Ambre Luguet

Sulfide petrology and highly siderophile element geochemistry of abyssal peridotites: A coupled study of samples from the Kane Fracture Zone (45°W 23°20N, MARK Area, Atlantic Ocean)

Nineteen samples from the Kane Fracture Zone have been studied for sulfide mineralogy and analyzed for S, Se, platinum-group elements (PGE), and Au to assess the effect of refertilization processes on the PGE systematics of abyssal peridotites. The lherzolites show broadly chondritic PGE ratios and sulfide modal abundances (0.01 to 0.03 wt%) consistent with partial melting models, although the few pyroxene-hosted sulfide inclusions and in situ LAM-ICPMS analyses provide evidence for in situ mobilization of a Cu-Ni–rich sulfide partial melt. The most refractory harzburgites (spinel Cr# > 29) are almost devoid of magmatic sulfides and show uniformly low PdN/IrN (<0.5) for variable PtN/IrN (0.8 to 1.2). The compatible behavior of Os, Ir, Ru, Rh, and Pt reflects the presence of primary Os-Ru alloys. Some harzburgites displaying petrographic evidence for refertilization by incremental melts en route to the surface are enriched in sulfides (up to 0.1 wt%). Some of these sulfides are concentrated in small veinlets of clinopyroxene and spinel crystallized from these melts. These S-rich harzburgites display superchondritic PdN/IrN (up to 2.04) positively correlated with sulfide modal contents. It is concluded that refertilization processes resulting in precipitation of metasomatic sulfides may significantly enhance Pd concentrations of abyssal peridotites while marginally affecting Pt (PtN/IrN ≤ 1.24) and Rh (RhN/IrN ≤ 1.23) as well. When the effects of such processes are screened out, our database suggests PGE relative abundances in the DMM (Depleted MORB Mantle; MORB: Mid-Ocean Ridge) within the uncertainty range of chondritic meteorites, without evidence of superchondritic Pt/Ir and/or Rh/Ir ratios.

In situ Os isotopes in abyssal peridotites bridge the isotopic gap between MORBs and their source mantle.

Abyssal peridotites are assumed to represent the mantle residue of mid-ocean-ridge basalts (MORBs). However, the osmium isotopic compositions of abyssal peridotites and MORB do not appear to be in equilibrium, raising questions about the cogenetic relationship between those two reservoirs. However, the cause of this isotopic mismatch is mainly due to a drastic filtering of the data based on the possibility of osmium contamination by sea water. Here we present a detailed study of magmatic sulphides (the main carrier of osmium) in abyssal peridotites and show that the 187Os/188Os ratio of these sulphides is of primary mantle origin and can reach radiogenic values suggesting equilibrium with MORB. Thus, the effect of sea water on the osmium systematics of abyssal peridotites has been overestimated and consequently there is no true osmium isotopic gap between MORBs and abyssal peridotites.

Laser-ablation microprobe (LAM)-ICPMS unravels the highly siderophile element geochemistry of the oceanic mantle

Abstract The highly siderophile element (HSE) contents of base-metal sulphides have been determined by laser-ablation microprobe (LAM)-ICPMS in abyssal peridotites from the Mid-Atlantic and South West Indian ridges. (Pd/Ir)N (0.007–505, N: CI-chondrite-normalised), (Pt/Ir)N (0.001–0.77) and (Rh/Ir)N (0.159–273) vary significantly between both grains and samples, irrespective of indicators of melt removal, but in line with bulk-rock platinum-group element (PGE) ratios and sulphide modal abundances. Positive deviations of PGE abundance ratios in whole-rock analyses are due to late-precipitated Cu–Ni-rich magmatic sulphides from incompletely extracted partial melts. These results contradict explanations of the HSE systematics of the oceanic mantle as reflecting global scale processes such as core–mantle exchange.

Sulfur and selenium systematics of the subcontinental lithospheric mantle: Inferences from the Massif Central xenolith suite (France)

Abstract Selenium has been analyzed in addition to S in 58 spinel peridotite xenoliths collected in Cenozoic alkali basalts from the Massif Central (France). The S concentration range now available for this suite, calculated from 123 samples, is the largest ever reported for alkali basalt-hosted xenoliths (

Enriched Pt-Re-Os Isotope Systematics in Plume Lavas Explained by Metasomatic Sulfides

To explain the elevated osmium isotope (186Os-187Os) signatures in oceanic basalts, the possibility of material flux from the metallic core into the crust has been invoked. This hypothesis conflicts with theoretical constraints on Earths thermal and dynamic history. To test the veracity and uniqueness of elevated 186Os-187Os in tracing core-mantle exchange, we present highly siderophile element analyses of pyroxenites, eclogites plus their sulfides, and new 186Os/188Os measurements on pyroxenites and platinum-rich alloys. Modeling shows that involvement in the mantle source of either bulk pyroxenite or, more likely, metasomatic sulfides derived from either pyroxenite or peridotite melts can explain the 186Os-187Os signatures of oceanic basalts. This removes the requirement for core-mantle exchange and provides an effective mechanism for generating Os isotope diversity in basalt source regions.

Clinopyroxene microtextures reveal incompletely extracted melts in abyssal peridotites

Textural evidence is interpreted to suggest that in regions where upwelling rates of the mantle are slow to very slow, a small amount (;2%) of melt was present when plagioclasefree abyssal peridotites entered the conductive regime at the base of the oceanic lithosphere. Upon crystallization, this melt appears to have been undersaturated in orthopyroxene, but precipitated clinopyroxene, Al-rich and Ti-poor spinel, and sulfides. Furthermore, the primary clinopyroxene grains have rare earth element patterns typical of residues of fractional melting, suggesting that the interstitial liquids were incremental partial melts rather than having mid-oceanic-ridge basalt compositions.

Platinum-Group Elements : A New Set of Key Tracers for the Earth's Interior

Due to their “iron-loving” properties, platinum-group elements (PGE) are expected to be stored in the Earth’s core. Although very low, at a few parts per billion, PGE concentrations measured in mantle-derived rocks are too high to be in chemical equilibrium with the core. The “late veneer” model offers the best explanation for this paradox—it postulates that a flux of primitive meteorites hit the early Earth after core formation had ceased. However, the inferred PGE composition of the hypothetical primitive mantle exhibits slight positive excesses of Ru, Rh, and Pd compared to the canonical chondritic signature. Such deviations have triggered considerable debate about the composition of the late veneer and the extent of reworking of PGE signatures by igneous processes within the Earth’s mantle.

Analysis of platinum group elements and gold in geological materials using NiS fire assay and Te coprecipitation; the NiS dissolution step revisited

Abstract The NiS fire assay–Te coprecipitation separation procedure for inductively coupled plasma mass spectrometry (ICP-MS) analyses of platinum-group elements (PGE) in silicate rocks has been revisited with the aim of reducing volatile PGE losses (Os, Pd). The NiS bead was dissolved in 20.2% HCl, in an open system allowing H 2 S to escape without loss of HCl. Yield, accuracy and reproducibility were tested by replicate analyses of a CANMET reference material (UMT1), a mantle lherzolite (FON B 93), prepared as in-house standard, and abyssal harzburgites, analysed as unknowns. The yields as estimated from UMT1 range from 97% (Ir) to 93–94% (Rh, Pt, Pd) and 91% (Au). The Os value (7.13±1.4 ppb) is equal within 1 sigma level, to the CANMET provisional value. The mean Os content of FON B 93 (3.42 ppb) fits the Os concentration inferred for the terrestrial mantle very well, as does the Os/Ir ratio (1.053 vs. 1.063) while Pd concentrations are increased by 18% compared to previous analyses after open-beaker dissolution steps. The abyssal harzburgites also yield consistent Os contents (3.28±0.19 ppb) and a perfectly chondritic Os/Ir ratio (1.07). Thus modified, the NiS fire-assay method allows Os to be nearly completely recovered, while greatly reducing the volatility of Pd. Moreover, the PGE analyses of coarse-grained rocks are highly reproducible (1% for Rh, Pt, Pd; 4% for Ir, 5% for Ru, 6% for Au), if performed from large-sized powder aliquots.

Minéralogie des sulfures de Fe-Ni-Cu dans les péridotites abyssales de la zone Mark (ride Médio-Atlantique, 20–24°N)

Abyssal peridotites from the MARK area (Mid-Atlantic ridge), studied from 27 samples, contain magmatic sulphides of mantle origin. The magmatic paragenesis (pentlandite + pyrrhotite + chalcopyrite) was protected in some inclusions. All the intergranular sulphides have been affected by hydrothermal transformations. In spite of these transformations, two populations of sulphides can be identified: (1) grains having survived the partial melting event in situ; (2) grains precipitated during or after the partial melting.

Mantle melt production during the 1.4 Ga Laurentian magmatic event: Isotopic constraints from Colorado Plateau mantle xenoliths

Plutons associated with a 1.4 Ga magmatic event intrude across southwestern Laurentia. The tectonic setting of this major magmatic province is poorly understood. Proposed melting models include anorogenic heating from the mantle, continental arc or transpressive orogeny, and anatexis from radiogenic heat buildup in thickened crust. Re-Os analyses of refractory mantle xenoliths from the Navajo volcanic field (NVF; central Colorado Plateau) yield Re depletion ages of 2.1–1.7 Ga, consistent with the age of the overlying Yavapai and Mazatzal crust. However, new Sm-Nd isotope data from clinopyroxene in peridotite xenoliths from NVF diatremes show a subset of xenoliths that plot on a ca. 1.4 Ga isochron, which likely reflects mantle melt production and isotopic resetting at 1.4 Ga. This suggests that Paleoproterozoic subcontinental lithospheric mantle was involved in the 1.4 Ga magmatic event. Our constraints support a subduction model for the generation of the 1.4 Ga granites but are inconsistent with rifting and anorogenic anatexis models, both of which would require removal of ancient lithosphere.