Cosmogenic 3 He dating of olivine with tightly retained mantle 3 He, Volcano Mountain, Yukon

<p>We present a step-heat method for isolating cosmogenic <span class="inline-formula">³</span>He (<span class="inline-formula">³</span>He<span class="inline-formula">_c</span>) from mantle He in olivine xenocrysts to date the eruption of four of the morphologically youngest nephelinite lava flows from Volcano Mountain (VM), the youngest cone in the Fort Selkirk volcanic field in Yukon, Canada. In these olivines, the standard procedure of powdering grains to <span class="inline-formula">&lt;30</span> <span class="inline-formula">µm</span> failed to effectively remove mantle helium prior to fusion: samples from four different flows yielded unusually high powder fusion <span class="inline-formula">⁴</span>He concentrations of 1.8 to 6.3 pmol g<span class="inline-formula">⁻¹</span>, with <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M12" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msup><mi/><mn mathvariant="normal">3</mn></msup><mi mathvariant="normal">He</mi><msup><mo>/</mo><mn mathvariant="normal">4</mn></msup><mi mathvariant="normal">He</mi></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="46pt" height="15pt" class="svg-formula" dspmath="mathimg" md5hash="95b1ad0337252b63fc3e0b1e9b5bcab3"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="gchron-7-255-2025-ie00001.svg" width="46pt" height="15pt" src="gchron-7-255-2025-ie00001.png"/></svg:svg></span></span> ratios of 7.9 to 9.6 <span class="inline-formula"><i>R</i>_A</span>. When combined with the <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">3</mn></msup><mi mathvariant="normal">He</mi><msup><mo>/</mo><mn mathvariant="normal">4</mn></msup><mi mathvariant="normal">He</mi></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="46pt" height="15pt" class="svg-formula" dspmath="mathimg" md5hash="9c8b2c5dfd0c647564f754850b5dfe5f"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="gchron-7-255-2025-ie00002.svg" width="46pt" height="15pt" src="gchron-7-255-2025-ie00002.png"/></svg:svg></span></span> ratios obtained by crushing (average <span class="inline-formula">8.1±0.2</span> <span class="inline-formula"><i>R</i>_A</span>), these measurements yield Holocene cosmogenic exposure ages but with very large uncertainties arising from the large mantle <span class="inline-formula">³</span>He correction. The inability to effectively isolate <span class="inline-formula">³</span>He<span class="inline-formula">_c</span> from these samples likely arises from the survival of small (<span class="inline-formula">≪30</span> <span class="inline-formula">µm</span>) fluid inclusions hosting mantle He through the powdering step. The presence of such unusually small fluid inclusions may relate to the origin of the olivines as disaggregated peridotite xenoliths rather than the more commonly analyzed olivine phenocrysts. We circumvented this problem by step-heating powdered olivine in steps of 800, 1000, and 1400 °C. Helium isotopic systematics indicate that 70 %–90 % of <span class="inline-formula">³</span>He<span class="inline-formula">_c</span> was released in the low temperature step, and the rest was released in the middle temperature step. By the highest temperature step, the released He had a mantle-like <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M24" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msup><mi/><mn mathvariant="normal">3</mn></msup><mi mathvariant="normal">He</mi><msup><mo>/</mo><mn mathvariant="normal">4</mn></msup><mi mathvariant="normal">He</mi></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="46pt" height="15pt" class="svg-formula" dspmath="mathimg" md5hash="3811f02652deb599b0f4b38260d1f649"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="gchron-7-255-2025-ie00003.svg" width="46pt" height="15pt" src="gchron-7-255-2025-ie00003.png"/></svg:svg></span></span> ratio. Combining results from the step-heating and crush–fusion methods, we determined that the four Volcano Mountain lava flows erupted approximately coevally, at <span class="inline-formula">10.5±1.7</span> ka.</p>

Abstract licence: CC BY

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