Collaborative Research: Non-peridotite Melting in Plume-influenced Extensional Environments: Lithological Heterogeneity of the African Superplume and African Lithosphere

智力上的功绩。该项目旨在确定在东非裂谷发育之前导致渐新世埃塞俄比亚大陆溢流玄武岩火山活动的地幔熔体的源岩性(如橄榄岩、辉石岩)。地幔熔体的渗透可以通过削弱和聚焦应变在岩石圈变薄的启动和演化中发挥核心作用。因此，了解地幔熔体来源的组成，特别是矿物学，对于限制熔体产生的条件至关重要。非橄榄岩岩性，如上升岩柱中的辉石岩或岩石圈地幔内的交代体，是潜在的富饶的地幔来源，能够产生大量的岩浆。相反，在这些条件下(即较小的地幔减压程度或较低的地幔潜在温度)，典型的橄榄岩岩性可能不会产生显著的熔融。此外，研究区域下的地幔具有显著的地震衰减特征，这归因于非洲超热地幔物质上涌的存在。这项拟议的研究旨在调查岩石圈与羽状地幔作为主要岩浆来源的相对贡献。具体地说，地幔源岩性(即橄榄岩与辉石岩)将通过对埃塞俄比亚西部高原保存完好的2000米厚的溢流玄武岩的地球化学特征进行评估，这是以前从未研究过的。将通过对中新世盾形火山喷发的不寻常的不饱和硅岩石进行平行研究，评估富集型岩石圈地幔对熔体生成的可能贡献，初步数据表明，岩浆来自古老的非洲岩石圈地幔。分析计划将包括测量橄榄石中的主要和选定的微量元素、锶-钕-铅-氢同位素和次要元素，并将在新的Ar/Ar地质年代学和古地磁研究建立的时间框架内进行解释，旨在限制这些矿床的时间、持续时间和喷发速度。这些数据将为玄武岩的形成过程提供新的见解。更广泛的影响。该项目产生的数据将首次从岩石学角度深入了解非洲超级烟柱的地球物理推断组成成分的来源，并将与该区域正在进行的地球物理项目直接相关。该项目将支持Tyrone Rooney(博士，2006)的早期职业研究，并通过与亚的斯亚贝巴大学研究人员的互动促进国际合作。该项目代表了五个美国机构(鲁尼-密歇根州立大学、赫兹伯格-罗格斯大学、Kappleman/Holt-UT奥斯汀、Spell-UNLV和Konter-UTEP)的科学家之间的新合作，他们都将为分析测量、解释和建模做出贡献。该项目利用并扩展了密歇根州立大学(激光消融电感耦合等离子体质谱仪)、罗格斯大学(电子探针)、得克萨斯大学奥斯汀分校(古地磁实验室)、UNLV(稀有气体MS)和UTEP(MC-ICPMS)最先进的分析设施的应用。该项目还将通过在埃塞俄比亚的五个美国机构和研究人员的广泛合作互动，支持对密歇根州立大学的一名博士生的指导。

Intellectual Merit. This project seeks to determine the source lithology (e.g., peridotite, pyroxenite) of mantle melts that contributed to Oligocene Ethiopian continental flood basalt volcanism that preceded development of the East African Rift. Infiltration of mantle melts can play a central role in the initiation and evolution of lithospheric thinning by weakening and focusing strain. Thus, understanding the composition, and in particular the mineralogy, of the sources of mantle melts is critical to constrain the conditions attending melt generation. Non-peridotite lithologies such as pyroxenite in an upwelling plume or metasomes within the lithospheric mantle are potentially fertile mantle sources capable of producing significant volumes of magmas. In contrast typical peridotite lithologies may not produce significant melt under these conditions (i.e., smaller degrees of mantle decompression, or lower values of mantle potential temperature). Moreover, mantle underlying the study area is characterized by significant seismic attenuation, which has been attributed to the presence of an African superplume upwelling of hot mantle material. The proposed study aims to investigate the relative contributions of lithospheric vs. plume-like mantle as primary magma sources. Specifically, mantle source lithologies (i.e. peridotite versus pyroxenite) will be evaluated via geochemical characterization of a well-preserved 2000 m thick section of flood basalts from the western Ethiopian Plateau that has never before been studied. The possible contribution of enriched lithospheric mantle to melt generation will be assessed through a parallel study of unusual Si-undersaturated rocks erupted in a Miocene shield volcano, for which preliminary data imply magma derivation from ancient African lithospheric mantle. The analytical program will involve measurement of major and selected trace elements, Sr-Nd-Pb-Hf isotopes, and minor elements in olivine, and will be interpreted within a temporal framework to be established by new Ar/Ar geochronology and paleomagnetic studies designed to constrain the timing, duration, and eruptive rates of these deposits. Together these data will provide new insights into processes of basalt generation. Broader Impacts. The data resulting from this project will offer the first petrologic insights into the origin of the geophysically-deduced compositional component of the African superplume and will be directly relevant to ongoing geophysical projects ongoing in the region. This project will support the early career research of Tyrone Rooney (Ph.D., 2006), and fosters international collaboration through interactions with researchers at Addis Ababa University. The project represents a new collaboration between scientists at five US institutions (Rooney-MSU, Herzberg-Rutgers, Kappleman/Holt-UT Austin, Spell-UNLV, and Konter-UTEP), all of whom will contribute to the analytical measurements, interpretations, and modeling. The project utilizes and extends the applications of state-of-the-art analytical facilities at Michigan State University (laser ablation ICP-MS), Rutgers University (electron microprobe), UT Austin (paleomagnetic lab), UNLV (Noble gas MS), and UTEP (MC-ICPMS). The project also will support mentoring of a Ph.D. student from MSU through the wide range of collaborative interactions at the five US institutions and researchers in Ethiopia.