Gas‐saturated crystallization and degassing in large‐volume, crystal‐rich dacitic magmas from the Altiplano‐Puna, northern Chile

Abstract

Melt inclusions hosted in quartz from large‐volume ignimbrites and related lava flows from the late Neogene to Pleistocene Altiplano‐Puna Volcanic Complex, northern Chile, record the magmatic volatile evolution and constrain conditions of magma storage. Glasses from pristine and rehomogenized inclusions have high‐Si rhyolitic compositions (average SiO2=77.5 wt%). Their host rocks range from dacite to rhyodacite (SiO2=63.9–72.5 wt%) and have a high abundance of phenocrysts (33–55%). Infrared spectroscopic analysis of inclusions from pumice samples typically yielded H2O contents between 3.0 and 4.0 wt% and relatively low and more variable CO2 contents <400 ppm. Increasing H2O contents were found in a series of successively trapped inclusions, and bubble‐free inclusions tend to have lower CO2 contents with increasing H2O. Inclusions from lava samples have lower but constant H2O contents of 2.0±0.3 wt% and CO2 close to the detection limit (∼10 ppm). Incompatible trace elements with high affinities to partition into a fluid phase (e.g., B, Cl) show minor enrichment in melt inclusions, whereas compatible trace elements (e.g., Sr, Ba) became strongly depleted due to feldspar‐dominated crystallization. Variations in H2O and CO2 contents as well as concordant preeruptive pressures inferred from volatile solubility and Al‐in‐hornblende barometry (150±50 MPa) also indicate volatile saturation and upper crustal magma storage for the ignimbrite magmas. The melt inclusion record is consistent with near‐isobaric cooling from ∼830° to 780°C (inferred from mineral thermometry) under gas‐saturated conditions in shallow (4–6 km deep) reservoirs. Model calculations for overpressures generated by closed‐system crystallization and gas exsolution either result in premature failing of the magma chamber walls or require the presence of large volumes of highly compressible magmatic foam. As an alternative, open‐system degassing of these magmas prior to their explosive and effusive eruption is proposed.