Co2+ as a chemical analogue for Fe2+ in high-temperature experiments in basaltic systems

William E. Coons, John R. Holloway, Alexandra Navrotsky

Research output: Contribution to journalArticlepeer-review

6 Scopus citations

Abstract

High-temperature experiments on ferromagnesian compositions have been hampered by the rapid absorption of up to 95% of the original iron by platinum and 40% by silver-palladium capsules. Molybdenum or iron capsule materials can decrease or alleviate iron loss, but restrict oxygen fugacities to values near the iron-wustite buffer. Because Co2+ is stable at f{hook}O2 = HM and because the solubility of Co in platinum in this range of f{hook}O2 is ∼0.05% at temperatures to 1350°C, its use as an analogue for Fe2+ is possible. In addition, experiments simulating various Fe2+ ratios can be easily performed by choosing appropriate Co2+/Fe3+ ratios. The cobalt phases produced possess brilliant and distinctive colors which are valuable aids in optical identification of minute phases. The cobalt analogue hypothesis was tested with atmospheric pressure experiments in air on the cobalt analogue of the 1921 Kilauea basalt at three simulated Fe2+/Fe3+ ratios. The results were compared with those of R.E.T. Hill (1969) for the natural 1921 basalt. The phase relations were the same, with the cobalt system stability fields systematically shifted by about +50°C. Microprobe analysis of olivines and the coexisting glasses indicate that the distribution of Co2+ between olivine and melt is independent of temperature and liquid composition. Although the analogue liquid composition differs from the equilibrium composition of the natural system, it may be corrected be employing distribution coefficients (KD = 0.61 for the Co system; KD = 0.33 for the Fe system) to closely approximate what the natural system would yield if iron loss did not occur.

Original languageEnglish (US)
Pages (from-to)303-308
Number of pages6
JournalEarth and Planetary Science Letters
Volume30
Issue number2
DOIs
StatePublished - May 1976

ASJC Scopus subject areas

  • Geophysics
  • Geochemistry and Petrology
  • Earth and Planetary Sciences (miscellaneous)
  • Space and Planetary Science

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