t. furuta, k. kobayashi, k. momose - deep sea …. furuta and k. kobayashi, ocean research...

41. MAGNETIC PROPERTIES OF IGNEOUS ROCKS OF THE PHILIPPINE SEA,DEEP SEA DRILLING PROJECT LEG 58

T. Furuta and K. Kobayashi, Ocean Research Institute, University of Tokyoand

K. Momose, Department of Geology, Shinshu University, Matsumoto, Japan

ABSTRACTThe Curie temperature and thermomagnetic behavior of whole-

rock samples were measured in basalts recovered from Sites 442, 443,and 444 of DSDP Leg 58 in the Shikoku Basin, and from Site 446 inthe Daito Basin, north Philippine Sea. Chemical composition andmicroscopic features of opaque oxides in the same samples were alsoinvestigated. Degree and mode of oxidation of titanomagnetite varyirrespective of site, lithology, or magnetic polarity, and no systematiccorrelation has been found between any two of these characteristics.Magnetic properties are systematically different between massiveflows recovered at Hole 444A (Shikoku Basin) and Hole 446A (DaitoBasin), although the controlling factor is unknown.

INTRODUCTION

Alternating sequences of normal and reversed mag-netic polarity were found in basaltic layers at Sites 442,443, and 446 of Leg 58. At Site 444, the entire basalt se-quence penetrated by drilling ( — 60 m) is reverselymagnetized, opposite to the positive magnetic anomalyat this site, attributed to normal magnetization of rocksbelow the depth of penetration (see Site reports, thisvolume). In this paper, the correlation between magnet-ic polarity and degree of oxidation has been carefullyexamined in view of the possible origin of reversed mag-netization by self-reversal.

Pillow basalts occur at Sites 442 and 443, whilebasalts at the other sites and those overlying and inter-calated with the pillows are massive flows. Some mas-sive basalts were identified as sills or sheets intruded in-to sediments. Possible differences in mineralogy of ironoxides between pillow and massive basalts—such as ti-tanium content and degree of oxidation—have been ex-tensively investigated.

THERMOMAGNETIC MEASUREMENTSThermal changes in saturation magnetization, /s,

were measured on 28 samples from Site 442, 27 samplesfrom Site 443, 10 samples from Site 444, and 42 samplesfrom Site 446. A small chunk of rock (~ 100 mgs) washeated at 6°C/min in a magnetic field of approximately4300 oe after the sample was heated at 620° C for 30minutes, it was cooled to room temperature at the samerate. The cycle of heating and cooling was accomplishedin a vacuum of 10~4 torr.

Two types of thermomagnetic curves were recog-nized: thermally reversible (Figure la) and irreversibletypes (Figures lb-d); both types are normally foundamong submarine basalts. In the reversible samples, theCurie temperature (Tcl) is easily determined, as shown

in Figure la. The Curie temperature markedly decreasesin some samples (e.g., 442B-8-1, 444-27-1) after heatingat 620° C. (The Curie temperature after heating at 620° Cis denoted by Tch).

With irreversible curves, more than two Curietemperatures, (Tcl, Tc2, Tc3,....Tcl) can be distinguished.The Curie temperature revealed in the cooling process isusually that which is nearly the same or slightly lowerthan the highest Tci seen in the heating process. It hasbeen concluded that the recognition of more than twoCurie temperatures in the heating process is not due tocoexistence of multiple magnetic phases in the originalrock, but is caused by Stepwise inversion of titanomag-hemite (spinel-cubic form of titanomagnetite oxidized ata low temperature) to less-titaniferous magnetite athigher temperatures (Ozima et al., 1968; Ozima andLarson, 1970). The Curie temperature of less-titanifer-ous magnetite is higher than that of magnetite richer intitanium (Akimoto and Katsura, 1959).

Ratios of intensities of saturation magnetization atroom temperature before and after heating at 620° C,(Jho/Jo) were calculated from the thermomagnetic mea-surements and are listed in Table 1. The ratio variesfrom 0.5 to 4.0, but most frequently has values ofaround 1.0 in the samples investigated. These values arelisted in Table 1 together with the Curie temperatures.

MICROSCOPY OF OPAQUE MINERALSOpaque minerals were examined in polished sections

by reflected-light microscopy. Titanomagnetite was dis-tinguished from ilmenite by its color and anisotropy,and from sulfide by its color. The majority of titano-magnetite and titanomaghemite grains are optically uni-form, as seen in Figure 2a,c. Some titanomagnetiteshave well-developed cracks (Figure 2d), similar to thosereported by Hall et al. (1976), Johnson and Hall (1978),and Kobayashi et al. (1979). Figure 2e shows titanomag-

923

T. FURUTA, K. KOBAYASHI, K. MOMOSE

1.0 -

442B = 8-7, 107-109 cm

100 200 300T< C)

200 300 400TCC)

500

Figure 1. Thermomagnetic curves of typical samples (W = 4300 oe;P = 10~4 ton).A. Thermally irreversible type, typical of submarine basalts; 442B-5-1, 116-118cm. B. Thermally reversible type, with evidence of high-temperature oxidation;442B-8-7, 107-109 cm. C. Thermally reversible type, unoxidized sample; 444A-20-1, 71-73 cm. D. Thermally irreversible type, common in Hole 446A; 446A-18-3, 83-85 cm.

netite containing ilmenite exsolution lamellae along{111} planes. Figure 2f shows a skeletal grain of titano-magnetite which appears to have been oxidized at a hightemperature.

ELECTRON-MICROPROBE ANALYSISOF OPAQUE MINERALS

The Ti/Fe ratio in titanomagnetite and ilmenite, ifthe grain size is larger than 10µm, was determined by aJXA-5 electron-probe microanalyzer. The compositionwas determined in more than 20 grains of one specimen,

and the results were averaged, although intergrain scat-ter is generally small.

The molecular percentage of Ti to Fe is shown inTable 1. The contents of other elements were measuredin the same grains. Figure 3 shows concentration pro-files across a grain of titanomagnetite. It is recognizedthat the TiO2 content is roughly proportional to that ofFeO, while that of A12O3 is inversely proportional tothat of FeO and TiO2. Contents of MnO, MgO, andV2O3 are less than 1 weight per cent in all tested titano-magnetite grains. The A12O3 content ranges from 0.7 to

924

MAGNETIC PROPERTIES OF IGNEOUS ROCKS

TABLE 1Summary of Magnetic Properties and Chemical Composition of Opaque Minerals in DSDP Leg 58 Basaltic Samples

Sample(interval in cm)

442B-3-3,118-1204-1,117-1195-1,116-1186-1,48-506-2,110-112

7-1,59-617-1,106-1088-1,47-498-2, 24-268-3, 55-57

8-5, 23-258-6, 84-868-7,107-1099-1,24-269-1,33-35

9-1,103-1059-2, 73-759-3, 32-3411-1,60-6211-3,18-20

13-1,60-6213-3,2-416-1,29-3116-1,34-3616-2,24-26

17-1,104-10619-2,62-6420-1,27-29

443-49-3,137-13949-4,102-10450-2, 30-3252-1,33-3552-4, 21-23

53-2,12-1454-2, 54-5654-3,125-12754-5, 146-148

54-7, 76-7855-1,99-10156-1,117-11956-3, 120-12258-1,61-63

58-3, 34-3658-4, 6-8

443-59-2,15-1759-3,12-1460-1,17-1960-2, 110-11260-4,125-127

61-2, 103-10562-1,90-9262-4, 62-6463-2, 71-7363-5,126-128

63-8, 84-86

444A-20-1, 27-2920-1,71-7320-4,46-4823-2, 87-8923-2,138-140

25-1,49-5125-3,92-9426-2, 5-727-1, 90-9227-5, 39-41

446A-2-1, 145-1472-3,41-433-3, 50-523-4, 34-364-1,32-34

Typea

(R)(R)R

(R)R

RR

(R)(R)(R)

(R)IIII

(I)R(I)I

(R)

IRII

RIIII

(R)I

I(I)RIR

R

RRRRR

RRRRR

I(1)IRR

Curie Temperature

^cl

295410360390470

345270300365470

250205550570570

450_

390365335

315380250285340

440220380

210155270325145

240100230290

105315390280410

515240

200170190185125

250240270240365

120

265180235250310

445460185425290

250435230240245

T ?

(

525555540540

-

440-—--_----

__

550555525

520550535550

-

535510560

310_-

490-

480-

470570

__

560460

-

_

410

425_-

525-

490420515475525

-

_

-__

450

____-

455_

460_-

T i,

C)

555570560570565

___—-_—_—-

590550570560555

580560580570580

555_

570

_

__

540-

550__-

___

490-

_

-

_

--_-__

575-

570

-

_

___-

__——-

_

___-

T h

530535525530545

370360245285410

195200535550550

550525490535525

530530510530545

530475535

525230540560210

535110505560

140535570540555

540460

450500200545160

520480560515575

125

215140200205300

275_

120210160

365260400200230

3.23.13.03.21.8

1.11.20.90.950.9

0.951.00.52.32.1

1.52.63.14.23.4

3.63.82.73.65.1

3.41.94.1

1.91.11.33.61.2

3.51.04.15.2

1.03.24.43.52.0

_

-

2.21.50.91.81.0

_

2.54.12.74.0

0.95

0.940.960.950.941.03

0.950.920.880.910.92

1.50.81.00.80.9

Ti/Fe(based onmol %)

21.826.8

_-_

26.223.9

_24.2

23.9_

29.432.4

-

25.5__

-__

25.724.9

-__-

_

28.4__-

_-

36.2-

27.7___-

_

-

_

29.927.1

_29.0

__

27.9--

28.3

22.325.9

_24.3

-

__

28.824.2

-

_

___

26.5

Titanomagnetite

x Valueb

0.5370.633

_-_

0.6330.579

_0.585

0.580_

0.7060.734

-

0.610___-__

0.6140.599

-_

-

_

0.663

_-

_-

0.797-

0.650___-

_

-

_

0.6910.637

_0.674

__

0.654_-

0.662

0.5460.617

_0.586

-

__

0.6700.585

-

_

_—

0.628

A12O3

(wt. %)

2.72.6_-

_

1.21.1_1.4

1.7_

0.71.1-

1.2__—-

_

_2.02.1-

_

--

_

1.0-_-

_

__-

1.6_-_-

_

-

_

1.51.8_1.5

_

_2.2_-

-

2.92.3_1.6-

_

_4.11.8-

_

___

2.1

Size(µm)

30-5010-20

1010-20

_

200-300150-200

_300400

100-200_

200150-200

-

100__

<IO<IO

_—

304050-70

70

<550-70

5-7

30030015090

300

12020

4003

3010103030

60150

300150-300

180250420

100-150180

10-30100-150

60

300

150150_

100150

500200500300200

70-100<IO

—_

150-200

IlmeniteTi/Fe

(based onmol%)

___-____-

86.4_

82.995.4

-

99.0__—-___—-_--

_

91.6-_-

_—

95.6-

_—-_-

-

_

84.288.1

—92.0

_----

91.4

89.492.2

__-

____-

_

_-_

88.1

Remarks

___-___—

11 lamella

_—

11 lamella--

__-

Sulfide-_----_--

_

----

--—-

_—-_-

-

-

_

_---_

--

-

_

-_--

11 lamella11 lamella

-Il-rich

-

_

Sulfide-rich-_-

Rock Texture0

pl-cpx hyalo-ophitic basaltpl-cpx subophitic doleritepl-cpx hyalo-ophitic basaltpl-cpx hyalo-ophitic basalt

_pl-cpx subophitic doleritepl-cpx subophitic dolerite

-pl-cpx subophitic dolerite

pl-cpx subophitic dolerite-

pl-cpx subophitic doleritepl-cpx subophitic dolerite

-

pl-cpx intersertal basalt--

pl-cpx intersertal basaltpl-cpx intersertal basalt

_-

pl-cpx intersertal basaltpl-cpx intersertal basaltpl-cpx hyalo-ophitic basalt

pl-cpx intersertal basaltpl-cpx hyalo-ophitic basaltpl-cpx intersertal basalt

pl-cpx*-ol ophitic doleritepl-cpx* ophitic doleritepl-cpx* ophitic doleritepl-cpx hyalo-ophitic basaltpl-cpx-ol hyalo-ophitic basalt

pl-cpx hyalo-ophitic basaltpl-cpx-ol intersertal basaltpl-cpx-ol ophitic doleritepl-cpx hyalo-ophitic basalt

pl-cpx hyalo-ophitic basaltpl-cpx hyalo-ophitic basaltaphyric basaltpl-cpx intersertal basaltaphyric basalt


pl-cpx-ol ophitic doleritepl-cpx-ol ophitic doleritepl-cpx ophitic doleritepl-cpx-ol subophitic doleritepl-cpx-ol ophitic dolerite

pl-ol intersertal basaltpl-cpx intersertal basaltpi intersertal basaltpl-cpx subophitic doleritepl-cpx subophitic dolerite

pl-cpx-ol ophitic dolerite

ρl-ol-cpx*-ka ophitic doleritepl-ol-cpx*-ka ophitic dolerite

-pl-cpx-ol subophitic doleritepl-cpx ophitic dolerite

pl-cpx ophitic doleritepl-cpx ophitic doleritepl-cpx-ol ophitic doleritepl-cpx-ol ophitic doleritepl-cpx-ol subophitic dolerite

pl-cpx*-ka subophitic doleriteaphyric basalt

--

pl-cpx*-ka subophitic dolerite

925


TABLE 1 - Continued

Sample(interval in cm)

446A-19-2,14-1619-5,18-2020-3,140-14221-1,82-8421-3, 79-81

21-6,16-1822-1,145-14722-4, 95-9723-2,43-4523-6,101-103

24-1,43-4525-1,13-1525-3,41-4326-3, 44-4626-3,142-144

27-1,77-7928-1,100-102

4-2, 44-465-1,16-185-2, 24-266-1, 10-126-3,109-111

7-4, 138-1409-1, 82-8410-5,110-11211-1,69-7111-2,23-25

12-1,53-5512-2,69-7113-1,107-10914-1, 102-10414-3,66-68

15-3,58-6016-2,44-4617, CC18-3,83-8519-1,69-71

Type*

(I)IIII

I(I)(I)(I)(I)

IR(I)

(

::

)

(i)(i)(i)

(i)RRI

(I)

II

(I)II

Curie Temperature

Tcl

505390_

315365

370405390460465

290290410305315

290380

280260280300280

285290445510350

265280150400350

310310515350-

T

_

460470440450

455460455

—-

455_

455435425

455450

440420450470445

440440510545460

435__

465440

445445

_450495

T Tc3 ch

:)

440405445415410

440435410365

- 425

430- 215

385400475

420- 360

- 415370430440430

425430520465

- 395

_ _

22070

435405

410- 395

500415455

•W o

0.91.01.11.11.15

1.10.91.00.91.0

1.40.90.91.01.3

1.20.9

1.31.21.31.41.2

1.21.20.90.90.9

1.00.91.01.00.9

1.01.00.81.11.0

Ti/Fe(based on

mol%)

_

___-

_

22.622.017.125.0

34.4___

23.4

_

-

20.6___

28.2

24.227.8

__

29.3

_

25.427.6

_-

_

_

_-

Titanomagnetite

x Valueb

_

___-

_

0.5540.5410.4370.604

0.768___

0.569

_

-

0.512___

0.659

0.5850.653

__

0.680

_

0.6080.649

-

_

-—_-

AI2O3(wt. %)

_

__-

_

3.13.12.51.9

2.8___

2.2

_

-

4.6___

3.0

2.92.6__

2.2

_

3.95.8—-

_

___-

Size(µm)

_

20-30__-

_

100100-20040-60

100-150

10-30___

70-100

10-2015-20

80-100__

80-100100-150

100-20050-100

_70-100

150-200

_

150-20070-100

--

50-70_

30-50150-200

IlmeniteTi/Fe

(based onmol %) Remarks

_

- -_ __ _-

_ _

79.676.374.184.5

Il-rich_ __ __ _

79.2

_ _

-

85.7_ -_ _- -

86.0

79.084.2

_ —- -

89.5

_ _

96.6— -- —-

_ _

_ -_ -

Il-rich- -

Rock Texture0

_

pl-cpx hyalo-ophitic basalt__-

_

pl-cpx intersertal basaltpl-cpx intersertal basaltρl-cpx*-ka intersertal basaltpl-cpx subophitic dolerite

pl-cpx intersertal basalt-_

pl-cpx ophitic dolerite

pi intersertal basaltpi intersertal basalt

pl-cpx*-ka subophitic dolerite—_

pl-cpx*-ka ophitic doleritepl-cpx subophitic dolerite

pl-cpx*-ka subophitic doleritepl-cpx intersertal basalt

-pl-cpx*-ka subophitic doleritepl-cpx dolerite ophitic

_pl-cpx*-ka subophitic doleritepl-cpx *-ka subophitic dolerite

--

pi intersertal basalt--


a R = reversible; I = irreversible; parentheses indicate atypical curve.bValue in the formula* Fβ2Tiθ4 (1 -x) Fβ3θ4.ccpx* denotes titanaugite; ka denotes kaersutite.

5.8 (Table 1); its presence would slightly modify the xvalue of titanomagnetite, if it occurs in a solid solutionof magnetite-ulvospinel.

RESULTS OF MEASUREMENTS

Hole 442B

Basalt in this hole is present in upper and lowerlayers, with an intercalated 3-m-thick sediment layer.The upper layer consists only of massive flows; most ofthe lower layer is pillow flows, but these are intruded bytwo massive flows. These rocks are all olivine-depletedplagioclase-clinopyroxene basalts or dolerites.

The upper half of the upper massive-flow unit hasirreversible thermomagnetic properties, with an initialCurie temperature (Tcl) ranging between 300 and470°C. Most samples from the lower half of the uppermassive-flow unit are reversible in a heating-coolingcycle, although some samples taken near the contactswith sediments are irreversible. Both pillows andmassive flows in the lower part of the hole are thermo-magnetically irreversible.

The molecular ratio of titanium to total iron intitanomagnetite was determined mostly in the upper,

massive basalts, because the grains of opaque mineralsare too small in the pillow lavas. The values obtainedare 21.8 to 32.4 (corresponding to x = 0.54 to 0.73).

The initial Curie temperature (Tcl) of the thermallyreversible samples ranges between 205 and 470° C, whilethe x value is concentrated around 0.6 (0.58-0.63).These results are summarized in Table 1 and Figure 4.The Curie temperature of stoichiometric titanomagne-tite with a composition of x = 0.6 is about 200°C (Aki-moto et al., 1957; Ozima and Sakamoto, 1971), which isroughly consistent with the observed values in two sam-ples (442B-8-5 and 442B-8-6). It is thus concluded thatthese two samples are unoxidized at low temperatures.

The observed Curie temperature for Sample 442B-8-3 is 470°C, although the measured x value is 0.59. InSamples 442B-8-7, 24-26 cm and 442B-9-1, 33-35 cm,Tcl = 520to570°C, and* = 0.71 to 0.73. If the titano-magnetite is uniform and stoichiometric, the composi-tion showing Tcl = 470°C is x = 0.2. Microscopic ex-amination of polished sections revealed the occurrenceof fine exsolution lamellae of ilmenite in the titanomag-netite of Samples 442B-8-3 and 442B-8-7. Exsolutionmay have been caused by high-temperature oxidation.The ilmenite lamellae are too fine to be analyzed by elec-

926


Figure 2. Photomicrographs of typical opaque minerals in reflected light.A. Unoxidized titanomagnetite (left) and discrete grains of ilmenite (right);442B-8-1, 47-49 cm; 50-µm scale. B. Titanomagnetite oxidized at a lowtemperature; 443-59-2, 15-17 cm; 50-µm scale. C. Unoxidized titanomagnetite;442B-8-5, 23-25 cm; 50-µm scale. D. Unoxidized titanomagnetite with well-developed cracks; 444A-27-5, 39-41 cm; 100-µm scale. E. Magnetite with il-menite exsolution lamellae along the {111} planes; 442B-8-7; 10-µm scale.F. Large skeletal grain of titanomagnetite oxidized at a low temperature;442B-9-1, 24-25 cm; 50-µm scale.

tron microprobe. The obtained content of titanium isprobably the average of that in titanomagnetite and il-menite. In some samples, discrete, idiomorphic ilmenitecrystals are seen under the reflecting microscope. Theseilmenite grains are sufficiently large for electron-micro-probe analysis, and the molecular percentage of titani-um was determined to be 83 to 99, which is close to sto-ichiometric ilmenite.

The initial Curie temperature of the thermally irre-versible samples varies from 220 to 470° C. The x value

for Sample 442B-5-1, having Tcl = 360°C, is 0.63, andfor Sample 442B-16-1, 29-31cm, having Tcl =250°C, is0.61. The ratio of saturation magnetization before andafter heating at 620°C in a vacuum, (Jho/Jo) is 5.1 forSample 442B-16-2, which may indicate a high degree oflow-temperature oxidation of titanomagnetite.

Hole 443More than seven massive flows and at least four pil-

low lavas were cored in this hole. The rocks are plagio-

927


wt. %

3CO

O 2<

1

24 h

O 22

20

72

9 70

68

170µm

Figure 3. Electron-microprobe profiles for principal ox-ides in one grain of titanomagnetite, Section 443-63-8. FeO represents 2ΔFeO + 1ΔFe2O3.

clase-clinopyroxene-olivine basalts and dolerites. Somecontain titanaugite.

Some samples from the massive flows show thermallyreversible magnetic properties, with Curie temperatureslower than 200°C. Sample 443-54-7, taken from a pillowlava, is also reversible, with a Tcl = 105°C and a Tch =140°C. Its x value is 0.65, corresponding to a Curie tem-perature of about 130° C if the titanomagnetite is stoi-chiometric. The observed values of 7\, and x indicate

that titanomagnetite in Sample 443-54-7 is completelyunoxidized. A slight discrepancy between the observedand expected Curie temperatures may be due to the ef-fect of alumina. The grain size of the titanomagnetite isaround 30 µm, which is relatively large for pillow lavas.Probably the sample is from the core of a large pillowstructure in which slow cooling caused large grain sizes.

Irreversible samples from this hole have Tcl rangingfrom 185 to 390°C, rc h from 450 to 575°C, and Jho/Jo

from 1.3 to 5.2—similar to samples from Hole 442B.Figure 5 summarizes these results. The irreversible changein magnetization is due to inversion of titanomaghemite(a cubic-spinel phase oxidized at low temperatures) toless-titaniferous magnetite.

Hole 444A

All igneous rocks recovered from this hole representmassive flows of plagioclase-clinopyroxene-olivinedol-erite. The upper layers, intruded into sediment, con-tain titanaugite and kaersutite. X-ray-fluorescence anal-ysis indicates that they are alkali olivine basalts (or dol-erites), containing more than 6 per cent total alkalis.

Thermomagnetic properties of these samples are allreversible, and their Curie temperatures range between175 and 445°C (Figure 6). Ilmenite lamellae were recog-nized in three samples (444A-25-1, 444A-25-3, 444A-27-1) showing a Curie temperature higher than 400°C. Thehigh Curie temperature is caused by low titanium con-tent in the titanomagnetite phase after the formationof ilmenite lamellae. The bulk content of titanium inthe opaque minerals is nearly the same as that in theother samples. The A12O3 content appears to be higherfor this hole than for the other two holes in the ShikokuBasin, but its effect on magnetic properties is unknown.

Hole 446A

Igneous rocks recovered from this hole representmassive intrusions of alkali-rich plagioclase-clinopyrox-ene basalts or dolerites. Five of 42 examined samplesshowed reversible thermomagnetic properties, Tcl rang-ing between 150 and 290° C.

Thermomagnetic properties of thermally irreversiblesamples are slightly different from those of samplesfrom other holes. Most remarkable is that the Curietemperature after heating Tch is lower than the highestCurie temperature (Tc2 or Tc3) revealed in the heatingprocess (Figure 7). Jho/Jo is near unity in irreversiblesamples.

The apparent decrease in the Curie temperature afterheating at 620° C is possibly due to inversion of the fer-romagnetic phase having Tc2 or Tc3 to non-ferromagnet-ic ilmenite, as seen in some terrestrial rocks formed bysubaerial eruption (Ozima and Larson, 1967). Althoughthe rocks examined in the present study were not formedin a subaerial environment, they appear to have beenoxidized at moderately high temperatures. The degreeof oxidation in these rocks seems to be great, in spite ofthe relatively large grain size. The causes of this oxida-tion of titanomagnetite are unknown. Magnetic rniner-

928


Hole442B

300

450 "

Curie Temp.(100°C)

0 1 2 3 4 5 6

Ti/Fe(based on mol %)Titanomagnetite

10 20 30 40

o

o

X

Ilmenite70 80 90

TitanomagnetiteGrain Size

(µm)10 50 100 500

(10-3 gauss)1 5 10

MedianDestructive

Field(oe)

50 100 500

xX

«<o o

A pillow lava

• thermally reversibleO thermally irreversibleX partly reversible

Figure 4. Down-hole plots of magnetic polarity, lithology, initial Curie temperature, Ti/Fe ratios in titanomagnetiteand ilmenite, grain size of titanomagnetite, intensity of natural remanent magnetization (in) and median destruc-tive field. Jn and MDF taken from Site 442 report.

als in the intrusive basalts of Hole 444A are almostunoxidized and thermally reversible.

The A12O3 content in titanomagnetite in the Hole446A rocks is 2 to 5.8 weight per cent, nearly twice ashigh as that for Holes 442B and 443. The A12O3 contentin titanomagnetite is richer in alkali rocks (446A and444A) than in tholeiitic rocks (442B and 443).

Because the grain size is sufficiently large, the Ti/Feratio of ilmenite coexisting with titanomagnetite wasprecisely determined by electron microprobe. As seen inFigure 8, there is a positive correlation between Ti/Fevalues for titanomagnetite and coexisting ilmenite. As-suming that titanomagnetite and ilmenite were in equi-librium with the environment and with each other dur-ing crystallization, the temperature and partial pressureof oxygen during crystallization were 930 to 1150°C and10-13 to 10-16 atm, according to the diagram of Bud-dington and Lindsley (1964). The Ti/Fe ratio in titano-magnetite increases as secondary oxidation proceeds atlow temperatures. The values given above thus may pro-vide only a rough estimate of the crystallization con-ditions.

CORRELATION OF THERMOMAGNETICPROPERTIES WITH LITHOLOGY, MAGNETIC

POLARITY AND MAGNETIC INTENSITY

To investigate possible correlations between themagnetochemical characteristics and other properties ofrocks, a histogram of initial Curie temperature (rcl) wascompiled for all samples of massive flows and pillowlavas examined in the present investigation (Figure 9).There seems to be no distinction in Tcl between thepillow lavas and massive flows, although the number ofpillow-lava samples is insufficient for statistical consid-eration. Low-temperature oxidation may be controlledby unknown factors, rather than the mode of extrusion.

Figure 10 shows histograms of Tcl for normal andreversed magnetic polarities for each hole of Leg 58. Nocorrelation can be recognized between Tcl and magneticpolarity, although reversed samples have significantlyhigher Tcl at Hole 446A and generally lower Tcl atHoles 442B and 443 than do normal ones. No evidenceindicating that the reversed polarity is due to self-reversal has been found.

929


Ti/FeCurie Temp. (based on mol %)

(100 C) Titanomagnetite Ilmenite


(µm)

0 1 2 3 4 5 6 10 20 30 40 70 80 90 10 50 100 500

( 1 0 3 gauss)

.5 1 5 10

MedianDestructive

Field(oe)

50 100 500

ISi


Figure 5. Down-hole plots of magnetic polarity, lithology, initial Curie temperature, Ti/Fe ratios in titanomagnetiteand ilmenite, grain size of titanomagnetite, intensity of natural remanent magnetization (Jn), and median destruc-tive field. Jπ and MDF taken from Site 443 report.

Ti/FeCurie Temp. (based on mol %)

(100 C) Titanomagnetite Ilmenite

0 1 2 3 4 5 6 10 20 30 40 70 80 90


(µm)

10 50 100 500

d O ' 3 gauss)

.5 1

MedianDestructive

Field(oe)

5 50 100 500

pillow lava


Figure 6. Down-hole plots of magnetic polarity, lithology, initial Curie temperature, Ti/Fe ratios in titanomagnetiteand ilmenite, grain size of titanomagnetite, intensity of natural remanent magnetization (Jn), and median destruc-tive field. Jn and MDF taken from Site 444 report.

The intensity of natural remanent magnetization (Jn)of each sample measured on board the Glomar Challen-ger (see site chapters, this volume) is plotted versus theinitial Curie temperature in Figure 11. The scatter ofvalues is so large that no correlation is recognized be-tween Jn and Tcl, although the Tcl of a sample having anextremely small Jn is relatively low, and the Tcl of asample having an extremely large Jn is generally high.The values of Jn are not related to the degree of oxida-tion of titanomagnetite in these samples. Low-tempera-

ture oxidation does not seem to systematically alter theintensity of remanent magnetization.

SUMMARY AND CONCLUSIONS

Comprehensive investigation of magnetic mineralogyof basalt from Leg 58, in combination with shipboardPaleomagnetism data for the same samples, suggests thefollowing conclusions:

1. The initial Curie temperature is highly variable, ir-respective of site, lithology, or magnetic polarity.

930


Curie Temp.(100-C)

0 1 2 3 4 5 6

Ti/Fe(based on mol %)Titanomagnetite llmenite

10 20 30 40 70 80 90

450

E 5002

550

600

×X

oo

XX

X

o ×


(µm)

) 50 100 500

X

O ×

(10~3 gauss)

.5 1 5 10

MedianDestructive

Field(oe)

50 100 500

o oo

× X

Sio

opillow lava

thermally reversiblethermally irreversiblepartly reversible

Figure 7. Down-hole plots of magnetic polarity, lithology, initial Curie temperature, Ti/Fe ratios in titanomagnetiteand ilmenite, grain size of titanomagnetite, intensity of natural remanent magnetization (in), and median destruc-tive field. Jn and MDF taken from Site 446 report.

2. No correlation exists between magnetic polarity andthermomagnetic properties, such as initial Curie tem-perature and thermal stability. This implies that thereversed magnetization seen in some layers is not due toself-reversal.

3. No correlation exists between intensity of naturalremanent magnetization and degree of oxidation.

4. Most of the massive flows cored in Hole 446A seemto have been oxidized at a moderately high temperature,while massive-flow samples from Hole 444A are eitherunoxidized or oxidized at a higher temperature. Basalts

from these two holes appear to have had somewhat dif-ferent thermal and chemical histories.

5. In rocks from Hole 446A, a systematic correlationis recognized between Ti/Fe ratios of titanomagnetite andcoexisting ilmenite. Both minerals seem to have beencrystallized at the closing stage of magma solidification.Temperature and oxygen partial pressure during crystal-lization are estimated at 930 to 1150 qC and 10~13 to10~16 atm, based upon a simplifying assumption.

6. Minor elements in titanomagnetite constitute lessthan 1 weight per cent, except for A12O3, which ranges

931


100o

o

_ 90

80

70

o ooo

o o oo

20 30I

40Ti/Fe in titanomagnetite (mol %)

Figure 8. Ti/Fe ratios in titanomagnetite and ilmenite.

from 1.0 to 5.8 weight per cent. The A12O3 content in ti-tanomagnetite seems to be higher in the alkali basalts.

ACKNOWLEDGMENTS

We are very grateful to H. Kinoshita and T. Ishii forvaluable suggestions and critical reading of this manuscript.

REFERENCES

Akimoto, S., Katsura, T., and Yoshida, M., 1957. Magneticproperties of TiFe2O4-Fe3O4 system and their change withoxidation. J. Geomagnet. Geoelec, 9, 165-178.

Akimoto, S., and Katsura, T., 1959. Magneto-chemical studyof the generalized titanomagnetite in volcanic rocks. / .Geomagnet. Geoelec, 10, 69-90.

Buddington, A. F., and Lindsley, D. H., 1964. Iron- titaniumoxide minerals and synthetic equivalents. J. Petrol., 5,318.

Hall, J. M., Fink, L. K., and Johnson, H. P., 1976. Petrogra-phy of opaque minerals, Leg 34. In Yeats, R. S., Hart, S.R., et al., Init. Repts. DSDP, 34: Washington (U.S. Govt.Printing Office), pp. 349-362.

20

2 io

100 200 300 400 500 600

CC)

Figure 9. Histogram of initial Curie temperatures (Tcl)for all Leg 58 sites. Data from massive basalts (hol-low bars) and from pillow lavas (dotted bars) aresuperposed.

Johnson, J. P., and Hall, J. M., 1978. A detailed rockmagnetic and opaque mineralogy study of the basalts fromthe Nazca Plate. Geophys. J. Roy. Astron. Soc, 197,45-64.

Kobayashi, K., Steiner, M., Faller, A., Furuta, T., Ishii,T., Shive, P., and Day, R., 1979. Magnetic mineralogy ofbasalts from Leg 49. In Luyendyk, B. P., Cann, J. R., etal., Init. Repts. DSDP, 49: Washington (U.S. Govt. Print-ing Office), pp. 793-805.

Ozima, M., and Larson, E. E., 1967. Study on irreversi-ble change of magnetic properties of some ferromagneticminerals. J. Geomagnet. Geoelec, 19, 117-128.

, 1970. Low- and high-temperature oxidation oftitanomagnetite in relation to irreversible changes in themag-netic properties of submarine basalts. J. Geophys. Res., 75,1003-1018.

Ozima, M., and Ozima, M., and Kaneoka, I., 1968. Potas-sium-argon ages and magnetic properties of some dredgedsubmarine basalts and their geophysical implications. J.Geophys. Res., 73, 711.

Ozima, M., and Sakamoto, N., 1971. Magnetic properties ofsynthesized titanomaghemite. J. Geophys. Res., 76, 7035.

932


200 300 400 500

Figure 10. Histogram of initial Curie temperatures (Td)for all examined rocks. Da-ta from normally magnetized samples (vertically ruled) and from reversely mag-netized samples (horizontally ruled) are superposed.

933


"» CO

ò

50

10

5

1

.5

.1

-

#

O

XD

_

-

—

-

Hole 442BHole 443Hole 444AHole 446A

O

O

o©

oD

×O9

° 6b

oo

cP

DD

V 9D

O o D

X θO J3 O

•D

DXO CD

XX

m

π

B

t

c

D

•

8D

Φ

• D "θ

1

xD

•• *D

X

O

π

100 200 300

rc1 c400 500 600

Figure 11. Relationship between intensity of naturalremanent magnetization (in) and initial Curie tem-perature (Tcl).

934

t. furuta, k. kobayashi, k. momose - deep sea …. furuta and k. kobayashi, ocean research...

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