METASOMATIC ECLOGITIC DIAMOND GROWTH - EVIDENCE FROM MULTIPLE DIAMONDINCLUSIONS

Diamond formation from metasomatic fluids, rather than from igneous me lts, remains controversial but is paramount to our understanding of di amonds' mantle origin(s). Physical and chemical properties of diamonds , their inclusions, and host eclogites from the Mir kimberlite, Yakuti a, Russia form the basis for our evaluation of diamond origin. Mir ecl ogitic diamonds and their multiple inclusions show a definite break in time and temperature between the formation of the core zones and the rims of the diamonds. Extreme changes in chemistry for multiple diamon d inclusions (DIs) between the cores and the rims cannot be accounted for by magmatic fractional crystallization. Evidence also exists for l arge temperature decreases (40 degrees to 140 degrees C) from the core s to the rims of some diamonds. The distinct changes in nitrogen conte nts and aggregation states from cores to rims of diamonds would appear to reflect different residence times for these portions of the diamon ds in the mantle-i.e., formation of cores and rims at vastly different times (e.g., 2 Gp). Many of the mineral-chemical characteristics, inc luding C and; N isotopes and N aggregation states of the diamond, can best be explained by crystallization of the diamonds after formation o f the eclogite host. This suggests that the formation of the eclogite and the nucleation and growth of some diamonds are not coeval and poss ibly not cogenetic. Most diamondiferous eclogite xenoliths probably ha ve never experienced a major magmatic epi sode (i.e., complete melt st age) after subduction of their crustal protoliths into the mantle. Car bon isotopes in diamond, sulfur isotopes from sulfide DIs, and oxygen isotopes from eclogite minerals all point to crustal pl otoliths for m any eclogites. All of the factors above, taken as a whole, indicate th at many eclogitic diamonds are the result of petrogenesis by metasomat ism over a prolonged period of lime. Introduction of metasomatic fluid s facilitates the precipitation of the diamonds, either in tote or as rims on previously formed diamonds. Inasmuch as some eclogites are con sidered to be igneous in origin-e.g., Group-A eclogites of Taylor and Neal (1989)-it is entirely possible that these eclogites may contain t ruly igneous diamonds. However, even some of these diamonds may have l ater metasomatic overgrowths.