Mass-independent fractionation of oxygen isotopes during thermal decomposition of carbonates

Abstract

Nearly all chemical processes fractionate 17 O and 18 O in a massdependent way relative to 16 O, a major exception being the formation of ozone from diatomic oxygen in the presence of UV radiation or electrical discharge. Investigation of oxygen threeisotope behavior during thermal decomposition of naturally occurring carbonates of calcium and magnesium in vacuo has revealed that, surprisingly, anomalous isotopic compositions are also generated during this process. High-precision measurements of the attendant three-isotope fractionation line, and consequently the magnitude of the isotopic anomaly (⌬ 17 O), demonstrate that the slope of the line is independent of the nature of the carbonate but is controlled by empirical factors relating to the decomposition procedure. For a slope identical to that describing terrestrial silicates and waters (0.5247 ؎ 0.0007 at the 95% confidence level), solid oxides formed during carbonate pyrolysis fit a parallel line offset by ؊0.241 ؎ 0.042‰. The corresponding CO2 is characterized by a positive offset of half this magnitude, confirming the massindependent nature of the fractionation. Slow, protracted thermolysis produces a fractionation line of shallower slope (0.5198 ؎ 0.0007). These findings of a 17 O anomaly being generated from a solid, and solely by thermal means, provide a further challenge to current understanding of the nature of mass-independent isotopic fractionation. Measurements have been conducted of the oxygen tripleisotope composition of calcium oxide, magnesium oxide, and mixed calcium-magnesium oxides formed by thermal decomposition of naturally occurring terrestrial carbonates under high vacuum conditions. The rationale was originally to devise a rapid and simplified procedure, compared with existing methods (1, 2), for high-precision measurements of oxygen isotopic anomalies in meteoritic carbonates. Carbonates were pyrolyzed by using infrared laser heating, with continuous pumping to ensure the removal of CO 2 as produced. Because calcium oxide readily hydrates and also reacts with CO 2 , it was important that samples remained isolated from atmosphere after pyrolysis. For this purpose, 3-5 mg of carbonate (consisting of one or two individual grains, except in the case of fine-grained international reference materials) was thermally decomposed in the same vacuum chamber as used for subsequent fluorination of the residual solid oxide. A 25-W infrared laser, with attenuated beam, provided the heat source. For carbonate decomposition, the temperature was increased gradually from ambient, in a carefully controlled manner, ensuring that the pressure as monitored in the reaction cell did not exceed Ϸ10 Ϫ4 mbar at any stage, until evolution of gas ceased and the residual solid was incandescent. It is difficult to estimate the final temperature, but it is likely to have exceeded 1,000°C. Released volatiles were continually removed during pyrolysis, to minimize the extent of back-reaction between the respective decomposition products. Oxygen triple-isotope analysis ...

Reformulated 17O correction of mass spectrometric stable isotope measurements in carbon dioxide and a critical appraisal of historic ‘absolute’ carbon and oxygen isotope ratios

“…For these reasons, they are unsuitable for establishing a representative massdependent relationship for CO 2 . Miller et al (2002) reported mass-dependent relationships with k s values between 0.520 ± 0.002 and 0.525 ± 0.002 for thermal decomposition of calcium and magnesium carbonates (including the reference materials NBS 18 and NBS 19) to the corresponding oxide and CO 2 , which were analysed in two different laboratories. The mass-dependent fractionation lines were displaced from the origin of a ln (1 + 17 d) versus ln (1 + 18 d) plot using VSMOW as a reference.…”

“…The difference cannot be due to a calibration error of the oxygen reference gases used by the two laboratories involved in the study because their reference gases were intercalibrated (Martin F. Miller, private communication, 2007). Miller et al (2002) attributed the variation in k s to varying kinetic isotope effects under different experimental conditions. It is therefore not clear to what extent the observed k s values reflect natural abundance variations or variations in experimental procedures.…”

Correction for the 17O interference in δ(13C) measurements when analyzing CO2 with stable isotope mass spectrometry (IUPAC Technical Report)

“…When investigating the δ( 17 O)-δ( 18 O) relationship in carbonates by thermal decomposition, Miller et al [15] found a λ value of 0.5247 for the CO 2 as well as for the associated residues (CaO and MgO) h . The δ( 18 O) values in these experiments covered a range of about 30 ‰.…”

Mass independently fractionated sulfur components in chondrites

“…As such, it is a sensitive probe of chemical reactions. Unlike oxygen, where more than one process can produce mass independent isotopic compositions (Thiemens and Heidenreich, 1983;Thiemens, 1999;Gao and Marcus, 2001;Miller et al, 2002), mass independent sulfur is in most cases produced by photochemical reactions (Farquhar et al, 2000c(Farquhar et al, , 2001. Precise measurements of sulfur isotopes in primitive meteorites may thus uniquely serve as a potential isotopic marker of photochemical processes in the early solar nebula and enhance our understanding of the origin of isotopic anomalies for other elements, particularly oxygen where it is likely that photochemistry is important.…”