MODELING OF VARIATIONS ACCOMPANYING MINERAL REACTIONS IN PELITIC XENOLITHS AT THE VOISEY'S BAY NI-CU-CO DEPOSIT, LABRADOR, CANADA
We have modeled the closed system destruction of garnet via a reaction that produces cordierite, orthopyroxene and K-feldspar:
K2Fe6Al2Si6O20(OH)4 + 2Fe3Al2Si3O12 + 9SiO2=Fe2Al4Si5O18(2H2O) + 10FeSiO3 + 2KAlSi3O8
Computed d18O values (‰) for a 11.3‰ protolith are: cordierite - 11.5, orthopyroxene - 10.3, and K-feldspar - 12.7.
Progressive dehydration of cordierite results in expulsion of 18O-enriched water and the production of hercynite and quartz (Fe2Al4Si5O18(2H2O)=2FeAl2O4 + 5SiO2 + 2H2O), with final values of 8.7 and 16.3‰, respectively. Xenoliths at Voisey’s Bay are quartz deficient, and characterized by bulk d18O values near 7‰. Final open system interaction between xenoliths and magma resulted in the loss of a siliceous, 18O-enriched partial melt, and the transfer of Fe2+ from the magma to yield hercynite that is pseudomorphic after corundum.