Cosmogenic and nucleogenic 3He in apatite, titanite, and zircon
Cosmogenic and nucleogenic 3He in apatite, titanite, and zircon
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DOI：
10.1016/j.epsl.2006.06.008
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发表时间：
2006-08
影响因子：
5.3
通讯作者：
K. Farley;J. Libarkin;S. Mukhopadhyay;W. Amidon
中科院分区：
文献类型：
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作者：
K. Farley;J. Libarkin;S. Mukhopadhyay;W. Amidon
在玻利维亚高原中新世熔结凝灰岩的2.7米长的钻孔岩芯中，在13个深度间隔测量了磷灰石、钛铁矿和锆石中的宇宙成因3 He和石英中的宇宙成因21 Ne。所有三个3 He深度剖面以及21 Ne剖面随深度呈指数衰减，表明这两种同位素都是宇宙成因的，没有来自其他来源的显着贡献。~ 3 He产生的衰减长度标度为Λ=180±11 g/cm ~ 2，与中子散射的预期值一致，也与石英中宇宙成因~（21）Ne的衰减长度标度一致。将测得的~ 3 He浓度归一化为~（21）Ne，并利用独立已知的宇宙成因~（21）Ne产生率，测得海平面和高纬度磷灰石、钛铁矿和锆石中的表观宇宙成因~ 3 He产生率分别为112、97和87原子/克/年。这些估计值的形式不确定度为1.20%（2σ），并且由测量的3 He/21 Ne比值的不确定度和21 Ne产生率的不确定度产生。然而，在这些阶段中，影响表观3 He产生率的另一个因素来自于散裂的3 He和氚（衰变为3 He）的长停止范围。由于所有三个副相的平均原子序数都比主要造岩矿物高，因此它们的3 He产生率将低于周围环境。因此，长的停止范围将导致3 He的净注入，因此比纯粹的原位生产更高的表观生产率。因此，这些表观生产率仅适用于所分析的特定粒度。对筛分的锆石等分试样的分析表明，粒度增加2倍（从100 μ m到100 μm横截面），表观生产率降低10%。虽然这种影响值得进一步研究，这里分析的粒度是典型的配件阶段，经常遇到的，所以表观速率提供了一个适当的起始位置，表面暴露定年使用3 He在这些矿物。
Cosmogenic3He was measured in apatite, titanite, and zircon and cosmogenic21Ne in quartz at 13 depth intervals in a 2.7-m long drill core in a Miocene ignimbrite from the Altiplano of Bolivia. All three3He depth profiles as well as the21Ne profile attenuate exponentially with depth, indicating that both of these isotopes are cosmogenic in origin with no significant contribution from other sources. The attenuation lengthscale for3He production of Λ=180±11 g/cm2is consistent with expectations for neutron spallation, and is identical to that found for the cosmogenic21Ne in quartz. By normalizing the measured3He concentrations to21Ne and using the independently known cosmogenic21Ne production rate, the apparent cosmogenic3He production rates in apatite, titanite, and zircon were respectively found to be 112, 97, and 87 atoms/g/yr at sea-level and high latitude. The formal uncertainty on these estimates is ∼ 20% (2σ), and arises in equal parts from uncertainties in the measured3He/21Ne ratios and the uncertainty in the21Ne production rate. However an additional factor affecting the apparent3He production rate in these phases arises from the long stopping range of spalled3He and tritium (which decays to3He). Because all three accessory phases have higher mean atomic number than major rock-forming minerals, they will have lower3He production rates than their surroundings. As a consequence the long stopping ranges will cause a net implantation of3He and therefore higher apparent production rates than would apply for purely in-situ production. Thus these apparent production rates apply only to the specific grain sizes analyzed. Analysis of sieved zircon aliquots suggests that a factor of 2 increase in grain size (from ∼ 50 to ∼ 100 μm cross-section) yields a 10% decrease in apparent production rate. While this effect warrants further study, the grain sizes analyzed here are typical of the accessory phases commonly encountered, so the apparent rates provide an appropriate starting place for surface exposure dating using3He in these minerals.