Showing papers by "Yong-Fei Zheng published in 1991"
TL;DR: In this article, a significant oxygen isotope fractionation is obtained between hematite and magnetite, which is attributed to the quantitative redox of iron in the experiments, that can in turn explain the nearly identical isotope composition of oxygen in coexisting hematites and magnetites pairs from the banded iron-formation in the Iron Quadrangle, Brazil.
Abstract: Thermodynamic oxygen isotope factors for hematite and magnetite are calculated using the modified increment method and incorporating the theoretical and experimental reduced partition function ratios determined for quartz and water in the temperature range 0 to 1200 o C. The obtained oxygen isotope fractionation factors between hematite, magnetite and water are expressed as 10 3 ln α Fe2O3-H2O =2.69×10 6 /T 2 -12.82×10 3 /T+3.78, 10 3 ln α Fe3 O 4- H 2 O=3.02×10 6 /T 2 -12.00×10 3 /T+3.31, and those between quartz and hematite or magnetite are expressed as 10 3 ln α Si O 2-FeO3 =1.55×10 6 /T 2 +9.05×10 3 /T-4.82, 10 3 ln α SiO2-Fe3O4 =1.22×10 6 /T 2 +8.22×10 3 /T-4.35. The present results are well in agreement with existing theoretical, experimental and empirical calibrations involving magnetite and hematite. A significant oxygen isotope fractionation is obtained between hematite and magnetite, whith hematite being depleted in 18 O relative to magnetite. This matches with the observation from Precambrian iron-formation in the Hamersley Range, Australia. The rule of effect of oxidation state on the oxygen isotope fractionation seems not to work for the two iron oxides. Previous experimental calibration of magnetite-water system by reduction of hematite to magnetite is concordant with the present result for hematite-water system. This is attributed to the quantitative redox of iron in the experiments, that can in turn explain the nearly identical isotope composition of oxygen in coexisting hematite and magnetite pairs from the banded iron-formation in the Iron Quadrangle, Brazil
64 citations
TL;DR: In this paper, the relative fractionation relationship of sulphur isotope data for coexisting sulphate and sulphide pairs from hydrothermal ore deposits on δ38S versus Δ34S diagrams are deciphered theoretically.
Abstract: Correlative fractionation relationships of sulphur isotope data for coexisting sulphate and sulphide pairs from hydrothermal ore deposits on δ38S versus Δ34S diagrams are deciphered theoretically. Taking into account dissolved H2S and SO42- in hydrothermal fluids during precipitation of both sulphate and sulphide minerals, a 4-species closed system is suggested for describing the conservation of mass among all sulphur-bearing species on the δ-Δ diagrams. The covariation in the δ34S values of both sulphate and sulphide is ascribed to isotopic exchange between oxidized and reduced sulphur species during mineral precipitation. The isotopic exchange could be a thermodynamic process due to simple cooling of high temperature fluids, which results in an equilibrium fractionation, or a kinetic process due to mixing of two sulphur reservoirs, which leads to a disequilibrium fractionation. The δ34S value of total sulphur in a hydrothermal system could change due to the precipitation of minerals, or due to the escape of H2S and/or SO42-. Sulphur isotope data for anhydrite and pyrite pairs from the Luohe porphyrite iron deposit in the Yangtze River Valley is used to illustrate the mixing responsible for the disequilibrium fractionation.
16 citations
TL;DR: In this article, the slope ≈ 1 arrays on δ − δ diagram are introduced as test for istotopic equilibrium between coexisting substance pairs out of closed system which are capable of providing an isotopic geothermometry.
Abstract: The slope ≈ 1 arrays on δ – δ diagram are introduced as test for istotopic equilibrium between coexisting substance pairs out of closed system which are capable of providing an isotopic geothermometry. Some examples are given for illustrating the applicability of this graphical method.
1 citations