This is a too general question that cannot be properly answered without additional data.

The obvious thing is to use the fractionation factor between dolomite and water.

But before you can consider that you need to care about some practica issues that will affect your results:

- Nature of the sample: is it 100% Dolomite or are there other additional mixed carbonates?

- Analytical technique employed (Acid reaction? Off-line? On-Line?)

- Conditions of the reaction (critically: Temperature of reaction; time allowed).

Depending on the answers to the questions above your results maybe useful or not at all.

Correcting measured oxygen isotope values from dolomite involves addressing potential isotopic fractionation effects that may have occurred during sample analysis. Dolomite, a carbonate mineral, can experience isotopic fractionation due to various processes. The most common correction involves the use of an appropriate fractionation factor or applying an offset to bring the measured values in line with a standard reference material.

I have few suggestions to correct measured oxygen isotope values from dolomite:

Choose a Reference Material:

Select a well-characterized carbonate reference material with known oxygen isotope values. Commonly used standards include Vienna Pee Dee Belemnite (VPDB) for marine carbonates and Vienna Standard Mean Ocean Water (VSMOW) for freshwater carbonates.

Calculate Fractionation Factor (Equilibrium Fractionation):

Determine the equilibrium fractionation factor between dolomite and the chosen reference material. This factor represents the isotopic fractionation during the formation of dolomite under equilibrium conditions.

The equation for correcting measured values (δ^(18)O) is:

Corrected δ 18O=Measured δ 18O+Fractionation Factor

Consider Diagenetic Effects:

Dolomite may undergo diagenetic alteration, leading to non-equilibrium fractionation. In such cases, additional corrections might be necessary based on the specific diagenetic processes involved.

Quality Control:

Regularly analyze reference materials along with your samples to monitor instrument stability and correct for any instrumental drift.

Standardize Analytical Techniques:

Ensure that consistent analytical techniques are used for sample preparation and analysis. This includes standardized methods for extracting and analyzing oxygen from carbonate samples.

If dealing with multiple samples, consider statistical approaches to assess the reliability of your corrected values, such as calculating reproducibility and precision.

Goodluck

Clemente Recio

I appreciate your answer and how to address such an issue

The rock samples are fully dolomitized and have been used to obtain O and C isotopic values. Powders were dissolved at 75 °C with 100% phosphoric acid. The produced CO2 was analyzed using a thermoelectron delta+ advantage stable isotope ratio mass spectrometer.

I have been advised to use the phosphoric acid fractionation factors from Rosenbaum and Sheppard (1986) for correction, and my concern is how to do the calculation. Although, I have been advised to multiply the fractionation factor obtained at a certain T, in my case 75 C, with the obtained O values. I am still curious to know if this is the right method.

Safdar Bashir

Thank you. This is a well-articulated answer, and yes, the correcting equation is what I was looking for.

I used Rosenbaum and Sheppard's (1986) fractionation factor (FF) for dolomite; however, I was advised to multiply the FF with obtained O values, and I was sceptical, and they were unsure how to do the correction. Anyway, could you please recommend a paper that addresses such a procedure? This will be really appreciated.

Dear Dr Al Sarmi,

Thanks for the technical details.

I will need, however, some additional details.

Basically, when we do instrumental isotope ratio determination, we "measure" **the difference** in isotopic ratios between a sample gas (in this case, the CO2 produced by acid reaction of a carbonate) and a working gas.

In the case of carbonate analysis, we "measure" simultaneously C and O isotopic ratios on the same gas.

Given the chemistry of carbonate - acid reaction, the CO2 produced has all C from the carbonate, but only 2/3 of the oxygen; the third O stays into water, and between CO2-O and H2O-O there is an isotopic fractionation that depends on temperature, and needs to be corrected for.

It is this correction that can be made applying Rosenbaum & Sheppard's fractionation factor.

But before you do that, you need to "correct" the raw delta values to values relative to the international standards (PDB, SMOW, or actually, V-PDB, V-SMOW2). This might have already been done by the Lab, although I doubt it; otherwise they would have given you the "true" dolomite values, and you would need no further calculations (at least, when I was in charge of the Isotope Lab, I always returned the values with all required corrections, and relative to the international reference standards).

So you need details of the analytical procedure:

- Does the Lab use a raw working gas? I used to do this, but in every run I would include known-value reference samples, that I used to correct the value relative to the working gas into values relative to reference standards.

- Does the Lab "calibrate" their working gas in advance? If such is the case, the values returned are already referred to international working samples. Your C values are already "dolomite-values", and you only need to correct the temperature-dependent value of oxygen (the dolomite fractionation factor of Rosenbaum & Sheppard, for example).

Please have a look at Zachary Sharp's book (principles of stable isotope geochemistry, 2007. Pearson Education, Inc.; this book can be "legally" downloaded for free thanks to Zach's generosity); pg. 19 . While doing so, it is well worth reading the whole section 2.3 "the delta value".

In any case, I would drop a message to the Lab; tell them that your samples were 100% dolomite, and ask them for the corrections; most likely they will have that automatized, and will be easy for them.

Despite of that, do not skip having a look at Zach Sharp's book; it is always nice to understand "the guts" of isotope analysis.

Good luck,

Clemente

Many thanks indeed Clemente Recio

I appreciate your time and effort in providing an in-depth explanation

Yes, our isotope data are quoted relative to V-PDB, and the step I inquired about is how to correct the temperature-dependent value of oxygen, which I should have clarified in the question. I thank you for providing the reference to read more about this!

Cheers