Disturbance of Sm-Nd, Rb-Sr and U-Pb Isochrons During Shock and Thermal Metamorphism - An Experimental Approach
Disturbance of Sm-Nd, Rb-Sr and U-Pb Isochrons During Shock and Thermal Metamorphism - An Experimental Approach
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冲击和热变质过程中 Sm-Nd、Rb-Sr 和 U-Pb 等时线的扰动 - 一种实验方法
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发表时间：
2007
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影响因子：
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通讯作者：
Y. Asmerom
中科院分区：
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作者：
A. Gaffney;L. Borg;Y. Asmerom
冲击和热变质作用都有可能扰乱陨石的放射性成因同位素系统，并重置、旋转或破坏等时线，否则等时线将记录陨石的结晶年龄和初始同位素组成。为了给陨石的同位素系统分配岩石成因意义，必须确定样品如何受到冲击或热变质作用的影响。为了量化冲击和热变质作用对等时线的扰动，我们对10017海母玄武岩样品进行了冲击和加热实验，并分析了冲击、加热和对照样品的等分，确定了它们的同位素组成。方法：采用高钛母玄武岩10017的3个等份进行研究。其中一份在1000℃真空（1 × 10 -5托）中加热一周。JSC的F. Horz的另一项研究结果震惊了55 GPa。作为实验对照，第三组不受电击，也不加热。采用磁选和手挑工艺分别制备了对照液和加热液的矿物分离液。手工拣选过程中被磁分选物剔除的矿物颗粒也进行了分析，并以后缀“-rej”命名。仅用磁选法制备了激波分选物。大多数矿物和所有岩石整体馏分在25℃的2N HCl中浸出10分钟。浸出过程中的残留物以后缀“(R)”命名，渗滤液以后缀“(L)”命名。所有馏分均加入同位素示踪剂，使用标准离子交换技术纯化，并通过热电离质谱分析。Sm-Nd测定结果：对照样品的6个矿物组分和整个岩石组分的Sm-Nd年龄为3.633±0.057 Ga。通过比较，确定的最佳年龄为3.650±0.140 Ga。相比之下，由Nyquist等人分析的在35 GPa震波下的10017等温线显示Sm-Nd等时线的旋转年龄更小，为2.68±0.12 Ga[2]。在我们加热的10017样品中，三个渗滤液馏分落在由对照等温线定义的等温线的左侧，而WR(R)馏分落在对照等温线的右侧（图1）。复合WR分数(WR（R) + WR(L)）落在控制等时线上，表明整个岩石在加热过程中保持封闭系统。5个被加热矿物组分（不包括WR(R)）的年龄为3.647±0.053 Ga，与对照样品的年龄几乎相同。
Introduction: Both shock and thermal metamorphism have the potential to disturb the radiogenic isotope systematics of a meteorite, and reset, rotate or destroy isochrons that would otherwise record the crystallization age and initial isotopic composition of the meteorite. In order to assign petrogenetic significance to the isotope systematics of a meteorite, it is necessary to determine how the sample has been affected by shock or thermal metamorphism. In the attempt to quantify disturbance to isochrons resulting from shock and thermal metamorphism, we undertook experiments to shock and heat a sample of 10017, a mare basalt, and analyzed the shocked, heated and control sample aliquots to determine their isotopic compositions. Methods: Three aliquots of 10017, a high-Ti mare basalt, were used in this study. One aliquot was heated at 1000 C, in a vacuum (1 x 10 -5 torr), for one week. A second aliquot was shocked at 55 GPa, by F. Horz at JSC. The third aliquot was left unshocked and unheated, as an experimental control [1]. Mineral separates of the control and heated aliquots were prepared by magnetic separation and hand-picking. Mineral grains rejected from the magnetic sepatrates during hand-picking were also analyzed, and are designated with the suffix ‘-rej’. Mineral separates of the shocked aliquot were prepared by magnetic separation only. Most mineral and all whole rock fractions were leached in 2N HCl, at 25 C, for 10 minutes. Residues from the leaching procedure are designated with the suffix ‘(R)’, and the leachates are designated with the suffix ‘(L)’. All fractions were spiked with isotope tracers, purified using standard ion exchange techniques, and analyzed by thermal ionization mass spectrometry. Sm-Nd results: The Sm-Nd age determined for the six mineral and whole rock fractions of the control sample is 3.633 ± 0.057 Ga. For comparison, the best age determined for the shocked aliquot is 3.650 ± 0.140 Ga. In constrast, an aliquot of 10017 shocked at 35 GPa and analyzed by Nyquist et al. showed rotation of the Sm-Nd isochron to a younger age of 2.68 ± 0.12 Ga [2]. In our heated 10017 sample, the three leachate fractions fall to the left of the isochron defined by the control aliquot, whereas the WR(R) fraction falls to the right of the control isochron (Figure 1). The recombined WR fraction (WR(R) + WR(L)) falls on the control isochron, indicating that the whole rock remained a closed system during heating. The age derived from the five heated mineral fractions (excluding WR(R)) is 3.647 ± 0.053 Ga, which is nearly identical to the age determined for the control sample.