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排出的主要机制。一个密切相关的过程是热液流体的循环，例如，它是大型矿床形成的原因。然而，关于岩浆体如何冷却的知识在很大程度上是基于模型计算的，但是基于冷却历史的自然数据的定量限制很少。在给定的温度区间内，不同的热传递和移出模式（如传导、热液循环、熔体中的对流）会留下明显不同的冷却速率模式。在这里，我们建议从入侵的不同层次确定冷却速率。这将使我们有可能区分在冷却侵入体的过程中，哪一种可能的过程占主导地位。定量的亚固体冷却速率将用不同的扩散计时器从天然岩石样品中确定；最近发展的斜长石中含镁和完善的钙橄榄石地速计。这两种独立方法的应用将保证冷却速率的鲁棒性和一致性。获得的数据将与分析和数值热传输模型的冷却速率预测进行比较，以确定热量排出的主要模式。作为第一步，我们将研究相对简单的单阶侵入，其中侵入体的几何形状和侵入历史是众所周知的。在单个井壁内不同位置的冷却速率（即从上到下与围岩接触的剖面）将使我们能够量化涉及井壁冷却的热液循环的影响/程度。在第二步中，我们将研究更复杂、更大的系统，以分层侵入为代表，由单批岩浆形成（如Skaergaard侵入）或由多个幕式侵入（如Bushveld复合体）。绘制这些岩浆系统的冷却速率将使我们能够量化不同层状侵入体亚固体冷却的时间尺度，并提供有关侵入体热结构和复杂热演化的信息。研究结果将更好地量化这些侵入火成岩体的热输运和热损失模式。这也将是了解矿床成因的重要一步，矿床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.