Stable Isotope Geochemistry

Magma-derived CO 2 emissions in the Tengchong volcanic field, SE Tibet: Implications for deep carbon cycle at intra-continent subduction zone

“…CO 2 / 3 He vs. 3 He/ 4 He (a) and CO 2 / 3 He vs. δ 13 C-CO 2 (b) for the TVF samples. (Sano and Fischer, 2013), δ 13 C-CO 2 = −6.5 ± 2.5‰ (Pineau and Javoy, 1983), CO 2 / 3 He = 2 × 10 9 (Marty and Jambon, 1987); ORG, 3 He/ 4 He = 0.02 R A (Lupton, 1983), δ 13 C-CO 2 = −18.45‰ (Cook-Kollars et al, 2014), CO 2 / 3 He = 10 13 (O'Nions and Oxburgh, 1988); CAR, 3 He/ 4 He = 0.02 R A (Lupton, 1983), δ 13 C-CO 2 = 0‰ (Hoefs, 2009), CO 2 / 3 He = 10 11 −10 14 (O'Nions and Oxburgh, 1988). He-C elemental and isotopic compositions of the EMW ( 3 He/ 4 He = 6.19 R A , δ 13 C-CO 2 = −13.4‰, CO 2 / 3 He = 2.49 × 10 10 ) are calculated according to best-fit ternary mixing in mantle wedge (See details in Fig.…”

“…Given that subduction zone metamorphism can transfer δ 13 C values of typical organic sediments to heavier values (Bebout, 1995), an average δ 13 C value (−18.45‰;Cook-Kollars et al, 2014) of metamorphosed organic matter is taken as that of subducted ORG end-member. On the other hand, subducted CAR end-member can be represented by limestones from the Indo-Burman Ranges (e.g., the Albian limestones in the Upper Irrawaddy basin; Mitchell, 1993), and their δ 13 C values theoretically vary in 0 ± 2‰ (Shields and Veizer, 2002;Hoefs, 2009 Lupton, 1983). Other reference values for associated parameters of the DMM, CAR and ORG end-members are listed in Supplementary Table S2. As shown in the He-C isotope coupling model (Fig.…”

“…Relative proportions of the DMM, ORG and CAR end-members can be calculated following equations proposed by Sano and Marty (1995), which are as follows: ( 3 12 3 12 3 12 3 12 12 13 12 13 12 13 12 13 (Sano and Marty, 1995;Sano and Williams, 1996;Zhang et al, 2015), which are DMM: δ 13 C = −6.5‰, CO 2 / 3 He = 2 × 10 9 ; ORG: δ 13 C = −18.45‰, CO 2 / 3 He = 10 13 ; CAR: δ 13 C = 0‰, CO 2 / 3 He = 10 13 . In our calculation, subducted limestone (as the agent for mantle wedge enrichment) and crustal limestone (as contaminant for the TVF samples) are assumed to have identical δ 13 C value (0‰; Hoefs, 2009) and CO 2 / 3 He ratio (10 13 ; O'Nions and Oxburgh, 1988).…”

Empirical calibration of the clinopyroxene–garnet magnesium isotope geothermometer and implications

“…Three approaches have been used to calibrate isotope thermometers: theoretical calculations, experimental measurements, and empirical calibrations by natural samples (Hoefs 2009). To date, experimental calibration of the clinopyroxene-garnet Mg isotope thermometer is still lacking, and empirical calibration is very limited.…”

“…The degree of equilibrium isotope fractionation is a function of temperature, which makes it possible to use equilibrium inter-mineral fractionations as isotope geothermometers (Urey 1947;Hoefs 2009). Minerals with lower coordination number of the cation of interest generally have shorter and stronger bonds, hence favoring heavier isotopes when thermodynamic equilibrium is achieved (e.g., Bigeleisen 1965;Young et al 2015).…”