Abstract
Context.Carbonaceous chondrites have undergone alteration in their parent bodies and display oxidized secondary phases, including sulfates in CI and CM chondrites. The cause of the formation of these sulfates is yet to be determined.Aims.This study investigates the potential of endogenous radiolysis of water (i.e., radiolysis caused by radionuclides present in the rock) on the parent bodies of carbonaceous chondrites. Radiolysis may have contributed to the enhanced degree of oxidation of CI and CM chondrites, and we also examined CV chondrites as a case with no measured sulfates.Methods.We quantified the oxidants produced by radiolysis and how much of the sulfur content could be oxidized to form sulfates by this method. The amount of oxidants was calculated using a radiolytic production model developed and used for Earth and planetary applications that takes into account relevant physical parameters (water-to-rock ratio, grain density) and composition (amount of radionuclides, sulfur content).Results.For CM and CI parent bodies, even using a very favorable set of assumptions, only slightly more than 1% of the available sulfur can be oxidized into sulfates by this process, significantly below the amount of sulfates observed in these chondrites.Conclusions.Endogenous radiolysis is unlikely to have significantly contributed to the abundance of sulfate in CI and CM meteorites. The hypothesis of oxidation of sulfur by large quantities of O2accreted with primitive ice, on the other hand, is quantitatively supported by measurements of O2in comet 67P/Churyumov-Gerasimenko.
Highlights
While carbonaceous chondrites (CC) are some of the most pristine materials available for studying of the origin of our solar system, they have still undergone alteration in their parent bodies as a result of exposure to fluids (Brearley 2006; Krot et al 2015) or elevated temperatures (Huss et al 2006)
We further explored the effect of an early onset of alteration, and find it is possible for the CM chondrites to match the observed 16% fSO4 if the alteration starts at 0.1 Myr after calcium-aluminum inclusions (CAI), an unrealistically early time (Jilly et al 2014)
We showed that for a variety of scenarios, endogenous radiolysis can only deliver a very small contribution to the oxidation of the parent bodies of carbonaceous chondrites, as indicated by the very modest amount of sulfates that could be created through this process
Summary
While carbonaceous chondrites (CC) are some of the most pristine materials available for studying of the origin of our solar system, they have still undergone alteration in their parent bodies as a result of exposure to fluids (Brearley 2006; Krot et al 2015) or elevated temperatures (Huss et al 2006). Oxygen isotopic ratios in CM sulfates point to a pre-terrestrial origin via aqueous processes (Airieau et al 2005). Reports in the literature change through time with regard to the presence of sulfate veins in the CI chondrite falls, suggesting that hypothetical pre-terrestrial sulfates present on the parent body were mobilized and altered once in terrestrial environments (Gounelle & Zolensky 2001). This interpretation is consistent with their oxygen isotopic composition, with a ∆17O that matches terrestrial values (Airieau et al 2005). Formation of sulfates by disproportionation of elemental sulfur with water is unlikely; experiments indicate it would increase the δ34S
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