How Old Are they?
Based on the analysis of long-lived and short-lived isotopic systems, it is thought that the differentiation of the HED parent body occurred only a few million years after the formation of the CAIs and chondrules. Some anomalous samples, such as Asuka 881394 which is apparently an ancient eucrite (Wadhwa et al., 2009), can provide information about the timing of differentiation on bodies that may be similar to 4 Vesta.
Long-lived chronometers
Although many of the long-lived chronometers have yielded ages that are younger than 4.4 Ga (e.g., Rb-Sr: Papanastassiou et al., 1974; Sm-Nd: Bogard et al., 1993; Galer and Lugmair 1996; Prinzhofer et al. 1992; Pb-Pb: Tera et al., 1997), both cumulate and non-cumulate eucrites yield ancient ages (Figure 15). A Lu-Hf isochron for 18 eucrites yields an age of 4.464 Ga (Blichert-Toft et al., 2002). These ancient ages have been supported by more recent U-Pb ages obtained on zircons, yielding a range of ages from 7 to 20 Ma after T0 (Figure 16; Misawa et al., 2005).
Figure 15: Histogram of ages for cumulate and non-cumulate eucrites dated using the Sm-Nd method. There are ancient ages recorded by both cumulate and non-cumulate eucrites, but some younger ages may be due to thermal resetting (from Pieters et al. (2005) summary).
Figure 16: Combined isochron for five eucrites from the study of Misawa et al. (2005)illustrating the very old age for these meteorites.
It is generally accepted that many younger ages (<4.4 Ga) may reflect resetting due to impact or thermal metamorphism (Bogard, 1995). However, the details of the cooling and metamorphism are not fully understood. For example, based on the cooling history of cumulate eucrites determined by Miyamoto and Takeda (1994), Tera et al. (1997) have argued that the young (4.40-4.48 Gyr) Pb-Pb ages of three cumulate eucrites analyzed by them are, in fact, their true crystallization ages. More work is no doubt required to sort out which ages are reset and which record crystallization ages.
The best evidence for resetting comes from the Ar-Ar systematics of eucrites. Ages determined by argon isotopic analyses can represent disturbed or reset ages by major impact events (Bogard, 1995). The clustering of Ar-Ar ages for eucrites between 4 and 3.5 Gyr (Figure 17) is highly suggestive of a period of heavy bombardment similar to the late heavy bombardment suggested for the Moon (Ryder, 2002). HEDs share several common age peaks (Figure 17), and the youngest at 3.5 Gyr may indicate that samples destined to become HED meteorites resided on Vesta until at least as recently as 3.5 Gyr, and likely much later (based on CRE ages from Welten et al., 1997). However, younger ages for the formation of Vestoids become increasingly less likely because of the rarity of giant impacts sufficient to form the south pole basin on Vesta.
Figure 17: Summary of Ar-Ar ages for eucrites indicating several possible peaks that are from thermal resetting due to impact and/or metamorphism on the HED parent body (from Bogard, 1995, review paper).
Short lived chronometers
Early igneous differentiation on the HED parent body is also supported by analysis of the Al-Mg, Mn-Cr, Hf-W, Fe-Ni and Ag-Pd systems.
Many previous attempts to find evidence for live 26Al in HEDs and other differentiated meteorites have been unsuccessful (e.g., Schramm et al. 1970; Lugmair and Galer 1992; Hsu and Crozaz 1996). However, a recent Al-Mg study of the Piplia Kalan basaltic eucrite suggest Al / Alof (7.5± 0.9)x10 , indicating possible formation 5 Myr after CAIs (Figure 18; Srinivasan et al. 1999). This suggests that igneous differentiation could have occurred very early on the HED parent body. This is important as it may add validity to previous suggestions that Al was an important heat source for melting and differentiation on asteroids.
Figure 18: Isochron for Mg-Al system in the Piplia Kalan eucrites, indicating the presence of live 26Al at the time of eucrites formation. From the study of Srinivasan et al. (1999).
Bulk rock Mn-Cr systematics of eucrites and diogenite samples define a good correlation line, indicating that the source regions for these HED meteorites were formed contemporaneously (Lugmair and Shukolyukov, 1998). When the Mn/ Mn ratio for the bulk HED isochron (53Mn/ Mn of (4.7 ± 0.5) x 10 ) is compared to the Mn/ Mn in angrites of (1.25 ± 0.07) x 10 (Lugmair and Shukolyukov 1998), whose absolute Pb-Pb age is established as 4557.8 ± 0.5 Myr (Lugmair and Galer 1992), a Mn-Cr model age of 4564.8 ± 0.9 Myr is obtained. Thus, the significance of the age defined by the bulk HED isochron lies in the implication that the HED parent body was accreted and underwent complete differentiation within ~4 Myr from the time of formation of the first known solids (i.e., CAIs) in the solar nebula (Figure 19).
Figure 19: Isochron for Mn-Cr system and HED meteorites measured by Trinquier et al. (2009) showing the ancient age for differentiation of the HED parent body.
Short timescales of accretion and subsequent core formation on asteroidal bodies are additionally supported by Hf-W systematics (summarized in Fig. 20; Lee and Halliday, 1996, Lee et al., 1997). Tungsten isotopic data for various iron meteorites indicate a deficit in W relative to carbonaceous chondrites (Fig. 20), which implies that metal segregation occurred before Hf had decayed completely to W. As a result, Hf-W systematics in eucrites also point towards rapid accretion, differentiation, and core formation on the HED parent body; that is, within 5-15 Myr of solar system history (Lee and Halliday 1996).
Figure 20: Epsilon W values for iron meteorites (top) and eucrites (bottom) compared to some chondrite groups (from Halliday, 2000). W deficit for irons compared to positive anomaly for eucrites illustrates that there was live Hf when the HED parent body differentiated.
Finally, the 60Fe-60Ni and the 107Pd-107Ag systems also show evidence for early differentiation. 60Fe, with a half-life of 1.5 Myr, has been shown to have been extant at the time of differentiation and basalt formation on the HED parent body (Shukolyukov and Lugmair 1993).
Summary
The chronologic information obtained from both the long and short-lived chronometers can be summarized as follows. The differentiation of the HED parent body was completed within 3 to 5 Ma after T0. Some crystallization ages of basaltic meteorites are as young as 10-20 Ma after T0, but ages younger than this can be attributed to resetting of various isotopic systems during impact or thermal metamorphism (Figure 21). Impact craters reworked the surface during and after formation of the primary zones, rearranging and mixing components and produced many rocks with younger ages. Deposits from the largest (and perhaps earliest) craters could have buried samples deeply enough to cause heating and moderate metamorphism.
Figure 21: Timeline of early events in the solar system, including accretion and core formation, magmatism, and then later thermal metamorphism (from Kleine et al. (2009); with data from Quitte et al. (2000), Touboul et al. (2008), Yin et al. (2002)).