[meteorite-list] Did an Impact Make the Mysterious Microscopic Magnetite Crystals in ALH 84001?

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Wed, 31 Oct 2007 14:09:36 -0800 (PST)
Message-ID: <200710312209.OAA17080_at_zagami.jpl.nasa.gov>

http://www.psrd.hawaii.edu/Oct07/magnetite-origin.html

Did an Impact Make the Mysterious Microscopic Magnetite Crystals in
ALH 84001?

Planetary Science Research Discoveries
October 30, 2007

--- Tiny crystals of magnetite in Martian meteorite ALH 84001 might have
been made when shock waves decomposed iron carbonate.

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

Fervent debate swirls around microscopic crystals of magnetite
(Fe3O4) in Martian meteorite ALH 84001. Some investigators
suggest that the crystals are evidence of past life on Mars, citing
magnetite crystals of similar chemical compositions and sizes made by
magnetotactic bacteria on Earth. Others cite assorted experiments and
observations to argue that the important little crystals formed entirely
by non-biological processes, hence say nothing about life on Mars. One
of those processes is the decomposition of iron carbonate (the mineral
siderite), which occurs in ALH 84001. Researchers argue that heating
this mineral causes it to decompose into magnetite and CO2 gas.
Experiments showing this were done by heating siderite and observing
that it decomposed and formed magnetite, but nobody had shock-heated
siderite to see if magnetite crystals formed. (Shock is a rapid, strong
rise and fall in pressure. It happens under many circumstances,
including meteorite impacts.)

The lack of shock experiments has been solved by Mary Sue Bell
(University of Houston and Jacobs Engineering). She experimentally
shocked samples of siderite at the Experimental Impact Laboratory at the
Johnson Space Center. She shows that magnetite crystals of the right
size and composition formed when samples were shocked to 49 GPa
(about 500,000 times the pressure at the
Earth's surface). This is more evidence for a non-biological origin for
the magnetite crystals in ALH 84001 and is consistent with what we know
about the impact history of the rock. There seems to be growing evidence
against a biological origin, but don't expect these results to
completely settle the debate!

Reference:

    * Bell, M. S. (2007) Experimental Shock Decomposition of Siderite
      and the Origin of Magnetite in ALH 84001. Meteoritics and
      Planetary Science, v. 42, p. 935-949.

PSRDpresents: Did an Impact Make the Mysterious Microscopic Magnetite
Crystals in ALH 84001? --Short Slide Summary
<http://www.psrd.hawaii.edu/Oct07PSRD-magnetite-origin.ppt>
(with accompanying notes).

------------------------------------------------------------------------

Tiny Crystals, Big Arguments

A group led by David McKay at the Johnson Space Center, with co-workers
elsewhere, made the startling claim in 1996 that there was evidence for
fossil life in Martian meteorite ALH 84001 (see our first PSRD article:
Life on Mars? <http://www.psrd.hawaii.edu/Oct96/LifeonMars.html>).
The evidence they cited has taken quite a beating during the past
decade as scientists enthusiastically tested each argument (that is, they
tried to prove the hypothesis wrong). The one line of evidence still open
is the presence of tiny crystals of compositionally-pure magnetite. A
vociferous debate has been raging about their origin. On one side is the
McKay group, with the magnetite studies spearheaded by Kathy Thomas-Keprta.
On the other side is almost everyone else who has studied the rock (plus some
who haven't!).

Kathy Thomas-Keprta and her colleagues cite five reasons for concluding
that at least a subset of the magnetite crystals in ALH 84001 formed
inside microorganisms. These include their shapes (mostly prisms), sizes
(4-100 nanometers), and their chemical purity (essentially pure iron
oxides). Their arguments stem from the properties of magnetite grains
produced by what biologists call magnetotactic bacteria on Earth.

[variety of magnetites from meteorite and from terrestrial bacteria]

Kathy Thomas-Keprta and her colleagues separated magnetite crystals from
the ALH 84001 carbonate globules. Many have well-formed crystal shapes
like those in the electron microscope images shown on the left. They
look a lot like magnetites formed by terrestrial magnetotactic bacteria,
such as the bacteria designated MV-1, shown on the right.

[graph comparing sizes of magnetites]

Graph of magnetite sizes in ALH 84001 and in terrestrial magnetotactic
bacteria. The overlap of the data shows that many of the small magnetite
crystals in the meteorite have similar sizes to those made by
terrestrial bacteria, such as strain MV-1.

Although the similarities in shape, size, and composition of magnetite
crystals in ALH 84001 and terrestrial magnetite-producing bacteria seem
convincing, most cosmochemists and astrobiologists considered the
evidence circumstantial. Similarity does not prove origin, they argued,
and proposed alternative ways of producing the magnetite crystals. The
most popular non-biological origin involves heating
inorganically-produced iron carbonate to form magnetite and carbon
dioxide gas. One interesting set of experiments was done at the Johnson
Space Center by D. C. Golden and his colleagues. Golden formed globules
of carbonate minerals by precipitation from solutions of salty hot water
(see the image on the left, below). They resembled those in ALH 84001
(see image on the right, below). He then heated the products and showed
that a variety of magnetite crystals formed from the iron carbonate
minerals in the laboratory products.

[experimentally produced carbonates compared to carbonates in ALH 84001]

These scanning electron microscope images show the carbonate globules
produced by precipitation from a hot (150 Celsius) water solution
(LEFT), compared to similar carbonate globules in ALH 84001 (RIGHT).

[experimentally produced magnetite crystals]

These are transmission electron microscope images (in reverse contrast)
of magnetite crystals formed by heat-treating chemically-zoned carbonate
globules made in D.C. Golden's laboratory at the Johnson Space Center.
Image c shows a variety of shapes: parallelepipeds (pp), cubes (cb), and
octahedral (oh). Image d shows tooth-shaped (th) and bullet-shaped (bt)
grains of magnetite. Image e shows a chain of small magnetite crystals
with tear-drop (td) shapes.

[rotating grain]

Shock damaged pyroxene crystal from ALH 84001 in the polarizing light
microsope. Image credit: Edward Scott (University of Hawaii.)

Other investigators presented evidence that the magnetite in ALH 84001
formed by carbonate decomposition. For example, see PSRD article:
Resolution of a Big Argument About Tiny Magnetic Minerals in Martian
Meteorite <http://www.psrd.hawaii.edu/May02/ALH84001magnetite.html>.

These results convinced many researchers that tiny magnetite crystals
could form when impacts heated the rock and caused carbonate minerals to
decompose, but none used an actual shock experiment to produce magnetite
from siderite. This is an important gap because ALH 84001 is strongly
shocked. In fact, Allan Treiman (Lunar and Planetary Institute, Houston)
has shown that the rock experienced several separate shock events. This
is not surprising for an old rock (it initially crystallized 4.5 billion
years ago) because planetary surfaces were severely bombarded until 3.8
or 3.9 billion years ago. Everyone agrees that the rock formed in a
large body of magma trapped deep in the Martian crust. It was excavated
at some point to a location near the surface where water evaporated and
precipitated carbonate minerals, including siderite, along grain
boundaries. The rock was shocked after that event, causing assorted
deformation of the carbonates, possibly including decomposition of the
siderite.

------------------------------------------------------------------------

Shocking Rocks in the Lab

[siderite rock sample]

All that makes a detailed story of shock events, yet there have been no
shock experiments done to show that shock and the instantaneous heating
accompanying it can cause decomposition of siderite. Mary Sue Bell has
fixed that problem. She obtained unshocked siderite from Natural Copper
Lake in Nova Scotia (see photo), and thoroughly characterized it with
electron microprobe analysis and transmission electron microscopy.
Because elements such as magnesium, manganese, and calcium readily
substitute for iron in siderite, it is rarely found as pure FeCO3. Bell
demonstrates this through x-ray imaging in the electron microprobe. The
siderite is composed mostly of iron, but also contains significant
amounts of magnesium and calcium. The composition is also not uniform
throughout the mineral grains.

Bell crushed the siderite sample in alcohol to prevent oxidation, and
sieved it to obtain a size fraction ranging from 120 to 250 micrometers.
She took small (70 milligram) samples of the crushed rock and packed
them into tungsten-alloy holders that contained no iron, and placed them
in the experimental shock apparatus. (see photo below, left). The
siderite powder samples were shocked at pressures ranging from 24 to 49
GPa in a chamber containing carbon dioxide to
simulate conditions at the Martian surface. The shock effects are
obvious even just looking at the samples in colorless vials (see photo
below, right).

[JSC impact lab Unshocked and shocked siderite powders]

[LEFT] Jerry Hanes is shown loading "the gun" in JSC's Impact
Laboratory. This is where the siderite powders were shocked to specific
pressures. [RIGHT] The original, unshocked siderite powder is reddish
brown, but became darker at progressively higher shock pressures.

The shock effects are more dramatic when given a closer look with an
electron microscope. Small magnetite crystals are present when shocked
to 49 GPa, indicating decomposition of some of the siderite to
magnetite. The magnetite crystals are small and resemble the
prism-shaped crystals in ALH 84001, although they are generally larger
than those in the meteorite. The magnetite crystals contain up to 19
mole percent magnesium, but some contain no detectable magnesium.

[TEM image of magnetite crystals]

This transmission electron microscope image shows the transformation of
siderite to magnetite when shocked to 49 GPa pressure. In the upper
right area, labeled "S", is siderite containing nucleating crystals of
magnetite. The other crystals in this image are magnetites of different
shapes. Arrow "a" points to an equant crystal, arrow "b" points to an
euhedral elongated crystal, and arrow "c" points to a subhedral
elongated magnetite crystal.

------------------------------------------------------------------------

Will the Debate Go On for Another Decade?

Thomas-Keprta and her colleagues call attention to the purity of the
magnetite in marshalling evidence for a biogenic origin. Terrestrial
magnetite formed by bacteria are essentially pure Fe3O4. However, there
seems to be disagreement about how pure the magnetite crystals are in
ALH 84001. For example, D. C. Golden and colleagues at the Johnson Space
Center argue that the magnetite crystals in the meteorite are not pure
iron oxide. They found, using the same transmission electron microscopy
and procedures used by Thomas-Keprta, that magnetite in ALH 84001
contains up to 3 mole percent magnesium, although they also found that
many contain no detectable magnesium. (Much of the hard-fought debate
about the origin of magnetite in ALH 84001 involves different groups of
investigators at the Johnson Space Center. I wonder if that puts a
damper on the annual office Christmas party!)

The compositions and sizes of the magnetite crystals produced in Bell's
shock experiments may not match perfectly with those in ALH 84001, but
they do show unequivocally that shock can produce magnetite by
decomposition of siderite. Thomas-Keprta and her colleagues have argued
in the past that magnetite in ALH 84001 formed by more than one process,
one of which was biogenic. They do not deny that the rock was shocked
and that the shock and other thermal events could cause decomposition of
siderite. But, they argue, the tiny, prismatic, pure magnetite crystals
formed by microscopic organisms living in water flowing along cracks in
rock.

Figuring all this out is clearly a tricky business. Searching
definitively for life on Mars requires samples without such a
complicated history. Such samples might be found by carefully examining
promising regions on Mars, obtaining samples, and bringing them back to
Earth. A Mars sample return mission might be launched as early as 2018.
Maybe after detailed study of those samples everyone will agree on
whether the rocks and soil returned contain evidence for present or past
life on Mars.

------------------------------------------------------------------------

ADDITIONAL RESOURCES

    * PSRDpresents: Did an Impact Make the Mysterious Microscopic
      Magnetite Crystals in ALH 84001? --Short Slide Summary
      <http://www.psrd.hawaii.edu/Oct07PSRD-magnetite-origin.ppt>
      (with accompanying notes).
    * Bell, M. S. (2007) Experimental Shock Decomposition of Siderite
      and the Origin of Magnetite in ALH 84001. Meteoritics and
      Planetary Science, v. 42, p. 935-949.
    * Golden, D. C., Ming, D. W., Schwandt, C. S., Lauer Jr., H. V.,
      Socki, R. A., Morris, R. V., Lofgren, G. E., and McKay, G. A.
      (2001) A Simple Inorganic Process for Formation of Carbonates,
      Magnetite, and Sulfides in Martian Meteorite ALH84001. American
      Mineralogist, v. 86, p. 370-375.
    * 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
    * Taylor, G. J. (October, 1996) Life on Mars? Planetary Science
      Research Discoveries. http://www.psrd.hawaii.edu/Oct96/LifeonMars.html
    * Thomas-Keprta, K. L., Bazylinski, D. A., Kirschvink, J. L.,
      Clemett, S. J., McKay, D. S., Wentworth, S. J., Vali, H., Gibson
      Jr., E. K., and Romanek, C. S. (2000) Elongated Prismatic
      Magnetite Crystals in ALH84001 Carbonate Globules: Potential
      Martian Magnetofossils. Geochimica et Cosmochimica Acta, v. 64, p.
      4049-4081.
Received on Wed 31 Oct 2007 06:09:36 PM PDT