Cosmogenic chlorine-36 from calcium spallation
Cosmogenic chlorine-36 from calcium spallation
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DOI：
10.1016/0016-7037(95)00429-7
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发表时间：
1996-02
影响因子：
5
通讯作者：
J. Stone;G. Allan;L. Fifield;R. Cresswell
中科院分区：
文献类型：
--
作者：
J. Stone;G. Allan;L. Fifield;R. Cresswell
钙是宇宙生成36cl的主要靶元素。因此，在暴露在地球表面的方解石和钙长石等矿物中，36cl迅速达到可检测的水平。在这些矿物中，钙同位素的散裂通常占36cl生成的80-90%，另外40ca的负介子捕获和35cl的热中子捕获也起到了辅助作用。为了给地表暴露定年提供依据，我们校准了17300年前的Tabernacle Hill玄武岩钙长石中的宇宙生36cl含量。在海拔1445米，有效地磁纬度40.9°时，钙的散裂率为152±11原子(g Ca)−1a−1。在海平面和高纬度对应的速率估计为48.8±3.4原子(g Ca)−1a−1。用于得出这些值的μ子捕获率在神幕山地点为8.8±2.2原子(g Ca)−1a−1，而在海平面和高纬度地区为4.8±1.2原子(g Ca)−1a−1。当根据绝对时间尺度重新计算时，本研究确定的钙碎裂率与之前在帐幕山的全岩校准测量结果非常吻合。钙36c1产量的校准是方解石暴露测年技术发展的基础。由于其高钙含量，在方解石36cl的生产速度高于任何其他常见的造岩矿物。用加速器质谱法测量方解石中的36cl，检测限为每克~5 × 103个原子，可以测定暴露在~102 - 106年期间的石灰石表面的年代。或者，在侵蚀喀斯特表面的情况下，可以确定从小于1到大于1000 μm /年的侵蚀速率。尽管钙长石中的36cl生成速率低于方解石，但对这种矿物的测量将为确定年轻玄武岩熔岩的年代提供一种有用的方法。
Calcium is a major target element for cosmogenic36Cl production. Consequently36Cl rapidly reaches detectable levels in minerals such as calcite and calcium feldspar exposed at the Earth's surface. Spallation of calcium isotopes typically accounts for 80–90% of36Cl production in these minerals, with subsidiary contributions from negative muon capture by40Ca and thermal neutron capture by35Cl. To provide a basis for surface exposure dating, we have calibrated cosmogenic36Cl production in calcium feldspar from the 17,300 year old Tabernacle Hill basalt. At an altitude of 1445 m and an effective geomagnetic latitude of 40.9 ° the calcium spallation rate is 152 ± 11 atoms (g Ca)−1a−1. The corresponding rate at sea level and high latitude is estimated at 48.8 ± 3.4 atoms (g Ca)−1a−1. The muon capture rate used to derive these values is 8.8 ± 2.2 atoms (g Ca)−1a−1at the Tabernacle Hill site, scaled from a value of 4.8 ± 1.2 atoms (g Ca)−1a−1at sea level and high latitude. The calcium spallation rate determined in this study is in excellent agreement with previous whole-rock calibration measurements at Tabernacle Hill, when these are recalculated with respect to the absolute timescale. The calibration of36C1 production from calcium underpins development of an exposure dating technique for calcite. Due to its high calcium content, the36Cl production rate in calcite is higher than in any other common rock-forming mineral. Measurement of36Cl in calcite, with an accelerator mass spectrometric detection limit of ~5 × 103atoms per gram, allows dating of limestone surfaces exposed for periods ranging from ~102–106years. Alternatively, erosion rates from less than 1 to greater than 1000 μm per year can be determined in the case of eroding karst surfaces. Though the36Cl production rate is lower in calcium feldspar than in calcite, measurements on this mineral will provide a useful means of dating young basalt lavas.