本文为国家自然科学基金项目（编号 41622301和41590623）资助的成果。
碳酸盐岩和碳酸盐化岩石是俯冲带中碳的主要地质载体。俯冲- 折返的超高压变质岩记录了俯冲隧道中流体- 岩石的相互作用与元素迁移，是研究地球内部元素循环的重要媒介。本文采集了大别造山带罗家岭互层状产出的大理岩和榴辉岩，建立了横跨岩性界面的矿物组成、显微结构和Mg- Fe- C- O同位素剖面。大理岩中主要组成矿物为白云石，其次为方解石、石英、黑云母和绿帘石等。靠近榴辉岩，大理岩中的方解石含量逐渐升高，在大理岩和榴辉岩界面中方解石是主要的碳酸盐矿物。榴辉岩经历了角闪岩相的退变质作用，主要组成矿物为石榴子石、角闪石、石英和云母，含少量方解石和白云石。罗家岭大理岩的δ26Mg（相比于DSM- 3标样）值为-1. 66‰~-1. 51‰。相对于玄武质原岩（δ26Mg=-0. 25‰），罗家岭榴辉岩具有轻Mg同位素组成（δ26Mg=-1. 48‰~-1. 25‰）。大理岩的δ56Fe（相比于IRMM- 14标样）值为-0. 62‰~-0. 56‰，榴辉岩具有较重的Fe同位素组成（δ56Fe=0. 16‰~0. 45‰）。大理岩和榴辉岩中的碳酸盐矿物具有显著的C- O同位素变化（δ13C=-3. 4‰~2. 2‰，δ18O=7. 6‰~14. 3‰），且δ13C与δ18O呈正相关关系。研究结果表明：大理岩和榴辉岩界面的机械混合作用使榴辉岩中含有孤立的碳酸盐矿物，并促进了大理岩和榴辉岩之间的反应，但是机械混合作用影响范围只有几厘米。俯冲带中的流体活动促进了大理岩中白云石分解和元素迁移，使榴辉岩获得大理岩的轻Mg同位素信号，甚至直接携带碳酸盐矿物进入地幔。从大理岩到榴辉岩Fe同位素的扩散迁移可以导致榴辉岩具有较重的Fe同位素组成，而大理岩具有较轻的Fe同位素组成。碳酸盐化榴辉岩是驱动地球深部碳循环和造成地幔Mg同位素不均一性的重要载体。
Carbonates and carbonated rocks are the main carbon carriers in subduction zones. Ultrahigh pressure (UHP) metamorphic rocks record fluid- rock interaction and element migration. In this study, we investigated interlayered marbles and eclogites from Luojialing in the Dabie UHP metamorphic belt, by analyzing the mineralogy and petrology, microstructure and Mg- Fe- C- O isotope compositions across the lithological contact. Marble samples are mainly composed of dolomite, with minor calcite, quartz, biotite and epidote. The volume fraction of calcite in marble samples increases with a decreasing distance to eclogites and becomes predominant at the lithological contact. Eclogite samples underwent amphibole facies retrograde metamorphism and mainly consist of garnet, amphibole, quartz and mica, with minor calcite and dolomite. Marble samples have light Mg isotope compositions with an average δ26Mg value of -1. 57‰ (δ26Mg=-1. 66‰ to -1. 51‰) and light Fe isotope compositions with an average δ56Fe value of -0. 58‰ (δ56Fe=-0. 62‰ to -0. 56‰), whereas eclogite samples show enrichment of light Mg isotopes (δ26Mg=-1. 48‰ to -1. 25‰), but heavy Fe isotope with an average δ56Fe value of 0. 34‰ (δ56Fe=0. 16‰ to 0. 45‰). Carbonate minerals in marbles and eclogites have large C and O isotopic variations (δ13C=-3. 4‰ to 2. 2‰, δ18O=7. 6‰ to 14. 3‰) and show a positive correlation between δ13C and δ18O. Mechanical mixing at the lithological contact promoted the reaction between marbles and eclogites, but the influence of mechanical mixing was limited to within a few centimeters. Fluid activity promoted the breakdown of dolomite and elemental migration between marble and eclogite. Eclogites can acquire light Mg isotopic signature from marble and further carry carbonate minerals down into the mantle during subduction. This process was also accompanied by the diffusion of light Fe isotopes from the eclogite to marble. Carbonation of eclogites during subduction, therefore, plays a pivotal role in deep carbon cycle and generation of Mg isotope heterogeneity in the mantle.
闫伟豪,王勤,李伟强.2022.俯冲带中大理岩与榴辉岩的Mg- Fe- C- O同位素迁移[J].地质学报,96(2):500-516.
Yan Weihao, Wang Qin, Li Weiqiang.2022. Behavior of Mg- Fe- C- O isotopes at the contact zone between marble and eclogite in subduction zones[J]. Acta Geologica Sinica,96(2):500-516.
