¹R.R. Fu, ¹E.A. Lima, ¹B.P. Weiss
¹Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA, USA

Magnetic fields in the solar nebula may have played a central role in mass and angular momentum transport in the protosolar disk and facilitated the accretion of the first planetesimals. Thought to be key evidence for this hypothesis is the high unblocking-temperature, randomly oriented magnetization in chondrules in the Allende CV carbonaceous chondrite. However, it has recently been realized that most of the ferromagnetic minerals in Allende are products of secondary processes on the parent planetesimal. Here we reevaluate the pre-accretional magnetism hypothesis for Allende using new paleomagnetic analyses of chondrules including the first measurements of mutually oriented subsamples from within individual chondrules. We confirm that Allende chondrules carry a high-temperature component of magnetization that is randomly oriented among chondrules. However, we find that subsamples of individual chondrules are also non-unidirectionally magnetized. Therefore, the high-temperature magnetization in Allende chondrules is not a record of nebular magnetic fields and is instead best explained by remagnetization during metasomatism in a

Reference
Fu RR, Lima, EA, Weiss BP (2014) No nebular magnetization in the Allende CV carbonaceous chondrite. Earth and Planetary Science Letters 404, 54–66
Link to Article [DOI: 10.1016/j.epsl.2014.07.014]

Copyright Elsevier

^1,2Thomas S. Kruijer, ¹Thorsten Kleine, ¹Mario Fischer-Gödde, ³Christoph Burkhardt, ²Rainer Wieler
¹Institut für Planetologie, Westfälische Wilhelms-Universität Münster, Wilhelm Klemm-Strasse 10, 48149 Münster, Germany
²ETH Zürich, Institute of Geochemistry and Petrology, Clausiusstrasse 25, 8092 Zürich, Switzerland
³Origins Laboratory, Department of the Geophysical Sciences, The University of Chicago, 5734 South Ellis Avenue, Chicago, IL

Ca–Al-rich inclusions (CAI) are the oldest dated objects formed in the solar system and are pivotal reference points in early solar system chronology. Knowledge of their initial 182Hf/180Hf and 182W/184W is essential, not only for obtaining precise Hf–W ages relative to the start of the solar system, but also to assess the distribution of short-lived radionuclides in the early solar nebula. However, the interpretation of Hf–W data for CAI is complicated by nucleosynthetic W isotope variations. To explore their extent and nature, and to better quantify the initial Hf and W isotope compositions of the solar system, we obtained Hf–W data for several fine- and coarse-grained CAI from three CV3 chondrites. The fine-grained CAI exhibit large and variable anomalies in ε 183W (εiWεiW equals 0.01% deviation from terrestrial values), extending to much larger anomalies than previously observed in CAI, and reflecting variable abundances of s – and r -process W isotopes. Conversely, the coarse-grained (mostly type B) inclusions show only small (if any) nucleosynthetic W isotope anomalies. The investigated CAI define a precise correlation between initial ε 182W and ε 183W, providing a direct empirical means to correct the ε 182W of any CAI for nucleosynthetic isotope anomalies using their measured ε 183W. After correction for nucleosynthetic W isotope variations, the CAI data define an initial 182Hf/180Hf of (1.018±0.043)×10−4(1.018±0.043)×10−4 and an initial ε 182W of −3.49±0.07−3.49±0.07. The Hf–W formation intervals of the angrites D’Orbigny and Sahara 99555 relative to this CAI initial is 4.8±0.6 Ma4.8±0.6 Ma, in good agreement with Al–Mg ages of these two angrites. This renders a grossly heterogeneous distribution of 26Al in the inner solar system unlikely, at least in the region were CAI and angrites formed.

Reference
Kruijer TS, Thorsten Kleine T, Mario Fischer-Gödde M, Christoph Burkhardt C, Wieler R (2014) Nucleosynthetic W isotope anomalies and the Hf–W chronometry of Ca–Al-rich inclusions. Earth and Planetary Science Letters 403,317–327.
Link to Article [DOI: 10.1016/j.epsl.2014.07.003]

Copyright Elsevier

¹Raúl O.C. Fonseca,²Guilherme Mallmann, ³Peter Sprung, ¹Johanna E. Sommer, ¹Alexander Heuser, ¹Iris M. Speelmanns, ¹Henrik Blanchard

¹Steinmann-Institut, Universität Bonn, 53115 Bonn, Germany
²School of Earth Sciences, The University of Queensland, Brisbane QLD 4072, Australia
³Institut für Planetologie, Westfälische Wilhelms-Universität Münster, 48149 Münster, Germany

The timing of core formation is essential for understanding the early differentiation history of the Earth and the Moon. Because Hf is lithophile and W is siderophile during metal–silicate segregation, the decay of 182Hf to 182W (half-life of 9 Ma) has proven to be a useful chronometer of core–mantle differentiation events. A key parameter for the interpretation of 182Hf/182W data is the Hf/W ratio of the primitive (i.e. undepleted) mantle. Since W is incompatible during mantle melting, its ratio relative to U and other similarly incompatible elements in basalts (e.g. Th, La) may be used as proxies for their mantle sources. However, the assumption that W and U are equally incompatible may be flawed for petrological systems that equilibrated over a large range of oxygen fugacity (fO2). Although W is typically perceived as being homovalent, evidence suggests that U is heterovalent over the range of fO2 inferred for the silicate mantles of the Earth and the Moon.

Here we report new partitioning data for W, U, high-field-strength elements (HFSE), and Th between clinopyroxene, orthopyroxene, olivine, plagioclase and silicate melt. In agreement with previous studies, we show that these elements behave as homovalent elements at fO2 characteristic of Earth’s upper mantle. However, both W and U become more compatible at low fO2, indicating a change in their redox state, with W becoming more compatible at progressively lower fO2. This result for W is particularly unexpected, because this element was thought to be hexavalent even at very low fO2. The much higher compatibility of W4+ (the species inferred here at low fO2) relative to W6+ means that even a small fraction of W4+ will have a significant effect on the overall compatibility of W. Our results imply that over the range of reducing conditions in which lunar differentiation is thought to have taken place (i.e. ∼IW-2 to IW-0.5), W is likely to become fractionated from U. When our partitioning data are applied to model the fractional crystallization of a lunar magma ocean, lunar trends for U/W, Hf/W and Th/W are well reproduced. The result of this model carries with it the implication that the Hf/W of the bulk silicate fractions that comprise the Earth and the Moon are virtually identical.

Reference
Fonseca ROC, Mallmann G, Sprung P, Sommer JE, Heuser A, Speelmanns IM, Blanchard H (2014) Redox controls on tungsten and uranium crystal/silicate melt partitioning and implications for the U/W and Th/W ratio of the lunar mantle. Earth and Planetary Science Letters 404, 1-13
Link to Article [DOI: 10.1016/j.epsl.2014.07.015]

Copyright Elsevier

¹Martin R. Lee, ¹Paula Lindgren, ¹Mahmood R. Sofe
¹School of Geographical and Earth Sciences, University of Glasgow, Glasgow G12 8QQ, U.K

Carbonate minerals in CM carbonaceous chondrite meteorites, along with the silicates and sulphides with which they are intergrown, provide a detailed record of the nature and evolution of parent body porosity and permeability, and the chemical composition, temperature and longevity of aqueous solutions. Fourteen meteorites were studied that range in petrologic subtype from mildly aqueously altered CM2.5 to completely hydrated CM2.0. All of them contain calcite, whereas aragonite occurs only in the CM2.5-CM2.2 meteorites and dolomite in the CM2.2-CM2.0. All of the aragonite crystals, and most of the calcite and dolomite grains, formed during early stages of parent body aqueous alteration by cementation of pores produced by the melting of tens of micrometre size particles of H2O-rich ice. Aragonite was the first carbonate to precipitate in the CM2.5 to CM2.2 meteorites, and grew from magnesium-rich solutions. In the least altered of these meteorites the aragonite crystals formed in clusters owing to physical restriction of aqueous fluids within the low permeability matrix. The strong correlation between the petrologic subtype of a meteorite, the abundance of its aragonite crystals and the proportion of them that have preserved crystal faces, is because aragonite was dissolved in the more altered meteorites on account of their higher permeability, and/or greater longevity of the aqueous solutions. Dolomite and breunnerite formed instead of aragonite in some of the CM2.1 and CM2.2 meteorites owing to higher parent body temperatures. The pore spaces that remained after precipitation of aragonite, dolomite and breunnerite cements were occluded by calcite. Following completion of cementation, the carbonates were partially replaced by phyllosilicates and sulphides. Calcite in the CM2.5-CM2.2 meteorites was replaced by Fe-rich serpentine and tochilinite, followed by Mg-rich serpentine. In the CM2.1 and CM2.0 meteorites dolomite, breunnerite and calcite were replaced by Fe-rich serpentine and Fe-Ni sulphide, again followed by Mg-rich serpentine. The difference between meteorites in the mineralogy of their replacive sulphides may again reflect greater temperatures in the parent body regions from where the more highly altered CMs were formed. This transition from Fe-rich to Mg-rich carbonate replacement products mirrors the chemical evolution of parent body solutions in response to consumption of Fe-rich primary minerals followed by the more resistant Mg-rich anhydrous silicates. Almost all of the CMs examined contain a second generation of calcite that formed after the sulphides and phyllosilicates and by replacement of remaining anhydrous silicates and dolomite (dedolomitization). The Ca and CO2 required for this replacive calcite is likely to have been sourced by dissolution of earlier formed carbonates, and ions may have been transported over metre-plus distances through high permeability conduits that were created by impact fracturing.

Reference
Lee MR, Lindgren P, Sofe MR (2014) Aragonite, breunnerite, calcite and dolomite in the CM carbonaceous chondrites: High fidelity recorders of progressive parent body aqueous Alteration. Geochimica et Cosmochimica Acta (in Press)
Link to Article [DOI: 10.1016/j.gca.2014.08.019]

Copyright Elsevier

¹Huan Y. A. Meng, ²Kate Y. L. Su, ^1,2George H. Rieke, ³David J. Stevenson, ^4,5Peter Plavchan, ^2,6,7Wiphu Rujopakarn, ⁸Carey M. Lisse, ⁹Saran Poshyachinda, ¹⁰Daniel E. Reichart

¹Lunar and Planetary Laboratory and Department of Planetary Sciences, University of Arizona, 1629 East University Boulevard, Tucson, AZ 85721, USA.
²Steward Observatory and Department of Astronomy, University of Arizona, 933 North Cherry Avenue, Tucson, AZ 85721, USA.
³Division of Geological and Planetary Sciences, California Institute of Technology, MC 170-25, 1200 East California Boulevard, Pasadena, CA 91125, USA.
⁴NASA Exoplanet Science Institute, California Institute of Technology, MC 100-22, 770 South Wilson Avenue, Pasadena, CA 91125, USA.
⁵Missouri State University, 901 South National Avenue, Springfield, MO 65897, USA.
⁶Department of Physics, Faculty of Science, Chulalongkorn University, 254 Phayathai Road, Pathumwan, Bangkok 10330, Thailand.
⁷Kavli Institute for the Physics and Mathematics of the Universe (WPI), Todai Institute for Advanced Study, University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, 277-8583, Japan.
⁸Space Department, Applied Physics Laboratory, Johns Hopkins University, 11100 Johns Hopkins Road, Laurel, MD 20723, USA.
⁹National Astronomical Research Institute of Thailand (Public Organization), Ministry of Science and Technology, 191 Siriphanich Building, Huay Kaew Road, Muang District, Chiang Mai 50200, Thailand.
¹⁰Department of Physics and Astronomy, Campus Box 3255, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599, USA.

The final assembly of terrestrial planets occurs via massive collisions, which can launch copious clouds of dust that are warmed by the star and glow in the infrared. We report the real-time detection of a debris-producing impact in the terrestrial planet zone around a 35-million-year-old solar-analog star. We observed a substantial brightening of the debris disk at a wavelength of 3 to 5 micrometers, followed by a decay over a year, with quasi-periodic modulations of the disk flux. The behavior is consistent with the occurrence of a violent impact that produced vapor out of which a thick cloud of silicate spherules condensed that were then ground into dust by collisions. These results demonstrate how the time domain can become a new dimension for the study of terrestrial planet formation.

Reference
Meng HYA, Su, KYL, Rieke GH, Stevenson DJ, Plavchan P, Rujopakarn W, Lisse CM, Poshyachinda S, Reichart DE (2014) Large impacts around a solar-analog star in the era of terrestrial planet Formation. Science 345, 6200, 1032-1035
Link to Article [DOI: 10.1126/science.1255153]

Reprinted with permission from AAAS