EVIDENCE FOR POSITIVE DILATION DURING SERPENTINIZATION OF A STILLWATER HARZBURGITE

The Stillwater Complex, Montana, is a considerably complex tripartite system that includes an ultramafic peridotite zone containing cumulate poikilitic harzburgites. A sampled metasomatically-altered harzburgite from this zone revealed sub-parallel veins of serpentine. The parent cumulate material now exists as a collection of ultramafic relicts within these serpentine veins, along with thin discontinuous veins of detrital FeOx and small amounts of chlorite staining. Differences between the specific gravities of the parent ultramafics and product serpentinites, comparison of component ratios, structural indicators, and serpentinization style suggest either significant mass transport and/or volume increase by nearly 22%. Characterization of the serpentinization in this study has been conducted with regard to calculations previously developed to quantify material gains, losses, and volume changes by R. Gresens, serpentine classification as suggested by F. Wicks and E. Wittaker, and structural volume relationships described by D. O’Hanley. N.J. Page has discussed serpentinization in a similar olivine cumulate from the peridotite member of the Stillwater ultramafic zone, but the dominant-direction, fracture controlled serpentine veins in this study’s sample represent slightly different system mechanics. Unserpentinized material has been broken, separated, and in some cases exhibits undulatory extinction relative to the veins- reflecting positive dilation of the sample. Through SEM analysis, these relationships were quantitatively defined. Specifically noted was lack of evidence for Mg and Si removal thus indicating non-constant volume processes. During serpentinization, olivine had the largest change in estimated original modal percentage; other observations include retrograde chlorite stains within the sample suggestive of slight retrograde alteration of a high-Al serpentinite and serpentine compositions of nearly 95% lizardite.