A RE-OS ISOTOPE STUDY OF KIMBERLITE DERIVED PERIDOTITE XENOLITHS FROM FINLAND: CONSTRAINTS FOR THE ORIGIN OF LAYERED CONTINENTAL MANTLE

Dating the events that have led to the stabilisation of the lithospheric mantle and the subsequent reworking processes remains a challenging task. Isotopic systems often show open behaviour at the ambient pressure-temperature conditions within the stability field of garnet peridotites, or are seriously disturbed by mantle metasomatism. Rhenium-osmium isotope systematics turn out to be less prone to such a disturbance and the system has been successfully applied to get age information on the depletion history and stabilisation of deep cratonic roots. Data has become available from the major Archean cratons of southern Africa, Siberia, USA, and Canada. In this study we will present the first Re-Os data from the mantle beneath the Fennoscandian Shield, more specifically of mantle xenoliths derived from the roots of the Archean Karelian craton by 600 Ma old kimberlite magmatism. The Karelian craton forms a coherent, mainly late Archean cratonic nucleus ~ 400 000 km2 in size in eastern and northern Finland and adjacent Russia (Fig. 1). Our mantle xenolith samples come from Group I kimberlites that intrude the margin of the Karelian craton margin setting. Earlier studies of mantle xenolith and xenocryst studies have indicated that the subcontinental lithospheric mantle at this locality is stratified into at least three distinct layers; cited A, B, and C. The Layer A extends from c. 60 km to ~110 km depth. This zone is characterised by pyropic garnets that are extremely depleted in Ti, Zr, and Y. Five fine-grained garnetspinel harzburgite xenoliths have similar garnet compositions and represent larger samples from this layer. Two spinel facies xenoliths originate from the narrow garnet-free layer between the most shallow garnet xenocrysts and the Moho at ~60 km depth, also included in layer A. The Layer B extends from 110 to 180 km. It is defined by the depth interval where ultradepleted garnets do not exist anymore, but which instead is characterised by the presence subcalcic G10 (diamond indicator) harzburgite garnets. Titanium content of pyropes at layer B are highly variable indicative of greater degree of lithological diversity compared to other layers. This feature is also evident in the variable modal composition of xenoliths, which include harzburgites, lherzolites, wehrlites and some websterites. The lowermost Layer C extends from 180 km to the base of the lithosphere at ~250 km. This layer is characterised by complete absence of subcalcic G10 garnets and lherzolitic pyropes are less depleted compared to layer B. Furthermore, layer C is characterised by abundance of orange, relatively Cr-poor and Ti-Zr-Y –rich, pyropes of megacrysts composition. Importantly, B–C boundary does not represent lithosphere– asthenosphere boundary because all mantle xenoliths from layer C still have coarse texture, with no evidence of shearing. In addition to peridotites, the layer C also contains diamondiferous eclogites, believed to occur as thin stretched layers or small pods among the layer C peridotites. Xenoliths derived from the middle layer B (at ~110–180 km depth), which is the main source of diamond-indicator garnets (G10) and diamonds in Finnish kimberlites, are characterised by unradiogenic Os isotopic composition. 187Os/188Os shows a good correlation with indices of partial melting implying an age of ~3.3. Ga for melt extraction. This age corresponds with the oldest formation ages of the overlying crust, suggesting that layer B represents the unmodified SCLM stabilised during the Paleoarchean. Underlying layer C (at 180–250 km depths) is the main source of Ti-rich pyropes of megacrystic composition but is lacking G10 pyropes. The osmium isotopic composition of layer C xenoliths is more radiogenic compared to layer B, yielding only Proterozoic TRD ages. Layer C is interpreted to represent a melt metasomatised equivalent to layer B. This metasomatism most likely occurred at c. 2.0 Ga when the present craton margin formed following continental break-up. Shallow layer A (at ~60–110 km depth) has knife-sharp lower contact against layer B indicative of shear zone and episodic construction of SCLM. Layer A peridotites have “ultradepleted” arc mantle -type compositions, and have been metasomatised by radiogenic 187Os/188Os, presumably from slabderived fluids. Since layer A is absent in the core of the craton, its origin can be related to Proterozoic processes at the craton margin. We interpret it to represent the lithosphere of a Proterozoic arc complex that became underthrusted beneath the craton margin crust during continental collision ~1.9 Ga ago.

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