Collaborative Research: Fluid Transport and Fluid-Rock Interactions Preserved in Two Serpentinite Melanges in the Guatemala Suture Zone

俯冲通道流体负责变质块体的变质、脉岩的结晶、上覆地幔楔体的蛇纹岩化以及最终产生弧状火山作用的熔剂熔融。我们建议扩展我们对危地马拉缝合带(GSZ)蛇纹岩混杂岩的研究，以阐明岩石中记录的流体质量传输的记录。GSZ包括两个蛇纹岩混杂岩，它们保存了上述元素。莫塔瓜断裂南侧的一个系统记录了俯冲通道内的非常寒冷和潮湿的条件，以大约2.6 GPa和450-500℃形成的劳钠闪石榴辉岩块为代表。另一个系统在断层以北的系统代表了绿帘石-榴辉岩块中保存的略温暖但仍然潮湿的条件，变质作用的峰值在500-600℃和~2 GPa.这两个混杂岩都携带了在上复的蛇纹化地幔橄榄岩中形成的脉系的碎片；其中包括翡翠岩、绿辉岩、钠长岩和云母占主导地位的岩石，这些岩石含有来自洋壳蚀变和俯冲沉积物的强烈流体特征。对具有代表性的岩石进行了详细的岩石学和地球化学表征，包括主、次、痕量元素地球化学和锶-钕同位素体系，重点研究了B、Li、Be的含量和同位素。结构、矿物学和化学成分将被用来对俯冲过程中发生的过程进行分类，而不是后来的折返过程。智力价值：俯冲是地球上一个基本的过程，它将地表物质循环到地幔内部，并有助于大陆地壳的生长。然而，关于俯冲带中流体介导的物质传输和蚀变的细节还知之甚少，需要进一步解决。仔细研究保存下来的俯冲带，在那里既保存了流体沉淀，又保存了水化的地幔楔形岩石，将有助于对这个问题的重要见解。建议以以前由NSF资助的一个项目的成果为基础，对GSZ蛇纹岩混杂岩中保存的岩石中的矿物进行详细的化学分析，目的如下：(1)通过研究残留相来表征地幔楔形原岩；(2)分析蛇纹岩矿物中的主要、次要和微量元素，特别注意B、Li和Be(及同位素特征)和结构环境，以评估原岩的流体变质作用以及俯冲过程中与折返过程中产生的变化；(3)分析脉岩中的矿物(如硬玉、绿辉石、云母岩等)。用同样的方法来确定穿过俯冲通道并进入地幔楔形的流体的溶质负荷，以及基于结构和矿物组合的相对时间。利用流体-矿物分配系数，我们将估计进入地幔楔形的流体成分。(4)利用锶-钕同位素数据评估岩性成分的来源(例如，地幔与再循环大陆或沉积来源)。这项拟议的研究将加强对流体从俯冲通道到地幔楔形的运输的理解。更广泛的影响：包括大学和博物馆的研究人员的合作，许多国家的学生和科学家的参与，包括中美洲的学生和科学家的参与，支持危地马拉的教师培训和地质培训，与加勒比地区玉器考古研究的联系，研究小组之间的联系，以及通过访谈、文章和博物馆网站向媒体和公众广泛宣传我们的活动、成果和数据。

Subduction channel fluids are responsible for modification of metamorphic blocks, crystallization of vein rocks, serpentinization of the overlying mantle wedge and ultimately the flux melting that spawns arc volcanism. We propose to extend our studies of the serpentinite mélanges of the Guatemala Suture Zone (GSZ) to elucidate this record of mass transport by fluid as recorded in the rocks. The GSZ encompasses two serpentinite mélanges that preserve the elements described above. One system on the south side of the Motagua fault records very cold and wet conditions within the subduction channel, represented by lawsonite eclogite blocks formed at ~2.6 GPa and 450-500ºC. The other system north of the fault represents somewhat warmer but still wet conditions preserved in epidote-eclogite blocks, with peak metamorphism at 500-600°C and ~ 2 GPa. Both mélanges carry fragments of vein systems that formed within the overlying serpentinizing mantle peridotites; these include jadeitite, omphacitite, albitite, and mica-dominant rocks that contain a strong signature of fluids from altered oceanic crust and subducted sediment. Detailed petrologic and geochemical characterization of representative rocks is proposed for major, minor, and trace-element geochemistry and Sr-Nd isotopic systematics for this research, with a focus on B, Li, Be concentrations and isotopes. Textures, mineralogy, and chemical composition will be utilized to sort between processes that occurred during subduction as opposed to later exhumation. Intellectual Merit: Subduction is a fundamental process on Earth that recycles surface matter into the interior mantle and contributes to growth of continental crust. However, details of fluid-mediated mass transport and alteration in subduction zones are poorly understood and require further resolution. Careful study of preserved subduction zones, where both fluid precipitates and hydrated mantle wedge rocks are preserved, will contribute important insights on this issue. It is proposed to build upon results of a previously NSF-funded project by detailed chemical analysis of minerals in rocks preserved in serpentinite mélanges in the GSZ with the following objectives: (1) Characterize the mantle wedge protolith by study of the relict phases; (2) Analyze serpentinite minerals for major, minor and trace elements with particular attention to B, Li, and Be (and isotopic signatures) and textural settings to assess fluid modification of protolith and distinction between changes produced during subduction versus during exhumation; (3) Analyze minerals from vein rocks (e.g., jadeitite, omphacitite, mica-rock, etc.) with the same approach to determine the solute load of fluids traversing the subduction channel and entering the mantle wedge, as well as the relative timing based on textures and mineral assemblages. Using fluid-mineral partition coefficients we will estimate fluid composition entering the mantle wedge. (4) Utilize Sr-Nd isotopic data to assess sources of lithologic components (e.g., mantle vs. recycled continental or sedimentary sources). The proposed research will enhance understanding of fluid transport from subduction channels to the mantle wedge. Broader Impacts: These include the collaboration of researchers at universities and museums, involvement of students and scientists in many countries, including those in Central America, support of teacher training and geological training in Guatemala, links with archaeological studies on Caribbean-area jade, connections among research groups, and extensive outreach to both the media and the public via interviews, articles and Museum web-sites aimed both at student education and dissemination of our activities, results, and data.