Robert N. Clayton

OXYGEN ISOTOPE FRACTIONATION IN DIVALENT METAL CARBONATES.

Equilibrium fractionation factors for the distribution of 18O between alkaline‐earth carbonates and water have been measured over the temperature range 0–500°C. The fractionation factors α can be represented by the equationsCaCO3–H2O, 1000 lnα = 2.78(106 T−2)− 3.39,SrCO3–H2O, 1000 lnα = 2.69(106 T−2)− 3.74,BaCO3–H2O, 1000 lnα = 2.57(106 T−2)− 4.73.Measurements on MnCO3, CdCO3, and PbCO3 were made at isolated temperatures. A statistical‐mechanical calculation of the isotopic partition function ratios gives reasonably good agreement with experiment. Both cationic size and mass are important in isotopic fractionation, the former predominantly in its effect on the internal vibrations of the anion, the latter in its effect on the lattice vibrations.

A Component of Primitive Nuclear Composition in Carbonaceous Meteorites

The oxygen of anhydrous, high-temperature minerals in carbonaceous meteorites is strongly depleted in the heavy stable isotopes 17O and 18O. The effect is the result of nuclear rather than chemical processes and probably results from the admixture of a component of almost pure 16O. This component may predate the solar system and may represent interstellar dust with a separate history of nucleosynthesis.

The effect of polymorphism and magnesium substitution on oxygen isotope fractionation between calcium carbonate and water

Abstract Calcite, aragonite and magnesian calcite were slowly precipitated from aqueous bicarbonate solutions, and oxygen isotope fractionation factors between the precipitate and water were measured. For calcite-water at 25°C and 0°C, 1000 ln α = 28.1 and 33.7, respectively, both values in excellent agreement with the Urey-Epstein paleotemperature scale. Aragonitewater at 25°C gave 28.7, implying a small but significant fractionation between aragonite and calcite. O18 also concentrates in magnesian calcite, relative to pure calcite precipitated under the same conditions, by 0.06%. for each mole-percent MgCO3 in calcite.

Oxygen isotope studies of achondrites

Abstract Oxygen isotope abundances provide a powerful tool for recognizing genetic relationships among meteorites. Among the differentiated achondrites, three isotopic groups are recognized: (l ) SNC (Mars), (2) Earth and Moon, and (3) HED (howardites, eucrites, diogenites). The HED group also contains the mesosiderites, main-group pallasites, and silicates from IIIAB irons. The angrites may be marginally resolvable from the HED group. Within each of these groups, internal geologic processes give rise to isotopic variations along a slope- 1 2 fractionation line, as is well known for terrestrial materials. Variations of Δ17O from one planet to another are inherited from the inhomogeneities in the solar nebula, as illustrated by the isotopic compositions of chondrites and their constituents. Among the undifferentiated achondrites, five isotopic groups are found: (1) aubrites, (2) winonaites and IAB-IIICD irons, (3) brachinites, (4) acapulcoites and lodranites, and (5) ureilites. The isotopic compositions of aubrites coincide with the Earth and Moon, and also with the enstatite chondrites. These bodies apparently were derived from a. common reservoir, the isotopic composition of which was established at the chondrule scale by nebular processes. Isotopic similarities between chondrites and achondrites are seen only for the following instances: (1) enstatite chondrites and aubrites, (2) H chondrites and HE irons, and (3) L or LL chondrites and IVA irons. The isotopic data also support the following genetic associations: (1) winonaites and IAB-IIICD irons, (2) acapulcoites and lodranites, and (3) ureilites and dark inclusions of C3 chondrites. An attempt to reconcile the whole-planet isotopic compositions of Earth, Mars, and the eucrite parent body with mixing models of their chemical compositions failed. It is not possible to satisfy both the chemical and isotopic compositions of the terrestrial planets using known primitive Solar System components.

OXYGEN ISOTOPE STUDIES OF CARBONACEOUS CHONDRITES

Abstract The carbonaceous chondrites display the widest range of oxygen isotopic composition of any meteorite group, as a consequence of the interaction of primordial isotopic reservoirs in the solar nebula. These isotopic variations can be used to identify the reservoirs and to determine conditions and loci of their interactions. We present a comprehensive set of whole-rock analyses of CV, CO, CK, CM, CR, CH, and CI chondrites, as well as selected components of some of these meteorites. A simple model is developed which describes the isotopic behavior during parent-body aqueous alteration processes. The process of thermal dehydration also produces a recognizable effect in the oxygen isotopic composition.

Oxygen isotope fractionations involving diopside, forsterite, magnetite, and calcite: Application to geothermometry

Oxygen isotope fractionations between diopside, forsterite, magnetite, and calcite have been studied experimentally at high pressures (P = 15–16 kbar) and temperatures (T = 600–1300°C) with the carbonate-exchange technique of Claytonet al. (1989). The fractionations determined for these minerals can be combined with the data of Claytonet al. (1989) to give an internally consistent set of mineralmineral fractionations of the form 1000 ln α = A × 106T−2, where the coefficient A is given in the following table: 7. Cc Ab An Di Fo Mt Qtz 0.38 0.94 1.99 2.75 3.67 6.29 Cc 0.56 1.61 2.37 3.29 5.91 Ab 1.05 1.81 2.73 5.35 An 0.76 1.68 4.30 Di 0.92 3.54 Fo 2.62 Full-size table Table options View in workspace Download as CSV The diopside-calcite and forsterite-calcite fractionations of the present study are in excellent agreement with the theoretically-derived fractionations of Kieffer (1982). Mineral-mineral fractionations obtained by the carbonate-exchange technique are also in fair agreement with those derived from hydrothermal experiments except where the fractionations involve quartz or calcite. In those cases, the results of the present study indicate that the experimentally-determined quartz-water and calcite-water fractionations are systematically too small. Application of the present calibrations to natural samples yields reasonable crystallization temperatures for volcanic rocks. In plutonic igneous rocks and granulites, however, thermometers involving magnetite indicate extensive retrograde re-equilibration. Using the quartz-pyroxene thermometer, it may be possible in favorable cases to recover high temperature data from granulites.

Oxygen-Isotope Fractionations in Systems Containing Dolomite

Oxygen-isotope fractionation was determined in the laboratory at various temperatures in the range 200°-800° C. in the following systems: dolomite-water, dolomite-calcite, dolomite-carbon dioxide, and calcite-carbon dioxide. Over the range 300°-510° C. the dolomite-water equilibrium fractionation can be represented by the empirical equation 1000 in

Measurement of O18O16 ratios of total oxygen of carbonates

a_{dolomite-water} = 3.20 \times 10^{6} T^{-2 } - 2.00