ANALYZING HYDRATION AS THE POTENTIAL CAUSE OF OLIVINE AND PLAGIOCLASE TROCTOLITES

The Stillwater complex is an Archean layered igneous intrusion located in southwestern Montana. It contains two thick bands of anorthosite topped by olivine and plagioclase troctolites. There are at least two proposed hypotheses to explain the reappearance of olivine in the stratigraphic section: (A) an orthomagmatic model in which primitive, olivine saturated magma was added to the evolved magma, thus re-introducing olivine into the system, or (B) a hydromagmatic model in which the plagioclase and pyroxene mush protolith was hydrated with water vapor by the degassing of the underlying mush to produce olivine rather than pyroxene by either incongruent melting of pyroxene or silica loss to the vapor, as has been previously suggested for olivine of olivine-bearing zone I (OB-I) of the Stillwater Complex (e.g., Boudreau, 1988 Can. Min. 26, 193-208).

Plagioclase zoning can test the two hypotheses: for hypothesis A, once olivine and plagioclase accumulates on the floor of the chamber, then closed system crystallization of interstitial liquid should lead to a modest normal zoning by the first mechanism, whereas the second (hypothesis B) can produce reverse zoning by silica and sodium loss. Several core/rim analyses were conducted by electron microprobe analysis for plagioclase grains from troctolites of olivine-bearing zone V (OB-V), and approximately a dozen grains were analyzed for each thin section. In total, 79 pairs were reversely zoned, and 26 were normally zoned. The most negative difference was -10.8, the most positive was 4.5, but the majority of pairs were clustered from -5 to 1. The average was -2.1. Core grains had an average An number of 77.6 ±5, typical for other plagioclase from the Middle Banded series.

The prevalence of reversely zoned grains relative to normally zoned grains is consistent with silica and sodium loss from pre-existing plagioclase, suggesting that hypothesis B is correct: the troctolites are the result of hydration. There is also no evidence of plagioclase core compositions being reset to more primitive, An-rich compositions. Finally, the presence of a green hornblende associated with plagioclase enclosed in olivine also supports a hydration hypothesis.