Cosmogenic <sup>10</sup>Be in pyroxene: laboratory progress, production rate systematics, and application of the <sup>10</sup>Be–<sup>3</sup>He nuclide pair in the Antarctic Dry Valleys
<p>Here, we present cosmogenic-<span class="inline-formula"><sup>10</sup></span>Be and cosmogenic-<span class="inline-formula"><sup>3</sup></span>He data from Ferrar dolerite pyroxenes in surficial rock samples and a bedrock c...
| Main Authors: | , , , , , , , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2023-07-01
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| Series: | Geochronology |
| Online Access: | https://gchron.copernicus.org/articles/5/301/2023/gchron-5-301-2023.pdf |
| Summary: | <p>Here, we present cosmogenic-<span class="inline-formula"><sup>10</sup></span>Be and cosmogenic-<span class="inline-formula"><sup>3</sup></span>He data
from Ferrar dolerite pyroxenes in surficial rock samples and a bedrock core
from the McMurdo Dry Valleys, Antarctica, with the goal of refining the
laboratory methods for extracting beryllium from pyroxene, further
estimating the <span class="inline-formula"><sup>10</sup></span>Be production rate in pyroxene and demonstrating the
applicability of <span class="inline-formula"><sup>10</sup></span>Be–<span class="inline-formula"><sup>3</sup></span>He in mafic rock. The ability to routinely
measure cosmogenic <span class="inline-formula"><sup>10</sup></span>Be in pyroxene will open new opportunities for
quantifying exposure durations and Earth surface processes in mafic rocks.
We describe scalable laboratory methods for isolating beryllium from
pyroxene, which include a simple hydrofluoric acid leaching procedure for
removing meteoric <span class="inline-formula"><sup>10</sup></span>Be and the addition of a pH 8 precipitation step
to reduce the cation load prior to ion exchange chromatography. <span class="inline-formula"><sup>10</sup></span>Be
measurements in pyroxene from the surface samples have apparent <span class="inline-formula"><sup>3</sup></span>He
exposure ages of 1–6 Myr. We estimate a spallation production rate for
<span class="inline-formula"><sup>10</sup></span>Be in pyroxene, referenced to <span class="inline-formula"><sup>3</sup></span>He, of 3.6 <span class="inline-formula">±</span> 0.2 atoms g<span class="inline-formula"><sup>−1</sup></span> yr<span class="inline-formula"><sup>−1</sup></span>. <span class="inline-formula"><sup>10</sup></span>Be and <span class="inline-formula"><sup>3</sup></span>He measurements in the bedrock core
yield initial estimates for parameters associated with <span class="inline-formula"><sup>10</sup></span>Be and
<span class="inline-formula"><sup>3</sup></span>He production by negative-muon capture (<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M23" display="inline" overflow="scroll" dspmath="mathml"><mrow><msubsup><mi>f</mi><mn mathvariant="normal">10</mn><mo>∗</mo></msubsup><mo>=</mo><mn mathvariant="normal">0.00183</mn></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="67pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="b8d80758c5229555d2f87ecc0009ae48"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="gchron-5-301-2023-ie00001.svg" width="67pt" height="14pt" src="gchron-5-301-2023-ie00001.png"/></svg:svg></span></span>
and <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M24" display="inline" overflow="scroll" dspmath="mathml"><mrow><msubsup><mi>f</mi><mn mathvariant="normal">3</mn><mo>∗</mo></msubsup><msub><mi>f</mi><mtext>C</mtext></msub><msub><mi>f</mi><mtext>D</mtext></msub><mo>=</mo><mn mathvariant="normal">0.00337</mn></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="83pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="9948fc960df7c711666d09b5b9e12773"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="gchron-5-301-2023-ie00002.svg" width="83pt" height="14pt" src="gchron-5-301-2023-ie00002.png"/></svg:svg></span></span>).</p>
<p>Next, we demonstrate that the <span class="inline-formula"><sup>10</sup></span>Be–<span class="inline-formula"><sup>3</sup></span>He pair in pyroxene can be
used to simultaneously resolve erosion rates and exposure ages, finding that
the measured cosmogenic-nuclide concentrations in our surface samples are
best explained by 2–8 Myr of exposure at erosion rates of 0–35 cm Myr<span class="inline-formula"><sup>−1</sup></span>. Finally, given the low <span class="inline-formula"><sup>10</sup></span>Be in our laboratory blanks
(average of 5.7 <span class="inline-formula">×</span> 10<span class="inline-formula"><sup>3</sup></span> atoms), the reported measurement precision, and
our estimated production rate, it should be possible to measure 2 g samples
with <span class="inline-formula"><sup>10</sup></span>Be concentrations of 6 <span class="inline-formula">×</span> 10<span class="inline-formula"><sup>4</sup></span> and 1.5 <span class="inline-formula">×</span> 10<span class="inline-formula"><sup>4</sup></span> atoms g<span class="inline-formula"><sup>−1</sup></span> with 5 % and 15 % uncertainty, respectively. With
this level of precision, Last Glacial Maximum to Late Holocene surfaces can
now be dated with <span class="inline-formula"><sup>10</sup></span>Be in pyroxene. Application of <span class="inline-formula"><sup>10</sup></span>Be in
pyroxene, alone or in combination with <span class="inline-formula"><sup>3</sup></span>He, will expand possibilities
for investigating glacial histories and landscape change in mafic rock.</p> |
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| ISSN: | 2628-3697 2628-3719 |