[meteorite-list] Formation of Carbonate Minerals in Martian Meteorite ALH 84001 from Cool Water Near the Surface of Mars

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Wed, 21 Dec 2011 13:03:00 -0800 (PST)
Message-ID: <201112212103.pBLL30Cw000270_at_zagami.jpl.nasa.gov>

http://www.psrd.hawaii.edu/Dec11/ALH84001_Carb.html

Formation of Carbonate Minerals in Martian Meteorite ALH 84001 from
Cool Water Near the Surface of Mars

Planetary Science Research Discoveries
December 15, 2011


--- A new approach to thermometry using isotopic compositions of carbon
and oxygen indicates that carbonate minerals in Martian meteorite ALH
84001 formed at 18 ?? 4 ^o C.

Written by G. Jeffrey Taylor
Hawai'i Institute of Geophysics and Planetology

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 ^o C, 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 CO_2 . This allowed the investigators to use the isotopic
abundances of both carbon and oxygen. The results indicate that the
carbonates formed at 18 ?? 4 ^o C 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.


Reference:

    * Halevy, I., Fischer, W. W., and Eiler, J. M. (2011) Carbonates in
      the Martian Meteorite Allan Hills 84001 Formed at 18 ?? 4 ^o C in a
      Near-Surface Aqueous Environment. /Proceedings of the National
      Academy of Sciences,/ v. 108, p. 16895-16899.
      doi:10.1073/pnas.1109444108.
    * *PSRDpresents:* Formation of Carbonate Minerals in Martian
      Meteorite ALH 84001 from Cool Water Near the Surface of Mars
      --Short Slide Summary <PSRD-ALH84001_Carb.ppt> (with accompanying
      notes).


Probing Early Martian Waters

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? <../Oct96/LifeonMars.html>).
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 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 ^18 O/^16 O
and low ^13 C/^12 C 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 ?? ^18 O
and ?? ^13 C.) 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:

    * Low-Temperature Origin of Carbonates Consistent with Life in ALH
      84001 <../May97/LowTempCarb.html>
    * Shocked Carbonates May Spell N-O L-I-F-E in ALH 84001
      <../May97/ShockedCarb.html>
    * Resolution of a Big Argument About Tiny Magnetic Minerals in
      Martian Meteorite <../May02/ALH84001magnetite.html>
    * Did an Impact Make the Mysterious Microscopic Magnetite Crystals
      in ALH 84001? <../Oct07/magnetite-origin.html>

One way to independently determine the temperature of formation is to
use clumped isotope thermometry.

Clumping Isotopes

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 CO_2 . Isotopologues have the same
chemical composition (CO_2 in this case), but differ in how much ^12 C,
^13 C, ^16 O, ^17 O, and ^18 O they contain. There are quite a few
combinations, but Halevy focused on CO_2 with a molecular mass of 47.
Mass 47 is particularly useful because it contains both ^18 O and ^13 C,
which show a temperature-dependent preference for sticking together
rather than residing in separate CO_2 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 ?? ^18 O and ?? ^13
C give direct information about the isotopic composition of the fluids.

The most common isotopologue of CO_2 has a mass of 44. It is composed of
the abundant isotopes ^16 O (two of them in each molecule) and ^12 C,
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 R^47 . 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, ??_47 , is calculated from the R values and reported
in parts per thousand. ??_47 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 CO_2 and the instrumentation to measure it
are even more daunting!)

Previous work on experimentally produced and natural samples has shown
that ??_47 is correlated with temperature of formation. This empirical
correlation agrees well with calculations of the temperature dependence
of ??_47 based on thermodynamic principles. Thus, once ??_47 is
determined, the temperature of formation can be calculated. The actual
correlation is between ??_47 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 ??_47 , just look up the corresponding temperature in
degrees Celsius.

Halevy and coworkers extracted CO_2 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 CO_2 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 ??_47 . The mean temperature
calculated from the graph above is 18 ?? 4 ^o C, 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.

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 <../May97/LowTempCarb.html>). 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 <../July05/Mars_paleotemp.html>).

Evaporating Fluids, Precipitating Carbonates

Previous measurements by ion microprobe of ?? ^18 O and ?? ^13 C 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 ^o C, precipitating carbonates and
losing carbon dioxide as a gas.

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...!

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.


Additional Resources Links open in a new window.

    * *PSRDpresents:* Formation of Carbonate Minerals in Martian
      Meteorite ALH 84001 from Cool Water Near the Surface of Mars
      --Short Slide Summary <PSRD-ALH84001_Carb.ppt> (with accompanying
      notes).

    * Ghosh, P., Adkins, J., Affek, H., Blata, B., Guo, W., Schauble, E.
      A., Schrag, D., and Eiler, J. M. (2006) ^13 C-^18 O bonds in
      carbonate minerals: A new kind of paleothermometer. /Geochimica et
      Cosmochimica Acta,/ v. 70, p. 1439-1456, doi:
      10.1016/j.gca.2005.11.014. [NASA ADS entry
      <http://adsabs.harvard.edu/abs/2006GeCoA..70.1439G>]
    * Halevy, I., Fischer, W. W., and Eiler, J. M. (2011) Carbonates in
      the Martian Meteorite Allan Hills 84001 Formed at 18 ?? 4 ^o C in a
      Near-Surface Aqueous Environment. /Proceedings of the National
      Academy of Sciences,/ v. 108, p. 16895-16899.
      doi:10.1073/pnas.1109444108. [abstract
      <http://www.pnas.org/content/108/41/16895>]
    * Knauth, L. P., Brilli, M., and Klonowski, S. (2003) Isotope
      Geochemistry of Caliche Developed on Basalt. /Geochimica et
      Cosmochimica Acta,/ v. 67, p. 185-195, doi:
      10.1016/S0016-7037(02)01051-7. [NASA ADS entry
      <http://adsabs.harvard.edu/abs/2003GeCoA..67..185K>]
    * Quade, J., Rech, J. A., Latorre, C., Betancourt, J. L., Gleeson,
      E., and Kalin, M. T. K. (2007) Soils at the Hyperarid Margin: The
      Isotopic Composition of Soil Carbonate from the Atacama Desert,
      Northern Chile. /Geochimica et Cosmochimica Acta,/ v. 71, p.
      3772-3795, doi: 10.1016/j.gca.2007.02.016. [NASA ADS entry
      <http://adsabs.harvard.edu/abs/2007GeCoA..71.3772Q>]
    * Scott, E. R. D. (May, 1997) Shocked Carbonates May Spell N-O
       L-I-F-E in ALH 84001. /Planetary Science Research Discoveries./
      http://www.psrd.hawaii.edu/May97/ShockedCarb.html
      <../May97/ShockedCarb.html>
    * Scott, E. R. D. and Barber, D. J. (May, 2002) Resolution of a Big
      Argument About Tiny Magnetic Minerals in Martian Meteorite.
      /Planetary Science Research Discoveries./
      http://www.psrd.hawaii.edu/May02/ALH84001magnetite.html
      <../May02/ALH84001magnetite.html>
    * Taylor, G. J. (Oct. 1996) Life on Mars? The Evidence and the
      Debate. /Planetary Science Research Discoveries./
      http://www.psrd.hawaii.edu/Oct96/LifeonMars.html
      <../Oct96/LifeonMars.html>
    * Taylor, G. J. (May, 1997) Low-temperature Origin of Carbonates
      Consistent with Life in ALH84001. /Planetary Science Research
      Discoveries./ http://www.psrd.hawaii.edu/May97/LowTempCarb.html
      <../May97/LowTempCarb.html>
    * Taylor, G. J. (Oct., 2007) Did an Impact Make the Mysterious
      Microscopic Magnetite Crystals in ALH 84001?. /Planetary Science
      Research Discoveries./
      http://www.psrd.hawaii.edu/Oct07/magnetite-origin.html
      <../Oct07/magnetite-origin.html>
Received on Wed 21 Dec 2011 04:03:00 PM PST