An Assessment of Clinopyroxene as a Recorder of Magmatic Water and Magma Ascent Rate

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Abstract

Clinopyroxene (Cpx) phenocrysts have the potential to record magmatic water contents and magma ascent rates through their concentration of H2O (incorporated as H+ and commonly referred to as water). Here we investigate three issues related to the fidelity and utility of the Cpx water record: the partitioning of water between Cpx and melt, the diffusivity of water in natural Cpx phenocrysts, and the possibility for water loss in Cpx erupted in lava flows. Samples studied are from volcanic ash of the 1974 eruption of Volcán de Fuego (Guatemala) and scoria and lava from the 1977 eruption on Seguam Island (Alaska). The partitioning of water was determined by analyzing melt inclusions (MIs) and the adjacent clinopyroxene host by ion microprobe. For seven Cpx-hosted MIs from Seguam, the partition coefficient is well predicted (within 0·1 wt % H2O on average) using the temperature-dependent parameterization of O’Leary et al. (2010; Earth and Planetary Science Letters297, 111–120). For the determination of diffusivity, H2O concentration profiles were measured in oriented Cpx from Fuego by ion microprobe. Water decreases toward the rim, consistent with diffusive re-equilibration during ascent-driven degassing. Using previously estimated durations of ascent (7–12 min), we determined the H+ diffusivity (10–9·7–10–10·3 m2 s–1 at a temperature of 1030 °C) that would satisfy these timescales. These calculated values are comparable with the medial values determined for natural Cpx (Mg# < 92·5) in laboratory diffusion studies. Tephra and lava co-erupted on Seguam bear similar Cpx populations in their major and trace element compositions, but the lava Cpx contain 80% lower H2O concentrations on average than the tephra Cpx. Using the values obtained from the Fuego Cpx, the difference in H2O between the lava and tephra Cpx can be attributed to post-eruption H2O loss during the estimated ∼20 min the sample remained above the H+ closure temperature. The results from this study indicate that clinopyroxene from slowly cooled basaltic lavas should not be used to reconstruct initial magmatic water contents. High ascent rates, rapid post-eruptive cooling, large phenocrysts, and cooler magma (e.g. andesites and rhyolites) favor better preservation of water in Cpx. In cases of H+ loss, Cpx zonation can be exploited as a chronometer for syn- and post-eruptive volcanic processes.

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