Crystallization in Hydrous Magmas

JE Hammer

Dept. Geology and Geophysics –SOEST

University of Hawaii

Honolulu, HI 96822

Magmas commonly become volatile-saturated at shallow crustal levels and begin to outgas while ascending during eruption. Exsolution of H₂O during ascent drives crystallization in the form of phenocryst growth and/ or the formation of a groundmass microlite population. The rheologic consequences of H₂O loss and increasing solid particle content may influence the style of subsequent eruption. A quantitative understanding of these processes is essential for improving conduit flow models and for correctly interpreting ascent history using the crystal textures of erupted materials. Insights from recent study of the kinetics of degassing-induced feldspar crystallization are applied to understanding the 1991 Pinatubo eruption, and are germane to arc systems in general.

• A multiplicity of disequilibria may be preserved in an erupted magma because the rates of relevant chemical reactions differ. For example, the rate of hornblende equilibration in response to a decrease in P_H2O is much slower than the rate of feldspar crystallization. The former requires coupled substitution of network-forming cations between the crystal and melt, while the latter is limited by comparatively rapid diffusion of mineral-forming components in the melt. As a result, feldspar contents and glass compositions may record a more recent equilibration to decreased P_H2O than amphibole crystals are capable of indicating. Moreover, comparison of different equilibria can yield information about the timing of magmatic processes if the rate laws of these reactions are known. For example, we compared our glass composition geobarometer with hornblende geobarometry in 1991 Pinatubo dacite, and found that partial magma equilibration occurred at lowP_H2O within three weeks of eruption. The results are interesting because a late-stage reduction in P_H2O of this magnitude implies either (1) magma ascent by 2700 m, or (2) an influx of other volatiles (CO₂, SO₂) into the system. Either of these processes could be related to the injection of basaltic magma to the dacite reservoir, which is generally accepted to have triggered the eruptive sequence.

• The crystal textures of erupted magmas result from a combination of factors, including the relative contributions of nucleation and crystal growth, the particular mechanisms of crystal growth, and the rate limiting step in crystal growth for each phase. In an experimental study of feldspar crystallization kinetics during decompression, we found that the rate of crystallization at any given step depends on the magnitude of the decompression step, the extent of crystallization during previous steps, and the transport properties of crystal-forming components in the melt at the new pressure (i.e., melt viscosity). The dispersion of crystal sizes and shapes are particularly indicative of solidification conditions. In the case of Pinatubo dacite, natural textures were most closely reproduced in constant-rate, multi-step decompression experiments of ~10 MPa/h, which corresponds to steady ascent at 0.12 m/s. In contrast to a previous interpretation, we now conclude that the ejection of dense, microlite-rich, partially-degassed juvenile material along with vesicular, microlite-free pumice during pulsatory eruptions at Pinatubo reflects variable magma travel times from the reservoir rather than a static process of shallow conduit degassing and plug formation.