µID-TIMS: spatially resolved high-precision U–Pb zircon geochronology
<p>We present a novel methodology for spatially resolved high-precision U–Pb geochronology of individual growth domains in complex zircon. Our approach utilizes a multi-ion-species (<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id=&q...
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| Main Authors: | , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2024-11-01
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| Series: | Geochronology |
| Online Access: | https://gchron.copernicus.org/articles/6/621/2024/gchron-6-621-2024.pdf |
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| Summary: | <p>We present a novel methodology for spatially resolved high-precision U–Pb geochronology of individual growth domains in complex zircon. Our approach utilizes a multi-ion-species (<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M3" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msup><mi mathvariant="normal">Xe</mi><mo>+</mo></msup><mo>/</mo><msup><mi mathvariant="normal">Ar</mi><mo>+</mo></msup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="44pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="08617a421c2c1c3d6f2ee873ebddf585"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="gchron-6-621-2024-ie00001.svg" width="44pt" height="14pt" src="gchron-6-621-2024-ie00001.png"/></svg:svg></span></span>) plasma focused ion beam (PFIB)–femtosecond (fs) laser system equipped with a scanning electron microscope (SEM). This system enables micrometer-resolution sampling of zircon growth domains with real-time monitoring by cathodoluminescence SEM imaging. Microsamples are then extracted, chemically abraded, dissolved, and analyzed by isotope dilution thermal ionization mass spectrometry (ID-TIMS) to obtain high-precision U–Pb dates. For its superior beam precision (<span class="inline-formula">∼</span> 8–20 <span class="inline-formula">µ</span>m diameter), cleaner cuts, and negligible nanometer-scale damage imparted on the zircon structure, PFIB machining (30 kV) is preferred for microsamples of sizes expected in most future studies focusing on texturally complex natural zircon (20–120 <span class="inline-formula">µ</span>m length scales). Femtosecond laser machining is significantly faster and therefore more appropriate for larger microsamples (<span class="inline-formula"><i>></i>120</span> <span class="inline-formula">µ</span>m length scales), but it is also coarser (<span class="inline-formula">≥20</span> <span class="inline-formula">µ</span>m probe size), produces rougher cuts, and creates a micrometer-scale-wide structurally damaged zone along the laser cuts (i.e., 2 orders of magnitude wider compared to PFIB). Our experiments show that PFIB machining can be conducted on zircon coated with carbon and protective metal coatings as neither offset the U–Pb systematics, nor do they introduce trace amounts of common Pb. We used a Xe<span class="inline-formula"><sup>+</sup></span> PFIB and femtosecond laser to obtain U–Pb dates for Mud Tank and GZ7 zircon microsamples covering a range of sizes (40 <span class="inline-formula">×</span> 18 <span class="inline-formula">×</span> 40–100 <span class="inline-formula">×</span> 80 <span class="inline-formula">×</span> 70 <span class="inline-formula">µ</span>m) and found that microsampling does not bias the accuracy of the resulting <span class="inline-formula">µ</span>ID-TIMS U–Pb dates. The accuracy and precision of <span class="inline-formula">µ</span>ID-TIMS dates for zircon of any given age depend, as for non-microsampled zircon, on the available mass of U and radiogenic Pb – both a function of sample size. Our accompanying open-source code can aid researchers in estimating the necessary microsample size needed to obtain accurate dates at precision sufficient to resolve the processes under study. <span class="inline-formula">µ</span>ID-TIMS bridges the gap between conventional bulk-grain high-precision dating and high-spatial-resolution in situ techniques, enabling the study of the timescales of a variety of processes recorded on the scale of individual growth zones in zircon. This method can be applied to zircon of any age and composition, from terrestrial systems to precious samples from other planetary bodies.</p> |
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| ISSN: | 2628-3697 2628-3719 |