Congratulations to Sarah Brehm, who successfully defended her dissertation on Friday, May 14, 2021
Advisor: Rebecca Lange
Abstract:
Basalts are windows into the mantle from which they were derived, and their eruption at the Earth’s surface provides an unparalleled opportunity to probe the conditions in the mantle that lead to their formation such as temperature, water content, and oxidation state. In this thesis, Quaternary basalts from three localities in the Basin and Range province (Western Great Basin, Central Great Basin, Yellowstone/Snake River Plain) are examined to evaluate how temperature and dissolved water content vary in basalts as a function of mantle source (i.e., subduction- modified lithosphere, asthenosphere, mantle plume). In Chapters 2 and 3, basalts were targeted that contain mantle xenoliths, which requires rapid transit through the crust and precludes prolonged storage in crustal magma chambers. Instead, the hypothesis of rapid phenocryst growth of olivine during ascent along fractures was evaluated for all samples. A second hypothesis that was tested is whether the most Mg-rich olivine analyzed in each sample matches the expected liquidus composition for a basalt with the whole-rock composition. For all samples that passed the olivine-melt liquidus test, olivine-melt thermometry and hygrometry were applied at the liquidus. This gives the temperature and melt water content at the onset of phenocryst growth, and the latter can be compared with direct analyses of olivine-hosted melt inclusions.
In Chapter 2, the hypothesis of rapid phenocryst growth during ascent was tested for a suite of 10 basalts from the Western Great Basin (Big Pine volcanic field, CA), several of which contain mantle xenoliths. Olivine and clinopyroxene phenocrysts display diffusion-limited growth textures, which is consistent with rapid phenocryst growth during ascent. When the most Mg-rich olivine composition in each sample is paired with the whole-rock composition, all olivine-melt equilibrium tests are passed. Application of olivine-melt thermometry and hygrometry at the liquidus gives temperatures (~1250-1100 °C) that vary with MgO content (~13-7 wt%), and water contents that range from ~1.5-3.0, which matches those analyzed in olivine-hosted melt inclusions from the literature.
In Chapter 3, several alkaline lavas from the Central Great Basin (Cima volcanic field and Dish Hill, CA) were targeted due to their mantle xenoliths. Several of the xenolith-bearing samples are notable for their low MgO content (> 5 wt%) and are not direct partial melts of mantle, and thus an outstanding question is how they were able to carry mantle xenoliths to the surface, if they first underwent fractional crystallization at depth. Although evidence of rapid phenocryst growth is present in all samples, the most Mg-rich olivine analyzed in each sample fails olivine-melt equilibrium liquidus tests. The only hypothesis that could not be disproven is that magma mixing between two (or more) melts occurred rapidly during ascent along fractures, one of which is a high-MgO melt with entrained mantle xenoliths.
In Chapter 4, Quaternary basalts adjacent to the active Yellowstone volcanic field and along the Snake River Plain were examined. Despite the absence of any mantle xenoliths, the most Mg-rich olivine analyzed in 15 of 16 basalts pass olivine-melt equilibrium liquidus tests. Application of olivine-melt thermometry and hygrometry give temperatures of ~1200-1130°C and water contents ≤ 1.8 wt% (which match H2O analyses in olivine-hosted melt inclusions from the literature). The Yellowstone basalts are thus relatively hot, given their modest melt water contents and relatively low Mg# values, which is consistent with their source in a fertile, hot mantle plume.
