Abstract
Majoritic garnet, characterized by an excess of silicon (>3 Si per formula unit), is considered one of the major phases of the Earthās transition zone from 410ā660 km depth. Quantifying the H2O content of nominally anhydrous mantle minerals is necessary to evaluate their water storage capacity from experiments and modeling the Earthās deep water cycle. We present mineral-specific infrared absorption coefficients for the purpose of quantifying the amount of water incorporated into majorite as hydroxyl point defects. A suite of majoritic garnet samples with varying proportions of Si, Fe, Al, Cr, and H2O was synthesized at conditions of 18ā19 GPa and 1500ā1800 Ā°C. Single-crystals were characterized using X-ray diffraction, electron microprobe analysis, secondary ion mass spectrometry (SIMS), IR, Raman, and Mossbauer spectroscopy. We utilize SIMS and Raman spectroscopy in combination with IR spectroscopy to provide IR absorption coefficients for water in majoritic garnets with the general mineral formula (Mg,Fe)3(Si,Mg,Fe,Al,Cr)2[SiO4]3. The IR absorption coefficient for majoritic garnet in the OH stretching region is frequency dependent and ranges from 10 470 Ā± 3100 Lmol ā1 cm ā2 to 23 400 Ā± 2300 Lmol ā1 cm ā2
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