Estimating erosion rates and exposure ages with 36Cl produced by neutron activation
Estimating erosion rates and exposure ages with 36Cl produced by neutron activation
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DOI：
10.1016/0016-7037(95)00267-4
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发表时间：
1995-09
影响因子：
5
通讯作者：
P. Bierman;A. Gillespie;M. Caffee;D. Elmore
中科院分区：
文献类型：
--
作者：
P. Bierman;A. Gillespie;M. Caffee;D. Elmore
从花岗岩矿物颗粒中提取的水溶性氯的同位素组成似乎反映了泥石流扇面的相对年龄和基岩露头的侵蚀速率。我们的数据表明，一些花岗岩地貌是稳定的景观元素，每百万年侵蚀几米到几十米。从三个不同相对年龄的泥石流扇表面收集的样品显示，模型年龄预期随着时间的增加而增加，但任何特定表面上巨石的36Cl-暴露年龄的差异超过五倍。我们用来提取氯的方法是为了分离35Cl(n，γ)36Cl的产生途径，并简化样品的化学制备。这种从流体包裹体中分离出来的方法可以直接解释测量到的36Cl/Cl值，但需要一个模型来预测岩石表面以下的热中子产生和吸收剖面。利用这个模型，我们发现在地表以下采集的8个样品的36Cl/Cl的实测值和预测值之间以及在彼此附近采集的几组样品之间的一致性都很好。利用12 9个花岗岩的元素丰度数据，建立了放射性成因中子支持的~(36)Cl/Cl的模型，其变化范围为4~12 5×10−15(平均值=2 1±18，1σ)。然而，从地表到10米深处，由Muon相互作用产生的热中子有可能比放射成因的中子产生更多的36Cl。地下产生的36Cl对从冰雹和泥石流或冲积扇中收集的年轻的低海拔样品进行地表暴露测年具有重要意义，特别是在巨石表面侵蚀严重或巨石源区的高度远高于待测年地形的年代。
The isotopic composition of water-soluble Cl, extracted from mineral grains in granitic rocks, appears to reflect the relative age of debris-flow fan surfaces and the erosion rate of bedrock outcrops. Our data indicate that some granitic landforms are stable landscape elements eroding on the order of meters to tens of meters per million years. Samples collected from debris-flow fan surfaces of three distinct relative ages show the expected increase of model age with time but the36Cl-exposure ages for boulders on any particular surface vary by a factor of more than five. The method we used for extracting Cl was designed to isolate the35Cl (n, γ)36Cl production pathway and to simplify the chemical preparation of samples. Such isolation from fluid inclusions allows direct interpretation of measured36Cl/Cl ratios but requires a model to predict the thermal-neutron production and absorption profile below the rock surface. Using such a model, we find good agreement between measured and predicted36Cl/Cl for eight samples collected below the ground surface as well as consistency between several groups of samples collected near one another. Using elemental abundance data for 129 granitic rocks, we modeled36Cl/Cl supported by radiogenic neutrons and find that it varies from 4 to 125 × 10−15(mean = 21 ± 18, 1 σ). However, from the surface to a depth of 10 m, thermal neutrons resulting from muon interactions have the potential to generate more36Cl than do radiogenic neutrons. The production of36Cl in the subsurface has important implications for surface-exposure dating of young, low-altitude samples collected from moraines and debris-flow or alluvial fans, especially at ages where erosion of the boulder surface is significant or where the source area for boulders is at altitudes much higher than the landform to be dated.