MARGINS: Mariana Forearc Geology and Early Arc Volcanism

马里亚纳弧前地质与早期弧火山：学术价值。要求提供资金，对伊豆-波宁-马里亚纳弧（IBM）早期俯冲相关火山岩进行地球化学分析，这些火山岩是由首席PI最近在关岛附近通过新海6500潜水或陆上调查收集的。目标是全面了解该弧系统南部的早期演化，以便通过时间和空间对岩浆来源和熔融过程进行比较，从原始弧到完全发育的火山弧。分析将包括整个岩石和枕状玻璃的主元素、微量元素和Sr-Nd-Pb-Hf同位素组成，以及橄榄石和其他矿物的玻璃状熔融包裹体中的主元素、微量元素和挥发性元素浓度。喷发年龄将受到40Ar/39Ar和U-Pb年代学的限制。对于地面原始电弧样品，挥发性元素的含量可以通过分析矿物承载的玻璃包裹体来确定。在关岛东南海沟最深处和最靠近海沟的海底火成岩中，有细粒辉长岩和淬质玄武岩。这些熔岩类似于海中脊玄武岩，但地球化学特征表明它们与俯冲有关。这些前弧玄武岩依次被低钙钙镁安山岩和镁质安山岩覆盖。三种岩石类型的主元素、微量元素和Sr-Nd-Pb同位素分析均完成。仍然需要Hf同位素分析。安山岩的年代为30-32 Ma，这表明它们的喷发大约与最初的IBM弧断裂同时发生。然而，较老岩石的年龄目前仅受地层学和与IBM弧其他地方类似岩石的年龄比较的限制。2008年7月将在该地区进行进一步的潜水。这里要求的大部分资金将用于分析这些潜水所得的岩石，并完成地球化学分析和放射性年龄测定。根据迄今为止所完成的工作，假设弧前玄武岩是俯冲开始后首先喷发的火山岩。它们的起源很大程度上与上升流的地幔的减压融化有关，因为它上升了，填补了板块最初灾难性下沉留下的空间。进一步的假设是，在富含水的板块衍生流体的强大通量存在下，这种现在较浅的哈兹伯尔质地幔残留物的持续融化导致了博宁质岩石的形成。从塞班岛45 ~ 46 Ma流纹岩和关岛41 ~ 42 Ma弧拉斑岩开始，形成了具有相对正常弧特征的熔岩。因此，从原弧地幔上升流到第一弧地幔逆流的转变似乎需要4-7 Ma，具体取决于位置。这项研究具有潜在的变革性，因为它可以重新定义由俯冲开始产生的岩浆序列，这将有助于理解俯冲开始的原因以及地幔如何对下沉的板块做出反应。这项工作将提供在电弧寿命期间从板到表面的元素质量通量的改进估计。更广泛的影响：这项研究的结果将有助于确定拟议的IODP“项目IBM”钻孔的替代地点。这项工作将继续日本和美国科学家之间富有成效的合作，参与RV横须贺/新海6500的几次潜水巡航，可能会带来切实的经济和政治利益。该项目还将涉及研究生和本科生的研究。

Mariana Forearc Geology and Early Arc VolcanismIntellectual Merit. Funding is requested to carry out geochemical analyses of early subduction-related volcanic rocks from the Izu-Bonin-Mariana (IBM) arc collected via recent Shinkai 6500 diving or on-land investigations by the lead PI near Guam. The goal is to develop a comprehensive understanding of the early evolution of the southern portion of this arc system so that magma sources and melting processes can be compared through time and space, spanning from a proto-arc to a fully developed volcanic arc. Analyses will include major element, trace element, and Sr-Nd-Pb-Hf isotope compositions of whole rocks and pillow glasses, and major, trace, and volatile element concentrations in glassy melt inclusions from olivine and other minerals. The ages of eruption will be constrained by 40Ar/39Ar and U-Pb geochronology.For subaerial proto-arc samples volatile element contents can be determined by analysis of mineral-hosted glass inclusions. Submarine igneous rocks collected deepest and closest to the trench southeast of Guam comprise fine-grained gabbroic rocks and quench textured basalts. These lavas are resemble mid-ocean ridge basalts, but have geochemical signatures suggesting that they are subduction-related. These forearc basalts are overlain in turn by low-Ca boninite and magnesian andesites. Major element, trace element, and Sr-Nd-Pb isotope analyses are complete for all three rock types. Still needed are Hf isotope analyses. The andesites have been dated at 30-32 Ma, which places their eruption at about the same time as the original IBM arc rifted. The ages of the older rocks, however, are presently only constrained by stratigraphy and comparison to ages of similar rocks from elsewhere in the IBM arc. Further diving will be done in this region during July, 2008. Most of the funds requested here will be used to analyze rocks from these dives, and to complete the geochemical analyses and radiometric age dating. Based on the work accomplished to date, it is postulated that the forearc basalts are the first volcanic rocks to erupt after subduction begins. Their genesis largely involved decompression melting of upwelling mantle as it ascended to fill space left by the catastrophic initial sinking of the slab. It is further hypothesized that continued melting of this now-shallow harzburgitic mantle residue in the presence of a robust flux of water-rich slab-derived fluid resulted in the genesis of the boninites. Beginning with the 45-46 Ma rhyolites on Saipan and 41-42 Ma arc tholeiites on Guam, lavas with relatively normal arc signatures started being generated. Thus the changeover from proto-arc mantle upwelling to first-arc mantle counterflow appears to take 4-7 Ma depending on location. The research is potentially transformative because it could redefine the succession of magmas resulting from subduction initiation, which will contribute to an understanding of why subduction starts and how the mantle responds to the sinking slab. This work will provide improved estimates for mass fluxes of elements from the slab to the surface over the life of the arc.Broader Impacts: Results from this study will contribute to identifying an alternate site for a proposed IODP "Project IBM" drill hole. This work will continue fruitful collaboration between Japanese and US scientists involved with several dive cruises of the RV Yokosuka/Shinkai 6500, possibly leading to tangible economic and political benefits. The project also will involve graduate and undergraduate student research.