Pt, Au, Pd and Ru Partitioning Between Mineral and Silicate Melts: The Role of Metal Nanonuggets

The partition coefficients of Pt and other Pt Group Elements (PGE) between metal and silicate D(sub Metal-Silicate) and also between silicate minerals and silicate melts D(sub Metal-Silicate) are among the most challenging coefficients to obtain precisely. The PGE are highly siderophile elements (HSE) with D(sub Metal-Silicate) >10(exp 3) due to the fact that their concentrations in silicates are very low (ppb to ppt range). Therefore, the analytical difficulty is increased by the possible presence of HSE-rich-nuggets in reduced silicate melts during experiments). These tiny HSE nuggets complicate the interpretation of measured HSE concentrations. If the HSE micro-nuggets are just sample artifacts, then their contributions should be removed before calculations of the final concentration. On the other hand, if they are produced during the quench, then they should be included in the analysis. We still don't understand the mechanism of nugget formation well. Are they formed during the quench by precipitation from precursor species dissolved homogeneously in the melts, or are they precipitated in situ at high temperature due to oversaturation? As these elements are important tracers of early planetary processes such as core formation, it is important to take up this analytical and experimental challenge. In the case of the Earth for example, chondritic relative abundances of the HSE in some mantle xenoliths have led to the concept of the "late veneer" as a source of volatiles (such as water) and siderophiles in the silicate Earth. Silicate crystal/liquid fractionation is responsible for most, if not all, the HSE variation in the martian meteorite suites (SNC) and Pt is the element least affected by these fractionations. Therefore, in terms of reconstructing mantle HSE abundances for Mars, Pt becomes a very important player. In the present study, we have performed high temperature experiments under various redox conditions in order to determine the abundances of Pt, Au, Ru and Pd in minerals (olivine and diopside) and in silicate melts, but also to characterize the sizes, density and chemistry of HSE nuggets when present in the samples

Genetic Relations Between the Aves Ridge and the Grenada Back-Arc Basin, East Caribbean Sea

The Grenada Basin separates the active Lesser Antilles Arc from the Aves Ridge, described as a Cretaceous‐Paleocene remnant of the “Great Arc of the Caribbean.” Although various tectonic models have been proposed for the opening of the Grenada Basin, the data on which they rely are insufficient to reach definitive conclusions. This study presents, a large set of deep‐penetrating multichannel seismic reflection data and dredge samples acquired during the GARANTI cruise in 2017. By combining them with published data including seismic reflection data, wide‐angle seismic data, well data and dredges, we refine the understanding of the basement structure, depositional history, tectonic deformation and vertical motions of the Grenada Basin and its margins as follows: (1) rifting occurred during the late Paleocene‐early Eocene in a NW‐SE direction and led to seafloor spreading during the middle Eocene; (2) this newly formed oceanic crust now extends across the eastern Grenada Basin between the latitude of Grenada and Martinique; (3) asymmetrical pre‐Miocene depocenters support the hypothesis that the southern Grenada Basin originally extended beneath the present‐day southern Lesser Antilles Arc and probably partly into the present‐day forearc before the late Oligocene‐Miocene rise of the Lesser Antilles Arc; and (4) the Aves Ridge has subsided along with the Grenada Basin since at least the middle Eocene, with a general subsidence slowdown or even an uplift during the late Oligocene, and a sharp acceleration on its southeastern flank during the late Miocene. Until this acceleration of subsidence, several bathymetric highs remained shallow enough to develop carbonate platforms

Trace element geochemistry of K-rich impact spherules from howardites

The howardite-eucrite-diogenite (HED) achondrites are a group of meteorites that probably originate from the asteroid Vesta. Howardites are complex polymict breccias that sometimes contain, in addition to various rock debris, impact melt glasses which show an impressive range of compositions. In this paper we report on the geochemistry and O isotopes of a series of 6 Saharan polymict breccias (4 howardites and 2 polymict eucrites), and on the trace element abundances of high-K impact spherules found in two of them, Northwest Africa (NWA) 1664 and 1769, which are likely paired. The high-K impact spherules found in the howardites NWA 1664 and NWA 1769 display remarkable trace element patterns. Compared to eucrites or howardites, they all show prominent enrichments in Cs, Rb, K, Li and Ba, strong depletion in Na, while the REE and other refractory elements are unfractionated. These features could not have been generated during impact melting of their host howardites, nor other normal HED target materials. The involvement of Na-poor rocks, and possibly rocks of granitic composition, appears likely. Although these lithologies cannot be well constrained at present, our results demonstrate that the surface of Vesta is certainly more diverse than previously thought. Indeed, despite the large number of available HED meteorites (about 1000 different meteorites), the latter are probably not sufficient to describe the whole surface of their parent body

Fluid-mobile element budgets in serpentinized oceanic lithospheric mantle: Insights from B, As, Li, Pb, PGEs and Os isotopes in the Feather River Ophiolite, California

International audienceSerpentinized oceanic lithosphere may be an important source for boron and other fluid-mobile elements that are anomalously enriched in arc volcanic rocks. However, the integrated water/rock ratios associated with different styles of serpentinization may be variable. For example, large water/rock ratios are involved in the serpentinization of abyssal peridotites exhumed to the seafloor, whereas much lower water/rock ratios are likely to dictate serpentinization along deep faults and fractures. To address how fluid-mobile element enrichments vary with serpentinization at different settings, we investigated serpentinized harzburgites from the Feather River Ophiolite (FRO) in northern California. Major and trace element systematics indicate that serpentinization of the FRO ultramafics involved seawater. However, FRO serpentinites have unradiogenic Os isotopic compositions and near-chondritic platinum group element relative abundances, contrasting with serpentinized abyssal peridotites, which have radiogenic Os isotopic compositions and disturbed platinum group element systematics. These observations indicate that the integrated water/rock ratio involved in FRO serpentinization was smaller than that involved in abyssal peridotite serpentinization. B concentrations in the FRO (5–15 ppm), while substantially higher than primitive mantle (< 0.1 ppm), are still lower than in abyssal peridotites (10–170 ppm). These low values are not due to metamorphic loss as there is no petrographic evidence for prograde metamorphism (the serpentine minerals are low temperature forms like chrysotile and lizardite) and there is no consistency between observed fluid-mobile element (B, As, Pb, and Li) contents and depletions predicted from metamorphic dehydration models. Low B and fluid-mobile element contents in the FRO may thus be an intrinsic feature of low water/rock ratio serpentinization. Such values may be more representative of serpentinized oceanic lithospheric mantle rather than abyssal peridotites, which sample only the top veneer of the lithosphere

Earth's volatile accretion as told by Cd, Bi, Sb and Tl core-mantle distribution

International audienceThe timing and origin of volatile elements accretion on Earth has been and continues to be key questions, despite intense research scrutiny. Two end-member scenarios are usually proposed in which (1) volatile elements were delivered during the main phases of Earth's accretion and underwent subsequent core-mantle differentiation, or (2) Earth accreted from largely dry and volatile-depleted material, with late addition of volatile-rich material after differentiation. Studying the behaviour of elements that are both volatile and siderophile in a metal-silicate equilibrium can help discriminate between those two scenarios by deconvolving the effect of siderophile processes such as the Earth's differentiation from the effect of volatile processes. We report high-pressure and high-temperature metal-silicate equilibrium experiments that are used to trace the behaviour of four moderately siderophile and volatile elements: Cd, Bi, Sb and Tl. Experiments were performed in piston cylinder and multianvil presses between 2 and 20 GPa, from 1700 to 2600 K, in order to study the partitioning behaviour of these elements, including the relative influence of pressure, temperature, oxygen fugacity (fO2), and composition. Our results indicate that Cd, Bi, Sb and Tl partitioning coefficients are largely controlled by changes in temperature, pressure, fO2 and the S content of the metal phase. The pressure effect on Tl and Bi partitioning is measured for the first time and improves significantly the knowledge of Bi and Tl behaviour during core formation. Core formation modelling was used to reconcile the experimental data with observed abundances for different accretion scenarios. Homogeneous accretion with full core-mantle equilibration induces a massive segregation of Bi, Sb and Tl in the core, preventing reproduction of observed present-day mantle abundances. We find that a scenario in which the volatile elements are accreted in the last 10-20 % of the Earth's accretion is the most suitable accretion process that is able to explain the abundances of Cd, Bi, Sb and Tl in Earth's mantle. Partial core-mantle equilibration is necessary to reproduce Bi and Tl abundances. Our partitioning data also suggests that a 0.5 % chondritic late veneer may account for the Bi abundance measured in the bulk silicate Earth. These observations corroborate a growing wealth of evidence in support of this schematic heterogeneous accretion pathway

Trace-element composition of Fe-rich residual liquids formed by fractional crystallization: Implications for the Hadean magma ocean

New isotopic studies of 142Nd, the daughter product of the short-lived and now extinct isotope 146Sm, have revealed that the accessible part of the silicate Earth (e.g., upper mantle and crust) is more radiogenic in 142Nd/144Nd than that of chondritic meteorites. The positive 142Nd anomaly of the Earth's mantle implies that the Sm/Nd ratio of the mantle was fractionated early in Earth's history and that the complementary low 142Nd reservoir has remained isolated from the mantle since its formation. This has led to the suggestion that an early enriched reservoir, formed within Earth's first hundred million years (the Hadean), resides permanently in the deep interior of the Earth. One hypothesis for a permanently isolated reservoir is that there may be an Fe-rich, and hence intrinsically dense, chemical boundary layer at the core-mantle boundary. The protoliths of this chemical boundary layer could have originated at upper mantle pressures during extreme fractional crystallization of a global magma ocean during the Hadean but testing this hypothesis is difficult because samples of this early enriched reservoir do not exist. This hypothesis, however, is potentially refutable. Here, we investigate a post-Archean magnetite-sulfide magma formed by extreme magmatic differentiation to test whether residual Fe-rich liquids of any kind have the necessary trace-element signatures to satisfy certain global geochemical imbalances. The magnetite-sulfide magma is found to have high Pb contents (and low U/Pb ratios), high Re/Os ratios, and anti-correlated Sm/Nd and Lu/Hf fractionations. Permanent segregation of such a magma would (1) provide a means of early Pb sequestration, resulting in the high U/Pb ratio of the bulk silicate Earth, (2) be a source of radiogenic 187Os in the source regions of plumes, and (3) provide an explanation for decoupled Hf and Nd isotopic evolution in the early Archean, which is not easily produced by silicate fractionation. However, the magnetite-sulfide magma is not highly enriched in K, and thus, at face value, this magma analog would not serve as a repository for all of the heat producing elements. Nevertheless, other Fe-O-S liquids reported elsewhere are enriched in apatite, which carries high concentrations of K, U and Th. Given some promising geochemical fractionations of the Fe-rich liquids investigated here, the notion of a Hadean Fe-rich residual liquid deserves continued consideration from additional experimental or analog studies.15 page(s

Remnant iron oxide/sulfide mattes from a Hadean magma ocean at the core-mantle boundary: Insights from a small scale post-Archean analog

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Tracing Earth's Volatile Delivery With Tin

International audienceEarth's accretion history for volatile elements, and the origin of their depletions with respect to the Sun and primitive meteorites, continue to be debated. Two end-member scenarios propose either that volatile elements were delivered during the main phases of accretion and differentiation, or that the Earth accreted from materials largely devoid of volatiles with late addition of volatile-rich materials. Experiments evaluating the effect of metal-silicate equilibrium on elemental and isotopic distribution of volatile and siderophile elements such as Sn can help to distinguish between these scenarios. In this study, we have systematically investigated the relative influence of temperature, pressure, oxygen fugacity, and metal and silicate composition on the metal-silicate partioning behavior of Sn, from 2 to 20 GPa and 1,700 to 2,573 K, indicating that Sn siderophility noticeably decreases with temperature and S content of the metal but increases dramatically with pressure. A resolvable isotopic fractionation factor between metal and silicate suggests that core-mantle equilibrium temperatures (∼3,000 K) could potentially generate a Sn isotopic composition of the mantle lighter than the core by 150-200 ppm/amu. Core formation modeling shows that the volatiles were added during the last 10% of the accretion history. A final core containing 2.5 to 3.5 wt.% S is required. Furthermore, modeling of the BSE isotopic composition argues for a late Sn delivery on Earth with carbonaceous chondrite-like material as the most likely source of volatiles. Therefore, both elemental and isotopic approaches converge toward an identical volatile accretion scenario, involving a late volatile delivery