Volcanic gases and the reaction of sulfur dioxide with aluminosilicate glasses

Volcanic gases are an important part of the volatile cycle in active planetary systems and contribute significantly to the mobilization and transport of metals to planetary surfaces. On Earth, Venus, Mars and Io, SO2 is the most abundant corrosive species in volcanic gases, and basalts are ubiquitous on these planetary bodies. The reaction between SO2 and silicate rocks forms oxidized sulfate and reduced sulfide. This reaction is a key process in the formation of porphyry deposits. In volcanic eruption plumes SO2 reacts with volcanic ash and is scavenged onto the surface of the ash particles. Knowledge of the reaction mechanisms between volcanic gas and rocks, minerals and glasses, and processes controlling the metal mobilization and transport in volcanic gas can constrain models of volatile and metal budgets of planetary crusts and surfaces. Using thermochemical modelling, I present a new model for the composition of volcanic gas on the Moon and compare it to a terrestrial volcanic gas from Erta Ale volcano (Ethiopia). The main species in lunar volcanic gas are H2, S2 and CO. This finding is in contrast to previous studies which suggested that CO was the sole driver of explosive volcanic eruptions on the Moon. This lunar volcanic gas has a lower capacity for metal transport compared to the Cl- and H2O-rich volcanic gas from Erta Ale volcano. To identify how SO2-glass reactions occur at high temperature and to investigate what might promote and limit these reactions, I present results from an experimental study. Pure SO2 was reacted with silicate glasses in the system anorthite-diopside-albite and with Fe-bearing natural basaltic glasses. The sulfate reaction products are relatively enriched in Ca compared to the silicate glass composition, in particular in experiments with Fe-free anorthite-diopside glasses. On these Fe-free glasses CaSO4 is the sole observed phase in the coatings at 800 °C, whereas at 600 °C minor amounts of MgSO4 were detected. At 800 °C, the flux of Ca from the silicate glass to the surface exceeds that of Mg by a factor of up to 330, whereas at 600 °C this factor is only 3. The rate of reaction is not constant, decreasing by an order of magnitude from 1 h to 24 h at 800 °C. The reaction of SO2 with tholeiitic basalt glasses produces coatings of CaSO4, MgSO4, Na2SO4 and oxides including Fe2O3 and TiO2. In addition, the reaction modifies the basalt glass because Ca, Mg and Na are lost to the coating. This results in the nucleation of crystalline spherulites and needles including SiO2, Al2O3, as well as Fe-Na-rich and Mg-rich pyroxenes. VIII The results suggest that the structural properties of the silicate glass substrate control the diffusive transport of Ca, Na, Mg, Fe and Ti to the surface which in turn controls the overall reaction rate and the formation of sulfates, oxides and silicates. These findings can be applied to predicting reactions on planetary surfaces and at shallow levels within their crusts

Replication Data for: Experimental and petrological investigations into the origin of the lunar Chang'e 5 basalts

Abstract: The origin of young Chang'e 5 (CE5) lunar basalts is highly debated. We present results from high-pressure, high-temperature (P-T) phase equilibria experiments, and from petrological modeling, to constrain the depth and temperature of the source of these unique mare basalts. The experimental results indicate that the CE5 basalts could have formed either by melting clinopyroxene and Fesingle bondTi oxide-rich cumulates in the shallow lunar mantle, or by extreme fractional crystallization of a hot Mg-rich parental melt. Our findings corroborate the local preservation of significant heat (of at least 1200 °C) in the lunar mantle that is needed to generate basaltic melts of CE5 compositions at 2 Ga. We argue that the CE5 basalts are most likely formed by melting of Fe and Ti-rich cumulates in the shallow lunar mantle as extreme fractional crystallization of olivine and plagioclase from picritic parental melts requires too high temperatures in the lunar mantle (> 1500 °C) at ∼2 Ga. Highlights: • The origin of the Chang'e 5 basalts was investigated by high-P and high-T experiments and fractional crystallization modeling. • Experiments show that theese basalts can be generated by melting of a Fe- and Ti-rich shallow lunar mantle cumulate. •The Chang'e 5 basalts can also be generated by extreme fractional crystallization of a parental Mg-rich lunar melt. • Based on thermal constraints the cumulate melting model is more likely than extreme fractional crystallization model