Constraints of Parent Magma on Altered Clay Minerals: A Case Study on the Ashes near the Permin-Triassic Boundary in Xinmin Section, Guizhou Province
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摘要: 目前对于粘土层中伊蒙混层矿物的堆垛结构、单元层含量与岩浆母质及环境条件关系的了解等，仍然十分匮乏.一定沉积环境下火山灰层中粘土矿物组合、以及伊蒙混层堆垛方式精细结构特征，可能记录了沉积(包括成岩作用) 环境对火山物质蚀变产物的影响.采用X射线衍射(XRD)、扫描电子显微镜(SEM)、元素地球化学分析、氧同位素分析方法等方法，对贵州新民深海相二叠系-三叠系(P-T) 界线附近蚀变火山灰层的地球化学特征、粘土矿物精细结构特征等进行了深入研究.结果表明，4个火山灰粘土层均含有2种具有R3结构、不同混层比的伊蒙混层矿物相，且均出现粘土矿物集合体取代原先的火山碎屑颗粒或在颗粒表面生长的现象，粘土矿物形成于沉积-成岩阶段因而其泥质结构被保留；样品XM-5-1和XM-5-2的Fe3+原子数分别为0.16和0.17个且具有明显Eu负异常和较低的K2O含量，而XM-5-3和XM-5-4的Fe3+原子数均为0.14个，说明相对于前2个粘土层，后2个的岩浆母质更加偏向酸性而表现为成岩蚀变程度更强.粘土矿物的氧同位素组成为17.3‰~18.1‰，与常温下蒙脱石与海水的平衡数值相近，表明粘土矿物化学组成与海底成岩蚀变环境有关，而不同火山灰层的粘土矿物学特征则主要取决于岩浆岩母质以及成岩蚀变强度.Abstract: The study on the influence of sedimentary environments on the altered clay mineralogy facilitates accurate stratigraphical correlation using volcanic ashes as marked beds. However, the relationships among the stacking ordering, illite layer contents, parent magma and environmental condition are still poorly known. A case study on altered vocalnic materials near the Permian-Triassic boundary in the Xinmin section, Guizhou province was performed using X-ray diffraction (XRD), scanning electron microscopy (SEM), geochemical analysis, and oxygen isotope analysis methods. Results show that all the four volcanic ash layers contain two phases of mixed-layered illite/smectite (I/S) with R3 structure and varied layer contents, and clay mineral aggregates were found having replaced the volcaniclastic particles or have grown on the surfaces of volcaniclastic particles. It can be inferred that clay minerals formed in sedimentary to early diagenesis periods since the materials preserve argillaceous texture when clay minerals were produced. The number of Fe3+ atoms are 0.16 and 0.17 respectively in the samples XM-5-1 and XM-5-2, and 0.14 in both samples XM-5-3 and XM-5-4, indicating that parent magmas of the latter ones were more of meta acid compared with the former two. It is indicative of more intense alteration of the latter two volcanic ash layers, together with the apparent Eu negative anomaly and less K2O contents. Oxygen isotopic composition of the volcanic ashes ranges narrowly from 17.3‰ to 18.1‰, consistent with the equilibrium value between smectite and ocean water at 25 ℃, suggesting that chemical composition of the Xinmin clay minerals is related to conditions of submarine diagenesis alteration since the clay mineralogy of different volcanic ash layer depends on its parent magma and degree of diagenesis alteration.
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图 2 新民剖面地层及采样位置描述
Fig. 2. Stratigraphic descriptions and sampling horizons of Xinmin section
图 4 新民剖面＜2 μm颗粒大小的粘土矿物混层相XRD拟合图
Fig. 4. Experimental and calculated XRD patterns of < 2 μm clay fractions from Xinmin ashes
图 5 新民火山灰层粘土矿物代表性(样品XM-5-2) 扫描电镜照片
Fig. 5. Representative SEM photographs of clay minerals from Xinmin volcanic ashes (XM-5-2)
图 6 新民剖面火山灰粘土层化学组成的K2O/Al2O3-Na2O/Al2O3关系
Fig. 6. The K2O/Al2O3-Na2O/Al2O3 relations of Xinmin volcanic ash beds
图 7 新民剖面火山灰粘土层微量元素蛛网图
Fig. 7. Primitive mantle and UCC normalized trace element spider diagrams of Xinmin ashes
图 8 新民剖面火山灰粘土层稀土元素蛛网图
Fig. 8. Chondrite-normalized REE distribution pattern of Xinmin volcanic ashes
表 1 新民剖面火山灰粘土矿物化学组成(%) 及晶体化学式
Table 1. Chemical compositions (%) of Xinmin clay minerals and their structural formulae
样品 SiO2 TiO2 Al2O3 Fe2O3 MnO MgO CaO Na2O K2O P2O5 LOI XM-5-1 51.95 0.50 25.06 3.10 0.04 3.39 0.56 0.45 6.24 0.19 8.93 XM-5-2 52.35 0.56 24.87 3.36 0.03 3.34 0.42 0.39 6.40 0.10 8.54 XM-5-3 52.50 0.44 25.30 2.81 0.01 3.36 0.26 0.11 6.82 0.09 8.75 XM-5-4 52.38 1.36 24.86 2.80 0.09 3.44 0.08 0.00 6.59 0.03 8.74 UCC* 65.89 0.50 15.17 4.49 0.07 2.20 4.19 3.89 3.39 0.20 PAAS* 62.80 1.00 18.90 6.50 0.11 2.20 1.30 1.20 3.70 0.16 样品 晶体化学式 XM-5-1 (K0.54Na0.06Ca0.04Mg0.03)(Al1.50Fe0.16Mg0.31Ti0.03)2(Si3.51Al0.49)4O10(OH)2 XM-5-2 (K0.55Na0.05Ca0.03Mg0.05)(Al1.50Fe0.17Mg0.29Ti0.04)2(Si3.53Al0.47)4O10(OH)2 XM-5-3 (K0.58Na0.01Ca0.02Mg0.04)(Al1.53Fe0.14Mg0.30Ti0.03)2(Si3.53Al0.47)4O10(OH)2 XM-5-4 (K0.55Ca0.01Mg0.06)(Al1.55Fe0.14Mg0.29Ti0.02)2(Si3.56Al0.44)4O10(OH)2 注：UCC*和PAAS*据Taylor and McLennan (1985). 
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表 2 新民剖面火山灰层微量元素和稀土元素含量(10-6)
Table 2. Trace element and rare earth element compositions of Xinmin volcanic ashes (10-6)
样品 XM-5-1 XM-5-2 XM-5-3 XM-5-4 UCC* PAAS* Li 15.4 11.4 9.6 11.5 20.0 75.0 Be 5.02 3.83 3.64 4.08 3.00 Sc 14.9 18.4 19.1 35.2 13.6 16.0 V 44.8 19.3 16.3 107.0 107.0 150.0 Cr 6.86 44.30 2.18 6.01 85.00 110.00 Co 13.80 2.09 4.47 23.10 17.00 23.00 Ni 50.4 22.8 11.8 15.6 44.0 55.0 Cu 36.0 14.0 14.3 23.7 25.0 50.0 Zn 103.0 77.6 48.8 42.3 71.0 85.0 Ga 29.2 31.7 30.0 28.8 17.0 20.0 Rb 264 294 293 278 112 160 Sr 49.5 40.1 25.9 29.0 350.0 200.0 Zr 406 483 567 713 190 27 Nb 21.0 30.8 19.0 29.8 12.0 210.0 Cs 7.84 9.49 9.49 11.50 4.60 19.00 Ba 256 240 230 284 550 15 Hf 14.2 14.7 16.4 19.7 5.8 210.0 Ta 2.96 2.66 2.61 2.57 1.00 Pb 48.9 31.0 19.6 18.9 17.0 20.0 Th 63.7 61.2 52.6 49.2 10.7 14.6 U 12.40 11.80 8.25 3.83 2.80 3.10 La 57.3 105.0 62.8 68.0 30.0 38.2 Ce 113 214 157 211 64 80 Pr 15.0 28.5 20.8 25.6 7.1 8.8 Nd 55.6 103.0 73.9 89.9 26.0 33.9 Sm 10.8 18.7 13.0 14.1 4.50 5.55 Eu 1.59 2.25 1.93 2.25 0.88 1.08 Gd 9.92 13.40 9.61 9.08 3.80 4.66 Tb 1.75 2.09 1.63 1.54 0.64 0.77 Dy 10.8 12.6 11.0 11.2 3.50 4.68 Ho 2.08 2.46 2.42 2.60 0.80 0.99 Er 6.10 7.19 7.76 8.43 2.30 2.85 Tm 0.88 1.07 1.18 1.35 0.33 0.41 Yb 5.65 6.78 7.46 9.06 2.20 2.82 Lu 0.84 0.95 1.10 1.36 0.32 0.43 Y 63.8 63.5 67.9 74.7 22.0 27.0 ΣREE 291.52 518.52 371.54 455.64 ΣLREE 253.49 471.92 329.34 411.09 ΣHREE 38.03 46.59 42.20 44.55 ΣLREE/ΣHREE 6.67 10.13 7.80 9.23 注：UCC*和PAAS*据Taylor and McLennan (1985).
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表 3 新民火山灰粘土层氧同位素组成
Table 3. O isotope compositions of Xinmin ashes
样品 矿物名称 δ18OV-SMOW(‰) 沉积相 XM-5-1 ＜2 μm伊蒙混层粘土矿物 17.8 深海相 XM-5-2 ＜2 μm伊蒙混层粘土矿物 17.3 深海相 XM-5-3 ＜2 μm伊蒙混层粘土矿物 17.5 深海相 XM-5-4 ＜2 μm伊蒙混层粘土矿物 18.1 深海相
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