Metachondrites


CV Metachondrites

Northwest Africa 1839

Morocco
Found 2003
Metachondrite (CV affinity)

A 121.8 g, partially crusted stone was purchased in Rissani, Morocco in June 2003. Description and classification (T. Bunch and J. Wittke, NAU): polygonal, equigranular (<0.75 mm grain size); no relict chondrules; prominent twinning in plagioclase. Mineral modes: orthopyroxene, 37 vol. %; olivine, 35 vol. %; plagioclase, 12 vol. %; Ca-rich pyroxene, 6 vol. %; troilite, 4.5 vol. %; metal (taenite only), 3.5 vol. %; chromite, 2 vol. %. Very homogeneous; silicate compositional range is within the analytical precision of ± 0.2 mol %. Orthopyroxene, Fs18.9Wo2.3, Fe/Mn = 41; olivine, Fa22.0, Fe/Mn = 63 g/g; plagioclase, An53.4Or2.2; clinopyroxene, Fs7.2Wo47; taenite, Ni = 16.7 wt. %; chromite, Cr/(Cr+Al) = 0.73. Shock level, S1; weathering grade is W2. Specimens: 20.1 g, NAU; main mass, Oakes.

NWA 1839. Image © T.E. Bunch, 2005

NWA 1839. left) Plane-polarized light; note lack of chondrules. (right) Cross polarized light image. © T.E. Bunch, 2005

NWA 1839. (left) Cross-polarized light; 4x higher magnification. Note 120-degree grain boundary junctions. © T.E. Bunch, 2005

Reference

A. J. Irving, T. E. Bunch, D. Rumble III & T. E. Larson, 2005, Metachondrites: Recrystallized and/or residual mantle rocks from multiple, large chondritic parent bodies: 68th Met. Soc. Meeting, #5218.


Northwest Africa 3133

Morocco
Purchased 2004 March/August
Metachondrite (CV affinity)

A single, 724 g stone was purchased in Zagora Morocco, March 2004. Description and classification (T. Bunch and J. Wittke, NAU; A. Irving and S. Kuehner UWS): equigranular texture of subhedral to anhedral grains with 120° triple junctions; mean grain size = 0.28 mm. Mineral modes in vol. %: olivine, 46; orthopyroxene, 28; metal (includes intact weathered oxides), 8; plagioclase, 8; diopside, 5, troilite 3 and chromite, 2. Highly equilibrated mineral compositions within the analytical precision of ± 0.2 mol %: olivine, Fa22.0 FeO/MnO = 64 g/g; orthopyroxene, Fs19.0Wo2.3, FeO/MnO = 42 g/g; diopside. Fs7.5Wo48.3; plagioclase, An53.5 Or2.3; metal. Ni = 17.4 - 20.2 wt %; troilite, Ni = 1.2 - 5.2 wt %.; chromite, TiO2 = 2.61 wt %; Cr/(Cr+Al) = 0.73. Oxygen isotope replicate analyses of acid-washed, hand picked olivine by laser fluorination gave δd18O = 0.92, δ17O = -3.65 and ΔD17O = -4.12 that plot near the CCAM line and within the region of CV compositions (T. Larson, UWO, Canada). Shock level is S2 and the weathering grade is W3. Specimens: type specimen 20.2 g, NAU; main mass, Hupé.

NWA 1839. (left) Plane-polarized light; note lack of chondrules. (right) Cross polarized light image. © T.E. Bunch, 2005


Other CV Metachondrites

IdLocationDateClass.Mass (g)StateFaFs Comments
NWA 2643Erfoud, Morocc2004Ach.26S2, W3 22.218.6paired with NWA 3133


CR Metachondrites

Northwest Africa 3100

Northwest Africa
Purchased 2004
Metachondrite (CR affinity)

A 136 g complete stone with weathered crust was purchased in Rissani, Morocco in 2003 June. Description and classification (T. Bunch and J. Wittke, NAU; A. Irving, UWS): polygonal-granular, fine-grained (avg. size 0.155 mm; range, 0.03 to 0.7mm); recrystallized (two very small relict chondrules were observed in one of two thin sections). Mineral modes (vol%): olivine, 78; orthopyroxene, 10; plagioclase, 5; troilite, 5; metal, 1; chromite, diopside and merrillite all <1; olivine is greatly enhanced and metal is depleted relative to LL6 chondrites. Olivine, Fa28.7 (FeO/MnO = 65.6 g/g; orthopyroxene, Fs23.5Wo0.8. (FeO/MnO = 48.8); plagioclase, An34.1Or3.1; diopside, Fs2.8Wo41; Ni-rich troilite, Ni = 4.2 wt%; metal (taenite only), Ni = 20.1 wt%; chromite, Cr/(Cr+Al) = 0.80, Mg/(Mg+Fe) = 0.21. Low shock level; weathering grade, W2. Oxygen isotopes: replicate analyses of an acid-washed whole rock sample by laser fluorination (T. Larson and F. Longstaffe, UWO) gave respectively: δ18O = 1.54, 1.15, δ17O = -0.92, -1.07, Δ17O = -1.72, -1.61 per mil, which imply that this specimen has affinities with CR chondrites (Bunch et al., 2005). Specimens: type specimens, 20.2 g, and two polished thin sections, NAU; main mass, Farmer.

NWA 3100. (left) Plane-polarized light; note relict chondrule indicated by arrow. (right) Cross polarized light image. © T.E. Bunch, 2005

References

A. J. Irving, T. E. Bunch, D. Rumble III & T. E. Larson, 2005, Metachondrites: Recrystallized and/or residual mantle rocks from multiple, large chondritic parent bodies: 68th Met. Soc. Meeting, #5218.

T. E. Bunch, A. J. Irving, T. E. Larson, F. J. Longstaffe, D. Rumble, III, & J. H. Wittke, 2005, "Primitive" and igneous achondrites related to the large and differentiated CR parent body. Abstr. 36th Lunar Planet. Sci. Conf., #2308.


En Metachondrites

Northwest Africa 2965

Algeria
Purchased August 2005
Metachondrite (enstatite chondrite affinities)

History: Hundreds of fragments that weigh >100 kg were collected in the Algerian desert in 2004. Some fragments retain a weathered fusion crust; smaller fragments ~100 g are weathered to a dark brown; larger pieces are medium gray with patchy staining, particularly heavy along fractures. Petrography (T. Bunch and J. Wittke. NAU): a completely recrystallized enstatite chondrite; polygonal to irregular grain outlines; grain size varies from 0.02 mm to 0.7 mm; faint round, curved, blocky objects, contain coarser-grained orthopyroxene than in the matrix and probably represent pre-existing chondrules, no relict chondrules were observed. Low-level shock, the weathering grade ranges from W2-W5. Mineralogy: orthopyroxene, Fs98.4±0.02; plagioclase, An17.8Or4.4, troilite, Ti = 0.91 wt. %, Cr = 1.10 wt. %, Zn = 0.6 wt. %; metal, daubreelite, and tiny, vermicular grains of graphite. Classification: metachondrite (enstatite chondrite affinities). Specimens: type specimen, 24 g, NAU; main mass holders, Turecki/Reed.


H Metachondrites

Northwest Africa 2353

Northwest Africa
Purchased 2004
Metachondrite (H affinity)

A stone of 580 g was purchased in Erfoud, Morocco. Description and classification (T. Bunch and J. Wittke, NAU): completely recrystallized into polygonal and subhedral grains; H-group chondrite affinities, but with different oxygen isotopes (H7, old classification), no relict chondrules; grain size <0.5 mm (mean = 0.2 mm); intensely fractured with abundance of subparallel compression fractures. Olivine and pyroxene analyses are within the precision of the electron microprobe. Olivine, Fa17.9 (FeO/MnO = 34); orthopyroxene, Fs15.6Wo3.1 (FeO/MnO = 19); plagioclase, An13.1Or2.6; chromite, Cr/(Cr+Al) = 0.87; FeS, Ni = 1.65 wt %; metal (taenite only), Ni = 21.6 wt %; troilite, Ni = 1.36 wt % Ni, trace amount of merrillite. Oxygen isotope replicate analyses (T. Larson and F. Longstaffe, UWO) of acid-washed, bulk sample by laser fluorination gave: δ17O = 3.16 and 3.3, δ18O = 5.10 and 5.51, and Δ17O = 0.44 and 0.51. Weathering grade, W3 and lightly shocked (S2). Specimens: type specimen, 20.1 g, NAU; main mass, Regelman.

NWA 2353. (left) Plane-polarized light. (right) Cross polarized light image. © T.E. Bunch, 2005


Northwest Africa 2635

Northwest Africa
Purchased 2004
Metachondrite (H affinity)

A 4.2 kg, partially crusted stone was purchased in Erfoud, Morocco. Description and classification (T. Bunch and J. Wittke, NAU, results from one thin section (2.4 cm2): mostly recrystallized into fine-grained (<0.5 mm, avg. = 0.185 mm) polygonal and subhedral grains with the exception of several medium to large (1.2-3.0 mm) orthopyroxene grains that contain very small olivine (<0.1 mm) inclusions. Olivine and orthopyroxene analyses are homogeneous within the precision of the electron microprobe (± 0.15 mol. %) and indicate H chondrite affinities. Olivine, Fa18.9 (FeO/MnO = 38.1); orthopyroxene, Fs16.8Wo2.9 (FeO/MnO = 20.0); plagioclase, An15.1Or4.7; metal (only taenite found), Ni = 18.9 wt. %; chromite (only one grain found), Cr/(Cr+Al) = 0.84; FeS, Ni = 1.42 wt. % and Co = 0.28 wt. %, minor merrillite. Most metal has been oxidized; specimen contains ~70% olivine implying it contained less metallic iron than an H chondrite when fresh. Oxygen isotope replicate analyses (D. Rumble, CIW of acid washed, bulk sample by lased fluorination gave δ17O = 3.23 and 2.98, δ18O = 5.03 and 4.37 and Δ17O = 0.614 and 0.708. Weathering grade, W4 with staining; shock level, low. Specimens: type sample, 23.2 g, NAU; main mass, Strope/Farmer.


Northwest Africa 3145

Morocco
Purchased 2004 November
Metachondrite (H affinity)

A single stone weighing 678.4 g was purchased by H. Strufe (Strufe) from a Moroccan dealer in Erfoud in 2004 November. Classification and mineralogy (A. Irving and S. Kuehner, UWS; T. Bunch, NAU): Metamorphic aggregate (average grain size 0.2 mm) consisting of olivine (Fa18.2 -18.7, FeO/MnO = 35.0-37.6), orthopyroxene (Fs16.2Wo4.0 to Fs16.6Wo4.2, FeO/MnO = 24.3-25.7), plagioclase (An12.3Or6.7 to An27.4Or2.8), clinopyroxene (Fs7.4Wo43.4 to Fs8.5Wo40.4, FeO/MnO = 15.8-21.7) and metal (mostly kamacite with 5% Ni and bands of taenite with 30% Ni, but heavily altered to limonite) with accessory Al-poor chromite, merrillite and Ni-bearing troilite. Limonite forms abundant veinlets throughout the sample (W2-3). This specimen is essentially identical in texture, mineral compositions and degree of alteration to a polished slice of NWA 2353 analyzed concurrently, and it is concluded that these are paired stones. Both specimens are modally heterogeneous (especially with respect to chromite, clinopyroxene and merrillite), and also with respect to plagioclase composition. Furthermore. it is likely that both of these stones are paired with NWA 2635. Specimens: type specimen, 28.6 g, and one polished thin section, NAU; one polished mount, UWS; 201 g, Hupé; main mass, Strufe.

NWA 3145. (left) Plane-polarized light. (right) Cross polarized light image. © T.E. Bunch, 2005