Siderophile Elements Research Articles

GEOCHEMICAL CHARACTERISTICS OF THE MAIN ORE COMPONENTS IN THE GOSHGARCHAY PORPHYRY-EPITHERMAL SYSTEM (LESSER CAUCASUS, AZERBAIJAN)

The article deals with the determining characteristics of ore elements in the Goshgarchay porphyry-epithermal system. The genetic and geochemical characteristics of the main ore components were studied, and close positive and negative relationships between pairs of elements were determined by correlation analysis on the selected elements. According to the results of the cluster analysis, the geochemical associations that allow the localization of the development area of mineralization at different stages of formation of the porphyry-epithermal system were analyzed. The following series of stable elements, which are typical for hydrothermal deposits, are distinguished among the ore elements of the host and ore-bearing intrusive rocks: Cu, Mo, Pb, Zn, Ag, Ni, Co, Mn, Ti, Cr, V, Sr, etc. This group of elements is considered to be the indicator elements of the copper-porphyry deposit within the porphyry-epithermal system. It has been determined that correlations between elements in copper-porphyry mineralization are characterized by a wide range of element impurities. It includes both chalcophile (As, Sb, Cu, Bi, Cd, Ga, In, Ge, Au, Ag, Te) and siderophile (Co, Ni, Mo, Fe, Cr) elements. The range of values of vertical geochemical zonality on individual sections of the deposit allows to assess the level of the erosion truncation of occurrences in the porphyry-epithermal system.

Geochemical Markers of Stagnant Zones in an Urban Heat Island

The paper presents data acquired by a comparative study of the vertical variability in the chemical composition and ratios of aerosol subdisperse fractions in snow layers chronologically correlated with the stratigraphically significant snowfall periods. These data highlight features of the concentrating of trace elements at reactive (geochemical) barriers in the snow profile. The ratio of three geochemically close groups of elements, such as siderophile, sulfophile, and lithophile elements, were found out to vary relatively little from one snow layer to another in the growing snow cover. Trajectory analysis of the transfer of air masses to which stratigraphically significant snowfalls were related to the observation site provides no evidence that the identified geochemical phenomenon can be explained by that the winter aerosol field that was formed above an urban area with different trajectories of air masses can be somehow inherited in the snow layers of the growing snow cover and thus affect the vertical distributions of trace elements. Evidence indicates that that the ratios of assemblages of trace elements in the discrete snow layers, which remain stable with the growth of the snow mass, can be employed as geochemical markers of stagnant zones in an urban heat island, and the method of geochemical sampling of a snow cover in its discrete layers at a rare network of urban meteorological observation sites is an efficient additional tool for studying microscale atmospheric processes and recovering information on characteristics of the transfer of pollutants in a spatially limited urban area.

Large-scale replacement of ancient mantle lithosphere during supercontinent assembly: Evidence from the South China Craton

The modification and destruction of the sub-continental lithospheric mantle (SCLM) has been identified in numerous studies. However, what principally controls the susceptibility and extent of destruction of the SCLM and how this leads to the corresponding variation of the overlying crust remain long-standing issues. Here we provide an integrated study of petrology, mineralogy, and geochemistry, including highly siderophile element (HSE) abundances and whole-rock and/or mineral Os-Hf-Sr-Nd isotopic compositions of mantle peridotite xenoliths from Hainan Island, together with a compiled dataset from the eastern South China Craton (SCC) to investigate the mechanism and timing of replacement and destruction of the SCLM. The mantle xenoliths from the eastern SCC are overall more fertile than cratonic lithospheric mantle, and have generally experienced various extents of metasomatic enrichment, including carbonate and silicate melt metasomatism. In addition, the absence of Archean ages and compositional or temporal stratification of the SCLM beneath the eastern SCC indicates that the Archean lithospheric mantle may have been replaced/reworked and has essentially vanished, hence the present underlying lithospheric mantle is a relatively new and young lithospheric mantle. The formation time of the SCLM underneath Hainan Island constrained by both the ReOs and LuHf isotopic systems is late Mesoproterozoic (∼1.2 Ga), largely consistent with the Meso- to Neoproterozoic peak of whole-rock TRD ages (0.9–1.3 Ga) of the refractory mantle peridotites in the compiled dataset. This age distribution can be reconciled with the assembly time of the Rodinia supercontinent, indicating a close linkage between the replacement of large-scale lithospheric mantle via mantle convection erosion and regional continental amalgamation. Meanwhile, the relative location to the plate margins influences the preservation of the cratonic mantle. The SCLM located at the plate margin (e.g., eastern SCC compared to western SCC), where destructive processes frequently occur, is susceptible to destruction, whereas that within the plate interior is likely to remain relatively intact.

Actinides Th and U in Atmospheric Particulate Matter in Yakutsk

The distribution of Th and U was studied in the near-ground atmosphere of the city of Yakutsk. The chemical compositions of particulate matter (PM) in the summer atmosphere and in the solid and soluble phases of the snowpack were studied in 2019–2020. In the summer atmosphere, Th and U are concentrated mostly in the dust fraction of PM, together with a group of siderophile elements. The wintertime PM is characterized by a more homogeneous size of its particles. PM minerals with which concentrations of actinides correlate are amphiboles and pyroxene for Th and carbonates for U. The Th and U concentrations were found out to systematically decrease in the sequence soil–summer PM (dust)–winter PM (snow), at the dominance of Th, but the aqueous phase of the snow contains U concentrations one order of magnitude higher than those of Th. The bulk (close to 90%) of actinides is deposited during the warm season. The total amount of actinides falling out in Yakutsk is 63 mg/(m2 year), with two-thirds of this amount being Th. This value may locally increase to 200 mg/(m2 year) and more in areas of contrasting anomalies, with the predominance of Th. The main sources of actinides entering the near-ground atmosphere of Yakutsk are dusting from the soil surface and emissions from vehicles, with lesser amounts coming from energy-producing and construction-industry facilities.

Impact‐induced devolatilization of four ungrouped ataxites and the formation of a silica glass spherule in ALHA77255

AbstractAllan Hills A77255, Babb's Mill (Blake's Iron), Nordheim, and Chinga are ungrouped ataxitic iron meteorites that are similar to the IAB group of noncarbonaceous‐type irons in their concentrations of common and refractory siderophile elements. Mo‐isotopic data show that ALHA77255, Nordheim, and Chinga are carbonaceous‐type (CC) irons. (The Mo‐isotopic composition of Babb's Mill [Blake's Iron] has not yet been measured, but it also seems likely to be a CC iron.) Relative to mean IAB irons, these four ataxites are severely depleted in moderately volatile elements: Ga, >99%; Ge, >99%; Cu, 79%–97%; As, 70%–96%; P, 76%–90%. These samples were probably devolatilized by major collisions on separate parent asteroids (consistent with fractional crystallization modeling showing they are unlikely to be derived from the same metallic core). Collisionally induced devolatilization of ALHA77255 likely facilitated the formation of a 5‐mm diameter silica–glass spheroid in this meteorite. The spheroid may have formed by a complex process involving impact‐induced vaporization of mantle material in its parent asteroid, followed by fractional condensation.

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.

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

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

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

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

Spinel harzburgite–derived silicate melts forming sulfide-bearing orthopyroxenite in the lithosphere. Part 2: Sulfide compositions and their chalcophile and highly siderophile trace element signatures

In the first article, we have reported petrological data for a new, glass-bearing orthopyroxenite vein cutting a sub-arc mantle xenolith from Kamchatka. As similar veins from the West Bismarck arc, this orthopyroxenite is sulfide-rich and formed by cooling of parental melts derived by partial melting of spinel harzburgite sources. Here, I report new data for the abundances of major base metals and chalcophile and highly siderophile trace elements in vein sulfides from the two localities. Kamchatka vein sulfides are all Cu-poor monosulfide solid solution (MSS). West Bismarck veins contain MSS and a ternary (Fe, Cu, Ni)S solid solution (“xSS”), which ranges between MSS and intermediate solid solution (ISS) in composition. Sulfides follow Ni and Cu enrichment trends and have chondrite-normalized platinum-group element (PGE) patterns with elevated Pt relative to Os, Ir, Ru, and Rh. Pt alloys are frequently associated with sulfides and vugs formed from hydrothermal fluids, which also contain metallic Fe and wüstite. Vein sulfides, ranging from Fe-rich MSS (ca. 1,050–1,100°C) to xSS (≤850°C) through Ni-rich MSS, were formed in a sulfide liquid line of descent under oxygen and sulfur fugacity conditions (fO2 and fS2) down to one log unit below the fayalite–magnetite–quartz and close to the Pt-PtS buffers, respectively. The Ni and Cu enrichment trends in MSS are consistent with cooling and fractionation of Ni-rich and Cu-poor sulfide liquids (original atomic ∑metal/S∼0.9), which will finally solidify as xSS or ISS. Chondrite-normalized Pt/Pd>1 in some of the sulfides is a signature of spinel harzburgite sources. Because it occurs at relatively low fS2, the crystallization sequence of these sulfide liquids is accompanied by the formation of abundant PGE alloys and other metallic phases. Melts derived from spinel harzburgite sources can be originally oxidized to carry up to ∼2,600 ppm S (predominantly as S6+) and follow a sulfide-undersaturated evolution trend, until they are rapidly cooled to crystallize as orthopyroxenite dykes or sills. There, S6+-Fe2+ redox reactions with host rocks, together with the production of high-Mg# andesite derivatives with low S solubility and high-temperature, hydrothermal fluids at decreasing fO2 and fS2, will lead to the local precipitation of abundant sulfides and alloys.

Generation of Continental Lithospheric Mantle by Tectonic Isolation of Oceanic Plate

AbstractContinental formation models invoke subduction or plume‐related processes to create the buoyant, refractory character of continental lithospheric mantle (CLM). From similarities in melt depletion, major element composition, modal clinopyroxene, and Os isotope systematics it has been proposed that oceanic mantle lithosphere is the likely protolith to non‐cratonic CLM, however, a direct link between the two has been difficult to ascertain. Using dredged mantle peridotite xenoliths from the Ferrel Seamount, off the west coast of Baja California, Mexico, we show that tectonic isolation of an oceanic plate may lead to formation of non‐cratonic CLM. Ferrel xenoliths are coarse‐grained spinel lherzolite, or rare harzburgite. Bulk‐rock and clinopyroxene trace element compositions reveal two‐stages of melt refertilization following melt depletion, with infiltration by mid‐ocean ridge basalt‐type melts, followed by melt addition from host alkali basalt. Melt depletion correlations with 187Os/188Os and highly siderophile element abundances indicate preserved melt depletion and refertilization processes are ancient. From these observations, the Ferrel xenoliths represent lithosphere from the abandoned Pacific‐Farallon ridge. The history of melt depletion, followed by MORB‐melt refertilization is consistent with the peridotites representing oceanic mantle lithosphere that was subsequently incorporated into the Baja‐Guadalupe microplate during “ridge jump.” These peridotites demonstrate that isolation of oceanic lithosphere that is rafted onto a continental margin provides a viable means for producing non‐cratonic CLM. We suggest that continuation of late‐stage, low degree melt refertilization may provide a link between oceanic lithosphere and non‐cratonic CLM and propose a tectonic model to preserve and facilitate this continued evolution.

Tellurium isotope fractionation during evaporation from silicate melts

As a moderately volatile, redox-sensitive chalcophile and siderophile element, Te and its isotopic composition can inform on a multitude of geochemical and cosmochemical processes. However, the interpretation of Te data from natural settings is often hindered by an insufficient understanding of the behavior of Te in high-temperature conditions. Here, we present the results of Te evaporation and isotopic fractionation in silicate melting experiments. The starting material was boron-bearing anorthite-diopside glass with 1 wt% TeO2. The experiments were conducted over the temperature range of 868–1459 °C for 15 min each, and at oxygen fugacities (logfO2) relative to the fayalite-magnetite-quartz buffer (FMQ) of FMQ−6 to FMQ+1.5, and in air. Evaporation of Te decreases with decreasing fO2. For high-temperature experiments performed at >1200 °C Te loss is accompanied by Te isotope fractionation towards heavier compositions in the residual glasses. By contrast, Te loss in experiments performed at temperatures <1200 °C typically resulted in lighter Te isotopic compositions in the residues relative to the starting material. In air, Te evaporates as TeO2, whereas at lower oxygen fugacities we predict the evaporation of Te2, using Gibbs free energy minimization calculations. In air, the experimentally determined kinetic isotopic fractionation factor for δ128/126Te at T > 1200 °C is αK = 0.99993. At reducing conditions, Te likely substitutes as Te2− for O2– in the melt structure and becomes increasingly soluble at highly reducing conditions. Consequently, Te evaporation is not predicted for volcanic processes on reduced planetary bodies such as the Moon or Mercury.

Cretaceous Meteorite Impact-Induced Initial Subduction: Records of highly Siderophile Element Abundances and Re-Os Isotopes in Ophiolites

Cretaceous Meteorite Impact-Induced Initial Subduction: Records of highly Siderophile Element Abundances and Re-Os Isotopes in Ophiolites

Origin of moderately volatile element depletion on differentiated bodies: Insights from the evaporation of indium from silicate melts

In comparison with the Sun and CI chondrites, moderately volatile elements (MVEs) are depleted in terrestrial planets and other small, rocky differentiated bodies in the inner solar system. The abundances of most MVEs in the bulk silicate Earth (BSE) fall on a trend that defines a near log-linear decrease with their 50% nebular condensation temperature (Tc50). This temperature scale has traditionally been used to infer elemental volatility during planetary formation and accretion, however, indium (In) deviates from this correlation. Despite being a siderophile element that could have been depleted by core formation, In is overabundant for its calculated Tc50 in the BSE, as well as in the silicate portions of other small bodies (e.g., Moon and Vesta). This overabundance of In indicates that Tc50, calculated under nebular conditions, may not be applicable to planetary evaporation that occurs at much higher oxygen fugacity (fO2) and pressure than nominal nebular conditions. Here, we conduct a series of evaporation experiments for basaltic melts to quantify the volatility of In under conditions relevant to planetary evaporation. Our results show that, when using the evaporation temperature (Te1, refers to the temperature at which 1% of element i has evaporated from liquid to gas phase under equilibrium) as the volatility scale, the abundances of volatile elements, including In, of the Moon and Vesta display a progressive depletion with increasing volatility (decreasing Te1). This smooth depletion pattern contrasts with the overabundance of In shown on the Tc50 scale, suggesting that volatile depletion on small bodies occurred under non-nebular environment instead of during nebular condensation. On the other hand, the volatile element composition of the BSE (including In) could be explained by integrating (i) early accreted precursor materials of the proto-Earth that underwent volatile loss under conditions more oxidizing than those of the solar nebula with (ii) late added volatile-rich materials.

The case for the angrite parent body as the archetypal first-generation planetesimal: Large, reduced and Mg-enriched

Angrites are silica-undersaturated achondrites formed very early in the history of the Solar System, and the most volatile-depleted known meteorites. As such, the study of angrites can provide critical insights into the early stages of planetary formation, melting and differentiation. Yet, understanding the origins of angrites and the nature of their parent body has long been hindered by the initially small number of specimens available. Here, we leverage (i) the rapidly growing number of known angrites, and (ii) equilibrium crystallization experiments at various pressure, temperature and oxygen fugacity conditions (P-T-fO2), to revisit the petrogenesis of angrites and constrain key features of the angrite parent body (APB), such as its composition and size.We observe that quenched (i.e., volcanic) angrites define two compositional groups, which we show are readily related by fractional crystallization. This crystallization trend converges on an olivine-clinopyroxene-plagioclase (Ol + Cpx + Plag) multiple saturation boundary, whose composition is sampled by D’Orbigny, Sahara 99555 and NWA 1296. Using the observation that some quenched specimens represent primitive angritic melts, we derive a self-consistent bulk composition for the APB. We find that this composition matches the proposed Mg/Si ratio of 1.3 derived from the angrite δ30Si values, and yields a core size (18 ± 6 wt%) in agreement with the siderophile elements depletion in the APB mantle. Our results support a primary control of nebular fractionation (i.e., partial condensation) on the composition of the APB. To establish the liquid phase equilibria of angrites, a series of 1 atmosphere and high-pressure crystallization experiments (piston cylinder and internally heated pressure vessel) were performed on a synthetic powder of D’Orbigny. The results suggest that the APB was a large (possibly Moon-sized) body, formed from materials condensed at relatively high-temperature (∼1300–1400 K), and whose fO2 changed from mildly reducing (∼IW-1.5) to relatively oxidizing (∼IW+1 ± 1) in the ∼ 3 Myr between its core formation and the crystallization of D’Orbigny-like (Group 2) angrites. Based on its timing of accretion and differentiation, its composition, redox, and size, we argue that the APB represents the archetype of the first-generation of refractory-enriched planetesimals and embryos formed in the innermost part of the inner Solar System (<1 AU), and which accreted in the telluric planets.

Re-healing cratonic mantle lithosphere after the world's largest igneous intrusion: Constraints from peridotites erupted by the Premier kimberlite, South Africa

Constraining the processes that drive the disruption and re-healing of cratonic mantle roots is essential in understanding how cratons evolve. A striking example of this “cycle” of disruption and re-healing is provided by the deep mantle root currently observed beneath the ∼2.05 Ga Bushveld-Molopo Farms Complex (BMFC) of the central Kaapvaal Craton. Magmatism that produced the BMFC involved considerable disruption of the craton mantle root, yet the diamondiferous nature of the root, as sampled by Mesoproterozoic kimberlite eruptions, indicates that following Paleoproterozoic disruption, the lithosphere was re-healed and a cool geotherm re-established. A snap-shot of this mantle root provided by 48 peridotite xenoliths from the ∼1.2 Ga Premier (now Cullinan) kimberlite allows us to examine the evolution of the central Kaapvaal mantle lithosphere to constrain this re-healing process.Premier peridotites can be classified into 3 petrographic groups: an olivine-rich group (>80% olivine), with predominantly Archean Re-depletion (TRD) ages (Group I), a more orthopyroxene-rich group (Group II; <80% olivine) with dominantly Paleoproterozoic TRD ages, whose mode at ∼2.1 Ga matches the age of the BMFC, and a deformed, metasomatized group, with TRD ages mostly ∼2.1 Ga. Paleoproterozoic Group II and deformed peridotites represent 70% of the peridotite population and have systematically lower olivine Fo (molar 100×Mg/(Mg+Fe)) contents and higher Al2O3 compared to Archean Group I peridotites. Re-Os and other highly siderophile element (HSE) systematics indicate that the Paleoproterozoic TRD ages reflect the time of melt extraction. The dominant Group II peridotites have Al-Yb systematics indicative of a low-P melt-residue origin. The combined data rule out the suturing of the mantle root by accumulation of melting residues associated with the BMFC plume event. Instead, we invoke a model in which rapid decay of a plume-flux associated with BMFC generation leaves a lithospheric “gap” in the central craton that becomes filled with buoyant slab-derived oceanic lithospheric mantle associated with the subduction system that generated the ∼2.1 to ∼1.8 Ga western Kheis-Okwa-Magondi margin to the Kaapvaal Craton. An evolution to flat-slab subduction rapidly shuts down BMFC magmatism, emplaces Paleoproterozoic eclogites and diamonds in the root and heals the lithosphere disrupted by the plume. Such plume-subduction interaction, also observed in the modern Earth, might be a common process affecting the lithosphere of other cratons.

Meteoritic component in impact breccias of the Jeokjung-Chogye structure, South Korea: Evidence from the HSE abundances and Re[sbnd]Os isotopic systematics

Geochemical compositions, including highly siderophile element (HSE) abundances and Re-Os isotope compositions, of nine polymict impact breccias in two drill cores and three target rocks from the Jeokjung-Chogye structure, South Korea, have been investigated to test the impact origin of the structure, and to infer the potential projectile type for the formation of the crater. The impactites have around 5 times higher abundances of platinum-group elements (PGE) compared to target rocks, and two orders of magnitude higher Re abundances. The weighted mean for 187Os/188Os ratios of impactites is 0.806, which is lower than this value for the target rocks (1.049), suggesting the presence of a meteoritic component. The higher PGE abundances and a broad negative correlation between Os contents and 187Os/188Os ratios show that the impactites incorporated ∼0.05 wt% of a chondritic component. The endogenic or chondritic relative abundances of the HSE argue against iron meteorites as a potential impactor for the Jeokjung-Chogye structure. Yet, the present data do not allow to unambiguously identify the projectile type (i.e., chondrite class) present in the impactites because of the high proportion of the indigenous component.

Highly siderophile element behavior in the subduction zone mantle: Constraints from data on the Yap Trench peridotites

The highly siderophile elements (HSEs) in peridotites from the Yap Trench are investigated to understand their behavior in the subduction zone mantle, which is characterized by high melt depletion and metasomatism. Results reveal that the abundance of HSEs in Yap Trench peridotites is extremely low compared with that in oceanic peridotites from the mid-ocean ridge and other subduction-zone environments, indicating the extensive extraction of the sulfide melt. Chondrite-normalized HSE patterns are heterogeneous and exhibit strong fractionation of platinum-group elements (PGEs) and slight enrichment of Re and Au. The Ru/Ir (1.64–16.12), Os/Ir (0.15–7.19), and Pt/Pd (0.53–13.17) ratios in most of the studied samples are higher than those in CI chondrite (Ru/Ir: 1.49; Os/Ir: 1.06; and Pt/Pd: 1.67). These suprachondritic PGE ratios result from the exhaustion of the base metal sulfide (BMS) and the formation of platinum-group minerals and alloys, such as Os–Ru-rich sulfides and PtFe alloys, that are selectively enriched with certain HSEs. Mantle metasomatism appears to have no systematic influence on PGEs, although it may increase the abundance of Re and Au in the Yap Trench peridotites. Moreover, compared to abyssal peridotites, subduction-related peridotites have fractionated PGE patterns, particularly for the iridium group, suggesting that subduction conditions may promote the fractionation of the PGEs. However, suprachondritic PGE ratios are generally observed in subduction-related peridotites with a low Al2O3 content (<1 wt%) instead of subduction-related fertile samples. This suggests that highly depleted and subduction-related conditions are required for the fractionation of PGEs. We recommend that the high oxygen fugacity, hydrous melting, and high degree melting of the subduction-zone mantle may result in the exhaustion of the BMS in highly depleted peridotites, promoting the formation of platinum-group minerals and HSE-rich alloys, in which the HSEs are significantly fractionated.

Geochemical records of the end-Triassic Crisis preserved in a deep marine section of the Budva Basin, Dinarides, Montenegro.

The end-Triassic extinction event (∼ 201.5 Ma) is one of the five major mass extinction events in Earth's history, however, considerable discussion continues on the exact causes and timing of the event. This is because, whilst certain geochemical data on T-J sections appears to be largely comparable globally, with for example a significant (up to 6‰) negative carbon-isotope (δ13C) excursion at the extinction horizon, more often than not other geochemical variations are neither uniform nor fully consistent between sections. Critical to this discussion is that the majority of the studied sections containing the end-Triassic extinction event are limited to shallow marine or terrestrial sections, which are prone to discontinuities and hiatuses. In this study, we present carbon isotopes (δ13Ccarb), total organic carbon (TOC), major and trace, mercury (Hg) and highly siderophile elements (HSE), osmium-isotope compositions and paleomagnetic data of a relatively less studied deep-marine T-J succession in the Budva Basin, Čanj, Montenegro. At Čanj, deep-marine Triassic limestones are abruptly interrupted by a ∼ 6 cm finely laminated clay layer, before transitioning to more argillaceous Jurassic red beds. The clay layer is interpreted to represent the end-Triassic extinction interval and is characterized by a negative carbon isotope excursion, relative heavy rare earth element (HREE) enrichment, Hg increase, HSE enrichment and a sharp shift to unradiogenic osmium-isotopic ratios. This establishes the Čanj section as a unique and well-preserved outcrop that exquisitely encapsulates the end-Triassic extinction in the Tethyan marine realm. The distinct geochemical markers recorded at Čanj are consistent with the Central Atlantic Magmatic Province as the main driver behind the end-Triassic extinction.

Compositions of carbonaceous-type asteroidal cores in the early solar system.

The parent cores of iron meteorites belong to the earliest accreted bodies in the solar system. These cores formed in two isotopically distinct reservoirs: noncarbonaceous (NC) type and carbonaceous (CC) type in the inner and outer solar system, respectively. We measured elemental compositions of CC-iron groups and used fractional crystallization modeling to reconstruct the bulk compositions and crystallization processes of their parent asteroidal cores. We found generally lower S and higher P in CC-iron cores than in NC-iron cores and higher HSE (highly siderophile element) abundances in some CC-iron cores than in NC-iron cores. We suggest that the different HSE abundances among the CC-iron cores are related to the spatial distribution of refractory metal nugget–bearing calcium aluminum–rich inclusions (CAIs) in the protoplanetary disk. CAIs may have been transported to the outer solar system and distributed heterogeneously within the first million years of solar system history.

Lithostratigraphy and sedimentology across the Cretaceous/Tertiary boundary in the Flyschgosau (Eastern Alps, Austria)

Detailed lithological profiles across the Cretaceous/Tertiary boundary (K/T) in the flyschoid Gosau Formation at Gosau (Elendgraben) and Gams (Knappengraben) are presented. The revised low sediment accumulation rates in the lowermost Danian correspond to those calculated by Smit &amp; Hertogen (1980) for the Caravaca section. The estimated longer time-span for the deposition of the K/T boundary layers of 6-11 ka seems to be more realistic. The sediment at the K/T in the Gosau area was deposited under reducing conditions (Reissner et al. 1985). Peak levels of the siderophile elements do not coincide at Gosau and Gams. A diachronism of events is therefore assumed. The gradual decline of CaCO3 and some major element oxides beginning 4 cm below the K/T boundary and also a 1 mm thin yellow clay layer 10 mm underneath the boundary clay at Gams suggest multiple events and could be the result of a period of increased volcanism.