For several years the PI has operated one of the major geochemical laboratories in the US, and he has concentrated on problems dealing with the origin and evolution of the Earth's mantle. During the previous funding period he and his associates have: a) determined Sm/Nd ages for sub-continental mantle by analysis of silicate inclusions in diamonds: b) delineated a golbal isotopic anomaly belt (DUPAL anomaly) lying between the equator and 50 degrees S. latitude; cf) delimited the range of isotopic signatures for the subcontinental lithosphere by analysis of xenoliths and xenolith-bearing baslats from southern Africa, Turkey, Hungary, Arizona, Poland, Spitzbergen and Nunivak; d) refined a model for the refractory incompatible element contents of the bulk silicate Earth; e) completed a variety of leaching experiments on clinopyroxenes from 2.7 by volcanic rocks to demonstrate the trustworthiness of this mineral for recording initial Sr and Nd isotopic mantle ratios; f) showed that while significant isotopic avriations exist on mantle scale lengths of 10-50 meters, intra-mineral disequilibria is only demonstratabel in mantle xenoliths carrying a metasomatic imprint; and g) provided access to state-of-the-art isotopic facilities to over 400 scientists from MIT and elsewhere. During the five-year tenure of this award, it is proposed to continue these same general themes. Particular effort will be placed on the Pb and Os isotopic variations in mantle rocks by direct measurement of both ultramafic xenoliths and of smaples from the Ronda ultramafic massif.
