The roles of decompression rate and volatiles (H2O+Cl±CO2±S) on crystallization in (trachy-) basaltic magma

The decompression rates (r) and the amount and types of volatile species (H2O + Cl± CO2 ± S) are crucial parameters for the physical behavior of ascending magmas because they affect the formation of crystals and bubbles, as well as bulk viscosities of volcanic suspensions and their eruptive styles (e.g., effusive vs. hazardous explosive eruptions). However, the roles of CO2, S and Cl, in addition to H2O, on decompression induced crystallization (DIC) are still poorly understood. In this study, we investigated the DIC of a trachybasaltic magma, as a function of volatile content (H2O≈5wt.%;Cl≈0.7wt.%;±CO2≈2000 ppm;±S≈3000 ppm)and r. Isothermal decompression experiments were conducted at T = 1030 °C and log(fO2) ≈ QFM + 2 (QFM: quartz–fayalite–magnetite buffer) by releasing pressure (P) continuously from 300 to 70 MPa at r = 0.01, 0.1 and 1 MPa/s. The phase assemblages at 300 MPa, before the onset of decompression were composed of 91 to 99 area% melt/glass, 2 to 9 area% clinopyroxene, ≪1 area% spinel and ≪1 area% bubbles; with higher cpx proportions in the CO2-bearing samples (i.e., at lower water activity). We compare our experimental results with numerical models of equilibrium degassing (using SolEx and DCompress), phase assemblages (using MELTS) and with literature data on phase stabilities in S-bearing and S-free systems. These comparisons reveal that near-equilibrium conditions are reached in all three investigated systems if decompressed at the lowest rate of 0.01 MPa/s before quenching. The crystallization of clinopyroxene during decompression at 0.1 and 0.01MPa/s is strongly enhanced by the presence of S,whereas spinel shows less significant variations. Plagioclase, biotite and olivine only occur in the S-bearing samples decompressed at a r of 0.01 MPa/s. At r=0.01 MPa/s (near-equilibrium) a crystallinity of ~60 area% was reached in the S-bearing system, while the S-free systems are characterized by a ~20 area% crystallinity. The strong influence of S on the crystallinity ismainly explained by an increasing thermal stability of clinopyroxene during decompression to 70MPa, which is probably due to the lower water activity in the presence of S at P ≤ 70 MPa and the increase of the liquidus T with decreasing water content in the melt (i.e., with decreasing P at volatile saturated conditions). Even though the viscosity of the S-bearing melt is relatively low during decompression, the increasing modal abundance of clinopyroxene (and other phases) can lead to a strong increase of the effective magma viscosity and, thus, can slow down magma ascent within the conduit and limit the escape of bubbles. Subsequently, the magma may either i) solidify at shallow depths or ii) new magma inputs and/or a P increase in the conduit owing to second boiling can induce an explosive eruption. Hence, the obtained experimental results indicate that S-bearing basaltic arc magmas have a higher chance to erupt explosively than S-poor basalts, especially if the ascent rate is relatively low.