82-2 A Novel Curvature-Based Approach to Assessing Grain Roundness from Low-Resolution Three-Dimensional Image Data

Abstract:

Grain roundness serves as a record of transport processes. Consequently, geologists have developed a variety of methods for measuring roundness, including both qualitative observations and quantitative metrics. In general, estimates of roundness derived from three-dimensional (3D) data, typically collected with X-ray Computed Tomography (CT), are preferable to observation-based and two-dimensional (2D) methods because the former can be subjective and the latter are subject to substantial measurement errors. However, even 3D roundness metrics become unreliable for small grains imaged at standard resolutions (e.g., grains <200 microns long imaged at 3 microns/voxel), which result in relatively low per-grain resolution (i.e., the average grain is represented by fewer voxels). In such cases, roundness metrics often exhibit significant dependence on grain size, leading to potentially biased measurements that make it difficult to compare roundness values across different grains. Therefore, new 3D roundness metrics are necessary for extracting accurate paleoenvironmental information from the shapes of small grains.

Detrital zircons (DZ) are one kind of sedimentary grain that exemplify this challenge. Researchers often use DZ for geochronometry, and the distribution of their radiometric ages can help determine sediment provenance. In cases where DZ provenance analyses provide evidence for continental-scale transport, grain roundness can encode the mode of transport, as aeolian transport rounds grains much more quickly than fluvial transport. However, accurately measuring the roundness of detrital zircons is challenging because of their microscopic size–usually tens to hundreds of microns long.

Here, we define two new 3D roundness metrics based on a grain's surface curvature. We test our new metrics on synthetic DZ grains and real DZ samples, comparing the performance of our methods to existing metrics. We show that, compared to other approaches, one of our metrics more reliably quantifies roundness for triangular mesh resolutions less than 5,000 vertices, while the other offers a resolution-invariant method of measuring roundness. These metrics will allow us to generate accurate detrital zircon roundness data, which will facilitate efforts to reconstruct past sediment transport networks with these grains. We further anticipate these new metrics will be useful for a variety of applications in the broader Earth sciences and engineering disciplines.

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