[meteorite-list] The Multifarious Martian Mantle

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Jun 24 18:55:47 2004
Message-ID: <200406242028.NAA11048_at_zagami.jpl.nasa.gov>

http://www.psrd.hawaii.edu/June04/martianMantle.html

The Multifarious Martian Mantle
Planetary Science Research Discoveries
June 23, 2004

--- Detailed analyses of Martian meteorites reveal that the planet's
interior preserves distinctive regions that formed 4.5 billion years ago.

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

Pieces of relatively young lava flows from Mars (all less than 600
million years old) preserve a record of the planet's initial segregation
into core, mantle, and crust. Research by Lars Borg (University of New
Mexico), his colleague David Draper, and his former colleagues at the
Johnson Space Center, Chris Herd (University of Alberta, Canada), and
Cyrena Goodrich (formerly at the University of Hawaii and now at
Kingsborough Community College in Brooklyn, New York) shows that there
are distinctive regions in the interior of Mars. These regions, or
reservoirs as cosmochemists like to call them, formed early, about 4.5
billion years ago, and come in two flavors. One, dubbed "enriched,"
contains high concentrations of trace elements, has a high ratio of
lanthanum to ytterbium (La/Yb), high strontium-87 to strontium-86
(87Sr/86Sr), a low ratio of neodynmium-143 to neodynmium-144
(143Nd/144Nd), and is relatively oxidized. The other, dubbed "depleted,"
contains lower levels of trace elements, has lower La/Yb and 87Sr/86Sr,
higher 143Nd/144Nd, and is relatively reduced (much less oxidizing than
the enriched reservoir). There are mixtures in between these extremes.
The reservoirs may have formed in a global magma ocean. Their
preservation for 4.5 billion years indicates that Mars, in contrast to
Earth, did not have active plate tectonics since the reservoirs formed.

References:

Borg, L. E., Nyquist, L. E., Weissman, H., Shih, C.-Y., and Reese, Y.
(2003) The age of Dar al Gani 476 and the differentiation history of the
martian meteorites inferred from their radiogenic isotopic systematics.
Geochimica Cosmochimca Acta, v. 67, p. 3519-3536.

Borg, L. E. and Draper, D. S. (2003) A petrogenetic model for the origin
and compositional variation of the martian basaltic meteorites.
Meteoritics & Planetary Science, v. 38, p. 1713-1731.

Goodrich, C. A., Herd, C. D. K., and Taylor, L. A. (2003) Spinels and
oxygen fugacity in olivine-phyric and lherzolitic shergottites.
Meteoritics & Planetary Science, v. 38, p.1773-1792.

Herd, C. D. K. (2003) The oxygen fugacity of olivine-phyric martian
basalts and the components within the mantle and crust of Mars.
Meteoritics & Planetary Science, v. 38, p. 1793-1805.

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

Samples of Martian Lava Flows

Much of the surface of Mars is made of volcanoes or lava plains, or
sedimentary rocks made from them. River valleys and canyons dissect
volcanoes and lava flows, and long ago, before about 4 billion years
ago, chunks of asteroids and comets blasted the surface to make the
cratered Martian highlands. Nevertheless, remnants of the lavas remain.
More recent impacts flung bits of lava flows and other types of igneous
rocks off the planet. Some of the ejected rocks made their way to Earth;
we've found about 30 of them. These free samples of Mars are invaluable
in helping us understand the geological history of the Red Planet.

The main evidence that the rocks actually come from Mars is from trapped
gases in three of them. The mixture of gases is a striking match for the
Martian atmosphere as measured by the Viking landers in the mid-1970s.
All of the Martian meteorites share a unique oxygen isotopic
composition, linking them to each other and to the three with trapped
Mars air in them.

There are several groups of Martian meteorites. The most common,
shergottites, are basalts, a common type of lava flow. The shergottites
might be chips of thick surface flows or solidified magma that cooled in
dikes below the surface. They have a wide range in chemical and
mineralogical compositions. They tend to have much less plagioclase
feldspar than do typical terrestrial basalts, but like basalts on all
the planets they formed by partial melting of the interior. Quite a few
processes can operate as a magma oozes through a hundred kilometers of
rock to the surface, but we can often see through these processes-in
fact, we can learn a lot about them-to deduce the chemical composition
and much about the origin of the rocks making up the interior, or
mantle. Lars Borg, Chris Herd, and their colleagues have been able to
determine the time of initial differentiation of Mars, hence the time
the mantle formed, and to deduce the variations in the composition
within the mantle.

[sample of olivine-phyric shergottite]

Polished sample of a shergottite, Sayh al Uhaymir 005, which was found
in northern Africa. The image is an x-ray map obtained in an electron
microprobe. Light green is olivine, dark green is pyroxene (actually two
different minerals, one higher in calcium than the other), and pink is
maskelynite (plagioclase feldspar shocked impact to a glass).
Meteoriticists call such olivine-rich shergottites "olivine-phyric
shergottites" to distinguish them from shergottites (similar but with
little or no olivine) and lherzolitic shergotittes (lots of olivine and
pyroxene, little plagioclase, and probably not lava flows).

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

Young Meteorites, Old Mantle

The shergottites come from lavas that erupted between about 150 million
years and 500 million years ago. Nakhlites (pyroxene-rich rocks that
formed in different types of lava flows from Shergottites) and the
meteorite Chassigny are older, 1300 million years (1.3 billion years).
The oldest Martian meteorite we have identified is Allan Hills 84001,
which formed from a magma intruded deep in the Martian crust 4.5 billion
years ago.

Although the shergottites are all quite young compared to the 4.55
billion years age of the planets, they nevertheless contain a clear
record of the time when their mantle source regions formed.
(Cosmochemists refer to the region in a planet's mantle that melted to
give rise to a magma as the magma's source region.) Lars Borg and his
colleagues show this in two ways. The first was to investigate the
isotopes of rubidium (Rb) and strontium (Sr). This requires painstaking
work in ultraclean laboratories to prevent contamination. The samples
are crushed, minerals separated, and each separate dissolved in assorted
acids, run through a process that separates Rb from Sr, and isotopes
measured by mass spectrometry.

[Dr. Borg in the clean lab LEFT: Lars Borg in the isotope clean lab at
the Johnson Space Center.]

Using their own and analyses from other cosmochemists, Borg and his
colleagues plotted the 87Rb/86Sr ratio in each rock against the
87Sr/86Sr ratio (see graph below). 87Rb is radioactive and as time
passes it decays to 87Sr. If all the mantle sources for the rocks had
the same 87Rb/86Sr but different amounts of Rb and Sr, they would have
formed a horizontal line initially. With time, 87Rb would decay to 87Sr,
leading to a line with a slope. The slope corresponds to the age of the
mantle sources. If the individual points all plot close to a single
line, it suggests that they had a simple history: All their mantle
sources (hence much, perhaps most of the Martian mantle) formed
simultaneously at the time indicated by the slope of the line. Each
source subsequently melted, but at different times, producing lavas 150
to 575 million years ago (the ages of the shergottites). The slope of
the line drawn between the point for Que94201 and Shergotty, which most
of the other points lie close to, indicates an age of 4.49 billion
years. (The points falling off the line are the nakhlites.) Note that
the values of 87Rb/86Sr, which reflect the elemental ratio of Rb/Sr,
range quite widely. I return to this important observation below.

whole rock Rb=Sr isotopic data Whole rock Rb-Sr isotopic data for
shergottites and nakhlites. The shergottites all fall along a line
suggesting an ancient age of 4.49 billion years. This age may represent
the time when distinct sources formed in the Martian mantle.

Cosmochemists get age information from samarium (Sm) and neodymium (Nd),
two rare earth elements. Sm has two radioactive elements. 147Sm decays
to 143Nd with a long half-life of 106 billion years. On the other hand,
146Sm decays to 142Nd with a much shorter half-life, only 103 million
years. 146Sm decays fast enough that it would be almost completely
transformed to 142Nd in about five half lives, about 500 million years
(0.5 billion years). The shergottites have clear evidence that this
short-lived isotope was present in their mantle sources, so the sources
formed before 4.0 billion years ago.

We have two speedometers here, one recording city driving, the other
highway driving. Combined they ought to give us a more complete record
of shergottite driving habits, i.e., the ages of their mantle sources.
In December, 2002, Lars Borg was pondering these Sm-Nd isotopes
systematics as he and his dog, Meka (who should have been named
Isochron), were driving from New Mexico to his wife's parents' home in
California. His wife and young child had flown to the west coast,
leaving him alone with his thoughts. (Meka is not particularly
talkative.) While driving through high, colorful deserts and low, hot,
barren ones, Borg worked out that he could use both the long-lived and
short-lived Sm isotopes to define the age of the shergottite mantle
sources even better than the Rb-Sr data had, or at least make an
independent assessment of it.

Borg realized he could use three parameters to assess the age of
differentiation of shergottite mantle sources. One is the initial ratio
of 143Nd/144Nd in each shergottite, expressed as epsilon-143Nd, a
measure of how the ratio deviates from the ratio in chondritic
meteorites, in parts per thousand. The other is the initial ratio of
142Nd/143Nd, or epsilon-142Nd, the deviation of the ratio from
chondritic meteorites, again in parts per thousand. He also needed to
know the ratio of 147Sm/144Nd in the source regions. Assuming that all
the epsilon values were initially like those in chondrites, that
formation of the sources involved formation of reservoirs with different
Sm/Nd ratios, and that the sources melted only once more (when each
shergottite magma formed), Borg knew he could calculate lines of equal
age (isochrons) for different times. By plotting data from shergottites
on the resulting complicated graph, Borg hoped to find the age of mantle
differentiation.

[two-stage Nd isotope evolution]

Lars Borg devised this plot of the variation in Nd isotopes with time
and ratio of Sm/Nd after mantle formation, assuming a simple two-stage
history for Martian meteorites (early mantle formation followed billions
of years later by magma formation). The circles are for shergottites
(except the one labeled "ALH," which is not a basalt) and the squares
are for the nakhlites and chassigny. The filled circles fall along an
isochron indicating an age of 4.51 billion years, which Borg and his
colleagues take as the age of formation of the shergottite source
regions. The points off the line, including Shergotty and the nakhlites,
probably had histories more complicated than the simple two-stage
history upon which the isochron lines are based.

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

Distinctive Mantle Reservoirs

The isotopic data shows that the mantle sources for the shergottites
formed at about 4.5 billion years ago, soon after Mars formed. To retain
a record of that important event, much of the mantle cannot have been
disturbed until shergottite magma formation 150 to 500 million years
ago. This suggests that, unlike the Earth, there was not extensive
mixing of the mantle or recycling of the crust back into it, unless that
took place before 4.5 billion years ago.

Other data allow Borg and his coworkers to identify other chemical
properties of the mantle sources, or reservoirs. A distinctive
characteristic of shergottites is that they have a range of rare earth
element concentrations, especially in the concentration of light rare
earth elements (those with lower atomic weights). This can be expressed
in a shorthand way by the ratio of lanthanum (La) to ytterbium (Yb).
Specifically, La/Yb correlates with initial 87Sr/86Sr (see diagram
below) and with epsilon-143Nd. The samples with high La/Yb (about the
same as in chondritic meteorites) come from "enriched" mantle
reservoirs. Those with low La/Yb come from "depleted" reservoirs. The
depleted reservoirs have low initial 87Sr/86Sr and high epsilon-143Nd.
Scott McLennan (State University of New York, Stony Brook) also
identified distinctive mantle reservoirs by comparing trace element
concentrations in Martian meteorites.

[initial Sr/Sr correlation with La/Yb]

Initial 87Sr/86Sr correlates with the extent to which La/Yb is depleted
compared to chondritic (C1) values. Enriched sources have high 87Sr/86Sr
because they had higher 87Rb/86Sr initially, hence produced more 87Sr by
decay of radioactive 87Rb.

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

Oxidation State of Mantle Reservoirs

The shergottites reveal another fundamental characteristic of the
distinctive mantle reservoirs: the enriched reservoirs are significantly
more oxidizing than the depleted ones. This is revealed by determination
of a parameter called the oxygen fugacity, which is related to the
partial pressure of oxygen available in a rock to oxidize elements that
can occur in more than one valence state. Iron is particularly affected.
Under highly oxidizing conditions it can be all Fe3+ (trivalent). Under
somewhat reducing conditions it might be all Fe2+. When conditions are
very reducing, much of the iron might be metallic (Fe0).

FeTi oxide pair Cosmochemists determine oxygen fugacity by measuring the
composition of iron-bearing minerals in a rock. Specifically, they
determine the amount of Fe2+ and Fe3+ in each mineral. Examples are two
iron-titanium oxide minerals (see photo on the left) or an iron-bearing
oxide called spinel associated with pyroxene and olivine (also both
iron-bearing minerals). It requires very careful observations of the
mineralogy and the way minerals mingle in rocks to choose the right
minerals to analyze. It also requires extremely accurate analyses by
electron microprobe because the amount of Fe3+ cannot be determined
directly-only total Fe can be measured. The amount of di- and tri-valent
iron is determined by calculating the precise formula for a mineral and
assuming the analysis is perfect (the total of all elements is exactly
100 percent).

LEFT: The distribution of Fe3+ and Fe2+ between iron-titanium oxide
minerals ilmenite and ulv?spinel can be used to determine oxygen
fugacity. This shows co-existing oxide and other minerals in the
shergottite EET 79001. The image is a backscattered electron image taken
with a scanning electron microscope. [Image is 150 microns across.]

It takes careful workers like Chris Herd, Cyrena Goodrich, and Larry
Taylor (University of Tennessee) to do that type of work. Their hard
work paid off. They found that oxygen fugacity also correlates with
La/Yb (see diagram below). The enriched reservoir (high La/Yb) is more
oxidizing than the depleted one, by a factor of about 1000. (The oxygen
is usually reported in logarithm units of atmospheres, but often
relative to the fugacity of a known equilibrium assemblage. In the plot
below, from Chris Herd's paper, it is reported relative to the
quartz-fayalite-magnetite (QFM) assemblage, or buffer.)

[Ba/Yb versus oxygen fugacity]

LEFT: Graph of La/Yb (normalized to the ratio in carbonaceous
chondrites) versus oxygen fugacity (compared to the
quartz-fayalite-magnetite QFM buffer assemblage). Enriched (high La/Yb)
mantle sources are substantially more oxidizing than depleted (low
La/Yb) mantle sources.

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

Formation of Mantle Reservoirs

Mars appears to have at least two distinct mantle reservoirs, and
probably others. The table below summarizes the properties of the two
distinct reservoirs tapped by magmas that created the shergottites.

Enriched Reservoir high La/Yb
low Sm/Nd (-?Nd)
high Rb (high 87Sr/86Sr)
oxidized
Depleted Reservoir low La/Yb
high Sm/Nd (+?Nd)
low Rb (low 87Sr/86Sr)
reduced

In papers published a couple of years ago, Borg, Herd, and company
called the enriched reservoir "crust-like." This followed our thinking
about planetary crusts being enriched in trace elements compared to the
underlying mantles. It is also consistent with observations of the Mars
Odyssey Gamma-Ray Spectrometer, which shows higher levels of K and Th
(both elements that behave like rare earth elements) compared to Martian
meteorites. However, placing the enriched component in the crust
required that depleted shergottite magmas interacted with the crustal
rocks to become more oxidized, enriched in trace elements and 87Sr/86Sr,
and lower in Sm/Nd in very precise ways that are require disparate
chemical properties and processes to precisely balance for each magma,
an unlikely event. The solution was to put the reservoirs into the mantle.

[basalt formation on Mars model]

Model for shergottite (and Martian basalts in general) formation in the
Martian mantle. Two reservoirs were produced early in Martian history.
One is depleted in trace elements and is highly reducing (almost at the
iron metal-iron oxide buffer "IW"). The other is enriched in trace
elements and oxidizing. It contains substantially more Fe3+ than does
the depleted reservoir, but might also contain H2O (though this is not
necessary to cause the oxidation state to be higher than in the depleted
reservoir).

How did the reservoirs form? The very old age of the reservoirs drive
Borg and others to suggest that Mars was surrounded by an ocean of magma
when it formed, as most planetary scientists think happened to the Moon
when it formed. [See magma ocean illustration in PSRD article The Oldest
Moon Rocks <http://www.psrd.hawaii.edu/April04/lunarAnorthosites.html>.]
If so, as the magma ocean began to crystallize, the first minerals would
not take up much rare earth elements (including La, Sm, Nd, and Yb) or
Rb. As crystallization continued the magma would become progressively
enriched in elements excluded from the crystallizing solids, perhaps
reaching values a hundred times higher. This could produce a large range
from depleted mantle to highly enriched. And, because Fe3+ tends to be
excluded from crystallizing minerals more than Fe2+ does, the ratio of
Fe3+ to Fe2+ increases, making the enriched reservoirs more oxidizing.
If H2O were present, it would also concentrate in the enriched reservoirs.

Lars Borg and Dave Draper, his colleague at the University of New
Mexico, modeled this process quantitatively. Such calculations have been
done to try to understand the lunar magma ocean. In the Martian case,
the depth of the magma ocean matters a lot because the deeper the magma
ocean, the higher the pressure near its base. The pressure difference
results in different minerals crystallizing and those minerals differ in
the extent to which they exclude or include trace elements. This
research is still in its early stages, but the basic approach is leading
to hypotheses testable by analyses of Martian meteorites.

[model of the interior of Mars]

A likely picture of the interior of Mars, based on experiments by
Constance Bertka and Yingwei Fei. The uppermost mantle of Mars consists
of olivine and pyroxene, with a small amount of garnet (shaded green).
These are fairly common minerals on Earth, the other planets, the Moon,
and asteroids. However, at a depth of about 1100 km, the olivine begins
to convert to a more dense form, called gamma-spinel, without changing
its chemical composition. The conversion is complete by 1300 km. Along
with the conversion of olivine to a spinel crystal structure, garnet and
pyroxene convert to a mineral called majorite, which has a crystal
structure like garnet, but is close to pyroxene in chemical composition
(shaded yellow). At higher pressures, hence deeper, there is a
relatively abrupt transition at 1850 km (shaded black) to a mixture of
perovskite (itself a mixture chemically of MgSiO3 and FeSiO3) and
magnesiowustite (a mixture of FeO and MgO). The metallic core (shaded
gray) begins at about 2000 km depth and continues to the center at a
depth of 3390 km.

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

[lunar magma crystallization models]

Cartoon showing the crystallization sequences for a lunar magma ocean
(after Gregory Snyder, then at the University of Tennessee) and three
Martian magma oceans with different depths. Higher pressure in the
deeper oceans results in different element distributions.

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

More Data Needed

It is astonishing that cosmochemists can determine so much about the
formation of the Martian mantle from a few rocks that formed four
billion years after the mantle did! Yet the unfolding story is
reasonable. One test of the ideas will come from analysis of the
composition of the Martian crust as determined by the Mars Odyssey Gamma
Ray Spectrometer (papers are being written during June and July, 2004).
The GRS data, particularly for the important trace elements K and Th,
will allow us to compare to the compositions of shergottites and to
assess how consistent the entire crustal composition is with the idea of
enriched and depleted mantle reservoirs.

To fully assess the reservoirs, their ages, and models for their
formation we need more samples, particularly basalts from the ancient
highlands. We can hope to find more meteorites (the search for Antarctic
meteorites now fields two teams each Antarctic summer), but we might
still be stuck to whatever was blasted off Mars during the past few
million years. Ideally, we will obtain more samples of Mars by sending
robotic (and eventually piloted) spacecraft to Mars to return samples
for study in labs here on Earth. Testing our ideas of mantle reservoirs
and their formation requires the highly precise analyses that can be
done only on earth, at least for now.

[paintings of two concepts for Mars vehicles]

NASA is tentatively planning to launch a sample-return mission to Mars
in 2013. These paintings show two concepts for how the return vehicle
will blast off. Note that both images have rovers. Recent experience
with the Spirit and Opportunity rovers shows the need for mobility to
collect the best possible samples.

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

ADDITIONAL RESOURCES

Borg, L. E., Nyquist, L. E., Weissman, H., Shih, C.-Y., and Reese, Y.
(2003) The age of Dar al Gani 476 and the differentiation history of the
martian meteorites inferred from their radiogenic isotopic systematics.
Geochimica Cosmochimca Acta, v. 67, p. 3519-3536.

Borg, L. E. and Draper, D. S. (2003) A petrogenetic model for the origin
and compositional variation of the martian basaltic meteorites.
Meteoritics & Planetary Science, v. 38, p. 1713-1731.

Goodrich, C. A., Herd, C. D. K., and Taylor, L. A. (2003) Spinels and
oxygen fugacity in olivine-phyric and lherzolitic shergottites.
Meteoritics & Planetary Science, v. 38, p.1773-1792.

Herd, C. D. K. (2003) The oxygen fugacity of olivine-phyric martian
basalts and the components within the mantle and crust of Mars.
Meteoritics & Planetary Science, v. 38, p. 1793-1805.

McLennan, S. M. (2003) Large-ion lithophile element fractionation during
the early differentiation of Mars and the composition of the martian
primitive mantle. Meteoritics and Planetary Science, v. 38, p. 895-904.
Received on Thu 24 Jun 2004 04:28:04 PM PDT