Geochemical Investigation of Xenoliths From the Central Rio Grande Rift and Colorado Plateau: Constraints on Lithosphere Evolution and Possible Delamination

“这项奖励是根据2009年美国复苏和再投资法案（公法111-5）资助的。”学术价值：本提案寻求资金，用于研究中部里约热内卢大裂谷和科罗拉多高原边缘下岩石圈地幔的演化。该项目将为该省构造和火山演化的各种相关问题提供见解，包括Farallon板块俯冲和脱水在里约热内卢大裂谷熔岩“富集”地幔源生成中的作用。然而，这一提议主要是为了回答一个简单的问题：是否像最近几项地球物理研究表明的那样，在中央b里约热内卢大裂谷和科罗拉多高原边缘下的原始元古宙岩石圈地幔被剥离或对流侵蚀积极地移走了？如果正确的话，这些研究表明，先前在内华达山脉下记录的岩石圈分层可能比目前认识到的更为普遍，可能是由伸展和挤压构造机制引发的。将橄榄岩捕虏体中的Hf-和os -同位素变化与熔体耗尽程度的矿物学约束进行对比，将对里约热内卢大裂谷和科罗拉多高原捕虏体中记录的熔体耗尽时间进行约束。此外，橄榄岩斜辉石微量元素丰度结合Sm-Nd同位素组成将限制科罗拉多高原和里约热内卢大裂谷部分地区观测到的交代叠印的年龄。总之，这些数据将使我们能够区分元古代，融化耗尽和可能交代的岩石圈地幔和年轻的，肥沃的软流圈。因此，这项研究将为Gao等人（2004）和Song & Helmberger（2007）提出的岩石圈分层模型提供直接检验，以解释在中央裂谷和高原边缘下的浅地幔中存在异常低的地震速度，以及在中原岩石圈下倾斜的板状结构中存在互补的异常高速度。在相对较浅的深度（~40-60 km）发现软流圈衍生的橄榄岩，整个地区以橄榄岩包体套为代表，这将支持所提出的分层模型，并与岩石圈因伸展而简单被动减薄相矛盾。相比之下，在这些深度识别古代岩石圈将表明岩石圈的移动明显较少，并且需要重新评估现有地震层析成像数据的当前解释。从这项研究中获得的关于岩石圈地幔在大陆裂陷之前和期间的化学和物理演化的见解，将扩大我们对大陆裂陷和大陆分裂的触发和机制的理解。这仍然是板块构造学中的一个基本问题。更广泛的影响：研究生和本科生将参与该项目的各个方面，从数据采集到在国家会议上以及通过同行评审的出版物上解释和传播结果。本科生参与实地考察和拟议研究的分析方面将使被选中的学生能够在课堂外发现科学研究的兴奋。一名本科生已经完成了一篇关于我们研究区域捕虏体岩石学的荣誉论文。我们计划通过将更多的本科生纳入研究计划的各个方面来扩大这一初步试点项目的成功。该提案将通过组织跨学科研讨会来促进跨学科合作和伙伴关系，重点关注里约热内卢大裂谷和科罗拉多高原的演变。

"This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5)."Intellectual Merit: This proposal seeks funding to study the evolution of the lithospheric mantle beneath the central Rio Grande Rift and Colorado Plateau margin. The project will provide insights into a variety of questions related to the tectonic and volcanic evolution of this province, including the role of Farallon plate subduction and dehydration in generation of "enriched" mantle sources of Rio Grande Rift lavas. However, this proposal seeks primarily to answer one simple question: Has the original Proterozoic-age lithospheric mantle beneath the central Rio Grande Rift and Colorado Plateau margin been actively removed by delamination or convective erosion, as suggested by several recent geophysical studies? If correct, these studies suggest that lithosphere delamination, previously recorded beneath the Sierra Nevada, may be more prevalent than currently recognized and may be triggered by extensional as well as compressional tectonic regimes. Correlation of Hf- and Os-isotope variations in peridotite xenoliths with mineralogic constraints on degree of melt depletion will allow constraints on the timing of melt depletion recorded in xenoliths from the Rio Grande Rift and Colorado Plateau. Also, peridotite clinopyroxene trace element abundances combined with Sm-Nd isotope compositions will constrain the age of metasomatic overprints observed in portions of the Colorado Plateau and Rio Grande Rift. Together, these data will allow us to distinguish between Proterozoic, melt-depleted and possibly metasomatized lithospheric mantle and young, fertile asthenosphere. This study will thus provide a direct test of the lithosphere delamination model proposed by Gao et al. (2004) and Song & Helmberger (2007) to explain the presence of anomalously low seismic velocities in the shallow mantle beneath the central rift and plateau margin and complementary, anomalously high velocities within a slab-like structure dipping beneath the Central Plains lithosphere. Identification of asthenosphere-derived peridotite at the relatively shallow depths (~40-60 km) represented by peridotite xenolith suites throughout the region would support the proposed delamination models, and would be inconsistent with simple passive thinning of the lithosphere in response to extension. In contrast, identification of ancient lithosphere at these depths would suggest significantly less lithosphere removal and would require reevaluation of current interpretations of the available seismic tomographic data. Insights gained from this study on the chemical and physical evolution of lithospheric mantle prior to and during continental rifting will expand our understanding of the triggers and mechanisms involved in continental rifting and continental break-up. This remains a fundamental question in plate tectonics.Broader Impacts: Students at the graduate and undergraduate level will participate in all aspects of this project, from data acquisition to interpretation and dissemination of results at national conferences and through peer-reviewed publications. Undergraduate student involvement in fieldwork and analytical aspects of the proposed research will allow the selected student to discover the excitement of scientific research outside the classroom. One undergraduate has already completed an honors thesis examining the petrology of xenoliths from our study area. We plan to extend the success of this initial pilot project by incorporating additional undergraduates into all aspects of the research program. This proposal will foster cross-discipline collaboration and partnerships through organization of interdisciplinary workshops focused on the evolution of the Rio Grande Rift and Colorado Plateau.