| Summary: | Shifts in ^(87)Sr/^(86)Sr and ^(143)Nd/^(144)Nd ratios measured in cumulates from the upper levels of the Muskox mafic intrusion indicate that isotopic and bulk chemical exchange were decoupled across a mafic-silicic liquid interface during crystallization of the intrusion. Modeling of diffusive exchange between liquid layers demonstrates that isotopic compositions of silicate liquids in layered magma chambers may be strongly affected by this process on time scales of 10^3 to 10^4 years. Diffusive contamination can be used to place constraints on the physical processes and time scales of magmatic systems. © 1992 American Association for the Advancement of Science. 11 October 1991; Accepted 2 December 1991. We thank T. N. Irvine for samples and field assistance. The comments of R. C. Capo, R. B. Hanson, F. M. Richter, F. J. Spera, and an anonymous reviewer improved the manuscript. This work was supported by NSF grants EAR84-15143 and EAR88-04609.
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