Boron isotope values in Paleozoic brachiopods and corals, collected from the Yunnan-Guizhou Plateau, China, can be used to constrain the boron isotope compositions of past oceans. All brachiopod shells and coral samples were screened for diagenetic recrystallization by cathodoluminescence microscopy, trace element geochemistry of B, Fe, Mn, Sr, and scanning electron microscopy. The boron isotope ratios for brachiopods in Silurian, Devonian, Carboniferous, and Triassic calcites are in the ranges 8.9‰-14.0‰, 8.8‰-13.8‰, 10.3‰-16.3‰, and 6.7‰-12.4‰, respectively. The boron isotope ratios of coral calcites in the Silurian, Devonian, and Permian are 9.1‰-12.2‰, 6.1‰-13.8‰, and 9.2‰-16.1‰, respectively. The δ11B values for both brachiopods and corals are significantly lower than those for modern biogenic carbonates, indicating that the Paleozoic oceans were depleted of δ11B by up to 10‰. Our results are consistent with previous published studies. The boron isotope compositions of corals and brachiopods show the consistent trends. The low δ11B values may be explained by an enhanced riverine flux of boron from the continents.
MA YunQiXIAO YingKaiHE MaoYongXIAO JunSHEN QuanJIANG ShengXiang
From the point of growth units, the growth mechanism of hydrotalcite (HT) crystal is investigated in this paper. Results show that the growth morphology of HT is consistent with the model of anion coordination polyhedron growth units. The Raman shift of growth solutions of HT, Cu-HTlc, and Cu-Zn-HTlc are monitored using Raman spectroscopy. In the experiment, the growth units of Mg-Al-hydrotalcite are [Mg-(OH)6]4- and [Al-(OH)6]3-, and the growth units of Cu-Htlc and Cu-Zn-HTlc are [Mg-(OH)6]4- and [Al-(OH)6]3-, respectively. The growth process of hydrotalcite is as follows: growth units first incorpo- rate into metal layers, then metal layers adsorb An- and H2O, and the growth units incorporate into layer compounds according to this rule. Growth units will have different incorporations and growth morphologies caused by different growth surroundings. Furthermore, the reason why Cu-HTlc is difficult to synthesize is also interpreted in this paper.
Brine from the saline Qarhan Lake was evaporated at 28±2°C in a clean environment.Two groups of experiments were conducted;one with complete separation of precipitate and brine at different stages of evaporation,and the other with continuous precipitation during the evaporation.Seventy-nine precipitate and brine samples were collected during the experiments,and the δ37 Cl values were determined using an improved thermal ionization mass spectrometry procedure for precise measurement of chlorine isotopes based on Cs2Cl+ ions.Based on the concentrations of Na+,K+,and Mg2+,evaporation was divided into three main precipitation stages as follows:halite dominant,carnallite dominant,and bischofite dominant.The δ37 Clsolid and δ37 Clliquid values of the precipitate and coexisting brine samples at different stages showed the following characteristics.The precipitates were enriched with 37 Cl relative to the coexisting brine samples,and the δ37 Cl of both the precipitate and brine samples decreased gradually during evaporation.The fractionation factors(αh) between halite and brine were the highest,followed by that(αc) between carnallite and brine,and then that(αb) between bischofite and brine.The αc and αb values of less than one,which indicate the precipitate is enriched in 35 Cl,were found when the evaporation process entered a new stage.However,the δ37 Cl values of carnallite,bischofite,and the coexisting brine samples decreased during evaporation.The residual brine is a 35 Cl reservoir.The experimental phenomena were consistent with the δ37 Cl values in saline deposits in the literature.δ37 Cl can be used as an indicator of brine evaporation processes,which is important in the exploration of sylvinite deposits.
LUO ChongGuangXIAO YingKaiMA HaiZhouMA YunQiZHANG YanLingHE MaoYong
