Collaborative Research: "Double-Double Dating" of Detrital Monazite and Detrital Zircon: Quantifying Sediment Recycling in Tectonic Studies

许多地球科学家认为，沉积岩很可能是“再循环”的，即沉积物首先从先前存在的沉积岩中被侵蚀，然后被重新置于水文循环中，然后再次沉积到盆地中，成为新的沉积岩。尽管这是一个重要的建议，但量化这一过程发生的程度的数据有限。这项研究将测试一种新的方法，使用一种新的和相对便宜的方法，利用铀和钍同位素的化学分析和测年，以提供所需的定量数据。如果这些新方法成功，各种各样的科学家可以使用它们来更好地了解沉积盆地的历史--这些信息不仅对学术科学家至关重要，对石油和经济矿产行业的人也是如此。除了研究的科学目标外，该项目还培训一门重要的STEM(科学、技术、工程和数学)学科的本科生和研究生，从而为培养下一代科学家做出贡献。此外，它还将在纽约中部和肯塔基州为高中学生提供进入地球科学的途径，并将促进根据研究结果为教师和学生开发教学模块。研究结果将作为同行评议的出版物和在专业协会会议上的介绍予以传播。分析数据将通过公众可访问的在线数据库提供。在构造研究中考虑沉积物再循环对于准确评估地体运动和碰撞历史等构造重建至关重要。初步研究表明，一些碎屑独居石颗粒显示出在较老的碎屑序列中以成岩作用形成的结构证据(即碎屑成岩独居岩)，因此必须回收。主要研究人员认为，根据结构和年龄，碎屑成岩颗粒很容易与源自源区结晶岩石的碎屑独居石区分开来。该项目将测试这种快速检测回收的方法的有效性，而不是成本更高、劳动密集得多的碎屑锆石“双测年”方法(即，铀-铅年代学与同一晶体的(铀-钍)/氦热年代学相结合)。主要研究人员将利用独居石晶体的背向散射电子成像进行结构分析，以表征被确认为成岩碎屑独居岩和碎屑变质/火成岩独居岩的颗粒。然后，这些颗粒将使用钍-铅地质年代学技术进行测年，然后是(铀-钍-钐)/氦测年，以定量地确定我们的快速、定性方法是否有效。研究人员还将确定单个独居石颗粒的钕同位素组成，以确定这是否是扩大沉积物源信息的一种新的有用方法。同位素方法将应用于美国东部的新元古代和古生代沉积地层。

Many geoscientists have suggested that it is likely that sedimentary rocks are "recycled", i.e. sediment is first eroded from pre-existing sedimentary rocks, gets placed back into the hydrologic cycle, and then is once again deposited into a basin to become a new sedimentary rock. Although this is an important suggestion, there are limited data that quantify to what degree this process occurs. This research will test a new approach, using a novel and relatively inexpensive method utilizing chemical analysis and dating of uranium and thorium isotopes in order to provide this needed quantitative data. If these new methods are successful, they can be used by a wide variety of scientists to develop a much better understanding of the history of a sedimentary basin - information critical not only to academic scientists but to those in both the petroleum and economic minerals industry as well. In addition to the scientific goals of the research, the project train undergraduate and graduate students in an important STEM (science, technology, engineering and mathematics) discipline, thus contributing to the education of the next generation of scientists. In addition, it will provide outreach to high schools for pathways for high school students to go into the earth sciences in both central New York and Kentucky, and will also facilitate the development of teaching modules for teachers and students based on the results of the research. Results of the research will be disseminated as peer-reviewed publications and as presentations at professional society meetings. Analytical data will be made available through publicly accessible online databases.Consideration of sediment recycling in tectonic studies is critical for accurate assessment of tectonic reconstructions such as terrane movement and collisional histories. Preliminary research indicates that some detrital monazite grains exhibit textural evidence of having formed diagenetically in an older clastic sequence (i.e. detrital diagenetic monazite) and therefore must be recycled. The principal investigators suggest that the detrital diagenetic grains are readily distinguishable, based on textures and ages, from detrital monazite derived from crystalline rocks in the source terranes. This project will test the effectiveness of this rapid approach for detecting recycling compared to the much more expensive and labor intensive method of "double dating" detrital zircon (i.e. Uranium-Lead geochronology combined with (Uranium-Thorium)/Helium thermochronology of the same crystal). The principal investigators will perform textural analysis, using backscatter electron imaging of monazite crystals, to characterize grains identified as diagenetic detrital monazite versus detrital metamorphic/igneous monazite. These grains will then be dated using Thorium-Lead geochronologic techniques followed by (Uranium-Thorium-Samarium)/Helium dating to quantitatively establish if our rapid, qualitative approach is valid. The researchers will also determine the Neodymium isotopic composition of the dated single monazite grains to determine if this is a new and useful approach to expand sedimentary provenance information. The isotopic methods will be applied to Neoproterozoic and Paleozoic sedimentary strata from the eastern United States.