Shergottite, basaltic
Fell October 3, 1962
11° 44' N., 7° 5' E.
An eyewitness account of this meteorite fall was provided to Robert "Meteoriteman" Haag at the actual site. Haag related in his Meteorite Field Guide (1991) that a man who was chasing cows out of his cornfield near the village of Zagami Rock, Nigeria, suddenly heard a loud explosion and was buffetted by a pressure wave. Seconds later there was a puff of smoke and a thud about ten feet away. Fearing that an artillery shell had landed, the man waited for a few minutes before approaching the two-foot hole. The black rock was recovered and placed in a museum in Kaduna.
A calcium-rich, basaltic rock, having a pre-atmospheric size of ≤0.5 m and a weight of at least 150 kg (Eugster et al., 2002), had just completed a 3-million-year journey from Mars. Following an atmospheric ablation of more than 90%, this rare martian rock was reduced to a single 18 kg mass. Evidence supporting a martian origin includes the following:
a young crystallization age of only 180 m.y. (however, this age may instead represent a shock event accompanied by maskelynite production [Jagoutz and Dreibus, 2002; Eugster et al., 1997], or perhaps metasomatic homogenation affecting the radiometric chonometers [Albarède et al., 2008], and the actual crystallization age may be 4.0 b.y.)
a mineral composition consisting of 0.043 wt% water (kaersutite), which contains high D/H ratios consistent with the martian atmosphere.
the presence of trapped martian atmospheric gases, with isotopic percentages matching those measured by the Viking and Pathfinder missions.
features of a weak gravity field acting on the crystallizing minerals.
residual magnetic properties.
unique Mn/Fe ratios in pyroxene.
a unique O-isotopic signature common to all SNC meteorites, but distinct from other meteorite classes.
a positive match between known basaltic shergottites (EETA79001B and QUE 94201) and a rock from the martian plains at Meridiani Planum, named ‘Bounce Rock’, from analyses of a suite of sophisticated instruments employed by the Mars Exploration Rover.
Along with Zagami, a few dozen other meteorites composing four major groups—the shergottite, nakhlite, chassignite, and orthopyroxenite groups—also fit the criteria for a martian origin.
The martian shergottite group was previously divided into two distinct subgroups, the basaltic and "lherzolitic" subgroups. In actuality, the "lherzolitic" shergottites do not contain the minimum abundances of olivine or orthopyroxene as those established for terrestrial lherzolites. Since there was no known petrologic relationship existing between the basaltic and "lherzolitic" shergottite subgroups, and these groups are resolved from each other on an O-isotope plot, the use of the term lherzolites was proposed by Eugster and Polnau (1997) to represent this unique group of martian meteorites. Thereafter, in an effort to resolve the discrepencies that exist between the official IUGS definition of lherzolites and the application of that term to the varied group of "lherzolitic" shergottites, Mikouchi (2009) addressed the need for changing the name of the "lherzolitic" shergottites to one that is more consistent and more broadly applicable. Since a texturally-based nomenclature was already employed for some shergottite subgroups, e.g., olivine-phyric, it was suggested that the term "pyroxene-oikocrystic" shergottites would be an appropriate name with which to encompass all of the various martian "lherzolitic" shergottites that exist in the worldwide collections. This would include both intermediate and enriched "lherzolitic" shergottites, as reflected by a geochemical classification scheme, as well as any depleted members that may be recovered in the future.
The division of the remaining shergottites have recently undergone a revision:
An olivine-poor basalt subgroup comprising those meteorites with a volcanic origin derived from a fractionated magma and consisting primarily of the clinopyroxenes pigeonite and augite, in addition to having a high abundance of feldspathic glass (actually, most basaltic shergottites are more accurately termed komatiites based on their low plagioclase content and depleted trace element composition).
A subgroup with olivine-porphyritic textures. The name picritic shergottite was suggested for this new subgroup by Barrat et al., 2002, while the name olivine-phyric shergottite was suggested by Goodrich, 2002. Goodrich suggests that the term picritic shergottite implies certain petrogenetic characteristics, such as mixing of two compositionally distinct magma sources, which is not necessarily the case for all members of this new subgroup; therefore the purely descriptive term olivine-phyric is favored.
Recently, several shergottites with both olivine and orthopyroxene megacrysts have been identified, which has led to the proposal of a new shergottite subgroup—the olivine–orthopyroxene-phyric shergottites.
The shergottite group as a whole contains a large proportion of plagioclase feldspar, which was shocked to pressures of ~30 GPa, creating maskelynite intergrowths. The presence of other high-pressure silica polymorphs, including cristobalite, high pressure glass, stishovite, and post-stishovite, the latter only identified by cathodoluminescence techniques, suggests that localized regions experienced higher pressures >40 GPa. The tiny, darkened melt veins in Zagami were formed as a result of shock-induced shear deformation, perhaps at low shock pressures (Bogert et al., 2003).
Similar to other highly shocked martian meteorites, Zagami contains a significant concentration of martian atmospheric Ar within melt pockets (ave. 19.7 ppb), with a minor component present within shock veins (ave. 1.2 ppb). The favored scenario explaining the existence of this trapped gas component within the melt pockets argues for the initial introduction of martian atmospheric gas into pre-existing cracks and pores. Following the passage of a shock wave, sudden decompression and pressure release occurred creating bubbles within sub-mm-sized, localized melt pockets. Finally, as pressures became equilibrated, the trapped atmospheric gases migrated into the vesicles of the melt phase from the surrounding cracks and pores (Walton et al., 2007).
The Zagami meteorite includes a range of lithologies formed through progressive fractional crystallization of the magma, which reflects increasing enrichment of FeO and incompatible elements (McCoy et al., 1999). Most prevalent (~80%) is the early-crystallizing normal lithology (NZ), composed of both fine-grained (FG) and coarse-grained (CG) components, which are crossed by shock-melt veins. The parent melt of the fine-grained lithology may have inherited a larger abundance of pre-existing pyroxene nuclei than did the coarse-grained lithology parent melt (Nyquist et al, 2006). Next in order to crystallize, constituting most of the remaining evolved, incompatible element-enriched rock, was an FeO-enriched component, the dark-mottled lithology (DML). The DML has been found to contain a small component (~10%) of late-stage shock melt pockets that are enriched in fayalite, phosphates, sulfides, oxides (e.g., ZrO2), and mesostases, and which contain a martian atmosphere component. The phosphates in these late-stage melt pockets, consisting primarily of whitlockite, contain ~0.5 wt% martian water, reflecting either a very dry magma source or efficient outgassing. One very large, highly evolved melt pocket was previously named after its finder, David New (DN).
The clinopyroxene crystals in Zagami are composed of pigeonite and augite with Mg-rich cores, which suggests that initial crystallization began inside a slowly cooling, fractionating magma chamber (0.1–0.5°C/hr) at a depth of ~7–15 km. This was followed by a nucleation hiatus in which pyroxene grain size increased at the expense of the smallest grains. The resumption of crystal nucleation and growth occurred during magma ascent and subsequent eruption onto the surface. Preliminary findings indicate that both olivine and pyroxene grains have a preferred orientation (Stephen et al., 2010), It was shown by Becker et al. (2011) that the weaker foliation associated with the coarse-grained pyroxene of the NZ lithology, in contrast to the fine-grained lithology, was not formed by a strong directional flow; perhaps more consistent with a shallow intrusive.
In a study of the light lithophile element concentrations in Zagami pyroxenes (Herd et al., 2005), particularly that of the incompatible element Li, it was found that Li exhibited significant zoning, decreasing from core to rim by 51% (and by 75% in Shergotty). This zoning has been attributed to possible postcrystallization partitioning of Li into magmatic water, which was subsequently degassed; however, shock-induced diffusion from the rims remains another possible alternative. In a related study of the Li systematics in NWA 480, Beck et al. (2004) found that Li in the pyroxenes reveals a large isotopic variation from core to rim, but that it also maintains a constant concentration. They suggest that these compositional trends for Li reflect the loss of Li (i.e., mass-fractionation) from the crystallizing melt through the degassing of water-rich fluids, a process similar to that proposed to have occurred in Zagami and Shergotty.
The oxidation state of Zagami suggests that its parent magma assimilated an LREE-enriched crustal component before it cooled. These conditions are consistent with an origin in a typical Tharsis-type, volcanic magma chamber, such as Olympus Mons (see photos below). Some studies suggest that a later period of rapid cooling in a lava flow 10 m thick produced the Fe-rich rims on the pyroxene cores and created the partial flow alignment of the crystals (exhibited in the NZ lithology only).
Using U–Pb, Rb–Sr, Sm–Nd, and Lu–Hf isotopic systematics, it was determined that the formation of Zagami occurred from one of the most highly fractionated and evolved magma sources very early in Solar System evolution, ~4.558 b.y. ago. It was further ascertained through concordant isotopic chronometers that crystallization of Zagami from the molten state took place as recent as ~166 m.y. ago (DML and CG), similar to the crystallization ages obtained for Shergotty and Los Angeles. The FG lithology gave an age of ~177 m.y. ago, while a cosmogenic correction resulted in an age of 223 (±6) m.y. A study of Ar systematics by Korochantseva et al. (2009) resulted in a whole rock age of 200–250 m.y. for Zagami and ~400 m.y. for Shergotty. The isotopic and lithologic heterogeneity within Zagami is thought to be a result of magma mixing or brecciation (Nyquist et al., 2010).
On the other hand, some believe that these chronometers do not reflect a recent crystallization on Mars, but instead, consider that the young ages represent a major impact resetting event associated with the observed shock effects ranging from mosaicism to maskelynization of plagioclase feldspar; features consistent with a shock stage of S4–5. It was found by Bouvier et al. (2007) that Pb–Pb and Ar–Ar isochrons of shergottites represent an old crystallization age of ~4–4.5 b.y. However, derivation of an ancient crystallization age for Zagami and other shergottites, based on the hypothesis that the young Ar–Ar age reflects isotopic resetting during a secondary event, has been shown to be inconsistent with the degree of shock-heating they have experienced. It is more likely that the isotopic anomalies are the result of impact ejection (~30 GPa for Zagami with a temperature increase of ~70°C). In other words, Zagami does not exhibit petrological features of post shock heating to the degree which would cause a loss of an amount of 40Ar commensurate with an age of ~4 b.y. (Bogard and Park, 2007). Moreover, high degrees of shock would have led to chemical and isotopic equilibration of olivine and pyroxene, and the observed igneous zoning would have been obliterated (J. Jones, 2007); in fact, very little shock melt is present.
It was proposed by Bouvier et al. (2005) that the Rb–Sr ages of shergottites were reset through groundwater dissolution of phosphates within the martian rock. However, the Rb–Sr age was obtained after removal of the phosphates, and it was demonstrated by Nyquist et al. (2009) for the shergottite NWA 1460 that the Rb–Sr isochron as well as other isochrons result from trace element partitioning during igneous crystallization, and that they accurately date this stage. In a similar way, Albarède et al. (2008) proposed that the resetting of radiogenic chronometers could have occurred as a result of extensive percolation of hot sulfate-rich fluids, and that the 4.0 b.y. crystallization age is the accurate age. However, no signs of such alteration are observed, and the distribution of radiogenic Ar present among different minerals is inconsistent with known diffusion mechanisms. Furthermore, the Sm–Nd data were shown to be unaffected by any such groundwater metasomatism processes, while a low-temperature alteration disturbance of the Pb–Pb chronometer was shown to be inconsistent with shergottite features.
In their Ar–Ar analyses of Zagami minerals representing distinct magma sequences, Bogard and Park (2008) concluded that the excess 40Ar was neither the result of shock-implantation of martian atmospheric gas nor the remnant of in situ decay of 40K from a rock that crystallized 4 b.y. ago. The former scenario is inconsistent with the correlation that is observed by Korochantseva et al. (2009) between the atmospheric Ar component, which was trapped during a secondary event, and the radiogenic Ar component, which chronicles the time of crystallization. In the latter scenario, a higher 40Ar concentration in plagioclase than in pyroxene would be expected, which is not the case. Bogard and Park (2008) concluded that the excess 40Ar present throughout Zagami was likely inherited from the source magma or by assimilation of K-rich crustal material during the early magma phase. Their finding that the 40Ar concentrations are the same among the various minerals in Zagami is thought to reflect the modulation of radiogenic gas through a pressure release mechanism as the magma rose from a depth >7 km toward the surface.
Other investigations of in situ U–Pb were conducted on baddeleyite (zirconium dioxide), a late-stage crystallization mineral that forms readily under high oxygen fugacity conditions (Herd et al., 2007; Misawa and Yamaguchi, 2007). They found that U–Pb systematics remain undisturbed at shock pressures up to 57 GPa and temperatures up to 1300°C, and accurately reflect a very young age for Zagami and other shergottites of ~166 m.y. Further studies utilizing Rb–Sr, Sm–Nd, Ar–Ar, and U–Pb ages all show consistently young ages for Zagami of approximately 166 m.y. (Borg et al., 2005; Park and Bogard, 2007). An Ar–Ar age study by Korochantseva et al (2009) was complicated by excess Ar, but they determined an age for Zagami of 200–250 m.y. In support of a young crystallization age for the shergottites, Walton et al. (2008) propose that the bias for young ages determined for the majority of martian meteorites reflects the selective influence of crystalline, consolidated, unaltered, unbrecciated, and unweathered material in the ejection process. This type of rock is typically located in fresh volcanic terranes at higher elevations (lower atmospheric density), favoring a successful impact ejection.
The age discrepancy of shergottites was addressed by Blinova and Herd (2009) in their study involving the petrogenesis of the depleted shergottites. They proposed a three-stage formation model in which the ancient age of 4.535 b.y. for the depleted shergottites, as shown by Pb–Pb and Rb–Sr data, corresponds to the initial crystallization of the magma ocean. The investigators envision that both a Deep Mantle Source and a more shallow Naklite Source were formed at that time. A partial melting event affecting the Nakhlite Source occurred 1.3 b.y. ago, leaving a slightly depleted Nakhlite Residue component; they designated this event the Nakhlite Event. About 500 m.y. after the Nakhlite Event, an ascending hot plume (and possibly many) brought the depleted Deep Mantle Source into contact with the Nakhlite Residue, producing the depleted shergottite parental source magma (for which Y-980459 is a close match to this parental melt composition). Differential mixing of the Deep Mantle Source and the Nakhlite Residue could have produced the various compositions of depleted shergottites, while the enriched and intermediate shergottites could have formed through lower degrees of partial melting of more volatile-rich material located at greater distances from the plume. Their model also explains the Sm/Nd isotopic signatures observed in shergottites, including the young ~1.3 b.y. age, and it provides an explanation for the various redox conditions observed in the shergottite suite; i.e., enriched shergottites were formed by partial melting of volatile- and incompatible-element-bearing trapped liquid in the parental source magma.
A subsequent shock event, which occurred ~3 m.y. ago, probably represents the ejection of this rock from Mars. Studies of the impact melt glass veins have identified fine-grained material that is enriched in a felsic component and depleted in a mafic component (Rao and McKay, 2002), as well as having an elevated sulfur abundance (Rao et al, 1999) and possibly an elevated Pb abundance (Borg et al, 2005). It is proposed that this impact glass material represents martian soil that was mechanically fractionated through impact gardening and metasomatism processes, and was then incorporated into the melt phase.
The mars ejection age for Zagami pyroxene based on 37Ar, determined by adding together the cosmic ray exposure (CRE) age and the terrestrial age (Eugster et al., 2002), is 2.2–3.0 m.y. The ejection ages of other basaltic shergottites, e.g., Shergotty (3.0 ±0.3 m.y.), Los Angeles (3.0 ±0.3 m.y.), NWA 480 (2.40 ±0.2 m.y.), and QUE 94201 (2.8 ±0.3 m.y.), suggest that they all may have experienced a simultaneous ejection event on Mars. Two other olivine-bearing shergottites, Dar al Gani 476 (and pairings) and Sayh al Uhaymir 005 (and pairings), have very similar ejection ages of 1.24 (±0.12) m.y. and 1.0–1.4 m.y., respectively. Their similar CRE ages, bulk chemical compositions, 26Al concentrations, and textures, may represent a common magma source and ejection event from Mars. The shergottite EETA79001 has an ejection age of 0.73 (±0.15) m.y., which may possibly be associated with the DaG/SaU ejection event. Alternatively, EETA79001 may represent a late breakup in space of the lherzolitic shergottite meteoroid, consistent with its content of lherzolitic-type xenocrysts. All of the lherzolitic meteorites found so far have ejection ages that closely coincide (3.8 ±0.7 to 4.7 ±0.5 m.y.), indicating that they were likely ejected from Mars ~1 m.y. before most of the basaltic shergottites, or possibly that they crystallized later at depth. One olivine-bearing shergottite, Dhofar 019, has the oldest ejection age measured so far with the exception of the orthopyroxenite ALH 84001—both have almost identical CRE ages of 15.7 (±0.7) m.y. and 15.0 (±0.8) m.y., respectively. After taking account of the ejection ages for the nakhlites (9.7 ±1.1 to 11.9 ±2.2 m.y.) and the similar chassignites (11.1 ±1.6 m.y.), a minimum number of five separate martian ejection events are represented in the worldwide collections.
Zagami is very similar to terrestrial basalts in even minor and trace element contents. Through studies involving O, N, and Cr isotopic compositions of martian meteorites, along with bulk planetary Fe/Si ratios, it is proposed that Mars accreted from early Solar System material comprising enstatite and ordinary chondrites in a ratio of 74:26, in accord with a core that contains 6.7 wt% Si (Mohapatra and Murty, 2003).
A study was made on Zagami in which calculations were made of shock veins in order to determine the duration of the shock wave at impact (~10 ms), and with that, the commensurate size of the impact crater that was formed. The results indicate a crater size of 1.5–5.0 km (Walton et al., 2008). Additionally, an excellent candidate for one of the source craters which led to the spallation of the late-forming shergottites is the 10.1-km-diameter rayed crater Zunil, which is the most conspicuous crater located in the young, Amazonian-aged (1.8 billion years ago to the present day), lava-covered Cerberus Planum within Elysium Planitia. Other shergottites may have been ejected from the young terrain at the base of the Ceraunius Tholus volcano in the Tharsis province, or from similar young craters reflecting ejection events consistent with CRE ages of <18 m.y. The specimen shown above is a 21.0 g cut fragment from the NZ lithology containing oriented pigeonite crystals and a segment of a shock-melt vein at the bottom edge.
Photo courtesy of NASA/JPL/Malin Space Science Systems
