http://www.jpl.nasa.gov/snc/news50.html
MARTIAN METEORITE NWA 1460 FOUND IN MOROCCO
Dr. Anthony Irving
University of Washington
February 12, 2003
NWA 1460
[Image] [Image]
Image 1: Shergottite NWA Image 2: Cut face of NWA
1460 complete stone, showing 1460, showing yellow
fresh fusion crust and part pigeonite grains and dark
of interior. Photo © Adam maskelynite laths. Photo ©
and Greg Hupe. Nelson Oakes.
A 70.2 gram meteorite completely coated with fusion-crust found in Morocco
in December 2001 is almost certainly paired with basaltic shergottite NWA
480 (acquired in November 2000 by Bruno Fectay and Carine Bidaut). Small
fragments from one end of the new stone were sent by a Moroccan dealer to
Adam and Greg Hupe, who submitted it for study by Drs. Anthony Irving and
Scott Kuehner at the University of Washington in Seattle. The complete stone
(referred to as "Black Beauty") was later purchased by Pennsylvania
collector Nelson Oakes. Like the smaller 28 gram NWA 480 stone, this new
stone is very fresh and is coarser grained than most other Martian
meteorites. It consists mainly of large grains of compositionally zoned,
low-Ca pyroxene (with orthopyroxene cores, augite mantles and pigeonite
rims) and glassy, shocked plagioclase (maskelynite), with lesser amounts of
two phosphate minerals (merrillite and chlorapatite), exsolved iron-titanium
oxides, ilmenite, chromite, pyrrhotite, potassium-rich glass, silica and
baddeleyite. A distinctive feature of both stones is the occurrence of
complex intergrowths consisting of varying amounts of
fayalite+hedenbergite+silica along the boundaries of pigeonite grains, which
probably represent breakdown products of former pyroxferroite (an iron-rich
pyroxene-like mineral first found in Apollo 11 lunar basalts). In addition,
the University of Washington scientists discovered crystals of baddeleyite
(zirconium dioxide) associated with the symplectites in NWA 1460.
Baddeleyite was recently confirmed by Dr. Albert Jambon in NWA 480 as well,
and the patterns of compositional zoning in the pyroxene grains of NWA 1460
match those found by Barrat et al. (2002) for NWA 480. The almost identical
morphologies, textures and mineral compositions of both specimens suggests
strongly that they are paired stones from the same fall, and the oxygen
isotopic composition measured earlier for NWA 480 at the University of Paris
confirms its Martian origin. The new, larger stone now will permit
scientists to confirm the cosmic ray exposure age (2.4 million years)
measured by Marty et al. (2001) for NWA 480, and to measure accurately the
age of its igneous formation on Mars. The unusually fresh fusion crust on
both stones suggests that they are part of a relatively recent fall (perhaps
even within the last century) of an ancient volcanic or shallow subsurface
igneous rock ejected from Mars by a large impact. There is a good
possibility that other stones from the same fall also landed in the western
Sahara desert, but they may be difficult to find because the exact locations
of the two existing stones are known only to the nomads who provided them to
Moroccan dealers.
[Image] [Image]
Image 3: False-color Image 4: False-color
backscattered electron image backscattered electron image
showing compositionally-zoned showing an exsolved
pyroxene grains (orthopyroxene iron-titanium oxide grain
- blue; augite - green; containing oriented lamellae
pigeonite - red to yellow), of ilmenite (green) within
maskelynite (dark blue), titanomagnetite (red). Photo ©
merrillite (red), and Anthony Irving and Scott
titanomagnetite and chromite Kuehner.
(white). Photo © Anthony
Irving and Scott Kuehner.
[Image] [Image]
Image 5: False-color Image 6: False-color
backscattered electron image backscattered electron image
showing a symplectitic showing a symplectitic
intergrowth of intergrowth of fayalite+silica
fayalite+hedenbergite+silica at a contact between pigeonite
at a contact between pigeonite and merrillite. Photo ©
and K-Al-Si-rich glass. Photo Anthony Irving and Scott
© Anthony Irving and Scott Kuehner.
Kuehner.
[Image] [Image]
Image 7: False-color Image 8: False-color
backscattered electron image backscattered electron image
showing prismatic grains of showing patchy compositional
baddeleyite (bright yellow) zoning in maskelynite (top)
associated with a and an interstitial grain of
fayalite+hedenbergite+silica silica (probably formerly
symplectitic intergrowth and shock-produced stishovite)
ilmenite near a contact containing irregular dendritic
between pigeonite and structures and surrounded by
maskelynite. Photo © Anthony radial cracks. Photo © Anthony
Irving and Scott Kuehner. Irving and Scott Kuehner.
[Image]
Image 9: Plot of compositions
of zoned pyroxene grains in
NWA 480 and NWA 1460. Diagram
© Anthony Irving and Scott
Kuehner.
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