Thermal effects of scanning electron microscopy on He diffusion in apatite
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Date
2013
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Publisher
Elsevier
Abstract
In order to investigate potential diffusive loss of He from apatites during SEM analysis, we performed
(1) single-grain (U\Th)/He dating for 47 Durango apatite fragments (from b90 μm to 150–250 μm) which
were previously examined using SEM under different analytical conditions, and (2) electron–matter interaction
simulation combined with diffusion modeling. The determined (U\Th)/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 (U\Th)/He ages of 31.02 ± 1.01 Ma (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 ~50 K within a depth of 0.3 μm 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 (b1%) under regular SEM operating conditions. The direct measurements
of (U\Th)/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 (U\Th)/He
dating does not cause any meaningful diffusive He loss for most of the apatite samples.
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Keywords
Scanning electron microscope (SEM), (U\Th)/He dating, Durango apatite, Temperature rise, He diffusion