Search

Earth and the Moon are shown here to have indistinguishable oxygen isotope ratios, with a difference in Δ'17O of -1 ± 5 parts per million (2 standard error). On the basis of these data and our new planet formation simulations that include a realistic model for primordial oxygen isotopic reservoirs, our results favor vigorous mixing during the giant impact and therefore a high-energy, high-angular-momentum impact. The results indicate that the late veneer impactors had an average D'17O within approximately 1 per mil of the terrestrial value, limiting possible sources for this late addition of mass to the Earth-Moon system. [ABSTRACT FROM AUTHOR]

Abstract: We modeled the Δ17O of atmospheric O2 using 27 ordinary differential equations comprising a box model composed of the stratosphere, troposphere, geosphere, hydrosphere and biosphere. Results show that 57% of the deficit in 17O in O2 relative to a reference water fractionation line is the result of kinetic isotope fractionation attending the Dole effect, 33% balances the positive Δ17O of O(1D) in the stratosphere, and 10% is from evapotranspiration. The predicted Δ′17O O2 relative to waters is −0.410‰ as measured at the δ18O of air. The value for Δ′17O O2 varies at fixed δ18O with the concentration of atmospheric CO2, gross primary production, and net primary production as well as with reaction rates in the stratosphere. Our model prediction is consistent with our measurements of the oxygen isotopic composition of air O2 compared with rocks if rocks define a fractionation line with an intercept in δ′17O=103ln(δ17O/103 +1) vs. δ′18O=103ln(δ18O/103 +1) space less than SMOW but more positive than some recent measurements imply. The predicted Δ17O is less negative than that obtained from recent measurements of O2 directly against SMOW. Underestimation of Δ′17O O2 can only be ameliorated if the integrated (bulk) Δ′17O for stratospheric CO2 is significantly greater than measurements currently allow. Our results underscore the need for high-precision comparisons of the 17O/16O and 18O/16O ratios of atmospheric O2, VSMOW, and rocks. [Copyright &y& Elsevier]

Abstract: Optically thin surfaces of the solar circumstellar disk were likely sites for generating 16O isotope variability in the early Solar System. Astrochemical reaction network calculations predict that a robust feature of these photoactive horizons of the disk was conversion of CO gas to 16O-poor (high Δ17O) H2O ice on a timescale of 105 yr. Within several AU of the central star ultraviolet fluxes were too great for the oxygen isotopic effects of CO photodissociation to be sequestered in H2O, meaning that the CO self shielding oxygen isotopic effect was an outer disk phenomenon. Calculations depicting transport in the circumstellar disk suggest that CO photodissociation at disk surfaces triggered a wave of high-Δ17O H2O that passed from surface regions through the outer disk and into the rocky planet-forming region on a timescale of 105 to 106 yr. [Copyright &y& Elsevier]

Iron is a ubiquitous element in terrestrial and extra-terrestrial settings and can provide clues as to the conditions during which planetary scale processes occurred. One example of this is determining the conditions accompanying metal core formation in rocky planets and planetesimals. For at least two decades there has been a growing database of experimental and natural data aimed at understanding whether iron isotopes fractionate during the separation of silicate and metal. While it has been argued that the data are not in agreement with one another, it is apparent that once criteria are established that prove equilibrated samples, there is good agreement amongst the different studies when looked at as a function of the metallic composition. Proving equilibrium is critical in these types of experiments. The three-isotope experimental technique for establishing equilibrium is found to be both mathematically and fundamentally sound. Further it is clear that the question of whether there is an equilibrium iron isotope fractionation between metal and silicate is not straightforward and that it can vary significantly as a function of temperature, pressure, metallic composition, oxygen fugacity, and silicate composition. • Iron isotope fractionation during core formation is a function of many variables. • High pressure and temperature isotope experiments are presented. • The three-isotope technique is mathematically shown to be an effective tool. [ABSTRACT FROM AUTHOR]

The abundances of molecules containing more than one rare isotope have been applied broadly to determine formation temperatures of natural materials. These applications of "clumped" isotopes rely on the assumption that isotope-exchange equilibrium is reached, or at least approached, during the formation of those materials. In a closed-system terrarium experiment, we demonstrate that biological oxygen (O2) cycling drives the clumped-isotope composition of O2 away from isotopic equilibrium. Our model of the system suggests that unique biological signatures are present in clumped isotopes of O2--and not formation temperatures. Photosynthetic O2 is depleted in 18O18O and 17O18O relative to a stochastic distribution of isotopes, unlike at equilibrium, where heavy-isotope pairs are enriched. Similar signatures may be widespread in nature, offering new tracers of biological and geochemical cycling. [ABSTRACT FROM AUTHOR]

Almahata Sitta (AhS), an anomalous polymict ureilite, is the first meteorite observed to originate from a spectrally classified asteroid (2008 TC3). However, correlating properties of the meteorite with those of the asteroid is not straightforward because the AhS stones are diverse types. Of those studied prior to this work, 70–80% are ureilites (achondrites) and 20–30% are various types of chondrites. Asteroid 2008 TC3 was a heterogeneous breccia that disintegrated in the atmosphere, with its clasts landing on Earth as individual stones and most of its mass lost. We describe AhS 91A and AhS 671, which are the first AhS stones to show contacts between ureilitic and chondritic materials and provide direct information about the structure and composition of asteroid 2008 TC3. AhS 91A and AhS 671 are friable breccias, consisting of a C1 lithology that encloses rounded to angular clasts (<10 μm to 3 mm) of olivine, pyroxenes, plagioclase, graphite, and metal‐sulfide, as well as chondrules (~130–600 μm) and chondrule fragments. The C1 material consists of fine‐grained phyllosilicates (serpentine and saponite) and amorphous material, magnetite, breunnerite, dolomite, fayalitic olivine (Fo 28‐42), an unidentified Ca‐rich silicate phase, Fe,Ni sulfides, and minor Ca‐phosphate and ilmenite. It has similarities to CI1 but shows evidence of heterogeneous thermal metamorphism. Its bulk oxygen isotope composition (δ18O = 13.53‰, δ17O = 8.93‰) is unlike that of any known chondrite, but similar to compositions of several CC‐like clasts in typical polymict ureilites. Its Cr isotope composition is unlike that of any known meteorite. The enclosed clasts and chondrules do not belong to the C1 lithology. The olivine (Fo 75‐88), pyroxenes (pigeonite of Wo ~10 and orthopyroxene of Wo ~4.6), plagioclase, graphite, and some metal‐sulfide are ureilitic, based on mineral compositions, textures, and oxygen isotope compositions, and represent at least six distinct ureilitic lithologies. The chondrules are probably derived from type 3 OC and/or CC, based on mineral and oxygen isotope compositions. Some of the metal‐sulfide clasts are derived from EC. AhS 91A and AhS 671 are plausible representatives of the bulk of the asteroid that was lost. Reflectance spectra of AhS 91A are dark (reflectance ~0.04–0.05) and relatively featureless in VNIR, and have an ~2.7 μm absorption band due to OH− in phyllosilicates. Spectral modeling, using mixtures of laboratory VNIR reflectance spectra of AhS stones to fit the F‐type spectrum of the asteroid, suggests that 2008 TC3 consisted mainly of ureilitic and AhS 91A‐like materials, with as much as 40–70% of the latter, and <10% of OC, EC, and other meteorite types. The bulk density of AhS 91A (2.35 ± 0.05 g cm−3) is lower than bulk densities of other AhS stones, and closer to estimates for the asteroid (~1.7–2.2 g cm−3). Its porosity (36%) is near the low end of estimates for the asteroid (33–50%), suggesting significant macroporosity. The textures of AhS 91A and AhS 671 (finely comminuted clasts of disparate materials intimately mixed) support formation of 2008 TC3 in a regolith environment. AhS 91A and AhS 671 could represent a volume of regolith formed when a CC‐like body impacted into already well‐gardened ureilitic + impactor‐derived debris. AhS 91A bulk samples do not show a solar wind component, so they represent subsurface layers. AhS 91A has a lower cosmic ray exposure (CRE) age (~5–9 Ma) than previously studied AhS stones (11–22 Ma). The spread in CRE ages argues for irradiation in a regolith environment. AhS 91A and AhS 671 show that ureilitic asteroids could have detectable ~2.7 μm absorption bands. [ABSTRACT FROM AUTHOR]

The Luliang Shan area of the North Qaidam high pressure (HP) to ultrahigh pressure (UHP) metamorphic terrane in northwestern China features thick, garnet- and phengite-rich metasomatic selvages that formed around gneiss-hosted mafic eclogite blocks during HP conditions. Here we present new 40 Ar/ 39 Ar, δ 18 O , and δ 11 B results from a previously studied 30 m, 18 sample traverse that extends from the host gneiss into a representative eclogite block. Previous thermobarometry and new mica-quartz oxygen isotope thermometry from the traverse reveal that the phengite-rich selvage formed at temperatures similar to those recorded by the eclogites at peak pressure. Quartz and white mica δ 18 O data from the selvage cannot be explained by simple mixing of gneiss and eclogite, and indicate a fluid/rock ratio >1 during regional-scale infiltration of high δ 18 O (ca. 14‰) fluids. Heavy δ 18 O overgrowths of metamorphic zircon over lighter δ 18 O detrital grains indicate that the gneiss was similarly affected. Starkly contrasting boron content and δ 11 B compositions for the host gneiss and the selvage also cannot be explained by local-scale devolatilization of the gneiss to form the selvage. Instead, the boron systematics are best attributed to two distinct phases of fluid infiltration: (1) low-boron selvage phengite with δ 11 B from −10 to −30‰ grew under HP conditions; and (2) tourmaline and boron-rich muscovite with generally positive δ 11 B crystallized in the host gneiss under subsequent lower pressure epidote–amphibolite facies conditions as the Luliang Shan gneiss terrane was exhumed past shallower portions of the subduction channel. Consistent with observations made worldwide, we were able to identify uptake of excess argon ( 40 Ar E ) in phengite as a high pressure phenomenon. Phengite 40 Ar/ 39 Ar ages from massive eclogite exceed the ca. 490 Ma zircon U–Pb age of eclogite metamorphism by a factor of 1.5. However, phengite ages from the more permeable schistose selvage were even older, exceeding the time of eclogite formation by a factor of 1.7. In contrast, lower pressure retrograde muscovite present within the host gneiss and in discrete shear zones cutting the selvage yield 40 Ar/ 39 Ar ages that were younger than the time of HP metamorphism and consistent with regional cooling age patterns. Our observation of high 40 Ar E concentrations in phengite from schistose rocks infiltrated by regionally extensive fluids at HP conditions runs contrary to widely held expectations. Conventional wisdom dictates that low phengite/fluid partition coefficients for argon ( D phg/fluid Ar = 10 − 3 to 10 − 5 ) coupled with the dry, closed systems conditions that are widely reported to characterize HP metamorphism of continental crust explains why high concentrations of 40 Ar E partitions are able to accumulate within phengite. We alternatively propose that phengite/fluid partition coefficients for argon increase linearly with pressure to values as high as 10 − 2 to allow phengites to accumulate large amounts of 40 Ar E from aqueous fluids under HP to UHP conditions. [ABSTRACT FROM AUTHOR]

Given the global dearth of Hadean (>4 Ga) rocks, 4.4–4.0 Ga detrital zircons from Jack Hills, Narryer Gneiss Complex (Yilgarn Craton, Western Australia) constitute our best archive of early terrestrial materials. Previous Lu–Hf investigations of these zircons suggested that felsic (low Lu/Hf) crust formation began by ∼4.4 to 4.5 Ga and continued for several hundred million years with evidence of the least radiogenic Hf component persisting until at least ∼4 Ga. However, evidence for the involvement of Hadean materials in later crustal evolution is sparse, and even in the detrital Jack Hills zircon population, the most unradiogenic, ancient isotopic signals have not been definitively identified in the younger (<3.9 Ga) rock and zircon record. Here we show Lu–Hf data from <4 Ga Jack Hills detrital zircons that document a significant and previously unknown transition in Yilgarn Craton crustal evolution between 3.9 and 3.7 Ga. The zircon source region evolved largely by internal reworking through the period 4.0–3.8 Ga, and the most ancient and unradiogenic components of the crust are mostly missing from the record after ∼4 Ga. New juvenile additions to the crust at ca. 3.9–3.8 Ga are accompanied by the disappearance of unradiogenic crust ca. 3.9–3.7 Ga. Additionally, this period is also characterized by a restricted range of δ 18 O after 3.8 Ga and a shift in several zircon trace element characteristics ca. 3.9–3.6 Ga. The simultaneous loss of ancient crust accompanied by juvenile crust addition can be explained by a mechanism similar to subduction, which effects both processes on modern Earth. The oxygen isotope and trace element information, although less sensitive to tectonic setting, also supports a transition in zircon formation environment in this period. [ABSTRACT FROM AUTHOR]

Abstract: Several lines of isotopic evidence – the most direct of which is from Hadean Jack Hills zircons – suggest a very early history of crust formation on Earth that began by about 4.5Ga. To constrain both the fate of the reservoir for this crust and the nature of crustal evolution in the sediment source region of the Jack Hills, Western Australia, during the early Archean, we report here initial 176Hf/177Hf ratios and δ18O systematics for <4Ga Jack Hills zircons. In contrast to the significant number of Hadean zircons which contain highly unradiogenic 176Hf/177Hf requiring a near-zero Lu/Hf reservoir to have separated from the Earth’s mantle by 4.5Ga, Jack Hills zircons younger than ca. 3.6Ga are more radiogenic than –13ε (CHUR) at 3.4Ga in contrast to projected values at 3.4Ga of –20ε for the unradiogenic Hadean reservoir indicating that some later juvenile addition to the crust is required to explain the more radiogenic younger zircons. The shift in the Lu–Hf systematics together with a narrow range of mostly mantle-like δ18O values among the <3.6Ga zircons (in contrast to the spread towards sedimentary δ18O among Hadean samples) suggests a period of transition between 3.6 and 4Ga in which the magmatic setting of zircon formation changed and the highly unradiogenic low Lu/Hf Hadean crust ceased to be available for intracrustal reworking. Constraining the nature of this transition provides important insights into the processes of crustal reworking and recycling of the Earth’s Hadean crust as well as early Archean crustal evolution. [Copyright &y& Elsevier]