James Day of Scripps Institute of Oceanography will give a talk at Speakers Club entitled "Oceanic peridotites and the nature and significance of depleted mantle domains."

ABSTRACT:

A fundamental geochemical tenet is that isotopes are not disturbed from natural ratios during high-T processes. The assumption in many studies is that basalt production therefore records the mantle isotopic signature. For some isotope systems, however, this is not a valid assumption, including the apparent “Pb paradox”, the "osmium isotopic gap” between basalts and peridotites, and the observation of ultra-depleted isotope compositions in mantle peridotites. These are long-standing problems that challenge current understanding of how chemical signatures are transferred from the mantle to the crust. In this contribution I will present new data on abyssal peridotites, dredged from the ocean floor to show that these complex rocks record an upper mantle that is even more depleted than previously thought and provide new composition constraints on mantle evolution.