Early Impact Events on Chondritic Parent Bodies: Insights From NWA 11004, Reclassified as an LL7 Breccia. Issue 5 (18th May 2020)

• Record Type: Journal Article
• Authors: Li, Y.; Rubin, A. E.; Hsu, W.; Ziegler, K.
• Is Part Of: Journal of geophysical research. Volume 125:Issue 5(2020)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2020-05-18
• Subjects: type 7 ordinary chondrites -- partial melting -- early solar system -- impacts; Planets -- Periodicals; Geophysics -- Periodicals; 559.9
• DOI: 10.1029/2019JE006360
• Languages: English
• ISSNs: 2169-9097

Abstract

Key Points: NWA 11004, reclassified as an LL7 breccia, records a partial melting process NWA 11004 records a late‐stage shock event at 4546 ± 34 Ma that produced undulose to weak mosaic extinction in pyroxene The presence of type 7 chondrites in the early solar system may be ascribed to impacts on already warm parent bodies Abstract: The NWA 11004 ordinary chondrite (OC) can provide insights into the complex petrogenetic processes of the early solar system. Although originally classified as an L7 chondrite, it is reclassified as LL based on kamacite Ni (4.9 ± 0.3 wt.%) and Co (3.6 ± 0.5 wt.%) and bulk O‐isotopic composition (δ 17 O = 3.76‰; δ 18 O = 5.39‰). NWA 11004 is characterized by (1) the occurrence of 3‐ to 5‐mm‐sized poikilitic pyroxene, (2) scattered low‐Ca pyroxene data in a TiO2 versus Al2 O3 diagram, (3) relatively magnesian olivine and low‐Ca pyroxene (Fa25.4, Fs21.3 ), (4) low abundances of high‐Ca pyroxene, plagioclase, troilite and Ca‐phosphate, and (5) low rare earth element contents in low‐Ca pyroxene. The geochemical features of olivine and low‐Ca pyroxene in NWA 11004 differ from literature data for grains that crystallized from a melt in an OC impact melt breccia. We suggest that in NWA 11004, a plagioclase‐phosphate high‐Ca pyroxene‐troilite melt migrated away during partial melting. Some high‐Ca pyroxene grains crystallized from the residual melt, as indicated by a positive linear trend in a TiO2 versus Al2 O3 diagram. Whereas poikilitic low‐Ca pyroxene in NWA 11004 exhibits undulose‐to‐weak mosaic extinction, the olivine chadacrysts exhibit sharp optical extinction; this implies that NWA 11004 experienced a late‐stage shock event (S4) followed by annealing. The Ca‐phosphate 207 Pb/ 206 Pb age of 4546 ± 34 Ma most likely dates this late‐stage shock event. We suggest that the presence of type 7 OC in the early solar system may be attributable to impacts on warm chondritic asteroids that were initially heated by the decay of 26 Al. Plain Language Summary: Unlike most equilibrated ordinary chondrites (classified as Types 4–6) that experienced solid‐state thermal metamorphism, a few ordinary chondrites underwent higher‐temperature processes that included (partial) melting. NWA 11004 is one of the rare chondrites (labeled type 7) that record low degrees of partial melting in an asteroidal body 4546 ± 34 million years ago. The presence of type 7 chondrites in early solar system history may be ascribed to impacts on already warm parent bodies that were heated previously by the decay of 26 Al.