¹⁸²Hf‐¹⁸²W chronometry and the early evolution history in the acapulcoite‐lodranite parent body

Abstract

Abstract-An acapulcoite, Northwest Africa (NWA) 725, a transitional acapulcoite, Graves Nunataks (GRA) 95209, and a lodranite, NWA 2235, have been studied with the short-lived chronometer 182 Hf-182 W system in order to better constrain the early evolution history in the acapulcoite-lodranite parent body. Unlike the more evolved achondrites originating from differentiated asteroids-e.g., eucrites and angrites-bulk rock acapulcoites and lodranite are characterized by distinct 182 W deficits relative to the terrestrial W, as well as to the undifferentiated chondrites, ε w varies from −2.7 to −2.4. This suggests that live-182 Hf was present during the formation of acapulcoites and lodranites, and their parent body probably had never experienced a global melting event. Due to the large uncertainties associated with the isochron for each sample, the bulk isochron that regressed through the mineral separates from all 3 samples has provided the best estimate to date for the timing of metamorphism in the acapulcoite-lodranite parent body, 5 (+6/−5) Myr after the onset of the solar system. It is thus inconclusive whether acapulcoites and lodranites have shared the same petrogenetic origin, based on the Hf-W data of this study. Nevertheless, the formation of acapulcoite-lodranite clan appears to have post-dated the metal-silicate segregation in differentiated asteroids. This can be explained by a slower accretion rate for the acapulcoite-lodranite parent body, or that it had never accreted to a critical mass that could allow the metal-silicate segregation to occur naturally.

Petrologic and Minerochemical Trends of Acapulcoites, Winonaites and Lodranites: New Evidence from Image Analysis and EMPA Investigations

“…Based on W and Sm isotope measurements for metals and silicates of non-magmatic iron meteorite groups and winonaites, their exposure history and parent body formation has been assessed [25]. A metal separation at 5.06 +0.42 /−0.41 Ma after the solar system formation was proposed, which agrees with the previously published 182 Hf-182 W ages for both the silicate melting (4.6 +0.7 /−0.6 Ma) and the formation of winonaites (4.8 +3.1 /−2.6 Ma) and corresponds to an accretion age of ~2 Ma after Solar System formation for the IAB/winonaite parent body [10]. The metamorphism of IAB silicate inclusions and winonaites has been reported to take place at significantly different times, between ~11 and ~14 Ma, via an impact triggered melt pool formation process [25].…”

“…These features are more common in acapulcoites than in lodranites, which experienced a higher metamorphic grade. Recently, several reports have focused on the 182 Hf-182 W chronometry to reveal the early evolution history in the acapulcoite-lodranite parent body [10,11], while others have focused on the I-Xe system to determine the accretion age of the acapulcoite-lodranite parent body, suggesting the existence of post-formation collision events [12]. The affinity of lodranites for acapulcoites appears evident from the isotopic distribution of oxygen, supporting the hypothesis that they originated from a common parent body.…”

Hf–W chronometry of primitive achondrites

“…(a) Acapulcoites and lodranites: these meteorites were investigated for their Hf-W systematics by Lee (2008). The authors obtained a rather imprecise age for parent body differentiation of 5 +6 / À7 Ma after solar system formation.…”