A new tetragonal silicate mineral occurring as inclusions in lower- mantle diamonds

Jeff Harris, Mark T. Hutchison, Mike Hursthouse, Mark Light, Ben Harte*

*Corresponding author for this work

Research output: Contribution to journalArticle

70 Citations (Scopus)

Abstract

Aluminium is one of a small group of elements - Si, Mg, Fe, Al, Ca and O-that form the bulk of the Earth's mantle. In the upper mantle, Al is largely contained in minerals with a garnet structure. But the mineral transformations associated with the breakdown of garnet structures in the uppermost lower mantle have been a matter of uncertainty. Here we report the discovery of a new aluminous mineral, present as inclusions in diamonds of lower-mantle origin found at Sao Luiz, Brazil. This phase has a stoichiometric garnet composition similar to pyrope-almandine but has a distinct tetragonal structure (space group I4̄2d), and is here referred to by the acronym TAPP (tetragonal almandine-pyrope phase). Although TAPP has not been recognized in experimental studies, we suggest that it is nevertheless a primary rather than retrogressive phase and has a limited stability field in relatively aluminous bulk compositions in the uppermost lower mantle. TAPP, like garnet, would lead to relatively low densities for basic rocks compared with peridotites (assumed to be the dominant mantle rocks), and we suggest therefore that it may play an important role in determining density differences and the dynamics of segregation between ultrabasic and basic compositions in the mantle.

Original languageEnglish
Pages (from-to)486-488
Number of pages3
JournalNature
Volume387
Issue number6632
DOIs
Publication statusPublished - 29 May 1997
Externally publishedYes

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