Conditions of magma mixing as recorded in amphiboles from Mount Hood, Oregon

Oregon’s Mount Hood erupts monotonous andesite-dacite lavas produced by mixing between silicic and mafic magmas shortly before eruption. This mixing process is recorded in mineral compositions, textures, and zoning, particularly in plagioclase and amphibole (two of the dominant phenocryst phases). Detailed compositional and textural studies of amphibole can be used to constrain magmatic conditions before and during magma mixing. Amphiboles from dacite domes erupted during the Old Maid (~220 years ago) and Timberline (1.8-1.4 ka) eruptive periods can be divided into two groups based on their compositional and textural characteristics. Group 1 amphiboles are pargasite and tschermakite, and Group 2 are edenite and magnesiohornblende. Group 1 amphiboles contain >10 wt% Al2O3, whereas Group 2 crystals contain <10 wt%. Breakdown rims are typically more extensive in Group 2 amphiboles than in Group 1. Volatile contents also differ between the two groups of amphiboles. Group 1 has lower Cl concentrations than Group 2, consistent with the crystallization of Group 1 amphiboles during or after magma mixing. Concurrent magma mixing and degassing may produce a free vapor ± liquid phase, into which Cl will heavily partition, particularly at pressures <2 kbar. This leaves the melt depleted in Cl, so amphiboles formed during concurrent mixing and degassing will have lower Cl concentrations than those formed prior to degassing. This free vapor ± liquid phase is also a possible means for increasing magmatic overpressure and initiating eruption during mixing episodes. Amphibole-plagioclase equilibria also provide constraints on the conditions of magma mixing. Calculated temperatures (determined using adjacent plagioclase crystals in textural equilibrium with amphibole) indicate that Group 1 amphiboles (927-960°C) were formed at temperatures approximately 50°C hotter than Group 2 amphiboles (880-921°C). We suggest that Group 2 amphiboles formed during the crystallization of a silicic magma prior to magma mixing, whereas Group 1 amphiboles formed once amphibole was stable after mixing between silicic and hotter mafic magma shortly before eruption. Higher REE concentrations and larger negative Eu anomalies in Group 2 amphiboles also suggest crystallization from a more silicic magma, and following precipitation of greater amounts of plagioclase, than the higher temperature late-crystallizing amphiboles of Group 1. Experimental studies at Mount St. Helens have found groundmass (the equivalent of Group 1) pargasite to be stable at pressures as low as 100 MPa. Similar studies at Mount Unzen have demonstrated pargasite stability at pressures as low as 70 MPa. Assuming comparable stability conditions under Mount Hood, the presence of Group 1 pargasite suggests that magma mixing below Hood occurs at depths greater than 3 km. Crystallization temperatures for the late-forming Group 1 amphiboles are approximately 50-70°C hotter than temperatures estimated at Unzen, indicating that pargasite may need even higher pressures to be stable and that magma mixing below Hood may occur even deeper than 3 km.