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

We have analyzed Os-187/Os-186 of MORBs and abyssal periodtites. Peridotites have unradiogenic Os-187/Os-186 (1.019-1.071), close to the carbonaceous chondrite average (almost-equal-to 1.06). MORBs have more radiogenic Os-187/Os-186 (1.117-3.534). We have used a leaching method to remove highly radiogenic Os-rich MnO2 precipitates that may have contaminated the samples. Leached North Atlantic MORBs have a small variation of Os-187/Os-186 (1.082-1.086 for 30-degrees-N and 1.095-1.113 for FAMOUS area samples), suggesting the removal of any external contamination. 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 indicating that a radiogenic component is still present after leaching. Our experiments do not demonstrate that leached lavas are free of contamination. However, leached MORBs from the North Atlantic seem to be free of external contamination, although they are still more radiogenic than abyssal peridotites from the same region. This discrepancy may result from assimilation of hydrothermal material, hotspot/ridge interaction, or the melting of a heterogeneous mantle. The unradiogenic nature of abyssal periodotites from the North Atlantic relative to the primitive mantle may reveal large-scale heterogeneities in the extraction and recycling of the oceanic crust in the mantle.