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

Podiform chromite deposits are a characteristic feature of the mantle sequences of harzburgitic ophiolites. The chromites usually have very low Re and 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 and evolution of oceanic lithosphere. The Re and Os contents and the Os isotopic compositions of seven podiform chromitites from Zunhua ophiolitic mélange belt, North China are reported in this study. The Re contents range from 0.019 to 0.128 ng/g, Os from 8.828 to 354.0 ng/g, and the 187 Os/ l88 Os ratio from 0.11003 to 0.11145. Three massive chromitites among the sample set have very high Os contents (> 300 ng/g), and their 187 Os/ 188 Os ratios range from 0.11021 to 0.11030, averaging 0.11026±0.00005 (σ), equivalent to a γOs= ∼-0.12 ±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.