[S-GC48] Volatiles in the Earth - from Surface to Deep Mantle

[SGC48-02] Multiple halogen components in subcontinental lithospheric mantle revealed by single-grain analysis of mantle-derived xenoliths

*Hirochika Sumino¹, Sota Niki², Ray Burgess³, Masahiro Kobayashi⁴, Hiroyuki Kagi² (1.Department of Basic Science, Graduate School of Arts and Sciences, The University of Tokyo, 2.Geochemical Research Center, Graduate School of Science, University of Tokyo, 3.School of Earth and Environmental Sciences, University of Manchester, 4.Tokyo Metropolitan Industrial Technology Research Institute)

Keywords:Halogen, Mantle, Heterogeneity, Subcontinental lithospheric mantle, Mantle xenolith

Volatile recycling back to the Earth’s mantle at subduction zones has a significant, yet poorly constrained impact to the volatile budget in the mantle. Halogens with marine pore-fluid signatures have previously been discovered in mantle wedge peridotites, suggesting that pore-fluid-derived volatiles can survive the subduction cycle to subarc depths and modify the subcontinental lithospheric mantle (SCLM) [1,2]. To better constrain how such subduction fluids modifies the halogen composition of SCLM, we analyzed halogens in single grains/ a few grains of olivine, ortho- and clino-pyroxene crystals separated from mantle xenoliths from Southern Patagonia, Kamchatka, and the Philippines, in which several halogen components in addition to a MORB-like one have been previously identified in bulk and mineral separates [2,3]: (A) a pore-fluid-like component with high Br/Cl and I/Cl ratios similar to bulk mantle wedge peridotites [1,2]; (B) a Cl-enriched component relative to MORB, similar to bulk altered oceanic crust (AOC) and metasomatised intraplate mantle xenoliths [4,5]; and (C) a component enriched in Br and moderately in I compared to MORB, which resembles fluids in AOC, diamonds and mantle xenoliths in Russian kimberlites [4,6,7]. Halogens in the mineral grains were measured with neutron-irradiation and noble gas mass spectrometry combined with crushing and laser heating extraction.

Whereas only component (A) was observed in bulk samples from Kamchatka and the Philippines containing abundant water-rich fluid inclusions [2], components (B) and (C) were also identified in their olivines. In contrast, the component (A) is a relatively minor component in Southern Patagonian samples. These suggest that mantle wedge metasomatism by pore-fluid derived volatiles would be obscured by fractionation processes in SCLM [5], while AOC-related signatures survive more robustly.

[1] Sumino et al. (2010) EPSL. [2] Kobayashi et al.(2017) EPSL. [3] Sumino et al. (2018) Goldschmidt abstract. [4] Chavrit et al., (2016) GCA. [5] Kobayashi et al. (2019) G-cubed. [6] Burgess et al. (2009) GCA. [7] Broadley et al. (2018) Nature Geo.