LAYERED MANTLE AT THE KARELIAN CRATON MARGIN: P-T OF MANTLE XENOCRYSTS AND XENOLITHS FROM THE KAAVI-KUOPIO KIMBERLITES, FINLAND

Abstract

Peridotitic clinopyroxene (cpx) and pyrope garnet xenocrysts from four kimberlite pipes in the Kaavi-Kuopio area of Eastern Finland have been studied using major and trace element geochemistry to obtain information on the vertical compositional variability of the underlying mantle. The xenocryst data, when combined with the petrological constraints provided by peridotite xenoliths, yield a relatively complete section through the lithospheric mantle. Single-grain cpx thermobarometry fits with a 36-mW/m 2 geotherm calculated using heat flow constraints and xenolith modes and geophysical properties. Ni thermometry on pyrope xenocrysts gives 700-1350 °C and, based on the cpx xenocryst/xenolith geotherm, indicates a wide sampling interval, ca. 80-230 km. Plotting pyrope major and trace element compositions as a function of temperature shows there are three distinct layers in the local lithospheric mantle: A low-temperature (<850 °C) harzburgite layer distinguished by Ca-rich but Ti-, Y- and Zr-depleted pyropes. The xenoliths originating from this layer are all fine-grained garnet-spinel harzburgites with secondary cpx. A variably depleted lherzolitic, harzburgitic and wehrlitic horizon from 950 to 1150 °C or 130 to 180 km. A deep layer from 180 to 240 km composed largely of fertile material. The peridotitic diamond window at Kaavi-Kuopio stretches from the top of the diamond stability field at 140 km to the base of the harzburgite-bearing mantle at about 180 km, implying a roughly 40-km-wide prospective zone.