Magmatic storage conditions, decompression rate, and incipient caldera collapse of the 1902 eruption of Santa Maria Volcano, Guatemala

Phase equilibria experiments and analysis of natural pumice and phenocryst compositions indicate the 1902 Santa Maria dacite was stored at similar to 140-170 MPa and 840-850 degrees C prior to eruption. H2O-saturated, cold-seal experiments conducted in vessels with an intrinsic log f(O2) of NNO + 1 +/- 0.5 show that the natural phase assemblage (melt + plagioclase + amphibole + orthopyroxene + Fe-Ti oxides + apatite) is stable from approximately 115-140 MPa at temperatures below similar to 825 degrees C, to similar to 840-860 degrees C at 150 MPa, to >850 and <875 degrees C at 200 MPa. Natural plagioclase phenocrysts have rim compositions that range from An(40)-An(45); this range of compositions defines a similar to 20 degrees C band that intersects the stable phase assemblage at similar to 150 MPa and 850 degrees C. Plagioclase hosted melt inclusions were analyzed using FTIR and commonly contain <5 wt.% H2O, which corresponds to a pressure of similar to 170 MPa at 850 degrees C, under pure H2O saturated conditions. Amphibole geothermobarometry (Ridolfi et al., 2010) applied to experimental samples suggest two populations of amphiboles, phenocrysts grown during the experiments and inherited xenocrysts, but the pressure-temperature conditions returned by the geothermobarometer are routinely >50 MPa and >50 degrees C greater than experimental run conditions; precise estimates of magmatic conditions based solely upon amphibole composition are likely inaccurate. The experimental results and analysis of natural crystals suggest that although the natural amphiboles likely record a broad range of magmatic conditions, only the lower bounds of that range reflect pre-eruptive storage conditions. Comparison of Santa Maria microlite abundances with decompression experiments examining other silicic systems from the literature suggests that the 1902 dacite decompressed at the rate of similar to 0.005 to 0.01 MPa/s during the eruption. Applying the decompression rate with the previously described eruption rate of approximately 2-3 x 10(8) kg/s (Williams and Self, 1983; Carey and Sparks, 1986) to the conduit model CONFLOW reveals that the eruption conduit was dike-like with an along-strike length >1 km. Despite depositing similar to 20 km(3) of dacite tephra (equivalent to similar to 8.5 km(3) magma), the 1902 eruption did not form an obvious caldera. This work suggests that collapse of the dike-like conduit terminated the eruption, preventing full caldera collapse. Published by Elsevier B.V.