ALBERT

Notes: Abstract Most of the Al3+ entering the pyroxenes does so by substituting for tetrahedral Si4+. This creates a charge imbalance that requires the simultaneous entry of Cr3+, Ti4+, Fe3+ or Al3+ into octahedral sites. Cr3+, because of its high crystal field stabilisation energy (CFSE), is the most important of these elements to enter the early-formed pyrosenes but it is replaced by Ti4+ later in fractionation when the Cr3+ content of the melt becomes depleted. The dependence of Cr3+ and Ti4+ on charge balance controls their partition between coexisting pyroxenes and olivines. Ca-rich pyroxene which contains more Al3+ than Ca-poor pyroxene also has more Ti4+ and Cr3+ whereas olivine, which contains negligible Al3+, has low Cr3+ and Ti4+. The Al3+ content of pyroxenes is influenced by changes in P, T, $$a_{{\text{SiO}}_{\text{2}} }$$ and $$a_{{\text{Al}}_{\text{2}} {\text{O}}_{\text{3}} }$$ of the magma and by the nature of the ion providing charge balance in the octahedral site. Of these $$a_{{\text{SiO}}_{\text{2}} }$$ is dominant and variations in the Al3+ content of the Jimberlana pyroxenes correspond closely with the expected changes in the $$a_{{\text{SiO}}_{\text{2}} }$$ of the melt. The substitution of divalent ions, such as Mn2+ and Ni2+, in the pyroxene lattice is by replacement of Fe2+ or Mg2+ in the octahedral M 3 and M 2 sites and is therefore independent of charge balance. If there are no size restrictions, the principal factor to be considered is the CFSE the ion receives in octahedral co-ordination. Ni2+, which receives a high CFSE, partitions strongly between the early-formed pyroxenes and olivines and therefore becomes depleted in the magma with fractionation. Conversely Mn2+, which receives zero CFSE, concentrates in the magma with fractionation and becomes a more important substitute in the later-formed pyroxenes. Its geochemical behaviour is controlled by its size. The narrow miscibility gap of the Jimberlana pyroxenes and the high En content of the Ca-poor pyroxenes at the bronzite pigeonite changeover suggest that these pyroxenes crystallised at a higher temperature than pyroxenes of comparable composition from other intrusions.