CONSTRAINTS ON THE TIMESCALES AND MIXING DYNAMICS OF THE MAGMA RECHARGE EVENT ASSOCIATED WITH THE 1915 ERUPTION OF LASSEN PEAK, CALIFORNIA

The 1915 eruption of Lassen Peak produced 4 rock types (light dacite, black dacite, dark andesite, and andesitic inclusions) that are products of variable mixing between a reservoir dacite and intruding basaltic andesite (Clynne, 1999). Crystal size distribution (CSD), Nomarski textural imaging, major-element and Sr isotopic data for plagioclase were integrated to constrain crystal residence times and mixing dynamics. At least three distinct plagioclase crystal populations were identified on the basis of distinct CSDs and An core contents: microlites (ml) (<0.1 mm, highest intercept, steepest slope, An_40-67), microphenocrysts (mp) (0.1-0.5 mm, An_59-82) and phenocrysts (ph) (>0.5 mm, lowest intercept, shallowest slope, An_29-40). Ml An contents broadly correlate with adjacent glass. Based on integration of these results, ph formed at low undercooling in the dacitic reservoir and mp formed at higher undercooling in the hybrid magma shortly following initial, turbulent mixing. Homogeneous An core and size data for the mp also indicate that the initial mixing event was fairly efficient. Ml crystallized at the highest degree of undercooling during eruption. Based on published plagioclase growth rates, crystal residence times of 150-1,500 yrs for the reservoir plagioclase ph, 4-30 days for mp and about 2 days for ml were calculated. Initiation of phreatic activity preceded the eruption by one year and may have coincided with the recharge event, which would indicate that rapid crystallization of the mp was not sustained throughout the interval between recharge and climatic eruption. Evidence in plagioclase phenocrysts of few resorption surfaces, few core-to-rim increases in An content, and little core-to-rim variation in ⁸⁷Sr/⁸⁶Sr (e.g. ~0.70411-0.70424±5, collected by laser-ablation MC-ICP-MS) suggests that mafic recharge was not a common process in the Lassen Peak chamber. The 1915 Lassen magma chamber is therefore an example of a reservoir that was relatively thermally and chemically isolated prior to the recharge event that initiated the 1915 eruption. The results of this study indicate that integration of CSD, textural and in situ chemical data can elucidate information about the physiochemical conditions of a particular magmatic system, thereby enhancing understanding of how magma plumbing systems work.