Petrology and Sr, Nd, and Pb isotope geochemistry of mid-ocean ridge basalt glasses from the 11°45′N to 15°00′N segment of the East Pacific Rise

Abstract:

Basaltic glasses from the geophysically well-studied section of the East Pacific Rise (EPR) between 11°45′N to 15°00′N range from normal mid-ocean ridge basalts (MORB) to transitional MORB and their major element variations correlate with isotopic and trace element indices of enrichment. To first order, basalts enriched in Na8.0, incompatible elements, 87Sr/86Sr, and 206Pb/204Pb but low in Fe8.0 and 143Nd/144Nd are more prevalent along the shallow portions of the ridge axis. In detail, the samples can be divided into two chemical and geographical Groups: the southern bathymetric dome, extending from the 11°45′N overlapping spreading center to ∼14°10′N, and the northern Group, extending from ∼14°10′N to the Orozco transform. The boundary between these two Groups is apparent in a change in isotopic composition. Results indicate that there are three mantle source components that produce the compositional variability observed among samples from the 11°45′N to 15°00′N segment of the EPR: a depleted mantle component, a seamount-type enriched mantle component, and an Indian MORB-like mantle component. South of ∼14°10′N, the geochemical variability is dominated by binary mixing between a depleted mantle component and an enriched component similar to near-ridge seamounts. North of ∼14°10′N, the low 206Pb/204Pb, high 207Pb/204Pb Indian MORB-like component exerts a major influence on the geochemical variability of the axial lavas. Regional averages of major element composition (e.g., Na8.0 and Fe8.0) show relatively limited variability consistent with the restricted range in depth for this region and plot within the Pacific field of the previously defined global trends. Major element variations among individual samples, however, parallel the global array, and their correlation with indices of mantle enrichment supports the idea that the “Pacific-type local trend” results from small-scale heterogeneities in the mantle beneath the EPR. Our results also indicate that tectonic segmentation and magmatic boundaries are probably not causally related in this study area and that the sizes of the present magma chambers are not a dominant factor in determining the compositional variability of erupted lavas.

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