Khatyrka, a new CV3 find from the Koryak Mountains, Eastern Russia

A new meteorite find, named Khatyrka, was recovered from eastern Siberia as a result of a search for naturally occurring quasicrystals. The meteorite occurs as clastic grains within postglacial clay-rich layers along the banks of a small stream in the Koryak Mountains, Chukotka Autonomous Okrug of far eastern Russia. Some of the grains are clearly chondritic and contain Type IA porphyritic olivine chondrules enclosed in matrices that have the characteristic platy olivine texture, matrix olivine composition, and mineralogy (olivine, pentlandite, nickel-rich iron-nickel metal, nepheline, and calcic pyroxene [diopside-hedenbergite solid solution]) of oxidized-subgroup CV3 chondrites. A few grains are fine-grained spinel-rich calcium-aluminum-rich inclusions with mineral oxygen isotopic compositions again typical of such objects in CV3 chondrites. The chondritic and CAI grains contain small fragments of metallic copper-aluminum-iron alloys that include the quasicrystalline phase icosahedrite. One grain is an achondritic intergrowth of Cu-Al metal alloys and forsteritic olivine ± diopsidic pyroxene, both of which have meteoritic (CV3-like) oxygen isotopic compositions. Finally, some grains consist almost entirely of metallic alloys of aluminum + copper ± iron. The Cu-Al-Fe metal alloys and the alloy-bearing achondrite clast are interpreted to be an accretionary component of what otherwise is a fairly normal CV3 (oxidized) chondrite. This association of CV3 chondritic grains with metallic copper-aluminum alloys makes Khatyrka a unique meteorite, perhaps best described as a complex CV3 (ox) breccia

Sources of unique rhenium enrichment in fumaroles and sulphides at Kudryavy volcano

Rhenium (Re) is one of the least abundant elements in Earth, averaging 0.28 ppb in the primitive mantle. The unique occurrence of rheniite ReS2 (74.5 wt% of Re) in Kudryavy volcano precipitates raises questions about recycling of Re-rich reservoirs within the Kurile-Kamchatka volcanic Island arc setting. The sources of this unique Re enrichment have been inferred from studies of Re-Os isotope systematic and trace elements in volcanic gases, sulphide precipitates and host volcanic rocks. The fumarolic gas condensates are enriched in hydrophile trace elements relative to fluid-immobile elements and exhibit high Ba/Nb (133-204), Rb/Y (16-406) and Th/Zr (0.01-0.25) ratios. They are characterised by high Re (7-210 ppb) and Os abundances (0.4-0.9 ppb), with 187Os/188Os ratios in a range 0.122-0.152. This Os isotopic compositional range is similar to that of the peridotite xenoliths from the metasomatised mantle wedge above the subducted Pacific plate, the radiogenic isotopic signature of which is probably due to radiogenic addition from a slab-derived fluid. Re- and Os-rich sulphide and oxide minerals precipitate from volcanic gases within fumarolic fields. Molybdenite (MoS2), powellite (CaMoO4) and cannizzarite (Pb4Bi6S13) contain 1.5-1.7 wt%, 10 ppm, and 65-252 ppb of Re, respectively. Both molybdenite and rheniite contain normal Os concentrations, with total Os abundances in a range from 0.6 to 3.1 ppm for molybdenite, and 2.3-24.3 ppb for the rheniite samples. Repeated analyses of osmium isotope ratios for two rheniite samples form a best-fit line with an initial 187Os/188Os ratio of 0.32 ± 0.15 and an age of 79 ± 11 yr, which is the youngest age ever measured in natural samples. The high Re contents in molybdenite and rheniite led to high radiogenic 187Os values, even in the limited period of time, with 187Os/188Os ratios up to 3.3 for molybdenite and up to 4.4 for rheniite. The Os isotopic compositions of andesite-basaltic rocks from the Kudryavy volcano (187Os/188Os up to 0.326) are more radiogenic than those of residual peridotites and fumarolic gas condensates that are mainly constituted from magmatic vapor. Such radiogenic values can be attributed either to the addition of a radiogenic Os-rich subduction component to the depleted mantle, or to the assimilation of older dacitic caldera walls (187Os/188Os = 0.6) during arc magma ascent and emplacement. The latter hypothesis is supported by the correlation between 187Os/188Os ratio and indicators of fractionation such as MgO or Ni, and by low contents of potentially hydrophile trace elements such as Ba, Rb and Th relative to fluid-immobile elements such as Nb, Zr and Y. The high Re flux in the Kudryavy volcano (estimated at ~46 kg/yr) can be explained by remobilisation of Re by Cl-rich water from an underplated mantle wedge and subducted organic-rich sediments of the Pacific plate. © 2007 Elsevier Ltd. All rights reserved

Behavior of highly-siderophile elements during magma degassing: A case study at the Kudryavy volcano

The capacity of natural vapor phase to transport metallic elements is not unambiguously established relative to that of a liquid hydrothermal phase. We measured highly-siderophile element (HSE) and Au abundances in gas condensates and mineralized rocks in order to examine the geochemical behavior of these elements during magma degassing at the Kudryavy volcano, Kurile Arc. Gas condensates of the Kudryavy volcano are enriched with Re, Os and Au (to 210 ppb Re, 0.907 ppb Os, 2.4 ppb Au, 0.49 ppb Pt, 0.4 ppb Pd, 0.04 ppb Ir, 0.07 ppb Rh, 0.009 ppb Ru). The measured enrichment factors demonstrate that Os is the element that is most strongly compatible with fluid. Fluid compatibility decreases in the sequence: OsNReNAuNPtNPd over the temperature range from 480 to 850 °C. The mobility of HSE and Au in fluid is confirmed by the sublimation of their compounds, amongst which rheniite ReS2 and K perrhenate KReO4, native Pt, Pt–Pd selenide and various Au alloys have been identified with a scanning electron microscope [Nature 369 (1994) 51; Miner. Deposita 40 (2006) 828]. In addition, new HSE compounds, including ReO2, ReO3, Pt(OH)2 and metal-chloro-organic complexes, were detected in the sublimates using X-ray photoelectron spectroscopy. In contrast to the chalcophile behavior of Pb, Re and Os exhibit a dual behavior in the gaseous phase, since both sulfide and oxide phases containing these metals precipitate throughout the entire temperature range. However, available mineralogical, experimental and thermodynamic modeling data indicate that Re and Os are preferentially transported as oxygen-bearing species. Data on metal contents in fumarolic crusts of the volcano confirm that a high-temperature low-density fluid can concentrate these metals to economic grade.Newly obtained data on the Pb and Sm–Nd isotopic composition of volcanic gas condensates and host rocks were correlated with available data on Re and Os abundances and with the Re–Os isotopic composition of the same sample set in order to identify the possible sources of the magmatic melts. The homogeneity of the Pb and Nd isotopic composition of volcanic rocks (206Pb/204Pb: 18.33–18.41, 207Pb/204Pb: 15.52–15.54, 206Pb/204Pb: 38.19–38.24; n=6; 143Nd/144Nd: 0.513067–0.513118; n=5)indicates that the main source of the melts was metasomatised depleted MORB mantle. This is consistent with the relatively low radiogenic 187Os/188Os isotope ratios of younger basaltic andesites and fumarolic gas condensates, but is inconsistent with theradiogenic Os isotope characteristics of the acid volcanic rocks and the high Re abundance in rocks and fluids [Geochem. Cosmochem. Acta 72 (2008) 889].The results of this study suggest that similar elemental and isotope HSE signature can be characteristic of HSE fractionation in other environments of low-density oxidizing fluid stability

Facies Variation in PGE Mineralization in the Central Platreef of the Bushveld Complex, South Africa

AfricaAabsStractThe lateral variation in platinum-group mineral (PGM) assemblages in the magmatic constituents of the Platreef, Bushveld Complex, have been investigated using whole-rock PGE data, scanning electron microscope observations and electron-microprobe data on the PGM. The Platreef comprises a series of mafic–ultramafic sills together with interlayers and xenoliths of metamorphosed sedimentary rocks variably mineralized in PGE, Ni and Cu. In the Central Platreef, the zone with the highest exploitable PGE grade (referred to as PGE reef) occurs at the top of the sequence and is hosted by feldspathic pyroxenite partially replaced by feldspathic harzburgite down-dip. Elevated PGE contents are also related to zones of massive and disseminated chromitites that occur as discontinuous layers within the top reef and at the bottom of the Platreef close to the footwall contact. The harzburgite-hosted reef and chromitites contain PGE mineralization with a high PGE tenor, enriched in Rh and Ru that is taken to indicate proximity to a feeder zone for the Platreef. Textural relationships show that an early succession of laurite ± PGE sulfarsenides ± sperrylite crystallized at a high temperature from a hot magma that was unsaturated with respect to sulfide. Once sulfide saturation was achieved, cooperite and eutectic mixtures of Pt3Fe, Pt sulfide and Fe–Ni–Cu sulfides exsolved first from the sulfide liquid. With decreasing temperature, sperrylite and hollingworthite continued crystallizing across a wide range of conditions from early magmatic to hydrothermal and formed a series of solid solutions and exsolution products. The composition of the PGM assemblages changes along strike within a discrete magmatic layer, and the variation is a function of both the footwall and magmatic rock lithologies. The proportions of Pt–Pd sulfide, Pt–Fe alloy and Rh–Ir–Ru minerals are the most useful in discriminating between different magmatic facies and associated mineralization, whereas Pt–Pd bismuthotellurides are more widespread, and their abundance is dependent on the PGE grade. Early magmatic crystallization of Rh–Ir–Ru-rich PGM distinguishes the PGE profile characteristic of harzburgite and chromitite from the PGE profile typical of the pyroxenite-hosted PGE mineralization that has exsolved from the PGE-rich sulfide liquid