[S-GC33] Solid Earth Geochemistry, Cosmochemistry

[SGC33-P04] Role of fluid rock reaction for the production of FOZO isotopic composition

*Gen Shimoda¹, Tetsu Kogiso², Katsuhiko Suzuki³, Akira Ishikawa⁴, Norikatsu Akizawa⁵ (1.Geological Survey of Japan, AIST, 2.Kyoto University, 3.Japan Agency for Marine-Earth Science and Technology, 4.Tokyo Institute of Technology, 5.University of Tokyo)

Keywords:FOZO, mantle, isotopic composition

It has been noted that Pb-Nd-Sr isotopic compositions of OIB converge on a small space. Definitions of the convergent space were proposed by several groups, such as the Focus Zone (FOZO; Hart et al., 1992), the Common component (C; Hanan and Graham, 1996), Prevalent Mantle (PREMA; Zindler and Hart, 1986), etc. The convergent space has relatively small Nd and Sr isotopic variation but relatively large Pb isotopic range, resulting in vertical trend in 3D isotopic plot with ¹⁴³Nd/¹⁴⁴Nd (x axis), ⁸⁷Sr/⁸⁶Sr (y axis) and ²⁰⁶Pb/²⁰⁴Pb (z axis). A recent geochemical study proposed cause of isotopic convergence, in which reaction between subducted oceanic crust and fluid released by serpentinite dehydration can produced chemical composition that is suitable for the production of FOZO isotopic compositions when the recycling age of oceanic crust was about 1.0 Ga (Shimoda & Kogiso, 2019).
In this presentation, we estimated isotopic range of recycled oceanic crusts modified by subduction process, including dehydration and fluid-rock reaction. Isotopic variation was estimated with wide variety of geochemical conditions, such as chemical variation of oceanic crust, variable degree of dehydration, fluid rock ratios, recycling ages and different subduction conditions. We calculated isotopic compositions of recycled oceanic crust about a million sets of Pb-Sd-Sr-Hf isotopic compositions. The calculated isotopic compositions distributed near FOZO and PREMA. Although the tested subduction conditions were varied widely, isotopic compositions of recycled oceanic crust show small isotopic variation, indicating that isotopic composition of FOZO simply reflect isotopic variation of subduction modified oceanic crust. It may follow that focusing isotopic distribution of OIB simply express isotopic variation of recycled oceanic crust with variable subduction conditions and recycling ages.