Siderophile Element Abundances Research Articles

Origin of iron production in the Eastern Mediterranean: Osmium isotope and highly siderophile element evidence from Iron Age Jordan

Osmium isotope and highly siderophile element (HSE: Os, Ir, Ru, Rh, Pt, Pd, Re, Au) abundance analysis provides a new methodology for examining the origins of ancient iron production in the Eastern Mediterranean, a current scholarly lacuna. Here we present results from a two-stage campaign of laser-ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) followed by thermal ionization mass spectrometry and solution ICP-MS to measure mixed iron-copper chunks and iron objects from the Faynan region of Jordan. Faynan is a copper-ore resource zone with a rich history of copper smelting. The combination of sophisticated copper metallurgy and iron artifacts in the archaeological record of Iron Age Faynan (10th-9th centuries BCE) offers an exceptional opportunity to investigate the hypothesis that iron production originally developed as an adventitious byproduct of advanced copper smelting technologies. Based on the results, we find no connection from HSE abundances and inter-element ratios, or osmium isotopes, between the iron objects (Pt/Os = 2–7; 187Os/188Os = 1.15 to 2.50) and mixed metal chunks from the furnaces (Pt/Os = 7.7–35; 187Os/188Os = 2.15 to 8.96) excavated in Faynan, indicating that the objects were not locally produced. This conclusion provides additional evidence that iron production in the Levant was probably not directly developed from copper smelting. In turn, osmium isotopes and siderophile element abundances in artifacts and products from metal production can also offer a new line of evidence for examining exchange networks in the Iron Age Eastern Mediterranean and elsewhere.

Genetics, age and crystallization history of group IIC iron meteorites

The eight iron meteorites currently classified as belonging to the IIC group were characterized with respect to the compositions of 21 siderophile elements. Several of these meteorites were also characterized for mass independent isotopic compositions of Mo, Ru and W. Chemical and isotopic data for one, Wiley, indicate that it is not a IIC iron meteorite and should be reclassified as ungrouped. The remaining seven IIC iron meteorites exhibit broadly similar bulk chemical and isotopic characteristics, consistent with an origin from a common parent body. Variations in highly siderophile element (HSE) abundances among the members of the group can be well accounted for by a fractional crystallization model with all the meteorites crystallizing between ∼10 and ∼26% of the original melt, assuming initial S and P concentrations of 8 wt.% and 2 wt.%, respectively. Abundances of HSE estimated for the parental melt suggest a composition with chondritic relative abundances of HSE ∼6 times higher than in bulk carbonaceous chondrites, consistent with the IIC irons sampling a parent body core comprising ∼17% of the mass of the body.Radiogenic 182W abundances of two group IIC irons, corrected for a nucleosynthetic component, indicate a metal-silicate segregation age of 3.2 ± 0.5 Myr subsequent to the formation of Calcium-Aluminum-rich Inclusions (CAI). When this age is coupled with thermal modeling, and assumptions about the Hf/W of precursor materials, a parent body accretion age of 1.4 ± 0.5 Myr (post-CAI) is obtained.The IIC irons and Wiley have 100Ru mass independent “genetic” isotopic compositions that are identical to other irons with so-called carbonaceous chondrite (CC) type genetic affinities, but enrichments in 94,95,97Mo and 183W that indicate greater s-process deficits relative to most known CC iron meteorites. If the IIC irons and Wiley are of the CC type, this indicates variable s-process deficits within the CC reservoir, similar to the s-process variability within the NC reservoir observed for iron meteorites. Nucleosynthetic models indicate that Mo and 183W s-process variability should correlate with Ru isotopic variability, which is not observed. This may indicate the IIC irons and Wiley experienced selective thermal processing of nucleosynthetic carriers, or are genetically distinct from the CC and NC precursor materials.

Ultra-depleted 2.05 Ga komatiites of Finnish Lapland: Products of grainy late accretion or core-mantle interaction?

Rhenium-Os, PtOs, SmNd, LuHf, and HfW isotope data, together with lithophile trace element and highly siderophile element (HSE: Re, Os, Ir, Ru, Pt, and Pd) abundances, are reported for 2.05 Ga Jeesiörova and Kevitsa komatiites from the Central Lapland Greenstone Belt, Fennoscandia, Finland. Both komatiites are closely genetically related, with the Kevitsa dikes having served as feeding magma conduits to the Jeesiörova pillowed and massive lavas. The parental komatiite magma is estimated to have contained ~25 wt% MgO and was, thus, derived from a mantle source at least as hot as those of some of its late Archean counterparts. A suite of Jeesiörova and Kevitsa whole-rock komatiite samples and olivine and chromite separates define an internal ReOs isochron with an age of 2049 ± 13 Ma and an initial γ187Os = −0.2 ± 0.2 (2SE), indicating long-term chondritic Re/Os in the mantle source. By contrast, PtOs data for a set of Jeesiörova chromite separates define an average initial μ186Os = +29 ± 2 (2SE), indicating a long-term history of suprachondritic Pt/Os in the mantle source. The absolute HSE abundances in the mantle source of the Jeesiörova-Kevitsa komatiite system are estimated to have been 120 ± 5% of the present-day Bulk Silicate Earth (BSE). This is the first komatiite system for which excess HSE in the mantle source, relative to modern BSE, has been documented.The 147Sm143Nd and 176Lu176Hf data yield isochron ages and initial ratios of, respectively, 2046 ± 22 Ma with ε143Nd = +3.7 ± 0.3, and 2072 ± 20 Ma with ε176Hf = +8.7 ± 0.4 (2SE), indicating a long-term history of depletions of Nd relative to Sm, and Hf relative to Lu. The measured μ182W = +1.5 ± 3.3 is indistinguishable from the modern mantle value. Despite being strongly depleted in highly incompatible lithophile trace elements, the Th-Nb-La systematics of the komatiites indicate ~1% crustal contamination of the original komatiite magma, assuming the contaminant was similar in composition to the calculated Fennoscandian Tonalite Average (FTA). This level of contamination would have also significantly modified the Nd, Hf, and W isotope compositions of the original komatiitic magma, but not the Os isotope compositions or HSE abundances. The calculated original komatiite magma, corrected for the effects of crustal contamination, would have had initial ε143Nd ~ +4.9, ε176Hf ~ +10.2, and μ182W ~ −10.Our modeling indicates that the initial 186,187Os/188Os isotopic compositions and suprachondritic HSE abundances, coupled with the projected negative μ182W, are best explained by either (1) derivation from a mantle domain characterized by an excess of late accreted, differentiated planetesimal core metal, i.e., “grainy” late accretion, or (2) addition of chemically fractionated terrestrial core metal to the mantle source domain of the komatiites. The presence of these characteristics in the Jeesiörova-Kevitsa komatiite mantle source provides further evidence for the early creation and long-term survival of chemically diverse domains within the mantle.

Forearc origin for Coast Range Ophiolites inferred from osmium isotopes and highly siderophile elements

The Jurassic Coast Range Ophiolites, California, provide evidence for tectonic processes that formed the western margin of North America. Whether the ophiolites were formed in a mid-ocean ridge, back arc or forearc setting, however, remains unconstrained. To address this issue, a systematic grid sampling approach was taken to examine the mantle section of an archetypal ~161 Ma Coast Range Ophiolite at Point Sal, California. The Point Sal peridotites are serpentinized, with 187Re187Os evidence for a localized Late Miocene alteration event in grids closest to the lowermost bounding fault of the ophiolite. This event is interpreted to be related to tectonic changes and hydration-dehydration reactions acting on the serpentinites in response to changes in the azimuth and rate of Pacific-North American plate motion. Alteration and serpentinization have not systematically affected highly siderophile element (HSE) abundances, especially Os, Ir or Ru which, together with rare earth element abundances, record significant melt depletion (>20%) of a fertile mantle protolith (187Os/188Os = ~0.129) during ophiolite formation. Peridotites subsequently experienced localized and heterogenous addition of Re, Pt, Pd and radiogenic 187Os/188Os from Cl-rich, oxidized (QFM) fluids. Geochemical features of both Point Sal peridotites, as well as associated basaltic lavas, are consistent with high degrees of partial melting and variable addition of fluids as also observed in global forearc peridotites. A forearc origin for the Point Sal Ophiolite Complex supports models for the dominant formation process of 153–166 Ma Coast Range Ophiolite infant Arc crust terranes within a single westward facing arc along the Sierra continental-margin.

High resolution osmium data record three distinct pulses of magmatic activity during cretaceous Oceanic Anoxic Event 2 (OAE-2)

Oceanic Anoxic Event 2 (OAE-2) occurred at the Cenomanian-Turonian boundary (∼94.1 Ma) and was a time of profound global changes in ocean chemistry and the carbon cycle. This event was characterized by a positive carbon isotope excursion (CIE) caused by massive organic carbon burial, global greenhouse temperatures, ocean deoxygenation, and changes in ocean life driven by large igneous province (LIP) activity. LIPs throughout the Phanerozoic have had dynamic magma flux, with episodes of major eruptions interspersed with periods of relatively less intense eruptions. A possible trigger for LIP activity throughout the Phanerozoic has been attributed to extraterrestrial impacts because there are multiple contemporaneous occurrences of large craters, LIP activity, and mass extinctions in the geologic record. At the Cenomanian-Turonian boundary, there is a 25 km diameter (rim-to-rim) complex crater in NW Alberta, Canada known as the Steen River impact structure dated at 91 ± 7 Ma (Carrigy and Short, 1968). An alternative explanation for those craters found contemporaneous with LIP activity and mass extinctions is that they were created by large explosive events related to LIP activity. Explosive events associated with mantle plume incubation beneath cratonic lithosphere have been suggested to create geologic features commonly attributed to impacts (e.g., shocked quartz, microspherules, etc.). Currently, the trigger for LIP activity during OAE-2, as well as the duration of LIP activity and the temporal variation and magnitude of eruption rates are not well constrained.To address the issue of LIP eruption rates and the trigger for LIP activity, we examined osmium (Os) abundances and isotopes as well as highly siderophile element (HSE; for this study: Re, Ru, Pd, Os, Ir, Pt) abundance data from a continuous sedimentary section spanning OAE-2. The section is from the Eagle Ford Group in the Iona-1 core, deposited in the Cretaceous Western Interior Seaway (KWIS). We found three high Os concentration intervals with mantle-like initial Osi isotope (initial 187Os/188Os) values of ∼0.16. These intervals are interpreted to reflect high-flux LIP magmatic pulses. Between the pulses, lower Os abundances with more radiogenic Osi values of ∼0.20 are observed, which we interpret as low-flux LIP activity between the high-flux periods. This trend of high-and-low flux Os concentration pulses with mantle-like Osi values during the high flux periods is found in another KWIS core (USGS Portland #1) deposited to the north of Iona-1, and in core Deep Sea Drilling Project (DSDP) Site 530 Hole A (hereafter DSDP 530A; drilled off-shore Namibia in the Angola Basin) deposited in the Southern Hemisphere. Before and throughout the Iona-1 core OAE-2 interval, HSE abundance patterns indicate a mantle source for the unradiogenic Os, and are not consistent with an extraterrestrial impact trigger or contribution to LIP activity during OAE-2.This evidence for multiple high-flux pulses of LIP activity driving ocean deoxygenation has implications for the modern ocean, which is currently experiencing deoxygenation. These results provide new constraints on subsequent high-flux periods that extended the event. The first high-flux period started ∼60 Kyr after our selection of the onset of the CIE. The second and third high-flux periods started ∼270 and ∼400 Kyr after the onset of the CIE, respectively. After the third high-flux period, δ13Corg and Os isotope ratios shifted back to pre-excursion values over ∼585 kyr. In the Iona-1 core, OAE-2 lasted for 1.04 ± 0.12 Myr based on our selection of the CIE.

The composition of Mars

Comparing compositional models of the terrestrial planets provides insights into physicochemical processes that produced planet-scale similarities and differences. The widely accepted compositional model for Mars assumes Mn and more refractory elements are in CI chondrite proportions in the planet, including Fe, Mg, and Si, which along with O make up >90% of the mass of Mars. However, recent improvements in our understandings on the composition of the solar photosphere and meteorites challenge the use of CI chondrite as an analog of Mars. Here we present an alternative model composition for Mars that avoids such an assumption and is based on data from Martian meteorites and spacecraft observations. Our modeling method was previously applied to predict the Earth’s composition. The model establishes the absolute abundances of refractory lithophile elements in the bulk silicate Mars (BSM) at 2.26 times higher than that in CI carbonaceous chondrites. Relative to this chondritic composition, Mars has a systematic depletion in moderately volatile lithophile elements as a function of their condensation temperatures. Given this finding, we constrain the abundances of siderophile and chalcophile elements in the bulk Mars and its core. The Martian volatility trend is consistent with ⩽7 wt% S in its core, which is significantly lower than that assumed in most core models (i.e., >10 wt% S). Furthermore, the occurrence of ringwoodite at the Martian core-mantle boundary might have contributed to the partitioning of O and H into the Martian core.

Sulfide mantle source heterogeneity recorded in basaltic lavas from the Azores

New highly siderophile element (HSE: Os, Ir, Ru, Pt, Pd, Re) abundance and Os isotopic compositions, along with major- and trace-element abundance and Sr-Nd-Hf-Pb isotope data are reported for high-MgO (>8–17 wt.%) lavas from the islands of São Miguel, Terceira, Pico and Faial in the Azores archipelago. The lavas span the range of Sr-Nd-Hf-Pb isotope and trace-element abundance variability reported previously for the islands. Pico and Faial lavas preserve distinct absolute and relative HSE abundances and 187Os/188Os (ƩHSE = 2 ± 2 ng g−1; 2SD; 187Os/188Os in lavas with > 50 pg g−1 Os = 0.1255 ± 31; 2SD) compared with São Miguel and Terceira lavas (ƩHSE = 5 ± 3 ng g−1; 187Os/188Os50pg = 0.1284 ± 48). Although HSE abundance variations in volcanic rocks from individual Azorean islands can be explained by sulfide crystallization, those between Pico and Faial and São Miguel and Terceira can only reasonably be attributed to partial melting of distinct mantle sources.Pico and Faial lava compositions are consistent with high degrees of sulfide melting (90–100%) from a sulfide-depleted mantle source (<100 ppm) with relatively unradiogenic 187Os/188Os (<0.126). São Miguel and Terceira lavas likely derive from a sulfide-enriched (300–600 ppm) mantle source that underwent lesser degrees of sulfide melting (1–15%), with more radiogenic 187Os/188Os (>0.128). Radiogenic sulfides are not solely linked to the enriched lithophile isotope signature found in eastern São Miguel. Instead, São Miguel and Terceira magmatism may collectively result from metasomatism associated with the recycling of old subducted oceanic lithosphere. Abundances of sulfide-hosted HSE, and 187Os/188Os, are decoupled from silicate-hosted Sr-Nd-Hf-Pb isotope compositions, in the mantle. Compositional variations of the HSE among global OIB suggest that, while distinct mantle domains are represented by heterogeneous sulfide mineralogy, comprising common metasomatic (enriched) and magmatic (depleted) compositions, these sulfide populations are distinct beneath different OIB due to their origin in different lithospheric materials (e.g., oceanic crust versus pelagic sediment), and to the degree of partial melting caused by variable mantle potential temperature or lithospheric thickness.

Mineralogy, petrology, and oxygen isotopic composition of Northwest Africa 12379, metal-rich chondrite with affinity to ordinary chondrites

Northwest Africa (NWA) 12379 is a new metal-rich chondrite with unique characteristics distinguishing it from all previously described meteorites. It contains high Fe,Ni-metal content (∼ 70 vol.%) and completely lacks interchondrule matrix; these characteristics are typical only for metal-rich carbonaceous (CH and CB) and G chondrites. However, chondrule sizes (60 to 1200 μm; mean = 370 μm), their predominantly porphyritic textures, nearly equilibrated chemical compositions of chondrule olivines (Fa18.1–28.3, average Fa24.9±3.2, PMD = 12.8; Cr2O3 = 0.03 ± 0.02 wt.%; FeO/MnO = 53.2 ± 6.5 (wt.-ratio); n = 28), less equilibrated compositions of low-Ca pyroxenes (Fs3.2–18.7Wo0.2–4.5; average Fs14.7±3.7Wo1.4±1.3; n = 20), oxygen-isotope compositions of chondrule olivine phenocrysts (Δ17O ∼ 0.2–1.4‰, average ∼ 0.8‰), and the presence of coarse-grained Ti-bearing chromite, Cl-apatite, and merrillite, all indicate affinity of NWA 12379 to unequilibrated (type 3.8) ordinary chondrites (OCs). Like most OCs, NWA 12379 experienced fluid-assisted thermal metamorphism that resulted in formation of secondary ferroan olivine (Fa27) that replaces low-Ca pyroxene grains in chondrules and in inclusions in Fe,Ni-metal grains. Δ17O of the ferroan olivine (∼ 4‰) is similar to those of aqueously-formed fayalite in type 3 OCs, but its δ18O is significantly higher (15–19‰, average = 17‰ vs. 3―12‰, average = 8‰, respectively). We suggest classifying NWA 12379 as the ungrouped metal-rich chondrite with affinities of its non-metal fraction to unequilibrated OCs and speculate that it may have formed by a collision between an OC-like body and a metal-rich body and subsequently experienced fluid-assisted thermal metamorphism. Trace siderophile element abundances and isotopic compositions (e.g., Mo, Ni, Fe) of the NWA 12379 metal could help to constrain its origin.

Pacific Lithosphere Evolution Inferred from Aitutaki Mantle Xenoliths

Abstract Highly siderophile element (HSE: Os, Ir, Ru, Pt, Pd, Re), major and trace element abundances, and 187Re–187Os systematics are reported for xenoliths and lavas from Aitutaki (Cook Islands), to investigate the composition of Pacific lithosphere. The xenolith suite comprises spinel-bearing lherzolites, dunite, and harzburgite, along with olivine websterite and pyroxenite. The xenoliths are hosted within nephelinite and alkali basalt volcanic rocks (187Os/188Os ∼0·1363 ± 13; 2SD; ΣHSE = 3–4 ppb). The volcanic host rocks are low-degree (2–5%) partial melts from the garnet stability field and an enriched mantle (EM) source. Pyroxenites have similar HSE abundances and Os isotope compositions (Al2O3 = 5·7–8·3 wt %; ΣHSE = 2–4 ppb; 187Os/187Os = 0·1263–0·1469) to the lavas. The pyroxenite and olivine websterite xenoliths directly formed from—or experienced extensive melt–rock interaction with—melts similar in composition to the volcanic rocks that host the xenoliths. Conversely, the Aitutaki lherzolites, harzburgites and dunites are similar in composition to abyssal peridotites with respect to their 187Os/188Os ratios (0·1264 ± 82), total HSE abundances (ΣHSE = 8–28 ppb) and major element abundances, forsterite contents (Fo89·9±1·2), and estimated extents of melt depletion (&amp;lt;10 to &amp;gt;15%). These peridotites are interpreted to sample relatively shallow Pacific mantle lithosphere that experienced limited melt–rock reaction and melting during ridge processes at ∼90 Ma. A survey of maximum time of rhenium depletion ages of Pacific mantle lithosphere from the Cook (Aitutaki ∼1·5 Ga), Austral (Tubuai’i ∼1·8 Ga), Samoan (Savai’i ∼1·5 Ga) and Hawaiian (Oa’hu ∼2 Ga) island groups shows that Mesoproterozoic to Neoproterozoic depletion ages are preserved in the xenolith suites. The variable timing and extent of mantle depletion preserved by the peridotites is, in some instances, superimposed by extensive and recent melt depletion as well as melt refertilization. Collectively, Pacific Ocean island mantle xenolith suites have similar distributions and variations of 187Os/188Os and HSE abundances to global abyssal peridotites. These observations indicate that Pacific mantle lithosphere is typical of oceanic lithosphere in general, and that this lithosphere is composed of peridotites that have experienced both recent melt depletion at ridges and prior and sometimes extensive melt depletion across several Wilson cycles spanning periods in excess of two billion years.

Abundance of highly siderophile elements in lunar basalts controlled by iron sulfide melt

The Moon accreted meteoritic material towards the end of Solar System formation. Quantification of this late accretion requires an estimation of the abundance of highly siderophile, or iron-loving, elements in the lunar mantle. As lunar mantle samples are not available, estimates are derived from lunar basalt compositions, but the melting phase relations needed to derive the mantle composition are poorly constrained. Here we present sulfur solubility measurements from laboratory experiments, combined with thermodynamic calculations, which show that the lunar basalt source is likely to be saturated in a sulfur-poor, iron-rich sulfide melt that concentrates some highly siderophile elements more than others. We found that the observed range in the ratios of highly siderophile elements in primitive lunar basalts is much smaller than expected from residual sulfide control alone. Instead, the elemental ratios are consistent with mixing between primary sulfide-saturated melts and minute (<1%) amounts of lunar regolith that contain impact debris. Although the composition of some samples suggests a highly depleted lunar mantle, the exact level of depletion is unclear, because mixing trajectories overlap at the inferred level of regolith contamination. We conclude that the composition of the lunar mantle is veiled by regolith contamination of the lunar basalts. If so, highly siderophile element abundances in lunar mantle-derived materials cannot be used to determine the mass of material accreted late onto the Moon. The abundance of iron-loving elements in Moon rocks cannot indicate the amount of late accretion onto the Moon, according to experiments and thermodynamic calculations constraining the behaviour of these elements under lunar mantle conditions.

Osmium isotope compositions and highly siderophile element abundances in abyssal peridotites from the Southwest Indian Ridge: Implications for evolution of the oceanic upper mantle

Abyssal peridotites are extensively exposed on the seafloor of amagmatic segments at the ultraslow spreading Southwest Indian Ridge (SWIR). This study presents abundances of highly siderophile elements (HSEs; Os, Ir, Ru, Pt, Pd and Re) and Re–Os isotope compositions as well as major and trace element concentrations of nine spinel-facies peridotites from the Dragon Bone amagmatic segment (~53°E) and the easternmost part (~63.5°E and 69.5°E) of the SWIR. These peridotites have experienced intense serpentinization and seafloor weathering in addition to variable degrees of melt depletion (bulk rock Al2O3 = 0.74–2.73 wt%). Low-temperature alterations, however, have a negligible effect on Os isotope compositions and concentrations of platinum-group elements. The 63.5°E peridotites displayed sub-chondritic Os/Ir and chondritic Pt/Ir ratios and slightly variable Re/Ir and Pd/Ir ratios. The observed positive covariance between 187Re/188Os ratios and γOs, Re and Pd abundances can be explained most likely by interaction of mantle residues with radiogenic basaltic melts, which is consistent with the petrographic observations indicative of melt infiltration. The 53°E peridotites, on the other hand, displayed larger variations in Os isotopes and relative HSE abundances, indicating a more complex evolutionary history. High Re abundances and 187Os/188Os ratios in some 53°E samples attest to Re and radiogenic Os addition, probably through reaction with S-undersaturated hydrous melts. Specifically, sample 21V-S9-D5‐2 exhibited a high 187Os/188Os ratio (0.148) and enrichment of Cu and Pt abundances, implying interaction with an oxidizing agent. Although the Re–Os isotopes of SWIR peridotites were superimposed by variable metasomatic processes, they preserve the ancient Os isotope signature (~1Ga) and record the melting episode at ~0.7 Ga during the closure of the Mozambique Ocean. We infer that ancient depleted and hence buoyant mantle exists beneath the SWIR, which probably supports the weakly magmatic rifted Marion Rise.

Trace element partitioning between sulfide-, metal- and silicate melts at highly reduced conditions: Insights into the distribution of volatile elements during core formation in reduced bodies

Chalcophile and siderophile element abundances are used to provide important constraints on the interior compositions of planetary bodies as well as the pressure (P) - temperature (T) conditions that prevailed during core formation. The oxygen fugacity (fO2) during core formation varied considerably between the various terrestrial planets and asteroidal bodies in our solar system. Mercury, the aubrite parent body (AuPB) and some terrestrial precursor bodies may have differentiated at highly reduced conditions.At present knowledge about how the metal liquid-silicate melt and sulfide liquid-silicate melt partitioning behavior of major and trace elements are affected by high S concentrations in the silicate melt at highly reducing conditions is incomplete. Here, we experimentally study the metal-silicate and sulfide-silicate partitioning behavior of trace elements in reduced silicate melts over a wide range of S contents as a function of redox state at 1 GPa and 1833–1883 K. Silicate melt S contents ranged between ~0.5 and ~20 wt%, with a corresponding silicate FeO range of ~0.4 to ~17.5 wt%, in a fO2 range between 1 and 9 log units below the iron-wüstite buffer. Our results reproduce the decrease of the S concentration at sulfide saturation (SCSS) with decreasing FeO contents down to ~3 wt%, as well as its strong increase at <3 wt% FeO. At S contents exceeding >6–9 wt% S, the FeO contents increase again.Results show that most elements (Mg, Ti, V, Cr, Mn, Cu, Zn, Se, Nb, Cd, Sb, Te, Ta, Tl, Pb and Bi) are more chalcophile than siderophile at reducing conditions, whereas Si, Co, Ni, Ga, Ge, Mo and W preferentially partition into Fe-rich melts instead of sulfide liquids. Silicon, Ti, Se, and Te preferentially partition into Fe-S over (Fe,Mg,Ca)-S liquids, whereas Mn, Zn and Cd are more compatible in the latter. As proposed by Wood and Kiseeva (2015), chalcophile elements such as Cu, Se and Te behave less chalcophile with increasing S concentrations of the silicate melt, whereas the opposite is observed for nominally lithophile elements such as Mg, Ca and Ti.The results can be used to improve interpretations of the observed trace element systematics of aubrites and other reduced achondrites. All of the volatile elements considered here behave chalcophile at the reducing conditions inferred for differentiation of the AuPB. A significant degree of the observed volatile element depletions in aubrites may therefore reflect their preferential partitioning into sulfide liquids, rather than degassing during or after differentiation of the AuPB. These results suggest that, depending of the extent of core merging, precursor body differentiation and the efficiency of sulfide liquid segregation, reduced precursor bodies that were incorporated in the early Earth were likely more rich in volatile elements than currently assumed.

Syneruptive incorporation of martian surface sulphur in the nakhlite lava flows revealed by S and Os isotopes and highly siderophile elements: implication for mantle sources in Mars

Martian lava flows likely acquired S-rich material from the regolith during their emplacement on the planet’s surface. We investigated five of the twenty known nakhlites (Nakhla, Lafayette, Miller Range (MIL) 090032, Yamato 000593, and Yamato 000749) to determine whether these lavas show evidence of regolith assimilation, and to constrain the potential implications that this process has on chemical tracing of martian mantle source(s). To establish the proportionate influence of atmospheric, hydrothermal, and volcanic processes on nakhlite isotopic systematics we obtained in situ sulphur isotope data (Δ33S and δ34S) for sulphide grains (pyrrhotite and pyrite) in all five nakhlite samples. For Nakhla, Lafayette, and MIL 090032, these data are integrated with highly siderophile element (HSE) abundances and Os-isotope compositions, as well as textural information constrained prior to isotopic analysis. This work thereby provides the first Re-Os isotope systematics for two different nakhlites, and also the first Re-Os isotope data for martian sample for which detailed petrographic information was constrained prior to digestion.We report the largest variation in δ34S yet found in martian meteorites (−13.20‰ to +15.16‰). The relatively positive Δ33S and δ34S values of MIL 090032 (δ34S = +10.54 ± 0.09‰; Δ33S = −0.67 ± 0.10‰) indicate this meteorite assimilated sulphur affected by UV-photochemistry. In contrast, the strongly negative values of Lafayette (δ34S = −10.76 ± 0.14‰; Δ33S = −0.09 ± 0.12‰) are indicative of hydrothermal processes on Mars. Nakhla, Yamato 000593, and Yamato 000749 sulphides have a narrower range of sulphur isotope compositions (Δ33S and δ34S ∼ 0) that is consistent with no assimilation of martian surface materials during lava flow emplacement. Consequently we used this second group of Δ33S values to approximate the Δ33S of the nakhlite source, yielding a Δ33S value of −0.1‰.Nakhlite HSE patterns result from a sulphide-saturated melt where Ru-Os-Ir alloys/sulphide were likely crystallized during earlier phases of magmatic processing in Mars to result in the fractionated HSE patterns of the nakhlites. Our data, alongside a synthesis of previously published data, suggest assimilation of an enriched component to the primary nakhlite melt, potentially a late-stage crystallization cumulate from the martian magma ocean stage. In the context of this model, and within large uncertainties, our data hint at perturbation and potential decoupling of nakhlite Re-Os isotope systematics from other isotopic systems as a result of small degrees of assimilation of a regolith component with highly radiogenic 187Os/188Os.

Ferrous oxide-rich asteroid achondrites

Ferrous oxide (FeO)-rich asteroid achondrites can be defined as asteroid-derived samples that experienced incipient partial melting processes in the early Solar System (>4.5 Ga) leading to melt-residues and cumulate and melt rocks that have high FeO in silicate grains (molar Mg/[Mg + Fe] < 80), implying relatively oxidative conditions (fO2 of IW +1 to +3). These achondrites include olivine-dominated brachinite and brachinite-like achondrite meteorites, ungrouped meteorites including Lewis Cliff 88763, Northwest Africa (NWA) 6693 and NWA 6704, Tafassasset, NWA 011/1296, and the oligoclase-rich meteorites Graves Nunataks (GRA) 06128 and GRA 06129. Ferrous oxide-rich asteroidal achondrites differ from other partially-melted achondrites, including ureilites and acapulcoite-lodranites in that the latter have higher molar Mg/ (Mg + Fe) in silicate grains, and lower fO2 (IW 0 to −2). New mineral chemical, whole-rock major- and trace-element and highly siderophile element (HSE: Re, Os, Ir, Ru, Rh, Pt, Pd, Au) abundance data, and O and Os isotope data are presented for FeO-rich achondrite meteorites Allan Hills 84025 (brachinite), Miller Range (MIL) 090206 and MIL 090405 (brachinite-like achondrites), and NWA 6693 (ungrouped). These results, combined with available data for FeO-rich asteroidal achondrites, reveal that these rocks include nearly-pure residues after partial melting, to samples formed by melt-rock reaction and as cumulates, requiring variable to extensive Fe-Ni-S partial melting, and between 1 and 20% silicate partial melting. The FeO-rich asteroidal achondrites originate from at least four distinct parent bodies, based on O-Cr-Ti isotope systematics, and occur for both carbonaceous and non-carbonaceous chondrite precursor sources. The initial water and volatile contents of FeO-rich asteroid achondrites were similar to carbonaceous chondrite groups, implying both carbonaceous and non-carbonaceous precursor materials generated water-rich partially-melted asteroidal bodies. The existence and recognition of FeO-rich asteroid achondrites explains the otherwise enigmatic nature of some iron meteorite groups (e.g., IVA, IVB) that require segregation from an oxidized asteroid parent body. The internal structure of some asteroid parent bodies was likely to be complex, reflecting early differentiation processes of nascent core formation, Fe-Ni-S melt pooling, variable silicate partial melting, igneous differentiation and the important role of melt-rock reaction, melt refertilization and late-stage C- (reduced bodies) or S-rich (oxidized bodies) fluid and vapor reactions.

Effects of multi-stage rifting and metasomatism on HSE-187Os/188Os systematics of the cratonic mantle beneath SW Greenland

We report highly siderophile element (HSE) abundances and Re–Os isotope compositions, obtained by isotope dilution inductively coupled plasma mass spectrometry, of olivine separates from a suite of multiply metasomatised peridotite xenoliths entrained in kimberlites from SW Greenland. Combined with petrographic and compositional observations on accessory base metal sulphides (BMS), the results reveal new insights into the chemical, physical and mineralogical effects of multi-stage rifting and associated melt percolation on the Archaean lithospheric mantle. Refertilised lherzolites are dominated by rare to frequent small (tens of µm) BMS inclusions in olivine, whereas modally metasomatised phlogopite-bearing lherzolite and wehrlites have higher proportions of more Ni-rich BMS, including abundant large interstitial grains (hundreds of µm). The olivine separates display depleted HSE systematics with Primitive Upper Mantle (PUM)-normalised Pd/Ir of 0.014–0.62, and have both depleted and enriched 187Os/188Os (0.1139–0.2724) relative to chondrite that are not correlated with 187Re/188Os. Four out of ten olivine separates retain similarly depleted Os corresponding to Re-depletion model ages of 2.1–1.8 Ga. They may reflect Palaeoproterozoic refertilisation (lherzolitisation) during Laurentia plate assembly, with re-introduction of clinopyroxene and Os-rich BMS into the originally refractory mantle lithosphere by asthenosphere-derived basaltic melts, followed by recrystallisation and occlusion in olivine. Unradiogenic Os is observed regardless of lithology, including from peridotites that contain abundant interstitial BMS. This reflects addition of Os-poor BMS (<< 1 ppm) during more recent wehrlitisation and phlogopite-introduction, and control of the Os isotopic signature by older Os-rich BMS that precipitated from the basaltic melt. Depletions in compatible HSE (< 0.5 × PUM for Ru, Ir, Os) in all but one olivine separate reflect nugget effects (amount of depleted vs. metasomatic BMS inclusions) and/or loss due to sulphide dissolution into oxidising small-volume melts that invaded the lithosphere during recurrent rifting, the latter supported by similar depletions in published bulk peridotite data. Combined, these multiple metasomatic events destroyed all vestiges of Mesoarchaean or older inheritance in the olivine separates investigated here, and highlight that caution is needed when interpreting Proterozoic Os model ages in terms of Proterozoic lithosphere stabilisation.

Age and composition of the subcontinental lithospheric mantle beneath the Xing'an–Mongolia Orogenic Belt: Implications for the construction of microcontinents during accretionary orogenesis

We present major- and trace-element concentrations, Sr–Nd–Hf isotopic compositions, ReOs isotopes, and highly siderophile element (HSE) abundances of 17 xenolithic mantle peridotites from the Nuomin volcanic field to constrain the assembly and evolution of the Xing'an–Mongolia Orogenic Belt (XMOB), Northeast China. The Nuomin peridotites are divided into five groups (harzburgite groups Haz-1, Haz-2, and Haz-3, and lherzolite groups Lhz-1 and Lhz-2). The Haz-1 peridotites are characterized by refractory chemical compositions (Al2O3 contents of 1.03–2.30 wt% and olivine forsterite (Fo) numbers of 90.7–92.5) that are best interpreted as residues after high degrees of mantle partial melting. The Haz-2 and Haz-3 peridotites exhibit slightly more fertile compositions (Al2O3 of 1.72–1.80 wt% and Fo of 89.0–89.9 for the Haz-2, and Al2O3 of 2.86 wt% and Fo of 91.2 for the Haz-3), resulting from refertilization by silicate melts. The Lhz-1 samples are more fertile (Al2O3 of 2.90–3.64 wt% and Fo of 89.1–90.3) than the Lhz-2 samples (Al2O3 of 1.64–2.16 wt% and Fo of 90.9–91.3). The combination of trace-element data (Ti/Eu ratio) and Sr–Nd–Hf isotopic compositions indicates the presence of at least three peridotite end-members (Haz-1, Haz-2, and Lhz-1–Lhz-2) beneath the Nuomin area, which can be explained by the infiltration of two distinct metasomatic agents (MA1: composition similar to potassic basaltic rocks in the Wudalianchi–Erkeshan–Keluo area, and MA2: Fe–Ti-enriched melt). HSE abundances and Os isotopic data show that the Os isotopic compositions of the Haz-1, Lhz-1, and Lhz-2 peridotites were not significantly affected by lithophile-element-dominated metasomatism and can be used to constrain the melt extraction ages of these peridotites. The lower 187Os/188Os ratios of Haz-1 peridotites define a prominent rhenium depletion age (TRD) of ~1.6 Ga and represent fragments of the Paleo–Mesoproterozoic lithospheric mantle rather than reflecting the heterogeneous characteristics of the asthenosphere or extraneous emplacement. In contrast, the Lhz-1 and Lhz-2 lherzolite groups together have a narrow range of 187Os/188Os ratios that give younger TRD ages (~0.5 to 1.0 Ga). Our data demonstrate that both the Paleo–Mesoproterozoic and Neoproterozoic subcontinental lithospheric mantle coexist beneath the XMOB and that they correspond respectively to the bimodal age peaks of crustal growth of ~1.6 Ga and ~0.8 Ga defined by the XMOB granitoids. The XMOB was assembled primarily during the Phanerozoic by multiple blocks that had formed from the Paleoproterozoic to Neoproterozoic. The similar chemical and Sr–Nd–Hf isotopic signatures of both young and old lithospheric mantle indicate that the large-scale metasomatism event involving potassic melts most likely occurred during the evolution of the Paleo–Asian Ocean. The crustal materials carried to the mantle by multiple subduction events could be an effective source of potassium for the origin of potassic basaltic rocks.

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.

Diamondiferous Paleoproterozoic mantle roots beneath Arctic Canada: A study of mantle xenoliths from Parry Peninsula and Central Victoria Island

While the mantle roots directly beneath Archean cratons have been relatively well studied because of their economic importance, much less is known about the genesis, age, composition and thickness of the mantle lithosphere beneath the regions that surround the cratons. Despite this knowledge gap, it is fundamentally important to establish the nature of relationships between this circum-cratonic mantle and that beneath the cratons, including the diamond potential of circum-cratonic regions. Here we present mineral and bulk elemental and isotopic compositions for kimberlite-borne mantle xenoliths from the Parry Peninsula and Central Victoria Island, Arctic Canada. These xenoliths provide key windows into the lithospheric mantle underpinning regions to the North and Northwest of the Archean Slave craton, where the presence of cratonic material has been proposed. The mantle xenolith data are supplemented by mineral concentrate data obtained during diamond exploration. The mineral and whole rock chemistry of peridotites from both localities is indistinguishable from that of typical cratonic mantle lithosphere. The cool mantle paleogeotherms defined by mineral thermobarometry reveal that the lithospheric mantle beneath the Parry Peninsula and Central Victoria Island terranes extended well into the diamond stability field at the time of kimberlite eruption, and this is consistent with the recovery of diamonds from both kimberlite fields. Bulk xenolith Se and Te contents, and highly siderophile element (including Os, Ir, Pt, Pd and Re) abundance systematics, plus corresponding depletion ages derived from Re-Os isotope data suggest that the mantle beneath these parts of Arctic Canada formed in the Paleoproterozoic Era, at ∼2 Ga, rather than in the Archean. The presence of a diamondiferous Paleoproterozoic mantle root is part of the growing body of global evidence for diamond generation in mantle roots that stabilized well after the Archean. In the context of regional tectonics, we interpret the highly depleted mantle compositions beneath both studied regions as formed by mantle melting associated with hydrous metasomatism in the major Paleoproterozoic Wopmay-Great Bear-Hottah arc systems. These ∼2 Ga arc systems were subsequently accreted along the margin of the Slave craton to form a craton-like thick lithosphere with diamond potential thereby demonstrating the importance of subduction accretion in building up Earth’s long-lived continental terranes.

Tungsten-182 in the upper continental crust: Evidence from glacial diamictites

Study of igneous and sedimentary rocks that may be representative of the chemical and isotopic characteristics of portions of the upper continental crust (UCC) has provided important insights to the origin and growth of the continents, as well as the large-scale chemical evolution of the silicate Earth. For example, changes in the major and trace element compositions, as well as long-lived radiogenic isotope systematics of (meta)sedimentary rocks through time have led to the conclusion that at least some portions of the Archean UCC were enriched in mafic-ultramafic components compared to UCC with younger provenance (Taylor and McLennan, 1985; Condie, 1993; Gaschnig et al., 2016; Chen et al., 2016; Tang et al., 2016; Garçon et al., 2017). Short-lived radiogenic isotope systems are an additional means of tracing the contributions of diverse mantle-derived components to the UCC. Tungsten-182 anomalies have been observed in both ancient and modern rocks. Although these anomalies were ultimately created by processes that occurred while 182Hf was extant during the first ~60 Ma of Solar System history, the causes of their incorporation and preservation in the rock record, as well as the frequency and distribution of rocks in the UCC with isotopic anomalies remain poorly understood. Here, μ182W values (deviations in 182W/184W from laboratory reference standard in parts per million) are reported for previously well-characterized, fine-grained glacial diamictites deposited between ~3.0 and ~0.3 Ga. Glacial diamictites deposited during the Mesoarchean as part of the Witwatersrand Supergroup in southern Africa are characterized by an average μ182W value of −12.5 ± 5.0 (2SD), yet a diamictite of similar age from the spatially associated Pongola Supergroup (Mozaan) is not isotopically anomalous. The isotopically anomalous diamictites are also characterized by comparatively high Ni and highly siderophile element (HSE) abundances, indicative of significant contributions from ultramafic, most likely komatiitic components. No resolvable 182W anomalies were found in glacial diamictites with lower Ni and HSE, deposited after 2.3 Ga. This in turn suggests that W isotopic anomalies in Archean UCC, at least in part, reflect contributions from deep mantle upwellings that produced some of the komatiites. The new results provide further evidence that the isotopic composition of W in the Archean crust was highly variable.

Chondritic late accretion to Mars and the nature of shergottite reservoirs

Mars is considered to have formed as a planetary embryo that experienced extensive differentiation early in its history. Shergottite meteorites preserve evidence for this history, and for late accretion events that affected their mantle sources within Mars. Here we report the first coupled 187Re–187Os, 87Sr/86Sr, highly siderophile element (HSE: Os, Ir, Ru, Pt, Pd, Re) and major element abundance dataset for martian shergottites that span a range of MgO contents, from 6.4 to 30.3 wt.%. The shergottites range from picro-basalt to basaltic-andesite compositions, have enriched to depleted incompatible trace-element compositions, and define fractional crystallization trends, enabling the determination of HSE compatibility for martian magmatism in the order: Os > Ir ≥ Ru ≫ Pt ≥ Pd ≥ Re. This order of compatibility is like that defined previously for Earth and the Moon, but the fractionation of strongly compatible Os, Ir and Ru appears to take place at higher MgO contents in martian magmas, due to early onset of sulfide fractionation. In general, enriched shergottites have lower MgO contents than intermediate or depleted shergottites and have fractionated HSE patterns (Re + Pd + Pt > Ru + Ir + Os) and more radiogenic measured 87Sr/86Sr (0.7127–0.7235) and 187Os/188Os (0.140–0.247) than intermediate or depleted shergottite meteorites (87Sr/86Sr = 0.7010–0.7132; 187Os/188Os = 0.127–0.141). Osmium isotope compositions, corrected for crystallization age, define compositions that are implausibly unradiogenic in some enriched shergottites, implying recent mobilization of Re in some samples. Filtering for the effects of alteration and high Re/Os through crystal-liquid fractionation leads to a positive correlation between age-corrected Sr and Os isotope compositions. Mixing between hypothetical martian crustal and mantle reservoirs are unable to generate the observed Sr–Os isotope compositions of shergottites, which require either distinct and discrete long-term incompatible-element depleted and enriched mantle sources, or originate from hybridized melting of deep melts with metasomatized martian lithosphere. Using MgO-regression methods, we obtain a modified estimate of the bulk silicate Mars HSE composition of (in ng g−1) 0.4 [Re], 7.4 [Pd], 9.6 [Pt], 6.2 [Ru], 3.7 [Ir], 4 [Os], and a long-term chondritic 187Os/188Os ratio (∼0.1312). This result does not permit existing models invoking high-pressure and temperature partitioning of the HSE. Instead, our estimate implies 0.6–0.7% by mass of late accretion of broadly chondritic material to Mars. Our results indicate that Mars could have accreted earlier than Earth, but that disproportional accretion of large bodies and a relative constant flux of accretion of available materials in the first 50–100 Ma of Solar System led to the broad similarity in HSE abundances between Earth and Mars.