R3: High-pressure science and deep Earth’s material

[R3-P-07] Experimental constraints on the coloration of ringwoodite synthesized at high pressure and high temperature.

*Steeve Greaux¹, Youyue Zhang¹, Hideharu Kuwahara¹, Ryoichi Nakada², Yuji Higo³ (1. Ehime University, 2. JAMSTEC, 3. JASRI)

Keywords:

High pressure and High temperature,Ringwoodite,Iron speciation,Mantle transition zone,Seismic velocities

Ringwoodite (Rw) is a high-pressure polymorph of olivine with a spinel structure that forms at the high P and T conditions of the lowermost part of the Earth’s mantle transition zone (MTZ) (Ringwood, 1958). Fe-bearing Rw is an important constituent mineral of the MTZ, where it represents about 60 vol.% to 80 vol.% of the pyrolitic mantle and subducted harzburgitic crust (Irifune & Ringwood 1987, Ringwood 1991). Rw crystals have a peculiar deep blue color, which contrast with colorless Mg₂SiO₄ Rw synthesized in the laboratory. The cause of the blue color in Rw has been extensively discussed by meteoritic studies, but its origin is still mostly unknown. Former studies proposed blue color originates from light scattering effect at grain boundaries (Nagy+ 2011) or due to glassy material found around Rw crystals in meteorites (Price 1979; Lingemann & Stöffler 1998) but it was later ruled out by transmission electron microscopy on Rw aggregates (Taran+ 2009) and synthesis of Rw single crystals arboring a deep blue color (Thompson+ 2021). Iron content has also been ruled out because there is no correlation between Rw’s color and iron-content in meteorites (Lingemann & Stöffler, 1998; Taran+ 2009). Alternatively, it was proposed that the blue color is related to the incorporation of water, which may push Fe to be surrounded by 4 oxygen ions rather than the usual 6 oxygens (Dobson 2019).Here we present electron probe, Raman spectroscopy, X-ray diffraction and XANES measurements at Fe L₃-edge on hydrous and dry Fe-bearing Rw with green, blue and brown colors, synthesized at 20 GPa and varying T and oxygen fugacity (fO₂) conditions. We found color of Rw does not depend on grain size, bulk Fe- and H₂O contents. Instead we found color change depends on synthesis temperature and fO₂ conditions although Fe³⁺/Fe_total in all Rw only varies by less than ±10%. These results suggest color of Rw depends not only on Fe speciation but also cation order-disorder in the spinel structure. The occurrence of cation order-disorder in Rw could have important implications for the seismic characteristics of the 520’ discontinuity (Muir+ 2025). Nevertheless, our results gives some critical constraints to infer the fO₂ and temperature conditions at which Rw crystals found as inclusion in deep diamonds or in meteorites were formed, based on their color.