US-Italy Collaboration: Determination of Boron Isotope Ratios in Subducted Sediments

俯冲带是地壳循环进入地幔的主要场所，对整个地球历史上的地幔和大陆地壳演化具有重要意义。俯冲沉积物为弧岩浆的形成提供了包括H2O和CO2在内的挥发物、微量元素和独特的同位素特征。通过评估逐渐脱水的海相粘土中硼浓度和同位素比值的共变，本文提出的研究将约束1)在单个俯冲带的良好约束段内约束海相粘土输入海沟成分的初始变异；2)进入次弧区前泥沙成分的递进脱水演化；3)流体的组成，这些流体最终被驱逐回海洋，或被带到更深的地方并释放到弧前地幔中。催化研究利用了从日本西南部的南开海槽和四国岛收集的一组新样品，该样品有效地对从海沟到内前弧的俯冲板块表面的连续样带进行了采样，并且对其峰值古温度进行了非常好的限制。样品组由20个来自第三系和白垩纪石曼托带的俯冲、下覆和发掘的页岩和泥岩组成，峰值古温度为100-285¢ªC。这组独特的样品提供了一个机会来限制在俯冲的前几十公里的成岩和变质作用的地球化学影响。了解通过俯冲带的元素循环对于理解地球大气、水圈和地圈之间复杂的相互作用和交换至关重要。例如，大气中的二氧化碳水平影响碳酸盐在海洋中的溶解度，而沉淀的碳酸盐沉积物的俯冲作用是大气碳的重要长期汇。此外，大量的海水在俯冲带俯冲到地球深处，随着地质时间的推移，可能会导致地球深部地幔中大量氢气的积累。硼和锂是具有不同同位素特征的轻元素，使我们能够追踪地表物质（如水和沉积物）进入地球深处的循环。这个催化研究项目由CNIC项目支持，将建立美国和意大利地球科学家之间的密切合作，并为早期职业的学生研究人员提供宝贵的经验，即在比萨国家地质研究中心（CNR）国际公认的同位素地球化学实验室接受培训。

Subduction zones are the primary site of crustal recycling into the Earth¡¯s mantle, with important implications for mantle and continental crustal evolution throughout Earth history. Subducted sediments contribute volatiles including H2O and CO2, trace elements, and unique isotopic signatures to the formation of arc magmas. By assessing the co-variation of boron concentrations and isotope ratios in progressively dehydrated marine clays, the proposed study will constrain 1) the initial variability in the composition of marine clay inputs to the trench within a well-constrained segment of a single subduction zone; 2) the evolution of sediment composition through progressive dehydration prior to entering the sub-arc region; and 3) the composition of fluids that are ultimately expelled back to the ocean, or carried deeper and released into the forearc mantle. The proposed catalytic investigation makes use of a novel sample set collected from the Nankai Trough and Shikoku Island in Southwest Japan, which effectively samples a continuous transect along the surface of the subducted slab from the trench to the inner forearc, and for which peak paleo-temperatures are extraordinarily well-constrained by previous studies. The sample set consists of twenty subducted, underplated, and exhumed shales and mudstones from the Tertiary and Cretaceous Shimanto Belts, (peak paleo-temperatures ~100-285¢ªC). This unique suite of samples provides an opportunity to constrain the geochemical effects of diagenesis and metamorphism during the first few tens of kilometers of subduction.Understanding elemental cycling through subduction zones is critical for understanding the complex interactions and exchanges between the Earth¡¯s atmosphere, hydrosphere, and geosphere. For example, carbon dioxide levels in the atmosphere influence carbonate solubility in the oceans, and subduction of precipitated carbonate sediments is an important long-term sink for atmospheric carbon. In addition, significant amounts of ocean water are subducted into the deep Earth at subduction zones, which over geologic time may lead to the accumulation of a significant reservoir of hydrogen in the Earth¡¯s deep mantle. Boron and lithium are light elements with distinct isotopic signatures that allow us to track the recycling of surface materials such as water and sediments into the depths of the Earth. This catalyzing research project, supported by the CNIC program, will establish a close collaboration between US and Italian geoscientists, and gives early-career student researchers the valuable experience of being trained in the internationally recognized isotope geochemistry laboratory at the Consiglio Nazionale delle Ricerche (CNR) in Pisa.