Experimental determination of pre-eruptive storage conditions and continuous decompression of rhyodacite magma erupted from Chaos Crags, Lassen Volcanic Center, California
A series of hydrothermal (high-temperature and -pressure) phase equilibrium experiments were performed on a natural rhyodacite pumice from the 1,050 calibrated years B.P. pyroclastic flow from the Chaos Crags, Lassen Volcanic Center, California. The pumice (LQ13-01, collected at the same location as LC84-417 by M.A. Clynne) is from the lower pyroclastic flow member of the group 1 lavas, the most silicic products known of Chaos Crags. Group 1 lavas are homogeneous (bulk rock containing 69-70 wt. % SiO2), petrographically and compositionally similar with rare to sparse mafic inclusions, and comprise the earliest emplaced units of Chaos Crags, the lower, middle, and upper pyroclastic flows, and domes A and B. Group 2 lavas are comparatively heterogeneous (bulk rock containing 67-69 wt. % SiO2), with increasing abundance (10-15%) of mafic inclusions throughout the emplacement sequence, and comprise domes C through F. The phase assemblage in the natural sample used as experimental starting material comprises phenocrysts of quartz, plagioclase feldspar with rims of ~An35, biotite, hornblende, and Fe-Ti oxides in a vesiculated glassy matrix. Trace mafic enclaves are also present, but were removed from experimental starting material. All experiments were performed in cold-seal furnaces at the Smithsonian Institution, under H2O-saturated conditions at pressures of 75 MPa to 200 MPa and temperatures of 750°C to 900°C, at oxygen fugacity (ƒO2) NNO+1 (±0.5-log-units), for 93 to 142 hours. SEM analyses of experimental products show quartz is stable at ≤200 MPa at 750°C to <175 MPa at 775°C and at 75 MPa at 800°C, and is not stable at temperatures ≥825°C, within the investigated pressure range. Amphibole is stable at >75 MPa at 750°C to >100 MPa at 775°C and at 200 MPa at 825°C, and is not stable at 75 MPa or ≥850°C. Biotite is stable at all investigated pressures at 750°C, stable >125 MPa at 800°C, and is not stable for any pressure at ≥850°C. Pyroxene, not present in the starting material is stable at 200 MPa at ≥800°C and all investigated pressures at ≥825°C. EPMA analyses of 16 anomagnetiteilmenite touching pairs have equilibrium temperatures of 760-775°C and fO2 of ~NNO+1.35 (model of Ghiorso and Evans, 2008). FTIR analysis of 34 quartz-hosted melt inclusions contain 3.99±0.40 to 6.48±0.65 wt.% (the bulk of analyses cluster between 3.99±0.40 and 5.42±0.54 wt. %) and up to 361±36 ppm CO2, suggesting saturation pressures of 92-297 MPa (model of Papale et al., 2006). EPMA analyses of amphibole rims show chemical equilibrium (model of Ridolfi et al., 2010) occurred at lower H2Omelt concentrations, lower pressure (71-89 MPa), slightly higher temperature (772-810°C), and more oxidizing (NNO+2.0 to NNO+2.6 log-units) conditions than melt inclusions and titanomagnetite-ilmenite pairs suggest. Comparison of the natural samples with experimental run products suggests H2O-saturated pre-eruptive pressure-temperature storage conditions of 145±25 MPa and 770±10°C. Continuous decompression experiments were conducted using experimental run product CCPb-34, equilibrated at 120 MPa and 775°C, with sufficient H2O to ensure H2O-saturated conditions, as a starting material. Decompression rates ranged from 8.5 MPa/hr to 0.35 MPa/hr, corresponding to total decompression times of 16 hours to 54 days. SEM and EPMA textural and geochemical comparisons between continuous decompression experimental run products and natural samples indicate that decompression and ascent of magma producing group 1 lavas was relatively quick, >2.6 MPa/hr (corresponding to minimum ascent velocity of 100 m/hr and total decompression times of <48 hrs), whereas magma that produced group 2 lavas (domes C-F) decompressed and ascended relatively slowly, <2.6 MPa/hr (corresponding to ascent velocity of <100 m/hr and total decompression times >48 hrs).