distribution of HSE and variability of Os isotopes in mantle sulfides. The main results of these studies are: 1, the extreme variability of the chondrite-normalised HSE patterns; 2, Base metal sulfides control the PGE budget of mantle rocks; 3, The HSE patterns define two sulfide groups, which reflect also differences in major element chemistry and microstructural position. Silicate-enclosed MSS show arch-shaped primitive mantle normalised HSE patterns with low PdIr, suggesting that they are residual after melting. Unradiogenic Os isotope compositions, agree indeed with ancient melt depletion and long-term isolation from the convecting mantle. In contrast, interstitial sulfides, typically S-poor and Cu-rich have lower Os-Ir contents but high PdIr. Petrographic and geochemical features indicate that they have been metasomatically introduced. Their 187Os/188Os display usually radiogenic values and show variable 187Re/188Os. Both types of sulfide often occur in the same sample; such samples have normal sulfur contents (e.g. S< 200ppm). The in-situ analysis of PGE and Os isotopes provides crucial information on the behaviour and distribution of the HSE and on the evolution of the lithosphere. Although the in-situ Re/Os method is subject to larger errors than TIMS analysis, its spatial resolution preserves petrographic information allowing the data to be interpreted with less ambiguity. Since many of the mantle samples studied contain the two types of sulfide we therefore suggest that many of the whole-rock analyses reflect mixing between several sulfide generations. It is thus hazardous to draw conclusions on Earth formation and differentiation processes based upon whole rock PGE abundances or Os isotopes without at least basic information on sulfides. These data suggest that sulfide could become a key phase in unravelling the formation and evolution of the mantle lithosphere.
During ODP Leg 209, eight sites were drilled along the Mid-Atlantic Ridge from 14\deg43 to 15\deg... more During ODP Leg 209, eight sites were drilled along the Mid-Atlantic Ridge from 14\deg43 to 15\deg44 N, allowing recovery of ca. 354 meters of residual mantle peridotite intruded by gabbroic rocks (up to 25% of recovered samples). We present here the results of a whole rock major and trace element study of 28 peridotites selected among the less altered samples (those which preserved enough primary features to allow detailed petrographic and mineralogical studies), at Site 1272 and Site 1274 (respectively S and N of the 15\deg20 fault zone). Site 1272 and Site 1274 peridotites are mainly harzburgites (cpx < 5%) with a few dunites. Harzburgites are characterized by highly variable opx contents (10-30 vol.%). Some samples show evidence of trapped melt (up to 3-5%). A few samples at Site 1274 show textural evidence of abundant cpx crystallization, resulting in the formation of lherzolites. All studied peridotites have been modified by alteration (>70% serpentinisation), leading to the addition of significant amounts of volatiles (loss on ignition >10 wt%). Nevertheless, except for alkali-earth elements and U, major and trace element compositions show no evidence of additional compositional changes during alteration. Site 1272 and Site 1274 peridotites are characterized by high Mg# (100 x molar Mg/[Mg + Fe]) (90.3- 92.2) and low Al2O3 contents (<0.3 wt% in dunites and ranging from 0.6 to 0.9% in harzburgites). Site 1272 and Site 1274 peridotites display flat to light Rare Earth Element (REE) depleted patterns (harzburgites : CeN<0.015; YbN<0.26 - dunites : CeN<0.001; YbN<0.04-0.08- N: chondrite normalised). The most depleted harzburgites are observed at the southernmost Site 1272 (CeN<0.003; YbN: 0.045-0.1). REE allow to distinguish two harzburgites groups at Site 1274. The first one shows patterns similar to those of Site 1272 peridotites yet with slightly higher REE content (CeN: 0.0015-0.015; YbN: 0.14-0.26). The second displays on average lower heavy REE contents (YbN: 0.1-0.18) and more variable light REE contents (CeN: <0.002-0.05). These samples are found at the bottom of Hole 1274A where cpx display the highest Na- and Ti contents among the studied peridotite suite. Site 1272 and Site 1274 peridotites composition is similar to that of the most depleted peridotites sampled in ophiolites (e.g., Oman, Cuba) and in oceans (e.g., Izu-Bonin-Marianna). They represent the most refractory peridotites yet sampled at a slow-spreading ridge.
Whole-rock concentrations of Se and Te were determined in 53 peridotite samples collected in orog... more Whole-rock concentrations of Se and Te were determined in 53 peridotite samples collected in orogenic bodies from the Eastern Pyrenees, including the Lherz massif (33 samples), the type-locality of lherzolite. Orogenic lherzolites tectonically emplaced into the continental crust escaped devolatilisation that may strongly alter concentrations of those volatile trace elements in basalt-hosted peridotite xenoliths. The Se and Te concentration ranges (< 3.5–100.7 ppb; < 1–11 ppb) are consistent with the few analyses of mantle samples yet available, yet extending to higher Se concentrations. In-situ analyses performed by LAM-ICPMS on base metal sulfides indicate that the Se budget is controlled by sulfides whereas a large fraction of Te (50 ± 30%) does not reside in sulfides. The missing fraction is accounted for by fractional crystallization of Pt–Pd tellurides from Cu–Ni sulfide melts. A maximum of one third of the S budget of Eastern Pyrenean peridotites was added by assimilation of crustal sulfides, which crystallized as pyrite. Disregarding those contaminated samples, some features of the Se and Te systematics (e.g. bivariate plots vs. fertility index) cannot be explained by partial melting processes. The best fit for 33 peridotites (mostly from Lherz) yields higher than chondritic S/Se (3.1213 ± 500; r2 = 0.96) for near chondritic Se/Te (8.6 ± 2; r2 = 0.9). Our study suggest that such superchondritic S/Se ratios are better explained by petrogenetic processes involving refertilization of harzburgitic lithosphere by metal (Cu,Ni)-rich sulfide melts precipitated along with clinopyroxene and Al-spinel from silicate melts en-route to the surface.►Selenium behaves as a chalcophile element in mantle samples. ►Tellurium in mantle samples is partitioned between tellurides and Cu–Ni sulfides. ►Se and Te systematics are good markers for magma–mantle interaction.
During ODP Leg 209, a magma-starved area of the Mid-Atlantic Ridge was drilled (Site 1274) in the... more During ODP Leg 209, a magma-starved area of the Mid-Atlantic Ridge was drilled (Site 1274) in the vicinity of the Fifteen-Twenty Fracture Zone that offsets one of the slowest portions of the spreading ridge. Bulk rock geochemistry indicates that Site 1274 peridotites represent the most depleted peridotites sampled so far at a slow spreading ridge. Their composition can be explained by open system partial melting and incomplete melt extraction; nevertheless, observation of interstitial clinopyroxene (Cpx) and local variations in bulk trace element contents suggests the occurrence of a late melt freezing reaction with melts from different mantle sources (Godard et al, 2008). Recent studies of mantle-derived peridotites have shown that several sulphide populations, characterised by different microstructural occurrences and elemental and isotopic compositions, coexist at the thin section scale. Thus by establishing the Re-Os isotopic systematic of the different sulphide populations together with an in situ trace element characterization of the associated silicates, we can shed some light on the intricacy of melt-extraction and melt-percolation processes beneath mid-ocean ridges. Site 1274 peridotites show several sulphide populations. Sulphides 1 (Sulf-1) are either enclosed in relict Ol1 and Opx1, or form isolated round blebs of sulphide within the serpentine matrix (Ol1). Their mineralogical and microstructural features are mostly characteristic of sulphide residual after melting. Sulphide-2 are partly embayed in Opx1 porphyroclasts and show an abnormal Cu-rich composition more akin to the solidification products of a sulphide partial melt. Finally, a third type of magmatic sulphide (Sulf-3) formed of pentlandite and primary bornite occurs as large (100-500 µm) convoluted patches intimately associated with Cpx2. Microstructural features suggest that the Sulf3-Cpx2(±Spl2) assemblage represents the crystallization product of a Cu-Ni-rich sulphide-bearing melt, trapped under lithospheric conditions and post-dating the crystallization of Sulf-2. Despite its obvious metasomatic origin, the extremely low light REE content of the Cpx2 suggests that they crystallized from an ultra depleted melt not in equilibrium with melt produced locally. 187Os/188Os obtained by LA-MC-ICPMS on c.a. 60 individual sulphide grain vary between 0.1097 and 0.1396 encompassing the range obtained on separated sulphide grains and whole-rocks (Harvey et al., 2006). Thanks to the spatial resolution of the LA-MC-ICPMS we were able to recognize that the sulphide populations described above are characterized by distinct 187Re/188Os -187Os/188Os systematics. Sulf-1 and Sulf-2 define a broadly positive correlation between 187Re/188Os and 187Os/188Os similar to the one described for the sulphides in Kane FZ abyssal peridotites (Alard et al., 2005). We interpret this relationship as a mixing line between sulphides residual after melting (Sulf-1) and sulphide (Sulf-2A) precipitated from a partial melt. The age of the first melting increment of this mantle section is thus given by the TRD model ages obtained on Sulf-1 (2.16 ±0.30 Ga). Sulf-3 shows constant non-radiogenic Os-isotope compositions (weighed mean = 0.11044±0.00019, n = 27) despite extremely variable 187Re/188Os (0.15-1.05). This indicates that Sulf-3 crystallization was recent, as no significant in-situ ingrowth of 187Os by 187Re decay has taken place. This is consistent with the microstructural occurrence of Sulf-3 and the quenched microstructure of Spl2-Cpx2 intergrowths. Although Sulf-3 are clearly metasomatic, they are significantly less radiogenic than Sulf-1. Such unradiogenic Os compositions require a long-term evolution in a Re-depleted reservoir (in agreement with the Cpx2'REE patterns). The minimum time required to develop such compositions is given by the TRD model ages of Sulf-3, c.a. 2.5±0.1 Ga. Thus in the case of Leg 209 the oldest ages and most depleted signature are found in the metasomatic assemblage (Sulf-3-Cpx2). Such metasomatic melt must have been derived from the partial melting of an old (≥2.5 Ga, Archean) and highly depleted mantle. REFERENCES: Alard, et al., 2005, Nature, 436: 1005-1008.; Godard et al., 2008, Earth & Planet. Sci. Lett., 267:410-425 ; Harvey et al., 2006, Earth & Planet. Sci. Lett., 244: 606-621.
During ODP Leg 209, a magma-starved portion of the Mid-Atlantic Ridge (MAR) was drilled in the vi... more During ODP Leg 209, a magma-starved portion of the Mid-Atlantic Ridge (MAR) was drilled in the vicinity of the Fifteen-Twenty Fracture Zone (FZ) that offsets one of MAR's slowest segments (~ 2.5 cm/yr full spreading rate). We present here the results of a multi-elemental study (ICPMS and LA-ICPMS) of 27 peridotites drilled at Sites 1272 and 1274 (respectively to the south and the north of the FZ). Hole 1272A and Hole 1274A peridotites comprise mainly harzburgites with minor dunites. Hole 1274A peridotites preserve abundant primary minerals (up to 50%). Clinopyroxene (cpx) have interstitial or/and reactional textures, which suggest they are secondary. Spinels measured in Hole 1274A peridotites have high Cr# ~ 45-50. The occurrence of refractory peridotites with high Cr# in spinels (>40) had been reported previously in the Fifteen-Twenty area, but only to the south of the FZ [e.g., Cannat et al., 1992]. The Fifteen Twenty FZ peridotites represent the most depleted peridotites yet sampled at a slow spreading ridge. They have low cpx primary modes, low Al2O3 contents (< 1 an. wt. %) and high Mg# (>91.5), bulk rock trace elements compositions are mostly below 0.1 x primitive mantle (PM), and cpx trace element contents <1 x PM. These compositions suggest high degrees of partial melting (>20%). Yet, standard (closed-system) melting models fail to account for two notable geochemical characteristics of the peridotites: (1) their relatively linear REE patterns, with no trace of strong LREE depletion and (2) their strong depletion in HREE. An open system model (melting plus percolation of melts produced by upwelling mantle at higher pressure) can explain their global depleted REE trend, their HREE depleted signature, plus the occurrence of secondary cpx. These results may imply a more actively convecting mantle than generally supposed below slow spreading centres. Cannat et al, Earth Planet. Sci. Lett., 109 (1-2), 87-106, 1992
The alternative hypotheses that mantle eclogites are either rocks produced by mantle processes or... more The alternative hypotheses that mantle eclogites are either rocks produced by mantle processes or fragments of subducted oceanic slabs have been intensively debated for 30 years, and the origin of these rocks is still not well constrained.
Abyssal peridotites represent residual mantle that has lost variable but generally large amounts ... more Abyssal peridotites represent residual mantle that has lost variable but generally large amounts of melt, which subsequently forms mid-oceanic ridge basalts (MORB). Evidence suggests that much of this melting is very recent, and isotope variations that do exist are usually explained by compositional heterogeneity in the source rather than by age differences. However, studies have indicated that many abyssal peridotites may have experienced multiple episodes of melt extraction. The 187Re-187Os isotope system offers some unique insights into the timing and nature of melt extraction from abyssal peridotites. This is because Os behaves as a compatible element during melting, whereas Re is moderately incompatible [1], hence mantle rocks may develop low 187Re/188Os ratios, and evolve to unradiogenic Os isotope compositions with time. Whole-rock model ages do reveal ancient melting events not preserved by other isotope systems, but perturbation of the Re-Os system means that these can only be minimum ages. However, if individual minerals such as sulphide remain closed to diffusion or reaction then, in principle, age information on ancient melt depletion may be accessible. This study presents Re-Os isotope data for abyssal peridotites, on both a whole-rock and individual sulphide grain scale, from ODP Leg 209 Hole 1274A in the 15°20'N fracture zone on the mid-Atlantic ridge. All of the samples possess 187Os/188Os isotope compositions that are less radiogenic than estimates for the primitive upper mantle [2]. Many are less radiogenic than previous estimates for the depleted MORB mantle and are lower than any yet reported for mid ocean ridge abyssal peridotites, and whole rock 187Os/188Os ratios are consistent with melt loss over, at least, the past 1.5 Ga. Many sulphide grains show evidence for recent recrystallisation or diffusional modification either due to partial melting or seawater alteration. However, some grains are extremely unradiogenic (187Os/188Os = 0.114) providing unequivocal evidence for at least some degree of melt depletion at ca. 2 Ga. Taken with recently published data [3] these results suggest that ancient melt depletion may be a widespread feature of the oceanic upper lithosphere, even though evidence for this depleted reservoir has not yet been observed in the Re-Os chemistry mid-ocean ridge basalts. References: [1] Burton et al., (2002). EPSL 198, 63-76. [2] Meisel et al., (1996). Nature 383, 517-520. [3] Alard et al., (2005). Nature 436, 1005-1008.
Osmium behaves as a highly compatible element during mantle melting and is preferentially retaine... more Osmium behaves as a highly compatible element during mantle melting and is preferentially retained in the residue, whereas Re is moderately incompatible and enters the melt. The Re-Os isotope system thus offers unique information on the timing and nature of melt depletion in the mantle, that is often considered to be immune from secondary metasomatic processes. For the system to be perturbed either Re or "exotic" Os must be added during some later event. However, the Os budget in mantle rocks is almost entirely controlled by sulphide, and thus the integrity of Os is fundamentally dependent upon the behaviour of sulphide. In this study we have reexamined the petrology and geochemistry of a classic suite of spinel lherzolite xenoliths from the Massif Central, France. These samples preserve a range of fertility (0.85 - 3.61 CaO wt %) attributed to variable melt extraction. Equilibration temperatures of 900^oC-1100^oC, and minimum equilibration pressures of 1.3Gpa (from geobarometry and fluid inclusions) indicate that these xenoliths last equilibrated under upper mantle conditions. Whole rock Re-Os data yields a range of 187Os/188Os ratios from 0.115 - 0.125 which correlate well with indices of fertility, consistent these rocks having been simply affected by melt depletion. However, A detailed study of secondary melt inclusions within the silicate phases has revealed that not only are the xenoliths contaminated by exotic, small percentage highly silicic melts but also that sulphide blebs are found in co-genetic relationships with CO_2 rich, silica rich and composite melt inclusions. That, at least some, of the Os in these rocks is hosted in metasomatic sulphide is consistent with in-situ Os isotope data from Mont Briancon (1). This shows that within a single rock only sulphides trapped in olivine preserve unradiogenic compositions, whereas interstitial, metasomatic sulphides are more radiogenic. Preliminary Re-Os data for olivine also yields unradiogenic values relative to the whole rock, which suggests that this phase may also have been affected by metasomatism. In this case, radiogenic values in the whole-rock samples reflect metasomatic enrichment, not primary depletion. (1) Alard, O. et al. Earth Planet. Sci. Letts., 203, 651-663 (2002).
During ODP Leg 209, a magma-starved area of the Mid-Atlantic Ridge was drilled (Site 1274) in the... more During ODP Leg 209, a magma-starved area of the Mid-Atlantic Ridge was drilled (Site 1274) in the vicinity of the Fifteen-Twenty Fracture Zone that offsets one of the slowest portions of the spreading ridge. Bulk rock geochemistry indicates that Site 1274 peridotites represent the most depleted peridotites sampled so far at a slow spreading ridge. Their composition can be explained by open system partial melting and incomplete melt extraction; nevertheless, observation of interstitial clinopyroxene (Cpx) and local variations in bulk trace element contents suggests the occurrence of a late melt freezing reaction with melts from different mantle sources (Godard et al, 2008). Recent studies of mantle-derived peridotites have shown that several sulphide populations, characterised by different microstructural occurrences and elemental and isotopic compositions, coexist at the thin section scale. Thus by establishing the Re-Os isotopic systematic of the different sulphide populations together with an in situ trace element characterization of the associated silicates, we can shed some light on the intricacy of melt-extraction and melt-percolation processes beneath mid-ocean ridges. Site 1274 peridotites show several sulphide populations. Sulphides 1 (Sulf-1) are either enclosed in relict Ol1 and Opx1, or form isolated round blebs of sulphide within the serpentine matrix (Ol1). Their mineralogical and microstructural features are mostly characteristic of sulphide residual after melting. Sulphide-2 are partly embayed in Opx1 porphyroclasts and show an abnormal Cu-rich composition more akin to the solidification products of a sulphide partial melt. Finally, a third type of magmatic sulphide (Sulf-3) formed of pentlandite and primary bornite occurs as large (100-500 µm) convoluted patches intimately associated with Cpx2. Microstructural features suggest that the Sulf3-Cpx2(±Spl2) assemblage represents the crystallization product of a Cu-Ni-rich sulphide-bearing melt, trapped under lithospheric conditions and post-dating the crystallization of Sulf-2. Despite its obvious metasomatic origin, the extremely low light REE content of the Cpx2 suggests that they crystallized from an ultra depleted melt not in equilibrium with melt produced locally. 187Os/188Os obtained by LA-MC-ICPMS on c.a. 60 individual sulphide grain vary between 0.1097 and 0.1396 encompassing the range obtained on separated sulphide grains and whole-rocks (Harvey et al., 2006). Thanks to the spatial resolution of the LA-MC-ICPMS we were able to recognize that the sulphide populations described above are characterized by distinct 187Re/188Os -187Os/188Os systematics. Sulf-1 and Sulf-2 define a broadly positive correlation between 187Re/188Os and 187Os/188Os similar to the one described for the sulphides in Kane FZ abyssal peridotites (Alard et al., 2005). We interpret this relationship as a mixing line between sulphides residual after melting (Sulf-1) and sulphide (Sulf-2A) precipitated from a partial melt. The age of the first melting increment of this mantle section is thus given by the TRD model ages obtained on Sulf-1 (2.16 ±0.30 Ga). Sulf-3 shows constant non-radiogenic Os-isotope compositions (weighed mean = 0.11044±0.00019, n = 27) despite extremely variable 187Re/188Os (0.15-1.05). This indicates that Sulf-3 crystallization was recent, as no significant in-situ ingrowth of 187Os by 187Re decay has taken place. This is consistent with the microstructural occurrence of Sulf-3 and the quenched microstructure of Spl2-Cpx2 intergrowths. Although Sulf-3 are clearly metasomatic, they are significantly less radiogenic than Sulf-1. Such unradiogenic Os compositions require a long-term evolution in a Re-depleted reservoir (in agreement with the Cpx2'REE patterns). The minimum time required to develop such compositions is given by the TRD model ages of Sulf-3, c.a. 2.5±0.1 Ga. Thus in the case of Leg 209 the oldest ages and most depleted signature are found in the metasomatic assemblage (Sulf-3-Cpx2). Such metasomatic melt must have been derived from the partial melting of an old (≥2.5 Ga, Archean) and highly depleted mantle. REFERENCES: Alard, et al., 2005, Nature, 436: 1005-1008.; Godard et al., 2008, Earth & Planet. Sci. Lett., 267:410-425 ; Harvey et al., 2006, Earth & Planet. Sci. Lett., 244: 606-621.
One of the main advantages of garnet geochronology is a possibility of establishing a direct link... more One of the main advantages of garnet geochronology is a possibility of establishing a direct link between isotopic ages and PT conditions. However, both Lu-Hf and Sm-Nd garnet dating can strongly be affected by submicroscopic inclusions capable of dominating the Sm-Nd and Lu-Hf budget. We investigated possible effects of various inclusions on Sm-Nd and Lu-Hf systems by combined isotope dilution and LA ICPMS studies. Internal isochrons obtained for 6 high-grade blocks of metabasites from the Franciscan complex yielded highly precise Lu-Hf ages ranging from 114 to 170 Ma, but failed to provide Sm-Nd dates. The main reason for failure of the Sm-Nd dating was a rather large amount of matrix silicate inclusions, which contained > 100 times more Nd than analyzed "impure" garnet fractions. The same inclusions had very limited influence on the Lu-Hf budget. This is mainly due to strong enrichment of garnets in heavy REE and due to much lower Hf concentrations in rock forming silicates. The 176Lu/177Hf ratios obtained for the analyzed garnets are typically between 1.5 and 8, but for two samples with spessartine-rich garnets, values range between 21 and 28 and are the highest yet reported. Small amount of zircon and rutile inclusions did not notably suppress 176Lu/177Hf ratios. This is mainly due to their small size (<10 μ m) and low abundance, but also because of their limited dissolution during sample digestion on a hotplate. Our results demonstrate that even samples with significant amounts of inclusions can yield precise and accurate Lu-Hf dates. For upper amphibolite facies metapelites from Vietnam, we obtained well defined Sm-Nd ages of 52.1+/-2.5 and 31.4+/-1.0 Ma, which are grossly discordant with the corresponding Lu-Hf dates of 84.1+/-1.6 Ma and 77.9+/-1.6 Ma, respectively. We therefore conducted LA ICP MS measurements on 4 selected samples. Sm/Nd ratios show rather flat zonation profiles throughout most of the crystal, with an up to 30% increase in the rims. The zonation closely follows Fe and Mn profiles. Lu/Hf ratios, on the other hand, show very large variations with low values in the rims reaching up to several hundreds in the cores and shows closer association with Y, Ca and Mg. Preservation of very strong zonation pattern of heavy REE together with Lu-Hf ages significantly older than corresponding Sm-Nd dates suggest higher closure temperature for the former system and possible inheritance problems in the analyzed samples. Sm/Nd ratios obtained by isotope dilution for garnets leached with sulphuric acid (SAL) were only about 10% lower than those estimated by LA ICPMS. This demonstrates high efficiency of SAL leaching in purifying garnets from monazite inclusions, which is of fundamental importance for achieving reliable ages. The same leaching technique had very different influence on Lu-Hf system. Garnets with 176Lu/177Hf ratios >20 after leaching yielded even higher ratios after leachcing. On the other hand, SAL applied to garnets with more typical 176Lu/177Hf ratios were brought down even by up to 50% relative to the unleached fractions. The latter case demonstrates removal of inclusions with Lu/Hf ratios higher than those in garnet. The most likely mineral with such high ratio in analyzed metabasites is apatite and in the case of metapelites- xenotime.
Analysis of trace elements in fossil foraminiferal tests is widely used by palaeoceanographers to... more Analysis of trace elements in fossil foraminiferal tests is widely used by palaeoceanographers to reconstruct past environmental conditions in the oceans. However, there remain a number of problems associated with the interpretation of these trace element data. Firstly, there is a lack of understanding as to the controls on the uptake of trace elements into foraminiferal tests. Secondly, fossil test chemistry may be influenced by post-depositional diagenesis, and thirdly, the impact of chemical cleaning techniques, used to remove contaminant detrital and authigenic mineral phases from the test surface, on test chemistry is uncertain. In order to address these problems, information is required as to the variability of trace elements within individual foram tests. To this end, we have developed a technique for the analysis of trace elements in foraminiferal calcite by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). This technique is reproducible (generally <12 % (1σ) for the elements studied), and has low detection limits. The accuracy of the technique is difficult to assess, but Mg/Ca, Sr/Ca, Ba/Ca and Li/Ca ratios for foraminiferal calcite are within the range reported elsewhere in the literature when normalised to a calcite standard. Analysis of different parts of a Gr. tumida test shows that Li/Ca, Sr/Ca and Ba/Ca of calcite pustules surrounding its aperture are distinctly different from the chamber and keel calcite. Successive analyses of the same Orbulina universa test throughout a chemical cleaning procedure show that the Mg/Ca ratio of the test falls significantly after removal of ferromanganese oxide phases, but is unchanged after oxidative cleaning. Rastering of relatively flat parts of G. sacculifer has enabled us to investigate changes in chemistry through the test wall. High Li/Ca on the outermost part of the test can be attributed to contaminant phases, but Li/Ca is relatively constant at >3 μm depth. B, Na, Mg, Mn, Zn, and Ba are also present in this contaminant phase.
The exceptional exposure of the Oman ophiolite allows to study in detail the mantle processes occ... more The exceptional exposure of the Oman ophiolite allows to study in detail the mantle processes occurring beneath a spreading centre. We have carried out a Re-Os pilot study focussed on the Maqsad diapir in the western part of the Sumail massif. The Maqsad diapir is roughly constituted of 3 petrographic units: 1, the main harzburgitic domain (MHD); 2, the diapir harzburgite (DH) showing plunging lineation; 3, a thick dunitic, mantle-crust transition zone (MTZ) topping the DH section. The MHD samples are characterised by Os content about 4 ppb and 187Os/188Os down to 0.1210. These features are in agreement with a >15% melt extraction. These unradiogenic Os compositions yield a model Re-depletion age ca. 1 Ga, similar to other Re-Os model age obtained for other oceanic related mantle samples worldwide. Samples from the DH domain despite similar range in Al2O3 content show more radiogenic Os composition (0.1240<187Os/188Os <0.1298). The MTZ dunites show even more radiogenic Os composition (0.1352<187Os/188Os <0.1399). The MTZ dunite are further characterised by low Os content (<1.5 ppb) not in agreement with the well know high compatible nature of Os. These observations are inconsistent with a residual origin after melting for these dunites. In contrast, Os systematic indicate than the MTZ dunites as well as the diapir harzburgites are not a mere product of melt depletion but rather result from extensive melt-rock reaction as previously demonstrated (e.g., Kelemen et al., 1995; Godard et al., 2000). Those authors have postulated that dunites are the product of melt-rock reaction leading to the crystallisation of olivine and Fe-enrichment (leading to low Mg-number) similar but less extensive process has been envisaged for the diapir harzburgite. 187Os/188Os is inversely correlated with the Mg-number of the MTZ and DH samples. Suggesting that indeed the petrographic and geochemical characteristics of both domains are due to the same process occurring at various extend. The processes leading to the overprinting of the Os isotopic signature in the DH and MTZ domain is currently under investigation but could be related to the dissolution and precipitation of sulphides and to the S-saturation and under-saturation of the percolating melt. These preliminary results suggest that the Re-Os system could be a very efficient tracer of melt-rock reaction occurring at large melt-rock ratios and indicate that a fair amount of melt-rock interaction occurred. Such processes may have important bearings for the Os-systematic of MORB. Kelemen et al., Nature, 375, 747-753, 1995; Godard et al, Earth Planet. Sci. Lett., 180, 133-148, 2000.
The decay of 187Re to 188Os provides an exceptional tracer of recycled crustal materials in the E... more The decay of 187Re to 188Os provides an exceptional tracer of recycled crustal materials in the Earth's convective mantle, and radiogenic Os isotope compositions of many mid-ocean ridge basalts (MORB) (with187Os/188Os isotope ratios ranging from 0.127 to 0.164) have been taken by some to indicate the presence of recycled oceanic crust in the upper mantle [1]. Others, however, have attributed the data to seawater contamination [2] or an age effect (due to the decay of 187Re following igneous crystallisation) [3]. Despite the controversy surrounding the MORB glass data, no attempt has been made to reproduce these measurements. This study presents new U-Th-Ra and ultra low blank Re-Os isotope data for MORB glass from the Atlantic, Pacific and Indian Oceans (including 18 samples previously measured [1]). Coupled Th and Ra excesses rule out an age effect, but surprisingly the new Os isotope measurements do not reproduce the radiogenic compositions previously observed. These data were most likely due to an analytical artefact. The range of 187Os/188Os is significantly smaller than that observed in previous work varying from 0.126 to 0.144 with an average value of 0.133 that is indistinguishable from estimates for the modern primitive upper mantle, and within the range of abyssal peridotites. Os isotope ratios are not correlated with Os abundances or with 187Re/188Os and hence no evidence for binary mixing can be inferred. The small but resolvable variations that do exist can not be easily explained by age or seawater contamination. Rather co-variations in Os concentration with rare earth elements, Cl and B, are most readily attributed to fractional crystallisation. While subtile but systematic covariations with Sr and Pb isotopes, and distinctive signatures between the Atlantic-Pacific and Indian Oceans, are consistent with some degree of chemical heterogeneity in the MORB mantle. [1] P. Schiano, J.-L. Birck, C.J. Allegre, EPSL 150, 363-379 (1997). [2] S.B. Shirey, R.J. Walker, Ann. Rev. EPS, 26, 423-500 (1998). [3] E.H. Hauri, Phil. Trans. Roy. Soc. Lon. 360, 2371-2382 (2002)
The bulk composition of highly refractory peridotites is characterised by a strong depletion in t... more The bulk composition of highly refractory peridotites is characterised by a strong depletion in trace elements together with relative enrichments in the most incompatible elements, such as Rb, Ba, Sr and U (considered as fluid-mobile), but also in Pb and LREE, Th, Nb and Ta (less/not affected by fluid-rock interactions). The origin of these trace element variations in the mantle exhumed on the seafloor has been little discussed: they may result either from mantle processes and/or serpentinization, but the later process is often considered as dominant in the highly altered abyssal peridotites. In order to better characterize the distribution of highly incompatible elements between primary and alteration minerals in these rock samples, we carried out an in situ multi-elemental study of a series of serpentinized harzburgites sampled on the Mid-Atlantic Ridge at ODP Site 920 (23°N, MARK) and at ODP Site 1274 drilled to the north of the Fifteen-Twenty Fracture Zone using ICP-MS and HR-ICP-MS coupled to laser ablation. Site 1274 peridotites preserve abundant primary minerals (up to 40%-50%). Site 920 peridotites are almost entirely serpentinized and few relics of primary minerals are observed (0%-<10%). At both sites, clinopyroxene (cpx) has interstitial or/and reactional textures, which suggest they are secondary. In spite of their highly variable degree of alteration, the peridotites sampled at Sites 920 and 1274 have the depleted yet spiked trace element patterns (mostly < 0.1 x PM) that is typical of refractory mantle peridotites, except for a few Site 920 serpentinites which are distinguished by high U and Pb contents (up to 7 x PM). Pyroxene and olivine have depleted compositions, although Site 920 pyroxenes are slightly enriched relative to that of Site 1274 (Yb: cpx = 1.6-3 x PM (Site 920) and 0.5-1 x PM (Site 1274), opx (orthopyroxene) ~ 0.5 x PM (Site 920) and 0.2-0.4 x PM (Site 1274), olivine ~ 0.02 x PM). Pyroxenes and olivine are characterized by selective enrichments in highly incompatible elements relative to REE (e.g., ~0.1 x PM up to ~1 x PM for U and Pb and Ce ~ 0.001 x PM in opx), similar to what observed in the bulk rock except for Sr. The bulk rock trace element content is mostly controlled the opx trace element budget. In detail, cpx is not in equilibrium with opx. These compositions cannot be solely explained by mantle partial melting and indicates melt rock interactions in the upwelling mantle, which is also marked by the late precipitation of cpx. Our results suggest that the bulk rock geochemistry of the serpentinized samples preserves the peridotite primary signature for most trace elements. Nevertheless, in situ analyses of alteration minerals indicate extensive chemical exchanges at the thin section scale (Andreani et al, this session). These results are discussed in terms of trace element re-distribution between phases and at the mineral boundaries during magmatic processes and consequences on the composition and type of alteration minerals during serpentinization.
The highly siderophile element (Platinum Group elements (PGE)+ Au+Re) form a coherent group which... more The highly siderophile element (Platinum Group elements (PGE)+ Au+Re) form a coherent group which behave very differently to lithophile elements and thus provide a different perspective on the formation and evolution of the lithospheric mantle. In addition, the Re-Os isotopic system may provide reliable melt depletion ages. However, the distribution and behaviour of the HSE in the mantle is poorly understood. Although base metal sulfides are thought to be the main host for HSE, little attention has been paid to sulfide abundance, distribution, mineralogy and microstructural sites in mantle. Further understanding of HSE behaviour in the mantle will depend on the studies of the relationships between HSE distribution, ReOs isotope systematic, and sulfide petrographic characteristics. In-situ techniques as LA-ICPMS or LA-MC-ICPMS allow to investigate the distribution of HSE and variability of Os isotopes in mantle sulfides. The main results of these studies are: 1, the extreme variability of the chondrite-normalised HSE patterns; 2, Base metal sulfides control the PGE budget of mantle rocks; 3, The HSE patterns define two sulfide groups, which reflect also differences in major element chemistry and microstructural position. Silicate-enclosed MSS show arch-shaped primitive mantle normalised HSE patterns with low PdIr, suggesting that they are residual after melting. Unradiogenic Os isotope compositions, agree indeed with ancient melt depletion and long-term isolation from the convecting mantle. In contrast, interstitial sulfides, typically S-poor and Cu-rich have lower Os-Ir contents but high PdIr. Petrographic and geochemical features indicate that they have been metasomatically introduced. Their 187Os/188Os display usually radiogenic values and show variable 187Re/188Os. Both types of sulfide often occur in the same sample; such samples have normal sulfur contents (e.g. S< 200ppm). The in-situ analysis of PGE and Os isotopes provides crucial information on the behaviour and distribution of the HSE and on the evolution of the lithosphere. Although the in-situ Re/Os method is subject to larger errors than TIMS analysis, its spatial resolution preserves petrographic information allowing the data to be interpreted with less ambiguity. Since many of the mantle samples studied contain the two types of sulfide we therefore suggest that many of the whole-rock analyses reflect mixing between several sulfide generations. It is thus hazardous to draw conclusions on Earth formation and differentiation processes based upon whole rock PGE abundances or Os isotopes without at least basic information on sulfides. These data suggest that sulfide could become a key phase in unravelling the formation and evolution of the mantle lithosphere.
During ODP Leg 209, eight sites were drilled along the Mid-Atlantic Ridge from 14\deg43 to 15\deg... more During ODP Leg 209, eight sites were drilled along the Mid-Atlantic Ridge from 14\deg43 to 15\deg44 N, allowing recovery of ca. 354 meters of residual mantle peridotite intruded by gabbroic rocks (up to 25% of recovered samples). We present here the results of a whole rock major and trace element study of 28 peridotites selected among the less altered samples (those which preserved enough primary features to allow detailed petrographic and mineralogical studies), at Site 1272 and Site 1274 (respectively S and N of the 15\deg20 fault zone). Site 1272 and Site 1274 peridotites are mainly harzburgites (cpx < 5%) with a few dunites. Harzburgites are characterized by highly variable opx contents (10-30 vol.%). Some samples show evidence of trapped melt (up to 3-5%). A few samples at Site 1274 show textural evidence of abundant cpx crystallization, resulting in the formation of lherzolites. All studied peridotites have been modified by alteration (>70% serpentinisation), leading to the addition of significant amounts of volatiles (loss on ignition >10 wt%). Nevertheless, except for alkali-earth elements and U, major and trace element compositions show no evidence of additional compositional changes during alteration. Site 1272 and Site 1274 peridotites are characterized by high Mg# (100 x molar Mg/[Mg + Fe]) (90.3- 92.2) and low Al2O3 contents (<0.3 wt% in dunites and ranging from 0.6 to 0.9% in harzburgites). Site 1272 and Site 1274 peridotites display flat to light Rare Earth Element (REE) depleted patterns (harzburgites : CeN<0.015; YbN<0.26 - dunites : CeN<0.001; YbN<0.04-0.08- N: chondrite normalised). The most depleted harzburgites are observed at the southernmost Site 1272 (CeN<0.003; YbN: 0.045-0.1). REE allow to distinguish two harzburgites groups at Site 1274. The first one shows patterns similar to those of Site 1272 peridotites yet with slightly higher REE content (CeN: 0.0015-0.015; YbN: 0.14-0.26). The second displays on average lower heavy REE contents (YbN: 0.1-0.18) and more variable light REE contents (CeN: <0.002-0.05). These samples are found at the bottom of Hole 1274A where cpx display the highest Na- and Ti contents among the studied peridotite suite. Site 1272 and Site 1274 peridotites composition is similar to that of the most depleted peridotites sampled in ophiolites (e.g., Oman, Cuba) and in oceans (e.g., Izu-Bonin-Marianna). They represent the most refractory peridotites yet sampled at a slow-spreading ridge.
Whole-rock concentrations of Se and Te were determined in 53 peridotite samples collected in orog... more Whole-rock concentrations of Se and Te were determined in 53 peridotite samples collected in orogenic bodies from the Eastern Pyrenees, including the Lherz massif (33 samples), the type-locality of lherzolite. Orogenic lherzolites tectonically emplaced into the continental crust escaped devolatilisation that may strongly alter concentrations of those volatile trace elements in basalt-hosted peridotite xenoliths. The Se and Te concentration ranges (< 3.5–100.7 ppb; < 1–11 ppb) are consistent with the few analyses of mantle samples yet available, yet extending to higher Se concentrations. In-situ analyses performed by LAM-ICPMS on base metal sulfides indicate that the Se budget is controlled by sulfides whereas a large fraction of Te (50 ± 30%) does not reside in sulfides. The missing fraction is accounted for by fractional crystallization of Pt–Pd tellurides from Cu–Ni sulfide melts. A maximum of one third of the S budget of Eastern Pyrenean peridotites was added by assimilation of crustal sulfides, which crystallized as pyrite. Disregarding those contaminated samples, some features of the Se and Te systematics (e.g. bivariate plots vs. fertility index) cannot be explained by partial melting processes. The best fit for 33 peridotites (mostly from Lherz) yields higher than chondritic S/Se (3.1213 ± 500; r2 = 0.96) for near chondritic Se/Te (8.6 ± 2; r2 = 0.9). Our study suggest that such superchondritic S/Se ratios are better explained by petrogenetic processes involving refertilization of harzburgitic lithosphere by metal (Cu,Ni)-rich sulfide melts precipitated along with clinopyroxene and Al-spinel from silicate melts en-route to the surface.►Selenium behaves as a chalcophile element in mantle samples. ►Tellurium in mantle samples is partitioned between tellurides and Cu–Ni sulfides. ►Se and Te systematics are good markers for magma–mantle interaction.
During ODP Leg 209, a magma-starved area of the Mid-Atlantic Ridge was drilled (Site 1274) in the... more During ODP Leg 209, a magma-starved area of the Mid-Atlantic Ridge was drilled (Site 1274) in the vicinity of the Fifteen-Twenty Fracture Zone that offsets one of the slowest portions of the spreading ridge. Bulk rock geochemistry indicates that Site 1274 peridotites represent the most depleted peridotites sampled so far at a slow spreading ridge. Their composition can be explained by open system partial melting and incomplete melt extraction; nevertheless, observation of interstitial clinopyroxene (Cpx) and local variations in bulk trace element contents suggests the occurrence of a late melt freezing reaction with melts from different mantle sources (Godard et al, 2008). Recent studies of mantle-derived peridotites have shown that several sulphide populations, characterised by different microstructural occurrences and elemental and isotopic compositions, coexist at the thin section scale. Thus by establishing the Re-Os isotopic systematic of the different sulphide populations together with an in situ trace element characterization of the associated silicates, we can shed some light on the intricacy of melt-extraction and melt-percolation processes beneath mid-ocean ridges. Site 1274 peridotites show several sulphide populations. Sulphides 1 (Sulf-1) are either enclosed in relict Ol1 and Opx1, or form isolated round blebs of sulphide within the serpentine matrix (Ol1). Their mineralogical and microstructural features are mostly characteristic of sulphide residual after melting. Sulphide-2 are partly embayed in Opx1 porphyroclasts and show an abnormal Cu-rich composition more akin to the solidification products of a sulphide partial melt. Finally, a third type of magmatic sulphide (Sulf-3) formed of pentlandite and primary bornite occurs as large (100-500 µm) convoluted patches intimately associated with Cpx2. Microstructural features suggest that the Sulf3-Cpx2(±Spl2) assemblage represents the crystallization product of a Cu-Ni-rich sulphide-bearing melt, trapped under lithospheric conditions and post-dating the crystallization of Sulf-2. Despite its obvious metasomatic origin, the extremely low light REE content of the Cpx2 suggests that they crystallized from an ultra depleted melt not in equilibrium with melt produced locally. 187Os/188Os obtained by LA-MC-ICPMS on c.a. 60 individual sulphide grain vary between 0.1097 and 0.1396 encompassing the range obtained on separated sulphide grains and whole-rocks (Harvey et al., 2006). Thanks to the spatial resolution of the LA-MC-ICPMS we were able to recognize that the sulphide populations described above are characterized by distinct 187Re/188Os -187Os/188Os systematics. Sulf-1 and Sulf-2 define a broadly positive correlation between 187Re/188Os and 187Os/188Os similar to the one described for the sulphides in Kane FZ abyssal peridotites (Alard et al., 2005). We interpret this relationship as a mixing line between sulphides residual after melting (Sulf-1) and sulphide (Sulf-2A) precipitated from a partial melt. The age of the first melting increment of this mantle section is thus given by the TRD model ages obtained on Sulf-1 (2.16 ±0.30 Ga). Sulf-3 shows constant non-radiogenic Os-isotope compositions (weighed mean = 0.11044±0.00019, n = 27) despite extremely variable 187Re/188Os (0.15-1.05). This indicates that Sulf-3 crystallization was recent, as no significant in-situ ingrowth of 187Os by 187Re decay has taken place. This is consistent with the microstructural occurrence of Sulf-3 and the quenched microstructure of Spl2-Cpx2 intergrowths. Although Sulf-3 are clearly metasomatic, they are significantly less radiogenic than Sulf-1. Such unradiogenic Os compositions require a long-term evolution in a Re-depleted reservoir (in agreement with the Cpx2'REE patterns). The minimum time required to develop such compositions is given by the TRD model ages of Sulf-3, c.a. 2.5±0.1 Ga. Thus in the case of Leg 209 the oldest ages and most depleted signature are found in the metasomatic assemblage (Sulf-3-Cpx2). Such metasomatic melt must have been derived from the partial melting of an old (≥2.5 Ga, Archean) and highly depleted mantle. REFERENCES: Alard, et al., 2005, Nature, 436: 1005-1008.; Godard et al., 2008, Earth & Planet. Sci. Lett., 267:410-425 ; Harvey et al., 2006, Earth & Planet. Sci. Lett., 244: 606-621.
During ODP Leg 209, a magma-starved portion of the Mid-Atlantic Ridge (MAR) was drilled in the vi... more During ODP Leg 209, a magma-starved portion of the Mid-Atlantic Ridge (MAR) was drilled in the vicinity of the Fifteen-Twenty Fracture Zone (FZ) that offsets one of MAR's slowest segments (~ 2.5 cm/yr full spreading rate). We present here the results of a multi-elemental study (ICPMS and LA-ICPMS) of 27 peridotites drilled at Sites 1272 and 1274 (respectively to the south and the north of the FZ). Hole 1272A and Hole 1274A peridotites comprise mainly harzburgites with minor dunites. Hole 1274A peridotites preserve abundant primary minerals (up to 50%). Clinopyroxene (cpx) have interstitial or/and reactional textures, which suggest they are secondary. Spinels measured in Hole 1274A peridotites have high Cr# ~ 45-50. The occurrence of refractory peridotites with high Cr# in spinels (>40) had been reported previously in the Fifteen-Twenty area, but only to the south of the FZ [e.g., Cannat et al., 1992]. The Fifteen Twenty FZ peridotites represent the most depleted peridotites yet sampled at a slow spreading ridge. They have low cpx primary modes, low Al2O3 contents (< 1 an. wt. %) and high Mg# (>91.5), bulk rock trace elements compositions are mostly below 0.1 x primitive mantle (PM), and cpx trace element contents <1 x PM. These compositions suggest high degrees of partial melting (>20%). Yet, standard (closed-system) melting models fail to account for two notable geochemical characteristics of the peridotites: (1) their relatively linear REE patterns, with no trace of strong LREE depletion and (2) their strong depletion in HREE. An open system model (melting plus percolation of melts produced by upwelling mantle at higher pressure) can explain their global depleted REE trend, their HREE depleted signature, plus the occurrence of secondary cpx. These results may imply a more actively convecting mantle than generally supposed below slow spreading centres. Cannat et al, Earth Planet. Sci. Lett., 109 (1-2), 87-106, 1992
The alternative hypotheses that mantle eclogites are either rocks produced by mantle processes or... more The alternative hypotheses that mantle eclogites are either rocks produced by mantle processes or fragments of subducted oceanic slabs have been intensively debated for 30 years, and the origin of these rocks is still not well constrained.
Abyssal peridotites represent residual mantle that has lost variable but generally large amounts ... more Abyssal peridotites represent residual mantle that has lost variable but generally large amounts of melt, which subsequently forms mid-oceanic ridge basalts (MORB). Evidence suggests that much of this melting is very recent, and isotope variations that do exist are usually explained by compositional heterogeneity in the source rather than by age differences. However, studies have indicated that many abyssal peridotites may have experienced multiple episodes of melt extraction. The 187Re-187Os isotope system offers some unique insights into the timing and nature of melt extraction from abyssal peridotites. This is because Os behaves as a compatible element during melting, whereas Re is moderately incompatible [1], hence mantle rocks may develop low 187Re/188Os ratios, and evolve to unradiogenic Os isotope compositions with time. Whole-rock model ages do reveal ancient melting events not preserved by other isotope systems, but perturbation of the Re-Os system means that these can only be minimum ages. However, if individual minerals such as sulphide remain closed to diffusion or reaction then, in principle, age information on ancient melt depletion may be accessible. This study presents Re-Os isotope data for abyssal peridotites, on both a whole-rock and individual sulphide grain scale, from ODP Leg 209 Hole 1274A in the 15°20'N fracture zone on the mid-Atlantic ridge. All of the samples possess 187Os/188Os isotope compositions that are less radiogenic than estimates for the primitive upper mantle [2]. Many are less radiogenic than previous estimates for the depleted MORB mantle and are lower than any yet reported for mid ocean ridge abyssal peridotites, and whole rock 187Os/188Os ratios are consistent with melt loss over, at least, the past 1.5 Ga. Many sulphide grains show evidence for recent recrystallisation or diffusional modification either due to partial melting or seawater alteration. However, some grains are extremely unradiogenic (187Os/188Os = 0.114) providing unequivocal evidence for at least some degree of melt depletion at ca. 2 Ga. Taken with recently published data [3] these results suggest that ancient melt depletion may be a widespread feature of the oceanic upper lithosphere, even though evidence for this depleted reservoir has not yet been observed in the Re-Os chemistry mid-ocean ridge basalts. References: [1] Burton et al., (2002). EPSL 198, 63-76. [2] Meisel et al., (1996). Nature 383, 517-520. [3] Alard et al., (2005). Nature 436, 1005-1008.
Osmium behaves as a highly compatible element during mantle melting and is preferentially retaine... more Osmium behaves as a highly compatible element during mantle melting and is preferentially retained in the residue, whereas Re is moderately incompatible and enters the melt. The Re-Os isotope system thus offers unique information on the timing and nature of melt depletion in the mantle, that is often considered to be immune from secondary metasomatic processes. For the system to be perturbed either Re or "exotic" Os must be added during some later event. However, the Os budget in mantle rocks is almost entirely controlled by sulphide, and thus the integrity of Os is fundamentally dependent upon the behaviour of sulphide. In this study we have reexamined the petrology and geochemistry of a classic suite of spinel lherzolite xenoliths from the Massif Central, France. These samples preserve a range of fertility (0.85 - 3.61 CaO wt %) attributed to variable melt extraction. Equilibration temperatures of 900^oC-1100^oC, and minimum equilibration pressures of 1.3Gpa (from geobarometry and fluid inclusions) indicate that these xenoliths last equilibrated under upper mantle conditions. Whole rock Re-Os data yields a range of 187Os/188Os ratios from 0.115 - 0.125 which correlate well with indices of fertility, consistent these rocks having been simply affected by melt depletion. However, A detailed study of secondary melt inclusions within the silicate phases has revealed that not only are the xenoliths contaminated by exotic, small percentage highly silicic melts but also that sulphide blebs are found in co-genetic relationships with CO_2 rich, silica rich and composite melt inclusions. That, at least some, of the Os in these rocks is hosted in metasomatic sulphide is consistent with in-situ Os isotope data from Mont Briancon (1). This shows that within a single rock only sulphides trapped in olivine preserve unradiogenic compositions, whereas interstitial, metasomatic sulphides are more radiogenic. Preliminary Re-Os data for olivine also yields unradiogenic values relative to the whole rock, which suggests that this phase may also have been affected by metasomatism. In this case, radiogenic values in the whole-rock samples reflect metasomatic enrichment, not primary depletion. (1) Alard, O. et al. Earth Planet. Sci. Letts., 203, 651-663 (2002).
During ODP Leg 209, a magma-starved area of the Mid-Atlantic Ridge was drilled (Site 1274) in the... more During ODP Leg 209, a magma-starved area of the Mid-Atlantic Ridge was drilled (Site 1274) in the vicinity of the Fifteen-Twenty Fracture Zone that offsets one of the slowest portions of the spreading ridge. Bulk rock geochemistry indicates that Site 1274 peridotites represent the most depleted peridotites sampled so far at a slow spreading ridge. Their composition can be explained by open system partial melting and incomplete melt extraction; nevertheless, observation of interstitial clinopyroxene (Cpx) and local variations in bulk trace element contents suggests the occurrence of a late melt freezing reaction with melts from different mantle sources (Godard et al, 2008). Recent studies of mantle-derived peridotites have shown that several sulphide populations, characterised by different microstructural occurrences and elemental and isotopic compositions, coexist at the thin section scale. Thus by establishing the Re-Os isotopic systematic of the different sulphide populations together with an in situ trace element characterization of the associated silicates, we can shed some light on the intricacy of melt-extraction and melt-percolation processes beneath mid-ocean ridges. Site 1274 peridotites show several sulphide populations. Sulphides 1 (Sulf-1) are either enclosed in relict Ol1 and Opx1, or form isolated round blebs of sulphide within the serpentine matrix (Ol1). Their mineralogical and microstructural features are mostly characteristic of sulphide residual after melting. Sulphide-2 are partly embayed in Opx1 porphyroclasts and show an abnormal Cu-rich composition more akin to the solidification products of a sulphide partial melt. Finally, a third type of magmatic sulphide (Sulf-3) formed of pentlandite and primary bornite occurs as large (100-500 µm) convoluted patches intimately associated with Cpx2. Microstructural features suggest that the Sulf3-Cpx2(±Spl2) assemblage represents the crystallization product of a Cu-Ni-rich sulphide-bearing melt, trapped under lithospheric conditions and post-dating the crystallization of Sulf-2. Despite its obvious metasomatic origin, the extremely low light REE content of the Cpx2 suggests that they crystallized from an ultra depleted melt not in equilibrium with melt produced locally. 187Os/188Os obtained by LA-MC-ICPMS on c.a. 60 individual sulphide grain vary between 0.1097 and 0.1396 encompassing the range obtained on separated sulphide grains and whole-rocks (Harvey et al., 2006). Thanks to the spatial resolution of the LA-MC-ICPMS we were able to recognize that the sulphide populations described above are characterized by distinct 187Re/188Os -187Os/188Os systematics. Sulf-1 and Sulf-2 define a broadly positive correlation between 187Re/188Os and 187Os/188Os similar to the one described for the sulphides in Kane FZ abyssal peridotites (Alard et al., 2005). We interpret this relationship as a mixing line between sulphides residual after melting (Sulf-1) and sulphide (Sulf-2A) precipitated from a partial melt. The age of the first melting increment of this mantle section is thus given by the TRD model ages obtained on Sulf-1 (2.16 ±0.30 Ga). Sulf-3 shows constant non-radiogenic Os-isotope compositions (weighed mean = 0.11044±0.00019, n = 27) despite extremely variable 187Re/188Os (0.15-1.05). This indicates that Sulf-3 crystallization was recent, as no significant in-situ ingrowth of 187Os by 187Re decay has taken place. This is consistent with the microstructural occurrence of Sulf-3 and the quenched microstructure of Spl2-Cpx2 intergrowths. Although Sulf-3 are clearly metasomatic, they are significantly less radiogenic than Sulf-1. Such unradiogenic Os compositions require a long-term evolution in a Re-depleted reservoir (in agreement with the Cpx2'REE patterns). The minimum time required to develop such compositions is given by the TRD model ages of Sulf-3, c.a. 2.5±0.1 Ga. Thus in the case of Leg 209 the oldest ages and most depleted signature are found in the metasomatic assemblage (Sulf-3-Cpx2). Such metasomatic melt must have been derived from the partial melting of an old (≥2.5 Ga, Archean) and highly depleted mantle. REFERENCES: Alard, et al., 2005, Nature, 436: 1005-1008.; Godard et al., 2008, Earth & Planet. Sci. Lett., 267:410-425 ; Harvey et al., 2006, Earth & Planet. Sci. Lett., 244: 606-621.
One of the main advantages of garnet geochronology is a possibility of establishing a direct link... more One of the main advantages of garnet geochronology is a possibility of establishing a direct link between isotopic ages and PT conditions. However, both Lu-Hf and Sm-Nd garnet dating can strongly be affected by submicroscopic inclusions capable of dominating the Sm-Nd and Lu-Hf budget. We investigated possible effects of various inclusions on Sm-Nd and Lu-Hf systems by combined isotope dilution and LA ICPMS studies. Internal isochrons obtained for 6 high-grade blocks of metabasites from the Franciscan complex yielded highly precise Lu-Hf ages ranging from 114 to 170 Ma, but failed to provide Sm-Nd dates. The main reason for failure of the Sm-Nd dating was a rather large amount of matrix silicate inclusions, which contained > 100 times more Nd than analyzed "impure" garnet fractions. The same inclusions had very limited influence on the Lu-Hf budget. This is mainly due to strong enrichment of garnets in heavy REE and due to much lower Hf concentrations in rock forming silicates. The 176Lu/177Hf ratios obtained for the analyzed garnets are typically between 1.5 and 8, but for two samples with spessartine-rich garnets, values range between 21 and 28 and are the highest yet reported. Small amount of zircon and rutile inclusions did not notably suppress 176Lu/177Hf ratios. This is mainly due to their small size (<10 μ m) and low abundance, but also because of their limited dissolution during sample digestion on a hotplate. Our results demonstrate that even samples with significant amounts of inclusions can yield precise and accurate Lu-Hf dates. For upper amphibolite facies metapelites from Vietnam, we obtained well defined Sm-Nd ages of 52.1+/-2.5 and 31.4+/-1.0 Ma, which are grossly discordant with the corresponding Lu-Hf dates of 84.1+/-1.6 Ma and 77.9+/-1.6 Ma, respectively. We therefore conducted LA ICP MS measurements on 4 selected samples. Sm/Nd ratios show rather flat zonation profiles throughout most of the crystal, with an up to 30% increase in the rims. The zonation closely follows Fe and Mn profiles. Lu/Hf ratios, on the other hand, show very large variations with low values in the rims reaching up to several hundreds in the cores and shows closer association with Y, Ca and Mg. Preservation of very strong zonation pattern of heavy REE together with Lu-Hf ages significantly older than corresponding Sm-Nd dates suggest higher closure temperature for the former system and possible inheritance problems in the analyzed samples. Sm/Nd ratios obtained by isotope dilution for garnets leached with sulphuric acid (SAL) were only about 10% lower than those estimated by LA ICPMS. This demonstrates high efficiency of SAL leaching in purifying garnets from monazite inclusions, which is of fundamental importance for achieving reliable ages. The same leaching technique had very different influence on Lu-Hf system. Garnets with 176Lu/177Hf ratios >20 after leaching yielded even higher ratios after leachcing. On the other hand, SAL applied to garnets with more typical 176Lu/177Hf ratios were brought down even by up to 50% relative to the unleached fractions. The latter case demonstrates removal of inclusions with Lu/Hf ratios higher than those in garnet. The most likely mineral with such high ratio in analyzed metabasites is apatite and in the case of metapelites- xenotime.
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Papers by Olivier Alard