講演者：Petri Peltonen 氏 (Geological Survey of Finland)
タイトル：
Mantle xenoliths and xenocrysts from the Eastern Finland kimberlites:
Composition, structure and origin of the 250-km-thick Fennoscandian
lithospheric mantle

（2004年09月22日）

要旨：
Most Archaean cratons are underlain by anomalously thick (typically ca. 200 km) cold mantle keels generally distinguished by fast and anisotropic seismic velocities relative to the underlying asthenospheric mantle. The evidence, based on more than 100 mantle xenoliths and thousands of mantle xenocrysts recovered from eastern Finland kimberlites, imply that the Karelian continental mantle is heterogeneous with respect to texture and chemical composition, and stratified into at least two distinct layers. The upper part of the mantle (down to 150 km) is composed mainly of fine grained, granoblastic, and highly depleted harzburgites. They show evidence for both cryptic and modal mantle metasomatism with secondary development of minor phlogopite and clinopyroxene. This upper part of the continental mantle is believed to have stabilized during Archean (~3.5?2.7 Ga), concomitant with the formation of the overlying crust.
The lower part of the Karelian continental mantle (from 150 to 230 km) mainly consists of diverse suite of lherzolites, wehrlites, and minor pyroxenites. Compared to the harzburgitic shallow mantle layer, these rocks are more fertile and have coarse texture typical for lithospheric peridotites in cratons elsewhere. However, they lack evidence for ancient metasomatism such as long-term enrichment of Nd over Sm and Rb over Sr. This is believed to result from reworking and melt metasomatism of the base of the continental lithosphere during post-Archean times. These peridotites host thin layers or small pods of diamondiferous Group A eclogites. Their bimineralic nature, high abundances of compatible trace elements, low lithium abundances, and pyrope-rich garnet and low jadeite content of clinopyroxenes imply that they represent cumulates of mantle derived melts rather than recycled Archean oceanic crust. The diamonds in these eclogites exhibit anomalous birefringence patterns due to intense plastic deformation and suggest that the eclogite seams also acted as sublithospheric shear zones at the time of diamond genesis.