Collaborative Research: A Novel Solution Model for Igneous Biotite: Improving MELTS and the Biotite-Sanidine-Magnetite Thermobarometer/Hygrometer with New Experiments

Understanding where large volumes of magma live in Earth’s crust before eruption is critical for hazard assessment at many volcanoes. Scientists can determine the magma storage conditions of some historic eruptions using minerals transported to the surface. Those mineral compositions reflect the temperature, pressures and water contents at depth.

But to tie minerals to those variables like temperature and pressure, researchers need models. This team proposes to create a new model for biotite, which occurs in many large-volume eruptions. To create this new model, the researchers will grow biotite in experiments at high temperatures and pressures, like those deep in the Earth.

They will then study the compositions of the minerals, fluids, and glasses made in those experiments. These experiments and resulting model will be a significant contribution to our understanding of these magmas. By implementing their solution model in MELTS, which is well known by many geochemists, they ensure that many users will be able to easily access their deliverables using accessible code in a Jupyter notebook.

This project will also train a PhD student in chemical analysis and high-temperature experiments. It will support a postdoctoral fellow and 3-6 undergraduates, and includes a partnership with the Mentoring and Teaching Creates Hope (MATCH) program, which is a literacy improvement program at NMT. This partnership will generate readings and experiment pairings for third grade students to improve their literacy and help them engage with science concepts.

Unlocking the record of temperature and pressure associated with the formation of voluminous silicic magmas recorded by minerals requires carefully designed experiments to provide a thermodynamic framework to establish how mineral-melt or mineral-mineral equilibria relate to P-T-X conditions. Biotite is a common mineral in silicic igneous rocks that has the potential to record pre-eruptive temperatures, pressures, H2O and fO2s.

However, biotite has not been exploited as a recorder of intensive variables in magmas owing to the paucity of experiments that generate analyzable biotite crystals, which has prevented development of solution models that accurately depict the activity-composition relationships for igneous biotite compositions. The researchers propose to develop a new solution model for igneous biotite that accounts for substitution of Fe3+ and Ti on octahedral sites based on a new set of buffered, fluid-saturated experiments they plan to conduct.

This new solution model will allow users to transform biotite compositions into activity values for biotite end members, which can be used to unlock the records of temperature, fO2 and/or H2O recorded in common, biotite-bearing assemblages in silicic magmas.

The researchers will generate a suite of biotite-bearing experiments (conducted in cold seal vessels at varying temperature, pressure and fO2) along with a suite of biotite standards, where biotite Fe3+/FeT values are assessed with micro-colorimetry, the flank method using electron microprobe, and through XANES spectroscopy. Minerals and glasses will be analyzed with electron microprobe.

Fluids in mixed volatile experiments will be analyzed with RAMAN spectroscopy and LA-ICPMS. These experiments and chemical data will be used to provide a calibration for the biotite solution model, which will enable researchers to utilize this mineral for understanding pre-eruptive fO2 and fH2O contents of natural samples or experiments and for assessing equilibrium in natural samples or experiments.

The solution model will be implemented in MELTS (an open-source thermodynamic model), so that it will be available to a wide user base. Additionally, the work will generate a suite of oriented biotite standards with known Fe3+/FeT and a new method for growing large biotite crystals. The scope of work includes training in chemical analysis (wet chemistry, microprobe, XANES analyses, Raman spectroscopy) and high temperature experiments for a PhD student, a postdoctoral fellow and 3-6 undergraduates, over the duration of the award.

The work also includes a partnership with the Mentoring and Teaching Creates Hope (MATCH) program, which is a literacy improvement program at NMT. This partnership will generate readings and experiment pairings for third grade students to improve their literacy and help them engage with science concepts.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.