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46th Lunar and Planetary Science Conference (2015)
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PETROLOGIC AND ISOTOPIC CLASSIFICATIONS OF UNGROUPED ACHONDRITE NWA 8186:
IMPLICATIONS FOR A CK/CV ASTEROIDAL ORIGIN. P. Srinivasan1, F. M. McCubbin1, C. B. Agee1, K.
Ziegler1, M. E. Sanborn2, Q.-Z. Yin2. 1Institute of Meteoritics and Dept. of Earth and Planetary Sciences, University
of New Mexico, Albuquerque, NM 87131, USA. 2Dept. of Earth and Planetary Sciences, UC Davis, CA 95616,
USA. Email: [email protected].
Introduction: The discovery of primitive
achondrites and models of partially differentiated
chondrite parent bodies have reintroduced the
hypothesis of a common origin for chondrites and
achondrites. This single-body hypothesis indicates that
both groups could coexist on the same parent body in
radial layers of increasing thermal grade towards the
core [1]. Thermal evolution models on early accreting
bodies predict the formation of an unmelted chondritic
crust as a consequence of melting by radiogenic heating
[2-7]. Therefore, achondritic components would form
within the inner, hotter regions of the parent body, and
chondritic components would remain on the outer,
cooler regions.
CK and CV chondrites share a resemblance in
mineralogy, oxygen and chromium isotopes [8-10], and
recent discoveries of CV-like (e.g. NWA 3133 [11-13])
and CK-like [14] metachondrites and achondrites
indicate a compellingly similar provenance that fits
with the single-body hypothesis. NWA 8186 is an
ungrouped achondrite showing geochemical and
isotopic similarities to CK-like carbonaceous
chondrites. This meteorite plots precisely on the CCAM
line for oxygen isotopes, and in the region where CK,
CV, and CO chondrites plot for Cr isotopes (Figure 1).
Preliminary analyses showed NWA 8186 is dominated
by olivine and plagioclase, with minor amounts oxides,
augite, and Cl-apatite. No traces of iron metal or troilite
were identified [14]. We have further examined textures
and compositions in this meteorite in order to obtain a
refined petrologic classification of this sample, and to
assess the likelihood that this achondrite is linked with
the CK/CV parent body.
Methodology: For the analysis of NWA 8186, a
~1 x 0.5 cm thin section was used that was carbon
coated for quantitative electron-beam procedures. A
FEI Quanta 3D FEG SEM was used to produce BSE
and EDS X-ray maps, and EPMA analyses were
completed on all observed phases using a JEOL 8200
superprobe.
Figure 2. BSE mosaic of NWA 8186 overlain by a sulfurmap (blue) and phosphorus-map (red). This sample is
roughly 1x0.5 cm, primarily composed of olivine, and is
highly fractured. Apatite (light red) and merrillite (dark
red) are found clustered in the left region of the section.
Sulfides tend to be larger in size towards the right region
of the section.
Figure 1. Plot of NWA 8186 and other achondrite and
chondrite meteorite groups in 17O-54Cr space.
Literature data are from [15,16] and references therein.
Results: Examination of the meteorite sample
showed significant fracturing in all mineral phases.
Many 120° triple junctions were also observed,
typically occurring between olivine and plagioclase
feldspar. Olivine is the dominant phase, comprising
>80% of the sample (Figure 2), and has a Mg# of
~0.65, Fe/Mn = ~127, and NiO ranges from 0.5-1.5
wt.%. Nanometer-sized Ni-rich metal (or Ni-rich oxide)
blebs were also found in the olivine. Plagioclase
feldspar, ~10% of the sample, is An51, and augite, <5%
of the sample, is Fs11En39. Modal abundances indicate
that this meteorite is a dunite, in accordance with IUGS
classification protocols [17-19].
Four oxides phases were observed totaling about
~5% of the meteorite. The dominant oxide phase is
magnetite, and some of the magnetite grains display
thick exsolution lamellae of hercynitic spinel
(FeAl2O4), thin ilmenite (FeTiO3) lamellae, and
micrometer-sized areas of titanomagnetite (Figure 3).
Sulfides were also identified from SEM X-ray
maps, which had previously not been recognized in
NWA 8186 [14]. The modal abundance of sulfides is
46th Lunar and Planetary Science Conference (2015)
Figure 3. BSE image of an exsolved magnetite (Mag) grain
with thick Fe-rich spinel (Sp) lamellae, thin ilmenite (I)
lamellae, and blocky areas of titanomagnetite (TM).
Ol=olivine, P=pyroxene, F= feldspar, Me = merrillite, Sf =
sulfide.
<1% of the sample, and they typically occur in close
relation to the oxides. Figure 2 displays an S X-ray map
overlain on a BSE image. These micrometer-sized
sulfides are monosulfide solid solution, mss, (Fe,Ni)1xS, with x values ranging from 0.02-0.11, and an
average composition of Fe=26.2%, Ni=29.6%,
S=33.3%, and Co=0.4%. Pentlandite solid solution,
Fe4.27Ni4.73S7.84, with composition of Fe=27.4%,
Ni=32.0%, S=33.3%, and Co=0.377% was also found
coexisting with mss.
The P X-ray map overlain on Figure 2 indicates the
phosphate distribution in NWA 8186. Cl-rich apatite
(Ca5(PO4)3Cl), composed of ~53.4% CaO, 41.5% P 2O5,
4.8% Cl, and 0.43% F, was seen surrounding olivine
grains. Figure 4 is a plot of X-site occupancy (mol.%)
in apatite grains. Apatite in NWA 8186 mainly consists
of chlorine in the X site with minor F and a missing
structural component that is likely OH. Merrillite
(Ca18Na2Mg2(PO4)14) was also found associated with
apatite, and composed of ~46.5% CaO, 46.7% P2O5,
3.5% MgO, 2.78% Na2O.
Figure 4.
Truncated
ternary plot
of X-site
occupancy
(mol.%) in
apatite
grains.
Apatite is Clrich, and
depleted in
OH(?) and F.
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Discussion: In addition to NWA 8186 (this study
and [14]), previous studies have noted the potential
association of heavily metamorphosed chondritic
samples (above petrologic grade 6) to CV and CK
chondrites [11-13]. Thermal evolution models have
been used to suggest that the CK/CV parent body could
have radially evolved into a layered body with a
metallic
core,
silicate
magma
ocean,
and
undifferentiated chondritic crust [2-7]. The crust of the
CK/CV parent body would increase in metamorphic
grade towards the mantle and might include reduced
CVs in the outer crust, oxidized CVs in the mid-crust,
and CK chondrites in the lower crust [2].
CK and oxidized-CV meteorites are some of the
most oxidized carbonaceous chondrites known.
Ilmenite-magnetite pairs in CKs indicate an fO2 of
QFM+3 – QFM+5 [20-22]. Exsolution of magnetite,
spinel, and ilmenite phases, as well as the presence of
NiO-rich olivine and absence of Fe-rich metal indicates
this meteorite also formed at high fO2.
Two formation mechanisms are being postulated
for CK-like achondrites. 1) NWA 8186 might have
formed as a residue from partial melting of CK
chondrite, indicating proximity to the silicate magma
ocean, or 2) NWA 8186 might represent a cumulate that
formed as the crystallization product of a CK partial
melt that erupted onto the surface of the CK/CV parent
body. Exploring the link between CK chondrites and
CK-like achondrites is currently being probed through
high-pressure/high-temperature experimental studies.
The connection between these two groups will help
constrain the thermal and magmatic evolution of the
CK/CV parent body.
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L.T. et al. (2011) EPSL, 305, 1-10. [3] Weiss B.P. et al.
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85, #P31C-02. [14] Agee C.B. et al. (2014) Met. Soc., 77,
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[17] Le Maitre et al. (1989) Black. Sci. Pub. 193 pp. [18] Le
Bas et al. (1991) J. Geol. Soc. London 148, 825-833 [19] Le
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Polar Sci., 1, 25-44. [22] Geiger T. et al. (1995) PSS, 43, 485498.