The geochemistry of pyroxenes from the lower layered series of the Jimberlana intrusion, Western Australia

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.