Decoding Reservoir Interactions and Pre-Eruptive Timescales : Olivine Insights into the Cracked Mountain Eruption, Mount Meager Volcanic Complex, Canada

Geochemical and textural analyses of 236 olivine phenocrysts reveal the dynamic magmatic processes that shaped the Cracked Mountain eruption (401 ± 38 ka), a glaciovolcanic event within the Mount Meager Volcanic Complex (Garibaldi Volcanic Belt, British Columbia, Canada). This study characterises the magmatic reservoirs involved, their interactions, and key processes such as magma recharge, mixing, storage, and eruption triggering. Two main populations of olivine crystals (Groups 1 and 2), distinguished by their compositions, zoning patterns, and inclusions, reveal interactions between two distinct storage zones prior to the eruption. The deeper reservoir, storing an olivine–plagioclase–augite (OPA)-bearing magma at ~8 km depth (~240 MPa), hosted Group 2 olivine phenocrysts (core compositions Fo79–81) with abundant spinel inclusions and no sulphides. At shallower depths (~5 km, ~160 MPa), an olivine–plagioclase (OP) reservoir hosted Group 1 olivine phenocrysts (core compositions Fo~83), which lack spinel inclusions but contain rare sulphides. The two magmas subsequently ascended, mixed, and stalled in a shallow reservoir, where skeletal Group 3 phenocrysts (Fo77–78) crystallised. Group 1 crystals, once entrained, developed pronounced normal zoning as they equilibrated with the more evolved mixed melt, while Group 2 displayed faint normal zoning due to minimal fractional crystallisation of their host melt (both populations exhibiting overlapping rim compositions within the Fo76–79 range). Diffusion modelling of olivine zoning, conducted on 160 profiles, indicates that this mixing occurred within a year of the eruption. Lastly, a late-stage episode of mixing and/or melt oxidation, evidenced by faint Fo reverse zoning, may have destabilised the system and triggered the eruption.