Two new eucrite breccias from Northwest Africa


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Abstract– This work describes two newly discovered eucrite breccias: three presumably paired meteorites, all named Northwest Africa (NWA) 6105, and NWA 6106. For each meteorite, major- and minor-element compositions of minerals were determined using the electron microprobe. Pyroxene Fe-Mn co-variations and bulk-rock oxygen isotope compositions confirm their classification as eucrites. Variations in mineral compositions and textures are attributed to differences in clast types present (i.e., basaltic or cumulate eucrite). The pyroxene compositions support the hypothesis that samples NWA 6105,1; 6105,2; and 6105,3 are paired polymict eucritic breccias, whereas sample NWA 6106 is a monomict basaltic eucritic breccia. Two-pyroxene geothermometry yields temperatures too low for igneous crystallization. The variation in temperatures among samples suggests that metamorphism occurred prior to brecciation.


Howardites, eucrites, diogenites (HEDs) meteorites represent the most extensive suite of achondrite samples from a proto-planetary body, generally accepted to be asteroid 4 Vesta based on spectroscopic similarities and orbital constraints (e.g., McCord et al. 1970; Drake 1979; Binzel and Xu 1993). Eucrites, the “E” in the HEDs, are basalts or gabbros and are believed to have crystallized as surficial lavas or within plutons at shallow crustal levels. Minerals present include pyroxenes and plagioclase as the dominant phases and smaller amounts of troilite, chromite, ilmenite, silica, or silica-rich glass, Fe-Ni metal, and phosphate (e.g., Stolper 1977; Mittlefehldt et al. 1998; Keil 2002; McSween et al. 2011). Depending on texture and mineral composition, the eucrites are subdivided into basaltic and cumulate eucrites. Basaltic eucrites are fine- to medium-grained with Fe-rich, exsolved pyroxenes with fine lamellae (zoned or not, depending on whether they have suffered later thermal metamorphism) and plagioclase of An75–96. Cumulate eucrites are coarse-grained with Mg-rich exsolved pyroxenes with coarse lamellae and plagioclase of An90–96 (e.g., Delaney and Prinz 1984; Mittlefehldt et al. 1998; McSween et al. 2011). Eucrites are often brecciated and occur as either monomict (having only one pyroxene type) or polymict (having two or more pyroxene types) rocks. Characteristics of the pyroxene types are outlined in detail elsewhere (i.e., Miyamoto et al. 1978; Delaney et al. 1982).

The focus of this study involves two newly discovered eucrite breccias represented by four meteorites: Northwest Africa (NWA) 6105,1 (approximately 12 g), 6105,2 (approximately 1 g), 6105,3 (approximately 9 g), and 6106 (approximately 302 g); all were recovered in near proximity in Morocco. The purpose of this study was to provide classifications and petrographic descriptions for all four meteorites. Previous work on these samples was only preliminary and reported in abstracts (McFerrin et al. 2010; Singerling et al. 2011). By analyzing these new eucrites, we can provide a more representative sampling of these basaltic lithologies on Vesta, which is essential to a better understanding of Vestan magmatic processes.


Polished thin sections of the four meteorites were observed with a petrographic microscope to describe their mineralogy and petrography. The imaging software Infinity Analyze was used to construct maps of the samples in reflected light at 2.5× magnification. Areas of interest were also imaged in plane-polarized and cross-polarized light at higher magnifications. Modal abundances were established using ImageJ (free image analysis software) on backscattered electron (BSE) images, following the method outlined by Liu et al. (2009). Mineral compositions were determined using the wavelength dispersive spectrometers of the CAMECA SX-100 electron microprobe (EMP) analyzer. These analyses were performed with an accelerating potential of 15 keV, a beam current of 20 nA (10 nA for feldspars and glass), and a 1–5 μm beam size (10 μm for plagioclase). Peak and background counting times were 20 s. Detection limits (3σ above background) were as follows: 0.03 wt% for SiO2, TiO2, Al2O3, Cr2O3, MgO, MnO, and CaO; 0.05 wt% for FeO, Na2O, and K2O; and 0.05–0.1 wt% for Co and Ni in metal.

Oxygen isotope analyses were performed with a laser-fluorination vacuum-preparatory line and MAT 253 mass spectrometer at ISEI. The δ18O measurements were calibrated against Vienna standard mean ocean water (VSMOW) which was fluorinated in the same vacuum-preparatory line and analyzed on the same mass spectrometer. The δ17O analyses were calibrated based on the analysis of terrestrial silicate minerals as △17O = 0, where △17O = 1000 × [ln (δ17O/1000 + 1) − 0.527 × ln (δ18O/1000 + 1)] (Miller 2002). The δ18O of reference garnet material, UWG-2 (Valley et al. 1995), gives a value of 5.65‰ relative to VSMOW. Analytical precision (1σ, N = 6), based on replicate analysis of UWG-2 garnet, is ±0.03‰ for δ17O, ±0.05‰ for δ18O, and ±0.024‰ for △17O.


Petrographic Descriptions

NWA 6105,1

NWA 6105,1 is a breccia containing numerous eucrite clasts (Figs. 1 and 2a). The mineral modes (vol%) of the sample are: pyroxene (low- and high-Ca) 56%, plagioclase 43%, with ilmenite, troilite, chromite, Fe-Ni metal, phosphate, and impact melt glass each <1%. The pyroxenes in the matrix are predominately pigeonite with some grains having fine exsolution lamellae of augite. Shock effects are relatively common, causing fractures and undulatory extinction in plagioclase and pyroxene grains. Opaque minerals include angular chromite and ilmenite. Most of the minor minerals (i.e., ilmenite, troilite, chromite, etc.) are enclosed by low-Ca pyroxenes; however, some grains occur as fragments within the matrix and as grains interstitial to pyroxene and plagioclase in clasts. The grain sizes of matrix minerals range from <5 μm to 4 mm.

Figure 1.

 Photomicrographs in plane-polarized light (PPL) of thin sections a) NWA 6105,1; b) 6105,2; c) 6105,3; and d) 6106. All images are at the same scale.

Figure 2.

 Backscattered electron images of thin sections a) NWA 6105,1; b) 6105,2; c) 6105,3; and d) 6106. All images are at the same scale. Note that these are different field-of-views than those in Fig. 1.

Within the matrix of NWA 6105,1, there are numerous eucrite clasts (seven identified in our thin section), varying in size, composition, and texture. The clasts range in size from 1 to 5 mm and vary in shape from round to angular; they exhibit a coarse-grained granular texture and resemble cumulate eucrites, although their pyroxene compositions are not magnesian enough (Mittlefehldt et al. 1998). All the clasts contain large pyroxene and plagioclase grains surrounded by a fine-grained clastic matrix. The clasts vary in texture from ophitic to hypidiomorphic granular and are predominately hypocrystalline containing crystals of pigeonite and plagioclase, within a matrix of pyroxene, plagioclase, opaque phases, phosphate, and in some cases, glass. Most of the pyroxene grains are pigeonite, with exsolution lamellae of augite about 5–15 μm thick.

NWA 6105,2

NWA 6105,2 is a breccia within a breccia, composed of two distinct lithologies, A and B, which make up approximately 75% and 25% of the sample, respectively (Figs. 1 and 2b). The larger fragment A is coarse-grained with ophitic to subophitic clasts. The modes for the clast are: pyroxene (low- and high-Ca) 55%, plagioclase 40%, and clastic matrix 5%. Individual clast sizes are typically 0.5–1 mm. The majority of the pigeonite grains are moderately fractured. Most of the plagioclase grains are twinned and show undulatory extinction. Evidence of shock metamorphism includes bent twin-lamellae, mosaicism, and pervasive fractures in plagioclase, although no maskelynite was observed. Ilmenite and chromite are nonuniformly distributed within fragment A, occurring as large (approximately 50–100 μm) anhedral grains within the matrix or as irregular blebs or elongate rods in pigeonite grains.

The smaller lithic fragment B is fine-grained and shares many textural similarities with NWA 6105,1. The modes in this clast are: pyroxene (low- and high-Ca) 54%, plagioclase 38%, silica 8%, with ilmenite, troilite, chromite, and Fe-Ni metal, each <1%. The matrix of B is enriched in glass and opaque phases which cause it to appear darker.

NWA 6105,3

NWA 6105,3 is a fine-grained eucrite breccia with textures indicative of shock metamorphism (Figs. 1 and 2c). There are two distinctive clasts: one brecciated and one sulfide-rich. The modes of the sample are: pyroxene (low- and high-Ca) 51%, plagioclase 30%, silica 15%, calcite 4%, with ilmenite, troilite (in the sulfide-rich clast approximately 3%), chromite, and Fe-Ni metal, each <1%. The higher abundance of silica in this sample compared with the others can be explained by the fact that the thin section studied had much less material. The presence of a large (about 200 μm) silica grain contributed greatly to the overall modal abundance of this phase. The pyroxene grains are subhedral to euhedral and range in size from <5 to 250 μm. The <5–50 μm pyroxene grains in the matrix are granoblastic polygonal, with exsolution. The largest (about 300 μm) pyroxene clast, located near the center of the thin section, displays slightly different compositions from the rest of the pyroxenes in the sample. This is referred to as 6105,3 Clast in the figures and tables in later portions of the paper. Plagioclase ranges from crystalline to polycrystalline grains to maskelynite. These grains range in size from 10 μm to 0.5 mm in the matrix. Ilmenite, troilite, chromite, and Fe-Ni metal occur as euhedral or anhedral grains that range in size from <5 to 130 μm. They are dispersed throughout the sample, with the exception of the sulfide-rich clast characterized by anhedral troilite blebs. Silica occurs as anhedral grains that range in size from <5 μm to 0.2 mm. Using Ca X-ray maps, calcite was also observed as a secondary (terrestrial alteration) mineral present in fractures along one edge of the sample.

NWA 6106

NWA 6106 is a breccia containing unbrecciated igneous lithic clasts (seven identified) which have an ophitic to subophitic texture (Figs. 1 and 2d). The modes of this sample are: pyroxene (low- and high-Ca) 47%, plagioclase 48%, silica 3%, with calcite, ilmenite, troilite, chromite, and Fe-Ni metal, each <1%. The pyroxenes contain exsolution lamellae ranging in apparent thickness from <5 to 12 μm and are subhedral to anhedral. Pyroxenes range in size from <0.1 to 1.7 mm. The chromite, ilmenite, and metal grains are anhedral, ranging from <0.1 to 0.3 mm. The plagioclase is mostly lath-shaped with Carlsbad and albite twinning. The grain size ranges from <0.1 mm in the crushed matrix to approximately 0.8 mm in some lithic clasts. A vein, which was determined to be calcite with the EMP, cuts through the thin section and is interpreted as a product of terrestrial weathering.

Mineral Compositions


NWA 6105 and 6106 are breccias, and as such it should come as no surprise that they have significant variations in the major-element compositions of pyroxene and plagioclase. Representative analyses were chosen to illustrate the ranges of compositions of pyroxene and plagioclase. The results for each sample are listed in Table 1. Only pyroxenes visibly free of small (<1 μm) inclusions of oxide minerals (i.e., ilmenite and chromite) were analyzed.

Table 1. Representative major- and minor-element data from electron microprobe analyses (oxides in wt. %, cations in nb ions).
NWA 6105,1Low-Ca PxaAugiteaPlagaNWA 6105,2Low-Ca PxaAugiteaPlaga
TiO20.280.100.390.18<0.03 <0.03TiO20.150.190.710.48<0.03<0.03
Cr2O3<0.030.56<0.030.15<0.03 <0.03Cr2O30.480.230.420.60<0.03<0.03
MgO17.912.313.711.3<0.03 <0.03MgO17.012.513.411.1<0.03<0.03
MnO0.861.130.440.41<0.03 <0.03MnO0.981.080.440.72<0.03<0.03
Ox Basis666688 666688
Crn.d.0.018n.d.0.004n.d. n.d.Cr0.0150.0070.0130.019n.d.n.d.
Mg1.0340.7390.7750.644n.d. n.d.Mg0.9810.7410.7560.644n.d.n.d.
Mn0.0280.0380.0140.013n.d. n.d.Mn0.0320.0360.0140.024n.d.n.d.
NWA 6105,3Low-Ca PxaAugiteaPlagaNWA 6106Low-Ca PxaAugiteaPlaga
  1. aRepresentative analyses.

  2. n.d. = not detected; Cum = cumulate.

Ox Basis6688 6688
Ti0.0040.018n.d.n.d.Ti0.0080.014n.d. n.d.
Cr0.0030.026n.d.n.d.Cr0.0040.028 n.d. n.d.
Mg0.7750.625n.d.n.d.Mg0.6650.666 n.d.n.d. 
Mn0.0340.018n.d.n.d.Mn0.0380.037 n.d.n.d.

Variations in pyroxene major-element composition mainly occur in Ca-content and, to a lesser extent, Mg- and Fe-content. Figure 3 shows pyroxene quadrilaterals for all analyzed pyroxene compositions in the four meteorites. Ternary diagrams of minor elements (Ti-Al-Cr) are included as well. As the pyroxene composition becomes more Ca-rich, it also becomes less Fe-rich. The trend observed here was also reported by Mayne et al. (2009) and is termed the Ca-Fe trend. Finely exsolved pyroxenes cause what appears to be a continuous range in composition along the Ca-Fe trend. In reality, the pyroxenes are either high-Ca (exsolved augite lamellae) or intermediate- to low-Ca (host pigeonite). The limitations of instrument resolution cause what appear to be intermediate compositions. Pyroxenes in NWA 6105,1 and 6105,2 (Figs. 3a and 3b) show two distinct Ca-Fe trends, one anchored at approximately En50 and another at approximately En40. These are symbolized according to the type of eucrite clast: cumulate (open circles) versus basaltic (closed circles). NWA 6105,1 shows more variation in the En50 trend, which may reflect several different cumulate compositions in this sample. NWA 6105,3 also has pyroxenes with several trends, as shown in Figs. 3c and 3d; one trend is characteristic of the groundmass pyroxenes (En40), and the other represents analyses of a single, large (approximately 300 μm) pyroxene clast (En46–40). NWA 6106 has only one Ca-Fe trend (En36), as shown in Fig. 3e. The variety of these major-element trends in pyroxenes reflects different formational histories (i.e., parent magmas, degree of thermal metamorphism, etc.)

Figure 3.

 Pyroxene quadrilaterals and minor-element ternary diagrams for a) NWA 6105,1; b) 6105,2; c) 6105,3; d) 6105,3 Clast; and e) 6106. Open circles = cumulate eucrite; closed circles = basaltic eucrite.

Minor elements such as Ti, Al, and Cr equilibrate more slowly than major elements in pyroxenes, so one can expect a greater spread in these data even after thermal metamorphism (Mayne et al. 2009). Figure 3 shows ternary diagrams of these three elements. Two trends are present, as described by Mayne et al. (2009): (1) constant Ti and variation in Cr-Al, as illustrated in Fig. 3c; (2) constant Cr and variation in Ti-Al, as illustrated in Fig. 3d. The other diagrams show mixtures of these two trends. There appears to be no consistent pattern among minor elements that distinguishes cumulate and basaltic eucrite clasts.


Plagioclase shows a range of compositions for the analyzed grains in each meteorite sample (Table 1). Figure 4 shows portions of an An-Ab-Or ternary diagram, depicting the compositions of plagioclase. The heterogeneous compositions of the plagioclase imply that it is not equilibrated in terms of major elements, unlike the pyroxenes. Plagioclase takes more time than pyroxene does to equilibrate in terms of major elements (e.g., Morse 1984).

Figure 4.

 An-Ab-Or ternary diagram depicting the plagioclase compositions in a) NWA 6105,1; b) 6105,2; c) 6105,3; and d) 6106. Basaltic and cumulate envelopes labeled (Mayne et al. 2009; McSween et al. 2011).



Data obtained from EMP analysis can aid in quantitatively determining the classification of these samples as eucrites. A useful method involves plotting Mn versus Fe in pyroxenes (Fig. 5). The figure includes reference lines for Vesta, Earth, and the Moon (Papike et al. 2003; Lentz et al. 2007).

Figure 5.

 Mn versus Fe contents of pyroxenes plot along the bold Vesta line defined by other howardites, eucrites, diogenites (Papike et al. 2003; Lentz et al. 2007) within standard error.

Oxygen isotopes are also useful in identifying the parent body of a given meteoritic sample. Table 2 lists oxygen isotope data, and Fig. 6 illustrates these values for the NWA samples studied. All samples plot approximately along the HED mass-fractionation line.

Table 2. Oxygen isotope data for NWA 6105,1; 6105,3; and 6106. The data are in good agreement with the HEDFL.
NWA 6105,1−0.2551.713.74
NWA 6105,3−0.2231.833.91
NWA 6106−0.2371.753.77
Standard Dev0.0150.040.06
Figure 6.

 Oxygen isotope plot illustrating that NWA 6105 and 6106 (gray x’s) are howardites, eucrites, diogenites. The terrestrial fractionation line is plotted for reference. Data are from Greenwood et al. (2005) and Franchi et al. (1999).


The QUILF (quartz-ulvospinel-ilmenite-fayalite) two-pyroxene geothermometer (Andersen et al. 1993) was used to estimate the equilibration temperatures of NWA 6105 and 6106. The exsolution lamellae of augites in host orthopyroxenes are suitable for geothermometry. Normally, the QUILF two-pyroxene geothermometer requires known Ca-contents of coexisting augite-orthopyroxene and the orientation of the two-pyroxene tie-line. For this work, the endmembers were chosen for the cumulate and basaltic pyroxene trends within samples containing both clast types, because the pyroxene values form discrete mixing lines for each trend. The QUILF program uses En and Wo values for augite-orthopyroxene pairs to calculate the equilibration temperatures. The data used for the calculations, as well as the equilibration temperatures obtained, are summarized in Table 3.

Table 3. Geothermometry data used for QUILF program and equilibration temperatures for NWA samples.
  6105,16105,26105,36105,3 Clast6106
  1. Cum = cumulate; Bas = basaltic.

AugiteEn (%) 36.3 32.9 38.6 32.8 32.3 33.1 29.4
 Wo (%) 45.0 44.9 42.4 32.1 41.8 36.0 41.8
OpxEn (%) 54.8 38.2 50.2 39.8 38.8 40.5 35.6
 Wo (%)  0.81  1.33  1.08  3.23  2.83  2.60  1.96
T (°C) 652 ± 52645 ± 32684 ± 32886 ± 39814 ± 46828 ± 40722 ± 30

The determined temperatures, in all cases, are too low to reflect igneous crystallization which requires crystallizations from a melt; for eucrites, this occurs at about 1060 °C (Stolper 1977). Instead, the calculated temperatures likely resulted from thermal metamorphism. The temperatures do not closely agree for the basaltic clasts in the NWA 6105 samples (6105,1 = 645 ± 32; 6105,2 = 886 ± 39; and 6105,3 = 814 ± 46 °C) indicating that metamorphism likely occurred before breccia assembly. It is important to note that the temperature obtained for NWA 6105,1 is very low for a basaltic eucrite. In fact, it is lower than the temperature calculated for the cumulate clasts (652 ± 52 °C) in this sample.

Some of the equilibration temperatures are in agreement with values reported in the literature within standard error. The exceptions include both basaltic and cumulate clasts in NWA 6105,1 and cumulate clasts in NWA 6105,2. Typical basaltic eucrite equilibration temperatures range from 800 to 950 °C (Delaney and Prinz 1984) with a more complete range of 700–1000 °C (Yamaguchi et al. 1996). For a comparison with other Vestan lithologies, the cumulate eucrite range is 765–992 °C, while the diogenite range is 719–840 °C (Takeda et al. 1976; Harlow et al. 1979; Mittlefehldt 1994). The fact that some of the NWA 6105 temperatures lie outside of literature values means some clasts of this sample experienced extremely slow cooling. The occurrence of inverted pigeonite would lend support to this slow-cooling hypothesis; unfortunately, the textures associated with inverted pigeonite were not observed although further study of these samples may discover them. While the equilibration temperatures have two values for the NWA 6105 samples (approximately 660 °C and approximately 850 °C), reflecting their polymict nature, the temperature for NWA 6106 is distinct (722 °C).


The NWA 6105 and 6106 samples were all found in close proximity, which may suggest pairing. If paired, these samples should display similar textures and mineral compositions. We also might expect the temperatures of equilibration to be similar, but the brecciated nature of the samples makes the utility of this characteristic questionable.

The pyroxene compositions and textures indicate that samples NWA 6105,1; 6105,2; and 6105,3 are polymict eucritic breccias containing either cumulate and basaltic clasts or basaltic clasts of differing compositions. The Ca-Fe trends of each are essentially equivalent (cumulate clasts with En50 and basaltic with En40) with the exception of the large pyroxene clast (En46–40) in 6105,3, which falls in between the two trends on the pyroxene quadrilateral. Although no cumulate eucrite clasts was found in 6105,3, it is clearly polymict and cumulate clasts may occur in a larger sample. Sample NWA 6106, on the other hand, is a monomict basaltic eucritic breccia. NWA 6106 has different textures and pyroxene compositions (Ca-Fe trend of En36) than any of the 6105 samples. The geothermometry results provide further evidence of pairing. The NWA 6105 samples have two distinct values, indicating their polymict nature, while the NWA 6106 sample has one value that is distinct from either of the temperatures obtained for 6105.

A combination of the above evidence implies that the NWA 6105 samples are paired although only further work, such as determining cosmic-ray exposure ages, can truly determine whether this is indeed the case. The evidence also suggests that the NWA 6105 samples and the NWA 6106 sample are not paired.


The following points summarize our findings related to NWA 6105 and 6106:

  • 1 All four meteorites are breccias composed of eucrite clasts set in a finely comminuted matrix.
  • 2 Classification of these meteorites as eucrites is supported by the pyroxene Mn versus Fe plot and oxygen isotope data.
  • 3 Pyroxene grains in NWA 6105,1 and 6105,2 define two Ca-Fe trends, one basaltic and one cumulate; 6105,3 pyroxenes also define multiple basaltic Ca-Fe trends, both basaltic; and those of 6106 define one basaltic Ca-Fe trend.
  • 4 We suggest that NWA 6105,1; 6105,2; and 6105,3 are paired samples of a polymict basaltic/cumulate eucrite breccia, and NWA 6106 is a monomict basaltic eucrite breccia.
  • 5 Geothermometry yields temperatures of metamorphic equilibration ranging from approximately 652 to 886 °C. These temperatures indicate metamorphism before the final assembly of the 6105 breccia.

Acknowledgments— We thank the following individuals for useful discussions: Andrew Beck, Andrea Patzer, Arya Udry, Allan Patchen, Yang Liu, Brian Balta, and Kevin Thaisen. A special thanks is extended to Ai-Cheng Zhang for his initial basic characterization of these samples. This work was partly funded by NASA Cosmochemistry grants NNX10AH48G to HYM and NNX11AG58G to LAT.

Editorial Handling— Dr. Edward Scott