Lawsonite: an exceptional geochemical archive in HP/LT rocks

水和其他对生命至关重要的化学物质从地球表面循环到地球内部深处，然后再循环回来，周期长达数百万年。水被带到很深的地方，在构造板块的矿物晶体中潜入地下（俯冲）。该旋回中最重要的矿物之一是lawsonite，它仅在俯冲板块上层特有的高压和相对低温条件下形成。Lawsonite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。虽然在俯冲板块中很常见，但在岩石记录中保存下来的lawsonite很少，特别是在古代俯冲带的高压岩石中。该项目使用了来自全球12个俯冲复合体的稀有新鲜lawsonite的成分，作为俯冲带元素循环过程的档案。花岗岩组成和分带是俯冲带流体-岩石相互作用的特殊记录者。组成和结构数据阐明了在亚弧深度主要挥发性物质分解之前，在俯冲洋壳和上覆地幔的界面上，铁榴辉石和伴生矿物在挥发性物质和元素循环中的作用。本研究首次确定了lawsonite中的氧同位素值，并对lawsonite及其伴生矿物的微量元素组成、分带和微观结构进行了全面表征。初步数据表明，滑石中的氧同位素丰度是俯冲板块（如蛇纹岩（低d18O）或海洋沉积物（高d18O））中流体来源的敏感指标，并表明m<s:1> <s:1> <s:1>物质交换体和更结构连贯的俯冲杂岩中的流体来源可能是不同的。样品组主要由铁榴辉岩、蓝片岩、含铁榴辉岩变质沉积岩（碳酸盐、石英岩）、交代岩和脉体组成，包括顺行、峰期和逆行几代铁榴辉岩。由于其高模态丰度和组成（12 wt% H2O, 1-2 wt% Fe2O3），至少从新元古代开始，lawsonite可能是地幔氧化还原条件和H2O含量的重要因素。该奖项反映了美国国家科学基金会的法定使命，并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，被认为值得支持。

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.