34. magnetic mineralogy of basalts from …. magnetic mineralogy of basalts from leg 49 k....
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34. MAGNETIC MINERALOGY OF BASALTS FROM LEG 49
K. Kobayashi,1 M. Steiner,2 A. Faller,3 T. Furuta,1 T. Ishii,1 P. Shive,2 and R. Day4
INTRODUCTION
The Curie temperatures and thermomagnetic behavior ofthe whole rocks, as well as the titanium content andmicroscopic features of titanomagnetite contained in therocks, were studied on the same samples used for shipboarddeterminations of paleomagnetic properties. At least one(usually much more than one) sample was available fromeach lithologic unit of the basement rocks. A particular aimof the studies was to find any intersite variations ofmagnetochemical properties, such as variations withdistance parallel to and perpendicular to the northernMid-Atlantic and Reykjanes ridges. We also looked forvariations with depth in the holes, such as might be causedby differences in magmatic differentiation or hydrothermalalteration. The major ferromagnetic phases, which arepresumably responsible for the natural remanentmagnetization and the marine magnetic anomalies, werecarefully identified. The findings of the thermochemicalmagnetic study are correlated with rock magnetic andpaleomagnetism results in the final chapter.
THERMOMAGNETIC MEASUREMENT
Changes in saturation magnetization (Js) with temperature(T) were studied on 120 samples from all the sites. The Js-Tcurves were measured at the Ocean Research Institute,Tokyo (predominantly Holes 407 and 409, indicated by K inthe last column of Table 1) and at the University ofWyoming (mainly Holes 409, 410, 410A, 411, 412A,designated M in Table 1). Two samples were investigated atthe University of Minnesota (MN in Table 1). The Tokyoinstrument is a horizontal torsion balance which eliminatesany effect of weight changes of sample on the automaticallyrecorded magnetization changes. A chip of rock of about100 to 150 milligrams was used for each measurement. Afield of approximately 4300 Oe was applied throughout theheating and cooling cycles. The rate of heating and coolingwas 6°C/min. The samples were kept at maximumtemperature (usually 620°C) for half an hour before cooling.All samples were heated in a vacuum of 10~4 Torr, using adiffusion pump without a liquid nitrogen trap.
The University of Wyoming instrument uses a Cahnmicrobalance. Small chips weighing about 180 milligramswere heated in a field of 4150 Oeat a rate of 25 to 30°C/min.Samples were taken to 700°C and cooled immediately at thesame rate. A vacuum between 10 ~4 to 10 ~5 Torr was
'Ocean Research Institute, University of Tokyo.2University of Wyoming, Laramie3University of Leeds, Leeds, England.4University of California, Santa Barbara.
maintained by a diffusion pump, also without a liquidnitrogen trap. In addition to the Tokyo and Wyomingdeterminations, two Js-T curves were obtained on avibrating sample magnetometer at the University ofMinnesota. The magnetic field used was 2500Oe, andheating was done in a vacuum of less than 1 × 10 ~6 Torr.The heating rate was about 10 to 15°C/min. The differencein heating and cooling procedures and in heating rates hascaused slight differences between the Js-T curvesdetermined at Tokyo and those determined at Wyoming. Inparticular, more of the reversible type curves were obtainedat Wyoming, and more frequently Tokyo curves had threeintensity drops before a completely paramagnetic state isreached than did Wyoming curves.
As already seen in previous work (Ozima et al., 1968),two types of samples can be distinguished from theirthermomagnetic behavior: a thermally reversible type(Figure la) and an irreversible one (Figure lb). Withirreversible samples, magnetization generally decreaseswith increasing temperature, to reach nearly zero or aminimum value at a certain temperature (Tci), and increaseswith further heating until it finally becomes paramagneticabove a higher Curie temperature. Several samples showtwo high-temperature peaks, as indicated in Figure lc (Tc2and Tc3 in Table 1). These peaks can be explained astitanomaghemite undergoing a two-stage inversion tolow-Ti spinel plus hemoilmenite.
The Curie temperatures of the high-temperature phases(TCH), which are determined from the cooling curves afterheating to 620° to 700°C, are as listed in Table 1. Someconsistency is evident, either within an entire site, or invarious levels at a site; this suggests a similar magneticphase through some thickness of basalt. The ratio ofmagnetization at room temperature before heating (Jo) tothat obtained after the heating-cooling cycle (JHO) is alsolisted. The ratio ranges from 0.75 to 7.53, but mostfrequently falls between 1 and 3 (Table 1). There seems tobe a general tendency for the ratio JHO/JO to increase with theCurie temperature of the high-temperature phase (TCH)(Figure 2). Decomposition of titanomaghemite to lesstitaniferous titanomagnetite plus hemoilmenite would causeJHO/JO values greater than unity.
There are several samples with unusual thermomagneticfeatures. One unusual curve is from a sample in a lower unitof Hole 409; it shows reversible thermomagnetism curvesbut a Curie temperature of roughly 540° (Figure 3a). Thissuggests an unoxidized titanomagnetite close to magnetite,such as is commonly found in island-arc volcanic belts. Theflow unit of this sample is petrographically different fromthe others, and is regarded as a product of highlydifferentiated magma (labeled as a tholeiitic andesite in the
793
K. KOBAYASHI ET AL.
TABLE 1Summary of Thermomagnetic Properties and Crystallographic and Chemical Characteristics of the Samples Treated in This Study
Core-Section
Hole 407
33 CC35-136-236-236-337-137-137-238-138-138-238-338-338-339-239-339-339-339-340-142-142-244-145-145-245-245-345-446-146-346-34 6 447-147-247-247-347-34 7 4
Hole 408
35-135-136-236-537-137-137-137-338-138-2
Hole 409
7-6B7-79-19-3
10-110-210-611-311412-115-115-215-415-515-616-116-217-118-221-122-123-124-1
Depth(m)
300.5319.9330.3331.3332.2338.8339.3340.8348.5348.6349.9352.4351.7351.8359.6360.7360.9361.0361.9367.6386.9388.8424.2434.2435.4436.4437.7438.5443.1446.7447.3448.2453.7454.2454.9455.7456.2457.4
323.8324.1335.4339.0342.1343.0343.2346.1352.2352.6
80.081.191.194.2
100.6102.3109.4113.8114.9120.9148.3149.9153.1154.7156.8158.7159.2167.4179.0206.0217.1224.3233.7
Thermomag.Reversibility
(R)I
n.d.
(R)n.d.n.d.RR1I1
n.d.III
(R)IR
n.d.I1RII1I1
(R)n.d.
III1I1IRI
n.d.III
n.d.R
n.d.R
n.d.(R)
(R)RI1RIRR
n.d.1
n.d.IIIII1
n.d.III1
(R)
Curie Temperature
Tcj
485213n.d.212n.d.n.d.145128213240235n.d.222200220170200140n.d.222190140295223345270220250n.d.340400340315220273235195310
n.d.360295335n.d.210n.d.185n.d.260
160200278310143200210143n.d.245n.d.255218245235222335n.d.222230208220295
T c 2
_
438
485
__
385435380
(390)453420
462_
370457
_508460553565462452
575605560560550477465
_555
500545530
_
-
445
_
_483
(490)-
513_-
465
475480495495480500
480450485500480
T c 3
_
_
_
__
530540535
485_____
540____
___
__
__
562__-
555__
_
-
_
__
547_
__
_
_55055 0
—550
_
550_-_-
T c H
440430
180
12580
505565490
440415430345420110
4904303305004355 25570430410
55 05255255805205 304501905 20
5255455 30
250
260
375
260200460515120480215120
560
540540570505540470
540495430460340
J Ho/Jo
0.751.39n.d.1.14n.d.n.d.1.121.004.272.303.17
2.131.431.311.181.181.00
4.401.721.201.960.764.555.661.531.02
5.012.155.906.964.154.811.710.924.85
3.733.89n.d.
0.94
1.13
0.85
1.101.101.472.501.003.300.891.00
3.08
2.447.536.192.215.661.81
6.482.512.262.730.92
a(A)
n.d.n.d.n.d.
8.437n.d.n.d.
8.442n.d.n.d.n.d.n.d.n.d.
8.405n.d.n.d.n.d.n.d.n.d.n.d.
8.402n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.
n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.
n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.
Fe/Ti(Mole)
27.7n.d.
27.726.429.430.429.431.7n.d.n.d.n.d.
27.834.134.7n.d.n.d.n.d.
28.325.533.3n.d.n.d.
31.8n.d.
30.4n.d.29.428.328.3n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.
30.4
40.222.0n.d.n.d.
28.3n.d.
22.426.126.6n.d.
27.4n.d.n.d.
33.328.535.3
n.d.26.5n.d.n.d.
26.1n.d.36.4n.d.n.d.
35.5n.d.n.d.n.d.n.d.
30.9n.d.
28.3
x Value
0.65n.d.
0.650.630.680.700.680.72n.d.n.d.n.d.
0.630.760.77n.d.n.d.n.d.
0.660.610.74n.d.n.d.
0.72n.d.
0.70n.d.
0.680.660.66n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.
0.70
0.860.54n.d.n.d.
0.66n.d.
0.550.620.63n.d.
0.64n.d.n.d.
0.750.660.78n.d.
0.630.70n.d.
0.62n.d.
0.80n.d.n.d.
0.78n.d.
0.62n.d.n.d.
0.71n.d.
0.67
Size of Mag OxidationMineral (µm) Stagea
10 ^ 2 0
0.5 ^ 3 0 2
10^50n.d.n.d.
% 100
0.5^5 3n.d.n.d.n.d.10 'v 100<IOn.d.n.d.n.d.n.d. 3<5
5 ~ 10n.d.n.d.<5n.d. 3n.d.n.d.n.d.n.d.n.d.n.d. 3 ^ 4n.d.n.d.n.d.n.d.n.d.n.d.n.d.<IO
10 Λ, 1005
n.d.n.d. 3 ^ 4n.d.n.d.n.d.10 'v 100n.d.n.d.
5Λ, 10n.d. 2n.d.5 'v 10
>IO>IOn.d.>IOn.d.n.d. 3n.d.n.d. 3>IOn.d. 3 ^ 4n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.>IO
Observer
KKFKFFKKKMKFKKKMKKFKKKKKKMKKFMKKMKKKMK
FKM
M N
FMFKFM
KMKKKKMKFMFMKMMKKFKMKMK
794
TABLE 1 - Continued
MAGNETIC MINERALOGY OF BASALTS
Core-Section
Depth(m)
Thermomag.Reversibility
Curie Temperature (CC)
T c l T c 2 T c 3 T c H 'Ho/Jo (A)Fe/Ti
(Mole) x ValueSize of Mag
Mineral (µm)Oxidation
Stage'1 Observer
24-124-3
24-4
24-5
24-7
25-1
25-3
26-1
27-1
31-1
31-131-2
Hole 410
39-1
39-1
39-139-1
39-2
39-2
39-2
39-3
39-5
41-1
Hole410A
1-72-12-5 M
2-5 G
2-53-23-24-24-44-44-45-15-46-1
6-2
Hole 411
1-12-12-25-1
Hole 412A
1-12-22-2}-2
4-15-1
7-2Y
7-2G
9-111-3
14-2
14-5
Hole 413
1-21-2
235.0
237.3
238.9
240.2
243.0
243.8
246.9
25 2.8
262.3
300.1
301.1
302.2
359.8
360.10
360.15
360.23
361.74
361.83
361.86
362.8
365.2
378.5
334.0
335.2
340.6
340.6
341.5
346.6
347.0
355.9
358.4
358.7
358.7
363.7
368.5
372.6
n.d.
74.8
82.8
83.1
110.2
156.3
167.1
167.4
177.2
184.8
194.5
215.1
215.1
232.5
254.7
282.0
285.8
110.6
111.0
RRn.d.
IRIIIIII1
540160n.d.
235
130
285
252
265
230
283
235
254
—-
495_
525
490
480
508
445
4905 00
—-
_
_
-
560
545-
-
55 0
5 20
185
500
100540515515490450475500
1.00
0.88
n.d.
1.00
1.47
5.71
4.79
3.28
2.00
2.20
3.49
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
21.6n.d.
29.9
n.d.
29.4
n.d.
n.d.
n.d.
24.5
n.d.n.d.
28.6
0.53n.d.
0.69
n.d.
0.68n.d.n.d.n.d.
0.59
n.d.n.d.
0.66
In.d.
I
(R)
II11
n.d.II11I1
Rn.d.RI
<R)
RRRR
265 425
295425425430365350
3 305
n.d.
300
200
270
310
405
275
n.d.
403
365
315
300
405
360
230
210
205
250
170
173
178
170
123
165
270
265
205
210
185
200
250
245
500 555 3.02
Small Basalt Chunks:570
560
5 85
600
545
525
290
375 510
520 n.d.
485
5 35
515
310
5 25
533
490
500
510
490
495
525
470
n.d.
405
n.d.
555
545
565
560
565
545
525
550
550
590
550
J3001625
470 545
610
525
- 545
465
500
590
- n.d.
555
460
495
495
25 0
n.d.
215
5 35
205
140
200
180
75
270
380
575
290
605
245
220
190
205
455
455
3.70
2.30
n.d.
3.32
4.19
2.28
2.34
n.d.
209
n.d.
1.41
1.22
2.61
2.32
2.60
2.53
1.71
2.20
2.63
1.58
2.35
2.53
0.95
n.d.
0.93
2.29
0.91
0.97
0.97
1.01
1.01
0.87
1.33
1.05
0.93
0.97
0.94
0.94
1.83
1.49
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
26.1
n.d.
n.d.
n.d.
n.d.
27.1
n.d.
n.d.
n.d.
20.0
n.d.
n.d.
n.d.
20.5
n.d.
24.0
n.d.
n.d.
n.d.
27.7
n.d.
n.d.
28.8
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
0.62
n.d.
n.d.
n.d.
n.d.
0.64
n.d.
0.61
n.d.
0.50
n.d.
n.d.
n.d.
0.51
n.d.
0.58
n.d.
n.d.
n.d.
0.65
n.d.
n.d.
0.67
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
5 Λ, 10n.d.n.d.n.d.>IOn.d.n.d.n.d.5 -V 10
n.d.n.d.5 Λ, 10
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
0.025
n.d.
n.d.
n.d.
0.025^5
0.5 % 1
<5
0.5 -N/ 5
n.d.
n.d.
5 'v 10
<0.025 Λ, 2
<0.025 % 2
<0.025
<5
10
7-v 15
>IO
10^40n.d.
>IO
5 'v 20
2Λ, 15
0.5^5
n.d.
0.5^5
n.d.
n.d.
2^5n.d.
3,IL
3
2, IL
3, IL
2, IL
K 2
aAfter Johnson and Hall (1978).Note: Observers: K, at Tokyo; M, at Wyoming; F, at Leeds; and Mjsj, at Minnesota, n.d. = no data, R = reversible, I = irreversible, (R) = partially rever-
sible, as discussed in the text. The sizes quoted are rough values of the most aboundant grain size.
795
K. KOBAYASHI ET AL.
1.0 n
0.5 H \
7X100 0 1(c)
6 x100°C
i.<H
0.5 H
5 x100°C(b)
Figure 1. Thermomagnetism curves of typical samples, (a) thermally reversible type, 412A-3-2. (b) thermally irreversibletype with one peak in Js-T, 407-35-1. (c) thermally irreversible type with two peaks in Js -T, 407-38-1.
shipboard nomenclature). It is likely that this distinctpetrologic circumstance produced opaque minerals formedunder conditions much different from those in the typicaloceanic basalts. This sample is also unusual in othermagnetic properties (see Day et al., this volume).
Another sample, the first piece of basalt recovered atHole 407 (33, CC of Table 1), also shows a reversible highCurie temperature. This rock was collected by the corecatcher at the sub-basement surface, and may be a fragmentof rock transported from an adjacent area. It will beexcluded in the following discussion.
An occasional sample shows a secondary Js-T peak in theheating, but the Js-T curve in the cooling process is moresimilar to the heating portion of the Js-T curve at the lowertemperatures before the peak appears (Figure le). This typeof sample (Figure 3b) is classified as partially reversible (R)in Table 1. Except for the one above-mentioned tholeiiticandesite sample, the Curie temperatures of the reversiblesamples are lower than the initial Curie temperatures (Tc) ofthe irreversible types.
Several characteristic features can be recognized in thethermomagnetic properties of each hole. In Hole 407, the
796
MAGNETIC MINERALOGY OF BASALTS
JHo/Jo
8 -
6 -
2 H
oGα
<SbD
JHO/JO
O ODo
OD
2 -
D2 o oDD
Δ
Δ
Δ
Δ
α α
(a)TcH
100 °C
(b)
3
Tc.
Figure 2. JHO/JO versus TCj^ diagrams, showing their correlation, (a) o Hole 407, Δ Hole 408, o /fofe 409. (b) < Hole
θHole410A, Hole 411, u Hole412A.
initial Curie temperature (Tci) in the lower part is about100°C higher than in the upper part (Figure 4a). Reversiblesamples occur in a few units at different depths in the upperpart of the hole, in both the normal and reversed magneticunits. Irreversible samples are frequently situated within 1to 2 meters of reversible ones. In the lower paleomagneticunit of the hole, however, below the gap in recovery, almostall samples are irreversible (Figure 4a). This differencecoincides with a change in inclinations at that point.
The upper half of Hole 408 (above the sub-bottom depth344 m) is wholly irreversible; the lower half is reversible(Table 1). The boundary coincides with a boundary in thepaleomagnetism results, in other rock magnetic studies(Day et al., this volume), in petrographic examination of theopaque minerals (discussed later), as well as in alteration ofthe whole rocks, obvious to the unaided eye.
Despite the excellent recovery of samples representing200 meters of basalt, no systematic variations inthermomagnetic properties with depth are evident in Hole409 (Figure 4b). The reversible high-Tc tholeiitic andesite
occurs at about 150 meters. Values of the initial Curietemperatures and frequency of occurrence of thermallyreversible samples from this hole (about 2 m.y. age) are notsignificantly different from Hole 407 (about 40 m.y.). Thisindicates that, in the North Atlantic, the thermomagneticproperties of the ocean floor basalts may be independent ofage between 2 m.y. and 40 m.y., in contrast to the previouspostulations of age-dependence of magnetic featuresattributable to gradual oxidation (Carmichael, 1970; Irvingetal., 1970).
Hole 410, a basalt limestone breccia, shows the highestvalues of the initial Curie temperature (around 380°C) ofany Leg 49 hole (Table 1). All samples from this hole haveirreversible thermomagnetism curves. The data shown inTable 1 pertain to samples from large chunks of basaltwithin the breccia and to samples from much smaller basaltpieces within the breccia (see Site Summary Chapter, Site410, this volume). Curie temperatures are notably moreuniform among the small breccia pieces than among thelarger ones. This probably reflects a hot origin of the breccia
797
K. KOBAYASHI ET AL.
1.0 "
0.5 "
1.0 -
0.5-
6 x100°C(a)
0 1
(b)
6 ×100°C
Figure 3. (a) thermally reversible with a high Curie temperature, 409-24-1 (tholeiitic andesite). (b) partially reversible type(see test) 409-24-1. The difference between the heating and cooling curves at low temperatures may reflect a thermallag problem.
and the higher temperatures attained by the smaller piecescompared with the larger ones. The Curie temperatures ofthe high-temperature phases of all samples are concentratedaround 540°C. The ratio JHO/JO is distributed around 3.0. Inno sample from Hole 410 does this ratio exceed 5.0 — incontrast to Holes 407 and 409, in which a notable fraction ofirreversible samples have a ratio of 4.5 to 7.0 (Figures 4d,3a, and 3b). This implies that the titanomagnetite in Hole410 has been extensively oxidized. It seems likely thatoxidation proceeded when the rock was brecciated andreacted with seawater at a high temperature.
The initial Curie temperatures of samples from Hole410A are very uniform with depth, and their average islower than that of Hole 410 by about 100°C. Nevertheless,all (except a glassy margin of a flow) display irreversiblethermomagnetism curves. A comparison measurement wasmade between a glass and the margin of the glass. As mightbe expected, the glass has a lower Curie temperature thanthe immediate margin rock (Figures 5c and 5d) and is morereversible, indicating that it has been less oxidized. Thevalues of TCH in the Hole 410A are more scattered, and theiraverage is slightly lower than for Hole 410. The differencebetween the two sites may be a result of different degrees ofoxidation (Hole 410A is less oxidized), althoughtitanomagnetites in both holes appear to be oxidized to alarge extent as far as the thermomagnetic behavior (andmicroscopic properties, discussed later) is concerned(Figure 4c).
Hole 411, which has the most recently formed basement(1.0 m.y.) among holes of Leg 49 (and even the youngestamong holes so far drilled by DSDP), has reversible
samples in the upper 10 meters and an irreversible samplebelow. Although only 35 meters were cored here, thisdifference in thermomagnetic characteristics (Table 1) isconsistent with the paleomagnetic boundary defined byinclination and median destructive field (see SiteSummary). The initial Curie temperatures are relativelyhigh compared with those of other reversible samples.
Samples from Hole 412A (1.6 m.y.) are all reversible,and have low initial Curie temperatures. The values of theCurie temperatures are uniform throughout the hole andlower than those of the younger Hole 411 (Figure 4d). Twosamples were measured from one paleomagnetic core whichcuts an oxidation front so that one half is yellowish tan andthe other gray. Curie temperatures are the same; the grayphase shows a nearly reversible curve (Figure 5a). Theyellow portion shows two Curie temperatures, and is moreirreversible (Figure 5b), but still not as irreversible as mostirreversible samples from this leg (as illustrated in Figureslb and lc). It is interesting that this alteration is not assevere as it appears from the yellow color of the rocks.Although it drastically changes the rock color, the degree ofoxidation is not as high as many of the irreversible samplesin other holes.
It is, nevertheless, the most oxidized sample from Hole412A, and the only sample displaying irreversibility. Itwould seem that the rocks at Hole 412A have been underless oxidizing conditions than those of the other holes ofsimilar age; both Site 411(1 m.y.) and Site 409 (2 m.y.) aremore oxidized than is Hole 412A. Normally, oxidation oftitanomagnetite proceeds very rapidly after basalt formation(see discussion in Steiner et al., this volume). The low
798
MAGNETIC MINERALOGY OF BASALTS
Depth(m) Tc x ioo°C × " v α l u e
0 1 2 3 4 5 6 0.5 .6 .7 .8
Depth(m) T c x100°C ×~ v α l u e
0 1 2 3 4 5 6 0.5 .6 .7 .8
320 -
340 -
360 -
380 -
400 -
420 -
440 -
1
2
-
3
a)
1
L•
R
T
DAT
o
R
i i
o
Θ
•cv
oδ
m <Po
o
ü
1
A
A
o
o °o
o
°°o
Δ
Δ
Δ
Δ Δ
Δ
Δ
Depth(m) Tc ×100°C1 2 3 4 5 6
3 6 0 -
370-
380 J
330
340 -
350 -
3 6 0 -
370-
°Q> AOUQ>
o
o
o
öU
100 -
120 -
140 "
160 "
180 "
200 -
220 -
240 -
260 -
260 -
300 -
320 -
1
2
3
4
O Δ
O Δ
O (O Δ
Depth (m) Tc x100°C x-value0 1 2 3 4 5 6 0.5 .6 .7 .8
i i i i i i
160-
180-
200-
220-
240-
260-
280-
1
2
3
4
5
6
7
8
9
10
11
12
13
14
(d)
• A
• A
• A
• A
3 Δ A
• A
• A
• A
Figure 4. Downhole plots of Tc (TCj indicated by circles and TCJJ by triangles, where solid marks denote reversiblesamples and hollow denote irreversible) and x value (ulvospinel mole fraction in xFe2Tiθ4 (l-x)Fe304 solidsolution), (a) Hole 407. (b) Hole 409. (c) Holes 410 (above) and 410A (below), (x value plots are not shownbecause of shortage of data), (d) Hole 412A.
799
K. KOBAYASHI ET AL.
b)
4 0 0 600
T,°CFigure 5. Comparison of Curie temperature curves of gray (a) versus yellow-brown (b) basalt across a color
change (oxidation front) within one paleomagnetic sample (2.5 cm diameter). Comparison of Curie tempera-ture curves in glass (c) and in the immediate margin rock (d).
800
MAGNETIC MINERALOGY QF BASALTS
Curie temperatures and the reversibility of Hole 412Asamples suggest that the oxidation-reduction environmentimmediately or shortly after formation of the basementrocks must have been different, perhaps because of itslocation within a fracture zone.
CHEMICAL COMPOSITION OFTITANOMAGNETITES CONTAINED
IN THE ROCKS
Fragments of rocks were polished and examined under areflecting light microscope. Several particles oftitanomagnetite or titanomaghemite were selected andanalyzed by electron microprobes. Two instruments wereused; one at the University of Tokyo (designated K in thelast column of Table 1), and the other at University of Leeds(marked F). The atomic ratio Ti/Fe was obtained, and themolecular fraction of ulvospinel (x) in theulvospinel-magnetite solid solution [xFe2TiO4 (1-x)FeβCM or its oxidized product was calculated on theassumption that the mineral is exclusively composed of Fe,Ti, and O. Detailed analysis of a few samples indicated thatcontent of metals other than Fe and Ti (Al, Mn, etc.) is lessthan 1 wt per cent, which is negligibly small in thefollowing consideration.
A significant correlation exists between the ulvospinelcontent (x value) and the initial Curie temperature, Tci;samples with higher Tci generally have a larger x value.This tendency is opposite to that with the stoichiometric(unoxidized) titanomagnetite series, in which the Curietemperature increases as the ulvospinel content (x value)decreases. The present data suggest that samples withhigher ulvospinel content are much more oxidized thanthose with lower ulvospinel content, since oxidationincreases the Curie temperature as well. This is more clearlyrecognized when the results are plotted in the ternarycompositional diagram (Tiθ2-FeO-Fe2θ3) with contours ofthe Curie temperature (Figure 6). Diagrams are plotted forthe three holes with the most samples. The solid and hollowcircles in the figure distinguish reversible and irreversiblethermomagnetic behavior. Distinct groups of the reversibleand irreversible samples are obvious in the ternarydiagrams.
Contours of the Curie temperature shown in this figureare after Ozima and Sakamoto (1971), whose data aresomewhat different from those given by Readman andO'Reilly (1972). Recent experimental study shows that bothcontours need some revision. The general tendencyconsidered here is, however, clearly demonstrated in eitherof the postulated diagrams. The circles in Figure 4 shouldtherefore be understood as a demonstration of thesemiquantitative correlation between Ti (or ulvospinel)content and the Curie temperature, and not as the preciserepresentation of degree of oxidation.
The observed result, that the Ti content is higher in themore oxidized samples, may be explained by either of twoprocesses: (1) The Ti content in the titanomagnetite wasoriginally variable when the rock was solidified, and thehigh-Ti titanomagnetite was more easily oxidized than thelow-Ti titanomagnetite in a similar environment. Thisexplanation assumes that spinel-cubic crystals of
FeTiO3Fe2Ti05
Fe2Ti04
Fe,O,
Fe2Ti05
FeO= 0.25 0.5
(b)Fe,O,
FeTiO3Fe2Ti05
Fe,Ti
FeO
(c)
Fe,O, Fe 2O 3
Figure 6. Ternary diagram showing distribution ofx valuesand the Curie temperatures. (See text for details.) Solid(hollow) circles indicate thermally reversible (irreversible)samples.
titanomagnetite are a closed system relative to Fe and Ti,and that only O atoms can get in and out of the system. (2)The Ti content in the titanomagnetite was originally uniformin most of the basaltic units, with an x value of about 0.6(the minimum asymptotic value in the least oxidizedsample), and later, Fe 3 + ions left the system to form thereddish staining in silicates (as fine hematite or goethite) asthe low-temperature oxidation proceeded (Johnson andHall, in press), causing gradual increase in Ti content of theoxidized titanomagnetite (titanomaghemite). In this case,the spinel-cubic phase is an open system relative to Fe 3 + .
Microscopic observation described in the followingsection indicates that the second explanation is more likely,since highly oxidized titanomagnetite is usuallyaccompanied by reddish brown staining composed of ironhydroxides. The titanomaghemites observed to have xvalues greater than 0.70 may be the result of oxidation of x= 0.60 titanomagnetites in which iron is removed from the
801
K. KOBAYASHI ET AL.
lattice to form the red stain (Johnson and Hall, in press),thereby increasing the Ti content relative to Fe. If so, theoceanic ridge basalts must have had a natural remanentmagnetization originally of thermal origin, because theCurie temperature of unoxidized stoichiometrictitanomagnetite with x = 0.60 is about 190°C, which issufficiently high to hold a thermoremanent magnetization.
If titanomagnetite with x greater than 0.80 were to formwhen the lava crystallized, it could not acquire athermoremanent magnetization, since its Curie temperatureis below room temperature. Natural remanent magnetizationobserved with such samples may be a chemical remanentmagnetization, acquired during oxidation soon afterformation of the rock. This remanence then may not recordthe geomagnetic field at the exact time of ocean floorspreading (Nagata and Kobayashi, 1963). Low-temperatureoxidation of oceanic basalts by warming in zero field (Parkand Irving, 1970) has, however, produced magneticdirections in the same direction as the samples had prior toheating. Laboratory experiments by Banerjee (1966) andSenno and Tawara (1967) have suggested the possibility ofcoupling of the remanences in the formation of a new phasefrom an original phase. Thus, it is not yet clear what themechanism of remanence formation would be intitanomagnetites with x values greater than or close to 0.80.
Although the formation of titanomagnetites with valuesof x 3* 0.80 at the time of crystallization cannot definitelybe excluded, our data suggest that they are the products ofoxidation. Most values are in the range of 0.60 to 0.70(average = 0.66), in agreement with a general abundance ofx values = 0.65 in previous studies (see Johnson and Hall,in press). This, coupled with the hypothetical mechanism ofoxidation which includes removal of iron from the lattice toform the red staining in the more oxidized basalts (Johnsonand Hall, in press), suggests the mechanism of Tienrichment by Fe removal to produce our observedcorrelation of greater x value with greater oxidation (higherTc).
OPAQUE MICROSCOPY
Forty-eight polished thin sections were examined(sections were made aboard the Glomar Challenger; ten ofthem were subsequently repolished on shore). Individualsections do not generally correspond to samples on whichthermomagnetic and Fe/Ti measurements were made (Table1). However, observations within the group of sectionsfrom each site seem to correlate with the general pattern ofthermomagnetic, rock magnetic, and paleomagneticmeasurements for that site. The reflected lightmeasurements have been categorized, as best as possible,into the oxidation stages 1 through 5 of Johnson and Hall,(1978).
Most of the observations were made using the ScrippsInstitution of Oceanography Reichert Microscope with100-power oil immersion lens and 6.3-power eyepiece.About one-fifth of the samples were later examined at theUniversity of Wyoming, using a Leitz microscope,100-power oil immersion lens and 12.5-power eyepiece.The size calibration of the Reichert field of view was poorlyestablished, and for that reason there is uncertainty in thenumbers quoted for grain size. These numbers should be
used only for internal comparison among samples; theabsolute value may be inaccurate. Where applicable,estimates of grain size are of effective grain size, and takeinto account cracking of grains.
Quantification of observations is difficult. We attemptedto present the material in tabular form, as given in Table 2.The color of the titanomagnetite grains (for ease ofdiscussion, the oxidized titanomagnetites described hereinare referred to simply as "titanomagnetites") ranges frommedium brown (MB in Table 2), defined by Hall (personalcommunication) as the color of unoxidized titanomagnetite,to bright gray-white (>GW), characteristic of fairly heavilyoxidized material. The intermediate colors used in Table 2are gray-brown (GB), gray (G), and gray-white (GW).Some grains have distinctive brown rims (Rims).Subdivision of the grains by cracking (see Johnson andHall, 1978) is described in terms of the average number ofcracks present per grain. Lightly oxidized grains have zeroto one crack per grain; the most heavily oxidized grainsobserved in these sites generally have six to eight cracks pergrain. Relative to other deep-sea basalts examined, thesebasalts are all fairly unaltered, and correspond at most to astage 3 or a stage 4 oxidation. Reddish or red-brownstaining of the silicates is another indication of the degree ofoxidation. Except samples from Holes 410 and 410A,staining is slight to absent, again indicating a low stage ofoxidation.
Description of the grain shape is difficult, because mostsamples contained several shapes in roughly equal amounts.The following terminology attempts to describe the averageshapes observed. "Skeletal" (SK) means grains in theshapes of crosses and fishbone patterns, reflectingpreliminary crystallization of the cubic magnetite structure(see grain shapes in Figures 1 and 2, Johnson, in press; fig.12 and 13b, Johnson and Hall, 1978). "Triangular" (Tr) isanother shape reflecting the cubic structure of magnetitecrystals, but "triangular" grains are more equidimensionalthan "skeletal" grains. Further growth of the grain givesmore boxlike equidimensional shapes as completion of themagnetite cubic crystal shape is approached; grains withsuch shapes are designated squarish (Sql and Sq2) in Table2. They are illustrated by the shapes in fig. 13a of Johnsonand Hall (in press) fig. 1, 2, and 5 of Hall et al. (1976), andare similar to fig. 3 of Johnson (in press). Sql still suggeststhe skeletal cross pattern to the observer (shape in fig. 1,Hall et al., 1976); Sq2 has gone to a more square-shapedgrain (fig. 5, op. cit.) Still other grains cannot be describedin terms of a regular pattern. These subhedral grains aredenoted "irregular" (IRR) in Table 2 (fig. 14 of Johnsonand Hall, 1978). Certain samples show irregular grains thathave very irregular boundaries resembling a "moth-eaten"(Moth) appearance. This is not the same as corroded grainboundaries. We advance no explanation here for that shape.Some samples contain grains too small to see accurately.These are termed dust (Dust), and are frequentlydisseminated within silicate grains along crystallographicplanes. Since cracking in most cases changes the effectivegrain shape, the dominant grain shape is designated: E =approximately equidimensional grains and L = elongategrains. Parentheses are used to indicate additional smallcomponent (<20%) of another shape.
802
MAGNETIC MINERALOGY OF BASALTS
TABLE 2Summary of Microscopic Features of Selected Samples
Core-Section
Hole 407
35-136-338-139-345-146-3
47-1
Hole 408
36-236-537-137-3
Hole 409
7-78-1
12-115-215-215-5
15-621-122-124-528-132-1
Hole 410
39-1
39-139-139-5
41-1
Hole 410A
1-72-2
2-4
2-5
3-1
3-2
4-1
4-25-15-46-2
Hole 411
1-12-25-1
Depth(m)
319.9332.6348.5361.0434.2446.7
453.0
334.1339.0343.4346.3
81.182.7
120.9149.9150.9154.7
155.8205.0215.5240.2273.0310.8
359.1
359.8360.2365.1
378.1
334.0336.9
339.4
341.4
344.8
346.6
354.0
355.9363.7368.5374.9
74.383.1
110.1
Color
MBMBG-GWGG-GWToo Small(rims)G
GWGGWGW
MBMBGBGBGGB
BGGWMB>GWG (Rims)
G, Rims
G, Rims- No VisibleG, Rims
G, Rims
G, RimsGB, Rims
>GW, Rims
GW, Rims
GW, Rims
GW, Rims
G, Rims
GW&BGGG, Rims
MBGBG
Cracking
0-10-13-81-20—
3-8
00-50-10
03-42
2-30-12-8
0-2
0-1001-2
0-1
0-1Magnetite -
0
0-1
Too smallToo smalljO-1 on1 large^ 0-1?1-3 onlarge
0-1 onlarge
2-3 onlaree
Too smallToo small
1-2?Too small
0-100
Staining
000
V. Slight0
Red Brn
0
Red BrnSlight
00
0Red Brn Areas
SlightV. Slight
0Red Brn in
heavily cracked areasSlight
Red BrnRed Brn
0Slight
0
Red
Red BrnSlightRed
Red Brn
RedYellow
V. Slight
Slight
Red Brn
Red Brn &Yellow
Slight
SlightBrownSlight
Red Brn
00
Slight
Size (µm)
0.51
0.51
0.5<0.25
0.5
0.23
0.51
0.5
0.250.5
1
13
0.521
0.5
2
1
0.25
0.5
0.25
0.25
0.5
0.5
0.5<0.25<0.25
<O
211
-30-40_ 5- 5- 5- 0 . 5
- 5
- 5- 4 0- 3- 15
-424- 0 . 5- 5-8
8- 10- 5- 8- 4- 3
2
_ 3
- 6
_ 3
0.025- 2
- 10
• 5
- 5
- 1
- 2
- 5- 2- 2
.25
- 5- 4- 4
Accessories
ilmenite,>ilmenite,ilmenite,
<ilmenite,
hydrox,hydrox, S,
>ilmenite,hydrox, S,
S
s,
<S
ss
<S
s<S
<S
s<S
ss
>S
sBS
sBSBS
BS?<S
s<S
« S
ilmenite,spinel?
>S
sBS
Shape
Sq2, Irr, MothSq2Sq2SK, Sq2IRR, MothToo small
Very Moth
Sql,Sq2,Irr, Sql,Sq2Sq2, Irr
Irr, (SK)Irr, (SK)Very SKSK, (Tr)SKSq2
SK (Tr)Sql,Sq2Sq2, (IRR)
Tr, Sq2SK, Sq2,
TR
Tr, (SK)No Crystalline Str. Glass —
ilmenite,spinel
spinel, S
spinel?,ilmenite,
ilmenite?,ilmenite?,
s
s
>S
s<S
, BS
ssss
sss
IRR
SK
DustDust
SK,IRR
SK and Dust
IRR, Tr, Dust
Tr
IRR
IRR, (Moth, SK)Dust, SKDust, SK, SqlDust
SK, IRRIRR, SKIRR, Moth
EEE
L, EE
—
1
E, LE, L
E?
EEL
L, ELE
E,(L)E,(L)
E
EE, (L)
E
E
E
L, E
E?E?
E, L
L?
E
E, (L)
E
E, LL, EE, L
L, EE, L
Stage
22333
34
3
3-43 433
23333
3-4
233233
4
4
4
4
43-4
3
4
4
4
4
3-4334
223
803
K. KOBAYASHI ET AL.
TABLE 2 - Continued
Core-Section
Depth(m) Color Cracking Staining Size (µm) Accessories Shape Stage
Hole412A1-12-25-17-2
11-3
Hole 4131-2
156.3167.3194.5215.1254.7
110.4
MBMBMBMBMB
GB
Rims
00-10-10-10-2
Slight000
Red Brn
7 - 1 51 0 - 4 05 - 2 02 - 1 5
0.5 - 5
2 - 5
>ilmenite,
>ilmenite,spinel,
>SS
ss
>S
s
Sq2Sq2SK, SqlSq2Sq2
Sq2, Tr
E,(
L,E,(E,C
DEE
L)L)
E
1-21-21-21-21-2
2-3
Note: Sizes quoted are only rough estimates of the most abundant grain sizes (see text). In large ranges, the most common size is denoted byitalic. In accessory description, < = small amount, > = large amount (as discussed in text). See text for explanation of other symbols.
The main accessory minerals noted are ilmenite, yellowsulfide (S), occasional spinel, and occasional blue sulfide(BS). Ilmenite usually occurs as discrete grains, or veryrarely as overgrowths on magnetite, but usually not aslamellae in magnetite. Yellow sulfide is very abundant, andoccurs separately, or attached to, or included in magnetitegrains. Johnson (in press) has suggested a relationshipbetween Curie temperature (and therefore oxidation stage)and the presence of sulfide grains (see fig. 15, Johnson,1978). Blue sulfide grains are usually much smaller thanyellow sulfide grains, but have the same occurrence habit.Spinel is only occasionally present; when present, it occursboth as separate grains and with magnetite overgrowths.Occasionally, the mammary shape of hydroxides occurs inaddition to red-brown staining, and is mentioned in Table 2(Hydrox). Hematite was not seen in this study. Relativeabundance of accessory minerals is indicated by thesymbols > and < . If an accessory mineral was notablylarge (>) or small (<) in amount (relative to the othersamples examined), it is so indicated.
The most noteworthy observations at each site will bediscussed in the following paragraphs. Hole 407 has at leasttwo different petrographic units. The uppermost samples inthe hole have much fresher-looking titanomagnetite grainsthan the ones below. The difference is most strikinglyindicated by the brown color of the magnetite in theseuppermost samples, as against gray below. The difference isalso indicated by the observations that the opaque grainsfrom higher samples are larger and less cracked than theirlower counterparts, and contain no ilmenite. This unitappears, from its fresher nature, to be much younger thanthe underlying units. Indeed, the magnetic polarity of thesamples is normal, relative to the reversed ones below, andmedian destructive fields (MDF) are very much lower thanthe reversed ones, particularly those reversed ones notapparently having a superimposed secondary normalmagnetization. Microscopic observation did not delineatethe thermomagnetic boundary in the lower part of the hole(see thermomagnetism discussion).
Site 408 shows a slight difference between the upper twosamples and the lower two. The difference is reflectedprimarily in the reddish staining in the upper flows, andagrees with the observation by petrologists (409 Hole
Summary) that the upper part is more oxidized (and is buff)whereas the lower part is more reduced (blue-gray).
At Site 409, optically fresh and oxidized specimensalternate in a manner similar to the alternation of Curie pointvalue, and of MDF and NRM intensity.
Holes 410 and 410A are striking in their high degree ofoxidation and fine grain size. Most specimens contain afine-grained titanomagnetite with brown rims on all grains.The titanomagnetite is gray to gray-white (occasionallybright gray-white), indicating a high degree of oxidation. Inmany cases, grains are too small to determine amount ofcracking or even grain shape. When cracking is estimated inTable 2, it is on the largest grains and is thus not necessarilyrepresentative. All samples have the reddish staining.Spinel is present occasionally, in contrast to other sites.
Opaque mineralogical variations at Site 411 reflect themagnetic divisions indicated by inclination and MDF.Reflected-light microscopy shows the lowermost sampleexamined to be more oxidized than the ones above (seeTable 2). Hole 412A, older than 411, appears from opaquemicroscopy and thermomagnetism measurements to containthe least oxidized material examined in Leg 49 basalts. Allof the titanomagnetite is still medium brown, little crackingis apparent, sulfides are abundant, and grains are large. Theone sample from Site 413, just across the fracture zone andolder, has a higher degree of oxidation.
We conclude from the observation of opaques in reflectedlight that, in general, the degree of oxidation predicted byCurie points and examination of opaques is the same foreach site or portion of a site. The lesser degree of oxidationtends to be associated with opaques of larger grain size.There is a general lack of high-temperature oxidation in allof these sites. The degree of oxidation is usually low (2 to 3on the stage 1 through 5 scale of Johnson and Hall, 1978),except for the Site 410 holes. There, the grain size ofopaques and the high degree of oxidation is consistent withthe pillow basalt and tip-of-flow volcanogenic brecciaorigin proposed in the hole summary (Part 1, this volume)for those holes.
SUMMARY AND CONCLUSION
Comprehensive investigation of ferromagnetic mineralscontained in basalt samples collected on DSDP Leg 49
804
MAGNETIC MINERALOGY OF BASALTS
(Reykjanes Ridge and northern Mid-Atlantic Ridgetraverse), using combined techniques of thermomagneticmeasurement, electron microprobe analysis, and reflectedlight microscopy, suggests the following conclusions:
1) The degree of low-temperature oxidation oftitanomagnetite is not necessarily related to the age of therocks.
2) The degree of low-temperature oxidation does notdirectly depend upon the sub-bottom depth, either. A250-meter column of basalt, penetrated in Hole 409, showssamples with alternating degrees of oxidation throughoutthe hole. The degree of oxidation does depend to a certainextent on lithology.
3) The degree of oxidation is surprisingly low at all ofthe Leg 49 sites except at Site 410, where the rocks are ofbreccia and closely related origins.
4) The Curie temperature of the basaltic layer wascertainly higher than room temperature and possibly higherthan 150°C when the rocks initially erupted at themid-oceanic ridge. Thus, at least the upper 1 km of oceanicLayer 2 acquired thermoremanent magnetization at the timethe layer was formed.
The lack of relationships between oxidation and age ordepth suggests that oxidation is probably more influencedby hydrothermal circulation in the basement. Suchcirculation is probably related to the amount of fracturing ofthe basement and to the grain size of the basalts. Theseconcepts and the correlation of the magnetochemistry,discussed here, with other rock magnetic properties such assusceptibility, saturation magnetization, and coercive force,as well as with the paleomagnetism results of inclination,intensity, and stability of the natural remanentmagnetization, will be discussed in the following article.
ACKNOWLEDGMENTS
The generous help of Dr. J. M. Hall, Department of Geology,Dalhousie University, in teaching one of the authors (Steiner) thetechniques of opaque microscopy, is greatly appreciated.
REFERENCES
Banerjee, S.K., 1966. Exchange anisotropy in intergrownmaghemite and haematite, Geophys. J., v. 10, p. 449-450.
Carmichael, C M . , 1970. The Mid-Atlantic Ridge near 45°N, 7Magnetic properties and opaque mineralogy of dredgedsamples, Canadian J. Earth Sci., v. 7, p. 239.
Hall, J.M., Fink, L.K., and Johnson, H.P., 1976. Petrography ofOpaque Minerals, Leg 34. In Hart, S.R., Yeats, R.S., et al.,Initial Reports of the Deep Sea Drilling Project, v. 34:349-362.
Irving, E., Park, J.K., Haggerty, S.E., Aumento, F., andLoncarevic, B., 1970. Magnetism and opaque mineralogy ofbasalts from the mid-Atlantic ridge at 45°N, Nature, v. 228,p. 974.
Johnson, H.P., in press. Opaque mineralogy of the igneous rocksamples from Hole 395A — DSDP Leg 45. In Melson, W.G.,Rabinowitz, P.D., et al., Initial Reports of the Deep SeaDrilling Project, v. 45: Washington (U.S. GovernmentPrinting Office).
Johnson, H.P. and Hall, J.M., 1978. A detailed rock magnetic andopaque mineralogy study of the basalts from the Nazca Plate,Geophys. J. Roy. Astron. Soc, v. 52, p. 45-64.
Nagata, T. and Kobayashi, K., 1963. Thermo-chemical remanentmagnetization of rocks, Nature, v. 197, p. 476-477.
Ozima, M. and Sakamoto, N., 1971. Magnetic properties ofsynthesized titanomaghemite, /. Geophys. Res., v. 76, p.7035.
Ozima, M., Ozima, M., and Kaneoka, I., 1968. Potassium argonages and magnetic properties of some dredged submarinebasalts and their geophysical implications, J. Geophys. Res.,v. 73, p. 711.
Park, J.K. and Irving, E., 1970. The Mid-Atlantic Ridge near45°N. XII. Coercivity, secondary magnetization, polarity, andthermal stability of dredge samples, Canadian J. Earth Sci.,v. 7, p. 1509-1510.
Readman, P.W. and O'Reilly, W., 1972. Magnetic properties ofoxidized (cation-deficient) titanomagnetite, J. Geomag.Geoelect., v. 24, p. 69-90.
Senno, H. and Tawara, Y., 1967. Exchange Anisotropy in themixed phse of y-Fβ2θ3 and α-Fe2θ3, Japanese J. AppliedPhys., v. 6, p. 509-511.
805