[meteorite-list] Asteroidal Lava Flows

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 10:14:08 2004
Message-ID: <200304290409.VAA10202_at_zagami.jpl.nasa.gov>

http://www.psrd.hawaii.edu/April03/asteroidalLava.html

Asteroidal Lava Flows
--- Meteorite studies indicate that we have pieces of lava flows
from at least five asteroids.

Written by G. Jeffrey Taylor
Hawai'i Institute of Geophysics and Planetology
April 28, 2003
   
Some meteorites are pieces of lava flows. They have the expected
minerals present and the crystals are intertwined in a
characteristic way indicative of crystallization in a lava flow.
This shows that lavas erupted long ago on at least some asteroids.
Planetary scientists have recognized three main groups of asteroidal
lava flows, each distinctive enough to show that they must come from
different asteroids. The most abundant are the eucrites, which might
actually hail from asteroid 4 Vesta. Mesosiderites are complex
mixtures of smashed up volcanic rock and metallic iron. Angrites
have a distinctive group of minerals in them, but also clearly
formed by volcanism. Recent studies increase the number of groups to
five.

David Mittlefehldt (Johnson Space Center) and colleagues Marvin
Killgore (Southwest Meteorite Lab) and Michael Lee (Hernandez
Engineering, Houston, Texas) show that the five known angrites
probably represent at least two different asteroids. Four of the
angrites are fairly similar to each other in chemical composition,
but a fourth, Angra dos Reis, was very different and may come from
an entirely different asteroid. (This is ironic as the group derives
its name from Angra dos Reis.) Akira Yamaguchi (National Institute
of Polar Research, Tokyo, Japan) and colleagues in Japan at the
University of Chicago show that a recently found eucrite, Northwest
Africa 011 (NWA 011 for short), has a quite different composition of
its oxygen isotopes than the rest of the eucrites. They suggest that
NWA 011 comes from a different asteroid than the other eucrites.
Thus, it appears that we have samples of lava flows from five
different asteroids.


References:
Yamaguchi, A., Clayton, R. N., Mayeda, T. K., Ebihara, M., Oura, Y.,
Miura, Y., Haramura, H., Misawa, K., Kojima, H., and Nagao, K.
(2002) A new source of basaltic meteorites inferred from Northwest
Africa 011. Science, vol. 296, p. 334-336.

Mittlefehldt, D. W., Killgore, M., and Lee, M. T. (2002) Petrology
and geochemistry of D'Orbigny, geochemistry of Sahara 99555, and the
origin of angrites. Meteoritics and Planetary Science, vol. 32, p.
345-369.

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

Lava Flows and Eruptions on Asteroids

Even big asteroids have fairly puny gravitational fields. This means that we
have to modify how we think magma is transported inside these little planets
and how lava is erupted. This is not a one-way street, however. Making the
adjustments sharpens our understanding of how magma transport and eruptions
happen on larger planets. These comparisons are one of the great benefits of
planetary science to understanding the Earth.

Lionel Wilson (Lancaster University, Brown University, and the University of
Hawai'i--we estimate Lionel's average position as somewhere in the Eastern
United States) has studied eruptive processes for many years. He has found
that low gravity can lead to explosive volcanism on asteroids if even
relatively small quantities of gases are present. The explosive eruptions
propel drops of disrupted magma so fast that they exceed the escape velocity
of many asteroids. This means that the lava is completely lost from the
asteroid.

There is a complicated interplay of the amount of volatile substances (which
produce the gases that drive the powerful eruptions) and the size of an
asteroid (which defines the escape velocity), as shown in the illustration on
the left. This process might explain why we have pieces of lava from so few
asteroids--it was lost from those smaller than about 200 kilometers in
diameter. Of course, there are always other explanations: it is also possible
that countless impacts chipped away thick accumulations of lava from asteroids
during the billions of years since they erupted.

LEFT: Lionel Wilson and Klaus Keil (Univ. of Hawai`i) have calculated the
velocities of lava droplets erupted on asteroids of various sizes. The
diagram shows that on small asteroids (x-axis shown in kilometers) with even
moderate amounts of volatiles such as carbon dioxide, sulfur dioxide, and
other gases (y-axis shown in ppm), the droplets will erupt so fast that they
will be lost into space (area shaded dark pink). The line labeled "ncrit"
defines the critical case when volatile content of the lava is enough to
disrupt the melt into a spray of droplets lost to space. The line labeled
"V" shows the escape velocity for reference.

Though modified by impact and metamorphism, the rocks that make up the eucrites
began as lava flows on an asteroid. Matches between the spectrum of light
reflected from eucrites in the laboratory and asteroid Vesta led Thomas McCord
(then at the University of Hawai`i) and his colleagues to propose in 1970 that
the eucrites hail from Vesta. This idea has been firmed up considerably by
Richard Binzel (Massachusetts Institute of Technology) and his colleagues.
Over the past decade or so, they have found numerous smaller asteroids that
also resemble Vesta and the eucrites and whose orbital parameters are related
to Vesta. So it seems we have samples of lava flows from a known asteroid.
Vesta is large (265 kilometers in radius), so would have retained the lava
unless it contained more than 3.8 wt% of volatiles, far higher than found in
eucrites. The eruptions would still produce high fountains, but these would
fall back to the surface and coalesce into lava flows.

RIGHT: Lava flows on asteroids would likely originate from fountains of lava.
The fountains would be higher and broader than on Earth. From the work by
Wilson and Keil, we'd expect the fire fountains on an asteroid to be
relatively wide, about double their height, because the materials move out
nearly ballistically.
 
Pieces of individual lava flows from Vesta look a lot like terrestrial lava
flows. In fact, Rachel Lentz (University of Tennessee) and I have found that
eucrites most closely resemble pahoehoe lava flows, rather than `a`a flows.
Pahoehoe flows are relatively smooth, whereas `a`a flows are very rough. These
are only the superficial differences, however. The important difference lies
in how they were emplaced on the surface. The lava inside a flowing pahoehoe
flow is insulated, which leads to formation of fewer but larger crystals than
in `a`a flows. In contrast, `a`a flows are rapidly flowing, usually
incandescent streams of lava. This leads to formation of a greater number of
crystals than in pahoehoe. The resemblance of eucrites to pahoehoe flows
suggests that lava flows on Vesta are insulated. They might flow slower or
be thicker than on Earth. Whatever the details, the striking similarity
between eucrites and terrestrial lava flows (see figures below) shows that
the eucrites almost certainly formed from lava on the surface of the asteroid.

BELOW: These two photomicrographs show the similarities between a eucrite
meteorite called Millbillillie (left) and a Hawaiian pahoehoe lava flow
(right). The grayish mineral is plagioclase feldspar; more colorful mineral
is pyroxene. The similarities indicate that eucrites formed in lava flows.
Scale bars are 1 millimeter.
 
Most eucrites did not simply form in a lava flow and then rest peacefully on
the eucrite parent body. The ones that erupted earliest were buried deeply by
subsequent flows and warmed up by heat escaping from the body below them,
forming metamorphic rocks. Only the latest lavas to erupt escaped this heating.
On top of that the surface of the flows were pummeled by projectiles early in
the history of the solar system, converting the original works of volcanic art
into debris piles. Because impacting and volcanism occurred at the same time,
the debris piles from the impacts were also buried by younger flows and
subsequently metamorphosed.

Sound complicated? Not so compared to the mesosiderites. These are baffling
rocks composed of rocky debris and metallic iron. Much, though not all, of
the rocky material is some kind of volcanic rock. The volcanic rock, which
formed on the surface of the mesosiderite asteroid, was mixed with metallic
iron, which made up its core, or the core of an impacting asteroid. Somehow
during all this, little or none of the mantle of either body was added to
the mix. These mysterious meteorites will be the subject of a future PSRD
article. For now, the important point is that there was another asteroid
with a surface covered with lava flows.

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

Not All Eucrites Come from Vesta

One of the distinctive features shared by all eucrites is their oxygen
isotopic compositions. They all fall on one line in a plot of
oxygen-17/oxygen-16 vs. oxygen-18/oxygen-16 (see diagram below). Other groups
of meteorites, the Earth and Moon (which share a common trend), and Mars (as
represented by Martian meteorites) also plot in specific locations on the
diagram. Planetary scientists use oxygen isotopic composition as a fingerprint.
Rocks that derive from the same body have the same composition. If two rocks
have different compositions they probably come from different asteroids or
planets. (This assumes that all parts of a body have the same oxygen
composition, which may not be strictly true. It is most likely to be true in
the case of bodies that melted substantially. The melting would help iron out
any differences in oxygen isotopic composition between different parts of
the body.) Of course, having the same oxygen isotopic composition does not
prove that two types of meteorites come from the same asteroid.

LEFT: The oxygen isotopic compositions of several groups of meteorites plot
in distinct regions on this plot of 17O / 16O vs. 18O / 16O. The lines for
each group are parallel because on each body the oxygen isotopes were
separated according to their masses, when the rocks formed. Cosmochemists
measure the 17O / 16O and 18O / 16O ratios in terms of deviations in parts
per thousand from a standard (delta 18O and delta 17O). The usual standard
is mean ocean water, abbreviated SMOW, for Standard Mean Ocean Water.

The star symbol shows where meteorite NWA 001 plots. The abbreviations:
HEDs = a class of differentiated meteorites (howardites, eucrites, and
diogenites), Ang = angrites, Mes = silicate inclusions in mesosiderites,
Pal = pallasites, SNC = Martian meteorites, CR and CM = carbonaceous
chondrites, TF line = terrestrial (Earth and Moon) fractionation line,
and CCAM line = carbonaceous chondrite anhydrous (water-free) mineral line.

Akira Yamaguchi and his colleagues used the oxygen isotope fingerprint idea
to identify a eucrite that is not a eucrite. They studied a meteorite
classified logically as a eucrite by its appearance, even in the microscope.
It was found in Northwest Africa, and given the name NWA 011. Although the
rock does indeed resemble eucrites, it has a distinctive oxygen isotopic
composition, falling far from the eucrites and other rocks associated with
them (nicknamed the HED meteorites, for howardites, eucrites, and diogenites).
NWA 011, Yamaguchi and his colleagues conclude, comes from a different
asteroid than the other eucrites. A distinct difference in the ratio of iron
oxide to manganese oxide in the mineral pyroxene also indicates a different
place of origin.

The new type of eucrite seems to have had a similar history to most of the
main group of eucrites. It formed in a lava flow, was smashed up to form an
impact deposit, then buried and metamorphosed. This suggests to Yamaguchi
that NWA 011 formed in a body about the size of Vesta, but perhaps no longer
exists. A team of astronomers led by D. Lazzaro (National Observatory, Brazil)
have found a Vesta-like asteroid whose orbital parameters show that it is
not related in any way to Vesta.

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

The Angrites and Their Odd Namesake

So now we are up to four asteroids that provide us with volcanic meteorites:
two types of eucrites, the baffling mesosiderites, and the angrites. However,
David (Duck) Mittlefehldt and his colleagues show that Angra dos Reis does
not seem to be related to the other five angrites, in spite of lending its
name to the entire group. Perhaps we have a fifth asteroid represented here.

Mittlefehldt and his coworkers made a detailed study of a new angrite named
D'Orbigny, a 16.6-kilogram meteorite found in Argentina. They also did
chemical analyses of Sahara 99555, a 2.7-kilogram meteorite found in northern
Africa, and compare these new data to what we knew previously about the
angrites. The new angrites appear to be volcanic rocks, as shown by the
shapes and intergrowths of minerals (see photograph below) and the way the
mineral olivine is chemically zoned and rich in calcium. Zoning usually
indicates that a crystal has grown fairly rapidly from a magma that cooled
too fast to allow formation of minerals uniform in composition.

ABOVE: This microscopic view is of the meteorite D'Orbigny. Light gray and
white mineral grains are plagioclase feldspar, blue grains are olivine, and
red, greenish and tanish ones are pyroxene. The colors are caused by
interference of light waves due to viewing a thin (35 micrometers) slice of
the meteorite in polarized light. As for eucrites, the shapes of the
individual crystals and the way in which they are intergrown indicates
that the rock crystallized in a lava flow.
 
The chemical composition in individual olivine crystals varies in a
complicated way. The interiors are uniform in composition, but about half
way to the rims the composition begins to change, with the ratio of
magnesium to iron and the concentration of chromium decreasing while
calcium increases. There are even some reversals in the chemical zoning
patterns. Mittlefehldt suggests that the complicated zoning patterns are
caused by the crystallization of pyroxene in the lava because pyroxene
incorporates different amounts of iron, magnesium, calcium, and chromium
compared to the amounts incorporated in olivine. There may be an added
effect due to the addition of fresh (uncrystallized) magma to the
crystallizing portion of the lava.

The chemical compositions of angrites are strikingly different from those
of the eucrites. Mittlefehldt and his coworkers illustrate this by drawing
attention to the ratio of calcium to aluminum. The angrites have Ca/Al
ratios larger than those found in carbonaceous chondrites, which are thought
to represent the relative abundances of most elements in the solar system.
The eucrites, in contrast, have Ca/Al ratios the same as in carbonaceous
chondrites. This shift in Ca/Al cannot be attributed solely to a mineral
with a low Ca/Al ratio separating from the angrite magma. For example, the
likely mineral is plagioclase feldspar. If this mineral separates, it also
affects the ratio of samarium (Sm) to europium (Eu). In eucrites, which
seem to have been affected by plagioclase removal somehow, Sm/Eu varies
widely. This ratio is quite uniform in angrites, suggesting that there is
some other cause for the high Ca/Al. Mittlefehldt suggests it might have
been caused by the presence of a mineral rich in aluminum (such as spinel)
in the interior of the angrite asteroid, where the angrite magmas were
created by partial melting. Angra dos Reis plots in a drastically different
place from the other angrites, suggesting it formed in a different asteroid
or in a very different region on the same asteroid.

ABOVE: This plot shows the ratio of samarium to europium (Sm/Eu) versus the
ratio of calcium to aluminum (Ca/Al) in eucrites and angrites. Both ratios
have been divided by the ratio in carbonaceous chondrites. Angrites are
clearly enriched in Ca relative to Al. This enrichment is not caused by
addition or separation of plagioclase feldspar, which would affect the
Sm/Eu ratio even more, as shown by the eucrites. Note that Angra dos Reis
is much different in both Ca/Al and Sm/Eu than the other angrites. This
suggests that Angra dos Reis may have formed on a different asteroid.
 
The angrites seem to consist of two different groups: Angra dos Reis is all
by itself in one group. The other five meteorites make up the other group.
Angra dos Reis has been metamorphosed, whereas the others have not been.
Angra dos Reis is also chemically distinctive. These factors point to an
origin on a different asteroid. If so, perhaps it has a different mix of
oxygen isotopes than do the other angrites, allowing us to use the same
argument for distinctive origin as Akira Yamaguchi used to identify a new
type of eucrite. No such luck. All six angrites have the same mix of oxygen
isotopes. Mittlefehldt and coworkers suggest that the oxygen isotope test
might not be definitive in this case because quite a few different meteorite
groups have oxygen isotopic compositions in this region of the oxygen
isotope plot. (Angrites are labeled "Ang" in the oxygen isotope plot above.)

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

Diverse Histories of Asteroidal Lava Flows

We cannot be sure that the angrites represent two separate asteroids, but
the case is pretty strong. This means we may have pieces of lava flows from
five different asteroids. All originated in volcanic eruptions, but some
groups seem to have been smashed up by impacts more than others, and some
have been metamorphosed by burial and others have not been, as shown in
the table below.


Meteorite Group Volcanic? Brecciated by Impacts? Metamorphosed?
Eucrites Yes Almost All Almost All
Eucrite-2 (NWA 011) Yes Yes Yes
Angrites (except one) Yes No No
Angra dos Reis Yes No Yes
Mesosiderites Yes Yes Yes

Why do the five asteroids have such different histories? One possibility
is that we do not have enough samples from some of them to be sure that
the observations apply to an entire asteroid. We have plenty of eucrites
and mesosiderites, but only one NWA 011 eucirte and six angrites,
including Andra dos Reis. Lets assume that these meteorites are
representative of the asteroids on which they formed. If so, why did the
two angrite asteroids escape impact bombardment? Or did just a surviving
remnant of it escape impact reworking and that is the piece providing us
with meteorites. Why are all of them except the angrites (excluding Angra
dos Reis) metamorphosed? Were their parent asteroids large enough for this
to occur on? If the angrite asteroid was smaller, why did it retain the
erupting lava? Answers to these questions await discoveries of additional
meteorites in each category, detailed astronomical observations of asteroids,
and perhaps missions (including sample-return missions) to a bunch of
asteroids.

--------------------------------------------------------------------------------
Additional Resources

Lentz, R. C. F. and Taylor, G. J. (2002) Petrographic textures and insights
into basaltic lava flow emplacement on Earth, the Moon, and Vesta. Lunar
and Planetary Science Conference XXXIII, abstract 1332.pdf.

Mittlefehldt, D. W., Killgore, M., and Lee, M. T. (2002) Petrology and
geochemistry of D'Orbigny, geochemistry of Sahara 99555, and the origin
of angrites. Meteoritics and Planetary Science, vol. 32, p. 345-369.

Wilson, L. and Keil, K. (1991) Consequences of explosive eruptions on
small Solar System bodies: the case of the missing basalts on the aubrite
parent body. Earth and Planetary Science Letters, v. 104, p. 505-512.

Yamaguchi, A., Clayton, R. N., Mayeda, T. K., Ebihara, M., Oura, Y., Miura,
Y., Haramura, H., Misawa, K., Kojima, H., and Nagao, K. (2002) A new
source of basaltic meteorites inferred from Northwest Africa 011.
Science, vol. 296, p. 334-336.
Received on Tue 29 Apr 2003 12:09:12 AM PDT