¹S. W. Lehner,²P. Németh,^3,4M. I. Petaev,¹P. R. Buseck
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12940]
¹School of Earth and Space Exploration, Arizona State University, Tempe, Arizona, USA
²Institute of Materials and Environmental Chemistry, Research Center for Natural Sciences, Hungarian Academy of Sciences, Budapest, Hungary
³Department of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts, USA
⁴Solar, Stellar, and Planetary Sciences, Harvard-Smithsonian CfA, Cambridge, Massachusetts, USA
Published by arrangement with John Wiley & Sons

Two new occurrences of porous, S-bearing, amorphous silica are described within metal-sulfide nodules (MSN) and as interchondrule patches in EH3 chondrites SAH 97072 and ALH 84170. This porous amorphous material, which was first reported from sulfide-bearing chondrules, consists of sinewy SiO2-rich areas containing S with minor Na or Ca as well as Fe, Mg, and Al. Some pores contain minerals including pyrite, pyrrhotite, and anhydrite. Most pores appear vacant or contain unidentified material that is unstable under analytical conditions. Niningerite, olivine, enstatite, albite, and kumdykolite occur enclosed within porous silica patches. Porous silica is commonly interfingered with cristobalite suggesting its amorphous structure resulted from high-temperature quenching. We interpret the S-bearing porous silica to be a product of silicate sulfidation, and the Na, Ca, Fe, Mg, and Al detectable within this material are chemical residues of sulfidized silicates and metal. The occurrence of porous silica in the cores of MSN, which are considered to be pre-accretionary objects, suggests the sulfidizing conditions occurred prior to final parent-body solidification. Ubiquitous S-bearing porous silica among sulfide-bearing chondrules, MSN, and in the interchondrule clastic matrix, suggests that similar sulfidizing conditions affected all the constituents of these EH3 chondrites.

¹Jérome Gattacceca,^2,3Agata M. Krzesińska,⁴Yves Marrocchi,⁵Matthias M. M. Meier,⁶Michèle Bourot-Denise,⁷Rob Lenssen
Meteoritics & Planetary Science (in Press) Link top Article [DOI: 10.1111/maps.12942]
¹CNRS, Aix-Marseille Univ, IRD, Coll France, CEREGE, Aix-en-Provence, France
²Department of Earth Sciences, Natural History Museum, London, UK
³Institute of Geological Sciences, Polish Academy of Sciences, Wrocław, Poland
⁴CRPG, CNRS, Université de Lorraine, UMR 7358, Vandoeuvre-les-Nancy, France
⁵ETH Zurich, Institute of Geochemistry and Petrology, Zurich, Switzerland
⁶IMPMC, MNHN, UPMC, UMR CNRS 7590, Paris, France
⁷Private meteorite collector, The Netherlands
Published by agreement with John Wiley & Sons

Polymict chondritic breccias—rocks composed of fragments originating from different chondritic parent bodies—are of particular interest because they give insights into the mixing of asteroids in the main asteroid belt (occurrence, encounter velocity, transfer time). We describe Northwest Africa (NWA) 5764, a brecciated LL6 chondrite that contains a >16 cm3 L4 clast. The L clast was incorporated in the breccia through a nondestructive, low-velocity impact. Identical cosmic-ray exposure ages of the L clast and the LL host (36.6 ± 5.8 Myr), suggest a short transfer time of the L meteoroid to the LL parent body of 0.1 ± 8.1 Myr, if that meteoroid was no larger than a few meters. NWA 5764 (together with St. Mesmin, Dimmitt, and Glanerbrug) shows that effective mixing is possible between ordinary chondrite parent bodies. In NWA 5764 this mixing occurred after the peak of thermal metamorphism on the LL parent body, i.e., at least several tens of Myr after the formation of the solar system. The U,Th-He ages of the L clast and LL host, identical at about 2.9 Ga, might date the final assembly of the breccia, indicating relatively young mixing in the main asteroid belt as previously evidenced in St. Mesmin.