Tec#onophysjcs, 184 (1990) 21-33
Elsevier Science Publishers B.V., Amsterdam
40Ar/39Ar laser dating of a single grain of magnetite
6zden 6zdemir and Derek York
Department of Physics, University of Toronto, Toronto, Ont. M5S
(Received November 25,1989; revision accepted March 7, 1990)
&demir, ii. and York, D., 1990. Ar/39Ar laser dating of a
single grain of magnetite. In: R. Van der Voo and P.W. Schmidt
(Editors), Reliability of Paleomagnetic Data. Tectonophysics, 184:
We have determined an 40Ar/39Ar age spectrum for a single grain
of magnetite using laser step-heating. A 300 pm
magnetite gram from a Grenville Front gneiss was heated in 14
steps to 16OOC. Excess 40Arf resulted in a saddle-shaped age
spectrum with an integrated age of 1962 f 39 Ma. However, the
minimum of the saddle gave an age of 1144 f 107 Ma, consistent with
typical Grenvillian cooling ages of llOO-900 Ma. We have also dated
a single gram of biotite from the same hand sample as the
may&e. The spectrum yielded an integrated age of 1261 + 3.6 Ma.
The thermal release patterns of *OAr* and of Ca/K ( 37Ar/3 Ar) were
very different for the magnetite and biotite. Thus 40Ar* can be
retained in measurable quantities in magnetite. Our results are a
fit step toward the attempted direct dating of different components
of NRM in paleomagnetism.
The iron-oxide minerals can carry a faithful record of the
geomagnetic field at the time of
original cooling of an igneous rock or deposition
of a sedimentary rock. The direction of the natural
remanent magnetization (NRM) derived from
paleomagnetic analysis may indirectly provide the age of the
magnetization by comparison with established apparent polar wander
paths (APWPs). However, it often happens that the APWP is am-
biguous (that is, paleopoles of different ages have similar
directions) or poorly established. This is particularly the case
for Paleozoic or Precambrian cratonic rocks and for rocks of any
age from accreted terranes. For these rocks, we need an independent
method of dating NRMs.
The K/Ar and more recently the ?Ar/39Ar radiometric methods,
have been widely used for the direct dating of potassium-bearing
minerals. Because of the parallels between magnetic and
argon blocking temperatures, it has sometimes been possible to
infer ages of magnetization from cooling histories determined from
analyses (Buchan et al., 1977; Berger et al., 1979; York, 1984;
Layer et al., 1989). All such compari- sons between paleomagnetic
analyses make the assumption that the iron oxides
and the potassium-bearing minerals such as bio-
tite, hornblende and K-feldspar formed effectively
simultaneously and experienced the same cooling
history. However, magnetite, which is the most common iron-oxide
mineral in continental ig-
neous, sedimentary or metamorphic rocks, is fre- quently
secondary, diagenetic, hydrothermal or
deuteric in origin. Hence the cooling curve derived from the
ages of the potassium-bearing minerals can not necessarily be used
to determine the age of formation and magnetization of
Magnetite can form at temperatures below the Ar blocking
temperature of micas and amphiboles, i.e. < 300C.
Ideally one would like to be able to check the formation age of
magnetite directly. Any mineral which contains enough potassium is
suitable for Ar/39Ar dating. The potassium may be either an
integral cation in the mineral lattice or an impurity in the
mineral (D~~mple and
0040-1951/90/$03.50 6 1990 - Elsevier Science Publishers
22 0.6ZDEMIR AN11 D. YORK
Lanphere, 1969). It should be possible to date
magnetite provided that sufficient potassium is
present, either in the lattice, or in the form of
small inclusions of another mineral within the
magnetite grain under the assumption that the
encapsulating mineral act as a barrier to Ar and K
The possibility of applying the 40Ar/39Ar
method to dating magnetite has attracted very
little interest previously. This lack of interest has
been due to magnetites very low potassium con-
tent and the likelihood of excess argon contamina-
tion. These two factors can have a dramatic effect
on the precision and accuracy of the ages mea-
sured. Harrison and ~cDougal1 (1981) reported
an anomalously old single-fusion K-Ar age of
3653 + 40 Ma for magnetite from mafic rocks
from Broken Hill, Australia. This magnetite is
contaminated by excess argon and, as a result, its
K-Ar date is much older than 1660 Ma, the age of
high-grade metamo~~sm in the Broken Hill
block. Harrison and McDougall studied the equi-
librium concentration of excess Ar in magnetite in
order to estimate the ambient argon pressure dur-
ing this event.
Magnetite occurs as a low-temperature mineral
in chondritic meteorites and may have trapped
noble gases during its formation (Larimer and
Anders, 1967; Mazor et al., 1970). Several labora-
tory experiments have been carried out on mag-
netite grown in an Ar environment in an effort to
understand the environmental conditions in the
early solar system when the meteorites trapped
rare gases (Lancet and Anders, 1973; Honda et al.,
1979; Yang et al., 1982).
The present investigation has a different pur-
pose from those mentioned above. In our study, a
single gram of magnetite has been examined by
the OAr/Ar laser step-heating technique in an
effort to test the dateability of magnetite, which as
far as paleomagnetism and rock magnetism are
concerned is a uniquely important mineral.
The sample used in the present study was from
a metamorphic gneiss of the Red Cedar Lake
formation from 3 km south of the Grenville Front.
The Grenville Front is one of the major features
of the Canadian Precambrian Shield and forms
the boundary between the Grenville and Superior
Structural Provinces. Metamo~~c cooling ages.
in the range 90%1100 Ma, are typical of the
Grenville Province. Rocks near the Grenville Front
tend to give the older ages in this range (Harper,
1967; Anderson, 1988). Harper (1967) and Baer
(1976) interpreted this age gradient to be a result
of differential uplift away from the Front. How-
ever, it is also known that rocks near the Front are
particularly affected by excess argon (Wanless et
The crushed and sieved gneiss sample was ul-
trasonically cleaned several times with distilled
water and methanol. Mineral separates of biotite
in the range 300-500 pm and magnetites in the
range 200-500 pm were hand picked under a
microscope. Composite grams, containing biotite
and quartz as well as magnetite, were removed,
leaving a 99.9% pure magnetite separate. Mag-
netite grains from this separate were used for the
Ar/39Ar, X-ray and SEM analyses. X-ray powder
pictures, using a Gandolfi camera with Cu-Ka:
radiation and a silicon standard, gave numerous
lines typical of magnetite. Best resolved were the
low-angle reflections (220), (311), (222), (400),
(422), (5ll), (440) and (533). The X-ray cell-edge
parameter was 8.39 + 0.01 A, in close a~eement
with the standard value of u = 8.396 A (ASTM
data file 19-629).
The &Ar/Ar analyses were made using a con-
tinuous laser fusion system at the University of
Toronto. The system consists of a continuous
argon-ion laser (Spectra-Physics 171-18, 18 W)
and a VG-1200 S mass spectrometer (Layer et al.,
1987). The single grains of magnetite and biotite
together with single grams of standard hornblende
Hb3gr within a high-purity aluminum sample
holder were irradiated at McMaster University to
transform a portion of the 39K atoms to 39Ar
through the interaction of fast neutrons. Follow-
ing the irradiation, indi~dual grams were heated
by the laser through a sapphire window for 30 s in
an ultra-high vacuum system evacuated to = lo-*
Torr. The beam size was larger than the grain in
order to obtain uniform heating. The single grains
of magnetite and biotite were heated in 14 and 17
4oAr/9Ar LASER DATING OF A SINGLE GRAIN OF MAGNETITE 23
steps and fused at 1600C and 1400C respec- tively. After the
extracted gas had been purified by a getter and cold finger for 3
mm it was introduced into the mass spectrometer. Five argon
isotopes (8 scans for biotite and 10 scans for magnetite) were
measured by peak-hopping. The ~imum detectable signal is about 2 X
lo- cm3 STP. Typical background values for the extraction line were
7 x lo-l4 cm STP for mass 36,1 X lo-l3 cm3 STP for mass 39, and 1.8
x lo-* cm3 STP for mass 40. During the analysis the background was
monitored (about every third and fifth step for magnetite and
biotite respectively) and the back- ground values were subtracted
from the fractions that followed. A Barnes RM-2A infrared micro-
scope was used to estimate the temperature during each laser
step-heating. The uncertainty in the
temperature estimate was + 5C. The e~ssi~ties of the biotite and
magnetite grams were measured and found to be 70% and 78%
respectively of the black-body emissivity. The technical aspects of
the OAr/3gAr dating are described in detail elsewhere (Layer et
The 40Ar/39Ar age spectrum of the single-grain magnetite is
shown in Fig. 1. The spectrum ex- hibits a saddle-shape and yields
an integrated age of 1962 rf 39 Ma. This age is much older than the
age of Grenvillian metamo~~sm (e.g., Dallmeyer and Rivers, 1983)
and must reflect the presence of an excess argon component. The
high low-temper- ature ages decrease to a minimum and then the
Argon data for magnetite
Hb3gr b, J = 2.313 x 1O-2 rf: 0.009 x 1O-2
0.0145 64.63 0.670 94.70 2093 + 824
0.0200 59.13 0.111 60.63 1581-f 630
0.0699 39.38 0.848 54.49 1472+ 264
0.0142 76.85 2.175 33.87 1044 f 1911
0.0341 56.97 0.651 39.97 1181 f 659
0.1703 52.59 1.4436 44.79 1283 rt 134
0.1933 52.90 1.0479 38.29 1144f 107
0.1249 35.53 0.1379 38.98 1159* 142
0.1776 12.73 0.4129 48.81 1363 + 117
0.3247 5.82 0.5709 60.27 1575 f 54
0.2285 1.06 0.2304 63.78 1635 ?z 85
0.0448 (32.63) 10.5621 89.52 2024 f 334
0.0134 48.27 2.9339 58.51 1544 f 1358
0.4014 0.181 0.2807 199.27 3111 f 28
37Ar/39Ar = 0.8335 f 0.0377 .^ Total atmospheric ?Ar vol. =
3.2121 X lo-cm3 STP
Total 39Ark vol. = 1.8318 X lo-l2 cm3 STP
mAr*/39Ar,c = 85.02 k 2.17
a Ages were cafcufated using the decay constants of Steiger and
b Hb3gr is a standard hornblende with an age of 1071 Ma (Turner
et al., 1971; Roddick, 1983). J = (eA* - l),/(40Ar*/s9Ar),; where s
refers to the standard.
The K concentration of the single gram magnetite was determined
from 3gAr, and age using the relation t = (l/h)ln[l +
(X/&)(taAr*/40K)J where h = 5.543 X lo- yr-, h, = 0.581 X 10-r
yr-, t = 1.9616 X lo9 yr, oAr* = 0.695 X lo-l4 mol and *K = 3.371 X
lo-l4 mol. The potassium content was 0.0018% or 18 ppm.
24 ii. 6ZDEMIR AND D. YORK
z & 2000
Fmtion of Oar rehosed Fig. 1. Age spectrum and Ca/K and Cl/K
ratio for the single-grain magnetite from a gneiss sampled 3 km
south of the Grenville
Front (Ontario, Canada). Shaded areas indicate the + lo
ages rise to a maximum of 3111 + 28 Ma in the argon (Lanphere
and Dalrymple, 1976). The final fraction released. This shape of
age spectrum minimum of the saddle (usually considered to give has
been observed previously in other mineral a maximum estimate for
the true age) gives an age types and attributed to the presence of
excess of 1144 f 107 Ma (Table 1 ). This is in reasonable
4VAr,yAr LASER DATING OF A SINGLE GRAIN OF MAGNETITE
agreement with typical Grenvillian cooling ages of 1100 Ma near
the Grenville Front.
Figure 1 also shows a plot of the variation in the Ca/K and Cl/K
ratios calculated from 3Ar/3Ar and 38Ar/3pAr ratios of the
magnetite grain during gas release. Ca/K values decrease after
about 30% of gas release, then remain rela- tively constant in the
30-75s release range. How- ever, the Ca/K ratio increases
dramatically at about 75% of 39Ar release (in the 115OC heating
step), then decreases to its original level. This effect may be due
to some impurity and is associ- ated with an age of 2024 + 334 Ma
in the age spectrum.
Figure 2 shows the OAr/39Ar age spectrum of the single grain of
biotite. The grain was extracted from the same hand sample as the
magnetite. The release spectrum exhibits a stepwise age increase
over the first 20% of gas release, presumably indi- cating a
partial loss of argon at some time during the samples history. The
age spectrum reaches a well defined plateau of age 1260 Ma (Table
2). The spectrum yields an integrated age of 1261 & 4 Ma. These
ages are somewhat in excess of ex- pected Grenvillian ages.
However, biotites located
just south of the Grenville Front have been found to have
accumulated excess quantities of radio- genie argon (Wanless et
al., 1970; Dallmeyer and Rivers, 1983; Anderson, 1988), so although
the spectrum yields an apparently well-defined plateau, it is most
probably a false plateau (Hy- odo, 1989), because of the excess
argon compo- nent. Figure 2 also shows the plot of Ca/K ratios of
the same biotite. The Ca/K values are almost constant up to 70% of
the gas release, whereupon an increase in the Ca/K ratio begins at
the 900C heating step, possibly because of argon release from new
sites in the biotite.
Thermal release patterns
The various argon isotopes 36Ar, 37Ar, *Ar and Ar together with
radiogenic ?Ar* degassed at each temperature step from the single
grain of magnetite are shown in Fig. 3. The release pat- terns of
36Ar, 37Ar, 38Ar, Ar and 40Ar* are very similar between 400C and
1150C and indicate that the isotopes are released in three distinct
phases with release peaks at approximately
I I I I I I , I ,
1400 - Age =I261 f3.6Aia
I , I
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.0 0.9 1.0
Fmfim OF 3s& [email protected]
Fig. 2. Age spectrum and G/K ratio for the single-grain biotite
from the same hand sample as the magnetite. Shaded areas
f 113 errors.
26 0. OZDEMIR ANI) D. YORK
Argon data for biotite
Temperature Vol. 39Ar,
(C) (X lo- cm3 STP)
Hb3gr b, J= 2.313 x lo-* & 0.009 x 1O-2
Ar/Ar *OAr*/9Ar, Age (Ma)
450 2.503 15.31 ( - 0.0031) 34.61 1061+ 8
500 1.105 11.86 0.0279 31.32 983 + 21 550 2.638 5.43 0.0034
39.47 1170 f 10
600 5.761 1.31 0.0056 42.81 1242 + 5
650 10.189 0.69 0.0046 43.38 1254rfr 4
700 17.464 0.49 0.0027 43.66 1259 + 3
750 22.839 0.35 0.0013 43.63 1259 + 2
800 22.142 0.18 0.0029 43.83 1263 f 3
850 13.948 0.51 0.0028 43.89 12645 4
900 5.748 0.55 0.006 44.44 1276 f 4
950 4.091 ( - 0.54) 0.0094 45.87 1305+ 7
1000 3.783 0.041 0.0115 46.34 1314+ 8
1050 3.643 0.055 0.0119 46.36 1315* 6
1100 3.587 0.81 0.0189 46.17 1311 f 9
1150 2.934 ( - 0.96) 0.0484 46.35 1315 + 10
1250 3.155 1.31 0.0960 45.01 1287f 8
1400 2.705 0.19 0.0938 45.76 1303+ 6
3Ar/39Ar, = 0.0095 f 0.0007
Total atmospheric 40Ar = 4.7615 X 10-l cm3 STP
Total 39Ar, = 1.2823 X lo- cm3 STP
40Ar*/9Ar, = 43.15 * 0.06
Age = 1261 + 4 Ma
a Ages were calculated using the decay constants of Steiger and
h Hb3gr is a standard hornblende with an age of 1071 Ma (Turner
et al., 1971; Roddick, 1983).
5Op, 700 and 1000C. The three pulses may also reflect varying
radii of diffusion or mechanisms for Ar release. These three phases
have three dis-
tinct corresponding linear segments in the Arrhenius plot (Fig.
A fourth large phase of 38Ar, 39Ar and 40Ar* is
released in the final step at about 1600C, whereas virtually no
36Ar is seen here.
The fractions of 38Ar, 39Ar and ?Ar* released at each
temperature step for the single-grain biotite are plotted against
temperature in Fig. 4. The
release pattern of biotite shows only one peak, located at about
75OC. The release patterns for @Ar*, 39Ar and 38Ar are almost
identical and indi-
cate that they are released from similar trapping sites.
The second maximum in the magnetite release pattern occurred at
7OOC, not far from that of biotite.
The diffusion parameters E and Do/a2 in the expression:
D Do -E/k7 -_=---_e a2 a*
were calculated from least-squares fitting of mea- sured
diffusion coefficients D/a* to a straight line in an Arrhenius
diagram (Fig. 5). In the equation, Do is the frequency factor, a is
the diffusion dimension, k is Boltzmanns constant and E is the
activation energy required to transfer
ulAr,yAr LASER DATING OF A SINGLE GRAIN OF MAGNETITE 27
%* I jgar
10000 - -200
e 7500-150- 2 2 u 5000-loo-
-._.&_ _ %Ar
Fig. 3. Thermal release patterns of the various argon isotopes
36Ar, Ar, 38Ar and 39Ar, together with %r*, for the
magnetite. %r* was obtained from the equation WAr* = (40Ar)r -
295.5 X ( 36Ar)A, where (%r)r and (36Ar)A are total and
atmospheric argon concentrations respectively. The number of
counts for *Ar*, 39Ar, *AI, Ar and 36Ar after fusion at 1600C were
S1290,243,110,16 and - 9 respectively. 1 count = 2 X IO-t5 cm3
an Ar atom from one site to another. The slope of frequency
factor Q/a*. Errors shown in the data the best fitting line yields
an experimental activa- points are f la. If Ar diffuses by a single
mecha- tion energy E and the Y-intercept gives the nism and all
sites are identical, diffusion can be
Fig. 4. Thermal release patterns of %r*, 39Ar and %r for the
single-grain biotite. %r* was obtained from the equation OAr =
(%r)r - 295.5 x (36Ar),. The rekase patterns for %r*. 39Ar and %r
are almost identical, but are quite different from the
release patterns for &&-grain magnetite (Fig. 3). 1
count = 2 X lo- cm3 STP.
0.6 0.6 I .o I .2 1.4 I .6 I.6
Fig. 5. Arrhenius diagram for the single-grain magnetite.
ing each linear segment as a distinct argon phase.
energies of 34 It 6, 66 f 8 and 68 + 5 kcal/mol were found
?Ar reteased in the temperature ranges 360-500. 550-750
and 800-1050C respectively. The numbers denote the release
temperatures (Ct. The slope of each best-fit line defines an
activation energy E and the Y-intercept yields a frequency
factor Do/a2. Errors in the data points are + lo.
modelled with a single E (Dalrymple and
Lanphere, 1969). However, this is not the case for
the present magnetite. Ar diffusion is a function
of several different activation energies and these
energies apply to different temperature ranges.
The graph of log(D/a) versus l/T for the single
grain of magnetite defines three straight lines (Fig.
5). The three lines are interpreted to represent
three distinct phases within the magnetite and are
therefore treated as three independent reservoirs.
The first three points, in the range 360-550C
0. OZDEMIR ANT) D. YORK
yield an activation energy of 34 + 6 kcal/mol
(Table 3). The second line, in the range 550-800C.
gives E = 66 t- 8 kcal/mol. These data points cor-
respond to the plateau in the age spectrum shown
in Fig. I. The ~~-temperature line segment is
approximately parallel to the second line and
yields an activation energy of 68 + 5 kcal/mol. As
noted above, the observed line segments in the
Arrhenius diagram of the single grain of magnetite
are associated with the three peaks in the thermal
release patterns and also correspond to three seg-
ments of the age spectrum (Fig. 1): the decreasing-
age low-temperature portion, the saddle minimum
and the mid high-temperature rising-age section,
The data points for the biotite grain in the
temperature range from 500 to 800C lie on a
line in an Arrhenius diagram and give an activa-
tion energy of 42 f 2 kcal/mol (Fig. 6). This value
is close to the hydrothermal value for E of 47 -+_ 2
kcal/mol for biotite (Harrison et al., 1985). A
transition occurs in the range 800-1000C where
log(D/aZ) decreases with increasing temperature.
This peculiar behaviour has been observed by
many workers and has been attributed to the
breakdown of biotites when heated in vacuum
(Berger and York, 1981) or to the two-phase na-
ture of biotite (York and Lopez-Martinez, 1986).
The data follow a straight line for the gas fractions
released between 950 and 1250C and yield an
activation energy of 34 I~I 3 kcal/mol. The release
Activation energies and diffusion coefficients for 39Ar, from
magnetite and biotite
Sample Step a Eb D,,/atc TB d n/SUMS
(C) (kcal/mol) (SK) (C)
Magnetite 360- 500 33.9 i 6 (2.9;&) x 10 53 f 34 3/0.004
550- 750 65.7 + 8 (4.61;3,,s) x IO 256 i 30 4/0.55
800-1050 68.2 + 5 (1.3+&J x 109 330 f 23 4/0.89
Biotite 500- 800 41.9 + 2 (2.6;:;) x IO6 151 i 11 X/1.9
950-1250 33.7 * 3 (5.9::) x 102 137 + 23 5/0.39
This column gives the temperature (C) of the first and Iast
steps of the range of those fractions fitted to a straight
b E is a experimental activation energy. Errors in this table
This is the pre-exponential frequency factor derived from an
d Ta is the isotopic btocking temperature calculated with
Dodaons (1973) formula. The blocking temperatures for the magnetite
biotite were calculated with a cooling rate of SW/Ma.
SUMS is a statistical parameter (York, 1969); n is the number of
points fitted to a straight line.
Ar/39Ar LASER DATING OF A SINGLE GRAlN OF MAGNETITE
of argon in this higher-temperature range may be due to the
decomposition or phase change of biotite or to a second diffusion
process. This sec- ond segment in the Arrhenius plot corresponds to
the high Ca/K ratios and high ages in the age spectrum (Fig.
The activation energy of the low-temperature segment of
magnetite is probably lower (E = 34 + 6 kcal/mol) than that of
biotite (E = 42 + 2 kcal/mol). It should also be pointed out that
the second linear segment in the Arrhenius plot of magnetite gave
an activation energy of 66 & 8 kcal/mol, which is much higher
than that of bio- tite apparently. Argon diffuses with different
activation energies in biotite and magnetite.
This suggests that the saddle region of the magnetite age
spectrum (Fig. 1) is not due to argon released from a biotite
inclusion. It does not prove this, however, because presumably
argon released from such an inclusion still has to diffuse through
the imprisoning magnetite lattice before finally escaping.
A single grain of magnetite 150 pm in size was prepared for
microstructural examination. The grain was mounted in transparent
epoxy on a glass slide. The epoxy and embedded magnetite were
mechanically polished to a thickness of - 80 pm for scanning
electron microscope (SEM, Hitachi S-570) and X-ray energy
dispersive analy- sis. Figure 7 a shows the scanning electron
micro- graph of the single magnetite grain. An X-ray microanalyser
(LINK AN-1~~ was used to ex- amine the grain in the SEM and then
perform rapid analysis of selected features within the imaged area.
The unique position beam mode conveniently combines the electron
image with spectral data on the single-system display. The beam
current was 100 pA at 20kV. The beam diameter could be adjusted as
small as 35-40 A. Figure 7b shows the results of the X-ray energy
dispersive analysis of the examined areas on the magnetite grain.
Two inclusions, about 7 x 25 pm and 5 X 40 pm, can be seen in the
electron micro- graph. X-ray microanalysis indicates that the in-
clusion labelled B contains minor amounts of Mg,
Si and Al and inclusion C is probably ilmenite with Mn impurity.
Neither of the inclusions con- tains significant quantities of
potassium as an impurity. However, this observation should not be
generalized to the single magnetite grain which we dated by
Locations for argon
We can speculate as to the trapping sites in magnetite.
Imperfections such as crystallographic point defects and
dislocations could act as traps for argon in the magnetite crystal.
Crystallo- graphic point defects are lattice errors at isolated
points in the crystal. They can be vacancies where atoms are
missing or impurity atoms occupying a lattice site (Wert and
Thompson, 1970). Vacancies are generally present in magnetite
crystals. The cation vacancies are mainly located in octahedral
sites @Donovan and OReilly, 1978) and are not large enough to
contain either a K or an Ar atom (Fig. 8). In contrast, anion
vacancies in magnetite seem more likely to hold a potassium or an
argon atom (DuFresne and Anders, 1962). Castle and Surman (1967,
1969) have studied the self-diffu- sion of oxygen in magnetite and
the effect of anion vacancies. They have determined that the
activation energy for creating a vacancy by oxygen
Fig. 6. Arrhenius diagram for the single-grain biotite. Treating
each linear segment as a distinct argon phase, activation en-
ergies of 42% 2 and 34& 3 kcal/mol were found for Ar released
in the temperature ranges 500-800 and 950-1250C respectively. The
error bars are flu. Note the different pat- terns and activation
energies compared to magnetite (Fig. 5).
diffusion is 7-8 kcal/mol, which leads to an equi- librium
concentration of 4 x 10 vacancies/g.
Crystallographic point defects can also be caused by bombardment
by fast neutrons in the nuclear reactor. The number of point
defects pro- duced in this way is dependent on the nature of the
magnetite crystal. A significant concentration
of displaced atoms can be created by this bombardment and it is
possible that significant changes in the properties of magnetite
can be produced (Hutch&on and Baird, 1967; McDougall and
Naturally occurring magnetite also contains dislocations. Heider
et al. (1987) studied a natural
2 4 4000-
Fig. 7. (a) Scanning eiectron micrograph of a single magnetite
grain from the same hand sample as the single grains of magnetite
biotite on which 40Ar/39Ar analysis was carried out. Light areas
(A) are magnetite, grey areas in the right corner of the grain (3
C) are inclusions. (b) X-ray energy dispersive analysis of areas
A, B and C shown in (a). The analysis was carried out with an
AN-10000 X-ray microanalyser at the University of Toronto.
ilAr,39Ar LASER DATING OF A SINGLE GRAIN OF MAGNETITE 31
Fig. 8. Crystal structure of magnetite, after Morrish
Magnetite has a spine1 crystal lattice. The large oxygen
form a close-packed face-centred cubic lattice. The ionic
of the oxygen anions is about 1.32 A which is much larger
that of metal cations (Fe+ and Fe3+, 0.83 and 0.67 A
respectively). In this oxygen lattice, two kinds of
sites occur, the tetrahedral (A) and the octahedral (B)
which are surrounded by 4 and 6 oxygen ions respectively. In
this cubic unit cell, 64 tetrahedral and 32 octahedral sites
present, of which only 8 and 16 respectively are occupied by
Fe*+ and Fe3+ cations. The cation distribution of magnetite
can be written Fe3+[Fe3* Fe 2+ IO,, where cations outside
bracket are in the tetrahedral sites and those inside the
are in octahedral sites. In addition to the octahedral and
tetrahedral sites, vacant lattice sites can also occur owing
missing oxygen ions in this cubic close-packed structure.
unoccupied octahedral and tetrahedral sites are not large
enough to contain an Ar atom (1.54 A).
single crystal of magnetite whose dislocation den- sity was (1.0
f 0.8) X 10 m-. The dislocation density, which is the number of
dislocation lines that intersect a unit area in the crystal, was
de- termined by counting etch pits. Stacking faults, associated
with partial dislocations have also been reported in magnetite
(Jakubovics et al., 1978). Figures 8 and 9 of their study show a
high density of stacking faults lying on (110) planes of mag-
netite crystals, with fault vectors of l/4 (110).
It seems reasonable to suppose that the prin- cipal trapping
sites for argon in magnetite are anion vacancies and dislocations.
Oxygen diffu- sion in water vapour seems likely to occur along
paths such as grain boundaries, dislocations or micropores (Castle
and Surman, 1969).
Summary and future study
In the single magnetite grain analysed, ages in the low- and
high-temperature portions of the gas release exceed typical
Grenvillian cooling ages. However, the central portion of the age
spectrum reveals a geologically not unreasonable age. Before making
a statement about whether or not mag- netite is suitable for
OAr/39Ar dating, there are two questions to be answered. First, is
the plateau age of Fig. 1 due to the magnetite itself? In other
words, is the associated potassium a cation impur- ity in the
magnetite lattice? Interestingly enough, a potassium ferrite of
composition KFe,,O,, has been reported (Gorter, 1954), showing that
potas- sium can be incorporated in a hexagonal ferrite lattice at
least. Secondly, is the potassium associ- ated with inclusions in
the magnetite grain? At present, both questions are difficult to
answer. Ideally, one would like to carry out microstruc- tural
studies before OAr/39Ar step-heating experi- ments. We have already
analysed one single grain of magnetite and did not discover any
biotite inclusions (Fig. 7). However, we were not able to do a
similar SEM study on the single magnetite grain on which the Ar/Ar
experiment was car- ried out. In the future, we will repeat our
experi- ments using larger single crystals of magnetite. This will
allow us to carry out *Ar/39Ar and SEM/X-ray energy dispersive
analyses on the same grain.
We draw the following conclusions: (1) @Ar* was trapped in
in a 300 pm magnetite grain. (2) The central portion of the age
gave an age of 1144 f 107 Ma which is consistent with typical
Grenvillian cooling ages of 1100-900 Ma.
(3) Our results are preliminary but they are a first step toward
the direct dating of magnetite and of its NRM.
We would like to thank Drs. Paul Layer and Chris Hall for their
experimental assistance and
Dr. Mike Easton of the Ontario Geological Survey for supplying
the gneiss sample. Malcolm Back of the Royal Ontario Museum and
Fred Neub of the MetaBurgy and Materiat Science Department kindly
carried out X-ray and SEM analyses. We are grateful to Drs. David
Dunlop and Minor-u Ozima for their inspiration and valuable discus-
sions and to Dr. Tulhs Onstott for reviewing and co~ent~g on the
manuscript. We also thank Carolyn Moon for typing the manuscript.
The project was supported by grants to D.Y. from the Ontario
Geological Survey and Natural Sciences and Engineering Research
Council of Canada.
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