HR: 10:20h
AN: V51K-01    [PDF]
TI: Ab initio molecular orbital calculations for boron isotope fractionations on boron acids and borates
AU: * Liu, Y
EM: yunliu@glue.umd.edu
AF: Dept. of Chem & Biochem. University of Maryland, UMCP, College Park, MD 20742 United States
AU: Tossell, J
EM: tossell@chem.umd.edu
AF: Dept. of Chem & Biochem. University of Maryland, UMCP, College Park, MD 20742 United States
AB: Based on the studies of Hemming and Hanson (1992) and Palmer et al. (1987), the stable B isotopic composition (d11B) of carbonate has been used as a pH-indicator to reconstruct the paleo-pH value of seawater and the paleo-CO2 in the atmosphere (e.g. Palmer and Pearson, 2003; Sanyal et al., 2000; etc.). This promising approach is based on the basic isotope exchange reaction between B(OH)3 and B(OH)4-: 10B(OH)3 + 11B(OH)4- -- 11B(OH)3 + 10B(OH) (1) The isotopic fractionation coefficient K for the above reaction is hence very important for building the relation between pH and d11B composition. However, for a long time, there has been only one calculated value given by Kakihana et al., (1977), which K=1.0194 at 25§C. Unfortunately, Kakihana et al. used a theoretical method to calculate this value without specifying its details. Hence we indeed don't know how accurate this value is. Recently, Oi (2000, 2001) has calculated the K value again by using the ab initio molecular method and a simple scaling method. His calculated K value is 1.0260. Compared to 1.0194 provided by Kakihana et al., this difference already can cause a change in predicted pH of at least 0.5 pH unit. Oi (2000) also suggested a wrong approximation method for the K values for polyboric acids and polyborates. Hence we think a careful re-calculating on this issue is needed. We calculate the isotopic fractionation K value by different approaches: the water-cluster and the simple scale factor method Oi has used. We also extend the calculations for polyborates and polyboric acids, which are important in some cases. Our results show the K value should be about 1.025 to 1.027 based on our water-cluster model. Our calculations on different temperatures and on 4-coordinated B in silicates (such as the BO4(Si(OH)3)4- cluster) can let us easily explain many experimental results.
DE: 1040 Isotopic composition/chemistry
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting