Differentiation and Compaction in the Skaergaard Intrusion
1Department of Earth Sciences, University of Aarhus, Høegh-Guldbergs GADE 2, DK-8000 Århus C, Denmark
2Department of Geology, University of California, One Shields Avenue, Davis, CA 95616, USA
3Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, UK
4Nordic Volcanological Centre, Institute of Earth Sciences, University of Iceland, Sturlugata 7, IS-101 Reykjavik, Iceland
RECEIVED SEPTEMBER 24, 2007; ACCEPTED MARCH 25, 2009
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Abstract |
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Igneous differentiation processes are constrained from bulk compositions, densities and mineral modes of 116 cumulate gabbro samples in a new reference profile through the Layered Series of the Skaergaard intrusion, East Greenland. The stratigraphic distribution of P, U and Rb in cumulates and residual magma, modeled by Rayleigh fractionation, constrains the final porosities or trapped liquid contents to 30–52% in LZa troctolites, decreasing to 4–12% at the top of LZb olivine-gabbros and remaining low (1–13%; 4·6% on average) in the oxide-gabbros of LZc, MZ and UZ. Local variations in trapped liquid content are associated with modal layering: leucocratic, low-density rocks have higher proportions of trapped melt than adjacent melanocratic, high-density rocks. These observations are explained by varying degrees of compaction. Compaction was most important after the onset of Fe–Ti oxide crystallization because of the high densities of the crystal matrix. Here computed rates of compaction exceed the rate of crystal accumulation in layers that are metres to a few tens of metres thick. In the basal section (LZa) the crystal pile was too thin and the density of the crystal matrix too low to drive the rate of compaction above the high rate of crystal accumulation promoted by initial cooling through the intrusion floor. In the overlying section (LZb) the efficiency of compaction gradually increased as a result of thickening of the crystal pile and lowering of the rate of crystal accumulation. The modeling constrains the P2O5 content of the residual magma to 
1·7 wt % at the level of apatite-in, suggesting that the magma contained 49 wt % SiO2 and followed a trend of iron enrichment. Compaction of the uppermost metres to tens of metres of crystal mush at the top of the cumulate pile was an efficient means of differentiation and resulted in layers with variable final porosities and trace element contents depending on the mineralogy and density of the crystal matrix.
KEY WORDS: layered intrusion; compaction; magma; differentiation; crystallization
*Corresponding author. E-mail: christian.tegner{at}geo.au.dk
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