Oxygen Isotope Anomalies Research Articles

Carbon Dioxide and Oxygen Isotope Anomalies in the Mesosphere and Stratosphere

Isotopic (δ 17 O and δ 18 O) measurements of stratospheric and mesospheric carbon dioxide (CO 2 ) and oxygen (O 2 ), along with trace species concentrations (N 2 O, CO, and CO 2 ), were made in samples collected from a rocket-borne cryogenic whole air sampler. A large mass-independent isotopic anomaly was observed in CO 2 , which may in part derive from photochemical coupling to ozone (O 3 ). The data also require an additional isotopic fractionation process, which is presently unidentified. Mesospheric O 2 isotope ratios differed from those in the troposphere and stratosphere. The cause of this isotopic variation in O 2 is presently unknown. The inability to account for these observations represents a fundamental gap in the understanding of the O 2 chemistry in the stratosphere and mesosphere.

Determination of oxygen self-diffusion in åkermanite, anorthite, diopside, and spinel: Implications for oxygen isotopic anomalies and the thermal histories of Ca-Al-rich inclusions

Oxygen self-diffusion coefficients have been measured for three natural diopsidic clinopyroxenes, a natural anorthite, a synthetic magnesium aluminate spinel, and a synthetic åkermanite for oxygen fugacities ranging from the NNO to IW buffers. The experiments employed a gas-solid isotopic exchange technique utilizing 99% 18O-enriched CO-CO2 gas mixtures to control both the oxygen fugacity and the isotopic composition of the exchange reservoir. Diffusion profiles of the 18O tracer were obtained by in-depth analysis with an ion microprobe. The experimental results, fit to the Arrhenius relation D = D0e(−QRT), yield the following: Do (m2 s−1)Q (kJ mol−1)diopside4.3− 3.8+32.6× 10 − 4457 ± 26åkermanite4.7− 4.4+83.5× 10 − 7457 ± 26278 ± 33spinel2.2− 1.8+8.7× 10 − 7404 ± 21anorthite8.4− 8.0+174× 10 − 13162 ± 36At a given temperature, oxygen diffuses about 100 times more slowly in diopside than indicated by previous bulk-exchange experiments (Connolly and Muehlenbachs, 1988). Our data for anorthite, spinel, and åkermanite agree well with prior results obtained by gas-solid exchange and depth profiling methods (Elphick et al., 1988; Reddy and Cooper, 1981; Yurimoto et al., 1989, respectively). Since these other experiments were conducted at different oxygen fugacities, this agreement indicates that diffusion of oxygen in these nominally Fe-free minerals is not greatly affected by fO2 in the range between pure oxygen and the iron-wüstite buffer. However, our diffusion coefficients for anorthite, melilite, and spinel are also uniformly lower than those obtained by bulk analysis of crushed powders at similar temperatures (Muehlenbachs and Kushiro, 1974; Hayashi and Muehlenbachs, 1986; Ando and Oishi, 1974).The oxygen diffusion data are used to evaluate the effects of three different types of thermal histories upon the oxygen isotopic compositions of minerals found in Type B Ca-Al-rich inclusions (CAIBs) in carbonaceous chondrites: 1.(1) gas-solid exchange during isothermal heating,2.(2) gas-solid exchange as a function of cooling rate subsequent to instantaneous heating, and3.(3) isotopic exchange with a gaseous reservoir during partial melting and recrystallization. With the assumptions that the mineral compositions within a CAIB were uniformly enriched in 16O prior to any thermal processing, that effective diffusion dimensions may be estimated from observed grain sizes, and that diffusion in diopside is similar to that in fassaitic clinopyroxene, none of the above scenarios can reproduce the relative oxygen isotopic anomalies observed in CAIBs without improbably long or unrealistically intense thermal histories relative to current theoretical models of nebular evolution. The failure of these simple models, coupled with recent observations of “disturbed” magnesium isotopic abundances and correlated petrographic features in anorthite and melilite indicative of alteration and recrystallization, suggests that the oxygen isotopic compositions of these phases may have also been modified by alteration and recrystallization possibly interspersed with multiple melting events. Because the modal abundance of spinel remains relatively constant for plausible melting scenarios, and its relatively sluggish diffusion kinetics prevent substantial equilibration, Mg-Al spinel is the most reliable indicator of the oxygen isotopic composition of precursor material which formed Type B CAIs.

High-Resolution Stable Isotope and Microfossil Record from the Gulf of Mexico

ODP Site 625B was hydraulically piston cored in the northeastern Gulf of Mexico providing an apparently complete sedimentary record for the last 5.35 my. A high-resolution chronology was derived by tuning the oxygen isotope record to variations in orbital obliquity. Ages for calcareous nannofossil and planktonic foraminiferal datums were determined. Eight of the 11 nannofossil last appearances occur during glacials or during the onset of glacial periods. Of the eight foraminiferal last appearances, half occur during glacials. Results from open-ocean records also show a preponderance of last appearances within glacial intervals for the last 2.5 my. The higher frequency of last appearances during glacial intervals may result from greater environmental stress. Late Pliocene (2.30-1.66 Ma) and early Pleistocene (1.66-0.73 Ma) oxygen isotope anomalies with magnitudes of {minus}1.4 to {minus}2.0{per thousand} indicate glacial meltwater discharge from the Mississippi River implying that an ice sheet has been present at midlatitudes in North America periodically since 2.3 Ma.

Extreme hydrogen, oxygen and carbon isotope anomalies in the pore waters and carbonates of the sediments and basalts from the Norwegian Sea: Methane and hydrogen from the mantle?

Abstract D H ratios in the pore waters of the sediments from the Norwegian Sea decrease as a function of depth to values as low as −14%. Oxygen isotope ratios in the pore waters and carbon and oxygen isotope ratios in carbonates both in the sediments and basalts are low. Extensive alteration of basalt has been given as the explanation for the low oxygen isotope ratios. Material balance calculations suggest that alteration of volcanic material and oxidation of organic matter cannot explain the hydrogen and carbon isotope anomalies. Arguments are presented suggesting that methane and hydrogen from the mantle are oxidized to carbon dioxide and water by sulfate and ferric iron in the basaltic crust to yield the low hydrogen and carbon isotope ratios.

The effect of pressure and excitation energy on the isotopic fractionation in the formation of ozone by discharge of O2

Studies were made to determine the effect of the variation of the isotopic composition of initial O_2, discharge energy increase and the pressure on the isotopic composition of the O_3 produced from molecular oxygen. An RF generator with several frequency modes, the highest one at 3 MHz was used for the excitation of O_2 molecules. The pressure of the initial O_2 gas was varied from 2 to 15 cm Hg. The isotopomeric patterns produced by this generator are moderately different from those by an RF generator of lower frequencies. We used oxygen gases of three different isotopic compositions. The slope of δ_233/δ_234 varied with composition and pressure continuously between 1.19 to -1/2, passing values of ±∞. The significance of this is that ^(17)O and ^(18)O enrichments in this reaction can take place independent of one another. All data are compatible with a model based on the anharmonic predissociation of vibrationally hot O_3. The theory has two parameters p and q which depend on the distribution of vibrational states of ozone as affected by the symmetry. Several investigations are suggested by our results, including the possible explanation for the oxygen isotope anomalies found in meteorites.

Stable isotope “anomalies” in Mediterranean Pleistocene records

Oxygen and carbon isotope records of twelve Mediterranean cores raised from west (Alboran sea) to east (off Israel), and of four Atlantic cores raised from the western side of Gibraltar and located respectively inside, below and beyond the modern Mediterranean outflow are compared to open ocean records of core 71–139C from the North Atlantic and core V19–30 from the East Pacific. Oxygen isotope stratigraphies can be established for deep Mediterranean records and the major events of the last 150 ka are easily identifiable. For instance, the five substages of stage 5 or the 18 ka event (stage 2) are identified but the 10 ka event (= Younger Dryas) is only recorded by Globigerina bulloides and benthic foraminifera, suggesting that the cooling affected only winter SST. 14C dates for the two terminations, I A and I B, of the last deglaciation which are recorded in sediment cores from areas with high sedimentation rates (Faro Ridge, Alboran Sea, Sicilian Strait), are in good agreement with the dates published for the same termination in Atlantic cores. An intermediate step, however, has been identified in some cores at 14 ka and divides termination I A into two phases. Oxygen isotope anomalies are recorded by the shallow water species Globigerinoides ruber. They correspond to (1) a higher amplitude glacial-interglacial change in δ 18 O values which exceeds that of the open ocean by 1‰ in the western basin and by 1.7‰ in the eastern basin; (2) large isotopic depletions during the deposition of sapropel S 1, S 3 and S 5. They are interpreted as the result of fresh water run-off responsible for a strong dilution in the shallower layers of the east Mediterranean waters. The isotopic effect of this dilution averages −0.75‰ for sapropel S 1 and −1.6‰ for sapropel S 5. During the last glacial maximum, the Mediterranean still acted as a concentration basin and a gradient of δ 18 O values similar to the modern one ( ≅ +0.75‰) existed from west to east in the deep waters. Apparently, deep water temperatures were lower during the last glacial but the decrease did not exceed 4°C in the eastern basin. Major anomalies are also contained in the 13C records of G. bulloides and benthic foraminifera. The 13C records from surface, intermediate and deep waters of the eastern basin and from intermediate and deep waters of the western basin are opposite to that of the open ocean. During glacial time δ 13 C values were higher in intermediate and deep waters of the whole Mediterranean, and also in surface water of the eastern basin. At the same time, the 13C gradient of surface to deep waters was low. This gradient increased during the last deglaciation while δ 13 C values of G. bulloides and benthic foraminifera decreased. This decrease, interrupted at 10 ka, may have been related to increased fresh water run-off from the bordering contintents due to increased precipitation, especially in the tropical belt. The 13C depletion is clearly recorded in the Levantine and deep water masses and can be traced westward in the Mediterranean outflow. Oxygen and carbon isotope anomalies recorded simultaneously in the east Mediterranean by the shallow water species Globigerinoides ruber during sapropel deposition indicate that increased precipitation on the bordering continents and strong fresh water run-off during the time of the last deglaciation, may have caused a density stratification and deep stagnation in the eastern basin. Uncertainties arise from the fact that, in the mixing model, deglacial Mediterranean waters are mixed with continental fresh waters, the isotopic characteristics of which are not entirely understood. Although increased surface water production of biogenic material can not be excluded as a possible cause of 13C depletion, it appears that the river discharge including the Nile, as well as Black Sea input, are also plausible sources for the fresh water layering.

Oxygen isotope, aeromagnetic, and gravity anomalies associated with hydrothermally altered zones in the Yankee Fork mining district, Custer County, Idaho

Epithermal Ag-Au vein and disseminated deposits in the Yankee Fork district are hosted in altered volcanic rocks having low delta 18 O values, low magnetic susceptibilities, low remanent magnetizations, and relatively high densities. These isotopic and physical quantities provide sensitive indices of rock alteration that can be contoured over areas that are much larger than zones of strong visible alteration. Such contour maps reveal a marked spatial association between the ore deposits, the steepest delta 18 O gradients, and the perimeter of zones where the rocks have extremely low total magnetizations. The oxygen isotope results indicate that the ore zones coincide with steep gradients in temperature and water/rock ratio in two fossil hydrothermal convective systems that involved fluids derived from Tertiary meteoric waters. Our magnetic measurements suggest that these same high-gradient zones also coincide with oxidation-reduction boundaries. Low aeromagnetic intensities and positive Bouguer anomalies are also associated with the altered zones in the Yankee Fork district. The intensities of these geophysical features are consistent with the size of the altered zones and with the contrast in physical properties between altered and unaltered rocks. Contour maps of the magnetic and isotopic properties of host rocks should find application in exploration for a variety of types of hydrothermal ore deposits, as well as in the study of ore controls and conditions of mineralization.

The last deglaciation in Orca Basin, Gulf of Mexico: High-resolution planktonic foraminiferal changes

Anoxic sediments in Orca Basin, northern Gulf of Mexico (water depth 2400 m) provide late Quaternary planktonic foraminiferal and paleoclimatic records of remarkably high resolution (∼ 250 yrs). Black, organic-rich, strongly laminated, unbioturbated muds of Orca Basin contrast with grayish, organic-poor, bioturbated sediments immediately outside of the basin. Planktonic foraminiferal assemblages contained within totally anoxic sediments exhibit superb preservation and probably have not been changed much, by dissolution or other processes, from the original living assemblage. A quantitative comparison of surface-sediment assemblages (> 100 μm and > 175 μm size fractions) from the anoxic area and from the oxygenated regime immediately outside the basin, nevertheless has revealed no significant differences between the assemblages. It is inferred, therefore, that the assemblages well above the lysocline in the northern Gulf have been subjected to little postmortem alteration, even though their preservation does not match those from the Orca Basin. Planktonic foraminiferal assemblages have been counted in two piston cores from Orca Basin that are as old as 29 kyrs B.P., and which contain continuous, high-resolution records of the last deglaciation. The location of these cores is 290 kmS of the modern Mississippi Delta and is known to have been within the influence of meltwater discharge from the Mississippi River system during the last deglaciation of the Laurentide ice sheet, an event which significantly decreased surface-water salinities in the Gulf of Mexico. This meltwater effect is strongly recorded in the Orca Basin sequence. Faunal and stable-isotope changes have been evaluated in relation to the Pleistocene-Holocene transition and are associated with two major processes related to deglaciation: (1) the well-known glacial-to-interglacial surface-water changes as cool Gulf waters were replaced by warmer water masses; and (2) severe open-ocean surface-water salinity decrease resulting from the meltwater discharge into the Gulf between about 16.5 and 12 kyrs ago. Species associations were dynamically changing throughout the late Quaternary in the northern Gulf in response to oceanic and continental influences. Factor analysis of the down-core assemblages reveal three major faunal groupings which account for 95% of the total variance. Factor one is by far dominated by Globigerinoides ruber which shows highest values closely associated with a major negative oxygen isotopic anomaly that resulted from the deglacial meltwater discharge to the Gulf. A severe surface-water salinity decrease created a harsh environment that was not favorable to the successful proliferation of most other planktonic foraminiferal species. This low-salinity fauna interferes with the normal faunal transition observed elsewhere between the latest Pleistocene and Holocene. Factor two is a warm-water (interglacial) assemblage dominated by Globorotalia menardii, Pulleniatina obliquiloculata and Neogloboquadrina dutertrei. Factor three is a cool-water (glacial) assemblage dominated by Globorotalia inflata, Globorotalia crassaformis and Globigerinella aequilateralis. In the sequence, the glacial assemblage is replaced about 16.5 kyrs ago by the low-salinity assemblage which itself persisted until about 12 kyrs ago when the meltwater ceased to flow into the Gulf and was replaced briefly (for 1.5 kyrs) by a return of the glacial assemblage until about 10.5 kyrs ago. This cool interval seems to be correlative with the Younger Dryas cold episode of Europe, and with the cold interval between Termination 1A and 1B in north Atlantic cores. After this, the warm-water (interglacial) assemblage migrated into the Gulf. Early Holocene (10.5-5 kyrs ago) assemblages are quantitatively distinct from late Holocene (5-0 kyrs ago) assemblages. Unlike previously documented associations, G. ruber rather than N. dutertrei, dominated during intervals of low surface-water salinity in the Orca Basin. We believe that surface-water masses were still too cool during the meltwater spike and salinities were too low for the proliferation of N. dutertrei. On the other hand G. ruber, a more euryhaline opportunistic species, survived the severe conditions, but did not prosper. N. dutertrei distinctly increased to moderate frequencies in the second half of the meltwater spike probably as surface waters warmed during deglaciation. In cores more distant from the Mississippi River outfall region, where oxygen isotopic evidence indicates the occurrence of less severe salinity reductions, N. dutertrei exhibits a distinct frequency peak, and G. ruber is less dominant. Species frequency oscillations provide intercore correlations at a resolution of only several hundred years.

Self-shielding in O 2—a possible explanation for oxygen isotopic anomalies in meteorites?

It is known that self-shielding of UV radiation by 16O 2 produces selective isotopic effects upon photodissociation. An analysis of this process and the chemical kinetics of isotopic exchange and trapping of O has been carried out for different gas compositions and temperatures. Calculations of the shift in 16O, 17O, 18O abundances due to self-shielding were carried out, and it is shown that the specific rates of photodissociation J i give J 17/ J 18 ∼ 1 for column densities of O 2 between ∼ 10 19 and 10 22 molecules/cm 2 with a maximum value of J 17/ J 16 = 11 at a column density of ∼ 10 21 molecules/cm 2. The O produced in this manner would have 17O/ 18O close to the starting material but be depleted in 16O. These large shifts are quenched at temperatures higher than 500 K, or if water is present in higher concentration than O 2. Following dissociation, fast isotopic exchange reactions (e.g. 17O + 16O 2 → 16O + 17O 16O) may destroy the effect unless the atomic oxygen is efficiently trapped. Competition between trapping and exchange is shown to be of a simple mathematical form which depends on the ratio of the rate of trapping of O to the rate of exchange. Using known rate constants it is shown that trapping of anomalous O by metal atoms, hydrogen, or the O 2 itself is inefficient at pressures lower than 10 −3 atm and normal solar abundances. Trapping on dust grains may be efficient if all oxides are present as sub-micron particles. However, the extinction of radiation by the dust is likely to quench the self-shielding effect itself. Self-shielding of radiation by O 2, may, under special conditions, lead to the production of isotopically anomalous products. However, in the light of the difficulties throughout many stages of the process, it appears to be an unsatisfactory explanation for the oxygen anomalies observed in meteorites. The above considerations are also applied to two experiments where O is trapped by O 2 to form ozone. The isotopic shifts found in experiments by Sander et al. [10] in O 3 are compatible with the criteria developed here. The shifts found by Thiemens and Heidenreich [11,12] using a discharge cannot be explained by self-shielding of UV radiation as the pressure is below the minimum needed for self-shielding to occur. The general nature of the pressure dependence of recent discharge experiments by Thiemens and others is in agreement with the prediction of a simple kinetic model based on the production of isotopically anomalous O during dissociation of O 2, isotopic exchange of O and O 2, and ozone formation. The observed isotopic effects in O 3 produced and trapped during discharge in O 2 thus appear to be understood phenomenologically but the mechanism for selective isotopic dissociation of O 2 is still obscure and remains a fundamental problem. Our analysis is directed toward O 2-rich environments and may only have limited applicability in the environment of the solar nebula. However, the general approach presented here is useful and may be applied to CO. It appears that selective isotopic shielding, or the mechanism which operates in the discharge experiments may still provide an explanation for some isotopic effects observed in meteorites but only if rapid trapping and isolation mechanisms (relative to exchange) can be found which apply in a low-pressure regime.

The last deglaciation in orca basin, gulf of Mexico: High-resolution planktonic foraminiferal changes

Anoxic sediments in Orca Basin, northern Gulf of Mexico (water depth 2400 m) provide late Quaternary planktonic foraminiferal and paleoclimatic records of remarkably high resolution (~250 yrs). Black, organic-rich, strongly laminated, unbioturbated muds of Orca Basin contrast with grayish, organic-poor, bioturbated sediments immediately outside of the basin. Planktonic foraminiferal assemblages contained within totally anoxic sediments exhibit superb preservation and probably have not been changed much, by dissolution or other processes, from the original living assemblage. A quantitative comparison of surface-sediment assemblages (>100 μ m and > 175 μ m size fractions) from the anoxic area and from the oxygenated regime immediately outside the basin, nevertheless has revealed no significant differences between the assemblages. It is inferred, therefore, that the assemblages well above the lysocline in the northern Gulf have been subjected to little postmortem alteration, even though their preservation does not match those from the Orca Basin. Planktonic foraminiferal assemblages have been counted in two piston cores from Orca Basin that are as old as 29 kyrs B.P., and which contain continuous, high-resolution records of the last deglaciation. The location of these cores is 290 km S of the modern Mississippi Delta and is known to have been within the influence of meltwater discharge from the Mississippi River system during the last deglaciation of the Laurentide ice sheet, an event which significantly decreased surface-water salinities in the Gulf of Mexico. This meltwater effect is strongly recorded in the Orca Basin sequence. Faunal and stable-isotope changes have been evaluated in relation to the Pleistocene—Holocene transition and are associated with two major processes related to deglaciation: (1) the well-known glacial-to-interglacial surface-water changes as cool Gulf waters were replaced by warmer water masses; and (2) severe open-ocean surface-water salinity decrease resulting from the meltwater discharge into the Gulf between about 16.5 and 12 kyrs ago. Species associations were dynamically changing throughout the late Quaternary in the northern Gulf in response to oceanic and continental influences. Factor analysis of the down-core assemblages reveal three major faunal groupings which account for 95% of the total variance. Factor one is by far dominated by Globigerinoides ruber which shows highest values closely associated with a major negative oxygen isotopic anomaly that resulted from the deglacial meltwater discharge to the Gulf. A severe surface-water salinity decrease created a harsh environment that was not favorable to the successful proliferation of most other planktonic foraminiferal species. This low-salinity fauna interferes with the normal faunal transition observed elsewhere between the latest Pleistocene and Holocene. Factor two is a warm-water (interglacial) assemblage dominated by Globorotalia menardii, Pulleniatina obliquiloculata and Neogloboquadrina dutertrei . Factor three is a cool-water (glacial) assemblage dominated by Globorotalia inflata, Globorotalia crassaformis and Globigerinella aequilateralis . In the sequence, the glacial assemblage is replaced about 16.5 kyrs ago by the low-salinity assemblage which itself persisted until about 12 kyrs ago when the meltwater ceased to flow into the Gulf and was replaced briefly (for 1.5 kyrs) by a return of the glacial assemblage until about 10.5 kyrs ago. This cool interval seems to be correlative with the Younger Dryas cold episode of Europe, and with the cold interval between Termination 1A and 1B in north Atlantic cores. After this, we warm-water (interglacial) assemblage migrated into the Gulf. Early Holocene (10.5–5 kyrs ago) assemblages are quantitatively distinct from late Holocene (5–0 kyrs ago) assemblages. Unlike previously documented associations, G. ruber rather than N. dutertrei , dominated during intervals of low surface-water salinity in the Orca Basin. We believe that surface-water masses were still too cool during the meltwater spike and salinities were too low for the proliferation of N. dutertrei . On the other hand G. ruber , a more euryhaline opportunistic species, survived the severe conditions, but did not prosper. N. dutertrei distinctly increased to moderate frequencies in the second half of the meltwater spike probably as surface waters warmed during deglaciation. In cores more distant from the Mississippi River outfall region, where oxygen isotopic evidence indicates the occurrence of less severe salinity reductions, N. dutertrei exhibits a distinct frequency peak, and G. ruber is less dominant. Species frequency oscillations provide intercore correlations at a resolution of only several hundred years.

Mass loss from the proto-sun - Formation and evolution of the solar nebula

We consider the formation and evolution of the solar nebula in the light of observations of T Tauri stars, oxygen-isotopic anomalies in meteorites, and the mass and angular momentum distribution in the present solar system. It is argued that the solar nebula formed from the mass lost by the proto-Sun. The outflow of initially partially ionized material in the presence of a strong proto-solar magnetic field would lead to the transfer of angular momentum from the central Sun to the outflowing matter. This explains the present angular momentum distribution between the Sun and the planetary system. When the outflowing matter cooled sufficiently, to less than 2000 K, approx. l0/sup 12/ cm from the Sun, the material would neutralize, and the magnetic field would then decouple from the outflowing matter. Further motion would be governed by the gravitational field of the proto-Sun, the gas pressure, and the centrifugal force. When these forces balance, the radial flow would stop, and a rotating solar nebula would form. Chemical condensation would occur in the outflowing matter when suitable pressure-temperature conditions would develop. The condensation of the refractory mineral Al/sub 2/O/sub 3/ would start at a distance of approx.2 x l0/sup 12/ cm from themore » Sun, where the pressure would be approx. 3 x l0 /sup 8/ atm, and temperature approx. l450 K. The condensation sequence of other lower temperature minerals would follow this. All the refractory minerals and iron would condense within the orbit of the planet Mercury. All the volatiles would condense before the outflowing matter crossed the asteroid region. The grains would move to the outer part of the nebula along with the outflowing gas.« less

The Blue Angel: I. The mineralogy and petrogenesis of a hibonite inclusion from the Murchison meteorite

A detailed mineralogie, chemical, and petrologic study of the Blue Angel, a relatively large (~1.5 mm) hibonite-containing inclusion from the Murchison meteorite, was performed in an attempt to understand the mechanisms of formation and modification of hibonite-rich inclusions. The Blue Angel inclusion is composed of roughly equal amounts of hibonite and calcite, with minor amounts of spinel, perovskite, diopside, and an Fe-rich silicate. The inclusion can be divided into three roughly concentric zones—a hibonite-rich core, a calcite-rich mantle, and a spinel-rich layered rim. The mineral chemistry and petrography of the Blue Angel are consistent with a three-stage formation history: 1. (1) an early stage of nebular condensation which produced the hibonite, perovskite, and spinel; 2. (2) a moderate temperature stage of aqueous alteration and metamorphism occurring on a small planetary body containing CO 2 and H 2O during which calcite was formed in the inclusion; and 3. (3) the final emplacement of the Blue Angel into its present position in Murchison. The study of the Blue Angel indicates that extensive alteration of Ca-Al-rich inclusions (CAI) may have occurred by aqueous alteration and thermal metamorphism followed by explosive mixing processes on a parent body. Such metamorphic reactions may involve formation and destruction of phases, such as melilite and diopside, which have been previously thought to be primary condensates. The mechanisms proposed for the formation and modification of the Blue Angel help to explain the secondary phases and oxygen isotope anomalies found in many CAI and eliminate the need for invoking kinetically-complicated back-reactions at very low pressures with a cool part of the solar nebula. The contribution of planetary metamorphism in the formation and alteration of CAI must be considered along with nebular processes in order to understand the formation of carbonaceous chondrites.

Periodic Freshwater Flooding and Stagnation of the Eastern Mediterranean Sea During the Late Quaternary

Major negative oxygen isotopic anomalies in planktonic foraminifera are associated with deep-sea anoxic mud layers (sapropels) deposited 9000 and 80,000 years ago in the eastern Mediterranean. The isotopic depletion in surface-dwelling foraminifera is significantly greater than in mesopelagic foraminifera. This difference in isotopic response suggests that surface-water salinities were drastically reduced during times of sapropel formation, possibly as the result of meltwater runoff from the Fennoscandian ice sheet into the eastern Mediterranean by way of the Black and Aegean seas.

Trace element distribution in mineral separates of the Allende inclusions and their genetic implications

Concentrations of the REE, Sc, Co, Fe, Zn, Ir, Na and Cr were determined by instrumental neutron activation and mass spectrometric isotope dilution analysis for mineral separates of the coarseand fine-grained types (group I and II of Martin and Mason's classification) of the Allende inclusions. These data, combined with data on mineral/liquid partition coefficients, oxygen isotope distributions and diffusion calculations, suggest the following: (1) Minerals in the coarse-grained inclusions (group I) crystallized in a closed system with respect to refractory elements. On the other hand, differences in oxygen isotope distributions among minerals preclude a totally molten stage in the history of the inclusion. Group I inclusions were formed by rapid condensation (either to liquid or solid) in a supercooled solar nebula; extrasolar pyroxene and spinel dust were included but not melted in the condensing inclusions, thus preserving their extrasolar oxygen isotope composition. REE were distributed by diffusion during the subsequent heating at subsolidus temperatures; because oxygen diffuses much more slowly at these temperatures, the oxygen isotope anomalies were preserved. (2) The fine-grained (group II) inclusions were also formed by condensation from a super-cooled nebular gas; however, REE-rich clinopyroxene and spinel were formed early and REE-poor sodalite and nepheline were formed later and mechanically mixed with clinopyroxene and spinel to form the inclusions. The REE patterns of the bulk inclusions and the mineral separates are fractionated, indicating that REE abundances in the gaseous phase were already fractionated at the time of condensation of the minerals. (3) Pre-existing Mg isotope anomalies in the coarse-grained inclusions must have been erased during the heating stage thus resetting the 26Al- 26Mg chronometer.

Correlated oxygen and magnesium isotope anomalies in Allende Inclusions, I: Oxygen

Two Ca‐Al‐rich inclusions from the Allende meteorite have been found to have undergone large mass‐fractionation of oxygen isotopes subsequent to incorporation of the nucleosynthetic 16O‐anomaly found in other Allende inclusions. The magnitude of the oxygen isotope fractionations is in constant ratio to the magnesium isotope fractionations found in the same inclusions by Wasserburg, Lee and Papanastassiou. The observations support earlier theories of the addition of supernova ejecta into the solar nebula just prior to collapse and condensation.

Neon and argon in the Allende meteorite

Trapped and cosmogenic Ne and Ar were measured in Ca,Al-rich aggregates and chondrules, mafic chondrules, and bulk and matrix samples from the Allende C3V chondritic meteorite to investigate the possible occurrence of anomalous isotopic compositions of noble gases that would correlate with oxygen or magnesium isotopic anomalies previously found in this meteorite. Large enrichments of both 22Ne and 36Ar were observed in low-temperature release fractions from several Ca,Al-rich inclusions, but the enrichments are consistent with galactic cosmic-ray production of 22Ne by spallation from sodium and 36Ar by neutron capture on chlorine. Trapped neon in matrix samples is comprised of two distinctive compositions, with ( 20Ne/ 22Ne) t equal to 8.7 ± 0.1 and 10.4 ± 1.0, that appear to correlate with the two gas-rich trace phases chromite/carbon and ‘Q’ described by Lewis et al. (1975). Several Ca,Al-rich aggregates which have high contents of the volatile elements Na, Cl, K, and Rb also contain trapped neon. However, no neon-E has been identified in any of the samples studied, including samples of several inclusions known to contain isotopically anomalous oxygen and magnesium.

Distribution of the pre-solar component in Allende and other carbonaceous chondrites

Excess 16O, 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 16O. The anhydrous silicates of C2 meteorites fall on the same line, but the hydrous silicate matrix of C2's define a mass-fractionation trend parallel to the terrestrial trend, but displaced towards higher 16O. All meteorites analyzed are isotopically heterogeneous on a sub-millimeter scale. Detailed analyses of separated phases of several Allende CaAl-rich inclusions reveal a consistent pattern of large 16O enrichments in spinel, pyroxene and sometimes olivine, and small16O enrichments in melilite, feldspathoids and grossular. The heterogeneous distribution of the 16O 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 16O. 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.