Angrites


Angrites, named for the Angra dos Reis meteorite, which fell in Rio de Janeiro, Brazil in early 1869, are probably the most beautiful of all meteorites with the exception of the Martian meteorites, Shergotty, Nakhla, and Lafayette. They are basaltic rocks and often contain porous areas and many round vesicles (small cavities). One angrite, SAH99555, contains the largest vesicles (gas bubbles) observed in any asteroidal meteorite, with diameters up to 2.5 cm. Angrites probably derive from a large, differentiated planetary body, Their textures, like that shown for the angrite D' Orbigny below, the presence of vesicles, and elemental distributions suggest that the angrites crystallized rapidly, either within a meter of the surface or as surface flows.

Angrites are evolved achondrites composed mainly of Ti-, Al-rich pyroxenes (Wo >50; "fassaite") with CaO-rich olivine (Fa10-100), plagioclase (An100), troilite, spinels, and unusual minerals, e. g., kirschsteinite and Ca-silicophosphates. Fassaite is uncommon in terrestrial and asteroidal rocks, but more common in lunar rocks. This pyroxene is reddish to purple, millimeters in size and ranges from 90 vol% in Angra dos Reis to 30 vol% in SAH 99555. Angrites are the most ancient igneous rocks known, with crystallization ages of around 4.55 Ga. They are thought to have formed on one of the earliest differentiated asteroids. By comparing the reflectance spectra of the angrites to that of several main belt asteroids, two potential parent bodies have been identified: 289 Nenetta and 3819 Robinson. Other researchers have suggested that angrites instead represent pieces of Mercury.

d'Orbigny. (left, plane light) Crystals of purple-brown fassaite and white plagioclase. (right, XPL). Height, 3 mm. Image © 2005 T. E. Bunch

Are Angrites From Mercury? in the absence of any “ground truth” the possibility that they might be from Mercury rests only on circumstantial arguments: (1) the virtual lack of Na implies a highly refractory planet (formed near the Sun?); (2) oxygen isotopic compositions are close to the terrestrial fractionation line (TFL); (3) corona textures in NWA 2999 require a body with sufficient heat/size to permit preservation of such reactions in progress; (4) each individual angrite specimen is texturally different with a unique cosmic ray exposure (CRE) age (5) the wide range in CRE ages (55 to <6.1 Ma) is consistent with a parent body large enough to be struck repeatedly (it may still be extant); (6) very ancient formation ages (>4.555 Ga) imply rapid core separation following accretion; (7) dynamical calculations predict that ~1% of material ejected from Mercury could reach Earth; (8) the limited shock effects may mean that some angrites were ejected by spallation and/or grazing impacts; others may be impact melts (could vesicles be from rock vapor?).

There are 11 known angrites (as of December 2010) and at least 7 pairings to NWA 2999. These are:

The images below are of selected angrites and show the dramatic differences in crystallization histories together with other unusual characteristics.

d'Orbigny. (left) Plane light photomicrograph showing compositional zoning of FeO and TiO2 in augite (darkest = highest concentrations) and lath to skeletal shaped anorthite, some have pyroxene cores. 6 mm base width. (right) Crossed polarized light photomicrograph showing subophitic to complex intergrowth textures of anorthite, olivine, and pyroxene suggest rapid cooling. Images © 2011 T. E. Bunch

LEW 87051. Cumulate texture of zoned Mg-olivine phenocrysts set in a microspinifex groundmass expressed as acicular anorthite aggregates in Al-Ti pyroxene. This texture implies supercooling of a mafic magma after the initial nucleation/growth of olivine phenocrysts and is similar to that oberved in terrestrial high magnesium boninites. Image © 2011 T. E. Bunch

NWA 1296. The finest-grained and most rapidly quenched angrite. Width = 33 mm.Image © 2011 T. E. Bunch

NWA 1296. BSE image of the delicate quench texture. Dark = anorthite; light gray = olivine; medium gray = Al-Ti pyroxene. Image © 2011 T. E. Bunch

NWA 1296. (left) Plane polarized light photomicrograph showing dendritic quench texture. Width = 9 mm. (right) Crossed polarized light photomicrograph. Width = 4 mm. Note change in scale between left nad right images. Images © 2011 T. E. Bunch

NWA 2999. Crossed polarized light photomicrograph of a brecciated protogranular plutonic angrite that crystallized at much greater parent body depth than the very shallow, quenched angrites. Width = 13 mm. Image © 2011 T. E. Bunch

NWA 4590. Crossed polarized light photomicrograph of an unbrecciated example of a deep-seated ptotogranular angrite. Width = 9 mm. Image © 2011 T. E. Bunch

NWA 4801. Crossed polarized light photomicrograph of a brecciated, mostly recrystallized cumulate angrite. Dark patch is a spinel. Partial crossed polarized light; width = 15 mm. Image © 2011 T. E. Bunch

SAH 99555. Crossed polarized light photomicrograph of ophitic basalt with acicular anorthite enclosed by Al-Ti augite and olivine. Width = 6 mm. Image © 2011 T. E. Bunch


Northwest Africa 2999

Morocco or Algeria
Find: 2005
Achondrite (angrite)

History: Twelve individual dark brown stones totaling 392 g, each with a thin fusion crust, were purchased from a Moroccan dealer in Tagounite by G. Hupé in August 2004.
Physical Characteristics: Grain size is predominantly from 0.1 to 0.5 mm, but all stones have irregularly distributed, larger yellowish plagioclase grains (up to 6 mm across) exhibiting an iridescent luster.
Petrography: (A. Irving and S. Kuehner, UWS; T. Bunch and J. Wittke, NAU) Based upon examination of thin sections of all separate stones, this meteorite is texturally heterogeneous. Terrestrial weathering has resulted in partial replacement of metal and minor grain boundary staining by iron hydroxides. The overall texture is protogranular, but there are large porphyroclasts of anorthite, spinel, and polygranular olivine. Anorthite also occurs as narrow (10-20 µm wide) coronas around spinel grains adjacent to clinopyroxene and both spinel and diopside are compositionally zoned away from the coronas. Texturally, this meteorite is very different from most angrites.
Geochemistry: The major minerals are Ca-rich olivine (Fa39.8-41.0; FeO/MnO = 77-97; CaO = 0.6-1.3 wt%), Al,Ti-bearing diopside (Fs9.6-11.3Wo53-54; FeO/MnO = 55-130; Al2O3 = 5-9, TiO2 = 0.5-2.4 [both wt%]), minor Cr-pleonaste spinel (Mg/(Mg+Fe) = 0.44-0.47, Al2O3 = 55-60, Cr2O3 = 4.7-8.7 [both wt%]), pure anorthite (containing Na2O <0.02 wt%), and kamacite, troilite, and S-bearing calcium silicophosphate. Oxygen isotopes: (D. Rumble, CIW) Triplicate analyses of acid-washed whole rock samples by laser fluorination gave, respectively, δ18O = 3.839, 4.093, 4.154; δ17O = 1.974, 2.054, 2.095; Δ17O = −0.041, −0.095, −0.086 (all ‰).
Classification: Achondrite (angrite).
Specimens: A 22 g type specimen and one polished thin section are on deposit at NAU. Three polished thin sections are on deposit at UWS. G. Hupé holds the main mass.

NWA 2999. Image © T. E. Bunch, 2008.


Northwest Africa 3164

Morocco or Algeria
Find: August 2004
Achondrite (angrite)

History: Many dark brown stones (totaling 928 g), were purchased from nomads by A. Aaronson in Rabat in August 2004.
Petrography (T. Bunch and J. Wittke, NAU; A. Irving and S. Kuehner, UWS): All stones have irregularly distributed larger yellowish plagioclase grains exhibiting a "schiller" luster. The major minerals are Ca-rich olivine, diopside, Cr-bearing pleonaste spinel with subordinate anorthite and spinel and accessory kamacite and troilite. No kirschsteinite or orthopyroxene was found. Primary metal is partly replaced by limonite, which also occurs along grain boundaries. The overall texture is protogranular, but there are large porphyroclasts of anorthite, spinel and polygranular olivine. There are clinopyroxene-spinel symplectites around anorthite porphyroclasts in contact with olivine, and anorthite also occurs as narrow (10-20 micron wide) coronas around spinel grains adjacent to clinopyroxene (Kuehner et al., 2006).
Geochemistry: Ca-rich olivine, Fa39.1-41.2, CaO = 1.2 to 1.8 wt%, FeO/MnO = 62-84; Al-Ti-bearing diopside, Fs10.3Wo52, Al2O3 = 6 to 7 wt%, TiO2 = 1 to 1.6 wt%, FeO/MnO = 130-142; Cr-bearing pleonaste spinel, Al2O3 = 59.7 wt%, Cr2O3 = 4.7 wt%, Mg/(Mg+Fe) = 45.7.
Classification: Achondrite (angrite). The characteristic textures and mineral compositions indicate that this material is paired with NWA 2999.
Type specimens: A total of 21 g and one polished thin section are on deposit at NAU. Mr. T. Boswell is the main mass holder.

Reference

Kuehner, S. M., Irving, A. J., Bunch, T. E., Wittke, J. H., Hupé, G. M. and Hupé, A. C., 2006, Coronas and symplectites in plutonic angrite NWA 2999 and implications for Mercury as the angrite parent body. Lunar. Planet. Sci. XXXVII, #1344.


Other Angrites

ID Location Date Class. Mass (g) State Fa Fs Comments
NWA 4569 Erfoud, Morocco 2005 ANG 484 S2, mod. 40.3 10.8 Wo53; An100; paired with NWA 2999