Storage Conditions of Large Silicic Magmatic Systems: Gauging Melt Evolution from Melt Inclusions Hosted in Different Phenocryst Phases

Friday, 19 December 2014
Stephanie B Grocke1, Shanaka L de Silva2, Paul J Wallace3, Adam JR Kent2, Richard L Hervig4, Benjamin James Andrews1 and Elizabeth Cottrell1, (1)Smithsonian, NMNH, Washington, DC, United States, (2)Oregon State University, College of Earth, Ocean, and Atmospheric Sciences, Corvallis, OR, United States, (3)University of Oregon, Eugene, OR, United States, (4)Arizona State University, Tempe, AZ, United States
Quartz- and sanidine-hosted melt inclusions from the 3.49 Ma rhyolitic Tara pumice fall deposit erupted from the Guacha II Caldera in SW Bolivia provide new insights into the melt evolution preceding a supereruption. Melt inclusions were analyzed for volatile contents using two different techniques, Fourier Transform Infrared Spectroscopy (FTIR) and ion microprobe (SIMS). Data from FTIR on quartz-hosted melt inclusions reveal pre-eruptive CO2 concentrations (maximum ~ 300 ppm), and H2O contents (average = 4.3 wt.%) that are similar to H2O contents derived from SIMS on the same inclusions (average = 4.2 wt.%). Melt inclusions in sanidine yield higher CO2 concentrations (maximum ~ 400 ppm) than those hosted in quartz, yet yield much lower H2O contents (average = 2.5 wt.% via FTIR; average = 2.7 wt.% via SIMS). The higher CO2 trapped in sanidine-hosted melt inclusions may suggest higher trapping pressures than are recorded by quartz, whereas the low H2O recorded by sanidine may signify preferential H loss from sanidine. SIMS and Laser-Ablation ICP-MS (LA-ICP-MS) trace element analyses of melt inclusions define a continuous liquid line of descent from sanidine-hosted inclusions that record high Sr and increasing Ba with crystallization, to quartz-hosted inclusions that record low Sr and decreasing Ba with crystallization. In the case of the Tara magmatic system, sanidine-hosted inclusions seem to record an earlier, deeper stage of the melt’s history. Assessing melt inclusions within multiple phenocryst hosts may provide insights into different stages of a melt’s history from storage to ascent and eruption.