The magnesium isotope (δ26Mg) signature of dolomites
The magnesium isotope (δ26Mg) signature of dolomites
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DOI：
10.1016/j.gca.2014.11.003
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发表时间：
2015-01-15
影响因子：
5
通讯作者：
Immenhauser, A.
中科院分区：
文献类型：
--
作者：
Geske, A.;Goldstein, R. H.;Immenhauser, A.
白云岩沉淀模式和动力学是有争议的和复杂的，由于复杂的和时间波动的流体化学和不同的成岩环境。利用完善的同位素系统（δ O-18，δ C-13，Sr-87/Sr-86）、流体包裹体和元素数据，以及详细的沉积学和岩相学数据，建立了一系列元古宙-更新世同沉积海相蒸发（萨布哈）橄榄岩的沉淀环境和后续成岩途径，同沉积非海相蒸发（湖相和沼泽相）碳酸盐岩、蚀变海相（“混合带”）碳酸盐岩和晚成岩热液碳酸盐岩。这些数据构成了系统调查白云石镁同位素比值（δ Mg-26（dol））的先决条件。此处记录的白云石δ Mg-26比率范围为-2.49份/千份至-0.45份/千份（δ Mg-26（平均值）= -1.75 +/-1.08份/千份，n = 42）。同位素最贫化的端员由最早的成岩海洋蒸发sabkha碳酸盐岩（-2.11 +/- 0.54份/千2 σ，n = 14）代表。在比较古代作品和现代作品时，有些变化可能是由于修改。海相蚀变岩（-1.41 +/-0.64/1000 2 sigma，n = 4）和最早成岩的湖相和沼泽相碳酸盐岩（-1.25 +/-0.86/1000 2 sigma，n = 14）的负性小于萨布哈碳酸盐岩，但在成分上并不明显。各种热液镁铝榴石的特征在于相对宽范围的δ Mg-26比率，其值为-1.44 +/-1.33ppm（2 σ，n = 10）。通过使用流体包裹体数据和聚集同位素测温法（Delta（47））来代表热液橄榄岩的沉淀温度，流体温度（类似于100至180 ℃）与白云石Mg同位素特征（R-2 = 0.14）之间没有相关性; δ Mg-26（dol）与δ O-18（dol）之间也没有相关性。不同类型白云岩的镁同位素值受一系列复杂的不同镁源和汇，溶解/沉淀和非平衡分馏过程的影响，并在成岩重置过程中叠加。使用δ Mg-26（dol）作为替代物的进一步进展将需要δ Mg-26（fluid-dol）的新的理论和实验数据，包括游离Mg水离子对流体温度的脱水效应。在古成岩系统中，必须考虑复杂的变量。这些包括流体化学和物理性质，镁源和汇，降水和降水后过程中的时间变化，包括开放和封闭系统与环境流体的地球化学交换。所有这些因素使三角洲Mg-26（dol）作为其沉积或成岩环境代用指标的应用复杂化。然而，这里显示的数据也表明，δ Mg-26（dol）原则上可以在详细的理解框架内解释。(C)2014爱思唯尔有限公司版权所有。
Dolomite precipitation models and kinetics are debated and complicated due to the complex and temporally fluctuating fluid chemistry and different diagenetic environments. Using well-established isotope systems (delta O-18, delta C-13, Sr-87/Sr-86), fluid inclusions and elemental data, as well as a detailed sedimentological and petrographic data set, we established the precipitation environment and subsequent diagenetic pathways of a series of Proterozoic to Pleistocene syn-depositional marine evaporative (sabkha) dolomites, syn-depositional non-marine evaporative (lacustrine and palustrine) dolomites, altered marine ("mixing zone") dolomites and late diagenetic hydrothermal dolomites. These data form the prerequisite for a systematic investigation of dolomite magnesium isotope ratios (delta Mg-26(dol)). Dolomite delta Mg-26 ratios documented here range, from -2.49 parts per thousand to -0.45 parts per thousand (delta Mg-26(mean) = -1.75 +/- 1.08 parts per thousand, n = 42). The isotopically most depleted end member is represented by earliest diagenetic marine evaporative sabkha dolomites (-2.11 +/- 0.54 parts per thousand 2 sigma, n = 14). In comparing ancient compositions to modern ones, some of the variation is probably due to alteration. Altered marine (-1.41 +/- 0.64 parts per thousand 2 sigma, n = 4), and earliest diagenetic lacustrine and palustrine dolomites (-1.25 +/- 0.86 parts per thousand 2 sigma, n = 14) are less negative than sabkha dolomites but not distinct in composition. Various hydrothermal dolomites are characterized by a comparatively wide range of delta Mg-26 ratios, with values of -1.44 +/- 1.33 parts per thousand (2 sigma, n = 10). By using fluid inclusion data and clumped isotope thermometry (Delta(47)) to represent temperature of precipitation for hydrothermal dolomites, there is no correlation between fluid temperature (similar to 100 to 180 degrees C) and dolomite Mg isotope signature (R-2 = 0.14); nor is there a correlation between delta Mg-26(dol) and delta O-18(dol). Magnesium-isotope values of different dolomite types are affected by a complex array of different Mg sources and sinks, dissolution/precipitation and non-equilibrium fractionation processes and overprinted during diagenetic resetting. Further progress on the use of delta Mg-26(dol) as a proxy will require new theoretical and experimental data for Delta Mg-26(fluid-dol) that includes dehydration effects of the free Mg aquo ion versus fluid temperature. In ancient diagenetic systems, complex variables must be considered. These include fluid chemistry and physical properties, Mg sources and sinks, temporal changes during precipitation and post-precipitation processes including open and closed system geochemical exchange with ambient fluids. All of these factors complicate the application of delta Mg-26(dol) as proxy for their depositional or diagenetic environments. Nevertheless, the data shown here also indicate that delta Mg-26(dol) can in principle be interpreted within a detailed framework of understanding. (C) 2014 Elsevier Ltd. All rights reserved.