PAC Group - Primitive Achondrites
The abbreviation "PAC" stands for "primitive achondrites", a group that comprises vastly different subgroups of meteorites that have only one thing in common - they all resemble their chondritic ancestors to varying degrees in both composition and mineralogy. They probably formed on small chondritic parent bodies and were only partially melted and differentiated through accretion processes or from impact events, and then rapidly cooled. Primitive achondrites then experienced varying degrees of melting, thermal processing, and recrystallization.
The name of this small group of primitive achondrites is derived from its only witnessed fall, the meteorite of Acapulco that fell in Mexico in 1976. Since its composition is nearly chondritic, Acapulco was initially classified as an anomalous chondrite, although it does not contain any relict chondrules. However, when similar meteorites were subsequently found, the acapulcoite group was formed to describe a distinct class of primitive achondrites. Acapulcoites mark the transition between primordial chondritic matter and more differentiated rocks. The acapulcoite group presently comprises 12 members, if one excludes probable pairings.
Acapulcoites are composed primarily of fine-grained olivine, orthopyroxene, minor plagioclase, nickel-iron metal, and the iron sulfide, troilite. They have mineral compositions that are intermediate between those of E and H chondrites, but they exhibit a characteristic oxygen isotopic pattern distinct from all other known chondrite groups. Importantly, some acapulcoites have been shown to contain a few relict chondrules such as the meteorite of Monument Draw, Texas, and one member actually exhibits an abundance of distinct chondrules - our recent find from Tissemoumine, Morocco, officially named NWA 725. This unique acapulcoite is actually a scientific sensation, and it is one of our best finds. The presence of distinct chondrules within NWA 725 attests to the fact that the acapulcoites represent an extremely primitive group, a true transition between chondrites and achondrites. It is thought that they originated on the same parent body as the closely related lodranites, another class of primitive achondrites.
This group of primitive achondrites is named for the type specimen, Lodran, a meteorite that fell in Pakistan in 1868. There are only
12 members in this group, and nearly all of them have been found in the blue-ice fields of Antarctica. It is somewhat of a mystery why
except NWA 2235 no new lodranite specimens have been discovered thus far as part of the wealth of new meteorites coming from the hot deserts of Africa and Asia.
Initially, the lodranites were grouped with the stony-iron meteorites since they contain components of both stony material, consisting of olivine, orthopyroxene, and minor plagioclase, and nickel-iron metal in nearly equal proportions. However, since the discovery of the closely related acapulcoite group, the lodranites have been classified as primitive achondrites. Because both groups share similar mineralogical and oxygen isotopic compositions, it is thought that they are derived from the same parent body, most likely an S-type asteroid that has not yet been identified. Lodranites have coarser-grained olivines and pyroxenes and experienced higher temperatures than acapulcoites; these facts indicate that the lodranites have their origin within the deeper layers of the acapulcoite/lodranite parent body where they were subjected to a more intense and prolonged thermal processing.
The brachinites form another small group of primitive achondrites. They are named for the type specimen, Brachina, a meteorite that was found in Australia in 1974. Originally, the olivine-rich Brachina was thought to be a second chassignite, a unique Martian meteorite that contains primarily olivine. However, further research revealed a distinct trace-element pattern as well as a unique oxygen isotopic composition for Brachina. Today, the brachinite group includes seven members.
Brachinites are composed primarily of small, equigranular olivine grains, but scattered among them we find small amounts of clinopyroxene, orthopyroxene, and minor plagioclase. Metal is rare or completely absent although the brachinites contain up to 20% total iron, mostly in the form of iron-rich olivine. Recent studies of the olivine compositions of different asteroids suggest that 289 Nenetta might be the parent body of the members of this group.
Still, there are those ungrouped primitive achondrites such as Divnoe and Zag (b) that are very close to the brachinite group even though they contain larger amounts of free nickel-iron, troilite, and chromite. It has not yet been determined if these ungrouped primitive members originated on the same parent body as the brachinites and simply represent different degrees of thermal processing. Alternatively, they may have originated on one or more different parent bodies that share a similar history and composition to the brachinites. We will briefly discuss those brachinite-like primitive achondrites in the "ungrouped" section below.
The winonaites represent another class of primitive achondrites, and they are named for a most unusual find. The meteorite of Winona was found in a stone cist in the ruins of the prehistoric Elden pueblo, Arizona, USA, in 1928. The circumstances of the find suggest that the builders of the pueblo kept and venerated the meteorite as a sacred object after they had actually seen it fall. Even modern science has to admit that Winona is indeed something special; it became the namesake for a rare group of primitive achondrites comprising about 10 members, after excluding probable pairings.
Winonaites are composed largely of fine-grained pyroxenes, minor magnesium-rich olivine, the iron-sulfide troilite, and nickel-iron metal. The total iron content ranges between 18% and 30% among members of this group, possibly a factor of the close relationship that exists between the winonaites and silicate inclusions in IAB iron meteorites. These silicate inclusions are very similar to the winonaites in chemistry and mineralogy, and they exhibit the same unique oxygen isotopic composition. Recent research suggests that both the winonaites and IAB irons originated on the same parent body - a partially differentiated asteroid that was disrupted just as it began to form an iron core and a silicate-rich crust. This disrupting impact mixed silicates into molten nickel-iron forming the silicated IAB irons, and mixed olivine-rich residues of partial melts into unmelted silicates, forming the winonaites.
Interestingly, three of the winonaites have such anomalous characteristics that they don't fit easily into the group; these include the only witnessed fall of the group, Pontlyfni, Yamato 74025, and NWA 516. The latter meteorite is a new winonaite from Morocco that might be paired with one of our new finds. When compared to the other winonaites, they form a distinct grouplet that shows textural, mineralogical, and chemical differences indicative of a lower degree of thermal processing. In fact, Pontlyfni was even found to contain relict chondrules. Further research will reveal whether these meteorites represent different degrees of thermal processing on a common parent body, as supposed for the acapulcoites/lodranites, or rather, are samples of a separate parent body that formed in a common region of the primordial solar nebula, experiencing a similar igneous history.
The ureilites are named for Novo Urei, a rural village in the Mordova Republic, Russia, where several meteorites fell in late 1886. It has been reported that one stone was soon recovered by local peasants - but not to preserve it for science. On the contrary, the stone was immediately broken apart and eaten! The report does not reveal the reason for this odd behaviour - maybe they ate it because the freshly fallen meteorite smelled good, or perhaps because it had the typical shape of a loaf of bread, which a ureilite often resembles. However, not all of the stones were eaten, and Novo Urei became the type specimen of one of the best-represented achondrite groups in our collections. The ureilite group comprises about 60 members, again, excluding all probable pairings from the hot deserts of Africa and the ice fields of Antarctica.
The ureilites are subdivided into two groups: the monomict main group and the less common polymict group. Main group ureilites are composed largely of coarse-grained olivine and minor pyroxene, mostly in the form of calcium-poor pigeonite, set in a dark carbonaceous matrix of graphite and diamond, nickel-iron metal, and troilite. Our recent find, El Gouanem, Morocco, is a rather typical member of the main group. Polymict ureilites consist of a mixture of different lithologies. Besides clasts from main group ureilites, they contain magmatic inclusions, dark carbonaceous clasts, chondritic fragments of different origins, and various other inclusions. This suggests a surface or regolith origin for the polymict ureilites, an assumption that is supported by the values for noble gases that have been implanted into the regolith by the solar wind.
However, both the origin and the formation history of the ureilites remain enigmatic. Their mineral and oxygen isotopic compositions suggest that they formed as residues from partial melting, and therefore represent primitive achondrites that probably formed on several parent bodies. On the other hand, rare-element patterns and other chemical characteristics indicate that ureilites are highly fractionated igneous rocks that formed in different regions of the same parent body; probably a moderately differentiated C-type asteroid that was disrupted by an impact event and then rapidly cooled. An impact history would also explain the occurrence of high-pressure minerals such as diamond and londsdaleite that are formed by intense shock metamorphism. Even this theory is not without its problems though. Recently, a new ureilite from the Libyan Sahara named DaG 868 was found to contain diamonds, but paradoxically, appears to be nearly unshocked. Other ureilites, like our new NWA 766, contain exotic minerals like chromium-spinel, chromium-rich garnet, and associated glasses. These unusual specimens present more questions than they answer regarding the ureilite puzzle. Further research is needed to unravel the mystery of the origin of the ureilites and the complex history of their parent body.
Ungrouped Primitive Achondrites
Some primitive achondrites don't easily fit into the existing groups. We already mentioned Divnoe and Zag (b), two ferroan achondrites closely related to the brachinites. They share similar compositions, but show different oxygen isotopic patterns suggesting that Zag (b) might represent a missing link between the brachinites, Divnoe, and the lodranites. The implications of this find are currently poorly understood, and further research will be required to show how these primitive achondrites are related to each other.
Some other interesting primitive achondrites are only represented by a single find, providing inadequate justification for the establishment of a new group. For example, two new primitive enstatite achondrites, Zaklodzie, Poland, and Itqiy, Western Sahara, are unique in mineral composition. Although they both are composed largely of the magnesium-rich pyroxene, enstatite, and nickel-iron metal, the texture of Zaklodzie resembles an acapulcoite while that of Itqiy is more like a lodranite. They probably both represent samples of moderately differentiated, E chondrite parent bodies that experienced similar differentiation processes such as that which occurred on the acapulcoite/lodranite parent body.