Toshiko K. Mayeda

Variation of O18 content of waters from natural sources

A number of marine water and fresh water samples were examined for the relative O18O16 ratio, and the variation of this ratio was determined to a precision of ± 0.1%. In the case of surface marine waters, for a range of salinity of 29.40%., the O16 content varies over a range of approximately 6%. The low O18O16 ratios were obtained from surface marine waters contaminated with meltwater from the ice fields, while the marine waters of high salinity were richest in O18. The observed relation between O18 content and salinity of the oceanic waters can be explained by a process of multiple stage distillation which produces a continuous loss of fresh water to the ice regions from the surface waters of the warm oceans. The lower salinities of cold ocean currents, such as the Alaskan and Californian currents, are due primarily to mixing with meltwater from cold regions. The effect of glaciation upon the isotopic method of measuring paleotemperatures is discussed. The results for deep oceanic samples and for non-typical water samples are also discussed.

The use of bromine pentafluoride in the extraction of oxygen from oxides and silicates for isotopic analysis

Abstract A technique has been developed in which bromine pentafluoride is used as a reagent for quantitative liberation of oxygen from oxides and silicates. For all of the rocks and minerals analysed, the oxygen yields are 100 ± 2 per cent of the theoretical amount. The advantage over techniques involving reduction with carbon lies in the consistently better oxygen yields, with consequent decrease in systematic errors in isotopic composition. Bromine pentafluoride has advantages over fluorine in being easier and safer to handle in the laboratory, in being readily purified, and in reacting with some minerals which do not react completely with fluorine. The results of isotopic analyses are compared with measurements made in other laboratories by other procedures.

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.

DISTRIBUTION OF THE PRE-SOLAR COMPONENT IN ALLENDE AND OTHER CARBONACEOUS CHONDRITES

Excess 16 O, relative to terrestrial abundances, has been found in all samples of C2, C3 and C4 carbonaceous chondrites which have been analyzed, amounting to nine meteorites thus far. Whole-rocks and mineral separates from all the C3 and C4 meteorites fall on a single mixing line consistent with admixture of 1–5% of excess 16 O. The anhydrous silicates of C2 meteorites fall on the same line, but the hydrous silicate matrix of C2s define a mass-fractionation trend parallel to the terrestrial trend, but displaced towards higher 16 O. All meteorites analyzed are isotopically heterogeneous on a sub-millimeter scale. Detailed analyses of separated phases of several Allende Ca Al-rich inclusions reveal a consistent pattern of large 16 O enrichments in spinel, pyroxene and sometimes olivine, and small 16 O enrichments in melilite, feldspathoids and grossular. The heterogeneous distribution of the 16 O excesses, together with their enhancement in minerals believed to be early solar nebular condensates, implies the existence of pre-solar “carriers” of the isotopic anomaly, probably grains or molecules with oxygen which was nearly pure 16 O. These carriers have not been unequivocally indentified, but pre-solar grains of corundum or spinel, and pre-solar molecules of SiO are possibilities. The isotopic anomalies may also have been disturbed by diffusion-controlled processes of exchange between early condensates and their surroundings, either in the nebula, or later in the parent body. No direct correlation has yet been observed between the oxygen isotope anomalies and recently observed isotope anomalies in neon, magnesium and xenon.

A classification of meteorites based on oxygen isotopes

Abstract On the basis of18O/16O and17O/16O ratios, meteorites and planets can be grouped into at least six categories, as follows: (1) the terrestrial group, consisting of the earth, moon, differentiated meteorites and enstatite chondrites; (2) types L and LL ordinary chondrites; (3) type H ordinary chondrites; (4) anhydrous minerals of C2, C3, C4 carbonaceous chondrites; (5) hydrous matrix minerals of C2 carbonaceous chondrites; (6) the ureilites. Objects of one category cannot be derived by fractionation or differentiation from the source materials of any other category.

The oxygen isotope record in Murchison and other carbonaceous chondrites

The ranges of δ18O and δ17O in components of the Murchison (C2) chondrite exceed those in all other meteorites analyzed. Previous authors have proposed that C2 chondrites are the products of aqueous alteration of anhydrous silicates. A model is presented to determine whether the isotopic variations can be understood in terms of such alteration processes. The minimum number (two) of initial isotopic reservoirs is assumed. Two major stages of reservoir interaction are required: one at high temperature to produce the16O-mixing line observed for the anhydrous minerals, and another at low temperature to produce the matrix minerals. The isotopic compositions severely constrain the conditions of the low-temperature process, requiring temperatures 44%. Extension of the model to the data on C1 chondrites requires aqueous alteration in a warmer, wetter environment.