MOAPA VALLEY: A SECOND NON-ANTARCTIC CM1
CHONDRITE FROM NEVADA., USA. A. J. Irving1, S. M.
Kuehner1, D. Rumble, III2, R. L. Korotev3 and S. Clary. 1Dept. of
Earth & Space Sciences, University of Washington, Seattle, WA
98195, USA, irving@ess. washington.edu; 2Geophysical Labora-
tory, Carnegie Institution, Washington, DC, 20015, USA; 3Dept.
of Earth & Planet. Sciences, Washington University, St. Louis,
MO 63130, USA.
Discovery: A single dark gray, flattened stone (698.8 g), ex-
hibiting subparallel contraction cracks and partially coated with
black, vesicular fusion crust was found in 2004 in the Moapa
Valley, southeast of Logandale, Nevada.
Petrography: Rounded to ellipsoidal objects (0.05 to 0.7 mm
across), exhibiting a preferred orientation of their long axes, are
set in a very fine grained, dark matrix containing small clusters
of sulfide grains. The larger objects (presumably former chon-
drules) are composed mainly of serpentine minerals and S-
bearing phase(s) (probably tochinilite), and in thin section range
in color from pale yellow-brown to red- brown to sepia brown;
some such objects have serpentine-rich rims and/or larger, sub-
hedral, dark brown grains composed of serpentine/cronstedtite.
The sulfides consist of finely intergrown pyrrhotite and pentland-
ite. Very rare magnetite and calcite are present in the matrix, and
the specimen is traversed by narrow open fractures and several
thin veinlets filled with chalcedony.
Broad electron beam analysis of different components gave
the following ranges in composition (in wt.%): ellipsoidal objects
SiO2 31.7-38.4, TiO2 0.07-0.25, Al2O3 1.8-7.2, Cr2O3 0.19-0.73,
FeO 15.9-26.3, MnO 0.07-0.18, MgO 22.7-28.8, CaO 0.06-0.25,
Na2O 0.04-0.29, SO3 0.03-0.87, SUM 85.7-87.4; matrix SiO2
27.1-28.4, TiO2 0.11-0.16, Al2O3 2.7, Cr2O3 0.48-0.57, FeO 34.1-
39.7, MnO 0.20-0.25, MgO 14.7-16.5, CaO 0.36-2.9, Na2O 0.21-
0.42, SO3 3.9-8.5, SUM 89.9-94.1. The specimen must be very
porous on a fine scale, judging from the great difficulty in fully
degassing small fragments under vacuum.
Oxygen Isotopes: Analyses of two acid-washed whole rock
fragments by laser fluorination gave, respectively, δ18O = 6.29,
6.08; δ17O = 0.93, 0.68; Δ17O = -2.382, -2.519 per mil. These
compositions plot on the CM chondrite trend [1], close to com-
positions for anomalous CM2 chondrite NWA 3340 [2] and an-
other, 19 gram non-Antarctic CM1 chondrite from Northwest
Africa [3].
Bulk Composition: INAA of two ~100 mg samples gave the
following mean abundances: FeO 28.7 wt.%, Na2O 0.11 wt.%,
Ni 1.6 wt.%; in ppm, Sc 8.9, Cr 3900, La 0.38, Sm 0.24, Eu 0.06,
Yb 0.28, Lu 0.041. The REE pattern is flat at ~1.3xCI chon-
drites, and is similar to those of other CM chondrites [4].
References: [1] Clayton R. N. and Mayeda T. K. 1999. Geo-
chim. Cosmochim. Acta 63:2089-2104. [2] Kuehner S. M. et al.
2007. Meteoritics & Planetary Science 32:A74. [3] Weisberg M.
K. et al. 2009. Meteorit. Bull. 96. [4] Wasson J. T. and Kalle-
meyn G. W. 1988. Phil. Trans. Roy. Soc. Lond. A325: 535-544.