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

围绕俯冲洋板块在构造质量转移中的作用有两个主要问题：1）俯冲如何有效地从表面储层中封存碳; 2）洋板块如何从深度>100公里的地方返回，以及为什么这种洋板块很少见。为了回答这些问题，本项目的主要目标是采用碳酸盐作为板片脱碳和俯冲带变质作用的压力（P）-温度（T）-时间（t）演化的新指标。碳酸盐分带的时间尺度，特别是，是至关重要的碳通量的估计，以及俯冲通道动力学。这些新的岩石学工具将被应用于西天山超高压（UHP）变质带，代表了仅有的两个大洋超高压板之一，在造山带折返。在该超高压带中，无论是变基性岩还是变岩中，碳酸盐矿物和碳酸盐脉都十分丰富。榴辉岩中的基质碳酸盐主要为镁铁白云石，核部Mg#增加，成分带明显。 该项目整合了实验研究，实地研究，岩石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-折返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个博士学位。和3个硕士论文项目以及本科生的参与，致力于从事那些来自科学代表性不足的少数群体。 在多伦多大学的高压实验室中，利用活塞-圆柱装置对铁白云石-铁白云石中的扩散Fe-Mg交换进行了实验研究。校准的扩散模型将应用于榴辉岩和高压脉中的分区碳酸盐，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）和携带P-T信息的矿物的放射性测年（例如，金红石、钛铁矿）。分析硅酸盐、碳酸盐和电解质流体之间的相关系，将碳酸盐带与变质阶段联系起来。在一个类似的计划，通常适用于分区石榴石，相平衡加上生长扩散模拟将揭示整体的P-T-t路径的超高压板，放置在俯冲通道的俯冲-折返动力学的岩石学约束。进一步分析开放体系特征，评价了流体-岩石相互作用引起的板片脱碳酸作用的程度，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，这些估计相结合的物质平衡分析的宏观地球化学变化的交代作用，以评估净碳通量从板。

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.