Storage Conditions of Pudahuel Rhyolitic Ignimbrite in Central Chile

Abstract:

Pudahuel Ignimbrite is a rhyolitic (~74% SiO₂), crystal poor deposit associated to the Maipo volcanic complex in the Southern Volcanic Zone in the Andes. Its genesis is associated with the formation of the Quaternary Diamante caldera. It’s inferred that ~450 km³ pumice and ash was emplaced during a single massive eruption. The present study aims to understand its genesis through characterization of its pre-eruptive magma storage conditions through application of equilibrium thermometry, hygrometry and laboratory experiments.

Optical microscopy and back-scattered electron imaging suggest that the mineral phases (plagioclase (~1.6%), biotite (0.5%), magnetite (>1%), ilmenite (tr), zircon (tr), monazite (tr), apatite (tr)) are homogeneous and are in chemical equilibrium with glass. Five phase equilibrium experiments were performed at H₂O-vapor saturated conditions in cold-seal pressure vessels in the Experimental Petrology Laboratory at University of Hawai’i at Manoa (UHM), and analyzed with electron microprobe. Run conditions were determined by application of the Fe-Ti oxide thermometer and the plagioclase-glass hygrometer, which was used to determinate the pressure-temperature-X_H2Ofl conditions that produced the glass and feldspar compositions observed in the natural sample. These results were combined with the water solubility in the rhyolitic melt obtained with an empirical formula and in that way narrow the formation conditions of the natural sample.

The experiment performed at 700ºC and 310 MPa shows the same phases that are present in the natural sample except for quartz. The other experiments present orthopyroxene. Plagioclase is present in all of them meanwhile biotite is not present in the experiments ran at 850ºC. Glass chemistry is similar between experiments made at 700ºC and 750ºC and the natural sample. Plagioclase composition of the experiment performed at 700ºC is quite similar to the natural composition.

Although none of the experiments have the same mineral phases as the original sample, the one performed at 700ºC, 310 MPa and 7.8 wt% H₂O is likely the only one that better could represent the pre-eruptive conditions. These T-P-X_H2O values could be reasonably obtained in geological conditions associated with partial melting of upper continental crust.