The effect of hydrothermal alteration and metamictisation on the U-Th-Pb- distribution and U-Pb-systematics of pyrochlore of the Sokli carbonatite complex, N-Finland

Pyrochlore is a common accessory mineral in granite pegmatites, nepheline syenites and carbonatites. Its ability to accomodate many different elements lead to a wide range of pyrochlore varieties. This is obvious for the U- and Th- contents. The Sokli carbonatite complex is part of the Kola alkaline province (KAP) in the northern Baltic Shield. The complex is situated in the area of Savukoski, Finish Lapland, close to the border to Russia. The emplacement age of 360 – 370 Ma falls in the range observed for other complexes in KAP. The magmatic core was build up in three phases, two magmatic phases separated by a hydrothermal phase.
Pyrochlore crystals of the Sokli carbonatite complex show a large chemical variation over the evolution of the magmatic stages, as well as in small rock samples within single stages. This is obvious for the U- and Th- contents. The pyrochlore crystals show a principle change in crystallinity caused by metamictisation due to high U- and Th- concentrations. These high concentrations make pyrochlore useful for age determinations, albeit the metamict state and its modified diffusibility for chemical elements can disturb this. The U- concentration decreases from the early stages of the magmatic evolution to the late stages, to be completely depleted in the last stage. The Th- concentration increases from the early stages until the middle stages, then it decrease to the late stages. Except pyrochlore of rock sample 387R170 all structural analyzed pyrochlor crystals show clear reflexes and present the cubic pyrochlore cell unit. During the structural analyses revealed for sample 387p1 no reflexes were detected, which demonstrates that the pyrochlore crystal lattice is fully broken down due to the metamictisation caused by the high U- content within a decay time of 370 Ma.
The analyzed U-Pb-system of the unaltered metamict pyrochlore shows no loss of those elements due to long term diffusion. The slightly over concordant U-Pb-ages show that pyrochlores of carbonatites of the northern Baltic shield are suitable for dating.

Warnsloh, J.M. (2006) Auswirkungen der hydrothermalen Alteration und der Metamiktisierung auf die U-Th-Pb-Verteilung und die U-Pb-Isotopensystematik in Pyrochlor des Sokli Karbonatit-Komplex, N-Finnland. Diss. RWTH Aachen.