[meteorite-list] Pretty Green Mineral -- Pretty Dry Mars?

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 10:27:45 2004
Message-ID: <200311090513.VAA00086_at_zagami.jpl.nasa.gov>

http://www.psrd.hawaii.edu/Nov03/olivine.html

Pretty Green Mineral -- Pretty Dry Mars?
Planetary Science Research Discoveries
November 7, 2003

Written by Linda M.V. Martel
Hawai'i Institute of Geophysics and Planetology

--- The discovery of olivine-bearing rocks on Mars
underscores the need to understand weathering rates of silicates in
the Martian environment.

Spectra of the Martian surface from the Mars Global Surveyor Thermal
Emission Spectrometer (TES) have been matched with laboratory spectra of
olivine. Todd Hoefen and Roger Clark (U. S. Geological Survey, Denver) and
colleagues at Arizona State University and NASA Goddard Space Flight Center
reported a 30,000-square-kilometer area of olivine-bearing rock in the Nili
Fossae region, northeast of Syrtis Major. Olivine is the common name for a
suite of iron-magnesium silicate minerals known to crystallize first from a
magma and to weather first in the presence of water into clays or iron
oxides. The occurrence of olivine on the surface of Mars and its
susceptibility to chemical weathering has geochemists busy investigating how
long it has been there and what that means about climate history.

Reference:

     Hoefen, T. M., Clark, R. N., Bandfield, J. L., Smith, M. D., Pearl, J.
     C., and Christensen, P. R. (2003) Discovery of olivine in the Nili
     Fossae region of Mars. Science, v. 302, p. 627-630.

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

Discovering Olivine on Mars

The TES instrument measures the infrared energy emitted by surface materials
and by CO2, water ice, dust, and water vapor in the Martian atmosphere.
Hoefen and his colleagues studied the surface of Mars in locations from 60oN
to 60oS and focused on TES data in the spectral range from ~300 to ~550
cm-1, corrected to eliminate the atmospheric components. Three diagnostic
spectral features of olivine were matched to the TES data: absorption
features centered near 400 and 510 cm-1 (due to the bending of
silicon-oxygen bonds) and a peak near 450 cm-1; see the plot shown below.

    [spectra comparison of lab olivine and TES surface]

The olivine spectrum is for a laboratory sample with Fo66
composition and particle size <60 microns. The TES spectrum,
from the olivine-bearing area in Nili Fossae, has been corrected
for atmospheric gas, dust, and water vapor. 600 to 800 cm-1 in
the TES spectrum is the location of an atmospheric CO 2 absorption
band, which gives no information about the surface and is
therefore not shown. Spectra have been offset vertically for easier
comparison.

Olivine (Mg,Fe)2SiO4 is a greenish-colored silicate mineral common in many
mafic igneous rocks (dark-colored with significant iron and magnesium
content). A piece of typical olivine basalt from Hawaii is pictured below.
Olivine is in fact a solid solution series ranging from the magnesium
end-member called forsterite, Mg2SiO4 (Fo100) to the iron end-member,
fayalite, Fe2SiO4 (Foo). The Fo value is a convenient shorthand for
describing olivine composition. Fo = mol%Mg / (mol%Mg + mol%Fe) x 100.
Spectroscopists are able to distinguish between olivine compositions because
the spectral absorption bands vary in position as a function of composition
(e.g. see also work by Jack Salisbury, Johns Hopkins University and Vicky
Hamilton, University of Hawaii ). As the Fo value increases (that is,
decreasing FeO content), olivine absorption bands shift toward higher
wavenumbers. Changing the particle size of an olivine laboratory sample does
not shift the absorption bands but does change the overall band depths.
Hence, the shapes, positions, and depths of the olivine fundamental bands
were used to map the distribution of olivine compositions on the Martian
surface.

     [Hawaiian olivine basalt rock, as an example]

     This is a photograph of a typical Hawaiian olivine basalt. The
     rock is 14 centimeters across and contains about 15 to 20%
     olivine. A weathered face oxidized to a brownish-red is just
     visible at the bottom.

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

Olivine in Nili Fossae

Hoefen and coauthors report the detection of olivine in small outcrops
distributed nearly globally between 60oS and 60oN, but the largest surface
exposure occurs in the Nili Fossae region. This region is a fractured and
cratered terrain thought to be of Noachian age (>3.5 billion years). It is
northeast of Syrtis Major--a broad, very low shield volcano with two summit
calderas whose lava flows form a plateau more than 1000 kilometers across.
Syrtis Major is thought to be of late Hesperian age (~3 billion years). Its
shape and surface features resemble Hawaiian shield volcanoes and suggest a
mafic composition consistent with a basaltic rock type. Its calderas are
thought to be located on extensions of ring fractures associated with the
Isidis impact crater basin (located to the northeast). The current
hypothesis is that the fractures, faults, and grabens (valleys between
faults) in Nili Fossae are also related to the formation of the Isidis
impact basin. Hoefen and collaborators consider that either a pre-existing,
subsurface unit of olivine-bearing basalt was exposed by the impact event
itself or by post-impact faulting and subsequent erosion, or olivine-bearing
basalts were erupted onto the surface during post-impact volcanic activity
in Nili Fossae. They favor the idea that the olivine-bearing basalt was
already in the target area before the Isidis impact and that post-impact
faulting exposed it.

The olivine mapped by Hoefen and team show a compositional range of Fo60 to
Fo70 in the southwest region of Nili Fossae. The northeast region shows
olivine ranging from Fo40 to Fo60, which corresponds to slightly higher iron
contents (see map below). This range in olivine composition is consistent
with compositions of olivine-rich Martian meteorites, such as Chassigny and
ALH A77005 (discussed further in a section below). Earlier work headed by
Jean-Pierre Bibring (l'Institut d'Astrophysique Spatiale, Orsay, France) and
John Mustard (Brown University) used the ISM imaging spectrometer
near-infrared data (with a spectral range from 0.7 to 3.1 microns) from the
Phobos 2 probe to show different spectral signatures for the eastern and
western regions of Syrtis Major, but it was not possible then, in 1990, to
determine the significance in terms of mineralogical differences.

     [Hoefen et al. olivine Mars map]

     Olivine composition mapped in the Nili Fossae region. Map on the
     left shows the location of the enlarged area shown on the right.
     Hoefen and coworkers see a trend toward lower Fo values (higher
     FeO content) to the northeast. They counted the pixels mapped as
     olivine in the map and concluded that the Nili Fossae olivine
     exposure covers about 30,000 square kilometers.

Based on the presumed age of ~3.6 billion years for the Nili Fossae region,
Hoefen and colleagues think this could be the upper limit to when the
olivine was exposed at the surface. Because olivine weathers rapidly to
clays and iron oxides, this implies that no water has flowed there since
then. Alternatively, the olivine may have been uncovered more recently, in
the past few thousand years or so, and the current cold and dry conditions
have slowed or limited chemical weathering. What's needed is a better
constraint on how long olivine can exist.

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

Olivine in Ganges Chasma in Valles Marineris

In addition to Nili Fossae, spectral signatures of olivine are also found in
the lower walls of Ganges Chasma--a several kilometers-deep side canyon at
the east end of Valles Marineris. Spectroscopists determine that the
composition of the Ganges Chasma floor is basaltic and that the walls
contain a discrete layer of basaltic rock with 10 to 15% olivine of
composition Fo68. This layer of olivine is being examined with Mars Odyssey
THEMIS data (see image below). It appears to be 50 to 100 meters thick and
is located 4.5 kilometers down from the top of the wall. Phil Christensen
and fellow THEMIS scientists have conjectured that the olivine-bearing rock
either erupted onto the surface or crystallized underground, was buried by
kilometers of rock units, and has since been exposed by erosion.
Alternatively, they say it could be a sedimentary layer enriched with
olivine. As in Nili Fossae, the presence of olivine at Ganges Chasma has
geologists pondering why it hasn't weathered away. Christensen and
colleagues working with the THEMIS data note that detection of olivine may
mean that significant subsurface weathering did not occur, despite the
potential for liquid water to be present and stable at the temperatures
expected at a depth of 4.5 kilometers and it indicates that significant
surface weathering has not occurred since the olivine-bearing layer was
exposed.

  [Christensen et al. olivine THEMIS map]

     In this false-color mosaic of Ganges Chasma (~13oS, 318oE), orange
     and red tones on the plateaus are dust, blue on the canyon floor
     is basalt, and the purple bands trending east-west in the canyon
     walls are olivine-bearing basalt. The black and white strips are
     THEMIS temperature images that lack compositional data. No
     atmospheric corrections were applied to the THEMIS data to make
     this mosaic.

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

Olivine-rich Martian Meteorites

Cosmochemists have been studying olivine in meteorites for years and are
joining with spectroscopists to look for possible source regions of the
Martian meteorites. A particularly successful collaboration involves
spectroscopists Vicky Hamilton (University of Hawaii), Phil Christensen and
Josh Bandfield (Arizona State University) with meteoriticist Hap McSween
(University of Tenneessee). Their work shows that olivine-bearing rocks in
Nili Fossae, Ganges Chasma, and other areas resemble the mineralogies of
meteorites ALH A77005 (~55% olivine) and Chassigny (~90% olivine) with ~Fo68
composition. But the age of the ancient terrains (>3.5 billion years for
Nili Fossae) is inconsistent with the ages of the meteorites (1.3 billion
years for Chassigny and 0.18 billion years for ALH A77005). The search for
meteorite-like spectra from the surface of Mars is an ongoing and exciting
endeavor.

           [Chassigny meteorite]

     Map of the mineral distribution in the Chassigny Martian
     meteorite. The map was made using an electron microprobe by
     measuring the intensities of X-rays from iron, aluminum, and
     calcium. Green is olivine, blue is pyroxene, and purple is
     feldspar. Other Martian meteorites also contain olivine, though
     not as much as in Chassigny.

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

First to Crystallize, First to Weather

Silicate minerals weather in the same sequence as they crystallize
(described by Bowen's Reaction Series). Olivine crystallizes first from a
magma (at temperatures around 1200 oC), and is the first to weather in the
presence of water. Like other silicate minerals, olivine is susceptible to
chemical weathering in the following ways: dissolution (minerals dissolve in
water), hydrolysis (minerals react with water forming clays), and oxidation
(iron-bearing minerals react with oxygen forming iron oxides or rust). The
chemical reactions occur only where the surface of the mineral and water
interact. So, the smaller the particle, the higher the ratio of surface area
to volume, and the faster the particle will weather chemically.

The reality of how susceptible olivine is to chemical weathering does not
seem to jive with its appearance on the Martian surface. Some alteration
minerals have been identified in TES spectra of the Martian surface, but not
necessarily in the olivine-bearing regions. So, the presence of olivine in
places such as Nili Fossae and Ganges Chasma apparently without a
corresponding abundance of alteration products seems inconsistent with what
we know about how fast olivine weathers. There is a demonstrated need to
better understand and quantify how long olivine has been exposed on the
surface.

Using published experimental data, University of Hawaii graduate student,
Julie Stopar, is taking the first step toward determining minimum olivine
residence times in water, that is, how fast olivine will dissolve in water.
The length of time needed to dissolve olivine grains depends on olivine
composition, particle size, temperature, and pH. The graph below shows how
long it takes olivine grains of different sizes to completely dissolve. The
grains have a composition of Fo65 and the water was assumed to be slightly
acidic with a pH of 5 (both values are thought to be typical of Mars). The
calculations show that even at low temperature, olivine should dissolve in
less than 10,000 years. Stopar and colleagues are calculating minimum
dissolution rates. Actual rates may be longer due to grain coatings or other
processes that work to slow dissolution.

[lab results of olivine dissolution rates]

     Graph shows the dissolution rates for olivine of composition Fo65,
     under pH=5 conditions, for three different temperatures, 5oC (blue
     line), 25oC (green line), and 100oC (red line).

Forthcoming PSRD articles will follow the research on Martian olivine: how
long it has been exposed on the surface, how it relates to the geochemistry
of Martian meteorites, and how it will teach us about the abundance or
longevity of water on the surface of the Red Planet. This research brings
together meteorite studies, spectroscopy of meteorites and Mars, and
experimental data on the rate and nature of olivine dissolution.

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

ADDITIONAL RESOURCES

     Bibring, J.-P. and many others (1989) Results from the ISM experiment.
     Nature, doi:10.1038/341591a0, v. 341, p. 591-593.

     Christensen, P. R. and many others (2003) Morphology and composition of
     the surface of Mars: Mars Odyssey THEMIS results. Science, v. 300, p.
     2056-2061.

     Hamilton, V. E., Christensen, P. R., McSween, Jr., H. Y., and
     Bandfield, J. L. (2003) Searching for the source regions of martian
     meteorites using MGS TES: Integrating martian meteorites into the
     global distribution of igneous materials on Mars. Meteoritics and
     Planetary Science, v. 38(6), p. 871-885.

     Hamilton, V. E., Christensen, P. R., and McSween Jr., H. Y. (1997)
     Determining the martian meteorite lithologies and mineralogies using
     vibrational spectroscopy. Journal of Geophysical Research,
     doi:10225593-25603.

     Hoefen, T. M., Clark, R. N., Bandfield, J. L., Smith, M. D., Pearl, J.
     C., and Christensen, P. R. (2003) Discovery of olivine in the Nili
     Fossae region of Mars. Science, v. 302, p. 627-630.

     Hoefen, T. M., Clark, R. N., Pearl, J. C., and Smith, M. D. (2000)
     Unique spectral features in Mars Global Surveyor Thermal Emission
     Spectra: Implications for surface mineralogy in Nili Fossae. DPS 2000
     abstract 332.

     Mustard, J. F., Bibring, J.-P., Erard, S., Fischer, E.M., Head, J. W.,
     Hurtrez, S., Langevin, Y., Pieters, C. M., and Sotin, C. J. (1990)
     Interpretation of spectral units of Isidis-Syrtis Major from
     ISM-Phobos-2 observations. LPSC XXI abstract, p. 835-836.

     Ruff, S. W. and Christensen, P. R. (2002) Bright and dark regions on
     Mars: Particle size and mineralogical characteristics based on Thermal
     Emission Spectrometer data. Journal of Geophysical Research, v. 107,
     doi: 10.1029/2001JE001580 (2002).

     Salisbury, J. W., Walter, L. S., Vergo, N., and D'aria, D. M. (1991)
     Infrared (2.1-25 microns) Spectra of Minerals. Baltimore: Johns Hopkins
     University Press, 267 p.

     Stopar, J. D., Taylor, G. J., Hamilton, V. E., Browning, L., and
     Pickett, D. (2003) Maximum rates of olivine dissolution of Mars. Sixth
     International Conference on Mars abstract no. 3151.

     Thermal Emission Spectrometer (TES) on Mars Global Surveyor (MGS).

     Thermal Emission Imaging System (THEMIS) on Mars Odyssey.
Received on Sun 09 Nov 2003 12:13:44 AM PST