Quartz and sanidine dissolution in silicic magmas

Kenneth Befus¹ , Benjamin Andrews²

• Affiliations: ¹University of Texas at Austin ²Smithsonian Institution 

• Presentation type: Poster

• Presentation time: Thursday 16:30 - 18:30, Room Poster Hall

• Poster Board Number: 218

• Programme No: 1.9.25

• Theme 1 > Session 9

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Abstract

Crystal textures record magmatic processes, but these records may be erased if the crystals dissolve in response to thermal or chemical disequilibrium. Many igneous rocks contain crystals that preserve evidence for partial dissolution. Crystal exteriors may be rounded or pocketed. Internal crystal zoning patterns may preserve cross-cutting relationships and unconformities. By analogy to their sedimentary counterparts, a crystal unconformity can be used to interpret the 'chronostratigraphy' of an otherwise unrecorded magmatic process. To explore the significance of the missing crystal record we performed a series of dissolution experiments using crystal-free rhyolite glass, to which we added sieved fractions of quartz and sanidine. Experiments were held at 775 to 875 °C and 100 to 250 MPa (P_total=P_H2O), producing conditions up to 100 °C above the liquidus, for up to 50 days. The resulting crystal textures were characterized using X-ray CT and petrography. Dissolution scales with both time and the degree of disequilibrium. At 100 °C disequilibrium both phases had fully dissolved in <300 hours, indicating average dissolution rates of ~1 µm h^-1. At 30 °C disequilibrium quartz and sanidine dissolved at ~0.1 µm h^-1. These dissolution rates are the same order of magnitude as crystal growth rates. Because natural crystals commonly preserve external and internal dissolution textures, the timescales of dissolution-inducing magmatic disturbances are often short enough to avoid full digestion. If dissolution textures are observed in mm-sized quartz and sanidine then episodes of thermal or chemical disequilibrium are unlikely to have lasted for more than a decade.