Evolution of the Angrite Parent Body and Concordancy of Isotope Chronometers in Angrite Meteorites

Seann J. McKibbin, Trevor R. Ireland, Yuri Amelin, Hugh St. C. O'Neill

Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia

Figure 1.

The small achondrite meteorite clan classified as angrites have unique mineralogy, chemistry and oxygen isotopic compositions that strongly suggest they share a single parent body. They come in two broad petrologic groups: quenched 'volcanics' such as D'Orbigny and Sahara 99555, and 'plutonics/metamorphics' like LEW 86010. They are particularly suited to isotopic dating using the Pb-Pb chronometer and the short-lived (extinct) Mn-Cr system (⁵³Mn → _⁵³Cr, t_1/2 = 3.74 My) from which age differences between meteorites can be obtained. The angrites have yielded ages in these systems spanning a period of Solar System history critical to understanding of protoplanet formation (~4564.5 to ~4557.5 Ma from Pb-Pb).

However, concordancy between the two decay systems has been poor, with Pb-Pb isotopes recording up to 7 My of evolution between early volcanic and later plutonic varieties, while the Mn-Cr ages preserve a difference in ages of only ~5.5 My for the same meteorites [1]. To investigate this discrepancy, the Mn-Cr systematics of D'Orbigny and Sahara 99555 have been reinvestigated using the SHRIMP-RG ion-probe. Results support an early Mn-Cr age for both meteorites, and reinforce the discordancy between D'Orbigny and LEW 86010 in the Mn-Cr/Pb-Pb systems. However, the new result for Sahara 99555, along with improved agreement in the community over its Pb-Pb crystallisation age, suggests that this meteorite is concordant when compared with the younger LEW 86010 (Figure 1). This means that the two meteorites sample a common isotopic reservoir evolving in time, with no complications from diffusive closure occurring in different minerals at different times, and no later disturbance.

The emerging consensus is that angrite basalts crystallised from lava flows or a magma ocean around 4564.5 Ma, and magmatism and thermal activity continued until at least 4557.5 Ma as the protoplanet cooled. Further work will centre on angrites NWA 4590 and NWA 4801, which have not yet been dated using the Mn-Cr system.

Wadhwa M, Amelin Y, Bizzarro M, Kita N, Kleine T, Lugmair G, Yin Q (2007) Comparison of short-lived and long-lived chronometers: Towards a consistent chronology of the early Solar System. Workshop on the Chronology of Meteorites and the Early Solar System pp. 173. LPI Contribution No. 1374, Lunar and Planetary Institute, Houston.