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Rider-Stokes, Ben
(2023).
DOI: https://doi.org/10.21954/ou.ro.00017156
Abstract
Angrite meteorites represent a group of ancient volcanic-plutonic mafic rocks that are thought to originate from a differentiated, early-formed planetesimal inward of Jupiter’s orbit. These meteorites have been reported to contain high quantities of hydrogen in accessory phosphates and have experienced variable amounts of shock deformation/impact mixing. They are therefore ideally placed to test the impact and volatile accretion histories of early-formed planetesimals.
To test the impact history of the angrite parent body (APB), nine angrites were investigated for whole-rock oxygen isotopic analysis. Mineral separations of the groundmass and ‘xenocrysts’ from three quenched angrites reveal an isotopic disequilibrium, indicative of isotopic mixing, suggesting that the xenocrysts are in fact relict olivine grains that survived impact melting. Northwest Africa (NWA) 12320 displays evidence of high-temperature processing, supportive of an impact-driven melting hypothesis, whereby the relict grain experienced high temperatures while situated in the impact-induced melt. Due to the ancient age of the quenched angrites, the large-scale mixing event may be related to the timing and/or migration of Jupiter. Through Pb-Pb dating of phosphates within NWA 12320 we provide the first empirical constraint on the timing of this major Solar System event. Phosphates record a crystallisation of ~4 Ma after Calcium Aluminium Rich Inclusions (CAI) formation, coinciding with Jupiter’s formation and/or migration.
Further Pb-Pb investigations of samples which have not been previously dated prior to this study were evaluated alongside NWA 12320. Most noticeably, a unique angrite, NWA 8535, records a much younger date in comparison to all other angrites reported in the literature. This date may record prolonged magmatism on the APB, which to this date, has not been deciphered.
To evaluate the role of impacts in terms of volatile accretion, the H isotopic compositions and H2O abundances of relict olivine and groundmass fractions within one quenched angrite that displays an oxygen isotopic disequilibrium (Asuka 12209) is investigated using in-situ nanoscale secondary ion mass spectrometry (NanoSIMS). Hydrogen abundance and isotopic compositions between the groundmass and olivine within this sample shows no discernible variation. This lack of variation implies that impacts in the very early Solar System played a limited role in the addition of volatiles to the inner Solar System. Furthermore, the H isotopic compositions and H2O abundances of later-forming angrites were evaluated to assess volatile accretion throughout the first 50 Ma of the inner Solar System history. These analyses hint at an increase in hydrogen contents in the latter periods of the APB’s history, suggesting impacts may have played a more significant role in the later history of the inner Solar System.
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