Unravelling H2O Contents of Glassy Volcaniclastics Recovered During Drilling in the Izu-Bonin Mariana Arc: Implications for Eruption Environment

Abstract:

The Izu-Bonin-Mariana arc system (IBM) extends over 2800 km south from the Izu Peninsula (Japan) to beyond Guam (USA). During 2014 the Izu segment was the focus of three IODP drilling expeditions (350, 351 and 352) that investigated different aspects of this intra-oceanic arc system. This work focuses on material recovered in IODP Expedition 350, which drilled at two sites. Site U1436 is at 1776 meters below sea level (mbsl), ~60 km east of the arc-front volcano Aogashima, in the fore-arc. A single hole reached 150 meters below seafloor (mbsf), recovering 71.6 m of Pleistocene to Pliocene tuffaceous mud (~60%) intercalated with ~150 layers of ash to lapilli-ash sized volcaniclastics. This was drilled as a geotechnical test hole for possible future deep drilling. Site U1437 at 2117 mbsl in the rear-arc was the main focus of the expedition. It is located in a volcano-bounded basin between the Manji and Enpo rear-arc seamount chains, ~90 km west of the arc-front volcanoes Myojinsho and Myojin Knoll. Drilling over three holes reached 1807 mbsf and recovered 1120.76 m of Pleistocene to Miocene tuffaceous mud and mudstone (~60%) intercalated with ~2500 layers of volcaniclastics that become coarser (lapilli-tuff to tuff-breccia) towards the bottom of the hole.

Glassy shards from volcaniclastic layers recovered at Site U1436 and the upper hole at Site U1437 have been analyzed for H₂O and CO₂ contents by Fourier-transform infrared spectroscopy (FTIR) to investigate whether the shards originated in submarine eruptions and, if so, at what depth, to help locate possible sources. Measured H₂O contents range from 0.12 to 6.37 wt% (CO₂ was below detection (~40 ppm) in all shards). Based on the solubility of H₂O in silicate melts, the high H₂O contents suggest some unrealistically deep eruptions. However, high H₂O contents may also reflect low-temperature, post-eruption hydration. Using H₂O speciation (H₂O is dissolved in silicate melts as molecular H₂O and OH groups), measured by FTIR, hydration has been recognized in many shards and corrected using H₂O speciation models (assuming OH remains unchanged during post-eruption hydration), reducing the range to 0.04 to 2.70 wt%. Assuming saturation, degassed shards from three layers suggest subaerial eruptions, with most shards suggesting submarine eruptions at 1,000 to 2,000 mbsl.