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Immiscibility of high salinity fluids in volcanic rocks and the mechanism of magma degassing in the Dongying sag, eastern China |
| CHEN Yong1, ZHOU Yaoqi1, REN Yongjun1, WANG Qiang1, YAN Shiyong1, LIU Chaoying1, XIAO Huanqin2, SUN Xinian3 |
| 1.Laboratory of Geochemistry and Lithosphere Dynamics, China University of Petroleum, Dongying 257061, China; 2.Exploration Department, Shengli Oil Field, Dongying 257015, China; 3.Earth Science Institute, Shengli Oil Field, Dongying 257015, China; |
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Abstract Fluid inclusions that bear halite daughter minerals were discovered in volcanic rocks at Pingnan area in the Dongying sag. The samples of the fluid inclusions collected from the BGX-15 well drill cores are hosted in quartz of diorite-porphyrite. The daughter minerals are identified as NaCl crystals after being observed under a microscope and analyzed by in situ Raman spectroscopy at -185°C. The results of micro-thermal analysis show that the homogenization temperatures of primary fluid inclusions are between 359 and 496°C, and the salinities of fluid inclusions are from 43.26 to 54.51 wt-%. All fluid inclusions in the studied samples can be divided into five types including primary fluid inclusions and secondary fluid inclusions. The fact that five types of fluid inclusions were symbiotic in the same quartz grain implies that immiscibility happened in magma. Due to the decrease in temperature and pressure during the ascent of magma, the fluids became intensively immiscible. This process accelerates the degassing of CO2 from magma, but the remnant fluids with high salinity are preserved in fluid inclusions. Thus, the primary fluid inclusions are mainly in NaCl-H2O fluids and poor in CO2. The results of our study indicate that the degassing of magma and accumulation of CO2 gas at the Pingnan area are relative to the immiscibility of high salinity fluids. This discovery is important because it can help us have a further understanding of the mechanism of magma degassing and accumulation of the inorganic CO2 in eastern China.
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Issue Date: 05 June 2007
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