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首页> 外文期刊>Earth and Planetary Science Letters: A Letter Journal Devoted to the Development in Time of the Earth and Planetary System >Cosmogenic and nucleogenic He-3 in apatite, titanite, and zircon
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Cosmogenic and nucleogenic He-3 in apatite, titanite, and zircon

机译：磷灰石，钛矿和锆石中的成因和成核He-3

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Cosmogenic He-3 was measured in apatite, titanite, and zircon and cosmogenic Ne-21 in quartz at 13 depth intervals in a 2.7-m long drill core in a Miocene ignimbrite from the Altiplano of Bolivia. All three He-3 depth profiles as well as the Ne-21 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 for He-3 production of Lambda=180 +/- 11 g/cm(2) is consistent with expectations for neutron spallation, and is identical to that found for the cosmogenic Ne-21 in quartz. By normalizing the measured He-3 concentrations to Ne-21 and using the independently known cosmogenic Ne-21 production rate, the apparent cosmogenic He-3 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 similar to 20% (2 sigma), and arises in equal parts from uncertainties in the measured He-3/Ne-21 ratios and the uncertainty in the Ne-21 production rate. However an additional factor affecting the apparent He-3 production rate in these phases arises from the long stopping range of spalled He-3 and tritium (which decays to He-3). Because all three accessory phases have higher mean atomic number than major rock-forming minerals, they will have lower He-3 production rates than their surroundings. As a consequence the long stopping ranges will cause a net implantation of He-3 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 similar to 50 to similar to 100 mu m 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 using He-3 in these minerals. (c) 2006 Elsevier B.V. All fights reserved.
机译：在玻利维亚Altiplano中新统火成岩中，在一个2.7米长的钻芯中，以13个深度间隔在磷灰石，钛矿和锆石中以及在石英中的成因Ne-21中测量了成因He-3。三个He-3深度剖面以及Ne-21剖面均随深度呈指数衰减，表明这两个同位素的起源都是宇宙成因的，其他来源均无明显贡献。 He-3产生Lambda = 180 +/- 11 g / cm（2）的衰减长度尺度与对中子散裂的预期一致，并且与在石英中产生宇宙的Ne-21的衰减长度尺度相同。通过将测得的He-3浓度标准化为Ne-21并使用独立已知的宇宙成因Ne-21生产率，发现磷灰石，钛矿和锆石中的明显宇宙成因He-3生产率分别为112、97和87。在海平面和高纬度时，原子/克/年。这些估计值的形式上的不确定性类似于20％（2 sigma），并且在同等程度上是由测得的He-3 / Ne-21比率的不确定性和Ne-21生产率的不确定性引起的。但是，在这些阶段中影响表观He-3产生速率的另一个因素是散乱的He-3和tri（衰减为He-3）的长终止范围。由于所有三个副相的平均原子序数均高于主要的成岩矿物，因此它们的He-3生产率将低于周围的环境。结果，较长的停止范围将导致He-3的净植入，因此比纯现场生产的表观生产率更高。因此，这些表观生产率仅适用于所分析的特定晶粒尺寸。筛分的锆石等分试样的分析表明，晶粒尺寸增加2倍（从类似的50到大约100微米的横截面）会使表观生产率降低10％。尽管需要进一步研究这种效应，但此处分析的晶粒尺寸是常见的辅助相的典型特征，因此表观速率​​为这些矿物中使用He-3的表面暴露定年提供了合适的起点。 （c）2006年Elsevier B.V.版权所有。

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