[S-GC32] Volatiles in the Earth - from Surface to Deep Mantle

[SGC32-P07] Hydrogen isotope of the mantle source in Pitcairn Island

*Takahiro Ozawa¹, Akira Ishikawa¹, Takeshi Hanyu², Kenji Shimizu³, Takayuki Ushikubo³, Morihisa Hamada², Hikaru Iwamori⁴ (1.Department of Earth and Planetary Sciences, Tokyo Institute of Technology, 2.Japan Agency for Marine-Earth Science and Technology, 3.Kochi Core Center, Japan Agency for Marine-Earth Science and Technology, 4.Earthquake Research Institute, the University of Tokyo)

Keywords:hydrogen isotope ratio, hotspot, Pitcairn Island

The origin of Earth’s water has long been debated based on the hydrogen isotope [deuterium/hydrogen (D/H)] ratio of the Earth. However, the D/H ratio of the bulk Earth estimate remains controversial since substantial amounts of water could be present inside the Earth. The relatively uniform D/H ratio for the upper mantle (δD of -60 ± 5 ‰ relative to VSMOW) is established by the analysis of submarine glass from mid-oceanic ridge basalts (Clog et al., 2013). Recently, Hallis et al. (2015) reported much lower D/H ratios from olivine-hosted melt inclusions in Baffin Island and Icelandic lavas, and suggested that the Earth’s deep mantle has much lower D/H ratio (δD < -218 ‰). They attributed such low-δD signature to a protosolar nebula origin which is derived from an isolated reservoir resided in the Earth’s deep interior. However, another possibility that such low-δD signature was created via dehydration of subducting slab (Shaw et al., 2008) needs to be evaluated.

The oceanic island basalt (OIB) of Pitcairn Island is typical of containing recycled material such as subducted slab (Eisele et al., 2002; Garapic et al., 2015). We found rapidly-quenched hyaloclastite containing abundant automorphic fresh olivine crystals and volcanic glass shards. Since the olivine crystals contain many crystal-free glassy inclusions, we analyzed concentration of volatiles (CO₂, H₂O, F, S and Cl) and P₂O₅, and hydrogen isotope ratio (δD) with SIMS, and major elements with EPMA for melt inclusions and glass shards, and compared them with the aim to evaluate the D/H fractionation during magma evolution and to constrain the D/H ratio of recycled components stored in the deep mantle.

Our results demonstrate that olivine-hosted melt inclusions have much higher H₂O and CO₂ contents than matrix-derived glass shards, consistent with that magma degassing is responsible for their volatile element abundances. Their δD values positively correlate with H₂O contents, also indicative of vapor-melt D/H fractionation during final stage of degassing. Since most measured melt inclusions were high in CO₂, such degassing of H₂O and D/H fractionation could be minor. With the exception of some inclusions depleted in volatiles, we obtained an average water content of 1.06 ± 0.16 wt.% and an average δD value of -89 ± 12‰ (2 SD) as the composition of primitive melt, which is significantly lower than the upper mantle value. This suggests that Pitcairn mantle plume tapped the material with a low D/H ratio, which could be derived from recycled slab components.