Rapid, informative crystal textural characterization using surface area to volume measurements

Hammer, J.E.

Department of Geology and Geophysics, SOEST, University of Hawaii, Honolulu, HI 96822

Frequency histograms of crystal sizes are the starting points for CSDs, which are used to infer decompression and/or cooling histories of magma. Full 3D size distributions obtained from 2D slices are rich in information but may be difficult or impossible to obtain, e.g., if individual features are intergrown with other phases or intersect the section plane more than once. It is possible to quantitatively assess crystal (or vesicle) morphology by counting intersections of feature boundaries with test lines to obtain the mean grain intercept length (L_m) and the overall surface area per unit volume (A/V). Here, we investigate with constant-rate cooling experiments the relationships between qualitative and quantitative textural parameters, modal mineralogy, and cooling rate in basalt. Cooling rate was varied over 2 orders of magnitude in 1 atm runs using a synthetic, crystal-free starting material. Key results include (1) clinopyroxene A/V values correlated with cooling rate and exhibited a similar magnitude in variation, and (2) a second, lower L_m/ higher A/V population of clinopyroxene and olivine crystals appeared in the lowest cooling-rate experiments. A/V ratio is an information-packed process indicator because it broadly reflects whether crystal growth is diffusion- or attachment controlled, which are in turn dependent upon the relative magnitudes of the thermodynamic driving force for phase transformation and various kinetic limitations. In this case, we infer that secondary nucleation occurs when the balance of these terms shifts due to rapidly changing liquid composition. Multistage thermal histories are not required to produce polymodal crystal size or shape distributions.