Oxygen reservoirs in the early solar nebula inferred from an allende CAI

Ultraviolet laser microprobe analyses of a calcium-aluminum-rich inclusion (CAI) from the Allende meteorite suggest that a line with a slope of exactly 1.00 on a plot of delta17O against delta18O represents the primitive oxygen isotope reservoir of the early solar nebula. Most meteorites are enriched in 17O and 18O relative to this line, and their oxygen isotope ratios can be explained by mass fractionation or isotope exchange initiating from the primitive reservoir. These data establish a link between the oxygen isotopic composition of the abundant ordinary chondrites and the primitive 16O-rich component of CAIs.     

Oxygen isotope variations at the margin of a CAI records circulation within the solar nebula

Micrometer-scale analyses of a calcium-, aluminum-rich inclusion (CAI) and the characteristic mineral bands mantling the CAI reveal that the outer parts of this primitive object have a large range of oxygen isotope compositions. The variations are systematic; the relative abundance of (16)O first decreases toward the CAI margin, approaching a planetary-like isotopic composition, then shifts to extremely (16)O-rich compositions through the surrounding rim. The variability implies that CAIs probably formed from several oxygen reservoirs. The observations support early and short-lived fluctuations of the environment in which CAIs formed, either because of transport of the CAIs themselves to distinct regions of the solar nebula or because of varying gas composition near the proto-Sun.     

Existence of an 16O-rich gaseous reservoir in the solar nebula

Carbonaceous chondrite condensate olivine grains from two distinct petrographic settings, calcium-aluminum-rich inclusion (CAI) accretionary rims and amoeboid olivine aggregates (AOAs), are oxygen-16 (16O) enriched at the level previously observed inside CAIs. This requires that the gas in the nebular region where these grains condensed was 16O-rich. This contrasts with an 16O-poor gas present during the formation of chondrules, suggesting that CAIs and AOAs formed in a spatially restricted region of the solar nebula containing 16O-rich gas. The 16O-rich gas composition may have resulted either from mass-independent isotopic chemistry or from evaporation of regions with enhanced dust/gas ratios, possibly in an X-wind environment near the young Sun.     

Oxygen isotope ratios in eclogites from kimberlites

The oxygen isotope compositions (delta(18)O) of eclogitic xenoliths from the Roberts Victor kimberlite range from 2 to 8 per mil relative to SMOW (standard mean ocean water). This surprising variation appears to be due to fractional crystallization: the eclogites rich in oxygen-18 represent early crystal accumulates; the eclogites poor in oxygen-18 represent residual liquids. Crystal-melt partitioning probably exceeded 3 per mil and is interpreted to be pressure-dependent. Anomalous enrichment of oxygen-18 in cumulate eclogites relative to ultramafic xenoliths suggests that crystal-melt partitioning increased after melt-formation but prior to crystallization.     

Short-lived oxygen diffusion during hot, deep-seated meteoric alteration of anorthosite

Heterogeneous oxygen isotope compositions of plagioclase from the Boehls Butte anorthosite include some of the most oxygen-18-depleted values (to -16 per mil) reported for plagioclase in meta-igneous rocks and indicate high-temperature (T > 500 degrees C) isotopic exchange between plagioclase and nearly pristine meteoric fluid. Retrograde reaction-enhanced permeability assisted influx of meteoric-hydrothermal fluids into the deep-seated anorthosite. Isotopic gradients of about 14 per mil over 600 micrometers in single crystals require short-lived (about 10(4) years) diffusional exchange of oxygen and locally large effective water:rock ratios, followed by rapid loss of water and cessation of oxygen diffusion in the anorthosite.     

Oxygen and carbon isotopic growth record in a reef coral from the Florida keys and a deep-sea coral from blake plateau

Carbon and oxygen isotope analysis through a 30-year (1944 to 1974) growth of Montastrea annularis from Hen and Chickens Reef (Florida Keys) shows a strong yearly variation in the abundances of both carbon-13 and oxygen-18 and a broad inverse relationship between the two isotopes. Normal annual dense bands are formed during the summer and are characterized by heavy carbon and light oxygen. "Stress bands" are formed during particularly severe winters and are characterized by heavy carbon and heavy oxygen. The isotopic effect of Zooxanthellae metabolism dominates the temperature effect on the oxygen-18/oxygen-16 ratio. The isotopic results on the deep-sea solitary coral Bathypsammia tintinnabulum, where Zooxanthellae are nonexistent, indicates that the abundance of the heavy isotopes carbon-13 and oxygen-18 is inversely related to the growth rate, with both carbon and oxygen approaching equilibrium values with increasing skeletal age.     

Chondrulelike objects in short-period comet 81P/Wild 2

The Stardust spacecraft returned cometary samples that contain crystalline material, but the origin of the material is not yet well understood. We found four crystalline particles from comet 81P/Wild 2 that were apparently formed by flash-melting at a high temperature and are texturally, mineralogically, and compositionally similar to chondrules. Chondrules are submillimeter particles that dominate chondrites and are believed to have formed in the inner solar nebula. The comet particles show oxygen isotope compositions similar to chondrules in carbonaceous chondrites that compose the middle-to-outer asteroid belt. The presence of the chondrulelike objects in the comet suggests that chondrules have been transported out to the cold outer solar nebula and spread widely over the early solar system.     

Isotopic Evidence on the Early Life History of Nautilus pompilius (Linne)

The ratios of oxygen-18 to oxygen-16 and of carbon-13 to carbon-12 in the early shell and septa of two Nautilus specimens are interpreted for the early ontogeny of the animals. Changes in the carbon isotope content are tentatively correlated with the end of the embryonic period and with environmental changes. Variations in the oxygen isotope content are in part ascribed to migrations from warm to cooler water after a certain stage of development. The size of the body chamber of the young animals can be determined by comparison of the isotope contents in outer shell and septa.     

Relationship between the oxygen isotope ratios of terrestrial plant cellulose, carbon dioxide, and water

The ratios of oxygen-18 to oxygen-16 ( (18)O/(16)O) of cellulose purified from two sets of wheat plants grown under conditions similar in all respects except for a large difference in the (18)O/(16)O ratios of the carbon dioxide supplied to them differ by only a small amount. The difference in the (18)O/(16)O ratios of the cellulose is similar to that observed for the (18)O/(16)O ratios of the water present in the plants. These results indicate that the oxygen derived from carbon dioxide undergoes complete exchange with the oxygen of the water in the plant during the synthesis of cellulose and that the (18)O/(16)O ratio of the water inside the plant is the primary influence on the (18)O/(16)O ratio of cellulose in terrestrial plants.     

Oxygen-18 content of atmospheric precipitation during last 11,000 years in the Great Lakes region

Freshwater lake marl and mollusks from Indiana show variations in ratios of oxygen-18 to oxygen-16 of a few per mil during the Postglacial. For the last 9000 years these variations are about 1 per mil for properly selected lake sediments. Thus the oxygen isotope composition of atmospheric precipitation appears relatively stable over this interval, indicating stationary atmospheric circulation patterns over the Great Lakes region. A reduction in oxygen-18 content around 10,000 years ago corresponds with the climate change indicated by pollen profiles. Large systematic differences were found between carbon-13 and oxygen-18 content of marl and mollusks.     

Oxygen isotope effect in high-temperature oxide superconductors

The effect of oxygen isotope substitution on the superconducting transition temperature, T(c), has been measured for BaBi(0.25)Pb(0.75)O(3) (T(c), approximately 11 K) and Lal(1.85) Ca(0.15)CuO(4) (T(c) approximately 20 K), and is compared to the shifts observed for La(1.85)Sr(0.15)CuO(4) (T(c) approximately 37 K) and YBa(2)Cu(3)O(7) (T(c) approximately 92 K). For all four materials, the transition temperature is shifted to lower temperature upon substitution of oxygen-18 for oxygen-16. The observed shifts demonstrate that phonons are involved in the electron-pairing mechanism in these oxide superconductors.     

The oxygen isotopic composition of the Sun inferred from captured solar wind

All planetary materials sampled thus far vary in their relative abundance of the major isotope of oxygen, (16)O, such that it has not been possible to define a primordial solar system composition. We measured the oxygen isotopic composition of solar wind captured and returned to Earth by NASA's Genesis mission. Our results demonstrate that the Sun is highly enriched in (16)O relative to the Earth, Moon, Mars, and bulk meteorites. Because the solar photosphere preserves the average isotopic composition of the solar system for elements heavier than lithium, we conclude that essentially all rocky materials in the inner solar system were enriched in (17)O and (18)O, relative to (16)O, by ~7%, probably via non-mass-dependent chemistry before accretion of the first planetesimals.     

Barium isotopes in chondritic meteorites: implications for planetary reservoir models

High-precision barium isotope measurements yielded differences of up to 25 parts per million in the 137Ba/136Ba ratio and 60 parts per million in the 138Ba/136Ba ratio between chondrites and Earth. These differences probably arose from incomplete mixing of nucleosynthetic material in the solar nebula. Chondritic meteorites have a slight excess of supernova-derived material as compared to Earth, demonstrating that the solar nebula was not perfectly homogenized upon formation.     

Osmium isotope evidence for an s-process carrier in primitive chondrites

Osmium extracted from unequilibrated bulk chondrites has isotope anomalies consistent with an insoluble s-process carrier, termed Os(i) here. Osmium from metamorphosed bulk chondrites does not have isotope anomalies, implying that the Os(i) carrier was destroyed by metamorphism. The isotopic homogeneity of metamorphosed bulk chondrites is consistent with extremely effective mixing of presolar grains from varied sources in the nebula. Osmium in the Os(i) carrier is likely from nucleosynthetic sites with a neutron density about two to four times as high as that of the average solar s-process Os.     

Banded corals: changes in oceanic carbon-14 during the little ice age

Radiocarbon analyses and stable isotope measurements are presented foro recent cores of banded corals from the Florida Straits. These values provide a record of variations in the ratio of carbon-14 to carbon-12 in the dissolved inorganic carbon in the surface waters of the Gulf Stream from A.D. 1642 to 1800. An increase in the carbon-14/carbon-12 ratio of 7 per mil for coral growth during the early 1700's was most likely induced by an increase in the carbon-14/carbon-12 ratio of 20 per mil in the atmospheric carbon dioxide that occurred at about 1700. The ratios of oxygen 18 to oxygen-16 in these coral bands show a small decrease of a water temperature ( approximately 1 degrees C) during the latter part of the Little Ice Age (1700 to 1725). These results support the hypothesis that the increase in atmospheric carbon-14 at about 1700, and possibly the temperature change as well, was caused by a decrease in solar activity (Maunder sunspot minimum).     

Ancient asteroids enriched in refractory inclusions

Calcium- and aluminum-rich inclusions (CAIs) occur in all classes of chondritic meteorites and contain refractory minerals predicted to be the first condensates from the solar nebula. Near-infrared spectra of CAIs have strong 2-micrometer absorptions, attributed to iron oxide-bearing aluminous spinel. Similar absorptions are present in the telescopic spectra of several asteroids; modeling indicates that these contain approximately 30 +/- 10% CAIs (two to three times that of any meteorite). Survival of these undifferentiated, large (50- to 100-kilometer diameter) CAI-rich bodies suggests that they may have formed before the injection of radiogenic 26Al into the solar system. They have also experienced only modest post-accretionary alteration. Thus, these asteroids have higher concentrations of CAI material, appear less altered, and are more ancient than any known sample in our meteorite collection, making them prime candidates for sample return.     

Neodymium isotope evidence for a chondritic composition of the Moon

Samarium-neodymium isotope data for six lunar basalts show that the bulk Moon has a 142Nd/144Nd ratio that is indistinguishable from that of chondritic meteorites but is 20 parts per million less than most samples from Earth. The Sm/Nd formation interval of the lunar mantle from these data is 215(-21)(+23) million years after the onset of solar system condensation. Because both Earth and the Moon likely formed in the same region of the solar nebula, Earth should also have a chondritic bulk composition. In order to mass balance the Nd budget, these constraints require that a complementary reservoir with a lower 142Nd/144Nd value resides in Earth's mantle.     

Cryptic grain-scale heterogeneity of oxygen isotope ratios in metamorphic magnetite

Oxygen isotope ratios measured by ion microprobe in magnetite from granulite-facies marble of the Adirondack Mountains, New York, range from +2 to +11 per mil (standard mean ocean water) across a single grain that measures 3 millimeters by 5 millimeters. Low values are concentrated in irregular domains near the grain boundary but also occur in the grain's interior. In contrast, grains 1 millimeter in diameter that are from a second nearby sample show no significant heterogeneity, except within 10 micrometers of the grain boundary. These data, including large gradients of up to 9 per mil per 10 micrometers, provide important new constraints on the nature and origins of intragrain isotopic heterogeneity and on oxygen isotope thermometry. The differences between these magnetite grains result from contrasting mechanisms of isotope exchange with fluids after the peak of regional metamorphism. Volume diffusion of oxygen through the crystal structure of magnetite contributed to isotope exchange in the rims of small grains, but larger grains are crosscut by healed cracks that are not readily detected and that short-circuited diffusion.     

Isotopic Composition of Cellulose from C3, C4, and CAM Plants Growing Near One Another

Cellulose from plants having crassulacean acid metabolism was enriched in deuterium but not in oxygen-18 in relation to cellulose from C(3) and C(4) plants growing in the same area, indicating that the deuterium enrichment is due to isotopic fractionation during biochemical reactions rather than during evapotranspiration. Hydrogen and oxygen stable isotope ratios of cellulose from the plants in this restricted area showed more variability than that observed in samples collected across an entire continent. Biological factors appear to be as important as environmental factors in determining the isotope ratios of plant cellulose.     

Carbon isotope fractionation in the Fischer-tropsch synthesis and in meteorites

Carbon dioxide and organic compounds made by a Fischer-Tropsch reaction at 400 degrees K show a kinetic isotope fractionation of 50 to 100 per mil, similar to that observed in carbonaceous chondrites. This result supports the view that organic compounds in meteorites were produced by catalytic reactions between carbon monoxide and hydrogen in the solar nebula.