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

From: M come Meteorite Meteorites <mcomemeteorite2004_at_meteoritecentral.com>
Date: Wed Oct 5 12:15:50 2005
Message-ID: <20051005161545.90772.qmail_at_web26203.mail.ukl.yahoo.com>

In Roberl Woolard site its write this:

H7, Metallic Melt Breccia (Primitive Achondrite)

Its ok?

Matteo

--- Frank Cressy <fcressy_at_prodigy.net> ha scritto:

> Matteo and all,
>
> I believe it is now classified as a "Metallic-melt
> Meteorite Breccia".
>
> Cheers,
> Frank
>
> M come Meteorite Meteorites
> <mcomemeteorite2004_at_yahoo.it> wrote:
> ok...in conclusion what new classification is
> portales
> valley?
>
> Matteo
>
> --- Ron Baalke 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
>
=== message truncated ===


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Received on Wed 05 Oct 2005 12:15:45 PM PDT