Relationship between Al-bearlng phases NAL and CF in the lower mantle

K. P. Litasov, Eiji Otani

Research output: Contribution to journalArticlepeer-review

7 Citations (Scopus)

Abstract

Alumina-bearing prhries with orthorhombic (phase CF) and hexagonal (phase NAL) structures have been identified in average MORB composition at pressures above 25 GPa. These phases are very similar in structure and composition and can coexist in a single experimental sample. In this paper, we report data on the chemical composition, structure, and stability fields of the CF and NAL phases and study their relationship in the lower mantle. It is shown that these phases are stable at a pressure of 20 GPa and temperatures below 1300°C as well as at a pressure of 25 GPa and temperatures below the solidus temperature. We have established that the stability of NAL and CF phases is determined mainly by the chemical composition of the system and, especially, the proportion of Na 2O and K2O. Phase NAL appears in the MORE system at a pressure of 24 - 25 GPa and contains (wt.%) Al2O3 (55-57), SiO2 (10 - 13), Na2O (5-6), and K2O (0.5-1.5). Phase CF appears at 26 GPa and completely replaces the NAL phase at 28-30 GPa. At 26-27 GPa, these phases have almost a similar composition (32-38 wt.% Al 2,O3 and 25-28 wt.% SiO2). However, the NAL phase contains less Na2O (6-7 wt.%) and more K2O (up to 1.5 wt.%) than the CF phase, which is free of K2O. We suggest that the CF phase is more stable in the lower mantle than the NAL phase. The stability field of the NAL phase can expand to higher pressures as a result of local K2O-enrichment of the mantle.

Original languageEnglish
Pages (from-to)1313-1325
Number of pages13
JournalGeologiya i Geofizika
Volume45
Issue number11
Publication statusPublished - 2004 Dec 1

Keywords

  • Al-bearing phases
  • Lower mantle
  • Oceanic crust

ASJC Scopus subject areas

  • Earth and Planetary Sciences (miscellaneous)

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