USING COMBINED GEOCHEMICAL AND TEXTURAL QUANTIFICATION OF CRYSTAL POPULATIONS TO CONSTRAIN MAGMA CHAMBER PROCESSES IN SILICIC VOLCANOES: AN EXAMPLE FROM MOUNT HOOD, OREGON

We present a study of Mount Hood, Oregon, where we combine quantitative characterization of crystal populations using crystal size distributions (CSD) with in-situ trace element analysis of individual crystalline phases, to constrain magma evolution and magma chamber processes. The novelty and utility of this approach is that CSD allow us to recognize crystal populations with common histories, and trace element measurements allow us to examine the detailed chemical records contained within individual crystal members of different populations.

Mount Hood consists primarily of andesitic lavas and associated rocks, and the majority of the current edifice has grown over the last ~500 ka. Eruptive products are remarkably homogeneous, with >80% of lavas having silica contents between 57-63 wt.%. Despite this textural and chemical trends suggest extensive magma mixing and mineral melt disequilibrium throughout the lifetime of the volcano.

CSD based on plagioclase (the most abundant phenocryst) are remarkably consistent through time. Five out the six lavas studied, which range in age from 475 ka to 200 years, show concave up CSD, with two distinct line segments characteristic of mixed magmas. We interpret this to reflect mixing of smaller crystals from a younger mafic magma with older larger crystals from a more evolved magma. Assuming a representative plagioclase growth rate, the estimated crystal residence times are several decades for the felsic population and < 10 years for smaller crystals. Trace element compositions of each population are also consistent with this, with larger crystals having lower Ti and Sr and higher LREE and Ba contents at a given anorthite. In addition narrow rims on felsic population crystals have trace elements compositions that match the mafic crystal population. We interpret these to have formed after mixing. The thickness of rims suggests eruption happened within only a few years of magma mixing events in all cases. We suggest that the magmatic system at Mount Hood has been characterized by repeated events of mixing of broadly similar composition felsic and mafic magma followed rapidly by eruption. The near constancy of this mixing over ~475 ka implies long term steady state behavior of the magma supply system.