NWA 7325 is a unique gabbroic achondrite which has sparked interest as possibly having originated from Mercury (Irving et al., 2013). In order to help understanding its origin, we investigated the chronology of this meteorite using both the U-Pb and Al- Mg systems. Due to the low U concentrations of NWA 7325, obtaining a precise Pb-Pb age and the U isotopic composition of this meteorite are difficult. Despite these daunting challenges, using an average Solar System 238U/235U ratio of 137.794 we
obtained a Pb-Pb isochron age of 4563.4 ± 2.6 Ma from six pyroxene residues. This age is consistent with our calculated Al-Mg (26Al/27Al0 = 3.03 ± 0.14 × 10-7) age of 4563.09 ±
0.26 Ma when anchored to the D’Orbigny angrite. Furthermore, the Al-Mg age is an order of magnitude more precise than the Pb-Pb isochron age, thus demonstrating the importance of using multiple isotopic systems in studies of early Solar System chronology. In addition to providing age data, the Al-Mg systematics of NWA 7325 also revealed an excess in δ26Mg*. We show that this excess can be attributed to Al/Mg
fractionation processes after planetary formation, without invoking the possibility of Mg isotopic heterogeneity in the early Solar System. Additionally, we show that the parental source of NWA 7325 most likely formed within 1.72 Ma of CAI formation.
The combined U-Pb and Al-Mg age data for NWA 7325 indicate its formation is almost contemporaneous with many of the oldest achondrites (e.g., quenched angrites) and only 4.2 Ma after CAIs. While this does not rule out the possibility of NWA 7325 originating from Mercury, it does make the prospect unlikely. Regardless of this however, NWA 7325 appears to be from a previously unsampled differentiated planetary body. With the addition of NWA 7325 to the achondrite data base, it is becoming apparent that a multitude of planetesimals formed, differentiated and crystallised from a chemically heterogeneous protoplanetary disk ~4-5 Ma after CAI formation.
79
4
U-Pb systematics of the anomalous achondrite
Asuka 881394
4.1
Introduction
Asuka 881394 (A881394) is a unique coarse grained eucrite like achondrite which was found in Antarctica in 1988. First thought to belong to the HED group, this small meteorite (total mass of 70.9 g) is now understood to have originated on a parent body separate from eucrites and other HED meteorites (Nyquist et al., 2003; Sanborn and Yin, 2014; Scott et al., 2009; Takeda et al., 1997; Wadhwa et al., 2009). Additionally, the currently reported ancient age of A881394 makes it the oldest achondrite studied so far (Wadhwa et al., 2009).
Although the minerology and chemistry of A881394 suggest this meteorite is similar to eucrites, there are a number of distinct features which make it unique. A991394 consists of roughly equal parts pyroxene (50%) and plagioclase (45%) with minor amounts of tridymite (5%) (Nyquist et al., 2003). In comparison to basaltic eucrites, the plagioclase crystals are very calcic (~An98), while the pyroxenes are high in magnesium (Nyquist et al., 2003; Takeda et al., 1997). Additionally, A881394 has a granulitic texture with pyroxene grains up to ~1.5 mm in length. The lack of pigeonite-to-orthopyroxene inversion textures and the very calcic plagioclase also suggest A881394 experienced a different cooling history to that of cumulate eucrites (Nyquist et al., 2003). Furthermore, differences in Mn-Cr systematics, stable Cr
80
isotopes anomalies and O isotope data indicate A881394 formed on a different body to eucrites and other HED meteorites (Sanborn and Yin, 2014; Scott et al., 2009; Wadhwa et al., 2009).
The currently reported Pb-Pb age of A881394 of 4566.51 ± 0.21 Ma using
238U/235U ratio of 137.88 (4565.33± 0.21 Ma when converted to the now measured
A881394 ratio of 238U/235U of 137.768 (Wimpenny et al., 2013)) makes it the oldest
achondrite studied so far (Wadhwa et al., 2009). However, the age interval between A881394 and D’Orbigny is inconsistent between the absolute and relative chronometer systems. The age difference between A881394 and D’Orbigny measured with Pb isotopes is 2.1 ± 0.3 (using 238U/235U ratio of 137.88), while the age
difference recorded by the 26Al-26Mg and 53Mn-53Cr systems are 1.0 ± 0.2 Ma and 0.9
± 0.4 Ma respectively (Wadhwa et al., 2009; Wimpenny et al., 2013). As a result, it is difficult to advocate the use of A881394 as one of the benchmarks of early Solar System chronology if we consider D’Orbigny to be an accurate age anchor (which based on the age consistencies among angrites and between angrites and a number of ungrouped achondrites appears to be true). Several possible causes for this age discrepancy have been discussed, including different closure temperatures of the isotopic clocks, heterogeneous distribution of 26Al/27Al and 53Mn/55Mn in the early
Solar System and later disturbances of one or both isotopic systems (Wadhwa et al., 2009; Wimpenny et al., 2013). Despite this, no definitive cause of this age discrepancy has yet been identified.
The previous U-Pb study on A881394 by Wadhwa et al. (2009) yielded a precise Pb-Pb isochron (4566.51±0.21 Ma) from the analysis of 16 pyroxene and 2 bulk rock fractions. Of the points used to make this isochron, 16 were highly radiogenic, with blank-corrected 206Pb/204Pb between 423 and 2675, while two were
less radiogenic, with blank-corrected 206Pb/204Pb of 97 and 135. Due to the small
fraction sizes (1.3 to 17.3 mg), thus large blank correction, the highly radiogenic analyses had large uncertainties which produced overlapping error ellipses. This resulted in the isochron effectively becoming a three point plot with the slope and intercept controlled by the two least radiogenic samples. If these two lesser radiogenic samples contain non-radiogenic Pb components different from the radiogenic group, the resulting isochron ages may be inaccurate. In this study we re- examine the Pb isotopic systematics of A881394 using an approach designed to evaluate the possible systematic uncertainty in the previous Pb isotopic isochron
81
age, while also helping us better understand the age discrepancy between the absolute and relative isotopic chronometers.