Noble gas isotopic compositions of deep carbonate rocks from the Tarim Basin

Abstract

Abundances and isotopic compositions of noble gases (He, Ne, Ar, Kr) with various existence states in carbonate rocks from the Tacan1 Well have been investigated by means of the stepwise heating technique. The elemental abundance patterns of noble gases in the samples show the enrichment of heavy noble gases and depletion of 20Ne relative to the atmosphere, which are designated as type-I and are similar to that observed in water, natural gases and sedimentary rocks. The 3He/4He ratios of deep carbonate samples at lower and medium temperature (300–700°C) and a majority of samples at higher temperature (1100 –1500°C) steps are very similar to those of natural gases in the same strata in this area, this feature of radiogenic crustal helium shows that the Tazhong Uplift is relatively stable. However, significant helium and argon isotopic anomalies are found at the 1100°C step in the Middle-Upper Ordovician carbonate rock, suggesting the incorporation of mantle-derived volatiles, this may be due to minor igneous minerals contained in sedimentary carbonate rocks. The 40Ar/ 36Ar ratios in the Cambrian carbonate rock are slightly higher than those in Ordovician carbonate rocks, which may reflect the influence of the chronologic accumulation effect of crust radiogenic 40Ar. Argon isotopes of various existence states in source rocks are much more different, both 38Ar/ 36Ar and 40Ar/36Ar ratios at the higher temperature steps are higher than those at the lower temperature steps.