MULTIPLE MANTLE MELTING REGIMES IN A SECONDARY FRONT VOLCANO, EL TIGRE, CENTRAL AMERICA
Samples are basalts and basaltic andesites and are chemically indistinguishable, except for two samples collected from a parasitic cone on the northern flanks of El Tigre. Samples from the main volcanic edifice show depletion of high field strength elements (HFSE) relative to large ion lithophile elements (LILE) and light rare earth elements (LREE). U-Th isotope analysis of one sample from the stratovolcano has secular equilibrium (230Th/238U), within error (activity ratio of=0.993), which may be due to the age of the flows. Nevertheless, trace element data show that these samples are U-enriched, similar to most arc lavas. In contrast, samples from the parasitic cone show lesser HFSE depletion relative to LILE and LREE compared to the samples from the main volcanic edifice. Remarkable for an arc lava, the one sample analyzed for U-series isotopic data shows 15% 230Th over 238U enrichment, suggesting a fundamentally different melting regime from that of the main cone.
Contrasting geochemical and isotopic data suggest the main stratovolcano was built by melts generated by flux melting, while melts that form the parasitic cone have been generated by decompression melting, with small slab contribution. 87Sr/86Sr and 143Nd/144Nd ratios of representative samples suggest that the same mantle wedge is experiencing both styles of melting, constraining source variability in evaluating mantle modification due to fluid and sediment addition.