Death of a Backarc Rift: A Petrologic Site Survey of Godzilla Mullion

与洋中脊相比，弧后扩张的生命周期较短，在向弧方向的脊跃之间平均伸展约15 Ma。弧后火山活动一旦形成，就会从相对弧状的成分演变为相对弧状的成分（Taylor and Martinez, 2003; Langmuir et al, 2006）。这两个观测结果都归因于从俯冲带上方的次级对流上升流到弧后扩张中心的距离过长（Taylor and Martinez, 2003; Martinez and Taylor, 2006）。这一假说对岩浆系统末期可能暴露的任何地幔岩石的地球化学作出了实质性的预测。随着弧后裂谷接近其生命周期的终点，融化过程会变得更加干燥，生产力会降低，岩石圈融化停滞会更加普遍。我们建议使用尖晶石和斜辉石的矿物化学，以及从Parace Vela裂谷（Mariana弧后前弧扩张中心）哥斯拉Mullion的大量深海橄榄岩中回收的整个岩石Os同位素来验证这一假设（19-20 Ma）。岩浆预算的减少将证实次级板块上升流是弧后扩张的驱动力的观点，并提出了一种触发脊跃的机制。初步的矿物化学数据表明，在哥斯拉-穆里安地幔岩石中存在连贯且可解释的熔融信号。Os同位素将用于确定发现的枯竭特征是代表最近的地球动力学配置还是继承了古代信号（例如，Liu et al., 2008）。这项工作还将为弧后和弧前地幔岩的原位对比提供重要依据，并为支持IODP 640-Full钻井方案的地幔岩岩石学表征提供依据。这进一步加深了对弧后核-杂岩型海洋地壳形成的基本认识。这是一种重要的新类型的地壳增生，但人们对它的了解相对较少。由于这个原因，它属于一类与海底形成有关的基本地质问题，可以通过直接的海底观测来进行最好的研究。该项目将支持一名优秀的年轻女科学家的职业发展，她将帮助指导本科生和研究生参与者。她还将扩展目前学习LA-ICP-MS和Os同位素技术的技能，并酌情参与研究和教学。研究生支持是为在抽样巡航中开始作为REU学生的本科生提供的。该项目是PI、日本研究所、JAMSTEC和日本海道测量局长期合作的一部分。已经为少数民族本科生获得了单独的资助，其中包括两名有兴趣在这一领域从事研究工作的女本科生。休斯敦大学是250所排名大学中第二多样化的大学（《美国新闻与世界报道》，2010年），服务于休斯顿周围的工薪阶层社区。这个项目提供了一个重要的接触点，来自传统上服务不足的群体（约60%）的学生在UH和科学研究的前沿。这项研究的结果也直接流入了PI的教学，2009年秋季，PI有105名矿物学新生，其中大多数是少数民族。因此，对传统上服务不足的社区的影响不成比例地大。

Compared to mid-ocean ridges, the life cycle of back-arc spreading is short, averaging ~15 Ma of extension between arc-ward ridge jumps. Once initiated, back-arc volcanism evolves from relatively arc-like to relatively MORB-like compositions (Taylor and Martinez, 2003; Langmuir et al., 2006). Both of these observations are ascribed to the over-increasing distance to the back-arc spreading center from secondary convective upwelling above a subduction zone (Taylor and Martinez, 2003; Martinez and Taylor, 2006). This hypothesis makes substantial predictions for the geochemistry of any mantle rocks that may be exposed during the end phase of the magmatic system. Melting should become drier, productivity lower and lithospheric melt stagnation more common as a back-arc rift approaches the end of its life cycle. We propose to test this hypothesis using mineral chemistry of spinel and clinopyroxene, and whole rock Os isotopes from a substantial suite of abyssal peridotites recovered from a time line from the Godzilla Mullion in the Parace Vela Rift, the former back arc spreading center behind the Mariana arc (19-20 Ma). A declining magmatic budget would confirm the notion that secondary slab upwelling is the driving force in back-arc spreading in general, and suggests a mechanism for triggering ridge jumps. Preliminary mineral chemical data suggests that a coherent and interpretable melting signal is present in the Godzilla Mullion mantle rocks. Os isotopes will be used to determine whether the depleted signatures found are representative of the recent geodynamic configuration or are inherited ancient signals (e.g., Liu et al., 2008). This work will also provide an important comparison between in-situ back-arc and fore-arc mantle rocks, and serve as a petrologic characterization of mantle rocks supporting IODP drilling proposal 640-Full.This proposal furthers the basic understanding of core-complex style ocean crustal formation in a back-arc. This is an important new and still relatively poorly understood type of crust accretion. For this reason, it belongs to a class of basic geologic problems related to the formation of the ocean floor that can best be investigated via direct ocean floor observations. This project will support the career development of an excellent young female scientist, who will help mentor undergraduate and graduate student participants. She will also extend her current skill set in learning LA-ICP-MS and Os isotope techniques, and participate in research and teaching as appropriate. Grad student support is for an undergrad who began as an REU student on the sampling cruise. This project is part of a collaboration of long standing between the PI, the Japanese Research Institute, JAMSTEC, and the Japanese Hydrographic Office. Separate funding has been acquired for minority undergraduates, including two female undergraduates interested in pursuing work in this area for later graduate school. The UH is the second most diverse among 250 ranked universities (US News and World Report, 2010), serving the surrounding working class Houston community. This project provides an essential point of contact between students from traditionally underserved groups (~60%) at UH and the cutting edge of scientific research. The results of this study also flow directly into the teaching of the PI, who had 105 beginning mineralogy students in Fall 2009, a majority of whom were minorities. The impact on traditionally underserved communities is thus disproportionately large.