Evolution of magmatic volatiles during drilling into a magma body, Krafla Iceland.

In 2009, the Iceland Deep Drilling Project IDDP-1 borehole unexpectedly intercepted a silicic magma body beneath Krafla volcano at a depth of approximately 2.1 km. Glass samples quenched in the drilling mud were directly sampled at ~15 minute intervals during drill circulation following magma interception at 15:17, providing a unique record of the changing state of the magmatic system over this period. The nature of the glass cuttings varied through time, with a decrease in the proportion of vesicular cuttings and an increase in dense glassy cuttings. The recovered glass cuttings provide a unique opportunity to investigate the magmatic response to a sudden decompression event, and in particular the response of magmatic volatiles over time. During decompression, volatiles diffuse through the magma and exsolve into growing bubbles. Conversely, pressure increases or temperature decreases can cause volatile species to diffuse back from bubbles into the surrounding melt. These processes create distinct diffusion gradients in magmatic glass surrounding the bubbles, and hence the time-dependent volatile response to magmatic decompression is uniquely preserved by the quenched IDDP-1 fragments. For this project an Electron Probe Micro-anaylser was used to determine the major element compositions of each sample, while Fourier transform infrared spectroscopy (FTIR) was used to generate maps of H2O and CO2 absorbance across broad areas. Average H2O and CO2 concentrations determined from the volatile maps are used to reconstruct the time-dependent evolution of magmatic H2O and CO2 at the magma interface. In addition, transects around bubbles were extracted to examine whether H2O and/or CO2 diffusion profiles developed, and if so how they responded with time. Results from the major element analyses suggest that the clear glass represents the boundary layer between the magma body and it’s surrounding rock, some of which appears to have been assimilated at the magma-rock interface. The brown glass cuttings (Melt-1b) appear to more closely represent the main magma body. Volatile results show that the concentration of total H2O did not change over time; however, OH/H2Om and CO2 both increase over time after initial intersection. These results are consistent with an increase in temperature that is interpreted to reflect deeper hotter magma rising into the borehole. The volatile distributions around bubbles show both resorption and bubble growth characteristics. This differs from an expected simple decompression (i.e. producing bubble growth) and suggests that the magma maintained a somewhat heterogeneous condition prior to quenching. This may reflect the relatively small borehole size, and together with the fact that no eruption was triggered suggests that very small decompression events can be moderated by melt immediately surrounding the region of low pressure