Wang Shui-Jiong, Li Shu-Guang
State Key Laboratory of Geological Processes and Mineral Resources, and Institute of Earth Sciences, China University of Geosciences (Beijing), Beijing 100083, China.
Natl Sci Rev. 2022 Feb 26;9(6):nwac036. doi: 10.1093/nsr/nwac036. eCollection 2022 Jun.
The lighter magnesium (Mg) isotopic signatures observed in intraplate basalts are commonly thought to result from deep carbonate recycling, provided that the sharp difference in Mg isotopic composition between surface carbonates and the normal mantle is preserved during plate subduction. However, deep subduction of carbonates and silicates could potentially fractionate Mg isotopes and change their chemical compositions. Subducting silicate rocks that experience metamorphic dehydration lose a small amount of Mg, and preserve the original Mg isotopic signature of their protoliths. When the dehydrated fluids dissolve carbonate minerals, they may evolve into lighter Mg isotopic compositions. The solubility of carbonate minerals in fluids decreases in the order of calcite, aragonite, dolomite, magnesite and siderite, leading to selective and partial dissolution of carbonate minerals along the subduction path. At the island arc depth (70-120 km), the metamorphic fluid dissolves mainly Mg-poor calcites, and thus the fluid has difficulty modifying the Mg isotopic system of the mantle wedge and associated arc basalts. At the greater depth of the back arc system or continental margin (>150 km), the supercritical fluid can dissolve Mg-rich carbonate minerals, and its interaction with the mantle wedge could significantly imprint the light Mg isotopic signature onto the mantle rocks and derivatives. Meanwhile, the carbonate and silicate remaining within the subducting slab could experience elemental and isotopic exchange, during which the silicate can obtain a light Mg isotopic signature and high CaO/AlO, whereas the carbonates, particularly the Ca-rich limestone, shift Mg isotopes and MgO contents towards higher values. If this isotopic and elemental exchange event occurs widely during crustal subduction, subducted Ca-rich carbonates can partially transform into being Mg-rich, and a portion of recycled silicates (e.g. carbonated eclogites) can have light Mg isotopic composition alongside carbonates. Both serve as the low-δMg endmember recycled back into the deep mantle, but the latter is not related to deep carbonate recycling. Therefore, it is important to determine whether the light Mg isotopic signatures observed in intraplate basalts are linked to deep carbonate recycling, or alternatively, recycling of carbonated eclogites.
在板块内玄武岩中观察到的较轻的镁（Mg）同位素特征通常被认为是深部碳酸盐再循环的结果，前提是在板块俯冲过程中，地表碳酸盐与正常地幔之间的镁同位素组成的显著差异得以保留。然而，碳酸盐和硅酸盐的深部俯冲可能会使镁同位素分馏并改变其化学成分。经历变质脱水的俯冲硅酸盐岩会损失少量镁，并保留其原岩的原始镁同位素特征。当脱水流体溶解碳酸盐矿物时，它们可能会演变成较轻的镁同位素组成。碳酸盐矿物在流体中的溶解度按方解石、文石、白云石、菱镁矿和菱铁矿的顺序降低，导致碳酸盐矿物在俯冲路径上发生选择性和部分溶解。在岛弧深度（70 - 120千米），变质流体主要溶解贫镁的方解石，因此该流体难以改变地幔楔和相关岛弧玄武岩的镁同位素体系。在弧后系统或大陆边缘的更大深度（>150千米），超临界流体可以溶解富镁的碳酸盐矿物，其与地幔楔的相互作用可能会将轻镁同位素特征显著地印在地幔岩石及其衍生物上。与此同时，俯冲板块内残留的碳酸盐和硅酸盐可能会经历元素和同位素交换，在此过程中，硅酸盐可以获得轻镁同位素特征和高CaO/AlO，而碳酸盐，特别是富钙石灰岩，会使镁同位素和MgO含量向更高值偏移。如果这种同位素和元素交换事件在地壳俯冲过程中广泛发生，俯冲的富钙碳酸盐可以部分转变为富镁，并且一部分再循环的硅酸盐（如碳酸榴辉岩）可以与碳酸盐一起具有轻镁同位素组成。两者都作为低δMg端元再循环回深部地幔，但后者与深部碳酸盐再循环无关。因此，确定在板块内玄武岩中观察到的轻镁同位素特征是与深部碳酸盐再循环相关，还是与碳酸榴辉岩的再循环相关，这一点很重要。
