The oxygen isotope effect in the earliest processed solids in the solar system: is it a chemical mass-independent process?

Abstract

Aims. An anomalous effect in the abundances of oxygen isotopes in the most refractory calcium-aluminum-rich inclusions (CAIs) was discovered some thirty years ago. The origin of these oxygen isotopic anomalies has hitherto remained unexplained. The origin is neither nuclear, nor has the recent photochemical self-shielding explanation been proven to be valid. We discuss a possible chemical mechanism to resolve these observed effects. Methods. By uniting the most recent laboratory observations of nanoclusters of silicates in beams and the first principles theoretical studies of their structure and properties with a major dynamical constraint recently described as the surface non-RRKM effect during SiO 2 formation on the growing grain, we show that the origin of the anomalous isotopic effect in high-temperature minerals in CAIs is chemical and strictly mass-independent. Results. We report that the surface non-RRKM effect would represent a major process in the formation of our own solar system and observable protoplanetary accretion disks, and the mass-independent isotope effects are directly associated with the formation of primary grains in the high temperature nebular environment. We expect that this chemical reaction mechanistic approach combined with future time-resolved studies on the kinetics of growth of silicates and a precise knowledge of the oxygen isotopic abundances of the sun would provide a very detailed understanding of the origins of formation of our solar system.

Mass-independent fractionation of oxygen isotopes in the mesostasis of a chondrule from the Semarkona LL3.0 ordinary chondrite

“…6. This scenario requires the hypothesis that the chemical reactions within the molten chondrule can mass-independently fractionate oxygen isotopes in an analogous way to known mechanisms discussed by Thiemens et al (1996), Thiemens (2006), Kimura et al (2007) and Ali and Nuth (2007). (d) 16 O-rich precursors are melted by a flash-heating event and cooled to produce phenocrysts, without any change in isotopic composition but with the lost of volatiles and reduction.…”

“…There is now good experimental evidence that the formation of solids can involve mass-independent fractionation of oxygen isotopes (Thiemens, 2006;Ali and Nuth, 2007;Kimura et al, 2007) so the formation of the chondrule could have induced mass-independent fractionation in its components with subsequent recondensation of some of the material bringing back more 16 O-enriched and volatile-enriched composition to the outer part of the chondrule. However, it is still difficult to envisage how the olivines acquired their 16 O-enriched composition if they were formed by crystallization from a melt, which was 16 O-poor, having acquired that composition from the chondrule-forming process.…”

“…Ways around this were proposed by considering photoionization of CO above the plane of the protosolar accretion disc (Young and Lyons, 2003;Lyons and Young, 2005;Young, 2007) or moving the self-shielding photochemical mechanisms to the progenitor molecular cloud (Yurimoto and Kuramoto, 2004). The self-shielding mechanism is appealing in that it has been observed to cause variable enhancements of the CO isotopomers in molecular clouds but the details of the mechanism and its possible applicability to the observation of mass-independent fractionation of oxygen isotopes in the early solar system is still vigorously debated (Krot et al, 2006;Ali and Nuth, 2007;Gounelle and Meibom, 2007;Chakraborty et al, 2008;Clayton, 2008;Davis et al, 2008;Lyons et al, 2008;Young et al, 2008;Yurimoto et al, 2008).…”

“…An alternative possibility is that the mass-independent fractionation occurred as the result of symmetry-selective chemistry or photochemical effects similar to processes that occur in the upper atmosphere and which can be reproduced in the laboratory (Thiemens, 1996(Thiemens, , 1999(Thiemens, , 2006Chakraborty and Bhattacharya, 2003;Romero and Thiemens, 2003;Savarino et al, 2003;Ali and Nuth, 2007;Kimura et al, 2007). This isotopic fractionation mechanism relies upon symmetry in molecules or density of quantum states being different for different isotopologues.…”

“…Rates of reaction for symmetric molecules such as 16 O and this provides the mechanism for mass-independent fractionation. Although much of the early research done upon such systems concentrated upon ozone and gas phase reactions, it has recently been shown that the chemical mass-independent fractionation effect can occur in a range of molecules and for reactions on solid surfaces (Marcus, 2004;Thiemens, 2006;Ali and Nuth, 2007;Kimura et al, 2007) and that (apart from water vapor), the oxygen in most atmospheric gases are fractionated mass-independently (Thiemens, 2006). Young et al (2008) state that the chemical mass-independent fractionation effect is attractive because of its simplicity but were unclear as to how the isotopic signature of the gas could be trapped in the solid and there is a lack of experimental evidence that such mechanisms can have significantly contributed to forming wide-spread mass-independent fractionation of oxygen isotopes in the early solar system.…”