Implications to garnet-clinopyroxene geothermometry of non-ideal solid solution in jadeitic pyroxenes

Garnet-clinopyroxene geothermometry has been applied to numerous rock types from the blueschist and eclogite terrain of the Sesia zone in the Western Alps. Discrepancies in apparent temperatures of up to 600° C have been found suggesting substantial deviation from ideal behaviour of the garnet-clinopyroxene equilibria in certain assemblages. Assemblages containing jadeitic pyroxenes rather than omphacitic or diopsidic pyroxenes yield very low K _D (2.0) values and correspondingly high apparent temperatures (> 1,000° C). The garnets coexisting with high-Na pyroxenes have compositions similar to those coexisting with omphacitic pyroxenes. Departure from ideal behaviour of these garnets is likely to be minor and is accomodated in the formulation of the geothermometer. Numerous examples of coexisting garnet-clinopyroxene pairs indicate that there is no obvious relationship of K _D with jadeite content in pyroxenes with jadeite content less than 70%, but at higher values, K _D varies inversely with the jadeite content. The dependence of K _D upon the jadeite content in the high-Na pyroxenes is believed to be due to the preference of M2 sites for Fe²⁺ in the enstatite-jadeite substitution (Fe²⁺MgNa_–1Al_–1). This substitution is usually very limited and has no demonstrable effect upon X _Fe ^Cpx when the M1 (Fe²⁺+Mg) content of the pyroxenes is high as in omphacitic and diopsidic pyroxenes. However, when the M1 (Fe²⁺+Mg) content is low, as in jadeitic pyroxenes, the non-ideality of the enstatite type exchange has a marked effect on X _Fe ^Cpx and consequently on garnet-clinopyroxene calculations.