[meteorite-list] More Evidence for Multiple Meteorite Magmas

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Fri, 20 Feb 2009 15:38:44 -0800 (PST)
Message-ID: <200902202338.PAA17817_at_zagami.jpl.nasa.gov>

http://www.psrd.hawaii.edu/Feb09/asteroidalMagmas.html

More Evidence for Multiple Meteorite Magmas
Planetary Science Research Discoveries
February 19, 2009

--- Cosmochemists show that a pair of meteorites formed in an asteroid
that erupted a newly-recognized type of asteroidal magma.

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

Cosmochemists have identified six main compositional types of magma that
formed inside asteroids during the first 100 million years of Solar
System history. These magmas vary in their chemical and mineralogical
make up, but all have in common low concentrations of sodium and other
volatile elements. Our low-sodium-magma diet has now changed. Two groups
of researchers have identified a new type of asteroidal magma that is
rich in sodium and appears to have formed by partial melting of
previously unmelted, volatile-rich chondritic rock. The teams, one led
by James Day (University of Maryland) and the other by Chip Shearer
(University of New Mexico), studied two meteorites found in Antarctica,
named Graves Nunatak 06128 and 06129, using a battery of cosmochemical
techniques. These studies show that an even wider variety of magmas was
produced inside asteroids than we had thought, shedding light on the
melting histories and formation of asteroids.

References:

    * Shearer, C. K., Burger, P. V., Neal, C. R., Sharp, Z., Borg, L.E.,
      Spivak-Birndorf, L., Wadhwa, M., Papike, J. J., Karner, J. M.,
      Gaffney, A. M., Shafer, J., Weiss, B. P. Geissman, J., and
      Fernandes, V. A. (2008) A Unique Glimpse into Asteroidal Melting
      Processes in the Early Solar System from the Graves Nunatak
      06128/06129 Achondrites. American Mineralogist, v. 93, p. 1937-1940.
    * Day, J. M. D., Ash, R. D., Liu, Y., Bellucci, J. J., Rumble, D.
      III, McDonough, W. F., Walker, R. J., and Taylor, L. A. (2009)
      Early Formation of Evolved Asteroidal Crust. Nature, v. 457, p.
179-182. doi:10.1038/nature07651.

PSRDpresents:More Evidence for Multiple Meteorite Magmas --Short Slide
Summary <PSRD-asteroidalMagmas.ppt> (with accompanying notes).

------------------------------------------------------------------------
Asteroidal Lava Flows

Lava flows are glowing works of performance art, both scary and
mesmerizing. Cosmochemists are impressed by the show, but also by what
lava flows represent geologically. Volcanic eruptions are an important
part of how planetary crusts form, and lava flow compositions tell us
about not only the composition of a planet's crust, but about the
composition of its interior as well. Lava flows, even those that
crystallized billions of years ago, are packed with information that can
be unraveled by cosmochemical detective work.

Lavas erupted on all the rocky planets and on Io, a moon of Jupiter
about the size of Earth's Moon. They also erupted on asteroids. See the
artist's depiction below. Cosmochemists have previously identified
samples of at least five of these (see PSRD article: Asteroidal Lava
Flows <../April03/asteroidalLava.html>), plus other meteorites that give
us information about the effects of melting inside asteroids. We now
have a distinctly different, sixth type of lava in the form of the
Graves Nunatak (GRA) meteorites.

artist's depiction of lava fountaining on an asteriod

Lava flows on an asteroid could have constructed its crust. They might
have erupted as broad, high fountains of lava, but also might have
stalled beneath the surface and solidified underground.

The five lava types identified previously tell stories of melting inside
their parent asteroids, eruptions as lava flows and magma intrusion
beneath the surface, assault by impacts, and metamorphism from heat
inside the asteroids or from impact--complicated stories. An interesting
part of the stories is that each type's history is different. One type
(called the angrites) was not metamorphosed or affected by impact
bombardment. Another, the mesosiderites, are fragmented basalt
lava flows mixed with metallic iron-nickel that then slowly cooled. The
one thing they have in common is low concentrations of volatile
elements like sodium and potassium. The
GRA 06128/9 meteorite pair is loaded with such moderately volatile
elements. It is a new type of asteroidal crust.

Meteorite GRA 06128 photo taken in Antarctica at time of collection by
ANSMET.

The ANSMET meteorite hunting team found GRA 06128 on blue ice in the
Graves Nunatak region of Antarctica during the 2006/2007 field-season.
This photograph was taken at the time of collection. The team gives each
meteorite a unique field number (shown on the number counter) that is
logged in the field notebook along with the rock's size and estimate of
fusion crust (black surfaces) and preliminary classification. The
meteorite's permanent name, GRA 06128, refers to its collection location
(GRA), season (06) and laboratory analysis number (128).

GRA 06128 and its pair, GRA 06129, are achondrite
meteorites with compositions unlike
any previously discovered Solar System materials. Image courtesy of
Ralph Harvey (Case Western Reserve University), the Principal
Investigator of the highly successful Antarctic Search for Meteorites
(ANSMET <http://geology.cwru.edu/~ansmet/>) program. See PSRD articles:
Searching Antarctic Ice for Meteorites <../Feb02/meteoriteSearch.html>
and Meteorites on Ice <../Nov01/metsOnIce.html> for descriptions of the
field work and some of the meteorite discoveries.

------------------------------------------------------------------------
A New Meteorite Magma Type

The two GRA samples are paired, by which meteoriticists mean they were
part of the same original meteoroid that blazed through Earth's
atmosphere. The original fragment broke apart and the fragments fell to
Earth. They usually fall in a relatively small area called a strewn
field, so finding two samples from the same original asteroid chunk is
not unexpected. Of course, cosmochemists pair them because the samples
have so many properties in common. The small differences reflect
heterogeneities in the rock, also not unexpected.

The first distinctive characteristic is that the GRA samples consist of
more than 75% sodium-rich plagioclase feldspar. Plagioclase ranges in
composition from a sodium-rich end member (albite, NaAlSi3O8) to a
calcium-rich one (anorthite, CaAl2Si2O8). The composition can be
anything between these two extremes, which mineralogists express as the
mole percentage of albite and anorthite. Plagioclase in GRA 06128/9
contains 85 mole percent albite and only 15 mole percent anorthite. In
contrast, plagioclase in all the other asteroidal lavas described so far
contains less than 25 mole percent albite, with most less than 15 mole
percent. This must reflect a drastic difference in the composition of
the interior of the GRA-type's parent asteroid compared to the others.

Back-scattered electron image in false color of GRA 06129

This is a false-color back-scattered electron image showing the
abundance and distribution of plagioclase (purple), pyroxene (green),
olivine (orange-yellow), phosphate minerals (blue-green), and in red,
iron oxides (mostly terrestrial weathering products), iron sulfides, and
metals.

The minerals in the GRA 06128/9 are uniform in composition. Their sizes
and the way their grain boundaries are rounded is indicative of either
slow cooling or, more likely, thermal metamorphism. It appears that they
were heated after they formed, causing the minerals to even out their
compositions (cosmochemists call this equilibration), and to change the
mineral shapes from their original igneous shapes.

Photomicrograph of a polished thin section of GRA 06129 in
cross-polarized light.

Photomicrograph of a polished thin section of GRA 06129 in
cross-polarized light. Both GRA 06129 and its pair, GRA 06128, have
granoblastic textures and are composed predominantly of sodium-rich
plagioclase (85 mole percent of the albite end member). White and grey
grains are plagioclase. Other grains are olivine and pyroxene.

An important measurement made by both groups of investigators is the
relative abundances of oxygen isotopes in
the two GRA samples. Oxygen isotopes can often be used as a fingerprint
to prove that samples of planetary materials come from the same body.
Cosmochemists measure all three oxygen isotopes (16O, 17O, and 18O; 16O
is most abundant). Plotting the ratio of 17O to 16O versus 18O to 16O
shows that the data for both Earth and Moon fall on the same line
(called the Terrestrial Mass Fractionation Line or TMFL). The line
slopes in the way expected for chemical processing (crystallization,
melting, alteration by water, and other processes). Data for meteorites,
however, fall on other lines, indicating differences in the abundance of
16O. Cosmochemists interpret this as evidence that oxygen isotopes were
not distributed uniformly throughout the Solar System. Martian
meteorites (SNC on the diagram below) are distinctly different from
lunar and terrestrial samples, containing less 16O than the Earth-Moon
system. Other igneous meteorites contain more 16O than do Earth and the
Moon. The GRA samples plot along a line with a group of meteorites
called brachinites, which are olivine-rich igneous rocks generally
thought by cosmochemists to be residues left over after partial melting
inside an asteroid.

Oxygen isotopic composition of GRA samples.

Oxygen isotopic composition of GRA samples (yellow symbols) measured in
two different laboratories (Zachary Sharp at the University of New
Mexico and Douglas Rumble at the Carnegie Institution of Washington).
The points for the GRA samples fall close to the same line, indicating a
common heritage. They appear to be related to a group of meteorites
called the brachinites, but are distinctly different from other igneous
meteorite groups, including Martian meteorites ("SNC"). The line labeled
"HED" shows where typical eucrites and related rocks plot. TMFL is the
Terrestrial Mass Fractionation Line on which data for Earth and Moon plot.

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

Processing an Asteroid

The connection between the GRA samples and the brachinite meteorites is
intriguing. It shows up in their concentrations of rare earth elements
(REE) as well. The diagram below shows the concentration of each rare
earth element, with the concentrations divided by the concentrations in
CI carbonaceous chondrites. (This normalization gets rid of the
inherently zig-zaggy pattern shown by the rare earth elements because
the abundances of elements with even atomic numbers are larger than
elements with odd atomic numbers.) In general, the GRA samples have
higher concentrations of REE than do the brachinites. This suggests a
complementary relationship in which an asteroid heated up by decay of
short-lived isotopes such as aluminum-26 (26Al), and began to melt,
forming a sodium-rich basaltic lava (the GRA 06128/9 samples) and
leaving behind an olivine-rich residue to later become the brachinites.
Alternatively, brachinites might have formed from the GRA magma by
accumulation of olivine. More work needs to be done to figure out the
details of brachinite formation.

Rare Earth Elements in GRA samples compared with other igneous rocks.
Rare earth element abundances (normalized to CI carbonaceous chondrites)
for GRA 06128/9 are higher than those in brachinites, hinting that
brachinites could be complementary solid residues formed by removal of
the GRA 06128/9 magma. Shown for comparison is average terrestrial
continental andesite, which has major
element concentrations somewhat like those of GRA 06128/9.

What was the starting material like? Two lines of evidence suggest it
was a previously unmelted chondritic rock of some sort. One piece of the
puzzle comes from melting experiments done by Rhian Jones, Chip
Shearer's colleague at the University of New Mexico, and reported in a
paper by Sharon Feldstein, Rhian Jones, and James Papike. They heated
samples of an L6 chondrite to various temperatures and times (1 hour to
21 days), quenched the hot samples, and studied the distribution and
composition of glass in the samples. The glass represents portions of
the rock that were molten during the experiment, which averaged about 13
weight percent of partial melting. Feldstein measured the concentrations
of major and trace elements in the glassy areas using electron
microprobe and ion microprobe analysis.

The Feldstein experiments show that partially melting the L6 chondrite
produces a sodium-rich melt with rare earth element concentrations in
the range of concentrations measured for the GRA samples. A difficulty
in the experiments was the loss of sodium and other volatiles during the
experiments. Nevertheless, sodium loss was small for the short, one-hour
experiments, showing that the melts were initially high in the molten
material. (We recently discussed the importance of sodium loss, or lack
of it, during chondrule formation in the PSRD article: Tiny Molten
Droplets, Dusty Clouds, and Planet Formation
<../Nov08/chondrule_sodium.html>.) The important point is that partial
melting of an ordinary chondrite produces magma not unlike the magma
represented by the GRA 06128/9 samples. The brachinites might represent
the left over, unmelted rock.

Rare Earth Elements in GRA samples compared with melting experiments.
This plot shows rare earth element abundances (normalized to CI
carbonaceous chondrites) in the GRA 06128/9 samples measured by James
Day and coworkers. The shaded area shows rare earth element abundances
in experimental partial melts of an ordinary chondrite from Feldstein,
Jones, and Papike.

The other clue to the composition of the GRA asteroid derives from the
abundances of highly-siderophile elements. Siderophile
means "iron loving," or, less
romantically, elements that concentrate in metallic iron if it is
present. Many siderophile elements will also concentrate in iron sulfide
if it is present. ("Highly siderophile element" means that an element is
obsessed with metallic iron, and that metal ought to take out a
restraining order on it.) The concentrations of highly siderphile
elements in GRA 06128/9 are not uniform as in chondritic meteorites.
They are fractionated, with palladium (Pd), Platinum (Pt), and Iridium
(Ir) depleted by about 70% compared to Rhenium (Re), Osmium (Os), and
Ruthenium (Ru). James Day and his coworkers suggest that the
fractionation among highly siderophile elements was caused by separation
of two different sulfide minerals during migration of the GRA 06128/9
magma inside its parent asteroid. Most important, the fractionation does
not reflect a widespread equilibration as would happen during core
formation. When the asteroid melted to produce the GRA 06128/9 magma, it
had not melted previously and contained metallic iron and sulfide
minerals distributed throughout its rocky portion. That is, it resembled
an ordinary chondrite.

On top of all the igneous processing to produce the GRA 06128/9 magma,
it must have formed a thick lava flow or pooled beneath the surface to
accumulate extra plagioclase. It consists of about 75 volume percent
plagioclase, larger than the amount expected from partial melting of an
ordinary chondrite, about 50 volume percent. The rock cooled slowly
enough for low-Ca pyroxene to form inside high-Ca pyroxene (a process
called exsolution). T. Mikouchi and M. Miyamoto (University of Tokyo)
estimate from the compositions of the two pyroxenes a cooling rate of 10
to 20 oC per year, implying a burial
depth of 15-20 meters beneath the surface.

------------------------------------------------------------------------
Many Meteorite Magmas

The GRA 06128/9 magma type is distinct from the others cosmochemists
have identified. GRA 06128/9 has a high sodium concentration, reflected
in its plagioclase having a high albite concentration (over 80 mole
percent, see table below). All the other magma types have plagioclase
low in albite, typically 15 mole percent or less.

Meteorite Group Volcanic? Albite in Plagioclase Brecciated by Impacts?
Metamorphosed?
Eucrites Yes 5-25 Almost All Almost All
Eucrite-2 (NWA 011) Yes 15 Yes Yes
Angrites (except one) Yes 0.3 No No
Angra dos Reis Yes 13 No Yes
Mesosiderites Yes 7-9 Yes Yes
GRA-type Sodic Magmas Possibly 82-85 Maybe Yes

Is the GRA 06128/9 type magma as rare as it seems? Possibly not. Groups
of igneous meteorites also contain solidified partial melts with
high-sodium plagioclase. Prominent examples are the
lodranites-acapulcoites, winonaites, and the silicate (rocky) parts of
IAB iron meteorites. An example of a rocky inclusion in a IAB iron
meteorite (from Caddo County, Oklahoma) is shown below. Gretchen Benedix
(University of Hawai???i at the time, now at the Natural History Museum,
London) and coworkers studied these interesting rocks in detail. The
large plagioclase is rich in sodium, like those in GRA 06128/9,
averaging about 84 mole percent albite. This magma probably crystallized
underground, not as a lava flow, but nevertheless shows that internal
melting of metal-bearing chondritic meteorites can yield sodium-rich magma.

Photomicrograph of a polished thin section of Caddo County in
cross-polarized light.

Photomicrograph of a polished thin section of Caddo County in
cross-polarized light showing large sodic plagioclase grain (plag)
enclosing and surrounded by olivine and pyroxene grains.

Cosmochemists have now identified at least six magma types in our
meteorite collections. The magma types have different chemical
compositions and impact and metamorphic histories, but the biggest
difference is in sodium concentration. Most have small concentrations of
sodium and other volatile elements. The GRA 06128/9 type has lots of it.
Furthermore, it appears that the low-sodium types also melted
substantially so that cores formed inside their parent asteroids. In
contrast, the sodic GRA 06128/9 type formed by partial melting of an
undifferentiated, previously unheated chondrite.

Why the difference? Timing might lead to big differences in asteroid
composition and melting history. For example, asteroids that accreted
early would contain more radioactive 26Al, leading to substantial
heating and core formation. Asteroids forming only a million years later
would have much less 26Al, thereby melting less. The temperature of the
accreting material makes a big difference, too. The hotter the average
temperature, the less volatiles an asteroid would contain. Thus, the GRA
06128/9 asteroid might have formed later and cooler than the asteroids
giving rise to the other magma types.

Cosmochemists hope that further work on GRA 06128/9 and searching for
additional magma types and new samples of the rarer ones will help us
figure out the details of asteroid formation and melting. For now, we
can just be amazed at the array of processes that produced magmas and
lavas in our Solar System.

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

ADDITIONAL RESOURCES

    * PSRDpresents: More Evidence for Multiple Meteorite Magmas --Short
      Slide Summary <PSRD-asteroidalMagmas .ppt> (with accompanying notes).

    * Benedix, G. K., McCoy, T. J., Keil, K., and Love, S. G. (2000) A
      Petrologic Study of IAB Iron Meteorites: Constraints on the
      Formation of the IAB-Winonaite Parent Body. Meteoritics and
      Planetary Science, v. 35, p. 1127-1141.
    * Day, J. M. D., Ash, R. D., Liu, Y., Bellucci, J. J., Rumble, D.
      III, McDonough, W. F., Walker, R. J., and Taylor, L. A. (2009)
      Early Formation of Evolved Asteroidal Crust. Nature, v. 457, p.
      179-182. doi:10.1038/nature07651.
    * Feldstein, S. N., Jones, R. H., and Papike, J. J. (2001)
      Disequilibrium Partial Melting Experiments on the Leedey L6
      Chondrite: Textural Controls on Melting Processes. Meteoritics and
      Planetary Science, v. 36, p. 1421-1441.
    * Mikouchi, T. and Miyamoto, M. (2008) Mineralogy and Pyroxene
      Cooling Rate of Unique Achondritic Meteorite GRA 06129. Lunar and
      Planetary Science XXXIX, abstract #2297. Lunar and Planetary
      Institute, Houston.
    * Shearer, C. K., Burger, P. V., Neal, C. R., Sharp, Z., Borg, L.E.,
      Spivak-Birndorf, L., Wadhwa, M., Papike, J. J., Karner, J. M.,
      Gaffney, A. M., Shafer, J., Weiss, B. P. Geissman, J., and
      Fernandes, V. A. (2008) A Unique Glimpse into Asteroidal Melting
      Processes in the Early Solar System from the Graves Nunatak
      06128/06129 Achondrites. American Mineralogist, v. 93, p. 1937-1940.
    * Taylor, G. J. (April 2003) Asteroidal Lava Flows. Planetary
      Science Research Discoveries.
      http://www.psrd.hawaii.edu/April03/asteroidalLava.html
      <../April03/asteroidalLava.html>
Received on Fri 20 Feb 2009 06:38:44 PM PST