Rare earth element characteristics of pyrope garnets from the Kaavi-Kuopio kimberlites - implications for mantle metasomatism

Abstract

Peridotitic garnet xenocr ysts from five kimberlite pipes in the Kaavi-Kuopio area of east ern Finland have been studied using major and trace element geochemistr y to obtain information on the stratigraphy, compositional variability and evolutionar y histor y of the underlying lithospheric mantle .N i thermometr y on garnet xenocr ysts gives 650–1350°C and, when extrapolated to the geotherm determined using mantle xenoliths, indicates a sampling inter val of c. 80–230 km.Three distinct mantle layers are recognized based on the xe nolith/xenocr yst record: (1) A shallow, 180 km, composed largely of fe rt ile material.The chondrite-normalized REE profiles of subcalcic harzburgitic garnet xenocr ysts origi nating from layer 2 bear evidence of an extensive ancient melt extraction event, similar to that obser ved in lithosphere underlying Archean cratons elsewhere .M emor y of this event has possibly also been preser ved in the REE N signatures of rare depleted garnets from layer 3 and in the CCGE pyropes from layer 1 despite their saturation in Ca. The lherzolitic and megacr yst garnet varieties exhibit LREE N depletion relative to MREE N and HREE N ,w ith the steady enrichment from Sm N to Yb N typical of Ca-saturated mantle garnets. The en richment of MREE and HREE probably derives from a metasomatic event caused by sili cate melts close in composition to megacr yst magma, which also imprinted a Ti-metaso matic overprint on many pyrope garnets. Harzburgitic and rare lherzolitic garnets, however, appear to have escaped this metasomatism.