Metasomatic coronas around hornblendite xenoliths in granulite facies marble, Ivrea zone, N Italy. II: Oxygen isotope patterns

Up to 20-cm-wide metasomatic reaction bands formed coronas around hornblend ite xenoliths in a marble matrix during high grade metamorphism in the Ivre a zone. The coronas are comprised of an innermost monomineralic clinopyroxe ne layer, a garnet-clinopyroxene layer and an outermost scapolite-clinopyro xene layer. The oxygen isotope composition of the original hornblendite cor e is 7%,, relative to V-SMOW and the oxygen isotope composition of the marb le matrix is 19.7 parts per thousand. The oxygen isotope transition across the corona is represented by a diffusion front with a step discontinuity at the inner margin of the corona. The systematics of the inter-mineral fract ionations indicates preservation of the oxygen isotope compositions from hi gh temperatures and maintenance of grain-scale oxygen isotope equilibrium d uring corona formation. The oxygen isotope pattern is interpreted in terms of a moving boundary diffusion problem. The growing reaction band and the r eactant hornblendite and marble represent a total of five media with differ ent transport properties and moving separation surfaces. Bulk oxygen diffus ion was at least three orders of magnitude faster then expected from volume diffusion, suggesting that transport was enhanced by relatively fast diffu sion along grain boundaries. Oxygen diffusivities in the individual layers correlate with the oxygen volume diffusivities in the major constituent min erals of the respective layers, suggesting mineralogical control on bulk ox ygen diffusion.