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

From: M come Meteorite Meteorites <mcomemeteorite2004_at_meteoritecentral.com>
Date: Wed Oct 5 01:43:33 2005
Message-ID: <20051005054330.97288.qmail_at_web26209.mail.ukl.yahoo.com>

ok...in conclusion what new classification is portales
valley?

Matteo

--- Ron Baalke <baalke_at_zagami.jpl.nasa.gov> ha
scritto:

>
>
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
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Received on Wed 05 Oct 2005 01:43:30 AM PDT