Find in Library

<p>Epidote – here defined as minerals belonging to the epidote–clinozoisite solid solution – is a low-<span class="inline-formula"><i>μ</i></span> (<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M4" display="inline" overflow="scroll" dspmath="mathml"><mrow><mi mathvariant="italic">μ</mi><mo>=</mo><mrow class="chem"><msup><mi/><mn mathvariant="normal">238</mn></msup><mi mathvariant="normal">U</mi><msup><mo>/</mo><mn mathvariant="normal">204</mn></msup><mi mathvariant="normal">Pb</mi></mrow></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="77pt" height="15pt" class="svg-formula" dspmath="mathimg" md5hash="f9542f8536e1bcbfb46039c49e1cad33"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="gchron-3-123-2021-ie00001.svg" width="77pt" height="15pt" src="gchron-3-123-2021-ie00001.png"/></svg:svg></span></span>) mineral occurring in a variety of geological environments and participating in many metamorphic reactions that is stable throughout a wide range of pressure–temperature conditions. Despite containing fair amounts of U, its use as a <span class="inline-formula">U−Pb</span> geochronometer has been hindered by the commonly high contents of initial Pb, with isotopic compositions that cannot be assumed a priori. We present a <span class="inline-formula">U−Pb</span> geochronology of hydrothermal-vein epidote spanning a wide range of Pb (3.9–190 <span class="inline-formula">µg g⁻¹</span>), Th (0.01–38 <span class="inline-formula">µg g⁻¹</span>), and U (2.6–530 <span class="inline-formula">µg g⁻¹</span>) contents and with <span class="inline-formula"><i>μ</i></span> values between 7 and 510 from the Albula area (eastern Swiss Alps), from the Grimsel area (central Swiss Alps), and from the Heyuan fault (Guangdong Province, China). The investigated epidote samples show appreciable fractions of initial Pb contents (<span class="inline-formula"><i>f</i>₂₀₆=0.7</span>–1.0) – i.e., relative to radiogenic Pb – that vary to different extents. A protocol has been developed for in situ <span class="inline-formula">U−Pb</span> dating of epidote by spot-analysis laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) with a magmatic allanite as the primary reference material. The suitability of the protocol and the reliability of the measured isotopic ratios have been ascertained by independent measurements of <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M13" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msup><mi/><mn mathvariant="normal">238</mn></msup><mi mathvariant="normal">U</mi><msup><mo>/</mo><mn mathvariant="normal">206</mn></msup><mi mathvariant="normal">Pb</mi></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="57pt" height="15pt" class="svg-formula" dspmath="mathimg" md5hash="e8ea2fa6ea740da8eb3d712f71595e48"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="gchron-3-123-2021-ie00002.svg" width="57pt" height="15pt" src="gchron-3-123-2021-ie00002.png"/></svg:svg></span></span> and <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M14" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msup><mi/><mn mathvariant="normal">207</mn></msup><mi mathvariant="normal">Pb</mi><msup><mo>/</mo><mn mathvariant="normal">206</mn></msup><mi mathvariant="normal">Pb</mi></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="63pt" height="15pt" class="svg-formula" dspmath="mathimg" md5hash="26657110764580c134f8bd874cd75509"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="gchron-3-123-2021-ie00003.svg" width="63pt" height="15pt" src="gchron-3-123-2021-ie00003.png"/></svg:svg></span></span> ratios, respectively, with quadrupole and multicollector ICP-MS applied to epidote micro-separates digested and diluted in acids. For age calculation, we used the Tera–Wasserburg (<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M15" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msup><mi/><mn mathvariant="normal">207</mn></msup><mi mathvariant="normal">Pb</mi><msup><mo>/</mo><mn mathvariant="normal">206</mn></msup><mi mathvariant="normal">Pb</mi></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="63pt" height="15pt" class="svg-formula" dspmath="mathimg" md5hash="a3bf5eed0d1719e37e21e765c332e5fd"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="gchron-3-123-2021-ie00004.svg" width="63pt" height="15pt" src="gchron-3-123-2021-ie00004.png"/></svg:svg></span></span> versus <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M16" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msup><mi/><mn mathvariant="normal">238</mn></msup><mi mathvariant="normal">U</mi><msup><mo>/</mo><mn mathvariant="normal">206</mn></msup><mi mathvariant="normal">Pb</mi></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="57pt" height="15pt" class="svg-formula" dspmath="mathimg" md5hash="0f1f1e1993a151568f62b2ede07afaa5"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="gchron-3-123-2021-ie00005.svg" width="57pt" height="15pt" src="gchron-3-123-2021-ie00005.png"/></svg:svg></span></span>) diagram, which does not require corrections for initial Pb and provides the initial <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M17" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msup><mi/><mn mathvariant="normal">207</mn></msup><mi mathvariant="normal">Pb</mi><msup><mo>/</mo><mn mathvariant="normal">206</mn></msup><mi mathvariant="normal">Pb</mi></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="63pt" height="15pt" class="svg-formula" dspmath="mathimg" md5hash="259d77b8370b2a09588d75072009f4c3"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="gchron-3-123-2021-ie00006.svg" width="63pt" height="15pt" src="gchron-3-123-2021-ie00006.png"/></svg:svg></span></span> ratio. Petrographic and microstructural data indicate that the calculated ages date the crystallization of vein epidote from a hydrothermal fluid and that the <span class="inline-formula">U−Pb</span> system was not reset to younger ages by later events. Vein epidote from the Albula area formed in the Paleocene (<span class="inline-formula">62.7±3.0</span> Ma) and is related to Alpine greenschist-facies metamorphism. The Miocene (<span class="inline-formula">19.2±4.3</span> and <span class="inline-formula">16.9±3.7</span> Ma) epidote veins from the Grimsel area formed during the Handegg deformation phase (22–17 Ma) of the Alpine evolution of the Aar Massif. Identical initial <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M22" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msup><mi/><mn mathvariant="normal">207</mn></msup><mi mathvariant="normal">Pb</mi><msup><mo>/</mo><mn mathvariant="normal">206</mn></msup><mi mathvariant="normal">Pb</mi></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="63pt" height="15pt" class="svg-formula" dspmath="mathimg" md5hash="e9e2aebc2a1913e72c4bc71ef173b36c"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="gchron-3-123-2021-ie00007.svg" width="63pt" height="15pt" src="gchron-3-123-2021-ie00007.png"/></svg:svg></span></span> ratios reveal homogeneity in Pb isotopic compositions of the fluid across ca. 100 m. Vein epidote from the Heyuan fault is Cretaceous in age (<span class="inline-formula"> 107.2±8.9</span> Ma) and formed during the early movements of the fault. In situ <span class="inline-formula">U−Pb</span> analyses of epidote returned reliable ages of otherwise undatable epidote–quartz veins. The Tera–Wasserburg approach has proven pivotal for in situ <span class="inline-formula">U−Pb</span> dating of epidote, and the decisive aspect for low age uncertainties is the variability in intra-sample initial Pb fractions.</p>