A NEW APPROACH TO IN-SITU K-Ar GEOCHRONOLOGY.
J.A. Hurowitz
1
, K.A. Farley
2
, N.S. Jacobson
3
,
P.D. Asimow
2
, J.A. Cartwright
4,2
,
J.M. Eiler
2
, G.R. Rossman
2
, Kathryn Waltenberg
2
,
1
Jet Propulsion Laboratory,
California Institute of Technology, Pasade
na, CA (joel.a.hurowitz@jpl.nasa.gov),
2
Division of Geological and Plan-
etary Sciences, California Institute of Technology, Pasadena, CA,
3
NASA Glenn Research Ce
nter, Cleveland, OH,
4
Abteilung Biogeochemie, Max Planck Institut für Chemie, Mainz, Germany.
Introduction:
The development of an in-situ geo-
chronology capability for Mars and other planetary
surfaces has the potential to
fundamentally change our
understanding of the evolution of terrestrial bodies in
the Solar System. For Mars specifically, many of our
most basic scientific questions about the geologic his-
tory of the planet require
knowledge of the absolute
time at which an event or
process took place on its
surface. For instance, what was the age and rate of
early Martian climate change recorded in the mineral-
ogy and morphology of surf
ace lithologies (e.g., [1])?
In-situ ages from a few select locations within the
globally established stratigraphy of Mars would be
transformative, enabling us to place
direct
chronologic
constraints on the timing and
rates of impact, volcanic,
sedimentary, and aqueous processes on the Martian
surface.
The current paradigm for establishing absolute ag-
es on the Martian surface is through statistical methods
based on lunar crater counting techniques. Progress has
been made in previous decades, improving the preci-
sion of such estimates (e.g., [2]). However, precision is
not equivalent to accuracy,
and two inescapable facts
regarding crater counting techniques remain: (1) “Any
estimate of the Martian absolute chronology involves,
implicitly or explicitly, an estimate of the Mars/Moon
cratering rate ratio” [3]; and (2) Mars is geologically
active, continually removing
the record of craters and
therefore causing bias towards younger ages. Thus,
uncertainties on the accuracy
of crater counting ages
can exceed a factor of 2 in portions of Martian geolog-
ic history where constraints
on crater flux are particu-
larly poor. These issues will continue to cast doubt on
crater age accuracy until radiometric age tie-points are
provided for Mars.
Previous and ongoing efforts at the design of in-
situ geochronology systems have targeted precisions of
± 15-20%. The new methodology we propose for
measuring in-situ potassium-argon (K-Ar) ages has the
potential to significantly improve on this measurement
precision. Such improvements would increase the utili-
ty of in-situ ages for early Martian history, where
crater counting methods are thought to be at their most
accurate [3], and enable us to meaningfully address
rates of processes on Mars.
The Issue of Excess Ar in Shergottite Meteorites
In discussions with our colleagues it is clear that
there is a general uneasiness regarding the feasibility
of accurately K/Ar dating Martian basalts. The reasons
for this uneasiness are most cl
early stated in a series of
papers by Bogard and colleagues [4-6]. The main con-
cern is that the Ar/Ar ages
of the most abundant class
of SNC meteorites – the shergottites – are substantially
older than their formation ages determined from other
radiometric systems such as
Sm/Nd, Rb/Sr and U/Pb.
For example, Zagami is thought to have crystallized at
170 Ma, yet its Ar/Ar “age”
(depending on what phase
and what type of analysis is being considered) is more
in the range of 300 Ma (and in some cases far older)
[4]. Several studies have demonstrated that the cause
of this discrepancy is excess Ar in the shergottites.
This Ar is thought to be derived from shock implanta-
tion of atmospheric gases and from trapping of mag-
matic Ar [5, 7]. There are several reasons that we still
believe that attempting K/Ar age determinations on
Martian basalts is worthwhile:
1)
Other classes of SNC’s give accurate K/Ar
ages. For example, nakhlites and Chassigny yield
Ar/Ar ages of about 1.35 Gyr, very similar to ages ob-
tained from other techniques [8]. Thus it is not clear
how pervasive this excess Ar
problem really is in terms
of the surficial coverage of Mars (as opposed to in the
very unusual subset that has been launched to Earth).
Specifically the shegottites are very shocked rocks
(possibly associated with the launch event) and so may
be expected to have larger amounts of shock implanted
gases than typical surface basalts.
2)
Bogard et al. [5] make the very curious obser-
vation that the amount of excess Ar in Shegottite min-
eral fractions is rather constant in their analyses, about
1x10
-6
cm
3
STP of
40
Ar. This is important because in a
basalt with a typical shegottite K concentration this is
equivalent to about 200 Myr of Ar accumulation (i.e.,
the K/Ar age would be about 400 Myr instead of the
“true” age of most shegotittes of ~200 Myr). However
in a 2 Gyr basalt with the same K content and amount
of excess Ar, the excess would
yield just a 6% error in
age (and this reduces to just 2% in a 3.5 Gyr basalt). In
other words, the fact that the shergottites are so young
is what makes this effect
so noticeable. In our opinion
an error of this magnitude
is acceptable, since the frac-
tion of extremely young basalts (<500 Myr) on the
surface of Mars is almost certainly very low.
3)
Bogard [6] notes the difficulty of obtaining
the very high temperatures necessary to completely
extract Ar from basaltic melts. Because we employ a
flux-assisted digestion technique this issue is not rele-
vant to our proposed approach (see below).
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A New Approach to K-Ar Geochronology
: We
recognize that there are two
major technical hurdles to
making an accurate and precise in-situ K-Ar age meas-
urement: (1) achieving melt
ing temperature for rocks
in order to quantitatively degas them of argon, and (2)
measuring sample mass as a means of relating K-
concentration to absolute abundance of argon. Here,
we describe a new approach to in-situ measurement of
K-Ar ages that solves both of these problems simulta-
neously.
In broad terms, our measurement protocol involves
four steps. In step 1, a crushed or powdered sample is
delivered to a crucible in a vacuum chamber, which
has been loaded on Earth (i.e., prior to flight) with a
lithium-based fluxing agent and a solid double-spike
containing known amounts of isotopically enriched
tracers of
39
Ar and
41
K. The
39
Ar
Spike
/
41
K
Spike
ratio is
thus known, forming the basis of our age calculation
(see below). In step two, the sample-flux-spike mixture
is melted by heating the cr
ucible with resistance heat-
ers. The flux agent contains LiBO
2
, which melts at
849
o
C and Li
2
B
4
O
7
, which melts at 920
o
C. Heating to
temperatures between 950-1000
o
C therefore readily
achieves melting and isotopic homogenization of the
sample-spike-flux mixture. In step three, the ratio of
radiogenic
40
Ar from the sample over
39
Ar from the
spike is measured on a ma
ss spectrometer. Argon is
quantitatively outgased by melting of the sample, al-
lowing us to solve for radiogenic
40
Ar in the sample by
measurement of the equilibrated sample/spike ratio. In
the final step (4), the
39
K
Sample
/
41
K
Spike
ratio is analyzed
via Knudsen Effusion Mass Spectroscopy (KEMS,
[9]), using the same mass spectrometer as that used for
the Ar-isotopic measuremen
t. The measured K- and
Ar-isotopic ratios are then used to solve for a whole
rock age. We have validated each of the individual
steps in this analytical pro
cedure, as described below.
Calculating K-Ar Ages using Double Isotope Dilution:
The K/Ar age equation is:
ൌݐ
1
ߣ
lnሺ
ߣ
ߣ
כݎܣ
ସ
ܭ
ସ
1ሻ
where
is the total
40
K decay constant,
e
is the decay
constant for the electron ca
pture decay mode that pro-
duces
40
Ar, t is the K/Ar age, and
40
K and
40
Ar* are
abundances in atomic units.
40
Ar* is the radiogenic
daughter product, and is thus the
40
Ar attributable to
in-situ
radioactive decay. Thus an age determination
requires measurement of the
40
Ar*/
40
K ratio.
K consists of three isotopes, with masses of 39
(93.3%), 40 (0.0117%), and 41 (6.73%). For isotope
dilution K measurements,
41
K of high isotopic purity
(>99%) is readily available.
There are three stable iso-
topes of Ar (of masses 36, 38, and 40) and isotope di-
lution is usually done using
38
Ar as a tracer. Since
38
Ar
is a useful indicator of cosmic ray exposure, we con-
sider instead the use of synthetic
39
Ar. This radioactive
isotope has a half-life of 269 years and is routinely
produced by neutron irradiation of
39
K bearing sub-
stances.
When a spike containing the isotopic tracers
41
K
and
39
Ar is added to the sample, and the combination
fused to release and measure Ar isotopes and K-
isotopes, the
40
Ar*/
40
K ratio can be determined as fol-
lows (all variables are as defined in Table 1).
Table
1.
Definition
of
Variables
40
Ar*
in situ produced radio
g
enic
Ar amount (unknown)
39
Ar
spk,
40
Ar
spk
amount of Ar isotope in
spike
40
Ar
m
,
36
Ar
m
,
39
Ar
m
measured Ar amounts
40
Ar
air
,
36
Ar
air
air-derived argon amounts
R
air
= (
40
Ar/
36
Ar )
air
Known
R
spk
=(
40
Ar/
39
Ar)
spk
independently determined
R
m
=(
40
Ar/
39
Ar)
m
Measured
39
K
u
,
40
K
u
,
41
K
u
amounts of K in unknown
39
K
spk
,
41
K
spk
amounts of K in spike
r
m
= (
39
K/
41
K)
m
Measured
r
spk
= (
39
K/
41
K)
spk
independently determined
r
nat
= (
39
K/
41
K)
nat
natural K isotopic composi-
tion (known)
r
40
= (
40
K/
39
K)
nat
natural K isotopic composi-
tion (known)
First, for Ar:
ݎܣ
ସ
ݎܣ ൌ
כ
ݎܣ
ସ
ସ
ݎܣ
ସ
௦
(1)
ݎܣ
ଷ
ݎܣ ൌ
ଷ
(2)
ݎܣ
ଷଽ
ݎܣ ൌ
ଷଽ
௦
(3)
Where equation (1) is the mass balance for
40
Ar and
indicates that some of the
40
Ar is derived from “con-
tamination” with (terrestrial or martian) atmospheric
argon. The the second and third equations indicate that
all
36
Ar is air-derived and all
39
Ar is spike-derived.
Combining these equations and the definitions in Table
1 yields:
ݎܣ
ସ
כ
ൌ ൬ܴ
ܴ െ
ቀ
యల
యవ
ቁ
ܴ െ
௦
ݎܣ൰
௦
ଷଽ
(4)
In the case of K:
ܭ
ଷଽ
ൌ ܭ
௨
ଷଽ
ܭ
௦
ଷଽ
(5)
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ܭ
ସଵ
ൌ ܭ
௨
ସଵ
ܭ
௦
ସଵ
(6)
ܭ
௨
ସ
ݎൌ
ସ
ܭ
௨
ଷଽ
(7)
Where equations (5) and (6) reflect mass balance and
equation (7) states that the isotopic composition of the
unknown sample is that of natural potassium. Combin-
ing these equations and the definitions in Table 1
yields:
ܭ
ସ
௨
ൌ ݎ
ସ
ቆ
ೞೖ
ି
ೝ
ೝ
ೌ
ିଵ
ܭቇ
ସଵ
௦
(8)
Combining equations 4 and 8 yields the ratio from
which the K/Ar age is determined:
రబ
כ
రబ
ൌ
൬
ோ
ି ோ
ೌೝ
൬
ಲೝ
యల
ಲೝ
యవ
൰
ି ோ
ೞೖ
൰
రబ
൭
ೝ
ೞೖ
షೝ
ೝ
ೝ
ೌ
షభ
൱
ቀ
యవ
రభ
ቁ
௦
(9)
It is important to note that by using equation 9, the
K/Ar age can be computed
directly from measured
isotopic ratios without knowledge of the mass of the
sample or of the spike glass. This is a fundamental
advantage of our age-dating technique, and is built on
the fixed
39
Ar
spk
/
41
K
spk
ratio. Furthermore, because we
only require the measurement of isotopic ratios with
our technique, instrument calibration becomes much
simpler or possibly even unnecessary.
Synthesis of a Double Isotope Spike:
We have prepared
an alkali-feldspar glass that contains the isotope dilu-
tion spikes
41
K and
39
Ar. The glass was prepared in a
2-step synthesis procedure. In the first step, pure oxide
(SiO
2
, Al
2
O
3
) and carbonate (Na
2
CO
3
) components
were fused at 1150
o
C in a Pt-crucible in an open-tube
gas mixing furnace to form a melt. This melt was held
above the liquidus for ~48 hours and then quenched to
form albite glass (NaAlSi
3
O
8
). The product glass was
then crushed with a mortar and pestle and mixed with
KCl (obtained from the Oak Ridge National Laborato-
ry) that is enriched in the
41
K isotope (99.17%
41
K).
This mixture was also loaded
in a Pt crucible and fused
at 1150
o
C into an open-tube
gas mixing furnace that
was continuously purged with a CO
2
-H
2
gas mixture.
The gas mixture was “tuned” to maximize pH
2
O at
1150
o
C and remove Cl from the mixture as HCl vapor,
leaving the K in the melt. Experiments at low pH
2
O
revealed that K is quantitatively lost from the melt
owing to the relatively high volatility of KCl. The melt
was held above the liquidus for ~96 hours and then
quenched to form a glass. Electron microprobe analy-
sis of this glass indicates typical K
2
O
Total
concentra-
tions of 6-8 wt%. Because the KCl contains a trace of
39
K (
≤
0.83%), the glass can be neuton irradiated using
typical Ar-Ar irradiation conditions. This produces
39
Ar in the glass by neutron capture, yielding our final
double-isotope solid spike. Our synthetic glass was
subjected to neutron irradiation for ~50 hours in the
Oregon State University TRIGA reactor.
Argon Isotopic Measurements:
We have performed a
variety of experiments designed to measure the Ar-
isotopic composition of gases released from silicate
materials fused in the presence of a Li-based fluxing
agent (50% Li-metaborate,
50% Li-tetraborate). In
early experiments, a K-feldspar sample was mixed
with a SrCl tracer, in order to determine whether flux-
assisted digestion of silicate minerals results in argon
release at low temperatures (1000
o
C), and whether or
not sample-tracer homogeneity is achieved by flux-
assisted digestion. The results shown in
Fig. 1
indicate
that Ar-release occurs during flux digestion. In addi-
tion, laser ablation ICP-MS analysis of the glass pro-
duced during this experiment has a homogeneous K/Sr
ratio (not shown), demonstrating sample-spike equili-
bration during flux-assisted melting.
Fig. 1
:
40
Ar/
36
Ar isotope ratio of K-feldspar measured
as a function of time on a quadrupole mass spectrome-
ter during flux-assisted digestion. The
40
Ar/
36
Ar ratio
is significantly elevated
relative to atmospheric
(
40
Ar/
36
Ar=296), indicating release of radiogenic
40
Ar
(i.e.,
40
Ar*) from the sample.
In subsequent experimentation, we mixed a 132
Ma basalt obtained from the Parana basin of Brazil
with our double-isotope spike and flux-melted the mix-
ture at 1000
o
C. This yielded measurements of 60%
radiogenic Ar from this young, low-K (<0.5 wt %
K
2
O) basalt.
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Potassium Isotope Measurements with KEMS:
We
have performed a series of experiments at the NASA
Glenn Research Center that were designed to deter-
mine whether the application of KEMS could provide
a suitable means of measuring the K-isotopic composi-
tion of vaporized K during low-temperature flux-
assisted melting. Briefly, KEMS involves melting of
the sample in a specially designed cell (in our case,
made of Mo), production of a vapor that exits the top
of the crucible in the Knudsen flow regime, line of
sight ionization of neutral vapor species, and meas-
urement of the ionized is
otopes on a mass spectrome-
ter, in our case a magnetic sector instrument [9]. The
high resolution of the magnetic sector instrument al-
lows separation of the
39
K peak from background hy-
drocarbons, and hence more accurate measurements.
Our experimental re
sults are shown on
Fig. 2
,
which is a plot of measured
39
K as a function of tem-
perature for: (1) Parana basa
lt, (2) Parana basalt melted
in the presence of flux, (3) non-isotopically enriched
spike glass melted in the pr
esence of flux, and (4) flux
as a control. The salient results of these experiments
can be summarized as follows: KEMS provides long
lived (2-3 hours), stable
39
K signals from all of the
analyzed samples (except fo
r the control, which con-
tains no measureable K); K measurements can be made
at low temperature (<1000
o
C) during flux assisted
melting, (3) the onset of K-vaporization occurs at the
same temperature for both the spike glass and the bas-
alt during flux-assisted melting, indicating that equili-
bration of K-isotopes from sample and spike should be
readily achievable.
Future Work:
Presently, we are synthesizing a second
batch of an isotopically-enriched spike that will be
used to produce our first age measurement using the
methodology we have developed. This will be accom-
plished in a 2-step procedure in which a mixture of
flux, Parana basalt, and spike glass are combined in a
Knudsen cell crucible and melted in a furnace attached
to a noble gas mass spectrometer in the Geological and
Planetary Sciences Department at Caltech. The gases
released following melting will be analyzed to measure
the Ar-isotopic composition. The melt will then be
quenched, and the Knudsen cell crucible containing the
glass will be sent to NASA
Glenn Research Center for
measurement of the K-isotopic composition of the
glass using a KEMS instrument. From these isotope
ratio measurements, the age of the Parana basalt sam-
ple will be computed.
Once we have successfully demonstrated an end-
to-end age measurement using this technique, we will
begin the construction of a single benchtop instrument
that can accomplish the en
tire measurement in a sin-
gle-step melting procedure. Due to the simplicity of
our measurement technique, the benchtop instrument
system will be relatively straightforward to build and
operate. Essentially what we require is a resistance
furnace, a line-of-sight ionizer, standard plumbing for
enrichment and sequestration of noble gases, and a
small mass spectrometer capable of measuring isotope
ratios. From this basic benchtop architecture, we can
design a suitably miniaturized instrument system for
field testing and eventual flight prototyping.
In summary, we have developed an inherently
simple experimental methodology that can be em-
ployed for the measurement
of K-Ar whole rock ages
on the surface of Mars, the Moon, and other Solar Sys-
tem bodies of interest. Our technique requires neither
high fusion temperatures fo
r Ar-release, nor a means of
weighing aliquots of sample to relate K-concentration
to Ar-abundance; both of which have been significant
technical hurdles that have
hampered previous at-
tempts to produce in-situ instruments for K-Ar geo-
chronology. This technique is applicable to materials
with low-K concentrations, and requires only the
measurement of isotope ratios, making in-flight mass
spectrometer calibration simple or perhaps even un-
necessary.
References:
[1] Bibring, J.P., et al. (2006)
Science
312,
400-404. [2] Hauber, E., et al.
GRL
38,
5. [3] Hartmann,
W.K. and Neukum, G. (2001)
Space Science Reviews
96,
165-194. [4] Bogard, D.D. and Park, J. (2008)
MAPS
43,
1113-1126. [5] Bogard, D., Park, J., and Garrison, D. (2009)
MAPS
44,
905-923. [6] Bogard, D.D. (2009)
MAPS
44,
3-14.
[7] Walton, E.L., Kelley, S.
P., and Spray, J.G. (2007)
GCA
71,
497-520. [8] Nyquist, L.E., et al. (2001)
Space Science
Reviews
96,
105-164. [9] Copland,
E. and Jacobson, N.,
Measuring Thermodynamic Prop
erties of Metals and Alloys
With Knudsen Effusion Mass Spectrometry
, in
NASA Tech-
nical Paper 2010-216795
. 2010. p. 54.
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Fig. 2
:
39
K signal measured on the NASA Glenn Research Cent
er KEMS as a function of temperature . Note that
the onset of
39
K signal for flux assisted melting of basalt and spike occurs at 845
o
C, which is the melting point of
the Li-metaborate flux.
1146
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International
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2012
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