Hf isotope geochemistry of the Galapagos Islands

Abstract

We report 176Hf/177Hf ratios determined by multicollector inductively coupled plasma mass spectrometry on 49 basalts from the Galapagos Islands. The ɛHf values range from about +8, a value typical of many oceanic island basalts, to about +15, a value typical of mid‐ocean ridge basalt. The ɛHf shows the expected correlation with ɛNd and inverse correlations with Sr and Pb isotope ratios. The range of values and correlations observed is consistent with Galapagos mantle plume‐asthenosphere mixing inferred from previous geochemical studies. While oceanic island basalt isotopic arrays suggest that multicomponent mixing is common in the sources of such basalts, the Galapagos is unusual in that depleted upper mantle appears to be a principal component. Thermal and dynamic interaction between the Galapagos mantle plume and the lithosphere and shallow asthenosphere is the most likely cause of mixing and of the geographic pattern of geochemical variation. The Galapagos Hf‐Nd‐Sr‐Pb isotopic array diverges toward the “plume” end, indicating the plume is heterogeneous. This heterogeneity is geographic, with distinct northern, central, and southern components. The central component and, to a lesser degree, the northern component are compositionally similar to the “common component” of plumes (variously called PHEM, FOZO, and “C”), while the southern component is unique with relatively high ɛHf and 206Pb/204Pb compared to 87Sr/86Sr and ɛNd.