Detrital rutile petrochronology: Assessing metamorphism through time at the global scale

重建地球历史上的变质条件对于理解热、动力和化学机制的长期演化具有重要意义。一个主要的和显然不可逾越的障碍是由破碎保存的结晶岩石记录标记的。关于板块构造是早还是晚在全球范围内开始的，从之前的非板块构造状态的过渡是突然的还是持久的，以及热状态是否从温暖状态演变到寒冷状态的解释，尤其受到从局部尺度外推到全球尺度的影响。相比之下，碎屑矿物颗粒具有显著的潜力，可以有效地从广泛的来源获取大型数据集，包括因侵蚀而丢失的造山带脱落的保存沉积物，从而更好地代表全球尺度的图景。然而，目前还没有一种工具能够从碎屑的角度可靠地结合年龄和P-T（压力-温度）信息。基于先进的统计方法在已编译的文献数据集上的初步应用，金红石中微量元素的多元考虑显示出很大的潜力来弥补这一差距。拟建项目分为两个阶段。在第一阶段，将通过从明确定义的结晶岩石样品中获取新数据并与已发表的数据相结合，编制金红石微量元素数据库。这将包括来自交代、火成岩、地幔和变质成因金红石的17种微量元素浓度。后者将被额外分析8种元素浓度。含金红石的变质岩样品涵盖了长英质和基性块状岩石组成以及广泛的P-T条件。建立了一种多变量判别模型(1)分离交代、火成岩、地幔和变质金红石，(2)预测变质金红石的T/P梯度。在第二阶段，开发的鉴别方案将与U-Pb定年相结合，并应用于来自南非、南美、加拿大东北部、印度南部和澳大利亚西部大部分地区河流的33个现代沙样品中的约10,000个金红石碎屑颗粒。这些河流的样品主要来自太古宙和元古代单元，以及来自记录早期侵蚀阶段的盆地的再加工沉积物，这些是通过结晶岩记录无法获得的。对于表明来自低T/P变质岩的样品，将对碎屑金红石的包裹体组合进行筛选，以寻找具有特征的低T/P矿物，以便通过矿物学证据证实地球化学预测。一个成功的项目将为变质梯度的演化提供新的线索，并支持或证伪我们的中心假设，即“古元古代全球范围内建立了现代风格的低T/P梯度”。

Reconstructing metamorphic conditions throughout Earth history is of fundamental importance to understand the long-term evolution of thermal, dynamic, and chemical regimes. A major and apparently insuperable obstacle is marked by the fragmentarily preserved crystalline rock record. Interpretations of whether plate tectonics globally initiated early or late, whether the transition from a preceding non-plate-tectonic regime was abrupt or long lasting, and whether thermal regimes secularly evolved from warm to cold conditions suffer in particular from extrapolating local-scale observations to the global scale. By contrast, detrital mineral grains have remarkable potential to efficiently acquire large datasets from a wide range of sources, including preserved sediments shed from orogens that are lost by erosion, and thus provide a much better representation of the global-scale picture. However, up-to-date, no tool exists which is able to reliably combine age and P–T (pressure–temperature) information from a detrital perspective. Based on a preliminary application of advanced statistical methods on a compiled literature dataset, a multivariate consideration of trace elements in rutile demonstrates high potential to close this gap. The proposed project is subdivided into two phases. In the first phase, a rutile trace-element database will be compiled by acquiring new data from well-defined crystalline rock samples and integration with published data. This will include 17 trace-element concentrations from rutiles of metasomatic, igneous, mantle, and metamorphic origin. The latter will be additionally analyzed for 8 element concentrations. Rutile-bearing samples from metamorphic rocks cover felsic and mafic bulk-rock compositions as well as a broad range of P–T conditions. A multivariate discrimination model will be developed that (i) separates metasomatic, igneous, mantle and metamorphic rutile, and (ii) predicts T/P gradients for metamorphic rutile. In the second phase, the developed discrimination scheme will be combined with U–Pb dating and applied to ~10,000 detrital rutile grains from 33 modern-sand samples of rivers draining considerable parts of S Africa, S America, NE Canada, S India, and W Australia. These rivers mainly sample material sourced from Archean and Proterozoic units as well as reworked sediments from basins recording earlier stages of erosion, which are inaccessible via the crystalline rock record. For samples indicating a contribution from low T/P metamorphic rocks, inclusion assemblages of detrital rutiles will be screened for characteristic low T/P minerals in order to confirm geochemical predictions by mineralogical evidence. A successful project will shed new light on the evolution of metamorphic gradients through time and support or falsify our central hypothesis that “modern-style low T/P gradients were established globally in the Paleoproterozoic”.