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Science 9 June 1989:
Vol. 244. no. 4909, pp. 1169 - 1174
DOI: 10.1126/science.244.4909.1169
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Articles

Rhenium-Osmium and Samarium-Neodymium Isotopic Systematics of the Stillwater Complex

DAVID D. LAMBERT 1, JOHN W. MORGAN 2, RICHARD J. WALKER 2, STEVEN B. SHIREY 1, RICHARD W. CARLSON 1, MICHAEL L. ZIENTEK 3, and MICHAEL S. KOSKI 4

1 Carlson, Department of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Road, NW, Washington, DC 20015.
2 U.S. Geological Survey, Mail Stop 981, Reston, VA 22092.
3 U.S. Geological Survey, 345 Middlefield Road, Mail Stop 901, Menlo Park, CA 94025.
4 Stillwater Mining Company, Star Route 2, Box 365, Nye, MT 59061.

Isotopic data for the Stillwater Complex, Montana, which formed about 2700 Ma (million years ago), were obtained to evaluate the role of magma mixing in the formation of strategic platinum-group element (PGE) ore deposits. Neodymium and osmium isotopic data indicate that the intrusion formed from at least two geochemically distinct magmas. Ultramafic affinity (U-type) magmas had initial egrNd of -0.8 to -3.2 and a chondritic initial 187Os/186Os ratio of sim0.88, whereas anorthositic affinity (A-type) magmas had egrNd of -0.7 to +1.7 and an initial 187Os/186Os ratio of sim-1.13. These data suggest that U-type magmas were derived from a lithospheric mantle source containing recycled crustal materials whereas A-type magmas originated either by crustal contamination of basaltic magmas or by partial melting of basalt in the lower crust. The Nd and Os isotopic data also suggest that Os, and probably the other PGEs in ore horizons such as the J-M Reef, was derived from A-type magmas. The Nd and Os isotopic heterogeneity observed in rocks below the J-M Reef also suggests that A-type magmas were injected into the Stillwater U-type magma chamber at several stages during the development of the Ultramafic series.

Submitted on February 28, 1989
Accepted on April 24, 1989


THIS ARTICLE HAS BEEN CITED BY OTHER ARTICLES:
Rhenium-Osmium Isotope and Platinum-Group Element Constraints on the Origin and Evolution of the 1{middle dot}27 Ga Muskox Layered Intrusion.
J. M. D. Day, D. G. Pearson, and L. J. Hulbert (2008)
J. Petrology 49, 1255-1295
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Science. ISSN 0036-8075 (print), 1095-9203 (online)