Hi Leonardo,

You can report the carbon and oxygen isotope composition of CO2 produced from dolomite by reaction with phosphoric acid on the VPDB scale in precisely the same as you would for CO2 produced from samples of calcite. The only change is that you need to apply a fractionation correction to take into account apparent differences between the partitioning of oxygen isotopes in the dolomite-phosphoric acid and calcite-phosphoric acid reaction system.

To implement this correction you can use the application developed by Kim et al. (Kim, S-T., Coplen, T.B. & Horita, J. 2015. Normalization of stable isotope data for carbonate minerals: implementation of IUPAC guidelines. Geochimica et Cosmochimica Acta, 158, 276-289). This allows you to normalize your dolomite data using results from the contiguous measurement of NBS19 and NBS18 and apply the oxygen isotope fractionation factor of Rosenbaum & Sheppard (Rosenbaum, J. & Sheppard S.M.F. 1986. An isotopic study of siderites, dolomites and ankerites at high temperatures, Geochimica et Cosmochimica Acta, 50, 1147-1150.) for a specific reaction temperature. There are alternative ways of applying this correction, but I would suggest that you stick with R&S and use the “consensus” temperature-dependent dolomite-water oxygen isotope fractionation relationship of Muller et al. for any geochemical interpretation. (Muller, I.A. and seven others. 2019. Calibration of the oxygen and clumped isotope thermometers for (proto-) dolomite based on synthetic and natural carbonates. Chemical Geology, 525, 1-17.). As far as I can tell your oxygen data will then be scaled in a way that’s consistent with the development of this fractionation equation.

In addition you will need to decide on the best way to react your dolomite samples. My feeling is that a reaction temperature of 70oC is a good compromise between reaction time and elevated vapour pressure of the acid (particularly if your acid has a specific gravity <1.91). Using the classical sealed vessel method I find that a reaction time of >8 h should guarantee complete dolomite decomposition. Of course this will be partially dependent on the particle size distribution of your sample powders, the sample mass, and whether you use a static or dynamic (stirred with continuous CO2 removal) gas preparation system.

You also ask about “reference” materials. As you probably know there are no authenticated calibration dolomite materials with assigned isotopic values. Values for NBS88b has been reported in several published studies, but NBS88b has no official status and, in the light of apparent isotopic exchange during long-term storage of calcite, there are questions about the maintenance of the isotopic composition of a material that is now almost 50 years old. Stefano Bernasconi (ETH, Zurich) advertised 3 dolomite (including NBS88b) “intercomparison materials” (see Muller et al. 2019). I also have small quantities of 3 dolomites/ferroan dolomites. Together these cover an oxygen isotope range of 27 permil (and about 7 permil for carbon). I can mail small quantities (say 100-200mg) for you to try. These materials have been run in 3 separate labs using both classical and modern automated procedures. I find that carbon data are in good agreement, but oxygen data differ by up to 1 permil and appears to be method dependent. It has proven difficult to identify the source(s) of these oxygen isotope discrepancies. In principle these data could be scaled to a dolomite-specific reference frame, but this would require someone to specify values for the reference dolomite and, since we don’t currently know the “right” value, which values do we choose? Unfortunately there’s no financial support to allow these problems to be investigated. I conclude that it would be necessary to devote substantial resources to produce, characterise, store, distribute and assign accepted isotopic values to any dolomite material that could be used as a reference for global distribution. 

I hope this might prove helpful.

I apologise for the long-winded message.

Best wishes,

Steve Crowley