Geochemical characteristics and source of pore fluids of the mud volcanoes in Shin-yan-ny-hu, Taiwan, China
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摘要:
陆地泥火山流体来源及其演化过程的研究，对于理解板块俯冲边界增生楔中流体特性及迁移途径有重要作用，对于油气等资源勘探具有指示意义。通过分析台湾西南部新养女湖（SYNH）泥火山喷出流体的离子组分、氢氧同位素组分以及泥质沉积物的矿物组分，探索泥火山喷出流体源区的化学特征、流体的来源及源区温度和深度条件；搭建组分运移、演化的概念模型。调查发现SYNH泥火山喷出泥质沉积物的矿物以石英、长石、伊利石、绿泥石为主，含少量方解石和白云石。流体中Na+、Cl−占主导，且Na+和Cl−呈显著正相关，表明泥火山流体起源于海洋沉积孔隙水。泥浆池中流体的Cl−浓度约为海水的1/5；氧同位素δ18O为6.24‰～6.59‰，明显富集18O；氢同位素δD的范围为−23.72‰～−12.9‰，显示贫化的特征。Cl−浓度和氢氧同位素特征指示蒙脱石脱水稀释流体组分、改变流体的化学特征。此外，氢氧同位素分布偏移大气降水线，表明大气降水对泥火山流体的影响较小。流体中富集Na+，严重贫化K+，可能与蒙脱石的伊利石化作用有关。根据Na/K、K/Na地温计和氢氧同位素数据推算深部流体的温度范围为79～181 ℃。结合地温梯度，推测SYNH泥火山喷出流体起源的深度为2.6～6.0 km，对应中新世长石坑组及乌山组的页岩-砂岩和上新世的开竹寮页岩。基于研究区构造背景、矿物组成及流体地球化学特征搭建SYNH泥火山深部流体喷出过程的简化模型，即原始海水组分被保留在孔隙中，塑性页岩的圈闭作用使得孔隙流体排出不均衡，随着上覆岩层的压实作用及横向构造挤压，孔隙内部流体压力急剧升高，当压力大于上覆岩层压力或存在高渗通道时，流体携带沉积物喷出地表。
Abstract:The sources and geochemical evolution of pore fluids of terrestrial mud volcanoes are critical to the understanding of the features and migrating paths of the fluids in subduction accretionary prisms. They are also important to oil and gas exploration. In order to get an answer to the problems mentioned above, we analyzed the ionic composition, and the hydrogen and oxygen isotopes of mud volcanic fluids as well as the mineral composition of the sediments of mud volcanos taking the Shin-yan-ny-hu (SYNH) mud volcano, Taiwan, China as a case. The analysis results suggest that the erupted minerals by the mud volcano are dominated by quartz, feldspar, illite and chlorite, with a small amount of calcite and dolomite, while in the fluids, Na+ and Cl− dominate and there is a significant positive correlation between Na+ and Cl−. Such a correlation suggests that the fluids are derived mainly from pore water of marine sediments. The Cl− concentration of the fluid is about one-fifth of the seawater, characterized by enriched oxygen isotopes (δ18O: 6.24‰～6.59‰) and depleted hydrogen isotopes (δD: −23.72‰～−12.9‰), indicating the dehydration of smectite. The hydrogen and oxygen isotopes are obviously deviated from meteoric precipitation, the results support the conclusion that the influences of meteoric precipitation on this area are limited. Comparing to seawater, the fluids are rich in Na+ and severely depleted in K+, owing to the increase in smectite by illite/smectite interaction. The geochemical signatures of the pore fluids further suggest that the fluids sourced from the deep with temperatures ranging between 79 ℃−181 ℃ responding to a source region 2.6 km to 6.0 km in depth, where, as we know, occur the Kai-tzu-liao shales of Pliocene, and the interbedded shale and sandstone sequence of Miocene Chang-chi-keng and Wu-shan Formations. Based on the tectonic background, mineral composition and fluid sources of the SYNH mud volcanos, we established a theoretical model in this paper: The pore fluids of marine sediments trapped by the plastic shales caused the uneven ejection of pore water. With the compaction by the overlying sediments and lateral extrusions, the pressures of pore fluids increased sharply. Finally, the fluids carrying sediments erupted out, when the pressures are greater than that of overlying rocks if there are high-permeability channels.
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Key words:
- mud volcano /
- pore fluid /
- smectite dehydration /
- geothermometer /
- fluid source
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表 1 SYNH泥火山4个喷口流体的地球化学组成及含量
Table 1. Geochemical compositions of discharged fluids from SYNH mud volcanoes.
样品编号 Cl−/
mMNH4+/
mMNa+/
mMK+/
mMMg2+/
mMCa2+/
mMMn2+/
μMCr/
μMBa2+/
μMSr2+/
μMU/
μMTi/
μMMo/
μMδ18OSMOW/
‰δDSMOW/
‰K/Na地
温计/℃Na/K地
温计/℃SYNH01-6 110.1 0.32 150.5 0.70 0.409 0.114 0.23 0.0029 2.27 17.35 0.050 0.28 1.82 6.45 −22.99 ND ND SYNH01-4 125.7 0.40 167.1 0.79 0.424 0.125 0.29 0.0038 2.72 20.53 0.053 0.26 2.02 6.56 −21.44 88.1 98 SYNH01-2 114.7 0.33 155.9 0.77 0.428 0.160 0.46 0.0015 1.80 19.43 0.055 0.33 2.15 6.34 −22.68 90.4 98.9 SYNH01-1 123.8 0.38 165.6 0.72 0.477 0.123 0.31 0.0038 2.80 19.40 0.050 0.26 1.74 6.54 −23.72 85.2 96.9 SYNH01-3 125.6 0.42 170.6 0.92 0.449 0.171 0.43 0.0019 2.30 24.05 0.068 0.38 3.16 6.59 −23.52 93.9 100.3 SYNH01-5 108.4 0.40 144.86 0.68 0.397 0.134 0.35 0.0023 2.61 19.69 0.045 0.32 1.60 6.59 −23.33 88.1 98 SYNH01-7 124.3 0.47 167.6 0.88 0.481 0.174 0.44 0.0027 2.53 23.53 0.057 0.38 2.42 6.28 −21.35 ND ND SYNH02-2 216.1 0.32 289.1 1.34 1.230 0.389 1.53 0.0142 0.85 42.02 0.081 1.00 4.16 5.58 0.4 ND ND SYNH03-1 129.3 0.33 176.5 0.81 0.531 0.151 0.37 0.0012 1.74 19.11 0.067 0.38 2.15 6.24 −12.90 87.2 97.6 SYNH03-2 198.1 0.49 254.5 1.10 0.621 0.127 0.13 0.0040 1.74 29.72 0.082 0.35 3.58 7.01 −7.45 ND ND SYNH04-1 107.8 0.45 135.9 0.64 0.361 0.205 0.27 0.0035 2.64 18.45 0.057 0.47 2.34 6.29 −21.20 88 97.9 SYNH04-2 150.1 0.30 186.6 0.84 0.634 0.280 0.47 0.0037 1.59 26.90 0.078 0.63 3.74 6.78 −9.88 ND ND 注：ND 表示未计算温度值。 
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表 2 SYNH泥火山喷出泥的矿物组分及含量
Table 2. Mineralogical compositions of the erupted mud by SYNH mud volcanoes.
% 样品编号 石英 长石 伊利石 绿泥石 方解石 白云石 SYNH01-6 40.9 15.2 19.0 21.7 2.2 0.9 SYNH01-4 39.7 14.8 20.6 21.5 1.4 1.9 SYNH01-2 38.6 16.0 19.5 22.3 0.5 3.0 SYNH01-1 42.5 16.8 16.7 21.6 1.5 0.9 SYNH01-3 47.6 14.5 15.1 18.4 3.1 1.4 SYNH01-5 39.7 16.2 17.6 19.9 4.2 2.4 SYNH01-7 45.4 18.5 12.4 16.4 5.4 1.9 SYNH02-2 42.5 15.2 17.5 19.3 3.4 2.1 SYNH03-1 44.0 14.9 17.7 20.0 1.4 2.0 SYNH03-2 47.2 19.8 12.9 15.0 1.1 3.9 SYNH04-1 43.0 16.3 17.6 19.7 1.2 2.2 SYNH04-2 37.5 12.7 20.7 22.6 2.0 4.4 背景土 49.8 16.7 12.5 17.5 1.2 2.2
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