SIMS depth profiling to determine volcanic conduit processes

Chemical and textural characteristics of eruption products can provide information on pre-eruptive volcanic conduit conditions. A secondary ion mass spectrometry (SIMS) depth profiling technique was developed to examine geochemical changes in the final stages of mineral growth at the sub-micrometer scale. Samples from three volcanic test cases were analyzed to demonstrate the applicability of the method, and resultant data provide information on physical processes in the conduit over the hours to days preceding eruption.;Plagioclase phenocrysts were extracted from juvenile clasts erupted during the 1997 and 2003 Vulcanian events at Soufriere Hills volcano, Montserrat. Although numerical models and petrologic experiments typically assume isothermal conduit conditions, anorthite (An) variations within SIMS depth profiles indicate that conduit temperatures increased by 50 to100 degrees Celsius prior to the 1997 Vulcanian events when contemporaneous overpressures approached 20 MegaPascals. Depth profiles also reveal lithium (Li) inflections that are interpreted to represent the onset of magma devolatilization and allow calculation of decompression-induced crystal growth rates in situ without the need for other petrologic lines of evidence. In the 2003 samples, Li inflections provide growth rates even when An compositions fail to record accurate pressure changes during short-duration (less than 10 hours) decompression events, providing a means to determine crystallization rates despite a lack of compositional equilibrium between crystals and melt.;Olivine phenocrysts from products of the monogenetic Lathrop Wells eruption were depth profiled in conjunction with analyses of groundmass magnetite crystallinities. Distances between symplectite lamellae in the olivine rims provide an iron-magnesium interdiffusivity rate, which allows calculation of the oxygen fugacity. Results indicate oxidation of the magma during the initial stages of activity, which resulted in crystallization and shallow conduit modification, increasing the magma ascent rate and changing the explosive style from weak Strombolian to violent Strombolian.;The results demonstrate that SIMS depth profiling provides in situ measurements of pressure/temperature variations, crystal growth rates, element diffusivities, and oxygen fugacity variations through analysis of phenocryst rim compositions. This allows quantification of pre-eruptive volcanic conduit phenomena at unprecedented physical and temporal scales, providing a previously unavailable view into the system immediately prior to explosive events.