Variation in ferrous iron-magnesium distribution coefficients of metamorphic pyroxenes from Quairading, Western Australia

Abstract

Distribution coefficients (K _D·Fe ⁺⁺ _−Mg) calculated for orthopyroxene-clinopyroxene pairs from 12 basic granulites of the Quairading district, Western Australia, range from 1.87 for magnesian pyroxenes (Opx Mg value=78.1) to 1.70 for iron-rich varieties (Opx Mg value 37.7). Field and petrographic evidence indicates that these pyroxenes have probably reached equilibrium within a narrow temperature range. In order to account for the observed variation of K _D values it is suggested that one (or both) of the pyroxene structures is not the ideal Fe⁺⁺-Mg “solution” proposed in the thermodynamic model of the pyroxene equilibrium exchange. After consideration of the geometry of the pyroxene cation sites, the relative bond energies of each site (especially crystal field stabilization energy) and structural ordering a model is proposed to explain the non-ideal behaviour of Fe⁺⁺-Mg in the pyroxene system. The distribution pattern in low-iron pyroxenes will probably show Fe⁺⁺ favouring the M _2·Opx site; competing unfavourably with Mg⁺⁺ for the M _1·Cpxsite; and probably excluded by Mg⁺⁺ from the M _1·Opxsite. As the iron content of the system increases the M _2·Opxsite will begin to become saturated with Fe⁺⁺ and this ion will enter the M _1·Opx site. Further increases in the iron content of the system will cause the Fe⁺⁺-Mg distribution to depend increasingly on the relative attraction of the M ₁ sites of both pyroxenes. Of these sites Fe⁺⁺ should show preference to the more distorted M _1·Cpxsite. The distribution coefficient reflects this swing towards a relative enrichment of Fe⁺⁺ in the clinopyroxene by decreasing regularly with increasing iron content. It is likely that this downward trend will not become evident until the iron content of the M _2·Opx site reaches saturation. This would explain why the K _D values for the magnesian pyroxenes remain practically unchanged until the orthopyroxene Mg value is approximately 60; from here on the iron-rich pyroxenes show a rapid decrease in K _D value with increasing iron content.

The Ca content of the pyroxenes is also significant since the Quairading pyroxenes show a marked increase in mutual solubility with increasing iron content. Calcium taken into the orthopyroxene structure will enter the M ₂ site ahead of Fe⁺⁺ so that this site will reach Fe⁺⁺ saturation at a lower iron content than when the orthopyroxene is Ca-free.

The application of K _D values to the regional study of basic granulites, particularly when establishing relative temperature zones on the basis of K _D variation, should only be attempted when pyroxenes which extend over a wide range of Fe-Mg content are available.