Lawsonite: an exceptional geochemical archive in HP/LT rocks

生命所必需的水和其他化学物质从地球表面循环到地球内部深处，再循环回来，循环周期长达数百万年。水被带到很深的矿物晶体在构造板块潜水（潜没）到地下。在这个循环中最重要的矿物之一是硬硅钙石，它只在俯冲板块上层的特征高压和相对低温条件下形成。劳森石很重要，因为它含有大量的水，还含有微量的其他元素，如铀，铅和稀土元素。尽管在俯冲板块中很常见，但硬毛钠石很少在岩石记录中存活下来，特别是在古代俯冲带的最高压力岩石中。该项目使用来自全球十二个俯冲复合体的稀有新鲜硬柱石的组成作为俯冲带元素循环过程的档案。硅铝石成分和分带是俯冲带流体-岩石相互作用的特殊记录者。成分和纹理的数据照亮的作用，硬毛钠石和伴生矿物的挥发性和元素循环在俯冲洋壳和上覆地幔的界面，在弧下深度的主要挥发分之前。本研究首次测定了硬柱石的氧同位素值，并对硬柱石及其伴生矿物的微量元素组成、分带和显微结构进行了综合表征。初步数据表明，硬铝石中的氧同位素丰度是俯冲板片中流体来源的敏感指标，如蛇纹岩（较低的d18 O）或海洋沉积物（较高的d18 O），并表明混合岩与结构连贯的俯冲复合体中的流体来源可能是不同的。该样品组包括硬柱石榴辉岩和蓝片岩，以及硬柱石轴承变质沉积岩（碳酸盐岩，石英岩）和交代岩和静脉，并包括硬柱石，峰，和逆行代。由于其高模态丰度和组成（1-2重量% H2O，1-2重量% Fe 2 O3），硬柱石可能是地幔氧化还原条件和H2O含量的一个重要因素，至少从新元古代开始。该奖项反映了NSF的法定使命，并被认为值得通过使用基金会的知识价值和更广泛的影响审查标准进行评估来支持。

Water and other chemical substances that are essential for life circulate from the Earth’s surface to the deep interior of the planet and back again in a cycle over many millions of years. Water is carried to great depths within mineral crystals in tectonic plates that dive (subduct) into the subsurface. One of the most important minerals in this cycle is lawsonite, which forms only under the characteristic high-pressure and relatively low-temperature conditions in the upper layer of subducting plates. Lawsonite is important because it contains a substantial amount of water and also contains trace amounts of other elements, such as uranium, lead, and rare earth elements. Although common in subducting plates, lawsonite rarely survives to be preserved in the rock record, particularly in the highest-pressure rocks of ancient subduction zones. This project uses the composition of rare, fresh lawsonite from a global suite of twelve subduction complexes as an archive of element-cycling processes in subduction zones. Lawsonite composition and zoning are exceptional recorders of fluid-rock interaction in subduction zones. Compositional and textural data illuminate the role of lawsonite and associated minerals in volatile and element cycling at the interface of the subducted oceanic crust and overlying mantle before major devolatilization at subarc depths. This research involves the first determination of oxygen isotope values in lawsonite and comprehensive characterization of the trace-element composition, zoning, and microstructures of lawsonite and associated minerals. Preliminary data indicate that oxygen isotope abundance in lawsonite is a sensitive indicator of the source(s) of fluids in subducted slabs, such as serpentinites (lower d18O) or oceanic sediments (higher d18O), and show that fluid sources in mélange vs. more structurally-coherent subduction complexes may be distinct. The sample suite consists of lawsonite eclogites and blueschists as well as lawsonite-bearing metasedimentary (carbonate, quartzite) and metasomatic rocks and veins, and includes prograde, peak, and retrograde generations of lawsonite. Owing to its high modal abundance and composition (12 wt% H2O, 1-2 wt% Fe2O3), lawsonite may be a significant factor in mantle redox conditions and H2O content since at least the Neoproterozoic.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.