Seven mafic and ultramafic rocks from the Stillwater Complex of Montana were analyzed for an Rb-Sr age determination. Five of these form a linear array on an isochron diagram. The slope of the isochron indicates an age of 2450 ± 210 m.y. for these samples, assuming Rb87 = 1.39 × 10−11 yr−1. The initial Sr87/Sr86 ratio is 0.7029 ± .0006 and the Rb87/Sr86 ratios range from 0.048 to 0.243. The other two rocks, serpentinized harzburgites from the basal zone, do not fit the isochron. One possible explanation for this anomalous behavior is that these rocks became contaminated with radiogenic Sr87 during serpentinization and had initial Sr87/Sr86 ratios of 0.7060 and 0.7094, respectively.

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