Collaborative Research: Constraining the Relative Importance of Fluid Fluxes and Lithospheric Metasomatism on the Evolution of the Rio Grande Rift, New Mexico

最近在美国西南部的裂谷模型表明，北美板块下的地幔已经经历了水化，可能是从大约8000万到4000万年前法拉隆板块在北美板块下的浅俯冲。水和其他挥发性元素（氯，硫，氟）的大陆地幔（岩石圈）的增加可能会对火山活动和构造过程产生深远的影响。水（沿着挥发性和流体流动的微量元素）加入地幔降低了熔融温度，使岩浆在较低的温度下生成。此外，地幔的水化作用削弱了岩石圈，潜在地允许更大程度的裂谷作用。尽管地幔水化作用的重要性很明显，但对岩石圈中的水量或水的来源的直接限制很少。因此，本研究的主要重点是测量玄武岩中的挥发物丰度，以确定挥发物在多大程度上被添加到大陆岩石圈。这项研究将集中在新墨西哥州的格兰德河裂谷-裂谷的范围从北到南各不相同，这使得这是一个理想的位置，以测试岩石圈的水合作用和裂谷和火山活动的范围之间的相关性。因为岩浆在地球表面喷发时脱气，精确测量挥发物丰度可能是有问题的。为了解决这一难题，熔融包裹体，小包裹体的岩浆结晶过程中被困在矿物和脱气之前，将进行分析，以提供原始玄武岩浆的挥发分含量的限制。本研究将从沿着和横跨格兰德河裂谷的南北和东西横断面收集原始熔岩，以充分比较挥发物浓度与裂谷程度，并寻找地幔水合程度的横向变化。此外，将检查不同年龄的熔岩，以解决岩石圈水合作用的潜在时间变化和从岩石圈到软流圈地幔熔融的潜在过渡。尽管先前有证据表明富集了流体流动的微量元素（例如，高Ba/Nb和Sr/Nd），其富集来源是地幔还是地壳一直存在争议。在这种新的方法中，流体流动的微量元素（俯冲的特征）和挥发性浓度与全岩同位素（Sr-Nd-Pb）和微量元素分析的熔融包裹体的组合将使我们能够区分潜在的地壳和地幔成分，积极识别地幔水合作用的一个重大障碍。熔体夹杂物的分析技术大大受益于微束技术的进步。主要元素和挥发物（S、Cl和F）将在俄勒冈州州立大学通过电子探针进行分析，而痕量元素丰度将在亚利桑那州州立大学通过离子探针进行测量。全同位素分析将使用奥斯汀得克萨斯大学新的TIMS设施进行，从而在几个机构进行合作研究。这项研究的更广泛的影响包括培养本科生的岩石学技术和早期职业科学家的持续发展。

Recent models of rifting in the Southwest United States have suggested that the mantle beneath the North American Plate has undergone hydration, potentially from shallow subduction of the Farallon Plate beneath the North American Plate from ~80-40 million years ago. The addition of water and other volatile elements (chlorine, sulfur, fluorine) to the continental mantle (lithosphere) may have a profound influence on volcanism and tectonic processes. Water (along with volatile and fluid-mobile trace elements) added to the mantle reduces the melting temperature, allowing magmas to be generated at lower temperatures. Additionally, hydration of the mantle weakens the lithosphere, potentially allowing for a greater extent of rifting. Despite the clear significance of hydration of the mantle, direct constraints on the amount of water in the lithosphere or the source(s) of this water are few. The primary focus of this research therefore is to measure volatile abundances in basaltic lavas to determine the extent to which volatiles have been added to the continental lithosphere. This study will concentrate on the Rio Grande Rift in New Mexico- the extent of rifting varies from N-S making this an ideal location to test for a correlation between hydration of the lithosphere and the extent of rifting and volcanism.Because magmas degas upon eruption on the Earth's surface, precise measurements of volatile abundances can be problematic. In order to address this dilemma, melt inclusions, small parcels of magma trapped within minerals during crystallization and prior to degassing, will be analyzed to provide constraints on the volatile content of primitive basaltic magmas. Primitive lavas will be collected for this study from N-S and E-W transects along and across the Rio Grande Rift to adequately compare volatile concentrations to the extent of rifting and to look for lateral variations in the degree of mantle hydration. Additionally, lavas of varying ages will be examined to address potential temporal variations in the hydration of the lithosphere and potential transition from lithospheric to asthenospheric mantle melting. Despite prior evidence of enrichment in fluid-mobile trace elements (e.g., high Ba/Nb and Sr/Nd), the source of the enrichment, either in the mantle or the crust, has long been debated. In this new approach, the combination of fluid-mobile trace elements (characteristic of subduction) and volatile concentrations from melt inclusions with whole rock isotope (Sr-Nd-Pb) and trace element analysis will allow us to distinguish potential crustal and mantle components, a significant obstacle to positively identifying mantle hydration. Analytical techniques for melt inclusions have greatly benefited from advancement in microbeam technology. Major elements and volatiles (S, Cl, and F) will be analyzed by electron microprobe at Oregon State University while trace element abundances will be measured by ion microprobe at Arizona State University. Whole isotope analyses will be conducted using the new TIMS facility at University of Texas at Austin resulting in collaborative research at several institutions. Broader impacts of this research include training of undergraduate students in petrologic techniques and the continued development of early career scientists.