Compositional and thermal evolution of olivine-hosted melt inclusions in small-volume basaltic eruptions: a “simple” example from Dotsero Volcano, NW Colorado

Abstract

Olivine-hosted melt inclusions have been analyzed from the young (4,150 ± 300 ybp) Dotsero basaltic (48.2 wt% SiO2) lava flow in Northwest Colorado, USA. Silicate melt-inclusion compositions have a bimodal distribution (41–46 wt% SiO2 and 47–50 wt% SiO2). Low-Si melt inclusions record high pre-eruptive sulfur concentrations (>1,000 ppm S) and variations in their major- and trace-element compositions appears to be related to shallow assimilation of local basement sandstone. Whole-rock compositions are modeled as a contamination of low-Si inclusion compositions with ~10 wt% sandstone. Host olivine crystallization may have accompanied magma injection into a shallow storage chamber. In contrast to the low-Si melt inclusions, the high-Si population is relatively degassed and records late-stage rapid crystallization either during or post-eruption. Hopper or skeletal olivine grains in conjunction with the bimodal inclusion compositions suggest relatively rapid cooling rates at the time of eruption and inclusion entrapment. Inclusion compositions, in conjunction with mineral textures, therefore provide a more complete picture of shallow magma processes, coupling the relative timing of undercooling and crystallization, assimilation and melt compositional evolution. Most of the inclusion and host textural and compositional data indicates late and very shallow petrogenetic processes and does not appear to record deeper (mid-, lower-crustal) processes.