Crustal Contamination of Mantle-derived Magmas within Piton de la Fournaise Volcano, Reunion Island

Abstract

Historical lavas from Piton de la Fournaise are ideally suited to investigate temporal changes in the magmatic processes at this active ocean-island volcano. Here we present a time series of the Sr, Nd, Hf, and U-series isotope ratios, and incompatible trace element abundances of lavas erupted from Piton de la Fournaise during the late 20th century. Most of the lavas display positive correlations on plots of ratios of trace elements that are more vs less incompatible during partial melting of the mantle (e.g. Th/Yb) against Sr/Sr or other trace element ratios (e.g. Nd/Sm).These trends can be explained by (1) melting of two compositionally distinct components intrinsic to a heterogeneous mantle plume beneath Re¤ union island or (2) incorporation of the ambient Indian mantle asthenosphere into the upwelling plume. The Sr/Sr and trace element (e.g. Nd/Sm) ratios of the lavas decreased systematically over three periods of time: 1953^1964, 1966^1973, and 1975^1991. Each of these trends may represent a single melting event within the mantle, such that the initial extraction of melt from a more fertile, traceelement enriched source (with relatively high Sr/Sr) was followed (as melting progressed) by the later extraction of melt from a more refractory, trace-element depleted (or less enriched) source (with relatively low Sr/Sr). The chemistry of some historical lavas from Piton de la Fournaise appears to be anomalous because (1) they depart from the trends defined by the majority of the lavas and (2) they have relatively low Th/U ratios and small Th^U disequilibria. The most likely explanation for these anomalous lavas is contamination of mantle-derived magmas with small amounts ( 3^6%) of low-degree (51%) partial melts of young gabbro and wehrlite cumulates within the volcanic edifice.The cumulates were probably altered to become enriched in Uprior to contamination (potentially as a result of recent hydrothermal alteration of the crust with U-rich fluids). Crustal contamination was most important between 1950 and 1964 and for the March 1998 lava from Hudson Crater, although moderate amounts of contamination may havebeen important throughout the late 20th century.