Melting and phase relations of carbonated eclogite at 9-21gpa and the petrogenesis of alkali-rich melts in the deep mantle

Ekaterina S. Kiseeva, Konstantin D. Litasov, Gregory M. Yaxley, Eiji Ohtani, Vadim S. Kamenetsky

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70 Citations (Scopus)

Abstract

The melting and phase relations of carbonated MORB eclogite have been investigated using the multi-anvil technique at 921GPa and 1100-19008C. The starting compositions were two synthetic mixes, GA1 and Volga, with the CO2 component added as CaCO3 (cc): GA1+10%cc (GA1cc) models altered oceanic crust recycled into the convecting mantle via subduction, and Volga+10%cc (Volgacc) models subducted oceanic crust that has lost some of its siliceous component in the sub-arc regime (GA1 minus 6•5 wt % SiO2).The subsolidus mineral assemblage at 9 and 13 GPa includes garnet, clinopyroxene, magnesite, aragonite, a high-pressure polymorph of TiO2 (only at 9 GPa) and stishovite (only at 13 GPa). At 17-21GPa clinopyroxene is no longer stable; the mineral assemblage consists predominantly of garnet with subordinate magnesite (only at 17 GPa), Na-rich aragonite, stishovite, Ca-perovskite (mostly at 21GPa), and K-hollandite (mostly at 17 GPa). Na-carbonate with an inferred composition (Na, K)2(Ca, Mg, Fe)(CO3)2 was present inVolga-cc at 21GPa and 12008C. Diamond (or graphite) crystallized in most runs in the GA1cc composition, but it was absent in experiments with the Volga-cc composition. In Volga-cc, the solidus temperatures are nearly constant between 1200 and 13008C over the entire pressure range investigated. In GA1cc, the solidus is located at similar temperatures at 9-13 GPa, but at higher temperatures of 1300-15008C at 17-21GPa. The difference in solidi between the GA1cc and Volga-cc compositions can be explained by a change in Na compatibility between 13 and 17 GPa as omphacitic clinopyroxene disappears, resulting in the formation of Na-carbonate or Na-rich melt in Volga-cc. The solidus temperature in GA1cc also increases with increasing pressure as a consequence of carbonate reduction and diamond precipitation, possibly brought on either via progressive Fe2+-Fe3+ transition in garnet at higher pressures or by a decrease of the activity of the diopside component in clinopyroxene. The lowdegree melts are highly alkalic (K-rich at 9-13 GPa and Na-rich at 17-21GPa) carbonatites, changing towards SiO2-rich melts with increasing temperature at constant pressure. The solidi of both compositions remain higher than typical subduction pressure-temperature (P-T) profiles at 5-10 GPa; however, at higher pressures the flat solidus curve of carbonated eclogite may intersect the subduction P-T profile in theTransition Zone, where carbonated eclogite can produce alkali- and carbonate-rich melts. Such subduction-related alkali-rich melts can be potential analogues of kimberlite and carbonatite melt compositions and important agents of mantle metasomatism and diamond formation in theTransition Zone and in cratonic roots. Melting of carbonated eclogite produces a garnet-bearing refractory residue, which could be stored in the Transition Zone or lower mantle

Original languageEnglish
Pages (from-to)1555-1583
Number of pages29
JournalJournal of Petrology
Volume54
Issue number8
DOIs
Publication statusPublished - 2013 Aug

Keywords

  • Carbonate metasomatism
  • Diamonds
  • High-pressure experiments
  • Kimberlite formation
  • MORB eclogite
  • Mantle
  • Transition Zone

ASJC Scopus subject areas

  • Geophysics
  • Geochemistry and Petrology

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