Stratigraphically controlled sampling captures the onset of highly fluid-fluxed melting at San Jorge volcano, Southern Volcanic Zone, Chile

摘要

Volcanological studies coupled with detailed geochemistry can reveal important aspects regarding the melting and ascent processes of a magmatic body. The explosive part of the eruption giving rise to scoria and tephra deposits can hold a wealth of information which can complement chemical analyses of lava flows, however, it is often poorly exposed. A well-exposed scoria deposit and lava flow at the small eruptive center (SEC) San Jorge near Pucon in the Southern Volcanic Zone (SVZ), Chile, provides an opportunity to examine melting and storage processes in a primitive magma body almost in 'real time' through sampling up the stratigraphy of the deposit. This dataset comprises whole rock major and trace element chemistry, Sr-Nd and U-Th-Ra isotopes, in addition to mineral data through the eruption sequence. San Jorge whole rock compositions are unique in the area (MgO 10-12 wt%, SiO2 50-52 wt%, Cr 600-900 ppm) and the studied tephras extend to the highest uranium- and radium-excesses yet measured in the SVZ. The unusual composition of the volcanic material displays similarities to rocks from the arc-front stratovolcano Villarrica but is distinct from other stratovolcanoes in the area, which exhibit greater influence from sediment input to the source, crustal assimilation and prolonged storage. We model the San Jorge magmas as a mixture of melts originating from highly fluid-fluxed, depleted mantle and fractionated basalt from the same source, with increasing amounts of the latter component with progression of eruption. This suggests that San Jorge-like magmas were the main feeder of the longer lived system building Villarrica. Abrupt major element variations up-sequence additionally show a rapid switch to more fractionated compositions, indicating that storage and evolution through crystallization may have taken place once the initially wet melts 'dried out'.