Current Research Projects
Magma decompression and crystallization
Determination of crystallization and mineral-melt reaction rates in controlled laboratory conditions is considered essential for establishing criteria by which ascent processes may be interpreted from volcanic rock samples.
Decompression, closed-system degassing, and crystallization of hydrous magma are studied in the UH Experimental Laboratory Facility using gas-medium and water-medium pressure lines. A custom-built pressure variator (shown below left) on the water-medium line enables continuous decompression over time scales relevant to magmas ascending at rates corresponding to natural dome-forming and subplinian eruptions.
For her dissertation research, Carrie Brugger quantified the kinetics of plagioclase crystallization in natural hydrous rhyodacite magma. She performed experiments that tracked variations in crystal nucleation and growth rates and the compositional and textural evolution during constant-rate decompression. This approach permitted results to be compared with natural samples, and also facilitated critical evaluation of crystal size distribution theory, as applied to natural volcanic products. In a follow-on project with the same starting material, Gabriele Lanzafame and Tom Shea are investigating the effects on microtexture of decompression step size.
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Custom built pressure variator. |
Back-scattered electron images of quenched experimental run products depict progressive plagioclase microlite texture during decompression from 130 MPa at 0.5 MPa h-1 and 850°C, under H2O-saturated conditions. |
Paths of effective undercooling and imposed by decompression of H2O-saturated rhyodacite and possible system responses. |
Funding: NSF EAR-0449888, EAR-0948728
This research complements prior work designed to evaluate crystallization in hydrous magmas in a theoretical context. Experiments in which isothermal decompression occurs instantaneously impose a discrete thermodynamic driving force for crystallization that can be estimated using established phase equilibria. Crystal number densities and morphologies can be related directly to nucleation rate and growth mechanism at the experiment pressure.
This style of experiment is uniquely suited for determining the instantaneous rates of crystal nucleation and growth (below left), which can be compared with predictions from available theories of nucleation and growth (below middle and right).
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Feldspar microlite habit, aspect ratio (length/width) and relative size shown in comparison with experiment pressure, degree of effective undercooling (DTeff), dissolved H2O content, melt viscosity, and relative rates of component diffusion (D), crystal growth (Y), and crystal nucleation (I). |
Experimental nucleation rate data (symbols) and model rates (curve) found using the classical nucleation theory, modified to include H2O-dependent crystal-melt interfacial free energy. |
Schematic cross section of the interfacial region between a subcritical cluster and the surrounding melt showing thermodynamic properties of a diffuse interface. |
Funding: NSF EAR-0087463
Tom Shea is quantifying the rates of plagioclase nucleation and growth in basaltic andesite (Mascota-22 of Western Mexico, studied by Moore and Carmichael, 1998) as a result of cooling and decompression. A first step in this process has been to address progressive oxidation inside experimental capsules, which is exacerbated in gas-medium cold-seal pressure vessel (CSPV) setups by the high temperatures and high H2 gradient across capsule walls.
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Simplified sketch of the various components and reactions occurring in the CSPV experiments. (b) Specific capsule design we employ for experimental charges. Key elements include an oxygen fugacity monitor (CoPd-CoO inner sheathed capsule) and O2 getter (crimped inner capsule with Ni powder). |
Funding: NSF EAR-0948728