Oxygen isotope fractionation between grossular-spessartine garnet and water: an experimental investigation
Lichtenstein, Uwe; Hoernes, Stephan
European Journal of Mineralogy Volume 4 Number 2 (1992), p. 239 - 250
published: Apr 21, 1992
manuscript accepted: Jul 5, 1991
manuscript received: Oct 30, 1990
ArtNo. ESP147050402017, Price: 29.00 €
Abstract Oxygen isotope fractionations have been determined between hydrothermaHy crystallized oxide mixtures of spessartine composition and water at 500-750oC and at 2-12 knar. Direct-exchange calibration of the fractionation factor α for natural spessartine- and grossular-rich garnets was performed only at 750°C and 16 kbar because the garnet-water exchange reaction takes place extremely slowly. At 750°C, both garnet compositions gave identical fractionation factors, indicating that the substitution of the octahedrally coordinated, divalent cations is of negligible influence on the oxygen-isotope fractionation. SEM studies showed that in the direct-exchange experiments the garnet-water exchange occurs largely by solution-precipitation mass-transfer mechanism. The spessartine-water fractionations based on the synthesis experiments fits the following equation (T in Kelvins): lOOOlnαspess-water = 1.27(±0.25)*106/T2-3.65(±0.31) in the temperature range from 500-750°C. Combining these fractionation factors with the quartz-water fractionations from Bottinga & Javoy (1973), the following quartz-garnet equation is obtained: lOOOlnctqz-gamet = 2.83*106/T2.