VOLCANIC GAS: EXPERIMENTALLY INVESTIGATING SUBLIMATION AND ROCK/GAS REACTIONS IN MULTI-COMPONENT SYSTEMS

Magmatic gas transports water, CO₂, and SO₂, as well as a variety of metal, metal oxide, sulfate and sulfide species from a magma to the surface. On Earth the high concentration of “non-condensable” gaseous species in volcanic gas limits the extent of vapor-deposited solids produced during cooling of the gas and inhibits our ability to identify the role of gas composition (and oxygen fugacity) and magma source on the nature of vapor-deposited solids. In order to better understand the controls on vapor-deposited phases that may contribute to surface fines, we have designed an experimental system in which a volatile supersaturated magma boils in a long evacuated silica glass tube and the gas released rises into a cooler part of the system in a strong thermal gradient. Investigation of mineral surface/magmatic gas interaction is done by placing a crystalline sample in the gas stream above a capillary drawn in the tube.

We investigated gas condensation from a Cl- and S enriched magma (0.4 wt% water; NNO; at 1150 ^oC). The boiling magma produced a multi-component gas that, in turn, produced vapor-deposits distributed along the thermal gradient. Fe/Na/K chlorides and silica were found throughout the temperature region ~800-300 °C. Within the temperature regime 500-300°C, abundant maghemite appeared as individual grains embedded in a matrix of chlorides (molysite+halite+sylvite). However, the greenish color of the chloride that adhered the maghemite to the glass tube prior to opening of the tube and exposure to air hinted that the primary iron chloride precipitated by the gas may have actually been lawrencite (FeCl₂). The maghemite was likely also secondary from oxidation of primary magnetite during cooling of the tube after removal from the furnace. Native sulfur was found at the cool end of the tube. For the sulfur-rich source, the pyrrhotite, and at lower temperature, pyrite, was added to the vapor-deposited assemblage. Placing a mineral grain (olivine, augite, plagioclase) in the gas stream additionally produced sulfates on the mineral surfaces presumably through interaction with the SO₂ component of the gas.

These experiments confirm that volcanic gas in magmatic systems such as Mars may differ significantly from subduction zone volcanic gas and influence explosivity and eruption products.