HSE Abundances Research Articles

Evidence for large-scale, long-term highly siderophile element heterogeneities in the Atlantic mantle from Leg 153 and 209 peridotites

Seafloor abyssal peridotites are products after partial melting and melt-rock interaction, providing constraints on upper mantle processes and heterogeneity. Despite extensive studies of ophiolitic peridotites and a burgeoning dataset for dredged abyssal peridotites, limited studies have been undertaken on abyssal peridotite drill core samples. This omission means a lack of comparison between modern ocean mantle lithosphere and that of ophiolites, where field context is possible in the latter, but not the former. Ocean Drilling Program (ODP) Leg 153 near the Kane fracture zone, and ODP Leg 209, near the Fifteen-Twenty fracture zone, both on the mid-Atlantic Ridge but ∼1000 km distant from one another, recovered serpentinized mantle peridotites. We report whole-rock major-, trace- and highly siderophile-element (HSE: Re, Pd, Pt, Ru, Ir, Os) abundances and 187Os/188Os ratios of peridotites from three cores (maximum depth of 200.8 m) from the two Legs to explore mantle heterogeneity within the Atlantic Ocean basin. Peridotites from ODP Leg 153 are relatively fertile, with Al2O3 contents ranging from 1.1 to 2.7 wt%. They are characterized by relatively radiogenic 187Os/188Os (0.1259 ± 0.0017; 2 S.D.) and uniform HSE abundances. The ODP Leg 209 peridotites are highly refractory, with Al2O3 contents ranging from 0.1 to 0.8 wt%, and imply that they record an extensively depleted mantle. They have 187Os/188Os of 0.1207 ± 0.0023 (2 S.D.) and fractionated HSE abundances, indicative of depletion occurring > 1 billion years ago. Marked heterogeneity both in the extent and timing of depletion are therefore evident along a large segment of the mid-Atlantic ridge that produces zero-age basalts. Measurement of Os isotopes in spatially associated mid-ocean ridge basalts (MORB) is challenging due to their sub ng/g Os contents. Instead, Nd isotopes of MORB are used to show that they are unlikely to have formed directly from abyssal peridotite-like sources. Abyssal peridotites therefore represent long-lived refractory residues during mantle upwelling with the sources of MORB being more enriched. Comparison between drilled abyssal peridotites and ophiolitic mantle reveals larger compositional variations in the latter. The overall higher melting extent in ophiolite mantle is likely reflective of a more complex life cycle than the modern oceanic lithosphere.

Highly siderophile element abundances and 187Re‐187Os systematics in the Tafassasset carbonaceous‐like primitive achondrite

AbstractHighly siderophile elements (HSE) strongly partition into metal phases over silicate minerals and so offer important constraints on nebular and core formation processes acting on early planetesimals. Abundances of the HSE are also an important tool for constraining relationships between metal‐rich meteorites. The first bulk rock and in situ HSE abundance and 187Re‐187Os data are reported for the ungrouped metal‐rich achondrite Tafassasset to examine models of its petrogenesis and origin. Bulk rock and metal grain HSE abundances are elevated at ~2 and ~15 times CI chondrite abundances, respectively, and are largely unfractionated from one another. Metal within Tafassasset is therefore likely to have quenched shortly after partial melting without significant fractional crystallization. Metal grain HSE abundances can be used to calculate a metal fraction of 14 ± 4 wt%, overlapping with the parent bodies of CC iron meteorites, which have also been related to Tafassasset using nucleosynthetic isotope anomalies. Despite such similarities, HSE systematics of bulk rock Tafassasset are not equivalent to any known chondrites, and metal grains do not overlap with iron meteorites or chondrite metal grains, precluding a direct genetic relationship.

A multi-siderophile element connection between volcanic hotspots and Earth's core

The existence of resolvable 182W/184W deficits in modern ocean island basalts (OIB) relative to the bulk silicate Earth has raised questions about the relationship of these rocks to Earth's core. However, because the core is expected to host high abundances of highly siderophile elements (HSE: Os, Ir, Ru, Pt, Pd, Re), it would be expected that such heterogeneity is accompanied by correlating variability in HSE abundances among OIB, but this has not been observed. We report instead a relationship between the W isotopic compositions and Ru/Ir ratios of Hawai‘i and Iceland OIB, which represent two of Earth's primary mantle plumes. Previous studies have highlighted the unique behavior of Ru relative to Os and Ir during metal-silicate fractionation, particularly when sulfide phases are segregated with metal. Using the information from these studies, we construct models predicting the consequences for HSE fractionation of various scenarios in which 182W/184W deficits can be created. These models show that the observed trends are likely inconsistent with modern, active core-mantle interaction at the CMB, and instead the observed low-Ru/Ir, low-182W/184W OIB are best explained by metal-silicate interaction that happened at significantly lower pressures. Such conditions may reflect what is expected for metal-silicate equilibration during the process of core formation itself, meaning that the deep mantle sources of OIB, such as ultra-low velocity zones, may instead reflect preserved relics of core formation. An ancient origin for distinct domains now residing at the core-mantle boundary is consistent with geophysical and petrological observations, for example that the Mg/Fe ratio of ferropericlase in the D″ layer is in significant disequilibrium with the modern core. Additional work is required to constrain the behavior of HSE during silicate differentiation processes that may also generate low 182W/184W ratios. However, if modern OIB represent a direct link to the ancient processes of core formation, future geochemical studies may be able to unlock new information about the formation and evolution of the Earth using OIB.

A Combined Re-Os and Pt-Os Isotope and HSE Abundance Study of Ru-Os-Ir Alloys from the Kunar and Unga Placer Deposits, the Taimyr Peninsula, Polar Siberia

In order to provide further insights into the origin of Ru-Os-Ir alloys, this study presents new highly siderophile element (HSE: Re, Os, Ir, Ru, Pt, and Pd) abundance and 187Re-187Os and 190Pt-186Os isotope data for detrital grains of native Ru-Os-Ir alloys in placer deposits of the Kunar and Unga Rivers, which display a close spatial association with the Kunar dunite–harzburgite complex in the northern part of the Taimyr Peninsula in the Polar Siberia. The study utilized electron microprobe analysis, negative thermal ionization mass-spectrometry (N-TIMS) and laser ablation multiple-collector inductively coupled plasma mass-spectrometry (LA MC-ICP-MS). The primary nature of the Ru-Os-Ir alloys is supported by the occurrence of euhedral inclusions of high-Mg olivine (Fo92–93) that fall within the compositional range of mantle olivine. The LA MC-ICP-MS data show similar average initial 187Os/188Os and γ187Os(740 Ma) values for PGM assemblages from the Kunar and Unga deposits of 0.1218 ± 0.0010, −0.18 ± 0.85, and 0.1222 ± 0.0025, +0.10 ± 2.1, respectively. These values are identical, within their respective uncertainties, to the initial 187Os/188Os value of the Ru-Os-Ir alloy grain measured by N-TIMS (0.1218463 ± 0.0000015, γ187Os(740 Ma) = −0.1500 ± 0.0012). The combined 187Re-187Os isotopic data for all studied grains (γ187Os(740 Ma) = −0.02 ± 1.6) indicate evolution of the Kunar and Unga mantle sources with a long-term chondritic 187Re/188Os ratio of 0.401 ± 0.030. In contrast to the 187Os/188Os data, the initial 186Os/188Os value of 0.1198409 ± 0.0000012 (µ186Os(740 Ma) = +34 ± 10) obtained for the same Ru-Os-Ir alloy grain by N-TIMS is suprachondritic and implies evolution of the Kunar and Unga mantle source(s) with a long-term suprachondritic 190Pt/188Os ratio of 0.00247 ± 0.00021. This value is ~40% higher than the average chondritic 190Pt/188Os ratio of 0.00180 and indicates long-term enrichment of the Kunar source in Pt over Os. Establishing the source of this enrichment requires further investigation.

Re-Os Isotope and HSE Abundance Systematics of the 2.9 Ga Komatiites and Basalts from the Sumozero-Kenozero Greenstone Belt, SE Fennoscandian Shield: Implications for the Mixing Rates of the Mantle

Re-Os Isotope and HSE Abundance Systematics of the 2.9 Ga Komatiites and Basalts from the Sumozero-Kenozero Greenstone Belt, SE Fennoscandian Shield: Implications for the Mixing Rates of the Mantle

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.

Evidence for a primitive deep mantle component in the source of Marquesas Islands Lavas from Os isotope and highly siderophile element abundance systematics

The Marquesas islands are an age-progressive volcanic chain in French Polynesia formed by partial melting above a mantle plume. Previous studies have demonstrated the presence of an enriched mantle (EM2) endmember mixed with a high-3He/4He (up to 14.4 RA) component in Marquesas post-shield lavas. In contrast, Marquesas shield lavas sample a dilute high-μ (HIMU) type component with lower 3He/4He (∼8 RA). We report new incompatible trace element and highly siderophile element (HSE: Os, Ir, Ru, Pt, Pd, Re) abundances and osmium isotope data for bulk rocks from the islands of Fatu Hiva, Hiva Oa and Nuku Hiva, for which He, Sr, Nd and Pb isotope compositions have been reported previously (Castillo et al., 2007). As the HSE are chiefly hosted in sulfide phases in the mantle and all but Re are compatible during partial melting, they offer information on both sulfide-rich and melt-depleted mantle lithologies within the Marquesas plume. Primitive mantle-normalized HSE abundances show depletions in Ru and Pd, and fractionated (Ir/Os)N in fourteen of the seventeen analyzed samples. Ruthenium depletions and Ir-Os fractionations are likely caused by parental magma fractional crystallization. Palladium depletions are not easily explained by magmatic or secondary alteration processes and are instead likely to be a feature of the source mantle which was partially depleted in the more incompatible HSE by previous melting events within the Marquesas plume. Osmium isotopic systematics are mildly radiogenic relative to chondrites and are identical within uncertainties for shield-building Fatu Hiva (γOs = +2.9 ± 1.6; 2 SD; n = 7) and post-shield Hiva Oa (+2.7 ± 2.2; n = 9), showing no evidence for significant assimilation of radiogenic oceanic crust. The osmium budget of Marquesas lavas is controlled by a volumetrically dominant peridotite endmember that is incompatible element depleted while the Sr, Nd, and Pb budgets are strongly influenced by a volumetrically minor, incompatible element enriched EM2 component. The Nd-Sr-Os isotope systematics of the lavas can be explained by mixing of 0.3–0.7% of an EM2 type melt into the Marquesas source. Marquesas lavas offer further support that the high-3He/4He “focus zone” (FOZO) mantle has 187Os/188Os mildly more radiogenic than the depleted upper mantle, similar to estimates of the primitive mantle. Osmium isotope systematics offer a useful tool with which to characterize more depleted components within the ocean island basalt-source mantle that may be difficult to detect using lithophile isotope signatures.

Petrogenesis of Northwest Africa 8686: A ferroan olivine‐phyric shergottite

AbstractNorthwest Africa (NWA) 8686 is an olivine‐phyric shergottite containing up to ~500 μm long olivine crystals in a fine‐grained groundmass of augite, pigeonite, and maskelynite, with accessory merrillite, pyrrhotite, and oxides. Bulk rock and mineral major and trace element concentrations are reported for NWA 8686, along with bulk rock highly siderophile element (HSE: Os, Ir, Ru, Pt, Pd, Re) abundances and 187Re‐187Os data. Based on its mineralogy, texture, and bulk rock rare earth element (REE) abundances, NWA 8686 is classified as an intermediate olivine‐phyric shergottite. It is notable for having some of the most ferroan olivine macrocrysts (forsterite content = 51.5 ± 3.5) of any olivine‐phyric shergottite. Olivine is absent from the NWA 8686 groundmass, which is unique for olivine‐phyric shergottites, and the calculated groundmass composition is also not olivine normative. The bulk meteorite has a low Mg# of 59 yet has HSE in broadly chondritic proportions (~0.005 × CI chondrite). The bulk rock REE pattern for NWA 8686 shows depletions in both the light and heavy REE relative to the middle REE. Compiled olivine and bulk rock data for olivine‐phyric shergottites indicate that olivine macrocrysts in almost all these meteorites are the result of entrainment of antecrysts or xenocrysts. The combination of low Mg# and chondritic HSE signatures in NWA 8686 means that it may have been formed either from the mixing between an evolved lava and early‐formed HSE‐rich phases such as Os‐Ir alloys or by the melting of a low Mg# mantle source with chondritic HSE abundances. The elevated Gd/Yb ratio in both NWA 8686 and other intermediate olivine‐phyric shergottites indicates that garnet was involved as either a residual or fractionating phase in their petrogenesis.

Chemical characteristics of iron meteorite parent bodies

The projected relative abundances of the highly siderophile elements (HSE; Re, Os, Ir, Ru, Pt, and Pd) for bulk parent bodies of 10 magmatic iron meteorite groups/grouplet (IC, IIAB, IIC, IID, IIF, IIIAB, IIIF, IVA, IVB, and South Byron Trio) are broadly similar and show no resolvable differences between noncarbonaceous (NC) and carbonaceous (CC) genetic heritage. The processes driving genetic isotopic heterogeneity in the early Solar System, therefore, evidently did not leave discernable chemical fingerprints with respect to HSE relative abundances on the bulk planetesimal scale. By contrast, the absolute abundances of HSE projected for parent body cores, which reflect core size, are more variable and, on average, higher in CC bodies compared to NC bodies. Overall, bulk core chemical compositions, as well as core size, are linked to the distribution of Fe within a parent body, which is controlled by its oxidation state. The CC parent bodies are constrained to have formed under heterogeneous oxidizing conditions which were, on average, more oxidizing than those of the NC environment.

Early global mantle chemical and isotope heterogeneity revealed by the komatiite-basalt record: The Western Australia connection

Although the heterogeneous nature of the chemical composition of Earth's mantle is now well established, the origin and longevity of the heterogeneities continue to be debated. In order to further study early-Earth heterogeneities, we present a set of Sm-Nd, Lu-Hf, Re-Os, and Hf-W isotope and lithophile and siderophile element abundance data for komatiites and basalts from the ∼3.53 Ga Coonterunah, ∼3.34 Ga Kelly, and ∼3.18 Ga Ruth Well and Regal systems of the Pilbara Craton in Western Australia. The Sm-Nd, Lu-Hf, and Re-Os isotope data yield isochrons consistent with the accepted emplacement ages of the respective komatiite-basalt lavas. The mantle sources evolved with long-term 147Sm/144Nd = 0.200 to 0.214 and 176Lu/177Hf = 0.0355 to 0.0395, spanning the entire range of the time-integrated Sm/Nd and Lu/Hf measured in the Archean and Proterozoic komatiite-basalt systems to-date. Unlike with the other early Archean komatiites and basalts, the coupled 143Nd-176Hf isotope systematics of the Pilbara lavas provide no evidence for the involvement of early magma ocean processes in the evolution of their mantle sources. Episodes of variable degrees of partial mantle melting and melt extraction can account for the observed large variations in the time-integrated Sm/Nd and Lu/Hf ratios in the early Archean mantle domains.In contrast to the highly variable Nd-Hf systematics, the initial γ187Os values vary within a narrow range from +0.9 to −0.4 indicating that the Pilbara mantle sources evolved with chondritic time-integrated Re/Os. The apparent discrepancy between the depletions in incompatible lithophile trace elements and near-chondritic Re/Os observed globally is reconciled via a model whereby early low-degree mantle melting events fractionated Sm from Nd and Lu from Hf, but had little effect on the Re/Os ratio. This in turn would imply early formation and long-term isolation of a basaltic crust highly enriched in incompatible lithophile trace elements.The calculated total HSE abundances in the komatiite mantle sources range from ∼30% in the Coonterunah to ∼70% in the Regal system, of those in the estimates for the modern BSE, indicative of a 2.4× increase in HSE abundances from 3.53 to 3.18 Ga.All four komatiite-basalt systems exhibit positive 182W anomalies ranging between +11.4 and +7.7 ppm. The 182W/184W compositions and calculated HSE abundances in the Pilbara komatiite-basalt sources are inversely correlated and are most consistent with grainy late accretion of large differentiated planetesimals. Regression of the combined 182W-HSE data for the komatiite systems allows an estimate of the W isotopic composition of the pre-late accretion BSE of +17 ± 7. This estimate is similar to that of the Moon of +25 ± 5 and lends further support to the notion regarding an initially common W isotopic composition in the Earth-Moon system.Regression of the available HSE abundance data for komatiite mantle sources worldwide provides an estimate for the time of complete homogenization of late accreted materials within the mantle by 2.5 ± 0.2 Ga. Calculations indicate an average survival time of late accreted planetesimals in the Earth's mantle of 1.9 ± 0.2 Ga, which constrains the average mantle stirring rates for the HSE in the Hadean and Archean.

A 187Re-187Os, 87Rb-87Sr, highly siderophile and incompatible trace element study of some carbonaceous, ordinary and enstatite chondrite meteorites

New 187Re-187Os, 87Rb-87Sr, triple O-isotope isotope, bulk rock highly siderophile- (HSE: Os, Ir, Ru, Pt, Pd, Re), major- and trace-element abundance data are reported for a variety of carbonaceous, ordinary and enstatite chondrite meteorites. In addition, new mineral chemical data are reported for the Chelyabinsk LL5 ordinary chondrite fall for comparison with existing chondrite data and to investigate element sequestration into metal and mineral phases within some chondrites. The focus of the study is to link the variations observed in the HSE abundances and Re-Os isotopes with other isotopic and elemental data to explore the relative roles of sample sizes, terrestrial alteration and parent body processes more fully on chondrite meteorite compositions. Trace element variations in Chelyabinsk silicate, oxide and metal grains highlight the importance of geochemical heterogeneity imparted by mineralogical variations and mode effects, as well as sample size. Using a range of sample powder aliquot sizes, it is possible to show that this becomes significant for the HSE at <0.1 g. Variations in high field strength elements relative abundances (HFSE: Ti, Zr, Nb, Ta, Hf) are also identified within individual aliquots of carbonaceous chondrite Ivuna, emphasizing the importance of complete dissolution of refractory phases. The range of fall and find meteorites examined here demonstrates that terrestrial alteration effects revealed for trace elements (e.g., Ba, U, Sr) do not correlate particularly well with Re/Os variations. Instead, the Re/Os ratios of carbonaceous chondrites are susceptible to disturbance, more so than indicated by incompatible trace element systematics, with the Murchison CM2 carbonaceous chondrite showing significant Re/Os fractionation between sample aliquots. For sample aliquots measured that do not show significant mode or terrestrial alteration effects, parent body processes appear to be largely restricted to thermal metamorphism and dehydration. Including data for this study, the combined published dataset for Re-Os isotope and HSE abundances now extends to 33 ordinary, 39 carbonaceous, 27 enstatite and 6 Rumuruti chondrites. The range in absolute HSE abundances among these meteorite groups is ∼30%, with all chondrites having, within uncertainties, the same average Os, Ir, Ru, Pt and Pd abundances. Notably, carbonaceous chondrites have long-term Re/Os ∼ 8% lower than for the other chondrite groups. If chondrite groups are representative of early planetary feedstocks, then the measured 187Os/188Os of ordinary chondrites make them a close match to the composition of the bulk silicate Earth. Assuming ∼0.5% late accretion of ordinary chondrites to Earth, this would result in a long-term Rb/Sr ratio ∼0.6% higher than from late accretion of carbonaceous chondrites, indicating that ordinary chondrites are a potentially attractive source for moderately volatile enrichment.

Effective global mixing of the highly siderophile elements into Earth’s mantle inferred from oceanic abyssal peridotites

Late accretion occurred through addition of massive impactors to Earth, leading to potential heterogeneity in the distribution of highly siderophile elements (HSE: Os, Ir, Ru, Pt, Pd, Re) within the mantle. Abyssal peridotites sample the present-day convecting mantle, which make them useful for examining the distribution of the HSE within the mantle. Here we report new HSE abundance data and 187Os/188Os ratios, in conjunction with mineral chemistry and bulk rock major- and trace-element compositions for abyssal peridotites from the fast-spreading Pacific Antarctic Ridge (PAR) and East Pacific Rise (Hess Deep), and for slow to intermediate spreading ridges from the Southwest Indian Ridge, Central Indian Ridge and Mid-Atlantic Ridge. These analyses expand the global abyssal peridotite Os isotope and HSE database, enabling evaluation of potential variations with spreading rate, from ultraslow (<20 mm/yr, full spreading rate) to fast (135–150 mm/yr). Accounting for likely effects from seawater modification and serpentinization, the Pacific data reveals heterogeneous and sometimes significant melt depletion for PAR (3–23% melt depletion; 187Os/188Os from 0.1189 to 0.1336, average = 0.1267 ± 0.0065; 2SD) and Hess Deep abyssal peridotites (15–20% melt depletion; 0.1247 ± 0.0027). Abyssal peridotites from fast to intermediate spreading ridges reveal no systematic differences in the distribution and behavior of the HSE or Os isotopes, or in degrees of melt depletion, compared with slow to ultraslow spreading ridges. These observations arise despite significant differences in melt generation processes at mid-ocean ridges, suggesting that the effects of ancient melt depletion are more profound on HSE compositions in abyssal peridotites than modern melting beneath ridges. Using global abyssal peridotites with Al2O3 content > 2 wt.%, the average composition of the primitive mantle is 0.3 ppb Re, 4.9 ppb Pd, 7.1 ppb Pt, 7.2 ppb Ru, 3.8 ppb Ir and Os, showing no Pd/Ir, but a positive Ru/Ir anomaly, relative to chondrites. There is ∼50% variation of the HSE abundances in the oceanic mantle, with much of this variation being observed at small length scales (<1 km) and due entirely to both modern and more ancient partial melting effects. Consequently, any significant HSE heterogeneities formed during late accretion or early Earth differentiation processes are no longer recognizable in the mantle sampled within ocean basins, implying generally efficient mixing of Earth’s mantle for these elements. By contrast, relatively ancient heterogeneity in Os and other radiogenic isotopes has been effectively preserved in the convecting mantle over the last ∼2 Ga, through recycling processes and through preservation and isolation of melt-depleted refractory residues.

Geochemical evidence for volcanic signatures in sediments of the Younger Dryas event

One of the prevailing hypotheses for the origin of the Younger Dryas (YD) cooling event is that it resulted from a bolide impact or airburst. Purported impact markers peak at or near the YD basal boundary layer at Northern Hemisphere locations. In this study, the 187Os/188Os ratios and highly siderophile element (HSE: Os, Ir, Ru, Pt, Pd, Re) abundances in a well-dated sediment section through the Younger Dryas at the Debra L. Friedkin site, Texas are reported. Unradiogenic 187Os/188Os peaks, which could be mantle-derived or extraterrestrial, have been found above, within, and below the YD basal boundary layer. Mass balance mixing models using chondrites or iron meteorites with upper continental crust fail to duplicate the chondrite-normalized HSE patterns of the sediment samples. These HSE signatures in the Friedkin site section replicate those found in Hall’s Cave, Texas. The new results here thus independently confirm that the HSE abundances in the unradiogenic Os layers are likely a fingerprint of volcanic gas aerosols derived from large Plinian eruptions and not extra-terrestrial materials. To better constrain the lithological origins of YD sediments from the Friedkin and Hall’s Cave sites, Texas, trace elements are presented here. The rare earth elements (REE) patterns and Ir, Ni, Ti and Zr abundances are also characterized with terrestrial signatures as opposed to impact melt rocks. An age profile correlation between the two study sites, further shows that three unradiogenic Os peaks overlap in time. The results are inconsistent with the extraterrestrial hypothesis and support instead an episodic and volcanic origin for the observed geochemical anomalies at the Debra L. Friedkin and Hall’s Cave sites, Texas.

High-precision Pb and Hf isotope and highly siderophile element abundance systematics of high-MgO Icelandic lavas

Ocean island basalts (OIB) exhibit significant variations in their 87Sr/86Sr, 143Nd/144Nd, 176Hf/177Hf, 186,187Os/188Os, and 206,207,208Pb/204Pb compositions that indicate contributions from multiple mantle sources. To further constrain the relationships between diverse long-lived isotopic systems in OIB, new double-spike Pb and Hf isotope data, as well as isotope dilution highly siderophile element (HSE: Os, Ir, Ru, Pt, Pd, and Re) abundances are reported for nineteen Icelandic picrites and basalts for which 3He/4He, 87Sr/86Sr, 143Nd/144Nd, and 186,187Os/188Os data have been reported previously (Brandon et al., 2007. GCA). Hafnium-Nd-He isotope systematics for the sample set reveal the presence of an ancient refractory source with low-3He/4He, while He-Pb isotope correlations indicate that recycled oceanic crustal components with high-3He/4He added from a less degassed mantle source are present in the Iceland plume. The highly depleted, low-3He/4He signature is sampled in the Northern and Western Volcanic Zones, likely due to higher degrees of partial melting at those localities enabling sampling of this refractory endmember. The 187Os/188Os ratios show correlations with Pb, Hf, and Nd isotope ratios indicating binary mixing between depleted and enriched sources, with no evidence for decoupling due to sulfide metasomatism. The spread of 187Os/188Os- 206,207,208Pb/204Pb data can be explained by addition of 1–11% young recycled oceanic crust to a depleted mantle source distinct from the source of Atlantic MORB. Abundances of the HSE in Northern and Western Volcanic Zone lavas overlap with those from other OIB with similar MgO contents, showing fractionated patterns that are enriched in the more incompatible Re, Pd, and Pt over the compatible Ru, Ir, and Os. Calculated parental melt HSE abundances of those lavas also overlap with those of other OIB. Eastern Volcanic Zone lavas are highly depleted in Pt and Pd relative to other Icelandic lavas. Abundances of Pt and Pd correlate with isotopic parameters, such as 206Pb/204Pb, in Icelandic lavas, indicating that the isotopically enriched endmember sampled in the Eastern Volcanic Zone is depleted in the HSE, consistent with it being mafic in nature. The abundances of MgO in the lavas also roughly correlate with isotopic parameters, supporting the identity of the depleted endmember as ultramafic and the enriched endmember as mafic. New and previously published double-spike Pb isotope data show chiefly binary mixing with an additional component present in the vicinity of the volcano Öræfajökull. Icelandic 207Pb/204Pb and 208Pb/204Pb data plot below and above the majority of Mid-Atlantic Ridge basalt data, respectively, indicating that both depleted and enriched end-members in the plume have a lower U depletion age and a higher Th/U than ambient mantle. This is consistent with the enriched and depleted end-members within the Iceland plume being <2 Ga recycled oceanic crust and lithospheric mantle, respectively.

Temporally limited late accretion after core formation in the Moon

AbstractHighly siderophile element (HSE: Au, Re, Pd, Pt, Rh, Ru, Ir, Os) abundances in planetary silicate mantles provide constraints on accretion and differentiation, as well as late accretion additions after core formation. The first in situ analyses of the HSE in mare basalt sulfide and metal phases enable the determination of the distribution of these elements during fractional crystallization processes. Metals have low Ni/Co (&lt;8) and strong HSE inter‐element fractionation (Pd/Ir = 18–100) and, with troilite (Ni/Co &lt; 4.2, Pd/Ir = 14–74), host 75–100% of the HSE in mare basalts. The compositions of these metal and sulfide grains are inconsistent with assimilation of impact‐contaminated HSE‐rich regolith materials. Furthermore, regolith contamination cannot explain the threshold of abundances of iridium (~50 ppt) in mare basalt bulk rock compositions. Instead, bulk rock mare basalts have HSE patterns consistent with inheritance from partial melting of mantle sources with no residual metal or sulfide. Mare basalt HSE and siderophile element abundances can be accounted for by their mantle sources recording prior separation of a small lunar core, fractionating Ni/Co and Hf/W and removing ~99% of the HSE after lunar formation. Following this event, late accretion of ~0.02% of lunar mass led to limited enrichment of the HSE in chondritic‐relative abundances. Signatures of core formation preclude elevated HSE abundances in the lunar silicate interior and indicate that disproportional late accretion in the Earth–Moon system may, in part, be due to later core formation in the Moon relative to Earth.

Crystallization histories of the group IIF iron meteorites and Eagle Station pallasites

AbstractThe group IIF iron meteorites and Eagle Station pallasites (PES) have highly siderophile element abundances (HSE; Re, Os, Ir, Ru, Pt, and Pd) of metal that are consistent with formation in planetesimal cores by fractional crystallization with minor to major solid metal–liquid metal mixing. Modeling of HSE abundances of the IIF irons indicates a complex formation history that included the mixing of primitive and evolved solid and liquid metals. By contrast, modeling of HSE abundances of PES metal suggests these meteorites formed mainly as equilibrium solids from a common liquid. Abundances of some of the siderophile elements in the IIF irons and PES are permissive of a common core origin; however, the abundances of W and Ni indicate the PES ultimately formed on a more oxidized body. The PES most likely formed by the injection of olivine present at the core–mantle boundary into a metallic core liquid as a result of impact. The core then crystallized inward, trapping the olivine.

Highly siderophile elements in shergottite sulfides and the sulfur content of the martian mantle

Shergottite meteorites are ultramafic to mafic igneous rocks derived from partial melting of distinct regions of the martian mantle. As such, they trace magmatic processes, including fractional crystallization and mixing processes in Mars. New chalcophile (Cu, Se, Zn, Pb), siderophile (Ni, Co, W), and highly siderophile element (HSE: Au, Re, Pd, Rh, Pt, Ru, Ir, Os) abundance data are reported for sulfide assemblages in a suite of thirteen incompatible trace-element depleted, intermediate and enriched shergottites, along with new whole-rock HSE abundance and 187Os/188Os data for seven shergottites. Sulfide grains in depleted and intermediate shergottites typically have the highest absolute abundances of HSE, with broadly flat CI-chondrite normalized patterns. Enriched shergottite sulfide grains typically have highly variable Au, elevated Pd and Rh and are relatively depleted in Zn, Ir and Os. The new HSE whole-rock data for enriched (Northwest Africa [NWA] 7397, NWA 7755, NWA 11043), and intermediate shergottites (NWA 10961, NWA 11065, NWA 12241, and NWA 12536) are generally consistent with existing 187Os/188Os and HSE abundance data for these geochemical groupings. Enriched shergottites with >1 ppb bulk rock Os have measured 187Os/188Os ranging between 0.1296 and 0.1471, with variable Pd and Pt contents. Intermediate shergottites with >1 ppb bulk rock Os have chondrite-relative proportions of the HSE at ∼0.01 to 0.001 × CI chondrites and 187Os/188Os from 0.1284 and 0.1295. Sulfides are the major host of the HSE, and they control the behavior of the HSE during petrogenetic processes in shergottite magmas, enabling the determination of HSE compatibility for martian magmatism in the order: Os > Ir ≥ Ru ≥≥ Rh ≥ Pd ≥ Re ≥ Pt ≥ Au. Fractionation models of removal of an olivine-dominated cumulate recreate HSE patterns for the whole-rock shergottites. Enriched shergottites are best reproduced by 25 to 30% of fractionation from a degassed parent melt (250 ± 50 ppm of S), whereas depleted and intermediate shergottites can be explained by slightly lower fractionation (10 to 15%) from higher S content parent melts (350 ± 100 ppm of S). Sulfur contents in the melt that are ∼50% higher than these estimates yield earlier S-saturation during fractional crystallization, leading to an abrupt decrease of the more compatible HSE (Ru, Ir, Os), which is not observed. These results indicate that the martian mantle and partial melts derived from it, are probably not anomalously enriched in S, and instead are similar to slightly higher than those of the terrestrial mantle and its partial melts.

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.

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.

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.