Carbonate zonation to reveal P-T-t-fluid history of fossil slab and carbon cycle in subduction zones

两个主要问题围绕着大洋板块俯冲作用在构造质量传递中的作用：1)俯冲作用如何有效地从地表储集层隔离碳；2)板块是如何从100公里深处返回的，为什么这样的板块很罕见。为了回答这些问题，本项目的主要目标是采用碳酸盐作为板块脱碳和俯冲带变质作用的压力(P)-温度(T)-时间(T)演化的新代表。特别是由碳酸盐分带得出的时间尺度，对于估计碳通量和俯冲通道动力学是至关重要的。这些新的岩石学工具将应用于西天山超高压变质带，该变质带是在造山带中出土的仅有的两个海洋超高压板块之一。在超高压带中，变质长岩和变质长岩中均含有丰富的碳酸盐矿物和矿脉。榴辉岩中基质碳酸盐以白云岩-铁白云岩为主，从岩心开始Mg#增加，组成带明显。该项目整合了实验研究、实地研究、热力学-动力学模拟和仪器分析，从岩石学的角度对俯冲带的碳循环和俯冲-挖掘动力学进行了总体描述。拟议的工作将发展HQP，因为它将支持2个博士和3个硕士论文项目以及本科生的参与，并致力于吸引那些在科学领域代表性不足的少数群体。利用多伦多大学高压实验室的活塞缸装置，对白云石-铁白云石中Fe-Mg扩散交换进行了实验研究。校正后的扩散模型将应用于榴辉岩和高压矿脉中的分带碳酸盐，以得出相对时间尺度，然后将其与基于其他矿物（例如石榴石中的Mn，磷灰石中的Sr）的估计和携带P-T信息的矿物（例如金红石，钛矿）的辐射定年法进行比较。通过分析硅酸盐、碳酸盐和电解质流体的相关系，将碳酸盐带与变质阶段联系起来。与带状石榴石相似，相平衡与生长-扩散模拟相结合将揭示特高压板块的整体P-T-t路径，为俯冲通道中的俯冲-掘出动力学提供岩石学约束。通过对开放体系行为的进一步分析，评价了流-岩相互作用对板状体脱碳化的程度及其对碳酸盐岩分带和碳酸盐岩带间同位素变化的影响。最后，将这些估算结果与由交代作用引起的宏观地球化学变化的质量平衡分析相结合，评估了从厚板流出的净碳通量。

Two major questions surround the role of subducting oceanic slab in tectonic mass transfer: 1) how efficiently subduction sequesters carbon from surface reservoirs; and 2) how slabs return from depths >100 km and why such slabs are rare. To answer these questions, the primary objective of this project is to adopt carbonate as a new proxy of both slab decarbonation and pressure (P)-temperature (T)-time (t) evolution of subduction zone metamorphism. The timescales derived from carbonate zonation, in particular, are critical for the estimates of carbon flux as well as subduction channel dynamics. These new petrologic tools will be applied to the Western Tianshan ultra-high pressure (UHP) metamorphic belt that represents one of only two oceanic UHP slabs having exhumed in orogens. In this UHP belt, carbonate minerals and veins are abundant in both metabasite and metapelite. The matrix carbonates in the eclogite are predominantly dolomite-ankerite, with increasing Mg# from the core and sharp compositional zones.The project integrates experimental studies, field-based research, thermodynamic-kinetic simulations, and instrumental analyses, toward a general picture of the carbon cycle in the subduction zone and subduction-exhumation dynamics from a petrologic perspective. The proposed work will develop HQP because it will support 2 Ph.D. and 3 M.Sc. thesis projects as well as undergraduate involvement, with a commitment to engaging those from under-represented minority groups in sciences.The diffusive Fe-Mg exchange in dolomite-ankerite will be investigated experimentally using piston-cylinder apparatus in the high-pressure laboratory at the University of Toronto. The calibrated diffusion model will be applied to zoned carbonate in eclogite and high-pressure veins to derive the relative timescales, which are then compared with the estimates based on other minerals (e.g., Mn in garnet, Sr in Apatite) and radiometric dating of minerals that carry P-T information (e.g., rutile, titanite). The phase relations among silicate, carbonate, and electrolyte fluid will be analyzed to correlate the carbonate zones to metamorphic stages. In a similar scheme as commonly applied to zoned garnet, phase equilibria coupled with growth-diffusion simulation will reveal the overall P-T-t path of the UHP slab, placing petrologic constraints on the subduction-exhumation dynamics in subduction channel. Further analysis of open-system behaviors evaluates the extent of slab decarbonation by fluid-rock interaction, and its influence on carbonate zonation and the isotopic variations among carbonate zones. Finally, these estimates are combined with the mass balance analysis of macroscopic geochemical variations by metasomatism to evaluate the net carbon flux from the slab.