Present-Day Temperatures of Magma Chambers in the Crust Beneath Tengchong Volcanic Field, Southwestern China : Estimation from Carbon Isotopic Fractionation between CO2 and CH4 of Free Gases Escaped from Thermal Springs
详细信息
- 责任者：Zhao Ciping (Earthquake Administration of Yunnan Province ; Kunming 650224 ; China) ; Ran Hua
- 刊名：Acta Petrologica Sinica
- 出版年：2011
- 卷期：27(10)
- 关键词：岩浆房 ; magma chambers ; 火山活动 ; volcanicity ; 云南 ; Yunnan Province
- 页码：p.2883-2897
- 图表：8 illus., 2 tables, 66 refs.
- 分类号：0801
- issnNo：ISSN1000-0569
- isbnNo：CN11-1922/P
摘要
Temperature is an important physical parameter of magma.It has important theoretical and practical significance to acquire and monitor temperature parameter of magma chambers for better understanding changes in the physical and chemical properties and behavior of a volcano,and for better assessing its activity and eruption risk.Here we report the carbon isotope composition data of CO2 and CH4 that collect from thermal springs in the Tengchong volcanic field(TVF) and the temperatures of the existing three magma chambers within crust beneath the TVF,which,are calculated with carbon isotope equilibrium fractionation equation T(K)=3880.3(1000lnα(CO2-CH4))-0.5984 between CO2 and CH4 that fit from Horita’s experimental correction of Richet’s theoretical fractionation factor data between them.Our results show that,the lowest carbon isotope equilibrium fractionation temperature between CO2 and CH4 of gas source area,is 397℃,the highest one,up to 1163℃,the average value,615℃ in the TVF and that,among the 3 crustal magma chambers in the TVF,the southern Wuhe-Longjiang-Puchuan one has highest present-day temperature,which ranges from 464℃ to 1163℃,with an average of 773℃;the central Tengchong-Heshun-Rehai one has higher temperature,which ranges from 438℃ to 773℃,with an average of 566℃;the northern Mazhan-Qushi-Yongan one has the lowest temperature,which ranges from 397℃ to 651℃,with an average of 524℃.We believe that the current temperature of the gas-rich region at the top of those 3 magma chambers in TVF varies from 400℃ to 1200℃,the actual average temperature of those 3 magma chambers should be higher than 615℃,the temperature of the core of those 3 magma chambers may be 700~1200℃,that of marginal region of those 3 magma chambers may be 400~600℃.The current temperature of the center of those 3 magma chambers has reached which rhyolitic magma(600~900℃),andesitic magma(800~1100℃) and basaltic magma(1000~1250℃) form in,suggesting further,in turn,that there exist objectively those 3 magma chambers in TVF.In addition,we find that,in the process of determining whether the carbon isotope fractionation equilibrium between CO2 and CH4 achieve or not with the δ-Δ diagram method,under the conditions that the slope sign of two fitting straight lines maintain always the opposite,no matter how exclud data points,one can not make the difference bΔ of the intercept bCO2 of δ13CCO2-ΔCO2-CH4 fitting straight line in the δ-axis and the intercept bCH4 of δ13CCH4-ΔCO2-CH4 fitting straight line in the δ-axis less than 0.1245,indicating that the intersection point of the two fitting straight lines can not fall in the δ-axis.And this value is near the constant item of Horita’s equation,sugestting that there exists really a constant item in the equation of carbon isotope fractionation equilibrium between CO2 and CH4,and that the carbon isotope fractionation between CO2 and CH4 exists allways at no matter how high temperature.The δ-Δ diagram guidelines of determining the carbon isotope fractionation equilibrium between CO2 and CH4 should be amended as follows:under the conditions that the sign of the slope of the two fitting lines keep allways opposite,the δ13CCO2-ΔCO2-CH4 fitting straight line and the δ13CCH4-ΔCO2-CH4 fitting straight line should intersect near the δ-axis with intercept difference 0.1245.