Determining Magma Mixing Duration Prior to the 1915 Eruption of Lassen Peak, California by Comparing Experimental Growth of Reaction Rims and Natural Olivine Crystals in Black Dacite

Lassen Peak, located in northern California, last erupted in 1915 producing hybrid black dacite containing xenocrystic olivine grains with morphologically complex reaction rims of orthopyroxene. These rims are interpreted to have grown during magma mixing/mingling of admixed basaltic andesite and dacite reservoir magma prior to eruption. Reaction rim growth rates were determined from a series of hydrothermal experiments performed on starting materials consisting of powdered natural dacite pumice from the 1915 eruption and ~5 wt. % of Fo85 olivine separated from a spinel lherzolite xenolith (UM-5) from Kilbourne Hole, NM to constrain the length of time between magma recharge and eruption. Time series experiments were performed with run durations of 50, 100, 200, 400, and 600 hours at 50 and 100 MPa, 825oC and 875oC. The experiments produced a range in reaction rim growth rates where rim thickness generally increased with time. Average rim growth rate for each series is as follows: 0.031 μm²h-1 for 50 MPa at 825oC, 0.010 μm²h-1 for 50 MPa at 875oC, 0.158 μm²h-1 for 100 MPa at 825oC, and 0.088 μm²h-1 for 100 MPa at 875oC. Overall, the 100 MPa experiments resulted in faster growth rates and thicker reaction rims than the 50 MPa experiments. At a given pressure, the higher temperature (875 oC) experiments show slower average growth rates, but thicker reaction rims than the 825oC equivalents. This suggests that growth rate is not constant over time, but likely is more rapid at the early stages of the experiments/heating event, and then slows over time. Reaction rim widths on 100 olivine grains from samples of black dacite were determined by analysis of SEM BSE images and average 26.1 ± 21.7 μm. This average rim width corresponds to a range of mixing durations of 5.8 months (100 MPa, 825oC) to 93 months (50 MPa, 875oC). Average reaction duration of 10.6 months (at 100 MPa, 875oC) is most consistent with our previous experimental work on the 1915 dacite. Reaction rim widths measured on olivine from andesitic enclaves in the black dacite tend to be ~2-3 times thicker than olivine imaged from black dacite samples. Histograms of reaction rim width abundance versus calculated mixing duration suggests mafic magma was episodically injected into dacitic crystal mush over a sustained period of time at roughly 3, 8, and 16 months prior to the 1915 eruption.