Chronometry in plutonic rocks: cooling rates of ancient oceanic crust

自板块构造开始以来，在大洋中脊形成新的洋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，在洋壳深成部分的形成过程中，如何有效地传递热量。关于洋脊深部的形成，人们提出了几种模型，它们主要围绕着洋壳通过流体的传导与平流来冷却。突出的端元模型包括辉长岩-冰川模型和片状岩床模型。到目前为止，测试这些模型的尝试导致了不确定的结果。例如，虽然在岩石中发现了深部热液流动的证据，但整个地壳的冷却似乎更符合传导性冷却。因此，有必要绘制出冷却速度作为与记录的流体通道距离的函数的分布图，同时量化流体流量。我们建议用一种综合的、多尺度的方法来解决这个问题，将扩散计时学和同位素地球化学结合起来，对阿曼蛇绿岩中Wadi Gideah参考剖面的层状辉长岩内的聚焦流体流动带(FFFZ)的样品进行研究。如果这些带具有有效冷却深部地壳的潜力，我们预计在FFFZ接触处的层状辉长岩将强烈冷却。此外，如果这种流体流动能够提高周围地壳的整体冷却速度，那么即使在相对于FFFZ的远场区域，也会记录到如此高的冷却速度。或者，如果只在近场区域获得高冷却速率，这可以解释为什么即使有流体流动的证据，也能获得整个地壳的传导冷却特征。因此，我们的目标是量化从与FFFZ接触开始的多个剖面的冷却速率，直到进入质量层状辉长岩。不同辉长岩样品中橄榄石、斜长石和辉石的冷却速度将使用多重扩散计时仪来确定。其中包括，例如，成熟的钙橄榄石地质测速仪，以及最近开发的基于斜长石(镁、钡、锶、镧、铈)中微量元素扩散的地质测速仪，以及任何可能使用辉石的计时仪(例如，铁-镁、锶或锂在单斜辉石中的扩散)。此外，我们建议应用快速扩散稳定的Li同位素体系，这是一种非常适合于测定橄榄石-辉石-斜长石体系中水热促进的快速冷却速度的工具，因此可以用来同时确定冷却速度和流体流量。除了提供对洋壳冷却机制的见解外，这项研究还将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。

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.