USING APATITE TO DISPEL THE “TRAPPED LIQUID” CONCEPT AND TO UNDERSTAND THE LOSS OF INTERSTITIAL LIQUID BY COMPACTION IN MAFIC CUMULATES: AN EXAMPLE FROM THE STILLWATER COMPLEX, MONTANA

Abstract

The size, paragenetic setting, distribution, and composition of apatite grains in 16 thin sections from one sample of an olivine gabbro from the Stillwater Complex, Montana were documented to determine whether they grew from trapped interstitial liquid or in an open system. The grains show extreme variability in cross-sectional area, ranging from <150 to >300,000 μm2. The apatite grains are not associated with quartz, Fe–Ti oxides, or evolved overgrowths on cumulus grains as expected for the crystallization of a trapped liquid, which suggests that they grew in an open system. The grains are very irregularly distributed, with a single cluster comprising almost 25% of the cumulative apatite cross-sectional area from all 16 slides. The spatial patterns of the apatite indicate that the interstitial liquid moved in tubular channels during the final stages of compaction. The trace-element composition of liquids calculated from apatite and silicate mineral compositions are similar, indicating that the apatite crystallized from liquids that were not dramatically enriched in incompatible elements. We conclude that the interstitial liquid never actually became trapped, that liquid was lost in an open system to essentially zero porosity, and that the model of cumulates as a combination of cumulus grains and trapped liquid should be discarded.