Publication Date:
2022-06-24
Description:
Four zircon Raman bands were previously calibrated to give consistent estimates of the accumulated self‐irradiation α‐dose in unannealed volcanic samples. Partial annealing of radiation damage produces inconsistent values because of differences in the relative annealing sensitivities. The damage estimates based on the external rotation band (DER) at ∼356 cm−1 and that based on the ν2(SiO4) band (D2) at ∼438 cm−1 are the most and least sensitive to damage annealing. The D2/DER‐ratio thus provides a numerical estimate of the extent of geologic annealing that a zircon sample has experienced. This ratio characterizes the thermal history of a zircon sample but also its state of radiation damage during the course of its geologic history, and thus the manner in which this state influences other thermochronologic methods. Meaningful interpretation of the zircon Raman age requires that the spectra are free of measurement artifacts. The major artifacts result from micrometer‐scale gradients of the damage densities within a zircon grain due to uranium and thorium zoning. The micrometer‐sized sampled volume may span different densities, producing overlapping spectra, causing apparent peak broadening, overestimated damage densities, and zircon Raman ages. The D3/D2‐ratio of the damage densities calculated from the ν3(SiO4) and ν2(SiO4) bands, most and least affected by overlap, is an efficient indicator of a meaningless signal. It reveals overlap in annealed and unannealed samples, because the used bands have similar responses to annealing. Multi‐band Raman maps can be converted to damage‐ratio maps for screening zircon mounts, and selecting spots for thermochronologic investigations.
Description:
Plain Language Summary:
Radioactive processes cause damage to the lattice of zircon crystals. This damage can be measured with a Raman instrument. Such measurements are important for methods determining the ages and thermal histories of zircon grains in rocks. Thus, the Raman measurements must be reliable and meaningful. This work proposes tools for detecting effects that hinder the interpretation of zircon Raman data. These effects are mixed signals and loss of damage due to exposure to elevated temperatures in the geologic environment. Zircon Raman spectra have different bands that respond differently to mixed signals and temperature. The ratio of the damage estimates from the least and most temperature‐sensitive bands thus indicates partial annealing. Raman spectra of zoned zircons often straddle areas with different lattice damage. Their overlapping signals cause artificial band broadening, and a damage overestimation. The ratio of the damage estimated from the least and the most affected bands identifies mixed signals and allows to reject unsuitable samples. The damage ratios can also be plotted in maps for damage screening and for selecting optimal spots for measurements.
Description:
Key Points:
Annealing and inhomogeneous damage are two main factors hindering radiation‐damage estimation for zircon Raman dating.
Comparison of internal and external Raman bandwidths allows to detect partial annealing of radiation damage in zircon.
Comparison of internal Raman bandwidths allows to detect artifactual broadening in zoned zircon.
Description:
Studienstiftung des Deutschen Volkes (Studienstiftung)
http://dx.doi.org/10.13039/501100004350
Description:
http://dx.doi.org/10.25532/OPARA-155
Keywords:
ddc:549
;
ddc:551.9
Language:
English
Type:
doc-type:article
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