Coexistence of enriched and modern-like 142 Nd signatures in Archean igneous rocks of the eastern Kaapvaal Craton, southern Africa
Coexistence of enriched and modern-like 142 Nd signatures in Archean igneous rocks of the eastern Kaapvaal Craton, southern Africa
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南非卡普瓦尔克拉通东部太古代火成岩中丰富且现代的 142 Nd 特征共存
DOI：
10.1016/j.epsl.2018.01.022
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发表时间：
2018
影响因子：
5.3
通讯作者：
Kröner
中科院分区：
文献类型：
--
作者：
Schneider;Hoffmann;Münker;Kröner
摘要146Sm-142Nd同位素系统的寿命短，是追踪太古宙壳幔分异过程和限制太古代克拉通年轻岩石印迹的重要工具。我们报道了对古片麻岩杂岩(AGC，斯威士兰)和Barberton绿岩带(BGB，南非)最古老的变质火山单元(BGB，南非)的花岗岩类和角闪岩进行的第一套全面的高精度142ND分析。所调查的样品的年龄范围从3.66Ga到3.22Ga，代表了AGC和BGB下部Onverwacht群的主要地质单位。相对于μ-1地面标准，测量样品的JND142Nd值在−8ppm到+3ppm的范围内，典型的误差小于4.4ppm。这17个样品的μ142Nd值呈双峰分布，其中10个样品的μ142Nd值有轻微负异常的趋势，而7个样品的142Nd值与陆地参考值相似。在3.44Ga Ngwane片麻岩样品和约3.45Ga Dwalile绿岩残留物的角闪岩中发现了唯一有把握可分辨的μ142Nd值，显示μ142Nd值在−7.9±4.4~−6.1±4.3ppm之间。μ142ND缺失与年龄、岩性单位或样品所在地无关。相反，我们的结果表明，AGC地壳的形成涉及两个不同的地幔区域。根据μ-142ND特征可以区分这两个储集层。地幔来源的岩石至少在3.46Ga以前具有负的μ142Nd，而花岗岩类至少在3.22Ga之前保留了由较老的μ地壳合并而形成的负AGC142Nd特征。未发现μ-142ND异常的最古老片麻岩的年龄可达3.64Ga，表明早太古宙已存在现代陆相142Nd储集层。
Abstract The short-lived 146 Sm–142 Nd isotope system is an important tool for tracing Hadean crust–mantle differentiation processes and constraining their imprint on much younger rocks from Archean cratons. We report the first comprehensive set of high-precision 142 Nd analyses for granitoids and amphibolites of the Ancient Gneiss Complex (AGC; Swaziland) and the oldest metavolcanic units of the Barberton Greenstone Belt (BGB; South Africa). The investigated samples span an age range from 3.66 Ga to 3.22 Ga and are representative of major geological units of the AGC and the lower Onverwacht Group of the BGB. Measured samples yielded μ 142 Nd values in the range from− 8 ppm to+ 3 ppm relative to the JNdi-1 terrestrial standard, with typical errors smaller than 4.4 ppm. The distribution of the μ 142 Nd values for these 17 measured samples is bimodal with ten samples showing a tendency towards slightly negative μ 142 Nd anomalies, whereas seven samples have 142 Nd similar to the terrestrial reference. The only confidently resolvable μ 142 Nd anomalies were found in a 3.44 Ga Ngwane Gneiss sample and in amphibolites of the ca. 3.45 Ga Dwalile Greenstone Remnant, revealing μ 142 Nd values ranging from− 7.9±4.4 to− 6.1±4.3 ppm. The μ 142 Nd deficits do not correlate with age, lithological unit, or sample locality. Instead, our results reveal that two distinct mantle domains were involved in the formation of the AGC crust. The two reservoirs can be distinguished by their μ 142 Nd signatures. Mantle-derived rocks tapped the enriched reservoir with negative μ 142 Nd at least until 3.46 Ga, whereas the granitoids preserved a negative μ 142 Nd signature that formed by incorporation of older AGC crust at least until 3.22 Ga. The oldest gneisses with no μ 142 Nd anomaly are up to 3.64 Ga in age, indicating that a modern terrestrial 142 Nd reservoir was already present by early Archean times.
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影响因子：
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作者：
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影响因子：
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影响因子：
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DOI：
10.1007/bf01292406
发表时间：
1987
期刊：
Zeitschrift für Physik A Atomic Nuclei
影响因子：
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作者：
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通讯作者：
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