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

围绕俯冲大洋板块在构造质量转移中的作用有两个主要问题：1)俯冲作用如何有效地从表层储存库中封存碳；2)板块如何从100公里深处返回，以及为什么这样的板块很少见。为了回答这些问题，本项目的主要目标是采用碳酸盐作为俯冲带变质作用的压力(P)-温度(T)-时间(T)演化的新的板岩脱碳指标。碳酸盐分带作用所得的时间尺度对于碳通量和俯冲通道动力学的估计尤其重要。这些新的岩石学工具将应用于西天山超高压变质带，该带是造山带中仅有的两个大洋超高压板片之一。在这条超高压带中，变质基性岩和变辉石中均含有丰富的碳酸盐矿物和矿脉。榴辉岩中的基质碳酸盐以白云石-铁角闪石为主，岩芯和陡峭的成分带中的镁含量增加。 该项目综合了实验研究、实地研究、热力学-动力学模拟和仪器分析，从岩石学的角度概括了俯冲带的碳循环和俯冲-折返动力学。拟议的工作将发展HQP，因为它将支持2名博士和3名硕士。论文项目以及本科生参与，致力于让那些来自代表不足的少数群体的人参与科学。 在多伦多大学高压实验室的活塞-圆柱体装置上，将对白云石-铁角闪石中的扩散Fe-Mg交换进行实验研究。校准后的扩散模型将应用于榴辉岩和高压矿脉中的碳酸盐带，以得出相对时间尺度，然后与基于其他矿物(例如石榴石中的锰、磷灰石中的锶)的估计值以及具有P-T信息的矿物(例如金红石、钛铁矿)的放射性测年进行比较。将分析硅酸盐、碳酸盐和电解液流体之间的相关系，将碳酸盐带与变质阶段进行对比。在通常应用于石榴石分带的类似方案中，相平衡结合生长-扩散模拟将揭示超高压板块的整体P-T-t路径，从而对俯冲通道中的俯冲-折返动力学施加岩石学约束。进一步的开放系统行为分析评价了流体-岩石相互作用对板岩脱碳的程度及其对碳酸盐分带和碳酸盐带间同位素变化的影响。最后，结合交代作用产生的宏观地球化学变化的物质平衡分析，估算了板坯的净碳通量。

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.