Storage Conditions of Large Silicic Magmatic Systems: Gauging Melt Evolution from Melt Inclusions Hosted in Different Phenocryst Phases
Friday, 19 December 2014
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.