Thermal effects of scanning electron microscopy on He diffusion in apatite: Implications for (U[sbnd]Th)/He dating

Abstract

In order to investigate potential diffusive loss of He from apatites during SEM analysis, we performed (1) single-grain (UTh)/He dating for 47 Durango apatite fragments (from <90μm to 150–250μm) which were previously examined using SEM under different analytical conditions, and (2) electron–matter interaction simulation combined with diffusion modeling. The determined (UTh)/He ages are internally consistent within their errors, and indistinguishable from the reported 40Ar/39Ar ages of 31.44±0.18 (2σ) Ma and the apatite (UTh)/He ages of 31.02±1.01Ma (Standard Deviation; McDowell et al., 2005). The results from the electron–matter interaction simulation suggest that “temperature rise” (ΔT=temperature increase during electron bombardment) peaks within a very thin layer at the outermost of the hypothetical apatite grain, and falls below ~50K within a depth of 0.3μm from the surface. Based on the simulated ΔT profile combined with available He diffusion parameters, the fractional loss of He (fHe) was calculated for different apatite grain dimensions. The numerical simulation supports that the He loss from apatite grains of typical physical dimensions is negligible (<1%) under regular SEM operating conditions. The direct measurements of (UTh)/He ages for SEM-treated apatites, as well as diffusion simulation using the electron–matter interaction model, indicate that SEM spot analysis or extensive chemical mapping prior to apatite (UTh)/He dating does not cause any meaningful diffusive He loss for most of the apatite samples.