MARGINS: Mariana Forearc Geology and Early Arc Volcanism

马里亚纳弧前地质学与早期弧火山作用请求提供资金，对Izu-Bonin-Mariana（IBM）弧区与俯冲有关的早期火山岩进行地球化学分析，这些火山岩是由牵头的PI在关岛附近通过最近的Shinkai 6500潜水或陆上调查收集的。其目标是全面了解该弧系南部的早期演化，以便可以在时间和空间上比较从原始弧到完全发育的火山弧的岩浆来源和熔融过程。分析将包括主要元素，微量元素，和Sr-Nd-Pb-Hf同位素组成的整个岩石和枕玻璃，和主要，微量和挥发性元素的玻璃熔体包裹体从橄榄石和其他矿物的浓度。火山喷发的年龄受40 Ar/39 Ar和U-Pb年代学的制约，而陆上原始弧样品的挥发性元素含量则可通过矿物玻璃包裹体的分析来确定。在关岛东南最深和最靠近海沟处采集的海底火成岩包括细粒辉长岩和淬火结构玄武岩。这些熔岩类似于大洋中脊玄武岩，但具有地球化学特征，表明它们与俯冲有关。这些弧前玄武岩依次被低钙玻安岩和镁质安山岩覆盖。所有三种岩石类型的主元素、微量元素和Sr-Nd-Pb同位素分析均已完成。仍然需要Hf同位素分析。安山岩的年龄为30-32 Ma，这表明它们的喷发与原始IBM弧的断裂大约在同一时间。然而，较老岩石的年龄目前仅受地层学的限制，并与IBM弧其他地方的类似岩石的年龄进行比较。2008年7月将在该地区进行进一步潜水。这里要求的大部分资金将用于分析这些潜水的岩石，并完成地球化学分析和放射性年龄测定。根据迄今所完成的工作，人们假定弧前玄武岩是俯冲开始后第一批喷发的火山岩。它们的成因主要涉及上涌地幔的减压熔融，因为它上升，以填补空间所留下的灾难性的初始下沉的板。它进一步假设，在存在一个强大的流量的富水板片衍生流体的继续熔融的现在浅方辉橄榄岩地幔残留物导致玻安岩的成因。从塞班岛45-46 Ma的流纹岩和关岛41-42 Ma的弧拉斑玄武岩开始，开始生成具有相对正常弧特征的熔岩。因此，从原弧地幔上涌到第一弧地幔逆流的转换似乎需要4-7 Ma，这取决于位置。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对下沉板块的反应。这项工作将提供改进的估计质量通量的元素从板到表面的生命的arc.Broader影响：从这项研究的结果将有助于确定一个替代网站的拟议IODP“IBM项目”钻孔。这项工作将继续日本和美国科学家之间的富有成效的合作，参与了RV Yokosuka/Shinkai 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.