Geochemical Variations in a Single Basaltic Flow at 9°30'N on the East Pacific Rise

Abstract

Basalts collected during Ocean Drilling Program Leg 142 from a single site at 9°3θ'N on the East Pacific Rise have been analyzed for Sr, Nd, and Pb isotope ratios by thermal ionization mass spectrometry, and trace element concentrations (rare earth elements, Ba, Y, Zr, Nb, Hf, and Ta) by inductively coupled-plasma mass spectrometry. Samples exhibit a small range in geochemical variations. Mean isotopic values for the data set are 0.702484 ± 0.000011 (lσ) for 87Sr/86Sr, 9.70 ± 0.27 (lσ) for eNd, 18.300 ± 0.025 (lσ), 15.477 ± 0.016 (lσ), and 37.705 ± 0.062 (lσ) for ^Pb/^Pb, ^Pb/^Pb, and ^Pb/^Pb, respectively. Trace element compositions are also typical of normal-type mid-ocean ridge basalt. Detailed statistical analysis indicates that the lavas are indistinguishable in terms of their isotopic ratios, Nb, Ta, Sr, Ni, Cu, and Zn concentrations, and most major oxide contents. Samples fall into two distinct units based on the concentrations of mostly incompatible elements, including the rare earth elements, Ba, Y, Zr, and Hf. The relationship between the units cannot be explained by shallow-level phenomena including fractional crystallization or crystal settling but must have originated below the magma chamber, which is consistent with other Leg 142 studies (Brophy and Allan, this volume; Brophy, this volume). Samples are derived from two or more similar but distinct parental melts. The parental melts share the same mantle source, but evolved via different melt generation paths. The precise temporal relationship between Units 1 and 2 cannot be determined owing to the lack of stratigraphic control during drilling. Interunit geochemical variations may represent two (or more) eruptive events, or heterogeneity within a single flow. In either case, this information is useful in evaluating geochemical results from axial studies, particularly from the immediate 9°-10°N ridge area. At a single site on the East Pacific Rise, variations in trace element concentrations are accompanied by isotopic homogeneity, suggesting that geochemical characteristics of and processes within the underlying mantle are the major factors controlling compositional variation at mid-ocean ridges, not shallow magma chamber processes. Large-scale phenomena such as mantle convection may be primarily responsible for the degree of isotopic heterogeneity at mid-ocean ridges, while complex mantle melting processes control trace element characteristics.