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      Magnesium and 54Cr isotope compositions of carbonaceous chondrite chondrules – Insights into early disk processes

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          Abstract

          We report on the petrology, magnesium isotopes and mass-independent 54Cr/ 52Cr compositions (μ 54Cr) of 42 chondrules from CV (Vigarano and NWA 3118) and CR (NWA 6043, NWA 801 and LAP 02342) chondrites. All sampled chondrules are classified as type IA or type IAB, have low 27Al/ 24Mg ratios (0.04–0.27) and display little or no evidence for secondary alteration processes. The CV and CR chondrules show variable 25Mg/ 24Mg and 26Mg/ 24Mg values corresponding to a range of mass-dependent fractionation of ~500 ppm (parts per million) per atomic mass unit. This mass-dependent Mg isotope fractionation is interpreted as reflecting Mg isotope heterogeneity of the chondrule precursors and not the result of secondary alteration or volatility-controlled processes during chondrule formation. The CV and CR chondrule populations studied here are characterized by systematic deficits in the mass-independent component of 26Mg (μ 26Mg*) relative to the solar value defined by CI chondrites, which we interpret as reflecting formation from precursor material with a reduced initial abundance of 26Al compared to the canonical 26Al/ 27Al of ~5 × 10 −5. Model initial 26Al/ 27Al values of CV and CR chondrules vary from (1.5 ± 4.0) × 10 −6 to (2.2 ± 0.4) × 10 −5. The CV chondrules display significant μ 54Cr variability, defining a range of compositions that is comparable to that observed for inner Solar System primitive and differentiated meteorites. In contrast, CR chondrites are characterized by a narrower range of μ 54Cr values restricted to compositions typically observed for bulk carbonaceous chondrites. Collectively, these observations suggest that the CV chondrules formed from precursors that originated in various regions of the protoplanetary disk and were then transported to the accretion region of the CV parent asteroid whereas CR chondrule predominantly formed from precursor with carbonaceous chondrite-like μ 54Cr signatures. The observed μ 54Cr variability in chondrules from CV and CR chondrites suggest that the matrix and chondrules did not necessarily formed from the same reservoir. The coupled μ 26Mg* and μ 54Cr systematics of CR chondrules establishes that these objects formed from a thermally unprocessed and 26Al-poor source reservoir distinct from most inner Solar System asteroids and planetary bodies, possibly located beyond the orbits of the gas giants. In contrast, a large fraction of the CV chondrules plot on the inner Solar System correlation line, indicating that these objects predominantly formed from thermally-processed, 26Al-bearing precursor material akin to that of inner Solar System solids, asteroids and planets.

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          Origin of Nucleosynthetic Isotope Heterogeneity in the Solar Protoplanetary Disk

          Stable-isotope variations exist among inner solar system solids, planets, and asteroids, but their importance is not understood. We report correlated, mass-independent variations of titanium-46 and titanium-50 in bulk analyses of these materials. Because titanium-46 and titanium-50 have different nucleosynthetic origins, this correlation suggests that the presolar dust inherited from the protosolar molecular cloud was well mixed when the oldest solar system solids formed, but requires a subsequent process imparting isotopic variability at the planetary scale. We infer that thermal processing of molecular cloud material, probably associated with volatile-element depletions in the inner solar system, resulted in selective destruction of thermally unstable, isotopically anomalous presolar components, producing residual isotopic heterogeneity. This implies that terrestrial planets accreted from thermally processed solids with nonsolar isotopic compositions.
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            Carbon, hydrogen and nitrogen in carbonaceous chondrites: abundances and isotopic compositions in bulk samples.

            Whole-rock samples of 25 carbonaceous chondrites were analysed for contents of C, H and N and delta 13C, delta D and delta 15N. Inhomogeneous distribution of these isotopes within individual meteorites is pronounced in several cases. Few systematic intermeteorite trends were observed; N data are suggestive of isotopic inhomogeneity in the early solar system. Several chondrites revealed unusual compositions which would repay further, more detailed study. The data are also useful for classification of carbonaceous chondrites; N abundance and isotopic compositions can differentiate existing taxonomic groups with close to 100% reliability; Al Rais and Renazzo clearly constitute a discrete "grouplet"' and there are hints that both CI and CM groups may each be divisible into two subgroups.
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              Widespread54Cr Heterogeneity in the Inner Solar System

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                Author and article information

                Journal
                9876074
                22847
                Geochim Cosmochim Acta
                Geochim. Cosmochim. Acta
                Geochimica et cosmochimica acta
                0016-7037
                5 August 2016
                15 October 2016
                15 October 2016
                : 191
                : 118-138
                Affiliations
                Centre for Star and Planet Formation, Natural History Museum of Denmark, University of Copenhagen, Øster Voldgade 5-7, DK-1350, Denmark
                Author notes
                [* ]Corresponding author. bizzarro@ 123456snm.ku.dk (M. Bizzarro).
                Article
                EMS69489
                10.1016/j.gca.2016.07.011
                4993235
                27563152
                720b049d-cb65-4b19-8e12-25acf9b1274d

                This is an open access article under the CC BY-NC-ND license ( http://creativecommons.org/licenses/by-nc-nd/4.0/).

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                Geosciences
                carbonaceous chondrites,chondrules,magnesium,chromium,isotopes,protoplanetary disk,mass transport

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