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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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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| author | J. B. Walters J. B. Walters J. B. Walters J. M. Garber A. Beranoaguirre A. Beranoaguirre L. J. Millonig L. J. Millonig A. Gerdes A. Gerdes T. Grützner T. Grützner H. R. Marschall H. R. Marschall |
| author_facet | J. B. Walters J. B. Walters J. B. Walters J. M. Garber A. Beranoaguirre A. Beranoaguirre L. J. Millonig L. J. Millonig A. Gerdes A. Gerdes T. Grützner T. Grützner H. R. Marschall H. R. Marschall |
| author_sort | J. B. Walters |
| collection | DOAJ |
| description | <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> |
| format | Article |
| id | doaj-art-d41b0aa79f25465b83844c0cbe2cb433 |
| institution | Kabale University |
| issn | 2628-3697 2628-3719 |
| language | English |
| publishDate | 2025-08-01 |
| publisher | Copernicus Publications |
| record_format | Article |
| series | Geochronology |
| spelling | doaj-art-d41b0aa79f25465b83844c0cbe2cb4332025-08-20T03:39:32ZengCopernicus PublicationsGeochronology2628-36972628-37192025-08-01730933310.5194/gchron-7-309-2025Zircon micro-inclusions as an obstacle for in situ garnet U–Pb geochronology: an example from the As Sifah eclogite locality, OmanJ. B. Walters0J. B. Walters1J. B. Walters2J. M. Garber3A. Beranoaguirre4A. Beranoaguirre5L. J. Millonig6L. J. Millonig7A. Gerdes8A. Gerdes9T. Grützner10T. Grützner11H. R. Marschall12H. R. Marschall13NAWI Graz Geocenter, Department of Earth Sciences, University of Graz, Graz, 8010, AustriaDepartment of Geosciences, Goethe University Frankfurt, 60438 Frankfurt am Main, GermanyFrankfurt Isotope and Element Research Center (FIERCE), Goethe University Frankfurt, 60438 Frankfurt am Main, GermanyDepartment of Geosciences, The Pennsylvania State University, University Park, PA 16802, USADepartment of Geosciences, Goethe University Frankfurt, 60438 Frankfurt am Main, GermanyFrankfurt Isotope and Element Research Center (FIERCE), Goethe University Frankfurt, 60438 Frankfurt am Main, GermanyDepartment of Geosciences, Goethe University Frankfurt, 60438 Frankfurt am Main, GermanyFrankfurt Isotope and Element Research Center (FIERCE), Goethe University Frankfurt, 60438 Frankfurt am Main, GermanyDepartment of Geosciences, Goethe University Frankfurt, 60438 Frankfurt am Main, GermanyFrankfurt Isotope and Element Research Center (FIERCE), Goethe University Frankfurt, 60438 Frankfurt am Main, GermanyDepartment of Geosciences, Goethe University Frankfurt, 60438 Frankfurt am Main, GermanyFrankfurt Isotope and Element Research Center (FIERCE), Goethe University Frankfurt, 60438 Frankfurt am Main, GermanyDepartment of Geosciences, Goethe University Frankfurt, 60438 Frankfurt am Main, GermanyFrankfurt Isotope and Element Research Center (FIERCE), Goethe University Frankfurt, 60438 Frankfurt am Main, Germany<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>https://gchron.copernicus.org/articles/7/309/2025/gchron-7-309-2025.pdf |
| spellingShingle | J. B. Walters J. B. Walters J. B. Walters J. M. Garber A. Beranoaguirre A. Beranoaguirre L. J. Millonig L. J. Millonig A. Gerdes A. Gerdes T. Grützner T. Grützner H. R. Marschall H. R. Marschall Zircon micro-inclusions as an obstacle for in situ garnet U–Pb geochronology: an example from the As Sifah eclogite locality, Oman Geochronology |
| title | Zircon micro-inclusions as an obstacle for in situ garnet U–Pb geochronology: an example from the As Sifah eclogite locality, Oman |
| title_full | Zircon micro-inclusions as an obstacle for in situ garnet U–Pb geochronology: an example from the As Sifah eclogite locality, Oman |
| title_fullStr | Zircon micro-inclusions as an obstacle for in situ garnet U–Pb geochronology: an example from the As Sifah eclogite locality, Oman |
| title_full_unstemmed | Zircon micro-inclusions as an obstacle for in situ garnet U–Pb geochronology: an example from the As Sifah eclogite locality, Oman |
| title_short | Zircon micro-inclusions as an obstacle for in situ garnet U–Pb geochronology: an example from the As Sifah eclogite locality, Oman |
| title_sort | zircon micro inclusions as an obstacle for in situ garnet u pb geochronology an example from the as sifah eclogite locality oman |
| url | https://gchron.copernicus.org/articles/7/309/2025/gchron-7-309-2025.pdf |
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