Re-Os isotopic systematics and platinum group element composition of the Tagish Lake carbonaceous chondrite

摘要

The Tagish Lake meteorite is a primitive C2 chondrite that has undergone aqueous alteration shortly after formation of its parent body. Previous work indicates that if this type of material was part of a late veneer during terrestrial planetary accretion, it could provide a link between atmophile elements such as H, C, N and noble gases, and highly siderophile element replenishment in the bulk silicate portions of terrestrial planets following core formation. The systematic Re-Os isotope and highly siderophile element measurements performed here on five separate fractions indicate that while Tagish Lake has amongst the highest Ru/Ir (1.63 +/- 0.08), Pd/Ir (1.19 +/- 0.06) and Os-187/Os-188 (0.12564-0.12802) of all carbonaceous chondrites, these characteristics still fall short of those necessary to explain the observed siderophile element systematics of the primitive upper mantles of Earth and Mars. Hence, a direct link between atmophile and highly siderophile elements remains elusive, and other sources for replenishment are required, unless an as yet poorly constrained process fractionated Re/Os, Ru/Ir, and Pd/Ir following late accretion on both the Earth and Mars mantles.The unique elevated Ru/Ir combined with elevated Os-187/Os-188 of Tagish Lake may be attributed to Ru and Re mobility during aqueous alteration very early in its parent body history. The Os, Ir, Pt, and Pd abundances of Tagish Lake are similar to Cl chondrites. The elevated Ru/Ir and the higher Re/Os and consequent Os-187/Os-188 in Tagish Lake, are balanced by a lower Ru/Ir and lower Re/Os and Os-187/Os-188 in CM-chondrites, relative to Cl chondrites. A model that links Tagish Lake with CI and CM chondrites in the same parent body may explain the observed systematics. In this scenario, CM chondrite material comprises the exterior, grading downward to Tagish Lake material, which grades to Cl material in the interior of the parent body. Aqueous alteration intensifies towards the interior with increasing temperature. Ruthenium and Re are mobilized from the CM layer into the Tagish Lake layer. This model may thus provide a potential direct parent body relationship between three separate groups of carbonaceous chondrites. Copyright (c) 2005 Elsevier Ltd.