HR: 14:45h
AN: V53D-05    [Abstracts]
TI: Searching for a better understanding of differentiation in the Skaergaard Intrusion
AU: * McBirney, A R
EM: mcbirney@uoregon.edu
AF: University of Oregon, Dept. of Geological Sciencee Dept. of Geological Sciences University of Oregon, Eugene, OR 97403, United States
AU: Sonnenthal, E L
EM: ELSonnenthal@lbl.gov
AF: Lawrence Berkeley National Laboratory, 1 Cyclotron Road, MS90-1116, Berkeley, CA 94720, United States
AB: The Skaergaard Intrusion is probably the most intensely studied body of igneous rocks on Earth, and yet it seems that the more we have learned about it the less confident we are that we understand the basic processes of magmatic differentiations that it so magnificently displays. The elegant model of differentiation by crystal settling that Lawrence Wager and his colleagues left us is in shambles and is yet to be replaced by a coherent alternative. Interpretations based on so-called cumulate textures have been discredited by our recognition that few if any of the rocks preserve their original textures, compositions, or modal proportions; they re-equilibrated during a long period of slow cooling. Many of these rocks have undergone extensive metasomatism that, in some instances, produced nearly mono-mineralic mafic and felsic assemblages. An abrupt change in the volatile components midway through the course of crystallization was accompanied by marked changes in the oxygen fugacity, partitioning of trace elements, and crystal-liquid equilibria. Equally important, our recognition that compositional changes had a greater effect on liquid densities than thermal expansion has forced us to reject the earlier interpretation of the convective regime, and this in turn led to the realization that the Layered Series did not crystallize from the main reservoir of liquid but from liquids that ponded on the floor after evolving during partial crystallization on the steep walls. In an effort to sort out all these new developments and find better explanations for the trends of compositioal evolution, we have abandoned all our previous assumptions in favor of letting the rocks themselves tell us how they reached their final state. To do this, we are constructing a numerical model that incorporates as many known physical-chemical processes as possible in the hope that it may lead to a better understanding of the fundamental principles of magmatic differentiation.
DE: 8429 Lava rheology and morphology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting