东太平洋隆IODP 1256钻孔蚀变洋壳的Ba同位素组成

Ba is a highly incompatible element during mantle partial melting. The Ba content of the crust is much higher than that of the mantle. Due to the obvious Ba isotopic fractionation in crustal samples, subduction of crustal materials with high Ba contents into the mantle could modify the Ba isotope composition of the mantle. Therefore, Ba isotopes have the potential to track recycled crustal materials. The oceanic crust altered by seawater and hydrothermal fluids is an important subduction endmember, which is essential for the study of crust-mantle material circulation. In addition, assessing the effect of fluid alteration on Ba isotope composition of the oceanic crust is of great significance for using Ba isotopes to constrain the oceanic Ba geochemical cycle. However, no Ba isotope data have been published for altered oceanic crust yet, which limits the development of Ba isotope geochemistry. . This project will study the Ba isotope composition of altered oceanic crust at Integrated Ocean Drilling Program (IODP) Hole 1256D from the East Pacific Rise, we are aiming to acquire the Ba isotope composition of the altered oceanic crust reservoir, and clarify the Ba isotope fractionation behavior during seawater and hydrothermal alteration. The results of this project will provide an important information for us to explore the application of Ba isotopes, and to constrain recycling of subducted material and the oceanic Ba geochemical circulation.

Ba在地幔部分熔融过程中是高度不相容元素，地壳的Ba含量远高于地幔。由于地壳中存在明显的Ba同位素分馏，高Ba含量的地壳物质俯冲进入地幔，将改造地幔的Ba同位素组成。因此，Ba同位素具有示踪俯冲物质再循环的应用潜力。经历了海水和热液蚀变的洋壳是重要的俯冲端元，对于研究壳-幔物质的循环至关重要。此外，评估海水和热液蚀变对洋壳Ba同位素组成的影响对利用Ba同位素制约大洋元素循环具有重要意义。但是目前为止还没有蚀变洋壳的Ba同位素数据发表，限制了Ba同位素地球化学的发展。本项目拟通过研究东太平洋隆大洋钻探计划IODP 1256钻孔的样品，用以限定蚀变洋壳储库的Ba同位素组成，和查明海水和热液蚀变过程中Ba同位素的地球化学行为。本项目的实施将为利用Ba同位素研究俯冲再循环和壳幔物质作用，制约大洋Ba元素循环奠定重要基础。

限定蚀变洋壳储库的Ba同位素组成，以及查明海水和热液蚀变过程对洋壳Ba同位素组成的影响，对于利用Ba同位素示踪俯冲物质再循环至关重要。对东太平洋隆大洋钻探计划IODP 1256钻孔样品的Ba同位素组成测量结果表明，蚀变洋壳具有高度不均一的Ba同位素组成，δ138/134Ba为-0.22 ~ 0.39‰（平均值为0.21±0.20‰, 2SD, n=34）。火山岩和席状岩墙部分都具有相对新鲜的洋中脊玄武岩（MORB）偏重Ba同位素组成，δ138/134Ba为-0.01‰ ~ 0.39‰，反映低温海水蚀变和高温热液蚀变过程对洋壳Ba同位素的影响，这可能归因于水岩交换过程中原生矿物的溶解。侵入体部分的Ba同位素组成偏轻， δ138/134Ba 为 -0.22‰ ~ -0.06‰,，可能继承于Ba同位素组成偏轻的高温岩浆热液流体。准确获得地幔的Ba同位素组成，是明确蚀变洋壳对地幔Ba同位素组成影响的前提。对洋中脊玄武岩的Ba同位素组成测量结果显示，其δ138/134Ba变化范围为-0.06 ~ +0.11‰。基于D-MORB样品估算出亏损地幔（DMM）的平均δ138/134Ba为+0.05±0.05‰ (2SD, n=16)。由于蚀变洋壳的Ba同位素组成变化范围远远大于DMM，因此，蚀变洋壳再循环对于幔源岩浆岩地幔源区的影响可以通过Ba同位素进行有效的识别。