Platinum-group elements and Os isotopic characteristics of the lower oceanic crust

We present osmium isotopic compositions and Os, Re and platinum-group eleme nt (PGE) contents in the lower oceanic crust gabbros from ODP Site 735B on the Southwest Indian Ridge. The average gabbros are very depleted in PGEs, e.g. the Os concentrations are very low and range from 0.5 to 38 ppt. One t roctolite sample is an exception, containing, e.g. 376 ppt Os, 396 ppt Ir a nd 8 ppb Pd. Re contents are extremely heterogeneous, ranging from 128 ppt to 28 ppb. The present day Os-187/Os-188 varies from 0.140 to 0.467, and co rrelates with Rb/Cs, suggesting that seawater alteration affected the Os is otopic system. The average Os content in the lower oceanic crust is about 9 ppt while median value for Re is about 487 ppt. Such very high Re/Os is re latively similar to the upper MORB-type oceanic crust, indicating that recy cled ocean crust basalts and gabbros will rapidly develop very radiogenic O s isotopic signatures. (C) 2000 Elsevier Science B.V. All rights reserved.