tt19 The Canadian M ine ralo g ist Vol. 34, pp. I | 79-1 193 (1996) TRACE.ELEMENT ZONING IN MANTLE MINERALS: METASOMATISM AND THERMAL EVENTS IN THEUPPER MANTLE WILLIAM L. GRIFFINI CSIRO Exploration and Mining, P.O. Box 136, North Ryde, New South Wales 21 Li, Australia, and Key Centre for Geochemical Evolution and Metallogeny of Continents (GEMOC,, School of Earth Sciences, Macquarie University, Syd.ney 2109, Australia DOUGLASSMITH Department of Geological Sciences, Universiry ofTexas, Austin, Texas 78713, U.S.A. CHRIS G. RYAN CSIRO Exploration and Mining, P.O. BoxI j6, North Ryde, New South Wales 2113, Awtralia SUZANNEY. O'REILLY Key Centre for Geochemical Evolution and Metallogeny of Continents (GEMOC), School of Eanh Sciences, Macquarie University, Sydney 2109, Awtralia TIN TINWIN CSIRO Exploration and Mining, P.O. Box136, Nonh Ryde, New SouthWales 2113, Awtralin Arsrnqc"r Studies of trace-element zoning in mantle-derived rninerals can provide important constraints onthe nature and time scales ofmetasomatic processes and thermal events in theupper mantle, and on diffusion rates. Zoning data ongarnet in peridotite xenoliths restrict some metasomatic processes to geologically short time-spans (10-1ff years) priorto the eruption of the host magmas, and suggest thatthemetasomatism is genetically connected to themagmatic events or theirimmediate prccursors. Such metasomatism effectsmajor changes in both the major-element and trace-element composition of the rocks, and these xenoliths should not be used to represent large volumes in the mantle. Modeling of heating and cooling rates, basedon Ni zoning in game! suggests that the heatingobserved in high-T xenoliths is a transient phenomenon associated with local intrusion of magrna bodies.The relative lengthsef 2ening profiles for various elements in single crystalsof gamet suggest that diffirsion coefficients, over the temperatffe range 1200-1400oC, decrease in the order Ni ) Fe, Mn > Ti, Zt > Y (andheavy^REE") = Ca > Cr, G4 V. Analyses of zo.ing profiles in gametcooledfrom a high temperature suggest that ratesof Ni dilhrsion are equivalent to thoseof Fe-Mg down to temperatures neax 600oC. Pyroxene equilibrationduring thermal and metasomatic events probablyinvolves volume diffrrsion over very short distances only, beween high-mobility pathssuchas cleavage planes. Kelwords : 2ening,mantle,difflrsion, metasomatism, pyrope-rich gamet. SoM:raans L'6tude de la zonation eil 6l6ments traces des min6raux du manteau peutpermettre de mieux cemerla nature des ptocessus et des 6v6nements m6tasomatiques du manteau sup6rieur, l'6chelle de temps n6cessaire pour leur d6veloppement, et les taux de diffrrsion.Les donn6es sur le gre4at desx6nolithes p6ridotitiques indiqueraient que certains processus m6tasomatiques se sont deroul6ssur une echelle de temps relativementcourte (10-1ff ann6es) pr6c6dant l'6ruption des magmas h6tes. Ainsi, la r E-mail address: bill.eriffin@mo.edu.au