The clustered nucleation and growth processes of garnet in regional metamorphic rocks from north‐west Connecticut, USA

Abstract

Serial sectioning and imaging with a flatbed scanner yielded the three‐dimensional size and spatial distribution of garnet porphyroblasts in two garnet schists and one staurolite‐bearing schist from the Everett Formation, north‐west Connecticut. The dominant garnet‐producing reaction in all samples was chlorite+quartz=garnet+H2O. The appearance of staurolite, and additional garnet growth in the staurolite‐bearing sample, was due to the reaction chloritoid=garnet+staurolite+chlorite. Statistical measures of garnet spatial distributions, using the pair correlation function (PCF), indicate that garnet crystals are weakly to strongly clustered at length scales between 2 and 10 mm. Such clustered nucleation may reflect minor bulk compositional variations. Covariance measures between garnet size and nearest‐neighbour distance, using the mark covariance function (MCF), suggest a very weak correlation between crystal size and nearest‐neighbour distance for length scales of 2 mm or less. These statistical data suggest that if diffusional gradients were present around growing garnet crystals, they did not influence nucleation and growth patterns at length scales greater than c. 2 mm. Compositional maps, through the garnet centres, show that the smaller crystals have lower Mn core compositions relative to larger crystals, consistent with progressive nucleation during pro‐grade metamorphism. Radius‐rate plots calculated from compositional X‐ray maps show similar growth rates for garnet crystals of different size, consistent with an interface‐controlled growth model for garnet. The presence of minor diffusional gradients around growing garnet cannot be entirely dismissed, but the lack of observable reaction rims, the clustered spatial distribution and the radius‐rate data are most consistent with an interface‐controlled garnet growth model.