Evaporative Minerals of the Upper 400m Sediments in a Core from the Western Qaidam Basin, Tibet
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摘要: 柴达木盆地位于青藏高原北部，沉积和储存了巨量的蒸发岩。其独特的地理位置、构造特征、盐类沉积和其它资源优势，成为地学领域各方向学者关注的焦点。2008年，中德合作在盆地西部取得千米深的钻孔，上部400 m表现为碳酸盐粘土层与盐层的交替，盐层的矿物种类是石盐（NaCl），石膏（CaSO4·2H2O）,芒硝（Na2SO4·10H2O），无水芒硝（Na2SO4），钙芒硝（Na2Ca(SO4)2），杂卤石（K2MgCa2(SO4)4·2H2O），尤钠钙矾（Na4Ca(SO4)3·2H2O）和白钠镁矾（Na2Mg(SO4)2·4H2O），以石盐和石膏为主。碳酸盐粘土层中还出现了少量重晶石（BaSO4）、碳酸钠钙石（Ca2Na2(CO3)3）、钠钙矾（Ca2Na3(SO4)3(OH)）、苏打石（NaHCO3）、斜碳钠钙石（Na2Ca(CO3)2·5H2O）、一水碳酸钠（Na2CO3·H2O）和水硼镁石（CaMg(B3O3(OH)5)2·6H2O）。碳酸盐粘土层中不同形态的石膏单晶、双晶和集合体，形成于次生浓卤水溶液，晶体形状与生长速度、溶液性质和环境条件有关。单盐矿物石盐、石膏和无水芒硝从卤水中直接结晶，复盐钙芒硝、白钠镁矾和尤钠钙矾则由单盐反应形成。尤钠钙矾为罕见的亚稳定矿物，柴达木盆地和青藏高原其它地区的研究中尚未见报道，该矿物形成温度远高于室温，可能是由石膏或无水芒硝在富NaSO4卤水溶液中反应形成。根据矿物种类、组合和阳离子含量推断，古卤水类型主要为NaCl型，其次为NaCaSO4、NaSO4、NaSO4Cl、NaCaSO4Cl，少数层位为CaSO4和NaMgSO4型卤水。盐层和碳酸盐粘土层的交替说明，1 Ma以来钻孔所在区域为干湿交替气候，18个成盐阶段或气候干旱阶段出现在0.97~0.03 Ma，最早的两个蒸发阶段为0.96~0.97 Ma和0.87 Ma，其它蒸发阶段出现在0.78~0.03 Ma，厚盐层集中出现在0.5~0.03 Ma。Abstract: Qaidam Basin is a tectonically controlled depression on the northern margin of the Tibetan Plateau. In 2008, a long core was drilled in Qahansilatu subbasin in the western Qaidam Basin. The sediment layers in the upper 400 m alternate between evaporative mineral layers and carbonaceous clay layers. The detailed mineralogical investigation focused on evaporative minerals including halite, gypsum, mirabilite, thenardite, glauberite, eugsterite, bloedite, barite, cesanite, shortite, thermonatrite, gaylussite, nahcolite and inderborite. Gypsum and halite make up the majority of the evaporative minerals. Environmentally induced variations in the mineralogy and crystal habit of the sulfates have been extensively investigated. Gypsum has prismatic and pyramid habits, such as disc pyramid, stubby prismatic, slender prismatic. Visible isolated gypsum and aggregates (rosette/radial and twins) are mostly scattered in carbonaceous clay layers, suggesting secondary gypsum well developed. Gypsum may be a precursor mineral of glauberite, and thenardite is the precursor of bloedite. As a metastable and rare mineral, eugsterite does not appear in other Tibetan areas. It forms at the expense of pre existing gypsum or thenardite in the core at an experimental temperature of higher than room temperature. The presence of eugsterite indicates a warm and/or hot climate at its deposition time. Mineralogical variations have been explained by the brine evolution of NaCl, NaCaSO4, NaSO4, NaSO4Cl, NaCaSO4Cl, CaSO4, and NaMgSO4. For instance, NaMgSO4 type corresponds to the bloedite, while NaCaSO4Cl type to the assemblage of halite, gypsum and glauberite. The evaporative minerals and carbonaceous clay layers alternation indicates the shift between dry and wet climate. According to the thickness, eighteen evaporative stages and/or dry climate stages were identified from 0.97 Ma to about 0.03 Ma. The two early dry climate stages are identified at 0.96~0.97 Ma and 0.87 Ma. The other sixteen evaporative stages occurred from 0.78 Ma to about 0.03 Ma. The evaporaterich stages suggested that evaporation was high and groundwater inflow was sufficient at the subbasin.
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Key words:
- evaporative minerals /
- geochemistry /
- halite /
- gypsum /
- Qahansilatu subbasin
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