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

俯冲带是地壳再循环进入地幔的主要场所，对整个地球历史上地幔和大陆地壳的演化具有重要意义。俯冲沉积物贡献挥发物，包括H2O和CO2，微量元素，和独特的同位素特征的弧岩浆的形成。通过评估逐渐脱水的海洋粘土中硼浓度和同位素比的共同变化，拟议的研究将限制：1）在单一俯冲带的一个良好限制段内，海洋粘土输入海沟的成分的初始变化; 2）在进入弧下区域之前，通过逐渐脱水，沉积物成分的演变; 3）最终被排回海洋或被带到更深处并释放到弧前地幔的流体的组成。拟议的催化调查利用一个新的样本集收集的南海海槽和四国岛在日本西南部，有效地采样连续断面沿着表面的俯冲板从海沟的内部前弧，峰值古温度是非常好的约束以前的研究。该样品集由第三纪和白垩纪四万十带的20个俯冲、底侵和折返的页岩和泥岩组成（峰值古温度约100-285 ℃）。这套独特的样品提供了一个机会，以限制在俯冲的最初几十公里的成岩作用和变质作用的地球化学效应。了解俯冲带中的元素循环对于了解地球大气圈、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，沉淀碳酸盐沉积物的俯冲是大气碳的一个重要的长期汇。 此外，大量的海水在俯冲带被俯冲到地球深部，随着地质年代的推移，这可能导致地球深部地幔中大量氢的积累。 硼和锂是轻元素，具有独特的同位素特征，使我们能够跟踪水和沉积物等表面物质向地球深处的循环。这个催化研究项目，由CNIC计划的支持，将建立美国和意大利地球科学家之间的密切合作，并给予早期职业生涯的学生研究人员在比萨的Consiglio Nazionale delle Ricerche（CNR）的国际公认的同位素地球化学实验室接受培训的宝贵经验。

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.