[meteorite-list] Two More Unique Martian Meteorites Found In Africa (NWA 998 and NWA 1195)

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 10:04:48 2004
Message-ID: <200205092321.QAA09417_at_zagami.jpl.nasa.gov>

http://www.jpl.nasa.gov/snc/news48.html

Two More Unique Martian Meteorites Found In Africa (NWA 998 and NWA 1195)
Dr. Anthony Irving
University of Washington
May 9, 2002

The fertile hunting grounds of the western Sahara have turned up two more
Martian meteorites, bringing the total number of unpaired examples to 26.
Both were recovered and confirmed by the same team of investigators based in
Seattle (USA). Adam and Greg Hupe, in collaboration with Drs. Anthony Irving
and Scott Kuehner at the University of Washington, acquired and analyzed a
new orthopyroxene-bearing nakhlite (NWA 998) and a new olivine-phyric
basaltic shergottite (NWA 1195), which were found by separate nomadic groups
in western Algeria or eastern Morocco (full descriptions submitted to the
Meteoritical Bulletin are given below). The new nakhlite (Images 1 and 2) is
the sixth known example and has unique features including the presence of
minor orthopyroxene accompanying the dominant subcalcic augite, intergrowths
of titanomagnetite and low-Ca pyroxene (Image 3; suggesting a
post-crystallization, high temperature oxidation event), and secondary
carbonate, sulfate and possibly hydrous minerals (which may be products of
weathering or other alteration processes on Mars). Oxygen isotope analyses
by Dr. Douglas Rumble at the Carnegie Institution (Washington, D.C.) have
confirmed its kinship with other Martian meteorites. The new basaltic
shergottite, which has an unusual elongated shape and a distinctive
weathering rind (Images 4 and 5), is the fourth known example of a subgroup
(including DaG 476, SaU 005 and NWA 1068, and respective paired samples)
characterized by relatively large olivine megacrysts in a finer grained
groundmass composed mainly of pigeonitic pyroxene and shocked plagioclase
(maskelynite). Scientists continue to debate whether some or all of the
olivine megacrysts in these samples represent phenocrysts grown from the
magma that produced the groundmass, or xenocrysts captured from subsurface
rocks or mixed in from possibly related, partially crystallized subsurface
magma batches. Most of the olivine crystals in NWA 1195 have well developed
crystal faces suggesting that they are phenocrysts (Images 6 and 7), and the
cores of the compositionally zoned olivine grains are the most
magnesium-rich yet found in Martian samples (indicative of a relatively
unfractionated magma from the interior of the planet).

The successful recovery of these rare and important meteorites required a
team approach among dealers, collectors and scientists sharing a common
interest in their pursuit, preservation and study. The Martian stone now
designated NWA 998 was rumored to exist in Morocco as early as September,
2001 at a time when international travel was out of the question. Tucson
dealers Mike Farmer and Jim Strope, trusting their sources, took this rumor
seriously and played a key role in the acquisition. Adam and Greg Hupe were
invited into the pursuit of this mysterious stone because of its extremely
high asking price, although they held little hope that it actually existed.
The Hupe brothers have invested considerable resources in the pursuit of
other such fantastic stones only to come back empty handed. Once the stone
was confirmed to really exist, a deal was finalized at the Tucson Gem and
Mineral Show in February, 2002, where it realized a record wholesale price,
many times more than any other meteorite purchased in this manner. This new
nakhlite is believed to have been found in western Algeria, but this is hard
to confirm because the Berber nomads will not disclose its exact location,
and, despite a promised thorough secondary search, no more has been
recovered.

The recovery of shergottite NWA 1195 followed a different path. Adam and
Greg Hupe spent months educating Moroccan dealers to look for unusual stones
among batches of meteorites, and also to document which nomads found good
stones so that additional pieces might be recovered. In the past, any stone
that was not magnetic or lacked a fusion crust was usually cast aside and
forgotten. The Hupe brothers financed the high costs of this untested
recovery method with few results until a shipment of suspected meteorites
arrived in Seattle in March, 2002. A 50 gram stone with no fusion crust was
pulled from a group of terrestrial stones by Adam Hupe, who suspected that
it might be a shergottite. The next day a thin section was prepared in
excited anticipation by Dr. Anthony Irving and David McDougall at the
University of Washington and revealed the characteristic presence of
maskelynite. Full confirmation required expert electron microprobe analysis
by Dr. Scott Kuehner. Since careful notes had been made on which nomadic
group found this stone, a search for more of this material was initiated and
within a week another 265 gram stone was recovered 15 meters from the
original in the Safsaf region of eastern Morocco. Greg Hupe then embarked on
a grueling 36-hour trip to Morocco and spent four days with nomads in the
desert to retrieve the new find. Greg brought along the end portion cut off
the 50 gram stone and was delighted when it fit perfectly to the newly
acquired 265 gram stone, thus reuniting this meteorite with its broken
counterpart after weathering separately for thousands of years.



Image 1: Main mass of nakhlite NWA 998. Photo Copyright Adam and Greg Hupe.

Image 2: Thin section image (11 mm by 6 mm) in partially cross-polarized
light of NWA 998 showing abundant prismatic augite, olivine (green),
orthopyroxene and titanomagnetite (black).
Photo Copyright Adam and Greg Hupe.

Image 3: False-color backscattered electron image of NWA 998 showing a
symplectitic intergrowth of titanomagnetite (beige) and low-Ca pyroxene
(green) at a contact between olivine (red) and titanomagnetite (beige)
adjacent to plagioclase (blue). Photo Copyright Adam and Greg Hupe.

Image 4: Assembled complete stone of shergottite NWA 1195 showing interior.
Photo Copyright Adam and Greg Hupe.

Image 5: Thin section image (29 mm by 19 mm) in plane light of NWA 1195
showing the thin terrestrial weathering rind (reddish brown) and porphyritic
texture, with large olivine phenocrysts (brown and yellow) in a groundmass of
pigeonite (pale yellow) and maskelynite (white). Photo Copyright Adam and
Greg Hupe.

Image 6: Thin section of NWA 1195 in cross-polarized light showing euhedral
olivine phenocrysts in a groundmass of pigeonite (yellow) and maskelynite
(black). Photo Copyright Adam and Greg Hupe.



Descriptions Submitted for Publication in the Meteoritical Bulletin

Northwest Africa 998
Algeria or Morocco
Purchased 2001 September
Martian meteorite (nakhlite)

A. and G. Hupe (Hupe) purchased from dealers at the Tucson Gem and Mineral
Show in 2002 February the main mass from a 456 g stone that had been
acquired at an unspecified site in western Algeria or eastern Morocco in
2001 September. Dimensions before cutting: 72 mm by 65 mm by 48 mm.
Classification and mineralogy (A. Irving and S. Kuehner, UWS): a friable,
dark green rock with minor orange-brown alteration products that probably
are of pre-terrestrial origin. Composed mainly of subhedral, olive-green,
complexly-zoned subcalcic augite (Fs22Wo39) with subordinate yellow olivine
(Fa64), orthopyroxene (Fs49Wo4), interstitial plagioclase (Ab61Or4
containing 0.1 wt.% SrO, and exhibiting normal birefringence),
titanomagnetite, chlorapatite and pyrrhotite. The overall texture is that of
a hypabyssal, adcumulate igneous rock, and the apparent crystallization
sequence is olivine, orthopyroxene, titanomagnetite, augite, apatite,
plagioclase. There is a weak preferred orientation of prismatic pyroxene
crystals, many of which have very distinctive zoning, with cores of augite
surrounded by irregular, inverted pigeonite rims (now consisting of
orthopyroxene with fine augite lamellae). Trains of tiny melt inclusions are
present along healed fractures within pyroxene; microprobe study confirms
that most of these are K-Na-Al-bearing silicate glass, but some are
intergrowths of glass and Fe-bearing carbonate, which may represent quenched
immiscible silicate- carbonate liquids. Symplectitic intergrowths of
titanomagnetite and low-Ca pyroxene are present at grain boundaries between
large, discrete olivine and titanomagnetite grains, but are not present
around chromian titanomagnetite inclusions within olivine. These
observations suggest that a pre-terrestrial oxidation process produced the
symplectites, and involved high temperature, deuteric fluid infiltration
along grain boundaries; such fluids also may have produced the irregular
pigeonitic rims on augite crystals. Secondary (probably pre-terrestrial)
ankeritic carbonate, K-feldspar (some Fe-bearing), (?)serpentine, calcite
and a Ca sulfate are present on grain boundaries and within cracks in
augite. Oxygen isotopes (D. Rumble, CIWGL*): replicate analyses of
acid-washed augite by laser fluorination gave _18O = +3.9 _ 0.2, _17O = +2.4
_ 0.1, _17O = +0.30 _ 0.02 per mil. Specimens: type specimens, 20 g, UWS, 20
g, FMNH, and two polished thin sections, UWS; main mass, Hupe.

[*Carnegie Institution of Washington, Geophysical Laboratory, Washington,
DC, U.S.A.]

Northwest Africa 1195
Morocco
Purchased 2002 March/April
Martian meteorite (basaltic shergottite)

A. and G. Hupe (Hupe) purchased a 50 g fragment of a broken stone with a
distinctive, thin weathering rind collected by nomads near Safsaf, Morocco
in 2002 March, and subsequently purchased the remainder of the same
elongated stone (total weight 315 g). Dimensions of reassembled stone: 133
mm by 43mm by 37 mm. Classification and mineralogy (A. Irving and S.
Kuehner, UWS): olivine megacrysts (up to 4 mm) are set in a groundmass of
low-Ca pyroxene and maskelynite (Ab37Or0.5 to Ab41Or0.7) with minor
Ti-chromite, pyrrhotite, ilmenite and Mg-bearing merrillite. The euhedral to
subhedral shapes of most of the olivine grains suggest that they are
phenocrysts rather than xenocrysts. Olivine exhibits strong compositional
zoning (cores Fa19, FeO/MnO = 54; rims Fa40, FeO/MnO = 62), and contains
abundant inclusions of chromite, clinopyroxene, orthopyroxene and
pyrrhotite. The groundmass low-Ca pyroxenes are zoned from cores of
pigeonite (Wo7Fs26, FeO/MnO = 37.1) or, less commonly, orthopyroxene
(Wo4Fs23, FeO/MnO = 37.0) to rims of more Fe-rich pigeonite (Wo12Fs33,
FeO/MnO = 36.6). Occurring very rarely on groundmass pyroxene grains are
patchy overgrowths of an Fe-rich mineral (possibly related to chamosite or
chlorite, with 35 wt.% FeO, 5 wt.% Al2O3, 1.5 wt.% MgO and a low oxide sum
of 85 wt.%, suggesting the presence of water or hydroxyl). Calcite occurs
sparsely along grain boundaries and as thin veinlets. Texture and mineral
compositions are similar to those in olivine-phyric basaltic shergottite DaG
476/670, but olivine is much more magnesian than in other olivine-phyric
basaltic shergottites SaU 005/008 and NWA 1068/1110. Specimens: type
specimen, 20 g, and two polished thin sections, UWS; main mass, Hupe.
Received on Thu 09 May 2002 07:21:55 PM PDT