MARGINS: Mariana Forearc Geology and Early Arc Volcanism

马里亚纳弧前地质学和早期弧火山的智力价值。请求提供资金，以便对伊祖-博宁-马里亚纳岛(IBM)早期俯冲相关的火山岩进行地球化学分析，这些火山岩是通过最近的新凯6500潜水或关岛附近的领头PI进行陆地调查而收集的。其目标是全面了解这一弧系南部的早期演化，以便能够在时间和空间上比较从原弧到充分发育的火山弧的岩浆来源和熔融过程。分析将包括全岩和枕头玻璃的常量元素、微量元素和锶-钕-铅-氢同位素组成，以及来自橄榄石和其他矿物的玻璃熔体包裹体中的常量、微量和挥发性元素浓度。喷发年龄受40Ar/39Ar年代学和U-Pb年代学的制约。对于陆上原弧样品，可通过矿物赋存玻璃包裹体分析来确定挥发性元素含量。在关岛东南最深和最接近海沟的海底火成岩由细粒辉长岩和淬火结构玄武岩组成。这些熔岩类似于大洋中脊玄武岩，但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。三种岩石类型的常量元素、微量元素和锶-钕-铅同位素分析均已完成。仍然需要进行Hf同位素分析。安山岩的年龄为30-32 Ma，这表明它们的喷发大约与原始的IBM弧裂开的时间相同。然而，较老岩石的年龄目前仅受地层学和与IBM弧形其他地方类似岩石的年龄比较的限制。2008年7月期间，将在该区域进行进一步潜水。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。根据迄今已完成的工作，推测弧前玄武岩是俯冲开始后第一次喷发的火山岩。它们的成因在很大程度上是由于上升的地幔上升时的减压熔融，以填补由灾难性的板块最初下沉留下的空间。进一步的假设是，在大量富水的板片流体存在的情况下，这种现在很浅的哈兹伯里质地幔残留物的持续熔融导致了玻安岩的形成。从塞班岛45~46 Ma流纹岩和关岛41~42 Ma弧拉斑玄武岩开始，开始形成具有相对正常弧形特征的熔岩。因此，从原弧地幔上升流到第一弧地幔逆流的转换似乎需要4-7 Ma的时间，具体取决于所处的位置。这项研究具有潜在的变革性，因为它可以重新定义俯冲引发的岩浆序列，这将有助于理解俯冲开始的原因以及地幔对下沉板块的响应。这项工作将改进对在弧形寿命内从平板到表面的元素质量通量的估计。广泛的影响：这项研究的结果将有助于为拟议的IODP“IBM项目”钻孔确定替代地点。这项工作将继续日本和美国科学家之间卓有成效的合作，参与了横须贺/新海6500号房车的几次潜水巡航，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.