http://www.psrd.hawaii.edu/Jan03/QUE93148.html
QUE 93148: A Part of the Mantle of Asteroid 4 Vesta?
Planetary Science Research Discoveries
January 23, 2003
--- A tiny meteorite tells a story of melting in the deep
mantle of a big asteroid.
Written by Christine Floss
Washington University in St. Louis
Meteorites recovered from Antarctica and other places on Earth are generally
first classified based on their mineralogies and textures. While this
approach works fairly well for large meteorites, it is quite a bit more
difficult to determine what group a meteorite belongs to when only a small
fragment is found. This is especially true when that fragment consists of
only one or two different coarse-grained minerals. Such was the case for QUE
93148 found in the Queen Alexandra Range, Antarctica in 1993. Although it
was originally classified as a lodranite, geochemical data soon showed that
it did not belong to this group. It currently appears that QUE 93148 is
related in some way to main group pallasites and may be a chip of the mantle
of the asteroid in which pallasites formed.
Reference:
Floss, Christine, (2002) Queen Alexandra Range 93148: a new type of
pyroxene pallasite? Meteoritics and Planetary Science, v. 37, p.
1129-1139.
--------------------------------------------------
Melted Meteorites and Asteroid Insides
Several types of meteorites formed by igneous (melting) activity in
asteroids. Some, such as eucrites, are pieces of lava flows. They formed
when the interior of an asteroid partially melted and the magma oozed to the
surface in dikes and erupted. (Rocks, like all complex substances, do not
melt at a single melting temperature. They partially melt over a range of
temperature and the initial melt is quite different in composition from the
rock that is melting. This leads to planetary differentiation--the formation
of a crust and underlying rocky mantle.) Some meteorites resemble the rocks
left behind inside an asteroid that melted. The magma migrated away,
traveling up to erupt, leaving behind a residue depleted in some minerals.
The lodranites, for example, are partial melting residues. In other cases
melting was very substantial and metallic iron fell to a core. At the
boundary between the core and the overlying rocky mantle, the mineral
olivine may have accumulated, producing olivine-metal mixtures called
pallasites. So, study of igneous meteorites (collectively called
achondrites) gives us a wealth of information about melting in asteroids.
One trick to all this, however, is to link the right meteorites to each
other. In the case of QUE 93148, the first step is to classify it correctly.
--------------------------------------------------
Classification of QUE 93148
QUE 93148 is a small meteorite--only 1.1 g. It is a coarse-grained
olivine-rich achondrite. In addition to olivine, it contains small amounts
of orthopyroxene and metal, with trace amounts of troilite, chromite and
phosphate. The photograph shows a thin section of QUE 93148, with two large
olivine grains partly surrounded by fusion crust, the thin layer of melted
rock formed when the meteorite blazed through Earth's atmosphere.
[Thin-section showing olivine grains]
Cross-polarized transmitted light photograph of thin section QUE93148,8.
The section contains two large olivine grains that are partly surrounded by
fusion crust. The field of view is about 6 mm across.
Based on this mineralogy and the major element compositions of the olivine
and orthopyroxene, this meteorite was initially classified as a lodranite.
However, studies done by myself and by colleagues Cyrena Goodrich from the
University of Hawaii and Kevin Righter at JSC soon showed that many of the
geochemical characteristics of QUE 93148 didn't fit with those of other
known lodranites. There are three key characteristics that indicate that
this meteorite is not a lodranite.
Trace Elements in QUE 93148
Trace elements (those which are only present in very small amounts in a
mineral) are useful for identifying what kinds of processes were involved in
forming a rock. They tend to be incompatible in most rock-forming minerals,
which means that as minerals crystallize from a magma, these elements will
accumulate in the left over magma until near the end of crystallization when
they are essentially forced into the last minerals crystallizing.
Conversely, when a rock starts to melt, incompatible elements will
preferentially go into the magma and the remaining residual rock will be
depleted in those elements. These behaviors lead to large differences in
incompatible trace element abundances in rocks and minerals that can provide
clues to the kind of processes that affected a given rock or meteorite.
I measured the concentrations of trace elements using an ion microprobe at
Washington University in St. Louis. This instrument works by focusing a
high-energy beam of oxygen ions onto a polished sample of QUE 93148. The
beam digs a hole while sputtering all the elements in the sample into a
cloud of neutral atoms and ions. The ions are accelerated into a mass
spectrometer, where they are magnetically separated by mass and then
counted. We determine the concentrations of elements in the sample by
comparison with minerals of known composition. There are some corrections to
be made, such as for interferences by assorted molecules, but analysts have
figured out how to make them accurately and routinely.
[ion microprobe]
Ion microprobe at Washington University in St. Louis.
I was initially interested in QUE 93148 because of its classification as a
lodranite. The lodranites and the related acapulcoites come from a common
parent body which experienced only a limited amount of heating. I have been
studying these meteorites to try to better understand how they are related
to each other and the types of melting processes they have undergone. The
silicate minerals in acapulcoites do not seem to have experienced any
melting, whereas some silicate melting and removal of those melts has
occurred in the lodranites. This is reflected in depleted abundances of
incompatible trace elements such as titanium (Ti) and zirconium (Zr) in the
pyroxenes of the lodranites compared to those of the acapulcoites, as shown
in the figure. When the lodranite precursor rocks (similar to acapulcoites)
melted, Ti and Zr were carried off with the magma, leaving behind rocks (the
lodranites) depleted in these elements compared to the original rock.
Plot of Ti vs. Zr abundances in QUE 93148
[Plot of Ti-Zr abundances] compared with the range of abundances observed
in acapulcoites and lodranites.
The pyroxenes from QUE 93148, however, have much lower abundances of these
elements than any of the lodranites. A simple calculation can be done to
show whether QUE 93148 is, like the lodranites, the residue of a melting
event on the acapulcoite-lodranite parent body. Tim McCoy of the Smithsonian
Institution and his colleagues have estimated that the lodranites are
residues of about 15% partial melting. As the figure below shows, much
higher degrees of melting (from about 80% to more than 100%) would be needed
to account for the Ti and Zr abundances in QUE 93148 orthopyroxene. The
mafic mineralogy (e.g., high abundance of olivine) of QUE 93148 does imply a
high degree of melting on the its parent body, but so much melting could not
have taken place on the acapulcoite-lodranite parent body, because these
meteorites still have a lot of primitive (i.e., chondritic) chemical and
mineralogical characteristics. QUE 93148 does not seem to be related to the
acapulcoites-lodranites.
[Percent partial melting curves]
The curves show the change in abundances of Ti and Zr in
residual pyroxene as a function of the amount of partial
silicate melt removed from the source. Lodranites are the
residues of about 15% melting whereas the Ti and Zr
abundances of QUE 93148 pyroxene, indicated by the arrows,
require from about 80% to more than 100% melting.
Oxygen Isotopes
The oxygen isotopic composition of QUE 93148 also indicates that this
meteorite is not a lodranite. One of the most significant observations in
meteorite research was the discovery 30 years ago, by Bob Clayton from the
University of Chicago and his colleagues, that the solar nebula has a
heterogeneous distribution of the three stable isotopes of oxygen. Since
that time, it has been established that most meteorite groups can be
distinguished on the basis of their oxygen isotopic compositions. Mass
dependent fractionation processes (such as melting and differentiation)
result in variations along a slope 1/2 line for a given planet or parent
body, whereas other deviations are inherited from inhomogeneities in the
solar nebula and are constant for a given differentiated parent body. Thus
oxygen isotopes are a powerful tool for recognizing relationships among
meteorites. Two meteorites with similar oxygen isotopic compositions share a
common oxygen reservoir, but may or may not come from the same parent body.
However, different oxygen isotopic compositions generally preclude an origin
on the same parent body. The figure below shows the oxygen isotopic
composition of QUE 93148, determined by Bob Clayton, compared to those of
several known meteorite groups.
[Oxygen isotopic compositions]
Oxygen isotopic compositions of QUE 93148 and other
achondrites.
QUE 93148 is clearly different from the lodranites (and related
acapulcoites), shown as small green squares. It's also different from other
meteorite groups such as the ureilites, the winonaites and the pyroxene
pallasites. However, notice that the oxygen isotopic composition of QUE
93148 does fall along the same mass fractionation line as several groups of
meteorites, such as the HED meteorites, main group pallasites, mesosiderites
and the brachinites.
Fe-Mn-Mg Relations
Finally, Cyrena Goodrich and her colleague Jeremy Delaney from Rutgers
University showed that the concentrations of iron (Fe), manganese (Mn), and
magnesium (Mg) can be used to determine relationships among groups of
meteorites. She and Kevin Righter compared the Fe/Mn/Mg data of olivine from
QUE 93148 with that of other achondrites (see figure below) and concluded
that QUE 93148 could not be related to the brachinites. The Fe-Mn-Mg
compositions of QUE 93148 olivine are similar to mesosiderite olivine, but
Goodrich and Righter noted that other data argue against QUE 93148 belonging
to this group. For example, QUE 93148 olivine has higher abundances of Ca
and Cr than does mesosiderite olivine. In addition, mesosiderites are
metal-silicate impact mixtures, while QUE 93148 shows clear igneous textures
in some thin sections.
[Plot of molar ratios]
Plot of the molar ratios of FeO/MgO vs. FeO/MnO in olivine
from QUE 93148 and other achondrite meteorites.
--------------------------------------------------
The HED-Pallasite-QUE 93148 Connection
The geochemical data I discussed above show that not only is QUE 93148 not a
lodranite, but it also doesn't belong to several other groups of igneous
meteorites--the ureilites, winonaites, pyroxene pallasites, brachinites or
mesosiderites. So what is QUE 93148? In the oxygen isotope diagram shown
above, QUE 93148 plots in the same region as the HED
(howardite-eucrite-diogenite) meteorites and the main group pallasites. The
HED meteorites are a large group of crustal igneous rocks consisting of
basalts, gabbros and orthopyroxenites. This is the only group of meteorites
for which a potential parent body has been identified. Tom McCord and
colleagues first showed in 1970 that the reflectance spectrum of asteroid 4
Vesta matched the spectrum of a basaltic eucrite. Since that time, there
have been countless debates over whether or not 4 Vesta is indeed the HED
parent body, but the issue remains unresolved. Nevertheless, this group of
meteorites did originate on an asteroid-sized body. The main group
pallasites have oxygen isotopic compositions similar to the HED meteorites
and Dave Mittlefehldt and his colleagues have suggested that they may have
originated on the same parent body. Pallasites are stony iron meteorites
that consist predominantly of large olivine crystals in a metal matrix (see
the photo below.) They are generally thought to represent material from the
core-mantle boundary of an asteroid. One of the hypotheses for the origin of
the HED meteorites, suggested by Kevin Righter and Michael Drake from the
University of Arizona, argues that this parent body differentiated in an
early magma ocean. Cyrena Goodrich and Kevin Righter noted that the major
element composition of olivine from QUE 93148 is similar to what would be
expected in olivine from the deep mantle of such a parent body. In addition,
the trace element data for QUE 93148 that I discussed earlier show that it
experienced high degrees of melting, as would be expected of a sample
originating on an asteroid that had melted substantially.
Pallasite sample showing rounded olivine crystals
[Pallasite sample] (brown) in a metal matrix (silver). The sample is about
8 centimeters across.
--------------------------------------------------
Is QUE 93148 a Pallasite?
In order to see if QUE 93148 might be related to the pallasites, I compared
its trace element data to that from two main group pallasites, Springwater
and Mount Vernon. I also compared my data to data obtained by Joseph
Boesenberg and his colleagues at the American Museum of Natural History for
two pyroxene pallasites (i.e., pallasites that contain small amounts of
pyroxene in addition to olivine and metal), Vermillion and Yamato 8451. The
oxygen isotope data show that QUE 93148 isn't related to the pyroxene
pallasites, but if their trace element distributions are similar this could
indicate that they formed by similar processes as the pyroxene pallasites.
[plots of Ti vs. Y and Zr]
Plots of Ti vs. Y and Zr in QUE 93148 pyroxene compared with
those in the pyroxene pallasites, Vermillion, and Yamato
8451.
The first set of figures here shows the Ti, Y (yttrium),and Zr abundances in
pyroxene from QUE 93148, with the acapulcoite and lodranite ranges shown for
reference, and also shows the abundances of these elements in pyroxene from
the two pyroxene pallasites. The second set of figures shows Mn, Y and Zr
abundances in olivine from QUE 93148, again with the acapulcoite and
lodranite ranges, and shows the abundances of these elements in olivine from
the pyroxene pallasites and also in olivine from two the main group
pallasites, Springwater and Mount Vernon.
[Plots of Mn vs. Y and Zr]
Plots of Mn vs. Y and Zr in QUE 93148 olivine compared with
those in the pyroxene pallasites, Vermillion and Yamato 8451
and in the main group pallasites Springwater (open circles)
and Mount Vernon (filled circles).
There are some differences, but both pyroxene and olivine in QUE 93148 have
elemental abundances similar to those in the pyroxene pallasites. The two
main group pallasites have elemental abundances that are quite different
from each other, and QUE 93148 olivine abundances fall between the two for
all three elements. Although it would be nice to have more data, the
available information seems to indicate that QUE 93148 is most like the
pyroxene pallasites.
--------------------------------------------------
A Piece of an Asteroid's Mantle?
QUE 93148 cannot be directly related to the pyroxene pallasites because of
their different oxygen isotopic compositions, but its oxygen isotopes are
similar to the main group pallasites (and the crustal HED meteorites). The
trace element data also show that it must have experienced a high degree of
melting, such as would occur in the deep mantle of a differentiated parent
body. Putting all the evidence together, it seems likely that QUE 93148 may
represent a new kind of pyroxene pallasite that is genetically linked to the
main group pallasites. If the main group pallasites and the HED meteorites
are indeed from the same parent body and if this parent body is in fact the
asteroid 4 Vesta (two big unresolved 'ifs') then QUE 93148 might be our
first sample from the mantle of this asteroid.
--------------------------------------------------
ADDITIONAL RESOURCES
Boesenberg J. S., Davis A. M., Prinz M., Weisberg M. K., Clayton R. N.,
and Mayeda T. K. (2000) The pyroxene pallasites, Vermillion and Yamato
8451: not quite a couple. Meteorit. Planet. Sci., v. 35, p. 757-769.
Floss C. (2002) Queen Alexandra Range 93148: a new type of pyroxene
pallasite? Meteorit. Planet. Sci., v. 37, p. 1129-1139.
Goodrich C. A. and Righter K. (2000) Petrology of unique achondrite
Queen Alexandra Range 93148. A piece of the pallasite
(howardite-eucrite-diogenite?) parent body? Meteorit. Planet. Sci., v.
35, p. 521-535.
Mittlefehldt D. W., McCoy T. J., Goodrich C. A., and Kracher A. (1998)
Non-chondritic meteorites from asteroidal bodies. In Planetary
Materials, Vol. 36 (ed. J. J. Papike), 195 pp. Mineralogical Society of
America.
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