High-Precision 26A1-26Mg Systematics of Basaltic Achondrites, Chondrites and Ultramafic Achondrites
Authors:   , Schiller Martin 
Published: November 2021

<p>A precise and accurate chronology of events that shaped the early Solar System is crucial in understanding its formation. One of the high-resolution chronometers that can be used to establish a relative chronology is the short-lived 26A1-to-26Mg clock (t1/2 = 0.73 Myr). This study developed new Mg chemical separation techniques for complex meteoritic matrices that produces Mg purities > 99% with > 99% yields. Mg was analysed by pseudo-high resolution multiple collector inductively coupled plasma mass spectrometry. These techniques make it possible to measure the mass-independent abundance of 26Mg (d26Mg*) that is related to 26A1 decay to very high-precision (+_ 0.0025 to 0.0050 per1000). These new techniques were then applied to three research objectives. The first part of this study presents Mg isotope data for thirteen bulk basaltic achondrites from at least 3 different parent bodies, as well as mineral isochrons for the angrites Sahara 99555 and D'Orbingy and the ungrouped NWA 2976. Model 26A1-26Mg ages based on bulk rock d26Mg* excesses for basaltic magmatism range from 2.6-4.1 Myr, respectively, after formation of calcium-aluminium-rich inclusions (CAIs) and the mineral isochrons for the angrites Sahara 99555 and D'Orbigny, and the ungrouped NWA 2976 yield apparent crystallisation ages of 5.06+0:06-0:05 Myr and 4.86+0:10-0:09 Myr after CAI formation. The elevated initial d26Mg* of the mineral isochron of NWA 2976 (+0.0175+ _0.0034h) likely reflects thermal resetting during an impact event and slow cooling on its parent body. However, in the case of the angrites, the marginally elevated initial d26Mg* (+0.0068 -0.0058h) could reflect d26Mg* in-growth in a magma ocean prior to eruption and crystallisation or in an older igneous protolith with super-chondritic A1/Mg prior to impact melting and crystallisation of these angrites, or partial internal re-equilibration of Mg isotopes after crystallisation. 26A1-26Mg model ages and an olivine+pyroxene+whole rock isochron for the angrites Sahara 99555 and D'Orbigny are in good agreement with age constraints from 53Mn-53Cr and 182Hf-182W shortlived chronometers. This suggests that the 26A1-26Mg feldspar-controlled isochron ages for these angrites may be compromised by the partial resetting of feldspar Mg isotope systematics. However, even the 26A1-26Mg angrite model ages cannot be reconciled with Pb-Pb ages for Sahara 99555/D'Orbigny and CAIs, which are ca. 1.0 Myr too old (angrites) or too young (CAIs) for reasons that are not clear. This discrepancy might indicate that 26A1 was markedly lower (ca. 40%) in the planetesimal- and planet-forming regions of the proto-planetary disk as compared to CAIs, or that CAI Pb-Pb ages may not accurately date CAI formation. The second part of this thesis focuses on investigating the homogeneity of (26A1/27A1)0 and Mg isotopes in the proto-planetary disk and to test the validity of the short-lived 26A1-to-26Mg chronometer applied to meteorites. Nineteen chondrites representing nearly all major chondrite classes were analysed, including a step-leaching experiment on the CM2 chondrite Murchison. d26Mg* variations in leachates of Murchison representing acid soluble material are <_30 times smaller than reported for neutron-rich isotopes of Ti and Cr and do not reveal resolvable deficits in d26Mg* (-0.002 to +0.118h). Very small variations in d26Mg* anomalies in bulk chondrites (-0.006 to +0.019h) correlate with increasing 27A1/24Mg ratios and d50Ti, reflecting the variable presence of CAIs in some types of carbonaceous chondrites. Overall, the observed variations in d26Mg* are small and potential differences beyond those resulting from the presence of CAI-like material could not be detected. The results do not allow radical heterogeneity of 26A1 (>_+_ 30%) or measurable Mg nucleosynthetic heterogeneity (>_+_ 0.005h) to have existed on a planetesimal scale in the proto-planetary disk. The data imply that planets (i.e. chondrite parent bodies) accreted from material with initial (26Al/27A1)0 in the range of 2.1 to 6.7 x 10-5. The average stable Mg isotope composition of all analysed bulk chondrites is d25MgDSM-3 = -0.152 +_ 0.079 per1000(2 sd) and is indistinguishable from that of Earth's mantle. The third part of this study comprises a high-precision Mg isotope and mineral major and trace element study of 24 diogenites. Diogenites are ultramafic pyroxene and olivine cumulate rocks that are presumed to have resulted from magmatic differentiation on the howardite-eucritediogenite (HED) parent body. There are, however, no precise and independent age constraints on the formation of diogenites and, in particular, their age relationships to the basaltic eucrites. Mg isotope analysis of diogenites showed significant variability in d26Mg* anomalies that range from -0.0108 +_ 0.0018 to +0.0128 +_ 0.0018 per1000. These anomalies generally correlate with the mineral major and trace element chemistry and demonstrate active 26A1 decay during magmatic differentiation. Furthermore, it also suggests that diogenites are products of fractional crystallisation from a large scale magmatic system. Heating and melting of the HED parent body was driven by 26A1 decay and led to diogenite formation 0.7 to 1.3 Myr after CAIs depending on whether a heterogeneous or homogeneous (26Al/27A1)0 distribution is assumed between the proto-planetary disk and CAIs. These data show that diogenite formation pre-dates eucrite formation and indicate HED parent body accretion and core formation occurred within the first Myr of the Solar System. The lifetime of the magmatic evolution is less well constrained. The data suggest that the complete range of diogenites may have formed as quickly as ~ 0.2 Myr.</p>