Quantitative 3-D Elemental Mapping by LA-ICP-MS of a Basaltic Clast from the Hanford 300 Area, Washington, USA

Laser ablation with inductively coupled plasma-mass spectrometry (LA-ICP-MS) was used to measure elemental concentrations at the 100-mu m scale in a 3-dimensional manner within a basaltic dast sample collected from the Hanford 300 Area in south-central Washington State, United States. A calibration method was developed to quantify the LA-ICP-MS signal response using a constant-sum mass fraction of eight major elements; the method produced reasonable concentration measurements for both major and trace elements when compared to a standard basalt sample with known concentrations. 3-Dimensional maps (stacked 2-D contour layers, each representing 2100 mu m X 2100 mu m) show relatively uniform concentration with depth for intrinsic elements such as Si, Na, and Sr. However, U and Cu accumulation were observed near the sample surface, consistent with the site's release history of these contaminants. U and Cu show substantial heterogeneity in their concentration distributions within horizontal slices, while the intrinsic elements are essentially uniformly distributed. From these measured U concentrations and published grain size distributions, gravel and cobbles were estimated to contain about 1% of the contaminant U, implicating the coarse fraction as a long-term release source.