Closed-system behaviour of the Re-Os isotope system recorded in primary and secondary platinum-group mineral assemblages : evidence from a mantle chromitite at Harold's Grave (Shetland ophiolite complex, Scotland)

This study evaluates in detail the mineral chemistry, wholerock and mineral separate Os-isotope compositions of distinct platinumgroup mineral (PGM) assemblages in an isolated chromitite pod at Harold's Grave which occurs in mantle tectonite in the Shetland Ophiolite Complex (SOC), Scotland. This was the first ophiolite sequence worldwide that was shown to contain ppm levels of all six platinum-group elements (PGE) in podiform chromitite, including the contrasting type localities found here and at Cliff. At Harold's Grave the primary PGM assemblage is composed mainly of laurite and/or Os-rich iridium and formed early together with chromite, whereas the secondary PGM assemblage dominated by laurite, Osrich laurite, irarsite, native osmium and Ru-bearing pentlandite is likely to reflect processes including in-situ serpentinization, alteration during emplacement and regional greenschist metamorphism. The osmium isotope data define a restricted range of 'unradiogenic' 187Os/188Os values for coexisting laurite and Os-rich alloy pairs from 'primary' PGM assemblage (0.12473-0.12488) and similar 'unradiogenic' 187Os/188Os values for both 'primary' and 'secondary' PGM assemblages (0.1242±0.0008 and 0.1245±0.0006, respectively), which closely match the bulk 187Os/188Os value of their host chromitite (0.1240±0.0006). The unprecedented isotopic similarity between primary or secondary PGM assemblages and chromitite we report suggests that the osmium isotope budget of chromitite is largely controlled by the contained laurite and Os-rich alloy. This demonstrates that closed system behaviour of the Re- Os isotope system is possible, even during complex postmagmatic hydrothermal and/or metamorphic events. The preserved mantle Os-isotope signatures provide further support for an Enstatite Chondrite Reservoir (ECR) model for the convective upper mantle and are consistent with origin of the complex as a Caledonian ophiolite formed in a suprasubduction zone setting shortly before obduction

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

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

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