Compaction of Igneous Cumulates Part I: Geochemical Consequences For Cumulates and Liquid Fractionation Trends

Abstract

The importance of crystallization during compaction of an 800 m section of cumulates in Olivine‐bearing zones III and IV of the Stillwater complex is assessed using whole‐rock and mineral compositions. Two sets of calculations provide estimates for the liquid fraction that crystallized in each sample. The first relies upon the effect that crystallized liquid has on the whole‐rock Mg#, and the second is based on the proportion of compatible and incompatible trace elements contributed by the crystallized liquid. Estimates based on trace‐elements vary with their bulk distribution coefficients (D) such that elements with higher Ds yield higher estimates of crystallized liquid. Comparison of estimates based on Y (average 9%) with those based on V (average = 45%) and on the whole‐rock Mg# (average 15%) show all three to be substantially different. The disparity in these estimates is interpreted to be a consequence of crystallization during compaction enriching the interstitial liquid in incompatible elements that were then expelled from the crystal pile prior to incorporation into late crystallizing minerals. Knowledge of the fate of liquid included in a growing crystal pile is of fundamental importance not only to interpreting the composition of cumulates but also to understanding how liquid compositions evolve by fractional crystallization. The return of evolved liquids from the crystal pile can decouple fractionation trends of major and trace elements producing apparently anomalous enrichment in the more incompatible trace elements.