Tec#onophysjcs, 184 (1990) 21-33

Elsevier Science Publishers B.V., Amsterdam

21

40Ar/39Ar laser dating of a single grain of magnetite

6zden 6zdemir and Derek York

Department of Physics, University of Toronto, Toronto, Ont. M5S IA7 (Canada)

(Received November 25,1989; revision accepted March 7, 1990)

ABSTRACT

&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: 21-33.

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 16OO’C. 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.

Introduction

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 49Ar/39Ar

analyses (Buchan et al., 1977; Berger et al., 1979; York, 1984; Layer et al., 1989). All such compari- sons between paleomagnetic and 4oAr/39Ar

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.

Magnetite can form at temperatures below the Ar blocking temperature of micas and amphiboles, i.e. < 300°C.

Ideally one would like to be able to check the formation age of magnetite directly. Any mineral which contains enough potassium is potentially

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 B.V.

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

exchange.

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 magnetite’s 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

al., 1970).

Experimental procedure

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 1600°C and 1400°C 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 + 5°C. 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 al., 1987).

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

TABLE 1

Argon data for magnetite

Temperature

(“0

Vol. “Ar

(X*O-1z~m3 STP)

Atmospheric

contamination (W)

Age (Ma) ’

*la

Hb3gr b, J = 2.313 x 1O-2 rf: 0.009 x 1O-2

360

400

500

550

600

700

750

800

900

1000

1050

1150

1400

1600

Integrated results

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

Age==l%2&39MaC

a Ages were cafcufated using the decay constants of Steiger and figer (1977).

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

Mognetfte

16

3000

-$ 2500

z & 2000

P 1500

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 errors.

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 3’Ar/3’Ar 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 115O’C 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 sample’s 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

25

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 900°C heating step, possibly because of argon release from new sites in the biotite.

Discussion

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 400°C and 1150°C 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& retawe@

Fig. 2. Age spectrum and G/K ratio for the single-grain biotite from the same hand sample as the magnetite. Shaded areas indicate

f 113 errors.

26 0. OZDEMIR ANI) D. YORK

TABLE 2

Argon data for biotite

Temperature Vol. 39Ar,

(“C) (X lo-” cm3 STP)

Hb3gr b, J= 2.313 x lo-* & 0.009 x 1O-2

Atmospheric

contamination (R;)

“Ar/“Ar *OAr*/‘9Ar, Age (Ma) ’

flu

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

Integrated results

3’Ar/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 J&ger (1977).

h Hb3gr is a standard hornblende with an age of 1071 Ma (Turner et al., 1971; Roddick, 1983).

5Op”, 700” and 1000°C. 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. 5).

A fourth large phase of 38Ar, 39Ar and 40Ar* is

released in the final step at about 1600°C, 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 75O’C. 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 7OO“C, not far from that of biotite.

Arrhenius diagram

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 Boltzmann’s 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-

2500- 50-

..+.. 3?Af

..“q..... ‘fiir

___o___ ?4,

-._.&_ _ %Ar

OL OL

Temperature PC1

Fig. 3. Thermal release patterns of the various argon isotopes 36Ar, “Ar, 38Ar and 39Ar, together with %r*, for the single-grain

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 1600°C were

S1290,243,110,16 and - 9 respectively. 1 count = 2 X IO-t5 cm3 STP.

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

600 -120-

500-IOO-

C

200-40-

100-20-

?T9mpem#un9 f”cl

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.

28

0.6 0.6 I .o I .2 1.4 I .6 I.6

1ooO/ TPK.l

Fig. 5. Arrhenius diagram for the single-grain magnetite. Treat-

ing each linear segment as a distinct argon phase. activation

energies of 34 It 6, 66 f 8 and 68 + 5 kcal/mol were found for

?Ar reteased in the temperature ranges 360-500”. 550-750°

and 800-1050°C 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 360-550°C

0. OZDEMIR ANT) D. YORK

yield an activation energy of 34 + 6 kcal/mol

(Table 3). The second line, in the range 550-800°C.

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,

respectively.

The data points for the biotite grain in the

temperature range from 500” to 800°C 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 800-1000°C 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 1250°C and yield an

activation energy of 34 I~I 3 kcal/mol. The release

TABLE 3

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 line.

b E is a experimental activation energy. Errors in this table are lo.

’ This is the pre-exponential frequency factor derived from an Arrhenius plot.

d Ta is the isotopic btocking temperature calculated with Dodaon’s (1973) formula. The blocking temperatures for the magnetite and

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. 2). 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,

29

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 40Ar/39Ar.

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 O’Reilly, 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 950-1250°C respectively. The error bars are flu. Note the different pat-

terns and activation energies compared to magnetite (Fig. 5).

30

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 Harrison, 1988).

Naturally occurring magnetite also contains dislocations. Heider et al. (1987) studied a natural

2 4 4000-

3

2000-

8000

Si

8000

6000

Fig. 7. (a) Scanning eiectron micrograph of a single magnetite grain from the same hand sample as the single grains of magnetite and

biotite on which 40Ar/39Ar analysis was carried out. Light areas (A) are magnetite, grey areas in the right corner of the grain (3 and

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.

il”Ar,‘39Ar LASER DATING OF A SINGLE GRAIN OF MAGNETITE 31

Fig. 8. Crystal structure of magnetite, after Morrish (1965).

Magnetite has a spine1 crystal lattice. The large oxygen anions

form a close-packed face-centred cubic lattice. The ionic radius

of the oxygen anions is about 1.32 A which is much larger than

that of metal cations (Fe’+ and Fe3+, 0.83 and 0.67 A

respectively). In this oxygen lattice, two kinds of interstitial

sites occur, the tetrahedral (A) and the octahedral (B) sites

which are surrounded by 4 and 6 oxygen ions respectively. In

this cubic unit cell, 64 tetrahedral and 32 octahedral sites are

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 the

bracket are in the tetrahedral sites and those inside the bracket

are in octahedral sites. In addition to the octahedral and

tetrahedral sites, vacant lattice sites can also occur owing to

missing oxygen ions in this cubic close-packed structure. The

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.

Conclusions

We draw the following conclusions: (1) @Ar* was trapped in measurable quantities

in a 300 pm magnetite grain. (2) The central portion of the age spectrum

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.

Acknowlqfgements

We would like to thank Drs. Paul Layer and Chris Hall for their experimental assistance and

32

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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