[meteorite-list] The Complicated Geologic History of Asteroid 4 Vesta

[Ron Baalke]

http://www.psrd.hawaii.edu/June09/Vesta.granite-like.html

*The Complicated Geologic History of Asteroid 4 Vesta*
Planetary Science Research Discoveries
June 25, 2008

--- Meteorites from asteroid 4 Vesta show that it contains patches of
granite-like rock.

Written by G. Jeffrey Taylor
Hawaii Institute of Geophysics and Planetology

Planetary scientists are pretty sure that almost all of the HED
meteorites come from the fourth-largest asteroid, 4 Vesta. HED stands
for the three types of rocks that make up the group. As cosmochemists
have studied the meteorites over the years, their view of the geologic
history of the asteroid has become progressively more complicated.
Jean-Alix Barrat and Marcel Bohn (CNRS and University of Brest, France),
Philippe Gillet (CNRS and Ecole Normale Supérieure de Lyon, France), and
Akira Yamaguchi (National Institute of Polar Research, Tokyo, Japan)
have found that Vesta is even more complicated--and interesting--than we
thought.

Barrat and his colleagues analyzed impact-produced glass spherules and
fragments in several howardites (mixtures of the two other main types,
eucrites and diogenites). Not surprisingly, most of the glasses have
compositions similar to eucrites, which are basalts, or mixtures of them
with diogenites, but a few are surprisingly rich in silicon and
potassium (expressed as the percentages of SiO_2 and K_2 O,
respectively). In fact, the concentrations of these oxides are similar
to granites, compositionally far from basalt. Their manufacture requires
extensive crystallization of magma. Combined with a more subtle
variation among the basalts, the emerging picture is one that includes
formation of a basaltic crust, partial melting of parts of the crust,
mixing of those melts with some (not all) magmas as they migrated
through the crust, and extensive crystallization of magma bodies to
produce residual magma resembling granites. On top of that, Vestan rocks
were thermally metamorphosed and battered and mixed by impacts. A pretty
complicated little planetary body!

*Reference:*

    * Barrat, J. A., Bohn, M., Gillet, Ph., and Yamaguchi, A. (2009)
      Evidence for K-rich Terranes on Vesta from Impact Spherules,
      /Meteoritics and Planetary Science,/ v. 44, p. 359-374.

*PSRDpresents:* The Complicated Geologic History of Asteroid 4
Vesta--Short Slide Summary <PSRD-Vesta.granite-like.ppt> (with
accompanying notes).

------------------------------------------------------------------------
*Howardites, Eucrites, Diogenites: Chunks of Vesta*

One group of meteorites from Vesta is called eucrites,
the E in HED. They are mostly basalts.
In the microscope, they resemble terrestrial lava flows, and for a long
time cosmochemists figured that eucrites formed the same way terrestrial
basalts form, by partial melting of the interior. However, assorted
geochemical arguments, especially the concentrations of siderophile
elements (they concentrate in
metallic iron), suggest that Vesta was totally melted (or nearly so)
when it formed. As it crystallized, the last 10-20% of magma would have
a composition like the eucrites. Of course, somehow that magma has to be
squirted onto the surface to make lava flows. The kinks have not been
ironed out of the idea. Nevertheless, it is clear that eucrites formed
as lava flows. Their ages indicate that it happened 4.5 billion years ago.

The other main igneous rock type from Vesta is diogenite. These are
composed almost entirely of orthopyroxene, so must represent
accumulations of this mineral from a slowly-cooling magma. This might
have been the magma ocean, but that is not certain. The howardites are
impact-produced mixtures of the other two rock types, completing the
Howardite, Eucrite, Diogenite suite, the HED meteorites.

I keep saying that the HEDs come from asteroid 4 Vesta. How do we know
that? The answer comes from astronomical measurements of the spectral
characteristic of light reflected from the asteroid's surface, which
looks very much like measurements of HED meteorites in the lab. See
*PSRD* article: Getting to Know Vesta <http://www.psrd.hawaii.edu/Nov07/HEDs-Vesta.html> for
more detail.

reflectance spectrum of Vesta from Earth-based telescope
This is a spectral plot of Vesta obtained at the NASA Infrared Telescope
Facility on Mauna Kea, Hawai'i by scientists in France using remote
control networks from l'Observatoire de Paris-Meudon. In particular, it
is the presence of the 0.9 and 1.9 micrometer absorption bands for
pyroxene in the spectra of Vesta that match spectra of the HED meteorites.

Over the years, cosmochemists have studied the HED meteorites in detail,
making subdivisions (as cosmochemists are want to do!). One of the most
interesting subdivisions is the identification of two distinct chemical
trends among the eucrites, illustrated in the diagram below. One is
called the main group trend, which is also called the Nuevo Laredo trend
after a prominent member of it. It is characterized by lower
concentrations of rare earth elements (such as lanthanum) than the
Stannen trend (named after the eucrite Stannern).

Plot showing two distinct trends, indicating two different suites of
igneous rocks from Vesta.
A plot of lanthanum (La) versus the ratio the oxides of iron to
magnesium (FeO/MgO). Note the two distinct trends among the eucrites,
indicating two different suites of igneous rocks from Vesta.

------------------------------------------------------------------------
*Glassy Spherules*

*T*he howardites are impact breccias. They are mixtures of almost all
the rocky components making up the crust of Vesta. Some of them contain
glassy spherules and glass fragments formed by impact. Jean-Alix Barrat
and his colleagues searched for them in eight howardites and found
numerous objects. Some are partly crystallized, but they found 61
fragments and spherules that were almost entirely glass and they report
the chemical compositions of them in their paper.

Glass spherule in the howardite NWA 1769. 	Glass spherule in the
howardite NWA 1769. It is surrounded by mineral fragments. The image is
of the intensity of backscattered electrons, taken with an electron
microprobe.

Most of the glassy objects have compositions like those of eucrites or
mixtures of them with diogenites, as shown in the diagram below.
(Cosmochemists and geochemists are fond of this diagram, which plots
weight percentages of total alkalis and silica (Na_2 O+K_2 O vs SiO_2 ).
It is called a TAS diagram and is used widely to classify igneous
rocks.) However, note that quite a few glass samples have elevated
alkalis compared to typical eucrite rocks (solid symbols in the
diagram). More importantly, note that two of them plot in the dacite
field, substantially enriched in alkalis and silica compared to the
basalts. In fact, they correspond roughly to terrestrial granites, but
formed in a different way.

TAS diagram to classify eucrites and impact glasses from howardites.
This TAS diagram shows total alkalis (oxides of potassium and sodium)
versus silicon dioxide concentration (all in weight percent) for
eucrites and impact glasses from howardites. The fields show where
different types of terrestrial rocks plot. Note that many of the glass
samples (open circle symbols) plot close to and on top of the eucrite
basalt samples (solid symbols), but some have elevated alkalis, and two
are drastically different with high SiO_2 and alkalis. These two
(indicated by blue arrows) plot in the dacite field, which along with
rhyolites correspond to the compositions of granite and similar rocks on
Earth. Data are from the literature for eucrites and from Barrat /et
al./ (2009) for the glass samples.

Barrat and his colleagues show that the glass compositions represent
impact melts, not unusual condensates from impact-produced vapor, and
that they have not lost any elements while they were hot droplets of
magma. They also show through calculations called chemical mixing models
that some of the glasses enriched in potassium but not silica could be
mixtures of typical eucrites and diogenites with granite, but many are
not. It appears that there is yet another chemical component lurking in
the crust of Vesta.

------------------------------------------------------------------------
*Making Granite the Hard Way*

*E*arth is home to almost all the granite in the solar system. Almost
all of it forms by remelting of pre-existing crust, either sediments or
by crystallization of somewhat silica-enriched magmas formed when an
oceanic plate subducts beneath a continental plate or another oceanic
plate. It makes vast bodies of rock making up major mountain ranges such
as the Sierra Nevada in the western United States.

The Vestan granites are not at all like terrestrial granites. They are
not abundant on Vesta and do not contain water-bearing minerals such as
mica that characterize terrestrial granite. The Vestan ones are more
like lunar granites. Like the Vestan samples, the lunar ones are also
rare and water-free. They are also small--the largest sample weighs only
1.8 grams! It is quite possible, however, that regions of the Moon tens
of kilometers across are made of granite-like rock. We need better
remote sensing data to know for sure.

Photomicrograph of felsite 14321,1047. 	This photomicrograph shows
graphic intergrowth of quartz (Qtz) and potassium feldspar (Kspar) in a
lunar granite, sample 14321,1047.

A lot of us have done a lot of work on lunar granites (also called
"felsites" to distinguish them from mountains of terrestrial granites).
Ideas for their origin fall into two main categories: Extensive
crystallization of basaltic magma and partial melting of the lunar crust
by intrusions of basalt causing partial melting. When magmas
crystallize, minerals are often removed by sinking or other processes
and so do not react with the magma any further. Removal of crystals
changes the composition of the magmatic system, a process called
fractional crystallization. Continued fractional crystallization leads
to progressively smaller amounts of magma that is progressively
different in composition from the original magma. Depending on the
chemical composition of the parent magma, the evolved magma might
resemble the granites. In other cases, the magma enters an unusual
compositional field that promotes separation of two magmas, one iron
rich, the other rich in silica and alkalis. In either case, extensive
crystallization can lead to formation of granite-like magma. Barrat and
his colleagues conclude that this is a feasible way to make the patches
of granite in the crust of Vesta.

Looking at it in the opposite way, if a rock is heated enough it will
partially melt. A pre-existing basalt could partially melt to form a
granite-like magma, although the composition of the rock has to be in
the right range. Heat for melting can come from the injection of large
amounts of hot magma, which heats a region of the crust, forming patches
of granites. There is clear evidence that partial melting of
pre-existing rocks occurred on Vesta. For example, Akira Yamaguchi and
colleagues (including me) showed in 2001 that eucrite EET 90020 had
clear evidence that it partially melted. So, the conditions for the
partial melting mechanism certainly existed, but Barrat and colleagues
do not think this is feasible for producing the granites. They point out
that when an eucrite is partially melted, the initial magma is not like
the granitic impact glasses. It is not rich enough in potassium.

So, we are not sure about how Vestan granite-like rocks formed, how
abundant they are, or the size of the outcrops. To answer those
questions cosmochemists need to make additional detailed studies of
howardites and eucrites.

------------------------------------------------------------------------
*Vesta's Rich Geological History*

*I*n their paper about the Stannern trend eucrites, Barrat and coworkers
present an interesting picture of the processes that affected Vesta's
crust. Tying together observations and interpretations by other
investigators and themselves, they propose that the crust formed by
eruption of basalt, the leftovers of magma ocean crystallization. The
specifics of this process are not clear cut yet, but Akira Yamaguchi and
colleagues showed earlier that it is likely that the crust was formed
rapidly and that the first basalts to erupt were buried by subsequent
flows. This led to creation of an insulating layer above the early
flows, causing the early flows to increase in temperature because of
heat flowing from the hot interior. This resulted in most eucrites being
metamorphosed. Intrusions of basalt magma into the crust caused
additional metamorphism, possibly even partial melting, of the rocks
surrounding the pools of magma. In addition, the pools fractionally
crystallized, forming what cosmochemists call cumulate eucrites and
perhaps in some extreme cases forming granites.

Diagram showing the model by Barrat and colleagues of the construction
of the crust of Vesta.
This diagram by Jean-Alix Barrat and colleagues shows a reasonable
snapshot of processes operating during the construction of the crust of
Vesta. Thermal metamorphism, driven by heat flowing from the hot
interior, is more severe as depth increases. Intrusions of basalt magma
cause addition metamorphism of the rock surrounding them, and allow for
fractional crystallization. Some areas, especially near the base of the
crust, become hot enough to partially melt, producing magma richer in
trace elements than typical eucrites. When normal eucrites interact with
these partially melted zones, they produce hybrid magmas (the Stannern
trend eucrites). In most cases, the basalt magma passes through the
crust without interacting with it, leading to eruption of normal, or
Nuevo Laredo trend eucrites.

In their 2007 paper, Barrat and coworkers suggest that the Stannern
trend eucrites (the ones richer in rare earth elements and geochemically
similar elements) form when normal eucrite magmas pass through regions
that were heated enough to partially melt. Such partial melts would
contain higher levels of elements that are not incorporated readily into
major minerals, so have higher concentrations in melted regions. Mixing
of normal eucrite basalt with relatively modest amounts of partial melt
could have produced the Stannern trend eucrites. Barrat and colleagues
back up this model with quantitative trace element modeling. If a magma
did not pass through a partially melting zone, a normal eucrite erupted.

This is a surprisingly complicated history for tiny planet about 530 km
across. It melted, formed a basaltic crust, and made an array of rocks
by accumulation in magma bodies. The crust remelted, and some of those
magmas mixed with normal eucrite basaltic magma to make the Stannern
trend lavas. The crust was metamorphosed by burial, and heated and mixed
by impacts while it was still hot. It will be fascinating to see what
the Dawn spacecraft finds when it spends half a year in orbit around
Vesta. Maybe its reflectance spectrometer will see little areas of HED
granite.

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

ADDITIONAL RESOURCES   *LINKS OPEN IN A NEW WINDOW.*

    * *PSRDpresents:* The Complicated Geologic History of Asteroid 4
      Vesta--Short Slide Summary <PSRD-Vesta.granite-like.ppt> (with
      accompanying notes).

    * Barrat, J. A., Bohn, M., Gillet, Ph., and Yamaguchi, A. (2009)
      Evidence for K-rich Terranes on Vesta from Impact Spherules.
      /Meteoritics and Planetary Science,/ v. 44, p. 359-374.
    * Barrat, M. A., Yamaguchi, A., Greenwood, R. C., Bohn, M., Cotton,
      J., Benoit, M., and Franchi, I. A. (2007) The Stannern Trend
      Eucrites: Contamination of Main Group Eucritic Magmas by Crustal
      Partial Melts. /Geochimica et Cosmochimica Acta,/ v. 71, p. 4108-4124.
    * Dawn Mission homepage <http://dawn.jpl.nasa.gov/index.asp>
    * Martel, L. M. V. (2007) Getting to Know Vesta. /Planetary Science
      Research Discoveries./
      http://www.psrd.hawaii.edu/Nov07/HEDs-Vesta.html
      <../Nov07/HEDs-Vesta.html>
    * Yamaguchi, A., Taylor, G. J., Keil, K., Floss, C., Crozaz, G.,
      Nyquist, L. E., Bogard, D. D., Garrison, D. H., Reese, Y. D.,
      Wiesmann, H., and Shih, C-Y. (2001) Post-crystallization Reheating
      and Partial Melting of Eucrite EET90020 by Impact into the Hot
      Crust of Asteroid 4 Vesta 4.5 Ga ago. /Geochimica et Cosmochimica
      Acta,/ v. 65, p. 3577-3599.