Heterogeneous magnesium isotopic composition of the upper continental crust
Heterogeneous magnesium isotopic composition of the upper continental crust
复制标题
上陆壳的异质镁同位素组成
DOI：
10.1016/j.gca.2010.08.030
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发表时间：
2010-12-01
影响因子：
5
通讯作者：
Chappell, B. W.
中科院分区：
文献类型：
--
作者：
Li, Wang-Ye;Teng, Fang-Zhen;Chappell, B. W.
高精度镁同位素数据报告了类似的100个特征良好的样品（花岗岩、黄土、页岩和上地壳复合材料），这些样品以前用于估计上大陆地壳组成。澳大利亚东南部的8种i型花岗岩（δ Mg-26 = -0.25 ~ -0.15）和中国东部的15种花岗岩类复合岩体（δ Mg-26 = -0.35 ~ -0.16）镁同位素组成变化有限，与SiO2含量无关，说明花岗岩岩浆分异过程中镁同位素分异不明显。同样，代表澳大利亚东南部s型花岗岩谱两端的2个s型花岗岩，其Mg同位素组成（δ Mg-26 = -0.23和-0.14）在其潜在烃源岩（δ Mg-26 = -0.20和+0.15）和i型花岗岩的范围内，表明地壳深熔过程中Mg同位素分选作用也不显著。而在东北地区的19块a型花岗岩中，δ Mg-26含量变化显著（-0.28 ~ +0.34），可能反映了物源的非均质性。与i型和s型花岗岩相比，沉积岩的Mg同位素组成高度不均匀，多数情况下Mg同位素组成较重，新西兰和美国的9个黄土中Mg-26 δ值在-0.32 ~ +0.05之间，20个后太古代澳大利亚页岩（PAAS）中Mg-26 δ值在-0.27 ~ +0.49之间，中国东部的20个沉积复合岩中Mg-26 δ值在-0.52 ~ +0.92之间。随着化学风化作用的增加，δ Mg-26值在页岩中的分散性大于黄土。此外，黄土中Mg-26与Li-7呈负相关。这些特征表明，化学风化作用对Mg同位素的分馏作用显著，并对沉积岩中Mg同位素组成的高度变化起着重要作用。根据上大陆地壳内主要岩石单元的估算比例及其平均MgO含量，得到上大陆地壳的加权平均Mg-26值为-0.22。研究表明，上地壳Mg同位素组成平均呈地幔状，但高度不均匀，δ Mg-26在-0.52 ~ +0.92之间。如此大的同位素变化主要是由于化学风化作用，在此过程中，轻Mg同位素丢失到水圈，而风化产物（如沉积岩）则留下重Mg同位素。(C) 2010 Elsevier Ltd.版权所有。
High-precision Mg isotopic data are reported for similar to 100 well-characterized samples (granites, loess, shales and upper crustal composites) that were previously used to estimate the upper continental crust composition. Magnesium isotopic compositions display limited variation in eight I-type granites from southeastern Australia (delta Mg-26 = -0.25 to -0.15) and in 15 granitoid composites from eastern China (delta Mg-26 = -0.35 to -0.16) and do not correlate with SiO2 contents, indicating the absence of significant Mg isotope fractionation during differentiation of granitic magma. Similarly, the two S-type granites, which represent the two end-members of the S-type granite spectrum from southeastern Australia, have Mg isotopic composition (delta Mg-26 = -0.23 and -0.14) within the range of their potential source rocks (delta Mg-26 = -0.20 and +0.15) and I-type granites, suggesting that Mg isotope fractionation during crustal anatexis is also insignificant. By contrast, delta Mg-26 varies significantly in 19 A-type granites from northeastern China (-0.28 to +0.34) and may reflect source heterogeneity.Compared to I-type and S-type granites, sedimentary rocks have highly heterogeneous and, in most cases, heavier Mg isotopic compositions, with delta Mg-26 ranging from -0.32 to +0.05 in nine loess from New Zealand and the USA, from -0.27 to +0.49 in 20 post-Archean Australian shales (PAAS), and from -0.52 to +0.92 in 20 sedimentary composites from eastern China. With increasing chemical weathering, as measured by the chemical index of alternation (CIA), delta Mg-26 values show a larger dispersion in shales than loess. Furthermore, delta Mg-26 correlates negatively with delta Li-7 in loess. These characteristics suggest that chemical weathering significantly fractionates Mg isotopes and plays an important role in producing the highly variable Mg isotopic composition of sedimentary rocks.Based on the estimated proportions of major rock units within the upper continental crust and their average MgO contents, a weighted average delta Mg-26 value of -0.22 is derived for the average upper continental crust. Our studies indicate that Mg isotopic composition of the upper crust is, on average, mantle-like but highly heterogeneous, with delta Mg-26 ranging from -0.52 to +0.92. Such large isotopic variation mainly results from chemical weathering, during which light Mg isotopes are lost to the hydrosphere, leaving weathered products (e.g., sedimentary rocks) with heavy Mg isotopes. (C) 2010 Elsevier Ltd. All rights reserved.