Two-End-Member Mixing in the Fluids Emitted From Mud Volcano Lei-Gong-Huo, Eastern Taiwan: Evidence From Sr Isotopes

Hung-Chun Chao,Chuan-Hsiung Chung,Chun-Chang Huang,In-Tian Lin,Chen-Feng You,Hou-Chun Liu,Ling-Ho Chung

doi:10.3389/feart.2021.750436

Hung-Chun Chao, Chuan-Hsiung Chung + Show 5 more

Open Access

https://doi.org/10.3389/feart.2021.750436

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Abstract

Mud volcano is one of the most important conduits for deep seated materials to migrate upward in sedimentary basins, convergent margins, and subduction zones. Understanding their temporal and spatial characteristics and variations provides us the important information on fluid sources and chemical compositions at depth. Mud volcano Lei-Gong-Huo (MV LGH) is a unique mud volcano, which is located on the mélange formation lying on the andesitic volcanic arc. Fluids emitted from 46 mud pools in MV LGH in eastern Taiwan were sampled and their major trace constitutes as well as H, O, and Sr isotopes (87Sr/86Sr and δ88Sr) were measured. Major constitutes of the fluids are Cl−, Na, and Ca. Compared with seawater, LGH fluids have lower Cl−, δD, δ18O, Na/Cl, K/Cl, and Mg/Cl but higher Ca/Cl ratios, indicating water–rock interaction of igneous rock and the ancient seawater at the source region. This interpretation is further supported by Sr isotopes, which show low value of 87Sr/86Sr ratio down to 0.70708. The result of spatial distribution showing strong negative correlation between Na and Ca concentration as well as Ca and 87Sr/86Sr ratios indicates that two end-member mixing is the major chemical characteristic. The fluids interacting with igneous rock carry high Ca, high δ88Sr, low Na, and low 87Sr/86Sr ratio, while those interacting with sedimentary rock carry low Ca, low δ88Sr, high Na, and high 87Sr/86Sr ratio. The source from the igneous region dominates the eastern and southeastern parts of the mud pools while sedimentary source dominates the western and northwestern parts. Most mud pools show mixing behavior between the two sources. Some of the sedimentary-dominated mud pools reveal existence of residual ancient water as indicated by 87Sr/86Sr. The major factor to fractionate the stable Sr isotopes in LGH waters is the source lithology. In summary, fluids emitted by mud pools in LGH originate from two sources, which are water–rock interactions of igneous rock with the ancient seawater from the east and sedimentary rock from the west at depth, resulting from the complex geologic background of mélange formation.

Highlights

Mud volcanoes (MVs) are one of the most efficient structures for the fluids at depth to migrate through thick sediments to the surface
MV fluids closed to marine environments denote similar chemical characteristics to deep marine pore fluids which originate from ancient seawater and have been altered by early diagenesis, clay dehydration, and water–rock interaction (e.g., Dia et al, 1999; Dählmann and de Lange, 2003; Hensen et al, 2004; You et al, 2004; Mazzini et al, 2007; Mazzini et al, 2009; Ray et al, 2013; Farhadian Babadi et al, 2019; Chen et al, 2020)
A total of 175 water samples emitted from 46 mud pools of Mud volcano Lei-Gong-Huo (MV LGH) and 1 mud pool from MV LS in eastern Taiwan were collected monthly in 10 campaigns, and their dissolved major and trace ion/element compositions as well as O, H, and Sr isotope ratios were analyzed

Summary

Introduction

Mud volcanoes (MVs) are one of the most efficient structures for the fluids at depth to migrate through thick sediments to the surface. They are a common diapiric feature in convergent margins where soft, thick, and fine-grained mud is rapidly deposited and sediment porosity is reduced by tectonic compaction. The development of the mud diapir or the fault provides the pathway for the fluid to migrate upward (Milkov, 2000; Dimitrov, 2002; Kopf, 2002; Mazzini and Etiope 2017). The origination of all three phases may decouple from each other (Sun et al, 2010; Mazzini et al, 2018)

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