PRECISE PT-RE-OS ISOTOPE SYSTEMATICS OF THE MANTLE FROM 2.7-GA KOMATIITES

Abstract

Precise Pt–Re–Os isotopic and highly siderophile element (HSE) data are reported for 10 drill core samples collected across a 2.7 m thick, differentiated komatiite lava flow at Pyke Hill in Ontario, Canada. The Re–Pt–Os isotopic data for five cumulate samples yield Re–Os and Pt–Os isochrons with ages of 2725±33 and 2956±760 Ma and initial 187Os/188Os=0.10902±16 and 186Os/188Os=0.1198318±18, respectively. These ages are consistent with the time of lava emplacement determined by other geochronometers. The initial 187Os/188Os isotopic composition of the Pyke Hill source is similar to the 187Os/188Os isotopic compositions of enstatite and ordinary chondrites and to that of PUM at 2725 Ma. Using the Pt–Os isotopic data for the Pyke Hill source, the present-day 186Os/188Os isotopic composition of PUM was calculated to be 0.1198387±18. The regression of global 187Os/188Os isotopic data for terrestrial materials shown to represent Os isotopic composition of the contemporary mantle indicates that the LILE-depleted mantle evolved from the solar system starting composition and, over Earth's history, was on average ∼8% depleted in Re vs. Os relative to PUM. From the HSE composition of the emplaced Pyke Hill komatiite lava, the mantle source was inferred to contain Re (0.30 ppb), Os (3.9 ppb), Ir (3.6 ppb), Ru (5.4 ppb), Pt (6.4 ppb), and Pd (6.3 ppb) in relative proportions similar to those in average enstatite chondrites, except for Pd, which was enriched over Pt by ∼30% in the Pyke Hill source compared to enstatite chondrites. From these data, the 190Pt/188Os of PUM was calculated to be 0.00157. If the HSE abundances in the terrestrial mantle were inherited from chondritic material of a late veneer, the latter should have had the HSE composition of ordinary or enstatite chondrites. Alternatively, if the reenrichment of the mantle with HSE was the result of an open-system transport of material across the core–mantle boundary, it was likely a global event that should have occurred shortly after core formation.