Petrogenesis and In Situ U-Pb Geochronology of a Strongly Shocked L-Melt Rock Northwest Africa 11042

Abstract

Northwest Africa (NWA) 11042, originally classified as a primitive achondrite with no chondritic relicts, is rather a unique L‐melt rock. It is a severely shocked, igneous‐textured ultramafic rock composed of euhedral to subhedral olivine (Fa25.1 ± 0.5) and pyroxenes (low‐Ca pyroxene Fs20.7 ± 0.8Wo4.2 ± 1.0 and Ca‐rich pyroxene Fs11.5 ± 0.5Wo37.6 ± 1.2) with interstitial albitic plagioclase (Ab80.7 ± 1.7Or5.0 ± 0.7) that has been completely converted to maskelynite. Mineral compositions are similar to those of equilibrated L‐chondrites. Melt pockets are scattered throughout the sample, containing high‐pressure minerals including ringwoodite, wadsleyite, jadeite, and lingunite. Merrillite and apatite in NWA 11042 contain significantly higher REE abundances than those of ordinary chondrites, indicative of igneous fractional crystallization. In situ U‐Pb dating of apatite in NWA 11042 reveals an upper intercept age of 4,479 ± 43 Myr and a lower intercept age of 465 ± 47Myr on the normal U‐Pb concordia diagram. The upper intercept age recorded the time when NWA 11042 initially crystallized. This age is much younger than when the decay of short‐lived nuclides (e.g., Al) would act as a major heat source, suggesting that melting and crystallization of NWA 11042 could be otherwise triggered by an impact event. The lower intercept age represents a reset age due to a later impact event, which is in coincidence with the disruption event of L‐chondrite parent body at ~470 Myr. NWA 11042 is an excellent example to link igneous‐textured meteorites with a chondritic parent body through shock‐induced melting. Plain Language Summary Achondrites are meteorites without any chondrules. There are two general categories: primitive achondrites and differentiated achondrites. Primitive achondrites are suggested to have formed from low‐degree partial melting of chondritic precursors, and differentiated achondrites are products of large degree of igneous fractionation on differentiated parent bodies. NWA 11042 exhibits an igneous texture with severe shock features similar to some Martian shergottites, but its mineral and oxygen isotopic compositions are most compatible with those of L‐chondrites. Based on its texture, chemical, and chronologic analyses, we found that NWA 11042 is closely related to L‐chondrites. The formation age of NWA 11042 is younger than when the decay of short‐lived nuclides would act as a major heat source. Thus, the formation of NWA 11042 would be possible due to impact heating. In addition, NWA 11042 also records a shock‐reset age that is in coincidence with the disruption event of L‐chondrite parent body. In conclusion, NWA 11042 is a melt rock derived from impact melting of L‐chondrite rocks, providing clues of melting and crystallization of chondritic materials with shock‐induced heating. And this is a good link between the impact history of the parent body and the petrologic, chronologic characteristics