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

合作研究：限制流体通量和岩石圈交代作用对新墨西哥州格兰德裂谷演化的相对重要性

• 批准号: 0810152
• 负责人: John Lassiter
• 金额: $ 5.8万
• 依托单位: University of Texas at Austin
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2008
• 资助国家: 美国
• 起止时间: 2008-07-01 至 2012-06-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0810152&HistoricalAwards=false
• 关键词: Collaborative; Research; Constraining; Relative; Importance

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.

美国西南部的最新裂谷模型表明，北美板块下方的地幔已经经历了水合作用，这可能是由于约 80-4000 万年前法拉隆板块浅层俯冲到北美板块下方所致。水和其他挥发性元素（氯、硫、氟）添加到大陆地幔（岩石圈）可能对火山活动和构造过程产生深远的影响。添加到地幔中的水（以及挥发性和可流动的微量元素）会降低熔化温度，从而使岩浆在较低的温度下产生。此外，地幔的水合作用削弱了岩石圈，可能导致更大范围的裂谷。尽管地幔水合作用具有明显的意义，但对岩石圈中水量或水源的直接限制却很少。因此，这项研究的主要重点是测量玄武岩熔岩中的挥发物丰度，以确定挥发物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奥格兰德裂谷 - 裂谷的程度从南北向变化，这使得这里成为测试岩石圈水合作用与裂谷和火山活动程度之间相关性的理想地点。因为岩浆在喷发时会脱气。在地球表面，精确测量挥发性丰度可能存在问题。为了解决这一难题，将分析熔融包裹体，即在结晶过程中和脱气之前被困在矿物中的小块岩浆，以提供对原始玄武岩浆挥发物含量的限制。本研究将从沿着里奥格兰德裂谷的南北和东西横断面收集原始熔岩，以充分比较裂谷范围内的挥发性浓度，并寻找地幔水合程度的横向变化。此外，还将检查不同年龄的熔岩，以解决岩石圈水合的潜在时间变化以及从岩石圈到软流圈地幔熔化的潜在转变。尽管先前有证据表明流体流动的微量元素（例如高 Ba/Nb 和 Sr/Nd）富集，但富集的来源（无论是在地幔还是地壳）长期以来一直存在争议。在这种新方法中，流体移动的微量元素（俯冲特征）和来自熔体包裹体的挥发物浓度与全岩石同位素（Sr-Nd-Pb）和微量元素分析的结合将使我们能够区分潜在的地壳和地幔成分，积极识别地幔水化的一个重大障碍。熔体夹杂物的分析技术极大地受益于微束技术的进步。主要元素和挥发物（S、Cl 和 F）将由俄勒冈州立大学的电子微探针进行分析，而微量元素丰度将由亚利桑那州立大学的离子微探针进行测量。整个同位素分析将使用德克萨斯大学奥斯汀分校的新 TIMS 设施进行，从而在多个机构进行合作研究。这项研究的更广泛影响包括对本科生进行岩石学技术培训以及早期职业科学家的持续发展。