Chronometry in plutonic rocks: cooling rates of ancient oceanic crust

深成岩的计时：古代洋壳的冷却速率

• 批准号: 439800555
• 负责人: Dr. Kathrin Faak
• 金额: --
• 依托单位: Institut für Geologie, Mineralogie und Geophysik
• 依托单位国家: 德国
• 项目类别: Research Units
• 财政年份: 2020
• 资助国家: 德国
• 起止时间: 2019-12-31 至 2022-12-31
• 项目状态: 已结题
• 来源: https://gepris.dfg.de/gepris/projekt/439800555?language=en
• 关键词: Chronometry; plutonic; rocks; cooling; rates

Since the onset of plate tectonics the formation of new oceanic crust at mid-ocean ridges is a principal mechanism of cooling of the Earth's interior. However, one of the central unsolved problems is how heat is efficiently transferred during the formation of the plutonic section of the oceanic crust. Several models for the generation of the plutonic section of oceanic ridges have been put forward, which mainly revolve around cooling of the oceanic crust by conduction vs. advection of fluids through the crust. Prominent end-member models include the gabbro-glacier model and the sheeted sill model. Attempts to test the models have led to inconclusive results so far. For example, while evidence of deep hydrothermal flow is found in the rocks, cooling of the overall crust appears to be more compatible with conductive cooling. Therefore, it is necessary to map out the distribution of cooling rates as a function of distance from documented fluid pathways, while simultaneously quantifying the fluid flux. We propose to address this problem with a comprehensive, multiscale approach, combining diffusion chronometry and isotope geochemistry on samples from focused fluid flow zones (FFFZ) within the layered gabbros of the Wadi Gideah reference profile in the Oman ophiolite. If such zones have the potential for an effective cooling of the deep crust, we expect strong cooling of layered gabbros at the contact of the FFFZs. Moreover, if such fluid flow is capable of enhancing the cooling rate of the surrounding crust as a whole, such high cooling rates would be recorded even in the far field region with respect to the FFFZ. Alternatively, if high cooling rates are obtained only in the near-field region, it may explain why signatures of conductive cooling for the crust as a whole are obtained even when evidence of fluid flow is present.Therefore, we aim to quantify cooling rates in multiple profiles beginning at the contact to a FFFZ and progressing away into the massive layered gabbro. Cooling rates will be determined on olivine, plagioclase and pyroxenes in various gabbro samples using multiple diffusion chronometers. These include, for example, the well-established Ca-in-olivine geospeedometer together with the recently developed geospeedometers based on trace element diffusion in plagioclase (Mg, Ba, Sr, La, Ce) as well as any possible chronometers using pyroxenes (e.g. Fe-Mg, Sr or Li diffusion in clinopyroxene). Additionally, we propose to apply the fast-diffusing stable Li isotope system, which is a highly suitable tool for the determination of hydrothermally promoted fast cooling rates in the system olivine-pyroxene-plagioclase and can as such be used to simultaneously determine cooling rates as well as fluid fluxes. In addition to providing insights into the cooling mechanism of the oceanic crust, the study would enable us to check the consistency and robustness of different diffusion chronometers applied on the same samples.

自从板块构造开始以来，在洋中脊处形成新的海洋地壳是地球内部冷却的主要机制。然而，尚未解决的核心问题之一是，在海洋地壳的深部构造部分形成过程中，热量是如何有效传递的。关于洋脊深成剖面的形成，人们提出了几种模式，主要围绕着流体通过地壳的传导和平流冷却洋地壳。主要的端元模型包括辉长岩-冰川模型和片状岩模型。到目前为止，对这些模型进行测试的尝试都没有得出决定性的结果。例如，虽然在岩石中发现了深层热液流动的证据，但整个地壳的冷却似乎更符合导电冷却。因此，有必要绘制出冷却速率随距离记录流体路径的函数分布，同时量化流体通量。我们建议用一种综合的、多尺度的方法来解决这个问题，结合扩散时计和同位素地球化学方法，对阿曼蛇绿岩Wadi Gideah参考剖面层状辉长岩中的聚焦流体流动带（FFFZ）样品进行分析。如果这些带具有有效冷却深部地壳的潜力，我们预计在fffz的接触处层状辉长岩会有强烈的冷却。此外，如果这种流体流动能够提高整个周围地壳的冷却速率，那么即使在远场区域，相对于FFFZ也会记录到如此高的冷却速率。或者，如果只在近场区域获得高冷却速率，这就可以解释为什么即使存在流体流动的证据，也能获得整个地壳的导电冷却特征。因此，我们的目标是量化多个剖面的冷却速率，从接触FFFZ开始，一直到块状层状辉长岩。用多重扩散计时器测定各种辉长岩样品中的橄榄石、斜长石和辉石的冷却速率。这些包括，例如，完善的ca -in-橄榄石地球速度计，以及最近开发的基于微量元素在斜长石中的扩散（Mg, Ba, Sr, La, Ce）的地球速度计，以及任何可能使用辉石的计时器（例如，Fe-Mg， Sr或Li在斜长石中的扩散）。此外，我们建议应用快速扩散的稳定Li同位素系统，该系统非常适合用于测定橄榄石-辉石-斜长石系统中水热促进的快速冷却速率，因此可以同时用于测定冷却速率和流体通量。除了提供对海洋地壳冷却机制的见解外，该研究还使我们能够检查应用于相同样品的不同扩散计时器的一致性和稳健性。