Comparative Trace Element Geochemistry of Peridotite Xenoliths from Siberian and Kaapvaal Craton Roots

N. Shimizu Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA, 02543, USA

N. P. Pokhilenko United Institute of Geology and Geophysics, Novosibirsk, Russia

F. R. Boyd Geophysical Laboratory, Carnegie Institute of Washington, Washington, D.C., USA

D. G. Pearson University of Durham, Durham, UK

Peridotite xenoliths in diamondiferous kimberlites are a unique window through which geocemical characteristics and evolution of the cratonic root lithosphere can be studied in detail. In contrast to the ancient Re depletion model ages reported on both Siberian and Kaapvaal craton peridotites, strong evidence for young metasomatic changes has also been reported. How and to what extent were bulk chemical compositions modified by metasomatism? This study documents metasomatic changes based on ion probetrace element data obtained fro clinopyroxene and garnet in peridotite xenoliths from Siberia (mainly from the Udachnaya pipe) and Kaapvaal (various pipes in South Africa and Lesotho), and attempts to answer the question. Trace element abundances and abundance patterns are similar in garnet and clinopyroxene in high-temperature ( >1150=B0C) red garnet-bearing peridotites between the two suites. Garnets are commonly zoned with Ti and Zr increasing rimward by a factor of 3 or more and HREE and Y by up to 50%. Pink garnets in low-temperature peridotites are also generally similar with sinusoidal REE patterns. The Siberian suite appears to be characterized by cryptic metasomatism (i.e., phlogopite and amphiboles are relatively rare), and the existence of garnet harzburgites (± cpx) with extremely depleted trace element abundances. These garnets show REE abundances as low as [La]n ~0.01, [Yb]n ~2 with some displaying indications of possible orthopyroxene precursors. Abundant evidence exists for reactions between mantle minerals and melts, including chromatographic fractionation of REE in both clinopyroxene and garnet. It is suggested that interaction between depleted (probably garnet and cpx free) harzburgites and melts (both carbonate and silicate) added new cpx and garnet, and produced the entire array of trace element variations in pink garnet-bearing peridotites. Some garnet cores of the high-temperature peridotites have similar trace element patterns with those in depleted garnet harzburgites, suggesting that they may also be metasomatic. The Kaapvaal suite represents more advanced stages of metasomatism in general and contains phlogopite, amphiboles and rare titanates. Trace element abundance patterns of clinopyroxene and garnet are also consistent with this notion. The observations suggest that bulk compositions of peridotites were modified for both trace and major elements, and caution against casual use of peridotite bulk chemistry in evaluating Archean processes.