Determination of the cooling history of magmatic intrusions usingdiffusion geochronometry

岩浆的冷却和结晶是地球内部热量转移的主要机制。一个密切相关的过程是热液流体的循环，例如，热液流体是形成大型矿床的原因。然而，关于岩浆体如何冷却的知识主要是基于模拟计算，但很少有基于自然数据的冷却历史的定量约束。不同的热传输和去除模式（例如传导，热液循环，熔体中的对流）在给定的温度间隔内留下明显不同的冷却速率模式。在这里，我们建议确定冷却速率从不同层次内的入侵。这将为我们提供可能性，以区分哪些可能的过程中参与冷却的侵入体dominant.Quantitative亚固相线冷却速率将确定从天然岩石样品使用不同的扩散计时器;最近开发的镁斜长石和完善的钙橄榄石geoseedometer。这两种独立方法的应用将确保冷却速率的鲁棒性和一致性。将获得的数据进行比较，从分析和数值热传输模型的冷却速率预测，以确定占主导地位的模式heat removal.As第一步，我们将研究相对简单的情况下，单岩床侵入体，侵入体的几何形状和侵入的历史是众所周知的。单个岩床内不同位置的冷却速率（即，在与围岩从上到下接触的剖面上）将允许我们量化岩床冷却中涉及的热液循环的影响/程度。在第二步中，我们将研究更复杂，更大的系统，以层状侵入体为代表，由一批岩浆（例如Skaergaard侵入体）或多个幕式侵入体（例如Bushveld杂岩）形成。在这些岩浆系统中映射冷却速率将使我们能够量化不同层状侵入体的亚固相线冷却的时间尺度，并提供侵入体的热结构和复杂的热演化信息。作为这项研究的结果，在这些侵入火成岩体的热传输和热损失的模式将更好地量化。这也将是理解矿床成因的重要一步，矿床成因与岩浆系统中热液流体的循环有关。

The cooling and crystallization of magmas is a major mechanism of heat removal from the interior of the Earth. An intimately linked process is the circulation of hydrothermal fluids that are, for instance, responsible for the formation of large ore deposits. Yet, the knowledge of how magma bodies cool is largely based on modelling calculations, but little quantitative constraints based on natural data on the cooling history are available.Different modes of heat transport and removal (e.g. conduction, hydrothermal circulation, convection in a melt) leave behind distinctively different patterns of cooling rate over a given temperature interval. Here, we propose to determine cooling rates from different levels within an intrusion. This will provide us the possibility to distinguish which of the possible processes involved in cooling the intrusive body were dominant.Quantitative sub-solidus cooling rates will be determined from natural rock samples using different diffusion chronometers; the recently developed Mg-in-plagioclase and the well-established Ca-in-olivine geospeedometer. Application of these two independent methods will insure the robustness and consistency of the cooling rates. The obtained data will be compared with cooling rate predictions from analytical and numerical heat transport models to determine the dominant mode of heat removal.As a first step, we will study the relatively simple case of single sill intrusions, where the geometry of the intrusive body and the intrusion history are well known. Cooling rates for different positions within a single sill (i.e. over a profile from the upper to the lower contact with the wall rock), will allow us to for quantify the effect/extent of hydrothermal circulation involved in cooling of sills. In a second step, we will study more complex, larger systems, represented by layered intrusions, that formed by a single batch of magma (e.g. the Skaergaard Intrusion) or by multiple episodic intrusions (e.g. the Bushveld Complex). Mapping cooling rates in these magmatic systems will allow us to quantify the timescales of sub-solidus cooling of different layered intrusions, and provides information on the thermal structure and complex thermal evolution of an intrusion. As a result of this study, the mode of heat transport and heat loss in these intrusive igneous bodies will be better quantified. This will also be an important step toward understanding the genesis of ore deposits, which are bound to the circulation of hydrothermal fluids in magmatic systems.