9315960 Gill The PIs propose an integrated four year study of: (1) the partition coefficients of Th, U, and Ra; (2) numerical models of the behavior of radionuclides during partial melting; and (3) Th, U, Ra, and Pa activities in volcanic rocks from mid-ocean ridge segments (Siqueros transform, 36oN and 33oS on the Mid Atlantic Ridge, and a propogating tip of the Galapagos Spreading Center at 86oW), two continental margin arcs (Japan and Kamchatka) and two oceanic arc-backarcs (tholeiites in central Izu and shoshonites in the northern Marianas), and two chemically- zoned continental felsic systems (Inyo-Mono in California and Baitoushan in China). The objective is to distinguish the effect of time from the effect of an open system in each case. The key to reaching this objective, apart from overcoming the difficult analytical problems, is choosing rock suites well-suited to thematic questions and for which abundant other geological and geochemical information will be available. Consequently, this proposal focuses on #3 above.
