Crystal chemistry of hydration in aluminous orthopyroxene

Abstract

Hydrogen incorporation in aluminous orthopyroxene may control the generation of melt and dominate the seismic properties at the base of the Earth.s lithosphere. To clarify the substitution mechanism of H, we have synthesized, characterized, and refined the crystal structure of this potentially significant variant of orthopyroxene. The experimentally produced crystals are small needles up to approximately 20 × 20 × 100 μm in size. Electron microprobe chemical analysis indicates about 11.7 wt% Al 2 O 3 . FTIR spectra indicate 7500 ppmw H 2 O with absorbance features qualitatively similar to natural mantle orthopyroxenes. TEM imagery indicates that the phase is pure orthopyroxene with low concentrations of defects and inclusions. Cell-parameter refinement from single-crystal X-ray diffraction gives a = 18.1876(7) Å; b = 8.7352(7) Å; c = 5.1789 (5) Å, V = 822.79(11) Å 3 , which is 1.2% smaller than pure Mg anhydrous orthoenstatite. The crystal structure has been refined from single-crystal X-ray intensity data measured using a rotating anode X-ray generator, micro-focused X-ray beam, and CCD detector system. The refined structure indicates about 5% vacancy in M2 and significant Al occupancy in both M1 and T2, consistent with its composition, (Mg 0.95,0.05 ) M2 , (Mg 0.79 Al 0.21 ) M1 , (Al 0.25 Si 0.75 ) T2 Si T1 O 6 . The existence of hydrous orthopyroxene in the mantle could absorb water released from olivine on decompression to delay the onset of melting in the spinel stability region in mantle peridotite compositions.