Collaborative Research: Diffusion of Helium in Calcite

同位素热年代学是各种地质学研究的重要工具，特别是包括研究抬升-侵蚀-变形和盆地加热。除了少数例外，这些研究一直集中在K或U浓度相对较高的矿物上。在这个项目中，研究人员将研究He在低U矿物方解石中的扩散，以更好地了解方解石作为(U+Th)/He热力学计时器的潜力。虽然典型的海相方解石具有5ppm的[U]，但与这种低浓度相关的问题可以通过增加样品大小来克服，而且在许多岩石(即石灰岩和大理石)中，直径大于1毫米的方解石晶体很常见。对He在火成岩、变质岩和沉积方解石中扩散系数的初步研究表明，在冷却速度为10C/m.y时，He的闭合温度为~65℃。这些非常有希望的初步结果将被更多的分析证实，并通过比较实验室诱导的扩散与自然界的扩散历史，以及方解石与白云石的扩散。这项研究的最终目的是确定他在方解石中的保留温度。这项工作具有巨大的潜力，因为方解石是石灰岩和大理石中的基本矿物，而这些岩石中没有其他可以用来确定时间-温度历史的矿物。这项技术可能用于的应用包括分析沉积盆地的历史(沉积盆地的温度是寻找石油的一个重要考虑因素)、山带的隆起和洞穴学。

Isotope thermochronology is an important tool in a variety of geologic studies including, in particular, investigations of uplift-erosion-deformation and basin heating. With a few exceptions, these studies have been focused on minerals with relatively high concentrations of either K or U. In this project, the investigators will study the diffusion of He in the low-U mineral calcite to better understand the potential for the use of calcite as a (U+Th)/He thermochronometer. Although typical marine calcites have [U]5ppm, problems associated with this low concentration can be overcome by increasing sample size and in many rocks (i.e., limestones and marbles) calcite crystals greater than 1 mm in diameter are common. Preliminary investigations on the diffusivity of He in igneous, metamorphic, and sedimentary calcite indicate a closure temperature for He of ~65C for a cooling rate of 10C/m.y. These extremely promising preliminary results will be confirmed by additional analyses and by comparing lab-induced diffusion with diffusive histories in nature, and calcite versus dolomite diffusion. The ultimate purpose of this research is to determine the temperature at which He is retained in calcite. This work has great potential because calcite is an essential mineral in both limestones and marbles and these rocks contain no other mineral which can be used to determine time-temperature history. The applications in which this technique might be used include analysis of the history of sedimentary basins (the temperature of sedimentary basins is an important consideration in the search for petroleum), the uplift of mountain belts, and speleology.