Determining the ages of young volcanic rocks is important for understanding the tectono-magmatic development of geologic terranes. Usually, if rocks are old enough the ⁴⁰Ar/³⁹Ar or K–Ar techniques can provide reliable ages. However, when rocks are younger, they often lack enough daughter product to resolve an age. Cosmogenic ³He methods provide an alternative for determining the eruption age of relatively recent mafic/intermediate lava flows. We sampled morphologically young basaltic andesite flows south of Miller Knoll, near Panguitch Lake on the Markagunt Plateau in southern Utah. We took two samples in close proximity from two different areas on the flows and separated both olivine and pyroxene. The typical protocol is to crush mineral separates on-line to determine an inclusion-hosted magmatic ³He/⁴He component. Then the powders are heated in a furnace to release the total ³He and ⁴He component and the crushed component is subtracted to determine the cosmogenic ³He component. Unfortunately, in the case of the Miller Lake flows, ³He yield from on-line crushes was below detection. An alternative isochron approach, which obviates the need for crush data, was first described by Cerling and Craig (1994) and more fully by Blard and Pik (2008). In this approach the ³He/⁴He of the total gas released from furnace heating is plotted vs. 1/⁴He. If the samples plot on a line, then the resulting y-intercept is the magmatic ³He/⁴He and the slope of the line is the cosmogenic ³He component, which determines the exposure age. Our data create good isochrons (MSWD = 0.76 and 0.14) with magmatic ³He/⁴He of 4.7–4.9 Ra. Concentrations of cosmogenic ³He are 2.13 and 2.61 × 10⁷ atoms g⁻¹after correction for radiometric ⁴He using the R correction factor and measured and estimated U and Th concentrations in whole rock and minerals, respectively. Using the LSDn scaling routine and an online exposure age calculator, we determine zero erosion exposure ages of 32 ± 3 ka for the upper part of the flow and 34 ± 4 ka for the lower part of the flow.

确定年轻火山岩的年龄对于了解地质地形的构造-岩浆发育很重要。通常，如果岩石年龄足够大，则⁴⁰ Ar / ³⁹ Ar或K–Ar技术可以提供可靠的年龄。但是，当岩石年轻时，它们通常缺乏足够的子产物来解决年龄问题。宇宙产生的³他的方法为确定相对较新的镁铁质/中间熔岩流的爆发年龄提供了一种替代方法。我们在犹他州南部Markagunt高原的Panguitch湖附近的Miller Knoll南部采样了形态学上年轻的玄武岩安山岩流。我们从流动的两个不同区域非常接近地采集了两个样品，并分离了橄榄石和辉石。典型的方案是在线破碎矿物分离物，以确定包含夹杂物的岩浆³ He / ⁴ He组分。然后将粉末在熔炉中加热，以释放出总的³ He和⁴ He组分，然后减去压碎的组分，确定产生宇宙的³他组成。不幸的是，就Miller Lake流量而言，在线压碎的³ He产量低于检测值。Cerling和Craig（1994）首先描述了一种替代等时同步方法，该方法无需获取压碎数据，而Blard和Pik（2008）则更全面地介绍了这种方法。在这种方法中，绘制了从炉子加热释放的总气体中的³ He / ⁴ He对^1/4 He。如果样本在一条线上绘制，则最终的y轴截距是岩浆³ He / ⁴ He，而线的斜率是宇宙成因³ He分量，它决定了暴露年龄。我们的数据在岩浆³ He /时产生了良好的等时线（MSWD = 0.76和0.14）⁴ He为4.7–4.9 Ra。在使用R校正因子对辐射⁴ He进行校正后，宇宙岩石中³ He的浓度分别为2.13和2.61×10 ^7个原子g ^-1 g ^-1，并且分别测量和估计了整个岩石和矿物中的U和Th浓度。使用LSDn缩放例程和在线暴露年龄计算器，我们确定流的上部零侵蚀暴露年龄为32±3 ka，下部为34±4 ka。