Strontium concentration,isotope composition and dolomitization fluids in the Feixianguan Formation of Triassic,Eastern Sichuan of China.

Three major dolomite types including micrite dolomite,grainy dolomite(with original texture)and crystalline dolomite, occur in the Feixianguan Formation of Triassic,Eastern Sichuan of China,in which the crystalline dolomite is the most important gas reservoir.Based on the strontium concentration,isotope composition of the dolomites,the dolomitization fluids were accordingly inferred here.The three types of dolomite have similar strontium content with higher ~(87)Sr/~(86)Sr ratios to some extent than that of the coeval seawater.The difterenee of distribution coefficient of Sr in calcite(or aragonite)and dolomite is the most important control factor on the strontium content in dolomites.Calculation by strontium mixing equation of seawater with meteoric water shows that the dolomitization fluid for crystalline dolomite is mixed by 94% meteoric water and 6% seawater;the fluid for grainy dolomite(with original texture)is mixed by 81% meteoric water and 19% seawater,and that for micrite dolomite is mixed by 92% meteoric water and 8% seawater.However,very low Mn contents(average 19.8ppm),relatively high Sr contents(average 137ppm)and the homogenization temperatures measured in fluid inclusions of crystalline dolomite do not support the hypothesis that meteoric water as high as 94% involved the dolomitization.The dolomitization fluid for this kind of dolomite was probably the trapped coeval seawater (with the age of 242 to 245Ma),which may have been released into the dolomite horizon through unidentified pathways.The Mn and Sr contents,as well as ~(87)Sr/~(86)Sr ratios of the mierite dolomite and grainy dolomite(with original texture)suggest the existence of meteoric water during the dolomitization.The mechanism of dolomitization for mierite dolomite could be explained by evaporative pumping,and that for grainy dolomite(with original texture)explained by mixing-zone model.