太古宙钾长石中复杂Pb库存

IF 3.2 2区地球科学 Q2 GEOSCIENCES, MULTIDISCIPLINARY

Precambrian Research Pub Date : 2025-06-17 DOI:10.1016/j.precamres.2025.107854

N.J. McNaughton

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引用次数: 0

摘要

铅同位素专家肯·路德维希和利昂·西尔弗在1977年写道：“……在应用前寒武纪长石的总样品铅（同位素）分析来解决地壳岩石中铅同位素演化的问题时，必须谨慎。”尽管如此，仍有已发表的研究假设太古宙火成岩样品中测量的钾长石Pb同位素组成代表了其寄主花岗岩（s.l.）和花岗岩源的初始Pb。根据澳大利亚已发表的例子，这一假设在很大比例的太古宙案例中是明显错误的，并且可能在相关测试数据可用时，一些随后提出太古宙中具有异常高模式μ (238U/204Pb)来源的解释将无法得到证实。太古宙钾长石的Pb库存量主要由三个部分组成：初始Pb、U-Th衰变生成的生长Pb和后期套印过程中获得的再活化Pb。将测量的钾长石和主岩Pb同位素组成的“两点Pb-Pb等时线”年龄与花岗岩的U/Pb年龄进行比较，建议作为识别太古宙钾长石中再活化Pb的简单方法。通过对同一k长石样品的TIMS和激光mc - icpms测量的Pb同位素数据进行比较，并将两者与其所在花岗岩的地层等时线进行比较，可以证明k长石数据不符合地层等时线：花岗岩的初始Pb应该位于地层等时线上。通过对岩石和钾长石进行最优精度的Pb同位素分析，大大提高了测试的灵敏度。太古宙钾长石的Pb储量复杂，不应在不测量寄主花岗岩及其年龄的情况下进行Pb同位素来源表征和演化研究。由于蚀变过程中的Pb迁移阻碍了太古宙钾长石对花岗岩初始Pb同位素组成的保存，因此需要对样品的场属性和岩石学进行评估，以确定可能的蚀变或套印事件。

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The complex Pb inventory in Archean K-feldspars

Lead isotope luminaries Ken Ludwig and Leon Silver in 1977 wrote: “… caution must be used in applying total-sample lead (isotopic) analyses to Precambrian feldspars to problems of lead-isotope evolution in crustal rocks”. Despite this, there continues to be published studies which make the assumption that the measured K-feldspar Pb isotopic composition in Archean igneous rock samples is representative of the initial Pb of their host granite (s.l.) and the granite source. Based on published Australian examples, this assumption is demonstrably wrong in a significant proportion of Archean cases, and it is likely that some subsequent interpretations proposing sources with unusually high model μ (²³⁸U/²⁰⁴Pb) in the Archean will not be confirmed when relevant testing data are available.

The Pb inventory in Archean K-feldspars comprises three main components: initial Pb, in-growth Pb from U-Th decay and remobilised Pb attained during later overprint events. The “two-point Pb-Pb isochron” age for the measured K-feldspar and hostrock Pb isotope compositions compared to the granite’s U/Pb age is recommended as a simple test for identifying remobilised Pb in Archean K-feldspar. Comparison of Pb isotope data measured by TIMS and Laser-MC-ICPMS on the same K-feldspar samples, and comparison of both to the formation isochron of their host granite, demonstrate examples where the K-feldspar data do not adhere to the formation isochron: the initial Pb of the granite should lie on the formation isochron. The sensitivity of this test is greatly improved by using optimum precision Pb isotope analyses for both the rock and K-feldspar. The Pb inventory of Archean K-feldspar is complex and should not be used for Pb isotope source characterisation and evolutionary studies without also measuring the Pb isotopes in the host granite and its age. The samples field attributes and petrography should be assessed to identify possible alteration or overprint events, as Pb mobility during alteration hinder preservation of the granite initial Pb isotope composition by K-feldspar in the Archean.

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来源期刊

Precambrian Research 地学-地球科学综合

CiteScore

7.20

自引率

28.90%

发文量

325

审稿时长

12 months

期刊介绍： Precambrian Research publishes studies on all aspects of the early stages of the composition, structure and evolution of the Earth and its planetary neighbours. With a focus on process-oriented and comparative studies, it covers, but is not restricted to, subjects such as: (1) Chemical, biological, biochemical and cosmochemical evolution; the origin of life; the evolution of the oceans and atmosphere; the early fossil record; palaeobiology; (2) Geochronology and isotope and elemental geochemistry; (3) Precambrian mineral deposits; (4) Geophysical aspects of the early Earth and Precambrian terrains; (5) Nature, formation and evolution of the Precambrian lithosphere and mantle including magmatic, depositional, metamorphic and tectonic processes. In addition, the editors particularly welcome integrated process-oriented studies that involve a combination of the above fields and comparative studies that demonstrate the effect of Precambrian evolution on Phanerozoic earth system processes. Regional and localised studies of Precambrian phenomena are considered appropriate only when the detail and quality allow illustration of a wider process, or when significant gaps in basic knowledge of a particular area can be filled.