Chemical and boron isotopic composition of magmatic and hydrothermal tourmalines from the Sinceni granite–pegmatite system in Swaziland
Chemical and boron isotopic composition of magmatic and hydrothermal tourmalines from the Sinceni granite–pegmatite system in Swaziland
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DOI：
10.1016/s0009-2541(98)00155-7
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发表时间：
1999
期刊：
影响因子：
3.9
通讯作者：
R. Trumbull;M. Chaussidon
中科院分区：
文献类型：
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作者：
R. Trumbull;M. Chaussidon
太古代Sinceni花岗岩是从花岗岩到伟晶岩的过渡的地质和地球化学充分研究的例子，随后在交代岩围岩中发展热液蚀变。在这个系统的所有环境中，从花岗岩到伟晶岩和热液岩中，都有旅游。利用电子探针和离子探针研究了电气石的化学和B同位素组成，以了解电气石成分在岩浆-热液发育阶段的变化。每种岩石类型（花岗岩、伟晶岩、变质岩中的热液脉）的电气石化学成分各不相同。就Na/Na+Ca和Fe/Fe+Mg比值而言，碧玺与相应寄主岩石的全岩比值相关。伟晶岩中的电气石显示出明显高于母体花岗岩的Al含量，以及相应的更大程度的碱位空位（约100%）。伟晶岩中30%，花岗岩中15-20%）。花岗岩中的电气石的同位素组成范围很窄（δ 11 B，平均值为−15.7‰，σ=1.3），这被认为与它们结晶时的岩浆成分近似。伟晶岩中细晶带的电气石具有与花岗岩相同的硼同位素组成范围（平均值为−15.2‰，σ=1.7）。相比之下，来自粗粒环带伟晶岩和热液电气石的电气石同位素较轻（δ 11 B值为−16.6至−23.0‰）。这些差异是一致的同位素分馏模型，由于流体出溶。电气石-流体分馏因子使我们能够估计出溶流体的δ 11 B组成约为−13‰。热液电气石的B同位素组成与350至500°C的温度下该流体的形成一致，这与流体包裹体显微测温的温度一致。
The Archean Sinceni granite is a geologically and geochemically well-studied example of the transition from granite to pegmatite, with subsequent development of hydrothermal alteration in metatonalite wallrocks. Tourmaline occurs in all settings of this system, from the granite to the pegmatites and in the hydrothermal rocks. The chemical and B-isotopic composition of tourmalines was studied by electron- and ion microprobe to understand how tourmaline composition varies during the stages of magmatic to hydrothermal development. The chemical composition of tourmaline from each rock type (granite, pegmatite, hydrothermal vein in metatonalite) is distinctive. In terms of their Na/Na+Ca and Fe/Fe+Mg ratios, the tourmalines correlate with the whole-rock ratios of the corresponding host rock. Tourmalines from the pegmatites show significantly higher Al contents than those from the parental granite, and a correspondingly greater degree of alkali-site vacancies (ca. 30% in pegmatite, 15–20% in granite). Tourmalines in granite have a narrow range of isotopic composition (δ11B , mean −15.7‰, σ=1.3), which is considered to approximate the composition of the magma from which they crystallized. Tourmalines from aplitic zones in pegmatite have the same range of boron isotopic composition as the granite (mean −15.2‰, σ=1.7). In contrast, tourmaline from a coarse, zoned pegmatite and hydrothermal tourmalines are isotopically lighter (δ11B values of −16.6 to −23.0‰). These differences are consistent with a model of isotopic fractionation due to fluid exsolution. Tourmaline–fluid fractionation factors allow us to estimate that the exsolved fluid had a δ11B composition of about −13‰. The B-isotopic compositions of hydrothermal tourmalines are consistent with formation from this fluid at temperatures of 350 to 500°C, which are in line with temperatures from fluid inclusion microthermometry.