Citation: Taylor, G. J. (December, 2011) Formation of Carbonate Minerals in Martian Meteorite ALH 84001 from Cool Water Near the Surface of Mars. Planetary Science Research Discoveries. http://www.psrd.hawaii.edu/Dec11/ALH84001_Carb.html (date accessed).PSRD-ALH84001_Carb.pdf
Carbonate minerals in the Allan Hills 84001 meteorite are important because they ought to contain information about the chemistry and temperature of the water they formed in. They are also an important part of testing the idea that the meteorite contains evidence of past life on Mars. Hypotheses for the origin of the carbonates are impressively varied. A key test of the ideas is to determine the temperature at which the carbonates formed. Estimates up to now range from a bit below freezing to 700 oC, too big a range to test anything!
To address the problem Itay Halevy, Woodward Fischer, and John Eiler (Caltech) used an approach that involves "clumped" isotope thermometry, which makes comparisons among different isotopic compositions of extracted CO2. This allowed the investigators to use the isotopic abundances of both carbon and oxygen. The results indicate that the carbonates formed at 18 ± 4 oC from a shallow subsurface (upper few meters to tens of meters) pool of water that was gradually evaporating. The wet episode did not last long, leading Halevy and his colleagues to conclude that the environment may have been too transient for life to have emerged here from scratch. On the other hand, if life already existed on the Martian surface this wet near-surface environment would have provided a happy home. An impact blasted the Martian home of ALH 84001, causing a transient heating event, perhaps disturbing the isotopic record...or perhaps not because the event was so short. In any case, the clumped isotope thermometry approach seems to have given a good measurement of the temperature at which the carbonate minerals formed.
The Martian landscape is decorated with features, such as valley networks, which indicate that surface water flowed in the distant past. The Allan Hills (ALH) 84001 meteorite is an igneous rock that consists of 1–2% carbonate minerals deposited inside the rock. These minerals hold the record of the concentrations of chemicals in the water, the temperature of the water, and possibly the amount of water that flowed through the rock. The meteorite also contains intriguing features that have been interpreted as evidence for past life on Mars (see the 1996 premier article in PSRD: Life on Mars?). Carbonates have also been observed from orbit using visible to near infrared spectroscopy, so the occurrence in ALH 84001 carries more general significance for the aqueous geochemistry of early Mars.
In spite of the overt evidence for water having played an important role in shaping the Martian surface before 3.5–3.8 billion years ago, climate models for Mars suggest that it is difficult to produce high enough atmospheric pressure and warm enough temperatures to have abundant liquid water on the surface. The carbonates in ALH 84001 may hold the key to determining how much water flowed on the surface, especially because they formed between 3.9 and 4.0 billion years ago in an igneous rock (represented by most of ALH 84001) that is 4.1 billion years old.
Carbonate globules in the ALH 84001 meteorite contain key information about the temperature at which they formed and the chemistry of the waters in which they precipitated. This NASA image is about half a millimeter across.
Carbonate concretions in ALH 84001 are chemically zoned, with central regions rich in calcium and iron, and rims rich in magnesium. Carbon and oxygen isotopes correlate with composition: Cores have low 18O/16O and low 13C/12C while the rims have higher values of both isotopic ratios. (Oxygen and carbon isotopic ratios are usually stated as ratios and compared to terrestrial standards, expressed by the notation δ 18O and δ 13C.) Previous research on oxygen and carbon isotopes in the Martian carbonates agree on neither the formation temperature, depth of formation, role of impacts, biological activity, or other factors. See, for example, PSRD articles presenting conflicting viewpoints:
One way to independently determine the temperature of formation is to use clumped isotope thermometry.
Previous estimates of the temperature of formation of carbonates in ALH 84001 required assumptions about the chemistry and isotopic compositions of the water from which the carbonates formed. To get around this problem, Halevy and coworkers focused on measuring the isotopic compositions of "isotopologues" of CO2. Isotopologues have the same chemical composition (CO2 in this case), but differ in how much 12C, 13C, 16O, 17O, and 18O they contain. There are quite a few combinations, but Halevy focused on CO2 with a molecular mass of 47. Mass 47 is particularly useful because it contains both 18O and 13C, which show a temperature-dependent preference for sticking together rather than residing in separate CO2 molecules. The advantage of this approach to deriving a temperature of formation is that it is based on thermodynamic equilibrium (chemical balancing) inside the carbonate minerals. Thus, we do not need to make assumptions about the chemical composition of the aqueous fluid to determine the temperature. Even better, once the temperature is known, measurements of δ 18O and δ 13C give direct information about the isotopic composition of the fluids.
The most common isotopologue of CO2 has a mass of 44. It is composed of the abundant isotopes 16O (two of them in each molecule) and 12C, which accounts for 98% of the abundance of all the isotopologues. After extensive chemical extraction, Halevy measured the abundances of mass 47 and 44 in a mass spectrometer. A particularly informative measurement is the ratio of mass 47 to mass 44, and expressed as R47. This value is compared to the value calculated from a probability calculation of the R value if the isotopes are randomly distributed among the isotopologues. A final parameter, Δ47, is calculated from the R values and reported in parts per thousand. Δ47 is a measure of the preference of the heavy isotopes for sticking to each other rather than to the lighter isotopes. (You have to know math to do this isotopic stuff, but the chemistry involved in the release of CO2 and the instrumentation to measure it are even more daunting!)
Previous work on experimentally produced and natural samples has shown that Δ47 is correlated with temperature of formation. This empirical correlation agrees well with calculations of the temperature dependence of Δ47 based on thermodynamic principles. Thus, once Δ47 is determined, the temperature of formation can be calculated. The actual correlation is between Δ47 and one divided by the temperature (in Kelvin), a common way to determine how a parameter varies with temperature. The graph below shows a good linear correlation with 1/T and the equivalent temperature in Celsius (shown along the top axis). Once you know Δ47, just look up the corresponding temperature in degrees Celsius.
|Value of Δ47 plotted against the inverse of the temperature in Kelvin (1/T). The temperature in degrees Celsius is shown along the top. The data form a reasonably straight line, allowing Halevy and coworkers to calculate the temperature at which the carbonates in ALH 84001 formed, once they had determined Δ47 for the Martian carbonates.|
Halevy and coworkers extracted CO2 from three samples of ALH 84001, using heated phosphoric acid. For each sample, they did three extractions by treating the samples for 1, 4, and 12 hours. This procedure separates the CO2 from the more readily dissolved calcium-rich carbonates from the more resistant magnesian carbonates. One sample turned out to be too small for accurate analyses, but the other two gave six good data points for Δ47. The mean temperature calculated from the graph above is 18 ± 4 oC, solidly in the cool temperature range and clearly not like the elevated temperatures some other techniques had given.
Halevy and coworkers explore the uncertainties in the temperature estimate, including whether it represents a single temperature or a range of temperatures. Considering that the rock was shocked by an impact after the carbonates formed, there is a chance that the temperature was disturbed. To search for effects on the carbonate concretions, Halevy used electron-backscattered diffraction to determine the orientations of the carbonate crystal lattice in different areas of a carbonate concretion. The data show that the structure fans out in a systematic way, rather than forming new crystals throughout the concretion as would be expected if the sample had experienced prolonged annealing at elevated temperature.
|Electron backscatter image of a carbonate concretion from one of the ALH 84001 samples analyzed by Halevy and co-authors. Colors indicate relative orientation. The small insert shows the direction of the primary crystallographic axis of the carbonate and indicates that it sweeps in a fan-shaped manner, implying radial growth of the crystal. A heating event that caused recrystallization of the carbonates (hence loss of original temperature information) would make the backscatter image and the map of crystal axis orientation blotchy. The lack of recrystallization is consistent with temperature spikes being small and rapid, not enough to significantly alter the record of the original crystallization of the carbonate.|
Although the textural evidence for localized, shock heating is strong, the electron backscattered diffraction measurement indicates that these heating events were localized and short. This is supported by the preservation of natural remnant magnetization measurements of the rock (see PSRD article: Low-temperature Origin of Carbonates Consistent with Life in ALH84001). Also, studies of argon retention in the rock indicates the rock was not heated significantly after the carbonates formed and the rock remained cold for the past 4 billion years (see PSRD article: Martian Meteorites Record Surface Temperatures on Mars).
Previous measurements by ion microprobe of δ 18O and δ 13C in carbonate concretions show that these parameters are correlated with each other and with carbonate major element composition. Combined with the modest, constant temperature determined from the clumped isotope approach, this suggested to Halevy and coworkers that the concretions may have formed by slow evaporation of a shallow subsurface water solution that had soaked cracks and other pore spaces in the rock that would become ALH 84001. The direct correlation between carbon and oxygen isotopic compositions indicates that the water was not in constant, rapid communication with the atmosphere, which would have led to more constant isotopic compositions. Halevy also shows that formation of the carbonates at a range of temperatures, not at constant temperature, is not consistent with the data. It appears that the water evaporated away at the constant temperature of 18 oC, precipitating carbonates and losing carbon dioxide as a gas.
|This diagram illustrates Halevy and co-authors' idea that the carbonates in ALH 84001 were formed when water flowed into the base of a rubbly upper surface of Mars and gradually evaporated. The left diagram shows the physical model, with the rock that would become ALH 84001 sitting in an underground, water-saturated area. The right hand diagram shows the chemical processes taking place: Formation of chemically zoned carbonate concretions from water that contains the appropriate elements in solution along with dissolved inorganic carbon (DIC), and loss of carbon dioxide to the atmosphere above. The water has only a feeble connection to the atmosphere, so both water chemistry and carbonate chemical and isotopic compositions change as the water evaporates.|
Similar evaporative deposits occur on Earth in cold or arid regions. Halevy and coworkers cite several examples of studies in which carbon and oxygen isotopes correlate, such as mine tailings in Canada, carbonate deposits in the Houghton impact structure in the Canadian Artic, carbonates in arid soils in the Mojave and Atacama deserts, and caliche formed on basalts in lava fields in Arizona. The isotopic variations are strikingly similar to those in ALH 84001, but Halevy points out that the terrestrial analogs are far from perfect because of the ubiquitous presence of organic compounds and living microorganisms on Earth. Biology is a key driver of chemical alteration on Earth, so the isotopic trends may be due partly to terrestrial life. Of course, if there was abundant life on Mars...!
|Jay Quade (University of Arizona) digs a test pit to sample carbonates formed in the arid soil of the Atacama desert, Chile. The region is almost as dry as Mars and has a landscape to match, although microorganisms live in the soil, only 10 inches or so below the surface. Quade and colleague Julio Betancourt (U.S. Geological Survey) and their students are studying the soil carbonates as biomarkers for terrestrial vegetation.|
|White caliche coats a cavity in a basalt flow in Sunset Crater, Arizona. The caliche may have formed by evaporation, but the green deposit appears to be biological in origin and its formation may have affected the trends produced in δ 13C.|
The carbonates in ALH 84001, as with other products of water solutions in Martian meteorites and with the aqueous products observed using spectroscopic techniques from orbit, are important for determining the chemistry and abundance of water in early Mars. The record in ALH 84001 seems to point towards a time when temperatures were mild enough that near-surface water was warm, certainly a nice temperature for life, but perhaps scarce or fleeting–conditions less suitable for a healthy lifestyle. We still do not know what made conditions warm. It might have been greenhouse gases in a denser atmosphere, a nearby impact, or nearby volcanic activity. We clearly need samples from many more places to fully understand the early climate history of Mars and its potential for life.
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