[meteorite-list] NWA 3133

From: David Weir <dgweir_at_meteoritecentral.com>
Date: Tue Nov 16 17:23:34 2004
Message-ID: <419A7DC8.702B5D6D_at_earthlink.net>

"stan ." wrote:

> I belive David Weir published the preliminary reclassification data > on the list not too long ago.

Actually I was only passing along a post from Nelson since he was having
e-mail trouble, and of course he is the owner who had this one
classified. Interestingly, a comparison of NWA 1839 with NWA 3133
doesn't appear to be a very close match in mineral composition, though
it does appear somewhat close in some elemental ratios. O-isotopic
ratios show for D17O: -3.81 to -4.18 for 1839, and -3.34 to -3.53 for
3133, but I have no idea of the significance of the spread in these
numbers. Suffice it to say they both plot near the CCAM line near CV
chondrites. So I'm just waiting for one of the primary scientists to
make a determination on their possible pairing.

The classification data for NWA 1839 and NWA 3133 is repeated below,
thanks to the Hupes and Oakes.

David

---------------------------------------------

Northwest Africa 1839
     Morocco
     Found 2003
     Primitive achondrite (ungrouped)
A 121.8 g, partially crusted stone was purchased in Rissani, Morocco in
June 2003. Description and classification (T. Bunch and J. Wittke, NAU):
polygonal, equigranular (<0.75 mm grain size); no relict chondrules;
prominent twinning in plagioclase. Mineral modes: orthopyroxene, 37 vol.
%; olivine, 35 vol. %; plagioclase, 12 vol. %; Ca-rich pyroxene, 6 vol.
%; troilite, 4.5 vol. %; metal (taenite only), 3.5 vol. %; chromite, 2
vol. %. Very homogeneous; silicate compositional range is within the
analytical precision of ? 0.2 mol %. Orthopyroxene, Fs18.9Wo2.3, FeO/MnO
= 41; olivine, Fa22.0, FeO/MnO = 63; plagioclase, An53.4Or2.2;
clinopyroxene, Fs7.2Wo47; taenite, Ni = 16.7 wt. %; chromite, Cr/(Cr+Al)
= 73. Oxygen isotope (D. Rumble, CIW) replicate analyses of acid-washed,
bulk sample by laser fluorination gave d18O = 0.51 and -0.73, d17O =
-3.54 and -4.57; D17O = - 4.18 and - 3.81 that plot near the CCAM line
and within the region of CV compositions. Shock level, S1; weathering
grade is W2. Specimens: 20.1 g, NAU; main mass, Oakes.

----------------------------------------------------

NWA 3133
found in Northwest Africa (as several stones totaling 4 kg) has a
metamorphic texture with 120deg triple grain junctions (mean grainsize =
0.28 mm); no chondrules are present. Olivine (46 vol.%;
Fa_{22}, FeO/MnO = 64) and orthopyroxene (28 vol.%; Fs_{18.9}Wo_{2.3},
FeO/MnO = 42) are the most abundant phases, with less intermediate
plagioclase (An_{53.5}Or_{2.3}), Cr-diopside (Fs_{7.5}Wo_{48.3},
Cr_{2}O_{3} = 0.71 wt.%), Al-Ti-bearing chromite (Cr/(Cr+Al) = 0.73,
TiO_{2} = 2.6 wt.%), Na-Mg-bearing merrillite, troilite (1-5 wt.% Ni)
and Fe-Ni metal (15-20 wt.% Ni). Clinopyroxene, chromite and merrillite
are inhomogeneously distributed as relatively large grains. Metal is
partially altered to limonite (W1-2), and minor limonite and calcite
occur along grain boundaries. Oxygen isotopic compositions (delta^17O,
delta^18O) determined in two laboratories by laser fluorination on
handpicked olivine (-3.67, +0.94; -3.62, +0.89; -2.91, +1.78 per mil)
and on acid-washed whole rock fragments (-2.25, +2.46; -1.75, +3.06 per
mil) plot on the mixing line for Allende and other CV3 chondrites. Thus,
NWA 3133 could be regarded as the first known "CV7 chondrite", and may
have been formed by metamorphic recrystallization (or perhaps igneous
processes) in the CV parent body. Other meteorites with oxygen and/or Cr
isotopic affinities to the CV3 chondrites are the three members of the
Eagle Station pallasite grouplet and the silicated irons Bocaiuva and
NWA 176 (Clayton and Mayeda., 1996; Liu et al, 2001; Shukolyukov and
Lugmair, 2001). Our inference from these data is that the CV parent body
was an at least partially differentiated (and relatively large) object
consisting of a metal+silicate core region surrounded by a presumably
silicate-rich mantle and a chondrule-CAI-rich regolith, the deepest
portions of which were metamorphosed and/or partially melted to form
primitive achondritic lithologies.
Received on Tue 16 Nov 2004 05:23:04 PM PST