What are they?
Lithologies, textures and chemistry
Eucrites are basaltic rocks comprised of pigeonite (low Ca clinopyroxene) and plagioclase feldspar, with minor phosphate, metal, troilite, silica, and ilmenite, and come in several different types based on their textures (Figure 1). First are basaltic textures such as coarse-grained ophitic and sub-ophitic, fine grained aphyric, and some even vesicular. These are all referred to as the non-cumulate eucrites. Second, are even coarser grained pigeonite and plagioclase rocks that have adcumulate or cumulate textures - these are classified as the cumulate eucrites. Eucrites can be unbrecciated, but most commonly they are brecciated and can be of one lithology (monomict) or many different lithologies (polymict).
Figure 1: plane and cross polarized light images of an unbrecciated eucrite LEW 85305 (left) and a brecciated eucrite, MIL 07004 (right)
Diogenites are coarse-grained orthopyroxene-rich rocks, generally ~ 90% orthopyroxene, and contain minor amounts of olivine, chromite, plagioclase, clinopyroxene, and opaque minerals such as troilite and metal. Diogenites are commonly brecciated, but there are unbrecciated samples as well (e.g., GRO 95555; Figure 2).
Figure 2: plane and cross polarized light images of a brecciated diogenite EETA79002 (left) and an unbrecciated diogenite, GRO 95555 (right)
Although olivine is typically < 10%, there is a growing group of olivine-rich diogenites that contain up to 50% olivine. These are of great interest to HED meteorite specialists because they may offer insight into the HED mantle, or to one end-member of magmatic evolution that had olivine and orthopyroxene crystallizing together.
Many eucrites and diogenites have been metamorphosed such that their pyorxenes and plagioclases have been equilibrated and lost any compositional record of the original igneous zoning (Reid and Barnard, 1979; Takeda and Graham, 1991). In this group are basaltic rocks that have been recrystallized into fine grained granulitic textures, and represent metamorphosed basalts (Yamaguchi et al., 1996, 1997). Diogenites have also experienced thermal metamorphism (e.g., Mori and Takeda, 1981; Yamaguchi et al., 2010) and this must be kept in mind when considering igneous formation models.
Howardites are brecciated mixtures of diogenite and eucrite material. As one might imagine, there is some difficulty in drawing the line between a howardite, a polymict eucrite, and a brecciated diogenite with some eucritic material. As such there are many cases in the literature of samples being characterized as "howardite or diogenite", or "howardite or polymict eucrites". Howardites are defined as having >10% diogenite or eucrite material (Figure 3), and they are also known to contain foreign material such as carbonaceous chondrite clasts (Mittlefehldt and Lindstrom, 1991; Buchanan and Reid, 1991; Buchanan et al, 1990, 1993, Metzler et al, 1995; Buchanan et al, 2000; Buchanan and Mittlefehldt, 2003). To get around the limitations of making a classification based on one thin slice through such a meteorite, some have attempted to classify based on bulk composition (Figure 4), somewhat analogous to the way lunar breccias are defined (e.g., Korotev, 2005).
Figure 3: plane and cross polarized light images of a howardite EET 87509 (left) and an polymict eucrite, EET 83227 (right)
Figure 4: Distinction between howardites and eucrites based on feldspar and pyroxene modal mineralogy from Delaney et al., 1984) (above); distinction between howardites and eucrites based on bulk compositional characteristics (from Warren et al., 2009)
This also works and has some advantages over petrography, but as always the divisions between fields must be defined well. Howardites also contain glassy spherules, perhaps from impact processes, and can contain dark fine grained clasts that may also be shock or impact related.
Mesosiderites are stony-iron meteorites that are mixtures of metal and brecciated silicates. The ratio of metal to silicate material is quite variable in mesosiderites, and together with the variation in silicate minerals and textures has led to a diverse number of mesosiderite types (e.g., Group A, B, C, D; Floran, 1978; Figure 5). Mesosiderites had in the past been linked to IIIAB irons and pallasites due to similarity of metal composition and also oxygen isotopic links (Clayton and Mayeda, 1983). However, with higher resolution oxygen measurements a distinction between mesosiderites and pallasites has been made (Greenwood et al., 2005). On the other hand, O isotopes for mesosiderites overlap completely with the HED meteorites, leading some to propose that they are from the same parent body, and perhaps 4 Vesta. This possibility is supported by the occurrence of mesosiderite clasts in howardites (Rosing and Haack, 2004). One question of interest is whether the DAWN mission will find any evidence for mesosiderites on the surface of 4 Vesta during is photographic / imaging orbit in 2011.
Figure 5: Sawn surface of mesosiderite RKP A79015 illustrating the high metallic FeNi content (left); Reflected light image of EET 87500 showing the relatively low metal contents (white phases) in this thin section
Compositional characteristics of the HED meteorites have been used for classification and also to better understand the origin of the different groups and subgroups. The Fe/Mn ratios of pyroxenes have been used to distinguish HED meteorites from other FeO-rich and pyroxene and plagioclase-bearing basalts such as lunar and martian basaltic meteorites (Figure 6; Karner et al., 2006; Goodrich and Delaney, 2000).
Figure 6: Mn and Fe2+ contents of pyroxenes from basalts from Earth, Moon, Mars and 4 Vesta (HED parent body) from the paper of Karner et al. (2006). The HED meteorite pyroxenes have the lowest Fe/Mn ratios of all samples. There is slight overlap with martian samples but these can be distinguished based on other data such as oxygen isotopes, and the presence of maskelynite and the lack of reduced FeNi metal in maritan basalts.
Similarly, oxygen isotopic measurements have shown that HED meteorites are offset slightly from the terrestrial fractionation line that defines samples from the Earth and Moon (e.g., Clayton and Mayeda, 1983; Wiechert et al., 2004; Figure 7). Major elements Ti and Mg have been used to distinguish two different trends among the eucrites - Stannern (partial melting) and Nuevo Laredo (fractionation) (Stolper, 1977; Barrat et al., 2007; Figure 8). These two trends can also be defined using different trace elements (e.g., Sm; Warren and Jerde, 1987).
Figure 7: Oxygen isotope data for HED meteorites compared to Mars, Moon, and the terrestrial fractionation line (TFL). Figure is from the study of Weichert et al. (2004) and illustrates the systematic offset between the TFL and HED meteorites, with the exception of Ibitira and a few others (see Figure 14).
Figure 8: La vs. FeO(t)/MgO for eucrites and defining the two trends - Stannern trend (partial melting) and main group/Nuevo Laredo trend (fractionation). Figure is from the study of Barrat et al. (2007)
Many HEDs were originally analyzed by INAA techniques, and now the prevalence of ICP-MS for bulk sample analysis has provided new elements that were not possible using previous techniques. Because chondritic and iron meteoritic material has high concentrations of siderophile elements like Ni and Ir, these elements are sometimes used to distinguish breccias from more pristine rocks (Warren et al., 2009). In addition the mixing between diogenite and eucrite end members can be monitored with trace elements such as Sc and Sm and major oxides such as MgO and CaO (Figure 9).
Figure 9 : Sc-CaO and Sm-MgO trends among howardite-eucrite-diogenite meteorites(from Warren et al. (2009). Howardites and polymict eucrites are mixtures of the two end members.