Heterogeneous mantle melting and magmatic processes at the East Pacific Rise (2.6-3.1 degrees S): Evidence from mid-ocean ridge basalt geochemistry and Sr-Nd-Pb isotopes

摘要

We present the major and trace elements and Sr, Nd, and Pb isotopes in mid-ocean ridge basalts (MORB) from the East Pacific Rise (EPR) at 2.6-3.1 degrees S. These samples are low-K tholeiites and show significant variation in their major element compositions (e.g. 4.60-8.18 wt% MgO, 8.34-12.12 wt% CaO, 9.78-14.25 wt% Fe2O3, and 0.06-0.34 K2O wt%). Trace element abundances of the 2.6-3.1 degrees S MORB are variably depleted (e.g. (La/Sm), (N) = 0.51-0.78, Zr/Y = 2.35-3.42, Th/La = 0.035-0.056, and Ce/Yb = 2.38-3.96) but closely resemble the average N-MORB. In the compatible elements (Ni and Cr) against incompatible element Zr plots, the 2.6-3.1 degrees S MORB show well-defined negative correlations, together with a liquid line of descent (LLD) modelling and petrographic observations, implying a significant role of olivine, plagioclase and clinopyroxene fractionation during magma evolution. When compared to global MORB and peridotites, the 2.6-3.1 degrees S MORB and most of the other axial lavas from the South EPR show similar Zn/Fe, Zn/Mn, and Fe/Mn ratios, attesting to a peridotite-dominated mantle lithology. However, the relationships between incompatible trace element ratios, such as Zr/Rb and Nb/Sm, and the negative correlation between Zr/Nb and Sr-87/Sr-86 indicate a geochemically heterogeneous mantle source. The mantle beneath the South EPR likely consists of two components, with the enriched component residing as physically distinct domains (e.g. veins or dikes) in the depleted peridotite matrix. In the Sr-Nd-Pb isotope space, the South EPR MORB lie along the mixing lines between the depleted MORB mantle (DMM) and the 'C'-like Pukapuka endmember. We infer that low-F melts derived from these enriched materials may cause localized mantle heterogeneity (veins or dikes) via an infiltration process. Subsequent melting of the refertilized mantle may impart an isotopically distinct characteristic to South EPR MORB.