Chondritic osmium isotopic composition of late Archean convecting upper mantle: Evidence from Zunhua podiform chromitites, Hebei, North China

摘要

Podiform chromite deposits are a characteristic feature of the mantle sequences of harzburgitic ophiolites. The chromites usually have very low Re high Os contents, which makes it the most resistant phase remaining from the primary magmatic history of the ultramafic sections of ophiolites. The podiform chromite is one of the robust indicators of initial Os isotopic compositions of the ophiolites where podiform chromites were derived from, which provides strong evidence for the origin evolution of oceanic lithosphere. The Re Os contents the Os isotopic compositions of seven podiform chromitites from Zunhna ophlolitic melange belt, North China are reported in this study. The Re contents range from 0.019 to 0.128 ng/g, Os from 0.019 to 0.128 ng/g, and the ~(187)Os/~(188)Os ratio from 0.11003 to 0.11145. Three massive chromitites among the sample set have very Os contents (>300 ng/g), their ~(187)Os/~(188)Os ratios from 0.11021 to 0.11030, averaging 0.11026 + - 0.1126 (a), eqoivalent to a gamma O_s = -4.12 + - 0.06 at 2.6 Ga, which means that the Os isotopic composition of convecting upper mantle is chondritic in late Archean. It is the Os isotopic composition of podiform chromitites that are derived from the oldest ophiolite in the world till now.