沉积条件下白云石与伴生方解石碳氧同位素组成的差别及影响因素

Differences and controls of carbon and oxygen isotope composition in dolomite and coexisting calcite under deposition conditions

以岩石学为基础,运用元素、X射线衍射和碳氧同位素分析等方法,在手标本中观尺度上探讨同一样品中白云石与伴生方解石的碳氧同位素组成及影响因素。研究表明,白云石比伴生方解石具有更高的δ13C和δ18O值。白云石的δ18O值高于伴生方解石可能与如下2种因素有关:(1)在白云石—水和方解石—水系统中,前者的氧同位素分馏系数大于后者,因此在白云石—方解石系统中,氧同位素分馏系数大于1;(2)白云石沉淀于比方解石盐度更高、更富18O的水体中。白云石的δ13C值高于伴生方解石可能与如下因素有关:在白云石—CO2和方解石—CO2系统中,前者的碳同位素分馏系数大于后者,因而对交代过程的白云石—方解石系统而言,碳同位素分馏系数大于1。准同生白云石与伴生方解石的氧同位素组成差值较大,而热液/深埋藏白云石与伴生方解石的氧同位素组成差值较小。

We discussed the composition and factors influencing carbon and oxygen isotopes in dolomites and coexis-ting calcites in hand specimen mesoscale based on petrography and combined with elemental, X-ray diffraction and carbon and oxygen isotope analyses. Results indicated that dolomites have higherδ13 C andδ18 O values than coexisting calcites. Two factors may account for the higherδ18O values of dolomites. （1） The oxygen isotope fractionation factors of a dolomite-water system are higher than those of a calcite-water system. Therefore, oxygen isotope fractionation factors are greater than 1 in a dolomite-calcite system. （2） The fluids of dolomite precipitation have higher salinity and higher 18 O abundance compared to coexisting calcite. In addition, the δ13 C values of dolomites show the same trends withδ18 O values. That is to say, the carbon isotope fractionation factors in a dolomite-CO2 system are higher than those in a calcite-CO2 system. Therefore, carbon isotope fractionation factors are greater than 1 in a dolomite-calcite system during replacement process. Oxygen isotope values varied obviously between syndepositional dolomites and coexisting calcites, while they were similar between hydrothermal/deeply burial dolomites and coexisting calcites.