Ti-in-quartz thermobarometry and TiO2 solubility in rhyolitic melts: New experiments and parametrization
Introduction
Pre-eruptive magma storage conditions are key information for understanding the mechanism of volcanic eruptions and evolution of the underlying magma plumbing systems (e.g., Bachmann and Huber, 2016; Putirka, 2017). Developments in petrological methods, particularly geothermobarometers based on mineral and melt compositions, have been widely employed to provide constraints on intensive variables (e.g., pressure, temperature and volatile activities) prevailing in magma reservoirs of volcanic systems (e.g., Andersen et al., 1993; Blundy and Cashman, 2008). Compared to basaltic and andesitic systems, much less thermobarometers are available for silicic systems, because suitable minerals (e.g., olivine, pyroxene and amphibole) that have been regularly involved in calibrating thermobarometers are lacking. In addition, due to potential loss of H from melt inclusion after entrapment (e.g., Severs et al., 2007; Tollan et al., 2019), gas saturation pressure estimated primarily based on measured H2O concentration in rhyolitic melt inclusions, in which CO2 concentrations are usually extremely low (e.g. Liu et al., 2006), may significantly underestimate the depth of magma pre-eruption storage if raw data are used. Alternatively, Myers et al. (2016, 2018, 2019) restored H2O contents in quartz-hosted melt inclusions based on an assumed linear correlation between H2O and incompatible elements, which would result in much appropriate estimation of gas saturation pressure.
As one of the major phases in silicic magmas, quartz accommodates various trace elements (such as Ti, Al, Fe, K; Götze, 2009), and several experimental studies showed that the concentration of Ti in quartz is pressure- and temperature-dependent (Wark and Watson, 2006; Thomas et al., 2010; Huang and Audétat, 2012; Thomas et al., 2015). Combined with the geospeedmeter based on diffusion of Ti in quartz (Cherniak et al., 2007), concentrations and distribution patterns of Ti in quartz can potentially offer spectacular information about magma storage conditions and timescales relevant to magmatic events (e.g., Wark et al., 2007; Gualda et al., 2018; Shamloo and Till, 2019).
Currently, there are two available independent calibrations of Ti-in-quartz thermobarometer, i.e. one from Thomas et al. (2010) and another one from Huang and Audétat (2012). However, they have large and irreconcilable discrepancies between each other, resulting in widespread confusion about the application of Ti-in-quartz thermobarometer for geological issues (e.g., Wilson et al., 2012). Both calibrations are based on experiments in which Ti-bearing quartz was crystallized from H2O-dominated fluids (i.e., hydrothermal quartz). Thomas et al. (2015) discussed critically the experimental design of Huang and Audétat (2012) and suggested that their approach may inevitably result in disequilibrium between newly formed quartz and fluid in terms of TiO2 activity. Other the other hand, the calibration of Thomas et al. (2010) was performed at pressures of 5–20 kbar, which does not cover the main range of storage pressures of silicic magmas in the continental crust that is usually below 5 kbar (e.g., Liu et al., 2006; Wilke et al., 2017). Considering that it is difficult to reconcile both datasets published so far, we decided to test another experimental approach, consisting in the crystallization of quartz from a silicate melt (rather than from a fluid). The experiments were designed to obtain quartz and co-existing Ti-bearing melt, as well as rutile in some cases. The experimental data are used to propose an improved Ti-in-quartz thermobarometer for rhyolitic magmas at crustal depths.
Section snippets
Starting materials
For investing the solubility of Ti in igneous quartz, we performed crystallization experiments using four synthesized high-silica glasses containing different TiO2 contents. Except for TiO2, these starting glasses have similar relative mass proportions for other present cations (Si, Al, Na and K). The normative Qz-Ab-Or (SiO2-NaAlSi3O8-KAlSi3O8) proportion is Qz70-Ab20-Or10, and the glasses are peraluminous with an aluminum saturation index [calculated as molar ratio Al2O3/(Na2O+K2O)] of
Experimental products
Experimental conditions and products using the two low-Ti starting glasses (S3 and S4) are listed in Table 1). Representative back-scattered electron (BSE) images are shown in Fig. 1. All experimental samples contained glass, quartz and an aluminosilicate phase. The ratio of Al/Si in the aluminosilicate phase clearly shows that the chemical formulae of the phase is Al2SiO5 and that should be sillimanite considering the P-T range of the experiments. For experimental runs using starting glass S3
Attainment of equilibrium
The calibration of a Ti-in-quartz thermobarometer requires that experimental data reflect equilibrium distribution of Ti between quartz and silicate melt. Different independent observations indicate that conditions close to equilibrium were reached.
In our experiments the Ti content of the melt is significantly higher than that of the quartz and possible problems leading to disequilibrium distribution of Ti may occur if this element cannot diffuse fast enough away from the interface quartz-melt.
Conclusions
Experiments designed to attain near-equilibrium conditions between felsic melt, quartz and rutile were used to re-calibrate the Ti-in-quartz thermobarometer at rutile-saturated conditions and the TiO2 solubility in melt, which are functions of both pressure and temperature. The combination of these two models allowed us to propose a new Ti-in-quartz thermobarometer without an independent estimation of provided that Ti concentrations in quartz and in a coexisting melt are available.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
We thank Colin Wilson and Jörg Hermann for their thoughtful reviews that have substantially improved this paper. We also thank Heather Handley for her editorial handling. This work was supported by German Research Foundation (DFG) project HO1337/40 in the frame of ICDP program.
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Present address: Department of Geology, Northwest University, 710069 Xi'an, China.