Zircon micro-inclusions as an obstacle for in situ garnet U–Pb geochronology: an example from the As Sifah eclogite locality, Oman
<p>Garnet is commonly used to calculate pressure (<span class="inline-formula"><i>P</i></span>)–temperature (<span class="inline-formula"><i>T</i></span>) histories of metamorphic rocks, as well as to monitor changes in bulk-r...
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| Main Authors: | , , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2025-08-01
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| Series: | Geochronology |
| Online Access: | https://gchron.copernicus.org/articles/7/309/2025/gchron-7-309-2025.pdf |
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| Summary: | <p>Garnet is commonly used to calculate pressure (<span class="inline-formula"><i>P</i></span>)–temperature (<span class="inline-formula"><i>T</i></span>) histories of metamorphic rocks, as well as to monitor changes in bulk-rock composition (<span class="inline-formula"><i>X</i></span>) and deformation (<span class="inline-formula"><i>d</i></span>). In situ U–Pb geochronology by laser ablation–inductively coupled mass spectrometry (LA-ICPMS) is a rapid and relatively high spatial resolution technique, which can be used to constrain the timing of the metamorphic <span class="inline-formula"><i>P</i></span>–<span class="inline-formula"><i>T</i></span>–<span class="inline-formula"><i>X</i></span>–<span class="inline-formula"><i>d</i></span> histories preserved in garnet. However, the low U contents (low <span class="inline-formula">µg g<sup>−1</sup></span> to <span class="inline-formula">ng g<sup>−1</sup></span> levels) of most common metamorphic garnet crystals present unique analytical challenges, including potential contamination of the U–Pb system by high-U inclusions, such as zircon, rutile, monazite, and apatite. Here we use laser ablation split-stream (LASS) ICPMS (LASS-ICPMS) analysis to simultaneously measure the U, Th, and Pb isotopes and trace-element contents of eclogite-facies garnet from metamafic rocks at As Sifah, Oman. We observe abundant zircon micro-inclusions (<span class="inline-formula"><2 µm</span>) in all five dated samples. Strong linear correlations in U vs. Zr contents in the analysed laser ablation spots plot along garnet–zircon mixing lines, the slopes of which can only be explained by zircon contamination. Despite clear zircon contamination in the trace-element data, the time-resolved laser ablation U and Pb signals show some irregularities but lack sharp diagnostic spikes typically indicative of inclusions. Instead, zircon micro-inclusions are sufficiently small, abundant, and dispersed over the scale of the laser spot site (193 <span class="inline-formula">µm</span> diameter) such that their contributions to the U, Th, and Pb signals are diluted to produce irregular time-resolved signals that have previously not been identified as inclusions.</p>
<p>Analyses affected by contamination result in well-defined U–Pb regression lines that give concordia intercept dates of 94–89 Ma. After screening, only one sample had sufficient inclusion-free analyses and spread in U–Pb ratios to calculate a statistically meaningful garnet U–Pb date. The calculated concordia intercept date of <span class="inline-formula">71±7 Ma</span> is consistent within uncertainty of previously published garnet–whole-rock Sm–Nd peak metamorphic ages. We suggest that the 94–89 Ma ages represent the growth of micro-zircons produced during low-grade metamorphism or hydrothermal alteration of the mafic tuff protolith during the submergence of and sediment deposition on the Arabian margin at this time. To obviate the effect of micro-inclusions in garnet LA-ICPMS U–Pb geochronology, we recommend a careful examination of garnet grains by electron microscopy prior to analysis and determination of background garnet U, Th, and Pb contents and Th<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M14" 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="539a58614ea8688159b8effbc6d3da8d"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="gchron-7-309-2025-ie00001.svg" width="8pt" height="14pt" src="gchron-7-309-2025-ie00001.png"/></svg:svg></span></span>U combined with the rejection of analyses with even slight or moderately irregular signals. We also demonstrate that LASS-ICPMS is a powerful tool to screen for inclusion contamination for in situ U–Pb garnet geochronology, providing confidence in the geologic meaning of the resulting ages.</p> |
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| ISSN: | 2628-3697 2628-3719 |