Magmatic processes and timescales of explosive volcanoes of the Chiapanecan and Central American volcanic arcs through zircon petrochronology

Abstract:

The three volcanoes El Chichón (Mexico), Santa María-Santiaguito (Guatemala) and Ilopango caldera (El Salvador) have produced the most voluminous and deadliest silicic eruptions within the active Chiapanecan and Central American volcanic arcs. Despite their importance, uncertainties remain regarding magmatic timescales and processes prior to explosive venting. Zircon ²³⁰Th-²³⁸U disequilibrium dating indicates that regardless of their eruption size, the sub-volcanic reservoirs of these volcanic centres accumulated silicic magma many 10s of thousands of years prior to eruption. During this time, zircon crystallized in a semi-continuous fashion, although zircon crystallization ages typically peak close to the eruption. Chemical data also indicate progressive magma differentiation and record secular changes in magma chemistry and temperature (e.g., δ¹⁸O and REE, Hf, Ti-in-zircon). Textural and chemical discontinuities are variably attributed to magma recharge and assimilation of crustal components superimposed on secular intervals of differentiation and cooling. Interaction of magma with supracrustal components (El Chichón volcano) and cannibalization of hydrothermally altered rocks (Ilopango caldera) as detected by high- and low-δ¹⁸O zircon values relative to mantle compositions, respectively, are identified as important magmagenetic processes. The preservation of zircon in the comparatively unevolved El Chichón and Santa María-Santiaguito magmas requires brief residence when mixed into and being homogenized within more primitive magmas than the more evolved melt environments where zircon originally crystallized. For El Chichón zircon, this environment is likely represented by magma mush forming an evolved boundary layer. Zircon petrochronology applied to these three active volcanic systems underscores the highly dynamic nature of arc volcanic magma reservoirs which experience hidden thermal and compositional changes over long and short timescales prior to eruption.