CALCULATING MELT COMPOSITIONS FROM MINERAL TRACE ELEMENT COMPOSITIONS: A CASE STUDY FROM THE KENTALLEN CLINOPYROXENE-OLIVINE MONZONITE

Partition coefficients between coexisting minerals and melt in a system control the trace element distribution between minerals in the resulting rock. In rocks where fractional crystallization or accumulation occurred, the melt composition may not reflect the bulk rock composition. The relationship between a mineral composition and its partition coefficients may be used to calculate the composition of the melt in which the mineral crystallized and help identify divergence between melt and bulk-rock composition and identify processes active during magma crystallization.

A series of appinites surround the Ballachulish Igneous Complex, Scotland. Field relationships show that these rocks predate intrusion of the Ballachulish Igneous Complex, but intruded after regional metamorphism of the host metasediments. A suite of olivine monzonites, Kentallenites contain the assemblage olivine + clinopyroxene + biotite + k-feldspar + plagioclase + oxides with accessory apatite. These rocks are thought to have crystallized from a melt composition similar to that of the entire appinite suite via fractional crystallization, accumulation, or a combination of the two. This study utilizes mineral compositions to calculate the melt that minerals crystallized from and the conditions in which the minerals in the crystallized.

The sequence of crystallization is olivine, clinopyroxene, apatite, biotite, and lastly feldspars. Crystallization temperatures for clinopyroxene phenocrysts put the minimum temperature and pressure of the liquidus at ~1285°C and ~14.5 kbar. Calculated melt compositions suggest the melt that crystallized clinopyroxene was more enriched in LREEs than the bulk rock and apatite crystallized in a melt that was more enriched in all REEs relative to the bulk rock composition. The calculated melts, coupled with mineral compositions and partition coefficients, indicate the clinopyroxenes and apatites are not in equilibrium with the bulk rock composition. This suggests the minerals that crystallized were fractionated from the system in a melt that was more enriched in the REEs than the currently observed bulk rock composition. The similarity of the calculated melts for apatite and clinopyroxene suggest these minerals crystallized at a similar time in the rock’s petrogenetic history.