John F. Kerridge

Isotopic characterisation of kerogen-like material in the Murchison carbonaceous chondrite☆

Isotopic data for C, H and N in acid-resistant residues from carbonaceous chondrites show substantial variability during stepwise pyrolysis and/or combustion. After subtraction of contributions due apparently to inorganic C grains, of probably circumstellar origin, considerable isotopic variability remains, attributable to the kerogen-like organic fraction. That variability may be interpreted in terms of three or four distinct components, based on C, H and N isotopes. The relative proportions of those components vary significantly from sample to sample. The different isotopic components are tentatively identified in terms of specific chemical/structural moieties within the kerogen-like material. This combination of chemical, structural and isotopic information suggests a complex for meteoritic organic matter. At least three components within the organic populations as a whole still carry a signature of apparently interstellar D-enrichment. Part, at least, of the interstellar carrier consisted of reactive entities, not solely polymers.

Magnetite in CI Carbonaceous Meteorites: Origin by Aqueous Activity on a Planetesimal Surface

The composition and morphology of magnetite in CI carbonaceous meteorites appear incompatible with a nebular origin. Mineralization on the meteorite parent body is a more plausible mode of formation. The iodine-xenon age of this material therefore dates an episode of secondary mineralization on a planetesimal rather than the epoch of condensation in the primitive solar nebula.

Deuterium in carbonaceous chondrites

Abstract Hydrogen isotopic compositions in seven carbonaceous chondrites lie in the range −70 to +771‰ relative to SMOW. These values decrease, to a range from −145 to +219‰, after low-temperature oxidation in an oxygen plasma. Deuterium enrichment is therefore concentrated in the organic matter, the hydrous silicates probably lying close to the terrestrial range for such material. Calculated values for δD of the organic fraction are +450 ‰ for Orgueil and Ivuna and up to +1600‰ for Renazzo. These enrichments, at least for Orgueil and Ivuna, suggest equilibration with protosolar hydrogen at very low temperatures. Assuming a value of 2.5 × 10−5 for the protosolar D/H ratio, nominal equilibration temperatures of 230°K for silicates and 180°K for organic matter may be derived.

Isotopic composition of carbonaceous-chondrite kerogen: evidence for an interstellar origin of organic matter in meteorites

Abstract Stepwise combustion has revealed systematic patterns of isotopic heterogeneity for C, H and N in the insoluble organic fraction (m-kerogen) from the Orgueil and Murray carbonaceous chondrites. Those patterns are essentially identical for both meteorites, indicating a common source of m-kerogen. The data cannot be reconciled with a single mass-fractionation process acting upon a single precursor composition. This indicates either a multi-path history of mass-dependent processing or a significant nucleogenetic contribution, or both. If mass-fractionation were the dominant process, the magnitude of the observed isotopic variability strongly suggests that ion-molecule reactions at very low temperatures, probably in interstellar clouds, were responsible. In any case, an interstellar, rather than solar nebular, origin for at least some of the meteoritic organic matter is indicated. This has interesting implications for the origin of prebiotic molecules, temperatures in the early solar system, and the isotopic compositions of volatiles accreted by the terrestrial planets.

Long‐term compositional variation in solar corpuscular radiation: Evidence from nitrogen isotopes in the lunar regolith

Implantation of solar corpuscular radiation into the lunar surface generates a population of solar atoms in the rims of lunar regolith grains. Laboratory analysis of those atoms can yield a measure of solar composition. Nitrogen trapped in the lunar regolith consists of at least two components, putatively originating in the Sun, differing in release temperature and therefore probably in implantation energy. The higher-energy component is depleted in 15N relative to the lower-energy component by amounts that range up to at least 20%. These components superficially resemble those identified previously in the solar-derived light noble gases, though with several marked differences. Thus the higher-energy noble gas components are depleted in the lighter isotope. Unlike the noble gas case, the 15N/14N ratios of both N components vary with antiquity in a complex fashion; the lower-energy component echoes the variations in the higher-energy component which dominate the isotopic evolution of the bulk samples. The magnitude of the bulk sample variation exceeds 30%; the higher-energy component varies by at least 25%. The bulk long-term trend in 15N/14N does not result from variations in mixing ratio of the two components. Both the compositional difference between the components and the long-term variations within them apparently originate in the Sun, though this conclusion is inconsistent with current understanding of solar structure and evolution. The nitrogen isotopic record therefore appears to represent a major challenge to solar physics.

Solar nitrogen: evidence for a secular increase in the ratio of nitrogen-15 to nitrogen-14.

Solar wind nitrogen, implanted in lunar soil samples, exhibits isotopic variations that are related to the time, although not to the duration, of implantation, with earlier samples characterized by lower ratios of nitrogen-15 to nitrogen-14. An increase in the solar nitrogen-15 content during the lifetime of the lunar regolith is probably caused by spallation of oxygen-16 in the surface regions of the sun.

Sulfur isotope systematics at the 21°N site, East Pacific Rise

Hydrogen sulfide in hydrothermal vent fluid at the 21°N site is enriched in 34S relative to Mid-Ocean Ridge basalts, probably by addition of H2S reduced from seawater sulfate by FeO-bearing basalt. Metalliferous sulfides are depleted in 34S relative to the fluid from which they apparently precipitated, the degree of depletion reflecting the microenvironment in which each mineral crystallised and/or kinetic effects. Isotopic compositions of coexisting sulfides in a basal mound are consistent with equilibration at around 445°C, though heating to such a high temperature seems unlikely. Similar sulfides in a black smoker and in a dead chimney are out of isotopic equilibrium at any temperature, apparently reflecting a complex series of replacement mineralisations and post-depositional oxidation, respectively.

Mafic silicates in the Orgueil carbonaceous meteorite

Abstract Iron-bearing olivines and pyroxenes occurring in Orgueil may represent a separate population distinct from the magnesian varieties previously reported. Compositions of these iron-bearing silicates are inconsistent with an origin by direct equilibrium condensation in the nebula. Such an origin is more plausible for the magnesian silicates but lacks conclusive evidence. An extra-solar system origin for either mafic population is possible though similarly lacking in evidence. About 15% of the olivines, randomly distributed with respect to iron content, retain particle track evidence of a pre-compaction irradiation.

Light element geochemistry of the Apollo 12 site

Abundances of C, N, S, and He and contents of CH4 and metallic Fe at Hadley-Apennine are typical of a mixed mare and highland environment. Isotopic compositions of C, N, and S similarly conform in most cases to patterns observed elsewhere on the moon. Systematic relationships between S abundance and total Fe content are apparent for crystalline rocks from most landing sites but their full significance is not yet understood.

Major element composition of phyllosilicates in the Orgueil carbonaceous meteorite

Abstract Phyllosilicates in Orgueil are depleted in Ca, Mn and Fe relative to the bulk meteorite. Partition of Fe and Mn was apparently established in the oxidized state and may reflect a late-stage redistribution, possibly after accretion of the parent body. Mineralogical identity of phyllosilicate species cannot be completely specified but an upper limit of 30% may be placed on the proportion of montmorillonite-like species.