2001/12/10 by Axel K. Schmitt · 1 citation
Earth and Planetary Sciences · Computer Science · #Geological and Geochemical Analysis #High-pressure geophysics and materials #Geochemistry and Geologic Mapping #Melt inclusions #Phenocryst #Geology #Geochemistry #Silicic #Fluid inclusions #Mineralogy #Magma #Lava #Rhyolite #Feldspar #Quartz #Volcanic rock #Volcano
paper · doi:10.1029/2000jb000089
openalex publication_date 2001/12/10 · openalex created_date 2025/10/10 · openalex updated_date 2026/07/23
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 SiO 2 =77.5 wt%). Their host rocks range from dacite to rhyodacite (SiO 2 =63.9–72.5 wt%) and have a high abundance of phenocrysts (33–55%). Infrared spectroscopic analysis of inclusions from pumice samples typically yielded H 2 O contents between 3.0 and 4.0 wt% and relatively low and more variable CO 2 contents <400 ppm. Increasing H 2 O contents were found in a series of successively trapped inclusions, and bubble‐free inclusions tend to have lower CO 2 contents with increasing H 2 O. Inclusions from lava samples have lower but constant H 2 O contents of 2.0±0.3 wt% and CO 2 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 H 2 O and CO 2 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.