Tourmaline B-isotopes as tracers of fluid sources in silicified Palaeoarchaean oceanic crust of the Mendon Formation, Barberton greenstone belt, South Africa
Tourmaline B-isotopes as tracers of fluid sources in silicified Palaeoarchaean oceanic crust of the Mendon Formation, Barberton greenstone belt, South Africa
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电气石 B 同位素作为南非巴伯顿绿岩带门登组硅化古太古代洋壳流体源的示踪剂
DOI：
10.1016/j.chemgeo.2015.10.009
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发表时间：
2015
期刊：
影响因子：
3.9
通讯作者：
Wiedenbeck
中科院分区：
文献类型：
--
作者：
Farber;Dziggel;Trumbull;Wiedenbeck
普遍硅化是早古生代火山-沉积序列中的一种普遍现象，反映了富硅流体与早古生代洋壳的相互作用。南非古太古代巴伯顿绿岩带硅化带局部含有丰富的电气石。为了限制流体的来源，负责的蚀变，6电气石含样品从3.3 Ga Mendon组进行了研究，通过原位分析矿物化学和硼同位素。样品包括一个硅化，精细层状沉积燧石和五个高度蚀变和硅化科马提岩。电子探针分析（EMPA）表明，电气石主要是镁铁云母，一个样品之间的镁铁云母和镁铁云母。旅游业有一个弱发展的斑片状的颜色和化学分带，在铝，铁和镁的组成变化。部分样品的铬浓度呈强烈的分带性，核部富集铬，局部高铬域与镁铬铁矿、铬白云母（“紫红石”）等其他富铬相接触。总体而言，硼同位素组成（δ11B）范围为-20.7至+10.2 ‰。根据δ11B比值的变化，可将电气石分为两类：一类是δ11B值变化范围小（< 4‰）的电气石（尖晶石结构的科马提岩和硅化沉积物），另一类是δ11B值变化范围大（可达18‰）的电气石（角砾化和片理化样品）。在一个尖晶石结构的科马提岩中发现了正的硼同位素值（+ 6至+ 10‰），并被解释为来自浅热液系统中海水与大洋地壳的相互作用。在所有其他样品中发现的中间硼同位素值（− 5至− 8‰）与MORB相似，可能代表来自大洋地壳的硼。在第2组火山岩的边缘发现了罕见且异常轻的硼同位素值（− 10至− 21‰）。这些值不能单独通过分馏产生，并且指向另一种流体源的存在。轻硼的来源尚未确定，但可能的候选者是来自花岗岩地壳或来自富含10 B的海水沉淀的海相硼化岩的再活动硼。在任何情况下，同位素轻硼似乎已经存在于海底热液蚀变和沉积的门登组在约。3.3佐治亚州
Pervasive silicification is a common phenomenon in Early Archaean volcano-sedimentary sequences, reflecting the interaction between Si-rich fluids and the Archaean oceanic crust. In the Palaeoarchaean Barberton greenstone belt, South Africa, the silicified zones locally contain abundant tourmaline. In order to constrain the source of fluids responsible for the alteration, six tourmaline-bearing samples from the 3.3 Ga Mendon Formation were investigated by in-situ analyses of mineral chemistry and boron isotopes. The samples comprise a silicified, finely laminated sedimentary chert and five highly altered and silicified komatiites. Electron microprobe analysis (EMPA) indicates that tourmaline is mostly dravitic, with one sample between dravite and Mg-foitite. Tourmaline has a weakly-developed patchy colour- and chemical zonation, with variations in Al, Fe and Mg composition. Some samples show strong zoning in Cr-concentration with Cr enrichment in the core, and locally the high-Cr domains are at contacts with other Cr-rich phases such as magnesiochromite and chromian muscovite (“fuchsite”).Overall, the boron isotope composition (δ11B) ranges from − 20.7 to + 10.2‰. Two groups of tourmaline are distinguished based on the variation in δ11B ratios within single samples: 1) those with small (< 4‰) range in δ11B values (spinifex-textured komatiites and silicified sediment), and 2) those with a large range in δ11B values of up to 18‰ (brecciated and foliated samples). Positive boron isotope values (+ 6 to + 10‰) are found in one spinifex-textured komatiite and are interpreted as being derived from seawater interacting with Archaean oceanic crust in shallow hydrothermal systems. The intermediate boron isotope values (− 5 to − 8‰) found in all other samples are similar to MORB and may represent boron derived from the Archaean oceanic crust. The rare and exceptionally light boron isotope values (− 10 to − 21‰) were found in rims of group 2 tourmaline. These values cannot be produced by fractionation alone, and point to the presence of another fluid source. The source of the light boron has yet to be identified, but possible candidates are remobilized boron either derived from granitic crust or from marine evaporites that precipitated from seawater enriched in10B. In any case, the isotopically light boron appears to have been present during seafloor hydrothermal alteration and deposition of the Mendon Formation at ca. 3.3 Ga.
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DOI：
--
发表时间：
2004
期刊：
影响因子：
--
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DOI：
--
发表时间：
2013
期刊：
影响因子：
--
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DOI：
--
发表时间：
2015
期刊：
影响因子：
--
作者：
D. Lowe;G. Byerly
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影响因子：
3.8
作者：
A. Kröner;E. Hegner;J. Wendt;G. Byerly
通讯作者：
G. Byerly
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影响因子：
5.3
作者：
A. Krüner;G. Byerly;D. Lowe
通讯作者：
D. Lowe