Collaborative Research: Diffusion of Helium in Calcite

同位素热年代学是各种地质研究的重要工具，特别是隆升-侵蚀-变形和盆地加热研究。除了少数例外，这些研究都集中在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温度计的潜力。虽然典型的海相方解石[U]含量为5ppm，但这种低浓度所带来的问题可以通过增大样本量来克服，而且在许多岩石（即石灰石和大理岩）中，直径大于1mm的方解石晶体是常见的。对He在火成岩、变质岩和沉积方解石中的扩散率的初步研究表明，He的封闭温度为~65C，冷却速率为10C/m.y。这些非常有希望的初步结果将通过进一步的分析和比较实验室诱导的扩散与自然界的扩散历史，方解石和白云石的扩散来证实。本研究的最终目的是确定He在方解石中保留的温度。这项工作具有巨大的潜力，因为方解石是石灰石和大理石中必不可少的矿物，而这些岩石中不含其他可用于确定时间-温度历史的矿物。该技术的应用包括沉积盆地的历史分析（沉积盆地的温度是寻找石油的重要考虑因素）、山带的隆升和洞穴学。

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.