EVIDENCE OF RAPID PHENOCRYST GROWTH OF OLIVINE IN BASALTS FROM SEVERAL VOLCANIC FIELDS IN NORTH AMERICA: IMPLICATIONS FOR APPLICATION OF OLIVINE-MELT THERMOMETRY AND HYGROMETRY AT THE LIQUIDUS

Although not all basalts erupt with entrained peridotite xenoliths, several do (e.g., basalts erupted from the Big Pine, CA volcanic field). In these cases, the basalts must have ascended rapidly through the crust without prolonged storage in a crustal reservoir, otherwise their mantle xenoliths would have dropped out. The question addressed in this study is whether olivine phenocryst in erupted basalts, including those with entrained mantle xenoliths, grow their olivine phenocrysts during ascent through the crust in dikes (versus growth in stalled magma chambers). This hypothesis has been tested on dozens of basalts from six different basaltic volcanic fields (Kings River, Big Pine, Long Valley, Yellowstone/Snake River Plain, Tancítaro, Colima) from the western United States and Mexico. Specifically, two related questions have been evaluated: (1) How often do erupted basalts have whole-rock compositions that represent liquid compositions from which their observed phenocrysts were grown? (2) How often does the most Mg-rich olivine that has been analyzed in each sample match the equilibrium olivine composition at the liquidus? To evaluate these two questions, both the Mn-Mg and Fe2+-Mg exchange partition coefficients have been used to test whether the most Mg-rich olivine analyzed in each basalt sample has the equilibrium composition at the liquidus of a melt with the whole-rock composition. We use the Blundy et al. (2020) lattice strain model for Mn-Mg Kd, and the Putirka (2016) model for Fe2+Mg Kd. The latter can only be employed when the oxidation state of a basaltic sample is known independently (e.g., two Fe-Ti oxybarometry, analyses of sulfur and vanadium valence ratios, etc.). When these two olivine-liquidus equilibrium tests are applied to 72 basalt samples from these six volcanic fields, 68 pass (94 %). This outcome enables application of the new Mg- and Ni-based olivine-melt thermometer/hygrometer of Pu et al. (2017; 2021) to the liquidus olivine composition in each of these 68 samples. The results give water contents that match those of directly analyzed in olivine-hosted melt inclusions, when available. These results collectively provide strong evidence that most erupted basalts grew their olivine phenocrysts during ascent, consistent with abundant evidence of diffusion-limited rapid growth textures.