The 2013 eruption of Chaparrastique volcano (El Salvador): Effects of magma storage, mixing, and decompression

Abstract

On December 29, 2013, an isolated vulcanian-type eruption occurred at Chaparrastique volcano (El Salvador) after 12 years of inactivity. The eruption was classified as VEI 2 and produced an ash plume with a maximum height of ~9 km. Textural and compositional data fromphenocrysts fromthe erupted products have been inte- grated with geochemical and isotopic information frombulk rocks to elucidate the magmatic processes respon- sible for the reawakening of volcanic activity. Phenocrysts consist of Fo-rich poikilitic olivines hosting high-Mg titanomagnetites, and Fo-poor olivines coexisting with low-Mg titanomagnetites.Mineral-melt equilibria suggest an origin for the distinct phenocryst populations by mixing between a high-T (~1130–1150 °C), basaltic magma with fO2 (NNO buffer) typical of the lower crust inarc systems and a low-T (~1060–1080 °C), basaltic andesiticmagma with fO2 (NNO+1 buffer) commonly encountered in shallower, more oxidized crustal reservoirs. Thermobarometry based on Fe-Mg ex- change between orthopyroxene and clinopyroxene constrains the crystallization before eruption at relative low-P (~150–250MPa) and low-T (~1000–1050 °C).Mixing between twochemically distinctmagmas is also ev- idenced by the occurrence of reverse zoned plagioclase phenocrysts with resorbed sodic cores and re-growth of sieve-textured calcicmantles. Conversely, plagioclase rims exhibit disequilibriumcompositions addressed to de- compression kinetics (~10−3 MPa/s) driven by rapidmagma ascent to the surface (~0.03 m/s). Major and trace element modelling excludes fractional crystallization as the primarymechanismcontrolling the bulk rock variability,whereas geochemical data align along amixing trend between two end-members represen- tative of the primitive basalt and the differentiated basaltic andesite. Trace element and isotope data indicate that the primary source of magmatism is an enriched MORB-like mantle with the contribution of fluxes ofmetaso- matic fluids and/ormelts produced by the subducted slab. The roleplayed by slab-fluid inputs of carbonate origin and slab-melts fromthe hemipelagic sediments seems to beminimal. Assimilation/contamination processes of magmas by crustal rocks are also negligible. In contrast, the geochemical signature of magmas is greatly influ- enced by slab-derived aqueous fluids produced prevalently by progressive dehydration of marine sediments and altered basaltic crust