Mafic Recharge to a Zoned Magma Reservoir Centuries Prior to the 1835 Cosigüina Eruption
Friday, 19 December 2014: 2:10 PM
The AD 1835 eruption of Cosigüina volcano, Nicaragua, produced ~6 km3 of tephra and had a global climate impact similar to that of the 1991 Pinatubo eruption. In this study, we aim to constrain the magmatic processes and their timescales that led to this eruption. Our results indicate that the 1835 eruption tapped a zoned, shallow (~4 km depth) magma reservoir; small volumes of crystal-poor dacite and silicic andesite were erupted first followed by crystal-rich andesite, which represents the bulk of the erupted magma. The compositions of bulk rocks, matrix glasses and melt inclusions are consistent with derivation of (1) the andesite from a basaltic andesite parent and (2) the silicic andesite and dacite from the andesite through liquid extraction and fractional crystallization. Bimodal crystal populations are interpreted to reflect a polybaric differentiation process in which calcic plagioclase (An75-90) formed from hot and hydrous andesitic to basaltic liquids in the lower crust whereas An50-65 plagioclase crystallized from cooler and partly degassed andesitic to dacitic liquids in the shallow magma reservoir. Mg diffusion chronometry applied to calcic plagioclase found in andesite and dacite constrains the residence time of these crystals in the silicic liquids to more than 100 and less than 2000 years, with detailed analysis of 3 crystals yielding 400 years. A mafic recharge event thus appears to have taken place a few centuries prior to 1835. Although the calculated timescales represent minimum differentiation times, we propose that magma reservoir zonation may have been established within 102-103 years at Cosigüina. Several thick, chemically zoned pyroclastic deposits are well exposed at a section 11 km west of the caldera, revealing the cyclic behavior of this volcano in the recent geologic past. If the oldest eruptions are of similar age to those of neighboring volcanoes (160-330 ka), a maximum recurrence time for repeated replenishment, magma reservoir zonation and eruption at Cosigüina is 1-2 x 104 years. Detailed geochronology is required to determine the frequency of explosive eruptions at this volcano.