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

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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.