The silicon isotope composition of Archaean continental crust from ~3.8 Ga West Greenland rocks

Included are locality, XRF, age, and Si isotope data for Eoarchaean orthogneisses, metabasalts, and metasediments from southern West Greenland. Samples were originally collected by Stephen Moorbath and given to Paul Savage at Oxford.

This dataset is in association with a project conducted by me (Madeleine Murphy) at the University of St Andrews from October 2018 to August 2019 towards fulfillment of the requirements for the degree of MSc Geochemistry under the supervision of Dr. Paul Savage. 08/08/2019

For additonal information about the MSc dissertation, please contact me at [e-mail].

Highlights:
Updated Si isotope analysis provides characterization of ancient continental crust
δ30Si of Archaean samples show systematic deviations from modern igneous rocks
Data suggest Archaean TTGs formed via horizontal tectonics and were heavily weathered

Abstract:
Studies of the ancient continental crust may illuminate Earth’s differentiation, tectonic regimes through time, and co-evolution of the atmosphere and continents. Though scarce, representative samples include the Archaean tonalite-trondhjemite-granodiorites (TTGs) and metamorphosed lithologies of southern West Greenland. Due to its potential to trace various geologic processes, Si isotope analysis of these rocks could illuminate the mechanisms behind TTG formation, the inputs that contaminated igneous melts, and the extent of ancient continental weathering. Here, we measured the Si isotope composition of ~3.8 Ga TTGs, metabasalts, felsic volcanics, and metasediments from the Itsaq Gneiss Complex, SW Greenland. A large range in Si isotopes was formerly reported for Archaean Greenland rocks to suggest hydrothermal involvement during the earliest crustal genesis. However, Si isotope analytical techniques have improved in the years since these findings, providing the opportunity to re-characterize the δ30Si of Archaean continental crust using an updated alkali fusion and MC-ICP-MS methodology.

For Archaean TTGs, we report an average δ30Si =－0.13 ± 0.03‰, isotopically heavier than Phanerozoic analogues, which may have resulted from the introduction of a seawater-derived contaminant to the TTG source melt. The fact that Archaean TTG Si signatures are not vastly different than Phanerozoic igneous rocks suggests they could have formed via magmatic differentiation in an arc setting. Our data for metabasalts and felsic volcanics closely parallel the Si signatures for modern analogues, pointing to their genesis in a subduction environment. For metasediments, we report δ30Si values from －0.19‰ to －1.2‰, much lighter than the range for heavily altered modern pelitic sediments, which implies their origin involved significant chemical weathering of a TTG protolith to form clay minerals. Combined, these interpretations suggest the formation of continental crust in the >3.8 Ga Archaean eon may have involved subduction and, crucially, the influence of an early hydrosphere.

本数据集包含西格陵兰南部始太古代（Eoarchaean）正片麻岩、变玄武岩及变沉积岩的产地信息、X射线荧光光谱（XRF）、年龄以及硅同位素数据。样品最初由Stephen Moorbath采集，并转交至牛津大学的Paul Savage。

本数据集关联本人Madeleine Murphy于2018年10月至2019年8月在圣安德鲁斯大学开展的研究项目，该项目为本人在Paul Savage博士指导下完成地球化学理学硕士学位的必修内容，完成日期为2019年8月8日。

如需了解该硕士学位论文的更多信息，请通过邮箱[e-mail]与我联系。

研究亮点：
1. 采用更新后的硅同位素分析方法，实现了古老大陆地壳的特征刻画；
2. 始太古代样品的δ³⁰Si值与现代火成岩呈现系统性偏差；
3. 数据表明，始太古代英云闪长岩-奥长花岗岩-花岗闪长岩（TTGs）形成于水平构造环境，并经历了强烈风化作用。

摘要：
对古老大陆地壳的研究可为地球分异作用、随时间演化的构造体制以及大气与大陆的协同演化提供重要启示。尽管样本稀缺，但西格陵兰南部的始太古代英云闪长岩-奥长花岗岩-花岗闪长岩（TTGs）及变质岩系是具有代表性的研究对象。鉴于硅同位素分析可用于示踪多种地质过程，对这些岩石开展硅同位素研究，能够揭示TTGs的形成机制、火成熔体的污染物质来源以及古大陆风化作用的强度。本研究对西格陵兰伊萨克片麻岩杂岩中约3.8 Ga的TTGs、变玄武岩、长英质火山岩及变沉积岩的硅同位素组成进行了测定。此前有研究报道了格陵兰始太古代岩石的硅同位素存在较大分馏范围，据此提出早期地壳形成过程中存在热液作用的参与。然而自该研究发表以来，硅同位素分析技术已取得长足进步，为我们采用更新后的碱熔-多接收电感耦合等离子体质谱（MC-ICP-MS）方法重新刻画始太古代大陆地壳的δ³⁰Si值提供了契机。

针对始太古代TTGs，本研究测得其平均δ³⁰Si值为-0.13±0.03‰，同位素组成较显生宙同类岩石更偏重，这一现象可能源于TTGs源区熔体混入了海水来源的污染物质。始太古代TTGs的硅同位素特征与显生宙火成岩并无显著差异，这表明其可能形成于弧环境的岩浆分异过程。本研究获得的变玄武岩与长英质火山岩的硅同位素特征与现代同类岩石高度一致，指示其形成于俯冲带环境。对于变沉积岩，本次测得的δ³⁰Si值范围为-0.19‰至-1.2‰，远低于经历强烈蚀变的现代泥质沉积物的同位素范围，这意味着其原岩为经历了显著化学风化作用的TTGs，并形成了黏土矿物。综合以上分析结果，本研究认为38亿年以上的始太古代大陆地壳形成过程可能包含俯冲作用，尤为关键的是，早期水圈的影响在此过程中发挥了重要作用。