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

知识价值。海洋环境（洋弧和洋中脊）中硅质岩浆成因的一个基本问题是，玄武岩岩浆的分离结晶与含角闪石的下地壳吸积岩或辉长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，并最终关系到它们对大陆生长的影响。PI最近提出了一种地球化学“测试”（基于SiO2和REE之间的共变），可能允许在海洋环境中区分这两种过程（Brophy, 2008,2009）。简而言之，母玄武岩大洋岛弧（OIA）或洋中脊（MOR）环境的分馏结晶应导致SiO2-REE正相关，而角闪岩脱水熔融（OIA环境）或辉长岩水化熔融（MOR环境）应导致SiO2-REE负相关。如果正确的话，这些预测的共变代表了对海洋环境中硅岩浆作用成因的直接地球化学测试。本提案旨在通过实验研究和天然岩石层的岩石学研究来评估该模型的稳健性。实验工作包括两个部分：(1)7.5 Kb（~地壳基底）洋弧壳角闪岩脱水熔融；(2) 2.0 Kb（~浅层地壳）洋中脊地壳辉长岩水化熔融。这些实验的设计将确保全球平衡和足够大的熔池的发展，以允许原位主要元素（EMP）和REE（和其他微量元素）（SIMS）分析。实验将在地球内部研究所、日本冈山大学、德国汉诺威大学、美国俄勒冈大学等3个地点进行。另一个好处是确定高SiO2液体（65% SiO2）与矿物角闪石、斜长石、辉长岩和Fe-Ti氧化物之间的REE和其他微量元素的分配系数。这类数据非常缺乏，将填补我们对固液分配系数整体知识的重要空白。野外工作包括对低OIA地壳角闪岩（日本本州岛Yakuno蛇绿岩的Asago体）脱水熔融和MOR辉长岩（加拿大新不伦瑞克省Fournier洋壳）水化熔融的自然例子的研究。通过对未熔化的寄主岩石和结晶硅熔体（即白色体）中主元素和微量元素丰度的直接分析，预计可以在自然环境中直接评价所提出的REE-SiO2系统。更广泛的影响。提出的研究有可能验证并因此提供一种直接的方法来确定海洋火山系统中硅岩浆的地壳熔融与分馏起源。在更实际的层面上，拟议的研究将启动（并希望延续）与几位国际科学家的合作研究，包括Eizom Nakamura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（新不伦瑞克大学）。这项工作将支持一名地质学研究生的培训。印第安纳大学地质科学系强烈鼓励本科生参与教师的研究。PI目前有两名本科生在他的实验室工作，并希望“开拓”更多适合本科生水平研究的个人项目。

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.