Ferric iron contents of clinopyroxene from cratonic mantle and partitioning behaviour with garnet

摘要

Together with garnet and spinel, clinopyroxene is known to be an important carrier of Fe~(3+) in mantle-derived peridotites. The systematics of Fe~(3+) incorporation in clinopyroxene and the Fe~(3+)-partitioning behaviour between clinopyroxene and garnet have been investigated from a large suite of garnet peridotite xenoliths (>50) mostly originating from the upper mantle beneath the Kaapvaal craton (Southern Africa). A correlation between Na and Fe~(3+) is apparent that is not observed for clinopyroxene from spinel peridotites. This indicates a change in how Fe~(3+) substitutes into clinopyroxene when going from the spinel to the garnet peridotite facies in the upper mantle. A general decrease in Fe~(3+)/∑Fe with increasing temperature is also observable, however, when considered in terms of Fe~(3+) cations per formula unit this negative correlation with temperature disappears. This partial decoupling Fe~(3+)/∑Fe values from the actual Fe~(3+) contents can be understood in terms of two factors: 1) Fe~(3+) incorporation is controlled by a number of parameters, including fO2, the degree of depletion of a given sample, as well as the composition of the clinopyroxene itself, and 2) Fe~(3+)/∑Fe in clinopyroxene can also be directly influenced by changes in Fe~(2+) content, which is known to be temperature dependent in the presence of opx and garnet The partitioning of Fe~(3+) between clinopyroxene and garnet can be considered in terms of two different Fe~(3+)-Al exchange equilibria involving Na-bearing components in clinopyroxene and either Ca-bearing or Fe~(2+)-bearing components in the coexisting garnet. Results from the natural samples suggest a temperature dependence, with Fe~(3+) being progressively partitioned into garnet with increasing temperature. However, the magnitude of this effect remains open due to uncertainties in available thermodynamic data. Analysis of the two exchange equilibria point to an error in the tabulated standard enthalpy of formation of NaFe~(3+)Si2O6 aegerine on the order of 25 kj/mol.