ApThermo and ApTimer: new online tools for the calculation of H2O budgets in magmas and the timescales of magma ascent based on apatite chemistry

Abstract:

Apatite is a ubiquitous mineral in igneous rocks that can incorporate various magmatic volatile elements (e.g., C, H, F, Cl, S). Recent studies have shown that the volatile chemistry of apatite can reveal not only the magma H₂O-CO₂ budgets but also the timescales and rates of magma ascent, allowing us to determine the two main factors that control eruptive styles using a single mineral. Here we propose two new online tools for the calculation of H₂O concentration in silicate melts (ApThermo; https://apthermo.wovodat.org), and the timescales of magma ascent (ApTimer; https://apthermo.wovodat.org). ApThermo has been developed for apatite-melt hygrometry calculation, based on the exchange coefficients for OH-Cl and OH-F between apatite and the melt derived from thermodynamic modelling and experimental determinations (Li and Costa, 2020). ApTimer has been developed for calculating the timescale required to produce F-Cl-OH zoning in natural apatite crystals, based on F-Cl-OH diffusivities determined by experiments and a three-component diffusion model (Li et al., 2020). Application of ApTimer to model the diffusion profiles of F-Cl-OH in groundmass apatite from volcanic rocks could give us timescales that correspond approximately to the duration of magma ascent. By applying the two methods to the 2006 effusive (VEI-1) and 2010 explosive (VEI-4) eruptions at Merapi volcano (Indonesia), we find that the pre-eruptive magmas of the two eruptions have similar H₂O-CO₂ concentrations, but magmas in 2010 ascended at least 5 to 10 times faster than those in 2006 (Li, 2019). Thus in this particular case, faster magma ascent is the main contributor to the larger and more explosive eruption. The two calculation tools proposed here have wide applications to study the pre-eruptive magma storage conditions, volatile contents and magma ascent rates, and the eruption dynamics, which can provide key information for volcano monitoring and eruption forecasting.