Enstatite chondrites meteorites

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... 

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

Weisberg M. K., Prinz M., and Fogel R. A. (1994) The evolution of enstatite and chondmles in unequilibrated enstatite chondrites. Meteoritics 29, 362-373. 

Figure 10 Normalized reflectance versus wavelength ( jLm) for M-type 16 Psyche versus iron and enstatite chondrite meteorites (lines) (Gaffey, 1976 Bell et al., 1988 Bus, 1999). All spectra are normalized to unity at 0.55 xm. Spectra for all three are relatively featureless with red spectral slopes and moderate albedos. While some M-class asteroids may be metallic core material, existing spectral and density data are inconsistent with this explanation for all M asteroids.

Silicon nitride crystallizes in two hexagonal forms (a- and fl-SisN and also a cubic y-modification with spinel structure that is stable at high pressure and temperature (>15GPa, >2000 K Zerr et al, 1999 Leitch et al, 2004). The a-phase occurs in nature as a rare mineral (nierite) in ordinary and enstatite chondritic meteorites (Lee et al, 1995). 

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.

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... 

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... 

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... 

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... 

Meteorites are divided into two broad categories chondrites, which retain some record of processes in the solar nebula and achondrites, which experienced melting and planetary differentiation. The nebular record of all chondritic meteorites is obscured to varying degrees by alteration processes on their parent asteroids. Some meteorites, such as the Cl, CM, and CR chondrites, experienced aqueous alteration when ice particles that co-accreted with the silicate and metallic material melted and altered the primary nebular phases. Other samples, such as the ordinary and enstatite chondrites, experienced dry thermal metamorphism, reaching temperatures ranging from about 570 to 1200 K. In order to understand the processes that occurred in the protoplanetary disk, we seek out the least-altered samples that best preserve the record of processes in the solar nebula. The CV, CO,... 

In Figure 3, sodium, zinc, and sulfur are representative of the abundances of moderately volatile elements (Figure 2 and Table 2). Abundance variations reach a factor of 5 for sulfur and 10 for zinc. All three elements show excellent agreement of solar with Cl abundances, in contrast to other groups of chondritic meteorites, except for the enstatite chondrites, which reach the level of Cl abundances. However, enstatite chondrites... 

Fagan T. J., McKeegan K. D., Krot A. N., and Keil K. (2001) Calcium, aluminum-rich inclusions in enstatite chondrites (2) oxygen isotopes. Meteorit. Planet. Sci. 36, 223 —230. 

Figure 2 Oxygen isotopic compositions of chondmles from all classes of chondritic meteorites ordinary (O), enstatite (E), carbonaceous (C), and Rumumti-type (R). The TF line and carbonaceous chondrite anhydrous mineral (CCAM) line are shown for reference in this and many subsequent figures. Equations for these lines are TF—5 = 0.525 and CCAM—5 = 0.9415 -4.00 (sources Clayton et al, 1983, 1984, 1991 Weisberg et al., 1991).

Chondritic meteorites, characterized by their relatively unfractionated chemical compositions, and usually consisting of chondrules and some matrix, can be subdivided into classes, as follows carbonaceous chondrites—Vigarano-type (CV), Ornans-type (CO), Mighei-type (CM), Renazzo-type (CR), Karoonda-type (CK), Bencubbin-type (CB), and ALH 85085-type (CH) (see Chapter 1.05 for details) ordinary chondrites—high-iron (H), low-iron (L), and low-iron, low-metal (FF) and enstatite chondrites—high-iron (EH), low-iron (EL) R-chondrites, characterized by olivine with very high ferrous iron content. 

Lin Y. and Kimura M. (1998) Petrographic and mineralogical study of new EH melt rocks and a new enstatite chondrite grouplet. Meteorit. Planet. Sci. 33, 501-511. 

Okazaki R., Takaoka N., Nakamura T., and Nagao K. (2000) Cosmic-ray exposure ages of enstatite chondrites. Antarct. Meteorit. Res. 13, 153—169. 

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