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Applications of Pb isotopes in granite K-feldspar and Pb evolution in the Yilgarn Craton
Geochimica et Cosmochimica Acta ( IF 4.5 ) Pub Date : 2021-12-03 , DOI: 10.1016/j.gca.2021.11.029
Andreas Zametzer 1 , Christopher L. Kirkland 1 , Michael I.H. Hartnady 1 , Milo Barham 1 , David C. Champion 2 , Simon Bodorkos 2 , R. Hugh Smithies 1, 3 , Simon P. Johnson 3
Affiliation  

The isotopic composition of Pb in a mineral or rock at the moment it formed – often referred to as common Pb – provides an important tool to track geological processes through time and space. There is a wide range of applications of common Pb isotopes including understanding magma sources, melt production, fractionation, contamination, and crystallization in the crust. Pb but not U is incorporated into the structure of K- feldspar during crystal growth, which, together with its widespread occurrence as a framework mineral, makes it an excellent common Pb tracer. Consequently, common Pb isotopes in granite K-feldspar crystals provide a potential signature of source composition and a link to crustal growth processes in the mid to lower crust. Hence, combining common Pb isotopes with Sm-Nd (or Lu-Hf) isotopic signatures from the same dated rocks allows assessment of the degree of isotopic communication from deep fractionation systems to those higher in the crustal column. In this contribution, we analyze common Pb isotopic signatures in K-feldspar from a granite sample transect through the Archean Yilgarn Craton in Western Australia. This transect crosses the major crustal-scale Ida Fault that is apparent on Nd and Hf isotopic maps and interpreted as a fundamental lithospheric boundary across which magma sources change. Our results yield a difference in median values of the Pb isotopes derivative parameters µ (238 U/ 204 Pb) and ω (232 Th/ 204 Pb) across the Ida Fault, with higher µ and ω associated with more evolved Nd and Hf isotopic signatures on the western side of the fault. Pb evolution in the Yilgarn Craton is distinct from the widely applied Stacey & Kramers (1975) model. New Yilgarn-specific Pb evolution models are developed with implication for common Pb correction. A correlation in the spatial trends of granite K-feldspar common Pb with those of upper crustal Pb ores and also the Sm- Nd and Lu-Hf systems reveals geochemical communication all the way through the crustal column, implying a common source for the entire lithospheric section on each side of the Ida Fault. Pb isotopes in granite K-feldspar are not an independent geochronometer but may yield important source context on major phase silicate growth that helps refine U-Pb geochronology interpretations (e.g., distinguishing magmatic versus metamorphic zircon growth).



中文翻译:

Pb同位素在花岗岩钾长石中的应用及Yilgarn克拉通的Pb演化

矿物或岩石形成时 Pb 的同位素组成(通常称为普通 Pb)提供了一种重要的工具,可以通过时间和空间跟踪地质过程。常见铅同位素有广泛的应用,包括了解岩浆来源、熔体产生、分馏、污染和地壳结晶。在晶体生长过程中,铅而不是 U 被纳入钾长石的结构中,这与它作为骨架矿物的广泛存在一起使其成为一种极好的常见铅示踪剂。因此,花岗岩钾长石晶体中常见的 Pb 同位素提供了源成分的潜在特征,并与中下地壳的地壳生长过程相关联。因此,将常见的 Pb 同位素与来自相同年代岩石的 Sm-Nd(或 Lu-Hf)同位素特征相结合,可以评估从深层分馏系统到地壳柱中较高部分的同位素通信程度。在这篇文章中,我们分析了来自西澳大利亚太古代伊尔加恩克拉通的花岗岩样本横断面的​​钾长石中常见的 Pb 同位素特征。该断面穿过主要地壳尺度的艾达断层,在 Nd 和 Hf 同位素图上很明显,并被解释为岩浆源变化的基本岩石圈边界。我们的结果产生了跨越艾达断层的 Pb 同位素衍生参数 µ (238 U/ 204 Pb) 和 ω (232 Th/ 204 Pb) 的中值差异,更高的 µ 和 ω 与更进化的 Nd 和 Hf 同位素特征相关在断层的西侧。Yilgarn Craton 中铅的演化不同于广泛应用的 Stacey & Kramers (1975) 模型。新的 Yilgarn 特定 Pb 演化模型的开发暗示了常见的 Pb 校正。花岗岩钾长石普通铅与上地壳铅矿以及 Sm-Nd 和 Lu-Hf 系统空间趋势的相关性揭示了整个地壳柱的地球化学通信,暗示整个岩石圈的共同来源艾达断层两侧的部分。花岗岩钾长石中的 Pb 同位素不是独立的地质年代计,但可能会产生关于主要相硅酸盐生长的重要来源背景,有助于完善 U-Pb 年代学解释(例如,区分岩浆和变质锆石生长)。Kramers (1975) 模型。新的 Yilgarn 特定 Pb 演化模型的开发暗示了常见的 Pb 校正。花岗岩钾长石普通铅与上地壳铅矿以及 Sm-Nd 和 Lu-Hf 系统空间趋势的相关性揭示了整个地壳柱的地球化学通信,暗示整个岩石圈的共同来源艾达断层两侧的部分。花岗岩钾长石中的 Pb 同位素不是独立的地质年代计,但可能会产生关于主要相硅酸盐生长的重要来源背景,有助于完善 U-Pb 年代学解释(例如,区分岩浆和变质锆石生长)。Kramers (1975) 模型。新的 Yilgarn 特定 Pb 演化模型的开发暗示了常见的 Pb 校正。花岗岩钾长石普通铅与上地壳铅矿以及 Sm-Nd 和 Lu-Hf 系统空间趋势的相关性揭示了整个地壳柱的地球化学通信,暗示整个岩石圈的共同来源艾达断层两侧的部分。花岗岩钾长石中的 Pb 同位素不是独立的地质年代计,但可能会产生关于主要相硅酸盐生长的重要来源背景,有助于完善 U-Pb 年代学解释(例如,区分岩浆和变质锆石生长)。花岗岩钾长石普通铅与上地壳铅矿以及 Sm-Nd 和 Lu-Hf 系统空间趋势的相关性揭示了整个地壳柱的地球化学通信,暗示整个岩石圈的共同来源艾达断层两侧的部分。花岗岩钾长石中的 Pb 同位素不是独立的地质年代计,但可能会产生关于主要相硅酸盐生长的重要来源背景,有助于完善 U-Pb 年代学解释(例如,区分岩浆和变质锆石生长)。花岗岩钾长石普通铅与上地壳铅矿以及 Sm-Nd 和 Lu-Hf 系统空间趋势的相关性揭示了整个地壳柱的地球化学通信,暗示整个岩石圈的共同来源艾达断层两侧的部分。花岗岩钾长石中的 Pb 同位素不是独立的地质年代计,但可能会产生关于主要相硅酸盐生长的重要来源背景,有助于完善 U-Pb 年代学解释(例如,区分岩浆和变质锆石生长)。

更新日期:2021-12-04
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