Geochemistry of basalts from the Hayes Transform region of the Mid‐Atlantic Ridge

Fresh glassy aphyric and phyric basalts and dolerites have been recovered from the Mid‐Atlantic Ridge spreading segments adjacent to the Hayes Transform (33°40′N) and within the transform valley. Glass and whole rock compositions exhibit Mg numbers that range from 44.3 to 70.6, CaO/Al 2 O 3 ratios of 0.57–0.87, Na 2 O contents of 1.84–3.49 wt %, (La/Yb) cn ratios of 0.40–1.39, 87 Sr/ 86 Sr ratios of 0.70270–0.70403, and 206 Pb/ 204 Pb 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 between two major mantle provinces based on major, trace, rare earth element, and Sr‐Nd‐Pb isotopic compositions of these basaltic samples. Modeling of partial melting and fractional crystallization from minor, trace, and rare earth elements indicates that the southern Hayes basalts could have been generated by ∼20% melting of a fertile normal mid‐ocean ridge basalt (NMORB) source region, efficient pooling of melts, and low‐pressure fractionation along a magmatically robust spreading segment (HA‐1). The northern Hayes basalts are estimated to have been generated by ∼13 to ∼20% partial melting of a heterogeneous source region (enriched MORB (EMORB) and infertile NMORB 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 ∼11 to ∼22% of a heterogeneous source region, efficient and inefficient pooling of melts, and moderate pressures of fractionation.