Hydrogen incorporation and the oxidation state of iron in ringwoodite: A spectroscopic study

Abstract

Ringwoodite [(Mg,Fe) 2 SiO 4 ] is the high-pressure polymorph of olivine stable in the upper mantle between ~525 to 660 km. Information on its temperature-dependent water content and Fe-oxidation state bears important implications on the hydrogen cycle and oxidation state of the Earth’s interior. We conducted several multi-anvil experiments to synthesize iron-bearing (0.11 ≤ x Fe ≤ 0.24) hydrous ringwoodite under oxidizing and reducing conditions. The experiments were performed at 1200 °C and pressures between 16.5 and 18.3 GPa. The incorporation of hydrogen and iron in ringwoodite was studied using Fourier transform infrared (FTIR), Mossbauer (MB), ultraviolet-visible (UV-VIS), and electron energy loss (EEL) spectroscopy. For MB spectroscopy, ringwoodite enriched in 57 Fe was synthesized. The IR spectra of ringwoodite show a broad OH band around 3150 cm −1 and two shoulders on the high-energy side: one intense at 3680 cm −1 and one weak at around 3420 cm −1 . The water content of the samples was determined using FTIR spectroscopy to have a maximum value of 1.9(3) wt% H 2 O. UV-VIS spectra display a broad band around 12 700 cm −1 and a shoulder at 9900 cm −1 representing the spin-allowed dd-transitions of VI Fe 2+ . The weaker band around 18 200 cm −1 is a distinct feature of Fe 2+ -Fe 3+ intervalence charge transfer indicating the presence of Fe 3+ in the samples. EEL spectra yield Fe 3+ fractions ranging from 6(3)% at reducing conditions to 12(3)% at oxidizing conditions. We performed heating experiments up to 600 °C in combination with in situ FTIR spectroscopy to evaluate the temperature-dependent behavior of ringwoodite, especially with respect to hydrogen incorporation. We observed a color change of ringwoodite from blue to green to brown. The heat-treated samples displayed hydrogen loss, an irreversible rearrangement of part of the hydrogen atoms (FTIR), as well as oxidation of Fe 2+ to Fe 3+ evidenced by the appearance of the spin-forbidden dd-transition band for Fe 3+ and the ligand-metal (O 2− -Fe 3+ ) transition band in the optical spectra. An increased Fe 3+ fraction was also revealed by EEL and MB spectroscopy (up to 16% Fe 3+ /∑Fe). Analyses of MB data revealed the possibility of tetrahedral Fe 3+ in the annealed ringwoodite. These results lead to a reinterpretation of the broad OH band, which is a combination of several bands, mainly [V Mg (OH) 2 ] x ), a weaker high-energy band at 3680 cm −1 ([V Si (OH) 4 ] x ) and a shoulder at 3420 cm −1 ([(Mg/Fe) Si (OH) 2 ] x ).