The ALH84001 meteorite was found in the Allan Hills area of Antarctica in 1984. It was identified as an SNC (martian) meteorite in 1994. The yellowish grains visible are carbonate.
The SNC meteorites (named after three representative members of the group: Shergotty, Nakhla, Chassigny) are a group of 33 achondrite meteorites. They have a diverse range of compositions but are known to be grouped and derived from the same parent body (which is almost certainly Mars, see below). This grouping has been recognised because of their oxidised mineral assemblages. Most meteorites contain Fe-Ni metal but in the SNCs, Fe and Ni have combined with oxygen and other elements so no metal is present. In addition, the SNC meteorites are grouped by their oxygen isotope compositions.
The SNC meteorites fall into 4 main groups (see Table below): shergottites, nakhlites, chassignites, orthopyroxenite. The shergottites are subdivided into basaltic and lherzolitic subgroups and the basaltic shergottites have an emerging subgroup (olivine-phyric).
Most of the SNC meteorites are finds (in hot deserts, Antarctica)
except Shergotty, Zagami, Nakhla and Chassigny, which are falls
(ie recovered at the exact time of, or shortly after the meteorites
hit the Earth). Shergotty fell in 1865 in Bihar State, India;
Nakhla near Alexandria, Egypt in 1911; Chassigny in Haute-Marne,
France in 1815; Zagami in Katsina province Nigeria in 1962.
Lafayette and Governador Valadares have less well understood
provenances and places of fall. They were found in a university
collection in Indiana, USA (Lafayette) and the city of Governador
|Shergotty 4kg||ALH77005 0.48kg||Nakhla 10kg||Chassigny 4kg||ALH84001 1.9kg|
|Zagami 18kg||LEW88516 0.013kg||Lafayette 0.8kg||NWA2737 0.61kg|
|QUE94201 12g||Y793605 0.02kg||Governador Valadares 0.16kg|
|NWA480 0.03kg||GRV9927 0.01kg||NWA817 0.1kg|
|NWA3171 0.51kg||Y1075 0.06kg||Y000593 13.7kg|
|Dhofar 378 0.02kg||NWA1950 0.8kg||NWA 998 456g|
|Los Angeles 7kg||MIL 03346 0.72kg|
This table contains the list of currently characterised SNC meteorites. However, more SNC's are in the hands of dealers and will gradually become better known to the scientific community. DAG Dar Al Gani (Libya), Dhofar (Oman), GRV Grove Hills Antarctica, SAU005 Sayh Al Uhaymir (Oman), NWA1068 etc. North West Africa e.g. Morocco, Algeria, Y Yamato (Antarctica), EETA Elephant Moraine (Antarctica), LEW Lewis Cliffs (Antarctica), ALH Allan Hills (Antarctica), QUE Queen Alexandria Range (Antarctica). Note that DAG476 is paired with DAG489, 735, 670, 876 (and the total weight of these finds is 12.5 kg); SAU005 with 008, 051, 094, 060, 090 (total weight 10.5kg); Y000593 paired with Y000749; NWA1068 (total weight 15 kg) with NWA1110 (total weight 0.77kg), NWA2373 (18g) and NWA1775; Los Angeles 001 and 002 are paired.
The SNC class of meteorites is distinct from primitive achondrites for a number of reasons. They have a range of crystallisation ages based on radiometric dating from 4.5 Ga (billion years) to 160 Ma (million years). In contrast chondrites and primitive achondrites all date from the earliest stages of the Solar System (4.5 - 4.6 Ga). The long range of ages over which the SNCs crystallised suggests that they formed on a large, geologically active planet. That is because smaller asteroidal bodies (e.g. hundreds of km in diameter) cooled within about the first ten million years of their formation. The mineral assemblages and chemical compositions of the SNC meteorites are also more fractionated than the primitive achondrites. The achondrites are mainly composed of Mg-rich olivine and pyroxene with Fe-Ni metal and feldspar. In contrast the SNC meteorites have a more varied and fractionated mineralogy, including Fe-rich pyroxenes and olivines. This requires that the SNC parent rocks crystallised from large bodies of silicate belt in which successive periods of crystal growth acted to change (fractionate) the composition of the remaining melt.
The age range and mineral compositions of the SNC meteorites both suggest that they formed on a planetary body larger than known asteroids. The possible planetary bodies can be considered. Venus (has a very thick atmosphere, which means that little material has been ejected from its surface into space. Some of the Galilean (Jupiter) satellites show volcanic activity but very little material could escape from the orbit of Jupiter. That leaves Mars as the most obvious parent body. However, in order to demonstrate their martian origin a final bit of geochemical evidence has been needed. The composition of gases trapped within the shock-melted glass of some of the SNCs (shergottites) is identical within experimental error to that of the martian atmosphere determined by the Viking landers. Since that relationship was demonstrated in the 1980s the martian origin of the SNCs has become generally accepted.
Shergotty (basaltic shergottite). The basaltic texture with grains of pyroxene and feldspar are evident. Shergotty fell in India in 1865.
Shergottites are the most abundant type of SNC meteorites. The basaltic shergottites are similar to terrestrial basalts consisting mostly of pyroxenes (augite and pigeonite) and relict plagioclase. The high degree of shock that the shergottites underwent at ejection from Mars changed the structure of the plagioclase into glass (known as maskelynite). Six of the shergottites are lherzolitic, containing olivine, orthopyroxene and low proportions of plagioclase. The minerals in the lherzolites have higher Mg/Fe ratios than those of most of the basaltic shergottites. However, clasts - or xenocrysts- within some of the shergottites, notably SAU005 and DAG476 (containing olivine, orthopyroxene and chromite) are similar to the lherzolitic shergottites.
SAU005. Thin section of chromite-bearing, basaltic shergottite. Most of the augite grains are cumulus, some of the olivines have been described as xenocrysts (ie grains carried into the melt from pre-existing mineral assemblages). Chromites are usually less than 50 microns and are located either within the olivine or in the mesostasis. ol olivine, m maskelynite, a augite. Plane Polarised Light, scale bar 1 mm. Inset shows a typical chromite grain arrowed.
Most of the shergottites formed as cumulates that settled out of basaltic magma. The cumulate origin is shown by preferred orientations of pyroxene grains. Some of the basaltic shergottites (Los Angeles and QUE94201) do not show cumulate textures. The minerals in these meteorites also have more fractionated compositions (e.g. lower Mg/Fe ratios), chromite is absent and olivine is only present in small amounts. The bulk chemical compositions of Los Angeles and QUE94201 are closer to their parental melts than the other basaltic shergottites are. The latters' (e.g. DAG476, SAU005) bulk compositions represent a mixture of trapped melt, cumulus grains and clasts from more primitive shergottites. EETA79001 is composed of two distinct lithologies A and B. EETA79001A contains a mixture of basalt and olivine-orthopyroxene-chromite xenocrysts whereas lithology B is more similar to other basaltic shergottites, like Shergotty itself. Glass inclusions within EETA79001 trapped some martian atmosphere. Zagami also has a mixture of basaltic lithologies with varying grain sizes and inclusions of shock melt that contain trapped martian atmosphere.
Augite and pigeonite (pyroxene) compositions in the shergottites. Fs Ferrosilite (FeSiO3), Wo Wollastonite (CaSiO3), En Enstatite (MgSiO3). The dashed areas enclose the pyroxenes of chromite-bearing SAU005 and DAG476. These are more Mg-rich and reflect the cumulus and xenocryst-bearing mineral assemblages of these basaltic shergottites. Natural History Museum, EPMA Cameca SX50, 20kV.
The basaltic composition of most shergottites is similar to the composition of the martian surface determined by space probes' spectroscopic measurements. Therefore these meteorites are the most representative samples we have of the martian crust. Their compositional features such as low Al contents and high Fe contents reflect the differences in chemical compositions of Mars and Earth.
The Nakhla meteorite fell in Egypt in 1911. The green colour is augite clinopyroxene. The black fusion crust on the exterior can also be seen.
Seven of the SNC meteorites are nakhlites (Nakhla, Governador Valadares, Lafayette ,NWA998, NWA817, MIL03346). These are clinopyroxenites containing augite, Fe-rich olivine, glassy mesostasis and Ti-rich magnetite. They are also thought to be cumulates. The nakhlites have preserved some of the clearest traces of aqueous alteration within the parent rocks on Mars (see electron microscope images below). Veins within the nakhlites contain clay, Fe-rich carbonate and evaporite salts. These veins are truncated by the meteorites' fusion crusts and so are clearly extraterrestrial ie they formed on Mars. Research on these secondary minerals points towards an origin through the evaporation of brines. This is particularly interesting because the MER Rovers in 2004/5 have shown evaporite minerals and nodules on the surface of Mars.Mars Missions.
Electron Microscope (Backscattered electron) image of clay and carbonate (siderite) vein in Lafayette section. ol olivine. See Bridges et al. EPSL, (2000) and Bridges et al. Chapter in Evolution of Mars, (2001)
Electron Microscope (Backscattered electron) image of salt minerals in Nakhla section. h halite NaCl, an anhydite CaSO4 p augite clinopyroxene, b baddelyite, il ilmenite, sd siderite, pl plagioclase. Scale bars 50 microns. See Bridges and Grady, MAPS, 1999.
Chassigny fell in France in 1815. It is composed of olivine and lesser amounts of chromite.
Chassigny is a dunite cumulate, being composed of over 90% olivine, with the remainder being pyroxene, chromite and plagioclase. The mineral compositions are equilibrated with little detectable variation in the olivine composition of Forsterite 68-70 (100 Mg/Mg+Fe atomic ratio = 68-70). A second chassignite (NWA2737) has recently been identified.
This meteorite (see figure at top of page) is a pyroxenite consisting of orthopyroxene with some chromite and maskelynite. The very limited variation in pyroxene and chromite compositions suggests that the parent igneous rock cooled slowly and at depth, homogenising any original compositional variability. Most attention on this meteorite has focused on the presence of about 1 vol. % of rounded carbonate grains or 'rosettes' of 0.1 - 0.2 mm diameter. Like the secondary assemblages in the nakhlites these may have formed through the rapid evaporation (e.g. within days) of brines at relatively low temperature ie less than 150 degrees centrigrade. One of the well-documented features of this carbonate is the variation in composition from cores to rims. Mg contents of the carbonate increase and Ca contents decrease towards the rosette rims. This is consistent with the expected compositional fractionation within a brine where later formed carbonate is progressively more Mg-rich. However, in this meteorite's case a complex series of shock alteration and fracturing appears to have altered the carbonate and, for instance, may have led to the formation of magnetite-rich rims.
In 1996 a group of NASA researchers published the results of a study into the carbonate rosettes in ALH84001. They controversially suggested that the carbonates had formed through bacterial action on Mars. Their evidence for this idea included the segmented rod-like morphologies seen at high magnifications on the carbonate and the morphology of some magnetite grains in the carbonate rims which they compared to magnetite grains formed as byproducts of bacterial action. Opponents of the biological theory have pointed out that none of the images show incontrovertible evidence in the form of cell walls or dividing bacteria for being fossilised remains. It has also been suggested that the rod-like structures could be artifacts of sample preparation e.g. gold coating prior to the electron microscope examinations that the NASA researchers made. McKay et al. also used chemical evidence as part of their theory. ALH84001 has been shown to contain polycyclic hydrocarbons (that is compounds such which are composed of multiple C6H6 rings) within its interior which probably originated on Mars. These compounds can, however, form without the influence of any life processes and so they are not proof of martian life. The argument for a biological origin has now largely settled down to a discussion of the origin of the magnetite grains in the carbonate rosettes' rims. After careful examination of the morphology of 5% of the grains which are whisker-shaped, some researchers have suggested that they formed at high pressure and temperature from a vapour during one of the shock events that seem to have affected the carbonates. In a contrary viewpoint, proponents of a biological origin have shown that many of the other magnetite grains contain structural defects, elongated shapes and pure Fe-oxide compositions that are consistent with a bacterial origin. However, these characteristics are also consistent with decomposition of the surrounding carbonate during shock. It seems that the carbonate-magnetite rosettes in ALH84001 may be explained through low temperature, inorganic precipitation from CO2-charged aqueous fluids, with subsequent shock-related alteration. However, the discussions about whether some of the magnetite grains might be bacterial in origin continue and so there remain advocates for a biological explanation of the ALH84001 carbonates. Whether true or not the idea has certainly stimulated much new work and thinking about Mars.
The ages at which the SNC meteorites crystallised on Mars have been determined in a variety of ways including Rb-Sr, Sm-Nd and 39Ar-40Ar. You can find out about radiometric dating by following the links at the bottom of this page. The crystallisation ages fall within 5 groups. ALH84001 is the oldest at 4.5 Ga (billion years), Chassigny is 1.35 Ga, the nakhlites 1.3 Ga, lherzolitic shergottites 180 Ma (million years) and basaltic shergottites 165-475 Ma. You can find the source of this data from the bibliography. From these ages it can be seen that ALH84001 is a fragment of Noachian crust, probably from the southern highlands. The other meteorites date from the Amazonian System. The young ages of the shergottites suggest that they were derived by impacts onto the northern hemisphere. Amazonian igneous rocks are widespread in the Tharsis region, in Elysium and Amazonis Planitia. You can follow a link from here to a map of Mars.
The Cosmic Ray Exposure (CRE) age of an SNC meteorite is the length of time from ejection off Mars until its fall onto the Earth's surface. Stable noble gas isotopes 3He, 21He and 38Ar are produced within the meteoroid by exposure to cosmic rays when it is in space. The production rate of these noble gas isotopes has been determined experimentally and so a CRE age can be calculated. The CRE ages can be used to determine when the parent rocks were ejected from Mars, although there is also a relatively short terrestrial age - which is the length of time that a meteorite has been on the Earth's surface - that can be added to the CRE age to give a more precise ejection age. See the links at the bottom of this page for more information about these topics and the source of the data quoted here. The Mars ejection ages of the SNC meteorites fall within 7 groups at 20, 15, 11, 4.5, 3, 1.3 and 0.7 Ma. The nakhlites cluster at 11 Ma. This, together with their similar crystallisation ages and mineralogy, suggests that they were derived from the same set of rocks on Mars and ejected by the same impact. As Chassigny has similar ejection and crystallisation ages it may also have been ejected in an 11 Ma impact event and some researchers have suggested that its parent rocks were related to those of the nakhlites. There is also a clustering of ejection ages around 3 Ma for 4 of the basaltic shergottites (Los Angeles, QUE94201, Shergotty and Zagami). ALH84001 has an ejection age of 15 May.