In order to investigate fluid fluxing and the time-scales of magma genesis in the mantle below Costa Rica, basaltic andesites erupted in 1963-1965 from Irazu volcano were analyzed for major and trace element compositions as well as thorium isotope and Ra-226 abundances. The 1963-65 eruption of Irazu produced two groups of tephras termed high-Ra, with (Ra-226)/(Th-230) = 1.13-1.21 and low-Ra with near equilibrium (Ra-226)/(Th-230) values. Low-Ra and high-Ra samples have similar overall compositions. For example, barium and HSF element concentrations are about the same in the two magma groups, and all tephras have (Th-230)/(Th-232) ratios of about 1.20 and (U-238)/(Th-230) congruent to 0.9. However, low-Ra samples have 6-16 % higher concentrations of many highly incompatible elements compared to high-Ra samples, whereas REE, Sr, and P2O5, concentrations are higher by about 3-8 % in the high-Iia samples. These compositional differences can be explained by variations in the degree of melting and in the compositions of fluids involved in magma genesis. Low-Ra magmas were generated by dehydration partial melting of garnet-bearing mantle that had been enriched previously by a slab fluid. High-Ra magmas were generated by a similar mechanism, but involved an additional mantle-derived fluid. Trace element compositions of the high-Ra magmas suggest that this fluid was similar to a carbonatite. The temperature and pressures during melting were appropriate for carbonatite-silicate melt miscibility, and the carbonatitic fluid was consumed or partly consumed during melting. The lower concentrations of many highly incompatible trace elements in the high-Ra samples resulted from a higher degree of partial melting of its mantle source caused by fluid-silicate melt miscibility. The approximately equal HFS element and Ba concentrations in the low- and high-Ra samples, and the relatively high REE, Sr, and P2O5 concentrations in the high-Ra samples reflects the addition of these elements to the high-Ra magma from the fluid. Ra-Ba-Th geochronometry suggests that the enrichment event occurred between a few centuries and eight millennia before the eruption. Copyright (C) 1998 Elsevier Science Ltd.
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