^1,2Seann J. McKibbin, ¹Trevor R. Ireland, ¹Yuri Amelin, ¹Peter Holden
¹Research School of Earth Sciences, Australian National University, Canberra, ACT, Australia
²Analytical, Environmental and Geo-Chemistry (AMGC), Vrije Universiteit Brussel, Brussels, Belgium

Angrite meteorites are suitable for Mn-Cr relative dating (53Mn decays to 53Cr with a half life of 3.7 Myr) using secondary ion mass spectrometry (SIMS) because they contain olivine and kirschsteinite with very high 55Mn/52Cr ratios arising from very low Cr concentrations. Discrepant Mn-Cr and U-Pb time intervals between the extrusive or ‘quenched’ angrite D’Orbigny and some slowly cooled or ‘plutonic’ angrites suggests that some have been affected by secondary disturbances, but this seems to have occurred in quenched rather than in slow-cooled plutonic angrites, where such disturbance or delay of isotopic closure might be expected. Using SIMS, we investigate the Mn-Cr systematics of quenched angrites to higher precision than previously achieved by this method and extend our investigation to non-quenched (plutonic or sub-volcanic) angrites. High values of 3.54 (±0.18) × 10-6 and 3.40 (±0.19) × 10-6 (2-sigma) are found for the initial 53Mn/55Mn of the quenched angrites D’Orbigny and Sahara 99555, which are preserved by Cr-poor olivine and kirschsteinite. The previously reported initial 53Mn/55Mn value of D’Orbigny obtained from bulk-rock and mineral separates is slightly lower and was probably controlled by Cr-rich olivine. Results can be interpreted in terms of the diffusivity of Cr in this mineral. Very low Cr concentrations in Ca-rich olivine and kirschsteinite are probably charge balanced by Al; this substitutes for Si and likely diffuses at a very slow rate because Si is the slowest-diffusing cation in olivine. Diffusion in Cr-rich Mg-Fe olivine is probably controlled by cation vacancies because of deficiency in charge-balancing Al and is therefore more prone to disturbance. The higher initial 53Mn/55Mn found by SIMS for extrusive angrites is more likely to reflect closure of Cr in kirschsteinite at the time of crystallisation, simultaneous with closure of U-Pb and Hf-W isotope systematics for these meteorites obtained from pyroxenes. For the younger angrites Northwest Africa (NWA) 4590 and 4801 we have found initial 53Mn/55Mn values which are consistent with more precise work, at 0.90 (±0.4) × 10-6 and 0.13 (±1.1) × 10-6 respectively. Our work shows that SIMS can usefully constrain and distinguish the ages of angrites of different petrologic groups. In reviewing the petrology of angrites, we suggest that NWA 2999, 4590, and 4801 underwent a secondary partial melting and Cr (+/-Pb) disturbance event that the sub-volcanic Lewis Cliff 86010, and perhaps the plutonic Angra dos Reis, did not. With our higher initial 53Mn/55Mn for D’Orbigny and Sahara 99555 as well as previous data, a combined quenched angrite initial 53Mn/55Mn of 3.47 (±0.12) × 10-6 (2-sigma, MSWD 1.00) yields consistent Mn-Cr and U-Pb intervals between these angrites and Lewis Cliff 86010. Discrepant Mn-Cr timescales for other plutonic and sub-volcanic angrites represents resetting during the secondary partial melting event at ∼4557.2 Ma and indicates a relative order of disturbance of isotope systems: Mn-Cr in olivine before U-Pb in pyroxene, with Hf-W in pyroxene being the most resistant.

Reference
McKibbin SJ, Ireland TR, Amelin Y, Holden P (2015) Mn-Cr dating of Fe- and Ca-rich olivine from ‘quenched’ and ‘plutonic’ angrite meteorites using Secondary Ion Mass Spectrometry. Geochimica et Cosmochimica Acta (in Press)
Link to Article [doi:10.1016/j.gca.2015.02.019]

Copyright Elsevier

¹Walter S. Kiefer, ²Justin Filiberto, ³Constantin Sandu, ^1,4Qingsong Li
¹Lunar and Planetary Institute, 3600 Bay Area Blvd., Houston TX 77058
²Dept. of Geology, Southern Illinois University, Carbondale IL 62901
³Now at Chevron Energy Technology, Houston TX
⁴Now at BP, Houston TX

At a given pressure, terrestrial peridotites of varying composition may have solidus temperatures that differ by up to 100 °C. Based on meteorite evidence, the mantle of Mars is believed to be enriched in Na and K and to have a higher Fe/Mg ratio (lower magnesium number, Mg#) than Earth. These differences all favor a mantle solidus temperature on Mars that is lower than on Earth and are important in understanding the volcanic history of Mars. We parameterize the peridotite solidus at 1 and 3 GPa as a function of Mg# and total alkali content, using existing measurements of peridotite melting at 1 and 3 GPa for Mg# between 75 and 91 and total alkali content between 0.06 and 1.17 weight percent. The solidus on early Mars was likely 30-40 °C lower than on Earth, which increases the predicted crustal production by about 20% over martian history, relative to a Mars model that uses a solidus calculated for terrestrial periodotite composition. Because Na is incompatible and migrates to the crust over time, the present-day martian solidus is higher than the primitive solidus but is still ∼15 °C less than on Earth. This enhances the present-day magma production rate at martian mantle plumes by a factor of 2-3.

Reference
Kiefer WS, Filiberto J, Sandu C, Li Q (2015) The Effects of Mantle Composition on the Peridotite Solidus: Implications for the Magmatic History of Mars. Geochimica et Cosmochimica (in Press)
Link to Article [doi:10.1016/j.gca.2015.02.010]

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¹Jennifer D. Newman ¹Christopher D. K. Herd
¹Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta, Canada

The Whitecourt meteorite impact crater, Alberta, Canada is a rare example of a well-preserved small impact structure, with which thousands of meteorite fragments are associated. As such, this crater represents a unique opportunity to investigate the effect of a low-energy impact event on an impacting iron bolide. Excellent documentation of meteorite fragment locations and characteristics has generated a detailed distribution map of both shrapnel and regmaglypted meteorite types. The meteorites’ distribution, and internal and external characteristics support a low-altitude breakup of the impactor which caused atmospherically ablated (regmaglypted) meteorites to fall close to the crater and avoid impact-related deformation. In contrast, shrapnel fragments sustained deformation at macro- and microscales resulting from the catastrophic disruption of the impactor. The impactor was significantly fragmented along pre-existing planes of weakness, including kamacite lamellae and inclusions, resulting in a bias toward low-mass ((<100 g) fragments. Meteorite mineralogy was investigated and the accessory minerals were found to be dominated by sulfides and phosphides with rare carlsbergite, consistent with other low-Ni IIIAB iron meteorites. Considerations of the total mass of meteoritic material recovered at the site relative to the probable fraction of the impactor that was preserved based on modeling suggests that the crater was formed by a higher velocity, lower mass impactor than previously inferred.

Reference
Newman JD, Herd CDK (2015) Mineralogy, petrology, and distribution of meteorites at the Whitecourt crater, Alberta, Canada. Meteoritics&Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12422]

Published by arrangement with John Wiley&Sons

¹Masarik, J., ¹Beňo, J.
¹Department of Nuclear Physics and Biophysics, Comenius University, Bratislava, Slovakia

The shape of meteorites is one of the major factors influencing the production of cosmogenic nuclides. Numerical simulations using the Los Alamos Code System (LCS) particle production and transport codes were done to investigate particle fluxes and production rates of cosmogenic nuclides 10Be, 26Al, and 60Co in meteoroids of spherical, ellipsoidal, and cylindrical shapes. The calculations show that fluxes of nuclear active particles and also production rates of cosmogenic nuclides are sensitive to the shape of the irradiated parent body.

Reference
Masarik J, Beňo J (2015) Effects of meteoroid shape on cosmogenic nuclide production processes. Meteoritics & Planetary Science (in Press)
Link to Article [doi: 10.1111/maps.12423]

Published by arrangement with John Wiley&Sons

^1,2,3Andrew M. Davis, ^1,3Frank M. Richter, ³Ruslan A. Mendybaev, ⁴Philip E. Janney, ⁴Meenakshi Wadhwa, ⁵Kevin D. McKeegan
¹Chicago Center for Cosmochemistry, The University of Chicago, Chicago, Il 60637
²Enrico Fermi Institute, The University of Chicago, Chicago, Il 60637
³Department of the Geophysical Sciences, The University of Chicago, Chicago, Il 60637
⁴Department of Geology, The Field Museum, Chicago, IL 60605
⁵Department of Earth and Space Sciences, University of California, Los Angeles, CA 90095

Magnesium isotope ratios are known to vary in solar system objects due to the effects of 26Al decay to 26Mg and mass dependent fractionation, but anomalies of nucleosynthetic origin must also be considered. In order to infer the amount of enhancement of 26Mg/24Mg due to 26Al decay or to resolve small nucleogenetic anomalies, the exact relationship between 26Mg/24Mg and 25Mg/24Mg ratios due to mass-dependent fractionation, the mass-fractionation “law”, must be accurately known so that the 25Mg/24Mg ratio can be used to correct the 26Mg/24Mg ratio for mass fractionation. Mass-dependent fractionation in mass spectrometers is reasonably well characterized, but not necessarily fully understood. It follows a simple power fractionation law, sometimes referred to as the “exponential law”. In contrast, mass fractionation in nature, in particular that due to high temperature evaporation that likely caused the relatively large effects observed in calcium-, aluminum-rich inclusions (CAIs), is reasonably well understood, but mass-fractionation laws for magnesium have not been explored in detail. The magnesium isotopic compositions of CAI-like evaporation residues produced in a vacuum furnace indicate that the slope on a log 25Mg/24Mg vs. log 26Mg/24Mg plot is ∼0.5128, and different from those predicted by any of the commonly used mass-fractionation laws. Evaporation experiments on forsterite-rich bulk compositions give exactly the same slope, indicating that the measured mass-fractionation law for evaporation of magnesium is applicable to a wide range of bulk compositions. We discuss mass-fractionation laws and the implications of the measured fractionation behavior of magnesium isotopes for 26Al-26Mg chronology.

Reference
Davis AM, Richter FM Mendybaev RA, Janney PE, Wadhwa M, McKeegan KD (2015) Isotopic mass fractionation laws for magnesium and their effects on 26Al-26Mg systematics in solar system materials. Geochimica et Cosmochimica Acta (in Press)
Link to Article [doi:10.1016/j.gca.2015.01.034]

Copyright Elsevier

^1,2Hitesh G. Changela, ⁴George D. Cody, ⁴Conel M.O’D. Alexander, ³Z Peeters, ⁴Larry R. Nittler, ²Rhonda M. Stroud
¹George Washington University, Department of Physics, 725 21st Street, NW, Washington, DC, 20052
²Naval Research Laboratory, 4555 Overlook Ave. SW, Washington, DC, 20375
³Geophysical Laboratory, Carnegie Institution of Washington, 5251 Broad Branch Rd NW, Washington DC, 20015
⁴Department of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Rd NW, Washington DC, 20015

The major form of organic material delivered to earth from an extraterrestrial origin is Insoluble Organic Matter (IOM). A morphological study of IOM in the CR (Renazzo-type) and CM (Mighei-type) carbonaceous chondrites was performed in order to constrain its origins and processing history. IOM residues from the following CR chondrites: GRO 95577 (CR 1), Al Rais (CR 1/2), EET 92042 (CR 2), QUE 99177 (CR 3) and the CM chondrites: MET 01070 (CM 2.2), Cold Bokkeveld (CM 2.3), Murchison (CM 2.4) and QUE 97990 (CM 2.5) were studied using Annular Dark Field STEM imaging. Characteristic features of the IOM, organic nanoglobules, were manually identified and measured for their abundances and size distributions. The IOM residues were also compared holistically for their degree of average ‘roughness’ or ‘coarsening’ using fractal image analysis. Manually identified nanoglobules have abundances making up less than 10% of the total IOM, which is consistent with previous studies. Their measured abundances do not correlate with petrologic grade. Thus parent body processing did not systematically deplete their abundances. The IOM is however on average ‘smoother’ or ‘coarser’ in the more altered chondrites, demonstrated by a lower fractal dimension using fractal box counting (DB). The DB values for the IOM in the CR chondrites are distinctive: QUE 99177 has the largest DB value (average = 1.54 ± 0.004) and GRO 99577 has the lowest (average = 1.45 ± 0.011). Al Rais and EET 92042 have IOM with average DB values within this range (average, 1.46 ± 0.009 and 1.50 ± 0.006). The CMs record a similar but less distinctive trend in DB, with QUE 97990 having the largest value (1.52 ± 0.004), MET 01070 the lowest (1.45 ± 0.019), and Cold Bokkeveld (1.50 ± 0.011) and Murchison (1.49 ± 0.017) equivalent to one another within error. The identified nanoglobules in the IOM of the CM chondrites are on average larger than those in the CR chondrites. The ‘coarsening’ or ‘smoother’ texture of the IOM (lower DB) in the more altered chondrites coupled with a tentative increase in the size of large features (identified nanoglobules) demonstrates that the aqueous processes leading to the lower petrologic types also formed the overall IOM morphology. In addition, observations of fluid-like textures more frequently found in the more altered carbonaceous chondrite residues suggests that organic and aqueous fluids determined at least some of these morphologies. The polymerisation of organic solutions is consistent with these morphologies. Their formation conditions are more favourable under containment in carbonaceous chondrite parent bodies.

Reference
Changela HG, Cody GD, Alexander CMOD, Peeters Z, Nittler LR, Stroud RM (2015) Morphological Study of Insoluble Organic Matter from Carbonaceous Chondrites: Correlation with Petrologic Grade. Geochimica et Cosmochimica Acta (in Press)
Link to Article [doi:10.1016/j.gca.2015.02.007]

Copyright Elsevier

¹Patrick N. Peplowski et al. (>10)*
¹The Johns Hopkins University Applied Physics Laboratory, Laurel, MD 20723, USA
Patrick N. Peplowski
*Find the extensive, full author and affiliation list on the publishers Website

The first map of variations in the abundances of thermal-neutron-absorbing elements across Mercury’s surface has been derived from measurements made with the anti-coincidence shield on MESSENGER’s Gamma-Ray Spectrometer (GRS). The results, which are limited to Mercury’s northern hemisphere, permit the identification of four major geochemical terranes at the 1000-km horizontal scale. The chemical properties of these regions are characterized from knowledge of neutron production physics coupled with elemental abundance measurements acquired by MESSENGER’s X-Ray Spectrometer (XRS) and GRS. The results indicate that the smooth plains interior to the Caloris basin have an elemental composition that is distinct from other volcanic plains units, suggesting that the parental magmas were partial melts from a chemically distinct portion of Mercury’s mantle. Mercury’s high-magnesium region, first recognized from XRS measurements, also contains high concentrations of unidentified neutron-absorbing elements. At latitudes north of ∼65° N, there is a region of high neutron absorption that corresponds closely to areas known to be enhanced in the moderately volatile lithophile elements Na, K, and Cl, and which has distinctly low Mg/Si ratios. The boundaries of this terrane differ from those of the northern volcanic plains, which constitute the largest geological unit in this region.
Reference
Peplowski PN et al. (2015) Geochemical terranes of Mercury’s northern hemisphere as revealed by MESSENGER neutron measurements. Icarus (in Press)
Link to Article [doi:10.1016/j.icarus.2015.02.002]

Copyright Elsevier

^1,2Jingao Liu, ¹Miriam Sharp, ¹Richard D. Ash, ³David A. Kring, ¹Richard J. Walker
¹Department of Geology, University of Maryland, College Park MD 20742 USA
²Department of Earth and Atmospheric Sciences, University of Alberta, 1-26 Earth Sciences Building, Edmonton AB T6G 2E3 Canada
³Center for Lunar Science and Exploration, Lunar and Planetary Institute, Universities Space Research Association, 3600 Bay Area Boulevard, Houston, Texas 77058, USA

Concentrations of highly siderophile elements (HSE) and 187Os/188Os isotopic compositions for eleven impact related rocks from the Apollo 15 and 16 landing sites are reported and combined with existing geochronological data to investigate the chemical nature and temporal changes in the large impactors implicated in the formation of the lunar basins. Data for the samples all define linear trends on plots of HSE versus Ir concentrations, whose slopes likely reflect the relative HSE compositions of the dominant impactors that formed the rocks.
The inferred Imbrium basin impactor that generated Apollo 15 impact melt rocks 15445 and 15455 was characterized by modestly suprachondritic 187Os/188Os, Ru/Ir, Pt/Ir and Pd/Ir ratios. Diverse impactor components are revealed in the Apollo 16 impact melt rocks. The 187Os/188Os and HSE/Ir ratios of the impactor components in melt rocks 60635, 63595 and 68416, with reported ages < 3.84 Ga, are within the range of chondritic meteorites, but slightly higher than ratios characterizing previously studied granulitic impactites with reported ages > 4.0 Ga. By contrast, the impactor components in melt rocks 60235, 62295 and 67095, with reported ages of ∼3.9 Ga, are characterized by suprachondritic 187Os/188Os and HSE/Ir ratios similar to the Apollo 15 impact melt rocks, and may also sample the Imbrium impactor. Three lithic clasts from regolith breccias 60016 and 65095, also with ∼3.9 Ga ages, contain multiple impactor components, of which the dominant composition is considerably more suprachondritic than those implicated for Imbrium and Serenitatis (Apollo 17) impactors. The dominant composition recorded in these rocks was most likely inherited from a pre-Imbrium impactor. Consideration of composition versus age relations among lunar impact melt rocks reveals no discernable trend.
Virtually all lunar impact melt rocks sampled by the Apollo missions, as well as meteorites, are characterized by 187Os/188Os and HSE/Ir ratios that, when collectively plotted, define linear trends ranging from chondritic to fractionated compositions. The impact melt rocks with HSE signatures within the range of chondritic meteorites are interpreted to have been derived from impactors that had HSE compositions similar to known chondrite groups. By contrast, the impact melt rocks with non-chondritic relative HSE concentrations could not have been made by mixing of known chondritic impactors. These signatures may instead reflect contributions from early solar system bodies with bulk chemical compositions that have not yet been sampled by primitive meteorites present in our collections. Alternately, they may reflect the preferential incorporation of evolved metal separated from a fractionated planetesimal core.
Pre-4.0 Ga ages for at least some impactor components with both chondritic and fractionated HSE raise the possibility that the bulk of the HSE were added to the lunar crust prior to the later-stage basin-forming impacts, such as Imbrium and Serenitatis, as proposed by Fischer-Gödde and Becker (2012). For this scenario, the later-stage basin-forming impacts were more important with respect to mixing prior impactor components into melt rocks, rather than contributing much to the HSE budgets of the rocks themselves.

Reference
Liu J, Sharp M, Ash RD, Kring DA, Walker RJ (2015) Diverse impactors in Apollo 15 and 16 impact melt rocks: evidence from osmium isotopes and highly siderophile elements. Geochimica et Cosmochimica Acta (in Press)
Link to Article [doi:10.1016/j.gca.2015.02.004]

Copyright Elsevier

^1,2Emmanuel Jacquet, ³Olivier Alard, ^2,4Matthieu Gounelle
¹Canadian Institute for Theoretical Astrophysics, 60 St George Street, Toronto, ON, M5S 3H8, Canada
²Institut de Minéralogie, de Physique des Matériaux et de Cosmochimie, CNRS & Muséum National d’Histoire Naturelle, UMR 7590, 57 rue Cuvier, 75005 Paris, France
³Géosciences Montpellier, UMR 5243, Université de Montpellier II, Place E. Bataillon, 34095 Montpellier cedex 5, France
⁴Institut Universitaire de France, Maison des Universités, 103 boulevard Saint-Michel, 75005 Paris, France

We report trace element concentrations of silicate phases in chondrules from LL3 ordinary chondrites Bishunpur and Semarkona. Results are similar to previously reported data for carbonaceous chondrites, with rare earth element (REE) concentrations increasing in the sequence olivine < pyroxene < mesostasis, and heavy REE (HREE) being enriched by 1-2 orders of magnitude (CI-normalized) relative to light REE (LREE) in ferromagnesian silicates, although no single olivine with very large LREE/HREE fractionation has been found. On average, olivine in type II chondrules is poorer in refractory lithophile incompatible elements (such as REE) than its type I counterpart by a factor of ∼2. This suggests that olivine in type I and II chondrules formed by batch and fractional crystallization, respectively, implying that type II chondrules formed under faster cooling rates (> ∼ 10 K/h) than type I chondrules. Appreciable Na concentrations (3-221 ppm) are measured in olivine from both chondrule types; type II chondrules seem to have behaved as closed systems, which may require chondrule formation in the vicinity of protoplanets or planetesimals. At any rate, higher solid concentrations in type II chondrule forming regions may explain the higher oxygen fugacities they record compared to type I chondrules. Type I and type II chondrules formed in different environments and the correlation between high solid concentrations and/or oxygen fugacities with rapid cooling rates is a key constraint that chondrule formation models must account for.

Reference
Jacquet E, Alard O, Gounelle M (2015) Trace element geochemistry of ordinary chondrite chondrules: the type I/type II chondrule dichotomy. Geochimica et Cosmochimica Acta (in Press)
Link to Article [doi:10.1016/j.gca.2015.02.005]

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¹Christa Göpel, ¹Jean-Louis Birck, ²Albert Galy, ³Jean-Alix Barrat, ⁴Brigitte Zanda
¹Institut de Physique du Globe de Paris, Sorbonne, Paris Cité, Univ Paris Diderot, UMR 7154 CNRS, F-75005 Paris, France
²Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge, CB2 3EQ, UK
³Université Européenne de Bretagne and CNRS UMR 6538, U.B.O-I.U.E.M., F-29280 Plouzané Cedex, France
⁴Laboratoire Minéralogie et Cosmochimie du Muséum, MNHN and CNRS UMR 7202, F-75005 Paris, France

Cr isotopic compositions have been measured on carbonaceous chondrites (CC): Tafassasset, Paris, Niger I, NWA 5958, NWA 8157 and Jbilet Winselwan. In bulk samples, the 54Cr/52Cr ratios (expressed as ε54Cr) range from 0.93 to 1.58 ε units. These values are in agreement with values characteristic for distinct petrologic types. Despite this 54Cr heterogeneity, the variability in the 53Cr/52Cr ratios (expressed as ε53Cr) of 0.2ε units and the Mn/Cr ratios is consistent with the previous finding of an isochron in the Mn-Cr evolution diagram.
The Mn/Cr ratio in CC corresponds to variable abundances of high-T condensate formed and separated at the beginning of the solar system, thus the canonical 53Mn/55Mn ratio can be defined. Based on a consistent chronology for U-Pb and Mn-Cr between the earliest objects formed in the solar nebula and the D’Orbigny angrite we define a canonical 53Mn/55Mn ratio and ε53Cri of 6.8 x 10-6 and – 0.177, respectively.
The internal Mn/Cr systematics in Tafassasset and Paris were studied by two approaches: leaching technique and mineral separation. Despite variable ε54Cr values (up to > 30 ε) linear co-variations were found between ε53Cr and Mn/Cr ratio. The mineral separates as well as the leachates of Tafassasset fall on a common isochron indicating that 1) cooling of the Tafassasset’s parent body occurred at 4563.5 +/-0.25 Ma, and that 2) 54Cr is decoupled from the other isotopes even though temperatures > 900 °C have been reached during metamorphism. In the case of Paris, the leachates form an alignment with a 53Mn/55Mn ratio higher than the canonical value. This alignment is not an isochron but rather a mixing line. Based on leachates from various CM and CI, we propose the occurrence of three distinct Cr reservoirs in meteoritic material: PURE54, HIGH53 and LOW53 characterized by a ε53Cr and ε54Cr of 0 and 25,000, -2.17 and 8, and 0.5 and -151, respectively. PURE54 has already been described and is carried by highly refractory nano-spinel; HIGH53 is Mn-rich and most probably carried by sulfides in the matrix, whereas LOW53 is characterized by low Mn/Cr ratios and it is sensitive to metamorphism. This component could correspond to mineral phases such as refractory oxides and carbide. Variable mixing proportions of HIGH53 and LOW53 would explain the larger-than-expected uncertainty (MSWD of 5.5) on the CC bulk regression line. A Monte Carlo simulation allows us to evaluate the impact of the dispersion of the initial Cr isotopic ratios (as a function of variable HIGH53). The co-variation of the Mn/Cr ratio and the ε53Cr defined by the mineral separates from Paris corresponds to an age of 4566.44 +0.66/-0.75 Ma, while their ε54Cr still differ by at least 0.42 ε. This age is likely to date the segregation of forsteritic olivines (most probably from type I chondrules) from fayalitic olivines (from type II chondrules) and, given the sampling procedure by handpicking of hundreds of grains, corresponds to the average age of chondrule formation.

Reference
Göpel G, Birck J-L, Galy A, Barrat J-A, Zanda B (2015) Mn-Cr systematics in primitive meteorites: insights from mineral separation and partial Dissolution. Geochimica et Cosmochimica Acta (in Press)
Link to Article [doi:10.1016/j.gca.2015.02.008]

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