PRE-ERUPTIVE STORAGE CONDITIONS OF 1915 LASSEN PEAK DACITE

A series of hydrothermal (high-pressure and –temperature) phase equilibria experiments have been performed on a natural dacite pumice from the 1915 eruption of Lassen Peak, California. The sample represents the silicic end-member (67.8 wt % SiO₂) of mixed dacite and andesite magmas present in the 1915 Lassen Peak eruptive products. Experiments were performed in Rene-style cold seal hydrothermal pressure vessels run in horizontal split tube furnaces. Run durations were 114-431 hours. Run conditions ranged from 50 MPa to 200 MPa and 700 to 875°C under H₂O-saturated conditions at the Ni-NiO buffer.

The H₂O-saturated liquidus of the dacite is >875°C and solidus <700°C in the investigated pressure range. All experimental run products contain glass, plagioclase, and Fe-Ti oxides. Other major stable phases include amphibole, pyroxene, and quartz. Biotite is seen in some experiments either displaying breakdown textures or with reaction rims of amphibole or pyroxene ± plagioclase. Quartz is stable at 50 MPa at all investigated temperatures and is present at or below 800șC at all investigated pressures. At about 850șC, quartz begins reacting with the melt to form reaction rims of pyroxene at 150 MPa and is absent at 200 MPa. At about 875șC, quartz begins to react out at 100 MPa and is absent at and above 150 MPa. Amphibole is stable at 800șC from 50 to 200 MPa, absent below 150 MPa at 850șC and absent below 175 MPa at 875șC. Electron microprobe analyses of glasses produced in experiments and in the natural pumice were collected and these analyses indicate that matrix glasses become progressively silica- and alkali-rich with decreasing P_H2O. Comparisons between experimental and natural phase assemblages and glass compositions suggest that the dacite equilibrated at approximately 50 MPa and 800 to 875șC prior to mixing and erupting with the andesite member. These storage conditions are the basis for a companion study on the kinetics of reaction rim growth on olivine xenocrysts found in the natural dacite and other 1915 eruption products. Mixing experiments are currently underway to reproduce olivine reaction rim textures, and thus, constrain the timing of magma mixing prior to the 1915 eruption of Lassen Peak.