ZIRCON MEGACRYSTS FROM KIMBERLITE - OXYGEN-ISOTOPE VARIABILITY AMONG MANTLE MELTS

The oxygen isotope ratios of Phanerozoic zircons from kimberlite pipes in the Kaapvaal Craton of southern Africa and the Siberian Platform v ary from 4.7 to 5.9 parts per thousand, VSMOW. High precision, accurat e analyses by laser reveal subtle pipe-to-pipe differences not previou sly suspected. These zircons have distinctive chemical and physical ch aracteristics identifying them as mantle-derived megacrysts similar to zircons found associated with diamond, coesite, MARID xenoliths, Cr-d iopside, K-richterite, or Mg-rich ilmenite. Several lines of evidence indicate that these delta(18)O values are unaltered by kimberlite magm as during eruption and represent compositions preserved since crystall ization in the mantle, including: U/Pb age, large crystal size, and th e slow rate of oxygen exchange in Ron-metamict zircon. The average del ta(18)O of mantle zircons is 5.3 parts per thousand, similar to 0.1 pa rts per thousand higher and in equilibrium with Values for olivine in peridotite xenoliths and oceanic basalts. Zircon megacrysts from withi n 250 km of Kimberley, South Africa have average delta(18)O = 5.32 +/- 0.17 parts per thousand, (n = 28). Small, but significant, difference s among other kimberlite pipes or groups of pipes may indicate isotopi cally distinct reservoirs in the sub-continental lithosphere or asthen osphere, some of which are anomalous with respect to normal mantle val ues of 5.3 +/- 0.3 parts per thousand,. Precambrian zircons (2.1-2.7 G a) from Jwaneng, Botswana have the lowest values yet measured in a man tle zircon, delta(18)O = 3.4 to 4.7 parts per thousand. These zircon m egacrysts originally crystallized in mafic or ultramafic rocks either through melting and metasomatism associated with kimberlite magmatism or during metamorphism. The low delta(18)O zircons are best explained by subduction of late Archean ocean crust that exchanged with heated s eawater prior to underplating as eclogite and to associated metasomati sm of the mantle wedge. Smaller differences among other pipes and dist ricts may result from variable temperatures of equilibration, mafic ve rsus ultramafic hosts, or variable underplating. The narrow range in z ircon compositions found in most pipes suggests magmatic homogenizatio n. If this is correct, these zircons document the existence of signifi cant quantities of magma in the sub-continental mantle that was region ally variable in delta(18)O and this information restricts theories ab out the nature of ancient subduction.