团簇同位素在白云岩化流体恢复中的应用与局限性

Application of clumped isotopes to restoration of dolomitizing fluids and its limitations

碳酸盐的团簇同位素(Δ47)可以直接确定矿物的形成温度,结合矿物的氧同位素,还可以根据传统的氧同位素温度计确定矿物生长的流体(水)的氧同位素(δ18 O),提供了一种潜在的确定白云化流体的δ18 O、分析白云岩化流体来源的新方法。受白云岩化之后的重结晶作用和高温固态重置作用的影响,白云岩的团簇同位素温度(TΔ47)会比白云化作用实际发生时期的温度偏高,给直接利用TΔ47来恢复白云岩化流体的δ18 O造成了一定的局限性。为了分析四川盆地东北部地区长兴组-飞仙关组白云岩化流体的来源,开展了普光气田和元坝气田的白云岩团簇同位素研究。普光气田和元坝气田长兴组-飞仙关组白云岩的TΔ47在70~130℃,是重结晶作用和固态重置之后的白云岩视平衡温度。根据地层温度演化史和白云石的固态重置模型,固态重置作用导致川东北地区白云岩的平衡温度增高了30~35℃。经固态重置恢复之后确定的白云化流体的δ18 O(SMOW)在0~4‰,表明白云岩化流体来源于高盐度的蒸发流体。

Clumped isotopes(Δ47)can be used to determine formation temperature of carbonate minerals.If combined with the conventional oxygen thermometer andδ18 O of the minerals,the temperature can be used further to calculate theδ18 O of fluids(water),in which the minerals are developed.This provides a potential new method for determining theδ18 O and sources of dolomitizing fluids.However,affected by recrystallization and solid-state reordering after dolomitization,the clumped isotope temperature(TΔ47)usually reads higher than the actual dolomitization temperature,thus limiting the application of TΔ47 to the restoration of fluidδ18 O.To determine the sources of the dolomitizing fluids from the Changxing(P 2 c)and Feixianguan Formations(T 1 f)in northeastern Sichuan Basin,clumped isotopes of dolostone samples from the Puguang and Yuanba gas fields were collected and measured.The TΔ47 of P 2 c and T 1 f dolostone samples from the two gas fields ranges between 70℃and 130℃,which,however,should be seen as apparent temperature of the samples considering the effect of recrystallization and solid-state reordering.The thermal history and solid-state reordering model results show that the apparent temperature of the samples would increase by about 30-35℃under high-temperature solid-state reordering and theδ18 O of the dolomitizing fluids is 0-4‰(SMOW)after restoring solid-state reordering,indicating a high-salinity evaporative sea water source.