Origin of primary PGM assemblage in сhromitite from a mantle tectonite at Harold’s Grave (Shetland Ophiolite Complex, Scotland)

In this paper we present textural and mineral chemistry data for a PGM inclusion assemblage and whole-rock platinum-group element (PGE) concentrations of chromitite from Harold’s Grave, which occurrs in a dunite pod in a mantle tectonite at Unst in the Shetland Ophiolite Complex (SOC), Scotland. The study utilized a number of analytical techniques, including acid digestion and isotope dilution (ID) ICP-MS, hydroseparation and electron microprobe analysis. The chromitite contains a pronounced enrichment of refractory PGE (IPGE: Os, Ir and Ru) over less refractory PGE (PPGE: Rh, Pt and Pd), typical of mantle hosted ‘ophiolitic’ chromitites. A ‘primary’ magmatic PGM assemblage is represented by euhedrally shaped (up to 60 μm in size) single and composite inclusions in chromite. Polyphase PGM grains are dominated by laurite and osmian iridium, with subordinate laurite + osmian iridium + iridian osmium and rare laurite + Ir-Rh alloy + Rh-rich sulphide (possibly prassoite). The compositional variability of associated laurite and Os-rich alloys at Harold’s Grave fit the predicted compositions of experiment W-1200-0.37 of Andrews and Brenan (Can Mineral 40: 1705–1716, 2002) providing unequivocal information on conditions of their genesis, with the upper thermal stability of laurite in equilibrium with Os-rich alloys estimated at 1200–1250 °C and f(S₂) of 10^?0.39–10^?0.07.     

PGE, Re-Os, and Mo isotope systematics in Archean and early Proterozoic sedimentary systems as proxies for redox conditions of the early Earth

Re-Os data and PGE concentrations as well as Mo concentrations and isotope data are reported for suites of fine clastic sediments and black shales from the Barberton Greenstone Belt, South Africa (Fig Tree and Moodies Groups, 3.25-3.15 Ga), the Belingwe Greenstone Belt, Zimbabwe (Manjeri Formation, ca. 2.7 Ga) and shales from the Witwatersrand, Ventersdorp and Transvaal Supergroups, South Africa ranging from 2.95 to 2.2 Ga. Moderately oxidizing conditions are required to mobilize Re and Mo in the environment, Mo fractionation only occurs in solution, and these parameters thus have potential use as paleoredox proxies for the early Earth.PGE + Re abundance patterns of Barberton Greenstone Belt sediments are uniform and very similar in shape to those of komatiites. This indicates (1) that the PGE came from a source of predominantly ultramafic composition and, (2) that PGE were transported and deposited essentially in particulate form. Sediments from the younger Belingwe Greenstone Belt show more fractionated PGE + Re patterns and have Re/Os ratios 10 to 100× higher than those of Barberton sediments. Their PGE abundance patterns and Re/Os ratios are intermediate between those of the mid-Archean shales and Neoproterozoic to Recent black shales. They reflect scavenging of Re from solution in the sedimentary environment.δ^98/95Mo values of black shales of all ages correlate with their concentrations. The Barberton Greenstone Belt samples have ∼1-3 ppm Mo, similar to a granitoid-basaltic source. This Mo has δ^98/95Mo between −1.9 and −2.4‰ relative to present day mean ocean water molybdenum, MOMO and is thus not isotopically fractionated relative to such a source. Similar to the PGE this indicates transport in solid form. Sediments from the Belingwe Greenstone Belt show in part enhanced Mo concentrations (up to 6 ppm) and Mo isotope fractionation (δ^98/95Mo up to −1.4‰ relative to MOMO). The combined PGE + Re and Mo data show mainly reducing conditions in the mid-Archean and suggest that by 2.7 Ga, the atmosphere and oceans had become more oxidizing.Substantially younger samples from the Transvaal Supergroup (to ca. 2.2 Ga) surprisingly have mainly low Mo concentrations (around 1 ppm) and show no significant Mo isotope fractionation relative to the continental source. Among possible explanations for this are a return to reducing atmospheric conditions after 2.7 Ga, reservoir effects, or Mo removal by sulfide precipitation following sulfate reduction in early Proterozoic oceans.     

Relics of eclogite facies metamorphism in the Austroalpine basement,Hochgrössen (Speik complex), Austria

Summary Retrograde eclogites and serpentinites from the Hochgr?ssen massif, Styria, are parts of the Speik complex in the Austroalpine basement nappes of the Eastern Alps. They are in tectonic contact with pre-Alpine gneisses, amphibolites, and Permo-Triassic quartz phyllites (Rannach Series). The eclogites are derived from ocean-floor basalts with affinities to mid-ocean ridge and back-arc basin basalts. Fresh eclogites are rare and contain omphacite with a maximum of 39 mol% jadeite content, garnet (Py_15–19) and amphibole. Retrograde eclogites consist of amphibole and symplectites of Na-poor clinopyroxene (5–8 mol% Jd) + albite ± amphibole. Amphiboles are classified as edenite, pargasite, tschermakite, magnesiohornblende and actinolite. In relatively fresh eclogite, edenite is a common amphibole and texturally coexists with omphacite and garnet. An average temperature of 700 °C was obtained for eclogite facies metamorphism using garnet-pyroxene thermometry. A minimum pressure of 1.5 GPa is indicated by the maximum jadeite content in omphacite. Thermobarometric calculations using the TWEEQ program for amphibole in textural equilibrium with omphacite and garnet give pressures of 1.8–2.2 GPa at 700 °C. The equilibrium assemblage of Na-poor clinopyroxene, albite, amphibole and zoisite in the symplectites gives a pressure of about 0.6–0.8 GPa at 590–640 °C. ⁴⁰Ar/³⁹Ar radiometric dating of edenitic amphibole in textural equilibrium with omphacite gave a plateau age of 397.3 ± 7.8 Ma, and probably indicates retrograde cooling through the closure temperature for amphibole (∼500 °C). The age of the high-pressure metamorphism thus must be pre-Variscan and points to one of the earliest metamorphic events in the Austroalpine nappes known to date. Received June 11, 2000; revised version accepted January 2, 2001     

Determination of Rare Earth Elements, Y, Th, Zr, Hf, Nb and Ta in Geological Reference Materials G-2, G-3, SCo-1 and WGB-1 by Sodium Peroxide Sintering and Inductively Coupled Plasma-Mass Spectrometry

Data are reported for rare earth elements (REE), Y, Th, Zr, Hf, Nb and Ta in four geological reference materials using sodium peroxide (Na₂O₂) sintering and inductively coupled plasma-mass spectrometry. The described procedure was used by students during their thesis work. A compilation of their reference material data acquired over one year of laboratory work demonstrates the ease and reliability of the method and the high reproducibility of the analytical results. Relative standard deviations of up to thirty six measurements of one reference material were lower than 5% for Y and the REE. Reproduciblities of Zr, Hf, Nb, Ta and Th were higher at between 5% and 10%, and can be attributed to the inhomogeneous distribution of zircon and other trace mineral phases and uncorrected drift effects. The concentration data are compared to reference and literature values and demonstrate that the procedure is also accurate. New data on G-3 show some systematic deviations from G-2, which are statistically significant.     

High resolution spectrophotometry of selected features in the 1.1 μm spectrum of Comet Kohoutek (1973f)

Fabry-Perot interferometry of Comet Kohoutek (1973f) at 1.1 μm with a resolution of 1.2 Å showed emission features identified as OH and CN lines in addition to a strong Fraunhofer continuum. Central intensities have been derived for three cases (uniform, gaussian, and gaussian plus ?^?1 law) of brightness profiles in the comet coma. Limits for CH₄, H₂O, HeI, SiL and CrI are also derived.     

Apparent Microfloral Response to Organic Degradation on Bathyal ­Seafloor: An Analysis Based on Sediment Fatty Acids

Abstract. Abyssal microfloral succession induced by experimental organic degradation was investigated. Notable changes in amounts and compositions of short-chain (C9-20) sediment fatty acids were observed, which indicated the shift of sediment microflora. Biomarker fatty acids for methanotrophs and sulfate-reducing bacteria dominated. Resultant fatty acid compositions were most closely related to those from a nearby methane seep harboring a dense Calyptogena colony; the clams were also seen in close vicinity of the deployed organic mass. These observations suggest that the organic degradation on the bathyal seafloor stimulates the formation of methanotrophic and thio­trophic microflora, resulting in the formation of a methane-seep-type benthic community.     

Diverse contributing sources to chromitite petrogenesis in the Shebenik Ophiolitic Complex, Albania: evidence from new PGE- and Os-isotope data

Summary Chromitites sampled from four different pseudostratigraphic levels of the Mesozoic Shebenik Ophiolite Complex, Albania, have low PGE totals <1 μg/g but show different types of PGE enrichment (Burgath et al., 2003) as well as differing mineral chemistry, PGM mineralogy and Os isotopic signatures. To circumvent analytical problems with low PGE abundances, representative samples were analyzed using HPA-digestion followed by isotope dilution ICP-MS. Osmium isotopes were determined by ICP-QMS and N-TIMS techniques. Podiform chromitites exposed in the mantle (Group I) and tabular chromitites exposed in the upper mantle (Group II) are Os-Ir-Ru-Rh enriched. In the upper mantle to mantle-crust transition zone, schlieren type chromitites (Group III) are enriched in Ru-Rh with low Os-Ir and low Pt-Pd. Within the mantle-crust transition zone disseminated chromitites in dunite are variably enriched in Ru-Rh-Pt, however, Os, Ir and Pd are low. IPGE rich chromites contain abundant small laurite inclusions whilst Rh and Pt are located in sulfarsenides marginally attached to transition zone chromites (see also Burgath et al., 2003). High Cr/Al ratios (>0.75) and low titanium contents of chromites throughout the sample suite are consistent with chromitite petrogenesis in a SSZ environment. Shebenik mantle chromitites with low ¹⁸⁷Re/¹⁸⁸Os ratios have an average, slightly suprachondritic initial osmium isotopic composition of 0.1285 ± 0.0022 (2s). Towards higher pseudostratigraphic levels, ¹⁸⁷Re/¹⁸⁸Os ratios increase and initial Os isotopies are very heterogeneous. Distinctly suprachondritic Os signatures require input of radiogenic source components, whereas subchondritic samples require assimilation of long term Re-depleted PGM.     

High Pressure Asher Digestion and an Isotope Dilution-ICP-MS Method for the Determination of Platinum-Group Element Concentrations in Chromitite Reference Materials CHR-Bkg, GAN Pt-1 and HHH

New concentration data for Ru, Rh, Pd, Re, Os, Ir and Pt are presented for three chromitite reference materials. A simple and very effective procedure was applied for the measurements. Samples were spiked with enriched isotopes and digested in a HNO₃/HCl (5+2) acid mixture at 300 °C and 125 bar (1.25 × 10⁷ Pa) pressure in a high pressure asher (HPA-S, Anton Paar). The programme settings were changed as a function of mass (0.5, 1, 2 and 4 g) and time (5 and 15 hours). Complete chromitite dissolutions for three digestions at each setting were monitored using XRD analyses of the amorphous residue after digestion. The osmium concentration was determined by sparging the OsO₄ that was formed during digestion into a quadrupole ICP-MS. After drying and re-dissolution of the remaining residue, the other PGEs were separated on-line from their matrix in a simple cation-exchange column that was coupled to the ICP-MS. The concentrations were determined through isotope dilution and external calibration (Rh). By using the on-line separation, we were able to control interference effects (isobaric and molecular), which resulted in highly reproducible data. Replicate measurements of the reference material CHR-Bkg (SARM CRPG-CNRS) with sample masses ranging from 0.5 to 4 g showed very small standard deviations compared to the results from the initial collaborative trials and published data (e.g., 3.2% RSD vs. 32% RSD for Ru). Results for platinum showed the largest scatter, which is currently attributed to the small size of the test portion. In addition to CHR-Bkg, the first results for two chromitite reference materials "platinumore" GAN Pt-1 and "chromiumore" HHH issued by the Central Geological Laboratory of Mongolia are presented.