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Geology; September 1997; v. 25; no. 9; p. 791-794; DOI: 10.1130/0091-7613(1997)025<0791:SCIAOO>2.3.CO;2
© 1997 Geological Society of America
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Stable Cl isotopes and origin of high-Cl magmas of the Stillwater Complex, Montana

A. E. Boudreau1, M. A. Stewart1 and A. J. Spivack2

1 Department of Geology, Duke University, Box 90227, Durham, North Carolina 27708
2 Center for Marine Science Research, 7205 Wrightsville Avenue, University of North Carolina at Wilmington, Wilmington, North Carolina 28403

The first Cl isotopic determinations on biotite from the Ultramafic series and from the J-M reef of the Lower Banded series of the Stillwater Complex, Montana, have {delta}37Cl values (normalized to seawater, 95% confidence limit error is 0.2{per thousand}) ranging from 0.27{per thousand} to –0.93{per thousand}; the average is –0.18{per thousand}. Assuming minimal fractionation on incorporation into biotite, these data imply an ultimate crustal source for the Cl. However, field, petrographic, and isotopic evidence suggests that high-Cl fluids did not infiltrate from the country rocks after emplacement of the Stillwater magma, nor was the Cl-isotopic signature the result of assimilation of Cl-rich crustal rocks. Stillwater parent magmas have boninitic affinities and can be modeled as second-stage melts from a metasomatically enriched, depleted mantle. We propose that the early melt-extraction event depleted the mantle in F and Cl. Subsequent metasomatism by a Cl-rich agent with a crustal Cl isotopic signature, probably a fluid, produced a source rock with a high Cl/F ratio. In modern magmas, high Cl/F weight ratios are associated with high overall volatile contents, suggesting that the Stillwater Complex and other high-Cl layered intrusions crystallized from magmas that may have contained in excess of 1 wt% water.




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