We present a comprehensive description of petrologic, chemical and spectroscopic features of thermally metamorphosed CI-like and CM (and CM-like) chondrites. Only two such CI chondrites have so far been discovered i.e. Y-86029 and Y-82162. Thermal metamorphism in these chondrites is apparent in their low contents of H2O, C and the most thermally labile trace elements, partial dehydration of matrix phyllosilicates and abundance of thermally decomposed Ca-Mg-Fe-Mn carbonates, which apparently resulted from heating of Mg-Fe carbonate precursors.The CM chondrites exhibit a wide range of aqueous and thermal alteration characteristics. This alteration was almost complete in Y-86720 and Y-86789, which also escaped alternating episodes of oxidation and sulfidization experienced by the others. Thermal metamorphism in the CM chondrites is apparent in loss of thermally labile trace elements and also in partial to almost complete dehydration of matrix phyllosilicates: heating was less uniform in them than in CI chondrites. This dehydration is also evident in strength and shapes of integrated intensities of the 3 mu m bands except in PCA 91008, which experienced extensive terrestrial weathering. Tochilinite is absent in all but Y-793321 probably due to heating. Textural evidence for thermal metamorphism is conspicuous in blurring or integration/fusion of chondrules with matrix in the more extensively heated (>= 600 degrees C) CM chondrites like PCA 91008 and B-7904. TEM and XRD analyses reveal that phyllosilicate transformation to anhydrous phases proceeds via poorly crystalline, highly desiccated and disordered 'intermediate' phases in the least and moderately heated (400-600 degrees C) carbonaceous chondrites like WIS 91600, PCA 91008 and Y-86029. These findings are significant in that they confirm that these phases occur in meteorites as well as terrestrial samples.Thermal alteration in these meteorites can be used to identify other carbonaceous chondrites that were thermally metamorphosed in their parent bodies. Combining RNAA trace element data for experimentally heated Murchison CM2 samples with petrographic and spectroscopic data, these thermally metamorphosed carbonaceous chondrites can be ordered by severity of open system heating as 400 degrees C <= Y-793321 < WIS91600 = EET90043 = A881655 < PCA91008 < B-7904 = Y-86029 < Y-82162 < Y-86720 = Y-86789 >= 700 degrees C. Nearly all heated carbonaceous chondrites discovered so far have been found in Antarctica, which is known to have sampled the flux of near-Earth material for much longer than exemplified by current falls. Published by Elsevier Ltd.

Isovolumetric replacement of euhedral and anhedral olivine by serpentine produced both centripetal and meshwork textures in the CM2 chondrites ALH 81002 and Nogoya. The compositions of these textural varieties of serpentine are uniform within narrow limits within each previously studied meteorite, independent of the composition of olivine being replaced, and different between the two meteorites. In QUE 93005 (CM2), coarse olivines of widely varying compositions (Fo(<76-99)) are replaced in a texturally similar manner by compositionally uniform serpentine (Mg0.73 +/- 0.05Fe0.27 +/- 0.05)(3)Si2O5(OH)(4). The narrow compositional range of serpentine replacing coarse olivine indicates that the aqueous solution from which the serpentine formed was compositionally uniform on scales at least as large as the meteorite (similar to 2.5 cm in longest dimension).Isovolumetric textures and compositional observations constrain elemental redistribution from coarse olivine to serpentine and to surrounding phases during serpentinization. Regardless of olivine's composition, isovolumetric replacement of coarse olivines by serpentine of the observed composition released more Mg and Si from olivine than was required to form the serpentine. Excess Mg and Si released by olivine destruction and not retained in serpentine were exported from the replaced volume.Olivines with different Fa/Fo proportions contributed different amounts of Fe and Mg to the serpentine. Ferroan olivines released more Fe than required to form the serpentines replacing them, so some of the Fe released from ferroan olivine was exported from the replaced volumes. Forsteritic olivines released less Fe than required to form the serpentines replacing them, so some Fe was imported into the replaced volumes augmenting the small amount of Fe released from forsteritic olivine. In QUE 93005 Fo(83.8) is the threshold composition between Fe-exporting and Fe-importing behavior in individual olivine-serpentine pairs, which released exactly the amount of Fe required to form serpentine of the observed uniform composition. Compositions of serpentines isovolumetrically replacing olivines, and threshold olivine compositions, in QUE 93005 differ from the corresponding values in Nogoya.Solvent and solute species diffused through the serpentine between the olivine-serpentine interface and the aqueous solution outside the isovolumetrically replaced volume. In QUE 93005, some of the Fe released from ferroan olivine in excess of the amount required to form serpentine reacted with S sourced from outside the pseudomorphs to form Fe-sulfide decorating the margins of the pseudomorphs of serpentine after fayalitic olivine. Such Fe-sulfide-decorated outlines after fayalitic olivine do not occur in ALH 81002 or Nogoya, indicating different Fe and S mass transfer regimes in different CM2 chondrites. Mg, Fe, Si, and S in the aqueous solution, including the excess Mg and Si exported from all serpentine pseudomorphs after olivine of any composition, were available to be incorporated into other phases spatially separate from the pseudomorphs after olivine, including regularly interstratified serpentine-tochilinite. Serpentines that replaced coarse olivines in QUE 93005 and ALH 81002 are less magnesian than those in Nogoya, indicating that the Nogoya aqueous-alteration environment was more evolved toward Mg-rich solutions. This easily located and characterized phase assemblage may be potentially useful for characterizing clasts of varying degrees of alteration in brecciated and heterogeneous CM chondrites, and future returned samples from mineralogically similar asteroids. (C) 2014 Elsevier Ltd. All rights reserved.

Coarse (chondrule and isolated) olivine in some CM chondrites is replaced by serpentine in both centripetal and meshwork replacement textures. Locally preserved textures formed by partial replacement of coarse olivine by serpentine in the carbonaceous chondrite Nogoya (CM2) establish unique associations between each individual mass of serpentine and the specific olivine from which that serpentine formed. Electron probe microanalyses show that the composition of serpentine replacing coarse olivine is uniform throughout all analyzed volumes of Nogoya, and is independent of the composition of the olivine being replaced. If, as previously proposed, late-stage alteration fluids were Mg-rich because Fe-source minerals were depleted in earlier stages, then the uniform Mg-rich composition of the serpentine replacing large silicate grains during advanced stages of alteration may indicate diffusional homogenization of the aqueous solutions over progressively larger spatial scales, enabled by long timescales and previously proposed stagnant or slow-moving fluids.The range of olivine compositions replaced in Nogoya is even larger than previously reported from ALH 81002 (CM2). This militates against hypotheses of strong primary-mineral control on the compositions of alteration products, at least at advanced stages of alteration. The serpentine formed by olivine replacement in Nogoya is more magnesian than the counterpart serpentine replacing all anhydrous primary silicates in ALH 81002. This intermeteorite heterogeneity of replacement-serpentine composition between ALH 81002 and Nogoya indicates that the aqueous solutions in which the olivine-serpentine replacement reactions occurred were of different compositions in the two different CM parent-body volumes sampled by ALH 81002 and Nogoya. The more magnesian character of serpentines in Nogoya than in ALH 81002 indicates that the Nogoya aqueous-alteration environment was even more highly evolved toward Mg-rich solutions than the environment indicated by the composition of the serpentine in ALH 81002.Persistence of primary-silicate remnants within centripetal and meshwork serpentine indicates that either the aqueous alteration episodes in the parent-body volumes represented by individual meteorites were too short to allow complete replacement of olivine by serpentine, or one or more reactants (most likely water) were completely consumed before the coarse primary silicate was completely replaced. Seemingly incompatible arguments for and against primary-mineral control of serpentine composition during CM chondrite alteration may be reconciled by considering the different grain sizes and reaction timescales that likely existed in different textural settings. (C) 2012 Elsevier Ltd. All rights reserved.

Here we compare new experimental studies with theoretical predictions of equilibrium iron isotopic fractionation among aqueous ferric chloride complexes (Fe(H2O)(6)(3+), FeCl(H2O)(5)(2+), FeCl2(H2O)(4)(+), FeCl3 (H2O)(3), and FeCl4-), using the Fe-Cl-H2O system as a simple, easily-modeled example of the larger variety of iron-ligand compounds, such as chlorides, sulfides, simple organic acids, and siderophores. Isotopic fractionation (Fe-56/Fe-54) among naturally occuring iron-bearing species at Earth surface temperatures (up to similar to 3 parts per thousand) is usually attributed to redox effects in the environment. However, theoretical modeling of reduced isotopic partition functions among iron-bearing species in solution also predicts fractionations of similar magnitude due to non-redox changes in speciation (i.e., ligand bond strength and coordination number). In the present study, fractionations are measured in a series of low pH ([H+] = 5 M) solutions of ferric chloride (total Fe = 0.0749 mol/L) at chlorinities ranging from 0.5 to 5.0 mol/L. Advantage is taken of the unique solubility of FeCl4- in immiscible diethyl ether to create a separate spectator phase, used to monitor changing fractionation in the aqueous solution. Delta Fe-56(aq-eth) = delta Fe-56 (total Fe remaining in aqueous phase)-delta Fe-56 (FeCl4- in ether phase) is determined for each solution via MC-ICPMS analysis.Both experiments and theoretical calculations of Delta F-56(aq-eth) show a downward trend with increasing chlorinity: Delta F-56(aq-eth) is greatest at low chlorinity, where FeCl2(H2O)(4)(+) is the dominant species, and smallest at high chlorinity where FeCl3(H2O)(3) is dominant. The experimental Delta F-56(aq-eth) ranges from 0.8 parts per thousand at [Cl-] = 0.5 M to 0.0 parts per thousand at [Cl-] = 5.0 M, a decrease in aqueous-ether fractionation of 0.8 parts per thousand. This is very close to the theoretically predicted decreases in Delta F-56(aq-eth), which range from 1.0 to 0.7 parts per thousand, depending on the ab initio model.The rate of isotopic exchange and attainment of equilibrium are shown using spiked reversal experiments in conjunction with the two-phase aqueous-ether system. Equilibrium under the experimental conditions is established within 30 min.The general agreement between theoretical predictions and experimental results points to substantial equilibrium isotopic fractionation among aqueous ferric chloride complexes and a decrease in Fe-56/Fe-54 as the Cl-/Fe3+ ion ratio increases. The effects on isotopic fractionation shown by the modeling of this simple iron-ligand system imply that ligands present in an aqueous environment are potentially important drivers of fractionation, are indicative of possible fractionation effects due to other speciation effects (such as iron-sulfide systems or iron bonding with organic ligands), and must be considered when interpreting iron isotope fractionation in the geological record. (c) 2009 Elsevier Ltd. All rights reserved.

The magnesium isotopic composition of Calcium-, Aluminium-rich Inclusions (CAIs) and chondrules from the CBb chondrites HH237 and QUE94411 was measured using MC-ICPMS coupled with a laser ablation system.CAIs from CBb chondrites exhibit limited mass-dependent fractionation (delta Mg-25' (DSM3) < 1.3 parts per thousand) and formed with undetectable Al-26 (Al-26/Al-27 <4.6 x 10(-6)). Petrographic observations suggest that CBb CAIs are igneous. The magnesium isotopic composition of CBb igneous CAIs contrast with that of CV3 igneous CAIs which are usually mass fractionated and formed with an elevated initial abundance of Al-26. We contend that the absence of Al-26 in CAIs is due either to a late formation in the case of a stellar origin of Al-26, or to a lack of exposure to impulsive flares in the case of an irradiation origin of Al-26. In both cases, it implies that a protoplanetary disk was present similar to 4563 Ma ago, when CBb chondrites agglomerated.Chondrules have delta Mg-25' (DSM3) varying from -0.80 to 0.95 parts per thousand. A rough negative correlation is observed between the delta Mg-25' of chondrules and their Mg-24/Al-27 ratio. This correlation is attributed to evaporation rather than mixing. Contrary to CAIs, chondrules from CBb chondrites have a magnesium isotopic composition similar to that of CV3 chondrules. This last result is surprising as CBb chondrules are significantly different from CV3 chondrules in mineralogy and chemistry.If chondrules from CBb chondrites formed in an impact-related vapour plume as proposed by Krot et at. [A.N. Krot, Y. Amelin, P. Cassen and A. Meibom, Young chondrules in CB chondrites formed by a giant impact in the early Solar System, Nature 436 (2005) 989-992], our data show that physical conditions in the vapour plume were similar to those of the solar accretion disk at the time and location of the fort-nation of CV chondrules. We note that the oxygen isotopic composition of CAIs is incompatible with their remelting in the putative impact vapour plume. Alternatively, it is possible that CBb chondrules formed in a protoplanetary disk as the differences between these and "normal" CV3 chondrules can also be explained in term of spatial and temporal variations of the protoplanetary disk. We show that their young Pb-Pb age is not an argument in favour of an impact origin as protoplanetary disks can last as long as 10 Myr around protostars. If CBb chondrites formed in the solar accretion disk, we speculate they might be the last formed chondrite group. Such a hypothesis might shed light on the unique properties of CBb chondrites. (C) 2007 Elsevier B.V. All rights reserved.

AND EVIDENCE FROM MAGNESIUM ISOTOPES IN CAIS. Justin. I. Simon, Sara. S. Russel, Eric. Tonui, and Edward. D. Young , Department of Earth & Planetary Sciences, University of California Berkeley, 477 McCone Hall, Berkeley, CA 94720, (simon@eps.berkeley.edu), Department of Mineralogy, Natural History Museum, Cromwell Road London, SW7 7BD, UK, Department of Earth & Space Sciences, University of California Los Angeles, 595 Charles E. Young Drive East, 2676 Geology Building, Los Angeles, CA 90095, Institute of Geophysics and Planetary Physics, University of California Los Angeles, 595 Charles E. Young Drive East, 2676 Geology Building, Los Angeles, CA 90095.

Most rocky objects in the solar system, including the primitive chondrites and the terrestrial planets themselves, formed at oxygen fugacities (fO2) near that of the Iron–Wüstite (IW) fO2 buffer. Conversely, the most ancient rocky objects of the solar system, the calcium aluminum-rich inclusions (CAIs), formed at fO2 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 fO2 for rims on CAIs show that this shift from ∼solar to protoplanetary (chondritic) fO2 occurred in 100,000 to 300,000 yr for these objects. Magnesium isotopes show further that the rise in fO2 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.