An integrated rock magnetic approach to the selection or rejection of ancient basalt samples for palaeointensity experiments

Physics of the Earth and Planetary Interiors, 75 (1993) 329-342

329

Elsevier Science Publishers B.V., A m s t e r d a m

An integrated rock magnetic approach to the selection or rejection of ancient basalt samples for palaeointensity experiments Neil T h o m a s Geomagnetism Laboratory, Olit'er Lodge l.aboratories, Unit'ersity of Lit'erpool, Oxford Street, Lit'erpool 1.69 3BX, UK (Received 20 December 1991; revision accepted 29 June 1992)

ABSTRACT Thomas, N., 1993. An integrated rock magnetic approach to thc selection or rejection of ancient basalt samples for palaeointensity experiments. Phys. Earth Planet. Inter., 75: 329-342. T h e rock magnetic properties of basalts are often useful indicators of suitability for use in palaeointensity experiments. The normal approach to sample selection is based on the use of a limited amount of rock magnetic data. possibly from only a single rock magnetic technique. This approach may result in the exclusion of otherwise suitable samples, or the inclusion of unsuitable samples. This study stresses the importance of analysing results from a n u m b e r of rock magnetic techniques before selecting or rejecting samples for use in palaeointensity experiments. A new integrated rock magnetic method of sample selection or rejection is proposed, which uses the combined results from 'thermal' (thermomagnetic and high-temperature susceptibility) and 'non-thermal' (low-temperature susceptibility, hysteresis and alternating field demagnetisation) rock magnetic techniques to define 'sets" of rock magnetic behaviour for Precambrian basalts, with a mixed grain size of pure magnetite. The behaviour of all samples in each 'set', during both Thellier and Shaw palaeointensity experiments, is categorised, allowing a set of selection or rejection criteria to be defined. This new approach to sample selection or rejection for palaeointensity experiments has value for basalts of all ages, provided the sample collection is large enough and all available rock magnetic techniques are used.

1. Introduction

There are many problems associated with palaeointensity work (e.g. Coe, 1967; Pesoncn, 1978) but the two which most dictate the success or failure of experiments are: (1) The failure of many rocks to comply with the fundamental requirement for palaeointensity work, namely, that the natural remanent magnetisation (NRM) of a rock must be a primary unaltered thermoremanent magnetisation (TRM) (Nagata, 1943). This happens as a result of physiCorrespondence to: N. Thomas, Geomagnetism Laboratory, Oliver Lodge Laboratories, University of Liverpool, Oxford Street, Liverpool L69 3BX, UK.

cal a n d / o r chemical changes which occur subsequent to the formation of the rock. (2) Alteration of the rock during laboratory heating, which results in changes in the chemistry of the rock. Palaeointensity experiments involve heating the rock and this has the drawback that it provides the opportunity to alter the chemistry (usually by oxidation) and, importantly, its magnetic mineralogy. The basis of sample selection for palaeointensity experiments is then, apparently, simple; a sample must have a stable NRM which is a primary TRM (acquired during or just after initial cooling) and have a magnetic mineralogy which is unlikely to undergo thermal alteration during laboratory heating. Several workers have attempted

0031-9201/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved

330

to provide criteria which can be used to isolate such samples. Pr6vot et al. (1985) and subsequently Derder et al. (1989) used two important criteria for determining the suitability of samples for use in palaeointcnsity experiments: repeatability of stable NRM directions for adjacent samples after alternating field (a.f.) dcmagnetisation and the possession of a single, high Curie temperature with a reversible curve (Type 2a of Mankinen et al., 1985). Only 18 out of 220 samples (8%) in the Derder et al. (1989) study satisfied these criteria and were subsequently used. Senanayake and McElhinny (1982) suggested a simple selection criterion based on low-temperature susceptibility (KLT) results only. They claimed that of the three types of behaviour described by Senanayake and McEIhinny (1981), Group 2 samples are the most suited because they are believed to contain deutcrically oxidised titanomagnetites with exsolved ilmenite lamcllae which effectively subdivide the grains into elongate single domain (SD) grains of magnetite. As a result of their advanced oxidation states (Classes II and Ili of Wilson and Watkins (1967)), Group 2 samples show resistance to alteration during laboratory heating. Senanayakc et al. (1982) presented palaeointensity results from Tertiary basalts selected on the basis of the KLT criterion. Out of 716 samples, only 149 (21%) were selected using this criterion. The remainder were discarded. The common complaint with the selection procedures of both Pr6vot et al. (1985) and Scnanayake et al. (1982) is that samples which did not pass their selection test were omitted, without trial, from palaeointensity experiments. This brings into question the validity of the selection procedure, which can be judged only by comparing the success of samples selected by the technique with that of samples which fail to fulfil the selection criteria. Rolph (1984) commented that rigorous application of the Senanayake et al. criterion to Carboniferous lavas would have resulted in the rejection of all but one of the 41 samples in his study. However, he investigated all samples and concluded that those which gave palaeointensity results showed no preference for any KLT group. This observation was supported

N. THOMAS

by Sherwood (1986) from studies of Miocene lavas from New Zealand. Furthermore, if this criterion had been applied to the samples of the present study, none would be selected for palaeointensity work, because of the absence of genuine Group 2 behaviour, as interpreted by Senanayake and McElhinny (1981). There is, then, evidence that (prc-)selection of samples, using only KLT resuits, is a wasteful approach. Rolph (1992) has found empirically that some historic lava samples which fail to give reliable palaeointensity results (based on agreement with observatory measurements) can be recognised by their a.f. demagnetisation characteristics. The aim of this paper is to present a new approach to the problem of sample selection or rejection for palaeointensity experiments, which utilises rock magnetic results from five techniques and relates them to palaeointensity results from the same samples. This is a more thorough approach than those outlined above. Comparison of the two sets of results for all samples in the collection is used to establish a set of selection or rejection criteria which, unlike previous selection criteria, takes into account the importance of mixed grain sizes, which frequently exist in ancient magnetite-bearing basalts (Radhakrishnamurty et al., 1991).

2. The material

A major problem encountered during palaeointensity experiments on ancient basalts is that ideal samples can rarely be found; the magnetic mineralogy is often complex and the thermal history is usually unknown. These factors, and the predominance of multi-domain (MD) grains of magnetite in such rocks, have led palaeomagnetists to believe that it is unlikely that reliable palaeointensity estimates can be obtained from ancient basalts. Consequently, work has concentrated on younger rocks, resulting in a shortage of field intensity data for ancient rocks, particularly those from the Precambrian. However, it is common knowledge that most ancient basalts, although dominated by MD magnetite, are in fact characterised by mixed grain sizes of pure mag-

BASAI.T SAMPLE SEI.E(q'ION FOR PAI.AEOINTENSITYEXPERIMENTS

netite, with SD grains also present in varying amounts (Radhakrishnamurty et al., 1991). This SD fraction is very important for palaeointensity work, as it is these grains that are more likely to carry a stable, primary TRM for which the blocking and unblocking spectra are identical and the law of additivity of TRM (Thellier, 1951) is obeyed. Thus, it is possible that consistent and acceptable palaeointensity estimates can be produced from ancient basalts, provided samples with a significant amount of SD material in the mixed grain size can be identified. It is necessary, then, to devise a method which can identify suitable samples so that palaeointensity results obtained from ancient basalts can be regarded as offering reliable estimates of the ancient geomagnetic field strength.

331

The suitability of the Gardar lavas for palaeomagnetic studies, allied with the large amount of rock magnetic data and the palaeointensity data of varying reliability, provides an opportunity to discuss sample selection and rejection criteria for palaeointensity work. The approximately 1320 m.a. Gardar lava succession, within the Eriksfjord Group of South Greenland, provides a collection of lava flows for which the geological history and palaeomagnetic stability is well documented. The thermal history of these flows was described by Upton et al. (1974), Larsen (1977) and Upton and Blundell (1978), who all showed that the last thermal event which could have affected the magnetic composition of the basalts was the intrusion of the llimaussaq central complex, dated at 1143 + 21 m.a. (Blaxland ct al., 1978). Palaeomagnetic

TABLE 1 Results of non-thermal rock magnetic tests for the l o w e r and Middle Lava Formations

Flow

LI L2 L3 L4 L5 L6 L7 L8 I_9 LI0 LI 1 LI2 LI3 LI4 L15 LI 6 L17 LI8 L19 L20 L21 L22 L23

KLT

Hysteresis

KLT group

RS

3b 3b 3b 3a 3b 3b 3b 3b 3b 3a 3b 3b 3a 3a 3b 3b 3b 3b 3a 3b 3b 3b 3b

[).62 11.66 0.76 0.46 [).56 0.89 [).85 0.75 0.82 0.53 0.93 0.88 {).42 [).56 0.87 1.00 [).57 0.68 11.45 0.61 0.80 0.75 0.81

Peak

M~

Mr~/ M,

( × 103A m 2 k g - t ) 1.37 1.34 1.15 1.37 1.37 1.51 1.43 1.33 1.52 1.24 1.46 1.51 1.20 1.35 1.48 1.63 1.32 1.14 1.39 1.16 1.68 1.52 1.33

2.9 0.7 1.1 4.0 2.1 3.6 1.0 I).9 1.2 1.5 0.6 1.1 2.6 1.8 0.6 0.8 1.0 11.4 2.1 0.4 ().9 1.2 3.7

It¢

XMD

(A m - I) 0.28 0.18 0.17 0.19 (I.16 0.21 0.12 0.29 0.17 0.16 0.20 0.14 0.31 0.10 0.22 0.22 0.119 0.21 0.10 0.14 0.08 [).18 0.22

24.3 12.9 11.7 8.3 11.3 11.9 7.8 I 1.4 12.9 1/).7 15.8 10.2 34.6 7.8 16.6 11.9 4.7 18.7 6.7 15.9 5.1 13.1 18.4

a.f. demagnetisation

RM

MDF (roT) 45.8 66.7 68.7 64.6 70.8 60.4 79.2 43.7 68.7 70.8 62.5 75.11 39.6 83.3 58.3 58.3 85.4 6[).4 83.3 75.(I 87.5 66.7 58.3

11 9 II 7 22 11 5 12 34 15 17 14 9 15 19 17 7 36 11 18 7 7 3(1

1 1 1 2 I I I/2 2 1 2 1 1/2 1/2 2 I 1 2 1 I 1/2 2 1/2 2

KLT, Ix)w-temperature susceptibility; RS, relative susceptibility ratio K i,u,/K30; Peak, ratio K~a k/K3o; Group 3a has a peak at - 150°C and an RS value of less than 0.5; Group 3b curves have a peak at - 150°C and an RS value between 0.5 and 1.0. Mrs, Saturation remanence; M s, Saturation magnetisation; He, coercivity; XMI), percentage of multi-domain material in a mixed grain size of pure magnetite; MDF, median destructive field; RM, rock magnetic set, as defined in Section 4.

332

N. T H O M A S

s t u d i e s o n all t h r e e lava f o r m a t i o n s o f t h e s u c c e s s i o n ( P i p e r , 1977; T h o m a s a n d P i p e r , 1992) h a v e i n d i c a t e d t h a t m a n y o f t h e flows h a v e s i n g l e - c o m p o n e n t r e m a n e n c e s a n d a r e v e r y s u i t a b l e for s u c h w o r k . F u r t h e r m o r e , T h o m a s (1992) a n d T h o m a s a n d P i p e r (1992) h a v e d o c u m e n t e d p a l a e o m a g n e t i c c o n t a c t tests for t h e U p p e r a n d L o w e r L a v a F o r m a t i o n s , w h i c h c o n s t i t u t e t h e s a m p l e set for t h e p r e s e n t study. T h e s e tests i n d i c a t e t h a t t h e m a g n e t i s a t i o n in t h e lavas p r e - d a t e s t h e e m p l a c e m e n t o f t h e I l f m a u s s a q i n t r u s i o n . T h u s , t h e flows o f t h e G a r d a r lava s u c c e s s i o n r e p r e s e n t o n e o f the most suitable sample collections of Proteroz o i c a g e o n w h i c h to p e r f o r m p a l a e o i n t e n s i t y s t u d i e s a n d to d e v i s e a n e w m e t h o d o f a s s e s s i n g t h e s u i t a b i l i t y o f a n c i e n t b a s a l t s a m p l e s for use in such studies.

3. The method T h e p r i n c i p l e o f t h e m e t h o d is to c a t e g o r i s e all s a m p l e s , b a s e d o n i n f o r m a t i o n p r o v i d e d by t h e i r r o c k m a g n e t i c p r o p e r t i e s , a n d t h e n to a n a l y s e how each sample behaves during palaeointensity e x p e r i m e n t s . P a t t e r n s o f b e h a v i o u r are i d e n t i f i e d f r o m t h e r o c k m a g n e t i c a n d p a l a e o i n t e n s i t y resuits, a n d a set o f c r i t e r i a a r e e s t a b l i s h e d w h i c h g o v e r n t h e s e l e c t i o n a n d r e j e c t i o n o f s a m p l e s for e i t h e r t h e m o d i f i e d T h e i l i e r ( C o e , 1967) o r t h e m o d i f i e d S h a w ( R o l p h a n d S h a w , 1985) p a l a e o i n t e n s i t y t e c h n i q u e . T h e s e c r i t e r i a t a k e into acc o u n t t h e i m p o r t a n c e o f m i x e d g r a i n sizes o f p u r e m a g n e t i t e , w h i c h is o f c o m m o n o c c u r r e n c e in a n c i e n t b a s a l t s ( R a d h a k r i s h n a m u r t y et al., 1991). A l s o , t h e i n c l u s i o n o f all s a m p l e s in the p a l a e o i n -

TABLE 2 Results of non-thermal rock magnetic tests for the Upper Lava Formation (see legend to Table I for explanation of symbols) Flow

KLT

Hysteresis Peak

a.f. demag-

M~

Mrs/M,

KLT group

RS

U23 U22 U21

3a 3a 3a

0.38 0.40 0.47

1.30 1.22 1.21

2.fi 2.3 .

.

.

U20

3b

0.82

1.16

.

.

.

U 19 U 18 U17 U 16 U 15 U14 UI3 UI2 U I1 UIO U9 U8 U7 U6 U5 U4 U3 U2 U1

3a 3b 3b 3b 3b 3a 3b 3a 3a 3a 3b 3a 3a 3a 3b 3a 3b 3a 3a

0.50 0.75 0.72 0.66 (].76 0.39 0.55 0.35 0.42 (I.43 0.59 0.45 0.38 (I.48 (].76 0.40 (].99 0.40 0.40

1.28 1.29 1.31 1.05 1.2(I 1.22 1.03 1.17 1.25 1.25 1.17 1.10 1.15 1.37 1.37 1.34 1.23 1.26 1.38

4.3 -

( X 10 ~ A m 2 kg-I)

. . 3.0 . 2.3 3.9 1.3 2.5 1.2 2.2 . 9.1 0.7 5.2 7.4

XMD

(A m - I ) 0.08 0.07

0.33 . .

. . 0.15

.

.

H,.

.

.

0.15 0.09 0.07 0.07 0.14 0.26 . 0.09 0.17 O.15 0.09

5.3 4.7 .

87.5 89.6

8 6

35.4

7 109 9

72.9

12

72.9 85.4 89.6

9 3 5 17 3 7 5 11 7

.

24.0 . . 10.2 . 9.4 5.8 4.9 4.4 12.7 19.8 5.7 11.1 11.3 5.0

RM

netisation MDF (roT) 2 2 2 3

89.6 75.0 50.0 85.4 68.7 72.9 85.4

13 8

2 3 3 3 3 2 3 2 2 2 3 2 2 2 3 2 3 2 2

BASALT SAMPI.E SI~I.EC'I'IONF()R PALAEOINTENSI'IY EXPERIMENTS

tensity analysis is a more thorough approach than previous methods. This approach minimises the possibility of overlooking suitable samples but increases the chances of selecting samples which satisfy the two requirements mentioned above, namely, that the NRM is a primary T R M and that the magnetic mineralogy is most resistant to alteration during laboratory heating. The new method involves the following steps: (I) Rock magnetic experiments (Table 1). Five major rock magnetic techniques are u s e d - - t h e r momagnetic analysis and high-temperature susceptibility (KHT) (the thermal methods); lowtemperature susceptibility (KLT), hysteresis measurements and a.f. demagnetisation (the nonthermal methods). The critical point of this new method is to combine results from all these techniques to build a complete picture of the magnetic composition and domain state of a sample. The thermal methods (which both involve heating the sample in air) identify the number of magnetic mineral phases present in a sample, their compositions, blocking temperature spectra, Curie temperatures and, importantly, their response to laboratory heating, as a measure of thermal alteration. The non-thermal methods can be used to calculate the composition and domain states of magnetic minerals but also, crucially, to calculate the relative amounts of MD and SD material present in samples with a mixed grain size. (2) Palaeointensity experiments (Table 2). Sister samples from each core are used to calculate palaeointensity values for each flow by the modified Thellier (Coe, 1967) and modified Shaw

333

(Rolph and Shaw, 1985) techniques. The reliability of results obtained from these experiments is assessed using the following criteria. For the Thellier data, at least four points were used to define a linear segment on the N R M - T R M plot and no anomalous data points within this segment were eliminated. The length of this segment must represent a 15% decrease in the NRM intcnsity, and the standard deviation of the slope on the straight-line fit must be less than 15%. Finally, the partial thermoremanent magnetisation (PTRM) checks taken at a minimum of two raised temperatures should be within 15% of the original PTRM value at those temperatures. For the Shaw cxperiments, similar constraints were placed upon the NRM-corrected T R M plots, with the exception of the PTRM checks, which are not performed in the modified Shaw method. (3) Correlation of behaviour. All rock magnetic and palacointensity results for each sample arc compared and categorised into "sets', which reflect patterns of similar behaviour within the sample collection. (4) Selection or rejection criteria. The sets defined in (3) above are used to establish criteria for the selection or rejection of samples for use in the modified versions of the original Thellier and Shaw techniques (Thellier and Thellier, 1959; Shaw, 1974). For this integrated approach, it is important that large sample collections are used to define fully the range of rock magnetic behaviour, and thereby the magnetic mineralogy, of the rocks used in the study. This criterion is satisfied by the present study as over I(X) palaeointensity determinations

TABLE 3 Definition of characteristics of each rock magnetic (RM) set RM set

1 2 3

Thermal tests

Non-thermal tests

Thermomagnetism

KIlT

KLT

Hysteresis

a.f. demag-

T~. (°(')

Curve

curve

RS

Peak

Group

M~/M~

nctisation MDF (roT)

580 580 600-620

3(2a) 2a 6

MD/SD(H) II(MD/SD) If'

> {I.5 < 0.5 > 0.5

1.311 1.15 1.215

3b 3a 3b

> 0.12 < 0.12 -

> 1(5 < 10 > 10

To, Curie temperature; RS, relative susceptibility ratio K_l,~c,/K3o; Peak, ratio Kocak/K3o; M,~/M~, ratio of saturation remanence to saturation magnetisation; MD, multi-domain; SD, single domain; H, reversible KHT curve: H', KIlT curve with a low temperature peak (see Fig. 1)

334

N. THOMAS

TABLE 4 P a l a e o i n t e n s i t y results f o r t h e L o w e r L a v a F o r m a t i o n Sample no.

AT

LI-02 " L2-01 L3-02 L4-02 -05 -06 L5-03 -06 -03 L6-02 L7-01 -02 -03 " L8-02 -03 -06 -07 L9-02 -03 " -04 -117 LI0-02

11-450 250-400 150-400 0-400 150-300 0-3511 200-450 . 250-500 250-560 100-450 150-560 0-300 150-450 150-450 0-450 . 150-450 100-350 100-450 . 150-5110

N

RM

9 5 6 8 5 4 8 .

1 1 1 2 2 2 1 .

.

6 7 9 111 6 7 5 8 .

1/2 1/2 1/2 2 2 2 .

7 5 9

Ba(Sh) (~T)

22 19 21 22 42 42 21

15.9 + 41.1 _+ 33.7 ± 32.7± 48.5 + 35.8 ± 26.2 ±

2.1 2.8 3.1 2.1 2.2 3.1 4.5

22 36 55 71 66 23 33 59

24.6± 22.7 ± 69.5 ± 78.4 ± 54.6 ± 33.5 ± 14.5 ± 49.9 ±

1.8 1.5 9.3 4.8 4.6 3.2 0.6 2.0

25 30 72

36.8 ± 3.4 30.3 ± 10.6 30.7 ± 5.7

. 1 I 1

.

Ba (#T)

. I 1

.

.

F

.

. 25

19.2±

1511-450 100-450

6 10

2 2

311 32

21.8 ± (1.7 21.3 ± 2.0

45.9 ± 3.11 4.4 ± O. l

LI 1-01 -02 -03 -115 1.12-01 L13-(11 416 ~ -07 L14-01 1.15-04 " L 1 6 - 0 4 :' -05 ~ -06 LI7-01 -02 -113 4)6

2110-500 150-40() 150-400 . 3110-450 0-450 1110-300 100-450 100-400 150-450 300-450 240-400 . 2110-450 380-5611 . 200-450

7 7 6

1 1 1

34 33 25

13.8 + 1.2 20.9 ± 2.11 2 7 . 7 ± 11.9

-05 -(17 L19-01 -02 L20-01 " -04 - 1 1 5 -07

2

1.0

1.8_*_0.1 50.3 ± 1.2 33.6 ± 1.6 39.11 ± 1.7 -

-03 -115

L18-02

6

7.2 ± 0.6 28.5 ± 2.3 31.0__+ 1.7 42.3 ± 0.8 3.5 ± 0.1 42.7 -+ 1.4 30.9 ± 3.3 28.7±0.3

6

2

31

19.4 ± 2.2

8.1 ± 0 . 1 4 0 . 9 ± 1.4 13.5_+0.2 8.2 ± 0 . 5 -15.8 +_0.5 -

11-4011

7

I

40

64.11+ 5.0

66.7 ± 3.5

21~1-450

5 . 6 8 4 5 . 8

1

26

84.9 ± 3.8

30 56 25 36

31.7± 29.4± 42.5 ± 80.8 _+

5.4 3.11 1.2 3.6

46

26.0±

1.6

3(I.2 ± 5.3 311.8± 1.3 26.5 + 0 . 4 84.0 ± 7.8 23.6_+0.3

0-300 150-5011 50(I-590 150-450 0-4511

.

.

.

5 6 5 4 6 4 4 5 .

.

.

38 21 13 27 9 13 26

.

6 6 .

. I/2 1/2 1/2 1/2 2 1 1 I

26.0± 22.2± 111.7± 16.5+ 411.2± 24.5± 10.0_+ 124.7 ±

1.6 1.3 1.0 1.11 3.6 3.0 (1.6 19.7

. 2 2

.

35 24 .

.

.

.

.

1 1 I/2 I/2 .

. 1/2

26.5 5- 2.2 27.5 _+ 1.3

.

335

BASAL'I" SAMPLE SEI.ECI'ION FOR PALAEOINTENSI'I'Y E X P E R I M E N T S

TABLE 4 (continued) Sample no.

AT

N

RM

F

Ba (p.T)

Ba(Sh) (~T)

L21-02 L22-02 1.23-01 a -02

1(X)-380 100-450 100-300 .

5 8 5

2 1/2 2 .

37 40 5

38.9 :t: 4.4 88.9 -£-_ 9.3 14.7+ 3.8

29.9 + 1.3

.

.

.

-

16.1 _+ 1.2 28.0 + 2.6

AT, Temperature range over which Ba was calculated; N, number of data points used to calculate Ba; RM, rock magnetic set, as defined in Section 4; F, percentage of NRM used in calculation of Ba; Ba(Th), modified Thellier palaeointensity value; Ba(Sh), modified Shaw palaeointensity value. " Sample eliminated from calculation of mean values.

were made from 41 lava flows (Table 2), each flow having a complete set of rock magnetic resuits see (see Table 1 and point (1), above). As a result of this large data set, the method devised in the present study can be confidently applied to basalt collections with similar magnetic mineralogy.

4. Correlation of b e h a v i o u r m t h e rock magnetic (RAM) sets

definition of

The results of the rock magnetic and palaeointensity experiments are listed and summarised in Tables 1-7 and Figs. 1-4. Samples from the Gardar lavas fall into one of three rock magnetic sets, whose characteristics are shown in Fig. 1 and defined in Table 3, based on their behaviour during the rock magnetic experiments. Very few samples from the collection show results which do not fall into one of these RM sets. Occasionally, a small number of samples (less than 5%) from RM set 1 have Type 2a instead of Type 3 thermomagnetic curves. The palaeointensity resuits for each RM set are discussed in detail below. (1) RM set 1. The majority of samples from this set give palaeointensity results which show consistency between the two techniques and between samples from the same flow. Eighteen flows (one from the Upper Lavas and 17 from the Lower Lavas) fall into this category, representing 38% of the entire collection. From these flows, 36 samples (two and 34 from the Upper and Lower Formations respectively) were used in Thellier experiments and 19 (two and 17) in Shaw experi-

ments. Twenty-seven and 12 of these samples give acceptable results using the Thellier and Shaw techniques respectively. Ten flows give palaeointensity values which are consistent between the two techniques for the same flow. Most set 1 samples pass at least two of the PTRM checks in the temperature region over which the palaeointensity was calculated during the modified Thellier experiments, and give linear NRM-corrected TRM plots (Rolph and Shaw, 1985) within the higher-coercivity (greater than 80 roT) region during modified Shaw experiments. An important feature of set 1 samples is that they mainly comprise flows from the Lower Lava Formation. These flows show the greatest amount of SD material in a mixed grain size of magnetite. Figure 2 shows modified Thellier and modified Shaw plots for two RM set 1 samples. The consistency of results shown by set 1 samples within flows and between techniques suggest that samples from this set are most likely to give acceptable palaeointensity results. (2) RM set 2. Generally, samples belonging to this set give a limited number of reliable palaeointensity results using the Thellier technique but do not tend to give reliable results using the Shaw technique. Specifically, 19 flows (13 from the Upper Lavas and six from the Lower Lavas) representing 40% of the entire collection, fall into this category. From these flows, 36 samples (23 from the Upper Lavas and 13 from the Lower Lavas) were used in Thellier experiments and 23 samples (13 and 10) in Shaw experiments. Eighteen samples give Thellier results which are considered to be reliable. On examination of the rock magnetic characteristics of set 2 samples, it

336

N. T H ( ) M A S

TABLE 5 Thellier palaeointensity results for the Upper Lava Formation; symbols as h)r Table 3 (note that no results were obtained for the Upper Lava Formation using the modified Shaw technique) Sample no.

AT

N

F

Ba (/xT)

U01-04 -05 U02-(11 -03 U03-06 -07 U04-02 -04 U05-02 -05 U06-02 -04 U07-01 -04 U08-02 -06

150-400 150-400 150-30(I 150-350 0+450 0 + 450 I)-350 200-400 250-450 300-450 0 + 370 0 + 370 0-450 0-2711 150-350 0-40(I

4 5 4 5 2 2 5 5 5 4 2 2 9 6 5 5

47 55 36 48 60 80 89 35 31 2(1 96 33 91 63 25 98

32.7 + 2.4 42.6 + 2.0 40.(I 4- 3.3 37.3 +_9.4 33.7_+ 1.7 " 34.8 _+4.5 " 60.8 _+2.9 34.7 _+2.5 13.8_+ 1.0 17.7_+3.2 31.9 _+2.3 ~' 34.6 + 0.1 ~ 81.4_+4.2 83.9 _+6.0 8.9 -+ 1.2 48.3 -+5.5 a

-08

0-200

4

86

26.7_+0.5

-03

-

U 111-03 -(15 Ull-01 -06 UI2-02 -07 U13-(12 U14-02 415

U17-05 -(17 U18-04 U19-05 -(17

0-20(I 0-200

-

0-200 150-400 0-40() 0-450 150-300 150-300 0-200 150-300 240-500 -

4 4 7 4 3 4 6 4 4 3 4 8 -

54 46 56 64 35 29 10 31 30 40 20 34 -

-

0-350

52.4 + 3.9 54.1 _+3.6 55.5_+9.0 ~ 47.2_+4.3 " 13.3+ 1.0 " 48.0 _+7.3 " 17.3 _+ 1.6 " 22.3_+4.2 30.3_+2.7 33.0_+3.9 J 14.1 _+0.8 15.9_+ 1.4 ~'

-

-

tj2(1-02

-

-

-03

-

-

-

-

6 5 5

24 31 42

11-400

7

80

4 9 . 4 _+ 6 . 6

0-40

4

84

4 5 . 4 _+ 3 . 9

U21-01

-02 U22-03 -06 U23-01 -02

0-300 150-35(I 0-200

~'

-

15.8_+2.7 a 48.0 _+3.6 50.9 _+1.9

RM set

1 2 3

No. of samples used (Thellier)

No. of samples used (Shaw)

Upper Lavas

Lower Lavas

Upper Lavas

la)wer Lavas

2 (2) 23(111 24 (0)

34 (25) 13(71 0 (01

2 (0) 13(0) 11 (01

17 (121 10(31 0 (0)

n e t i t e is u n s u i t a b l e f o r p a l a e o i n t e n s i t y w o r k , as t h e b l o c k i n g a n d u n b l o c k i n g s p e c t r a o f T R M in MD grains are not necessarily equivalent. Those s a m p l e s f r o m R M s e t 2 w h i c h d o give a c c e p t a b l e T h e i l i e r r e s u l t s all s h o w r o c k m a g n e t i c c h a r a c t e r istics w h i c h i n d i c a t e t h e p r e s e n c e of small a m o u n t s o f S D m a t e r i a l in t h e M D - r i c h s a m p l e . A n y s a m p l e s w h i c h d o give a m o d i f i e d S h a w r e s u l t h a v e v e r y low i n t e n s i t y v a l u e s . T h e s e low v a l u e s o c c u r as a r e s u l t o f t h e t o t a l r e m o v a l o f t h e N R M at low d e m a g n e t i s i n g f i e l d s as a r e s u l t of the predominance o f M D m a g n e t i t e in t h e samples, effectively leaving no NRM in t h e higher-coercivity region. Figure 3 indicates the typical modified Theilier and modified Shaw plots for acceptable and unacceptable samples from R M s e t 2. (3) R M s e t 3. S a m p l e s f r o m t h i s s e t d o n o t give a n y a c c e p t a b l e p a l a e o i n t e n s i t y r e s u l t s . A t o tal o f 24 s a m p l e s ( f r o m 11 flows), all f r o m t h e U p p e r L a v a F o r m a t i o n , w e r e u s e d a n d all f a i l e d to provide a consistent palaeointensity value, with either technique.Non-thermal rock magnetic data

TABLE 7 Results of modified Thellier and modified Shaw palaeointensity experiments

a Sample eliminated from calculation of mean values.

b e c o m e s c l e a r w h y s o m e fail t o give r e l i a b l e l i e r r e s u l t s . T h e m a g n e t i c m i n e r a l o g y is nated by MD magnetite with little evidence m a t e r i a l . It is w e l l k n o w n t h a t p u r e M D

TABLE 6 Success rate statistics for palaeointensity experiments (figures in parentheses are the numbers of samples for which results were accepted)

Theldomiof SD mag-

Lava group

N/n

Mean Thellier Ba (/zT)

Mean Shaw Ba (tzT)

Upper Lx)wer

13/0 32/15

31.4+3.0 34.3-t- 1.0

33.8+2.1

N/n, Number of accepted samples used in modified Thellier experiments/number of accepted samples used in modified Shaw experiments. Errors are quoted to lcr.

BASALT SAMPLE SELE(YI'ION FOR PAI.AEOINTENSI'rY

suggest that such samples contain a significant SD fraction, which should make them suitable for palaeointensity studies. However, the thermal resuits show that significant alteration has occurred during heating. The inflection in the thermomagnetic heating curve at about 450°C suggests that the dominant magnetic phase present in samples showing set 3 behaviour is maghaemite (Readman and O'Reilly, 1970). The presence of a large peak in the KHT heating curves probably reflects the destruction of a narrow range of SD maghaemite grains and the creation of haematite, which probably forms as a coat around the magnetite core of the grain. These peaks in the KHT curves have been witnessed by P.W. Schmidt (personal communication, 1992), who also interprets them as being caused by the presence of

(a)

337

EXPERIMENTS

maghaemite. His data are supported by transmission electron microscopy (TEM) images which clearly indicate the presence of maghaemite in samples with the peak and its absence in samples without the peak. The presence of maghaemitc indicates that some degree of low-temperature oxidation (maghaemitisation) has occurred in the samples showing RM set 3 behaviour. This produces a chemical remanence (CRM) which contaminates the Primary TRM. This explains the inability of set 3 samples to produce results and immediately invalidates the use of such samplcs for palaeointensity work, as the NRM is not a pure TRM. Examples of modified Thellier palaeointensity plots for these unsuitable samples are shown in Fig. 4; no results were obtained using the modified Shaw technique.

RM

1

(2 ,)

Gr.~q~ 31)

Type MD/SD

~ ~ " ' ~ N$>05

60O TEMPERATURE ( C) Type 2.

(b)

600

-~

-1~

TEMPERATURE ( C) TEMPERATURE( C) Type H (MD/SD) Group 3a

,o

0

Mrs/Ms

0.12

>

RM

t it t, :,. f,

100

2

12

~

Mrs/Ms

0.12~~.~

<

RS ~ ' <' =O~~ ' ~ - ~

(c) Type 6

TVI~- H'

sop -~o

-Ibo

Group 3b

~

RM3 o

Mrs/Ms

10o

1so

-

RS > 0.5

0

00O

0

(UX) -200

-100

O

tOO

Fig. 1. Characteristics of the three rock magnetic (RM) 'sets' defined for the Gardar lavas: (a) RM set 1, (b) RM set 2, (c) RM set 3.

338

N. T | I O M A S

SAMPLE 3

Ba = 26.7___0.9pT

SAMPLE 4

'~1 (J

"v..

Ba = 39.3__+ 1.4pT

10(I

;t10

NI(M

~0~)110

~ . ~

~

NRM

IRM

SAMPLE 3

C.R

o

R

TRM

Ba = 25.5___0.4.uT

SAMPLE 4

Ba = 3 8 . 7 + 1.6uT

60 NRM

NRM

IRM'(AI/A2~

TRM'IA1/A2"~

Fig. 2. Modified Thellier and modified Shaw palaeointensity results for two samples exhibiting RM set I characteristics. Sample 1, consistent result with both methods; sample 2, consistent result with both methods.

5. Sample selection and rejection The selection and rejection of samples for use in palaeointensity experiments has both advantages and disadvantages. The results of the present study can be used to highlight the major disadvantage, namely, the use of an inadequate rock magnetic data set, which leads to the erroneous selection or rejection of samples. Figure 1 illustrates three cases where use of a limited amount of available rock magnetic data would lead to an erroneous selection or rejection of a sample. In Fig. l(a), the thermomagnetic curve shows that a small amount of thermal alteration has occurred during laboratory heating, which would suggest that these samples may not be suitable

for palaeointensity work. However, the results of the non-thermal RM tests indicate the presence of a significant amount of SD material in a mixed grain size and a stable a.f. demagnetisation curve. These samples belong to RM set 1 and have produced reliable and consistent results with both palaeointensity techniques. In Fig. l(b), the thermomagnetic curve which characterises RM set 2 behaviour indicates a single, high Curie temperature and a reversible curve. This evidence suggests that the sample does not alter significantly on heating, and would be likely to give an acceptable palaeointensity result (e.g. Pr6vot et al., 1985; Derder et al., 1989). Although in some cases this assumption would be correct, the results of the non-thermal RM tests indicate that such samples are, in fact,

339

BASALT SAMPLE SEI.I-~C'I'ION FOR PAI.AEOINTENSITY EXPERIMENTS

SAMPLE 1

NRM

Ba = 49.3_.+ 2.5pT

300

~

SAMPLE 2

_

REJECTED

NRM

r-r-"

TRM

TRM

SAMPLE1

I

SAMPLE 2

Ba = 1.8___0.1.uT

NRM

Ba = 3.4-t- 0.2.uT

NRM

150

....

0

1 5 0 ~ - - - - O -

0

TRM'(AI/A2)

TRM'(AllA2)

Fig. 3. Modified Thellier and modified Shaw palaeointensity results for two samples exhibiting RM set 2 characteristics. Sample 3, Thellier accepted, Shaw rejected; Sample 4, no result obtained with either method.

SAMPLE

5

SAMPLE

REJECTED

6 REJECTED

NRM

NRM

r - ~

•

0

700 • • 700

TRM

TRM

Fig. 4. Modified Thellier and modified Shaw palaeointensity results for two samples exhibiting RM set 3 characteristics. Sample 5, no result obtained with either method; sample 6, no result obtained with either method.

340 completely dominated by MD magnetite and are thus unsuitable for both the modified Thellier and modified Shaw palaeointensity techniques. In Fig. l(c), the results of the non-thermal RM tests for set 3 suggest that these samples have a mixed grain size with a significant amount of SD material present. On this evidence alone, the samples would be expected to give an acceptable palaeointensity result with both methods. However, thc results of the thermal RM tests show that maghacmite is present, indicating that lowtemperature oxidation has occurred. Thus, sampies that show this behaviour would not give an acceptable palaeointensity result with either technique. These three examples clearly indicate the danger of using a limited amount of rock magnetic data to assess the suitability of samples for use in palaeointensity experiments. A better approach is to consider results from as many RM tests as possible, at least one of which should be thermal and at least one non-thermal. From the rock magnetic and palaeointensity results described, the lollowing set of selection or rejection criteria is proposed for identifying suitable samples for use in the modified versions of the Thellier and Shaw palaeointcnsity techniques. These criteria apply to basalts whose magnctic mineralogy comprises pure (Ti <0.1) magnetite with a mixed grain size, with varying amounts of SD material in MD-dominated samples, and arc thus designed for usc with ancient basalts. (1) Selcction (samples most likely to give acceptable results using Thellier or Shaw methods) Samples with RM set 1 characteristics. (2) Rejection (samples which fail to give acceptable results with either method) Samples with RM set 3 characteristics. Samples with all the following non-thermal RM characteristics, regardless of their thermal RM characteristics: Saturation remancnce to saturation magnetisation (MrJM.,) ratio less than 0.08; Group 3a KLT curve (RS (i.e. relative susceptibility K_ 19JK3o) less than 0.5); Median destructive field (MDF) less than 8 mT. (3) Rejection from Shaw experiments (samples

N.THOMAS which invariably are unstable to a.f. demagnetisation) Samples with RM set 2 characteristics. (4) Possiblc selection (samples having rock magnetic characteristics which do not fall into one of the three defined RM sets) Samples with all the following RM characteristics: Type 2a thermomagnetic curves; Group 3a or 3b KLT curves; MrJM s ratios between 0.08 and 0.12. The fourth criterion listed above represents a grey area in the method, where certain samples possess rock magnetic characteristics which do not fall into any of the three defined RM sets (Section 4). In the present study, such samples rcpresent only 7% of the collection (10 samples) and hence are relatively unimportant. Of these 10 samples, six gave palaeointensity estimates which comply with the reliability criteria given in Section 3 and four did not. It must be stressed that caution should be exercised when applying selection or rejection criteria to basalt collections. The criteria listed above arc applicable only to basalts with a mixed grain size of pure magnetite. The magnetic mineralogy of youngcr basalts and their behaviour during palacointensity experiments is likely to be very different. Thus, although the technique of grouping rcsults into characteristic 'rock magnetic sets' is a valid approach for basalts of any agc, the conclusions reached from such a classification, and hence thc resulting selection or rejection criteria, arc likely to differ for basalts of different ages.

6. Discussion and conclusions

The large amount of data accumulated in the present study has demonstrated that, provided an integrated rock magnetic approach is used, sample selection or rejection provides a useful addition to palaeointensity studies. The behaviour of each sample during rock magnetic experiments is correlated with that during palaeointensity experiments so that patterns of behaviour can be identified within the rock collection. These patterns

341

BASAI.T SAMPLE SEI.E(*I'ION FOR P A L A E O I N I ' E N S I T Y E X P E R I M E N T S

can t h e n be u s e d to establish c r i t e r i a which define the sets o f s a m p l e s which give the most r e l i a b l e results in t h e p r e s e n t study a n d are thus most s u i t a b l e for f u t u r e p a l a e o i n t e n s i t y work. T h e m a g n e t i t e - b e a r i n g a n c i e n t b a s a l t s which a r e most likely to give c o n s i s t e n t a n d a c c e p t a b l e field intensity d e t e r m i n a t i o n s a r e those which b e l o n g to R M set I, which is c h a r a c t e r i s e d by s a m p l e s c o n t a i n i n g significant a m o u n t s o f S D grains in a mixed grain size of p u r e m a g n e t i t e , i.e. t h o s e with the smallest a m o u n t o f M D grains. A l t h o u g h a large v o l u m e o f rock m a g n e t i c d a t a is used in the new m e t h o d , the results can be o b t a i n e d speedily, as the test t h e m s e l v e s are quick a n d a n u m b e r of tests can be p e r f o r m e d s i m u l t a neously. T h e r e are, o f course, as in most scientific t e c h n i q u e s , l i m i t a t i o n s to the use of the new selection o r rejection m e t h o d . T h e m a j o r limitation at p r e s e n t is the lack o f p r e c i s e q u a n t i t a t i v e e s t i m a t e s o f the rock m a g n e t i c p a r a m e t e r s which c h a r a c t e r i s e a s a m p l e t h a t is s u i t a b l e for p a l a c o i n t e n s i t y work. Specifically, we r e q u i r e a precise k n o w l e d g e of the Mr.,/M,, RS a n d M D F values which a s a m p l e should have b e f o r e it can be classed as suitable. Ideal b e h a v i o u r in basalts, especially a n c i e n t ones, is rare. Thus, any selection or r e j e c t i o n m e t h o d is subject to a c e r t a i n d e g r e e of inconsistency. H o w e v e r , the p r e s e n t study has shown that an i n t e g r a t e d rock m a g n e t i c a p p r o a c h is a t h o r o u g h , effective a n d a c c u r a t e m e t h o d o f assessing the suitability o f samples, with mixed grain sizes o f p u r e m a g n e t i t e , for p a l a e o i n t e n s i t y work. P a l a e o i n t e n s i t y t e c h n i q u e s t e n d to be l a b o u r intensive a n d time c o n s u m i n g . P r o v i d e d t h a t the i n t e g r a t e d a p p r o a c h is followed, t h e a d v a n t a g e s of s a m p l e selection o r rejection a r c clear. L a r g e rock collections can be rapidly i n v e s t i g a t e d a n d the s u i t a b l e s a m p l e s i d e n t i f i e d , thus saving a significant a m o u n t o f l a b o r a t o r y time. This is a g o o d e x a m p l e o f a positive use o f rock m a g n e t i s m in p a l a e o m a g n e t i s m to i n c r e a s e the efficiency a n d reliability o f p a l a e o i n t e n s i t y t e c h n i q u e s . F u r t h e r m o r e , the a d v a n t a g e s over p r e v i o u s m e t h o d s are important. The previous methods described here use a limited a m o u n t of rock m a g n e t i c data; they reject s a m p l e s on limited p r a c t i c a l criteria, a n d consequently perform palaeointensity experi-

m e n t s only on s e l e c t e d samples. In the i n t e g r a t e d m e t h o d , m a n y rock m a g n e t i c t e c h n i q u e s a r e used a n d p a l a e o i n t e n s i t y e x p e r i m e n t s are p e r f o r m e d on all samples. C o n s e q u e n t l y , s a m p l e s are rej e c t e d on e m p i r i c a l a n d t h e o r e t i c a l grounds, which is a m u c h m o r e t h o r o u g h a p p r o a c h .

Acknowledgements This work was c o m p l e t e d d u r i n g the t e n u r e o f an N E R C s t u d e n t s h i p . I a m grateful to my coll e a g u e s T i m R o l p h , Del A t k i n s o n a n d G r a h a m S h e r w o o d for t h e i r v a l u a b l e suggestions a n d discussions d u r i n g the p r e p a r a t i o n o f this p a p e r . A special t r i b u t e goes to my late father, whose s u p p o r t a n d e n c o u r a g e m e n t t h r o u g h o u t has b e e n i m m e a s u r a b l e , but who sadly did not quite see the finish.

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342 Pr~vot, M., Mankinen, E.A., Coe, R.S. and Gromm6, C.S., 1985. The Steens Mountain (Oregon) Geomagnetic Polarity Transition 2. Field intensity variations and di~ussion of reversal models. J. Geophys. Res., 90:10 417-10 448. Radhakrishnamurty, C., Likhite, S.D. and Sahasrabudhe, P.W., 1991. Domain states of magnetic grains in basalts and palaeointensity techniques. J. Geomagn. Geoelectr., 43: 1-15. Readman, P.W. and O'Reilly, W., 1970. The synthesis and inversion of non-stoichiometric titanomagnetites. Phys. Earth Planet. Inter., 4: 121-128. Rolph, T.C., 1984. The determination of ancient, historic and modern geomagnetic field intensities from thermally altered lavas. Ph.D. Thesis, University of Wales, Cardiff, pp. 1-169. Rolph, T.C., 1992, High field intensity results from recent and historic lavas. Phys. Earth Planet. Inter., 70: 224-230. Rolph, T.C. and Shaw, J., 1985. A new method of palaeofield magnitude correction for thermally altered samples and its application to Lower Carboniferous lavas. Geophys. J.R. Astron. Soc., 80: 773-781. Senanayake, W.E. and McEIhinny, M.W., 1981. Hysteresis and susceptibility characteristics of magnetite and titanomagnetites: interpretation of results from basaltic rocks. Phys. Earth Planet. Inter., 26: 47-55. Senanayake, W.E. and McElhinny, M.W., 1982. The effects of heating on low temperature susceptibility and hysteresis properties of basalts. Phys. Earth Planet. Inter., 30: 317321. Senanayake, W.E., McEIhinny, M.W. and McFadden, P.L.. 1982. Comparisons between the Thelliers' and Shaw's palaeointensity methods using basalts less than 5 million years old. J. Geomagn. Geoelectr., 34: 141-163.

N. "t~OMAS Shaw, J., 1974. A new method for determining the magnitude of the palaeomagnetic field. Application to five historic lavas and five archaeological samples. Geophys. J.R. Astron. Soc., 39: 133-144. Sherwood, G.J., 1986. The Middle to Late Miocene geomagnetic field: Implications of new results from New Zealand lavas. Ph.D. Thesis, University of Wales, Cardiff, pp. 1183. Thellier, E., 1951. Propri6t6s magn&iques des terres cuites et des roches. J. Phys. Radium, 12: 205-218. Thellier, E. and Thellier, O., 1959. Sur I'intensit6 du champ magn6tique terrestre dans le pass6 historique et g6ologique. Ann. G~ophys., 15: 285-376. Thomas, D.N., 1992. Rock magnetic and palaeomagnetic investigations of the Precambrian Gardar lava succession, South Greenland. Ph.D. Thesis, University of Liverpool, pp. 1-513. Thomas, D.N. and Piper, J.D.A., 1992. A revised magnetostratigraphy for the Mid-Proterozoic Gardar lava succession, South Greenland. Tectonophysics, 201: 1-15. Upton, B.G.J. and Blundell, D.J., 1978. The Gardar igneous province: evidence for Proterozoic continental rifting. In: E.R. Neumann and I.B. Ramberg (Editors), Petrology and Geochemistry of Continental Rifts. D. Reidel, Dordrecht, pp. 163-172. Upton, B.G.J., Macdonald, R. and Pinkerton, H., 1974. Early lavas of the Precambrian Eriksfjord Formation--South Greenland. Bull Geol. Soc. Den., 32: 123-141. Wil~m, R.L. and Watkins, N.D., 1967. Correlation of magnetic polarity and petrological properties in Columbia Plateau basalts. Geophys. J.R. Astron. Soc., 12: 405-424.