| Popis: |
Traditionally oxygen isotope studies are only concerned with the relationship between one rare isotope, oxygen-18, and the common isotope, oxygen-16. In these cases, the abundance of the third stable isotope, oxygen-17 is ignored because for almost all terrestrial processes the 17O-16O relationship roughly scales with the 18O-16O relationship through a fractionation processes and is thought to not provide any new information. However, the discovery of large mass independent isotope effects for ozone chemistry has driven a multitude of uses for triple isotope relationships. Here, the boundaries and behaviors of mass-dependent isotope fractionation are investigated from a theoretical perspective. Previous approximations to the statistical-mechanical models for predicting isotope effects have led to the notion that mass fractionation laws are constant, and later, constrained to a canonical range of possible values. In contrast to this widely held view, it is here demonstrated generically that the mass fractionation exponent, θ, can take any value for small fractionations, that these deviations are measurable. In addition, the use of ∆∆M or change/difference in cap-delta is suggested as a necessary and more reliable descriptor of multiple isotope fractionation relationships. Driven by the temperature dependence of this new parameter theoretical calculations for both two and three isotope fractionations between several common minerals and liquid water are presented for the purpose of calibration the three oxygen isotope geothermometer which can be applied in a similar way to the traditional two isotope thermometer. As an example, the new quartz-water fractionation curves are applied to triple oxygen isotope date from previously published 2.5 Ga marine chert samples. These results indicate that the water which those chert samples formed from had a temperature of < 1C and a δ18O of < -23.8. |
