HIGH TEMPERATURE MAGMATIC GAS: MINERAL DEPOSITION AND GAS/WALLROCK REACTION

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The association of volcanic activity with ore deposits has been long recognized, but exactly how volcanic activity leads to ore deposit formation remains unknown. Since lavas and ash are generally metal-poor, volcanologists have speculated that metal transport must occur by the gas given off during rise of the magma towards the surface. Volcanic gas in eruptive plumes has been studied extensively because it can be directly sampled. However, volcanic gas has already lost much of its metal load to the volcanic subsurface structure through precipitation of vapor-deposited minerals and by reaction with rock deep within the volcanic edifice. Access to the natural gas before it has lost much of its dissolved load is very limited because of the scarcity of high temperature fumaroles and their extreme hazard. These problems have, thus far, made it impossible to bridge the knowledge gap between volcanic activity and formation of associated ore deposits. This project will utilize a new unique experimental design to simulate this process of gas release from magma in the laboratory. This technique will be used to characterize the chemical nature of magmatic gas just as it is released from magma and to observe its chemical modification as it produces vapor-deposited phases and reacts chemically with rock. Characterization of the new minerals produced will take advantage of the strength of synchrotron X-rays at Brookhaven National laboratory, that will allow analysis of micron-sized particles and provide information of how the metals are incorporated into the crystalline structures. The results of this research will provide a basis for new understanding of barren vs. economically important subvolcanic regions that will aid in future economic deposit exploration on Earth and other planetary bodies. They will further our understanding of vapor-deposited mineral stability and metal incorporation– a subject applied heavily in the materials science industry. As part of outreach to this industry, the geological community, and planetary science, a publically accessible database will be developed to store electronic versions of synthesis methods for a variety of minerals and materials found in nature. This project also has an educational component geared to providing experimental experience to undergraduate geology students as a first step in ensuring that the next generation of geoscientists are prepared with an entire tool bag of approaches with which to further our scientific understanding of natural processes.This work focuses on the compositional evolution of high temperature F-Cl-S-bearing magmatic gases in the high-temperature subvolcanic region (or just above shallowly emplaced plutons). In this regime, magmatic gas begins its transformation to volcanic gas through loss of much of its dissolved solute load by precipitation of minerals and reaction with wallrock. Laboratory-scale experiments will be conducted in order to elucidate (i) the nature of mineral phases produced during cooling of the magmatic gas, (ii) the trace elements these minerals incorporate, and (iii) the crystallographic controls on trace element incorporation into the vapor-deposited minerals. These experiments will simulate the formation of natural magmatic gas by decompression boiling of volatile-saturated rhyolite and phonolite, and simulate the production of vapor-deposited phases by allowing this gas to cool in a strong thermal gradient. Gas/rock reaction will be investigated by reacting a synthetic basalt and rhyolite glass in the stream of gas given off by the boiling magma. These reaction experiments will yield the products of magmatic gas/wallrock reaction and indicate how magmatic gas/wallrock reaction affects trace element mobilization. Analysis will combine bulk chemistry, SEM, and Synchrotron X-ray microprobe techniques (hard and tender-energy X-ray fluorescence microanalysis, microbeam X-ray absorption spectroscopy, and microdiffraction) to characterize bulk chemical transport, microscopic phase distribution in the vapor-deposited material and on reaction surfaces, and localized chemical speciation in the vapor-deposited minerals.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.