OLIVINE XENOCRYSTS IN PICRITIC MAGMAS - AN EXPERIMENTAL AND MICROSTRUCTURAL STUDY

Experimental partial melting of plagioclase lherzolite at 0.7 MPa confining pressure has produced euhedral olivine crystals by corrosion and overgrowth during cooling. The particular conditions of this experiment allow observation of both processes recorded in the crystal shape: corrosion boundaries are rounded, or straight when parallel to [001] intersection of {110} planes; (010) and {110} facets are developed by fast overgrowth during quenching. These observations support the contention that phenocrysts in basaltic or picritic magmas are, in part, xenocrysts. The possible mantle origin of olivine crystals in two natural occurrences of ultramafic magmas; the picritic pillow lavas of the Troodos, and a wehrlitic intrusion of the oman ophiolite, is investigated. In both cases discriminant characteristics are deduced from detailed microstructural study. The mantle origin of olivine megacrysts in the investigated picrites raises the question about the existence of picritic magmas in the mantle.