沉积碳酸盐岩地幔再循环过程中的物理化学变化及其地球化学响应

Our work will focus on the middle part along the boundary of the North China Craton and the Central Asian Orogenic Belt, where a lot of basalt erupted during the Neogene time. Combining with the high-pressure and high temperature experiments, the carbonatic xenoliths and carbonated peridotite xenoliths carried by the basalts will be studied in terms of petrography and geochemistry to investigate that if carbonate can be recycled back into the deep mantle (>150km) by subduction in the form of sedimentary carbonate rock in addition to carbonated eclogite and carbonated pelite, and such physical changes and chemical reactions of the sedimentary carbonate rock as decarbonization, deformation and metamorphism, partial melting, carbon reducing and carbonate-peridotite reaction under the conditions of subduction zone and the geochemical responses of these processes in terms of trace element migration and Sr-Nd-Pb-Ca-Os isotopic variations and C-O isotopic fractionations. The major work to be done include: (1) seeking and identifying the ultrahigh-pressure minerals and highly reduced minerals using laser Raman spectrometer; (2) analyzing the major and trace element compositions and Sr-Nd-Pb-Ca-Os and C-O isotopic compositions of bulk rocks and some minerals using LA-(MC-)ICP-MS, TIMS and SIMS; (3) obtaining the rheological and partial melting parameters of the sedimentary carbonate rock under the conditions of subduction zone by the high-P-T experiments.

以华北克拉通北缘和兴蒙造山带结合部位中段为主要研究区。通过对区内玄武岩所捕获的碳酸盐质包体和碳酸盐化橄榄岩包体的岩相学和地球化学研究并结合高温高压实验，揭示地球表层碳能否直接以“沉积碳酸盐岩”的形式（而不仅是碳酸盐岩化榴辉岩）被俯冲板片携带到深部地幔（>150km）和在俯冲带发生的物理、化学变化（如，脱CO2作用、变质变形和部分熔融作用、碳还原作用、碳酸盐+橄榄岩反应作用）及其相应的地球化学响应（微量元素迁移和C-O同位素分馏）。主要工作包括：（1）利用激光拉曼光谱仪查找超高压、超还原矿物的类型及其分布；（2）利用LA-(MC-)ICP-MS、TIMS和SIMS等元素和同位素分析技术，获得全岩和部分单矿物样品中微量元素和Sr-Nd-Pb-Ca-Os同位素及C-O同位素的组成和变化特征；（3）通过高温、高压实验，获得碳酸盐岩在俯冲带条件下的流变学特征和部分熔融条件等。

深部碳循环不仅影响全球气候变化，同时会影响壳-幔物质组成和演化，是壳-幔物质循环研究的重要组成部分。本项目对兴蒙造山带达里湖地区和华北克拉通北缘汉诺坝、四子王旗、繁峙及阳原地区出露的碳酸岩包体、侵入体和脉体进行了详细的岩石学和地球化学研究，提供了沉积碳酸盐地幔再循环的证据，揭示了沉积碳酸盐岩俯冲循环的过程、机理。对华北克拉通北缘大同、繁峙和四子王旗新生代玄武岩中的地幔包体开展了岩石学和微区地球化学研究，揭示了与古亚洲洋壳俯冲有关的沉积碳酸盐再循环作用引起了广泛的华北克拉通北缘岩石圈地幔活化或者减薄。通过高温高压和实验和天然样品研究发现碳酸盐脱碳反应会引起巨大的C同位素分馏，并形成局部超还原环境，少量水的加入会降低碳酸盐矿物的黏度，促使碳酸盐在地幔矿物边界间迁移等。完成了沉积碳酸盐岩地幔再循环的地球化学响应工作，发现沉积碳酸盐岩再循环可显著升高地幔氧逸度、促进PGE迁移、改变地幔Sr-Ca-Pb-OS-Mg同位素组成。