Highly siderophile element geochemistry of peridotites and pyroxenites from Horní Bory, Bohemian Massif: Implications for HSE behaviour in subduction-related upper mantle

Highly siderophile element (HSE – Re, Os, Ir, Ru, Rh, Pd, Pt, and Au) and S abundances, sulphide petrography and 187Os/188Os compositions of two distinct suites – Mg-lherzolite and Fe-dunite/wehrlite accompanied by pyroxenites from Horní Bory, Bohemian Massif, Czech Republic are reported. The Mg-lherzolite suite is enriched in IPGE (Os, Ir, and Ru), moderately depleted in Pd, Pt and S, and has osmium isotopic compositions similar to that of the Phanerozoic convecting mantle. Such a composition can be best explained by reaction between previously depleted peridotite and sulphur-undersaturated melt at low melt-rock ratios. This reaction was associated with precipitation of IPGE alloys and Pt–As. The Fe-dunite/wehrlite suite is depleted in IPGE, enriched in Pd, Pt, and S and has radiogenic osmium isotopic compositions. The origin of this suite reflects melt-rock reaction between peridotite and subduction-related melt at high melt/rock ratios resulting in breakdown of primary sulphides and their replacement by HSE-poor but 187Os-enriched sulphides. Mass-balance calculations and relations between HSE and S contents show that, while most of the Re is trapped in base metal sulphides (BMS), only up to 28% of the rest of the HSE (Os, Ir, Ru, Pd, Pt) are accommodated in BMS. Several Pt–As phases discovered in Mg-lherzolite and Fe-dunite/wehrlite and Os–Ru phase found in Mg-lherzolite suggesting complex behaviour of HSE during melt-rock reactions depending on melt composition and melt/rock ratios. The Re–Os data for the Fe-wehrlite/dunite and pyroxenite rocks yield an errorchon age of 334 ± 19 Ma, which is similar to the age of other high temperature/pressure rocks from Gföhl Nappe (Moldanubian Zone, Bohemian Massif); this age likely corresponds to the time of melt-rock reactions and subduction.