碳酸盐矿物氧同位素分馏的理论研究

应用增量方法系统地计算了碳酸盐矿物的同位素分馏系数，得到不同结构和成分的碳酸盐矿物的18O富集顺序为：菱铁矿〉铁白云石〉菱镁矿≥白云石≥方解石〉文石〉菱锶矿〉白铅矿〉碳钡矿。在0℃～1200℃范围内获得了一组内部一致的碳酸盐－水体系的理论分馏系数，这些计算结果与已知的实验和／或经验数据之间存在良好的一致性，因此本文对碳酸盐矿物氧同位素分馏系数的理论校准不仅可应用于共生矿物组合形成温度的确定，而且能够应用于其形成机理的示踪。 计算结果表明，白云石的氧同位素分馏行为与方解石相似，在25℃下白云石与方解石之间的平衡分馏为0.56‰ 。理论预测文石相对于方解石显著地亏损 δ18O，在25℃时方解石与文石之问的平衡分馏为4.47‰ 。文石向方解石的同质多相转变可能是通过一种没有同位素再造的惰性氧结构单元CO3]2- 进行的，即只涉及Ca2+ 与C03]2- 基团之间键的断裂和再组台而未出现CO3]2- 基团内部C－O键的断裂和再组合。结果在自然界和实验室实验中，文石中氧同位索配分的温度关系能够传递副方解石中来。这种在同质多相转变形成方解石过程中的氧同位素继承性对于了解白云石－方解石－水体系分馏的难题至关重要。理论预测也能够用来解释对方解石分馏的经验估算与实验测定之间的分歧。

THEORETICAL STUDY OF OXYGEN ISOTOPE FRACTIONATION IN CARBONATE MINERALS

The increment method is adapted to systematically evaluate oxygen isotope fractionations in carbonate minerals and the following sequence of 18O-enrichment is obtained: siderite>ankerite>magnesite> dolomite>calcite>aragonite>strontianite>cerussite≥witherlte．The internally consistent fractionation factors for the systems quartz carbonate and carbonate-water are acquired for a temperature range of 0 to 1200℃．There is good agreement between the present calculations and known experimental and/or empirical data．Therefore, this theoretical calibrations involving the carbonate minerals are applicable not only to determination of the formation temperature of mineral assemblages but also to tracing their formation mechanisms． The present calculations suggest that dolomite should behave isotopically like calcite, and the cauilibrium fractionation between dolomite and calcite is 0.56‰ at 25℃．Aragonite is predicted to be significantly depleted in 18O relative to calcite; equilibrium fractionation between calcite and aragonite is 4.47％ at 25℃．It is possible that polymorphic transition from aragonite to calcite could proceed through an essentially intact oxygen structure without isotopic resetting．In other words, the polymorphic transition involves only the breaking and rebinding of bonds between cation and carbonate complex rather than bonds between C and O inside the carbonate complex．It is hypothesized that the temperature dependence of oxygen isotope partitioning in aragonite could be conveyed to calcite in nature and in laboratory experiments．However, it remains to be tested whether oxygen isotope inheritance in calcite formation by the polymorphic transition is of critical importance in attempts to resolve dilemma involving fractionations in aragonite-calcite-dolomitewater systems.