MONUMENT DRAW


Acapulcoite, primitive subgroup
('A Chondrite')
Acapulcoite–Lodranite Clan

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Found 1985, recognized 1990
32° 30.2' N., 102° 44.6' W.

While plowing, Mr. Joe D. Nevill unearthed a 524.5 g, moderately weathered stone about 18 miles NW of Andrews, Texas (McCoy et al., 1996). Five years later, in 1990, Dr. Vestal Yeats of Texas Technical University removed a 123.9 g end section and sent it to Glenn Huss for analysis, with the main mass being retained by Mr. Nevill. The 123.9 g end piece was cut into three sections, and these were distributed among Yeats (H498.2), Huss (H498.1), and Texas Tech (H498.3). The photo below shows this original end piece with lines drawn to show the three-way split. A photo showing the main mass mirror image of the Yeats end section can be seen here, while a different complete slice can be seen here.

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Photo courtesy of Walter Zeitschel

Dr. Yeats subsequently traded his portion to Walter Zeitschel in Germany. Polished thin sections were made for analyses, three from the Huss specimen and one from the Zeitschel specimen. The photo below shows an excellent petrographic micrograph of the Zeitschel polished thin section, shown courtesy of Peter Marmet. In the micrograph the abundant FeNi–FeS–phosphate veins are displayed as dark areas.

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click on photo for a magnified view
Photo courtesy of Peter Marmet

In 2000, the main mass of Monument Draw was sold by Mr. Nevill to International Meteorite Brokerage, with the vast majority of the material slated for trade to research institutions. The photo at the top of this page shows a 1.55 g thin partial slice acquired from International Meteorite Brokerage, while the photo below shows a slightly more complete section still in the possession of International Meteorite Brokerage. The location on the main mass from which this slice was taken contains a richer portion of the FeNi-metal vein than that which was employed for the thin section shown above.

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The parent body of Monument Draw had a unique chondritic composition comprising various chondrule types, high concentrations of planetary-type noble gases, and a chondritic mineralogy (A. Rubin, 2007). Many features of acapulcoites are similar to CR carbonaceous chondrites, including a high abundance of refractory lithophiles, a high modal abundance of FeNi-metal, chondrule diameters ~700 µm, and, along with lodranites, O-isotopic compositions that overlap the CR field. However, differences do exist between CR chondrites and acapulcoites, including both a higher FeNi-metal and FeS abundance in acapulcoites. Notably, the H4 chondrite GRV 020043 is both mineralogically and O-isotopically similar to acapulcoites and could 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, could be related to specific characteristics of the precursor material (Hidaka et al., 2012). Conversely, based on siderophile element abundances in magnetic components and 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. Schrader et al. (2017) found an apparent absence of chromite grains in relict chondrules from acapulcoite GRA 98028. They reasoned that since chromite grains are only present in type II chondrules, and that these occur in greatest abundance in ordinary chondrites but less so in carbonaceous and enstatite chondrites. Based on this as well as other data, they consider it likely that the precursor of acapulcoites was similar to a carbonaceous chondrite of a type unknown in our collections.

Monument Draw has a recrystallized texture with abundant 120° triple junctions, and has an average grain size of ~150–230 µm, the full range of which is similar to that of other acapulcoites (McCoy et al., 1996). In contrast, the genetically related lodranites experienced higher temperatures (up to 1200°C and ~20% partial melting), slower cooling, and greater diffusion rates leading to more efficient grain growth during contact with a silicate partial melt. Accordingly, they have a larger average grain size of ~540–700 µm. However, with the many new members available for 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.

Compared to ordinary chondrites, the ACA–LOD parent body was heated to higher temperatures, reflecting either an earlier accretion with a proportionately higher abundance of radiogenic elements such as 26Al, or, as elucidated by A. Rubin (2007) and J. T. Wasson (2016), by impact heating. Since some acapulcoites have ages younger than that which is attributed to the complete extinction of radiogenic 26Al, the former theory would be less plausible given a scenario involving primarily a radiogenic heat source.

By applying the Hf–W chronometer and integrating its relatively higher closure temperatures, an age of ~6 m.y. after CAI formation is derived for acapulcoite differentiation, corresponding to an absolute age of 4.5635 (±0.0007) b.y. A slightly older Hf–W age of 3.84 (+3.6/–3.1) m.y. after CAI formation was calculated by Schulz et al. (2010). With other factors considered, they concluded that the metal melting point, or the cooling point at which redistribution of Hf and W between metal and silicate ended, occurred 4.1 (+1.2/–1.1) m.y. after CAIs.

When coupled with Pb–Pb isochron data, it is evident that cooling rates were similar to those of H4 chondrites (Kleine et al., 2007); cooling rates of 65°C/m.y. and 100°C/m.y. were determined by Touboul et al. (2008) and Schulz et al. (2008), respectively. Using multiple chronometers and the Kelvin system, Schulz et al. (2010) determined a cooling rate of ~40K/m.y. as temperatures cooled down to ~720K, and then declined at a rate of ~3K/m.y. below that. Considering the similarity of these cooling rates to those of H4–H5 chondrites (although comparatively higher than cooling rates of interior H6 material) and the significantly higher degree of thermal metamorphism experienced by acapulcoites compared to H4 chondrites, an earlier accretion period (1.5–2 m.y. after CAIs) might be demonstrated for the acapulcoites. Furthermore, a smaller acapulcoite parent body size (~40–80 km diameter) with rapid cooling near the surface could be reasonably inferred. The young ages of other acapulcoites might reflect resetting of these radiometric chronometers through impact shock heating.

In a similar manner, the I–Xe system was employed by Crowther et al. (2009) to calculate the closure age of a sampling of lodranites and an acapulcoite. Having formed deeper and in hotter conditions, lodranites are considered to have cooled slower and experienced phosphate closure later than acapucoites. However, the samples revealed an average absolute closure age of ~4,558.2 m.y. relative to Shallowater; one lodranite had a younger closure age of 4,551.9 m.y. When coupled with Pb–Pb isochron data from phosphates, rapid cooling was calculated at a rate of 100 (±40)K/m.y. from peak temperatures, while slower cooling ensued below ~720K. This scenario is consistent with petrographic data showing that very rapid cooling was initiated at high temperatures as a collisional disruption event occurred on the parent asteroid, affecting the cooling of both acapulcoite and lodranite source regions. In consideration of the I–Xe and Pb–Pb systems, the earliest this disruption could have occurred is 9.4 m.y. after CAI formation.

Another model for the cooling history of the acapulcoites demonstrates its complexity. Initially, acapulcoites experienced moderate cooling sustained from a high temperature of at least 988°C (Fe–FeS melting point), possibly as high as ~1100°C, down to ~500°C. Thereafter, the acapulcoites entered a period of rapid cooling down to ~400°C, as evidenced by the older Pb–Pb age of Acapulco (4.557 b.y.; Zipfel et al., 1995). Below ~300°C, it began a slow cooling phase as indicated by its younger Ar–Ar age (~4.51 b.y.). Rapid cooling indicates that either the ACA–LOD parent body was smaller than the ordinary chondrite parent bodies, and/or that it experienced a collisional disruption early in its history, forming sub-km- to multi-km-sized fragments, and eventually succumbing to gravitational reassembly. Rubin (2007) suggests that the abundance of planetary-type noble gases is consistent with a rapid cooling phase. The slow cooling period may represent an event which covered the rock with an insulating ejecta blanket.

The mm- to cm-sized metal veins in Monument Draw are almost troilite-free, likely reflecting the segregation of immiscible melt phases. They contain associated phosphates, mostly in the form of whitlockite, along with lesser amounts of chlorapatite and minor fluorapatite. An interesting feature of Monument Draw is the preservation of at least one relict chondrule, an ~2 mm radial pyroxene chondrule. Chondrules are very rare among acapulcoites, and have been observed only in the acapulcoites GRA 98028 (~6 vol%), Dhofar 1222 (~4 vol%), and Y-74063. The presence of chondrules is a feature which suggests that the acapulcoites are actually metachondrites, based on terminology associated with several newly recognized groups of chondrule-free, texturally evolved chondrites with elemental ratios and O-isotopic compositions showing affinities to existing chondrite groups (Irving et al., 2005). Relict chondrules are only found in the primitive subgroup of acapulcoites, consistent with this subgroup experiencing the lowest equilibration temperatures (950°C) of all acapulcoites, while Acapulco, a member of the typical subgroup, experienced the highest (1170°C). Monument Draw also has the least equilibrated REE distribution within the studied group, and it has a 21Ne-based CRE age (~6.8–7.3 m.y.), which is typical of most other acapulcoites and lodranites.

Metal veins, like those present in Monument Draw, have been intensively studied (e.g., McCoy et al., 1996, 1997, 2006) to gain insight into the early stages of metal segregation and formation of a core on an asteroid. With a lack of shock indicators and brecciation in acapulcoites, and evidence for a significant degree of heating on its parent body, it has commonly been considered that it was heated noncollisionally by either short-lived radiogenic nuclides (e.g., 26Al, 60Fe), or by electrical conduction by the T-Tauri solar wind. Monument Draw is unshocked (S1), and most other acapulcoites have shock stages of S1 to S2, or rarely higher; these shock effects are attributed to post-annealing shock (Rubin, 2007).

In light of these disparate results, an in-depth review of a shock-melting model for acapulcoites and lodranites was conducted by Rubin (2007). He has clearly demonstrated that impact-heating played a significant role in the petrogenesis of the acapulcoites, and that post-shock annealing has erased all evidence of the shock indicators. For example, Rubin (2006, 2007) has propounded an impact heating theory to explain the diverse content of opaque phases and relict chondrules found among different acapulcoites, and he attributes these features to impact-shock heating followed by rapid cooling. He also ascribes a shock melt origin to the presence of the FeNi–FeS veins, as well as other relict shock features including the following:

  1. curvilinear trails of troilite blebs within silicate grains
  2. metal–sulfide veins
  3. polycrystalline metallic phases
  4. rapidly crystallized metal–troilite assemblages
  5. metallic Cu precipitates
  6. Cr veinlets within silicates
  7. Cr-plagioclase assemblages
The rapid cooling that ensued is consistent with both an impact scenario and a parent body breakup scenario, while post-shock annealing below an insulating ejecta blanket could account for the appearance of uncorrupted olivine crystal lattices.

The Monument Draw acapulcoite experienced a low degree of partial melting, ~2–3 vol%, and only a few other meteorites exhibit this same low degree of partial melting and melt migration. One such meteorite is the acapulcoite LEW 86220, which contains a component of trapped FeNi–FeS+plagioclase–pyroxene partial melt. It is inferred that this metallic+basaltic partial melt phase migrated from a great distance given the fact that its lodranite source rock would likely have been a few hundred degrees hotter than the LEW 86220 acapulcoite host rock (McCoy et al., 1997). Notably, the silicate-rich melt having the highest temperature, FRO 93001, was 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).

Another meteorite that preserves the stage of low degree partial melting and melt migration is the lodranite GRA 95209. While the GRA 95209 meteorite is classified as a lodranite based on petrography, it has a bulk composition more consistent with the characteristics of acapulcoites (high metal content of silicates, high Zn content of chromite, etc.), and so has been designated a transitional member of the clan along with two other similar meteorites EET 84302 and ALHA81187 (Chikami, 2001). By utilizing X-ray computed tomography techniques, W. Carlson (University of Texas at Austin) constructed a 3-D map of a 554 g mass of GRA 95209 presented as a rotational movie). It shows that the mass comprises three separate lithologies; 1) a predominant metal–silicate matrix, 2) metal-poor zones, and 3) metallic veins. A particularly large metallic sheet, the supposed product of a low-degree partial melt, is considered to have intruded the matrix region from an outlying melt source, perhaps migrating from as far away as hundreds of meters. Poorly graphitized carbon rosettes, measuring up to ~3 mm in diameter, have been identified within the metallic veins and sheet, as well as phosphates such as chladniite and various mineral phases constituting Fe–Mg–Mn-phosphates (McCoy et al., 2006).

This same study revealed that an extreme heterogeneity exists in the 13C composition of poorly graphitized carbon in the metal-poor lithology, even within a single metal grain. The investigators speculated that this carbon may have been formed in one of two ways: 1) by parent body processes involving an initial formation of carbonates involving aqueous alteration processes, their subsequent thermal decomposition, the production of elemental carbon via the Fischer–Tropsch-type process (i.e., outgassing CO dissociates at the surface of a metal grain to form a range of carbon-bearing products), and finally, the absorption of poorly graphitized carbon into molten metal grains; or 2) formation of carbon by the Fischer–Tropsch-type process in the nebula, isotopic fractionation and enrichment of 13C (or alternatively, utilizing 13C-rich precursor material), culminating in accretion to the ACA–LOD parent body, before or after its incorporation into a metallic melt phase. In contrast to the isotopically heterogeneous carbon within the metal-poor lithology, the 13C isotopic values of poorly graphitized carbon in the metallic sheet indicate a high degree of thermal equilibration occurred, at a minimum temperature of ~1150°C. This poorly graphitized carbon is consistent with an origin on the parent body.

In his study, Rubin (2007) found that carbon from primary graphite was probably responsible for an inconstant degree of reduction among the acapulcoites and lodranites, which occurred at high temperatures. Among the indications that reduction occurred are increased metal/troilite ratios, decreased Fa/Fs ratios, decreased FeO/MnO ratios, reversed zoning of some silicate grains (Monument Draw exhibits no reversed zoning), and increased orthopyroxene/olivine ratios.

Basaltic partial melt material complimentary to the residual lodranite rock has not been identified so far, probably owing to the fact that it was removed from the ACA–LOD parent body during an early period of its history. This scenario would be consistent with either explosive volcanism, given a small (~200 km diameter), volatile-rich asteroid, or impact-induced erosion of basaltic surface deposits.

Irving et al. (2005) have described this meteorite as a metamorphosed chondrite probably representing the regolith of the acapulcoite–lodranite parent body. Furthermore, they argued that the occurrence of distinct chondrules precludes the use of the term achondrite to describe this meteorite group, and they suggest that the term metachondrite or 'A chondrite' would be a more appropriate term to describe this texturally evolved meteorite. For more information on formation scenarios for the acapulcoite-lodranite parent body, please visit the Lodran page.