Osmium isotope systematics of the Proterozoic and Phanerozoic ophiolitic chromitites: In situ ion probe analysis of primary Os-rich PGM

摘要

In situ Os-187/Os-188 ratios are determined on Os-rich platinum-group minerals in podiform chromitites both in the Proterozoic ophiolite, Eastern Desert, Egypt, and in the Phanerozoic Oman ophiolite. Because they have very low Re/Os, these primary minerals reflect the initial Os-187/Os-188 ratios of their parental magmas. The platinum-group minerals (PGM) in the central Eastern Desert chromitites exhibit sub-chondritic to chondritic Os-187/Os-188 ratios, 0.1226 on average, which is lower than the primitive upper mantle evolution trend of a comparable age. Those of the southern Eastern Desert chromitites have more radiogenic Os, with supra-chondritic Os-187/Os-188 ratio of about 0.1293 on average, which could be due to crustal contamination. The three chromitite types in the northern part of the Oman ophiolite are almost indistinguishable in terms of their Os-187/Os-188 ratios; they have overlapping values ranging from sub-chondritic to supra-chondritic ratios. The PGE-rich, mantle chromitite samples have a wide range Of Os-187/Os-188 ratio from 0.1230 up to 0.1376, with an average of 0.1299. The values of the PGE-poor mantle chromitites overlap in their Os-187/Os-188 ratios with PGE-rich chromites, but are less variable and have a significantly higher average ratio. The Moho transition zone (MTZ) chromitites are highly variable in the Os-187/Os-188 ratio, ranging from 0.1208 up to 0.1459.