Zircon megacrysts from kimberlite: oxygen isotope variability among mantle melts |
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Authors: | John W Valley Peter D Kinny Daniel J Schulze Michael J Spicuzza |
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Institution: | (1) Dept. of Geology and Geophysics, University of Wisconsin, 1215 W. Dayton St., Madison, WI 53706, USA; Fax: 608/262-0693; E-mail: VALLEY@GEOLOGY.WISC.EDU, US;(2) Tectonics Special Research Center, School of Applied Geology, Curtin University of Technology, GPO Box U1987, Perth 6001, Australia, AU;(3) Dept. of Geology, University of Toronto, Erindale College, Mississauga, ON LL 1C6, Canada, CA |
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Abstract: | The oxygen isotope ratios of Phanerozoic zircons from kimberlite pipes in the Kaapvaal Craton of southern Africa and the
Siberian Platform vary from 4.7 to 5.9‰ VSMOW. High precision, accurate analyses by laser reveal subtle pipe-to-pipe differences
not previously suspected. These zircons have distinctive chemical and physical characteristics identifying them as mantle-derived
megacrysts similar to zircons found associated with diamond, coesite, MARID xenoliths, Cr-diopside, K-richterite, or Mg-rich
ilmenite. Several lines of evidence indicate that these 18O values are unaltered by kimberlite magmas during eruption and represent compositions preserved since crystallization in
the mantle, including: U/Pb age, large crystal size, and the slow rate of oxygen exchange in non-metamict zircon. The average
18O of mantle zircons is 5.3‰, ∼0.1 higher and in equilibrium with values for olivine in peridotite xenoliths and oceanic basalts.
Zircon megacrysts from within 250 km of Kimberley, South Africa have average 18O=5.32±0.17 (n=28). Small, but significant, differences among other kimberlite pipes or groups of pipes may indicate isotopically distinct
reservoirs in the sub-continental lithosphere or asthenosphere, some of which are anomalous with respect to normal mantle
values of 5.3±0.3. Precambrian zircons (2.1–2.7 Ga) from Jwaneng, Botswana have the lowest values yet measured in a mantle
zircon, 18O=3.4 to 4.7‰. These zircon megacrysts originally crystallized in mafic or ultramafic rocks either through melting and metasomatism
associated with kimberlite magmatism or during metamorphism. The low 18O zircons are best explained by subduction of late Archean ocean crust that exchanged with heated seawater prior to underplating
as eclogite and to associated metasomatism of the mantle wedge. Smaller differences among other pipes and districts may result
from variable temperatures of equilibration, mafic versus ultramafic hosts, or variable underplating. The narrow range in
zircon compositions found in most pipes suggests magmatic homogenization. If this is correct, these zircons document the existence
of significant quantities of magma in the sub-continental mantle that was regionally variable in 18O and this information restricts theories about the nature of ancient subduction.
Received: 8 August 1997 / Accepted: 6 May 1998 |
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