Reconstructing magma storage depths from olivine-hosted melt inclusions: Do vapor bubbles matter?

Abstract:

The depths at which magmas are stored, their pre-eruptive volatile contents, and the rates at which they ascend to the Earth’s surface are important controls on the dynamics of volcanic eruptions. Basaltic magmas are likely to be vapor undersaturated as they begin their ascent from the mantle through the crust. Once vapor saturation is achieved and the magma begins to degas, its pre-eruptive volatile content is determined largely by the depth at which it resides within the crust. Olivine-hosted melt inclusions are believed to retain their pre-eruptive volatiles because the strength of the host crystal insulates them from the decompression experienced by the host magma. This suggests that the concentrations of H₂O and CO₂ in vapor-saturated olivine-hosted melt inclusions can be used as an indicator of entrapment depth [1-3]. The common occurrence of vapor bubbles in melt inclusions adds significant uncertainty to such depth estimates, however, because they contain a significant proportion of the total CO₂. We have examined the distribution of volatiles in olivine-hosted melt inclusions from basaltic pillow lavas of two late Pleistocene subglacial eruptions in south Iceland: Miðfell and Skuggafjöll. We reconstructed the H₂O and CO₂ contents at the time of inclusion entrapment using two different techniques. In the first, a Vernadsky heating stage was used to homogenize the inclusions prior to analysis by secondary ion mass spectrometry (SIMS). In the second, we combined X-ray tomography, Raman spectroscopy and SIMS. First, X-ray microtomography is used to quantify the volumes of melt inclusion and vapor bubble. Next, the density of CO₂ in each vapor bubble is determined by confocal Raman spectroscopy. Finally, the concentrations of H₂O and CO₂in the included glass are determined by secondary ion mass spectrometry. We conclude that reconstructing H₂O and CO₂ concentrations has a significant advantage over the heating stage because the former approach retains information in the adjacent olivine on post-entrapment processes – such as cooling and crystallization – that is largely erased by the homogenization process.

References: [1] Wanless and Shaw (2012) Nat Geosci 5: 651-655; [2] Colman et al. (2015) Earth Planet Sci Lett 412:122-131; [3] Wanless et al. (2015) Geochem, Geophys, Geosys 16:126-147.