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，在向弧脊跳跃之间平均延伸约15 Ma。一旦开始，弧后火山作用就从相对类似于弧的成分演变为相对类似于MORB的成分（Taylor和Martinez，2003; Langmuir等人，2006年）。这两个观测结果都归因于俯冲带上方次级对流上升流到弧后扩张中心的距离过度增加（Taylor and Martinez，2003; Martinez and Taylor，2006）。这一假设对岩浆系统末期可能暴露的任何地幔岩石的地球化学进行了实质性预测。当弧后裂谷接近其生命周期的终点时，熔融过程会变得更干燥，生产力会降低，岩石圈熔体停滞会更常见。我们建议测试这一假设，使用矿物化学的尖晶石和单斜辉石，和全岩Os同位素从一个时间线从哥斯拉竖框的Parace Vela裂谷，前弧后蔓延中心马里亚纳弧（19-20马）后面的一个实质性的深海橄榄岩恢复。下降的岩浆预算将确认这一概念，即次生板片上涌是弧后扩张的驱动力，并提出了一种触发脊跳跃的机制。初步的矿物化学数据表明，一个连贯的和可解释的熔融信号是目前在哥斯拉Mullion地幔岩石。Os同位素将用于确定所发现的贫化特征是代表最近的地球动力学配置还是继承的古老信号（例如，Liu等人，2008年）。这项工作还将提供一个重要的比较，在原位弧后和弧前地幔岩石，并作为地幔岩石的岩石学表征支持IODP钻探建议640-Full。该建议进一步的核心复杂的风格在弧后洋壳形成的基本认识。这是一种重要的新的地壳增生类型，但人们对它的了解仍然相对较少。因此，它属于与海底形成相关的一类基本地质问题，最好通过直接海底观察来研究。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。她还将扩展她目前在学习LA-ICP-MS和Os同位素技术方面的技能，并酌情参与研究和教学。格拉德学生的支持是为本科生谁开始作为一个REU学生的采样巡航。该项目是PI、日本研究所、JAMSTEC和日本水文局之间长期合作的一部分。为少数民族本科生单独获得了资金，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。UH是250所大学中第二大多元化大学（《美国新闻与世界报道》，2010年），服务于休斯顿周围的工人阶级社区。该项目提供了从传统上服务不足的群体（约60%）在UH和科学研究的前沿学生之间的接触的重要点。这项研究的结果也直接流入PI的教学中，PI在2009年秋季有105名矿物学学生，其中大多数是少数民族。因此，对传统上得不到充分服务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.