Unravelling crustal formation in the Early Earth through the mass-dependent stable Ti isotope composition of Archaean Tonalite-trondhjemite-granodiorites (TTGs)

所谓的tonalites - trondhjemites - granodiorites （TTG）系列的岩石是一种主要局限于太古宙的钠质花岗岩，对我们理解早期地球大陆地壳的形成至关重要。虽然大多数人认为TTG是由含水基性岩性部分熔融引发的多阶段过程的结果，但关于主持这一部分熔融事件的确切地球动力学背景仍存在争议。假设范围从在类似俯冲背景下变厚的蚀变洋壳和榴辉岩的熔融，到在海洋高原背景下基性地壳的部分熔融。通过10-25%的TTG熔体萃取，留下含有钛铁矿和金红石等副相的致密残留物。在后期的TTG分异过程中，岩浆房中的熔体在结晶过程中形成堆积。这些矿物含有钛铁矿和角闪石。含钛矿物参与岩浆过程（即熔融和后来的结晶），可能导致可分解的钛同位素分馏。此外，致密矿石进入地幔的分层可能是后来科马铁矿熔体的来源，可能携带其独特的Ti同位素特征。通过测量古太古代的科马长岩，这些遗迹的命运有可能被追踪。此外，为了限制分数结晶的影响，我们计划研究太古宙层状斜长杂岩样品的Ti同位素。当代岩石记录清楚地表明，与羽状柱相关的岩浆样本和与岛弧相关的岩浆样本在岩浆分化过程中表现出不同的Ti同位素组成，与之相比，Ti同位素可能为TTG形成的确切地球动力学环境提供了见解。根据这一建议，我们的目标是开展详细的分析和实验活动，处理TTG岩石成因的主要方面，从其含水基性前体的部分熔融到其母岩浆的分异结晶和分异。为此，我们将结合特征良好的TTG、西格陵兰岛南部层状斜长岩体和南非科马地岩样品的Ti同位素数据，以及处理TTG岩石成因的活塞圆柱体实验的类似数据。然后将这两个数据集结合起来，揭示导致TTG形成的确切条件。这一建议可能使Ti同位素成为重建地球大陆地壳最早形成历史的新工具，并有助于我们对太古宙地球动力学的理解。

Rocks from the so-called Tonalite-Trondhjemite-Granodiorites (TTG) series, a type of sodic granitoid that is largely restricted to the Archean, are pivotal to our understanding of continental crust formation in the early Earth. While most agree TTG formed as a result of a multi-stage process that is initiated by the partial melting of hydrous mafic lithologies, controversy remains about the exact geodynamic setting that presided over this partial melting event. Hypothesis range from the melting of thickened altered oceanic crust and eclogite in a subduction-like setting, to the partial melting of mafic crust in an oceanic plateau setting. Through 10-25% of TTG melt extraction dense residues which contain accessory phases like ilmenite and rutile are left behind. Later during the differentiation of the TTG melts in the magma chamber cumulates form during crystallization processes. These contain ilmenite and hornblende. The involvement of Ti-bearing minerals in either magmatic process (i.e. melting and later crystallization), likely results in resolvable Ti isotope fractionation. Moreover, delamination of the dense restites into the mantle may contribute to the sources of later komatiite melts, possibly carrying their unique Ti isotope signature. By measuring Paleoarchean komatiites, the fate of these restites can potentially be traced. Moreover, to place constraints on the effect of fractional crystallization, we plan to investigate samples from an Archean layered anorthosite complex for Ti isotopes. In comparison to the contemporary rock record, which clearly shows that plume-related magmatic samples, and those associated with island arcs, display disparate behaviour concerning their Ti isotope composition during magmatic differentiation, Ti isotopes may provide insights into the exact geodynamic environment where TTG formed. With this proposal we aim to carry out a detailed analytical and experimental campaign, dealing with the main aspects of TTG petrogenesis, starting from the partial melting of their hydrous mafic precursors to the fractional crystallization and differentiation of their parental magmas. To do this we will combine Ti isotope data of well characterized TTG, rocks from layered anorthosite intrusions from southern West Greenland and komatiite samples from South Africa, with similar data that results from piston cylinder experiments dealing with TTG petrogenesis. Both datasets will then be combined to unravel the exact conditions that lead up to TTG formation. This proposal may establish Ti isotopes as a new tool to reconstruct the earliest formation history of Earth’s continental crust and may contribute to our understanding of Archean geodynamics.