Early Impact Events on Chondritic Parent Bodies: Insights From NWA 11004, Reclassified as an LL7 Breccia

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 TiO 2 versus Al 2 O 3 diagram, (3) relatively magnesian olivine and low-Ca pyroxene (Fa 25.4 , Fs 21.3 ), (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 TiO 2 versus Al 2 O 3 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 asTypes 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.

A Non‐Magnetized Chondrite Parent Body Revealed by Paleomagnetic Investigation of LL6 Chondrite NWA 14180

“…Hence, in light of the paleomagnetic results for other LL6 chondrites, we propose that no dynamo field existed during tetrataenite formation in NWA 14180 (Figure 11). Moreover, LL7 chondrites such as NWA 6990 and NWA 11004 are products of shock heating rather than endogenic thermal metamorphism within their parent planetesimal (Edwards & Blackburn, 2020; Li et al., 2020). This contradicts the assumption that type 7 chondrites represent a transitional zone between chondrites and achondrites from a partially differentiated planetesimal (Weisberg et al., 2006), which indicates that no melting occurred in the parent planetesimal of LL chondrites.…”

“…Hence, in light of the paleomagnetic results for other LL6 chondrites, we propose that no dynamo field existed during tetrataenite formation in NWA 14180 (Figure 11). Moreover, LL7 chondrites such as NWA 6990 and NWA 11004 are products of shock heating rather than endogenic thermal metamorphism within their parent planetesimal (Edwards & Blackburn, 2020;Li et al, 2020). This contradicts the assumption that type 7 chondrites Note.…”

Multiple shock events recorded in the Northwest Africa 2139 LL6 chondrite: Implications for collisional histories of the LL chondrite parent body

“…LL chondrites, which we focus on in this study, are not the exception. Although LL chondrites are less investigated than H and L chondrites, certain authors suggested that LL chondrite parent body(ies) underwent a shock event before 4.5 Ga (Dixon et al., 2004; Friedrich et al., 2017; Li et al., 2020; Rubin, 2002) and a major shock event at 4.2 Ga (Dixon et al., 2004; Friedrich, Perrotta, et al., 2014; Hugo et al., 2019).…”