Phosphorus Zoning in Cumulus Olivine: A History from the Start to the End of Solidification

Abstract

This paper presents data on the distribution of phosphorus and other elements (Ti, Al, Cr, Ni, Ca, Fe, Mg) in olivine crystals in meso- and adcumulates from the Yoko-Dovyren and Monchegorsk layered intrusions, including dunites, troctolites, anorthosites, and olivine-bearing chromitites. Olivine was found to contain skeletal and oscillatory cores, which could be formed (1) in the upper boundary layer of the magma chamber, (2) due to local mixing of magmas with different temperatures during magma emplacement process, and (3) at the boundary between the cumulate pile and the main magma volume of the chamber. Specific boundaries between olivine grains cutting the compositional zoning of one of the contacting crystals were observed. They were interpreted as indicators of pressure solution, which is one of mechanisms of compaction of original cumulate pile. Olivine grains from dunites of the Yoko-Dovyren massif and Mt. Travyanaya of the Monchegorsk massif, which demonstrate a lognormal crystal size distribution (CSD), often display signatures of resorption and subsequent overgrowth of small olivine crystals. The examination of phosphorus zoning of olivine confirmed an experimentally based conclusion that the lognormal CSD could be produced due to more extensive dissolution of smallest grains. In the case of the Yoko-Dovyren massif, this dissolution was attributed to the reaction of nonequilibrium intercumulus melt with the original cumulate crystals, whereas the overgrowth of the olivine grains occurred during further cooling. In the case of Mt. Travyanaya (Monchepluton), the dissolution was probably related to the peritectic reaction of olivine with melt producing orthopyroxene, and the additional growth of the olivine rim was caused by the inverse reaction caused by the diminishing the pyroxene stability field during magma ascent and decompression. In all types of the examined cumulates, except for chromitites, widespread relicts of intercumulus pores filled with phosphorus-rich olivine were discovered. These pores were suggested to form near the lower solidification front, when the compaction of the evolving cumulate pile was stopped. The enrichment of the pore olivine in phosphorus may indicate an increase in the degree of supersaturation/supercooling of the porous melt with respect to olivine accompanied by an increase in the olivine growth rate at the late stages of solidification.