Resolving magmatic time scales with single-crystal diffusion modeling
Can we distinguish between crystal growth from post-growth diffusive equilibration in controlling compositional zonation?
Shea et al. (2015) perform a detailed multi-element (Fe, Mg, P, Al, and Ni) examination of zoning in a Kilauea olivine with skeletal morphology. The two processes are easily discriminated in this case because olivine crystals initially grow rapidly and diagonally from corner locations, whereas diffusion influences mutually perpendicular crystal lattice orientations. The zoning patterns are produced using numerical simulation of diffusion modeling, requiring at least 4–5 months of diffusive reequilibration at magmatic temperatures. Results indicate that crystal morphologies produced by rapid growth can survive at magmatic temperatures for extended periods.
Shea T, Lynn, K.J., Garcia, M.O., (2015) Cracking the olivine zoning code: Distinguishing between crystal growth and diffusion. Geology 43:935-938.
Funding: EAR-1321890 to Shea and EAR-1347915 to Garcia
What errors are associated with cut effects? How many sections and transects are needed to constrain a magma mixing event?
Shea et al. (2015) use numerical 3D diffusion models of Fe-Mg, incorporating anisotropy, to evaluate and quantify the uncertainties associated with 1D models. The 3D model crystals were sectioned along ideal or random planes, which were used to perform 1D models and timescale comparisons. Timescales can vary between 0.2–10 times the true 3D diffusion time due to crystal shape and sectioning effects. Transects in face-orthogonal orientations located far from corners containing compositional plateaus, have the lowest the misfit between calculated and actual diffusion times. With appropriate a priori filtering, about 20 concentration profiles and associated 1D models suffice to achieve accuracy and precision of ~5% and ~15–25% relative, respectively.
Shea, T., Costa, F., Krimer, D., Hammer, JE. (2015) Accuracy of timescales retrieved from diffusion modeling in olivine: A 3D perspective. Am. Mineral. 100, 2026-2042.
Funding: EAR-1321890 and EAR-0948728
Volcán Quizapu: a testbed for improving petrologic tools
Magmatic phases record environmental conditions in crustal reservoirs and conduits-- but what are the response times between a heating event or magma mixing event and the ensuing mineralogical changes?
Sub-systems within a magma respond at different rates to a change in dissolved H2O content, temperature, or absolute pressure. For example, amphibole breakdown occurs in response to declining melt H2O content and the compositions of Fe-Ti oxide pairs are sensitive to changes in temperature. Ruprecht et al. employ Mg-diffusion in plagioclase crystals to constrain the timing of magma recharge at Volcán Quizapu (Chilean Andes). Magma heating and recharge provide an explanation for the low intensity (effusive) eruption of 5 km3 magma in 1846-47, which stands in stark contrast to the explosive eruption of a similar volume of dacite magma in 1923. In collaboration with Ruprecht, Emily First is performing static and dynamic experiments to track the effects of late-stage changes in temperature, pressure, water content, and composition of Quizapu dacite. Watch this space for developments...
Funding: EAR13-47887
Philipp Ruprecht led a Quizapu-focused workshop and field trip in February, 2016. Attended by graduate students from Chilean universities, Lamont, and UH, along with faculty Adam Simon (U. Michigan) and Einat Lev (Lamont), and Hammer. The group collected samples (photo) and appreciated the outstanding volcanic landscape.
Funding: President's Global Innovation Fund, Columbia Global Centers/ Latin America
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