Je. Snow et L. Reisberg, OS ISOTOPIC SYSTEMATICS OF THE MORB MANTLE - RESULTS FROM ALTERED ABYSSAL PERIDOTITES, Earth and planetary science letters, 133(3-4), 1995, pp. 411-421
Abyssal peridotites are fragments of the oceanic upper mantle. Previou
s studies have indicated that their Os isotopic compositions span a wi
de range, including values more radiogenic than mast estimates of the
average bulk Earth Os-187/Os-188 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 hi
gher values, may be of secondary origin. Comparison of altered rims an
d unaltered cores of two mylonitic peridotites reveal that submarine w
eathering can increase Os-187/Os-188 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 SI and Nd isotopi
c measurements, as well as major element compositional changes, that a
llow strongly altered samples to be identified. Applying these criteri
a, we find 10 samples, including literature data as well as new analys
es, that have apparently suffered only minimal alteration. These sampl
es range in Os-187/Os-188 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 depl
eted mantle rocks more enriched in Os-187/Os-188 than the preferred va
lue of similar to 0.127. Finally, if Os is lost from peridotites durin
g weathering, it may provide a significant source of mantle Os to the
world's oceans.