X-ray tomography verification for determining phase proportions in volcanic rocks

Abstract

Traditionally, phase proportions in igneous rocks have been estimated by optical microscopy of thin-sectioned rocks to study the mineralogy and identify the proportions of equilibrium phases. For example, an in-depth investigation of basaltic igneous rocks recovered from the oceanic crust at Hess Deep, 110Km west of the Galapagos Islands used phase proportions estimated from petrography to verify results calculated from chemical analyses of the basaltic samples. Chemical analysis divided these samples into two groupings based on their relative FeO content. Those with low-FeO contents dominated the volcanic rock subset and those with higher-FeO contents dominated the intrusive subset. Calculations based on the chemical analyses suggested that plagioclase (an Fe-poor phase) had accumulated in magmas and resulted in the low-FeO group. Petrographic microscopy supported the calculations by verifying that plagioclase exist in relatively greater abundance in the low-FeO group compared to the high-FeO group. Optical microscopy requires the rock samples be sectioned and polished, and then each section is individually imaged in only two dimensions by the petrographic microscope. From these images modal percent of phases is determined for a threedimensional sample. Using nondestructive x-ray tomography and three-dimensional image analysis, we produce a more accurate assessment of phase proportions using an intact lava sample from the low-FeO group, and provide a threedimensional perspective on the relationship between phases in the rocks unavailable with standard petrographic microscopy techniques.