Petrologic and geochemical investigation of carbonates in peridotite xenoliths from northeastern Tanzania

C. T. Lee, R. L. Rudnick, W. F. McDonough, I. Horn

Research output: Contribution to journalArticlepeer-review

78 Citations (Scopus)

Abstract

Primary carbonates in peridotite xenoliths from the East African Rift in northeastern Tanzania occur as intergranular patches with accessory minerals (olivine and spinel), as patches with accessory magmatic minerals (nepheline), and as round monomineralic inclusions in primary olivine grains. All are characterized by calcitic compositions (Ca/Ca + Mg + Fe from 0.83 to 0.99), extremely low SiO2 + Al2O3 + Na2O + K2O, low trace element abundance [total rare-earth element (REE) abundance <25 ppm], uniform extinction, and lack of reaction textures with the host xenolith. Calculated Fe-Mg exchange coefficients between carbonate and primary olivine indicate disequilibrium in most samples. Combined with the lack of significant reaction textures, this suggests that the carbonates were introduced shortly before or during eruption of the host magma. A global compilation of electron microprobe analyses of mantle-derived carbonates (in xenoliths, xenocrysts, and megacrysts) reveals compositional clusters near end member calcite, end member magnesite, and stoichiometric dolomite. Eutectic liquid compositions are less common, suggesting that many carbonate inclusions reported worldwide may be crystalline precipitates. Likewise, the calcites in this study are not interpreted to represent quenched carbonatitic melts, but are interpreted instead to be crystalline cumulates from such melts. These inferences are consistent with recent experiments, which show that carbonatitic melts cannot become more calcitic than CaCO3~80 wt%. Low trace element abundance may be a diagnostic feature of cumulate carbonate, and in combination with petrography and major element composition, serve to distinguish it from quenched carbonated liquid.

Original languageEnglish
Pages (from-to)470-484
Number of pages15
JournalContributions to Mineralogy and Petrology
Volume139
Issue number4
DOIs
Publication statusPublished - 2000 Jan 1
Externally publishedYes

ASJC Scopus subject areas

  • Geophysics
  • Geochemistry and Petrology

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