Highly Siderophile Elements Concentrations Research Articles

Precambrian history of the Pacific Mantle Domain: New constraints from Woodsreef and Port Macquarie serpentinized spinel harzburgites of the New England Orogen, Australia

Abstract The present-day mantle is divided into the African and Pacific domains by the circum-Pacific subduction girdle. Very little is known about the mantle composition of the Pacific Domain before 120 Ma due to the scarcity of the oceanic record, having mostly been destroyed by subduction processes. Accreted oceanic lithosphere (ophiolites) in orogens along the Paleo-Pacific margins provide rare opportunities to partially fill this knowledge gap. The early Cambrian (530–505 Ma) Weraerai ophiolite mélange in the New England Orogen in Eastern Australia represents fragments of the now-consumed Paleo-Pacific oceanic plate, predecessor of the Panthalassa and Pacific oceanic plates, accreted to east Gondwana during ca. 410–277 Ma. Early work revealed the presence of an accreted volcanic island(s) of possible mantle plume origin. However, due to their heavy alteration and weathering, the geochemical signature of the mafic rocks in the Weraerai ophiolite cannot be used to certify their plume origin with confidence. Therefore, mantle rocks found in the ophiolitic belt offer an alternative way to decipher the origin of the oceanic lithospheric fragments in the Weraerai ophiolite mélange. Here we report the petrographic, major, and trace elements composition, including highly siderophile elements (HSE), and Re-Os isotope composition for 16 serpentinized spinel harzburgite samples from Woodsreef and Port Macquarie. The observed spinel-orthopyroxene symplectite intergrowth textures are interpreted as garnet-breakdown textures due to cooling from an initial high temperature of >1200°C. Silicon and Al contents and Mg# of serpentinized spinel harzburgite, as well as heavy rare earth element modelling results, suggest a high-degree of melt extraction of 20–30% in the garnet stability field and in an anhydrous environment, probably in ocean island or oceanic plateau environments. The samples in this study have HSE concentrations interpreted to have resulted from post-melting processes rather than a melt extraction feature, indicating that their Re-Os model ages need to be interpreted with caution. Nevertheless, the melt depletion ages obtained from the Woodsreef and Port Macquaries samples range between 1.4 and 1.1 Ga, consistent with previous studies on other Pacific-rim ophiolitic rocks. We argue that these ages might be related to a major depletion event during the transition between supercontinents Nuna and Rodinia. Such depletion events affected a large proportion of this section of the mantle before the incorporation of the peridotites into the oceanic lithosphere in the Paleo-Pacific associated with rapid ascent of mantle plumes. This interpretation is consistent with the occurrence of accreted volcanic islands in the Werearai ophiolite mélange, as shown by the OIB chemical signatures of some of the mafic rocks, and their association with shallow water limestones that formed in near-equatorial latitudes. The proposed ca. 530–510 Ma Paleo-Pacific Ocean mantle plume event coincides with a global peak of oceanic mantle plume events that may record the legacy of a circum-Rodinia subduction girdle driving antipodal mantle superplume episodes.

Isotopic and trace element data of Tissint indicate a homogeneous strewn field, mobilization of Sr, REE, and Pb during shock metamorphism, and limits on the incorporation of martian surficial materials in impact melt glass

The Tissint strewn field has produced over 16 kg of material that has minimal terrestrial weathering and/or contamination. Tissint, along with 16 other incompatible trace element (ITE)-depleted shergottite specimens with igneous crystallization ages spanning 327–2403 Ma, were ejected together from Mars 1.1 m.y. ago. Despite the geochemical similarities of fragments from the Tissint strewn field, there are reported discrepancies in age determinations from different fragments that raise the possibility that the strewn field may be heterogenous. There are also questions about whether the shock ejection event incorporated martian soil components into impact glass, and the sources of radiogenic Sr and Pb that have been measured from leachate fractions in previous studies. An impact melt-rich fragment of Tissint was analyzed by LA-ICPMS for rare-earth element (REE) and highly siderophile element (HSE) concentrations and Pb isotopic compositions. Leachate and residues from 8 specimens representing separate individual fragments collected from the strewn field were analyzed for Rb-Sr. Unleached fractions of the 8 specimens were also analyzed for Sm-Nd and Lu-Hf.The measured REE and HSE concentrations of impact melt glass and associated sulfide measured by LA-ICPMS are consistent with bulk rock compositions of Tissint and show no evidence for incorporation of more ITE-enriched martian surface components. Measured Pb isotopic compositions confirm that the impact melt glass and associated sulfide contain no evidence for incorporation of more radiogenic materials than the Pb compositions inherited from the primary magma. In situ Pb isotopic data from sulfide likely represents the most robust method for constraining initial Pb isotopic compositions of shergottites whereas approaches that rely on leaching and digestion may not remove all mineral and/or crack surface contaminants.Rubidium-strontium analyses of the 8 Tissint specimens indicate that labile components hosting HCl-soluble Rb and Sr are not in isotopic equilibrium with the igneous assemblage and that the washed residues are in isotopic equilibrium with the igneous assemblage. The Sr isotopic compositions of the leachate are within the range of ‘more ITE-enriched’ depleted shergottites, perhaps indicating sources from the igneous pile on Mars. The radiogenic Sr component could represent crack and mineral surface coatings of volatilized materials derived from nearby depleted shergottite rock units during the impact ejection process but are not radiogenic enough to represent ITE-enriched crust or mantle components.The Lu-Hf isotopic data from the specimens indicate no evidence of contamination or element mobility, whereas the Rb-Sr and Sm-Nd isotopic systems show evidence for element mobility and potential mixing with an isotopic component not in equilibrium with the igneous phases. The calculated ages using data compiled from Brennecka et al. (2014), and Grosshans (2013) for Lu-Hf, Rb-Sr, and Sm-Nd are 571 ± 84 Ma, 590 ± 49 Ma and 559 ± 39 Ma, respectively. These data indicate that the specimens analyzed here are cogenetic and the Tissint strewn field appears to be homogeneous.

Search for a meteoritic component within the impact melt rocks of the Chicxulub impact structure peak ring, Mexico

Constraining the degree of preservation of a meteoritic signature within an impact structure provides vital insights in the complex pathways and processes that occur during and after an impact cratering event, providing information on the fate of the projectile. The IODP-ICDP Expedition 364 drilling recovered a ∼829 m continuous core (M0077A) of impactites and basement rocks within the ∼200-km diameter Chicxulub impact structure peak ring. No highly siderophile element (HSE) data have been reported for any of the impact melt rocks of this drill core to date. Previous work has shown that most Chicxulub impactites contain less than 0.1% of a chondritic component. Only few impact melt rock samples in previous drill cores recovered from the Chicxulub might contain such a signal. Therefore, we analyzed impact melt rock and suevite samples, as well as pre-impact lithologies of the Chicxulub peak ring, with a focus on the HSE concentrations and Re–Os isotopic compositions.Similar to the concentrations of the other major and trace elements, those of the moderately siderophile elements (Cr, Co, Ni) of impact melt rock samples primarily reflect mixing between a mafic (dolerite) and felsic (granite) components, with the incorporation of carbonate material in the upper impact melt rock unit (from 715.60 to 747.02 meters below seafloor). The HSE concentrations of the impact melt rocks and suevites are generally low (<39 ppt Ir, <96 ppt Os, <149 ppt Pt), comparable to the values of the average upper continental crust, yet three impact melt rock samples exhibit an enrichment in Os (125–410 ppt) and two of them also in Ir (250–324 ppt) by one order of magnitude relative to the other investigated samples. The 187Os/188Os ratios of the impact melt rocks are highly variable, ranging from 0.18 to 2.09, probably reflecting heterogenous target rock contributions to the impact melt rocks. The significant amount of mafic dolerite (mainly ∼20–60% and up to 80–90%), which is less radiogenic (187Os/188Os ratio of 0.17), within the impact melt rocks makes an unambiguous identification of an extraterrestrial admixture challenging. Granite samples have unusually low 187Os/188Os ratios (0.16 on average), while impact melt rocks and suevites broadly follow a mixing trend between upper continental crust and chondritic/mantle material. Only one of the investigated samples of the upper impact melt rock unit could also be interpreted in terms of a highly diluted (∼0.01–0.05%) meteoritic component. Importantly, the impact melt rocks and pre-impact lithologies were affected by post-impact hydrothermal alteration processes, probably remobilizing Re and Os. The mafic contribution, explaining the least radiogenic 187Os/188Os values, is rather likely. The low amount of meteoritic material preserved within impactites of the Chicxulub impact structure may result from a combination of the assumed steeply-inclined trajectory of the Chicxulub impactor (enhanced vaporization, and incorporation of projectile material within the expansion plume), the impact velocity, and the volatile-rich target lithologies.

Highly Siderophile Elements and Coupled Fe-Os Isotope Signatures in the Temagami Iron Formation, Canada: Possible Signatures of Neoarchean Seawater Chemistry and Earth's Oxygenation History.

Banded iron formations (BIFs) were deposited before and concurrent with the Great Oxidation Event at ∼2.33 Ga. They provide useful archives that document the transformation of the Precambrian hydrosphere from anoxic to progressively oxygenated conditions. Their formation involves removal of oceanic Fe by either inorganic or biologically promoted Fe2+ oxidation, or both. To evaluate depositional settings, elemental sources that affect seawater chemistry, and oxidation pathways, we present the first combined highly siderophile element (HSE) and Fe-Os isotope study for the ∼2.7 Ga Temagami BIF, Abitibi Greenstone Belt, Ontario (Canada). HSE abundances and 187Os/188Os ratios show no systematic variation between alternating magnetite and (meta)chert bands of the Temagami BIF. Whereas HSE concentrations mostly resemble modern crustal values, present-day 187Os/188Os ratios range from ∼0.17 to ∼10.8. Magnetite samples define a regression line corresponding to an age of 2661 ± 126 Ma. A chondrite-like 187Os/188Os initial value is in agreement with earlier studies on Neoarchean marine sediments and is thought to reflect seawater composition, which, unlike modern oceans, is dominated by mantle-like 187Os inventory most likely derived from deep-sea hydrothermal vents. Our δ56Fe data vary from about +0.6‰ to +0.9‰ and define a sawtooth-like pattern between alternating magnetite and (meta)chert layers. Partial oxidation of hydrothermally sourced Fe(II) and a lack of microbially mediated dissimilatory iron reduction provide the most plausible explanation for the positive δ56Fe values. Notably, our δ56Fe data for Temagami are in accord with trends defined by literature results for other Algoma-type BIFs that were deposited throughout the Archean.

Tracing Highly Siderophile Elements through Subduction: Insights from High-pressure Serpentinites and ‘Hybrid’ Rocks from Alpine Corsica

Abstract The highly siderophile elements (HSE) include the economically critical platinum group elements (PGE; Os, Ir, Ru, Rh, Pt, Pd, Au and Re), gold and rhenium. The HSE are redox sensitive in mantle and seafloor environments and have a strong affinity to iron and sulphur, therefore their distribution within the subducted mantle lithosphere record changes to oxidation state and sulphidation. The mobility of the HSE during subduction has important implications for Re–Os isotopic signatures in the mantle, and the formation of Cu–Au arc-related ore deposits. In this study, subducted rock samples from Alpine Corsica are used to track the HSE in serpentinites and hybrid ultramafic–mafic rocks through the subduction cycle. A comparison of bulk-rock HSE concentrations with those in pre-subduction analogues provides insights into the transfer of the HSE throughout the subduction cycle. Serpentinites subducted to blueschist–eclogite-facies conditions have similar HSE concentrations to primitive upper mantle (PUM) concentrations, therefore it is concluded that the HSE are not mobilized from serpentinites on the scale of the whole-rock or greater. Therefore, as suggested in previous studies, crustal lithologies may be more important contributors of the HSE to the sub-arc mantle, particularly Pt, Pd and Re. In contrast, HSE concentrations in hybrid rocks (talc schist and chlorite schist) deviate from protolith concentrations. Rhenium is higher in the talc schist, and Ir and Ru are lower in the chlorite schist than in the PUM, or possible mafic protoliths. Mineral parageneses place temporal constraints on the growth of hosts to the HSE (sulphides, oxides and metal alloys), and record changes to the activities of oxygen and sulphur (aO2–aS2), and hence redox conditions, from pre-subduction to exhumation. Laser ablation inductively coupled plasma mass spectrometry was used to determine the HSE concentrations in sulphides and oxides, and the detection of small (∼2–25 µm2) platinum group minerals utilized high-resolution SEM mapping techniques. The prograde and retrograde sulphides have lower HSE concentrations compared with sulphides from pre-subduction settings. Therefore, the redistribution of the HSE on a mineral scale from sulphides to alloys and/or other sulphides has occurred within the serpentinites, which may reflect more reducing conditions during serpentinization or subduction, consistent with the results of thermodynamic modelling. In contrast, the mineral assemblages in the hybrid rocks imply an increase in the extent of sulphidation and oxidation, and higher fluid:rock ratios during exhumation, coincident with Re enrichment in the talc schist, and a decrease in the concentrations of Ir and Ru in the chlorite schist, at length scales greater than those of the rock samples. Therefore, hybridization of lithologies at the slab–mantle interface may enhance the transfer of the HSE to the sub-arc mantle. If Re transfer from the slab to the sub-arc mantle is possible, this questions the robustness of Re–Os isotope signatures as tracers of crustal recycling.

The Zhamanshin impact structure, Kazakhstan: A comparative geochemical study of target rocks and impact glasses

The Zhamanshin impact structure, which is about 1 Myr old, has a diameter of 14 km and is situated in the semi-arid region of Kazakhstan (48°24′N, 60°58′E). It has a heterogeneous suite of target rocks, including predominantly crustal lithologies (e.g., clays and siltstones) with minor ultramafics.Zhamanshin is known for its unique association of impact glasses, including basic and acidic varieties of zhamanshinites and (tektite-like and a few aerodynamically-shaped) irghizites. The origin of both of these impact glasses has long been debated, which is complicated by incomplete sampling of target lithologies at the Zhamanshin site and a limited number of isotopic analyses. However, such studies are a prerequisite for a comprehensive discussion of the mechanisms that formed the unique association of different impact glasses in one impact event.We present major and trace element contents, as well as combined Sr-Nd isotope data for target rocks and impact glasses from the Zhamanshin impact structure. These data, for the first time, include Paleogene clays and siltstones from a core drilled in the vicinity of the crater and cutting through all major target lithologies. The core samples contain important source lithologies for the impactites from the Zhamanshin area.Mixing calculations, based on geochemical data and Sr-Nd isotope signatures, indicate that irghizites and Si-rich zhamanshinites can be produced from variously homogenized mixtures of mainly clays and siltstones with minor additions of ultrabasic rocks. Based on highly siderophile element (HSE) and Os isotope data (including the first analyses of the clay and siltstone lithologies) we calculated a hypothetical Os composition of the irghizite precursor, allowing us to approximate a chondritic admixture to the irghizites of roughly 1% of a chondritic component. This confirms previous suggestions about the amount of extraterrestrial components. A new HSE and Os isotope dataset for five zhamanshinites reveals, on average, crust-like HSE concentrations and Os isotope compositions, confirming earlier suggestions of a lack of meteoritic admixtures to these impact glasses.

Genetics, crystallization sequence, and age of the South Byron Trio iron meteorites: New insights to carbonaceous chondrite (CC) type parent bodies.

The nucleosynthetic Mo, Ru, and W isotopic compositions of the South Byron Trio iron meteorite grouplet (SBT) are consistent with all three meteorites originating on a single parent body that formed in the carbonaceous chondrite (CC) isotopic domain within the Solar nebula. Consistent with a common origin, the highly siderophile element (HSE) concentrations of the SBT can be related to one another by moderate degrees of fractional crystallization of a parental melt with initially chondritic relative abundances of HSE, and with initial S and P contents of ~7 and ~1 wt. %, respectively. Tungsten-182 isotopic data for the SBT indicate the parent body underwent metal-silicate differentiation 2.1 ± 0.8 Myr after calcium aluminum rich inclusion formation, and thermal modeling suggests the parent body formed 1.1 ± 0.5 Myr after CAI formation. This accretion age is not resolved from the accretion ages of other CC and most noncarbonaceous (NC) type iron meteorite parent bodies. Comparison of the projected parental melt composition of the SBT to those projected for the IVA and IVB iron meteorite groups suggests that at least some portions of the CC nebular domain were more oxidized compared to the NC domain. In addition, comparison of the SBT parental melt S content to estimates for parent bodies of the IIAB, IIIAB, IVA, IID, and IVB "magmatic" iron meteorite groups suggests that CC type iron meteorite parental melts were characterized by a general depletion in S, in addition to depletions in some other moderately volatile elements. Based on chemical and O isotope similarities, prior studies have suggested the possibility of a common parent body for the SBT and the Milton pallasite. Molybdenum and Ru isotopic compositions of Milton also provide permissive evidence for this. The HSE concentrations in the Milton metal, however, cannot be related to the SBT by any known crystal-liquid fractionation or mixing path. Thus, Milton more likely formed on a different, chemically distinct, but genetically identical parent body present in the CC nebular domain.

The importance of sulfur for the behavior of highly-siderophile elements during Earth’s differentiation

The highly siderophile elements (HSEs) are widely used as geochemical tracers for Earth’s accretion and core formation history. It is generally considered that core formation strongly depleted the Earth’s mantle in HSEs, which were subsequently replenished by a chondritic late veneer. However, open questions remain regarding the origin of suprachondritic Ru/Ir and Pd/Ir ratios that are thought to be characteristic for the primitive upper mantle. In most core-formation models that address the behavior of the HSEs, light elements such as S entering the core have not been taken into account and high P–T experimental data for S-bearing compositions are scarce. Here we present a comprehensive experimental study to investigate the effect of increasing S concentration in the metal on HSE metal–silicate partitioning at 2473K and 11GPa. We show that the HSEs become less siderophile with increasing S concentrations in the metal, rendering core-forming metal less efficient in removing the HSEs from the mantle if S is present. Furthermore, we investigated the FeS sulfide–silicate partitioning of the HSEs as a function of pressure (7–21GPa) and temperature (2373–2673K). The sulfide–silicate partition coefficient for Pt increases strongly with P, whereas those for Pd, Ru and Ir all decrease. The combined effect is such that above ∼20GPa Ru becomes less chalcophile than Pt, which is opposite to their behavior in the metal–silicate system where Ru is always more siderophile than Pt. The newly determined experimental results are used in a simple 2-stage core formation model that takes into account the effect of S on the behavior of the HSEs during core formation. Results of this model show that segregation of a sulfide liquid to the core from a mantle with substantial HSE concentrations plays a key role in reproducing Earth’s mantle HSE abundances. As Ru and Pd are less chalcophile than Pt and Ir at high P–T, some Ru and Pd remain in the mantle after sulfide segregation. Addition of the late veneer then raised the concentrations of all HSE to their current levels. Suprachondritic Ru/Ir and Pd/Ir ratios of the mantle can thus be explained by a combination of sulfide segregation together with the addition of a late veneer without the need to invoke unknown chondritic material.

Analytical Methods for the Highly Siderophile Elements

The highly siderophile elements (HSE) include the fifth-period transition metals ruthenium (Ru), rhodium (Rh), palladium (Pd), and the sixth-period transition metals rhenium (Re), osmium (Os), iridium (Ir), platinum (Pt) and gold (Au). In addition to being iron-loving, these elements are also resistant to oxidation, have high melting temperatures and are important as industrial catalysts. HSE abundances in geologic materials vary significantly, ranging from ~1 mg/g in ore materials down to a few pg/g in basalts (Table 1). These elements comprise two long-lived radiometric decay schemes: 187Re decays to 187Os, and 190Pt decays to 186Os. View this table: Table 1 Range of HSE abundances, in ng/g, in selected rock types and in model upper mantle. The HSE have been targeted to address a wide variety of geochemical and cosmochemical questions. Early work suggested HSE concentrations can constrain Hadean mantle evolution (Chou 1978) and showed the geochronologic potential of the Re/Os isotope system (Herr and Merz 1955; Herr et al. 1961; Markey et al. 1998). More recent applications combine the Re–Os decay system with abundance data for the HSE to investigate the evolution of the planets and the moon (Day et al. 2010, 2016, this volume), the terrestrial mantle (Rehkamper et al. 1997; Aulbach et al. 2016, this volume; Harvey et al. 2016, this volume; Luguet and Reisberg 2016, this volume), impact craters (Koeberl and Shirey 1993), geochemistry and geochronology of ore formation (Markey et al. 1998; Barnes and Ripley 2016, this volume), tektites (Koeberl and Shirey 1993), as well as the formation and evolution of the continental curst (e.g., Peucker-Ehrenbrink and Jahn 2001). Non-mass dependent isotope variations in Re, Os, Ru, Pt and Pd are also present in some meteorites and lunar samples and arise from nucleosynthesis and cosmogenic radiation (Yokoyama and …

Diverse impactors in Apollo 15 and 16 impact melt rocks: Evidence from osmium isotopes and highly siderophile elements

Concentrations of highly siderophile elements (HSE) and 187Os/188Os isotopic compositions for eleven impact related rocks from the Apollo 15 and 16 landing sites are reported and combined with existing geochronological data to investigate the chemical nature and temporal changes in the large impactors implicated in the formation of the lunar basins. Data for the samples all define linear trends on plots of HSE versus Ir concentrations, whose slopes likely reflect the relative HSE compositions of the dominant impactors that formed the rocks.The inferred Imbrium basin impactor that generated Apollo 15 impact melt rocks 15445 and 15455 was characterized by modestly suprachondritic 187Os/188Os, Ru/Ir, Pt/Ir and Pd/Ir ratios. Diverse impactor components are revealed in the Apollo 16 impact melt rocks. The 187Os/188Os and HSE/Ir ratios of the impactor components in melt rocks 60635, 63595 and 68416, with reported ages <3.84Ga, are within the range of chondritic meteorites, but slightly higher than ratios characterizing previously studied granulitic impactites with reported ages >4.0Ga. By contrast, the impactor components in melt rocks 60235, 62295 and 67095, with reported ages of ∼3.9Ga, are characterized by suprachondritic 187Os/188Os and HSE/Ir ratios similar to the Apollo 15 impact melt rocks, and may also sample the Imbrium impactor. Three lithic clasts from regolith breccias 60016 and 65095, also with ∼3.9Ga ages, contain multiple impactor components, of which the dominant composition is considerably more suprachondritic than those implicated for Imbrium and Serenitatis (Apollo 17) impactors. The dominant composition recorded in these rocks was most likely inherited from a pre-Imbrium impactor. Consideration of composition versus age relations among lunar impact melt rocks reveals no discernable trend.Virtually all lunar impact melt rocks sampled by the Apollo missions, as well as meteorites, are characterized by 187Os/188Os and HSE/Ir ratios that, when collectively plotted, define linear trends ranging from chondritic to fractionated compositions. The impact melt rocks with HSE signatures within the range of chondritic meteorites are interpreted to have been derived from impactors that had HSE compositions similar to known chondrite groups. By contrast, the impact melt rocks with non-chondritic relative HSE concentrations could not have been made by mixing of known chondritic impactors. These signatures may instead reflect contributions from early solar system bodies with bulk chemical compositions that have not yet been sampled by primitive meteorites present in our collections. Alternately, they may reflect the preferential incorporation of evolved metal separated from a fractionated planetesimal core.Pre-4.0Ga ages for at least some impactor components with both chondritic and fractionated HSE raise the possibility that the bulk of the HSE were added to the lunar crust prior to the later-stage basin-forming impacts, such as Imbrium and Serenitatis, as proposed by Fischer-Gödde and Becker (2012). For this scenario, the later-stage basin-forming impacts were more important with respect to mixing prior impactor components into melt rocks, rather than contributing much to the HSE budgets of the rocks themselves.

A Synthesis of Instrumental Analytical Techniques for Examination of the Thermal History of Pallasite Meteorites

Pallasites are a unique group of meteorites consisting of a mixture of approximately equal proportions of olivine grains in a matrix of Fe-Ni metal. These meteorites provide physical samples of the interior of a differentiated planetary body. As such, they provide direct geochemical clues to planetary differentiation processes. In this study, one large collaborative effort is undertaken to analyze a suite of 14 main-group pallasite specimens by several different instrumental analytical techniques in an attempt to provide a comprehensive picture of formation processes, temperatures and timescales. The suite of specimens chosen come from both the main group pallasites and the Eagle Station trio, and encompass a range of fayalite composition and of olivine shape and distribution. Samples have been optically imaged and then mapped EDS using a JEOL JSM-6500F field-emission SEM equipped with an Oxford X-Max 80 mm 2 silicon drift detector to determine major element composition and distribution. The Oxford AZtec software allows large-format montaged mapping so that the entire specimen can be mapped to the same scale. Smaller region EDS mapping highlights minor mineral phases and the interfaces between the mineral grains and the metal phases. These maps quantitatively verify the mineral phases present and the Fe/Mg ratio of the olivine phase and the Fe/Ni ratio of the metal phase. Electron microprobe WDS analyses of mineral phases are being done on a JEOL 8900 to verify major and minor element composition in regions of interest. These analyses also highlight areas that are targeted for focused ion beam (FIB) analysis. The FIB is used to examine the interface between olivine and metal and create a 3-D reconstruction. Nano-scale phases are then highlighted for future TEM preparation. Between the SEM and microprobe results, areas were highlighted for mass spectrometry analysis. Several samples were analyzed with laser ablation ICPMS for trace element content. Pallasites can provide information about highly-siderophile elements (HSE) present in both mantle and core of differentiated bodies. Many differentiated planetary bodies show enrichment in HSE concentration in their mantle relative to their core. Our study indicates that both HSE concentrations and the O-isotope fractionation share a common trend with iron III-AB meteorites. The HSE concentrations are also comparable to Mars and the Angrite parent body. In addition, diffusion profiles are being obtained in the microprobe for both the metal and olivine phases, which indicate the cooling rates of these meteorites. The iron metal region contains regions of taenite interlaced with kamacite, in which Ni zonation and diffusion has traditionally been used to determine the metallographic cooling rate within the taenite [1]. More recently, zonation of the olivine rim has been used to determine cooling rates [2], however these indicate a much faster cooling rate than that estimated by the metallic rates. This study aims to examine diffusion in both phases to better

Characterization of the dominant impactor signature for Apollo 17 impact melt rocks

Concentrations of the highly siderophile elements (HSE) Re, Os, Ir, Ru, Pt, and Pd and 187Os/188Os isotopic compositions are reported for seven Apollo 17 impact melt rocks. These data are used to examine the dominant chemical signature of the impactor that formed the melts. Six of the samples (72355, 72435, 72535, 76035, 76055, and 76135) have poikilitic textures; one sample (73235) has an aphanitic texture. Data for the samples define linear correlations when Ir is plotted versus other HSE concentrations, with y-intercepts indistinguishable from zero for most HSE in most rocks. Scatter about some of the trends, and occasional trends with positive y-intercepts, indicate either mixing of additional components that are heterogeneously distributed within several rocks, or modest fractionation of some HSE by volatilization, crystal fractionation, or other processes, during formation and evolution of the melt sheet. There is no statistical difference between the aphanitic and poikilitic samples in terms of HSE ratios after visible granulitic clasts were removed from aphanite 73235. Hence, earlier speculations that the two types of impact melt rocks at this site may have been generated by different impactors are not supported by our data.Most Apollo 17 samples examined here and in prior studies are characterized by very similar HSE signatures, consistent with a common impactor. These samples are characterized by elevated Ru/Ir, Pd/Ir, and Re/Os, relative to most chondrites. Collectively, the data indicate that the impactor was characterized by the following HSE ratios (2σ): Re/Ir 0.093±0.020, Os/Ir 1.03±0.28, Ru/Ir 1.87±0.30, Pt/Ir 2.36±0.31, Pd/Ir 1.85±0.41, and present-day 187Os/188Os of 0.1322±0.0013. The results most likely mean that the impactor was a body with a bulk composition that was just outside the range of meteoritic compositions currently sampled on Earth.

Effects of melt percolation on highly siderophile elements and Os isotopes in subcontinental lithospheric mantle: A study of the upper mantle profile beneath Central Europe

The effects of melt percolation on highly siderophile element (HSE) concentrations and Re–Os isotopic systematics of subcontinental lithospheric mantle are examined for a suite of spinel peridotite xenoliths from the 4 Ma Kozákov volcano, Bohemian Massif, Czech Republic. The xenoliths have previously been estimated to originate from depths ranging from ∼32 to 70 km and represent a layered upper mantle profile. Prior petrographic and lithophile trace element data for the xenoliths indicate that they were variably modified via metasomatism resulting from the percolation of basaltic melt derived from the asthenosphere. Chemical and isotopic data suggest that lower sections of the upper mantle profile interacted with melt characterized by a primitive, S-undersaturated composition at high melt/rock ratios. The middle and upper layers of the profile were modified by more evolved melt at moderate to low melt/rock ratios. This profile permits an unusual opportunity to examine the effects of variable melt percolation on HSE abundances and Os isotopes. Most HSE concentrations in the studied rocks are significantly depleted compared to estimates for the primitive upper mantle. The depletions, which are most pronounced for Os, Ir and Ru in the lower sections of the mantle profile, are coupled with strong HSE fractionations (e.g., Os N/Ir N ratios ranging from 0.3 to 2.4). Platinum appears to have been removed from some rocks, and enriched in others. This enrichment is coupled with lithophile element evidence for the degree of percolating melt fractionation (i.e., Ce/Tb ratio). Osmium isotopic compositions vary considerably from subchondritic to approximately chondritic ( γ Os at 5 Ma from –6.9 to +2.1). The absence of correlations between 187Os/ 188Os and indicators of fertility, as is common in many lithospheric mantle suites, may suggest significant perturbation of the Os isotopic compositions of some of these rocks, but more likely reflect the normal range of isotopic compositions found in the modern convecting mantle. Osmium isotopic compositions correspondingly yield model Re-depletion ( T RD) ages that range from essentially modern to ∼1.3 Ga. Our data provide evidence for large-scale incompatible behavior of HSE during melt percolation as a result of sulfide dissolution, consistent with observations of prior studies. The degree of incompatibility evidently depended on melt/rock ratios and the degree of S-saturation of the percolating melt. The high Pt contents of some of these rocks suggest that the Pt present in this pervasively metasomatized mantle was controlled by a phase unique to the other HSE. Further, high Os concentrations in several samples suggest deposition of Os in a minority of the samples by melt percolation. In these rocks, the mobilized Os was characterized by similar to the 187Os/ 188Os ratios in the ambient rocks. There is no evidence for either the addition of Os with a strongly depleted isotopic composition, or Os with suprachondritic isotopic composition, as is commonly observed under such circumstances.

Low abundances of highly siderophile elements in the lunar mantle: evidence for prolonged late accretion

Abundances of the highly siderophile elements (HSE) Re, Ir, Os, Ru, Pt and Pd were determined for etchates and residues of highly purified lunar orange (74220 and 74001,1217 and 1218) and green (15421,65 and 15426,164) volcanic glasses. Most etchates have >100 pg/g Os and Ir, and in some instances have HSE concentrations that approach chondritic relative abundances. Some etchates also have 187Os/ 188Os within the range of chondritic meteorites. In contrast, most residues have Os concentrations of <50 pg/g and have considerably suprachondritic 187Os/ 188Os ratios. The disparities in Os isotopic compositions between etchates and residues reflect the presence of at least two components in each of the glass fractions. The components are interpreted to be the indigenous lunar glasses and a meteoritic contaminant. Mixing models suggest the indigenous components in orange and green glasses have Os concentrations ≤25 pg/g. Compared to terrestrial rocks with comparable MgO contents, the concentration estimates suggest the lunar mantle sources of the volcanic glasses were depleted in HSE by at least a factor of 20 relative to the terrestrial mantle, consistent with Os and Ir concentrations in the lunar mantle of ≤0.15 ng/g. The HSE depletion in the lunar mantle indicates that it may be missing a substantial share of late accreted materials. The depletion is best explained as a consequence of the early formation of the lunar crust and subsequent isolation of the lunar mantle from late accreted materials. Given that the lunar crust was largely formed by 4.4 Ga, this suggests a preponderance of late accreted materials were added to the Earth–Moon system subsequent to 4.4 Ga, and requires a substantial influx of mass into the inner solar system more than 150 Ma after the formation of the solar system.

Melt percolation monitored by Os isotopes and HSE abundances: a case study from the mantle section of the Troodos Ophiolite

Combined siderophile and lithophile element systematics in mantle rocks can be used to monitor melt percolation processes in the Earth’s mantle. Here we present a coherent dataset from a single melt channel from the mantle section of the Troodos Ophiolite Complex on Cyprus. The melt channel is composed of a dunite vein that is surrounded by harzburgite. Dunite and harzburgite both have refractory Cr-spinel (Cr/(Cr+Al) of 0.58–0.60). Likewise, clinopyroxenes in both the dunites and harzburgites have strongly depleted REE patterns with (Gd/Yb) N values varying from 0.03 to 0.07. Such consistent lithophile element patterns suggest that the harzburgite and dunite interacted with the same melt during the melt percolation process. The distribution of the highly siderophile elements (HSEs) (Os, Ir, Ru, Pt, Pd and Re) in the melt channel cannot be explained by conventional partial melting models, but can be explained by melt-peridotite reaction. The harzburgites have slightly suprachondritic Os isotope ratios ( 187Os/ 188Os t=90 Ma =0.1288–0.1311) compared to the 187Os/ 188Os t=90 Ma of the carbonaceous chondrite reference (0.1264), and their HSE concentrations overlap with the range observed for lherzolites and harzburgites world-wide. In contrast, the dunites are significantly enriched in 187Os ( 187Os/ 188Os 90 Ma=0.1335–0.1374), like volcanic rocks from island arcs world-wide. HSE patterns in the dunites are also typical for mantle melts, in that they are enriched in Pd, Pt and Re relative to Ir, Os and Ru, which are lower than in the primitive mantle. Hence, the harzburgites and dunites have complementary HSE concentrations and ratios. In addition, HSE ratios such as Ir/Os, Re/Os, systematically increase from the harzburgite towards the dunite ((Ir/Os) N: 0.36–1.8; (Re/Os) N: 0.14–9.5). This implies that Ir, Os and Ru behave incompatibly and become fractionated from each other during the melt percolation process. These features are interpreted to reflect the progressive reaction of a mantle melt with spinel–lherzolite to form harzburgite and eventually dunite. We suggest that an upper mantle peridotite was infiltrated by a radiogenic mantle melt typical for subduction-related volcanism. At low melt/rock ratios a harzburgite residue is left behind and its HSE distribution and the REE pattern of cpx can be explained by open-system melting if one assumes the HSEs to behave incompatibly. Continued melt percolation eventually produces dunites, and all mantle sulfides are removed from the peridotite. Thus, the sulfides and the HSE distribution in the dunites are not of residual origin but are dominated by sulfides that segregated from a sulfide-saturated melt with a radiogenic Os signature. The HSE variation in harzburgites and dunites from the melt channel can be interpreted as a mixing line that has HSE-bearing sulfides from the melt and from the residual mantle as end members. We conclude that HSEs become significantly mobilized and fractionated during melt percolation processes, thus providing useful proxies for melting and enrichment processes in the Earth’s mantle.