文石-水体系氧同位素分馏系数的低温实验研究

采用缓慢分解法和“两步法”的附晶生长法，在低温(0℃～70℃)下实验合成纯文石型碳酸 钙矿物，以XRD和SEM技术对合成矿物的相组成和形貌进行了鉴定。将XRD与SEM及氧同位素分 析技术相结合，研究了文石的生成速率与氧同位素分馏之间关系。对0℃、25℃和50℃条件 下采用缓慢分解法合成的文石进行SEM观察发现，随着温度升高，矿物生成速率加快，氧同 位素分馏逐渐趋于不平衡，导致50℃条件下获得的文石-水体系氧同位素分馏是一种不平衡 分馏，而0℃和25℃条件下获得的低值代表平衡分馏。将0℃和25℃以下采用缓慢分解法获得 的文石-水体系分馏低值与采用“两步法”的附晶生长法在50℃和70℃条件下获得的文石- 水体系平衡分馏数据相结合，得到0℃～70℃范围内文石-水体系氧同位素平衡分馏方程为 ：103lnα=20.41×103T－41.42。这个实验结果不仅与增量方法理论计算结 果一致，而且与前人低温实验获得的文石或文石与方解石混合相碳酸钙-水体系，以及生物 成因文石-水体系的氧同位素分馏结果相近。这是首次根据实验确定的无机成因文石-水体 系热力学平衡氧同位素分馏系数，因此对于无机成因文石在古沉积环境和古气候研究中的应 用具有重要参考价值。

Experimental Studies of Oxygen Isotope Fractionation Factors between Aragonite and Water at Low Temperatures

Aragonite at 0 to 70℃ was synthesized by slow decomposition a nd “two-step” overgrowth approaches. The phase compositions and morphology we re detected by XRD and SEM techniques. The effect of aragonite precipitation rat e on oxygen isotope fractionation between aragonite and water was studied by com bining XRD and SEM techniques with oxygen isotope analysis. For the calcium carb onates synthesized in the temperature range of 0 to 50℃, the XRD and SEM analyses show that with increasing temperature the rate of aragonite precipitati on increases and oxygen isotope fractionations between aragonite and water are p rogressively out of equilibrium. As a result, the experimentally measured oxygen isotope fractionations for the aragonite-water system at 50℃ is at disequilib rium, and the lower fractionation values obtained at 0℃ and 25℃ are proxy for equilibrium fractionations. Taking the lower values at 0℃ and 25℃ together wit h the data obtained from the “two-step” overgrowth technique at 50℃ and 70℃ , it yields the following fractionation equation for the aragonite-water system : 103lnα=20.41×103／T－41.42 This equation not only agrees well with the theoretic calculation by the increme nt method, but is close to the previous experimental results for the aragon ite-water and aragonite/calcite mixture-water systems as well as the empirical estimates on the biogenic aragonite-water system. It provides the first experi mental calibration of oxygen isotope fractionation between inorganically precipi tated aragonite and water at the low temperatures and is thus responsible for th ermodynamic equilibrium fractionation.