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: 1051807
• 负责人: Michael Rowe
• 金额: $ 10.48万
• 依托单位: Washington State University
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2010
• 资助国家: 美国
• 起止时间: 2010-07-15 至 2012-06-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1051807&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.

美国西南部最近的裂谷模型表明，北美板块之下的地幔经历了水化作用，可能是大约8000万至4000万年前北美板块下方的法拉隆板块浅俯冲所致。水和其他挥发性元素(氯、硫、氟)加入大陆地幔(岩石圈)可能会对火山作用和构造过程产生深远影响。向地幔中加入水(以及挥发性和流动的痕量元素)可以降低熔融温度，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，潜在地允许更大程度的裂谷。尽管地幔的水化作用具有明显的意义，但对岩石圈中的水的数量或水的来源(S)的直接制约很少。因此，这项研究的主要重点是测量玄武岩熔岩中的挥发分丰度，以确定挥发分在大陆岩石圈中的添加程度。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N-S使这里成为测试岩石圈水化与裂谷和火山活动程度之间的相关性的理想地点。由于岩浆在地球表面喷发时会喷发，准确测量挥发性丰度可能是有问题的。为了解决这一困境，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，以限制原始玄武岩岩浆的挥发含量。为进行这项研究，将从沿格兰德裂谷和横跨里奥格兰德裂谷的N-S和E-W断面收集原始熔岩，以充分比较裂谷程度的挥发分浓度，并寻找地幔水化程度的横向变化。此外，还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流体活动的痕量元素(如高Ba/Nb和Sr/Nd)具有丰度，但地幔或地壳中的丰度来源长期以来一直存在争议。在这一新方法中，熔体包裹体的流体活动性微量元素(俯冲特征)和挥发分浓度与全岩同位素(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，这是正确识别地幔水化的重要障碍。熔体包裹体的分析技术大大得益于微束技术的进步。主要元素和挥发物(S、氯和氟)将在俄勒冈州立大学用电子探针进行分析，而微量元素丰度将在亚利桑那州立大学用离子探针进行测量。整个同位素分析将使用德克萨斯大学奥斯汀分校的TIMS新设施进行，从而在几个机构进行合作研究。这项研究的更广泛影响包括对本科生进行岩石学技术培训，以及对早期职业科学家的持续发展。