Electron Microprobe Analyses and Magnetic Properties of Non

Geophys. J . R. ustr. SOC. (1970) 21.485-511.

Electron Microprobe Analyses and Magnetic Properties of Non-Stoichiometric Titanomagnetites in Basaltic Rocks

I(. M. Creer and J. D. Ibbetson

(Received 1970 July 1)

Summary

Chemical analyses have been made of non-stoichiometric (i.e. slightly oxidized) titanomagnetites in basaltic rocks from Argentina, Turkey, the Azores, the Canary Islands, Japan, Bulgaria, Aden and the Pacific area. The compositions do not differ significantly from the average compositions of titanomagnetite grains which contain ilmenite lamellae (usually taken as evidence of deuteric oxidation) in other basalts from the same regions and also from Oregon.

The shapes of the thermomagnetic curves are considered to provide a rapid and useful approximation to the degree of non-stoichiometry. This has been determined more precisely from observations that the measured Curie points are invariably higher than the compositional ones, calculated for stoichiometric titanomagnetites of the same composition. The degree of non-stoichiometry so obtained has been represented by plotting the estimated compositions of the grains studied on a triangular plot.

1. Introduction

Interest in the chemical composition of titanomagnetites occurring in basalts and diabases has developed rapidly in recent years principally from considerations of the origin and stability of the natural magnetization of these rocks. Electron probe microanalyses of optically homogeneous basaltic titanomagnetites have been reported by Ade-Hall (1964), Ade-Hall, Wilson & Smith (1965), Carmichael & Nicholls (1967), Schult (1968) and Smith (1967) and of exsolved material by Smith (1968) and Wilson, Haggerty & Watkins (1968). Some of these authors compared the chemical and magnetic properties. Ade-Hall (1964) found a limited range of compositions for which the Curie points should be between 0" and 100°C whereas the measured Curie points lay between 0" and nearly 600°C. Attention was drawn to the fact that the Curie points of rocks tend to be higher than the value predicted by the mole per cent of ulvospinel present assuming a stoichiometric pure titanomagnetite (O'Reilly & Banerjee 1967; Wright & Lovering 1965). Several suggestions were made for this discrepancy including: (a) oxidation resulting in cation vacancies and some order-disorder phenomena or (b) submicroscopic exsolution of titanium and iron-rich phases. Pouillard (1950) found that the substitution of aluminium and chromium for iron in magnetites slightly depresses the Curie point and Frolich, Loffler & Stiller (1965) discussed how impurity cations may affect the Curie point. In the microprobe analyses of Ade-Hall (1964), Ade-Hall et al. (1965) and Smith (1967), A1 and Mg were not determined although it is known from chemical analyses

485

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486 K. M. Creer and J. D. Ibbetson

that they are often present to the extent of a few per cent in natural titanomagnetites (Carmichael & Nicholls 1967; Vincent 1960; Wright 1964; Wright & Lovering 1965). However, their presence cannot account for the size of the discrepancy reported. Schult (1968) found that magnetic properties of oxidized titanomagnetites (titano- maghemites) agreed with those predicted by Verhoogen (1956, 1962), a result which differs from that of Akimoto, Katsura & Yoshida (1957) whose samples however have since been shown not to have been titanomaghemite as originally thought (Ozima & Larson 1969). It has been suggested that self-reversal of magnetization may occur in titanomagnetites which contain more than about 50 mole per cent of ulvospinel (O'Reilly & Banerjee 1967; Schult 1968), but up to the present time reversible self-reversal connected with N-type thermomagnetic curves seems to be always below room temperature.

Chemical analyses of titanomagnetite grains separated from igneous rocks generally show that they are oxidized (Nagata 1961). A correlation between polarity and the state of oxidation has been reported by several authors, e.g. Ade-Hall & Wilson (1963), Ade-Hal1 (1964), Wilson & Watkins (1967) and Wilson (1966) but this correlation appears not to be general (Larson & Strangway 1966; Ade-Hall & Watkins 1970).

2. Experimental method

2.1 Principles The theory and experimental techniques involved in electron probe micro-

analysis of titanomagnetites have been discussed by Smith (1965) and Heinrich (1968a, b). The instrument employed by us was a Cambridge Instrument Company ' Geoscan ' which has two linear spectrometers capable of examining elements in the atomic number range 11 to 95. The high take-off angle of 75" of this instrument minimizes the overall absorption corrections (Yakowitz & Heinrich 1968). Never- theless, care must be taken in the preparation and mounting of samples (Heinrich 1963; Long & Sweatman 1969), since errors in flatness and orientation cause variations in the emergence angle of the X-rays generated.

2.2 Preparation and selection of samples Rock discs of 2-5cm diameter were prepolished in several stages using wetted

silicon carbide papers on the rotating discs of a Strauer ' Rotor ' machine. They were finished with a series of diamond pastes using fibre covered laps on a Strauer ' D P ' machine. Between stages the samples were cleaned by ultrasonics in a water bath to prevent contamination of laps with coarser paste from the previously used lap.

As the conductivity of basalts is rather low, the samples were coated with a 100 A thick conducting layer of copper to avoid electrostatic charging of the sample. This would deflect the electron beam and cause unsteady probe conditions. With an accelerating voltage of 20 kV, the electron beam current was adjusted to produce specimen currents of the order of 0.04pA. Considerable time and care was taken in the selection of grains suitable for analysis. The following criteria were adopted: (i) that if possible the grains should be larger than 20microns to avoid edge effects, (ii) they should have been identified as titanomagnetites under the ore microscope, (iii) they should exhibit low topographical relief, and (iv) preliminary analysis should have indicated little or no silica, the presence of which was taken to indicate that the grain was ' wedging out ' and that silicates beneath were being analysed.

2.3 Procedure and corrections to measurements The X-ray intensities generated in pure metallic samples of the elements to be

determined in the grains were first measured. Then having selected a good spot or


Non-stoicbiomelric titaeomagnetites in basaltic rocks 481

area on a grain, the intensities of the same X-ray lines generated in the rock were measured. Finally the standards were remeasured in order to detect and allow for instrumental drift (Heinrich 1968a; Smith 1967). This rarely amounted to more than 2 per cent over a three-hour period. As a check, samples of pure ulvospinel and magnetite were analysed together with the unknowns during most runs. If the analyses of these ‘substandards’ were out by more than 3 per cent, having made corrections for linear drift, all the analyses made during that run were rejected.

The ‘ raw results ’ obtained directly from the probe readings were corrected (i) for dead time of the electronics, (ii) for mass absorption under Philibert’s (1963) model modified by Duncumb and Shields (1966), (iii) for fluorescence (Reed 1965), and (iv) for back-scattered and stopped electrons (factors R and S respectively, Duncumb & Reed 1967).

Heinrich (1968b) has discussed the common sources of error in electron probe microanalysis. His comparison of experiment and theory showed Philibert’s equa- tion to be satisfactory. The effect of errors in the input parameters in the absorption (Philibert 1963) and fluorescence schemes (Reed 1965) shows that significant errors can arise if Philibert’s function has a value less than 0.8 or if wavelengths greater than 10 A are used (see also Long & Sweatman 1969).

In the grains we examined, iron, titanium and oxygen constitute the main elements although the latter could not be measured at the time these measurements were made. The concentrations of Mg, Al, Cr, Ca, P, Zn, Ni and Si were also measured. Si was measured in order to detect whether the silicates surrounding the ore grain under examination were being excited by the probe. The apparent mass concentration of the copper coating film was also determined. An average composition of a given grain was found by rastering, that is by scanning the electron beam over an area (up to about 20microns square) lying within the boundaries of the grain. For those samples containing only small ore grains which were too small to raster, spot analyses were made.

2.4 Objects of study-required accuracy We have selected grains which appear homogeneous in colour and reflectivity

when observed under the ore microscope and start with the supposition that we are dealing with unoxidized, stoichiometric (albeit slightly impure) titanomagnetites. We then test the validity of this supposition by measuring the Curie point and com- paring this with that expected for stoichiometric titanomagnetites of the measured composition.

2 . 5 Normalization of results Since we did not measure the oxygen content, we could not normalize the sum

of the analysed weights per cent of cations and anions to 100 per cent, and so we developed a graphical method for making the sum of the analysed weights per cent of cations equal to the value (which we call C) expected for a stoichiometric, though chemically impure titanomagnetite.

The proportion by weight of cations in a given titanomagnetite depends on the fraction of titanium and the ‘impurities’ present. If the series is expressed as IdFeJ-a-xTi,04, then the ratio Tix/Fe3-a-x depends only on the value of 6 and not on the nature cf the impurity element. If it is assumed that the impurities replace iron then the x value or position on the magnetite-ulvospinel line can be deduced from the Ti/Fe ratio and the associated value of 6 (Fig. 1).

However, the cation sum depends on the atomic weight of the impurity elements. This is illustrated in Fig. 2 for different values of S for Mg and A1 and means that if the effective value of 6 can be estimated from the observed weight per cent of Mg and A1 (e.g. 0.1 atoms of Mg per molecular formula unit, i.e. 6 = 0.1, corresponds to


488 K. M. Creer and J. D. Ibbesson

/

- 0.4

- 0.3

- 0.2

- 0.1

t I I I I I I I

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 X

FIG. 1. Curves of iron-titanium ratios vs. molecular proportion (x) of ulvospinel for different amounts of ' impurity ' cation 1. 6 is defined in the molecular formula

I b Fe3-b-x Ti,04. I = Mg or Al.

approximately 1.1 per cent by weight of Mg and the same molecular proportion of A1 impurity corresponds to 1.2 per cent by weight of Al), then the cation sum expected can be estimated for an observed Ti/Fe ratio. In general, the cation sum obtained from the analysis will differ slightly from the expected value due to experimental error and then the factor f required to obtain agreement can be obtained by simple division. This will not change the ratio of Ti/Fe but will change the estimate of the expected cation sum. First, a factorfis applied and the effect on 6 noted. It is then usually possible to obtain a revised value of .f that gives an estimate of 6 and cation sum consistent with each other. Elements whose atomic number is close to that of Fe and which are present to only a minor extent can be added to Fe in this procedure (0.5 per cent by weight of Mn or Cr correspond to 6 = 0.02).

Normalization factors f are given in the Tables 1 and 2 where it is seen that they are usually greater than unity. Many of our earlier analyses gave low values because we deposited too thin a conducting film as we subsequently proved. Pre-analyses of some samples revealed that although the weight per cent of cations analysed in some of the earlier runs were low, the relative proportions were correct. Normalization factors f greater than about 1.1 which remained high for subsequent analyses suggest that the ore grains under examination were not titanomagnetites but rather titano-


Ti,

1.0-

0.9

0.8

0.7 -

%&-X

-

-

0.6 -

0.5 - 0.4-

0.3 -

0.2-

0.1 -

Non-stoichiometric titanomagnetites in basaltic rocks

Cation sum for lines of constant S i\ . \ i \

489

Cation sum expected

FIG. 2. Total weight per cent of cations expected for different amounts of impurity cation (Mg or AI). Titanomagnetite series - Mg substituted; - - - - - A1 substituted. Haematiteilmenite series .................. Mg substituted; - - - Lines

of constant Ti molecular proportions.

maghemites, ilmohematites or pseudobrookites or else that they contained sub- micron-sized holes which would produce the same effect of total low cation content. In such cases we made further observations with the ore microscope.

Random errors were estimated by making independent measurements over about five rasters on different grains in each polished section. For moderately good analyses, the standard deviation was usually of the order of the difference in percentage of cations in the unoxidized spinel series and that in the oxidized rhombo- hedral series (2.5 per cent if expressed as equivalent iron). A generalization may be made that in the Argentinian and Turkish basalts there were as many grains with apparent excess as with apparent deficiency of cations as expressed byf, so it seems likely that these ' titanomagnetite ' grains must in fact be relatively unoxidized.

2 . 6 Estimation of ' compositional' Curie points ' Compositional ' Curie points were estimated from the electron microprobe

measurements by calculating the effective x value, i.e. the compositional position along the magnetite-ulvospinel join of the ternary diagram, magnetite being denoted by x = 0, and ulvospinel by x = 1. The Curie point of pure magnetite was taken as 575"C, that of ulvospinel as - 120°C and a linear variation was assumed to hold between the two extremes.

The x value was found in two ways, (i) Figs 1 and 2 were used to deduce x from the ratio of Ti/(Fe+ Mn+Cr), expressed as weights per cent, for an estimated amount (6) of Mg and Al, and (ii) the Ti content alone was used: pure ulvospinel contains 21.4 per cent by weight of Ti but replacement of Fe by lighter atoms increases this percentage by reducing the effective atomic weight, so that if we divide the weight per cent of Ti by 21.4 we obtain an upper limit to the compositional x value. The Curie point deduced from this x value will therefore be low. In Fe2-,Y,TiO4 the


Sam

ple

Loco

litY

No.

Arg

entin

a 11

7 11

8 13

9 15

2 20

3 29

3 37

6 T3

3 T3

4

Turkey

A57

A

41

A10

6 D

123

D12

7 G

143

H12

2 T9

1 K

27

BT3

1

Tabl

e 1

Ana

lysi

s of

app

aren

tly u

nexs

olve

d* ti

tano

mag

netit

e gra

ins

No. o

f an

alys

es

Fe

Ti

Mg

Al

Mo

Ni

Zn

Cr

Weig

ht p

er c

ent o

f cat

ions

5 5 5 5 5 5 5 5 5 3 2 5 2 2 2 5 2 2 2

50-9

15

.1

1-3

1.8

0.6

0.0

0.1

1.0

49-0

17

.3

1.8

1.4

0-6

0.1

0.1

0.1

52.1

11

.6

2.5

1.8

0.5

0.1

0.1

1.9

50.5

16

.1

1.6

2-0

0.6

0.1

0.1

0.1

524

12.8

2.

6 1.

7 0.

5 0.

0 0.

1 0.

3 51

.8 13

.6

2-3

1.7

0.5

0.0

0-1

0.2

51-7

13

.4

2.7

1-9

0.5

0.0

0.1

0.2

50.1

16

.1 1.9

0

9

0.5

n.d.

0.

1 0.

1 50

-5

15.1

2.

0 1.

9 0-

5 n.

d.

0.4

0.1

52.9

15

.0

0.7

0.8

0.5

0.0

0.2

0.1

52.1

14

.5

1.7

1.5

0.6

n.d.

n.

d.

n.d.

53

.9

13.7

1-

1 0.

9 0.

5 0-

0 0.

1 0.

5 52

.9

15.9

0.

6 0.

8 0

5

0.0

0.2

0.1

50.3

18

.3

1.1

0.5

0-1

0.1

0.4

0.0

54.0

15

.5

0.6

0.6

0.5

0-0

0.1

0-2

51.5

15

.9

1.4

0.7

0.5

0-1

0.1

0.2

54.8

13

.7

0.4

1.2

0.9

0.0

0-2

0.0

49.6

12

.4

3.2

3.0

0.6

0.1

0.1

0.1

50.2

18

.2

0.5

0.6

0-7

0.0

0.2

0.1

Cal

cula

ted

quan

titie

s C

fR

xT

,

70.8

1.

12

0.29

0.

68

100"

70

.4

1-08

0.

35

0.74

60"

70.4

1.

04

0.20

0.

51

220"

70

9 1.

07

0.29

0.

70

85"

70.4

1.1

0 0-

22

0.51

22

0"

70.2

1-

09

0-26

0.

61

150"

70

-4

1.07

0.

24

0.61

15

0"

70.9

1.

26

0.32

0.

74

60"

70.5

1.

09

0.27

0.

65

120"

71.1

1.

06

70.7

1.

06

71.1

1-

25

71.3

1.

01

71.0

1.

04

71.3

1.

03

70.9

1.

06

71.3

1-

05

69.6

1.

07

71-2

1.

07

0.28

0.

69

0.26

0.

63

0.24

0.

61

0.30

0.

74

0.35

0.

81

0.28

0.

66

0.29

0.

71

0.24

0-

62

0.24

0.

52

0.35

0.

84

75"

135"

15

0"

140"

10

" 11

0"

80"

140"

16

0"

-25"

4 4 2 2 2 1 1 1' 2 2 1 3 1 1 1 1 1 1 1'

-

520

550

250

&50

0 20

0 &

550

300

&53

0 18

0-25

0 -

150-

300

-

100-

150

-

350

-

-

200-

350

300

300?

2w

-400

10

0-20

0 30

0 1 2

0 18

0-50

0 10

0-20

0 23

0


Tab

le 1

(con

tinue

d)

Azo

res

Ade

n

Japa

n

Ger

man

y

Paci

fic

Bul

garia

cana

ries

FL

lFl

FL2A

l F

LU

2

FL2B

4 FL

2D2

FL7A

S

H2l

A

J93

RK

TP38

A

B1

B3

B6

B9

B10

Ic6

Ic16

Ic

l Ic

3

3 52

.6

12.0

2-

5 2-

0 0.

7 n.

d.

0.3

0.1

5 53

.2

14.9

0.

7 1.

2 0.

9 n.

d.

0.1

0.0

5 53

-4

14.9

0-

7 1.

1 0.

8 n.

d.

0.1

0.0

6 50

.9

16.3

1.

6 1.

0 0.

6 n.d

. 0.

4 0.

1 5

49.8

16

.2

1.8

1.7

0.8

n.d.

0.

1 0.

1 4

50.8

17

.3

0.6

0.7

0.6

n.d.

0.

1 0.

1

2 53

.3

14.2

1.

5 1.

9 0.

5 n.

d.

0.1

n.d.

4 51

.1

16.7

1.

1 0.

9 0.

5 0.

1 0.

1 0.

3

5 52

.5

10.2

2.

0 3.

6 0-

6 n.

d.

0.3

0.7

5 49

.5

17.6

1.

1 1.

6 0.

6 n.d

. 0.

2 0-

0

3 53

.5

10.3

1.

9 3.

6 0.

5 n.

d.

n.d.

0.

3 3

47.9

13.9

2.

9 4.

1 0.

5 n.

d.

n.d.

0.

4 3

52.1

11

.6

3.2

2.1

0.7

n.d.

n.

d.

0.4

4 50

.0

13.3

1.

2 3.

0 0.

5 n.

d.

n.d.

2.

4 1

51.6

14

.4

0.7

3.0

0.5

n.d.

n.

d.

0.1

2 49

.4

14.0

2.

8 3-

1 0

5

n.d.

n.

d.

0.1

2 52

.0

14.1

1.

8 1.

5 1.

0 n.

d.

n.d.

0.

2 3

60.0

8.

2 0.

7 0.

6 0-

8 n.

d.

n.d.

1.0

4

49.4

14

.2

3.0

2.3

0.7

n.d.

n.

d.

0-5

70.2

1.

08

0.22

0.

53

205"

71

.0

1.11

0.

27

0-68

lo

o"

71.0

1.

05

0.27

0.

68

100"

70

.7

1.06

0.

32

0.73

65

" 70

.5

1.10

03

2 0.

74

60"

70.2

1.

03

0.34

0-

81

10"

70.2

1.

01

0.27

0.

61

150"

71.0

1-

10

0.31

0.

72

70"

69.9

1-

86

0.19

0-

46

250"

71.6

1.

01

0.34

0.

76

90"

70.1

1.

06

0.19

0.

47

250"

69

.7

1.06

0.

28

0.61

15

0"

70.1

1-

09

0.22

0.

54

200"

70

.4

1-05

0.

25

0.60

16

0"

70.3

1.

05

0.28

0.

64

125"

69.9

1.

07

0.25

0.

58

170"

70

.6

1.08

0.

25

0.64

13

0"

71.3

1.

16

0.13

0-

38

310"

70

.1

1.09

0.

23

0-63

14

0"

* sam

ples

cont

aini

ng ti

tano

mag

netit

e gra

ins

exhi

bitin

g si

gns o

f m

aghe

mat

izat

ion

or g

ranu

latio

n ar

e in

clud

ed in

thi

s ta

ble.

2 3 1' 2 4 1' 1 1 1 2 1 1 1 1 1 2 1 4 1

350

&47

0 (k)

300

&56

0 (k)

250,

400

&53

0 (f)

400

&67

0 (j

) -

520

ti) 5

150-

250

&54

0 (f

) n


Tabl

e 2

Ana

lyse

s of

exs

olve

d* t

itano

mag

netit

e gra

ins

Loca

lity

Arg

entin

a

Turk

ey

Azo

res

Ore

gon

Japa

n

Sam

ple

No.

of

Wei

ght p

er cent

of c

atio

ns

Cal

cula

ted

quan

titie

s N

o.

anal

yses

Fe

T

i Mg

A1

Mn

Ni

ZD

Cr

C

f R

x

T,

115

5 49

.9

15.3

2.

1 2.

2 0.

3 0.

0 0.

0 0.

6 70

.4

1.15

0.

30

0.72

70

" 16

3 5

49.5

18

.2

1.6

0.6

0.4

0.1

0.1

0.1

70.6

1-

12

0.36

0.

83

0"

289

5 51

.1

18-2

0.

6 0.

6 0.

4 0.

0 0.

1 0.

2 71

.2

1.12

0.

35

0.79

20

" 31

7 5

51.1

11

.6

2.2

1.8

4.2

0.0

0.1

0-1

71.0

1.

08

0.20

0.

52

210"

B21

5

52.5

15

.6

1.0

0.8

0.4

0.1

0.1

0.1

71.1

1.

16

0.29

0.

73

50"

KY

9A

3 57

.6

9.5

1.4

1.0

0.5

0.1

0.1

0.1

70-7

1.

04

0.16

0.

45

235"

FLlA

6 5

48.3

19

.7

1.1

0.9

0.8

n.d.

0-

1 0-

0 70

.9

1.11

0.40 0.90

-20"

FL

lOC

4 4

48.6

17

.7

1.7

1-5

0.7

n.d.

0.

1 0.

1 70

.4

1.24

0.

36

0.78

30

"

319

3 56

.2

5.1

0.7

1.6

0.4

0.1

0.2

7.0

71.4

1.

05

0.08

0-

26

390"

32

6 2

55.6

7.

9 1.

0 1.

8 0.

4 0.

1 0.

1 4.

4 71

.3

1.22

0.

13

0.36

30

5"

359

3 62

.7

6.7

0.6

1.2

0.4

0.1

0.1

0.0

71.6

1.

08

0.11

0.

32

350"

36

6 3

50.5

18

.2

0.7

0.4

0.2

00

0.

1 0.

0 71

.2

1.02

0.

35

0.84

-11

0"

370

3 54

.3

12.6

0.

8 1.

9 0.

4 0.

1 0.

1 0.

0 71

.0

1.18

0.

23

0.58

18

5"

377

4 50

.2

17.0

1.

2 0.

7 0.

2 0.

1 0.

1 0.

0 71

.0

1-06

0.

33

0.78

30

"

J10

4 59

.0

9.8

0.8

0.9

0.3

n.d.

0.

2 0-

2 71

.2

1-10

0.

17 0.46

240"

J2

0 4

52.5

15

.7

1.0

1.1

0.4

n.d.

0.

1 0.

1 70

.9

1-04

0.

30

0.73

50

" 53

0 3

58.3

11

.0

0.4

1.0

0.3

0-0

0.2

0.1

71.3

1.

09

0.19

0.

52

200"

J4

0 4

56.8

13

.1

0.2

0.8

0.3

0.0

0.1

0.1

71-4

1.

11

0.23

0.

62

130"

J8

6 4

57.8

12

.0

0.4

0.4

0.7

n.d.

0.

2 0-

0 71

.5

1-11

0.

21

0.57

18

0"

Type

4 4 4 4 4 4 4 4 4 4 4 4 4 4 2 4 4 4 4

Js(T

) cur

ve

Tc ("c)

520

580

580

550

560

540

550

570

570

570

570

550

560

530

350

&57

0 52

5 56

0 55

0 53

0

* ilmenite-titanomagnetite

lam

ella

e pr

oduc

ed b

y de

uter

ic o

xida

tion.


Non-stoichiometdc titanomagnetites in basaltic rocks

I N&I points of impure magnetites

493

X

FIG. 3. Neelpointsof impuremagnetites. Data replotted fromFrohlich eraf. (1965).

weight per cent of Ti for element Y, with say, mean atomic weight = 25 and E = 0.5, is 23.0, so that dividing the weight per cent of Ti by 23 gives an estimate of the lower limit to the x value, thus enabling us to make a better estimate of the Curie point. The data of Frolich et al. (1965), illustrated in Fig. 3, have been used to estimate the depression of the Curie point due to the measured weights per cent of ' impurity ' cations below that for a pure titanomagnetite with the same x value. (But note that Frohlich et aZ.'s data are for doped magnetite, not titanomagnetite for which no data exist.) The discrepancy between the two estimates varies by up to 20"C, but the accuracy of any individual estimate is hardly likely to be better than 10°C.

3. Results

The results of our microprobe analyses are presented in Tables 1 and 2. The weights per cent of cations analysed, corrected as described in Section 2 are listed, together with the number of analyses made. Table 1 lists measurements made on homogeneous, nearly homogeneous or slightly maghematized or granulated titanomagnetite grains, which did not contain exsolved ilmenite lamellae which are considered to be characteristic of high temperature (deuteric) oxidation. Table 2 lists measurements made on exsolved ore grains. These have been placed in oxidation classes 2 to 5 on the scale familiar to rock magnetists (Wilson 8t Watkins 1967) and the listed compositions are integrated values for whole grains, having been obtained by scanning the electron beam over representative areas containing several lamellae.

Expected cation sums, expressed as weights per cent, and the normalization factors f by which the analysed weights per cent corrected as outlined in Section 2.3, are also listed in Tables 1 and 2. The calculated values of x, i.e. the molecular percentage of ulvospinel contained in the titanomagnetite grains and the mean Curie point estimated for a stoichiometric titanomagnetite grain with the calculated proportion of ulvospinel and with the ' impurity ' cations present (see Sections 2.5 and 2.6) are also listed.


494 K.M.CreernndJ.D.Ibb&~~~

Our first observation is that the average value of x for grains listed in Table 1 is not significantly different from that for grains listed in Table 2, the two average values and their standard errors being 0.67 k0.02 and 0.62+0.05, respectively. The standard deviations for the two distributions and the numbers of samples studied are presented in Table 3. Thus it would seem that the overall composition of the ‘ titanomagnetites ’ has no bearing on the question of whether or not deuteric oxidation and its attendant exsolution has occurred.

Table 3 Average composition of the ‘ titanomagnetites ’ expressed as ‘ molecular proportions x

of ulv6spinel in the basalts studied’ Standard Standard Number of

Classification of grains Mean value deviation error samples

Homogeneous, slightly maghematized or granulated (from Table 1) 0.67 0.1 1 0.02 38

With exsolved ilmenite lamellae (Table 2) 0.62 0.21 0.05 19

Table 4

Average composition of the ‘ titanomagnetires ’ expressed as molecular proportion of ulv6spinel according to place of origin

Standard Standard Number of Place of origin Mean value deviation error samples

Argentine 0.66 0.1 1 0.03 13 Turkey 0.67 0.11 0.03 12 Azores 0-73 0.10 0.03 8 Oregon 0.56 0.24 0.11 6 Japan 0.60 0.11 0.05 6 Bulgaria 0.57 0.07 0.04 5 Canaries 0.56 0.1 1 0.06 5

There do, however, appear to be small and possibly signicant differences between the distribution of compositions o f , titanomagnetites ’ from different places of origin. The standard deviations of these distributions (assumed to be normal) together with their mean values and standard errors are listed in Table 4. The mean values and standard error bars are also presented graphically in Fig. 6 so that the reader may judge whether the differences are significant. The two horizontal lines in this figure at x = 0.67 and x = 0.62 represent the average values for samples in Tables 1 and 2 respectively.


Non-stoichiometric titanomagnetites in basaltic rocks 495

Temperature "C

FIG. 4. Evolution of thermomagnetic curve of basalt initially containing almost unoxidized titanomagnetite with progressive oxidation. (Data from Creer & Petersen 1969). Thermomagnetic curve types 1, 2 & 4 are defined in this figure.

We now investigate the relationship between the thermomagnetic curves and the chemical composition of the ore grains studied. Our classification of thermomagnetic curves is based on the results of oxidation experiments carried out in the laboratory by prolonged heating in air at 400°C (Creer & Petersen 1969). The evolution of the thermomagnetic curves obtained after progressive oxidation of initially homogeneous titanomagnetite bearing basalt is illustrated in Fig. 4 where the shapes adopted as typical of different degrees of oxidation (all within deuteric oxidation class 1) are shown.

We have estimated the Curie (NCel) points for each sample studied from the thermomagnetic curves illustrated in Figs 5(a)-(s) and have tabulated these in Tables 1 and 2. We have also tabulated compositional Curie points determined as described in Section 2. In Fig. 7 we plot the cumulative per cent of samples for which the differences between measured and ' compositional ' Curie points indicated on the abscissa have been observed. Samples have been grouped according to the shape of the thermomagnetic curve. There would appear to be a significant difference between the curves obtained for samples yielding type 1 or type 2 curves. (Note: examples of type 1' curves are provided by samples FL2a2, T33, FL7a or A41, Figs 5(f) and (h), which are characterized by their tendency to oxidize rapidly on heating in air-see Creer, Ibbetson & Drew 1969; Sanver & O'Reilly 1970).

Thus the elevation of Curie temperature taken together with the shape of the thermomagnetic curve provides information about the early stages of oxidation of ' titanomagnetites ' in basalts. Creer & Petersen (1969) concluded (i) that the elevation of the lower Curie point was due to the formation of a cation deficient spinel in the early stages of oxidation, and (ii) that the higher Curie point was due to the exsolution on a sub-microscopic scale of an iron-rich magnetic phase. All thermomagnetic curves measured for basalts containing optically two-phase ore grains were of type 4, exhibiting a single high Curie point above 500 "C.

6



(C 1 139

\ \

. 0 1 2 3 4 5 d

I I I I I 1 2 3 4 5


Non-stoichiomeMc titanolljngnetitea In baseltic rocks 497

0 I 2 3 4 5 6


498 K. M. Crew and J. D. Ibbetaon


Non-stoicbiamtMc titanomqptitea in ba~l t ic rock8 499

I 2 3 4 5 6

I 2 5 6 3 4

0 I 2 3 4 5 6

FIG. 5 (a)+). Measured thermomagnetic curves for basalt samples analysed by electron microprobe.



FIG. 6. Average compositions of ' titanomagnetite * grains expressed as molecular proportion of ulvospinel for different places of origin. a = Argentine; b = Turkey; c = Azores; d = Oregon; e = Japan; f = Bulgaria and g = Canaries. See also

Table 4.

I I ' 2

FIG. 7. Cumulative curves of per cent of samples having Curie points elevated by amount indicated along x axis. These differences between the measured and

compositional ' Curie points are taken from Table I . The three curves are for samples exhibiting type 1 thermomagnetic curves (points represented by circles), type 1' thermomagnetic curves (points represented by triangles) and type 2

thermomagnetic curves (points represented by squares).



4. Correlation of optical and thermomagnetic properties

Polished sections of the samples studied under the microprobe were also observed under the ore microscope at magnification of 160x (low power objective) and 600x (high power objective under oil). The results of these observations are sum- marized in Tables 5 and 6, which correspond to Tables 1 and 2 respectively.

All samples listed in Table 5 contain ‘ titanomagnetites ’ which have been assigned to oxidation class 1 (except A57 to Icl which contain a few grains containing ilmenite lamellae, oxidation class 2 and which are therefore classified at oxidation state 1.5). We note that the thermomagnetic curves of these samples show considerable variations in shape (see Table 1, right-hand columns for classification).

There appeared to be no relationship between the shape of thermomagnetic curve and ‘ titanomagnetite ’ or ilmenite grain size (cols. 4 and 10, Table 5 ) or to the shapes of these grains (cols. 5 and 10) or to their colour (cols. 8 and 11). The characteristics of maghematization did not seem to be particularly important either: possibly because these are difficult to describe quantitatively in a satisfactory manner.

But the colour of the matrix does seem to provide a useful indication of the extent of the initial stages of oxidation, i.e. within oxidation class 1 (col. 13, Tables 5 and 6). We have observed the matrix under partly crossed nicols at high-power and have made a four-fold classification of the colours observed, namely red, brown-

)

100

O ’ O K , 0

a b c d

FIG. 8. Cumulative distribution curves of colour of staining of matrix for different thermomagnetic curve types 1, l‘, 2, 3 & 4. Colour (as seen at HP under partly crossed nicols) plotted along abscissa as follows: a = red; b = red + brown-mange; c = b+grey-brown; d = c+grey. Ordinate shows cumulative percentage of

samples studied (listed in Tables 3 and 4) showing the colours indicated.


Tabl

e 5

Sam

ples

con

tain

ing

hom

ogen

eous

and

slig

htly

mag

hem

atiz

ed o

r gra

nula

ted

titan

omag

netit

es

(4)

Arg

entin

a 29

3 l(

e)

376

l(e)

T3

3 l’(

f)

117

3(a)

118

3(a)

13

9 2(

c)

152

2(d)

203

2(c)

T3

4 2(

g)

350-

2 10

-5

30-2

10

0-2

40

4

130-

1 30

00-2

50

-2

50-2

50

-2

100-

2 10

0-10

SE

sk

SE

SE

sk

SE

sk

SE

E

sk

E SE

sk

E

- PB

r G

Y

pP-B

r b

Gr-

Br

c

GY

b

PBr

Br-

Gy/

P a

P-Br

C

P-Br

P-

Br

- G

Y

P-Br

a

Gy-

Br

b

nil

4-l(2

0)

P-Br

30

4(15

) P-

Br

2&3(

10)

Gy-

Br

6-2(

10)

Gy-

Br

7-l(1

0)

P-Br

nil

0.5%

FeS

R

(sil)

fm

0.5%

FeS

fm 2

F

Br-

R s

il i &

fm

Or-

Br

st,

R s

il &

i pB

r st

(R s

t)

1% F

eS

(R-O

r st

) 1%

FeS

R

sil fm

P 1%

FeS

t 9

1%

TM

Cr c

ores

i P 0.

5% F

eS fm

2

8

A

A

(AX

W

A

6-2(

10)

P-Br

40

-1(1

5)

P-Br

Turk

ey

A57

2(

0)

A41

l’(

g)

D12

3 l(

n)

D12

7 l(

e)

G14

3 l(

i)

H12

2 l(

i)

T91

l(i)

K2

7 l(j

) A

106

3(d)

BT3

1 l’(

o)

100-

2 20

-10

4 100-

2 15

0-2

100-

2 45

0-2

250-

2 20

-2

100-

20

2 100-

4

SE

E sk

SE

E E E

E E SE

sk

SE

BD

F A

C

1.5

1 P-

Br

P-Br

b b

10-3

(5)

P-Br

PB

r

P-Br

PB

r pB

r pP

-Br

PBr

Cr-

Br

PBr

P-Br

R i

fm

Or

(Or s

il)

Or

sil

GY

(R fm

) GY

G

Y

GY

Gy

Or s

il (R

i)

Gy-

Or

st

C a 1%

FeS

FeS

0.5%

FeS


Tabl

e 5

(con

tinue

d)

Gy

(R-B

r fm

) A

zore

s F

LlF

l l‘(

k)

300

E

(BH

F)

1 pP

-Br

a 20

(3)

P-B

r G

y 2

SE

FL7A

5 l’

(f)

250-

2 SE

A

1 P-

Br

a 12

-3(5

) P-

Br

Gy

Gy

Br s

t (R

sil)

FL

2Al

2(k)

50

-2

SE

AC

1

P-B

r b

20-2

(5)

pBr

GY

GY

(R f

m)

FL2A

2 2(

k)

50-2

SE

CD

1

P-B

r b

154(

10)

pBr

GY

G

Y (R

fm

)

FL2B

4 20

) 60

-2

SE

AC

1

pP-B

r/G

y a

10-2

(5)

pBr

GY

G

Y (R

i)

2 sk

D

2 sk

FL2D

2 4(

j) 30

0-2

SE

AB

CD

1

P-B

r a

40-1

(5)

pBr

GY

Br-

Or

st,

R

R s

il fm

& i

Ade

n H

21A

l’(

k)

150-

2 SE

AE

1’

P-Br

b

30-5

(5)

Gy-

Br

Gy

pBr

fm

Japa

n J9

3 l(

c)

200-

10

sk

BC

(F)

1 P-

Br

b 80

-5(5

) P-

Br

Gy

R-B

r fm

Ger

man

y R

K

l(1)

80

-2

EO

1

pP-B

r a

3-2(

8)

P-B

r G

y G

y fm

DE

G

5’

Paci

fic

TP3

8 2(

b)

1000

-2

SE

AE

(F)

1 P-

Br

b 60

0-25

(10)

P-

Br

Gy

GY

(R) f

m

Bul

garia

B1

l(

1)

100-

2 SE

0

1 G

y-B

r -

nil

GY

G

y-B

r B3

l(

1)

160-

2 SE 0

1 G

y-B

r -

nil

GY

(B

r fm

) B6

l(

m)

500-

2 SE

(C)

1 G

y-B

r -

nil

GY

(R f

m)

B9

l(m

) 20

0-2

SE 0

1 G

y-B

r a

25-l(

4)

GY

G

Y

B10

l(

m)

350-

2 SE 0

1 G

y-B

r a

25-l(

4)

GY

GY

Canary I

s. IC

3 l(

b)

100-

2 SE

0

1 C

r-Br

- ni

l G

Y

Gy-

Br

st

IC6

l’(a)

23

00-2

SE 0

1 C

r-Br

- ni

l GY

(R

) IC

16

l(a)

10

0-2

SE

AC

E

1 C

r-Br

- ni

l cy

pB

r st

ICl

4(g)

60

0-1

SE

AC

(F)

1.5

P-B

r b

3-l(1

0)

P-B

r G

y-B

r

0 a 0.

5% F

eS

1% F

eS

1% F

es. C

r cor

es in

TM

1%

FeS

1%

FeS

Cr cores i

n 1%

of

TM

Cr

cores in

50%

of T

M

Cr cores in

5%

of

TM

~

1

0

w


Tabl

e 5

(con

rinue

d)

s P

Not

es

Col

. (1

): C

ount

ry of

orig

in.

Col

. (2

): Sa

mpl

e nu

mbe

r

Col

. (3

): Ty

pe o

f th

erm

omag

netic

cur

ve a

nd f

igur

e ke

y.

Col

. (4

): R

ange

of

grai

n sizes i

n m

icro

ns.

Col

. (5

): S

hape

of grains:-E

= eu

hedr

al; S

E =

sub-

euhe

dral

; sk

= sk

elet

al.

Col

. (6

): M

aghe

mat

izat

ion a

nd g

ranu

latio

n of

tita

nom

agne

tite g

rab

. A:

- co

ntai

ning

pat

ches

of

gre.

y/w

hite

mat

eria

l in

the

regi

on o

f cr

acks

/ble

mis

hes,

smal

l in

area

com

pare

d w

ith th

e gr

ain.

B:

- co

ntai

ning

pat

ches

of

grey

/whi

te m

ater

ial n

ot o

bvio

usly

ass

ocia

ted

with

cra

cks o

r oth

er fa

ults

. C:

- ba

nds

of g

rey/

whi

te a

long

edge

s and

/or

inte

rior t

o th

e gr

ain.

D

:- so

me

grai

ns h

ave

an o

vera

ll w

hitis

h/gr

ey c

olou

r cas

t. E:

- some gr

ains S

een

to c

onta

in ir

regu

lar v

einl

ets

of gr

ey/w

hite

mat

eria

l sur

roun

ding

isla

nds o

f ap

pare

ntly

una

ltere

d tit

anom

agne

tite.

F:-

occa

sion

al grain e

xhib

its il

men

ite la

mel

lae.

G

:- pr

esen

ce o

f tit

anom

agne

tite g

rain

s co

ntai

ning

gran

ules

of

rutil

e.

(XI.

(7):

Oxi

datio

n nu

mbe

r (d

eute

ric).

Col

. (8

): C

olou

r of

titan

omag

netit

es.

Col

. (9

): Pr

opor

tion

of i

lmen

ite in ore m

iner

als:-

(a

) les

s th

an 1

0 pe

r cen

t; (b

) bet

wee

n 10

and

50

per c

ent;

(c) m

ore

than

50

per

cent

.

Col

. (10

): Range

of g

rain

siz

es o

f ilm

enite

: ave

rage

leng

th-b

read

th r

atio

incl

uded

in

brac

kets

.

Col

. (11

): C

olou

r of

ilmen

ites.

Col

. (12

): G

ener

al c

olou

r of

pol

ished

sec

tion

view

ed a

t low

pow

er m

agni

ficat

ion

in u

npol

ariz

ed li

ght.

&I.

(13)

: G

ener

al c

olou

r and

des

crip

tion w

hen

view

ed u

nder

par

tly c

ross

ed n

icol

s at h

igh

pow

er m

agni

ficat

ion

( x 6

50).

Abb

revi

atio

ns ha

ve m

eani

ng a

s fol

low

s:-

frn =

ferr

omag

nesi

anq,

sil =

silic

ates

. i =

inte

rstit

ial,

st =

stai

ns.

Bra

cket

s in

dica

te th

at th

e fe

atur

es d

escr

ibed

do

not o

ccur

freq

uent

ly.

Col

. (14

): ad

ditio

nal n

otes

- Fe

S =

pyr

ites,

Cr =

chro

mile

, TM =

tita

nom

agne

tite.

ID L

4 P

Col

our a

bbre

viat

ions

(col

s 8.

11.

12 a

nd 1

3)

Gy,

gre

y; P

, pin

k; B

r, br

own;

Cr,

crea

m; R

, red

; Or,

oran

ge; p

, pal

e.



orange, grey-brown and grey. We have plotted in Fig. 8 curves showing the cumulative number of samples exhibiting a given type of thermomagnetic curve with the colour of the matrix observed as described above and note that these curves exhibit a notable correlation in the early stages of oxidation (within class 1) for samples which yield thermomagnetic curves of types 1, 1’ or 2. For more highly oxidized samples which yield thermomagnetic curves of types 3 or 4, no appreciable distinction can be drawn from observations of the colour of the matrix (including ferromag- nesians and feldspars) as it invariably exhibits patches of red rather than orange or brown.

5. Degree of oxidation of ‘ titanomagnetites ’ in basalts

Ideally this would be measured directly by the microprobe. But, in practice, difficulties arise in the accurate determination of oxygen when accompanied by titanium. This is because oxygen has a K absorption edge at 23.3 A while titanium has an L absorption edge in the same region. Thus when oxygen occurs in a sample containing titanium, the apparent concentration of oxygen depends on the amount of titanium present. The difference in oxygen content of ulvospinel and magnetite is small: the former contains 28.6 per cent by weight and the latter 27.6 per cent. However, the difference as ‘ seen ’ by the probe is enhanced by the titanium in ulvospinel. The nature of the corrections to be applied to a more complex natural sample become very important. They are large and at present are not known sufficiently well to permit the determination of oxygen with sufficient accuracy to determine the degree of non-stoichiometry. Hence, in the work described in this paper, we have adopted the method described below, involving a combination of microprobe and magnetic studies.

From the microprobe measurements we determine x, the molecular proportion of ulvospinel in the ‘ titanomagnetite ’ having made allowance for the ‘ impurity ’ cations contained as described in Section 2. We then note the difference between the measured and compositional Curie points, both of which are listed in Tables 1 and 2. This observed elevation of Curie temperature we attribute to oxidation to a cation deficient spinel. The commonly quoted data about the Curie points of these com- pounds are those published by Akimoto et al. (1957) but recently it has been shown that their samples were not single phase, i.e. they were not homogeneous cation deficient titanomagnetites as they originally supposed.

Recently, Readman & O’Reilly (1970, in preparation) have carried out new determinations of the Curie points of synthetic non-stoichiometric titanomagnetites by oxidizing wet-ground material in air at temperatures of 200-300”C. Their results are illustrated in Fig. 9. We used these contours to locate the points plotted in Fig. 10 where the compositions of our natural ‘ titanomagnetites ’ are represented.

On the triangular compositional diagrams of Figs 9 and 10, oxidation lines for constant Ti/Fe ratio run parallel to the base of the triangle; thus having obtained our x values from the microprobe analyses, we use the elevation of Curie temperature and the data of Fig. 9 to locate the compositions of the grains we studied and hence their degree of non-stoichiometry.

6. On the importance of cation deficient titanomagnetites in rock magnetism

Many investigations by numerous workers have been made into the question of whether a relationship exists between the polarity of remanent magnetization in rocks and their petrology, in particular to the state of oxidation of the ‘ titanomagnetites ’. While some workers claim to have found such a correlation (e.g. Wilson & Watkins 1967; Ade-Hall &Wilson 1969), others have failed to do so, e.g. Larson & Strangway (1966), Ade-Hall & Watkins (1970).


Tabl

e 6

Sam

ples

con

tain

ing

titan

omag

netit

es w

ith il

men

ite la

mel

lae

Tita

nom

agne

tite g

rain

s Ilm

enite

gra

ins

Gen

eral

(1

) (2

) (3

) (4

) (5

) (6

) (8

) (9

) (1

0)

(11)

(1

2)

(13)

(1

4)

Arg

entin

a 11

5 4(

n)

60-2

SE

4.5

Gy-

Br

b 14

-3(8

) G

Y-l

k G

Y

Br-O

r-R

fm &

i 2"

ov

Turk

ey

Ore

gon

FlO

m

Ja

w

163

4(n)

12

0-10

28

9 4(

n)

60-2

31

7 4(

d)

50-2

4

B21

4(o)

50

-2

KY

9a2 401

) 20

0-1

319

4(s)

20

-1

326

4(s)

40-2

35

9 4(@

100-

2 36

6 4(

r)

50-2

37

7 4(

r)

60-2

20

-2

370

4(r)

20

-2

FLlA

6 2(

p)

50-2

FL

10C

44(h

) 20

0-2

J10

4(p)

30

-2

J20

4(p)

50

-2

J30

4(r)

60-2

J4

0 4(

r)

300-

2 J8

6 4(

r) 40

0-2

SE

5 SE

5 E

2.5

sk

E4

SE

3.

5

SE

4 SE

3.

5 SE

4.5

SE

3 E

2

sk

1

SE

5

E 3.

5 SE

3.

5

E 3.

5 E

2-5

E2

E

5.5

E3

Gy-

Br

Gy-

Br

pP-B

r P-

Br/G

y-B

r

P-Br

P-

Br

PBr

PBr

PBr

Cr-B

r C

r-Br

Cr-B

r PB

r

P-Br

P-

Br

P-Br

P-

Br

Cr-B

r P-

Br

Cr-B

r

b 18

0-5(

5)

PlB

r*

c 30

-2(1

0)

Gy-

Br*

b

14-2

(10)

P-

Br

b 15

-5(8

) P-

Br

a ve

rysm

all

P-B

r

a 10

-2(5

) G

y b

8-3(

10)

Cr-B

r b

9-3(

10)

Br*

b

u)-2

(4)

pBr*

b

40-5

(8)

b 50

-2(5

) G

y*

a 25

-2(5

) P-

Br*

a

25-2

(5)

P-B

r*

b 25

-5(8

) P-

Br*

a

15-5

(10)

pB

r*

a 15

-5(1

0)

pBr

b 40

-2(5

) pB

r-G

y ni

l G

y

Br-

Or-

R f

m, s

il &

i B

r-O

r R

fm, s

il &

i (R

i)

(2"

ov)

(2" ov)

0.5%

FeS

, (2"

ov)

F

g k cl

Br-

Or-

R

i 1

Br-

Or

sil

muc

h 2"

ov

I..

Or

st.

R i

& si

l R

i&fm

R

sil &

i O

r st

R i,

R s

il

P f B

r R

fm, s

il &

i

GY

R

fm, s

il &

i 2"

ov

GY

R

fm

muc

h 2"

ov

Br-

Gy

(R i

& si

l) G

Y

Rfm

&i

GY

GY

.

Ri&

fm

muc

h 2"

ov

(R i,

sil

& fr

n)

Or-B

r st

sil


Tabl

e 6

(con

tinue

d)

Not

es

Col

. (1

): C

ount

ry of

orig

in.

Col

. (2

): S

ampl

e nu

mbe

r.

Col

. (3

): T

ype

of th

enno

mag

netic

curv

e an

d figure k

ey.

Col

. (4

): R

ange

of g

rain

sizes i

n microns.

Col

. (5

): S

hape

of grai

ns:-

E =

euhe

dral

; SE

= su

b-eu

hedr

al; s

k =

skel

etal

.

Col

. (6

): O

xida

tion

num

ber

(deu

teric

).

Col

. (8

): C

olou

r of t

itano

mag

netit

es.

Col

. (9

): P

ropo

rtio

n of

ilmen

ite in

ore m

iner

als;

(a) l

ess

than

10 per

cent

, (b)

bet

wee

n 10

and

50

per

cent

, (c)

mor

e th

an 5

0 pe

r cen

t.

Col

. (10

): R

ange

of

grai

n siz

es o

f ilm

enite

: ave

rage

leng

th-b

read

th r

atio

incl

uded

in b

rack

ets.

Col

. (11

): C

olou

r of

ilmen

ites:

*in

dica

tes p

rese

nce

of a

ssoc

iate

d ha

emat

ite.

Col

. (12

): G

ener

al c

olou

r of

polis

hed

sect

ion

view

ed a

t low

pow

er m

agni

ficat

ion

in u

npol

ariz

ed li

ght.

Col

. (13

): G

ener

al c

olou

r and

des

crip

tion

whe

n vi

ewed

und

er p

artly

cro

ssed

nic

ols

at h

igh

mag

nific

atio

n x

650)

.

Col

. (14

): 2

" ov

= se

cond

ary

ore

exso

lved

in p

yrox

enes

or o

livin

es.

Surr

ound

ing

brac

kets

indi

cate

infr

eque

nt o

ccur

renc

e.

Abb

revi

atio

ns a

s fol

low

s:-

fm =

ferr

omag

nesi

ans,

sil =

silic

ates

, i =

inte

rstit

ial;

infr

eque

nt f

eatu

res i

nclu

ded

in b

rack

ets.

B C

ols (

8),

(ll)

, (12

) and

(13)

: abb

revi

atio

ns fo

r col

ours

as

in T

able

3.



Ti 0,

1/3

' '2 Fe20s / I I

t I '

Fe 0 113 Feg, I /

FIG. 9. Curie (Nkl) temperature of cation deficient ' titanomagnetites ' (titano- maghematites). Curves obtained from experimental studies of wet-ground synthetic

material (after Readman).

One reason for this apparent lack of agreement may be that the critically important properties were not being observed. Thus, the state of deuteric oxidation has been described in terms of a five- or six-fold classification of increasing oxidation. We believe, however, that the more important magnetic transformations may occur within the oxidation class 1 and that they may be attributable to low temperature rather than deuteric oxidation. This is discussed at greater length by Creer & Petersen (1 969) and Creer er al. (1 969).

But we stress that on our model the (partial) self-reversal is produced by (magneto- static) interaction between a mother phase (the titanium-rich titanomagnetite initially present) and a daughter phase (an iron-rich titanomagnetite with Curie point above 50O0C, exsolved on a submicroscopic scale). Our model thus differs from those in which self-reversal occurs due to cation migration when titanomagnetites are oxidized to homogeneous cation deficient spinels. This latter process may also be important in nature, though we note that of the 20 points representing the integrated chemical composition of the oxidized titanomagnetites we have studied (Fig. lo), only one, viz. sample number 18 falls within the area of the ternary diagram where self-reversal of TRM occurs, i.e. where the net magnetization is due to the tetrahedral sites rather than to the oxtahedral sites as in an unoxidized or moderately oxidized titanomagnetite, assuming the O'Reilly-Banerjee (1967) model of cation distribution (Fig. 1 1 (a)). Thus the high states of oxidation required for self-reversal of TRM on this cation distribution model may have been achieved only rarely in nature.


Non-stoichiometric titanomagnetites in basaltic rocks

TiOe

509

FIG. 10. Compositions of ' titanomagnetites ' contained in rocks studied in this paper determined from microprobe analysis and data from Fig. 8. Key to points asfol1ows:- 1 = sample293;2 = 376;3 = A41;4 = D123;5 = D127;6 = H122; 7 = T91; 8 = K27; 9 = RK; 10 = B1; 11 = B3; 12 = B6; 13 = B9; 14 = B10; 15 = Ic16:16 = Ic3;17 = T33;18 = BT31;19 = FL2a2(phasewithTc = 250°C);

19' = FL2a2 (phase with T, = 400 "C) and 20 = FL7a5.

( 0 ) (b)

FIO. 11. The range of compositions of oxidized titanomagnetites susceptible to self-reversal of NRM is indicated by the dotted shading. (a) For the O'Reilly- Banerjee (1967) model of cation distribution, and (b) for the Verboogen (1962) model. For the former model the compositional range in which the first-stage of oxidation only has occurred (i.e. in which only the Fez+ in B sites have been oxidized

to Fe3++) is shown by the diagonal shading.



On the Verhoogen (1962) model in which self-reversal is produced due to re- ordering of the cations as they migrate into their preferred sites from an initially random distribution, a rather wider range of compositions of cation deficient spinels is susceptible to self-reversal of remanence (Fig. ll(b)). In this model all the Fe3+ ions preferentially occupy A sites in the equilibrium distribution whereas in the O'Reilly-Banerjee model the lowest energy state is considered to be one in which B sites are occupied by a certain proportion of Fez+ as well as Fe3+.

We think that the Occurrence of partial and possibly of complete self-reversal due to progressive oxidation of titanomagnetite at normal temperatures over geo- logical time may be of importance in nature and in particular must be taken account of in the interpretation of marine magnetic anomalies (Creer, Petersen & Petherbridge 1970). But the mechanism may be one of sub-microscopic exsolution rather than of cation migration within a material which remains homogeneous.

Acknowledgments

The work was supported by a research grant for Rock and Mineral Magnetism from N.E.R.C. to which we express our thanks. We also take much pleasure in expressing our thanks and appreciation to Mr W. Davison who carefully and patiently carried out most of the microprobe measurements and who kept our Geoscan in working order.

Department of Geophysics and Planetary Physics,

University of Newcastle upon Tyne. School of Physics,

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Electron Microprobe Analyses and Magnetic Properties of Non