Efficacy of assessing magmatic storage depth using natural samples, experiments, and thermodynamic models: a case study from Valles Caldera, NM (USA) Supplemental Tables

Description

Estimates of magmatic storage are typically made using mineral assemblages in natural samples, experiments and thermodynamic models (e.g., MELTS), where each method has limitations. The use of mineral assemblages (and to some extent experiments) to determine magmatic storage conditions requires that the minerals are genetically related to the host magma, while models are limited by their calibrations, which are composed of experiments. Here we compare each of these methods to assess their utility in estimating storage conditions for post-collapse, two feldspar high-silica rhyolites sourced from Valles Caldera, NM (USA). Equilibrium experiments that overlap with magmatic storage conditions determined from sample petrology have glass and mineral compositions that match those in the natural samples suggesting a phenocryst assemblage that is an equilibrium assemblage. Outputs from rhyolite-MELTS (RMELTS) reproduce differing aspects of the natural samples and experiments, but generally confirm storage conditions (~750-770°C; ~130-165 MPa) recorded by the petrology of the post-collapse high-silica rhyolites. We find RMELTS accurately predicts saturation conditions and compositions of felsic phases and glasses in natural samples and experiments, further confirming its cotectic barometry is robust for two-feldspar saturated high-silica rhyolites. We ran 31 phase equilibrium experiments using an HSR-obsidian from Valles Caldera: 77.1 wt% SiO2, 0.09 wt% TiO2, 12.5 wt% Al2O3, 1.04 wt% FeOT, 0.06 wt% MnO, 0.05 wt% MgO, 0.38 wt% CaO, 4.15 wt% Na2O, and 4.59 wt% K2O (sample CM; Grismer & Waters, 2024). Experiments were conducted in cold-seal pressure vessels, which have an intrinsic fO2 of ~i-NiO (NNO) buffer, over a range of temperatures 700-875°C and pressures 25-300 MPa (where PTotal=PH2O), at the New Mexico Institute of Mining and Technology. All experiments were steady state runs held at experimental conditions for 72 to 168h.Backscatter electron images, and mineral and glass compositions of polished experimental charges, mounted in one-inch epoxy pucks, were collected on a Cameca SX-100 electron microprobe, at the New Mexico Bureau of Geology and Mineral Resources at NMT and a JXA-8530FPlus hyperprobe in the Department of Mineral Sciences at the National Museum of Natural History. This supplemental dataset includes all microprobe standards and settings (Supplemental Table S-1). Average phase compositions and standard deviations (Supplemental Table S-2). All microprobe analyses for glass, sanidine, and anorthoclase in Supplemental Tables S-3, S-4, S-5 respectively. And a complete comparison between experimental results and MELTS v.1.2.0, MELTS v.1.0.2, and MELTS v.1.1.0. This dataset is intended to be used as supplemental information to add to available experimental datasets of high-silica rhyolites saturated in two-feldspars.

Institutions

Categories

Funding

Licence