The nature and role of degassing of basaltic magmas: Terrestrial and lunar applications

Volatiles play a prominent role in many aspects of basaltic volcanism. In this experimental investigation we explore the effects of generating a gas phase in an ascending basaltic magma in both terrestrial and lunar magmatic systems. The influence of degassing on plagioclase-silicate melt phase relations is used to constrain magma ascent rates in the Crater Flat volcanic zone, which has been the topic of recent interest due to its proximity to the proposed Nuclear Waste Repository at Yucca Mountain, Nevada. Unlike the Earth, the lunar interior is volatile depleted. However, evidence indicates that explosive fire-fountain eruptions, such as the Apollo 17 volcanic glasses, were volatile assisted. The oxidation of graphite is investigated as the gas generating mechanism ultimately propelling these eruptions. And finally, the gas/silicate melt distribution behavior of sulfur and chlorine is examined over a range of oxidation states.; Experiments on natural and/or synthetic starting materials were conducted at pressures ranging from 15 to 200 MPa, for temperatures between 900°C-1360°C, and oxidation states of NNO -4.2 to +2. A series of decompression experiments were designed to simulate magma ascent through the shallow upper crust, and to generate a gas phase.; A comparison of natural plagioclase microlite size and composition with plagioclase crystallized experimentally during decompression suggests that the volatilerich (H2O ≤4.6 wt. %; CO2 ≤900 ppm) magmas erupted in the Crater Flat volcanic system ascend at a minimum rate of 0.04 m/s.; Isothermal decompression experiments indicate that the initial gas phase in the ascending Apollo 17 orange glass magma would have been generated at a crustal depth of ∼8.5 km (double previous estimate) through the oxidation of graphite and not the exsolution of dissolved volatiles as occurs on Earth.; Gas/melt partitioning experiments indicate that a C-O or C-H-O gas will evolve into a sulfur-rich composition under oxidizing conditions (NNO ≥+2). The partition coefficients for these oxidizing conditions range from 85 to 157, under more reduced conditions partitioning decreases to 1 (NNO -4.2). These results suggest that sulfur released by basaltic arc magmas may play a fundamental role in the excess sulfur degassing conundrum detected at volcanic arc systems.