86th Annual Meeting of the Meteoritical Society 2024 (LPI Contrib. No. 3036)

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ACCRETION AND THERMAL HISTORY OF THE ANGRITE PARENT BODY
A. Bouvier1, E. Obengo1, P. M. Reger2, W. Neumann3, W. H. Schwarz4, T. Ludwig4, M. Trieloff4, 1University ofBayreuth, Germany (audrey.bouvier@uni-bayreuth.de), 2University of Western Ontario, Canada, 3 German Aerospace Center (DLR), Berlin, Germany 4 Heidelberg University, Germany
Introduction: Quenched (~4564 Ma) and plutonic (~4557 Ma) angrite meteorites generally lack reports of major
shock effects, which could have altered their primary chemical and isotopic compositions [1]. Angrites have therefore
been used as anchors for short-lived radiochronometers [2]. The angrites constituting the intermediate-aged ~4560 Ma
group (e.g., NWA 2999, NWA 6291) are however brecciated meteorites with metal-rich compositions [3]. Recently,
moderate to high shock effects were reported for NWA 1670 (containing olivine xenocrysts with fractures, mosaicism,and undulatory extinction) and NWA 7203 angrites (containing melt shock veins associated with amorphous anorthiteand younger Pb-Pb ages of silicophosphates at 4543 ± 19 Ma) [4]. The diabasic angrite NWA 12320 has pyroxene
with a Pb-Pb age of ~4564 Ma that is similar to quenched angrites, and 1.3±0.6 Myr younger U-Pb phosphate ages,
which can be interpreted as a rather slow cooling (consistent with its texture) or later reheating [5]. Additionally,
impact processes may have modified their triple O isotopic compositions by crystal assimilation and brecciation processes [6]. We also identified mixed 26Al-26Mg isotopic signatures in anorthite fractions of NWA 10463 [7]. We herecombine petrological observations and geochronological records of angrites to constrain the accretion and thermal
evolution history of the angrite parent body.
Study: We investigated the mineral compositions, textures, and U-Pb geochronology of Ca-(silico)phosphates inNWA 8535, NWA 10463, and NWA 12004 angrites. We combine new and literature data (long-lived and short-lived
chronometers in mineral phases and estimates for corresponding closure temperatures as in [8]) to constrain a numerical thermal model for the angrite parent body.
Results & Discussion: NWA 10463 has intermediate textures comprising slow-cooling textures like exsolution
lamellae in Ca-rich olivine. We also observe reverse zoning in olivine, reaction rim coronas, and significant brecciation indicating a metamorphic alteration. The diabasic angrite NWA 12004 possesses fine to medium-grained textures
but its chemical composition is akin to quenched angrites such as D´Orbigny and NWA 12320. NWA 8535 olivinepreserved its cumulate texture including large olivine grains with triple junctions indicating equilibration due to slowcooling. Olivine grains however bear strong concentric zoning suggesting rapid cooling.
Chronology. We obtained Ca-(silico)phosphate Pb-Pb isotopic compositions by SIMS Cameca 1280 HR as in [9].
The average 204Pb/206Pb and Pb-Pb isochron mineral-Canyon Diablo Troilite ages (all 2SE) are 0.00016 ± 0.00008 and4552.8 ± 4.6 Ma (n=7) for NWA 10463, 0.0007 ± 0.0002 and 4554 ± 8 Ma (n=8) for NWA 12004, and 0.0003 ±0.0002 and 4515 ± 16 Ma (n=2) for NWA 8535, respectively. Our Pb-Pb age of phosphates for NWA 10463 is significantly younger than 4563.6 ± 7.9 Ma (n=6) reported by [10]. The reported age of NWA 8535 is identical to [10].
We are undertaking micro-structural analyses of the phosphate grains that were analyzed by SIMS and of other minerals to interpret their chronological records. The younger ages of the Ca-phosphates in angrites, compared to reported
pyroxene ages for quenched, plutonic, and intermediate angrites, indicate that magmatic, metamorphic, and metasomatic activity continued in the angrite parent body for an extended period of ~50 Myr after CAIs.
Numerical modeling. A total of 29 age dates from U-Pb (pyroxene and phosphate), Hf-W, Mn-Cr, and Al-Mg
systems for 13 quenched and plutonic angrites were considered in the thermal evolution modeling using a model from[8]. The data were fitted for each meteorite at a separate depth. The fit quality calculated indicates a parent bodyaccretion time at around 0.6-0.8 Ma after CAIs and a radius of at least 100 km. The parent body has a fully differentiated interior with a metallic core and a silicate mantle. A transient magma ocean forms in the mantle, while theangrites originate from a fully differentiated upper mantle above the magma ocean. The layering depths of the angrites
fitted spread typically between 2 and 20 km. Based on the U-Pb phosphate ages and closure temperatures, our modeling suggests an early extrusion at a time of ~10-12 Ma after CAIs from a deeper interior to a shallower depth forAngra Dos Reis and NWA 4590. Our results indicate that the angrite parent body formed after the parent body of Erg
Chech 002 at ~0.1 Ma after CAIs [8], but older than chondrite parent bodies and than the proposed accretion ages of1.7 and 1.1 Ma for primitive achondrite Acapulco-Lodran [11] and the Tafassasset [9] parent bodies, respectively.
References: [1] Amelin Y. (2008) Geochim. Cosmochim. Acta, 72: 221-232. [2] Wadhwa M., Treatise on
Geochemistry, Elsevier, Editor. 2006. [3] Kleine T. et al. (2009) Lunar Planet. Sc. Conf. XL, A#2403. [4] Hayashi H.
et al. (2022) Meteoritics & Planetary Science, 57: 105-121. [5] Datta C. et al. (2024) Icarus, 412: 115979. [6] RiderStokes B. et al. (2023) Nature Astronomy, 7: 836-842. [7] Reger P. M. et al. (2021) 84th Annual Meeting of theMeteoritical Society, 84, 6235. [8] Neumann W. et al. (2023) The Planetary Science Journal, 4: 196. [9] Ma M. et al.
(2022) Geochemical Perspectives Letters, 23: 33-37. [10] Rider-Stokes B. G. et al. (2024) Meteoritics & PlanetaryScience, 59: 23-39. [11] Neumann W. et al. (2018) Icarus, 311: 146-169.