Variation of gypsum morphology along deep core SG-1, western Qaidam Basin (northeastern Tibetan Plateau) and its implication to depositional environments

Abstract

Abstract The Qaidam Basin is an arid, closed, intermontane basin located on the northern margins of the Tibetan Plateau in China. A 938.5 m-long core (SG-1), dated between ∼2.8 Ma–0.1 Ma, was obtained from the western Qaidam Basin. Gypsum is one of major evaporative minerals in the core. A total of 201 gypsum samples were selected and their crystal morphologies were carefully investigated. The crystal habits are lenticular, prismatic, tabular, twinned, stubby, and aggregate. Prismatic crystals occur in the upper 723 m, while lenticular crystals exist almost entirely in the upper 523 m. Tabular crystals are the most common morphology. The total abundance of gypsum crystal (including tabular, lenticular, and prismatic forms) tends to increase with decreasing depth. The relationship between crystal habits and depositional environments is complex, because same morphology can appear in different environments, and different morphologies can also occur in the same depositional environment. However, the occurrence of different morphologies can be attributed to the variable growth rates caused by the inhibition effect of additional cations (e.g. Mg2+, Na+, K+ and Sr2+) on particular lattice parameters and the variation in precipitation experiments. The lattice parameters of the unit cell of gypsum, measured by X-ray diffraction method, are: a = 5.854–12.57 A; b = 9.654–16.231 A; and c = 5.163–14.024 A; β = 113.54–119.88°; Volume = 601.55–712.65 A3. The substitution of K+, Na+ and Mg2+ for Ca2+ can reduce c-axis, while Sr2+ can reduce a and b axes. a/c, b/c, a/b tend to increase with K+, Na+ and Mg2+ and decrease with Sr2+. The Mg2+, Na+, K+ and Sr2+ contents in gypsum show significant variability, with K/Ca molar ratios varying from 1.03 × 10−4 to 32.08 × 10−4, Mg/Ca molar ratios from 0.0023 to 0.1629, Na/Ca molar ratios from 0.031 to 0.575, and Sr/Ca molar ratios from 5.59 × 10−4 to 19.67 × 10−4. Sr2+, Mg2+, Na+, and K+ concentrations in gypsum mainly depend on growth rate and brine concentration.