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

与大洋中脊相比，弧后扩张的生命周期较短，平均向弧脊跳跃的伸展时间约为15 Ma。一旦启动，弧后火山活动就从相对弧形的成分演变为相对类似MORB的成分(Taylor和Martinez，2003；Langmuir等人，2006)。这两个观测结果都归因于俯冲带上方的次级对流上升流到弧后扩张中心的距离过大(Taylor和Martinez，2003；Martinez和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。随着弧后裂谷接近生命周期的结束，熔融应该变得更加干燥，生产率降低，岩石圈熔融停滞更加常见。我们建议使用尖晶石和单斜辉石的矿物化学，以及从贝拉裂谷哥斯拉山脉的时间线上发现的大量深海橄榄岩的全岩Os同位素来验证这一假说，贝拉裂谷是马里亚纳弧(19-20 Ma)后面的前弧后扩张中心。岩浆预算的下降将证实这样的观点，即次生板块上涌总体上是弧后扩张的驱动力，并暗示了触发山脊跳跃的机制。初步的矿物化学数据表明，哥斯拉地幔岩石中存在一个连贯的、可解释的熔融信号。将使用OS同位素来确定所发现的耗尽的特征是代表最近的地球动力学构型还是继承的古代信号(例如，Liu等人，2008年)。这项工作还将提供弧后和弧前地幔岩石的重要对比，并作为支持IODP钻井建议640-Full的地幔岩石岩石学特征。这一建议加深了对弧后核杂物式海洋地壳建造的基本认识。这是一种重要的新的地壳吸积类型，但人们对它的了解仍然相对较少。因此，它属于一类与海底形成有关的基本地质问题，最好的方法是通过直接海底观测来研究。该项目将支持一位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。她还将扩展她目前学习LA-ICPMS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邮轮上作为REU学生开始的本科生提供的。该项目是日本国际和平研究所、日本科学技术研究所和日本水文厅长期合作的一部分。已经为少数族裔本科生获得了单独的资金，包括两名有兴趣在这一领域从事工作的女本科生，以便在以后的研究生院学习。休斯敦大学在250所排名的大学中排名第二(《美国新闻与世界报道》，2010年)，服务于周围的工薪阶层休斯顿社区。该项目为UH传统弱势群体(约60%)的学生与前沿科学研究领域的学生提供了一个重要的接触点。这项研究的结果也直接流入了PI的教学中，在2009年秋季，他们有105名矿物学初学者，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是不成比例的大。

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.