ASSESSING THE ROUNDNESS OF 3D CRYSTALS: IMPLICATIONS FOR TEXTURAL COARSENING OF ALKALI-FELDSPAR BY DISSOLUTION-GROWTH

Of the three essential minerals in granitic rocks only alkali-feldspar (AF) commonly occurs as crystals that are markedly larger than surrounding groundmass grains (>1 cm long). Inclusions in these “megacrysts” indicate they grew at magmatic temperatures over periods of 10⁴-10⁶ years, and concentric Ba-rich zones record alternating episodes of dissolution and growth (D/G) during their development.

If AF megacrysts are products of textural coarsening caused by D/G, modeling predicts the crystal populations they are part of will exhibit (1) hump-shaped crystal size distributions, (2) preferential coarsening relative to coexisting plagioclase and quartz, and (3) an inverse correlation between crystal size and roundness. Measurements made on porphyries, which preserve magmatic mineral assemblages and textures, accord with the first two of these predictions. The third is not testable using randomly oriented rock sections, however, because roundness is a 2D parameter that depends on the section’s orientation through a 3D crystal.

To obtain a consistent measure of roundness for AF crystals, complete untwinned megacrysts weathered from porphyries associated with the Mount Whitney Intrusive Suite were sectioned through their centers parallel to (010). Radii of curvature measured at the intersections between the {110} and {001} or {-201} faces for each zone were divided by that zone’s c-axis length to calculate its roundness. A series of such measurements demonstrates that for most zones roundness is inversely related to size as expected for textural coarsening caused by D/G.

Some zones are anomalously rounded for their sizes, however, and have peak Ba concentrations that are higher or lower than those of most other zones in the crystals. Because the amount of dissolution that precedes a zone’s growth correlates with its peak Ba concentration (Rout et al., 2021), these anomalous zones are inferred to record unusually strong or weak dissolution events that punctuated the crystal’s growth history. These events are exceptional, however, and the inverse correlation between roundness and size defined by most zones suggests moderate dissolution events play a dominant role in coarsening AF crystals grown from felsic magmas.