Mantle-derived Helium Release Characteristics And Deep Magma Charmber Activities Of Present Day In The Tengchong Volcanic Area

1. QUESTIONS AND METHODS1.1. QuestionsThe existing research results suggest that massive heat energy and mantle-derived volatiles are still releasing present day in theTengchong volcanic area under which more than one magma chambers still exist, and that the Tengchong volcano eruption, possibly not related with the tectonic plate subduction, is resulted from local mantle uplift. But there are some problems unsolved, such as the intensity and the spatial distribution characteristics of heat energy and mantle-derived volatiles release present day in the Tengchong volcanic area, relationship between them and underground magma chambers, and their number, spatial distribution, size, temperature, activities as well as the mechanism model. These are the scientific questions which this dissertation plans to deal with.1.2. Train of thoughtThis dissertation, taking revealing nowadays magmatic activities in the Tengchong volcanic area as a goal, the hot spring as the object of research, chemical geothermometer, isotopic tracer, isotopic geothermometer as the means, based upon the modern chemical and the isotopic analysis technology, adreesses three specific scientific questions: They are the present release field of mantle-derived volatiles, temperature field of the most upper crust and the temperature of nowadays magma chamber of the Tengchong volcanic area. In conjunction with the predecessor's achievements of deep sounding, the activities monitoring and the origin research, this work proposes a machanism model for the Tengchong volcano generation and eruptions.1.3. PrinciplesThe volcanic eruption is one way by which the earth interior material and the heat energy release to the surface. When magma migrates upward or is detained in crust, the normal geothermal gradient formed by the heat conduction can create the partial perturbation. In turn, partial perturbation quantity of the normal geothermal gradient may reflect the existence of magma chambers. The hot spring formation is related to the local geothermal gradient, and the concentration of chemical substance dissolved in the geothermal water depends upon the temperature of the heat reservior. The hydrochemical geothermometer of a hot spring, can be used to reveal the partial perturbation quantity of the normal geothermal gradient at depth that can penetrate and circulate, thus may reflect existence of a magma chamber or its thermal aureole(or cap) indirectly.The gas dissoluted in the magma is the driving force of volcanic eruption, and is also the tracer of volcano magmatic activity. When migrating to the surface, erupting, cooling or crystallizing in underground media, magma may release gasses. The gasses play an influential role in magma migration and volcano eruption, particularly a dominant role in the explosive eruption. The component and the isotopic composition of gasses, being used for calculating thermodynamic parameters, thus may be sused to study the physic and/or chemical conditions of deep magma cambers.The helium is of stablest, lightest inert gas of which both volatility and proliferation seepage ability are strongest. When magma rises to the surface, the helium may arrive at the surface earlier it. Therefore, during the rising process or active period after being invaded, through analyzing helium isotopic composition of gasses which escaped from it, the mass origin of magma chamber can be determined by using tracing principle of helium isotope: for invading ones, the sudden change of helium isotopic composition ought to be able to observed; for cooling ones, helium isotopic composition should be steady, and their relative ages can be reflected with the specific value of helium isotopic composition.Carbon isotope equilibrium fractionation between homologous paragenetic CO2-CH4 molecular couple from magma chamber is controlled by its temperature to which their equilibrium time is related. The temperature is higher, the equilibrium time is shorter; and the temperature is lower, the equilibrium time is longer. The equilibrium time deponds exponentially upon the reciprocal of the equilibrium temperature. The carbon isotopic equilibration between CO2-CH4 molecular pair at high temperature (for instance 1000℃) which takes order of magnitude of several hours or yaears can attain new re-equilibration at low temperature (for instance 300℃) which needs to take order of 1,000,000 years. Therefore, when rising from the high temperature magma chamber area to the surface, the gasses reduce rapidly their temperatures, for the isotopic equilibrium which achieves in the high temperature area to be very difficult to change in the low temperature area, namely for isotopic composition to be"freezen". Isotopic equilibrium fractionation factor is related to temperature. Therefore, by analyzing the carbon isotope composition of CO2-CH4 molecular pair at low temperature surface area which arrives at, and if their isotopic exchange equilibration can be judged, then the equilibrium temperature can be calculated through the isotope fractionation equation. It dosen't represent the temperature at which a new isotopic re-equilibration is attained at the surface, but the"freezen"high temperature of the source area, namely of magma chamber.1.4. Technique approachFirst, using the massive hot spring hydrochemical data that predecessores already have analyzed, according to the geochemical thermodynamics principle, this work studies the temperature field in the most upper crust under the Tengchong volcano and adjacent areas by chemical geothermometer. The temperature field may reflect indirectly existence and activities of magma chambers: the geothermal gradient is higher, the possibility of existence of a magma chamber is bigger, and the activity of magma is stronger. The goal of this work is to obtain the evidence of existence of the magma chamber and the base of the field sampling design and the implementation by which another evidence of existence of magma chamber can be found using the isotopic tracer principle and the today's temperature of magma chamber can be calculated using the isotope geothermometer principle.Secondly, this work collectes samples of gasses escaped from hot springs which are distributed widespread in the Tengchong volcanic area, analyzes conventional component content and He, Ne isotopic compositions of them. It gains helium isotopic composition data in the wide range of the Tengchong volcano petrographic province and periphery (for example east to the Nujiang fault zone), and using helium isotope 3He/4He ratio tracing principle, promulgates whether there exists unified one or how many mantle-derived mass release centers in entire Tengchong volcanic area. This one or several mantle-derived mass release centers then correspond to one or several magma chambers.Thirdly, under the premise above mentioned, using principle that temperature controls carbon isotopic equilibrium fractionation between CO2 and CH4 molecular pair, through analyzing CO2 and CH4 carbon isotope composition of gasses from hot springs above this one or these magma chanmbers (spacial range already largely narrowed), this work calculates the temperature of this one or these magma chambers. The temperature is higher, the activity of magma chamber is stronger.Then, on the basis of above observed data and the research results of this dissertation above mentioned, it demonstrates the existence of magma chambers, and measures the size and depth of this one or these several magma chambers by unifing deep sounding data that the predecessors obtained by magnetotelluric sounding(MTS) and deep seismic sounding(DSS). It then discusses the nowadays activity of this one or these several magma chambers using observed long-term data of intensity of mantle-derived mass release, that the predecessors and the author of this dissertation have accumulated for more than 2 decades, the data of temperature of magma chambers, to unify the data of the Tenchong volcanic activity from measurements of crust deformation and earthquake monitoring.Finally, it proposes a model for the Tengchong volcanic eruption and origin based on synthesising observation data and research results mentioned above.2. MAIN CONTENTS OF THE THESIS2.1. Developing sample collecting tool for atmosphere-contaminating-prevention with enrichment functionIn order to guarantee that the gas sample collected is not contaminated by the atmosphere so as to improve the credibility of the observation result, and at the same time, to overcome a series of sampling problems in field such as that hot spring gas is of hyperpyrexia, that it is diffculty to collect sample over a short distance because operation space is too narrow, and that it takes a too long time because outgassing is too less, or that certain minor components need to concentrate by chemical reaction, and so on. This research has designed a set of gas sample collection installments which integrate atmosphere exhausting device, gas store device, enrichment device of trace component, the output interface of sample all in one.2.2. Sample-collecting, analysis of conventional and isotopic compositionThis work collectes gas samples escaped from hot springs in the Tengchong volcanic area using the sample collecting instrument with atmosphere-contaminating-prevention and enrichment function. The gas samples which are used to analyze conventional component and carbon isotope composition are packed in aluminum-film-covered plastic bags, the ones for helium isotope compostion analysis are packed in the steel cylinder or glass bottle. With the gas chromatograph it analyzes conventional component, with the mass spectrometer (VG5400) it analyzes 3He/4He, 4He/20Ne. This work collectes CO2 and CH4 gas samples escaped from hot springs in overlaped abnormal areas (magmatically active region), where both the intensity of mantle-derived mass release and relative geothermal gradient are high, using the sample collecting instrument this research developed. This work carries on enrichment to CH4 of the CH4 carbon isotope sample at site, analyzes carbon isotope composition both CO2 and CH4 sample with the mass spectrometer (MAT251).2.3. Analysis of temperature field in upper crust of the Tengchong volcanic areaUsing predecessor's hydrochemical data of hot springs, selecting unified geothermometer, this work calculates the temperature of thermal reservoirs. Assuming that the depth of these thermal reservoirs are same, then the temperature difference between hot springs and thermal reservoirs is defined as relative geothermal gradient. According to this definition, this work calculates the values of relative geothermal gradient in entire the Tengchong volcanic area.Using the values of relative geothermal gradient, the map of relative geothermal gradient in the entire Tengchong volcanic and adjacent areas is obtained with Kriging interpolation. According to this space distribution map and the data, this work studies spatial distribution characteristic of the temperature field of the most upper crust in the Tengchong volcano area, and discusses the relationship between the existence and the activity of magma chambers and the spatial distribution characteristic of the temperature field.2.4. Analysis of spatial-temporal distribution of release of mantle-derived gasAtomsphere contamination correction is made for 3He/4He ratio through the 4He/20Ne value. Then this work calculates percentage proportion of atmosphere, crust and mantle of helium isotope compostion. Finally the correction for percentage proportion of different sources is performed.Using Kriging interpolation, the plane distribution maps of primitive helium isotopic compostion 3He/4He (Ra) ratio values, of corrected helium isotopic compostion 3He/4Hec (Ra) ratio values, of percentage M which express contribution of mantle reservoir to helium isotopic compostion, and of corrected percentage Mc in entire the Tengchong volcanic and adjacent areas are obtained. These maps can reflect the spatial intensity level of mantle-derived mass release in the entire Tengchong volcanic and adjacent areas, and time series of these values of helium isotopic compostion at the same localities can reflect time variations of mantle-derived mass release. Synthesising specific data and these maps, this work studies spacial and temporal distribution characteristics of intensity of mantle-derived mass release in the entire Tengchong volcanic and adjacent areas, discusses the relationship between the existence and activity of magma chambers and these spacial and temporal distribution characteristics.2.5. Calculating of temperature of magma chambersTo search, read and analyze literatures mainly about the thermodynamics principle of isotopic exchange reaction, data of equilibrium fractionation factor and equilibrium fractionation equation of carbon isotope exchange reaction between CO2 and CH4 and temperature range of their utilization, so that this work can select the most appropriate formula and the data explanation principle for this research. Choosing the most resonable equilibrium fractionation equation or formula regressed from carbon isotope equilibrium fractionation factors of CO2 and CH4 which were theoretically calculated or experimentally determined, this work finally calculates the temperature of gas source area (magma chambers).2.6. Integrated analysis of existence and activities of magamsAccording to observed data and the research results of this dissertation mentioned above, unifing deep sounding data that the predecessors obtained by magnetotelluric sounding(MTS) and deep seismic sounding(DSS), it demonstrates the existence of magma chambers, and measures the size and depth of this one or these several magma chambers. It discusses the nowadays activity of this one or these several magma chambers using observed long-term data of intensity of mantle-derived mass release, which this thesis researches and the predecessors have accumulated for more than 2 decades, the data of temperature of magma chambers, to unify the data of Tenchong volcanic activity from crust deformation measurements and earthquake monitoring.2.7. Proposing of model of mechanism of Tengchong volcanoAccording to results on temperature field in upper crust, mantle-derived mass release field, temperature of magma chambers of the Tengchong volcanic area, unifing data of magnetotelluric sounding(MTS) and deep seismic sounding(DSS), this work determines the number, spatial distribution, size and depth of magma chambers which exist present day in the Tengchong volcanic area. Finally, in conjunction with the related information of petrology and the rock geochemistry research, this work proposes a model of Tengchong volcano origin.3. MAIN WORKLOADSIn order to achieve the above research aim and to complete above contents, it taken 5 years for this dissertation to complete the field sample-collecting, analysizing and synthetic research work, from July, 2003 to May, 2008. Field sample-collecting, analysizing for hot spring gas is implemented altogether 3 times in 2003, 2004, 2006, respectively, in the Tengchong volcanic area and the periphery. Employing 4 vehicles successively, traveling 16200 kilometers, We have surveyed 159 hot springs, measured temperature 145 times, altogether gathered 254 gas samples, obtaining 1229 data of chemical and isotopic compostion of them in the range of 12350 square kilometers with the radius of 63 kilometers. Among these samples, 117 are for the conventional component and carbon isotope composition of CO2, 88 for isotope composition of Helium and Neon, 49 for carbon isotope composition of CH4. We have obtained 724 conventional gas component data (of 96 samples), 181 conventional gas component data(of 21 CH4-enriching samples), 264 isotope composition data of Helium and Neon(88 samples), 36 carbon isotope composition data of CO2 (36 samples), 24 carbon isotope composition data of CH4 (24 samples). Moreover we have collected and compiled the numerous data of the temperatures and hydrochemistry ingredient of 304 hot springs in research area and the periphery which analyzed and measuered by fomer investigators.4. MAIN CONCLUSIONSBased on the above data, by resolving theses 3 issues such as temperature field in most upper crust, mantle-derived mass release field, and temperature of magma chamber, and combining achievements in deep sounding, activity monitory and origin research that the former investigators obtained, this research obtains the following 5 conclusions:4.1. There exist 3 magma chambers in crust present day in the Tengchong volcano area. The 1st one is located Tengchong county-Heshun township-Rehai resort area where is in middle of the Tengchong volcano area. The 2nd one is located Qushi-Mazhan where is in northern of the Tengchong volcano area. The 3rd one, lies in Wuhe-Puchuan-Xinhua area where is in southern of the Tengchong volcano area. The relative geothermal gradients of most upper crust above 3 magma chambers are 140℃, 120℃, 130℃, respectively. The mantle-derived proportions of volatile mass release of 3 magma chambers are 70%, 60% and 30%, respectively.4.2. The sizes (the horizontal diameter) of 3 magma chambers are approximately 20km, 19km and 23 km (long 45km), respectively, the depths of them are different: the 1st at 5-25km, the 2nd is 10-25km, and the 3rd is 7-14km.4.3. The temperature of the 1st magma chamber is within 324-789℃, with average 555℃. That of the 2nd, within 402-663℃, with average 532℃, That of the 3rd is within 320-1194℃, with average 679℃. It is believed that variation range of the present temperature of the gas enrichment region which is crown of magma chamber below the Tengchong volcano area is 320-1200℃, the actual temperature of magma chamber should be higher than the mean value 600℃. The temperatures of edge area of these 3 magma chambers possibly are in 300-600℃, the central temperature possibly in 700-1200℃. The nowadays temperatures of these 3 magma chamber's center has achieved the formation temperature of Rhyolite magma (600-900℃), the Andesite magma (800-1100℃) and basalt magma (1000-1250℃), further suggesting the objective existence of these 3 magma chambers at present in the Tengchong volcano area.4.4. The activities of 3 magma chambers are different. The 1st one, collecting relative geothermal gradient, the mantle-derived mass release, the deformation and the seismic activity and so on abnormal in one, being accepting the supplement from mamtle-derived magma, is of strongest activity, and is located directly under the Tengchong county. Its eruption will cause the most serious losses, and needs the key surveillance. The 2nd one, the release intensity of mantle-derived mass is also noticeable, which is also possibly accepting the supplement from mamtle-derived magma, should be strengthened the monitoring. The 3rd one, which has the biggest size, and is shallowest, where the mantle-derived mass releases is weakest (30%), is possibly accepting weakly the supplement of the mantle-derived magma at present, but with temperature is still very high, must be paid some attention.4.5. The formation of the Tengchong volcano, not related with the tectonic plate subduction, is the result of local mantle uplift which results in crust extension. The local mantle uplift is possibally related with the slab break off of the ancient Nujiang oceanic lithosphere or/and the ancient Myitkyina oceanic lithosphere, But the eruption of Tengchong volcano is far later than the subduction of the two ancient oceanic lithosphere. Therefore it can be considered that the formation of the Tengchong volcano is not related with the normal tectonic plate subduction. The spatial distribution image of mantle-derived mass release intensity present day in the Tengchong volcanic area is the direct reflection of size and strength of this mantle uplift which is 100km long from north to south, 50km wide from west to east approximately. This local uplift mantle either provides matter source of magma for the Tengchong volcano to prepare or resultes in the crust extension which provides structural conduits for magma to migrate up. The crust extension in the Tengchong volcanic area predicted with this model is also supported by today's local geomorphological investigation and deformation observation.5. MAIN INNOVATIONSThis work has quite explicitly answered the questions about the activity of the Tengchong volcano. Combining the related information which the former investigators obtained by petrology and rock geochemistry research, a model of the Tengchong volcano origin machism is proposed and 5 aspects of innovations are created as follows:3.1. A new concept of relative geothermal gradient is put forward and the temperature field in upper crust of the Tengchong volcanic area is demonstratedBy deeply mining to the predecessor's data of basic elements of hot springs, this work proposed a new concept, the relative geothermal gradient, and its calculation method. It is discovered that there exist 3 abnormal areas where relative geothermal gradient is higher than 100℃in most upper crust in the Tengchong volcano area. The abnormal higher areas of relative geothermal gradient, resulting from thermodiffusion of magma chamber underneath them, may be regarded as thermal aureole(or cap) of magma chamber, and reflects the existence of magma chamber indirectly.3.2. Present day spatial distribution of relesse of mantle-derived gas in the Tengchong volcanic area is demonstratedUsing the isotopic tracer technology, this work carries out survey of intensity of mantle-derived mass release in the region east to Nujiang and west to the Sino-Burman boundary, that covers the entire Tengchong volcano area. The spatial distribution of intensity of mantle-derived mass release in the Tengchong volcanic area suggests that there exist 3 separable different regions where intensity of mantle-derived mass release is abnormally higher. These 3 mantle-derived mass strong release regions overlap on the 3 high relative geothermal gradient areas, further suggestes that there exist magma chambers beneath the Tengchong volcano area.3.3. Three magma chambers are discovered in the Tengchong volcanic area, the important parameters such as sizes, depths and temperatures of them are estimated, and the activities of them are assessedBy analyzing the temperature field in most upper crust, mantle-derived mass release field in the Tengchong volcanic area, unifing data of magnetotelluric sounding and deep seismic sounding, this work discovered that there exist 3 magma chambers in crust present day in the Tengchong volcano area. The discovery of these 3 magma chambers in crust has established the foundation to reply the key scientific question about the nowadays activity of the Tengchong volcano. By synthetic studies, the sizes and depths of these 3 magma chambers are estimated. By appling the principle of isotopic geothermometer, nowadays temperatures have been estimated for these 3 magma chambers. By synthetic studies, the activities of these 3 magma chambers are assessed.3.4. A model of origin mechanism of Tengchong volcano is proposed According to existence of 3 magma chambers which this dissertation discovers in crust in the Tengchong volcano area, combining the related information which the former investigators obtained by petrology and rock geochemistry research, a model of the Tengchong volcano origin machism is proposed. It is considered that the formation of the Tengchong volcano, not related with the tectonic plate subduction, is the result of local mantle uplift which results in crust extension. The spatial distribution image of mantle-derived mass release intensity present day in the Tengchong volcanic area is the direct reflection of size and strength of this mantle uplift. This local uplift mantle either provides matter source of magma for the Tengchong volcano to prepare or resultes in the crust extension which provides structural conduits for magma to migrate up. The crust extension in the Tengchong volcanic area predicted with this model is also supported by today's local geomorphological investigation and deformation observation.