Os isotopic systematics of the MORB mantle: results from altered abyssal peridotites
Abyssal peridotites are fragments of the oceanic upper mantle. Previous studies have indicated that their Os isotopic compositions span a wide range, including values more radiogenic than most estimates of the average bulk Earth 187Os 188Os ratio. This is difficult to reconcile with their derivation as residues of MORB partial melting. We present results that suggest that some of this variation, particularly the higher values, may be of secondary origin. Comparison of altered rims and unaltered cores of two mylonitic peridotites reveal that submarine weathering can increase 187Os 188Os ratios by more than 5%. Leaching experiments demonstrate the presence of a radiogenic, easily leachable phase that is most easily explained as a seawater-derived component. Thus, Os isotopic results from abyssal peridotites must be interpreted with some caution unless it can be shown that the effects of seawater alteration were minor. We develop criteria based on Sr and Nd isotopic measurements, as well as major element compositional changes, that allow strongly altered samples to be identified. Applying these criteria, we find 10 samples, including literature data as well as new analyses, that have apparently suffered only minimal alteration. These samples range in 187Os 188Os from 0.1221 to 0.1270, with a mean of 0.1246 and a standard deviation of 0.0014. Thus, we see no evidence for depleted mantle rocks more enriched in 187Os 188Os than the preferred value of ∼ 0.127. Finally, if Os is lost from peridotites during weathering, it may provide a significant source of mantle Os to the world's oceans. © 1995.
Publication Source (Journal or Book title)
Earth and Planetary Science Letters
Snow, J., & Reisberg, L. (1995). Os isotopic systematics of the MORB mantle: results from altered abyssal peridotites. Earth and Planetary Science Letters, 133 (3-4), 411-421. https://doi.org/10.1016/0012-821X(95)00099-X