Fluid inclusion, isotopic and elemental geochemistry studies of cave-filling calcite in Lower-Middle Ordovician Yingshan Formation in Tahe Oilfield, NW China: Implication for karstification in non-exposed limestone
Description
Sixteen groups of FIAs are recognized for microthermometry in CFC. Homogenization temperature (Th), ice-melting temperatures (Tm-ice) and metastable freezing temperatures (Tf) were measured on 92 L-D biphase inclusions. All the microthermometric data are demonstrated in the Table 1. L-O aqueous inclusions were supercooled to -180 ℃ to stretch and induce vapors, and some of them generated vapors once or several times during supercooling. The L-D biphase aqueous inclusions (including the L-O aqueous inclusions with vapor appeared during supercooling) coexisted with the L-O aqueous inclusions were tested for Th values in six FIAs (n = 21). These inclusions exhibit inconsistent Th values, spreading from 85 ℃ to higher than 200 ℃ (Table 1). The L-O aqueous inclusions were subsequently heated and stretched for Tm-ice measurement. The overall range of Tf values in CFC cement is from -65 to -40 ℃, with most values greater than -50 ℃. These aqueous inclusions have Tm-ice values ranging from -6 to 0 ℃ (salinities of 0¬ to 9.4 wt% NaCl) with bi-modal at -0.9 ℃ and 0 ℃, respectively. Only Seven inclusions in this type have Tm-ice values below -1.9 ℃ (Table 1). From the data for carbon (C-) and oxygen (O-) isotopes (Table 2), the δ13C values of the micrite limestones have narrow ranges, typically varying from -1.0 to -0.4 ‰ (n = 5). In comparison, the δ18O values spread from -8.6 to -6.4 ‰, with an average value of -7.5 ‰. For CFC, the δ13C values (-2.3 to 0 ‰, n = 13) overlap with those of the micrite limestones, while the δ18O values (-15.3 to -12.5 ‰) are notably lower than those of micrite limestones. The 87Sr/86Sr ratios of the MC samples range from 0.70880 and 0.70894, with an average 0.70888 (n = 5). CFC has the 87Sr/86Sr ratios varying between 0.70955 and 0.70990, which are significantly higher than those of the micrite limestones. The individual LA spot elemental values (n = 11) are summarized in Table 3 and their REEY patterns are illustrated in Fig. 6. The total rare earth element (ΣREE) contents of CFC samples vary between 0.65 and 1.60 ppm (mean 0.95 ± 0.30 ppm) (Table 3). CFC samples have moderate Sr, Mn and Fe concentrations of 115.48–263.51 ppm, 126.42–176.09 ppm, and 386.16–428.60 ppm, respectively (Table 3). The calculated Mn/Sr ratios spread from 0.48 to 1.47 with an average value of 0.76 ± 0.36 (Table 3). All the CFC samples have low Al and Zr concentrations, with Al contents of 0.017–0.318 ppm (n = 9), and Zr contents of 0.011–0.030 ppm (n = 11) (Table 3). These samples exhibit poor correlations among REE, Al and Zr contents (Fig. 8A).