TABLE 16 Sample No.2
FIG. 30 SAMPLE NO.I
1.6 • 1.6
C.
SEPARATED AT 350°Cd.
SEPARATED AT55OC1.2
041
Tc
SAMPLE No. 1 (Fig.30)
(a) Rook specimen (b) Magnetic powder before T.M.S.
HEATING COOLING HEATING COOLING
J T°C K/Ko T°C K/Ko T°C K/ko T°C K/Ko. T°0 K/Ko T°C Kfto
20 1.00 510 1.21 605 0.01 20 1.00 585 0.06 615 0.02 40 1.01 525 0.96 590 0.01 60 1,00 595 0.04 595 0.03 60 1.02 540 0.67 570 0.01 80 1,01 605 0.03 580 0.06 85 1.01 550 0.16 505 0.22 110 1.01 615 0.02 560 0.15 95 1.02 560 0.08 480 0.55 140 1.02 545 0.23 115 1.01 570 0.04 455 1.16 165 1.03 525 0.47 140 1.03 585 0.01 435 1.20 190 1.04 510 0.68 170 1.08 605 0.01 400 1.19 235 1.09 480 0.84
200 1.11 380 1.16 290 1.16 455 0.78
220 1.12 350 1.06 365 1.21 420 0.76
245 1.15 310 1,12 320 1.03 400 0.74
280 1.15 280 1.16 420 1.02 370 0.72
300 1.12 235 1.12 440 1.04 340 0.67
320 1.12 200 1.07 480 0.98 300 0.67
340 1.12 175 1.05 500 0.99 250 0.65
380 1.13 160 1.03 510 1.05 210 0.62
400 1.15 135 1.00 520 1.04 185 0.61
440 1.17 120 0.99 540 0.80 155 0.59
465 1.22 100 0.99 555 0,40 110 0.62
485 1.22 80 0.98 570 0.12 80 0.59
(o) Non-magnetio at 350°C (d) Non-magnetic at 550°C
HEATING COOLING HEATING COOLING
T°C K/Ko T° C K/ko T°C K/I<o T°C K/Ko T°0 K/Ko T°C K/Nc 20 1.00 372 0.83 605 0.01 20 1.00 395 1.21 600 0.00 45 1.01 385 0.82 580 0.02 40 1.01 405 1.23 575 0.01 70 1.01 410 0.83 560 0.07 70 1.00 420 1.24 550 0.01 80 1.01 425 0.85 540 0.17 95 1.02 440 1.25 540 0.02 90 1.02 450 0.82 520 0.20 120 1.02 465 1.32 530 0.07 110 1.03 470 0.81 505 0.31 130 1.01 480 1.34 520 0.15 125 1.02 490 0.83 490 0.46 155 1.02 495 1.30 505 0.31 140 1.03 505 0.81 470 0.70 170 1.00 505 1.15 495 0.41 155 1.03 515 0.56 440 0.73 195 1.00 518 0.72 483 0.61 175 1.02 525 0.38 405 0,70 200 1.00 527 0.42 470 0,86 195 1.00 535 0.30 380 0.65 220 1.01 535 0.16 460 0.97 215 1.00 553 0.26 345 0.61 230 1.01 545 0.10 445 1.02 235 1.00 570 0.12 310 0.71 250 1.00 557 0.07 430 1.02 255 1.00 500 0.02 280 0.55 265 1.00 565 0.03 405 0.95 265 1.00 590 0.01 250 0.52 280 1.00 572 0.01 385 0.92 285 1.01 600 0.01 220 0.53 290 1.03 590 0.00 355 0.87 305 1.03 605 0.01 200 0.54 320 1.03 600 0.00 325 0.86
320 1.06 160 0.52 340 1.10 300 0.82
330 1.17 140 0.53 355 1.13 270 0.74
350 1.02 120 0.52 365 1.15 240 0.71
360 0.93 100 0.53 380 1.19
210 0.68
•150 0.66
(e) Magnetic at 55000
(Fig. 31)
(f) Residue SAMPLE No. 1
HEATING COOLING HEATING CO0LIiG
T°C K/Ko T°C K/ko T°C K/Ko T°C K/Ko T°C K/Ko T°C K/Ko J 20 1.00 305 1.04 595 0.00 20 1.00 545 0.67 605 0.00 40 1.01 325 1.08 580 0.01 40 1,00 565 0.39 580 0.01 50 1.01 340 1.11 570 0.01 50 1.00 570 0.31 535 0.40 80 1.00 355 1.15 560 0.01 60 0.99 580 0.21 500 0.58 90 1.01 370 1.17 545 0.05 85 0.99 587 0.13 480 0.68 100 1.00 390 1.19 535 0.07 100 0.97 595 0.04 455 0.67 110 1.00 410 1.24 520 0.13 120 0.98 600 0.01 425 0.67 130 1.00 420 1.25 505 0.22 140 1.00 605 0.00 400 0.67 140 1.01 450 1.28 492 0,39 I 165 0.97 375 0,66 150 1.01 465 1.32 475 0.66 200 0.97 350 0,67 165 1.01 485 1.36 460 0.93 230 0.98 325 0„07 180 1.01 495 1.40 450 1.00 275 0.99 310 0.65 195 1.00 510 1.34 430 1.00 300 0.99 275 0.66 205 1.01 520 0.90 400 0.98 330 1.00 255 0.67 215 1.01 527 0.57 350 0.94 360 1.03 235 0,66 225 1.01 535 0.30 325 0.92 380 1.08 195 0.66 235 1.01 550 0.14 300 0.88 415 1.07
250 1.01 560 0.11 275 0,87 440 1.01 270 1,02 570 0.07 240 0.83 470 0.93 285 1.03 595 0.00 200 0.80 505 0.85 295 1.04 600 0.00 150 0.77 527 0.77
110 0.74
of the rock itself since it contains all the representative ferromagnetic minerals that are contained in the rock.
The two main Curie points are at 370° and 570°0 for heating.
In the cooling curve, however, the lower Curie point
disappears leaving it with one Curie point at 550°C. The disappearance of the lower Curie point minerals could be due to the presence of an unstable mineral such as maghemite ( Fe203) that is easily transformed into the non-magnetic
state of hematite ( 0C - Fe203). A similar result has been (35)
reported elsewhere for the case of a magnetic gabbro that contains an appreciable amount of maghemite. Fig. (30c) shows the behaviour of the specimen containing the magnetic material that was non-magnetic at 350°C. In this curve the presence of this constituent is seen quite clearly from the peak around 340° and the Curie point at 35b°C. In the ideal case, when the specimen contains only this constituent, one would expect a drop of the curve to zero at this
temperature. But, due to the imperfectness of separation, other subsidiary materials in smaller amounts are present, giving Curie points of 520° and 570°C. The fact that the 350°C component is predominant could be seen from the
maximum value of K/Xo (= 1.2) at this temperature. The cooling curve has two Curie points also, but the maximum
value of the 3500C component has been decreased. It confirms the above explanation that this component is magnetically
81.
unstable. In this case, it would appear that not all this material is altered.
Figures (30d) and (3le) represent the curves for the constituents that have been separated at higher temperature (550°C). The former is for the Materials that have Curie
points between 3500 and 55000, and the other contains Curie points above 550°C. Here there is one Curie point in each case, both for cooling and heating, the first at 530° and the other at 560°C and the drop of K/ko at the Curie points is rather sharp.
It has been mentioned that a 'residual' powder was left after extracting the magnetic minerals and this was slightly magnetic due to the presence of some magnetic materials
that could not be extracted. The thermal variation of K/Ko of this 'residue' is shown in fig. (31f). The suscepti-bility remains nearly constant till 400°C where there is a little rise then it drops gradually to zero at 600°C. The cooling curve has the same shape except that there is no corresponding peak and below 480° it remains constant but at a value lower than the corresponding value on heating.
If this is compared with the magnetic materials extracted (fig.30b) it will be seen that there is a general
corres-pondence, suggesting similar constituents but possibly the less important ones are in different proportions.
The corresponding graphs for the individual constituents
1.4
1.0
0-6
200 400 600 200 400 600
b
160
475 300
530 550
1°C .
1.4
I.0
0.6
6.2
1.4
1.0
0.6
02
KlK„
d.
MAGNETIC AT 570°C550 0
TC
200
400
600
200 400 600
SAMPLE NO. 2 FIG. 32
SAMPLE No. 2 (Fig.32)
(a) Rook sample
(b) Non-Magnetio at 30000
HEATING COOLING HEATING COOLING
ToC K/Ko ToC KA° ToC K/Ko ToC K/ko Toe K/ko TOO K/ko 20 1.00 590 0.05 605 . 0.02 25 1.00 475 1.02 605 0.00 50 0.99 600 0.03 590 0.05 60 1.01 485 0.93 595 0.01
90 1.03 605 0.02 565 0.19 100 1.11 500 0.89 580 0.04 135 1.07 555 0.30 125 1.37 515 0.99 565 0.10 160 1.25 540 0.59 145 1.40 540 0.88 550 0.21 180 1.38 520 1.10 165 1.30 550 0.43 540 0.30 205 1.45 500 1.16 180 1.20 570 0,31 530 0.11 225 1.51 460 1.09 200 1.18 575 0.18 520 0.60 240 1.57 435 1.06 225 1.21 590 0.04 495 0.74 265 1.56 400 1.02 275 1,29 600 0.01 470 0.83
280 7,62 365 1.01 290 1.33 445 0.86
310 1.67 340 0.96 300 1.35 420 0.87
350 1.49 300 0.94 315 1.38 390 0.84
370 1.14 270 0.91 335 1.49 370 0.81
420 0.96 225 1.00 345 1.24 345 0.75
475 1.01 200 1.05 360 0.91 320 0.72
510 1.12 160 0.99 385 0.94 300 0.98
535 1.15 130 0.97 405 1.00 285 0.98
560 0.77 90 0.94 435 1.20 270 0.88
570 0.32 465 1.29 230 0.71
190 0.68 150 0.70 100 0.73
SAMPLE No. 2 (Fig. 32)
(0) Non-Magnetic at 5000C (d) Magnetic at 570°0
HEATING COOLING HEATING COOLING I
T°C K/ko TOG K/ko T°C K/ko T°0 K/Ko T C Kik° T00 I , K/Ko 20 1.00 495 1.22 600 0.01 20 1.00 555 0.19 605 0.00 30 0.98 510 0.81 595 0.01 50 1.01 570 0.09 595 0.00 65 0.96 530 0.54 580 0.01 75 1.01 585 0002 575 0.02 100 0.94 650 0.31 565 0.13 100 1.02 595 0.00 555 0.06 135 0.95 565 0.16 545 0.22 140 1,04 605 0.00 535 0.14 155 0.94 575 0.07 520 0,35 160 1.07 520 0.34 175 0,94 585 0.01 500 0.53 180 1.09 510 0.61 200 0.93 595 0.02 470 0.73 200 1.09 490 1.07 220 0.93 600 0.01 430 0.84 220 1.10 460 1.20
255 0.89 415 0.90 280 1.21 425 1.14
280 087 380 0.70 310 1.23 385 1,09
310 0.85 360 0.63 330 1.24 325 1,C5
335 0.79 340 0.66 355 1.25 285 1.00
360 0.80 310 0.66 380 1.27 250 0.99
380 1.01 295 0.67 420 1.27 220 0.96
400 1.08 270 0.66 440 1.29 195 0.97
415 0.85 240 0.65 460 1.38 170 0.95
430 0.75 205 0,66 490 1.43 145 0.95
450 0.85 180 0.66 510 1.49 120 0.94
475 1.04 160 0.66 535 1.08 95 0.93
125 0.66 545 0.47 70 0.91
82.
etc., from rock sample Nod 2, are shown in fig.(32) graph (a) for the rock itself shows very clearly two groups of magnetic materials with Curie points at about 300°C and 580°C. The rise of K/Ko at 300°C is quite remarkable reaching about 1.60 Here again, as has been observed beforel 'the cooling curve shows the peak on a very reduced scale, possibly due to the instability of the components.
The behaviour of the constituents that have Curie points less than 3000C, also, has a peculiar character (graph 6) showing four pronounced peaks corresponding to four differ-ent Curie points. The presence of the first two (1600 and 34000) is quite natural, since the thermomagnetic separation was carried out at about 3000C. For the higher two Curie points (namely at 4750 and 5500) this is due to the im-perfectness of the separation but nevertheless, the 3000 component is predominant. During the cooling, the four
components are reduced to two of Curie points 5300 and 30000.
The same behaviour is seen in graph (c) where the specimen contains the materials that have Curie points
between 30d3 and 500°C. Two Curie points appear at 420°C and 520°C in the heating curve while during cooling there is one, at 500°C.
On the other hand, the component which was still magnetic at 5700C has one Curie point (graph d) and the susceptibility K rises to a value of 1.5 K0 and then drops
FIG. 33 SAMPLE NO. 3
b.
EXTRACTED FRACTIONS 1.81.4
0-6
602
600.
K/K.
U.
SEPARATED AT 5SO0C200 400 600 11'.8
200 400 600
TC
e.
SEPARATED AT 5704' CTQC
0 200
400 600
K MAGNETIC AT 570° C
TIC
0
200 400 600
SAMPLE NO. 3 FIG. 34
103
1.4
I.0
0•6
0.2
1.8
1.4
1•0
0.6
0.2
(a) Rook oube (b) Magnetic Powder before T.M.S.
HEATING COOLING HEATING COOLING
T°C K/ko T°C K/ko VC K/ko T°C K/Ko T°C K Ko T°C K/Ko 40 1.00 530 1.04 600 0.01 40 1.00 457 0.86 595 0.01 70 1.03 545 1.05 565 0.15 60 1.04 472 0.77 580 0.01 80 1.01 555 1.06 550 1.00 80 1.05 478 0.75 560 0,03 110 1.02 575 0.71 535 1.07 120 1.09 495 0.70 540 0.06 125 1.02 580 0.24 510 1.09 140 1.12 505 0.67 525 0.13 140 1.02 587 0.03 480 1.11 170 1,08 512 0.61 515 0.32 165 1.01 595 0.02 430 1.11 190 1.14 527 0.31 505 0.60 185 1.02 600 0.01 410 1.09 210 1.16 540 0.09 495 0.83 210 1.03 355 1.07 225 1.18 560 0.07 480 0.85 230 1.03 330 1.07 245 1.21 570 0.05 470 0.82 250 1.04 300 1.06 260 1.24 580 O„03 453 0,77 270 1.05 270 1.06 275 7-28 587 0.02 430 0.75 290 1.05 230 1.04 295 1.35 600 0.01 400 0.70 320 1.05 195 1.03 315 1.46 610 0.00 360 0.62
360 1.02 160 1.01 340 1.55 340 0.60
390 1.04 130 1.01 355 1.62 325 0.58
425 1.03 115 1.00 385 1.63 300 0.55
450 1.04 100 1.01 405 1.69 240 0.48
470 1.05 90 1.00 430 1.42 200 0.47
485 1.04 80 1.00 440 1,13 160 0.46 i
1
510 1.03 60 1.00 450 0.95 130 0.45
'
(c) Non-Magnetic at 53000 (d) Non Magnetic at 550°C SAMPLE No. 3 (Fig. 33)
HEATING COOLING HEATING COOLING
T°0 Kik° T°C Kik° T°C K/Ko T°C K/Ko T°0 K/ko T°0 K/ko 20 1.00 545 0.15 600 0.01 20 1.00 575 0.02 610 0.01 40 1.02 560 0.02 580 0.02 60 1.09 610 0.01 570 0.02 60 1.02 580 0.03 560 0.03 105 1.14 560 0.05 100 1.07 590 0.02 550 0.12 130 1.14 515 0.54 120 1.07 600 0.01 535 0.14 180 1.16 500 0.84
140 1.09 510 0.54 210 1,19 470 0.92
165 1.10 500 0.77 265 1.20 450 0.97
180 1.10 485 0.94 280 1.23 390 0.86
200 1.10 460 0.86 305 1.31 365 0.84
220 1.12 445 0.85 320 1.35 345 0.79
250 1.14 410 0.77 345 1.42 313 0.73
280 1.17 360 0.69 365 1.51 270 0.72
300 1.20 320 0.65 390 1.58 230 0.70
330 1.23 295 0.62 415 1.67 180 0.69
375 1.40 245 0.59 435 1.72 150 0.68
395 1.49 200 0.59 465 1.51 120 0.66
460 1.54 180 0.58 495 1.01 100 0.64
480 1.14 140 0.54 530 0.44
490 0.95 125 0.53 550 0.14
510 0.72 100 0.52 560 0.05
525 0.54 80 0.52
MAPLE No. 3 (Fig. 34)
(e) Non Magnetic at 57°C (f) Magnetic at 57°C
HEATING COOLING HEATING COOLING
T°C K/Ko. T°C K/Ko T°C K Ko T°C K/Ko T°C K/Ko T°C K Ko 20 1.00 480 0.90 605 0.01 20 1.00 470 1.72 600 0.00 30 1.01 490 0.70 580 0.01 40 1.01 485 1.66 580 0.02 50 1.01 535 0.17 555 0.02 75 1.01 495 1.43 550 0.06 70 1.01 560 0.02 535 0.09 90 1.02 505 1.28 540 0.11 90 1.05 580 0.01 520 0.15 120 1.03 525 1.21 530 0.27 100 1.05 590 0.01 500 0.40 150 1.06 545 0.28 515 0.56 140 1.06 600 0.01 475 0.98 180 1.07 560 0.10 500 0.92 160 1.05 465 1.08 200 1.08 580 0.03 492 1.11 190 1.06 455 1.06 235 1.09 590 0.01 480 1.08 210 1.06 440 1.03 250 1,12 600 0.00 470 1.03
230 1.06 425 1.01 200 1.10 450 0.98
255 1.10 405 0.99 300 1.08 430 0.89
270 1.14 380 0.98 320 1.11 400 0.80
295 1.18 340 0.89 350 1.12 360 0.72
320 1.31 310 0.86 375 1.29 325 0.66
330 1.37 290 0.83 385 1.34 285 0.61
340 1.42 250 0.81 395 1.40 250 0.61
350 1.47 235 0.77 410 1.53 180' 0.59
390 1.58 220 0.75 422 1.58 160 0.57
440 1.59 195 0.74 427 1.61 125 0.57
470 1.22 135 0.73 455 1.68 90 0,58
100 0.73
83.
more sharply than the previous components, both in the heating and the co,I'ling processes.
The corresponding set of graphs for the third rock (No.3) are shown in figures (33) and 34): In the case of
the rock itself (graph a), the susceptibility remains constant till 55000 then drops to zero at about 580°C.
The behaviour of the components that could be separated at 530°, 550°, 570° and above 570°C are shown in graphs
(c)--p.(f) respectively. Two general characteristics are apparent in the last four graphs. In the first, is the pronounced peak just before the Curie point and in the second, the substantial reduction of susceptibility when the specimen is cooled.
Reviewing all these experimental observations, the
susceptibility K usually rises appi, eciably to a pronounced peak just before its Curie point is reached. The rise
varies from one specimen to another, ranging over a factor of 1.4 - 1.8 of its initial value K0 at room temperature.
This rise has been found in other magnetic materials and (36) it appears to correspond to the "Hopkinson Effect", a particular phenomenon associated with weak magnetic fields.
In the rock itself, it is not to be found, possibly because of the effects of the various magnetic minerals overlapping each other to give the resultant curve.
Also, in all cases when the artificial specimens have been subjected to the first heat treatment, the cooling
curve does not coincide with the corresponding heating
curve. The former is lower than the latter with the result that at room temperature, after heating and cooling, K
is less than the original value before heat treatment.
The change here is far greater in the constituents than in the original rock. On repeating heating and cooling
several times, the susceptibility changes reversibly with
temperature along nearly the same K-T curve as in the previous cooling as shown in fig.(35), graphs (a), (b) and (c).
This was done by heating and cooling an artificial sample three times consecutively. In drawing their corresponding graphs the value of the susceptibility at room temperature, after the first cooling was taken as the initial suscepti- bility Ko for the second heating (graph b). Similarly the room temperature value of K after the second cooling was
taken as the initial susceptibility for the third heating.
This phenomenon may suggest that the ferromagnetic minerals in the specimens are mineralogically unstable, being able to approach a more stable composition by thermal agitation during the comparatively short time of the (37) experiment. Similar results have been found in Japan.
To summarise the results of observations in this chapter, it can be said that the ferromagnetic minerals contained in igneous rocks consist of various constituents of different Curie points. Each component acquires normal thermoremanent magnetization then cooled down from above
SECOND HEATING . F11?3T HEATING
400 600 0 0 200
1.4
1.0
0.6
0.2
TZ TC
200 400 600
"THIRD HEATING
it
FIG. 35
(a) 1st heating
HEATING COOLING
T°C K/ko T°C K/ko T°C K/ko T°0 K/ko
20 1.00 340 1.16 590 0.01 270 0.68
40 0.98 375 1.22 576 0.02 235 0.67
45 0.98 400 1.03 560 0.08 205 0.66
60 0.99 415 0.99 555 0.15 175 0.65
80 0.99 430 1.00 540 0.18 150 0.64
100 1.02 460 0.99 530 0,28 130 0.64
115 1.03 480 1.01 520 0.46 100 0.63
125 1.03 510 1.02 500 0.75 90 0.63
155 1.05 525 1.05 490 0.81 75 0.63
175 1.05 545 0.60 470 0.82 185 1.05 555 0.35 450 0,79 200 1.05 565 0.19 435 0.76 215 1.05 570 0.11 425 0.76 225 1.05 580 0.06 400 0.72 240 1.05 590 0.03 385 0.71 266 1.06 595 0.02 II 365 0.70 275 1.08 600 0.01 350 0.70
300 1.10 330 0.69
310 1.12 315 0.68
1
320 1,13 300 0.68
SAMPLE No. 4 (Fig. 35)
(b) 2nd heating (0) 3rd heating
HEATING COOLING HEATING COOLING
T°C K/Ko T°C K/Ko T°C K/Ko T °C K/Ko T°C K/Ko T°C K/Ko
its Curie point. The Hopkinson effect could be observed but was not so pronounced as with some pure elements. This may suggest an explanation, in part, of the way in which the rock acquires its thermoremanent magnetism: one peculiar feature is that the thermoremanent intensity is far greater
than any induced magnetism during the 000ling4 Thus, if Km is the maximum observed susceptibility during cooling
and Jo is the thermoremanent magnetism acquired in a field H than Jc ,>" Km H. With a series of constituents with a range of Curie temperatures and "blocking temperatures"
defined by Neel, the thermoremanence will be added as each constituent passes through its blocking temperature without the necessity of the rock to show a high susceptibility.
86.
PART V
THE ORIGIN OF THE ,INVERSE REMANENT MAGNETIZATION OF ROCKS
Neel s' theories
, (14)
Quite recently, in 1951, Neel advanced several
mechanisms by which an eruptive rock can be magnetized in the opposite direction to the magnetic field in which it is cooled. It is of importance, here, to explain these mechanisms in some detail and to see whether they apply to the inversely magnetized Mull rocks in the light of the laboratory observations.
In his first mechanism, which will be referred to as (NI), Neel attributed the inverse magnetization to the
curious properties of some types of ferrimagnetic substances (21)
of spinal structure. In these substances, the magnetic ions are situated in a network of two lattices: sub-lattice A in which the positive ion is surrounded by four oxygen ions (tetrahedral), and sub-lattice B (octahedral) in which the positive ion is surrounded by six oxygen ions.
Each of these sub-lattices possesses spontaneous magnetiza-tion opposite to the other. In other words, this is an
imperfect case of antiferromagnetism. By means of a certain distribution of ions in these sub-lattices and with
convenient values of the different coefficients of the molecular field, the thermal variation of the total
spontaneous magnetization results in a type N having the following characteristics: Increasing the temperature from the absolute zero, the spontaneous magnetization decreases, becoming zero then being negative to a minimum value and then increases again to zero when the temperature reaches
the Curie point. This behaviour is due to the difference
in thermal variation of the partial spontaneous magnetizations of the two sub-lattices A and B.
Such a substance can possess, by cooling to the ordinary
temperature, remanent magnetization in the opposite direction to the magnetic field that produces it at high temperature.
The second mechanism (NII) suggests that the remanent magnetization of ferrimagnetics can, at least theoretically, be reversed by a substitution mechanism. For instance, in magnetite a molecule of Fe304 contain: an ion
of
Fe+++ at the sub-lattice A and two ions, one Fe ++ and one Fe ++at B. Therefore, the resultant spontaneous magnetization is in the direction of magnetization of B. Now, suppose that, after cooling and acquiring normal magnetization, due mainly to sub-lattice B, certain chemical changes are
produced in the substance - e.g. the substitution of the strongly magnetic ions Fe +++ in sub-lattice B by weakly or non-magnetic ions as AI++ or Ti'+++ This, reduces the spontaneous magnetization of B to a value less than that of A and hence, produces a reversal of the resultant
spontaneous magnetization and of the original thermoremanence.
88.
Summing up these two mechanisms, ferrimagnetic substances furnish two possible explanations of inverse magnetization based on the existence of ferrimagnetic substances, of the N type, in the rock. Although such type has been discovered
(38)
very recently, yet there is no evidence of its existence in rocks. On the other handy the possibility of chemical
substitution is still hypothetical. Moreover, the mechanism NI cannot be the explanation for the natural inverse magneti-zation of rocks, since normal magnetimagneti-zation is obtained on cooling. If mechanism NII 18 responsible, there is no direct test of this, since the mode of magnetization "in situ"
cannot be reproduced experimentally.
In Neelfs third mechanism (NIII), the ferromagnetic
materials, which are responsible for the remanent magnetization of igneous rocks, contain two ferromagnetic constituents A and B of widely different Curie points, the Curie point of A being higher than that of B. In fact, the presence of more than one constituent in igneous rocks has been shown
(32) (11)
by Nagata and Bruckshaw and Rao. Even more than two constituents have been found in the present investigation (Part IV). Neel suggested the presence of two constituents A and B concentrated in local aggregates called amas -which are, in turn, scattered among the non-magnetic materials of the rock. Each of A and B, if individually cooled
down in a magnetic field to a temperature below its Curie point, will acquire magnetization in the direction of the
field. However, an aggregate containing A and B, on cooling, passes first through the Curie point of A which, consequently, becomes permanently magnetized in the direction of the external field. Theny the magnetic field in the rock will be the resultant of the external field and of the
demagnetizing field of constituent A. Due to a large negative temperature coefficient of the intensity of magnetization, which is now independent of the external field, the moment increases as the temperature decreases.
As a result of this, while the external field remains
constant, the internal demagnetizing field increases until, under certain conditions, it exceeds the external field just before the Curie point of the second constituent is reached.
This will, then, acquire thermoremanence in the opposite
direction of the applied field and, uliaer certain conditions, this latter polarization can predominate. By calculation, it has been shown in this mechanism that the concentration of each of the two constituents should be of the order of 0.35 for producing an inverse magnetization. Since in igneous rocks the ferromagnetic materials are scattered in the non-magnetic matrix with a very much lower concentration (0.01 - 0.05), it is necessary in this mechanism that the
two ferromagnetic constituents must be closely associated inside an amas of a complex structure. In this way, the
"local" concentration of the ferromagnetic constituents in the amas can be much higher than the average concentration.
90.
Moreover, in order that the two constituents are joined in the amas, they should come from a common origin. They could come, for instance, by a segregation into two phases by cooling a homogeneous solid solution which has been stable at high temperatures.
In the fourth mechanism of Neel (Niv), it is suggested that, even if an amas (and so the resultant magnetization) is not initially inverse after cooling, it can become so at a later stage following some chemical changes. If these chemical changes only affect the constituent A, its
magnetization decreases or disappears and the rock is left with the magnetization of B which is opposed to the
external field. Alternately the constituent A may lose its magnetism more rapidly with time than in the case of B, again leading to an inverse polarity.
Discussion of Neel s' mechanisms
Neel s'first two mechanisms (NI and Nu) depend on the presence of certain types of ferrimagnetic minerals, either naturally or by substitution as a result of some chemical changes. These were, so far, not found in the ferromagnetic minerals of igneous rocks in general, and particularly those possessing natural inverse magnetization. Even in the Haruna
(17)
dacite rock that has been shown to acquire self reversal in (34)
the laboratory, it has been reported by Nicholls that such material could not be found, when its ferromagnetic minerals
were analysed. So, these two mechanisms can be rejected to explain the natural reverse magnetization in igneous rocks.
In the third mechanism (N111) the process of acquiring self reversal requires a concentration of the two consti-tuents much higher than normally found for the ferromagnetic contents in igneous rocks. While it requires 35 per cent concentration of both A and B constituents, the actual concentration is from 3-5%. The difficulty of accounting for the local concentration or amas, for which there is no petrological evidence, can be overcome by assuming that the two constituents are present in the same grain either as an intergrowth during crystallization from the original magma or as an alteration product of an initial single material.
In the latter case the alteration would have to occur before the temperature cooled to the Curie point of B. The
present evidence of rocks does not support (N111) since they would exhibit inverse magnetization under laboratory
conditions, and there is ample evidence from many sources (7),(20),(39).
that this is not the case.
Further, a number of specimens of natural inverse (7)
magnetization, then examined petrologically, showed no sign of secondary chemical changes as has been suggested in the fourth mechanism (Ni v). Chemical changes involving removal or replacement of part of the magnetic minerals should, in general, be evident. Indeed, with most minerals which are
92.
transparent in thin sections such changes are readily recognized, but the problem is more difficult with opaque minerals such as the ferromagnetic minerals concerned here.
Against this suggestion is the fact that the magnetic susceptibility at room temperature is of the same order of magnitude for both normally and inversely magnetized rocks of the same age and so is the intensity of magnetization.
Thus, for Icelandic lavas the mean values of the suscepti-bility and natural intensity are listed below.
Susceptibility Intensity Inverse 4.27 x 10-3
c.g.s. 1.14 x 10-3
c.g.s.
Normal 3.02 x 10-3 tt 1.18 x 10-3 "
Further, the change of susceptibility with temperature is
the same in both kinds, being independent on the polarization.
Finally, the rock specimens that possess natural inverse magnetization should acquire negative polarization when they are suitably heated in the laboratory. This requirement is entirely not satisfied in the Mull igneous rocks. They all, without exception acquired normal magnetization after cooling from their Curie points in different magnetic fields (from
(2),(7),(39) 0.02 - 0.9 gauss) cart 111.7. Other investigators,
also, when cooling rock specimens in the earth's field, have shown that in every case the rock acquired normal thermo-remanence, whether its natural magnetization was normal or inverse.
The only exception is the Haruna dacite rock by which (17),(40)
Nagata has advanced a modified process of Ne611 s third mechanism to account for the self reversal.
In fact, there are more than two constituents in the rocks of Mull, each has its Curie point, between 30000 and 600°C as seen in Part IV. But every constituent, after cooling down to room temperature in the earthls field, acquired normal thermoremanent magnetization.
It would appear, then, from the experimental evidence assembled in this work that Ne(ills theories cannot account for the natural inverse magnetization of the Tertiary lava flows of North of England.
Other possible explanations
Other theories relating inverse magnetization with
factors such as rock textures, grain size of the ferromagnetic minerals, presence of special minerals as titanium oxide
and rate of cooling of the rock yielded no correlation.
Also, there is the suggestion that mechanical stresses, set up by tectonic crustal forces, have a bearing on the inverse magnetization. It is known that an applied stress on a
solid modifies the state of magnetization, e.g. a nickel wire under tension and torsion may exhibit reverse magnetization.
(41)
Welo has shown that mechanical stresses on rocks increases
Welo has shown that mechanical stresses on rocks increases