The effects of diagenesis and dolomitization on Ca and Mg isotopes in marine platform carbonates: Implications for the geochemical cycles of Ca and Mg
The effects of diagenesis and dolomitization on Ca and Mg isotopes in marine platform carbonates: Implications for the geochemical cycles of Ca and Mg
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DOI：
10.1016/j.gca.2014.07.025
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发表时间：
2014-10
影响因子：
5
通讯作者：
M. Fantle;J. Higgins
中科院分区：
文献类型：
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作者：
M. Fantle;J. Higgins
本文给出了ODP Site 1196A海相灰岩和白云岩的Ca、Mg、O和C同位素和微量元素组成，它们的深度（~ 58 ~ 627 mbsf）和沉积年龄（~ 5 ~ 23 Ma）不等。本研究的目的是探索非传统同位素系统在成岩作用指纹图谱中的潜力，量化成岩作用过程中地球化学指标的改变程度，并研究成岩作用在全球Ca和Mg地球化学旋回中的重要性。结果表明，钙具有较高的固液质量比，在成岩作用中可能发生同位素蚀变。此外，Ca的蚀变与Mg的蚀变具有相关性，两者都可以作为破译古代岩石成岩作用的有用工具。整体碳酸盐岩δ44Ca值变化范围为0.60 ~ 1.31‰（SRM-915a标度）；灰岩δ44Ca平均值为0.97±0.24‰（1SD），与白云岩δ44Ca平均值（1.03±0.15 1SD‰）在误差范围内相同。镁同位素组成（δ26Mg， DSM-3标度）在−2.59‰~−3.91‰之间，灰岩（−3.60±0.25‰）和白云岩（−2.68±0.07‰）同位素差异明显。碳同位素组成（δ13C， PDB标度）变化范围在0.86‰~ 2.47‰之间，灰岩平均（1.96±0.31‰）与白云岩平均（1.68±0.57‰）略有偏移。灰岩的氧同位素组成（δ18O， PDB标度）（- 1.22±0.94‰）明显低于白云岩（2.72±1.07‰）。1196A的同位素数据表明，根据成岩轨迹，同位素和元素组成有明显的一致趋势。数值模拟支持这样一种观点，即这种趋势可以解释为成岩作用，并表明与石灰岩成岩作用相关的适当分布系数（KMg）为~ 1至5 × 10−3，明显低于实验室研究报告的值（>0.015）。Mg同位素的成岩分选因子分别为~ 0.9955（- 4.5‰）和0.9980（- 2‰），适合于灰岩成岩作用和白云化作用。
The Ca, Mg, O, and C isotopic and trace elemental compositions of marine limestones and dolostones from ODP Site 1196A, which range in depth (∼58 to 627 mbsf) and in depositional age (∼5 and 23 Ma), are presented. The objectives of the study are to explore the potential for non-traditional isotope systems to fingerprint diagenesis, to quantify the extent to which geochemical proxies are altered during diagenesis, and to investigate the importance of diagenesis within the global Ca and Mg geochemical cycles. The data suggest that Ca, which has a relatively high solid to fluid mass ratio, can be isotopically altered during diagenesis. In addition, the alteration of Ca correlates with the alteration of Mg in such a way that both can serve as useful tools for deciphering diagenesis in ancient rocks.Bulk carbonate δ44Ca values vary between 0.60 and 1.31‰ (SRM-915a scale); the average limestone δ44Ca is 0.97 ± 0.24‰ (1SD), identical within error to the average dolostone (1.03 ± 0.15 1SD ‰). Magnesium isotopic compositions (δ26Mg, DSM-3 scale) range between −2.59‰ and −3.91‰, and limestones (−3.60 ± 0.25‰) and dolostones (−2.68 ± 0.07‰) are isotopically distinct. Carbon isotopic compositions (δ13C, PDB scale) vary between 0.86‰ and 2.47‰, with average limestone (1.96 ± 0.31‰) marginally offset relative to average dolostone (1.68 ± 0.57‰). The oxygen isotopic compositions (δ18O, PDB scale) of limestones (−1.22 ± 0.94‰) are substantially lower than the dolostones measured (2.72 ± 1.07‰).The isotopic data from 1196A suggest distinct and coherent trends in isotopic and elemental compositions that are interpreted in terms of diagenetic trajectories. Numerical modeling supports the contention that such trends can be interpreted as diagenetic, and suggests that the appropriate distribution coefficient (KMg) associated with limestone diagenesis is ∼1 to 5 × 10−3, distinctly lower than those values (>0.015) reported in laboratory studies. With respect to Mg isotopes, the modeling also suggest that diagenetic fractionation factors of ∼0.9955 (−4.5‰) and 0.9980 (−2‰) are appropriate for limestone diagenesis and dolomitization, respectively.