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

美国西南部最近的裂谷模型表明，北美板块下的地幔经历了水化作用，可能是8000万至4000万年前法拉龙板块在北美板块下的浅层俯冲造成的。向大陆地幔（岩石圈）添加水和其他挥发性元素（氯、硫、氟）可能对火山作用和构造过程产生深远影响。添加到地幔中的水（以及挥发性和可流动的微量元素）降低了熔融温度，从而使岩浆在较低的温度下产生。此外，地幔的水化作用削弱了岩石圈，可能导致更大程度的裂谷作用。尽管地幔水化作用具有明确的意义，但对岩石圈中水量或这些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熔岩中挥发物的丰度，以确定挥发物被添加到大陆岩石圈的程度。这项研究将集中在新墨西哥州的里约热内卢大裂谷上——裂谷的程度从南北走向不同，这使得它成为测试岩石圈水化作用与裂谷和火山作用程度之间相关性的理想地点。由于岩浆在地球表面喷发时会脱气，因此对挥发物丰度的精确测量可能存在问题。为了解决这一难题，将分析熔融包裹体，即在结晶和脱气之前被困在矿物中的小块岩浆，以提供原始玄武岩岩浆挥发性含量的约束。本研究将从沿里约热内卢大裂谷和横跨里约热内卢大裂谷的N-S和E-W样带收集原始熔岩，以充分比较挥发性浓度与裂谷的程度，并寻找地幔水化程度的横向变化。此外，将研究不同年龄的熔岩，以解决岩石圈水化作用的潜在时间变化以及岩石圈向软流圈地幔熔融的潜在转变。尽管先前有证据表明流体流动微量元素（如高Ba/Nb和Sr/Nd）富集，但富集的来源是地幔还是地壳，长期以来一直存在争议。在这种新方法中，流体可移动的微量元素（俯冲特征）和熔融包裹体挥发性浓度与全岩石同位素（Sr-Nd-Pb）和微量元素分析相结合，将使我们能够区分潜在的地壳和地幔成分，这是确定地幔水化作用的一个重大障碍。熔体夹杂物的分析技术得益于微束技术的进步。主要元素和挥发物（S， Cl和F）将在俄勒冈州立大学用电子探针分析，微量元素丰度将在亚利桑那州立大学用离子探针测量。整个同位素分析将使用德克萨斯大学奥斯汀分校的新TIMS设备进行，从而导致几个机构的合作研究。这项研究的更广泛的影响包括培养岩石学技术的本科生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.