[meteorite-list] Portales Valley: Not Just Another Ordinary Chondrite

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Tue Oct 4 23:36:36 2005
Message-ID: <200510030518.j935IJP18073_at_zagami.jpl.nasa.gov>

http://www.psrd.hawaii.edu/Sept05/PortalesValley.html

Portales Valley: Not Just Another Ordinary Chondrite
Planetary Science Research Discoveries
September 30, 2005

--- A melted meteorite gives a snapshot of the heat and shock that
wracked an asteroid during the first stages of differentiation.

Written by Alex Ruzicka and Melinda Hutson
Department of Geology, Portland State University

Soon after the Portales Valley meteorite fell in 1998, it was classified
as one of the most common types of meteorites, an H6 ordinary chondrite.
Although researchers quickly recognized that Portales Valley is not a
typical H6 chondrite, there was little agreement about how the meteorite
formed. A recent study of Portales Valley by Ruzicka and colleagues
suggests that the textures, mineralogy, and chemistry of the meteorite
are best explained as the first good example of a metallic melt breccia.
This meteorite represents a transitional stage between chondrites and
various classes of differentiated meteorites, and offers clues as to
how differentiation occurred in early-formed planetary bodies.

Reference:

    * Ruzicka, A., Killgore, M., Mittlefehldt, D.W. and Fries, M.D
      (2005) Portales Valley: Petrology of a metallic-melt meteorite
breccia. Meteoritics & Planetary Science, v. 40, p. 261-295.

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

Differentiation: a widespread but poorly-understood process

Most solar system material underwent differentiation, a process
involving melting and separation of liquids and solids of varying
density and chemical composition. However, chondritic meteorites escaped
this process and are believed to be pieces of undifferentiated
asteroids. All other meteorites, and probably all rocks from planets and
large moons, melted when the parent bodies differentiated to form cores,
mantles, and crusts. The heat source for differentiation is uncertain,
as are the exact physical processes and conditions that allowed
differentiation to proceed in small planetary bodies with weak gravity.
Proposed sources of heat include internally-generated heat from
short-lived radioactive materials such as aluminum-26 (26Al), external
heating from our young active Sun, and heating resulting from collisions
between planetary bodies (shock heating). A detailed study of the
Portales Valley meteorite suggests that differentiation of small
planetary bodies involved a combination of an internal heat source and
shock. Shock heating was not the major heat source involved in
differentiation, but the stress waves associated with even modest shock
events played a critical role in helping materials to separate and
reconfigure during differentiation.

illustration of differentiation by Granshaw

A sequence of images showing stages in the differentiation of a
planetesimal, an early-formed planetary body. The image in the left hand
side shows a chondritic planetesimal becoming hot enough for melting to
begin. The middle image shows that the heavier metallic liquid sinks
toward the center, while the less dense rocky material rises toward the
surface. The result is a differentiated object with a crust, mantle and
core, as shown in the image in the right hand side. (Images created by
Frank Granshaw of Artemis Software for the Cascadia Meteorite
Laboratory, Portland State University.)

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

Not an ordinary H6 ordinary chondrite

Three features link Portales Valley to H-group ordinary chondrites.
These are (1) the presence of rare chondrules with a rather typical
chondritic texture present in silicate-rich areas, (2) the compositions
of most minerals, and (3) the
bulk oxygen isotopic composition of the meteorite. Nonetheless, Portales
Valley contains unusual features that distinguish it from any other
ordinary chondrite. Even in a cut section, the differences between
Portales Valley and a typical H-chondrite are readily apparent (see
figures below).

comparison to H chondrite

A comparison of a typical H-chondrite and Portales Valley. Bright areas
are mainly metallic; dark areas are mainly silicates. Left: A slice of a
meteorite that is paired with the Franconia (H5) chondritic meteorite.
The small lines on the ruler are one millimeter apart. Right: A slice of
the Portales Valley meteorite showing that the chondritic, silicate-rich
material occurs as angular clasts floating in metallic veins. Tiny
bright spots in silicate-rich clasts consist of troilite (FeS) and
smaller amounts of fine-grained metal. A large graphite nodule is visible.

Besides the obvious differences between Portales Valley and a typical H
chondrite, Portales Valley is also unusual in several other ways. It is
the only known ordinary chondrite that contains coarse (cm-sized)
graphite nodules as well as metal that shows a Widmanst??tten texture (an
intergrowth of high- and low-Ni metal, see left image below), both of
which are common in iron meteorites. Another notable feature is that
different sections of Portales Valley vary widely in their proportion of
metal, ranging from silicate-rich areas almost devoid of metal to areas
that are almost entirely metal. Finally, Portales Valley is also unusual
in having coarse (0.5-1 mm across) and abundant phosphate minerals,
which are usually found at the contact between metal and silicate-rich
areas (see right image below).

fig3

These are back-scattered electron images of areas in Portales Valley.
Left: Metal vein showing parallel kamacite (low-Ni metal) lamellae
surrounded by higher Ni-metal (zoned taenite and plessite), representing
a Widmanst??tten texture similar to that found in iron meteorites. The
entire metal grain is swathed by kamacite. Right: Coarse phosphate
(merrillite) intergrown with silicates (plagioclase, orthopyroxene,
olivine) next to coarse FeNi-metal (white).

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

Varied interpretations of Portales Valley

Portales Valley has been alternately interpreted as an annealed (heated)
impact breccia, a primitive achondrite, or a meteorite transitional between
chondrites and silicate-bearing iron meteorites. It is important to
determine which, if any, of these ideas is correct, as each implies a
different heat source and formation mechanism for the meteorite. We will
consider each of these ideas below.

Annealed impact melt breccia?

One possibility is that Portales Valley represents an impact melt
breccia that has undergone slow cooling from high temperatures
(annealing). When two objects collide, they may generate enough heat to
create a melt, which can form veins that cut through unmelted portions
of the meteorite. One model for Portales Valley is that the metal veins
were produced by this type of shock melting process. There are some
problems with this model. Studies show that shock deforms minerals in a
characteristic fashion producing features such as planar fractures and
mosaic extinction in olivine and structural changes in feldspars (see
PSRD article: Asteroid Heating: A Shocking View
<http://www.psrd.hawaii.edu/April04/asteroidHeating.html>.) None of
these deformation features are observed in Portales Valley. It has been
proposed that deformation features were present originally but were
removed by annealing. Even so, there are other problems with this model.
Typically when a chondrite is partially melted, glassy silicate veins
are produced which cut through unmelted material (see right image
below). With more extensive shock melting, larger areas of melt are
produced, with metal and troilite forming globules embedded in silicate
melts (see left image below). In neither case are large metallic veins
produced. Another problem is that in order to produce the large metal
veins observed in Portales Valley, temperatures would have to have been
so high that the silicates would be substantially melted and there would
be no visible chondritic texture left.

shock melting

These meteorites show features not seen in Portales Valley. The bright
areas are mainly metallic; the dark areas are mainly silicates. Left:
"Gao melt" showing clasts of H6 material surrounded by metal-bearing
silicate shock melt. The metal in the melted portion forms droplets
instead of veins (field of view is ~4.5 cm wide). Right: Peekskill (H6)
chondrite, which consists of angular clasts surrounded by thin, dark
glassy melt veins.


Primitive achondrite?

Another possibility is that Portales Valley is a primitive achondrite,
such as an acapulcoite, lodranite, or winonaite. These are meteorites
that are approximately chondritic in chemical composition, but which
have been raised to high enough temperatures to be melted. The primitive
achondrites are believed to have been heated solely or primarily by
internal heating. They generally do not have any significant vein
structure, although one acapulcoite (Monument Draw) contains a coarse
phosphate vein and a thin metal vein. Neither of these closely resemble
the large vein structure in Portales Valley. Additionally, experiments
show that melting chondrites by internal heat alone generally produces
isolated patches or globules of metallic melt, not veins.

comparison with achondrite

Bright regions in these meteorites are mainly metallic; dark areas are
mainly silicates. Note that the textures of the two meteorites are
completely different. Left: NWA 1058, an ungrouped primitive achondrite.
Right: Portales Valley. The medium gray object inside the bright metal
is one of two graphite nodules found in the meteorite.

Silicate-bearing iron meteorite?

Yet another possibility is that Portales Valley represents a
silicate-bearing iron meteorite. Some iron meteorites, such as Campo del
Cielo (IAB iron), contain regions that are texturally similar to
Portales Valley, as shown in the pictures below. The silicate regions in
IAB iron meteorites are often approximately chondritic in composition
and mineralogy, but it is clear that Portales Valley is not simply
another IAB iron as its oxygen isotope composition is completely different.

comparison with an iron

This comparison between a silicate-bearing iron meteorite and Portales
Valley shows that they are texturally similar. Bright areas are mainly
metallic; dark areas are mainly silicates. Left: Campo del Cielo - IAB
silicate-bearing iron. Right: Portales Valley.

On the other hand, the oxygen isotope composition of Portales Valley is
the same as that of H-group ordinary chondrites and resembles the IIE
group of silicate-bearing iron meteorites, as shown in the diagram
below. The silicate-bearing IIE iron meteorites are a diverse group.
Some contain silicates that are not similar to chondrites in composition
at all. Three others (Netscha??vo, Techado, and Watson) contain silicates
that are approximately chondritic in mineralogy and bulk chemical
composition. Two of these (Netscha??vo and Techado) also contain a few
recognizable chondrules, similar to Portales Valley. Despite these
similarities, there are significant differences in mineral compositions
and abundances between Portales Valley and IIE irons such as Netscha??vo.
These differences suggest that Portales Valley is not simply another
silicate-bearing IIE iron meteorite.

oxygen isotopes comparisons

Standard three-isotope oxygen diagram showing the compositions of
Portales Valley, H-, L-, and LL-group ordinary chondrite, and IIE iron
meteorites. TF is the terrestrial fractionation line. The y-axis plots
the ratio of oxygen-17 to oxygen-16 compared to mean sea water. The
x-axis is the ratio of oxygen-18 to oxygen-16, also normalized to sea
water. This figure shows that there is an overall resemblance between
Portales Valley, H-group chondrites, and IIE iron meteorites.

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

So how did Portales Valley form?

The best model for producing Portales Valley involves a blend of the
three possibilities described above. A shock process was probably
responsible for producing the obvious coarse vein structure, as
experiments show that such veins cannot be produced easily by static
heating. However, such a shock event could not have been very intense,
as the minerals in the meteorite are not significantly deformed. On the
other hand, as with primitive achondrites and iron meteorites, there is
clear evidence that portions of Portales Valley melted, probably by an
internal heating mechanism unrelated to shock. The reason that Portales
Valley is difficult to pigeonhole is that it is transitional between
primitive H-chondrite-like material and more evolved (achondrite, iron)
meteorite types. Because it is transitional, Portales Valley provides a
snapshot of the first stages of differentiation in asteroids.

Many unusual features of Portales Valley provide clues as to how it
formed, and any story of its formation has to explain them. These
features include:

   1. the coarse metal veins that appear to have formed when molten
      metal flowed around silicate clasts
   2. composition of the metal in Portales Valley is different from that
      found in H-chondrites and differs between the coarse-veined and
      silicate-rich areas
   3. troilite (sulfide phase) is not present in the coarse metal veins,
      but is concentrated in the silicate areas
   4. the presence of large graphite nodules in the coarse metal veins
   5. the presence of coarse phosphate preferentially located at the
      contact between the metal-rich and silicate-rich areas
   6. the amount of high-calcium pyroxene (clinopyroxene) in Portales
      Valley is less than half the amount expected in an H-group
      ordinary chondrite
   7. ratio of olivine to low-calcium pyroxene (orthopyroxene) is lower
      than that found in H-group ordinary chondrites

Ruzicka and his colleagues have proposed a model for the formation of
Portales Valley to explain the features listed above.

lightbulb Melted and mobilized metal and sulfide
There is good evidence that at least some of the metal and troilite in
Portales Valley was molten. The prominent metal-veining textures imply
that metal was substantially molten and that it sometimes entrained
silicate fragments (clasts) that floated in the metal. Moreover, the
composition of the metal is somewhat different than that found in
H-chondrites, and this difference can be explained by a partial melting
process.

The trace-element composition of the metal in Portales Valley varies
between the metal present in the coarse veins, and that found as finer
grains in silicate-rich areas. A model involving partial melting of
metal and sulfide with incomplete separation of melted and solid
portions can explain this variation and also provides constraints on the
maximum temperature during melting. The graphs below compare the
composition of metal in coarse veins and silicate areas (filled squares
and open circles, respectively) to model compositions (lines) produced
by melting either 33% or 50% of the metal and sulfide in an H-chondrite,
while keeping the melt in chemical equilibrium with the solid. Shock
processes cause the melt to move through the meteorite, relative to the
solid. Solid and liquid metal can be mixed in different proportions in
various places in the meteorite. In the figure below, the different
lines indicate various proportions of solid metal fractions (given by
Xsolid, where Xsolid can be as low as zero or as high as one) relative
to the total amount of metal (liquid or solid). The compositions are
normalized to the abundance of nickel and to H-chondrites. With 33%
melting, a solid-liquid metal mixture containing ~20-40% solid agrees
with the observed composition of fine metal in Portales Valley, and a
mixture containing ~40-80% solid substantially agrees with the
composition of the coarse vein metal. The Ga/Ni ratio of the latter is
too high to be explained by the model, but the concentration of Ga is
known to be affected by shock, which could account for this discrepancy.
In contrast, with 50% melting, the models clearly fail to match the
observed compositions. The model results imply that metallic melt
fractions of less than or equal to 40% provide acceptable matches to the
chemistry of Portales Valley. This amount of melting corresponds to
temperatures of ~940-1150 oC. With these temperatures, as much as 13% of
the silicates could have melted as well.

trace elements comparisons

These complicated, but informative, diagrams combine data from the
Portales Valley meteorite (symbols) with theoretical calculations. The
graph on the left shows how the ratios of the concentrations of several
elements in the metallic iron vary with the amount of solid and liquid
metal present. The data match reasonably well for 20-80% solid metal if
the amount of melting was 33% (as shown in the left graph). The data do
not match at all if 50% of the metal was molten (as shown in the right
graph).

The inferred temperatures of ~940-1150 oC for Portales Valley are
somewhat higher than the maximum temperatures of ~800-960 oC reached by
H-chondrites (type 6 metamorphic grade) due to internal heating. The
most likely explanation for the features seen in Portales Valley is that
it experienced a shock event while it was already warm from internal
heating. The shock would have provided only a small temperature
increase. More importantly, the shock wave was necessary for moving melt
through the meteorite. Portales Valley probably formed at depth in the
parent body below an impact crater in a slowly-cooling environment,
enabling the Widmanst??tten structure to form in metal and allowing other
mineralogical reactions to proceed.

When a chondrite is partially melted, all of the sulfide phases will
melt and the sulfur will be concentrated in the metallic melt. The
modeling results shown in the graphs above indicate that the metallic
regions in the silicate-rich areas had a higher proportion of liquid
metal, and thus should have incorporated more sulfur than the coarse
metal veins. In agreement with this, troilite, the major sulfur-bearing
phase in Portales Valley, is indeed concentrated in silicate-rich areas
and not found in coarse metal vein areas. However, the models imply that
at least some sulfur should have been present in the coarse veins
initially, when metallic liquid was still present. The absence of
troilite in the coarse veins implies that sulfur-bearing metallic liquid
must have been expelled from the coarse veins, and moved into the
silicate-rich areas, before Portales Valley finished solidifying.

lightbulb Changes in mineralogy due to mobilization of metal and sulfide
The presence of metallic liquids and the transport of this liquid
through the Portales Valley source region had a dramatic effect on the
distribution and amount of minerals in the remaining rock. For example,
carbon, like sulfur, will dissolve in metallic liquids during partial
melting. In order to account for the centimeter-sized graphite nodules
found in the meteorite, carbon must have been scavenged from large
volumes and concentrated locally in the metallic liquid, where it
crystallized to form the nodules. A similar crystallization process was
probably responsible for making graphite nodules in iron meteorites. The
scavenging effect inferred for Portales Valley would have been
facilitated by the presence of moving metallic liquids, which passed
through large volumes of the rock, picking up carbon along the way.

Moreover, at the high temperatures inferred for Portales Valley,
phosphorus would have dissolved in metallic alloy. Upon cooling,
phosphorus-bearing metal will react with high-calcium pyroxene in the
silicates to produce phosphates such as merrillite and apatite. This
reaction can explain why phosphate minerals are preferentially
concentrated at metal-silicate interfaces in Portales Valley, and why
phosphate is enriched and clinopyroxene is depleted in Portales Valley
compared to ordinary chondrites. Thermodynamic analysis suggests that
the phosphate would have been produced between temperatures of 975-725
oC as the meteorite cooled slowly from high temperatures. Finally, the
phosphate-forming reactions redistributed oxygen, which changed the
proportions of the major silicate minerals (olivine and pyroxene) in the
meteorite. The figure below shows the olivine-to-pyroxene ratio and
metal content in Portales Valley compared to other meteorites. Typically
the olivine/pyroxene ratio in Portales Valley is lower than that found
in H-chondrites. As shown by the dashed line, there is a relationship
between metal content and olivine/pyroxene ratio in Portales Valley,
forming a trend which is unlike that found in H-chondrites and
acapulcoites (a primitive achondrite). Areas in Portales Valley that
contain more metal are areas in which there was a greater amount of
phosphorus available for reaction, resulting in lower olivine contents.

olivene/pyroxene and metal comparisons

This diagram shows that the ratio of olivine to low-calcium pyroxeme is
lower in Portales Valley than that found in H-chondrites.

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

This table summarizes the major features of the Portales Valley
meteorite and how Ruzicka and coauthors explain them.
Feature in Portales Valley Proposed Formation
silicates trapped in metallic veins metal melted and flowed because of
shock heating
metal composition different from H-chondrites metal compositions were
modified by partial melting
FeS concentrated in silicate areas sulfur-bearing metal was expelled
from coarse metal veins
large graphite nodules in metallic veins carbon was scavenged from wide
area and concentrated in metal
big phosphate grains next to metal phosphorus in the metal moved to the
contact between the metal and silicate and was oxidized
low content of high-calcium pyroxene formation of calcium phosphate
occurred by reaction with high-calcium pyroxene
low ratio of olivine to low-calcium pyroxene formation of phosphate
minerals redistributed oxygen in the silicates, leading to the
destruction of olivine and the formation of low-calcium pyroxene

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

The complexities of planet formation

Portales Valley is transitional between primitive (chondrite) and
evolved (achondrite and iron) meteorites. It formed by a shock event
while the parent body was being heated internally. Portales Valley can
be considered an achondrite in the sense that it was partially melted.
It also bears striking resemblances to silicate-bearing iron meteorites,
which formed by differentiation. The main importance of Portales Valley
may ultimately lie in what the meteorite has to tell us about the
formation of other kinds of meteorites and the parent bodies from which
they were derived. Portales Valley may be telling us that simultaneous
impact and internal heating events could have been important in the
overall process of differentiation. The meteorite gives us a glimpse at
the nature of the complex processes that operated in even small bodies
as the planets were forming.

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

ADDITIONAL RESOURCES

    * Cascadia Meteorite Laboratory <http://meteorites.pdx.edu/>

    * Rubin, A. E. (2004) Postshock annealing and postannealing shock in
      equilibrated ordinary chondrites: implications for the thermal and
      shock histories of chondritic asteroids 1. Geochimica et
      Cosmochimica Acta, v. 68, p.673-689.

    * Ruzicka, A., Killgore, M., Mittlefehldt, D.W. and Fries, M.D
      (2005) Portales Valley: Petrology of a metallic-melt meteorite
      breccia. Meteoritics & Planetary Science, v. 40, p. 261-295.

    * Taylor, G. J. (2004) Asteroid Heating: A Shocking View. Planetary
      Science Research Discoveries.
      http://www.psrd.hawaii.edu/April04/asteroidHeating.html
Received on Mon 03 Oct 2005 01:18:19 AM PDT