1981 Volume 31 Issue 168 Pages 213-233
Electron microprobe analyses of olivine, chromite, pentlandite, and pyrrhotite in 51 samples representing a 753-foot section of mineralized drill core from the ultramafic basal cumulates of the La Perouse layered gabbro reveal that cumulus processes were complex and dominantly of a mechanical, sedimentary nature.
Analyzed olivine grains are rounded and of clearly cumulus origin. Chromite occurs predominately as rounded 100- to 250-micrometer grains trapped within these olivine grains. Pentlandite and pyrrhotite occur both within interstitial (net-textured) sulfide aggregates and as phases exsolved from droplets (to 150 micrometers) of immiscible sulfide melt that were trapped in growing cumulus olivine.
Compositions of olivine grains range from Fo 77.5 a to Fo 82.5 and by as much as 1.7 mole percent Fo within a single thin section. Cr/(Cr+Al) for chromite most commonly falls in the range 0.23 to 0.56; Mg/(Mg+Fe2+) in the range 0.20 to 0.45; and Fe 3+/(Fe2++Fe 3+) in the range 0.10 to 0.25. Within a thin section Cr/(Cr+Al) may differ by as much as 0.24 among individual chromite grains. Ni/(Ni+Fe+Co) for pentlandite typically ranges between 0.33 and 0.48, may differ by over 0.1 for grains within individual thin sections, and is typically distinct for interstitial and included grains.
All analyzed minerals show compositional ranges within individual samples that can best be explained by accumulation from a number of distinct magmatic environments. Wider compositional variation and the repetitive nature of this variation are revealed by the full sample suite. Active processes such as multiple injection, convective overturn, and/or density currents are considered essential to evolution of the studied rocks.
