2 The ureilite group
2.2 Mineralogy
2 The ureilite group
2.1 Introduction
Ureilites are well equilibrated carbon‐rich ultramafic rocks. They represent the second most abundant group of achondrites with 251 known specimens (GROSSMAN 2009). The first ureilite fell in 1886 near Novo‐Urei in Russia (NORTON 2002). Although clearly achondritic in terms of their texture and mineralogy, ureilites show features of primitive meteorites, such as high abundances of noble gases, high siderophile trace element abundances and CV‐ and CM‐chondrite‐like bulk oxygen isotopic composition (MITTLEFEHLDT et al. 1998).
Ureilites are divided into two subgroups: monomict and polymict ureilites: Polymict ureilites are complex breccias containing clasts of typical monomict ureilites (which are discussed later in this chapter) and a variety of other components, including carbon, suessite, sulfides and minor apatite (GOODRICH et al. 2004). Some of the lithic clasts in polymict ureilites resemble material from a variety of other meteorites, such as enstatite chondrites, angrites, aubrites and feldspathic melt rocks (MITTLEFEHLDT et al. 1998). The composition of olivine and pyroxene in polymict ureilites is consistent with a derivation from monomict ureilites
(GOODRICH et al. 2004). Plagioclase is common in polymict ureilites, but very rare in monomict ureilites (MITTLEFEHLDT et al. 1998). Twenty polymict ureilites are currently known (GROSSMAN 2009). Monomict ureilites will be reviewed in greater detail in the remainder of this chapter.
2.2 Mineralogy
2.2.1 Silicates
Ureilites typically consists of olivine and pyroxene and about 10 wt% of an interstitial carbonaceous material referred to as vein material (MITTLEFEHLDT et al. 1998). Three types of ureilites are distinguished as based on their mineralogy. In the majority of all ureilites (~80 %), pigeonite (Wo~7‐13) is the sole pyroxene (MITTLEFEHLDT et al. 1998). Augite‐bearing ureilites are rare (~10 %) and contain augite in addition to orthopyroxene or pigeonite. The third type of ureilites are olivine‐orthopyroxene‐ureilites and show orthopyroxene (Wo4.5‐5) instead of or in addition to pigeonite.
Modal pyroxene/(pyroxene+olivine) ratios range from 0 to ~0.9, averaging at ~0.3
(MITTLEFEHLDT et al. 1998). Typical ureilite textures show coarse grained, anhedral olivines and pyroxenes that meet in triple junctions. Olivines may contain small, rounded inclusions of pyroxenes (MITTLEFEHLDT et al. 1998).
A pronounced elongation in olivines and pyroxenes has been described in several ureilites, such as Kenna, Novo‐Urei, Dingo pup Donga, RC027 and Dyalpur (BERKLEY et al. 1976, BERKLEY et al. 1980, GOODRICH et al. 1987b). Fabric analyses (BERKLEY et al. 1976, BERKLEY et al. 1980) show that this elongation reflects a foliation of the {100} crystal face of the olivine and a lineation defined by the crystallographic [001] axis of the olivine and pyroxene. Some ureilites show a mosaicized texture and much smaller grain sizes, which has been interpreted as a result of shock (MITTLEFEHLDT et al. 1998).
The olivine geochemistry is similar for all ureilites except for the amount of fayalite (fa), which ranges from fa2 (ALH84136) to fa23 (e.g. CMC04044, GROSSMAN 2009), with a maximum in fa distribution at ~fa20 (MITTLEFEHLDT et al. 1998). Within each ureilite fa is constant in olivine cores. Coexisting pyroxenes span a similar range with respect to FeO concentrations, which indicates olivine/pyroxene equilibrium (MITTLEFEHLDT et al. 1998). Ureilite olivines are characterized by high CaO (0.3 ‐ 0.45 wt%) and Cr2O3 (~0.56‐0.85 wt%) concentrations (MITTLEFEHLDT et al. 1998).
A characteristic feature of ureilite olivines and pyroxenes are reduction rims usually
associated with carbonaceous material (e.g. BERKLEY et al. 1976, BERKLEY et al. 1980, GOODRICH et al. 1987b). These rims are almost FeO‐free (fa0‐fa2) and riddled with tiny inclusions of low‐
Ni metal. Most of these rims are narrow (10‐100µm), although some ureilites (ALH82130, HaH126) show very large reduction rims with almost completely reduced olivine grains (MITTLEFEHLDT et al. 1998). Reduction rims are smaller and less frequently found in pyroxene grains. These reduced rims have been attributed to a solid state reaction, where the fayalite‐
component of the olivine has reacted with intergranular carbon (Eq. 1) to form high mg#
residual olivine and Ni‐poor metal (e.g. WASSON et al. 1976, SINGLETARY and GROVE 2003).
Eq. 1
2.2.2 Vein material
The vein material occurs along silicate grain boundaries but can also intrude the silicates along cracks and cleavage planes (MITTLEFEHLDT et al. 1998). It contains carbon phases, FeNi‐
metal and interstitial silicates. The carbon phases are mainly graphite, but other minerals such as chaoite, µm‐sized grains of lonsdaleite and diamond have also been identified in several ureilites (VDOVYKIN 1972, VDOVYKIN 1975, BERKLEY et al. 1976, MARVIN and WOOD 1972).
Carbon concentrations in ureilites range from 0.2 wt% (e.g. Goalpara) to up to ~6 wt% (e.g.
North Haig) (GRADY et al. 1985, WIIK 1972, JAROSEWICH 1990, MCGALL and CLEVERLY 1968, TAKEDA 1987) with an average carbon content of ~3 wt%. The ureilite carbon content does not correlate with fa. The graphite is usually fine grained, although large mm‐sized euhedral graphite crystals have also been found within several ureilites (BERKLEY and JONES 1982, TREIMAN and BERKLEY 1994).
The origin of the carbon has been subject to discussion. WLOTZKA (1972) pointed out, that ureilite olivines were not in equilibrium with carbon in the vein material, which led to the assumption that carbon was introduced into the system by a “late event”. Subsequent work on silicate reduction in the presence of carbon (e.g. BERKLEY and JONES 1982, WALKER and
GROVE 1993) has shown that the reduction (Eq. 1) is strongly pressure dependent, as large
volumes of CO‐gas are formed. Reduction therefore may be stabilized by the depth in which the ureilite material is positioned on the ureilite parent body. WASSON et al. (1976), HIGUCHI et al. (1976) and WILKENING and MARTI (1976) proposed a carbon‐rich impactor to account for the high noble gas and carbon concentrations in ureilites. The presence of large graphite crystals in relatively unshocked ureilites would contradict the impact‐hypothesis (BERKLEY and
JONES 1982, WACKER 1986). WEBER et al. (1976) pointed out that carbon in carbonaceous
chondrites occurs mostly in form of hydrocarbons. It would therefore be unlikely that large amounts of noble gases, which are typical for ureilites, could be retained during
graphitization of the hydrocarbons. Instead, a model with graphite as nebular condensate was invoked to account for carbon in ureilites (WEBER et al. 1976). A similar model had been proposed by JANSSENS et al. (1987). RAI et al. (2003) argues, that if ureilite noble gases were implanted into the carbon phases from a plasma, the (132Xe/36Ar)0‐ratio is a function of the temperature of the plasma. KALLEMEYN et al. (1996) argues that Δ17O (the deviation of 17O from the TFL) of any meteorite is a function of nebular temperature and therefore of
formation location. The lack of correlation of (132Xe/36Ar)0 and Δ17O in ureilites led RAI et al.
(2003) to suggest that carbon in ureilites is not related to ureilite silicates. A strong argument for a primary origin of the carbon are cohenite‐bearing metal‐sulfide spherule inclusions in ALHA77257, ALHA78262 and ALHA78019 described in GOODRICH and BERKLEY (1986). These spherule inclusions were interpreted as droplets of a primary metal liquid that has been entrapped during olivine crystallization (GOODRICH and BERKLEY 1986). The presence of cohenite within these droplets suggests that at least some of the carbon in ureilites is primary in origin.
Published Ni‐concentrations in vein metal range from <1 to ~9 wt%. Cobalt concentrations range from 0.1 to 0.6 wt% and P abundances range from 0.1 to 0.4 wt%. Silicon ranges from 0 to ~4 wt% (BERKLEY et al. 1980, BERKLEY 1986). Several authors proposed that ureilite vein metal represents residual metal formed by the removal of a S‐rich melt (e.g. TAKEDA 1987,
JONES and GOODRICH 1989, HUMAYUN et al. 2005, WARREN et al. 2006, RANKENBURG et al. 2008).
However, as VAN ORMAN et al. (2009) pointed out, the removal of only S‐rich melt is not a perfect explanation of siderophile element abundances in ureilite vein metal.
Other minor phases that have been identified in the vein metal include schreibersite (MITTLEFEHLDT et al. 1998), suessite (KEIL et al. 1982) and interstitial silicate. These interstitial silicates are fine grained and common in ureilites. They usually consist of Low‐Ca pyroxene (Wo1‐14) and augite (Wo26‐43) (MITTLEFEHLDT et al. 1998). Interstitial silicates show much higher mg# than the core of the main pyroxene grains and lower Mn/Mg, Cr/Mg, Na/Mg, Ti/Mg and Al/Mg‐ratios (GOODRICH et al. 1987a).
2.2.3 Spherule inclusions in olivine
Spherule inclusions have been reviewed in great detail in GOODRICH and BERKLEY (1986). They have been found in ALHA78262, ALHA77257, ALHA82130, ALHA78019 and PCA82506.
Minerals found in spherule inclusions are cohenite, Fe‐Ni‐metal, sulfides and rare phosphorus‐bearing minerals (GOODRICH and BERKLEY 1986). GOODRICH and BERKLEY (1986) report Ni concentrations of bulk spherule inclusions ranging from 2.7 wt% in ALH82130 (fa6) to 6.6 wt% in ALHA78262 (fa21), Co‐concentrations range from 0.37 wt% in ALH82130 (fa6) to 0.62 wt% in ALHA78262 (fa21). Reported Ni concentrations in spherule Fe‐Ni‐metal range from 5.8 wt% in ALH82130 (fa6) to 12.2 wt% in ALHA78262 (fa21), while Co‐concentrations in
distribution of C and Ni, as well as the present mineral phases, GOODRICH and BERKLEY (1986) concluded that spherule inclusions represent a Fe‐Ni‐S‐C‐liquid. The presence of cohenite and sulfide within the same spherule indicates that the liquid has been entrapped prior to the separation of an S‐rich and a C‐rich liquid melt.
2.3 Ureilite bulk rock chemistry