Stable Cl isotopes and origin of high-Cl magmas of the Stillwater Complex, Montana

Journal Article (Journal Article)

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 δ37Cl values (normalized to seawater, 95% confidence limit error is 0.2‰) ranging from 0.27‰ to -0.935‰; the average is -0.18‰. 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.

Full Text

Duke Authors

Cited Authors

  • Boudreau, AE; Stewart, MA; Spivack, AJ

Published Date

  • January 1, 1997

Published In

Volume / Issue

  • 25 / 9

Start / End Page

  • 791 - 794

International Standard Serial Number (ISSN)

  • 0091-7613

Digital Object Identifier (DOI)

  • 10.1130/0091-7613(1997)025<0791:SCIAOO>2.3.CO;2

Citation Source

  • Scopus