Isotopic fractionation during acid digestion of calcite: A combined ab initio quantum chemical simulation and experimental study

Abstract

Rationale:Carbonate clumped isotope analysis involves the reaction of carbonate minerals with phosphoric acid to release CO 2 for measurement in a gas-source isotope ratio mass spectrometer. Although the clumped isotope proxy is based on the temperature dependence of 13 C-18 O bonding preference in the mineral lattice, which is captured in the product CO 2 , there is limited information on the phosphoric acid reaction mechanism and the magnitude of clumped isotopic fractionation (mass 63 in CO 3 2− to mass 47 in CO 2 ) during the acid digestion. Methods:We studied the reaction mechanism for the phosphoric acid digestion of calcite using first-principles density functional theory. We identified the transition state structures for each reaction involving different isotopologues and used the corresponding vibrational frequencies in reduced partition function theory to estimate the Δ 47 acid fractionation. Experimental Δ 47 data were acquired by processing the sample CO 2 gas through the dual-inlet peripheral of a ThermoFinnigan MAT253 isotope ratio mass spectrometer. Results:We showed that the acid digestion reaction, which results in the formation of CO 2 enriched with 13 C-18 O bonds, began with the protonation of calcium carbonate in the presence of water. Our simulations yielded a relationship between the Δ 47 acid fractionation and reaction temperature as Δ 47 = −0.30175 + 0.57700 × (10 5 /T 2 ) -0.10791 × (10 5 /T 2 ) 2 , with T varying between 298.15 and 383.15 K. Conclusions:We propose a reaction mechanism that shows a higher slope (Δ 47 acid fractionation vs. 1/T 2 curve) for the phosphoric acid digestion of calcite than in previous studies. The theoretical estimates from the present and earlier studies encapsulate experimental observations from both "sealed vessel" and "common acid bath" acid digestion methods.

Ab initio quantum chemical studies of isotopic fractionation during acid digestion reaction of dolomite for clumped isotope application

“…Starting with a dolomite dimer (Figure 1A), the reaction mechanism at the mineral–acid interface is modeled. We propose a mechanism where the acid digestion reaction commences with protonation of the carbonate dimer 17,18 . The proton is produced by deprotonation of the acid in the presence of water.…”

“…The reaction scheme for 2a is shown in Figure 2 and the QST3 calculation is based on this particular reaction step of the acid digestion reaction. It was shown that the first protonation on the bridging oxygen of the carbonate dimer releases product CO 2 19 and this is the deciding step of the type of the fractionation that follows 17 (Equation ). After the first protonation, the protonated oxygen atom remains attached to the cations of the dolomite dimer (Figure 2).…”

“…The numerator is determined from the relative reaction rates using transition state frequencies and reduced partition functions, as discussed in section 2.1. We designate our carbonate for acid reaction with abundances of 12 C 18 O 16 O 2 2− , 12 C 17 O 16 O 2 2− and 12 C 16 O 3 2− as 0.001968797, 0.000373003 and 0.981845503 (VPDB and VSMOW absolute values), respectively, in the denominator, which translates into the δ 13 C VPDB = 0 ‰ and δ 18 O VSMOW = 0 ‰ 13 scale to keep the theoretically determined Δ 47 values (Table 1) in the same reference frame as previous theoretical studies 13,14,17 …”

“…Isotopic fractionation associated with the initial stages of the carbonate and acid reaction was discussed by Pramanik et al, 17 based on the protonation of the calcite dimer, where post‐digestion isotopic exchanges are avoided, and this provided a new way to explain the experimental observations in the carbonate–acid digestion reaction using the common acid bath (CAB) method where the post‐digestion isotopic exchange is limited.…”

“…To study the acid fractionation of dolomite, we apply a reaction mechanism where the acid digestion reaction is initiated by the protonation of the oxygen atoms 17–19 in the CO 3 2− anion of the dolomite dimer 19,20 structure, and proceeds through the transition state structure along its reaction coordinate. The transition state structure obtained in this study contains the cations of the carbonates and thus it directly affects the vibrational frequencies and imaginary frequency of the transition state, unlike previous theoretical studies 13,14 .…”