"Rock of the Month #197, posted for November 2017" ---

LODRANITES are a rare class of achondrites,

represented by just 70 confirmed examples, out of more than 57,000 classified meteorites (Meteoritical Bulletin, as of 03 November 2017). There are a total of 149 meteorites in the extended grouping of acapulcoite plus lodranite achondrites (70 lodranites, 69 acapulcoites and 10 examples which, thus far, have defied efforts at subdivision within the larger clan). Only eight lodranites and seven acapulcoites exceed 1 kg in TKW, the largest lodranite being NWA 10265 (3360 grams).

Table 1 shows paired meteorites with similar mineral chemistry (data from Meteoritical Bulletin: median values quoted here - spread in quoted values is <1 mol.% in each case). The Met.Bull. entries note that each of these four meteorites is a breccia of coarse crystal fragments, and that each one displays (iron-) stained kamacite and bright green clinopyroxene, the latter very eye-catching in the often monochrome world of meteorite hand specimens.

Here is a brief summary of facts and current ideas concerning the nature and origin of lodranites. The source for much of the data presented here is Grady et al. (2014).

Lodranites tend to be coarser than acapulcoites (a rule-of-thumb cut-off being 0.5 mm diameter), while the latter may contain rare, relict chondrules, taken to represent relict, unmelted precursor material. An extended high-temperature period has led to extensive recrystallization, with annealing indicated by 120-degree grain-boundary triple junctions. This interval may have been longer for the coarser lodranites, versus the acapulcoites. Rare-earth elements (REE) are in low abundance in lodranites, with light REE (LREE) depletion and a strong negative europium anomaly, indicating the likely removal of LREE carriers (phosphates?) and of plagioclase feldspar (the key repository for europium) from the melt. The oxygen isotope signature of acapulcoites and lodranites is distinct from those of other achondrites, but overlaps with the metal-rich CR chondrites (a class, currently 179-strong, including Kaidun, Miller Range 90001, NWA 801, Renazzo and Shisr 033). A likely source for these rare achondrites is likely to be a relatively small (<100 km diameter) S-type asteroid. Another lodranite, NWA 10155 can be seen on the Turnstone web site.

Returning to the mineralogy of the slice, and of the four paired finds in Table 1, the bright green clinopyroxene is chrome diopside (circa En₅₂Wo₄₄Fs₄), with contents of sodium and chromium near the high end for achondrites (ibid., p.201) at almost 1% Na₂O and 1-2% Cr₂O₃. Chrome diopside is known in a range of generally alkaline to ultramafic lithologies on Earth, but occurs also in silicate inclusions in iron meteorites (e.g., Park et al., 1966; Takeda et al., 2003; Pravdivtseva et al., 2013), and in a number of achondrites (e.g., Li et al., 2011). The most notable terrestrial occurrence is in kimberlites, thus Cr diopside is a diamond indicator mineral used in exploration, along with certain compositions of garnets, ilmenite, chromite and enstatite. Cr diopside is also known in ultramafic alkaline intrusions, such as the Batbjerg complex on Kangerdlugssuaq Fjord in eastern Greenland. See also an illustrated example from Russia.

References

Grady,MM, Pratesi,G and Moggi-Cecchi,V (2014) Atlas of Meteorites. Cambridge University Press, 373pp.

Li,S, Wang,S, Bao,H, Miao,B, Liu,S, Coulson,IM, Li,X and Li,Y (2011) The Antarctic achondrite, Grove Mountains 021663: an olivine-rich winonaite. Meteoritics & Planetary Science 46, 1329-1344.

Park,FR, Bunch,TE and Massalski,TB (1966) A study of the silicate inclusions and other phases in the Campo del Cielo meteorite. Geochim. Cosmochim. Acta 30, 399-414.

Pravdivtseva,O, Meshik,A, Hohenberg,CM and Kurat,G (2013) I-Xe ages of Campo del Cielo silicates as a record of the complex early history of the IAB parent body. Meteoritics & Planetary Science 48, 2480-2490.

Takeda,H, Hsu,W and Huss,GR (2003) Mineralogy of silicate inclusions of the Colomera IIE iron and crystallization of Cr-diopside and alkali feldspar from a partial melt. Geochim. Cosmochim. Acta 67, 2269-2288.