Experimentally testing embayments as record keepers of magmatic ascent
Melt pockets in volcanic crystals, called embayments, have been used to determine magmatic ascent rate. As magma ascends the conduit, it decompresses, dropping volatile solubility and creating a concentrational gradient between the embayment interior and surrounding melt. This time-dependent, diffusion-limited gradient is recorded in embayment glass. But how faithfully do embayments record decompression? Can small datasets provide robust constraints on ascent rate? These questions require experimental verification. I'm working to simulate magmatic decompression in natural and synthetic embayments in the lab to establish the fidelity of embayment geospeedometry.
The jargon-free takeaway: we need to put embayments "back in the volcano" to see how good they are at telling us how fast magma travels!
Determining a partition coefficient for water in feldspar for rhyolitic eruptions
Melt inclusions are commonly used to determine magmatic water concentration. This approach is limited by abundance and quality of melt inclusions in a given system, and complicated by the need for experimental homogenization and diffusive loss of H. For my undergraduate research thesis, I analyzed OH concentration in feldspar phenocrysts from the Bishop Tuff and Huckleberry Ridge Tuff. With this data, I determined an empirical partition coefficient between [OH] in feldspar and water in quartz-hosted melt inclusions. This expands the range of volcanic rocks that can be analyzed for water.
The jargon-free takeaway: this coefficient lets you use the amount of water in the mineral feldspar to find out how much water was in the magma it came from!
Zeeman Crater's Anomalous Massif
A mysteriously tall massif looms over the edge of Zeeman Crater on the Moon's South Pole Aitken Basin (SPA). The composition of this anomalous feature lends insight into its formation and the catastrophic history of the SPA, the solar system's largest impact structure. Using Nettleton's method, I calculated bulk density across Zeeman crater and two neighboring craters with a linear regression between free air gravity and free air gravity expected from topography. Grain density can be inferred by accounting for the effect of porosity. The composition of Zeeman Crater's massif is noritic and largely uniform with the surrounding rock, invalidating an excavated mantle source. This finding supports melt sea differentiation resulting in an overlying, noritic residuum in the SPA.
The jargon-free takeaway: the mountain at the edge of Zeeman Crater is not a blob of the Moon's dense interior, but possibly a remnant of a cooling magma ocean!
Figure modified from Moriarty and Pieters (2018) and Yamamoto et al. (2010)
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Anthology of quartz-hosted embayment textures
Embayments used to reconstruct magmatic ascent rate in literature are typically described as glassy, bubble-free, microlite-free pockets within volcanic phenocrysts, isolated from highly vesiculated exterior glass. While pristine embayments are ideal recorders of disequilibrium, they do not represent the full range of textures common in embayments. Here, we conduct a survey of quartz-hosted embayments in ten different rhyolitic eruptions: the Bandlier, Bishop Tuff Fall, Bishop Tuff Ignimbrite, Mesa Falls, Oruanui, Huckleberry Ridge Tuff, La Primavera, Younger Toba Tuff, Lava Creek Tuff, and Tuff of Bluff Point. 300-1500 quartz crystals from each eruption are picked, counted, and classified as containing clear glass, intruding, elongate discrete, or small discrete bubbles. Continued research will explore what these diverse textures reveal about magmatic processes.
The jargon-free takeaway: embayments are bubbly, and those bubbles may have exciting stories to tell!