An Article In Meteorite-Times Magazine
by Tom Phillips

NWA 5480

This beautiful thin section of NWA 5480 was a generous loan from Greg Hupe. I really enjoyed looking at this slide.

I try to include some information on the material I am sharing but my writing skills are limited. I am happy to have such a cool write up by Greg on this incredible material.


Greg Hupe:

NWA 5480; a fresh, and incomparable new Olivine Diogenite. Several stones were found in Arg Shash, Mali in July 2008 by nomadic meteorite hunters. Samples from two different stones were sent to the University of Washington (UofW) where they were analyzed and determined to be meteoritic. Eventually seven stones with a combined Total Known Weight (TKW) of 4912 grams were purchased in October/November 2008, and the full type sample was analyzed at UofW and multiple oxygen isotope studies were performed at the Carnegie Institution, Washington, DC. Cosmic-ray exposure (CRE) analysis is currently being performed by scientists in Japan to determine how long this meteorite had been in space before landing on Earth. With the addition of NWA 5480, there are now seven known members of the “Olivine Diogenite” group (three specimens are from Antarctica and four are from Northwest Africa). NWA 5480 is texturally very different from the others and is the most olivine-rich, and may represent the deepest specimen of the Vesta mantle yet discovered!


Scientists are very excited about these specimens because they exhibit unique textural characteristics not previously found in any other known meteorites! The material must have been heated to a point where it became
"plasticized" and mixed, presumably in the deep interior (mantle) of the NWA 5480 parent body (presumably Vesta-related). By analogy with deformed olivine-orthopyroxene-rich samples of the Earth's mantle (known as harzburgites), it appears likely that temperatures were high enough that NWA 5480 represents a solid residue after partial melting to produce molten magma, which presumably ascended towards the surface of the parent body. The "swirl" texture that can be seen on the interior and exterior graphically displays the flowing and mixing of the olivine and orthopyroxene grains, with intermingled chromite crystals.


Image of 108.2-gram complete slice displaying mottled appearance of matrix:
http://www.lunarrock.com/nwa5480/nwa5480slice.jpg


Close-up image of NWA 5480 showing gemmy crystals (image measures 25mm wide):
http://www.lunarrock.com/nwa5480/nwa5480closeup.jpg


NWA 5480 is a very dense and hard meteorite but takes an extremely nice polish, which highlights the dazzling crystals that make up the matrix of this remarkable material. Each professionally prepared specimen comes with a color ID from The Hupé Collection.

Image of large 1.5cm chromite grains on 507-gram stone:
http://www.lunarrock.com/nwa5480/nwa5480chromite.jpg


False-color BSE image of NWA 5480 showing olivine grain distribution:
http://www.lunarrock.com/nwa5480/nwa5480bse.jpg


Classification submitted to the Meteoritical Bulletin for NWA 5480:
Northwest Africa 5480
Mali
Find: July 2008
Achondrite (olivine diogenite)

History: Found near Arg Shash, Mali in July 2008 and purchased by Greg Hupé in October/November 2008 from Moroccan dealers in Erfoud and Tagounite.

Physical characteristics: Seven very fresh, dense, dark brown to pale green mottled stones lacking obvious fusion crust with a total weight of 4912 g. Sporadic large, black chromite grains (up to 1.5 cm across) are visible on the exterior of some stones.

Petrography: (A. Irving and S. Kuehner, UWS) Medium to coarse grained, and composed mainly of olivine (57 vol.%) and orthopyroxene (42 vol.%), with minor chromite (1 vol.%) and accessory troilite and Ni-free metal. Plagioclase is absent. The distribution of olivine and orthopyroxene is very heterogeneous, with regions composed of up to 90% of either mineral, some in irregular, schlieren-like polygranular arrays. Olivine-rich regions (brown) are composed of clusters of small, polygonal to rounded grains intergrown with chromite and orthopyroxene; orthopyroxene-rich zones (pale green) consist of interlocking, larger oikocrysts of orthopyroxene containing numerous tiny chadacrysts of olivine.

Geochemistry: Orthopyroxene (Fs24.8Wo1.8, FeO/MnO = 25.6), olivine (Fa30.2, FeO/MnO = 46.6), chromite [(Cr/(Cr+Al) = 0.828, Mg/(Mg+Fe) = 0.181-0.207, TiO2 = 0.71 wt.%]. Oxygen Isotopes (D. Rumble, CIW): replicate analyses of acid-washed bulk silicates by laser fluorination gave, respectively, d18O = 3.215, 3.509; d17O = 1.459, 1.611; D17O = -0.232, -0.234 per mil.

Classification: Achondrite (olivine diogenite).

Specimens: A total of 21.4 g of sample and one polished thin section are on deposit at UWS. Mr. G. M. Hupé holds the main mass.



This is the thin section viewed in cross polarized light with no magnification.


This set is taken with a field of view of 0.95mm. The shots break down like this:

1: plane polarized light
2: partially cross polarized light (-90 degrees)
3: fully cross polarized light
4: fully cross polarized light with a 1/4 wave retardation plate + 45 degrees
5: fully cross polarized light with a 1/4 wave retardation plate - 45 degrees


1: plane polarized light


2: partially cross polarized light (-90 degrees)


3: fully cross polarized light


4: fully cross polarized light with a 1/4 wave retardation plate + 45 degrees


5: fully cross polarized light with a 1/4 wave retardation plate - 45 degrees


These next five shots were taken with a Field of view of 0.40 mm with the addition of a wave plate.


This last set is the magnification level I really like to work in. It's Field of view is 0.25 mm or approximately 400X "At the eye piece" magnification.

 

Tom Phillips can be reached by email at:/font>
STARSANDSCOPES@aol.com

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