Journal of Petrology Advance Access originally published online on April 20, 2009
Journal of Petrology 2009 50(4):661-684; doi:10.1093/petrology/egp016
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Crustal Contamination of Mantle-derived Magmas within Piton de la Fournaise Volcano, Réunion Island
1Department of Geological Sciences, San Diego State University, 5500 Campanile Dr., San Diego, CA 92182-1020, USA
2Department of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Rd, N.W., Washington, DC 20015, USA
3Laboratoire De Sciences De La Terre, CNRS UMR 5570, Ecole Normale SupÉRieure De Lyon, 46, AllÉe D’Italie, 69364 Lyon Cedex 7, France
RECEIVED JUNE 10, 2008; ACCEPTED MARCH 16, 2009
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Abstract |
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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 87Sr/86Sr 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 Réunion island or (2) incorporation of the ambient Indian mantle asthenosphere into the upwelling plume. The 87Sr/86Sr 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, trace-element enriched source (with relatively high 87Sr/86Sr) 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 87Sr/86Sr). 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 230Th–238U disequilibria. The most likely explanation for these anomalous lavas is contamination of mantle-derived magmas with small amounts (
3–6%) of low-degree (<1%) partial melts of young gabbro and wehrlite cumulates within the volcanic edifice. The cumulates were probably altered to become enriched in U prior 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 have been important throughout the late 20th century.
KEY WORDS: crustal contamination; geochemistry; isotopes; Piton de la Fournaise; source heterogeneity
*Corresponding author. Telephone: (619) 594-2648. Fax: (619) 594-4372. E-mail: apietruszka{at}geology.sdsu.edu