Facies dependence of the mineralogy and geochemistry of altered volcanic ash beds: An example from Permian-Triassic transition strata in southwestern China
Facies dependence of the mineralogy and geochemistry of altered volcanic ash beds: An example from Permian-Triassic transition strata in southwestern China
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蚀变火山灰层矿物学和地球化学的相依赖性——以中国西南二叠纪-三叠纪过渡地层为例
DOI：
10.1016/j.earscirev.2018.12.007
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发表时间：
2019-03
影响因子：
12.1
通讯作者：
Shi Cheng
中科院分区：
文献类型：
--
作者：
Hanlie Hong;Thomas J. Algeo;Qian Fang;Lulu Zhao;Kaipeng Ji;Ke Yin;Chaowen Wang;Shi Cheng
控制不同类型沉积环境下火山灰床黏土矿物和地球化学组成变化的因素尚不完全清楚。华南二叠纪-三叠纪界线（PTB）层序对比良好的蚀变火山灰层（凝灰岩或k -膨润土）为更好地了解陆相、海相和海相火山物质的蚀变和成岩作用过程提供了机会。自生粘土矿物类型因相而异，以深海相、浅海相和湖相的混合层伊利石/蒙脱石（I/S）粘土为主，以古滩相和海陆混合相的高岭石和混合层I/S或高岭石/蒙脱石（K/S）粘土为主。此外，灰层中I/S粘土层序具有明显的相依赖性：深海相以R3层序为主，R1层序次之，浅海相以R1层序为主，湖相和古滩相以R3层序为主。I/S粘土的排序似乎受孔隙水K+有效性、溶液pH和沉积环境能量水平的控制。高ph条件有利于长石的形成，抑制蒙脱石的钝化，早期灰分蚀变过程中K+的损失与环境能量水平有关。而陆相I/S粘土层序主要受埋藏条件控制。蚀变火山灰的MgO和K2O含量与沉积物黏土矿物组成有关，具有相依赖性。海相灰岩的MgO含量（MgO/K2O > 0.30;MgO/Al2O3> 0.056）高于陆相灰岩（MgO/K2O < 0.30;MgO/Al2O3< 0.056）。MgO在灰层蚀变过程中普遍保留，其浓度主要受母岩浆类型和沉积成岩条件的影响。火山玻璃中K2O几乎完全浸出，与蒙脱石反应形成I/S粘土。稀土元素（REE）在华南地区总体上表现为单一剖面内的变化不大，而具有强烈的相依赖性。深、浅海灰层REE分布相似，均表现为轻稀土富集、重稀土微亏、Eu负异常等特征。陆相灰岩的稀土元素分布差异显著，湖相的Eu呈弱负异常，而古滩相的Eu几乎没有异常。稀土元素的分布受黏土矿物的吸附-解吸过程和辅助矿物的溶解-沉淀反应的影响。因此，利用火山灰的稀土元素模式作为源岩浆化学指纹必须考虑其沉积和成岩历史。Nb、Cu、Zn、Co、Cr、Ta、Mo、V、La、Th、U等微量元素在不同相灰分样品之间甚至同一剖面样品之间呈现出不同的富集规律。火山玻璃对粘土矿物的脱硝作用可导致TiO2的再分配，这与蚀变灰层（尤其是陆相）的粘土组分密切相关。石英灰的TiO2/ al2o3比值反映了物理改造和化学蚀变的影响。提出TiO2/ al2o3值<0.055为原生灰分，0.055 ~ 0.140为中等次生套印， > 0.140为强次生套印。使用Zr/TiO2vs Nb/Y图进行烃源岩鉴别必须考虑到这种改造和蚀变过程。在陆相和浅海相中，火山灰床…
Factors controlling variations in the clay mineralogy and geochemical composition of volcanic ash beds deposited in different types of depositional environments remain incompletely understood. The well-correlated, altered volcanic ash beds (tuffs or K-bentonites) of Permian-Triassic boundary (PTB) successions in South China provide an opportunity to better understand processes of alteration and diagenesis of volcanic materials in terrestrial, paralic, and marine sedimentary facies. The types of authigenic clay minerals produced vary among facies: mainly mixed-layer illite/smectite (I/S) clays in deep-marine, shallow-marine, and lacustrine facies, and kaolinite and mixed-layer I/S or kaolinite/smectite (K/S) clays in paludal and mixed marine-terrestrial facies. Furthermore, the stacking sequences of I/S clays in the ash beds are notably facies-dependent: mainly R3 with minor R1 in deep-marine facies, mainly R1 in shallow-marine facies, and mainly R3 in lacustrine and paludal facies. Ordering of I/S clays seems to have been controlled by porewater K+availability, solution pH, and energy levels of the depositional environment. High-pH conditions favored the formation of feldspar and inhibited the illitization of smectite, and loss of K+during early alteration of ash was related to environmental energy levels. However, the stacking sequences of I/S clays in terrestrial facies were dominantly controlled by burial conditions. The MgO and K2O contents of altered volcanic ash were related to sediment clay-mineral composition and thus also facies-dependent. Marine-facies ash beds contain relatively more MgO (i.e., MgO/K2O > 0.30; MgO/Al2O3> 0.056) than terrestrial-facies ash beds (MgO/K2O < 0.30; MgO/Al2O3< 0.056). MgO was generally retained during alteration of ash beds, and its concentration was largely a function of parent magma type and depositional and diagenetic conditions. K2O was almost completely leached out of volcanic glass, leading to formation of I/S clays by reaction with smectite.Rare earth elements (REE) generally exhibit little variation within a single section but strongly facies-dependent distributions across South China. The REE distributions of deep- and shallow-marine ash beds are similar, characterized by enrichment of light REEs, slight depletion of heavy REEs, and negative Eu anomalies. The REE distributions of terrestrial ash beds are notably different, with weak negative Eu anomalies in lacustrine facies and little to no Eu anomaly in paludal facies. REE distributions can be influenced by adsorption-desorption processes on clay minerals and by dissolution-precipitation reactions involving accessory minerals. Thus, the use of REE patterns of volcanic ashes as fingerprints of source-magma chemistry must take into consideration their sedimentary and diagenetic histories.Trace elements such as Nb, Cu, Zn, Co, Cr, Ta, Mo, V, La, Th, and U exhibit different concentration patterns between ash samples of different facies and even between samples from a single section. Devitrification of volcanic glass to clay minerals can lead to redistribution of TiO2, which shows a close association with the clay fraction of altered ash beds, especially in terrestrial facies. TiO2/Al2O3ratios of felsic ashes reflect the effects of physical reworking and chemical alteration. It is proposed that TiO2/Al2O3values <0.055 are indicative of primary ash composition, 0.055–0.140 of moderate secondary overprints, and > 0.140 of strong secondary overprints. The use of Zr/TiO2vs Nb/Y diagrams for source rock discrimination must take into account such reworking and alteration processes. In terrestrial and shallow-marine facies in which ash-bed …
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影响因子：
2.3
作者：
T. Kiipli;T. Kallaste;V. Nestor
通讯作者：
T. Kiipli;T. Kallaste;V. Nestor
影响因子：
5
作者：
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通讯作者：
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影响因子：
5.3
作者：
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通讯作者：
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影响因子：
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作者：
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通讯作者：
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影响因子：
2.2
作者：
D. Lowe;A. Hogg
通讯作者：
D. Lowe;A. Hogg