¹Kun Wang (王昆), ¹Stein B. Jacobsen, ¹Fatemeh Sedaghatpour, ²Heng Chen, ²Randy L. Korotev
¹Department of Earth and Planetary Sciences, Harvard University, 20 Oxford Street, Cambridge, MA 02138, USA
²Department of Earth and Planetary Sciences, Washington University in St. Louis, One Brookings Drive, St. Louis, MO 63130, USA

Recent high-precision isotopic measurements show that the isotopic similarity of Earth and Moon is unique among all known planetary bodies in our Solar System. These observations provide fundamental constraints on the origin of Earth–Moon system, likely a catastrophic Giant Impact event. However, in contrast to the isotopic composition of many elements (e.g. , O, Mg, Si, K, Ti, Cr, and W), the Fe isotopic compositions of all lunar samples are significantly different from those of the bulk silicate Earth. Such a global Fe isotopic difference between the Moon and Earth provides an important constraint on the lunar formation – such as the amount of Fe evaporation as a result of a Giant Impact origin of the Moon. Here, we show through high-precision Fe isotopic measurements of one of the oldest lunar rocks (4.51±0.10 Gyr4.51±0.10 Gyr dunite 72 415), compared with Fe isotope results of other lunar samples from the Apollo program, and lunar meteorites, that the lunar dunite is enriched in light Fe isotopes, complementing the heavy Fe isotope enrichment in other lunar samples. Thus, the earliest olivine accumulation in the Lunar Magma Ocean may have been enriched in light Fe isotopes. This new observation allows the Fe isotopic composition of the bulk silicate Moon to be identical to that of the bulk silicate Earth, by balancing light Fe in the deep Moon with heavy Fe in the shallow Moon rather than the Moon having a heavier Fe isotope composition than Earth as a result of Giant Impact vaporization.

Reference
Wang (王昆) K, Jacobsen SB, Sedaghatpour F, Chen H, Korotev RL (2015) The earliest Lunar Magma Ocean differentiation recorded in Fe. Earth and Planetary Science Letters 430, 202–208
Link to Article [doi:10.1016/j.epsl.2015.08.019]
Copyright Elsevier

¹Alexandra Abrajevitch, ²Eric Font, ³Fabio Florindo, ⁴Andrew P. Roberts
¹Institute of Tectonics and Geophysics, Kim-Yu-Chen 65, Khabarovsk 680000, Russia
²IDL-FCUL, Instituto Dom Luís, Faculdade de Ciências, Universidade de Lisboa, Campo Grande, 1749-016, Portugal
³Istituto Nazionale di Geofisica e Vulcanologia, Via di Vigna Murata 605, 00143-Rome, Italy
⁴Research School of Earth Sciences, Australian National University, Canberra, Acton 0200, Australia

The respective roles of an asteroid impact and Deccan Traps eruptions in biotic changes at the Cretaceous–Paleogene (K–Pg) boundary are still debated. In many shallow marine sediments from around the world, the K–Pg boundary is marked by a distinct clay layer that is often underlain by a several decimeter-thick low susceptibility zone. A previous study of the Gubbio section, Italy (Lowrie et al., 1990), attributed low magnetization intensity in this interval to post-depositional dissolution of ferrimagnetic minerals. Dissolution was thought to be a consequence of downward infiltration of reducing waters that resulted from rapid accumulation of organic matter produced by mass extinctions after the K–Pg event. We compare the magnetic properties of sediments from the Gubbio section with those of the Bidart section in southern France. The two sections are similar in their carbonate lithology and the presence of a boundary clay and low susceptibility zone. When compared to background Cretaceous sediments, the low susceptibility zone in both sections is marked by an absence of biogenic magnetite, a decrease in total ferrimagnetic mineral content, and a preferential loss of magnetite with respect to hematite – features that are consistent with reductive dissolution. However, unlike the Gubbio section, where the low susceptibility zone starts immediately below the boundary clay, the low susceptibility zone and the clay layer at Bidart are separated by a ∼4-cm carbonate interval that contains abundant biogenic magnetite. Such separation casts doubt on a causal link between the impact and sediment bleaching. More likely, the low susceptibility layer marks a different environmental event that preceded the impact. An episode of increased atmospheric and oceanic acidity associated with Deccan Traps volcanism that occurred well before the K–Pg impact is argued here to account for the distinct magnetic properties of the low susceptibility intervals.

Reference
Abrajevitch A, Font E, Florindo F, Roberts AP (2015) Asteroid impact vs. Deccan eruptions: The origin of low magnetic susceptibility beds below the Cretaceous–Paleogene boundary revisited. Earth and Planetary Science Letters 430, 209–223
Link to Article [doi:10.1016/j.epsl.2015.08.022]
Copyright Elsevier

^1,2Yves Marrocchi, ³Marc Chaussidon
¹CNRS, CRPG UMR 7358, Vandoeuvre-lès-Nancy, F-54501, France
²Université de Lorraine, CRPG UMR 7358, Vandoeuvre-lès-Nancy, F-54501, France
³Institut de Physique du Globe de Paris (IPGP), CNRS UMR 7154, Paris, F-75238, France

Primitive meteorites are characteristically formed from an aggregation of sub-millimeter silicate spherules called chondrules. Chondrules are known to present large three-isotope oxygen variations, much larger than shown by any planetary body. We show here that the systematic of these oxygen isotopic variations results from open-system gas–melt exchanges during the formation of chondrules, a conclusion that has not been fully assessed up to now. We have considered Mg-rich porphyritic chondrules and have modeled the oxygen isotopic effects that would result from high-temperature interactions in the disk between precursor silicate dust and a gas enriched in SiO during the partial melting and evaporation of this dust. This formation process predicts: (i) a range of oxygen isotopic composition for bulk chondrules in agreement with that observed in Mg-rich porphyritic chondrules, and (ii) variable oxygen isotopic disequilibrium between chondrule pyroxene and olivine, which can be used as a proxy of the dust enrichment in the chondrule-forming region(s). Such enrichments are expected during shock waves that produce transient evaporation of dust concentrated in the mid-plane of the accretion disk or in the impact plumes generated during collisions between planetesimals. According to the present model, gas–melt interactions under high PSiO(gas) left strong imprints on the major petrographic, chemical and isotopic characteristics of Mg-rich porphyritic chondrules.

Reference
Marrocchi Y, Chaussidon M (2015) A systematic for oxygen isotopic variation in meteoritic chondrules. Earth and Planetary Science Letters 430, 308–315
Link to Article [doi:10.1016/j.epsl.2015.08.032]
Copyright Elsevier

¹Gerrit Budde, ¹Thomas S. Kruijer, ¹Mario Fischer-Gödde, ²Anthony J. Irving, ¹Thorsten Kleine
¹Institut für Planetologie, Westfälische Wilhelms-Universität Münster, Wilhelm-Klemm-Straße 10, 48149 Münster, Germany
²Department of Earth and Space Sciences, University of Washington, Seattle, WA 98195, USA

Determining the timescales of the accretion and chemical differentiation of meteorite parent bodies provides some of the most direct constraints on the formation of planetesimals and the earliest stages of planet formation. We present high-precision Hf–W isotope data for a comprehensive set of ureilites, ultramafic mantle restites derived from a partially melted and incompletely differentiated asteroid. All samples are characterized by strong 182W deficits, indicating that silicate melt extraction on the ureilite parent body at 3.3±0.7 Ma3.3±0.7 Ma after CAI formation postdated core formation in iron meteorite parent bodies by ∼2–3 Ma. Thermal modeling of planetesimal heating by 26Al-decay combined with the new Hf–W data indicates that the ureilite parent body accreted at ∼1.6 Ma after CAI formation and, therefore, more than ∼1 Ma later than iron meteorite parent bodies, but more than ∼0.5 Ma earlier that most chondrite parent bodies. Due to its relatively ‘late’ accretion, the ureilite parent body contained too little 26Al to cause complete melting and, therefore, would have probably remained incompletely differentiated even without exhaustion of 26Al by silicate melt segregation. Our results show that both in terms of degree of differentiation and accretion timescale the ureilite parent body is intermediate between fully differentiated and undifferentiated bodies, implying that there is an inverse correlation between extent of melting and metal–silicate separation versus time of accretion and differentiation.

Reference
Budde G, Kruijer TS, Fischer-Gödde M, Irving AJ, Kleine T (2015) Planetesimal differentiation revealed by the Hf–W systematics of ureilites. Earth and Planetary Science Letters 430, 316–325
Link to Article [doi:10.1016/j.epsl.2015.08.034]
Copyright Elsevier