GEOCHEMISTRY OF BASALTS FROM THE HAYES TRANSFORM REGION OF THE MID-ATLANTIC RIDGE

Fresh glassy aphyric and phyric basalts and dolerites have been recove red from the Mid-Atlantic Ridge spreading segments adjacent to the Hay es Transform (33 degrees 40'N) and within the transform valley. Glass and whole rock compositions exhibit Mg numbers that range from 44.3 to 70.6, CaO/Al2O3 ratios of 0.57-0.87, Na2O contents of 1.84-3.49 wt %, (La/Yb)(cn) ratios of 0.40-1.39, Sr-87/Sr-86 ratios of 0.70270-0.7040 3, and Pb-206/Pb-204 ratios of 17.95-18.79. These basalts are some of the most primitive and most fractionated basalts from the Mid-Atlantic Ridge. The Hayes Transform appears to mark a geochemical boundary bet ween two major mantle provinces based on major, trace, rare earth elem ent, and Sr-Nd-Pb isotopic compositions of these basaltic samples. Mod eling of partial melting and fractional crystallization from minor, tr ace, and rare earth elements indicates that the southern Hayes basalts could have been generated by similar to 20% melting bf a fertile norm al mid-ocean ridge basalt (NMORB) source region, efficient pooling of melts, and low-pressure fractionation along a magmatically robust spre ading segment (HA-1). The northern Hayes basalts are estimated to have been generated by similar to 13 to similar to 20% partial melting of a heterogeneous source region (enriched MORE (EMORB) and infertile NMO RB source domains), incomplete pooling of melts, and moderate pressure fractionation along a magmatically starved spreading segment (OH-3). Transform valley basalts show estimated extents of melting from simila r to 11 to similar to 22% of a heterogeneous source region, efficient and inefficient pooling of melts, and moderate pressures of fractionat ion.