ISSN:
1432-0967
Source:
Springer Online Journal Archives 1860-2000
Topics:
Geosciences
Notes:
Abstract Ferric iron contents of coexisting ortho- and clinopyroxene from spinel lherzolite xenoliths were measured with Mössbauer spectroscopy and found to be significant. In orthopyroxene, the range in Fe3+/ΣFe is from 0.04 to 0.14; in clinopyroxene, the range is from 0.12 to 0.24. Reactions involving coexisting olivine, orthopyroxene, and clinopyroxene, where either the esseneite (CaFe3+ AlSiO6) or the acmite (NaFe3+Si2O6) component in the clinopyroxene is considered, are used to calculate oxygen fugacities. These oxygen fugacities agree well with those calculated with the olivine-orthopyroxene-spinel oxybarometer. Because these reactions do not involve garnet, spinel, or plagioclase, they may be applied to lherzolites to give internally-consistent oxygen fugacities across the pressure-dependent facies boundaries between plagioclase, spinel, and garnet lherzolite. Another application of this method is to predict the Fe3+/ΣFe in clinopyroxene coexisting with olivine and orthopyroxene given pressure, temperature, $$f_{o_2 }$$ , and the compositions of the coexisting phases in either experimental or natural assemblages. At values of $$f_{o_2 }$$ equal to those of the synthetic fayalite-magnetite-quartz buffer, for example, ∼15–35% of the iron in the clinopyroxenes from these xenoliths would be ferric. The simplifying assumption that all Fe is divalent in silicate phases at ‘geologically — reasonable’ oxygen fugacities must be re-evaluated.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1007/BF00283231
