TEXTURAL CHARACTERIZATION OF THE CIMA VOLCANIC FIELD (CALIFORNIA) USING TRADITIONAL AND NOVEL APPROACHES

The purpose of this study is two-fold, firstly to characterize the textures found in the volcanic rocks of the Cima Volcanic Field, and secondly to develop a new method of calculating Crystal Size Distributions (CSDs). CSDs are a statistical analysis of the number, shape, and size of a given phase in a sample and can allow for the crystallization history of the sample to be determined without destroying it. CSDs have classically been constructed by hand over the course of many hours but as technology has progressed over the past decade, faster methods are available with the use of high-resolution Computed Tomography (CT) scanning. The traditional CSD method requires the construction of thin sections and hours of tracing crystals (between 200-1000 per sample) by hand. However, the CT method requires only the cutting of samples into small cubes (~1 cm³ for our samples). The non-destructive CT scans can be performed in minutes to hours depending on sample size. Additionally, The CT method allows for all phases (including vesicles) to be imaged at the the same time. The scans are output as a 3-dimensional grey-scale model of the sample in which various shades of grey represent different densities such that dense phases appear bright and less dense phases, such as vesicles, appear black. Our purpose here was to try to replicate CSDs obtained by hand with those obtained by the CT method. Initial results indicate that the CT scanner can indeed replicate the CSDs performed by hand, although careful consideration must be taken when choosing the grey values (thresholding) for each phase. We found that having a sample’s corresponding thin section on hand is necessary in order to accurately threshold a sample. The CT method is a quick and accurate way to calculate CSDs. Because data on all phases is collected at the same time, one scan can be used for a variety of different calculations such as CSDs or vesicle size analysis.

We chose to analyze samples from the Cima Volcanic field in Eastern California because samples from different flow depths can be easily identified. We analyzed the top, middle, and bottom sections of basaltic flows that erupted from various cinder cones between 7 million and 15,000 years ago. Initial results indicate a complex crystallization history in the flows with evidence of crystal accumulation and textural coarsening.