NWA 1460 |
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 Image 1: Shergottite NWA 1460 complete stone, showing fresh fusion crust and part of interior. Photo © Adam and Greg Hupe. |
 Image 2: Cut face of NWA 1460, showing yellow pigeonite grains and dark maskelynite laths. Photo © 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 3: False-color backscattered electron image showing compositionally-zoned pyroxene grains (orthopyroxene - blue; augite - green; pigeonite - red to yellow), maskelynite (dark blue), merrillite (red), and titanomagnetite and chromite (white). Photo © Anthony Irving and Scott Kuehner. |
 Image 4: False-color backscattered electron image showing an exsolved iron-titanium oxide grain containing oriented lamellae of ilmenite (green) within titanomagnetite (red). Photo © Anthony Irving and Scott Kuehner. |
 Image 5: False-color backscattered electron image showing a symplectitic intergrowth of fayalite+hedenbergite+silica at a contact between pigeonite and K-Al-Si-rich glass. Photo © Anthony Irving and Scott Kuehner. |
 Image 6: False-color backscattered electron image showing a symplectitic intergrowth of fayalite+silica at a contact between pigeonite and merrillite. Photo © Anthony Irving and Scott Kuehner. |
 Image 7: False-color backscattered electron image showing prismatic grains of baddeleyite (bright yellow) associated with a fayalite+hedenbergite+silica symplectitic intergrowth and ilmenite near a contact between pigeonite and maskelynite. Photo © Anthony Irving and Scott Kuehner. |
 Image 8: False-color backscattered electron image showing patchy compositional zoning in maskelynite (top) and an interstitial grain of silica (probably formerly shock-produced stishovite) containing irregular dendritic structures and surrounded by radial cracks. Photo © Anthony Irving and Scott Kuehner. |
 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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