Statistical analysis of bubble and crystal size distributions: Application to Colorado Plateau basalts

A new analytical technique for the statistical analysis of bubble populations in volcanic rocks [Proussevitch, A.A., Sahagian, D.L. and Tsentalovich, E.P., 2007-this issue. Statistical analysis of bubble and crystal size distributions: Formulations and procedures. J. Volc. Geotherm. Res.] has been applied to a collection of Colorado Plateau basalts (96 samples). A variety of mono- and polymodal distributions has been found in the samples, all of which belong to the logarithmic family of statistical functions. Most samples have bimodal log normal distributions, while the others are represented by mono- or bimodal log logistic, and Weibull distributions. We have grouped the observed distributions into 11 groups depending on distribution types, mode location, and intensity. The nature of the curves within these groups can be interpreted as evolution of vesiculation processes. We conclude that within bimodal log normal distributions, the mode of smaller bubbles is the result of a second nucleation and growth event in a lava flow after eruption. In the case of log logistic distributions the larger mode results from coalescence of bubbles. Coalescence processes are reflected in growth of a larger mode and decreasing bubble number density. Another style of population evolution leads to a monomodal Weibull (or exponential) distribution as a result of superposition of multiple log normal distributions in which the modes are comparable in size and intensity. These various population distribution styles can be interpreted with an understanding of vesiculation processes that can be gained through appropriate numerical models of coalescence and population evolution. The applicable vesiculation processes include: a) a single nucleation-growth event, b) continuous multiple nucleation-growth events, c) coalescence, and d) Ostwald ripening.