Volatile Siderophile Elements Research Articles

Abundances of siderophile elements in H-chondrite metal grains: Implications for the origin of metal in unequilibrated ordinary chondrites

Understanding the evolution of metal in the protoplanetary disk is necessary to constrain the first steps of metal-silicate segregation and the early stages of the evolution of the protoplanetary disk. We measured the siderophile elemental compositions (PGE, Ni, Co, Fe, Cu, Ga, Ge) of individual metal grains in H ordinary chondrites by laser ablation inductively coupled plasma mass spectrometry to investigate their formation. We analyzed unequilibrated ordinary chondrites (H3) to constrain processes affecting the metal before accretion, and inferred the effects of metamorphism by comparing their elemental compositions to those of equilibrated chondrites (H4–H6). Our results highlight large variations of refractory (Re, Os, W, Ir, Ru, Mo, Pt) and moderately volatile siderophile element (Pd, Au, Ga, Ge) concentrations among metal grains in H3 samples that permit to classify them according to their Ge/Ir ratios and HSE contents. These intergrain variations are progressively homogenized in H4–H6 samples due to their increasing degrees of metamorphism. To constrain the origin of the metal, we modeled its evolution during melting and crystallization. Our melting model of a single metallic precursor containing 1.5 wt% C and up to 12 wt% S reproduces well the observed range of siderophile element compositions in the metal. Metal grains show a range of W, Mo, and Ga compositions that we interpret to reflect various local (grain-scale) oxidation states during the melting event(s) due to the heterogeneous distribution of various oxidizing components within the precursors. The very similar HSE compositions of H and L/LL metal grains suggests that the variations of bulk metal abundance and HSE concentrations observed among the different classes of ordinary chondrites (H, L, LL) result from the heterogeneous physical distribution of a relatively chemically homogeneous metal component among OC parent bodies, and not from a chemical (sensu lato) gradient between H and LL chondrites.

Genetics, age, and crystallization history of group IC iron meteorites

The IC iron meteorite group is characterized utilizing nucleosynthetic mass-independent isotopic compositions and 182W age constraints, coupled with siderophile element concentration measurements and modeling of crystal-liquid fractionation processes. The six IC irons analyzed, Arispe, Bendego, Chihuahua City, Nocoleche, NWA 2743, and Winburg have indistinguishable Mo and W genetic isotopic compositions and are consistent with derivation from the same parent body, which formed in the non-carbonaceous (NC) nebular reservoir. A pre-exposure µ182W value (parts-per-million deviations in isotopic ratios from terrestrial standards) for the six IC irons of −337 ± 5 corresponds to a metal-silicate segregation age of 1.0 ± 0.4 Myr after calcium-aluminum-rich inclusion (CAI) formation. This age is similar to those determined for other NC iron groups. Siderophile element abundances of the IC irons are generally similar and characterized by minor depletions in the more volatile siderophile elements. Highly siderophile element (HSE) distributions among the IC group suggest that the initial parent body core was S-rich, with preferred model results indicating an initial melt composition with ∼ 18 wt% S, 2 wt% P and 0.03 wt% C. Processes in addition to fractional crystallization, such as late-stage parent body modification, possibly as a result of impacts, and subsequent metal-melt mixing, are required within the first 100 Myr of Solar System history to explain the range of HSE abundances.

Trace element volatility and the conditions of liquid-vapor separation in the proto-lunar disk

The Moon is thought to have formed from material ejected by a giant impact that took place at the end of Earth's accretion. The material ejected to space generated a large hot structure where material beyond the Roche limit accreted to form the Moon. It has long been known that the Moon is characterized by abundances in moderately volatile elements (MVE) lower than that of the Earth, while more recent studies have established that the concentrations in refractory elements are similar to the bulk Silicate Earth. The thermodynamic conditions that prevailed after this impact are poorly known and understanding the origin of the Moon-Earth differences in MVE requires a knowledge of the volatility of elements under these conditions. In this study, we reexamine the volatility of a large set of geochemically relevant elements and attempt to determine the P-T conditions under which volatiles were putatively separated from the liquid material. Our model predicts very different condensation temperatures due to higher pressures, compared with the conditions of the Solar Nebula and we extend the values of these temperatures to a wide number of trace elements (Se, Ag, Pt, Mo, W, Zn, Sn, Sb, Rb, Cs, U, Th, Cr, Ni, Co, Ga, Ge, Cu, and P). Our modeling shows that the observed lunar compositions cannot be explained by a single set of P and T conditions. Rather, it is best explained by a mixture between high-temperature condensates (~4000 K) and low temperature condensates (2000-2500 K). An important constraint is that for the low temperature condensates, liquid metal must have been stable and this is crucial for matching the abundance of volatile siderophile elements in the bulk Moon.

Nickel-rich, volatile depleted iron meteorites: Relationships and formation processes

Ungrouped iron meteorites Tishomingo, Willow Grove, and Chinga, and group IVB iron meteorites, are Ni-rich. Similarities include enrichments of 10–100 × CI for some refractory siderophile elements, and equivalent depletions in more volatile siderophile elements. Superimposed on the overall enrichment/depletion trend, certain siderophile elements (P, W, Fe, Mo) are depleted relative to elements of similar volatility. All three ungrouped irons derive from parent bodies formed in the early Solar System. Willow Grove and Chinga are characterized by cosmic ray exposure corrected 182W/184W consistent with metal-silicate segregation on their parent bodies within 1–3 Myr of Solar System formation, within the age range determined for segregation of magmatic iron meteorite parent bodies, including group IVB irons. Tishomingo is characterized by a younger model age 4–5 Myr subsequent to Solar System formation, reflecting either late stage melting resulting from 26Al decay, or an impact resetting. The discovery of stishovite in Tishomingo, indicating exposure to a minimum shock pressure of 8–9 GPa, is consistent with the latter.Stishovite in Tishomingo and chromite included in troilite-daubréelite in IVB irons allows oxygen isotopic composition comparison between these meteorites. Different mass independent oxygen isotopic compositions of IVB irons and Tishomingo indicate genetically distinct parent bodies. By contrast, mass independent Mo isotopic compositions overlap within analytical uncertainties, indicating a similar, carbonaceous chondrite (CC) type genetic heritage. Molybdenum and 183W isotopic data for Chinga and Willow Grove indicate derivation from CC type parent bodies. Willow Grove shares Mo and 183W isotopic compositions with the Ni-rich South Byron Trio (SBT) grouplet and the Milton pallasite. These Ni-rich meteorites likely formed in the same general nebular environment as other CC planetesimals, likely the outer Solar System.Highly siderophile element (HSE) abundances of Willow Grove and Tishomingo are similar to some IVB meteorites, consistent with formation by moderate degrees of fractional crystallization from initial metallic melts with low S and P, and modestly fractionated HSE. The comparable HSE abundances of Tishomingo and Willow Grove to some IVB irons, yet substantially higher Ni concentrations, indicate formation on parent bodies with lower bulk HSE abundances or HSE concentration in proportionally smaller volumes of metal. HSE abundances in Chinga are considerably lower than in IVB irons, highly fractionated, and processes responsible for these remain elusive.For IVB irons and these ungrouped irons, high temperature condensation likely dominated the enrichment and depletion of the refractory and volatile siderophile elements, respectively. Parent body degassing may have also played a role. Relative depletion of volatile siderophile elements is not, however, a universal feature of high-Ni meteorites. The SBT and Milton pallasite are Ni-rich, but less depleted in the more volatile siderophile elements. Nickel enrichment was likely driven by oxidation of Fe metal during parent body accretion or core segregation. Oxidation of the Tishomingo and Willow Grove parent bodies may have occurred at ∼ IW + 1, indicated by relative Mo and W depletions due to metal/water reaction during differentiation. Late-stage reduction, indicated by the presence of Cr-bearing sulfides in Tishomingo and IVB irons, may have resulted from exhaustion of the oxidant.

The relationship of CH, CB, and CR chondrites: Constraints from trace elements and Fe-Ni isotope systematics in metal

Due to similarities in chemical composition and common Cr, Ti, N and O isotope trends, the metal-rich CR, CH and CB chondrites, often referred to as CR clan chondrites, are thought to be related to each other. This study aims to shed light on this relationship by the investigation of Fe and Ni isotope and trace element compositions of metal grains from CR and CH chondrites, in order to compare the results with previously reported data from CB chondrite metal. In situ trace element and Fe and Ni isotope analyses were conducted by femtosecond-laser ablation-(multicollector-)inductively coupled plasma-mass spectrometry (fs-LA-(MC-)ICP-MS). Furthermore, bulk CB metal and silicate separates were analyzed by solution MC-ICP-MS.Chemical compositions of metal grains in metal-rich chondrites are depleted in moderately volatile siderophile elements relative to the solar values with the exception of Pd. Such element abundance patterns are consistent with models of incomplete condensation from a gas with solar composition. Both, zoned and unzoned metal grains from CH and CB chondrites display very similar Fe and Ni isotopes compositions, indicating they likely formed within the same event, during non-equilibrium fractional condensation from an impact-induced vapor plume. This scenario is also supported by non-equilibrium Fe isotope signatures between bulk CB metal and silicate. Zoned metal grains likely formed in the fast-cooling outer shell region of the plume and are dominated by kinetic fractionation, resulting in isotopically light cores, while unzoned metal grains condensed under nearly equilibrium conditions, likely in the slow-cooling interior of the plume. Variability in Fe and Ni isotope compositions among different unzoned grains may be explained by (1) a kinetic component during their condensation and/or (2) evaporation and condensation-driven reservoir effects in the plume, which resulted in light and heavy isotope signatures, respectively. Textural differences between CH and CBb are most pronounced in the mean grain size, which may be attributed to grain-size sorting. Such a process could also explain the lack of zoned metals in CBa chondrites, as zoned metal grains in CBb and CH chondrites are by more than a magnitude smaller than the mean metal grain size of CBa chondrites. In CR chondrites, metal within chondrules likely formed by fractional condensation from a solar type gas followed by subsequent melting leading to equilibration with chondrule silicates. Larger isolated metal grains from the matrix are less processed, and apparently escaped silicate equilibration. Those metal grains are indistinguishable from unzoned grains in CH and CB chondrites in trace elements and Fe and Ni isotopic compositions albeit with a slightly narrower compositional range. Based on these findings we conclude that metal precursors in CR chondrites are strongly related to unzoned metal in CH, and CB chondrites and possibly share a common origin. This metal component would be smallest in CR chondrites, larger in CH and dominant in CB chondrites which is also consistent with age constraints and isotopic anomalies observed in CR clan chondrites.

Geochemical constraints on core-mantle differentiation in Mercury and the aubrite parent body

Differentiation of Mercury and the aubrite parent body (AuPB) at extremely reducing conditions is implied from the low FeO and high S contents of their mantles. Here, the mantle abundances of these elements as derived from remote sensing (in the case of Mercury) and aubrite meteorite analysis are used in conjunction with new experimentally determined metal-silicate partition coefficients at reducing conditions presented in Steenstra et al. (2020a) to quantify the geochemical consequences of core formation in highly reduced planetary bodies. Plausible core compositions of Mercury and the AuPB are assessed, and the distribution of Si and other elements during core formation are quantified.Combining the new results with previous remote sensing observations of the surface of Mercury it is found that its core is likely to be Si-rich (>4.5–21 wt%). The estimated core Si contents depend mostly on the assumption of C saturation in Mercury's mantle and the type of bulk composition considered. Calculations for C-free conditions also yield significant quantities of Si in the AuPB core (>6–20 wt%, depending on bulk composition and redox state). The amount of Si in the AuPB core is greatly decreased if C-saturation is assumed.The new experimental data and related thermodynamic parameterization for predicting the C contents at graphite saturation of FeSi alloys (CCGS) shows that Mercury's mantle was likely C-saturated following formation of a Si-rich core, allowing for the separation of a graphitic flotation crust. Due to the lower Si contents calculated for the AuPB core under C-saturated conditions, a graphitic flotation crust is unlikely to form in smaller-sized reduced asteroids such as the AuPB.The Si-rich nature of the AuPB core for C-undersaturated scenarios is expected to have resulted in preferential partitioning of the majority of the volatile siderophile elements (VSE) into the AuPB core. However, depletions for the most volatile VSE cannot be reconciled with core formation depletion only. Their depletions in aubrites require additional depletion from (I) segregating sulfide liquids during AuPB differentiation and/or (II) compatible behavior during mineral-melt fractionation and/or (III) loss of these elements in a vapor phase during and/or after AuPB differentiation.

Effect of silicon on activity coefficients of Bi, Cd, Sn, and Ag in liquid Fe‐Si, and implications for differentiation and core formation

AbstractThe depletion of volatile siderophile elements (VSE) Sn, Ag, Bi, Cd, and P in mantles of differentiated planetary bodies can be attributed to volatile‐depleted precursor materials (building blocks), fractionation during core formation, fractionation into and retention in sulfide minerals, and/or volatile loss associated with magmatism. Quantitative models to constrain the fractionation due to core formation have not been possible due to the lack of activity and partitioning data. Interaction parameters in Fe‐Si liquids have been measured at 1 GPa, 1600 °C and increase in the order Cd (~6), Ag (~10), Sn (~28), Bi (~46), and P (~58). These large and positive values contrast with smaller and negative values in Fe‐S liquids indicating that any chalcophile behavior exhibited by these elements will be erased by dissolution of a small amount of Si in the metallic liquid. A newly updated activity model is applied to Earth, Mars, and Vesta. Five elements (P, Zn, Sn, Cd, and In) in Earth's primitive upper mantle can largely be explained by metal‐silicate equilibrium at high PT conditions where the core‐forming metal is a Fe‐Ni‐S‐Si‐C metallic liquid, but two other—Ag and Bi—become overabundant during core formation and require a removal mechanism such as late sulfide segregation. All of the VSE in the mantle of Mars are consistent with core formation in a volatile element depleted body, and do not require any additional processes. Only P and Ag in Vesta's mantle are consistent with combined core formation and volatile‐depleted precursors, whereas the rest require accretion of chondritic or volatile‐bearing material after core formation. The concentrations of Zn, Ag, and Cd modeled for Vesta's core are similar to the concentration range measured in magmatic iron meteorites indicating that these volatile elements were already depleted in Vesta's precursor materials.

Significant depletion of volatile elements in the mantle of asteroid Vesta due to core formation

Vesta, the second-largest body in the asteroid belt, is believed to be a prime example of a rocky protoplanet. Studies of the HED (Howardite–Eucrite–Diogenite) meteorite suite, considered to be sourced from the mantle of Vesta, provide insight into protoplanet differentiation processes. Eucrites and diogenites are depleted in many volatile elements, but the depletions of many volatile siderophile elements are not well constrained. Although previous work indicated that the Vestan core equilibrated with its mantle in a global melting event, the possible contribution of core formation to the volatile budget of HED meteorites has not been assessed to date. This prohibits an assessment of the overall volatile depletion systematics of Vesta.Here, we use a compilation of published volatile siderophile element (VSE; C, S, Zn, As, Se, Cd, In, Sb, Te, Tl, Pb, Bi) abundances in diogenites and eucrites to constrain their depletions in the Vestan mantle. We assess to what extent these depletions may be volatility-related, caused by partitioning into sulfides in the mantle, and/or a result of their preferential partitioning into the Vestan core during core-mantle equilibration. Our new estimates for VSE depletions in the Vestan mantle show no correlation with condensation temperature, suggesting the depletions are difficult to reconcile with partial condensation or degassing during accretionary processes only. Sulfide saturation of the diogenite and eucrite source region is also unlikely because of low indigenous S abundances in these samples compared to calculated S contents at sulfide saturation. Consideration of the metal-silicate partitioning behaviour of the VSE shows that most of these elements are sufficiently siderophile to explain a significant part of their observed depletions in the Vestan mantle by their preferential partitioning into a S-rich core.Moderately volatile elements As and Sb both appear more abundant in the Vestan mantle than expected from their highly siderophile behaviour in systems with basaltic melts. These apparent overabundances may be related to strong silicate melt compositional effects on their metal-silicate partitioning behaviour. On the other hand, Zn, and to a lesser extent Te and Tl are significantly more depleted than expected from metal-silicate partitioning alone. This could be evidence for volatile loss of these elements from Vesta through kinetic loss in a hard vacuum. Alternatively, the perceived loss of Zn may be the result of a Zn-poor Vestan bulk composition and/or partly by preferential partitioning of Zn into spinel. The additional depletion of Tl and by extension other highly chalcophile elements such as Se and Te may be explained by segregation of sulfides in the eucritic melt source region(s), if significant amounts of S degassed from eucritic melts after sulfide segregation.Our results underline the feasibility of planetary cores being a major reservoir for many volatile elements that are depleted in planetary mantles, and that are currently assumed to have suffered incomplete condensation during accretion or (partial) degassing after accretion.

The dependence of metal-silicate partitioning of moderately volatile elements on oxygen fugacity and Si contents of Fe metal: Implications for their valence states in silicate liquids

The volatile siderophile elements are important tracers of the delivery of volatile elements to the Earth. Their concentrations in the bulk silicate Earth are a function of the relative timing of their accretion and their sequestration into the core: a comprehensive understanding of their metal-silicate partitioning behaviour is therefore required in order to infer the volatile element accretion history. We present new partitioning data between liquid metal and liquid silicate at 11 GPa for a suite of volatile siderophile elements: Ag, As, Au, Cu, Ge, P, Pb, Sb, Sn. We focus particularly on determining their valence states and the effects of Si on partitioning, which are required in order to extrapolate from experimental conditions to core-formation conditions. It was found that all elements have weak to strong positive interaction parameters with Si. At low fO2, redox equilibria dictate that the siderophile elements should become more siderophile. However, at low fO2, Si also partitions more strongly into the metal. Given the repulsive nature of the interaction between Si and the elements of interest, the increased Si concentration at low fO2 will counteract the expected increase in the partition coefficient, making these elements less siderophile than expected at very reducing conditions. This causes the linear relationship between fO2 and log(D) to become non-linear at low fO2, which we account for by fitting an interaction parameter between Si and the elements of interest. This has implications for the interpretation of experimental results, because the valence cannot be determined from the slope of log(D) vs. logfO2 if low fO2, high Si metal compositions are employed without applying an activity correction. This also has implications for the extrapolation of experimental partitioning data to core-formation conditions: reducing conditions in the early stages of core formation do not necessarily result in complete or even strong depletion of siderophile elements when Si is present as a light element in the core-forming metal phase.

Effect of silicon on activity coefficients of siderophile elements (Au, Pd, Pt, P, Ga, Cu, Zn, and Pb) in liquid Fe: Roles of core formation, late sulfide matte, and late veneer in shaping terrestrial mantle geochemistry

Earth’s core contains ∼10% of a light element that may be a combination of Si, S, C, O or H, with Si potentially being the major light element. Metal-silicate partitioning of siderophile elements can place important constraints on the P-T-fO2 and composition of the early Earth, but the effect of Si alloyed in Fe liquids is unknown for many of these elements. In particular, the effect of Si on the partitioning of highly siderophile elements (Au, Re and PGE) is virtually unknown. To address this gap in understanding, we have undertaken a systematic study of the highly siderophile elements Au, Pd, and Pt, and the volatile siderophile elements P, Ga, Cu, Zn, and Pb at variable Si content of metal, and 1600 °C and 1 GPa. From our experiments we derive epsilon interaction parameters between these elements and Si in Fe metallic liquids. The new parameters are used to update an activity model for trace siderophile elements in Fe alloys; Si causes large variation in the magnitude of activity coefficients of these elements in FeSi liquids. Because the interaction parameters are all positive, Si causes a decrease in their metal/silicate partition coefficients. We combine these new activity results with experimental studies of Au, Pd, Pt, P, Ga, Cu, Zn and Pb, to derive predictive expressions for metal/silicate partition coefficients which can then be applied to Earth. The expressions are applied to two scenarios for continuous accretion of Earth; specifically for constant and increasing fO2 during accretion. The results indicate that mantle concentrations of P, Ga, Cu, Zn, and Pb can be explained by metal-silicate equilibrium during accretion of the Earth where Earth’s early magma ocean deepens to pressures of 40–60 GPa. Au, Pd, and Pt, on the other hand become too high in the mantle in such a scenario, and require a later removal mechanism, rather than an addition as traditionally argued. A late reduction event that removes 0.5% metal from a shallow magma ocean can lower the Au, Pd, and Pt contents to values near the current day BSE. On the other hand, removal of 0.2–1.0% of a late sulfide-rich matte to the core would lower the Au, Pd, and Pt concentrations in the mantle, but not to chondritic relative concentrations observed in the BSE. If sulfide matte is called upon to remove HSEs, they must be later added via a late veneer to re-establish the high and chondritic relative PUM concentrations. These results suggest that although accretion and core formation (involving a Si, S, and C-bearing metallic liquid) were the primary processes establishing many of Earth’s mantle volatile elements and HSE, a secondary removal process is required to establish HSEs at their current and near-chondritic relative BSE levels. Mn and P - two siderophile elements that are central to biochemical processes (photosynthesis and triphosphates, respectively) - have significant and opposite interactions with FeSi liquids, and their mantle concentrations would be notably different if Earth had a Si-free core.

Cooling rates of type I chondrules from Renazzo: Implications for chondrule formation

AbstractCooling rates are one of the few fundamental constraints on models of chondrule formation. In this study, we used Cu and Ga diffusion profiles in metal grains to determine the cooling rates of type I chondrules in the Renazzo CR2 chondrite. To improve previous estimations of cooling rates obtained using this method, we used CT scanning and serial polishing of our sections to analyze equatorial sections of large metal grains. Through the cores of these metal grains situated at the surface of chondrules, the cooling rates calculated range from 21 to 86 K h−1 for a peak temperature Tp ~ 1623–1673 K. A metal grain embedded in the core of a chondrule exhibits a cooling rate of 1.2 K h−1 at a Tp ~ 1573 K. We also measured Cu‐Ga diffusion profiles from nonequatorial sections of metal grains and calculated a lower range of cooling rates of 15–69 K h−1 for Tp ~ 1473–1603 K compared to our results from equatorial sections. The high cooling rates inferred from the lightning model (several thousand K h−1) are clearly at odds with the values obtained in this work. The X‐wind model predicts cooling rates (~6–10 K h−1) lower than most of our results. The cooling rates calculated here are in close agreement with those inferred from shock wave models, in particular for temperatures at which olivine crystallizes (from ~10 to several hundreds K h−1 between 1900 and 1500 K). However, the chemical compositions of metal grains in Renazzo are consistent with the splashing model, in which a spray of metal droplets originated from a partially molten planetesimal. Volatile siderophile element depletion is explained by evaporation before metal was engulfed within silicate droplets. Liquid metal isolated from the liquid silicate crystallized during cooling, reacted with the ambient gas, and then re‐accreted within partially molten chondrules.

Volatile element signatures in the mantles of Earth, Moon, and Mars: Core formation fingerprints from Bi, Cd, In, and Sn

AbstractVolatile element concentrations in planets are controlled by many factors such as precursor material composition, core formation, differentiation, magma ocean and magmatic degassing, and late accretionary processes. To better constrain the role of core formation, we report new experiments defining the effect of temperature, and metallic S and C content on the metal‐silicate partition coefficient (or D(i) metal/silicate) of the volatile siderophile elements (VSE) Bi, Cd, In, and Sn. Additionally, the effect of pressure on metal‐silicate partitioning between 1 and 3 GPa, and olivine‐melt partitioning at 1 GPa have been studied for Bi, Cd, In, Sn, As, Sb, and Ge. Temperature clearly causes a decrease in D(i) metal/silicate for all elements. Sulfur and C have a large influence on activity coefficients in metallic Fe liquids, with C causing a decrease in D(i) metal/silicate, and S causing an increase. Pressure has only a small effect on D(Cd), D(In), and D(Ge) metal/silicate. Depletions of Bi, Cd, In, and Sn in the terrestrial and Martian mantles are consistent with high PT core formation and metal‐silicate equilibrium at the high temperatures indicated by previous studies. A late Hadean matte would influence Bi the most, due to its high D(sulfide/silicate) ~2000, but segregation of a matte would only reduce the mantle Bi content by 50%; all other less chalcophile elements (e.g., Sn, In, and Cd) would be minimally affected. The lunar depletions of highly VSE require a combination of core formation and an additional depletion mechanism—most likely the Moon‐forming giant impact, or lunar magma ocean degassing.

Distribution of Sb, As, Ge, and In between metal and silicate during accretion and core formation in the Earth

A large number of siderophile (iron-loving) elements are also volatile, thus offering constraints on the origin of volatile elements in differentiated bodies such as Earth, Moon, Mars and Vesta. Metal–silicate partitioning data for many of these elements is lacking, making their overall mantle concentrations in these bodies difficult to model and origin difficult to distinguish between core formation and volatile depletion. To address this gap in understanding, we have undertaken systematic studies of four volatile siderophile elements – Sb, As, Ge and In – at variable temperature and variable Si content of metal. Several series were carried out at 1GPa, and between 1500 and 1900°C, for both C saturated and C-free conditions. The results show that temperature causes a decrease in the metal/silicate partition coefficient for all four elements. In addition, activity coefficients for each element have been determined and show a very strong dependence on Si content of Fe alloy. Si dissolved in metal significantly decreases the metal/silicate partition coefficients, at both 1600 and 1800°C. The combination of temperature and Si content of the metal causes reduction of the metal–silicate partition coefficient to values that are close to those required for an origin of mantle As, Sb, Ge, and In concentrations by metal–silicate equilibrium processes. Combining these new results with previous studies on As, Sb, Ge, and In, allowed derivation of predictive expressions for metal/silicate partition coefficients for these elements which can then be applied to Earth. The expressions are applied to two scenarios for continuous accretion of Earth; specifically for constant and increasing fO2 during accretion. The results indicate that mantle concentrations of As, Sb, Ge, and In can be explained by metal–silicate equilibrium during an accretion scenario. The modeling is not especially sensitive to either scenario, although all element concentrations are explained better by a model with variable fO2. The specific effect of Si is important and calculations that include only S and C (and no Si) cannot reproduce the mantle As, Sb, Ge, and In concentrations. The final core composition in the variable fO2 model is 10.2% Si, 2% S, and 1.1% C (or XSi=0.18, XS=0.03, and XC=0.04. These results suggest that core formation (involving a Si, S, and C-bearing metallic liquid) and accretion were the most important processes establishing many of Earth’s mantle volatile elements (indigenous), while post-core formation addition or re-equilibration (exogenous) was of secondary or minor importance.

New geochemical models of core formation in the Moon from metal–silicate partitioning of 15 siderophile elements

We re-examine the conditions at which core formation in the Moon may have occurred by linking the observed lunar mantle depletions of 15 siderophile elements, including volatile siderophile elements (VSE) to predictive equations derived from a database compilation of metal–silicate partition coefficients obtained at lunar-relevant pressure–temperature–oxygen fugacity (P–T–fO2P–T–fO2) conditions. Our results suggest that at mantle temperatures between the solidus and liquidus the depletions for all elements considered can be satisfied, but only if the Moon was essentially fully molten at the time of core formation while assuming a S-rich (>8 wt%) core comprising 2.5 wt% of the mass of the Moon. However, we observe that at temperatures exceeding the mantle liquidus, with increasing temperature the core S content required to satisfy the element depletions is reduced. As a S-poor core is likely from recent lunar mantle estimates of S abundance, this suggests much higher temperatures during lunar core formation than previously proposed. We conclude that the VSE depletions in the lunar mantle can be solely explained by core formation depletion, suggesting that no significant devolatilization has occurred in later periods of lunar evolution. This is in agreement with the discovery of significant amounts of other volatiles in the lunar interior, but hard to reconcile with current lunar formation models.

Siderophile and chalcophile element abundances in shergottites: Implications for Martian core formation

AbstractElemental abundances for volatile siderophile and chalcophile elements for Mars inform us about processes of accretion and core formation. Such data are few for Martian meteorites, and are often lacking in the growing number of desert finds. In this study, we employed laser ablation inductively coupled plasma–mass spectrometry (LA‐ICP‐MS) to analyze polished slabs of 15 Martian meteorites for the abundances of about 70 elements. This technique has high sensitivity, excellent precision, and is generally accurate as determined by comparisons of elements for which literature abundances are known. However, in some meteorites, the analyzed surface is not representative of the bulk composition due to the over‐ or underrepresentation of a key host mineral, e.g., phosphate for rare earth elements (REE). For other meteorites, the range of variation in bulk rastered analyses of REE is within the range of variation reported among bulk REE analyses in the literature. An unexpected benefit has been the determination of the abundances of Ir and Os with a precision and accuracy comparable to the isotope dilution technique. Overall, the speed and small sample consumption afforded by this technique makes it an important tool widely applicable to small or rare meteorites for which a polished sample was prepared. The new volatile siderophile and chalcophile element abundances have been employed to determine Ge and Sb abundances, and revise Zn, As, and Bi abundances for the Martian mantle. The new estimates of Martian mantle composition support core formation at intermediate pressures (14 ± 3 GPa) in a magma ocean on Mars.

Highly siderophile element (HSE) abundances in the mantle of Mars are due to core formation at high pressure and temperature

AbstractHighly siderophile elements (HSEs) can be used to understand accretion and core formation in differentiated bodies, due to their strong affinity for FeNi metal and sulfides. Coupling experimental studies of metal–silicate partitioning with analyses of HSE contents of Martian meteorites can thus offer important constraints on the early history of Mars. Here, we report new metal–silicate partitioning data for the PGEs and Au and Re across a wide range of pressure and temperature space, with three series designed to complement existing experimental data sets for HSE. The first series examines temperature effects for D(HSE) in two metallic liquid compositions—C‐bearing and C‐free. The second series examines temperature effects for D(Re) in FeO‐bearing silicate melts and FeNi‐rich alloys. The third series presents the first systematic study of high pressure and temperature effects for D(Au). We then combine our data with previously published partitioning data to derive predictive expressions for metal–silicate partitioning of the HSE, which are subsequently used to calculate HSE concentrations of the Martian mantle during continuous accretion of Mars. Our results show that at midmantle depths in an early magma ocean (equivalent to approximately 14 GPa, 2100 °C), the HSE contents of the silicate fraction are similar to those observed in the Martian meteorite suite. This is in concert with previous studies on moderately siderophile elements. We then consider model calculations that examine the role of melting, fractional crystallization, and sulfide saturation/undersaturation in establishing the range of HSE contents in Martian meteorites derived from melting of the postcore formation mantle. The core formation modeling indicates that the HSE contents can be established by metal–silicate equilibrium early in the history of Mars, thus obviating the need for a late veneer for HSE, and by extension volatile siderophile elements, or volatiles in general.

Volatile fractionation in the early solar system and chondrule/matrix complementarity.

Bulk chondritic meteorites and terrestrial planets show a monotonic depletion in moderately volatile and volatile elements relative to the Sun's photosphere and CI carbonaceous chondrites. Although volatile depletion was the most fundamental chemical process affecting the inner solar nebula, debate continues as to its cause. Carbonaceous chondrites are the most primitive rocks available to us, and fine-grained, volatile-rich matrix is the most primitive component in these rocks. Several volatile depletion models posit a pristine matrix, with uniform CI-like chemistry across the different chondrite groups. To understand the nature of volatile fractionation, we studied minor and trace element abundances in fine-grained matrices of a variety of carbonaceous chondrites. We find that matrix trace element abundances are characteristic for a given chondrite group; they are depleted relative to CI chondrites, but are enriched relative to bulk compositions of their parent meteorites, particularly in volatile siderophile and chalcophile elements. This enrichment produces a highly nonmonotonic trace element pattern that requires a complementary depletion in chondrule compositions to achieve a monotonic bulk. We infer that carbonaceous chondrite matrices are not pristine: they formed from a material reservoir that was already depleted in volatile and moderately volatile elements. Additional thermal processing occurred during chondrule formation, with exchange of volatile siderophile and chalcophile elements between chondrules and matrix. This chemical complementarity shows that these chondritic components formed in the same nebula region.

Formation of metal and silicate globules in Gujba: a new Bencubbin-like meteorite fall

Gujba is a coarse-grained meteorite fall composed of 41 vol% large kamacite globules, 20 vol% large light-colored silicate globules with cryptocrystalline, barred pyroxene and barred olivine textures, 39 vol% dark-colored, silicate-rich matrix, and rare refractory inclusions. Gujba resembles Bencubbin and Weatherford in texture, oxygen-isotopic composition and in having high bulk δ 15N values (∼+685‰). The 3He cosmic-ray exposure age of Gujba (26 ± 7 Ma) is essentially identical to that of Bencubbin, suggesting that they were both reduced to meter-size fragments in the same parent-body collision. The Gujba metal globules exhibit metal-troilite quench textures and vary in their abundances of troilite and volatile siderophile elements. We suggest that the metal globules formed as liquid droplets either via condensation in an impact-generated vapor plume or by evaporation of preexisting metal particles in a plume. The lower the abundance of volatile elements in the metal globules, the higher the globule quench temperature. We infer that the large silicate globules also formed from completely molten droplets; their low volatile-element abundances indicate that they also formed at high temperatures, probably by processes analogous to those that formed the metal globules. The coarse-grained Bencubbin-Weatherford-Gujba meteorites may represent a depositional component from the vapor cloud enriched in coarse and dense particles. A second class of Bencubbin-like meteorites (represented by Hammadah al Hamra 237 and QUE 94411) may be a finer fraction derived from the same vapor cloud.

Geochemistry of the ungrouped carbonaceous chondrite Tagish Lake, the anomalous CM chondrite Bells, and comparison with CI and CM chondrites

Abstract— I have determined the composition via instrumental neutron activation analysis of a bulk pristine sample of the Tagish Lake carbonaceous chondrite fall, along with bulk samples of the CI chondrite Orgueil and of several CM chondrites. Tagish Lake has a mean of refractory lithophile element/Cr ratios like those of CM chondrites, and distinctly higher than the CI chondrite mean. Tagish Lake exhibits abundances of the moderately volatile lithophile elements Na and K that are slightly higher than those of mean CM chondrites. Refractory through moderately volatile siderophile element abundances in Tagish Lake are like those of CM chondrites. Tagish Lake is distinct from CM chondrites in abundances of the most volatile elements. Mean CI‐normalized Se/Co, Zn/Co and Cs/Co for Tagish Lake are 0.68 ± 0.01, 0.71 ± 0.07 and 0.76 ± 0.02, while for all available CM chondrite determinations, these ratios lie between 0.31 and 0.61, between 0.32 and 0.58, and between 0.39 and 0.74, respectively. Considering petrography, and oxygen isotopic and elemental compositions, Tagish Lake is an ungrouped member of the carbonaceous chondrite clan. The overall abundance pattern is similar to those of CM chondrites, indicating that Tagish Lake and CMs experienced very similar nebular fractionations.Bells is a CM chondrite with unusual petrologic characteristics. Bells has a mean CI‐normalized refractory lithophile element/Cr ratio of 0.96, lower than for any other CM chondrite, but shows CI‐normalized moderately volatile lithophile element/Cr ratios within the ranges of other CM chondrites, except for Na which is low. Iridium, Co, Ni and Fe abundances are like those of CM chondrites, but the moderately volatile siderophile elements, Au, As and Sb, have abundances below the ranges for CM chondrites. Abundances of the moderately volatile elements Se and Zn of Bells are within the CM ranges. Bells is best classified as an anomalous CM chondrite.

Siderophile element constraints on the formation of metal in the metal-rich chondrites Bencubbin, Weatherford, and Gujba

Laser ablation inductively coupled plasma mass spectrometry was used to measure abundances of P, Cr, Fe, Co, Ni, Cu, Ga, Ge, As, Mo, Ru, Rh, Pd, Sn, Sb, W, Re, Os, Ir, Pt, and Au in metal grains in the Bencubbin-like chondrites Bencubbin, Weatherford, and Gujba to determine the origin of large metal aggregates in bencubbinites. A strong volatility-controlled signature is observed among the metal grains. The refractory siderophiles Ru, Rh, Re, Os, Ir, and Pt are unfractionated from one another, and are present in approximately chondritic relative abundances. The less refractory elements Fe, Co, Ni, Pd, and Au are fractionated from the refractory siderophiles, with a chondritic Ni/Co ratio and a higher than chondritic Pd/Fe ratio. The moderately volatile siderophile elements Ga, Ge, As, Sn, and Sb are depleted in the metal, relative to chondritic abundances, by up to 3 orders of magnitude. The trace siderophile element data are inconsistent with the following proposed origins of Bencubbin-Weatherford-Gujba metal: (1) condensation from the canonical solar nebula, (2) oxidation of an initially chondritic metal composition, and (3) equilibration with a S-rich partial melt. A condensation model for metal-enriched (×107 CI) gas is developed. Formation by condensation or evaporation in such a high-density, metal-enriched gas is consistent with the trace element measurements. The proposed model for generating such a gas is protoplanetary impact involving a metal-rich body.