Using MicroCT to Quantify 3D Zoning in Sanidine: Implications for Magma Reservoir Processes

Studies of chemical zoning within igneous minerals are critical to many petrologic studies. Zoning in minerals, however, is commonly observed in thin sections or grain mounts, which are random 2D slices of a 3D system. Use of 2D sections to infer 3D zoning requires a set of assumptions, often not directly tested, and can introduce a number of issues such as incorrect width of zones and gradients between zones.

Micro Computed Tomography (microCT) offers a way to assess 3D zoning in minerals at high resolution. Use of microCT, however, requires that zoning is observable as differences in x-ray attenuation. Sanidine, with its affinity for Ba in the crystal lattice, can display large, abrupt, variations in Ba from core to rim that are related to various magma reservoir processes. These changes in Ba also significantly change the X-ray attenuation coefficient of sanidine, ultimately allowing for mineral zones to be mapped in 3D using microCT.

Here we utilize microCT to show 3D chemical zoning within sanidine grains from a suite of large caldera forming eruptions throughout the geologic record. Starting with 3D mineral reconstructions, we simulate thin section making by generating random 2D slices across a mineral zone to show that slicing orientation alone can drastically change the gradient width and slope of the same zone. These findings have important implications for tools that rely on the interpretation of chemical zoning within minerals (e.g. diffusion chronometry, timing of eruption triggers, rates of magma mixing/assimilation, etc.) and illustrate a new way to investigate caldera forming igneous systems, helping to further our understanding of their temporal evolution.