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

Bulletin of the Geological Society of Finland

关键词: mantle;    kimberlite;    garnet group;    xenocrysts;    peridotites;    xenoliths;    geochemistry;    rare earths;    geologic thermometry;    Kaavi;    Kuopio;    Finland;   

DOI  :  10.17741/bgsf/77.1.002

来源: DOAJ

【 摘 要 】

Peridotitic garnet xenocrysts from five kimberlite pipes in the Kaavi-Kuopio area of eastern Finland have been studied using major and trace element geochemistry to obtain information on the stratigraphy, compositional variability and evolutionary history of the underlying lithospheric mantle. Ni thermometry on garnet xenocrysts gives 650-1350 °C and, when extrapolated to the geotherm determined using mantle xenoliths, indicates a sampling interval of c. 80-230 km. Three distinct mantle layers are recognized based on the xenolith/xenocryst record: (1) A shallow, 180 km, composed largely of fertile material. The chondrite-normalized REE profiles of subcalcic harzburgitic garnet xenocrysts originating from layer 2 bear evidence of an extensive ancient melt extraction event, similar to that observed in lithosphere underlying Archean cratons elsewhere. Memory of this event has possibly also been preserved in the REEN signatures of rare depleted garnets from layer 3 and in the CCGE pyropes from layer 1 despite their saturation in Ca. The lherzolitic and megacryst garnet varieties exhibit LREEN depletion relative to MREEN and HREEN, with the steady enrichment from SmN to YbN typical of Ca-saturated mantle garnets. The enrichment of MREE and HREE probably derives from a metasomatic event caused by silicate melts close in composition to megacryst magma, which also imprinted a Ti-metasomatic overprint on many pyrope garnets. Harzburgitic and rare lherzolitic garnets, however, appear to have escaped this metasomatism.

【 授权许可】

Unknown