Oxygen isotope compositions of lavas from the Galapagos archipelago: geochemical contributions from modern crustal sources
New oxygen isotope compositions of olivine phenocrysts collected across the Galapagos archipelago show a larger range of δ18Oolivine values than previously reported (4.74–5.40‰) in the region. Olivines from Fernandina, Floreana, and Pinta, which represent the main radiogenic isotope end-members of the Galapagos hotspot, have δ18O values of 5.02 ± 0.08‰ (1σ), within the accepted range of the oxygen isotope composition of mantle olivines. In general, δ18Oolivine values do not correlate with radiogenic isotope compositions of hosting lavas. Instead, the span of δ18Oolivine values is more consistent with widespread lithospheric contamination. δ18Oolivine values at the higher end of the range in the Galapagos are correlated with indices of crustal assimilation including Sr/Sr*. Values below the normal mantle range can be explained by assimilation and fractional crystallization processes. Olivines that have δ18O values below the normal mantle range come from the western part of the archipelago with the thickest lithosphere. This is consistent with melt interacting with crust that underwent hydrothermal alteration at elevated temperature, causing a decrease in δ18O values. In contrast, the highest δ18O values of the Galapagos come from areas underlain by thin lithosphere in the eastern part of the archipelago. This is consistent with shallow crust/melt interaction that is generally associated with δ18O values above the normal mantle range. These results suggest that while the three end-member components of the Galapagos mantle have a generally homogenous δ18O value indistinguishable from “normal” upper mantle, there is a more widespread effect of lithospheric contamination in melts than previously thought.
KeywordsGalapagos Mantle plume Ocean Island Oxygen isotopes Assimilation AFC process
This work was supported by the National Science Foundation Graduate Research Fellowship (Grant no. DGE-1058262 to M.E.P.), the National Science Foundation Division of Ocean Sciences (Grant no. 0962195) and the NSF support that allowed collection of the samples. We would like to thank T. Prissel, B. Parks and E. Chin for their thoughtful discussions during the preparation of this manuscript and Dennis Geist for sharing some of the samples in this study. We’d also like to thank Dr. Chris Harris and an anonymous second reviewer as well the editor, Dr. Jochen Hoefs, for their thoughtful reviews. Their suggestions greatly improved the quality of the manuscript.
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