| Summary: | <p>Magnetite and spinel thermochronological (U–Th) <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M2" display="inline" overflow="scroll" dspmath="mathml"><mo>/</mo></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="8pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="527256ea34e0af356380afd605ccefc0"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="gchron-4-665-2022-ie00003.svg" width="8pt" height="14pt" src="gchron-4-665-2022-ie00003.png"/></svg:svg></span></span> He dates
often display significantly dispersed values. In the present study, we
investigated the contribution of analytical (and standardization) errors to
this dispersion. U and Th content of magnetite (natural and synthetic) and
natural Al spinel samples with U and Th concentrations between 0.02 and 116 <span class="inline-formula">µg</span> g<span class="inline-formula"><sup>−1</sup></span> were analyzed using both wet chemistry and in situ laser
ablation sampling methods. New magnetite reference samples (NMA and NMB)
were synthesized, consisting of U- and Th-doped nano-magnetite powders, whose U and Th concentrations were determined using a wet chemistry method (U and Th
of NMA and NMB are <span class="inline-formula">∼40</span> <span class="inline-formula">µg</span> g<span class="inline-formula"><sup>−1</sup></span> and <span class="inline-formula">∼0.1</span> <span class="inline-formula">µg</span> g<span class="inline-formula"><sup>−1</sup></span>, respectively). We show that, for both U and Th analyses, the reproducibility obtained
with the wet chemistry protocol depends on the U and Th concentration. It is below 11 % for U–Th values higher than 0.4 <span class="inline-formula">µg</span> g<span class="inline-formula"><sup>−1</sup></span> and reaches 22 % for U–Th content lower than 0.1 <span class="inline-formula">µg</span> g<span class="inline-formula"><sup>−1</sup></span>. This result implies that (U–Th) <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M15" display="inline" overflow="scroll" dspmath="mathml"><mo>/</mo></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="8pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="7572a9d7afeaa92ba0e8bb6f686362bd"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="gchron-4-665-2022-ie00004.svg" width="8pt" height="14pt" src="gchron-4-665-2022-ie00004.png"/></svg:svg></span></span> He thermochronological ages cannot be more reproducible than 24 % for magnetite containing less than 0.1 <span class="inline-formula">µg</span> g<span class="inline-formula"><sup>−1</sup></span> of U and Th, thus
explaining part of the natural ages variability. U and Th data obtained by
laser ablation ICP-MS on natural magnetite and Al spinel samples were
calibrated using both silicate glass standards and synthetic magnetite
samples. The U and Th contents determined using NMA are consistent with
those obtained by means of the wet chemistry method, but they are overestimated by 30 %
when using the glass standard samples only. These results highlight the
impact of the matrix effect on the determination of the U–Th content in
magnetite. We thus recommend the use of a well-characterized magnetite reference
for the calibration of the U–Th signals obtained by laser ablation. The
scatter in the (U–Th) <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M18" display="inline" overflow="scroll" dspmath="mathml"><mo>/</mo></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="8pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="62d2c8208bbdf49afb8db19c9f7b6b50"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="gchron-4-665-2022-ie00005.svg" width="8pt" height="14pt" src="gchron-4-665-2022-ie00005.png"/></svg:svg></span></span> He magnetite ages can be expected to be
<span class="inline-formula">∼20</span> % if the U and Th contents are determined by laser
ablation. This level of precision is actually not significantly different
from that obtained using the wet chemistry method, which paves the way for the
use of laser ablation for determining (U–Th) <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M20" display="inline" overflow="scroll" dspmath="mathml"><mo>/</mo></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="8pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="78c74280a32911099c6aadbec3864e34"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="gchron-4-665-2022-ie00006.svg" width="8pt" height="14pt" src="gchron-4-665-2022-ie00006.png"/></svg:svg></span></span> He ages. In the absence of a
spinel reference for U and Th calibration using LA-ICP-MS, silicate glass
references, along with NMA, were used. U and Th contents were found to be
<span class="inline-formula">∼30</span> % lower than the values obtained using wet chemistry.
This discrepancy underlines the importance of using a standard with a
composition close to that of the mineral of interest. Although magnetite and
Al spinel have related crystal structures, the magnetite standard is not
appropriate for U and Th analysis in Al spinel using LA-ICP-MS.</p>
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