CAREER: Investigating Controls on Arc Flare-ups and the Growth of Lower Continental Crust

拟开展的研究项目主要研究新西兰峡湾地区白垩纪岩浆弧的岩浆和热演化，主要目的是确定弧下地壳岩浆构造的速度和机制。地质年代学和地球化学数据将用于验证三个假设：1)进入下地壳和中地壳的岩浆通量是幕式的，空间上是短暂的，并在一个短暂的，大约300万年的高通量事件中达到顶峰；2)深部岩浆既来源于地幔楔源，也来源于原有的地壳源，形成大量高压榴辉岩堆积；3)弧根经历了非均匀的加热和冷却过程，这一过程不容易用简单的单调冷却模型来解释，而可能与岩石圈弧根下方软流圈上升流驱动的基性底板相容。解决这些假设将用于评估弧根的构建和修改过程，并创建白垩纪峡湾弧的四维岩石学模型。岩浆弧是地球上的俯冲工厂，在那里形成了大量的中间或安山岩大陆地壳。地球科学的一个基本问题是，弧环境中地幔部分熔融产生的主要岩浆是缺乏硅质的玄武岩，而不是中间安山岩。长期以来，人们一直推测地幔岩浆从玄武岩到安山岩的演化可能发生在弧的最深处（25-60公里深），在那里发生了致密矿物的分离结晶、岩浆混合和先前存在的地壳同化等基本过程。最近，对弧岩进行的高精度同位素定年使人们认为，在以弧活动剧烈期为特征的爆发事件期间可能会产生大量岩浆。这些观点表明，岩浆弧中的大陆地壳构造可能是高度偶然性的，我们在现代弧中看到的大部分岩浆的地球化学多样化可能发生在岩浆弧根部的短时间内。这个项目的重点是在新西兰峡湾大陆弧的一个独特的、被挖掘出来的深地壳部分，那里的深地壳暴露在25-65公里的古深度。初步数据表明，超过70%的岩浆弧根部是在一个短暂的高通量事件中形成的，该项目将有助于评估该岩浆弧深部大陆地壳构造的时间和地球化学过程。除了该项目的科学目标外，它还强调了研究与动态教学和指导计划的整合，旨在传播从K-12到研究生教育的所有学术水平的知识。加州州立大学北岭分校（CSUN）的本科生将在美国和新西兰参加实地和实验室研究。拉丁裔少数民族和女性学生（在地球科学和STEM中都是代表性不足的群体）将通过“ROCs”外展计划（CSUN学生的研究机会）进行招募和指导。ROCs计划旨在早期参与和指导代表性不足的CSUN本科生参与研究项目，并通过研讨会和每周会议为研究生和地球科学职业道路提供指导。PI和CSUN的学生还将与峡湾国家公园（联合国教科文组织世界遗产）的深湾户外教育信托基金合作，为来自新西兰南岛的K-12学生创造动手的地球科学活动。突出峡湾地质独特方面的活动和短视频将在csun托管的网站、YouTube和Vimeo上提供，以便向公众广泛传播。

The proposed research program focuses on investigating the magmatic and thermal evolution of an exhumed, Cretaceous magmatic arc in Fiordland, New Zealand with the primary goal of determining the tempo and mechanisms of magmatic construction of lower arc crust. Geochronologic and geochemical data will be used to test three hypotheses: 1) magmatic fluxes into the lower and middle crust were episodic, spatially transient, and culminated in a brief, approximately 3 million year high flux event; 2) deep-crustal magmas were derived from both mantle wedge and pre-existing crustal sources and generated voluminous high-pressure eclogitic cumulates; and 3) the arc root experienced heterogeneous heating and cooling that is not easily explained by simple monotonic cooling models, but instead may be compatible with mafic underplating driven by asthenospheric upwelling beneath the lithospheric arc root. Addressing these hypotheses will be used to evaluate processes of arc root construction and modification, and to create a 4-dimensional petrologic model for the Cretaceous Fiordland arc. Magmatic arcs are the subduction factories on earth where large volumes of intermediate, or andesitic, continental crust are created. A fundamental problem in Earth sciences is that the dominant magmas generated from partial melting of the mantle in arc environments are silica poor basalts, rather than intermediate andesites. It has long been speculated that the evolution of mantle-derived magmas from basalt to andesite likely occurs in the deepest roots (25-60 kilometers depth) of arcs where fundamental processes such as fractional crystallization of dense minerals, magma mixing and assimilation of preexisting crust occur. Recently, high-precision isotopic dating of arc rocks has led to the idea that large volumes of magmas may be generated during flare-up events, characterized by intense periods of arc activity. These ideas suggest that continental crustal construction in magmatic arcs may be highly episodic and that much of the geochemical diversification of magmas that we see in modern arcs may occur during brief time periods within the roots of the magmatic arc. This project focuses on a unique, exhumed deep-crustal section of a continental arc in Fiordland, New Zealand where the deep crust is well exposed to 25-65 kilometers paleodepth. Preliminary data suggests that over 70% of the arc root was emplaced during a brief, high-flux event, and this project will allow the evaluation of the temporal and geochemical processes of continental crust construction in the deep roots of this magmatic arc.In addition to the scientific goals of the project, it also emphasizes the integration of research with a dynamic teaching and mentoring program that aims to disseminate knowledge at all academic levels from K-12 to graduate education. California State University Northridge (CSUN) undergraduate students will participate in field and laboratory research in the United States and New Zealand. Latino minority and female students (both underrepresented groups in the geosciences and STEM) will be recruited and mentored through the 'ROCs' outreach program (Research Opportunities for CSUN students). The 'ROCs' program is aimed at early involvement and mentoring of underrepresented undergraduate CSUN students in research projects and providing guidance on graduate and geosciences career paths through workshops and weekly meetings. The PI and CSUN students will also work with the Deep Cove Outdoor Educational Trust in Fiordland National Park (a UNESCO World Heritage site) to create hands-on, geoscience activities for K-12 students from across the South Island of New Zealand. Activities and short videos highlighting unique aspects of Fiordland geology will be available on a CSUN-hosted website, YouTube and Vimeo for broad dissemination to the public.