[meteorite-list] A Younger Age for the Oldest Martian Meteorite (ALH 84001)

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu, 13 May 2010 12:28:05 -0700 (PDT)
Message-ID: <201005131928.o4DJS51B020114_at_zagami.jpl.nasa.gov>

http://www.psrd.hawaii.edu/May10/YoungerALH84001.html

A Younger Age for the Oldest Martian Meteorite
Planetary Science Research Discoveries
May 12, 2010

--- New isotopic analyses show that famous Martian meteorite ALH 84001
formed 4.09 billion years ago, not 4.50 billion years ago as originally
reported.

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

The Allan Hills (ALH) 84001 Martian meteorite is famous for containing
fiercely-disputed evidence for fossil life. Equally important to many
cosmochemists, the meteorite also contains important information about
the construction of the Martian crust by magmas derived from the interior,
and the subsequent modification of those igneous
rocks by large impacts and circulating water. A surprising feature of
ALH 84001 has been its extremely ancient age, 4.50 billion years, as
determined by samarium-neodymium (Sm-Nd) and rubidium-strontium (Rb-Sr)
isotopic dating. If correct, the
ancient age implies that the magma in which ALH 84001 formed intruded
the primordial crust, perhaps forming in a deep ocean of magma that
surrounded Mars during its initial differentiation into metallic core,
rocky mantle, and primary crust.

New age determinations by Thomas Lapen (University of Houston) and
colleagues there and at the Johnson Space Center, the Lunar and
Planetary Institute, the University of Wisconsin, and the University of
Brussels, Belgium, indicate that the rock crystallized in a magma 4.091
billion years ago. They used lutetium-hafnium (Lu-Hf) isotopes in
determining the new age. This isotopic system has the advantage of not
being affected as readily by impact heating and water alteration as are
Sm-Nd and Rb-Sr. The new age is consistent with igneous activity
throughout Martian history and with a period of heavy bombardment
between 4.2 and 4.1 billion years as inferred from the ages of large
impact basins on Mars.

Reference:

    * Lapen, T. J., Righter, M., Brandon, A. D., Debaille, V., Beard, B.
      L., Shafer, J. T., and Peslier, A. H. (2010) A Younger Age for ALH
      84001 and its Geochemical Link to Shergottite Sources in Mars.
      /Science,/ v. 328, p. 347-351.

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

The Tortured History of an Excellent Igneous Rock

At the risk of revealing my deep-seated biases, I have to say that ALH
84001, whether it ever teemed with microorganisms or not, is one great
igneous rock. And igneous rocks are wondrous products that reveal the
composition of planetary interiors, the processes that operated in
magmas as they migrated to the crust, and how the magmas crystallized
far beneath the surface or in lava flows that erupted onto it, shaping
the landscape. Igneous rocks are fundamentally important. And thin
sections of them look great in a polarizing microscope.

ALH 84001 is a piece from a mass of magma that crystallized inside the
Martian crust. As the magma crystallized, one of the first minerals to
form was orthopyroxene (iron-magnesium silicate), accompanied by small
amounts of chromite (iron-chromium oxide). These early-crystallizing
minerals accumulated at the base of the magma chamber, forming a rock
consisting of 97% orthopyroxene, 1% chromite, and other minerals that
crystallized from magma trapped between the accumulated pyroxene crystals.

Thin section in cross-polarized light of orthopyroxene crystals in ALH
84001. Artist's drawing of igneous rock formation and origin of ALH
84001 on Mars.
[LEFT] Large crystals of orthopyroxene in ALH 84001 show that this rock
formed in a magma chamber deep inside Mars. Dark areas are chromite (an
oxide of chromium and iron). The photograph is of a thin slice of the
rock as viewed in polarized light. [RIGHT] ALH 84001 formed 4.09 billion
years ago, according to new age dating by Tom Lapen and his colleagues.
The rock crystallized in a relatively large magma body inside the crust
of Mars. Its high abundance of one mineral (orthopyroxene) indicates
that this mineral must have accumulated in the magma, probably near the
bottom of the magma body, eventually forming the original igneous rock
with large crystals of orthopyroxene.

Martian geological processes did not leave the ALH 84001 cumulate to
rest in peace. Early bombardment by huge projectiles reworked the crust,
heating, melting, and mixing pre-existing rocks into a jumbled, cratered
surface. ALH 84001 shows the wounds from the bombardment in the form of
shock-damaged mineral grains, some melted and squirted into veins, and
areas where the large crystals of orthopyroxene have been crushed (see
photograph below). Detailed study of the meteorite, particularly those
by Alan Treiman (Lunar and Planetary Institute, Houston), show that the
rock was affected by more than one impact, further complicating the
interpretation of isotopic data.

[Thin section in cross-polarized light of crushed crystals in ALH
84001.] Rotating microscope stage
Rotating microscope stage shows uneven color changes in ALH 84001
pyroxene crystal damaged by shock.
[LEFT] Crushed portion of the ALH 84001 meteorite, as seen in a thin
section. Such impact-induced crushing and the accompanying heating
likely affected the record of the original igneous crystallization age
of the rock. [RIGHT] A shock damaged pyroxene crystal from ALH 84001 in
the polarizing light microscope. The uneven way the mineral color
changes as a section is rotated in cross-polarized light is indicative
of deformation, in this case by shock. Image courtesy of Ed Scott
(University of Hawaii).

ALH 84001 also contains deposits of carbonate minerals. These clearly
formed from water flowing through the rock, though after it had been
affected by one or more impacts. The fractures caused by passage of
impact-induced shock waves may have created path ways for water and
sites of carbonate deposition. This wet event, although not wet enough
to alter the main silicate minerals, may have chemically changed
phosphate minerals, the hosts of samarium and neodymium, thereby
affecting age measurements. In spite of the complexity of carbonates and
how water affected some of the original minerals in the rock, in 1999
Lars Borg and colleagues at the NASA Johnson Space Center managed to
painstakingly separate different mineral phases by differential
dissolution and then measure the isotopic composition of rubidium and
strontium in the rock. The resulting Rb-Sr age is 3.90??0.04 billion
years. They also measured the age using lead isotopes, which gave the
same value within experimental uncertainty, 4.04??0.10 billion years.
Apparently the carbonates precipitated from water that had soaked the
rock about 3.9 billion years ago, showing that water could flow through
the rock for at least a short time. For more details about carbonate
formation and the shock effects on them and the rest of the rock, see
PSRD articles: Carbonates in ALH 84001: Part of the Story of Water on
Mars <../July04/carbonatesALH84001.html> and Shocked Carbonates may
Spell N-O L-I-F-E in Martian Meteorite ALH84001
<../May97/ShockedCarb.html>.

Photomicrograph of thin section of ALH 84001 carbonate globules.
Artist's drawing of geologic setting on Mars about 3.9 billion years ago
when ALH 84001 rock was altered by water.
[LEFT] Carbonate assemblages in ALH 84001. Carbonates are orange, clear,
and dark, (in the center) and are surrounded by orthopyroxene (lighter
colored). Small dark grains are chromite. The carbonates were deposited
from water that soaked the rock for short periods of time. They have
been the focus of the argument about the evidence for fossil life in ALH
84001. Field of view is about 0.5 millimeters across. [RIGHT] Sometime
around 3.9 billion years ago ALH 84001 had been excavated from its deep
original position in the Martian crust and was part of the upper crust,
probably in rubbly crater ejecta. Mars was wetter then than it is now,
and water flowed through its crust and across its surface, including
through the deposit containing the shock-damaged rock that would become
ALH 84001.

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

Messed-Up Ages

One result of smashing impacts and flowing water is to mess up the
distribution of Rb and Sr, Sm and Nd, thereby compromising our ability
to determine a reliable age for the original crystallization of ALH
84001. Until now, the only age measurements were reported by Larry
Nyquist and his colleagues at NASA Johnson Space Center, in an abstract
in the Lunar and Planetary Science Conference. They reported a good
isochron for the Sm-147/Nd-143 system, giving an age of 4.50??0.13
billion years, and a slightly older age of 4.56 billion years by the
Rb-Sr system. This is the age that has been cited for the rock since the
abstract was published in 1995. Most of us have accepted it, especially
because the isotope experts were not arguing about it as they often do
about ages, but most cosmochemists also felt vaguely uneasy about it.
Nyquist and his colleagues had even raised a warning in their 1995
report, noting that data for the short-lived Sm-146/Nd-142 system hinted
at a younger age: radioactive Sm-146 (half-life of 103 million years)
had substantially decayed away before the rock formed.

Tom Lapen and his colleagues draw attention to the particular
sensitivity of Sm-Nd and Rb-Sr to shock heating and to alteration by
water. For example, they point out that 58% of the Sm and 78% of the Nd
reside inside phosphate minerals, which are subject to alteration by
water. A bit of alteration results in redistribution of the parent and
daughter isotopes, giving incorrect age results. We need a system that
is less subject to alteration by water, shock, and heat. The
lutetium-hafnium system fills this tall order.

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

Seeing through the Damp Rubble

In contrast to most of the Sm and Nd being housed in alteration-prone
phosphate minerals, 97% of the Lu and 96% of the Hf reside in
orthopyroxene. The remaining 4% of the Hf is in chromite, and an
insignificant 3% of the Lu is in phosphate minerals. Orthopyroxene and
chromite are not prone to resetting by either shock or reactions with
water, so Lapen and his colleagues figured the analyses had a good
chance of giving the true igneous age of the rock. In addition, they
selected samples that had well-preserved igneous textures, not much
granulation of the samples, and no visible carbonates or other alteration.

Photo of small sample of Martian meteorite ALH 84001 used in analyses.
One of the igneous-textured fragments used by Tom Lapen and his
colleagues to determine the crystallization age of ALH 84001. Samples
like this are less likely to have their isotopic systems disturbed by
shock.

Lapen and coworkers separated the fragments into four samples: S1, 100%
chromite; S2, pure orthopyroxene; S3, an unseparated bulk rock sample;
and S4, the bulk sample after separating chromite. They also did some
other chemical separations for new measurements of Sm and Nd isotopes,
but we will concentrate on the Lu-Hf data here. Lapen separated Lu and
Hf from the other elements and from each other by a series of chemical
processes using ultra-pure chemicals, and the resulting solutions were
measured for their isotopic compositions using a inductively-coupled
plasma mass spectrometer at the University of Wisconsin. The results are
shown in the isochron diagram below. The four Lu-Hf data points define a
precise line on the diagram and indicate an age of 4.091??0.03 billion
years (uncertainty is at the 95% confidence level). This probably
represents the time when the ALH 84001 cumulate orthopyroxenite
crystallized from a magma inside the Martian crust. The age is
substantially younger than the previous age measurements (4.50 billion
years).

Age for ALH 84001 determined by Lutetium-Hafnium isotopic dating.
Four different mineral and rock samples from ALH 84001 fall on a precise
isochron line. Lutetium-176 is the radioactive parent isotope (half-life
of 37.8 billion years). It decays to Hafnium-176. Both are divided by
Hafnium-177, a stable isotope of Hf. The slope of the line defines the
age (the steeper the slope, the older the age) and the intercept on the
y-axis provides important information about the history of the region of
the Martian interior where the ALH 84001 magma formed.

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

Martian Bombardment

Herb Frey (NASA Goddard Space Flight Center) has developed two
imaginative techniques to identify old, degraded impact basins on Mars.
Both use the detailed topographic data obtained by the Mars Observer
Laser Altimeter. One approach uses ghostly
outlines of ancient, eroded basins, which Frey calls "Quasi-Circular
Depressions," or QCDs for short. The other identifies even more ghostly
(but real) circular features, which he identifies on maps of inferred
crustal thickness. On these maps the circular depressions are shown by
thin crust surrounded by narrow rims of thicker crust. Frey calls these
structures "Circular Thin Areas," or CTAs. (Crustal thickness is
determined from the combination of topography, variations in the
gravitation field around the planet, and geophysical calculations.) The
locations of QCDs and CTAs, hence of impact basins, larger than 1000
kilometers across are shown in the maps below.

Topographic maps of Mars showing locations of impact basins identified
by Herb Frey.
Impact basins larger than 1000 kilometers diameter are shown on a series
of Mars topographic maps. On the maps, reddish and white colors are high
in elevation, blues and purples are low. Solid circles indicate QCDs;
dashed circles indicate CTAs. The three maps on top are views of the
equator at 60^o west longitude (left), 180^o west (center), and 300^o
west (right). The maps on the bottom are views looking down on the north
pole (left) and the south pole (right).

Herb Frey estimated the ages of each of the 20 basins larger than 1000
kilometers by counting the number of craters (QCDs and CTAs) larger than
300 kilometers inside each of the basins. This gives a solid measure of
their relative ages. Converting these relative ages into absolute ages
is much trickier. The method, uncertainties, and issues are discussed in
detail in a review paper by Bill Hartmann and Gerhard Neukum. For
younger surfaces, such as the lunar maria, the number of craters on them
is directly proportional to the age, and lunar samples have allowed a
good calibration of the number of craters as a function of time for the
past 3.5 billion years or so. The problem arises when the rate of impact
was higher, before about 3.9 billon years. Not only is the flux of
impactors higher, but smaller craters are eroded faster than larger
ones. This led Bill Hartmann, back in 1966, to devise a measure called
the "crater retention age," or CRA, which takes into account the extent
of retention versus crater size. Herb Frey used this approach and the
correlation of CRA with age given by Hartmann and Neukum. (He actually
used the average of the Hartmann's and Neukim's estimates because they
do not agree with each other!)

Considering these complexities, determining absolute ages by counting
craters might seem to be a pretty flaky business. In fact, alert readers
might think, considering how hard it has been to determine a precise
igneous age for ALH 84001 in state-of-the-art laboratories on Earth,
that dating distant planetary surfaces is hopeless. Fortunately, it is
not. The relative ages are well established, and the uncertainty in the
age of a specific impact basin is on the order of 0.1-0.2 billion years.
This uncertainty is larger than the uncertainty of 0.03 in the Lu-Hf age
of ALH 84001, but possibly smaller than the uncertainty in the age
derived from Sm-Nd dating. Recognizing the uncertainties, the 20 impact
basins identified by Herb Frey have a range of ages from about 3.85 to
4.25 billion years, with a sharp peak in the range 4.1-4.2 billion years
(see diagram below).

Histogram showing distribution of ages of 28 Martian impact basins.
Histogram showing the distribution of ages of 28 Martian impact basins
larger than 1000 kilometers across, determined by Herb Frey from crater
counting. These are from his 2008 paper with updates in 2010. (Red =
highland basins. Blue = lowland basins. Green = Tharsis basins. Gray =
recently-identified basins using a revised map of crustal thickness
created by Greg Neumann and colleagues at Goddard Space Flight Center.)
Note the distinct peak in the 4.1 to 4.2 billion year range. The igneous
age of ALH 84001, 4.09 billion years, is close to the lower boundary of
this range, and the age of carbonate deposition is lower, between 3.9
and 4.0 billion years. Note the startling lack of large impact basins
before 4.25 billion years. Rocks formed that early in Martian history
might exist as boulders in basin ejecta, if their ages were not reset by
the spike in impacts between 4.1 and 4.2 billion years. (Uncertainties
in the relation of crater counts to absolute age might shift the
distribution to younger or older ages, but will not change the shape of
the distribution--Mars experienced a peak in bombardment rate after a
post-accretion lull.)

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

Searching for the Oldest Mars Rocks

Isotopic dating and crater counting indicate that ALH 84001 formed as
an igneous intrusion during a period of intense bombardment of the
Martian surface. After cooling in an intrusion 4.09 billion years ago,
it was excavated by a large impact, exposed to flowing water (probably
beneath the surface), and smashed by another impact event as shown by
effects on the carbonate assemblages. This is an impressive bit of
forensic geology. Are all these geologic events consistent with the
record of basin formation revealed by Herb Frey's work? The formation
age of 4.09 billion years is near the tail end of intense cratering on
Mars, according to the crater count calibration. Perhaps the entire
distribution in basin ages should be shifted to the right by 0.2 billion
years (about the uncertainty in basin ages), to make a distinct peak at
3.9 to 4.0 billion years. If so, then ALH 84001 formed as the late heavy
bombardment was beginning, but managed to survive. Or, perhaps formation
of a huge impact basin triggered the melting that led to formation of
ALH 84001. Testing the possibilities requires more samples from the
ancient surface of Mars.

A downside of the new, younger age of ALH 84001 is that we no longer
think we have a rock that formed in the primary Martian crust. Can we
find 4.5 billion year old pieces of primary crust? Perhaps the fierce
bombardment around 4.1-4.2 billion years reset all the ages. It
certainly resurfaced the ancient Martian crust, but that does not mean
it erased all record of earlier events. The Moon has been similarly
bombarded, but igneous rocks returned from the lunar highlands record a
range of ages from 4.4 to 3.9 billion years, revealing a long and
compositionally-diverse magmatic history. As in ALH 84001, the record in
lunar samples is not always particularly crisp. For example, the oldest
lunar samples, the anorthosites, almost certainly make up the primary
crust of the Moon, but their isotopic systems have been disturbed by
impacts after their formation. See PSRD article: The Oldest Moon Rocks
<../April04/lunarAnorthosites.html> for details.

Dating and understanding the formation of the primary crust of Mars will
require obtaining direct samples of the crust. Finding the right
samples, or the places to search for them, will require remote sensing
data to pick appropriate sites and detailed geologic field work of the
sites. It is not clear that such intensive work can be done effectively
by rovers, so studying the ancient crust might have to wait until people
roam the surface of Mars for extended periods of time.

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

ADDITIONAL RESOURCES

    * *PSRDpresents:* A Younger Age for the Oldest Martian Meteorite
      --Short Slide Summary <PSRD-YoungerALH84001.ppt> (with
      accompanying notes).

    * ALH 84001 sample photos and microscope images
      <http://curator.jsc.nasa.gov/antmet/marsmets/alh84001/photos.cfm>
      from NASA Johnson Space Center.
    * Borg, L. E., J. N. Connelly, L. E. Nyquist, C.-Y. Shih, H.
      Wiesmann, and Y. Reese (1999), The Age of the Carbonates in
      Martian Meteorite ALH84001, /Science,/ v. 268, p. 90-94. [NASA ADS
      entry <http://adsabs.harvard.edu/abs/1999Sci...286...90B>]
    * Corrigan, C. M. (2004) Carbonates in ALH 84001: Part of the Story
      of Water on Mars, /Planetary Science Research Discoveries./
      http://www.psrd.hawaii.edu/July04/carbonatesALH84001.html
      <../July04/carbonatesALH84001.html>.
    * Frey, H. (2008) Ages of Very Large Impact Basins on Mars:
      Implications for the Late Heavy Bombardment in the Inner Solar
      System, /Geophys. Res. Lett.,/ v. 35, L13203,
      doi:10.1029/2008GL033515. [NASA ADS entry
      <http://adsabs.harvard.edu/abs/2008GeoRL..3513203F>]
    * Frey, H. (2010) Lessons Learned from Impact Basins on Mars, /41th
      Lunar Planet. Sci. Conf.,/ abstract #1136
      <http://www.lpi.usra.edu/meetings/lpsc2010/pdf/1136.pdf>.
    * Hartmann, W. K., and Neukum, G. (2001) Cratering Chronology and
      the Evolution of Mars, /Space Science Reviews,/ v. 96, p. 165-194.
      [NASA ADS entry <http://adsabs.harvard.edu/abs/2001SSRv...96..165H>]
    * Lapen, T. J., Righter, M., Brandon, A. D., Debaille, V., Beard, B.
      L., Shafer, J. T., and Peslier, A. H. (2010) A Younger Age for ALH
      84001 and its Geochemical Link to Shergottite Sources in Mars,
      /Science,/ v. 328, p. 347-351. [NASA ADS entry
      <http://adsabs.harvard.edu/abs/2010Sci...328..347L>]
    * Norman, M. (2004) The Oldest Moon Rocks, /Planetary Science
      Research Discoveries/.
      http://www.psrd.hawaii.edu/April04/lunarAnorthosites.html
      <../April04/lunarAnorthosites.html>.
    * Nyquist, L.E., Bansal, B.M., Wiesmann, H., Shih, C.-Y. (1995)
      "Martians" Young and Old: Zagami and ALH84001. /Proc. 26th Lunar
      Planet. Sci. Conf.,/ p. 1065-1066 ( abstract
      <http://www.lpi.usra.edu/meetings/lpsc1995/pdf/1533.pdf> ).
    * Scott, E. R. D. (1997) Shocked Carbonates may Spell N-O L-I-F-E
      in Martian Meteorite ALH84001, /Planetary Science Research
      Discoveries/. http://www.psrd.hawaii.edu/May97/ShockedCarb.html
      <../May97/ShockedCarb.html>.
Received on Thu 13 May 2010 03:28:05 PM PDT