Enstatite chondrites

Other meteorite classes like C2, CO and ordinary chondrites contain much smaller inclusions less than 1 mm (MacPherson et al. 1988) and require ion microprobe techniques to evaluate the isotopic compositions. On the least metamorphosed side. Cl have very few inclusions or oxide grains, but are the carrier of the greatest amounts of stellar nanodiamond and other carbides (Anders and Zirmer 1993). As will be shown for Cr anomalies in carbonaceous chondrites, the survival of the mineral carriers of the anomalies also depends on the metamorphic grade of the meteorites. Nevertheless, isotopic anomalies have also been formd in higher metamorphic grade from other classes, especially in the reduced enstatite chondrites. 

Figure 8. Figure (a) after Clayton et al. (1976, 1977). The scales are as in Figure 1. The O isotopic compositions of the different meteorite classes are represented ordinary chondrites (H, L, LL), enstatite chondrites (EFl, EL), differentiated meteorites (eucrites, lAB irons, SNCs) and some components of the carbonaceous chondrites. As the different areas do not overlap, a classification of the meteorites can be drawn based on O isotopes. Cr (b) and Mo (c) isotope compositions obtained by stepwise dissolution of the Cl carbonaceous chondrite Orgueil (Rotaru et al. 1992 Dauphas et al. 2002), are plotted as deviations relative to the terrestrial composition in 8 units. Isotopes are labeled according to their primary nucleosynthetic sources. ExpSi is for explosive Si burning and n-eq is for neutron-rich nuclear statistical equilibrium. The open squares represent a HNOj 4 N leachate at room temperature. The filled square correspond to the dissolution of the main silicate phase in a HCl-EIF mix. The M pattern for Mo in the silicates is similar to the s-process component found in micron-size SiC presolar grains as shown in Figure 7.

Virag A, Zinner E, Lewis RS, Tang M (1989) Isotopic compositions of H, C, and N in C8 diamonds from the Allende and Murray carbonaceous chondrites. Lunar Planet Sci XX 1158-1159 Volkening J, Papanastassiou DA (1989) Iron isotope anomalies. Astrophys J 347 L43-L46 Volkening J, Papanastassiou DA (1990) Zinc isotope anomalies. Astrophys J 358 L29-L32 Wadhwa M, Zinner EK, Crozaz G (1997) Manganese-chromium systematics in sulfides of unequilibrated enstatite chondrites. Meteorit Planet Sci 32 281-292... 

The aubrites are the most reduced achondrites (Keil et al., 1989). Their silicates are essentially free of iron, and they contain minor metallic iron. A variety of unusual sulfides of calcium, chromium, manganese, titanium, and sodium - all usually lithophile elements -occur in aubrites. These unusual sulfides also characterize the highly reduced enstatite chondrites, which may have been precursors for these rocks. 

The most abundant mineral in aubrites is coarse-grained orthopyroxene (as it is in enstatite chondrites), and only a small amount of plagioclase is present. Aubrites are commonly brecciated, and several clasts of related basalt have been observed. The aubrites are depleted in siderophile and chalcophile elements relative to chondritic abundances,... 

Rubin, A. E., Scott, E. R. D. and Keil, K. (1997) Shock metamorphism of enstatite chondrites. Geochimica et Cosmochimica Acta, 61, 847-858. 

Reynolds, J. H. (1960b) Isotopic composition of xenon from enstatite chondrites. Zeitschrift... 

Wadhwa, M., Zinner, E. and Crozaz, G. (1997) Mn-Cr systematics in sulfides of unequilibrated enstatite chondrites. Meteoritics and Planetary Science, 32, 281-292. 

Chondrules from CV3 chondrites give a confusing picture. A high-precision Pb-Pb whole-rock isochron for Allende chondrules gives an age of 4565.32 0.81 Ma, almost 3 Myr after CAIs (Connelly and Bizzarre, 2009). The I- Xe system in enstatite should have closed... 

As already noted, spectral similarities between the various asteroid classes and specific types of meteorites provide a way to identify possible meteorite parent bodies. The Tholen and Barucci (1989) asteroid taxonomy has been interpreted as representing the types of meteorites shown in Table 11.1. Using the Bus et al. (2002) taxonomy, the C-complex asteroids are probably hydrated carbonaceous chondrites (e.g. Cl or CM). These carbonaceous chondrite asteroids probably accreted with ices and will be considered in Chapter 12. Some S-complex asteroids are ordinary chondrite parent bodies, but this superclass is very diverse and includes many other meteorite types as well. The X-complex includes objects with spectra that resemble enstatite chondrites and aubrites, and some irons and stony irons, although other X-complex asteroids are unlike known meteorite types. A few asteroid spectra are unique and provide more definitive connections, such as between 4 Vesta and... 

Compositional variations among chondrites, (a) Lithophile and (b) siderophile and chalcophile elements in ordinary (H, L, LL), enstatite (EH, EL), R, and chondrites. In (c) and (d), the same data are shown for anhydrous carbonaceous chondrite groups. Elements are plotted from left to right in order of increasing volatility. Lithophile elements are normalized to Cl chondrites and Mg, siderophile and chalcophile elements are normalized to Cl chondrites. Modified from Krot et al. (2003). 

The diamonds first recognized by Lewis et al. (1987) have now been traced to every class of chrondritic meteorite (Alexander et al. 1990 Huss 1990 Russell et al. 1991) and are known to occur as a component within the matrix. The amounts available for extraction reach a maximum of 900 ppm but decline to nothing in higher petrologic grades. The cut-off point in unequilibrated ordinary chondrites is about type 3.6 but for enstatite chondrites the type 4s still contain diamond. Progressive... 

In all but one type of chondrite, small millimeter-sized spherical silicate grains are trapped in the mineral matrix. These small grains consist mainly of amorphous silicates and are called chondrules. This latter term gives its name to the meteorite class containing chondrules, i.e. the chondrite class. The chondrites themselves are divided into three subclasses enstatite, ordinary chondrites and carbonaceous chondrites. Carbonaceous chondrites themselves are classified into four groups I (for Ivuna), M (for Mighei), O (for Omans) and V (for Vigarano). 

To obtain a quantitative measure of the CL intensity of individual CL emissions, an optical multichannel analyzer was coupled to the optical system of an electron microprobe allowing simultaneous collection of CL spectra and minor element data from a single point (Steele, I.M. Meteoritics. submitted). For CL spectra obtained with a 15 kV focused beam, enstatite from both enstatite chondrites and achondrites showed three distinct peaks (Fig.l) centered at about 742, 664, and 483nm. To allow assignment of these peaks, spectra from synthetic Mn and Cr doped enstatite are shown in Fig. 2 and the emissions from these two samples closely match the two red peaks of meteoritic enstatite neither synthetic sample shows a blue peak. The peak positions of Cr and Mn are not constant for different meteoritic enstatites and are not the same as for the Cr and Mn doped standards. The variation is about 20nm... 

Figure 1. Typical CL spectrum from enstatite in the enstatite chondrite ALHA 77295. Peak positions for this and other spectra are estimated by eye and the spectra are not corrected for instrumental transmission.

Feldspar Unlike enstatite and forsterite, feldspar is present in most meteorites. The meteoritic feldspars are dominated by plagioclase which are those feldspar compositions in the continuous solid solution series ranging from albite (NaAlSi308) to anorthite (CaAl2Si2C>8) compositions near the ends of this series are most common. The earliest CL studies of feldspar were prompted by the lunar program about 1970 and a series of papers compared the CL of lunar, meteoritic and doped synthetic plagioclase. Later observations were made for anorthite in the carbonaceous chondrites. 

Forsterite Possibly the most common coarse-grained phase present in all carbonaceous and unequilibrated ordinary chondrites, as well as a minor phase in enstatite chondrites and achondrites, is olivine ((Mg,Fe)2Si04). Most olivine contains significant Fe and does not show CL however a small fraction of the olivine is Mg-rich (forsterite) and shows brilliant CL. This same phase is present in... 

Other Minerals Up to this point, reference has been made to other luminescing phases within meteorites but in all cases no systematic studies have been reported to 1) determine the cause of the CL or 2) to relate the CL to the genesis of the mineral or meteorite. The following comments are made to draw attention to the possible significance of CL observations in other meteoritic minerals most observations are from the author s personal experience and have not been documented. Oldhamite is a rare mineral in the enstatite chondrites but is known to carry appreciable quantities of rare earth elements (REE). The CL spectra have not been studied in detail but visual CL is yellow and a CL emission at 580nm was reported (H) but with rapid intensity change under the electron beam. A REE activator of the CL is possible. Hibonite is a relatively common... 

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