Acapulcoite–Lodranite Clan

standby for lodran photo
click on photo for a magnified view

Fell October 1, 1868
29° 32' N., 71° 48' E.

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.

Lodran is an igneous, coarse-grained meteorite that formed as a residue of a partial melt from which an incompatible-element-rich fraction was extracted. The silicate component is thought to have initially formed as a cumulate harzburgitic rock (Prinz et al., 1978). Lodran is composed primarily of approximately equal proportions of magnesian olivine (37 vol%) and pale yellow orthopyroxene (36 vol%), along with abundant FeNi-metal (25 vol%) that was injected and surrounded the silicate grains. Minor amounts of black chromite (1 vol%, commonly included in olivine), green augite, and troilite occur, with accessory schreibersite, phosphate (apatite), and a K-rich phase. A single green Cr-diopside grain was identified by Takeda et al., 1994). Benedix et al. (2009) described chromite in lodranites as occurring in two different forms: one consisting of rounded blebs embedded within olivine grains having a composition similar to primary chromite in acapulcoites, and the other consisting of Al-depleted (from plagioclase depletion) crystals associated with the metal–silicate interface.

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, attesting to a separate phosphate-dominated partial melting phase (Dobrica et al., 2008; Hidaka et al., 2013). Lodranites all have coarse granular–granoblastic textures, but vary in their respective grain sizes. Lodran experienced FeO reduction processes and exhibits minor zoning in olivine grains (Fukuoka et al., 1978).

Differences in mineral composition among lodranites led K. Yanai (2001) to tentatively place them into one of three subgroups, labeled A, B, and C; Lodran, with a high Fa% in olivine, is a member of subgroup A. In another study of lodranites, D. Mittlefehldt (2003) divided them into two distinct groups: 1) magnesian lodranites, comprising Gibson, Y-75274, and Y-8002; and 2) ferroan lodranites, comprising all others known at that time. Based on the Mg# and Ti-content of orthopyroxenes, he concluded that only Gibson was consistent with having an origin as a partial melt residue of acapulcoite-like precursor material, while 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, with each having a similar range of oxygen isotopic compositions. The discovery of the acapulcoite/lodranite breccia NWA 5782 provides empirical evidence for a common parent body (Bunch et al., 2010, #1281). 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. The acapulcoite–lodranite parent body was a unique chondritic object calculated by Golabek et al. (2014) to have been 50–130 km in diameter. They determined that the acapulcoites and lodranites were formed at different depths ranging from 9–19 km and 14–25 km, respectively—a more accurate depth determination being constrained by several unknown factors such as the initial temperature and porosity at the time of formation (where an initially porous parent body allows a more shallow formation location for both meteorite types), and even the actual shape of the planetesimal.

A petrogenetic model for the acapulcoite–lodranite parent body was ascertained by Henke et al. (2014), the calculations for which were based on an onion-shell structure. The stratigraphy was considered to be ordered from the surface inward (i.e., increased metamorphism) as chondrule-bearing acapulcoites ⇒ chondrule-free acapulcoites ⇒ lodranites. Their best fit model was consistent with a parent body 540 km in diameter that accreted 1.66 m.y. after CAIs, and in which acapulcoites and lodranites formed at depths of 4 and 8 km, respectively. The precursor material is thought to have been similar to CR or K chondrites, with radiogenic heating within the parent body reaching temperatures high enough to produce partial melting without significant differentiation. Schrader et al. (2017) observed an absence of chromite grains in relict chondrules from acapulcoite GRA 98028. They recognized that chromite grains are only present in type II chondrules, and that these occur in greatest abundance in ordinary chondrites but are less abundant in carbonaceous and enstatite chondrites. Based on this reasoning as well as other data, they consider it likely that the precursor of the acapulcoite–lodranite clan was similar to a carbonaceous chondrite of a type unknown in our collections. On the other hand, Keil and McCoy (2017) consider an S-type asteroid to be the most likely parental source object.

The total Fe content and Fe/Ni ratios in mafic silicates are intermediate between those of ordinary and enstatite (EL) chondrites. Likewise, trace element compositions were found to support an origin on a body having characteristics of both H and E chondrites (Fukuoka et al., 1978). In a study of C in lodranites and acapulcoites, Charon et al. (2010, 2012) found that both C and N isotopic systematics for lodranites and acapulcoites, as well as the degree of C ordering among them, resembles that present in the insoluble organic matter (IOM) of CI–CM chondrites. This suggests an exogenous origin for the IOM of lodranites and acapulcoites from impact of a CI or CM body.

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 might have begun its accretion very early while radiogenic 26Al was most prevalent, within a few m.y. of CAI formation. The Hf–W isochron indicates that the age of differentiation was ~5–6 m.y. after CAI formation, corresponding to an absolute age of 4.563 (±0.0009) b.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 (<1–3% silicate partial melting) as compared to the lodranites. The lodranite material formed within a hotter, deeper layer, experiencing a moderate degree of silicate partial melting (~5–20%) with loss of an FeS and a basaltic component (Bild and Wasson, 1976). The transitional acapulcoites exhibit features (e.g., HSE-rich metal) consistent with extensive melting of metal and sulfide phases, including melt migration and pooling, representing a continuum between the formation of acapulcoites and lodranites, or alternatively, representing formation at greater depths associated with core formation (Dhaliwal et al., 2017).

Based on multi-dimensional numerical models, a plausible thermal evolution for the acapulcoite/lodranite parent body was presented by Golabek et al. (2014). Accretion of the planetesimal began ~1.3 m.y. after CAI formation accompanied by the onset of radiogenic heating, primarily by 26Al. Over the succeeding ~3–4 m.y. temperatures increased from both radiogenic and impact heating until incipient metal–silicate segregation began as reflected in the lodranites. In contrast to the relatively near-surface formation of the acapulcoites and lodranites, higher temperatures at greater depth may have led to the formation of a metallic core. A rapid cooling stage is recorded in these meteorites at 4.555 b.y. ago, considered to represent the collisional disruption of the planetesimal. This was followed by re-accretion and the begining of an extended period of slow cooling.

The O-isotopic composition of the lodranite group is variable, and 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 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 the 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%). 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).

  1. primitive acapulcoites: near-chondritic (Se >12–13 ppm [degree of sulfide extraction])
  2. typical acapulcoites: Fe–Ni–FeS melting and some loss of sulfide (Se ~5–12 ppm)
  3. transitional acapulcoites: sulfide depletion and some loss of plagioclase (Se <5 ppm)
  4. lodranites: sulfide, metal, and plagioclase depletion (K <200 ppm [degree of plagioclase extraction])
  5. enriched acapulcoites (addition of feldspar-rich melt component)

Accretion of the acapulcoite–lodranite planetesimal 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 parent body 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 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 body. 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 4.5582 b.y. relative to the Shallowater aubrite (4.5627 ±0.0003 b.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 to produce 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 breakup 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, 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 (and other primitive achondrites) shows similarities to the H4 chondrite GRV 020043 both mineralogically and in O-isotopes, and H-chondrite-like material may represent the precursor chondrite of the primitive acapulcoite–lodranite achondrites (Li et al., 2010). The differences that do exist, such as in the elements V, Cr, and Se, may be related to specific characteristics of the precursor phase (Hidaka et al., 2012). Conversely, based on siderophile element abundances in magnetic components in acapulcoites, as well as lithophile element abundances in non-magnetic components, Hidaka et al. (2012) concluded that the precursor material of the acapulcoite–lodranite group was most similar to EL chondrites. In addition, a linkage may exist between acapulcoite–lodranite achondrites and the CR clan meteorites, which exhibit similarities related to their formation age, Hf–W systematics, and O-isotopic range (Lee, 2008).

For more information regarding the formation scenario of the acapulcoite–lodranite parent body, visit the Monument Draw page. The specimen shown above is a partial end section of the Lodran meteorite weighing 0.92 g and measuring 15 mm in its longest dimension. The photo shows a coarse grained aggregate with triple junctions. Below is a photo of a much larger specimen of Lodran curated at the National Museum of Natural History, Smithsonian Institution.

smithsonian lodran
click on photo for a magnified view
Photo courtesy of Martin Horejsi, c/o Smithsonian Institution