REE-SiO2 Systematics for Distinguishing a Fractional Crystallization Versus Crustal Melting Origin for Intra-Oceanic Silicic Melts: A Combined Experimental and Field Investigation

用于区分大洋内硅质熔体的分步结晶与地壳熔融起源的 REE-SiO2 系统学：实验和现场调查相结合

• 批准号: 0946284
• 负责人: James Brophy
• 金额: $ 24.46万
• 依托单位: Indiana 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: 2010-03-15 至 2014-02-28
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0946284&HistoricalAwards=false
• 关键词: REE; SiO2; Systematics; Distinguishing; Fractional

Intellectual Merit.  A fundamental problem in the origin of silicic magmas in oceanic environments (both oceanic arc and mid-ocean ridge) concerns the relative roles of fractional crystallization of basaltic magmas vs. partial melting of hornblende bearing lower crustal grabbro or gabbroic cumulate rocks. This issue bears on basic interpretations of the inner workings of oceanic magmatic systems and, ultimately, their impact on continental growth. The PI has proposed recently a geochemical "test" (based on the co-variation between SiO2 and the REE) that might permit discrimination of these two processes in oceanic environments (Brophy, 2008, 2009). In short, fractional crystallization of parental basalt oceanic island arc (OIA) or mid-ocean ridge (MOR) settings should lead to a positive SiO2-REE correlation while dehydration melting of amphibolite (OIA environment) or hydration-induced melting of gabbro (MOR environment) should lead to a negative SiO2-REE correlation. If correct, these predicted co-variations represent a straightforward geochemical test for the genesis of silicic magmatism in oceanic environments. This proposal aims to evaluate the robustness of this model via both experimental studies and petrologic studies of natural rock suites. The experimental work comprises two parts: (1) dehydration melting of oceanic arc crustal amphibolite at 7.5 Kb (~base of crust); and (2) hydration-induced melting of mid-ocean ridge crustal gabbro at 2.0 Kb (~shallow crust). These experiments will be designed to insure global equilibrium and development of sufficiently large melt pools to permit in situ major element (EMP) and REE (and other trace element) (SIMS) analysis. Experiments will be conducted at three different locations including The Institute for Study of the Earth's Interior, Okayama University, Japan, Hannover University in Hannover, Germany, and the University of Oregon. An added benefit will be the determination of partition coefficients for the REE and other trace elements between high-SiO2 liquid ( 65% SiO2) and the minerals hornblende, plagioclase, augite, and Fe-Ti oxides. Such data are sorely lacking and will fill an important void in our overall knowledge of solid-liquid partition coefficients. The field based work comprises studies of natural examples of both dehydration melting of lower OIA crustal amphibolite (the Asago Body of the Yakuno Ophiolite, Honshu Island, Japan) and hydration melting of MOR gabbro (the Fournier Oceanic Crust, New Brunswick, Canada). Through the direct analysis of major element and trace element abundances in both unmelted host rock and crystallized silicic melt (i.e. leucosomes) it is anticipated that the proposed REE-SiO2 systematics can be directly evaluated in the natural setting.Broader Impacts. The proposed study has the potential to verify and, therefore, provide a straightforward means of determining a crustal melting versus fractionation origin for silicic magmas in oceanic volcanic systems. At the more practical level, the proposed study will initiate (and hopefully perpetuate) collaborative research with several international scientists including Dr. Eizom Nakamura (Director, Institute for Study of the Earth's Interior), Dr. Akira Ishiwatari (Tohoku University), Dr. Juergen Koepke (University of Hannover) and Dr. John Spray (University of New Brunswick). The work will support the training of one graduate student in geology. The Department of Geological Sciences at Indiana University strongly encourages the participation of undergraduate students in faculty research. The PI currently has two undergraduate students working in his lab and expects to "carve out" additional individual projects suitable for undergraduate level research.

学术成就：海洋环境（包括大洋弧和洋中脊）中玄武岩岩浆起源的一个基本问题涉及玄武质岩浆的分离结晶与含角闪石的下地壳地堑或辉长岩堆晶岩的部分熔融的相对作用。这个问题关系到对海洋岩浆系统内部运作的基本解释，以及它们最终对大陆生长的影响。PI最近提出了一种地球化学“测试”（基于SiO2和REE之间的协变），可能允许区分海洋环境中的这两个过程（Brophy，2008年，2009年）。总之，在大洋岛弧（OIA）或大洋中脊（莫尔）环境中，玄武岩的分离结晶作用导致SiO2-REE正相关，而角闪岩的脱水熔融作用（OIA环境）或辉长岩的水化熔融作用（莫尔环境）导致SiO2-REE负相关。如果正确的话，这些预测的共同变化代表了一个简单的地球化学测试在海洋环境中的岩浆成因。该建议的目的是通过实验研究和天然岩套的岩石学研究来评估该模型的鲁棒性。 实验工作包括两部分：（1）7.5Kb（~地壳底部）洋弧壳斜长角闪岩的脱水熔融和（2）2.0Kb（~地壳浅部）洋中脊壳辉长岩的水化熔融。这些实验的设计将确保全球平衡和发展足够大的熔池，允许在现场的主要元素（EMP）和稀土元素（和其他微量元素）（西姆斯）的分析。实验将在三个不同的地点进行，包括日本冈山大学地球内部研究所、德国汉诺威的汉诺威大学和俄勒冈州大学。一个额外的好处将是稀土元素和其他微量元素之间的分配系数的测定高SiO2液体（65%SiO2）和矿物角闪石，斜长石，普通辉石，和Fe-Ti氧化物。这样的数据是非常缺乏的，将填补一个重要的空白，在我们的整体知识的固-液分配系数。野外工作包括对下OIA地壳斜长角闪岩（日本本州岛矢久野蛇绿岩的阿萨戈体）脱水熔融和莫尔辉长岩（加拿大新玩法的Fournier大洋地壳）水化熔融的自然实例的研究。通过直接分析未熔融的寄主岩石和结晶的硅质熔体（即白色体）中的主要元素和微量元素丰度，预计可以在自然环境中直接评估拟议的REE-SiO2系统学。拟议中的研究有可能验证，因此，提供了一个简单的方法来确定地壳熔融与分馏起源大洋火山系统中的岩浆。在更实际的层面上，拟议的研究将启动（并有望保持）与几位国际科学家的合作研究，包括Eizom中村博士（地球内部研究所所长）、Akira Ishiwatari博士（东北大学）、Juergen Koepke博士（汉诺威大学）和John Spray博士（新玩法大学）。这项工作将支助一名地质学研究生的培训。印第安纳州大学地质科学系强烈鼓励本科生参与教师研究。PI目前有两名本科生在他的实验室工作，并希望“开拓”适合本科水平研究的额外个人项目。