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Distinct from the ilmenites described above are the extremely Cr-rich examples from Pipe 9. Abundant apatite occurs as acicular grains commonly in radiating stellate clusters in the groundmass and as larger more prismatic grains grown primarily within calcite segregations. They are relatively Si-rich 0. The majority of the perovskite occurs as euhedral to subhedral discrete grains rarely as aggregates that are 0. Our limited data on perovskite shows them also to be typical of Group I kimberlites op.

Compositions of phlogopite, ilmenite, titanomagnetite, apatite and perovskite in this sample all fit within the ultramafic lamprophyre spectrum of minerals. Preparation of the thirty samples analyzed in this study involved crushing approximately g of each into small chips with hammer, hand-picking under binocular microscope approximately g of chips least contaminated by crustal xenoliths and powdering the picked material in an trace element-free iron grinding vessel.

The Dike 16 lamproite has a bulk composition that is very near that of the average olivine lamproite given by Mitchell and Bergman Only the relatively high concentration of CO 2 is Dike 16 seems to be anomalous Table 5. Considering that all olivine has been converted to serpentine, it is quite possible some of this carbonate is secondary. The ultramafic lamprophyre aillikite is also carbonate-rich, some of which may also be secondary. Dike 16 has a Sr and Nd isotopic composition, recalculated for Ma, that is very near the field for S.

African Group II kimberlites Fig. The age estimate comes from somewhat similar dikes near Kostamuksha, in Russian Karelia Proskuryakov et al. Clearly intrusion ages for all of these samples are necessary before firm initial isotopic compositions can be calculated and for this reason Ar-Ar and Pb dating of perovskite on Dike 16 and five kimberlites is presently being undertaken.

A basic tenet of kimberlite petrology is that kimberlites are very complex and represent mixes of various batches of magma and their phenocrysts along with abundant mantle xenocrysts Mitchell, They therefore do not represent true magmatic liquids. There is abundant evidence for this kind of mixing in the kimberlites discussed here, best exemplified by spinels in Pipe 1 and ilmenites in Pipe The groundmass spinels in Pipe 1 record a pleonaste crystallization event that is also recorded on the rims of some olivines and in some orbicules where pleonaste forms mantles and partially replaces olivine.

The change from chromite to pleonaste is rather abrupt but does not appear to represent a resorbed surface and may simply represent the consequences of fractional crystallization.

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Large amounts of olivine fractionation without contemporaneous crystallization of an aluminous phase may have caused the build up in Al 2O 3 that led to pleonaste saturation. This aluminum-rich magma is not the same one that crystallized the MUM mantles after the pleonaste. The most reasonable mechanism to explain this is the mixing of the chromite-pleonaste cores into a MUM saturated magma of a somewhat evolved, but more typical kimberlite composition. In some atoll spinel grains late magnetite also occurs on the inside of the MUM mantle within the zone of altered pleonaste Fig.

This probably indicates reaction of the late-stage groundmass forming magma with the pleonaste and contemporaneous crystallization of magnetite. A similar case for several transient liquids can be made concerning the zoning patterns in picroilmenite described above. It appears the high Mg, high Cr cores in trend a equilibrated with a liquid with similar Cr content as that which caused the trend b zonation Fig. The trend c zonation without an increase in Cr however indicates another transient liquid, one low in Cr. After this zonation occurred, all of these crystals were mixed into the Pipe 14 magma at which point the final spinel reaction rims formed.

The Finnish kimberlites in terms of trace element and isotopic compositions do not match completely with the S. African examples but instead are closer to the Koidu kimberlites from W. The latter have, as described by Taylor et al. The Koidu and Finland kimberlites overlap completely in all of these parameters.

Notably phlogopite phenocrysts are not abundant, and there are no indications of Group II kimberlite orangeite affinities in these rocks. Detailed studies of the complex zoning in constituent minerals reveal wide variations in the compositions of the magmas that mixed to form the kimberlites discussed here. Major elements indicate that there is a crustal component in some of the samples although trace elements show that this assimilation has had mostly a dilutive effect.

The Finnish kimberlites form a compositionally cohesive group of magmas more akin geochemically to the Koidu type kimberlites of West Africa than the Group I kimberlites from South Africa. Sr-Nd isotopic compositions either reflect derivation from a near bulk-earth mantle or represent mixing of Group I kimberlite isotopic sources and a more radiogenic Karelian Craton lithospheric mantle. Dike 16 has characteristics of both olivine lamproite and Group II kimberlite and may be an intermediate rock type that represents the Group II kimberlite equivalent from the Karelian Craton, i.

The authors gratefully acknowledge Ashton Mining Ltd. Helpful reviews by E. Skinner and A. Fung are greatly appreciated. Clark T. Isotopic and geochemical variation in kimberlites from the southwestern Craton margin, Prieska area, South Africa. Fifth Int.

Clement, C. A comparative geological study of some major kimberlite pipes in the northern Cape and Orange Free State.

W Bruce Graham

Cape Town. Griffin B. A new diamondiferous eclogite-bearing kimberlitic occurrence from Finland. Sixth Int. Kontinen A. K-Ar ages of hornblende and biotite from Late Archean rocks of eastern Finland — interpretation and discussion of tectonic implications.

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Acta 59, pp. Petrology and mineralogy of kimberlites and related rocks from Finland: Preliminary results, In Papunen, H. Balkema, Rotterdam. Pasteris J. Peltonen P.

Garnet peridotite xenoliths from kimberlites of Finland: Nature of the lithospheric mantle at an Archaean craton-Palaeoproterozoic mobile belt transition. This volume. Proskuryakov V. Lamproity Karelo-Kol'skogo regiona. Reid A. The Igwisi hills extrusive "kimberlites". Earth 9, pp. Rock, N. Skinner, E. In Ross, J. Australia Spec. Smith, C. Pb, Sr, and Nd isotopic evidence for sources of African Cretaceous kimberlites. Nature , pp.

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Smith C. Geochemical character of southern African kimberlites: a new approach based on isotopic constraints. SUN S.

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Chemical and isotopic systematics of oceanic basalts: implications for mantle composition and processes, In Saunders A. London 42, pp. Taylor W. Comparative geochemistry of West African kimberlites: Evidence for a micaceous kimberlite endmember of sublithospheric origin. Acta 58, pp. Tyni, M.