Cosmogenic 3He production rate in ilmenite and the redistribution of spallation 3He in fine-grained minerals
Cosmogenic 3He production rate in ilmenite and the redistribution of spallation 3He in fine-grained minerals
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钛铁矿中的宇宙成因 3He 生产率以及细粒矿物中散裂 3He 的重新分布
DOI：
10.1016/j.gca.2019.08.025
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发表时间：
2019
影响因子：
5
通讯作者：
Lamb, Michael P
中科院分区：
文献类型：
--
作者：
Larsen, Isaac J;Farley, Kenneth A;Lamb, Michael P
宇宙成因核素表面暴露定年和玄武岩景观侵蚀速率测量主要依赖于橄榄石或辉石中的3 He测量。然而，地球化学调查使用3 He一直是不可能的，在相当大的一部分玄武岩，缺乏可分离的橄榄石或辉石晶体，或这样的晶体存在，但已被化学风化。纹理细密的玄武岩通常含有小颗粒的钛铁矿，这是一种耐风化的矿物，是宇宙成因3 He生产的目标，具有良好的He保留和简单的矿物分离，但生产率受到很大限制。在这里，我们通过测量来自美国西部哥伦比亚河和斯内克河平原玄武岩省的细粒（直径20 μm横截面）钛铁矿和共存的辉石或橄榄石的玄武岩中的3 He浓度，经验地校准钛铁矿中宇宙成因3 He的产生率。钛铁矿与辉石和橄榄石的浓度比为0.78 ± 0.02，产生了93.6 ± 7.7 atom g− 1 yr − 1的表观宇宙成因3 He生产率，比先前对成分相似的矿物的理论和经验估计高出20-30%。产生率的差异源于宇宙射线溅射反应释放氚和3 He的高能量以及相关的长停止距离，导致它们在岩石中重新分布。具有低宇宙成因3 He产率的细粒相，如钛铁矿，由于从周围的3 He产率较高的基质中净注入3 He，将具有非常高的产率。半定量模型表明，散裂3 He的注入随着钛铁矿晶粒尺寸的减小而增加，导致当晶粒半径<150 μm时，生产率超过大晶粒中的生产率约10%。模型预测，对于我们的样品中的钛铁矿粒度，注入导致生产率比大颗粒的预期高20%，并且在不确定性范围内解决了我们校准的生产率，理论和以前工作的速率之间的差异。当寄主岩石和晶体的平均原子序数相差很大时，再分配效应最大化，就像全岩玄武岩和钛铁矿之间一样。
Cosmogenic nuclide surface exposure dating and erosion rate measurements in basaltic landscapes rely primarily on measurement of3He in olivine or pyroxene. However, geochemical investigations using3He have been impossible in the substantial fraction of basalts that lack separable olivine or pyroxene crystals, or where such crystals were present, but have been chemically weathered. Fine-textured basalts often contain small grains of ilmenite, a weathering-resistant mineral that is a target for cosmogenic3He production with good He retention and straightforward mineral separation, but with a poorly constrained production rate. Here we empirically calibrate the cosmogenic3He production rate in ilmenite by measuring3He concentrations in basalts with fine-grained (∼20 μm cross-section) ilmenite and co-existing pyroxene or olivine from the Columbia River and Snake River Plain basalt provinces in the western United States. The concentration ratio of ilmenite to pyroxene and olivine is 0.78 ± 0.02, yielding an apparent cosmogenic3He production rate of 93.6 ± 7.7 atom g−1yr−1that is 20–30% greater than expected from prior theoretical and empirical estimates for compositionally similar minerals. The production rate discrepancy arises from the high energy with which cosmic ray spallation reactions emit tritium and3He and the associated long stopping distances that cause them to redistribute within a rock. Fine-grained phases with low cosmogenic3He production rates, like ilmenite, will have anomalously high production rates owing to net implantation of3He from the surrounding, higher3He production rate, matrix. Semi-quantitative modeling indicates implantation of spallation3He increases with decreasing ilmenite grain size, leading to production rates that exceed those in a large grain by ∼10% when grain radii are <150 μm. The modeling predicts that for the ilmenite grain size in our samples, implantation causes production rates to be ∼20% greater than expected for a large grain, and within uncertainty resolves the discrepancy between our calibrated production rate, theory, and rates from previous work. The redistribution effect is maximized when the host rock and crystals differ substantially in mean atomic number, as they do between whole-rock basalt and ilmenite.
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