The ancient Martian orthopyroxenite ALH84001experienced a complex history of impact and aqueous alteration events. Here we summarize Sm-147-Nd-143 and Sm-146-Nd-142 analyses performed at JSC. Further, using REE data, we model the REE abundance pattern of the basaltic magma parental to ALH84001 cumulus orthopyroxene. We find the Sm-146-Nd-142 isotopic data to be consistent with isotopic evolution in material having the modeled Sm/Nd ratio from a time very close to the planet's formation to igneous crystallization of ALH84001 as inferred from the Sm-Nd studies

Nyquist, Laurence E.

Shih, Chi-Yu

NASA Technical Reports Server

PEERING THROUGH A MARTIAN VEIL: ALHA84001 Sm-Nd AGE REVISITED.  L. E. Nyquist
1
 and C-Y. 
Shih
2
, 
1
KR/NASA Johnson Space Center, Houston, TX 77058 (E-mail: laurence.e.nyquist@nasa.gov), 
2
Mail code: 
JE-23, ESCG/Jacobs Technology, P.O. Box 58477, Houston, TX 77258-8477 (chi-yu.shih-1@nasa.gov). 
 
Introduction:  The ancient Martian orthopyroxe-
nite ALH 84001experienced a complex history of im-
pact and aqueous alteration events. Treiman [1] identi-
fied petrographic evidence for its involvement in four 
or five crater-forming impacts following its initial 
crystallization “in a body of magma somewhere be-
neath Mars’ surface”. Adopting his relative chronology 
of events, fractured, granular bands present in 
ALH84001 were formed in an early (first?) impact 
event. The accompanying thermal metamorphism ho-
mogenized mineral compositions and probably was 
accompanied by production of feldspathic glass from 
igneous feldspars. In a later event, fractures in the 
granular bands became hosts to carbonate rosettes that 
often are found in association with the feldspathic 
glass. Sm-Nd studies [2,3] yielded ages of ~4.5 Ga, 
and carbonate formation was dated at 3.90±0.04 Ga by 
the Rb-Sr method and 4.04±0.10 Ga by the U-Th-Pb 
method [4]. The Sm-Nd ages have been cited as giving 
the time of igneous crystallization of ALH84001, an 
interpretation challenged by [5] on the basis of an ~4.1 
Ga Lu-Hf age. 
Here we summarize 
147
Sm-
143
Nd and 
146
Sm-
142
Nd 
analyses performed at JSC. Further, using REE data 
[6-8],  we model the REE abundance pattern of the 
basaltic magma parental to ALH84001 cumulus ortho-
pyroxene. We find the 
146
Sm-
142
Nd isotopic data to be 
consistent with isotopic evolution in material having 
the modeled Sm/Nd ratio from a time very close to the 
planet’s formation to igneous crystallization of 
ALH84001 as inferred from the Sm-Nd studies. 
147
Sm-
143
Nd: Fig. 1 shows results for 
147
Sm-
143
Nd 
analyses at JSC of 22 bulk samples and mineral sepa-
rates. An isochron fit (Isoplot model 1 [9]) gives an 
age of 4.568±0.088 Ga (2) and Nd = +1.2±0.8 rela-
tive to a Chondritic Uniform Reservoir (CHUR, [10]). 
These values are within uncertainty of those originally 
reported for five bulk samples and a pyroxene separate 
[3]. We attribute an apparently irreducible scatter in 
the 
147
Sm-
143
Nd data (MSWD ~ 100) to post-magmatic 
disturbance of the Sm-Nd system. 
A more restricted set of Sm-Nd data from [5] is in 
good agreement with our own. An isochron fit to four 
data presented by [5] (S2-S3-S4-R1) gives an age of 
~4.63 Ga. (Their bulk rock leachate datum (L1) is 
omitted from the regression). Tentatively adopting the 
~4.09 Ga Lu-Hf age as the crystallization age and or-
thopyroxene as an end-member component on a hypo-
thetical mixing line results in a calculated Nd ~+5 for 
orthopyroxene data from both labs implying a source 
of the ALH84001 parental magma depleted in LREE. 
146
Sm-
142
Nd: Fig. 2 shows Isoplot model 1 results 
for data for 16 samples yielding  initial 
146
Sm/
144
Sm 
(I(Sm)) = 0.0031±0.0009 and 142Nd = -0.36±0.12 at 
CHUR 
147
Sm/
144
Nd = 0.1967 [10]. Elevated values of  
142Nd >0 for the pyroxenes and 142Nd <0 for samples 
of low 
147
Sm/
144
Nd, particularly for leachate Opx(L) 
(~phosphates) and bulk rock samples, are inconsistent 
with the ~4.09 Ga Lu-Hf age. MSWD = 5.1 shows 
these data to be much less disturbed by post-magmatic 
reheating than the 
147
Sm-
143
Nd data, probably because 
events later than ~4.1 Ga are not registered. The age 
calculated relative I(Sm)=0.0076 and T=4.558 Ga for 
angrite LEW 86010 (equivalent to I(Sm)=0.0081 at T 
Orthopyroxenite ALH84001
147
Sm/
144
Nd
0.1 0.2 0.3 0.4 0.5
1
4
3
N
d
/1
4
4
N
d
0.510
0.512
0.514
0.516
0.518
0.520
CHUR
Opx(l)
Opx
WR(r)
WRs
Opx(r)
R1
S3 (WR)
S2
(pure Px)
S4 (dirty Px)
L1
T=4.568±0.088 Ga
Nd=+1.2±0.8
Opx1
JSC data
Lapen et al. (2010)
WR
T
Lu-Hf, WR
=4.09 Ga
Nd= -1.6
(Lapen et al., 2010)
T
Lu-Hf, Px
=4.09 Ga
Nd= +4.8
(Lapen et al., 2010)
Figure 1. 147Sm-143Nd isochron plot for bulk samples and 
mineral separates of ALH84001. The isochron is fit to JSC 
data. Data from [5] adjusted for differing normalizations 
are shown for comparison. 
Figure 2. 146Sm-142Nd data for bulk samples and mineral 
separates of ALH84001. 
147
Sm/
144
Nd
0.15 0.20 0.25 0.30 0.35 0.40 0.45
  
1
4
2
N
d
 (
re
l 
to
 E
a
rt
h
)
-2
-1
0
1
2
3
WR(l)
Opx(r)
Orthopyroxenite ALH84001
WR
WR(r)
WR
CHUR
Earth
Opx(l)
T=4.568 Ga
T
Lu-Hf
=4.091 Ga
(Lapen et al., 2010)B1 (Lapen et al., 2010)
Age rel to I(Sm)=0.0076 at T=4.558 Ga for LEW86010
I(Sm)=0.0031±0.0009
(142Nd)
Earth
= -0.36±0.12
T
LEW
=4.43±0.05 Ga
(4.47±0.04 Ga new  t1/2)
https://ntrs.nasa.gov/search.jsp?R=20130011255 2019-08-31T00:19:42+00:00Z
= 4.568 Ga) is T=4.425 +0.039/-0.054 Ga.  Alterna-
tively, for the newly determined   halflife of 
146
Sm (t1/2 
= 68 Ma [11]) and I(Sm)=0.0084 at 4.568 Ga, the cal-
culated relative age of ALH 84001 is 4.470 +0.035/-
0.026 Ga. 142Nd = -0.23±0.05 reported by [5] for a 
large (~1 gm) bulk sample (B1) is consistent with this 
isochron if 
147
Sm/
144
Nd (not measured), is estimated 
from its measured 
143
Nd/
144
Nd ratio. 142Nd = 
+0.19±0.13 previously reported by [12] for a bulk 
sample is not plotted because 
147
Sm/
144
Nd was not 
measured. However, we do not consider this analysis 
to be inconsistent with the 
146
Sm-
142
Nd isochron be-
cause bulk (“WR”) samples range up to 147Sm/144Nd 
~0.3 as required by this analysis (Fig. 1). Taken to-
gether, the 
142
Nd analyses of [5] and [12] are incon-
sistent with a nearly flat isochron as required by an age 
of ~4.1 Ga. 
Modeled REE abundances in parent melt: In 
Fig. 3, solid symbols represent REE patterns for ortho-
pyroxene samples from AHL 84001, taken from ion 
microprobe analyses of mineral grains [7, 8] and ICP-
MS analyses of orthopyroxene separates [6]. Open 
symbols represent REE patterns for melts parental to 
the orthopyroxene as calculated using the average of 
seven sets of REE distribution coefficients in Opx [6, 
13, 14]. The calculated parental melts are high in REE 
abundances, are LREE-enriched, and have an average 
147
Sm/
144
Nd of 0.17±0.01. The REE pattern of Martian 
crust [15] and NWA 7034 [16] are also plotted for 
comparison. The calculated REE abundances in the 
parental melts match those estimated for the Martian 
crust very well. Similarly high REE abundances occur 
in NWA 7034 [16], but differ by being slightly higher 
in overall REE abundances and having a negative Eu 
anomaly. Interestingly, 
147
Sm/
144
Nd = 0.171 in NWA 
7034 [16] equals that in the estimated Martian crust. 
Nd isotopic evolution prior to the parent melt: 
Fig. 4 shows the 
147
Sm-
143
Nd (upper) and 
146
Sm-
142
Nd 
(lower) results (yellow parallelograms). Red and green 
(for [11]) curves show Nd isotopic evolution from an 
HED (Howardite-Eucrite-Diogenite) or Earth-like par-
ent body at T = 4.568 Ga for 
147
Sm/
144
Nd = 0.17±0.01. 
Because the 
147
Sm-
143
Nd isochron was disturbed, the 
age it suggests may be biased to a value somewhat too 
old. The 
146
Sm-
142
Nd isochron probably represents a 
better estimate of a magmatic age of 4.4-4.5 Ga for 
ALH 84001. The modeled 
142
Nd evolution is consistent 
with a LREE-enriched source as calculated for the par-
ent magma from the REE abundances in pyroxene. 
Conclusions: We tentatively identify disturbances 
evident in the Sm-Nd data with the early, strong de-
formation and thermal metamorphic event identified 
texturally [1]. This event likely reset the Lu-Hf age, 
and other ages for radiometric systems less robust than 
Sm-Nd. The Sm-Nd system is particularly robust as 
demonstrated most recently for chondrites [17] be-
cause both the isotopic parent and daughter are REE. 
References: [1] Treiman A. H. (1998) Meteoritics & Planet. 
Sci., 33, 753–754. [2] Jagoutz E. et al. (1994) Meteoritics 29, 478-
479. [3] Nyquist L. E. et al. (1995) LPS XXVI, 1065–1066. [4] Borg 
L. E. et al. (1999) Science 286, 90-94. [5] Lapen T. J. et al. (2010) 
Science, 328, 347-351. [6] Barrat J.-A. and Bollinger C. D. (2010) 
Meteoritics & Planet. Sci., 45, 495–512. [7] Papike, J.J. et al. (1994) 
LPSC XXV, 1043-1044. [8] Wadhwa, M. and Crozaz, G. (1998) 
Meteoritics & Planet. Sci., 33, 685-692. [9] Lugwig K.R. (2008) 
Isoplot/Excel ver. 3.7. [10] Jacobsen S. B. and Wasserburg G. J. 
(1984) EPSL 67, 137-150. [11] Kinoshita N.  et al. (2012) Science 
335, 1614-1617. [12] Wadhwa, M. and Borg, L. (2006) LPS XXXVII, 
CD-ROM #2045. [13] Frei, D. et al. (2009) Contrib. Mineral. Petrol. 
157, 473-490. [14] van Kan Parker, M. et al. (2010) Contrib. Miner-
al. Petrol. 159, 459-473. [15] Taylor S.R.and McLennan, S.M. 
(2009) Planetary crusts: their composition, origin and evolution, 
Cambridge Univ. Press. [16] Agee C. et al. (2013) Science Express. 
Science DOI: 10.1126. [17] Martin C. et al. (2012) Ant. Met. 35, 38. 
Figure 3. Modeled REE abundances in ALH84001 parent 
melt. 
La             Ce        Nd   Sm Eu      Tb Dy Ho       Yb Lu
Orthopyroxenite ALH 84001
REE Ionic Radii
S
a
m
p
le
s
 /
 C
I
0.01
0.1
1
10
100
ALH 84001 Opx
(Papike et al., 1994; Wadhwa & Crozaz, 1998;
Barrat & Bollinger, 2010)
NWA 7034
Cal. Parental Melts
(
147
Sm/
144
Nd=0.17±0.01)
Martian crust
(Taylor & McLennan, 2009)
Figure 4. Modeled Nd-isotopic evolution between Mars' 
formation and crystallization of ALH84001. 
Orthopyroxenite ALH 84001
 
1
4
3
N
d
(r
e
l 
to
 C
H
U
R
)
-1
0
1
2
3
Time before Present (Ga)
4.3 4.4 4.5 4.6 4.7
 
1
4
2
N
d
(r
e
l 
to
 E
a
rt
h
)
-0.8
-0.4
0.0
0.4
ALH 84001
=
147
Sm/
144
Nd
=0.1967
=0.1967
CHUR
Earth
CHUR (CIT)
ALH 84001
Age rel to I(Sm)=0.0076 at T=4.558 Ga for LEW86010
t1/2=68Ma
t1/2=103Ma
=0.17
=0.16
=0.17
=0.18
=0.18
=0.16
HED Y98
(+1.41±0.51)


Peering Through a Martian Veil: ALHA84001 Sm-Nd Age Revisited

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