| 中地壳的地球化学动力学和矿石成因 |
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| 关键词:chemical kinetics critical state reaction of mineral in water middle crust |
| 基金项目:国土资源部项目(编号:20010302); 科技部项目(编号:2001DEA20023,2001DEA30041,2003DEA2C021,G1999043212); 国家自然科学基金项目(编号:4003011,49473196,20373064) |
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| 摘要: |
| Geochemical Kinetics and Ore Genesis in Middle Crust |
| The authors performed lots of kinetic experiments on silicate-water reactions at temperatures > 300℃ and under the conditions from a sub-critical state to a super-critical state of water. The dissolution kinetic experiments of such minerals as albite, actinolite and diopside and basalt were carried out by using packed bed reactor, in the temperature range of 250 to 400℃ and at 22MPa. Experiments show that dissolution rates behave a fluctuation in dissolution rates, in the case of crossing the critical state of water, i.e., over the temperature range of 300 to 400℃. When a multi-oxide silicate mineral was dissolved in water, different dissolved aqueous metals behaved different release rates, which is called an incongruent dissolution. Usually congruent dissolution in water occurred at 300℃, for example, congruent dissolution of albite appeared at 300℃. Experiments also show that the maximum dissolution rates of silicate minerals always occur at 300℃, for example, the maximum release rate of Si is at 300℃. When temperature was lower than 300℃, the release rates of Na, K, Mg, Ca, Fe and Al were higher than those of Si. On the contrary, the release rates of Si of silicate minerals were higher than those of other metals when the temperature was above 300℃. The bonding properties of the metal-oxygen bridge accurately determined the reaction rates between metal-oxide and water. Generally speaking, under the room condition, the sequence of Na-Obr,Ca-Obr,Mg-Obr,Al-Obr and Si-Obr exhibited the properties of relative change from the ionic bond to the polar bond. Water properties changed with temperature and pressure from the room condition to a sub-critical region (300～374℃,22 MPa), and then to a supercritical region (>374℃,22 MPa). As a result, water molecules were gradually not liable to break ionic bonds but liable to break polar bond. Besides, the authors performed the kinetic experiments of magnetite and cassiterite in water in the temperature range of 250 to 400℃, and obtained the same significant results. Those experiments found the fluctuation of reaction rates of minerals in water in the case of crossing the critical state of water. All of the experiments can be used to interpret the mechanism of water-rock interactions in the middle crust. Water-rock interactions mostly occur at temperatures up to 300～450℃ and under pressures about or higher than 20～50 MPa in the middle crust. The thickness of the earth crust is different form place to place, and hence the physical conditions in middle crust are not completely the same everywhere. In simulation of water/rock interactions in the middle crust, we should carry out the experiments of minerals (rocks) in water over the temperature range of 300～450℃. The fluid inclusion data demonstrate that the majority of hydrothermal ores are precipitated in the temperature range of 300 to 500℃, lower than the critical line of NaCl-H2O. The fluids in middle crust lie in the region from a sub-critical state to a supercritical state. The fluctuation of water properties in that case will lead to the fluctuation in reaction rates of water-rock interactions, resulting in the precipitation of large quantities of ores. |
| ZHANG Rong-hua,HU Shu-min,ZHANG Xue-tong.2006.Geochemical Kinetics and Ore Genesis in Middle Crust[J].Acta Geoscientica Sinica,27(5):460-470. |
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