LYING IN WAIT: EVOLUTION OF DACITE AT VOLCAN SANTA MARIA, GUATEMALA

To advance understanding of the chemical evolution and volcanic hazards associated with predominantly mafic composite volcanoes that typify the Central American Volcanic Arc in Guatemala we have begun a multi-pronged study of Volcan de Santa Maria. We aim to build upon the pioneering work at Santa Maria-Santiaguito by Bill Rose which he published in several influential papers between 1972 and 1987. 40Ar/39Ar dating indicates that (1) the composite cone of Volcan Santa Maria grew via addition of basaltic andesitic lavas and pyroclastic flows during at least three eruptive pulses beginning 103 ka, and (2) the volcano was most likely dormant for 35 ka prior to the catastrophic eruption of 8 cubic km of dacitic magma in 1902 A.D that included also a basaltic andesitic component. Our overall aim is to test the hypothesis, using 230Th-238U isotope disequilibria, that the 1902 dacite reflects incubation in the lower crust for at least 35 ka prior to eruption. To set the stage for the U-series investigation, we have measured the composition of 45 newly collected samples of lava and tephra for major element (XRF) and trace element (ICP-MS) composition; Sr-Nd-Pb isotope ratio measurements are underway on a subset of these. In parallel, major and trace element zoning of plagioclase, melt inclusion compositions, and T-fO2 estimates constrain the pre- to syn-eruptive P-T-X of the 1902 magma body. Findings thus far include: (1) calcic spikes in plagioclase of the 1902 tephra record mixing of basaltic andesitic and rhyodacitic melts prior to the explosive eruption, (2) H2O contents of plagioclase-hosted melt inclusions range from 2-6%, suggesting plagioclase growth during decompression of dacitic melt from a depth of 9-12 km, (3) trace element concentrations and ratios are consistent with generation of the 1902 dacite via crystal fractionation of a basaltic parent magma, but (4) 87Sr/86Sr ratios range from 0.70382 to 0.70403 and exhibit a subtle decrease with increasing SiO2 content and thus a decline with time over the past 103 ka, including growth of the Santiaguito dacite dome complex. This may reflect partial melting and assimilation of relatively unradiogenic middle to lower crust by basalt during creation of the large body of dacitic magma that erupted in 1902 and continues to erupt at Santiaguito today.