CRYSTAL SIZE DISTRIBUTION - CORRECTION FOR GLOMEROCRYSTS

The presence of glomerocrysts in igneous rocks results in irregular Crystal Size Distribution (CSD) patterns by producing crystal sizes that are related to processes other than nucleation and growth. A new technique that corrects for glomerocrysts in CSD analysis has been applied to a porphyritic rhyolite dike from the Wichita Mountains (O'Donnell and Hogan, 2004). A total of 339 feldspar crystals and 522 quartz crystals in an area of 1450 mm2 were analyzed. Using CSD Corrections 1.36 (Higgins 2000) without correcting for clusters produce a slope of -0.462 ± .027 and an intercept of -3.36 ± 0.15 (R2=0.316) for feldspar and a slope of -0.928 ± 0.052 and an intercept of -1.37 ± 0.14 (R2=0.1917) for quartz. Correction for glomerocrysts using the following process removes this bias and yield meaningful CSD results: 1) scan a microscopic high resolution image of a thin section 2) use high magnification micrograph images to distinguish feldspar from the matrix and borders of the crystals that agglomerated during the crystallization process 3) outline individual crystals and overlay the outlines on the scanned thin section image. This technique greatly reduces the error that subjective interpretations of identifying the borders of individual crystals of the glomerocrysts will report and gives a more accurate portrayal of the size distribution. The lower limit of crystal sizes can also be resolved using the high magnification micrographs to distinguish between the matrix and small phenocrysts that have been resorbed. Feldspar phenocrysts' major axes, which were previously measured as 0 - 5.0 mm, are now measured as 0 - 3.4 mm while quartz phenocrysts' major axes, which were previously measured at 0 - 3.2 mm in are now measured at 0 – 1.4 mm. Regression lines for feldspar now yield a slope of -0.677 ± 0.029 and an intercept of -1.87 ± 0.11 (R2=0.94) and for quartz now yield slope of -1.32 ± 0.06 and an intercept of -0.04 ± 0.12 (R2=0.001). The quartz R2 value is caused by a downward tailing at the smaller length sizes and if the smallest two bins are ignored to produce a straight CSD line an R2 value of 0.938 is produced.