Quantitative bounds on the size spectrum of isotopic heterogeneity within the mantle

Abstract

Isotopic heterogeneities within the mantle, as sampled by oceanic volcanism, are reported to exist on all length scales; unfortunately, this fundamental observation has progressed little beyond the qualitative stage. The isotopic systematics of mid–ocean–ridge basalts (MORBs) and ocean–island basalts (OIBs) are now being used to constrain the efficiency of convective mixing; one datum which could constrain convective mixing calculations is the size spectrum of chemical heterogeneity, but unfortunately this datum does not exist. Here I place quantitative bounds on the size spectrum of mantle heterogeneities, using the variations in ^(87)Sr/^(86)Sr ratios in oceanic basalts over different length scales. The spectrum is consistent with a flat spectrum filtered by magmatism. Such a white–noise spectrum indicates that there are many sources of anomalies at different wavelengths (with about equal magnitude), or that there is a global–scale source of anomalies operating continuously over thousands of millions of years, or some combination of both models.

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