Stable Cl Isotopes Of the Stillwater Complex, Montana
A. E. Boudreau, M. Stewart A. Spivack
Geology, 1997, v. 25, p. 791-794
ABSTRACT
The Stillwater complex is one of a few large mafic/ultramafic layered intrusions in which the halogen-bearing minerals are unusually rich in Cl. The first Cl isotope determinations from intercumulus biotite from the Ultramafic series and the J-M reef of the Lower Banded series have d37Cl values (normalized to seawater, 95% confidence limit error is 0.2 per mil) ranging from 0.27 to -0.93 and average -0.18, the lighter values possibly resulting from partial serpentinization in the J-M reef samples. Assuming minimal fractionation on incorporation into biotite, these data imply an ultimate crustal source for the Cl. However, field, petrographic and oxygen isotopic evidence suggests that high Cl fluids did not infiltrate from the country rocks after emplacement of the Stillwater magma, and lack of correlation between Cl isotopic and Cl/F stratigraphic trends with radiogenic isotopic trends suggest that Cl enrichment was not the result of assimilation of Cl-rich crustal rocks.
It has been previously shown that Stillwater parent magmas have boninitic affinities, and can be modeled as second stage melts from a depleted mantle that was subsequently enriched by a metasomatic event. It is proposed that an early melt extraction event depleted the mantle in fluorine. Subsequent metasomatism by a Cl-rich agent, probably a fluid, produced a source rock with a high Cl/F ratio, aided the transport of the light REE and alkalis and may have initiated melting. The Cl isotopic data suggests that the metasomatic agent was derived from subducted crustal material. Comparison with modern magmas suggests that high Cl/F contents are associated with magmas with high overall volatile contents and that, contrary to most existing interpretation, the Stillwater and other similar high-Cl layered intrusions crystallized from relatively "wet" magmas.
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