The classification of meteorites is commonly determined using isotopes, modal mineralogy, and bulk compositions [1]. Bulk rare earth elements (REEs) in meteorites are additionally utilized to understand parent body processes. Numerous authors have shown that chondritic groups exhibit REE patterns that may be attributable to their parent bodies [e.g. 2-4], and variations in abundances and concentrations of REEs may reflect early nebular processes, thermal metamorphism, and aqueous alteration on the parent body [5-6]

Agee, C. B.

Lapen, T. J.

McCubbin, F. M.

Righter, M.

Srinivasan, P.

NASA Technical Reports Server

Reassessing the formation of CK7 Northwest Africa (NWA) 8186.  P. Srinivasan1,2, F. M. McCubbin2, T. J. Lapen3, 
M. Righter3, C. B. Agee1. 1Institute of Meteoritics and Dept. of Earth and Planetary Sciences, University of New 
Mexico, Albuquerque, NM 87131, USA. 2NASA Johnson Space Center, Mail Code X12, 2101 NASA Parkway, Hou-
ston, TX 77058. 3Department of Earth and Atmospheric Sciences, University of Houston, Houston, Texas 77204. 
Email: psrinivasan@unm.edu. 
 
Introduction: The classification of meteorites is 
commonly determined using isotopes, modal mineral-
ogy, and bulk compositions [1]. Bulk rare earth ele-
ments (REEs) in meteorites are additionally utilized to 
understand parent body processes. Numerous authors 
have shown that chondritic groups exhibit REE patterns 
that may be attributable to their parent bodies [e.g. 2-4], 
and variations in abundances and concentrations of 
REEs may reflect early nebular processes, thermal met-
amorphism, and aqueous alteration on the parent body 
[5-6].  
Chondritic meteorites can further be classified based 
on their increasing degree of thermal metamorphism: 
petrologic type 3 to 6. One of the leading ideas for the 
increase in petrologic type is prograde-metamorphism, 
in which undifferentiated material is increasingly heated 
by the decay of 26Al on the parent body [e.g. 1,7]. Any 
continuation of petrologic sequence should therefore 
have the same requirement. The petrologic ‘type 7’ clas-
sification, which was first introduced by [8], still re-
mains controversial because of ambiguous heat sources 
associated with these meteorites (i.e. decay of 26Al or 
impact-related heating). A handful of ordinary and en-
statite chondrites are classified as ‘type 7 chondrites’ 
[e.g. 1-2, 8-9], but carbonaceous chondrites have yet to 
be regarded under this classification.  
The CK (Karoonda-like) chondrites are a highly ox-
idized (>FMQ) group of carbonaceous chondrites 
[2,10]. All of the CKs plot on/near the CCAM line for 
oxygen isotopes, contain refractory lithophile abun-
dances ~1.2x greater than CIs, have little to no metal 
grains, and NiO-rich olivine [2,11]. CKs are the only 
group of carbonaceous chondrites that have experienced 
the full range in chondritic thermal metamorphism (i.e. 
types 3 to 6). Geochemical analyses show this tempera-
ture range equates to ~576-867 °C [12]. Some CKs have 
also exhibited shock and melt veins, indicating the CK 
parent body has possibly experienced higher tempera-
tures and/or pressures [2,13].  
Northwest Africa (NWA) 8186 was originally clas-
sified as an ungrouped achondrite that showed similari-
ties to the CK chondrites [14-15]. It was later shown that 
an ‘achondritic’ classification may be unfitting [16], and 
this meteorite more closely resembles a recrystallized 
chondrite. NWA 8186 is isotopically and geochemically 
similar to the CKs [14-15], contains similar rare mineral 
phases like arsenides [17], and is comparable in oxygen 
fugacity (>FMQ) [15] to the CKs. While this meteorite 
strongly resembles a CK, it texturally appears to be of a 
higher petrologic type than CK6.  
In this current study, we present bulk REE abun-
dances in NWA 8186 and U-Pb ages to assess the rela-
tionship between NWA 8186, CK3-6 chondrites, and 
the CK parent body.  
Methodology:  A 1 x 0.5 cm polished mount of 
NWA 8186 was used to obtain rare earth element (REE) 
abundances in silicates and phosphates, and U, Th, and 
Pb concentrations from merrillite and apatite by laser 
ablation-inductively coupled plasma-mass spectrometry 
(LA-ICP-MS) at the University of Houston. The Varian 
810 ICP-MS was equipped with a 193 nm excimer laser 
that we focused to a 50 µm diameter and operated at a 
repetition rate of 8 Hz. We used U.S.G.S. basalt glasses 
BHVO-2G and BIR-1G as standards for the silicates, 
and a NIST glass and three apatites (Yates Mine, Mad-
agascar, and Bear Lake) as standards for the phosphates. 
Trace element concentrations were processed using the 
Glitter Software, and MgO and CaO concentrations 
were linked to previously obtained electron microprobe 
data [15]. Bulk REE values were calculated through 
modal recombination, excluding oxides and sulfides 
since the elements of interest are negligible in these 
phases. U-Pb and Pb-Pb ages were obtained using the 
Wavemetrics Igor Pro software, and Concordia dia-
grams were plotted with IsoPlot. Corrections for com-
mon Pb used solar system initial Pb isotope composi-
tions of 206Pb/204Pb = 9.307 and 207Pb/204Pb = 10.294. 
Random and systematic uncertainties follow the meth-
ods in [18].      
Results:  Bulk REE patterns from NWA 8186 and 
CK chondrites [4,6] normalized to  CI chondrites [19]  
are given in Figure 1. CK meteorites from different pet-
rologic types (3-6) are given to compare the composi-
tional effect of heating. With the exception of Y-82191, 
NWA 8186 contains a higher abundance of REEs com-
pared to the majority of the listed meteorites, especially 
light REEs. An Eu anomaly was not observed, and there 
was a slightly positive Tm anomaly (Tm/Tm*=1.166) in 
NWA 8186.  
U and Pb isotope analysis of seven merrillite grains 
yield a weighted average 207Pb/206Pb date of 4553 ± 21 
Ma (Figure 2).  
Discussion: The concentration of REEs in NWA 
8186 is within the range of the lower REE-bearing CKs 
and Y-82191, which contains an exceptionally high 
REE concentration [6]. NWA 8186 contains a higher 
https://ntrs.nasa.gov/search.jsp?R=20170001338 2019-08-29T15:40:48+00:00Z
abundance of phosphates (1 vol.%) compared to other 
higher petrologic type CKs, so the elevated light REE 
concentrations may be due to modal variations. Petro-
logic type does not correlate with REE concentrations 
[6], as the two CK6 chondrites overall have the lowest 
and highest REE values, and NWA 8186 plots between 
these two samples. CKs typically display a negative Eu 
anomaly [4,6] or none at all [6], comparable to NWA 
8186. The positive Tm anomaly seen in NWA 8186 has 
also been observed in other CK and CV chondrites [4,5]. 
Geochemically, NWA 8186 plots within the range of 
CK meteorites with respect to REE abundances.  
The CK parent body is thought to have 
accreted ~2.6 ± 0.2 Ma after CAI formation 
[20]. Merrillite can be easily reset by im-
pacts, so the old merrillite date of 4553 ± 21 
Ma rules out heating by late impact pro-
cesses. We interpret this to be a crystalliza-
tion age. However, this age can imply that 
either: 1) NWA 8186 was heated by the de-
cay of short-lived isotopes on the CK parent 
body, or 2) NWA 8186 was heated by early 
impacts immediately following formation 
on the parent body.  
Due to the similarities in bulk composi-
tion (major, minor, and trace), mineralogy, 
and isotopes [14-17], we are classifying 
NWA 8186 as a CK chondrite. Texturally, 
NWA 8186 differs from all known CK6 
meteorites, and it appears to have under-
gone a higher degree of heating. Therefore, 
we are further classifying NWA 8186 as a 
type 7 CK chondrite. The age we obtained 
of 4553 ± 21 Ma leads us to believe that this 
meteorite crystallized and was heated on the CK parent 
body. Although an impact origin may be possible as a 
heat source, NWA 8186 lacks any clear evidence for 
melting or high pressure phases. All of the silicate 
phases are homogeneous (including plagioclase), and 
many triple junctions are present between mineral 
phases, indicating slow cooling. Although the ‘type 7’ 
classification has not previously been used to describe 
carbonaceous chondrites, we believe it is more fitting 
than an ‘achondritic’ or ‘CK6’ classification.  
References: [1] Weisberg et al. (2006) Meteorites and 
the Early Solar System. Arizona Press. 19-52. [2] Kallemeyn 
et al. (1991) GCA, 55, 881-892. [3] Friedrich et al. (2014) 
GCA, 139, 83-97. [4] Noronha & Friedrich (2014) MAPS, 49, 
1494-1504. [5] Dauphas et al. (2015) GCA, 163, 234-
261. [6] Hirota et al. (2002) Geochem. Journal, 36, 309-
322. [7] Huss et al. (2006) Meteorites and the Early So-
lar System. Arizona Press. 567-586. [8] Dodd et al. 
(1975) GCA, 39, 1585-1594.  [9] Tait et al. (2014) 
GCA, 134, 175-196. [10] Righter K. and Neff K. E. 
2007. Polar Sci.:1:25–44. [11] Noguchi (1993) Proc. 
NIPR Symp. Ant. Met., 6, 204-233. [12] Chaumard N. 
and Devouard B. 2016. MAPS:51:547–573. [13] 
Schmieder et al. (2016) LPSC XLVII, Abstract #1646.  
[14] Agee C.B. et al. (2014) MetSoc., 77, #5385. [15] 
Srinivasan et al. (2015) LPSC, 46, #1472. [16] Sriniva-
san et al. (2016) LPSC, 47, Abstract #1620. [17] Srini-
vasan et al. (2016). MetSoc, 47, #6455. [18] Shaulis et 
al. (2010) G-cubed, 11, 1-12. [19] McDonough W. F. 
and Sun S.-s. (1995) Chem. Geo., 120, 223-253. [20] 
Sugiura N. and Fujiya W.  (2014) MAPS, 49, 772-787. 
Acknowledgements: Special thanks to the 
UNM Meteorite Museum for allocations of NWA 
8186.  
Figure 1. NWA 8186 and CK chondrite [4,6] REE patterns normalized to 
CI chondrite. NWA 8186 has higher REE abundances than the other CKs, 
with the exception of Y-82191 [6]. A Tm anomaly is also observed in NWA 
8186, similar to some of the other CKs.   
Figure 2. Weighted average plot of 207Pb/206Pb from merrillite 
grains in NWA 8186 giving a date of 4553 ± 21 Ma. 


Reassessing the Formation of CK7 Northwest Africa (NWA) 8186

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