Determination of the cooling history of magmatic intrusions usingdiffusion geochronometry

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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.