OS-187 OS-186 RATIOS OF MIDOCEAN RIDGE BASALTS AND ABYSSAL PERIDOTITES/

We have analyzed Os-187/Os-186 of MORBs and abyssal periodtites. Perid otites have unradiogenic Os-187/Os-186 (1.019-1.071), close to the car bonaceous chondrite average (almost-equal-to 1.06). MORBs have more ra diogenic Os-187/Os-186 (1.117-3.534). We have used a leaching method t o remove highly radiogenic Os-rich MnO2 precipitates that may have con taminated the samples. Leached North Atlantic MORBs have a small varia tion of Os-187/Os-186 (1.082-1.086 for 30-degrees-N and 1.095-1.113 fo r FAMOUS area samples), suggesting the removal of any external contami nation. On the other hand, for leached aliquots of a single lava from the Central Indian Ridge, Os-187/Os-186 ranges from 1.191 to 2.071 ind icating that a radiogenic component is still present after leaching. O ur experiments do not demonstrate that leached lavas are free of conta mination. However, leached MORBs from the North Atlantic seem to be fr ee of external contamination, although they are still more radiogenic than abyssal peridotites from the same region. This discrepancy may re sult from assimilation of hydrothermal material, hotspot/ridge interac tion, or the melting of a heterogeneous mantle. The unradiogenic natur e of abyssal periodotites from the North Atlantic relative to the prim itive mantle may reveal large-scale heterogeneities in the extraction and recycling of the oceanic crust in the mantle.