Vesta formation

Myr after CAIs formed (Bizzarre et al., 2005). Taken together, these data imply that differentiation and crust formation began on the HED parent body 2.5-3 Myr after CAIs formed (Fig. 9.9). This, in turn, implies very rapid accretion, heating, and melting in Vesta, probably within 1 Myr of the origin of the solar system. 

Kleine, T., Mezger, K., Mtinker, C., Palme, H. andBischoff, A. (2004) Hf- W isotope systematics of chondrites, eucrites, and Martian meteorites chronology of core formation and early mantle differentiation in Vesta and Mars. Geochimica et Cosmochimica Acta, 68, 2935-2946. 

Righter, K. and Drake, M. J. (1996) Core formation in the Earth s Moon, Mars and Vesta. Icarus, 124, 513-529. 

Figure 9.2 Fraction of stars with excess emission at IRAC wavelengths (between 3.6 and 8 um) as a function of the age of the stellar group. In addition to the data presented in Hernandez et al. (2008) and references therein, we have included the disk frequencies in the TW Hya association (Weinberger et al. 2004), and from the FEPS sample of Sun-like stars (Silverstone et al. 2006). The dot-dashed line is the least-squares fit to the L-band data from Haisch et al. (2001b). Above the plot we show a comparison to the formation timescale of CAIs, chondrules, and the asteroid Vesta in the Solar System. As we discuss in Section 9.4 there is evidence that CAIs formed early, in the first Myr of disk evolution.

Cosmochemical analysis of meteorites provides important constraints on the timing of planet formation. The first differentiated bodies formed <1.5 Myr after the start of the Solar System. However, processing of dust grains in the nebula and the formation of planetesimals continued for at least the first 2-3 Myr. Vesta formed within 5 Myr, while Mars was fully grown and differentiated by 10-20 Myr. Earth took longer to grow, and it was not until 50-150 Myr that the planet was fully formed and the Moon was present. 

Most meteorites are depleted in moderately volatile and highly volatile elements (see Figures 2-4). The terrestrial planets Earth, Moon, Mars, and the asteroid Vesta show similar or even stronger depletions (e.g., Palme et aL, 1988 Palme, 2001). The depletion patterns in meteorites and in the inner planets are qualitatively similar to those in the ISM. It is thus possible that the material in the inner solar system inherited the depletions from the ISM by the preferential accretion of dust grains and the loss of gas during the collapse of the molecular cloud that led to the formation of the solar system. There is, however, little support for this hypothesis ... 

Righter K. and Drake M. J. (1997) A magma ocean on Vesta core formation and petrogenesis of eucrites and diogenites. Meteorit. Planet. Sci. 32, 929-944. 

The least fractionated rocks of the Earth are those that have only suffered core formation but have not been affected by the extraction of partial melts during crust formation. These rocks should have the composition of the PM, i.e., the mantle before the onset of crust formation. Such rocks are typically high in MgO and low in AI2O3, CaO, Ti02, and other elements incompatible with mantle minerals. Fortunately, it is possible to collect samples on the surface of the Earth with compositions that closely resemble the composition of the primitive mantle. Such samples are not known from the surfaces of Moon, Mars, and the asteroid Vesta. It is, therefore, much more difficult to reconstruct the bulk composition of Moon, Mars, and Vesta based on the analyses of samples available from these bodies. 

Abundances of nonrefractory incompatible lithophile elements (potassium, rubidium, caesium, etc.) or partly siderophile/chalcophile elements (tungsten, antimony, tin, etc.) are calculated from correlations with RLE of similar compatibility. This approach was first used by Wanke et al. (1973) to estimate abundances of volatile and siderophile elements such as potassium or tungsten in the moon. The potassium abundance was used to calculate the depletion of volatile elements in the bulk moon, whereas the conditions of core formation and the size of the lunar core may be estimated from the tungsten abundance, as described by Rammensee and Wanke (1977). This powerful method has been subsequently applied to Earth, Mars, Vesta, and the parent body of HED meteorites. The procedure is, however, only applicable if an incompatible refractory element and a volatile or siderophile element have the same degree of incompatibility, i.e., do not fractionate from each other during igneous processes. In other words, a good correlation of the two elements over a wide... 

The formation of planetesimals may have been very rapid after the initial formation of the solar nebula. Objects as large as Mars would have grown within 10 yr (Weatherill, 1990). The core of the asteroid Vesta may have formed within only 3-4 Myr, and lavas flows on its surface may have occurred at this time also (Yin et al, 2002). Bodies like Vesta would have collided rapidly, aggregating their cores to form larger planetoids and then planets. The date of core formation in the Earth remains controversial but may have been as little as 30 Myr or less after the birth of the solar system (Kleine et al, 2002). Yin et al (2002) suggest that the aggregation of the Earth s core took place within 29 Myr. The core of Mars may have formed as early as within 13 Myr. 

Core formation model ages for Vesta, Mars, Earth and Moon... 

Then the moon s geological development stopped as rock formation ceased. The moon was too small it didn t hold enough heat to keep its insides liquid, and it didn t have enough volatile gases to keep pushing out its lava. The crust froze together and the volcanoes stopped. Mercury and the asteroid Vesta are so small and dry that they are frozen together in this point of time. 

Nonetheless, MC-ICP-MS has been used successfully for Nd isotopic analysis in cosmochemistry. A good example is the combined use of the Sm-Nd and Lu-Hf systems by Blichert-Toft et al. [70], They analyzed 18 eucrites (Table 10.1) and found an Sm— " Nd age of 4464 + 75 Ma, with three cumulative eucrites defining a better constrained isochron of 4470 + 22 Ma. This suggests that cumulative eucrites have a crystallization age that postdates the formation of the solar system by about 100 Ma and implies a fairly protracted crystallization history for the eucrite parent body, which is thought to be the asteroid 4 Vesta (Table 10.1). Combined " Nd and Nd ages of eucrites [71, 72] give a some-... 

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