ALLAN HILLS 84001


Orthopyroxenite
(with geochemical affinities to shergottites)

standby for alh84001 photo
NASA photo #S85-39570

Found December 27, 1984
76° 43' S., 159° 40' E.

Allan Hills 84001 was found during the 1984 annual field season of the Antarctic Search for Meteorites (ANSMET) by Robbie Score during a search by snowmobile in the Allan Hills region of the Far Western Icefield, Antarctica. The 1,931 g stone was originally classified as a diogenite with a weathering category of A/B (minor to moderate rustiness), and a fracturing category of B (moderate cracks). After a more thorough analysis, it was found that the meteorite was unusual in its Fe/Mn ratio, its Fe+3 content in chromite, the presence of pyrite instead of troilite, and an oxygen isotope analysis unlike that of diogenites, and it is now recognized as having a martian origin. Although ALH 84001 is an orthopyroxenite, and as such was characterized by the Planetary Chemistry Laboratory at Washington University as a subgroup of the nakhlites, its parental source magma has a composition that is consistent with the same mixtures of depleted and enriched REE end-member components that are used in a geochemical classification of the shergottites (Lapen et al., 2012). It was determined that the source magma of ALH 84001 contained a higher proportion of the enriched REE component than all other shergottites studied thus far. Therefore, ALH 84001 might be most appropriately classified as a subgroup of the shergottites.

Based on Sm–Nd and Rb–Sr data, ALH 84001 appears to have the oldest crystallization age of all martian meteorites at 4.470 (+0.035/–0.026) b.y. (Nyquist and Shih, 2013). However, calculations made by others utilizing the U–Pb and Pb–Pb isotopic systems defined a younger combined mineral age of 4.117 (±0.016) b.y., consistent also with the maskelynite Ar–Ar age (4.163 [±0.035] b.y.) and the Lu–Hf age; this is probably also the time of acquisition of the natural remanent magnetization component that is observed in the meteorite. A Lu–Hf age determination by Righter et al. (2009) resulted in a young crystallization age of 4.086 (±0.030) b.y., and an age derived by Lapen et al. (2010) reflects an age of 4.091 (±0.030) b.y. Reportedly, the Lu–Hf system is less subject to alteration by water, shock, and heat than the Sm–Nd system, and is thought to accurately reflect the crystallization of cumulate orthopyroxene derived from an LREE-enriched, late-stage parental source magma. This younger crystallization age may be associated with the Late Heavy Bombardment period of the solar system. Contrariwise, Nyquist and Shih (2013) consider the Sm–Nd radiometric system to be the more robust of the two, and they argue that it was the Lu–Hf age that reflects a resetting event.

Various radiometric dating systems indicate a younger carbonate formation age of 3.9–4.0 b.y., while a Th–Pb isochron at 2.9 b.y. attests to the late addition of Th to the ALH 84001 lithology, likely through metasomatic processes by a phosphate-rich liquid (Jagoutz et al., 2009; Albarède et al., 2009). The 4.12 b.y. crystallization age is consistent with that of a large number of enriched shergottites, including Zagami, Shergotty, RBT 04262, and NWA 1068, thought to be derived from a late-stage incompatible-element-rich residual liquid. Notably, strong similarities exist for the high Hf/Sm and Zr/Sm ratios and the trace element patterns among these enriched shergottites and ALH 84001 (Barrat and Bollinger, 2010). Furthermore, the similar values for ε142Nd and ε182W, and the Lu/Hf and U/Pb ratios, reflect a close affinity between shergottites and ALH 84001 and raise the possibility of a shergottite precursor. Another isochron observed at 4.3 b.y. is the same as another grouping of shergottites, including QUE 94201 and NWA 1195.

ALH 84001 consists of 97% cumulate, coarse-grained, magnesian orthopyroxene. The rock is thought to have crystallized at low pressure (<0.5 GPa) at a depth of several tens of km under slightly reducing conditions (QFM –2.7), and under the influence of carbon buffering (Righter et al., 2008). It is considered likely to be a sample from the 4+ b.y. old southern hemisphere of Mars, rather than the youger, lava-covered, northern hemisphere. Alternatively, a rayed crater 6.9 km in diameter named Gratteri, located near Memnonia Fossae southwest of the Tharsis volcano, could be the source of ALH 84001 (Tornabene et al., 2006). This crater, exhibiting more than 30 rays having lengths up to 595 km, was formed by an oblique impact ~20 m.y. ago. The impact excavated Noachian-aged rock (a period extending from the birth of Mars to ~3.5 b.y. ago) is consistent with the ancient age of ALH 84001. Based on data from the Infrared Mineralogical Mapping Spectrometer aboard the Mars Express orbiter, orthopyroxene-rich terrain of appropriate Noachian age has only been identified in regions of Syrtis Major and in the northwest region of Hellas basin (Ody et al., 2013).

Following the formation of carbonate and maskelynite, a rapid, localized, high-temperature (>1400°C) shock event occurred 1.158 (±0.110) b.y. ago (Cassata et al., 2010). From models of the time period following this localized event, temperatures for the ALH 84001 rock were maintained at either ~80°C for 10 m.y. if residing under an ejecta blanket, or ~330°C for several days if residing near the surface (Cassata et al., 2010). Remagnetization occurred during this shock heating event, causing the heterogeneous pattern of magnetization observed. Following the ejection of the rock from Mars ~12 m.y. ago, temperatures were calculated to be a maximum ~75°C or ~320°C, corresponding to a duration of 10 m.y. and several days, respectively.

A shock metamorphic event is recorded in the U–Th–Pb age of the phosphates, in the Ar–Ar shock age from maskelynite, and in other isochrons, each indicating an age of ~4.0 m.y (Terada et al., 2003). Treiman (1998) determined that ALH 84001 experienced 4–5 separate impact events. During the third impact event, which succeeded carbonate formation, the rock experienced its greatest heating, increasing to a temperature of at least 529°C, and perhaps as high as 800–900°C; the latter high temperature is consistent with the feldspathic melts present in the rock (Domeneghetti et al., 2007). The maskelynite has a refractive index that corresponds to a shock pressure of 32 (±1) GPa (Fritz et al., 2005). It may be presumed that during this significant impact event, the rock was launched into a suborbital trajectory and then covered by an ejecta blanket after landing. It is likely that this is the period when the Fe-rich carbonate globules were decomposed to fine-grained, whisker-shaped magnetite.

The rock was then rapidly cooled over a short duration of time, probably measured in minutes for a rock situated very close to the surface, preserving the carbonate and the observed chemical zoning. Thereafter, a cooling rate of ~35°C/year was established, as indicated by the orthopyroxene geospeedometer (Domeneghetti et al., 2007). At a depth of ~6 m, cooling proceeded over the next couple of hundred years. During one or two subsequent impact events, a post-shock temperature increase of 100–110°C (to at most 350–500°C) was attained, after which the ALH 84001 lithological unit did not experience high metamorphic temperatures again, not even during its subsequent ejection from the planet.

Recently, thermal emission spectrometry performed by the Mars Global Surveyor has located a region in Eos Chasma that contains orthopyroxene compositionally similar to that in ALH 84001, hinting at a possible point of origin. Within the orthopyroxene, mm- to cm-wide crushed and annealed fracture zones are present, indicating that this meteorite was subjected to an intense, localized shock event of short duration after it was cooled in an igneous plutonic environment. No evidence of subsequent metamorphism was obseved. The fracture zones contain µm- to sub-µm-sized angular, rounded, and euhedral chromite grains; some appear to be stringers of shock-dispersed larger grains, while others likely exsolved from a Cr-rich melt phase. In addition, orange-colored, rosette-, slab-, or disk-shaped carbonate inclusions are present, some of which have rims consisting of two Fe-rich black zones sandwiching a white Mg-rich zone.

These chromites and carbonates are thought to have precipitated ~4 b.y. ago from a heated, shock mobilized solution, consistent with the observed chemical heterogeneities and microstructures (Barber and Scott, 2006). However, other investigators believe the larger carbonates precipitated from a low-temperature, saturated solution. In support of a low temperature environment for carbonate formation, Theis et al. (2008) constrained the temperature of carbonate precipitation to 83°C (±67°C), based on the degree of Fe isotope fractionation affecting the zoned carbonates relative to the Fe isotopic composition of martian silicates with which they were initially in equilibrium. Nucleation or redistribution of these phases occurred within these impact-generated fracture zones, with the slab-shaped carbonates relegated to formation within the larger fractures. Various minerals were subsequently deposited upon these carbonates, which are designated "magnesite–siderite–magnesite" layers and "post-slab magnesites", while chromium oxide (eskolaite) was deposited on rare silica glass. The final phase in the post-shock assimilation sequence was the mobilization of molten feldspathic glass (Corrigan, 2004) and phosphate.

Usui et al. (2016) conducted a H,C,O-isotopic study of the carbonates in ALH 84001. They obtained a more precise range for the δD value of ~500–1,000‰; this value represents the H-isotopic composition of the water reservoir during carbonate formation in the Noachian period. In addition, they ascertained that the H-isotopic composition is positively correlated with the C- and O-isotopic compositions.

Nuclear track data suggest that ALH 84001 had a pre-atmospheric radius of ~10 cm, and suffered atmospheric ablation of over 85%. Among martian meteorites, ALH 84001 has the oldest cosmic-ray exposure age of 14.7 (±0.9) m.y. It has a terrestrial age of ~13,000 years, but spent less than 500 of those years exposed on the surface of the Antarctic icefields, consistent with its low terrestrial weathering effects.

In a paper published in Science, McKay et al. (1996) presented evidence of possible biogenic activity on early Mars that was found in this martian meteorite. The evidence supporting their theory included the following:

They wrote that although alternative explanations exist for each of these factors taken individually, when considered together in such close association, they constitute evidence for primitive life on early Mars.

According to some, an important constraint on this theory is the extremely small size of the proposed organisms, thought to be below the size needed to contain the framework of a living organism. Recently however, nannobacteria in the 50 nm size-range, with shapes resembling those found in ALH 84001, were cultured in the lab. Results of tests for DNA and cell walls were positive. Similar microfossil-like features have also been found in the Nakhla and Shergotty meteorites.

Magnetite particles extracted from carbonate globules in ALH 84001 have been characterized as irregular, prismatic, or whisker-like. The prismatic magnetites (see image below) were found to be indistinguishable from terrestrial magnetites produced by the bacteria strain MV-1 in several properties that are not associated with inorganic magnetites. Rather, these properties, regarding attributes of size, morphology, and chemistry, are uniquely characteristic of biogenic bacteria. Therefore, this particular type of magnetite may be interpreted as having a biogenic origin. In contrast, an inorganic process involving the thermal decomposition of siderite has been proposed as the source of the single-domain magnetites, which M. S. Bell (2017) has experimentally demonstrated under shock conditions of 49 GPa and temperatures >470°C.

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A TEM-based image showing different views of a magnetite (Fe3O4) nanocrystal from a magnetotactic bacterium.

Another important constraint on this theory is the temperature of formation. McKay et al. think it was no higher than 80°C while others place it closer to 700°C. Subsequent research in this area demonstrated that high-temperature impact-produced brecciation on ultra-mafic host rock can mobilize CO2 to form the carbonates observed. A subsequent rapid cooling of the carbonates, consistent with an impact event, would be necessary to preserve the fine-scale zoning features observed. However, the vast majority of investigations into the temperature issue concludes that the carbonates were formed at temperatures below 100°C. Of particular interest is an investigation into the natural remanent magnetism (NRM) of carbonate-containing fractures, which has found that the original magnetic field is present. Therefore, the occurrence of adjacent fragments having differently oriented magnetic fields attests to the fact that the temperature of the fractured rock was never greater than 325°C; at this point an identical magnetic orientation would form upon cooling. It can also be presumed that the carbonates formed in the fractures after the fractures were formed, and therefore, the carbonates formed at temperatures below 325°C.

Other researchers have investigated the distribution of PAHs by studying them in the martian meteorite EET 79001. This meteorite has a far younger formation age of 180 m.y., and as such, formed after the surface of Mars became devoid of water. In addition, analyses of PAHs from a carbonaceous meteorite that was recovered near ALH 84001, and of PAHs from Antarctic ice meltwater, have revealed that all of these sources of PAHs were similar. This study suggests that these sources probably reflect thousands of years of contamination from terrestrial or extraterrestrial PAHs that are present in the Antarctic ice. In support of the contamination theory, results of a time-of-flight secondary ion mass spectrometry (TOF-SIMS) analysis revealed a correlation exists between the PAHs and terrestrial lead (Stephan et al., 2003). This study also demonstrated that there is no spatial association between the PAHs and the carbonate globules as previously claimed. Moreover, the effects of oxidation from Antarctic weathering on the PAHs, resulting in a reduced alkylation, can be readily explained without reference to possible fossil martian life. Studies of other biomarkers of biologic activity including carbon, oxygen, and sulfur isotopes, suggest that they reflect cycling through the martian atmosphere or terrestrial contamination, and are not indicative of production through any biological system.

ALH 84001 Geologic History

Recent observations of the formation history of ALH 84001 point to a complex history of impact deformation, metamorphism, and chemical change, which places further constraints on the presence of biogenic fossils. In the beginning, crystallization of a basaltic magma containing ~2.5–5% interstitial melt was followed by contractive deformation and chemical equilibration. A period of aqueous alteration and dehydration of orthopyroxene, resulting in the production of olivine, may have also occurred. Following next was a series of deformations and intense thermal metamorphism, which produced and annealed, respectively, the granular bands. This suggests ALH 84001 was part of a large crater basement in which multiple compression and rarefaction shock waves, approaching 75 gigapascals (GPa), affected the rock. Afterwards, a shock event, with pressures of at least 30 GPa, created fractures that cut across the granular bands and formed feldspathic glass. The next event to occur was the deposition of zoned carbonates (having the morphology of an inverted conic frustum) in the fractures within the granular bands replacing the feldspathic glass (see following photo). A third impact deformation event of ~60 GPa fragmented the carbonate globules and injected melt veins of feldspathic glass into the globules. Evidence for a forth deformation event is also present. This event has reorientated the fragments surrounding a series of fractures, resulting in different magnetic signatures. This event could also represent the launch of ALH 84001 from Mars ~12 m.y. ago.

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Image of a thin section micrograph showing a carbonate globule (white arrow) within a pyroxene microfracture. The inset shows a close-up of the orange-colored carbonate globule.
Image credit: Ed Scott and Cyrena Goodrich

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Image showing sectioned carbonates exhibiting Oreo-like zoning.
Image credit: Ed Scott and Cyrena Goodrich

"Extraordinary claims require extraordinary evidence." –Carl Sagan

In a publication by Scott and Barber (2002) and one by Brearley (2002), the authors describe their investigations of magnetite grains embedded within carbonates in ALH 84001, which perhaps represents the most compelling evidence for a biosignature. Employing a transmission electron microscope (TEM), they examined the atomic alignment across the carbonate–magnetite boundary and discovered that the two minerals had identical crystal lattice spacings and orientations in three dimensions, indicating that the embedded magnetite crystal must have formed inside the carbonate crystal. It was argued that the magnetite grains nucleated within nm-scale voids and fractures following the thermal decomposition of Fe–Mg–Ca-carbonate (primarily the more thermally unstable siderite component) into magnetite and carbon dioxide; the voids were created by the loss of this carbon dioxide. Fragmentation and decomposition of the carbonates was probably initiated by a pulse of impact-shock heating (>35 GPa), which occurred ~4 b.y. ago. As such, any possible biogenic magnetite crystals that may have existed in the carbonates prior to the impact would have been altered by the high temperatures (~900°C). It was concluded that all of these spatially-associated magnetite morphologies, including the 27% proposed to be of biogenic origin, were likely formed by the thermal decomposition of carbonates.

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TEM image showing identical alignment between atomic lattice planes (arrows) of faceted magnetite and host carbonate.
Image credit: Barber and Scott, 2002

Just when it appeared that the case for early biogenic activity on Mars was lost, new evidence for biogenic markers has surfaced. Through the application of the Magnetite Assay for Biogenicity (MAB), K. L. Thomas-Keprta of NASA's Johnson Space Center, leading an international research team supported by the NASA Astrobiology Institute, reports that 25% of the nanometer-sized magnetites present in carbonates meet the MAB criteria for biogenicity. Additionally, the Mars Global Surveyor has found evidence for a strong ancient magnetic field present on Mars during the time of carbonate formation. These conditions are consistent with the evolution of a magnetotactic bacterial strain similar to the MV-1 type.

Utilizing a scanning electron microscope (SEM), scientists have determined that clusters or colonies of nannobacteria are abundant on Fe–Mg silicates and on a chromite grain studied in ALH 84001, confirming the original statement by McKay et al. in 1996. The present study ruled out the possibility that these objects were artifacts resulting from an excess in gold coating. These nanobodies occur in many shapes (balls, ellipses, ovoids, spheroids, and worm-like), and sizes (20–500 nm, but mainly 30–120 nm). They exhibit no crystal faces, but have rounded shapes consistent with lifeforms rather than mineral crystals. Terrestrial analogs of these nannobacteria having similar dimensions have been identified. It will now be neccesary to determine if these nannobacteria originated on Mars, or are the result of terrrestrial contamination by a previously unknown population of nannobacteria in Antarctica.

Through their continuing studies, Golden et al. (2003) have sought to compare the three-dimensional morphologies of ALH 84001 magnetites to those from the MV-1 bacteria strain and to those produced naturally by the thermal decomposition of organic matter, such as by impact shock metamorphism. In the laboratory they synthesized zoned, Fe-rich carbonate globules similar to those from ALH 84001 through hydrothermal precipitation. Next, they conducted shock experiments to 49 GPa which heated these carbonates to 450+°C and initiated thermal decomposition and the production of magnetite from Mg-rich siderite. The resulting magnetites had similar sizes (~50–100 nm), compositions (100% magnetite to an 80:20 ratio of magnetite and magnesioferrite), and 3-D morphologies to those in ALH 84001, and different from those in MV-1 bacterium, inferring a non-biogenic origin for magnetites in ALH 84001 (Bell 2007).

In contrast to the findings of Golden et al. (2003), which were based on conventional transmission electron microscopy, the team of Thomas-Keprta et al. (2003) utilized electron tomography back-projection techniques to image the nanophase magnetites from both ALH 84001 and MV-1 bacteria. They concluded that the magnetites from both sources were of an equivalent 3-D morphological type—{111}-truncated hexa-octahedral. However, in their independent studies of these magnetites, Golden et al. (2006) found that most do not have the {111}-truncated hexa-octahedral morphology characteristic of biogenic activity, nor are they chemically pure as would be required for a non-decompositional origin postulated by Thomas-Keprta team.

Thereafter, Thomas-Keprta et al. (2008) conducted observational experiments and established thermodynamic models involving the partial thermal decomposition of sideritic carbonates under various heating scenarios. They determined that a vast majority of both chemically pure and impure magnetites present in ALH 84001 carbonates cannot have been produced through thermal decomposition processes, but instead, were introduced into the carbonates from a different source region through some method of transport mechanism. They demonstrated that preferential decomposition of siderite will not produce pure magnetite, and that the magnesite-calcite component of the carbonate would undergo decomposition along with the siderite component, which is not observed (Thomas-Keprta et al., 2009). Furthermore, they determined that graphite would be among the decomposition products under most all oxygen fugacity conditions, but this is also not observed. Perhaps even more telling is the fact that the magnesite layer contained no siderite, and thus could not have decomposed to form magnetite.

A study was conducted by the Carnegie Institution's Geophysical Laboratory on the macromolecular carbon (MMC) associated with the carbonate globules, and a comparison was made to similar material from the Bockjord Volcanic Complex on Svalbard, a territory of Norway. Results were presented at the Astrobiology Science Conference 2006 held in Washington D.C., and it was found that magnetite was always associated with the MMC, and that it likely served as a catalyst in the production of MMC—a non-biological synthesis of organic matter on Mars. A similar abiotic origin for the submicron magnetite grains at appropriate temperatures remains most plausible to Treiman and Essene (2011).

and the investigation continues....

The photo shown above is the original NASA photo #S85-39570 showing the complete mass of ALH 84001 as found. Approximately 217 g of this meteorite has been allocated for research. This meteorite is not available to the private collector.


See the PSRD article by Linda M. V. Martel, "Did Martian Meteorites Come From These Sources?", January 29, 2007.