Waters et al. (2011) Perspective on the genesis of E-MORB from chemical and isotopic heterogeneity at 9-10ºN East Pacific Rise.

The study of chemically and isotopically enriched mid-ocean ridge basalts (E-MORB) has offered substantial insight into the origin of mantle heterogeneity, but the exact processes involved in producing this E-MORB enrichment are vigorously debated. Additionally, because the ages of E-MORB are not well constrained, the temporal petrogenetic and geological relationships between E-MORB and normal (N)-MORB are not known. To investigate these relationships, we measured major and trace elements, and Sr, Nd, Hf, Pb, and U-Th-Ra isotopes for a suite of E-MORB and N-MORB lavas that were collected off-axis from 9-10ºN along the East Pacific Rise (EPR).

 

These data show coherent mixing trends, implying that mixing of melts from different sources occurs at different depths. Forward modeling of the trace element data is consistent with mixing of deep, low-degree, enriched pyroxenite melts with shallow, high-degree, depleted peridotite melts. Previous studies of E-MORB at 9-10ºN have suggested that E-MORB compositions could be explained by off-axis magma transport and eruption. However, our U-series model age constraints, determined using U-series data available at PetDB for zero-age MORB from 9-10ºN EPR, indicate these E-MORB are old and were not erupted in situ. The asymmetric, off-axis distribution of E-MORB revealed by PetDB and ancillary data compiled for 9-10ºN EPR is more simply explained by a model in which E-MORB erupted within the axial summit trough (AST), flowed down the ridge flanks (~0-3 km), spread away from the AST, and were preserved on the seafloor through asymmetric construction of the extrusive layer. Taken together, the range of E-MORB ages and the geochemical mixing trends suggest that enriched melts are continuously supplied to the ridge axis, but because of their small proportions relative to the volumetrically and volcanically dominant N-MORB, E-MORB preservation and exposure is comparatively scarce.”

–C.L. Waters

Waters, C.L., Sims, K.W.W., Perfit, M.R., Blichert-Toft, J., Blusztajn, J. (2011), “Perspective on the genesis of E-MORB from chemical and isotopic heterogeneity at 9-10ºN East Pacific Rise.” Journal of Petrology, v. 52, p. 565-602.

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