This rare meteorite fell at 2:00 P.M. about 12 miles east of Lodhran, Pakistan. About 1 kg was preserved but the ~700 g main mass in the Museum of Geology in Calcutta, India has been missing for some years. About 150 g is distributed among a few major museum collections, and a small amount exists in private collections.
Lodranites are igneous rocks that formed as residues of a partial melt. They are composed of 37% magnesian olivine, 36% low-Ca pyroxene, 25% metallic iron, ~1% high-Ca pyroxene, and 1% chromite present as two different groups: one chromite group comprises rounded blebs embedded within olivine grains and having a composition similar to primary chromite in acapulcoites, while the other chromite group comprises Al-depleted, subhedral crystals associated with the metal–silicate interface, reflecting a depletion in plagioclase (Benedix et al., 2009). Lodranites contain little to no plagioclase since it was mostly depleted from the residue during the partial melt phase (up to at least 26% partial melting). In a similar manner, lodranites are depleted in REE abundances, which are associated with the phosphate whitlockite (Dobrica et al., 2008). Lodranites also contain accessory troilite and schreibersite. They all have coarse granular–granoblastic textures, but vary in their respective grain sizes. Lodran exhibits minor zoning of olivines.
Differences in mineral composition has led to their tentative placement into one of three subgroups, labeled A, B, and C; Lodran, with a high Fa% in olivine, is a member of subgroup A (Yanai, 2001). In a different study, another investigator (Mittlefehldt, 2003) divided the lodranites into two distinct groups: 1) magnesian lodranites, comprising Gibson, Y-75274, and Y-8002; and 2) ferroan lodranites, comprising all other members. He concluded, based on the Mg# and Ti-content of orthopyroxenes in lodranites, that only Gibson was consistent with having an origin as a partial melt residue of acapulcoite-like precursor material; the other lodranites are restites from more ferroan sources than those which formed the parental melts of the known acapulcoites.
Lodranites and acapulcoites were almost certainly formed on a common parent body, and they have a similar range of oxygen isotopic compositions. The discovery of the acapulcoite/lodranite breccia, NWA 5782, provides empirical evidence for a common parent body. The two meteorite types also have similar mineralogies, thermal histories, and cosmic ray exposure ages. In addition, lodranites and acapulcoites have identical cosmogenic nuclide abundances and similar shielding conditions. Their parent body was a unique chondritic object, as small as 80 km in diameter, containing mafic silicates intermediate between those of the ordinary and enstatite chondrite groups. In a study of C in lodranites and acapulcoites, Charon et al. (2010) found that both C and N isotopic systematics for Lodran, as well as the degree of C ordering, resembles that in the insoluble organic matter of CI–CM chondrites; this suggests a CI or CM precursor body for lodranites and acapulcoites.
In a synthesis of available data for acapuloites and lodranites, Eugster and Lorenzetti (2005) developed a model of the structure of the acapulcoite/lodranite parent body. They propose a layered "onion-shell" structure not unlike that proposed for the ordinary chondrite asteroids, but one which was larger and experienced higher temperatures necessary for partial melting. Alternatively, a relatively small parent body may have begun its accretion very early, within a few m.y. of CAI formation. The Hf–W isochron indicates that the age of differentiation was ~6 m.y. after CAI formation, corresponding to an absolute age of 4,563 (±0.9) m.y., which may be indicative of a much larger diameter body that retained its heat until well after most of the 26Al had decayed, at least ~3 m.y. after CAI formation. The typical acapulcoite material is thought to have originated in the outermost layer of the asteroid, which cooled earlier and faster consistent with its older gas retention age, finer-grain size, and less intense metamorphism as compared to the lodranites. The transitional acapulcoites formed at an intermediate level and exhibit the corresponding features. The lodranite material formed within a hotter, deeper layer, experiencing silicate partial melting and loss of a basaltic component.
The O-isotopic composition of the lodranite group is variable, and it plots between the terrestrial fractionation line and the carbonaceous chondrites. The lodranite MAC 88177, as well as the acapulcoites that have been analyzed, have O-isotopes, chemistry, and mineralogy very similar to that of the CR chondrite group, but some differences exist. Relict chondrules have been identified in several acapulcoites: a 2 mm radial pyroxene chondrule was found in Monument Draw, a few recrystallized barred olivine chondrules were found in Y-74063, numerous POP and PP chondrules are present in GRA 98028, and many radial pyroxene, granular olivine–pyroxene, and porphyritic olivine chondrules occur in Dhofar 1222; in addition, evidence exists for the presence of relict porphyritic chondrules in ALHA77081.
Acapulcoites represent the likely precursor material that existed prior to the collisionally-induced melting phase, in which temperatures became high enough to produce a 1–3 vol% basaltic partial melt—short of silicate partial melting, but high enough to mobilize melts of metallic FeNi–FeS and phosphate. This was followed by up to 5% melt removal, with most of the partial melt being retained in its source region, manifested by µm- to cm-sized metal veins (as present in Monument Draw). Other pockets were subjected to more intense shock, heating the rock to the point where plagioclase and clinopyroxene formed silicate basaltic partial melts. After a 12–20+ vol% basaltic, magnesian partial melt was extracted from the source rock, the residual melt, now depleted in FeS, FeNi, plagioclase, and incompatible trace elements, cooled to form the lodranite material. The acapulcoites crystallized with a fine grain structure (~0.15–0.23 mm) due to both rapid cooling near the surface and to a lack of silicate partial melt for continued grain growth. With graphite as a likely reducing agent, the acapulcoites became enriched in metallic Fe at the expense of FeO (Rubin, 2006). A coarser grain structure (~0.54–0.70 mm) was formed in the lodranites due an extended cooling period at depth, aided by an abundance of silicate partial melt. However, with the many new members available to study, it is now evident that a continuum exists for the grainsizes of these two groups, and it has been proposed by Bunch et al. (2011) that an arbitrary group division is no longer justified; the term acapulcoite–lodranite clan should therefore be applied to all members of the combined group.
In some rocks, varying degrees of melting, melt removal, and melt mixing occurred, forming such transitional acapulcoites as EET 84302, ALH A81187, and GRA 95209, each containing lithotypes intermediate between the acapulcoite and lodranite groups. The lodranite LEW 86220 contains two distinct lithologies representing an acapulcoite host that was intruded by a basaltic partial melt from the lodranite layer. The highest temperature silicate-rich melt, FRO 93001, formed through a high-degree partial melt (at least 35 wt%). It contains coarser grains with abundant enstatite, and preserves lodranitic xenoliths (Folco et al., 2006). The lodranite MAC 88177 represents a lithology that has been intruded by an FeS melt following the removal of a partial melt. Apparently, the acapulcoite–lodranite clan represents a continuum of thermal histories not easily partitioned into only two groups. A division of the acapulcoite–lodranite meteorites based on metamorphic stage was proposed by Floss (2000) and Patzer et al. (2003).
primitive acapulcoites: near-chondritic (Se >12–13 ppm [degree of sulfide extraction])
typical acapulcoites: Fe–Ni–FeS melting and some loss of sulfide (Se ~5–12 ppm)
transitional acapulcoites: sulfide depletion and some loss of plagioclase (Se <5 ppm)
lodranites: sulfide, metal, and plagioclase depletion (K <200 ppm [degree of plagioclase extraction])
enriched acapulcoites (addition of feldspar-rich melt component)
Accretion of the acapulcoite–lodranite asteroid began ~4.565 b.y. ago and was complete after only ~1 m.y. This was followed by heterogeneous heating by radioactive elements and impact shock heating to temperatures ranging from 980°C (Monument Draw) to 1170°C (Acapulco) to ~1250°C (Lodran). The cooling history of the acapulcoite–lodranite PB is varied and complex. Following the heating phase, which resulted in somewhat higher temperatures for lodranites (more deeply buried), both groups experienced a moderate cooling rate from peak metamorphic temperatures to about 600°C, at which time a rapid cooling ensued until about 350°C, likely reflecting emplacement near the surface. At 300°C, a drastic decrease in the cooling rate was initiated until the temperature reached about 290°C. At this point a further sharp decrease in the cooling rate occurred until ~90°C was reached. This drastic change to very low cooling rates suggests an increase in the insulating regolith. In consideration of the I–Xe and Pb–Pb systems utilized by Crowther et al. (2009), the earliest this disruption could have occurred is 9.4 m.y. after CAI formation.
This complex cooling history may reflect the impact removal of a significant overburden, or the collisional breakup and reassembly of the acapulcoite–lodranite asteroid. The parent body cooled to the Ar closure temperature ~4.51 b.y. ago for the acapulcoites and ~4.48 b.y. ago for the lodranites, reflecting the higher temperature and correspondingly longer cooling period of the lodranites. Identical I–Xe closure ages were calculated for both acapulcoite and lodranite samples by Crowther et al. (2009) to be 4,558.2 m.y. relative to the Shallowater aubrite (4,562.3 m.y.). This similarity in I–Xe closure ages is consistent with a very rapid cooling phase on the entire asteroid. A subsequent period of annealing resulted in recrystallization, producing equigranular textures and a loss of shock indicators (Rubin, 2007).
A cosmic-ray exposure (CRE) age of ~6.5 (±0.7) m.y. was calculated for all but two of the lodranites, and this age is considered to represent a common break-up event. This CRE age also coincides with that of virtually all of the acapulcoites (4–7 m.y.), consistent with a single ejection event for both groups. Coincidentally, the H-chondrite CRE ages coincide with the acapulcoite–lodranite CRE ages, possibly demonstrating disruption by common impactors. Another potentially consequential finding is that the acapulcoite–lodranite parent body may possibly be linked to the CR clan meteorites, exhibiting similarities related to their formation age, Hf–W systematics, and O-isotopic range (Lee, 2008).
For a more complete amd current formation scenario of the acapulcoite–lodranite parent body, visit the Monument Draw page. The specimen shown above is a partial slice with crust of the Lodran meteorite weighing 0.92 g and measuring 15 mm in its longest dimension. The photo exhibits the light-reflecting FeNi-metal grains embedded among the yellow-green silicates. Below is a photo of a much larger specimen of Lodran curated at the National Museum of Natural History, Smithsonian Institution.
