High-pressure behavior of Fe-bearing silicate garnets up to 100 GPa

Abstract:

Silicate garnets are key phases not only because of their petrological importance in thermobarometry and oxybarometry, but also due to the intriguing relationship between their structure and physico-chemical properties. Natural silicate garnets can accommodate a variety of divalent and trivalent cations in their crystal structure and form many solid solutions. While most major cations in garnet occur in only a single oxidation state (Al³⁺, Ca²⁺, Mg²⁺, Si⁴⁺), iron occurs as both Fe²⁺ and Fe³⁺.

Skiagite garnet (Fe²⁺₃Fe³⁺₂Si₃O₁₂) contains iron in two oxidation states and as a component in peridotitic garnet, it can be used as a redox sensor to determine mantle ƒO₂ from Fe³⁺/∑Fe. With increasing pressure/depth the Fe³⁺/∑Fe ratio increases due to the higher solubility of Fe³⁺ in garnet, hence expanding the stability field of skiagite. At greater depth skiagite garnet is expected to accommodate an excess of Si, forming a solid solution with the iron majorite endmember (Fe₄Si₄O₁₂). Studying the high-pressure behavior and redox relations of Fe-bearing silicate garnet can therefore provide important insight into the chemical composition and physical properties of the Earth’s mantle.

Single-crystals of skiagite-majorite solid solution were synthesized at 9.5 GPa and 1100 °C in a multianvil apparatus and found to have the composition Fe²⁺₃(Fe²⁺_0.234(2)Fe³⁺_1.53(1)Si_0.234(2))Si₃O_12. We performed a room temperature investigation of the equation of state up to 100 GPa using synchrotron single crystal XRD at ESRF, Grenoble. Fitting the data to a 3^rd order BM EoS gives K₀ = 166(3) GPa with K`=3.8(1) and V₀ = 1606.9(8) ų. In the pressure range between 50 and 60 GPa the overall unit cell volume drops by about 4 % and we observe a collapse of about 7 % in FeO₆ octahedral volume. Synchrotron Mössbauer source spectra at ambient pressure show two doublets assigned to dodecahedral Fe²⁺ and octahedral Fe³⁺ that remain essentially unchanged up to 50 GPa. However at ~52 GPa the hyperfine parameters of Fe³⁺ change due to an inferred spin-pairing transition. The presentation will discuss the implications of these observations for Fe-bearing garnets in the upper mantle and transition zone.