Hematite and Mn oxide (U-Th)/He dates from the Buckskin-Rawhide detachment system, western Arizona: Gaining insights into hematite (U-Th)/He systematics
Hematite and Mn oxide (U-Th)/He dates from the Buckskin-Rawhide detachment system, western Arizona: Gaining insights into hematite (U-Th)/He systematics
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赤铁矿和氧化锰 (U-Th)/He 源自亚利桑那州西部的鹿皮-生皮分离系统：深入了解赤铁矿 (U-Th)/He 系统学
DOI：
10.2475/10.2014.01
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发表时间：
2014
影响因子：
2.9
通讯作者：
D. Shuster
中科院分区：
文献类型：
--
作者：
N. S. Evenson;P. Reiners;J. Spencer;D. Shuster
在本文中，我们使用几种分析方法，努力更好地了解系统的（U-Th）/He计时器在赤铁矿和锰氧化物。从多晶赤铁矿样品的4 He扩散数据是一致的扩散从赤铁矿晶体的尺寸范围类似于那些直接观察到的样品材料。结合从以前的研究汇编的赤铁矿他扩散数据，这支持的解释，在一般的晶体尺寸是一个主要的控制，他在赤铁矿的保留。4 He/3 He扩散数据从一个较大的赤铁矿晶体的单个片段意味着存在多个扩散域小于所观察到的晶体片段的大小，这可能与裂纹，夹杂物，或其他内部功能，以及更高的浓度4 He在较小的域。我们使用在本研究和其他研究中确定的动力学值以及每个测年样品中赤铁矿晶体尺寸的测量值来估计每个样品的近似闭合温度，其中大部分在140至240 °C之间。微量元素和母核素浓度和（U-Th）/He的一些分析的赤铁矿样品的等分试样中测得的日期之间的关系表明，在某些情况下，U和Th集中在间隙相以外的赤铁矿。我们确定了两个过程负责大部分分散在赤铁矿日期从单个样品。在样品制备或分析过程中，从被分析材料中去除富含U和Th的间隙相会留下未被支撑的4 He注入赤铁矿晶体。这种去除的结果在明显的（U-Th）/他的日期人为年龄比的时间赤铁矿他关闭。在某些样品中可能仍然显著的较小分散源可归因于观察到的间隙相的高He扩散率，其不保留注入的He。对亚利桑那州西部Buckskin和Rawhide拆离断层系上下板块岩石中的赤铁矿进行的（U-Th）/He测年结果与拆离断层沿着快速延伸的时间相吻合。估计的赤铁矿闭合温度和赤铁矿样品年龄的数据从其他研究的鹿皮，生皮剥离系统的比较导致我们得出结论，赤铁矿日期记录快速冷却，随后剥离带矿化。锰氧化物（U-Th）/He年龄被解释为形成矿物的形成年龄的热液流体循环通过上板岩石后，停止伸展沿着拆离断层。总体而言，我们发现，（U-Th）/He测年的赤铁矿和锰氧化物矿物是很有前途的方法，获得这些常见的第二相的形成和冷却的时间信息。
In this paper, we use several analytical methods in an effort to better understand the systematics of the (U-Th)/He chronometer in hematite and manganese oxides. 4He diffusion data from a polycrystalline hematite sample is consistent with diffusion from hematite crystals with a range of sizes similar to those directly observed in sample material. Combined with a compilation of hematite He diffusion data from previous studies, this supports the interpretation that in general crystal size is a primary control on He retentivity in hematite. 4He/3He diffusion data from a single fragment of a larger hematite crystal imply the presence of multiple diffusion domains smaller than the observed size of the crystal fragment, which may be related to cracks, inclusions, or other internal features, as well as higher concentrations of 4He in smaller domains. We use kinetic values determined in this and other studies and measurements of hematite crystal size in each dated sample to estimate approximate closure temperatures for each sample, most of which range from 140 to 240 °C. Relationships between minor element and parent nuclide concentrations and (U-Th)/He dates measured in aliquots of some analyzed hematite samples suggest that, in some cases, U and Th are concentrated in interstitial phases other than hematite. We identify two processes responsible for much of the dispersion in hematite dates from single samples. Removal of U- and Th-rich interstitial phases from analyzed material during sample preparation or analysis leaves behind unsupported 4He implanted in hematite crystals. This removal results in apparent (U-Th)/He dates artificially older than the time of hematite He closure. A smaller source of dispersion that is likely to still be significant in some samples can be attributed to the high He diffusivities of observed interstitial phases, which do not retain implanted He. (U-Th)/He dating of hematite from upper and lower-plate rocks in the Buckskin and Rawhide detachment fault system of western Arizona yields ages that coincide with the timing of rapid extension along the detachment fault. Comparisons of estimated hematite closure temperatures and hematite sample ages to data from other studies of the Buckskin-Rawhide detachment system lead us to conclude that hematite dates record rapid cooling that followed detachment zone mineralization. Mn oxide (U-Th)/He dates are interpreted as formation ages of minerals formed by hydrothermal fluids that circulated through upper-plate rocks after cessation of extension along the detachment fault. Overall, we find that (U-Th)/He dating of hematite and Mn oxide minerals are promising methods for obtaining temporal information about the formation and cooling of these common secondary phases.