HED meteorites

Fig. 3.3 Three oxygen isotope plot of lunar and Martian rocks and HED meteorites supposed to be fragnments of asteroid Vesta (after Wiechert et al. 2003)...

Mesosiderites are a highly enigmatic group of differentiated meteorites. They are breccias composed of iron-nickel metal and silicate in roughly equal proportions. The metal represents molten material from the deep interior of an asteroid, whereas the silicate fraction consists of basalts and pyroxene cumulates similar to HED meteorites that formed near the surface. It is difficult to construct models that allow mixing of such diverse materials, but these disparate materials are generally thought to have been violently mixed by impact. 

The majority of HED meteorites show evidence of isotopic resetting over a limited period of time between 3.4 and 4.1 Ga ago. Most of the evidence comes from the system, but the 87Rb-87Sr system was also disturbed at this time in some meteorites... 

A peak in the shock ages for HED meteorites also corresponds to the period of late heavy bombardment on the Moon. This is unlikely to be a coincidence, and provides further support for this important event in solar system history. 

HED meteorites. Determining the presence of minerals with specific compositions from asteroid spectra can provide more quantitative correlations with meteorites (Gaffey et al., 2002). 

In most respects, asteroid 4 Vesta is geochemically similar to the Moon. As judged from howardite-eucrite-diogenite (HED) meteorites (see Chapter 6), Vesta is an ancient, basalt-covered world (Keil, 2002). Its rocks are highly reduced, and its depletions in volatile and siderophile element abundances resemble those of lunar basalts. And like the Moon, Vesta is hypothesized to have had an early magma ocean. The exploration of Vesta is now in progress, and within a few years we may have enough data to discuss it in a similar way that we have considered the Moon. 

Geologic data for asteroid 4 Vesta, (a) Spectral map of a part of the surface of Vesta as it rotates, showing the distribution of HED-like lithologies, modified from Binzel et at. (1997). (b) Interpretive geologic cross-section of Vesta, illustrating how HED meteorites might be distributed in the interior,... 

Usui, T. and McSween, H. Y. (2007) Geochemistry of 4 Vesta based on HED meteorites prospective study for interpretation of gamma-ray and neutron spectrometer for the Dawn mission. Meteoritics and Planetary Science, 42, 255-269. 

HED parent body the parent asteroid of the HED meteorites, thought to be the asteroid 4 Vesta based on the spectral similarities between this object and the HED meteorites. A known dynamical mechanism exists that can deliver material from Vesta to Earth-crossing orbits, supporting this assumption. 

HED meteorites howardites, eucrites, diogenites Mesosiderites Pallasites... 

Differentiated (planetary) Achondrites Angrites Aubrites Brachinites HED meteorites Eucrites Howardites Diogenites Ureilites Stony-irons Pallasites... 

Figure 28 Bulk oxygen isotopic compositions of SNC meteorites, lunar meteorites, and HED meteorites (source Clayton and Mayeda, 1996).

Mesosiderites are polymict breccias, as are many HED meteorites, thus presenting evidence for impact mixing on the surfaces of their parent bodies. Impact mixing on asteroidal bodies is expected to be parent-body-wide (e.g., Housen et al., 1979), leading some researchers to argue that mesosiderites and HEDs were formed on different parent bodies (see Mittlefehldt et al., 1998 Rubin and Mittlefehldt, 1993). 

Kimura M., Ikeda Y., Ebihara M., and Prinz M. (1991) New enclaves in the Vaca Muerta mesosiderite petrogenesis and comparison with HED meteorites. Proc. NIPR Symp. Antarct. Meteorit. 4, 263-306. 

Wiechert U., Halliday A. N., Palme H., and Rumble D. (2003) Oxygen isotopes in HED meteorites and evidence for rapid mixing in planetary embryos. Earth Planet. Sci. Lett, (in press). 

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... 

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