Australia: The Land Where Time Began

A biography of the Australian continent 

Lower-Mantle Water Reservoir Implied by the Extreme Stability of a Hydrous Aluminosilicate

It is often inferred that the source rocks of the basaltic lavas that form ocean islands have arisen as part of a thermal plume from the lower mantle, and compared with average rocks from the upper mantle these rocks are rich in water. It has been indicated by experiments, however, that the water solubility of the dominant lower mantle phases is very low, which has prompted suggestions that plumes may be sourced from reservoirs of water-rich primordial material in the deep mantle that have not yet been identified. In this paper Pamato et al. present the results of their high-pressure experiments which show that Al₂SiO₄ (OH)₂, the aluminium-rich endmember of dense, hydrous magnesium silicate phase D, is stable at temperatures that extend to more than 2,000^oC at 26 GPa. It was found that Al-rich phase-D is stable within mafic rocks under these conditions, implying that a significant long-term water reservoir in the convecting lower mantle could be oceanic crust that has been subducted. It is suggested here that melts which form in the lower mantle as a result of dehydration of hydrous minerals present in dense ultramafic rocks will migrate into mafic lithologies and crystallise to form Al-rich phase D. Water will be locally distributed into minerals that are nominally anhydrous when mantle rocks upwell. A potential source for ocean island basalts that does not require reservoirs of water-rich primordial material in the deep mantle is provided by this material that is upwelling.

Sources & Further reading

1. Pamato, M. G., R. Myhill, T. Boffa Ballaran, D. J. Frost, F. Heidelbach and N. Miyajima (2015). "Lower-mantle water reservoir implied by the extreme stability of a hydrous aluminosilicate." Nature Geosci 8(1): 75-79.