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Geomagnetic intensity between 100 million and 2500 million years ago
1969, Phys. Earth Planet. Interiors 2, 11-18, North-Holland Publishing Company, Amsterdam
G E O M A G N E T I C I N T E N S I T Y B E T W E E N 100 M I L L I O N A N D 2500 M I L L I O N Y E A R S A G O
E. J. SCHWARZ and D. T. A. SYMONS Geological Survey of Canada, Ottawa
Received 15 October 1968
The decay of the NRM of 146 specimens (including the 41 specimens reported on earlier) of igneous rocks during heating in steps of 25 or 50 °C between 200 or 300 °C and 550 °C was compared to the acquisition of TRM (H = 0.35 Oe) during subsequent cooling to 20 °C. The K-Ar ages of the specimens are distributed between 100 m.y. and 2500 m.y. The selection of results suitable for intensity determination was based on constancy of the rate of decay of the NRM and acquisition of TRM determined from at least 3 successive determinations over a temperature interval of at least 100 °C, an appreciable decay of the supposed original NRM in that temperature interval, and constancy in the NRM direction in that temperature interval. This selection yielded a total of 41 equatorial paleointensities. The intensity results obtained for several specimens collected from the same rock unit show a reasonable degree of internal con-
sistency. The results indicate that the mean equatorial geomagnetic intensity during most of the Phanerozoic and the Precambrian eras as far back as 2.5 billion years was 0.25 Oe with a standard deviation of 0.13 Oe. The large scatter in the equatorial intensities may be due to various possible errors and/or fluctuations in geomagnetic moment of periods shorter than the experimental error (5 to 10 %) of the radiometric ages of the specimens, possibly including a period as short as that suggested by the archeomagnetic results for the last five millenniums. The dipole representation of the paleomagnetic field as far back as 2500 m.y. is not contradicted by the variation of the paleointensity values with paleomagnetic latitude. Thus, several characteristics of the present geomagnetic field seem to have been present as far back as 2500 m.y. ago.
1. Introduction
CHAEL (1968) employed alternating field demagnetization. The Tertiary a n d late Mesozoic intensity values presented by BRIDEN (1966), SMITH (1967), a n d CARMICHAEL (1968) are generally somewhat smaller than the present value. The early Mesozoic a n d late Paleozoic geomagnetic intensity was substantially
I n tracing the history of the geomagnetic field - - one of the m a i n objectives of p a l e o m a g n e t i s m - i n f o r m a t i o n o n the E a r t h ' s magnetic m o m e n t is indispensable. Such i n f o r m a t i o n can be o b t a i n e d by the d e t e r m i n a t i o n of the intensity of stable original r e m a n e n t m a g n e t i z a t i o n of artifacts a n d rocks. The results of work o n artifacts a n d recent lava flows from several widely spread localities suggest that the geomagnetic equatorial intensity was approximately 1.5 times its present value (Fp) a b o u t 1500 years ago, a n d a p p r o x i m a t e l y 0.5 Fp a b o u t 5500 years ago (e.g. Cox, 1968). I n recent years, several attempts have been made to trace the geomagnetic intensity back t h r o u g h geologic time. Most attempts were based o n stepwise t h e r m a l d e m a g n e t i z a t i o n of the n a t u r a l r e m a n e n t m a g n e t i z a t i o n ( N R M ) of rock samples as suggested by KOENIGSBERGER (1938) a n d initiated by Tt-IELLIER (1938). NESBITT (1966) o b t a i n e d relative intensity values by d e t e r m i n i n g the ratio of the N R M intensity a n d the susceptibility o f red sediments while CARUl11
smaller than, or approximately equal to, Fp according to results reported by VAN ZIJL et al. (1962), BRIDEN (1966), NESBITT (1966), KRS 0 9 6 7 ) , a n d SCHWARZ and SVMONS (1968). Further, CARMICHAEL(1968) suggested a low geomagnetic intensity t h r o u g h o u t the Paleozoic and especially during early Paleozoic. The scattered P r e c a m b r i a n results o b t a i n e d by SCHWARZ a n d SYMONS (1968) and by CARMICHAEL (1968) suggest an average intensity c o m p a r a b l e to the present value. The earlier paper (SCHWARZand SVMONS, 1968) lists the paleointensity results based on radiometrically dated samples of igneous rocks selected from collections made by geologists of the Geological Survey of C a n a d a . The selection was based on the absence of evidence for m e t a m o r p h i s m a n d c o n c o r d a n c e in the radiometric and geologic ages. These earlier results
12
E.J.
SCHWARZ
A N D D, T. A. S Y M O N S
suggest a maximum equatorial intensity* at about 200 m.y. ago and show strong scatter in the Precambrian. It was thought worthwhile to extend the selection over the full collection of radiometrically dated samples to obtain a better idea of the possible range in intensity values between 100 m.y. and 2500 m.y. ago. The major drawback of using the collection is that, in general, only one specimen per rock unit can be obtained so that the consistency of the results cannot be checked for many of the sampled units. For that reason, duplicate and replicate samples of some rock units were added. In this manner, 105 specimens were selected in addition to the 41 specimens discussed in the earlier paper.
other as done in fig. 1, and the results obtained after stepwise heating were joined by a line. The following criteria were set in selecting specimens suitable for paleointensity determination: 1. Constancy of the rates of decay of the N R M and acquisition of p T R M determined for a temperature interval comprising at least three successive heating steps above 300 °C. A straight line through 3 or more N R M / p T R M = J N / J T plots means that this requirement is met (fig. 1). JN
Specimen I10
20 300 X "~400 \ y
2. Experimental procedure THELLIER'S (1938) method of comparing the thermal decay of the N R M with the acquisition of thermoremanent magnetization (TRM) during cooling of the specimens in a weak constant magnetic field was used in this study. The purpose of this method is to single out a N R M component that may correspond to a T R M component acquired at the time of formation of the igneous rocks sampled. Such a T R M component must be present for the determination of paleointensities. Therefore, the specimens were heated in steps of 25 or 50 °C from 200 or 300 °C up to 550 °C. After each step in the heating process, the specimens were cooled to 20 °C in a constant field of 0.35 Oe ± 5 700, and the direction and intensity of the total remanent magnetization of each specimen was measured with maximum errors of respectively 2 ° degrees of arc and 5~o using a biastatic magnetometer. This procedure was repeated at each step with the specimens in the reversed position to allow computing of the residual N R M and the acquired partial T R M (pTRM) as the components of both total remanence vectors measured. Care was taken to keep the heating and cooling rates constant, and the specimens were kept at the selected temperature for 10 minutes. The residual N R M and the p T R M components were then plotted against each * These paleoequatorial intensity values were based on a measured value o f 0.49 Oe for the laboratory field. This value probably was erroneous; subsequent measurements yielding 0.35 Oe. Moreover, experiments on p T R M acquisition of a selection o f the original specimens indicated that the original cooling in the laboratory had taken place in a field of about 0.35 Oe instead of 0.49 Oe.
0 0
Fig. l.
Intensity o f N R M
I0
. 20
J'l"
(,IN) withstanding heating to suc-
cessively higher temperatures (numbers near points, °C) and o f P T R M (JT) acquired during subsequent cooling to 20 °C in a constant field, plotted against each other. RI is defined as x/y. The deviation o f the 250 and 300 °G points from the line are within the possible measurement error. Units in gauss" cm 3" 10- 3
2. The direction of the remaining N R M must be constant within 10 ° of arc in the temperature interval for which the first criterion is satisfied. 3. The temperature interval must be at least 100 °C. It may be argued that a temperature interval of 100 °C is small compared to a total range of about 550 °C. However, most Paleozoic and Precambrian rocks bear secondary components of magnetization which are stable against heating to 300-350°C. Moreover, heating to temperatures of 500 °C or more probably results in either physico-chemical changes affecting the N R M bearing minerals in many samples or complications in the T R M acquisition mechanism. Table 1 gives the interval for each accepted sample. 4. The straight line joining the points defining the temperature interval of constant rate of decay of the N R M and acquisition of p T R M may be extrapolated, if necessary, to both axes in the JN-JT diagrams (see fig. 1). If the points in the temperature interval are taken to reflect part of the original T R M then it may be supposed that the total line would have joined the J N / J T points from 20 °C up to the Curie point in the
GEOMAGNETIC
INTENSITY
BETWEEN
100
MILLION
TABLE
2500 MILLION
AND
YEARS
13
AGO
1 -.___
N
S
3 C 7 C 8 A II A 13 B 16 A 19 C 23 C 26 C 27 C 28 C 30 A 40 C 43 C 44 C 52 C 57 C 58 B 59 B 64 A 74 A 86 B 88 C 92 A 93 A 95 A 97 A 98 C 105 A 1 IO A 122 C 125 B 129 B 130 B 131 B 133 B 138 C 143 B 144 C 145 C 146 A ~
ROCK
F F v v A M F v M A A F A M M M V F A M c M V F M v A C M M c F F F F F F M M F v
Gabbro Andesite Dacite Lamprop* Basalt Diorite Diabase Gabbro Norite Basalt Andesite Diabase Diabase Norite Norite Diabase Diabase Diabase Diabase Diabase Anortho* Diabase Basalt Ciabase Diorite Gabbro Basalt Anortho* Lamprop* Dacite Diabase Diabasc Diabase Diabase Diabase Diabase Diabase Gabbro Norite Gabbro Dacite _~~
K-AR
SITE
G
LONG
LAT
DATE
73W, 68W, 15w, 59w, 118W, 71W. 79W; 109w, 81W, 109W, 114W, 8OW, 86W, 81W, 82W, 78W, SOW; 82W, 8OW, 97W, 74w. 65W; 85W. 63W; 102W, lI5W, 115W, 64W, 82W, 74W, 73w, 76W, 76W, 76W. 76W; 16W. 76W; 86W, 8lW, 73W, 75w,
46N 5lN 56N 55N 68N 48N 48N 60N 47N 60N 66N 47N 49N 47N 47N 48N 47N 49N 49N 64N 46N 44N 73N 50N 66N 57N 67N 54N 46N 56N 59N 44N 44N 44N 44N 44N 44N 49N 47N 46N 56N
126 225, 285 570, 863, 1015, 1240, 1410 1625 1630, 1740, 2095 2320 1620 1620 I825 2095 2485 1240, 899, 805 366, 903. 138; 1800. 1095 12co 1400, 1395 300 2060 434, 8 17, 817, 8 17, 817. 751; 1705, 1625 126 285
N
specimen
number
S
specimen
from same sample
ROCK
rock type (*Lamprophyre,
G
grain
SITE
specimen
K-AR
AGE
size: A-aphanitic, collection
AGE ER
30 35 115 60 50
180 200
50 90 16 140 28 60
50
15 70 70 70 70 75 140
RL
JN/JT
REF.
RATIO
ER
TEMP.1.
P
60-117 64-127 63-154 62-181 64-54 62-l 45 63-l 18 63-98 FAl960 64-16 64-47 61-157 63-124 FA1960 FA1960 63-149 61-157 63-l 19 63-l 18 65-75 63-140 65-I 32 6432 65-129 63-65 63-90 63-91 63-164 61-148 63-153 62-126 64-l 19 64-l 22 64-122 64-122 64-122 65-111 64-105 FA1960 CO-1 17 63-l 54
1.00. 1.5, 1.0, 0.6; 1.2, 1.O, 2.0, 0.9, 1.50, 1.7, 2.0, 1.2. 1.0; 1.2, 1.1, 0.5, 1.17, 1.7, I .O, 0.6, 0.80, 0.23, 0.68, 0.3, 0.8, 0.5, 0.7, 0.8, 1.20, 1.34, 0.10, 0.15, 0.6, 0.42, 0.72, 0.84, 0.5 0.57, 1.7, 0.2. 0.5,
0.02 0.2 0.1 0.2 0.1 0.04 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.1 0. I 0. I 0.03 0.1 0.1 0.1 0.06 0.05 0.04 0.1 0. I
40&625 400-500 400-675 400-525 165-450 475-625 300-475 300-500 20-550 400-525 350-500 300-500 525-625 240-550 425-550 240-450 300-475 350-530 300-450 375-500 300-550 300-450 350-500 450-530 300-500 300-475 400-530 350-500 375-475 20-550 375-415 300-475 350-530 4255530 350-450 350-500 350-530 300-500 375-500 300-500 300-530
5 4 6 5 5 4 5 6 11 5 5 6 3 9 5 6 6 5 5 4 5 4 5 4 6 4 4 5 4 7 4 4 6 4 3 5 6 6 5 5 4
(A), same outcrop
0. I 0. I 0.1 0.05 0.03 0.04 0.07 0.2 0.03 0.02 0.08 0.1 0.04 0.2 0. I
0.I
(B), or an adjacent
outcrop
85 81 90 52 39 62 45 30 20 25 21 34 47 36 82 32 30 24 26 43 53 60 45 44 90 31 48 60 53 51 32 44 21 71 42 40 77 28 27 57 56
POLE
PL
LONG
LAT
REF.
157w, 104E, 132E. SW, 29E. 180, 134w, 109w, 107w, 107W, 115w, 74E, 94w, 107w, 107W, 122w, 74E, 61E, 167W, 29E. 29E; 135E. 178W, 173w, 147w, I75W, 176W, 109w, 9E, 132E, 14E, 153E,
65N 69N 36N 12N 8N 23N 27N 36N 47N 47N 13N 12N 28N 47N 47N 19N 12N C3N 4s 8N 8N 31N 11N 69N 29N 4N 1N 36N 2N 36N 12N ?ON 8N 8N 8N 8N 8N 29N 47N 65N 36N
LA1962 LC1967 lRI964 IRl964 CR1960 DU1962 LA1966 FE1965 SO1963 SO1963 LA1966 SY1967 SY1966 SO1963 SO1963 LA1966 SY1967 FE1965 LA1965 CR1960 CR1960 lR1964 DU1962 LA1968 FE1965 R01964 R01964 FE1965 LA1967 BL1964 SY1967 CR1960 CR1960 CR1960 CR1960 CR1960 CR1960 FE1965 SO1963 LA1962 IRl964
29E; 29E, 29E, 29E. 147W, lOlW, 157w, 132E,
of the same formation
40 28 3 33 11 6 38 66 71 78 37 28 67 71 71 43 28 23 4 7 2 16 8 39 44 16 13 54 14 4 16 22 3 3 3 3 3 42 71 40 3
as radiometrically
FEQ VAL
ER
.254, .409, .350, ,154, ,450, .344, .483, .164, .273, .304, .483, .327, .193, .218, .200. ,112; .319, ,494, ,348, ,204; .276, .012, ,231, ,071, .179, .157, .228, ,162. .391; .463, .032; .044, .208; ,145, .249, ,291, .173, ,130, .309. .047; ,173,
.005 .050 .032 .05 1 .038 ,020 .048 .037 .029 .036 .048 .054 .039 .018 ,018 .023 .009 .030 ,035 .034 .021 .016 .015 .024 .023 ,016 .033 .020 .016 .OIO .013 .022 ,069 .OlO .007 ,028 .035 .009 .037 .023 .035
dated
Anorthosite) V-very
site specified
fine, F-fine,
M-medium,
by longitude
C-coarse
west (LONG)
and latitude
north
(LAT)
potassium-argon radiometric age date (DATE) with its quoted error (ER) in millions of years as given in the source references (REF): GSC age determinations starting 60- are in Lowdon (1961), 61- in Lowdon et al. (1963), 62- in Leech et al. (1963), 63- 64- and 65- in Wanless et al. (1965) (1966) and (1967) respectively; FA is Fairbairn et al. (1960)
JN/JT
the value determined temperature interval
(RATIO) for the ratio of NRM intensity (JN) to TRM intensity (JT) with its technical (TEMP.1.) in centigrade degrees, and the number of determination points (P) defining
RL
per cent ratio
POLE
paleomagnetic pole used to determine the equatorial field intensity specified by longitude (LONG) east (E) or west (W) and latitude (LAT) north (N) or south (S) with its reference (REF) specified by BL for Black, CR for Collinson and Runcorn, DU for Dubois, FE for Fahrig et al., IR for Irving, LA for Larochelle, LC for Larochelle and Currie, RO for Robertson, SO for Sopher, and SY for Symons followed by the year of publication
PL
paleolatitude
FEQ
computed
of the determined
to the inferred
length
of the JN/JT
straight
line relationship
in degrees paleo-equatorial
field intensity
value (VAL)
and error
error (ER), defined the ratio
(ER) in oersteds
14
E.J.
SCHWARZ
AND
ideal case of a magnetically unaltered rock. Thus, the ratio (RL) of the line joining the J N / J T points in the temperature interval and the length of the total line may be taken as another criterion (fig. 1). The smaller the number of points in the temperature interval, the larger this ratio should be. In the present case a sample was accepted if the temperature interval contained 3 points and RL >~40%; 4 points and RL >~30%; 5 or more points and RL >~20% (table 1). 5. The specimens showing a knee (ScHWARZ and CHRISTIE,1967) or a step (K1TAZAWAand KOBAYASHI, 1968) in their J N / J T curves were eliminated from further consideration. These specimens probably bear more than one stable N R M component which cannot yet be classified so that the intensity of the paleomagnetic field cannot be uniquely determined. 3. Results
The N R M component of 41" specimens out of the total of 146 satisfied the criteria given above, and consequently it is interpreted as a primary T R M . If the reasonable assumption (NAGATA, 1961) is made that the T R M intensity is directly proportional to the weak ( < 1 0 e ) magnetic field acting on the specimens both during the original cooling of the rocks and during cooling of the specimens in the laboratory, then the absolute value of the slope (JN/JT in table 1) of the straight line joining the plotted N R M / p T R M intensities in the temperature intervals selected, equals the ratio of the field (F) in which the natural (p) T R M was acquired and that (Fe) in which the laboratory p T R M was acquired by each of the 41 specimens. The J N / J T ratio and its standard error is listed for each specimen in table 1. A measure for the consistency of the J N / J T ratios within rock units could only be obtained for the Sudbury norite and Gananoque diabase dike. Table 2 shows that the agreement between the ratios within each of these units is reasonable although the variation is too large to be due solely to measurement errors. Two J N / J T ratios were determined for Abitibi E N E trending diabase dikes (samples 19 & 59), the Nipissing diabase sill (samples 30 & 59), the Clearwater andesite (samples 8 & 146), and the * In our earlier paper, the results for 25 of the 41 specimens were considered satisfactory. However, only the results for ll of the specimens satisfy the criteria set in the present study. The data for these ll specimens are added to table l because only part of these data were given in the earlier paper.
D.
T. A. S Y M O N S
TABLE 2
Mean JN/JT ratios, standard deviation, and maximum deviation of the individual ratios from their means Rock unit
Samples
Mean St. Max. JN/JT Dev. Dev.
Sudbury norite Gananoque diab.
25, 43, 44, 144 (0)* 129, 130, 131, 133 (1)
1.4 0.2 0.3 0.65 0.17 0.23
* The figure between parenthesis indicates the number of samples discarded for the JN/JT determination. Manicouagan andesite (samples 7 & 145). The JN/ JT ratios for the two units mentioned first are not inconsistent but they differ markedly for each of the other two units. The ancient local geomagnetic intensity values (F) were reduced to the corresponding values (Feq) at the contemporary paleomagnetic equator by making use of the paleomagnetic pole positions listed in table 1 and assuming that the paleomagnetic field may be represented by that due to a geocentric dipole. The Feq values are plotted against the corresponding specimen ages on fig. 2. 4. Discussion
Fig. 2 shows that the Feq values, including KoBAVASHI'S 0968) four results for North American Precambrian rocks, are scattered between 0.03 and 0.49 Oe. The frequency distribution of the Pea values is almost normal. The mean equatorial intensity comes to 0.25 Oe with a standard deviation of 0.13 Oe which is not significantly different from the present geomagnetic equatorial intensity (about 0 . 3 0 e ) . The following factors may be considered in the explanation of the scatter in the Feq values. 1. Data obtained with the Tellier method may be misinterpreted. The temperature intervals given in table 1 show that the N R M of only a few specimens consists of essentially one component. For these few specimens, the quality of the results is comparable to that obtained by the same method for potteries and bricks. The N R M of the other specimens contain additional components of magnetization that are lost during heating up to about 300 °C. It may be safely assumed that these components are due to a viscous secondary remanence. The interpretation is more doubtful if the temperature interval in which the N R M decays in proportion to the T R M acquired is small or is well below the Curie point.
G E O M A G N E T I C I N T E N S I T Y B E T W E E N 100 M I L L I O N AND 2500 M I L L I O N YEARS AGO
15
Feq(oe) 0.8
0.6
+
+
o.,. 1Lt.
I
- . 4 ....
!
- 4 -
+
÷
0.2.
+
+
+i+,. .__
.
-
......... i . . . . . .
" .........
I ....... t .......
0
t 0
÷I 0'5
..... ~---
(o
t~)
zlo
2.5
AGE x 109(yr.)
Fig. 2. Equatorial paleointensity values (F=q) plotted against k-Ar ages for the specimens. The solid lines indicate the standard error limits, the dashed lines representing the results reported by KOBAYASHI (1968). Dotted lines indicate general error limits (ages only) as given in the corresponding references listed in table 1.
Minor non-linearity in the JN/JT plots may be due to a variety of causes as discussed by COE (1967). The possible error in the intensity estimates due to such causes cannot be evaluated as the JN/JT diagrams of most samples are only partially understood. However, the reasonable consistency in the results for specimens collected from the same unit (table 2) is encouraging. 2. Errors in the radiometric (K-Ar) determination of the ages of the specimens may be divided into the following types: a) Systematic errors due to loss or enrichment of radiogenic argon in the specimens. Although care was taken in selecting fresh looking specimens, errors of this type may have biased the age determined for some specimens. b) The spot readings of the ancient geomagnetic intensity cannot be dated more exactly than the experimental error limits on the radiometrically determined ages. Thus, it is impossible to detect relatively short period fluctuations in intensity such as those revealed by the archeomagnetic results with the present heterogeneous collection of samples. Moreover, the maximum values of Feq as indicated on fig. 2 are comparable to those shown by the archeomagnetic results for about 1500 years before present. Thus, relatively short period fluctuations o f Feq m a y be responsible for much of the scatter in the present F~q values.
3. In the reduction of the ancient local field strength (F) to contemporary equatorial values (Feq), use was made of known paleomagnetic pole positions. Four sources of errors may have been introduced by this procedure: a) The paleomagnetic pole positions were determined for a number of specimens covering a time interval sufficiently long to largely eliminate the effect of secular variation. Such a pole position, even if it is correct, may differ by as much as 20 ° from the position of the geomagnetic pole at the time of acquisition of original T R M by the individual specimens. Consequently, a random error of up to 20 ~ may be introduced in the determination of the Feq values. The validity of using a paleomagnetic pole position determined for another rock unit far from the collection site of some of the present samples is questionable because movements may have occurred between different parts of the Canadian Shield. However, three pole positions determined for the Muskox intrusion, the Coppermine lavas, and the Sudbury dykes of contemporaneous age and situated in two different geological provinces agree very well (ROBERTSON, 1964). c) It was assumed that the contemporary paleomagnetic field may be represented by that due to a geocentric dipole. Paleomagnetic pole positions for the Precambrian and the early part of the Paleozoic are
16
E. J. S C H W A R Z
AND
scarce, and they can be checked against each other in only a few cases (e.g. ROBERTSON, 1964, as noted in 2b above). Moreover, it seems unlikely that random errors in the pole positions used, are of sufficiently large magnitude to affect the majority of the Feq values by much more than 10~. The dipole hypothesis is open to question but the relevant paleomagnetic data yield no evidence contrary to this hypothesis. The dipole hypothesis is discussed in a later part of this paper. d) Local variations of the paleomagnetic field from that expected for a geocentric dipole is yet another potential source of error in the Feq results. At present, such deviations are appreciable and very common on the Canadian Shield, and are due to inhomogeneous magnetization of the Shield. Thus, several factors may have contributed to or caused the observed scatter in Feq values (fig. 2) by introducing errors which cannot be evaluated in the individual value. However, taking the values as essentially correct, it appears that relative short period fluctations in geomagnetic intensity as suggested by archeomagnetic results may have been present as far back as 2.109 years ago. If this tentative conclusion is accepted as a working hypothesis, the following suggestions for further work may be made. 1. A detailed paleointensity investigation of lava flows extruded in rapid succession with emphasis on the degree of consistency within each flow. 2. A detailed paleointensity investigation of bodies of igneous rocks with emphasis on the within-site consistency. The maximum F,q values are comparable to those determined in archeomagnetic studies on materials of ages of about 2000 years. This observation suggests that the maximum value reached by the equatorial intensity during the Phanerozoic and Precambrian was approximately equal to the equatorial intensity of about 0.45 Oe suggested by these archeomagnetic results. Very low F~q values may have occurred during transitional periods of polarity changes of the geomagnetic field during the Phanerozoic and Precambrian as far back as 2500 m.y. Paleomagnetic results show that geomagnetic polarity changes have occurred in the major part of the geologic time scale involved in the present study, but the information is too scanty to attempt a correlation between pole locations and intensity data. The occurrence of many periods of polarity changes in the time span considered would
D. T. A. S Y M O N S
result in a positively skewed frequency distribution of the Feq values if the proposed fluctuations in Feq are periodic. It is felt that the present results do not warrant an extensive statistical analysis. Comparison of the present results with the initial ones plotted in a similar manner (ScHWARZ and SYMONS, 1968) show the following differences: 1. The scatter in the present results is somewhat less. Among others, the samples yielding a high Feq value in the initial study, did not meet the stricter requirements for acceptability set in the present study. 2. The support given by the initial results to the tentative suggestion by BRIDEN (1966) and NESBITT(1966) for a maximum in field strength at about 200 m.y. ago is very considerably weakened if present at all. The present data do not suggest any long period fluctuations in geomagnetic intensity between 800 m.y. and 1900 m.y. Outside this interval, relevant information is generally scarce.
5. Dipole hypothesis A test for the validity of the hypothesis that the paleomagnetic field on the average may be represented by that due to a geocentric dipole is provided by plotting the JN/JT values against their corresponding paleomagnetic latitudes as determined independently from paleomagnetic pole positions. Such a plot shows strong scatter but also a definite tendency for the local paleomagnetic intensity to increase with distance from the paleomagnetic equator. The latter is in accord with the dipole hypothesis which requires the local intensity (F) to change with magnetic latitude (P1) according t o F--Feq (1 + 3 sin2P/) ~. To obtain a measure of the significance of the results, the cumulative distribution was used of the JN/JT ratios for the specimens with a paleomagnetic latitude (Pl) smaller than 25 ° (21 specimens) and larger than 25 ° (20 specimens). The mean JN/JT ratio (fig. 3a) for the group with Pl > 25 ° (1.09) is appreciably larger than that for the group with Pl < 25 ° (0.75) but the standard deviations for both groups are large (0.52 and 0.41 respectively). However, the application of the null hypothesis test suggests that the probabilities that the JN/JT ratios of both groups have the same mean and the same dispersion are about 0.02 and 0.06 respectively. On the other hand, the means of the Feq values computed from the JN/JT ratios of the specimens in
GEOMAGNETIC
INTENSITY
BETWEEN
100 MILLION
Cl
% IO0B06040200
,
0
,
0.5
1.0
•
,
1.5
2.0
AND
2500
MILLION
YEARS AGO
17
It may be concluded that the results of the test as discussed above are not conclusive but do yield independent support for the hypothesis that the paleomagnetic field may be approximated by that due to a geocentric dipole as far back as 2500 m.y. Much of the scatter in the JN/JT ratios and therefore much of the uncertainty in the results of this test is due to a rapid fluctuation in the paleoequatorial intensity according to the working hypothesis. Further work, as suggested in the last paragraph, may clarify the situation.
JN/JT
6. Conclusions % I00 80 60, 4O 20 0
o
oi,
62
o'.3
0.4
o~, Feq
Fig. 3. C u m u l a t i v e distribution o f (a) the J N / J T ratios for all samples with paleomagnetic latitudes smaller t h a n 25 ° (open circles) a n d larger t h a n 25 ° (dots), a n d (b) the Feq values calculated f r o m the J N / J T ratios for the s a m e groups o f samples u n d e r a s s u m p t i o n o f a geocentric dipole field.
both groups on the basis of the dipole hypothesis (fig. 3b) are in reasonable agreement (0.25 Oe and 0.24 Oe respectively with a standard deviation of 0.13 Oe in both cases). The null hypothesis test suggests that the probabilities that the Feq values of both groups have the same mean and dispersion are about 0.24 and 0.99 respectively. Thus the present results show that (1) the local paleomagnetic intensities (oc JN/JT) probably increase with paleomagnetic latitude according to some function, and (2) that if this function is taken to be the specific relation between the local intensity and the magnetic latitude valid under the geocentric dipole hypothesis, there is an obvious possibility that the resulting Feq values do not depend on the paleomagnetic latitudes of the sampling sites.
1. The results suggest that the average geomagnetic equatorial intensity as far back as 2.5 billion years was 0.25 Oe with a standard deviation of 0.13 Oe, and consequently has not differed significantly from the present value of about 0 . 3 0 e . 2. The scatter in the equatorial paleointensity values may be tentatively explained by assuming the occurrence of fluctuations in geomagnetic intensity with maximum values up to about 0 . 5 0 e and with periods shorter than the experimental error limits of the radiometric ages of the specimens. These periods may include ones as short as the period suggested by the archeomagnetic results to have occurred during the last five millenniums. 3. The hypothesis that the paleomagnetic field as far back as 2.5 billion years may be approximated by that due to a geocentric dipole is not in contradiction with the variation of the local paleointensity values with paleomagnetic latitude. Thus, several characteristics of the present geomagnetic field seem to have been present as far back as 2.5 billion years ago. The validity of the hypothesis that rapid fluctuations in geomagnetic intensity have occurred, may be tested by carrying out detailed paleointensity studies on sequences of rapidly extruded lava flows and slowly cooled igneous intrusions.
Acknowledgement We thank the geologists of the Geological Survey of Canada and the Ontario Department of Mines who generously put samples from their collections at our disposal.
E. J. SCHWARZ
18
AND
References Nature
202, 945.
BRIDEN, J. C. (1966),
Nature
212, 246.
CARMICHAEL, C. M. (1968), COE, R. S. (1967), A. (1968),
J. Geophys.
Geoelec.
Sci. Letters
3, 351.
19, 2, 157.
Ann.
Res. 73, 3247.
Bull. Geol.
P. M.
Progr.
IRVING, E. (1964),
Surv. Canada
Rep.
HURLEY and Dept.
Geol.
W.
1938,
Terrest.
H. PINSON (1960),
New
MIT,
7.
Nature
Magnetism
Atmospheric
215, 697.
K. and K. KOBAYASHI (1960),
J. Geomag.
Geoelec.
20, 1, 7. LAROCHELLE, A. (1966),
Canad.
J. Earth Sci. 3, 1671.
LAROCHELLE, A. (1967),
Canad.
J. Earth Sci. 4, 323.
LAROCHELLE, A. (1967),
Geol.
LAROCHELLE, A., (1968), LAROCHELLE, A., 208,
R.
F.
Geol.
Surv. Canada
67-39,
Surv. Canada,
BLACK and
R.
K.
LAROCHELLE, A. and K. L. CURRIE (1967),
12 pp.
Geol.
NESBITT, J. D. (1966),
63-17,
Surv. Canada
Surv. Canada
Nature
Interiors
K.
R.
K.
127 pp.
TIPPER and
62-17,
140 pp.
210, 618.
J. Geophys. Nature
E. J. and K. W.
R.
140 pp.
61-17,
Res. 66, 1941. 204, 66.
CHRISTIE (1967),
72, 12, 3263. SCHWARZ, E. J. and D. T. A. SYMONS, (1968),
J. Geophys.
Res.
Phys. Earth Planet.
1, 122. Geophys.
SOPHER, S. R. (1963),
THELLIER, E. (1938),
J. 12, 239.
Bull. Geol.
SYMONS,D. T. A. (1967), SYMONS,D. T. A. (1966), SYMONS,D. T. A. (1967),
RIMSAITE (1966),
Paper. WANLESS (1965),
J. Geophys.
R. K. WANLESS (1966),
Geol.
STOCKWELL and
Ann.
Surv. Canada
90, 34 pp.
Canad. J. Earth Sci. 4, 204. Econ. Geol. 61, 1336. Canad.
J. Earth Sci. 4, 1161.
Inst. Phys. Globe,
Paris, 16, 157.
RIMSAITE (1965), Geol. Surv. Canada, 64-17, 126 pp. WANLESS, R. K., R. D. STEVENS, G. R. LACHANCE and J. Y. H.
Res. 72,
16, 4163. and
H.
C. H. STOCKWELL, H. W.
WANLESS (1963),
SCHWARZ,
C.
Surv. Canada
WANLESS, R. K., R. D. STEVENS, G. R. LACHANCE and J. Y. H.
179.
LAROCHELLE, A.
LOWDON,
Geol.
SMITH, P. J. (1967),
York).
Elec. 43, 119 and 299. KRS, M. (1967),
71, 4949.
B., J. A.
ROBERTSON, W. A. (1964),
71, 75 pp.
Geophys.
Paleomugrwtism (Wiley,
KOENIGSBERGER, J. G.
Nature
LEECH, G.
OPDYKE, N. D. (1966),
J. Earth Sci. 2, 278.
FAIRBAIRN, H. W.,
KITAZAWA,
SYMONS
LOWDON, J. A. (1961),
FAHRIG, W. F., E. H. S. GAUCHER and A. LAROCHELLE (1965),
Eighth
A.
LOWDON, J. A.,
Earth Planet.
J. Geomag.
Du Bars, P. M. (1962), Canad.
T.
WANLESS (1963),
BLACK, R. F. (1964),
Cox,
D.
J. Geophys.
Res.
WANLESS, R. K.,
Geol. R. D.
EDMONDS (1967),
Geol.
Surv. Canada STEVENS, G.
65-17,
101 pp.
R. LACHANCE and C. M.
Surv. Canada,
66-17,
120 pp.
VAN ZIJL, J. S. V., K. W. T. GRAHAM and A. L. HALES (1962), Geophys.
J. 7, 23.
G E O M A G N E T I C I N T E N S I T Y B E T W E E N 100 M I L L I O N A N D 2500 M I L L I O N Y E A R S A G O
E. J. SCHWARZ and D. T. A. SYMONS Geological Survey of Canada, Ottawa
Received 15 October 1968
The decay of the NRM of 146 specimens (including the 41 specimens reported on earlier) of igneous rocks during heating in steps of 25 or 50 °C between 200 or 300 °C and 550 °C was compared to the acquisition of TRM (H = 0.35 Oe) during subsequent cooling to 20 °C. The K-Ar ages of the specimens are distributed between 100 m.y. and 2500 m.y. The selection of results suitable for intensity determination was based on constancy of the rate of decay of the NRM and acquisition of TRM determined from at least 3 successive determinations over a temperature interval of at least 100 °C, an appreciable decay of the supposed original NRM in that temperature interval, and constancy in the NRM direction in that temperature interval. This selection yielded a total of 41 equatorial paleointensities. The intensity results obtained for several specimens collected from the same rock unit show a reasonable degree of internal con-
sistency. The results indicate that the mean equatorial geomagnetic intensity during most of the Phanerozoic and the Precambrian eras as far back as 2.5 billion years was 0.25 Oe with a standard deviation of 0.13 Oe. The large scatter in the equatorial intensities may be due to various possible errors and/or fluctuations in geomagnetic moment of periods shorter than the experimental error (5 to 10 %) of the radiometric ages of the specimens, possibly including a period as short as that suggested by the archeomagnetic results for the last five millenniums. The dipole representation of the paleomagnetic field as far back as 2500 m.y. is not contradicted by the variation of the paleointensity values with paleomagnetic latitude. Thus, several characteristics of the present geomagnetic field seem to have been present as far back as 2500 m.y. ago.
1. Introduction
CHAEL (1968) employed alternating field demagnetization. The Tertiary a n d late Mesozoic intensity values presented by BRIDEN (1966), SMITH (1967), a n d CARMICHAEL (1968) are generally somewhat smaller than the present value. The early Mesozoic a n d late Paleozoic geomagnetic intensity was substantially
I n tracing the history of the geomagnetic field - - one of the m a i n objectives of p a l e o m a g n e t i s m - i n f o r m a t i o n o n the E a r t h ' s magnetic m o m e n t is indispensable. Such i n f o r m a t i o n can be o b t a i n e d by the d e t e r m i n a t i o n of the intensity of stable original r e m a n e n t m a g n e t i z a t i o n of artifacts a n d rocks. The results of work o n artifacts a n d recent lava flows from several widely spread localities suggest that the geomagnetic equatorial intensity was approximately 1.5 times its present value (Fp) a b o u t 1500 years ago, a n d a p p r o x i m a t e l y 0.5 Fp a b o u t 5500 years ago (e.g. Cox, 1968). I n recent years, several attempts have been made to trace the geomagnetic intensity back t h r o u g h geologic time. Most attempts were based o n stepwise t h e r m a l d e m a g n e t i z a t i o n of the n a t u r a l r e m a n e n t m a g n e t i z a t i o n ( N R M ) of rock samples as suggested by KOENIGSBERGER (1938) a n d initiated by Tt-IELLIER (1938). NESBITT (1966) o b t a i n e d relative intensity values by d e t e r m i n i n g the ratio of the N R M intensity a n d the susceptibility o f red sediments while CARUl11
smaller than, or approximately equal to, Fp according to results reported by VAN ZIJL et al. (1962), BRIDEN (1966), NESBITT (1966), KRS 0 9 6 7 ) , a n d SCHWARZ and SVMONS (1968). Further, CARMICHAEL(1968) suggested a low geomagnetic intensity t h r o u g h o u t the Paleozoic and especially during early Paleozoic. The scattered P r e c a m b r i a n results o b t a i n e d by SCHWARZ a n d SYMONS (1968) and by CARMICHAEL (1968) suggest an average intensity c o m p a r a b l e to the present value. The earlier paper (SCHWARZand SVMONS, 1968) lists the paleointensity results based on radiometrically dated samples of igneous rocks selected from collections made by geologists of the Geological Survey of C a n a d a . The selection was based on the absence of evidence for m e t a m o r p h i s m a n d c o n c o r d a n c e in the radiometric and geologic ages. These earlier results
12
E.J.
SCHWARZ
A N D D, T. A. S Y M O N S
suggest a maximum equatorial intensity* at about 200 m.y. ago and show strong scatter in the Precambrian. It was thought worthwhile to extend the selection over the full collection of radiometrically dated samples to obtain a better idea of the possible range in intensity values between 100 m.y. and 2500 m.y. ago. The major drawback of using the collection is that, in general, only one specimen per rock unit can be obtained so that the consistency of the results cannot be checked for many of the sampled units. For that reason, duplicate and replicate samples of some rock units were added. In this manner, 105 specimens were selected in addition to the 41 specimens discussed in the earlier paper.
other as done in fig. 1, and the results obtained after stepwise heating were joined by a line. The following criteria were set in selecting specimens suitable for paleointensity determination: 1. Constancy of the rates of decay of the N R M and acquisition of p T R M determined for a temperature interval comprising at least three successive heating steps above 300 °C. A straight line through 3 or more N R M / p T R M = J N / J T plots means that this requirement is met (fig. 1). JN
Specimen I10
20 300 X "~400 \ y
2. Experimental procedure THELLIER'S (1938) method of comparing the thermal decay of the N R M with the acquisition of thermoremanent magnetization (TRM) during cooling of the specimens in a weak constant magnetic field was used in this study. The purpose of this method is to single out a N R M component that may correspond to a T R M component acquired at the time of formation of the igneous rocks sampled. Such a T R M component must be present for the determination of paleointensities. Therefore, the specimens were heated in steps of 25 or 50 °C from 200 or 300 °C up to 550 °C. After each step in the heating process, the specimens were cooled to 20 °C in a constant field of 0.35 Oe ± 5 700, and the direction and intensity of the total remanent magnetization of each specimen was measured with maximum errors of respectively 2 ° degrees of arc and 5~o using a biastatic magnetometer. This procedure was repeated at each step with the specimens in the reversed position to allow computing of the residual N R M and the acquired partial T R M (pTRM) as the components of both total remanence vectors measured. Care was taken to keep the heating and cooling rates constant, and the specimens were kept at the selected temperature for 10 minutes. The residual N R M and the p T R M components were then plotted against each * These paleoequatorial intensity values were based on a measured value o f 0.49 Oe for the laboratory field. This value probably was erroneous; subsequent measurements yielding 0.35 Oe. Moreover, experiments on p T R M acquisition of a selection o f the original specimens indicated that the original cooling in the laboratory had taken place in a field of about 0.35 Oe instead of 0.49 Oe.
0 0
Fig. l.
Intensity o f N R M
I0
. 20
J'l"
(,IN) withstanding heating to suc-
cessively higher temperatures (numbers near points, °C) and o f P T R M (JT) acquired during subsequent cooling to 20 °C in a constant field, plotted against each other. RI is defined as x/y. The deviation o f the 250 and 300 °G points from the line are within the possible measurement error. Units in gauss" cm 3" 10- 3
2. The direction of the remaining N R M must be constant within 10 ° of arc in the temperature interval for which the first criterion is satisfied. 3. The temperature interval must be at least 100 °C. It may be argued that a temperature interval of 100 °C is small compared to a total range of about 550 °C. However, most Paleozoic and Precambrian rocks bear secondary components of magnetization which are stable against heating to 300-350°C. Moreover, heating to temperatures of 500 °C or more probably results in either physico-chemical changes affecting the N R M bearing minerals in many samples or complications in the T R M acquisition mechanism. Table 1 gives the interval for each accepted sample. 4. The straight line joining the points defining the temperature interval of constant rate of decay of the N R M and acquisition of p T R M may be extrapolated, if necessary, to both axes in the JN-JT diagrams (see fig. 1). If the points in the temperature interval are taken to reflect part of the original T R M then it may be supposed that the total line would have joined the J N / J T points from 20 °C up to the Curie point in the
GEOMAGNETIC
INTENSITY
BETWEEN
100
MILLION
TABLE
2500 MILLION
AND
YEARS
13
AGO
1 -.___
N
S
3 C 7 C 8 A II A 13 B 16 A 19 C 23 C 26 C 27 C 28 C 30 A 40 C 43 C 44 C 52 C 57 C 58 B 59 B 64 A 74 A 86 B 88 C 92 A 93 A 95 A 97 A 98 C 105 A 1 IO A 122 C 125 B 129 B 130 B 131 B 133 B 138 C 143 B 144 C 145 C 146 A ~
ROCK
F F v v A M F v M A A F A M M M V F A M c M V F M v A C M M c F F F F F F M M F v
Gabbro Andesite Dacite Lamprop* Basalt Diorite Diabase Gabbro Norite Basalt Andesite Diabase Diabase Norite Norite Diabase Diabase Diabase Diabase Diabase Anortho* Diabase Basalt Ciabase Diorite Gabbro Basalt Anortho* Lamprop* Dacite Diabase Diabasc Diabase Diabase Diabase Diabase Diabase Gabbro Norite Gabbro Dacite _~~
K-AR
SITE
G
LONG
LAT
DATE
73W, 68W, 15w, 59w, 118W, 71W. 79W; 109w, 81W, 109W, 114W, 8OW, 86W, 81W, 82W, 78W, SOW; 82W, 8OW, 97W, 74w. 65W; 85W. 63W; 102W, lI5W, 115W, 64W, 82W, 74W, 73w, 76W, 76W, 76W. 76W; 16W. 76W; 86W, 8lW, 73W, 75w,
46N 5lN 56N 55N 68N 48N 48N 60N 47N 60N 66N 47N 49N 47N 47N 48N 47N 49N 49N 64N 46N 44N 73N 50N 66N 57N 67N 54N 46N 56N 59N 44N 44N 44N 44N 44N 44N 49N 47N 46N 56N
126 225, 285 570, 863, 1015, 1240, 1410 1625 1630, 1740, 2095 2320 1620 1620 I825 2095 2485 1240, 899, 805 366, 903. 138; 1800. 1095 12co 1400, 1395 300 2060 434, 8 17, 817, 8 17, 817. 751; 1705, 1625 126 285
N
specimen
number
S
specimen
from same sample
ROCK
rock type (*Lamprophyre,
G
grain
SITE
specimen
K-AR
AGE
size: A-aphanitic, collection
AGE ER
30 35 115 60 50
180 200
50 90 16 140 28 60
50
15 70 70 70 70 75 140
RL
JN/JT
REF.
RATIO
ER
TEMP.1.
P
60-117 64-127 63-154 62-181 64-54 62-l 45 63-l 18 63-98 FAl960 64-16 64-47 61-157 63-124 FA1960 FA1960 63-149 61-157 63-l 19 63-l 18 65-75 63-140 65-I 32 6432 65-129 63-65 63-90 63-91 63-164 61-148 63-153 62-126 64-l 19 64-l 22 64-122 64-122 64-122 65-111 64-105 FA1960 CO-1 17 63-l 54
1.00. 1.5, 1.0, 0.6; 1.2, 1.O, 2.0, 0.9, 1.50, 1.7, 2.0, 1.2. 1.0; 1.2, 1.1, 0.5, 1.17, 1.7, I .O, 0.6, 0.80, 0.23, 0.68, 0.3, 0.8, 0.5, 0.7, 0.8, 1.20, 1.34, 0.10, 0.15, 0.6, 0.42, 0.72, 0.84, 0.5 0.57, 1.7, 0.2. 0.5,
0.02 0.2 0.1 0.2 0.1 0.04 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.1 0. I 0. I 0.03 0.1 0.1 0.1 0.06 0.05 0.04 0.1 0. I
40&625 400-500 400-675 400-525 165-450 475-625 300-475 300-500 20-550 400-525 350-500 300-500 525-625 240-550 425-550 240-450 300-475 350-530 300-450 375-500 300-550 300-450 350-500 450-530 300-500 300-475 400-530 350-500 375-475 20-550 375-415 300-475 350-530 4255530 350-450 350-500 350-530 300-500 375-500 300-500 300-530
5 4 6 5 5 4 5 6 11 5 5 6 3 9 5 6 6 5 5 4 5 4 5 4 6 4 4 5 4 7 4 4 6 4 3 5 6 6 5 5 4
(A), same outcrop
0. I 0. I 0.1 0.05 0.03 0.04 0.07 0.2 0.03 0.02 0.08 0.1 0.04 0.2 0. I
0.I
(B), or an adjacent
outcrop
85 81 90 52 39 62 45 30 20 25 21 34 47 36 82 32 30 24 26 43 53 60 45 44 90 31 48 60 53 51 32 44 21 71 42 40 77 28 27 57 56
POLE
PL
LONG
LAT
REF.
157w, 104E, 132E. SW, 29E. 180, 134w, 109w, 107w, 107W, 115w, 74E, 94w, 107w, 107W, 122w, 74E, 61E, 167W, 29E. 29E; 135E. 178W, 173w, 147w, I75W, 176W, 109w, 9E, 132E, 14E, 153E,
65N 69N 36N 12N 8N 23N 27N 36N 47N 47N 13N 12N 28N 47N 47N 19N 12N C3N 4s 8N 8N 31N 11N 69N 29N 4N 1N 36N 2N 36N 12N ?ON 8N 8N 8N 8N 8N 29N 47N 65N 36N
LA1962 LC1967 lRI964 IRl964 CR1960 DU1962 LA1966 FE1965 SO1963 SO1963 LA1966 SY1967 SY1966 SO1963 SO1963 LA1966 SY1967 FE1965 LA1965 CR1960 CR1960 lR1964 DU1962 LA1968 FE1965 R01964 R01964 FE1965 LA1967 BL1964 SY1967 CR1960 CR1960 CR1960 CR1960 CR1960 CR1960 FE1965 SO1963 LA1962 IRl964
29E; 29E, 29E, 29E. 147W, lOlW, 157w, 132E,
of the same formation
40 28 3 33 11 6 38 66 71 78 37 28 67 71 71 43 28 23 4 7 2 16 8 39 44 16 13 54 14 4 16 22 3 3 3 3 3 42 71 40 3
as radiometrically
FEQ VAL
ER
.254, .409, .350, ,154, ,450, .344, .483, .164, .273, .304, .483, .327, .193, .218, .200. ,112; .319, ,494, ,348, ,204; .276, .012, ,231, ,071, .179, .157, .228, ,162. .391; .463, .032; .044, .208; ,145, .249, ,291, .173, ,130, .309. .047; ,173,
.005 .050 .032 .05 1 .038 ,020 .048 .037 .029 .036 .048 .054 .039 .018 ,018 .023 .009 .030 ,035 .034 .021 .016 .015 .024 .023 ,016 .033 .020 .016 .OIO .013 .022 ,069 .OlO .007 ,028 .035 .009 .037 .023 .035
dated
Anorthosite) V-very
site specified
fine, F-fine,
M-medium,
by longitude
C-coarse
west (LONG)
and latitude
north
(LAT)
potassium-argon radiometric age date (DATE) with its quoted error (ER) in millions of years as given in the source references (REF): GSC age determinations starting 60- are in Lowdon (1961), 61- in Lowdon et al. (1963), 62- in Leech et al. (1963), 63- 64- and 65- in Wanless et al. (1965) (1966) and (1967) respectively; FA is Fairbairn et al. (1960)
JN/JT
the value determined temperature interval
(RATIO) for the ratio of NRM intensity (JN) to TRM intensity (JT) with its technical (TEMP.1.) in centigrade degrees, and the number of determination points (P) defining
RL
per cent ratio
POLE
paleomagnetic pole used to determine the equatorial field intensity specified by longitude (LONG) east (E) or west (W) and latitude (LAT) north (N) or south (S) with its reference (REF) specified by BL for Black, CR for Collinson and Runcorn, DU for Dubois, FE for Fahrig et al., IR for Irving, LA for Larochelle, LC for Larochelle and Currie, RO for Robertson, SO for Sopher, and SY for Symons followed by the year of publication
PL
paleolatitude
FEQ
computed
of the determined
to the inferred
length
of the JN/JT
straight
line relationship
in degrees paleo-equatorial
field intensity
value (VAL)
and error
error (ER), defined the ratio
(ER) in oersteds
14
E.J.
SCHWARZ
AND
ideal case of a magnetically unaltered rock. Thus, the ratio (RL) of the line joining the J N / J T points in the temperature interval and the length of the total line may be taken as another criterion (fig. 1). The smaller the number of points in the temperature interval, the larger this ratio should be. In the present case a sample was accepted if the temperature interval contained 3 points and RL >~40%; 4 points and RL >~30%; 5 or more points and RL >~20% (table 1). 5. The specimens showing a knee (ScHWARZ and CHRISTIE,1967) or a step (K1TAZAWAand KOBAYASHI, 1968) in their J N / J T curves were eliminated from further consideration. These specimens probably bear more than one stable N R M component which cannot yet be classified so that the intensity of the paleomagnetic field cannot be uniquely determined. 3. Results
The N R M component of 41" specimens out of the total of 146 satisfied the criteria given above, and consequently it is interpreted as a primary T R M . If the reasonable assumption (NAGATA, 1961) is made that the T R M intensity is directly proportional to the weak ( < 1 0 e ) magnetic field acting on the specimens both during the original cooling of the rocks and during cooling of the specimens in the laboratory, then the absolute value of the slope (JN/JT in table 1) of the straight line joining the plotted N R M / p T R M intensities in the temperature intervals selected, equals the ratio of the field (F) in which the natural (p) T R M was acquired and that (Fe) in which the laboratory p T R M was acquired by each of the 41 specimens. The J N / J T ratio and its standard error is listed for each specimen in table 1. A measure for the consistency of the J N / J T ratios within rock units could only be obtained for the Sudbury norite and Gananoque diabase dike. Table 2 shows that the agreement between the ratios within each of these units is reasonable although the variation is too large to be due solely to measurement errors. Two J N / J T ratios were determined for Abitibi E N E trending diabase dikes (samples 19 & 59), the Nipissing diabase sill (samples 30 & 59), the Clearwater andesite (samples 8 & 146), and the * In our earlier paper, the results for 25 of the 41 specimens were considered satisfactory. However, only the results for ll of the specimens satisfy the criteria set in the present study. The data for these ll specimens are added to table l because only part of these data were given in the earlier paper.
D.
T. A. S Y M O N S
TABLE 2
Mean JN/JT ratios, standard deviation, and maximum deviation of the individual ratios from their means Rock unit
Samples
Mean St. Max. JN/JT Dev. Dev.
Sudbury norite Gananoque diab.
25, 43, 44, 144 (0)* 129, 130, 131, 133 (1)
1.4 0.2 0.3 0.65 0.17 0.23
* The figure between parenthesis indicates the number of samples discarded for the JN/JT determination. Manicouagan andesite (samples 7 & 145). The JN/ JT ratios for the two units mentioned first are not inconsistent but they differ markedly for each of the other two units. The ancient local geomagnetic intensity values (F) were reduced to the corresponding values (Feq) at the contemporary paleomagnetic equator by making use of the paleomagnetic pole positions listed in table 1 and assuming that the paleomagnetic field may be represented by that due to a geocentric dipole. The Feq values are plotted against the corresponding specimen ages on fig. 2. 4. Discussion
Fig. 2 shows that the Feq values, including KoBAVASHI'S 0968) four results for North American Precambrian rocks, are scattered between 0.03 and 0.49 Oe. The frequency distribution of the Pea values is almost normal. The mean equatorial intensity comes to 0.25 Oe with a standard deviation of 0.13 Oe which is not significantly different from the present geomagnetic equatorial intensity (about 0 . 3 0 e ) . The following factors may be considered in the explanation of the scatter in the Feq values. 1. Data obtained with the Tellier method may be misinterpreted. The temperature intervals given in table 1 show that the N R M of only a few specimens consists of essentially one component. For these few specimens, the quality of the results is comparable to that obtained by the same method for potteries and bricks. The N R M of the other specimens contain additional components of magnetization that are lost during heating up to about 300 °C. It may be safely assumed that these components are due to a viscous secondary remanence. The interpretation is more doubtful if the temperature interval in which the N R M decays in proportion to the T R M acquired is small or is well below the Curie point.
G E O M A G N E T I C I N T E N S I T Y B E T W E E N 100 M I L L I O N AND 2500 M I L L I O N YEARS AGO
15
Feq(oe) 0.8
0.6
+
+
o.,. 1Lt.
I
- . 4 ....
!
- 4 -
+
÷
0.2.
+
+
+i+,. .__
.
-
......... i . . . . . .
" .........
I ....... t .......
0
t 0
÷I 0'5
..... ~---
(o
t~)
zlo
2.5
AGE x 109(yr.)
Fig. 2. Equatorial paleointensity values (F=q) plotted against k-Ar ages for the specimens. The solid lines indicate the standard error limits, the dashed lines representing the results reported by KOBAYASHI (1968). Dotted lines indicate general error limits (ages only) as given in the corresponding references listed in table 1.
Minor non-linearity in the JN/JT plots may be due to a variety of causes as discussed by COE (1967). The possible error in the intensity estimates due to such causes cannot be evaluated as the JN/JT diagrams of most samples are only partially understood. However, the reasonable consistency in the results for specimens collected from the same unit (table 2) is encouraging. 2. Errors in the radiometric (K-Ar) determination of the ages of the specimens may be divided into the following types: a) Systematic errors due to loss or enrichment of radiogenic argon in the specimens. Although care was taken in selecting fresh looking specimens, errors of this type may have biased the age determined for some specimens. b) The spot readings of the ancient geomagnetic intensity cannot be dated more exactly than the experimental error limits on the radiometrically determined ages. Thus, it is impossible to detect relatively short period fluctuations in intensity such as those revealed by the archeomagnetic results with the present heterogeneous collection of samples. Moreover, the maximum values of Feq as indicated on fig. 2 are comparable to those shown by the archeomagnetic results for about 1500 years before present. Thus, relatively short period fluctuations o f Feq m a y be responsible for much of the scatter in the present F~q values.
3. In the reduction of the ancient local field strength (F) to contemporary equatorial values (Feq), use was made of known paleomagnetic pole positions. Four sources of errors may have been introduced by this procedure: a) The paleomagnetic pole positions were determined for a number of specimens covering a time interval sufficiently long to largely eliminate the effect of secular variation. Such a pole position, even if it is correct, may differ by as much as 20 ° from the position of the geomagnetic pole at the time of acquisition of original T R M by the individual specimens. Consequently, a random error of up to 20 ~ may be introduced in the determination of the Feq values. The validity of using a paleomagnetic pole position determined for another rock unit far from the collection site of some of the present samples is questionable because movements may have occurred between different parts of the Canadian Shield. However, three pole positions determined for the Muskox intrusion, the Coppermine lavas, and the Sudbury dykes of contemporaneous age and situated in two different geological provinces agree very well (ROBERTSON, 1964). c) It was assumed that the contemporary paleomagnetic field may be represented by that due to a geocentric dipole. Paleomagnetic pole positions for the Precambrian and the early part of the Paleozoic are
16
E. J. S C H W A R Z
AND
scarce, and they can be checked against each other in only a few cases (e.g. ROBERTSON, 1964, as noted in 2b above). Moreover, it seems unlikely that random errors in the pole positions used, are of sufficiently large magnitude to affect the majority of the Feq values by much more than 10~. The dipole hypothesis is open to question but the relevant paleomagnetic data yield no evidence contrary to this hypothesis. The dipole hypothesis is discussed in a later part of this paper. d) Local variations of the paleomagnetic field from that expected for a geocentric dipole is yet another potential source of error in the Feq results. At present, such deviations are appreciable and very common on the Canadian Shield, and are due to inhomogeneous magnetization of the Shield. Thus, several factors may have contributed to or caused the observed scatter in Feq values (fig. 2) by introducing errors which cannot be evaluated in the individual value. However, taking the values as essentially correct, it appears that relative short period fluctations in geomagnetic intensity as suggested by archeomagnetic results may have been present as far back as 2.109 years ago. If this tentative conclusion is accepted as a working hypothesis, the following suggestions for further work may be made. 1. A detailed paleointensity investigation of lava flows extruded in rapid succession with emphasis on the degree of consistency within each flow. 2. A detailed paleointensity investigation of bodies of igneous rocks with emphasis on the within-site consistency. The maximum F,q values are comparable to those determined in archeomagnetic studies on materials of ages of about 2000 years. This observation suggests that the maximum value reached by the equatorial intensity during the Phanerozoic and Precambrian was approximately equal to the equatorial intensity of about 0.45 Oe suggested by these archeomagnetic results. Very low F~q values may have occurred during transitional periods of polarity changes of the geomagnetic field during the Phanerozoic and Precambrian as far back as 2500 m.y. Paleomagnetic results show that geomagnetic polarity changes have occurred in the major part of the geologic time scale involved in the present study, but the information is too scanty to attempt a correlation between pole locations and intensity data. The occurrence of many periods of polarity changes in the time span considered would
D. T. A. S Y M O N S
result in a positively skewed frequency distribution of the Feq values if the proposed fluctuations in Feq are periodic. It is felt that the present results do not warrant an extensive statistical analysis. Comparison of the present results with the initial ones plotted in a similar manner (ScHWARZ and SYMONS, 1968) show the following differences: 1. The scatter in the present results is somewhat less. Among others, the samples yielding a high Feq value in the initial study, did not meet the stricter requirements for acceptability set in the present study. 2. The support given by the initial results to the tentative suggestion by BRIDEN (1966) and NESBITT(1966) for a maximum in field strength at about 200 m.y. ago is very considerably weakened if present at all. The present data do not suggest any long period fluctuations in geomagnetic intensity between 800 m.y. and 1900 m.y. Outside this interval, relevant information is generally scarce.
5. Dipole hypothesis A test for the validity of the hypothesis that the paleomagnetic field on the average may be represented by that due to a geocentric dipole is provided by plotting the JN/JT values against their corresponding paleomagnetic latitudes as determined independently from paleomagnetic pole positions. Such a plot shows strong scatter but also a definite tendency for the local paleomagnetic intensity to increase with distance from the paleomagnetic equator. The latter is in accord with the dipole hypothesis which requires the local intensity (F) to change with magnetic latitude (P1) according t o F--Feq (1 + 3 sin2P/) ~. To obtain a measure of the significance of the results, the cumulative distribution was used of the JN/JT ratios for the specimens with a paleomagnetic latitude (Pl) smaller than 25 ° (21 specimens) and larger than 25 ° (20 specimens). The mean JN/JT ratio (fig. 3a) for the group with Pl > 25 ° (1.09) is appreciably larger than that for the group with Pl < 25 ° (0.75) but the standard deviations for both groups are large (0.52 and 0.41 respectively). However, the application of the null hypothesis test suggests that the probabilities that the JN/JT ratios of both groups have the same mean and the same dispersion are about 0.02 and 0.06 respectively. On the other hand, the means of the Feq values computed from the JN/JT ratios of the specimens in
GEOMAGNETIC
INTENSITY
BETWEEN
100 MILLION
Cl
% IO0B06040200
,
0
,
0.5
1.0
•
,
1.5
2.0
AND
2500
MILLION
YEARS AGO
17
It may be concluded that the results of the test as discussed above are not conclusive but do yield independent support for the hypothesis that the paleomagnetic field may be approximated by that due to a geocentric dipole as far back as 2500 m.y. Much of the scatter in the JN/JT ratios and therefore much of the uncertainty in the results of this test is due to a rapid fluctuation in the paleoequatorial intensity according to the working hypothesis. Further work, as suggested in the last paragraph, may clarify the situation.
JN/JT
6. Conclusions % I00 80 60, 4O 20 0
o
oi,
62
o'.3
0.4
o~, Feq
Fig. 3. C u m u l a t i v e distribution o f (a) the J N / J T ratios for all samples with paleomagnetic latitudes smaller t h a n 25 ° (open circles) a n d larger t h a n 25 ° (dots), a n d (b) the Feq values calculated f r o m the J N / J T ratios for the s a m e groups o f samples u n d e r a s s u m p t i o n o f a geocentric dipole field.
both groups on the basis of the dipole hypothesis (fig. 3b) are in reasonable agreement (0.25 Oe and 0.24 Oe respectively with a standard deviation of 0.13 Oe in both cases). The null hypothesis test suggests that the probabilities that the Feq values of both groups have the same mean and dispersion are about 0.24 and 0.99 respectively. Thus the present results show that (1) the local paleomagnetic intensities (oc JN/JT) probably increase with paleomagnetic latitude according to some function, and (2) that if this function is taken to be the specific relation between the local intensity and the magnetic latitude valid under the geocentric dipole hypothesis, there is an obvious possibility that the resulting Feq values do not depend on the paleomagnetic latitudes of the sampling sites.
1. The results suggest that the average geomagnetic equatorial intensity as far back as 2.5 billion years was 0.25 Oe with a standard deviation of 0.13 Oe, and consequently has not differed significantly from the present value of about 0 . 3 0 e . 2. The scatter in the equatorial paleointensity values may be tentatively explained by assuming the occurrence of fluctuations in geomagnetic intensity with maximum values up to about 0 . 5 0 e and with periods shorter than the experimental error limits of the radiometric ages of the specimens. These periods may include ones as short as the period suggested by the archeomagnetic results to have occurred during the last five millenniums. 3. The hypothesis that the paleomagnetic field as far back as 2.5 billion years may be approximated by that due to a geocentric dipole is not in contradiction with the variation of the local paleointensity values with paleomagnetic latitude. Thus, several characteristics of the present geomagnetic field seem to have been present as far back as 2.5 billion years ago. The validity of the hypothesis that rapid fluctuations in geomagnetic intensity have occurred, may be tested by carrying out detailed paleointensity studies on sequences of rapidly extruded lava flows and slowly cooled igneous intrusions.
Acknowledgement We thank the geologists of the Geological Survey of Canada and the Ontario Department of Mines who generously put samples from their collections at our disposal.
E. J. SCHWARZ
18
AND
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