Hydrogen Incorporation in Bridgmanite and Water in Earth's Lower Mantle

Abstract:

We have synthesized aluminum-bearing MgSiO₃ bridgmanite samples at 25GPa and various temperatures from 1600 to 2200ºC in a multi-anvil press and have characterized the samples by single-crystal X-ray diffraction, Raman spectroscopy, electron microprobe (EPMA), transmission electron microscopy (TEM), and secondary ion mass spectroscopy (SIMS). Bridgmanite crystals range up to 100 micrometers in size and contain 1 – 2.5% by weight Al₂O₃. SIMS analysis indicates up to several thousand ppm by weight H₂O within probe spot areas of approximately 10 x 10 mm, which corresponds to one H per Al atom in the structure. Microprobe chemical analysis and crystal structure refinement from single crystal X-ray data indicates that Al substitutes in the Si site, but not in the Mg site. However, Raman spectroscopy indicates the presence of micro-inclusions of brucite, super-hydrous phase B (Shy-B), possible magnesite, and stishovite in most of the runs. Some of the bridgmanite crystals in the samples synthesized at temperatures above 2100º C exhibit rounded inclusions that are interpreted to be melt that quenched to bridgmanite plus brucite, phase D, and/or stishovite.

These results indicate that hydrogen may have a small but significant solubility in at least the upper part of Earth’s lower mantle related to the presence of Al in bridgmanite. The solubility of H in the lower mantle is thus likely to be much lower than in the phases of the transition zone at depths less than 660 km. This raises the possibility that significant amounts of H may be trapped and stored in the transition zone.