Potassium isotope systematics of the LL4 chondrite Hamlet: Implications for chondrule formation and alteration

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^1,2Piers Koefoed,^1,3Olga Pravdivtseva,^1,2Heng Chen,⁴Carina Gerritzen,⁵Maxwell M. Thiemens,^1,2Kun Wang
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13545]
¹McDonnell Center for the Space Sciences, Washington University in St. Louis, St. Louis, Missouri, 63130 USA
²Department of Earth and Planetary Sciences, Washington University in St. Louis, St. Louis, Missouri, 63130 USA
³Department of Physics, Washington University in St. Louis, St. Louis, Missouri, 63130 USA
⁴Institut für Mineralogie und Geologie, Universität zu Köln, Köln, Germany
⁵Laboratoire G‐Time, CP 160/02, Université Libre de Bruxelles, Av. F. Roosevelt 50, Bruxelles, Belgium
Published by arrangement with John Wiley & Sons

Here, we apply recently developed high‐precision K isotope analyses to individual components of the LL4 chondrite Hamlet in order to investigate key processes which occurred during chondrite formation. The K isotopic compositions of all Hamlet chondrules range from −1.36‰ to −0.24‰ δ41K while the matrix and bulk samples show ranges of −0.89‰ to −0.80‰ and −0.86‰ to −1.08‰ δ41K, respectively. This range of δ41K values is significantly less than what was seen by in situ K isotopic analysis of Semarkona and Bishunpur chondrules, a likely effect of the different chondrite petrologic types, analytical artifacts in the SIMS analyses, and chondrule rim effects. Strong evidence for secondary parent‐body alteration effects within Hamlet suggests its K fractionation and distribution are dominantly controlled by these processes. Interestingly, the strong correlation between δ41K and chondrule mass suggests that chondrule size played a significant role in the K isotopic distribution within Hamlet. This trend is likely a result of either inherited initial differences in the chondrule K isotopic ratios which were not completely overprinted or mechanisms involved in the metamorphism processes creating variations. This K isotope correlation with chondrule mass could also be suggestive of chondrule‐forming nebular processes; nevertheless, it is currently unable to definitively favor any specific model. The K isotopic similarities between Hamlet and bulk ordinary chondrites suggest that all LL chondrites, if not all ordinary chondrites, may have formed via the same processes. Nevertheless, analysis of more pristine chondrules from chondrites of lower metamorphic grade is required to further assess any nebular processes of chondrule formation.

Partial core vaporization during Giant Impacts inferred from the entropy and the critical point of iron

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¹Zhi Li,^1,2Razvan Caracas,¹François Soubiran
Earth and Planetary Science Letters 547, 116463 Link to Article [https://doi.org/10.1016/j.epsl.2020.116463]
¹CNRS, Ecole Normale Supérieure de Lyon, Laboratoire de Géologie de Lyon UMR 5276, Centre Blaise Pascal, 46 allée d’Italie, 69364 Lyon, France
²The Center for Earth Evolution and Dynamics (CEED), University of Oslo, Oslo, Norway
Copyright Elsevier

Giant impacts are disruptive events occurring in the early stages of planetary evolution. They may result in the formation of a protolunar disk or of a synestia. A central planet and one or several moons condense upon cooling bearing the chemical signature of the silicate mantles of the initial bodies; the iron cores may partly vaporize, fragment and/or merge. Here we determine from ab initio simulations the critical point of iron in the temperature range of 9000-9350 K, and the density range of 1.85-2.40 g/cm³, corresponding to a pressure range of 4-7 kbars. This implies that the iron core of the proto-Earth may become supercritical after giant impacts and during the condensation and cooling of the protolunar disk. We show that the iron core of Theia partially vaporized during the Giant Impact. Part of this vapor may have remained in the disk, to eventually participate in the Moon’s small core. Similarly, during the late veneer a large fraction of the planetesimals have their cores undergoing partial vaporization. This would help mixing the highly siderophile elements into magma ponds or oceans.

Hydrothermal alteration associated with the Chicxulub impact crater upper peak-ring breccias

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¹S.L.Simpson,¹G.R.Osinski,¹F.J.Longstaffe,²M.Schmieder,²D.A.Kring
Earth and Planetary Science Letters 547,116425 Link to Article [https://doi.org/10.1016/j.epsl.2020.116425]
¹Department of Earth Sciences, Institute for Earth and Space Exploration, The University of Western Ontario, ON, N6A 3K7, Canada
²Lunar and Planetary Institute, Universities Space Research Association, Houston, TX, 77058 United States of America
Copyright Elsevier

The 66 Ma, ∼180 km Chicxulub impact structure in the northern Yucatán peninsula and southern Gulf of Mexico is the best-preserved large impact crater on Earth with a well-developed peak ring. The most recent drilling campaign took place offshore during the joint International Ocean Discovery Program – International Continental Scientific Drilling Program (IODP–ICDP) Expedition 364 at site M0077A (21.45°N, 89.95°W) and recovered ∼830 m of continuous core. Initial examination revealed that the peak-ring comprises four main lithological units (from the base upwards): crystalline basement granitoid rocks (Unit 4); a thin layer of impact melt rocks (Units 3A and B); melt-bearing breccias (Units 2A–C); and post-impact sedimentary rocks (Unit 1). Preliminary analysis of the drill core indicated that hydrothermal alteration has affected all lithologies and is especially pervasive in the melt-bearing breccias of Unit 2 (721.6 to 617.33 meters below sea floor, mbsf). Here we present the first detailed investigation of hydrothermal alteration within the melt-bearing breccias. Alteration phases are predominantly Fe-Mg clay minerals, zeolites, alkali feldspars, calcite and minor sulfides, sulfates, opal and Fe-Ti oxides. Alteration is especially intense proximal to lithologic contacts, particularly at the base of subunit 2B where there is an abrupt increase in host rock porosity ∼30 m above the impact melt rocks. The pervasiveness of clay minerals and zeolites is attributed to the high amounts of devitrified silicate glass throughout Unit 2. The phases preserved here are consistent with the findings of previous hydrothermal studies in other areas of the Chicxulub structure, and suggest an evolving water-rock system that was alkaline-saline, comparable to seawater-volcanic glass alteration.

Fumarolic-like activity on carbonaceous chondrite parent body

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¹Clément Ganino,^2,3Guy Libourel
Science Advances 6, eabb1166 Link to Articles [DOI: 10.1126/sciadv.abb1166]
¹Université Côte d’Azur, OCA, CNRS, IRD, Géoazur, 250 rue Albert Einstein, Sophia-Antipolis, 06560 Valbonne, France.
²Université Côte d’Azur, OCA, CNRS, Lagrange, Boulevard de l’Observatoire, CS 34229, 06304 Nice Cedex 4, France.
³Hawai’i Institute of Geophysics and Planetology, School of Ocean, Earth Science and Technology, University of Hawai’i at Mānoa, Honolulu, HI i 96821, USA.