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The short-lived radionuclide aluminium-26 (26Al) isotope is a major heat source for early planetary melting. The aluminium-26 – magnesium-26 (26Al-26Mg) decay system also serves as a high-resolution relative chronometer. In both cases, however, it is critical to establish whether 26Al was homogeneously or heterogeneously distributed throughout the solar nebula. Here we report a precise lead-207 – lead-206 (207Pb-206Pb) isotopic age of 4565.56 ± 0.12 million years (Ma) for the andesitic achondrite Erg Chech 002. Our analysis, in conjunction with published 26Al-26Mg data, reveals that the initial 26Al/27Al in the source material of this achondrite was notably higher than in various other well-preserved and precisely dated achondrites. Here we demonstrate that the current data clearly indicate spatial heterogeneity of 26Al by a factor of 3-4 in the precursor molecular cloud or the protoplanetary disk of the Solar System, likely associated with the late infall of stellar materials with freshly synthesized radionuclides. The homogeneity of Aluminium-26 (Al-26) isotope distribution in the accreting solar nebula is debated. Here, the authors show that the age determination of meteorite Erg Chech 002, compared with other igneous meteorites, indicates that Al-26 was heterogeneously distributed in the early Solar System. [ABSTRACT FROM AUTHOR]

The trapped melt fractional crystallization model for the IIIAB iron meteorites put forward by J. T. Wasson two decades prior is revisited. The basic precepts upon which the model was based remain true, and the model can be implemented using Ir and Au solid/liquid distribution coefficients that are broadly consistent with experimental data. For this reason, the difference between the Wasson model and some more recent trapped melt models lies mainly with inferences about the S concentrations of the core of the IIIAB iron meteorite parent body. For the Wasson model, S bulk concentrations of about 2 wt% are implied. For the more recent model, much greater concentrations of between about 12–15 wt% are indicated. The two different trapped melt models profoundly influence the interpretation of high δ57Fe values relative to chondrites in the IIIAB irons. The Wasson model suggests that there should be more variations in δ57Fe than are observed among these meteorites, while the more recent trapped melt model relies on the crystallization of FeS from the trapped melt to raise the δ57Fe of the latter, thus minimizing the variability. The interpretation of Fe isotope ratios in the IIIAB meteorites therefore depends critically on the S concentration of the parent body core. [ABSTRACT FROM AUTHOR]

High-temperature partitioning of the stable isotopes of rock-forming elements like Mg, Si, Fe, Ni and others are useful new tools in geochemistry and cosmochemistry. Understanding the fundamental driving forces for equilibrium inter-mineral fractionation comes from basic crystal chemistry and is invaluable for interpreting data from natural systems. Both charge and coordination number are key factors affecting bond length and bond stiffness and therefore the relative proclivity of a mineral phase for concentrating heavy or light isotopes. Quantitative interpretation of the plethora of new data relies on refinements of equilibrium fractionation factors through a feedback between crystal chemical reasoning, ab initio predictions, experiments, and analyses of well-characterized natural samples. This multifaceted approach is leading to a rapid rate of discovery using non-traditional stable isotopes in high temperature systems. For example, open-system mass transfer in the mantle is becoming increasingly evident from departures from equilibrium Mg and Fe isotope ratio partitioning between minerals, and differences in isotope ratios between bulk silicate Earth and meteorites are elucidating the conditions for Earth's core formation quantitatively. These applications rely critically on accurate equilibrium fractionation factors. [ABSTRACT FROM AUTHOR]

Abstract: Silicon has long been considered a possible light element in Earth''s core. If differences in 30Si/28Si ratios between metal (core) and silicate (mantle and crust) can be quantified, silicon isotopes may be used to constrain the amount of this element in the core, and in so doing elucidate the conditions that attended Earth''s differentiation. We investigate Si-isotope fractionation between metal and silicate in metal-rich enstatite meteorites as an analogue for Earth''s differentiation. We report here a 5 to 6‰ difference in the 30Si/28Si ratio between Si in metal and Si in silicate in the aubrites (enstatite achondrites) Mount Egerton and Norton County. The meteorites are believed to have derived from enstatite chondrites by melting and thermal metamorphism with final equilibration at 1200 and 1130±80K, respectively. Using the measured silicate–metal Si-isotope fractionation in these rocks we obtain a temperature dependence for fractionation of , in agreement with independent experimental and theoretical determinations. The measured silicate–metal fractionation suggests a ∼0.8‰ difference in the 30Si/28Si ratio between Earth''s core and mantle at P/T conditions relevant to core formation. Our results, based on thermodynamic calculations for Si solubility in iron-rich metal and the measured Si-isotopic silicate–metal fractionation, imply at least ∼6wt.% Si in the core (depending on the exact Δ 30SiBSE-chondrite value). The Si-isotope data also require that oxygen fugacity in the lower mantle increased during or after the process of core segregation by 1 to 2 log units. [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]

Abstract: Silicon isotope ratios of a typical CAI from the Leoville carbonaceous chondrite, obtained in situ by laser ablation MC-ICPMS, together with existing 25Mg/24Mg data, reveal a detailed picture of the astrophysical setting of CAI melting and subsequent heating. Models for the chemical and isotopic effects of evaporation of the molten CAI are used to produce a univariant relationship between P H2 and time during melting. The result shows that this CAI was molten for a cumulative time of no more than 70 days and probably less than 15 days depending on temperature. The object could have been molten for an integrated time of just a few hours if isotope ratio zoning was eliminated after melting by high subsolidus temperatures (e.g., >1300 K) for ∼500 yr. In all cases subsolidus heating sufficient to produce diffusion-limited isotope fractionation at the margin of the solidified CAI is required. These stable isotope data point to a two-stage history for this igneous CAI involving melting for a cumulative timescale of hours to months followed by subsolidus heating for years to hundreds of years. The thermobarometric history deduced from combining Si and Mg isotope ratio data implicates thermal processing in the disk, perhaps by passage through shockwaves, following melting. This study underscores the direct link between the meaning of stable isotope ratio zoning, or lack thereof, and the inferred astrophysical setting of melting and subsequent processing of CAIs. [Copyright &y& Elsevier]

Abstract: Most rocky objects in the solar system, including the primitive chondrites and the terrestrial planets themselves, formed at oxygen fugacities (f O2) near that of the Iron–Wüstite (IW) f O2 buffer. Conversely, the most ancient rocky objects of the solar system, the calcium aluminum-rich inclusions (CAIs), formed at f O2 values 5 orders of magnitude lower than the IW buffer in an environment more closely resembling a solar gas. High-resolution Mg isotope data and estimates for f O2 for rims on CAIs show that this shift from ∼solar to protoplanetary (chondritic) f O2 occurred in 100,000 to 300,000 yr for these objects. Magnesium isotopes show further that the rise in f O2 was accompanied by a rise in the partial pressure of Mg. These results establish that CAIs entered a region resembling where planet progenitors formed within 3×105 yr of their formation in the solar nebula. [Copyright &y& Elsevier]

The canonical initial [sup 26]Al/[sup 27]Al ratio of 4.5 x 10[sup-5] has been a fiducial marker for the beginning of the solar system.Laser ablation and whole-rock multiple-collector inductively coupledplasma-source mass spectrometry magnesium isotope analyses ofcalcium-and aluminum-rich inclusions (CAIs) from CV3 meteoritesdemonstrate that some CAIs had initial [sup 26]Al/[sup 27]Al values atleast 25% greater than canonical and that the canonical initial[sup 26]Al/[sup 27]Al cannot mark the beginning of solar systemformation. Using rates of Mg diffusion in minerals, we find that thecanonical initial [sup 26]Al/[sup 27]Al is instead the culmination ofthousands of brief high-temperature events incurred by CAIs during a10[sup -5]-year residence time in the solar protoplanetary disk. [ABSTRACT FROM AUTHOR]

Presents research which studied a calcium-aluminum-rich inclusion (CAI) from the Allende meteorite. Use of ultraviolet laser microprobe; Representation of the primitive oxygen isotope reservoir of the early solar nebula; Oxygen isotope ratios in most meteorites; Link between the oxygen isotopic composition of chondrites and CAIs.

Abstract We determined the Fe isotope fractionation between the metal and silicate phases of two aubrite meteorites, Norton County and Mount Egerton. We find that the metallic phase is high in 57Fe/54Fe with respect to the silicate phase, with Δ57Fe metal-silicate = 0.08‰ ± 0.04 for Mount Egerton and 0.09 ± 0.02‰ for Norton County. These data, combined with new measurements of 57Fe/54Fe of IIIAB iron meteorites, are used to constrain the origins of the high 57Fe/54Fe exhibited by all classes of iron meteorites. We find that if the parent bodies of the iron meteorites had chondritic bulk 57Fe/54Fe values, their cores must have been unusually small (≤8% by mass). Relaxing the constraint that the bodies were chondritic in their bulk iron isotope ratios allows for larger core mass fractions commensurate with usual expectations. In this case, the elevated 57Fe/54Fe values of iron meteorites are due in part to evaporation of melt during the accretion stages of the parent bodies and not solely the result of metal-silicate differentiation. [ABSTRACT FROM AUTHOR]