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

来自所谓的Tonalite-Trondhjemite-GranoDiorites(TTG)系列的岩石，是一种主要局限于太古宙的苏打花岗岩类，对我们理解早期地球的陆壳形成至关重要。虽然大多数人认为TTG是由含水镁铁质岩性的部分熔融引发的多阶段过程的结果，但对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。从俯冲环境中增厚的蚀变洋壳和榴辉岩的熔融，到大洋高原环境中镁铁质地壳的部分熔融，都有假说。通过10-25%的TTG熔体萃取，留下含有钛铁矿、金红石等副相的致密残留物。后来，在TTG的分异过程中，熔体在岩浆室中堆积形成结晶过程。它们含有钛铁矿和角闪石。含钛矿物参与了岩浆作用(即熔融和后期结晶)，可能导致可分辨的钛同位素分馏。此外，致密残石向地幔的剥离可能有助于后来科马提石熔体的来源，可能带有其独特的钛同位素特征。通过测量古太古代科马提岩，这些残留物的命运可能会被追踪。此外，为了限制分离结晶的影响，我们计划调查太古宙层状斜长岩杂岩体中的钛同位素。与当代岩石记录相比，当代岩石记录清楚地表明，与羽流有关的岩浆样品以及与岛弧有关的样品在岩浆分异过程中的钛同位素组成表现出不同的行为，钛同位素可能提供对TTG形成的确切地球动力学环境的洞察。根据这一建议，我们旨在开展一项详细的分析和实验活动，处理TTG岩石成因的主要方面，从其含水镁铁质前体的部分熔融到其母岩浆的分离结晶和分异。为此，我们将结合特征良好的TTG、来自格陵兰南部层状斜长岩侵入体的岩石和来自南非的科马提岩样品的钛同位素数据，以及活塞筒实验中关于TTG岩石成因的类似数据。然后，这两个数据集将被组合起来，以揭示导致TTG形成的确切条件。这一建议可能会确立钛同位素作为一种新的工具来重建地球最早的陆壳形成历史，并可能有助于我们对太古代地球动力学的理解。

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.