Magma Mixing Chronometry: Quantitative 3D Tomographic Analysis of Biotite Breakdown in Heating Experiments

Abstract:

Dacite lavas from Chaos Crags, Lassen Volcanic Center, CA contain inclusions of more mafic magmas, suggesting that mixing or mingling of magmas occurred just prior to lava dome extrusion, and perhaps triggered the eruption. The timescales between the mixing event and eruption are unknown, but reaction rims on biotite grains hosted in the Chaos Crags dacite may provide a record of the timescale (i.e., chronometer) between mixing and eruption. To quantify the effect of pre-eruptive heating on the formation of reaction rims on biotite, we conducted isobaric (150 MPa), H₂O-saturated, heating experiments on the dacite end-member. In heating experiments, we held the natural dacite at 800°C and 150MPa for 96 hours and then isobarically heated the experiments to 825 and 850°C (temperatures above the biotite liquidus, <815°C at 150MPa) for durations ≤96 hours. We analyzed run products using high-resolution SEM imaging and synchrotron-based X-ray tomography, which provides a 3-dimensional rendering of biotite breakdown reaction products and textures. X-ray tomography images of experimental run products reveal that in all heating experiments, biotite breakdown occurs and reaction products include orthopyroxenes, Fe-Ti oxides, and vapor (inferred from presence of bubbles). Experiments heated to 850°C for 96 h show extensive breakdown, consisting of large orthopyroxene crystals, Fe-Ti oxide laths (<100μm), and bubbles. When the process of biotite breakdown goes to completion, the resulting H₂O bubble comprises roughly the equivalent volume of the original biotite crystal. This observation suggests that biotite breakdown can add significant water to the melt and lead to extensive bubble formation. Although bubble expansion and magma flow may disrupt the reaction products in some magmas, our experiments suggest that biotite breakdown textures in natural samples can be used as a chronometer for pre-eruptive magma mixing.