MANTLE HETEROGENEITY NEAR THE EAST PACIFIC RISE: PB, SR, AND ND ISOTOPIC DATA FROM THE 8°20' N SEAMOUNT CHAIN

Upper mantle source compositions and melting systematics beneath mid-ocean ridges (MORs) are inferred from basalts derived from melts that focus, mix/re-equilibrate, and homogenize beneath ridge axes prior to eruption – thus obscuring the primary characteristics of MOR mantle. To determine the compositions and melting parameters of the upper mantle beneath the East Pacific Rise (EPR), compositions of lavas erupted along the 8°20’N seamount chain, a ~200 km long volcanic lineament oriented perpendicular and extending west from the EPR axis, were examined. High-resolution bathymetric maps obtained using the AUV Sentry are combined with geochemical analyses of ~350 basalts collected using the human-operated vehicle Alvin during research cruises in 2016 and 2018 on the R/V Atlantis (AT37-05 and AT42-06). Major and trace element concentrations and radiogenic isotope ratios are extremely variable along the seamount chain and often within a single seamount. There is no systematic spatial variability in compositions along the chain. Initial Ar⁴⁰/Ar³⁹ ages of ca. 0.1 to 2 Ma are consistent with paleomagnetic data from surface towed magnetics, and indicate seamount volcanism initiated near the EPR axis but in some cases persisted out to ~100 km from the ridge crest. The suite of samples represents a geochemical continuum of depleted, normal, and abundant enriched MOR basalts (MORB), spanning the full range of EPR and seamount compositions observed in northeast Pacific lavas. Pb, Nd, and Sr ratios indicate mixing of melts derived from enriched MOR mantle sources and an extremely depleted MOR mantle source (DDMM) near the EPR. Petrologic modeling of the 8°20’ N sample suite suggests multiple lherzolite mantle sources melt to variable extents to produce the heterogeneity observed in the seamount suite. DMORB lavas are produced by ~5 - 15% melting of a depleted MOR mantle, NMORB form from 5 – 15% melting of a slightly more enriched mantle, and EMORB range from <1% melting of a 10% enriched mantle to >15% melting of an end-member enriched mantle. These results confirm that the sub-ridge mantle is much more heterogeneous than previously proposed based on geochemical studies of on-axis basalts alone. Our findings require a substantive revision of models of upper mantle source variability and melting systematics at fast-spreading MORs.