Viscous magnetization at 300 K in a profile through Troodos type oceanic crust

PHYSICS OFTHE EARTH AND PLAN ETARY INTERIORS Physics of the Earth and Planetary Interiors 88 (1995) 101-116

ELSEVIER

Viscous magnetization at 300 K in a profile through Troodos type oceanic crust J a m e s M . H a l l *, C h a r l e s C. W a l l s , S. L a t a H a l l Department of Earth Sciences, Dalhousie Uniuersity, Halifax, N.S., B3H 3J5, Canada

Received 13 December 1993; revision accepted 27 July 1994

Abstract

This study examines the question of the relative importance of viscous magnetism, (VM), at 300 K, to the total magnetization of older oceanic crust. A section through the Troodos, Cyprus, ophiolite, now recognized as a good proxy for in-situ oceanic crust, has been used as the source of samples. The section extends downwards continuously from little altered submarine extrusives through greenschist facies sheeted dikes to mafic and variously serpentinized ultramafic cumulate intrusives. The principal result of the investigation is that VM is not expected to be the dominant magnetization, and is often relatively negligible, at all crustal levels. VM acquisition varies irregularly with depth, with predicted maximum values equivalent to about one-third of the total magnetization, in the extrusives and sheeted dikes. The contribution of VM in two samples of serpentinized ultramafics is insignificant. VM acquisition does not show any simple relationships with primary lithology, alteration history, or magnetic properties or history. The occurrence of two-stage VM growth mechanisms is widespread. The second stage typically has an onset time of about 103 rain and an acquisition rate three times that of the initial growth mechanism. The physical origin of this two-stage mechanism, and its significance in predicting VM growth over geologic time intervals, are considered to be important problems for future work.

I. Introduction

The nature of the magnetization of oceanic crust has been a subject of intense interest since Vine and Matthews (1963) suggested that oceanic crust contained a continuous record of the polarity history of the geomagnetic field. Whereas early attention was reasonably focused on the ability of oceanic crustal rocks to acquire rema-

* Corresponding author.

nent magnetization on initial cooling, more recent studies indicate that oceanic crust and mantle rocks become magnetized in a variety of ways and at different times. It follows that there is a need to investigate each of these mechanisms, to estimate the net magnetization of oceanic crust. In this context, viscous magnetization (VM), or time-dependent magnetization in the presence of an external field, and viscous remanent magnetization (VRM), or time-dependent magnetization after removal of the external field, have only received modest dedicated attention. Investigations were initiated by Lowrie (1973), with subse-

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ZM. Hall et aL /Physics Or'the Earth and Planetao, Interiors' 88 (1995) 101 1 l~

quent work by Lowrie et al. (1973a,b), (Lowrie, 1974), Lowrie and Kent (1976), Plessard and PrEvot (1977), Ozdemir and Banerjee (1981), Dunlop and Pr6vot (1982), Moskowitz (1985) and others. Most of these studies examined the ability of submarine volcanics from the extrusive layer to acquire VRM, although Dunlop and Pr6vot (1982) also considered the VRM acquisition properties of submarine intrusives. The objective of the investigation reported here is to extend these earlier studies toward a systematic survey of VRM acquisition through a complete section of oceanic crust. The section is that represented in the Troodos, Cyprus, ophiolite which includes all the principal lithologies of this type of crust. These lithologies contain a variety of magnetic phases, differing substantially in composition, grain size and time of formation. The results of this investigation, carried out at ambient temperatures, provide only partial information on VM in oceanic crust, that is, bearing on acquisition over the last 7 × 105 years in older oceanic crust. Older crust is taken here as being crust now well away from spreading ridges in which the heat flow, assumed to be entirely conductive, is 50 /xW m 2 or less. Using this value, typical values of conductivity, and allowing for sediment cover, temperatures at various crustal depths can be estimated. Thus, at about 0.8 km, below which depth in the Troodos section magnetite with Curie temperatures of 540°C or higher is the only magnetic phase, a temperature of apprQximately 40°C is obtained. Again,at 3.5 km, the depth of the bottom of the Troodos section, a temperature of about 100°C is expected. As Curie temperature increases with depth in this way, with, in addition, probable high blocking temperatures for the fine-grained secondary magnetite of serpentinized ultramafics at the greatest depths, measurements of VM at ambient temperatures (about 25°C or about 300°K) are likely to be representative of in-situ values. A heat flow of 50 mW m 2 corresponds to a crustal age of about 100 m.y. and, conservatively, from a quarter to one-third of the present area of ocean crust is included in this definition of older crust. As such crust has experienced many reversals of the geomagnetic field, only estimates of the VM ac-

quired since the last reversal can reasonably be attempted. Laboratory attempts to obtain geophysically meaningful estimates of in-situ VM in younger, hotter crust will be increasingly difficult to carry out as spreading ridge conditions are approached. Not only will measurements have to be made at well above ambient temperatures but also, at near ridge crest conditions, magnetizations acquired while hydrothermal mineralogic transitions are taking place will have to be determined. This follows, for example, as in the lower part of the volcanics and the sheeted dikes of the Troodos ophiolite, replacement of primary titanomagnetite (PM) by secondary magnetite (SM) took place early in crustal history at temperatures of 200°C or higher (Hall and Fisher, 1987, etc.). Thus, a full description of VM, VRM and associated types of magnetization--thermo-and chemico-viscous remanent magnetization (TVRM, CVRM)--will require an extensive range of further experimentation. The profile through Troodos type oceanic crust used in this investigation has been described in detail by Hall et al. (1991), together with a justification for its use as a reliable proxy for in-situ oceanic crust. In terms of the strongly magnetized upper interval, on average about 500 m thick, and moderately magnetized sheeted dikes, the section resembles magnetically oceanic crust, so far as it is known, and the Hole 504B magnetic profile, in particular. In summary, the section consists of three broad magnetic-petrological units. From the sediment-volcanic interface downwards, these are as follows: (1) The upper extrusives and dikes (Magnetic Unit I). These contain a range of magnetic mineralogies, representing various stages of low-temperature oxidation of primary igneous titanomagnetites. Both single-phase, cation-deficient titanomagnetites and variously phase split phases occur, with the phase present depending on both burial history and degree of contact with downwards-flowing cold seawater. Small primary igneous grain sizes, resulting from rapid initial cooling, are retained. In this unit, remanent magnetization everywhere exceeds induced magnetization. (2) The lower extrusives and dikes, the sheeted

J.M. Hall et al. / Physics of the Earth and Planetary Interiors 88 (1995) 101-116

103

Table 1 Characterization of samples used in VM and VRM studies Sample

Approximate crustal depth ( + ) (km)

Lithologic type

Characterization Jo(SI)

K(SI) ( x 10 -4)

Tc (°C)

Magnetic history

Magnetic Unit I ND CY-1, 94.91 0.095 UPL sheet flow 0.24 1.07 ND Fairly abundant anhedral-euhedral mt.; 1-5/xm range, 3/~m av. CY-1,103.45 0.103 UPL sheet flow 4.97 0.46 540 ND Gray-brown anhedral-skeletal mt.; 10/~m av.; red stained background common CY-1,159.18 0.159 UPL pillow flow 9.48 0.66 485 ND Very fine anhedral gray mt.; 2-5/zm range CY-1, 324.45 0.324 LPL sheet flow 1.25 1.27 525 ND Slightly cracked brown-gray mt.; mostly 3-10/~m, a few to 30/zm CY-la, 60.08 0.330 LPL sheet flow 0.29 2.51 445 D Light brown-gray skeletal-subhedral slightly cracked and granulated mt.; range 1-80/~m, 20-30/zm av. CY-1,353.90 0.359 LPL pillow flow 0.05 0.61 490 ND Gray-brown anhedral-subhedral mt.; 5-15/~m range CY-la, 100.75 0.371 LPL pillow flow 4.06 2.03 440 ND Brown-gray skeletal-subhedral slightly cracked mt.; range 1-30 ~m, 15/xm av. CY-1, 375.72 0.376 LPL dike 4.09 1.17 340, 490 ND Slightly cracked brown anhedral-subhedral mt.; 15-20/zm on average up to 70 ~m L20.66 0.490 LPL pillow flow 13.5 3.50 600 ND N/A CY-la, 239.85 0.510 LPL pillow flow 2.88 2.11 480 D mr., probably skeletal; range from < 3 to < 171zm, av. 8/xm L8.05 0.661 LPL sheet flow 13.4 1.59 440, 570 ND N/A CY-la, 398.60 0.669 BG pillow flow 0.75 1.24 ND ND N/A CY-la, 520.05 0.790 BG dike 0.99 3.16 555 D Largely secondary, brown, anhedral subhedral partly hematized mt.; range 1-5 ~tm, 10 p,m av. CY-la, 529.80 0.800 BG sheet flow 1.97 1.80 555 D Partly hematized subhedral-skeletal secondary mt.; < 1-15 ~m CY-la, 569.80 0.840 BG dike 0.68 2.02 570 ND Partly hematized medium gray secondary mt., common; up to 70 ~m CY-la, 629.55 0.899 BG sheet flow 1.90 3.33 580 ND Partly hematized mt., all secondary, very common; av. 30/xm, up to 100/xm SC25, la 1.591 SC dike 1.30 7.80 590 ND Rims or peripheral zones of very common brown-gray secondary mt., cores of mt. with much granular material and sphene pods; range 2-100 p~m, 30/zm av. 221.6 1.622 SC dike 0.84 7.93 570 D Largely uniform brown secondary mt.; a few pods of sphene and porous zones; range 2-24 p~m, 12 p~m av. 220.1 1.637 SC dike 3.46 7.92 560 D Part hematized secondary mt.; some uniform mt. peripheral areas, variable included granular material; range 5-130 p~m, 40 p.m av. 219.16 1.646 SC dike 0.47 6.78 550 D Partly hematized at least largely secondary mt.; broad peripheral areas of uniform mt., distinctly less included material than 217.2, 220.1; range 5-75/zm, 25/xm av. 217.2 1.661 SC dike 3.72 7.09 580 D Partly hematized secondary mt., small grains uniform, larger grains with uniform rims and relic and altered material in interiors; range 2-150/xm, 20/zm av. 217.3 1.661 SC dike 1.74 7.12 590 D Partly hematized secondary mt.; smaller grains uniform, larger grains contain sphene pods and granules of hematite in their interiors; range < 30-40/xm CY-4, 599.38 2.089 GDP dike 2.32 11.6 545 ND Partly hematized, light brown secondary mt.; range 1-100/.tm, 5 ~m av. CY-4, 984.22 2.474 GDP dike 2.94 7.52 540 D Light brown, largely secondary mt.; range 5-100 p,m, 30/~m av. CY-4, 999.08 2.489 GDP plagiogranite 0.043 1.03 N/A D N/A CY-4, 1340.49 2.830 GDP gabbro 0.057 0.23 550 D Rare anhedral mt.; range < 1-5 p.m, 1 p,m av.

104

J.M. Hall et al. /Physics of the Earth and Planetary Interiors 88 (1995) 101-116

Table 1 (continued) Sample

Approximate crustal depth ( + ) (km)

Lithologic type

Characterization Jo(Sl)

K(SI) (× 10 -4)

T¢ (°C)

CY-4, 1610.35 3.100 GDP gabbro 0.080 0.55 545 Mt. most commonly as chains of < 1-5/xm grains secondary after the serpentinization of olivine CY-4, 1938.59 3.429 CU highly serp. 13.6 7.51 N/A N/A ultramafic CY-4, 2087.30 3.577 Cu low serp. 0.038 < 0.04 N/A ultramafic Rare small anhedral-euhedral Mt.; smaller grains uniform brown, larger slightly granulated; range 1-13 Izm, 4/zm CY-4, 2250.65 3.741 CU highly serp. 0.26 4.52 570 ultramafic Abundant uniform pale brown mt. of several types; relatively rare small (_< 5/zm) apparently primary mt.; abundant, partly coalesced chains of small mt. probably after olivine; large patches (up 100/tm) with many inclusions of an acicular silicate

Magnetic history D ND D av. D

ND, Not previously demagnetized; D, previously demagnetized. UPL, Upper Pillow Lavas; LPL, Lower Pillow Lavas; BG, Basal Group; SC, Sheeted Complex; GDP, gabbro-diabase-plagiogranite; CU, cumulate ultramafies.

complex and the dikes occurring in the upper gabbros (Magnetic Unit II). Lithologies in this unit have generally experienced hydrothermal alteration at temperature of 200°C or higher, probably while still within the zone of active crustal accretion. As a consequence of this process, several major changes have taken place in the primary Ti-bearing magnetites of the rocks. After phase separation into Fe-rich and Ti-rich phases, extensive leaching of the Fe-rich phase took place. Leaching was followed by the apparently concurrent annealing of any remaining Fe-rich magnetite, in which exsolved phases were expelled, and the deposition of SM. The result was the formation of pseudomorphs after primary magnetites and vein fillings both consisting of multidomain, nearly pure, stoichiometric magnetite. This SM differs considerably in its magnetic properties from its PM precursors in its high induced magnetization and poor ability to retain remanence. As a consequence of these processes, induced magnetization exceeds remanent magnetization in this unit. (3) The cumulate intrusive below the Sheeted Complex (Magnetic Unit liD. This unit includes the upper gabbros, with restricted intrusives of plagiogranite, and deeper ultramafics. Magnetization is generally weak except where serpentinization is strongly developed in the ultramafics. In

this unit neither remanence nor induced magnetization are consistently dominant.

1.1. Sample Selection (Table 1 and Figs. 1 and 2) The profile through Troodos types of oceanic crust which all the samples are from is indicated as the drill-hole profile in Fig. 1. It has been selected for study as a result of the high density of sampling, much from well-described, continuous diamond drill core sections. A set of 31 samples was selected to represent the three magnetic units and, within these units, the main lithological and alteration types present (Fig. 2). For example, the subset from Magnetic Unit 1 includes several representatives of each of the three major lithological types present--pillowed flows, massive flows and dikes. Duplication of lithological types is to insure that the two main alteration facies present are also represented. These are the Upper Pillow Lava facies, in which all lithologies have been strongly affected by the extensive drawdown of cold bottom water, and the subjacent Lower Pillow Lava facies, in which low-temperature alteration is much more subdued. Most samples are from the continuous drillcores obtained during the research drilling investigations of the Cyprus Crustal Study Project

105

J.M. Hall et aL / Physics of the Earth and Planetary Interiors 88 (1995) 101-116

Sheeted Complex are from recently excavated deep road cuts. To obtain a comprehensive sample set, some compromises had to be made regarding the previous history of magnetic tests made on samples. In

(Gibson et al., 1989, 1991). The value of these samples is that they are known not to have experienced subaerial weathering, with attendant possible ambiguities in the interpretation of results. A small number of samples from in or near the

.-..L • "

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Profile

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Fig. 1. Location of the section (drill-hole profile) through the Troodos ophiolite from which samples were selected. Contours are of dike density, i.e. the fraction made up of dikes in outcrop sections. SE[, sediment-extrusive interface; SC, Sheeted Complex, (ZY-1, GY-la, CY-4, etc., location of drill-holes. Insert: map of C~prus showing the location of the Troodos ophiolite and the study area (from Hall ct al., ]99]).

106

J.M. Hall et al, / Physics ~( the Earth and Planetao' Interiors 88 (1995) 101 116 Sedimelnt Upper Pillow Lavas facies-flows strongly altered by drawn down sea water

=~CY-I, 103.45 ~CY-I, 159.18 C'¢-I, 324.45 CY-Ia, 60.08 -~CY-I, 353.9O

~;--205----

l Lower Pillow Lavas facies-flows sometimes with fresh glass-alteration minimum subordinant dikes

T-"

~ - ---(`~.1,.,0o.,5 CY-I, 375.72 _~'~CY-I a, 120.00

i--r-7--] _~CY-Ia,

520.05 _--~"-CY-la, 529.80 , - - 0 . 5 0 _ + - ~_~'~CY-[a. 569.80

I

l

Basal G r o u p facies-flows and dikes.

1

Top of interval corresponds to the top of the epidote-chlorite- quartz alteration zone

1

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ii

i,

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(

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,__n.-l~._ ~ _ ~ . ~ , - c~.~. 599.~0 ~ - 0 . 5 0 - I--I '-0.25-2-

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G a b b r o s with dikes and plagiogranites near top

of interval

1

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= ~ CY-4, 984.22

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0

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Cumuhnle

ultramalics

Boltoln of

section

(

~-4, 1938.59

( ~t-4+2087.311 \ ~ "1-4.2250.65

P

Fig. 2. Distribution with depth in the drill-hole profile of the samples chosen for study. Annotated broken horizontal lines indicate dike density. particular, 15 of the 31 samples had previously been subjected to partial alternate field (a.f.) demagnetization of natural remanence, usually to peak fields of either 40 or 50 roT. Interpretation of the results from these demagnetized samples will require careful consideration in view of the experience of Tivey and Johnson (1981), who found for samples containing coarser-grained

magnetite that viscous acquisition constants were significantly reduced by demagnetization. 2. E x p e r i m e n t a l

method

Samples were stored in near-zero field in a Helmholz coil system for between 103 and l 0 4 rain before being subjected to VM acquisition. Acquisition tests were made in a field of 6.3 × 103 A - l M (average Earth's field) using a modified Schonstedt Instrument Company (Reston, VA, USA) SVM-1 Susceptibility/Viscosity Magnetometer. Two strategies were used to minimize field variations experienced by samples over the approximately 104 rain acquisition intervals. The first was to shield the m a g n e t o m e t e r by placing it within a six-layer, nested set of mu-metal magnetic shields. The second was to determine empirically the location in the laboratory where time variations were at a minimum. Initially, runs were made using an X - Y plotter to record the output of the magnetometer. Measurements of viscous growth were made at approximately equal time intervals on a log-time basis. Modification of the SVM-1 system involved replacing the X - Y plotter output with a data logging system interfaced with a microcomputer. This system recorded the output from the magnetometer at 20 s intervals. An important consequence of the use of this high time-resolution measuring system was the recognition of the wide range of apparent magnetic field transients present in laboratory conditions. Such transients limit the definition of acquisition constants, both for low values of constants and for long measurement periods. Such transients were not recognized in earlier investigations that were limited to a small number of measurements at widely spaced times. Examples of these transients are given below, and consideration is given to their significance and possible means of suppressing them. On the completion of acquisition runs, samples were taken from the SVM-1 system, placed in a mu-metal box and carried directly to a Schonstedt DSM-1 magnetometer, at which time a decay run was started. Samples were left undisturbed in the near-zero field within the mu-metal shields of the magnetometer for about 104 min. Measure-

107

J.M. Hall et aL / Physics of the Earth and Planetary Interiors 88 (1995) 101-116

ments were made at approximately equal steps on a log-time basis of the component of magnetization in which the V R M had been acquired. It was noted that smooth decay was seriously interrupted by small movement of the sample within the near-zero magnetic field conditions along the axis of the shields. As such movement is required to reinitiate the m e a s u r e m e n t cycle if the magnetometer is switched off and on again, decay runs were made with the m a g n e t o m e t e r continuously switched on. This observation, together with the observations made concerning VM acquisition, emphasize the different requirements of longterm V R M studies, and the less demanding instrumental and field stability requirements of most paleo- and rock-magnetic studies. These more common measurements require stable conditions for only from 0.1 min to about 10 2 min, whereas VM or V R M studies require stable magnetic conditions for 10 4 min or more. One of the objectives of the investigation was to predict VM magnitude after 7 x 10 5 years. This required a protocol for modeling VM growth, to allow extrapolation from laboratory times (about 10 4 min) to between 1011 and 10 u min. The protocol utilized the observation that growth on a log-time base generally showed one or more linear segments. Conservative estimates were made of the onset and termination of each segment. Data for these intervals were averaged in 100 m e a s u r e m e n t (33 min) blocks, and best-fitting functions of the form

I

I

I

I

/

I

0

t

-I

I

4~0

80~

(b)

f

Jt = J 0 + Sa l°gl0 t

were fitted to the data for each interval using the Systat statistical software package. (.It is magnetization at time t, J0 initial magnetization and S a the acquisition constant as magnetization acquired per log-cycle of time). Fig. 3 shows examples of the goodness of fit of log VM growth functions to parts of growth curves plotted on a linear-time base, together with apparent transients in the magnetization of samples.

3. R e s u l t s

The results of the investigation are described under three headings--acquisition behavior, ex-

lO t

102

lOs

104

Time ( min )

Fig. 3. (a) A range of typical VM growth data obtained using the data logging system. Thin continuous curves are for logarithmic functions fitted over time intervals selected from examination of the logarithmic plots of (b). Upper curve: basalt sample CY-la, 239.85; fit is for the interval 700-7600 min. Middle curve: andesitic basalt sample CY-la, 520.05; fit for 1800-8100 rain. Lower curve: andesitic basalt sample CY-la, 529.80; fit for 500-10000 min. (b) Growth curves plotted on a log-time basis for the samples shown in (a). trapolation in time, and decay behavior. Thirty samples are included in the study, of which acquisition data are available for 29 and decay data for 20.

108

J.m. Hall et al. /Physics of the Earth and Planetary Interiors 88 (1995) 101-116

3.1. Acquisition behauior (Tables 2 and 3, Figs. 3 and 4) VM acquisition generally showed rapid initial growth for intervals of up to 10 2 min followed by a gradual reduction in the rate of growth (Fig. 3(a)). In this way, the behavior generally resembled the logarithmic form found in many other investigations and predicted theoretically (e.g. by Thellier, 1938; NEel, 1950; Stacey, 1963). For 15 samples, or about half the sample set, growth did not follow a single logarithmic law. For these samples, growth could usually be approximated well by two successive linear segments (e.g. Figs. 3(b) and 4(b)). Growth is always faster for the later segment. Thus either a second growth mechanism has replaced the first or a second mechanism, involving delayed activation, operated in addition to the first. This two-component growth differs from the continuous change in acquisition over 20 min time intervals described by Dunlop (1983) for dispersed magnetite powders. The time of onset of the second mechanism can be estimated from the first clear appearance

of a second linear segment in log-growth plots. This time ranges from 5 × 10 2 min to 3 × 103 min, essentially identical with the range found by Lowrie (1974) for in-situ oceanic crustal rocks, and has an average of 1.3 × 103 min. A small number of repeat runs, for example, with Sample 219.16, were indistinguishable in form and time of onset of the second segment from the first runs. There is no correlation between single or double growth mechanisms and lithological type. An identical ratio of single to double acquisition characteristics is found for little altered submarine flows and for dikes that have been subjected to greenschist facies hydrothermal alteration. Neither is there any correlation between single or double growth mechanisms and the previous a.f. demagnetization history of a sample. This contrasts with the results of Lowrie and Kent (1976), who found double growth mechanisms only after a.f. demagnetization. Two (or more) growth segments have frequently been described, or are evident in data for submarine volcanics (e.g. see Lowrie et al., 1973b; Lowrie and Kent, 1976; Plessard and Prfvot, 1976; Kent and Lowrie, 1977;

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Time (Inin}

Fig. 4. (a) Examples of single-stage V M growth from earlier measurements obtained using an X - Y recorder. Samples are from little-altered extrusives of Magnetic Unit 1. (b) Examples of single-stage and multistage growth for samples from greenschist facies dikes of Magnetic Unit II.

J.M. Hall et al. / Physics of the Earth and Planetary Interiors 88 (1995) 101-116

109

Table 2 VM growth and VRM decay characteristics Sample

Growth Time interval (min)

Decay G/log cycle (SI units)

R2

CY-1, 94.91

CY-1, 103.45 CY-1, 159.18 CY-1, 324.45 CY-la, 60.08 CY-1,353.90 CY-la, 100.75 CY-1,375.72 L20.66 CY-la, 239.85 L8.05 CY-la, 398.60 CY-la, 520.05 CY-la, 529.80 CY-la, 569.80 CY-la, 629.55 SC25-1a

221.6 220.1 219.16

217.2 217.3

CY-4, 599.38 CY-4, 984.22

50-200: 500-1000: pooled 50-10000 50-200 2000-5800 500-2000 2000-5900 50-200 500-6000 100-5400 50-500 2000-7000 Not available 50-300 0-6000 40-500 700-7600 50-2000 [0-9000: ~ 0.02] (only small piece available) 60-1400 1800-8100 60-500 500-10000 [1-10000

0.012 0.023 0.021 0.0035 0.090 0.023 0.130 0.007 0.025 0.032 0.006 0.021

0.988 0.937 0.974 0.964 0.986 0.970 1.000 1.000 0.984 0.980 0.999 0.940

0.014 0.048 0.034 0.094 0.016

0.886 0.996 0.970 0.988 0.951

0.018 0.078 0.010 0.013 0.005

0.850 0.961 0.515 0.515 0.243]

100-2000 1.1-20 30-1000 1500-11000 1.1-11000 50-3800 50-4500

0.013 0.014 0.012 0.027 0.021 0.004 0.012

0.802 0.973 0.895 0.921 0.929 0.954 0.972

100-600 4-20 20-200 3000-14000 50-500 2000-7340 60-200 [60-200

0.045 0.063 0.044 0.230 0.023 0.102 0.028 0.015

9.953 0.994 0.994 0.946 0.984 0.989 0.780 0.418]

0.019 0.110 = 0.007 0.003 0.150

0.962 1.000

50-200 1000-6000 1-30 10-100 500-2500

0.999 0.985

Time interval (rain)

D / l o g cycle (SI units)

R2

20-500 500-5800

0.011 0.030

0.996 0.952

0-8600

No. significant decay

10-300

0.049

0.992

5-2000 5-6000 5-50 1000-6000 5-500 5-5600 N/A 5-100 200-9000 N/A N/A

0.009 0.040 0.005 0.024 0.008 0.018

0.966 0.988 0.991 0.921 0.772 0.912

0.022 0.120

0.938 0.972

5-4265

0.020

0.972

N/A 5-20 0.029 No further decay > 20 min N/A N/A

0.990

10-9300 100-200 300-6000 5-20 20-13000

0.011 0.003 0.089 0.056 0.052

0.449 0.724 0.984 0.941 0.458

5-1250

0.033

0.997

50-1000 2000-6000 10-500 1000-40000 5-100 1000-10000 N/A

0.0066 0.0260 0.0007 0.0120 0.0240 0.1400

0.998 0.957 0.346 0.998 0.623 0.815

110

J.M. Hall et aL / Physics of the Earth and Planetaly Interiors 88 (10951 101-116

Table 2 (continued) Sample

CY-4, 999.08 CY-4, CY-4, CY-4, CY-4, CY-4,

1340.49 1610.35 1938.59 2087.30 2250.65

Decay

Growth Time interval (min)

G/log cycle (SI units)

R2

Time interval (min)

D / l o g cycle (SI units)

R2

110-1400 2000-9000 [200-7500 50-10000 _< 0.006 [500-10000 50-9500 < 0.007 100-100 [ 1500-10000

0.0065 0.0190 0.0015

0.757 0.486 0.750]

10 10000

0.0050

0.969

0.0004

0.851

0.016

0.564]

0.0130

0.724

0.017 0.043

0.9160 0.4510]

20-8700 N/A 10-10000 N/A N/A

Moskowitz, 1985). Lowrie (1974) and Kent and Lowrie (1976) suggested that VRM growth for Deep Sea Drilling Project (DSDP) basalts can commonly be described in terms of three linear segments, with the third showing an acquisition rate approximately equal to the first. VM growth for only two of the Troodos samples could be described in terms of three segments. Both (SC25-1a and 219.16) are basic dikes that have been altered in the greenschist facies. In further contrast with Lowrie's experience, for both sampies, two short, early, low growth rate segments are followed by prolonged higher growth rate segments. Reconciliation of the results of Lowrie Table 3 Sat and Sa2 for samples showing two segments of VM growth Sample

S~,1

S,e ( = Slate - Sal )

Ratio Sa2/Sal

CY-1, 94.91 CY-1, 103.45 CY-1,159.18 CY-1,324.45 CY-1,353.90 CY-la, 239.85 CY-la, 520.05 CY-la, 529.80 SC25-1a 219.16 217.2 CY-4, 599.38 CY-4, 984.22 CY-4, 999.08 CY-4, 22500.65

0.0120 0.0035 0.0230 0.0070 0.0060 0.0340 (I.0180 0.0100 0.0130 0.0510 0.0230 0.0190 0.0030 0.0065 0.0170

0.0080 0.0865 0.1070 0.0180 0.0150 0.0600 0.0600 0.0030 0.0140 0.1790 0.0790 0.0910 0.0120 0.0125 0.0260

0.67 24.7 4.65 2.57 2.50 1.76 3.33 0.300 1.08 3.51 3.43 4.79 4.00 1.92 1.53

Units of Sal , Sa2 are growth in VM in SI units per log-time cycle.

and those for the Troodos samples may lie in the longer measurement periods (up to 3.6 × 10 4 min) used by Lowrie, which possibly allowed a return to a low rate of acquisition to be seen. If it is assumed, as suggested above, that late rapid growth in VM is the result of the superposition of two logarithmic growth laws, then growth constants for the second mechanism can be separated from, and compared in magnitude with, constants for the first mechanism. Thus: Sa2 = Slate - Sal

where Sa2 and Sat, are, respectively, the growth constants for the late and early mechanisms, and Slate is the measured overall late growth rate. Ratio Sa2/Sat values range from 0.3 to 24.7 (Table 3). If three extreme values are set aside on the grounds of marginal separation of the two segments (two values) and a poorly defined initial segment (one value), the remaining 12 values are confined to the interval from 1.5 to 4.8, with an average value of 2.9. That is, on average, the second, late-onset mechanism results in a three times more rapid growth in VM then does the early-onset mechanism. Interestingly, this ratio for the Troodos samples is indistinguishable from that found for V R M growth for DSDP Leg 34 basalts by Lowrie and Kent (1976). However, whereas Lowrie and Kent found the double segment growth property to be at least in some instances the result of previous a.f. demagnetization, it is independent of such treatment in the Troodos samples. Additionally, similar average values of the ratio Sa2/Sal are obtained for the

J.M. Hall et al. / Physics of the Earth and Planetary Interiors 88 (1995) 101-116

undemagnetized (2.7) and demagnetized (3.2) Troodos subsets. Further indication of the similar behavior of very differently altered Troodos samples is in the indistinguishable average values of 3.0 and 3.1 for the subsets from Magnetic Units I and II, respectively.

3.2. Extrapolation in time (Table 4, Figs. 5-7) Although the extrapolation of growth of VM from laboratory times to geological times is likely to involve considerable uncertainties (see, e.g. Dunlop, 1983, pp. 684-685), it may still be a worthwhile exercise in that an estimate of the contribution of V M or V R M to total magnetization can be obtained. A check on the reasonable-

111

ness of such estimates can be made from the analyses of remanences during a.f. or thermal cleaning. The protocol adopted for this exercise is to assume a linear extrapolation of VM growth over the period of the Brunhes, taken as having a duration of 7 × 105 years or 11.56 log-time cycles, at the late, highest acquisition rate, observed in the laboratory. The possible sources of error involved in this exercise are considered before the results are examined. Two known sources of error are expected to be partly compensatory. Thus, the reduction in VM growth capacity following a.f. demagnetization reported by Tivey and Johnson (1981) appears to average about a factor of

Table 4 Summary of contributions to total magnetization after 7 X 105 years together with NRM stability indicators Sample

Remanent

CY-1, 94.91 CY-1, 103.45 CY-1, 159.18 CY-1,324.45 CY-la, 60.08 CY-1,353.90 CY-la, 100.75 CY-1,375.72 L20.66 CY-la, 239.85 1,,8.05 CY-la, 398.60 CY-la, 520.05 CY-la, 529.80 CY-la, 569.80 CY-la, 629.55 SC 25-1a 221.6 220.1 219.16 217.2 217.3 CY-4, 599.38 CY-4, 984.22 CY-4, 999.08 CY-4, CY-4, CY-4, CY-4, CY-4,

1340.49 1610.35 1938.59 2087.30 2250.65

Induced

Viscous

Total

Viscous as fraction of total

MDF (mT)

$200

0.72 4.97 9.48 1.25 26.40 0.64 4.06 4.09 13.50 2.88 13.40 0.75 0.99 1.97 0.68 1.90 1.30 0.84 3.46 0.47 3.72 1.74 2.32 2.94 0.043

0.67 0.29 0.41 0.80 1.58 0.38 1.28 0.74 2.20 1.32 1.00 0.78 1.99 1.13 1.27 2.09 4.90 4,98 4,98 4.26 4.45 4.47 7,29 4.72 0.65

0.15 0.16 0.13 0.12 0.01 0.19 N/A 0.03 0.03 0.21 0.01 0.12 0.23 0.05 0.03 0.04 0.05 0.01 0.02 0.36 0.13 0.05 0.12 0.18 0.10 [0.24] 0.08 _<0.14 0.01 < 0.40 0.06 [0.14]

34.0 15.6

0.24 0.17 0.11 0.16 0.025 0.14 0.042 0.15 0.44 0.14 0.13 0.34 0.006 0.16 0.094 0.42 0.15 0.10 0.052 0.20 0.44 0.18 0.16 0.35 0.28

0.14 0.35 4.72 < 0.04 2.84

1.63 6.30 11.40 2.340 28.40 1.26 N/A 4.99 16.30 5.29 14.60 1.73 3.88 3.25 2.01 4.14 6.51 5.87 8.58 7.39 9.35 6.53 10.90 9.39 0.77 [0.91] 0.21 < 0.50 18.50 < 0.13 3.30 [3.59]

12.1 8.9 11.0 8.4 1.9 9.2 1.8 8.6 18.0 2.2 12.0 15.5 11.7 9.7 8.5 19.0 17.5 6.6 5.5 12.2 19.4 10.0

0.057 0.080 13.600 0.038 0.260

0.24 1.00 1.50 0.29 0.37 0.24 N/A 0.16 0.55 1.09 0.18 < 0.23 0.90 0.15 0.06 0.15 0.31 0.05 0.14 2.66 1.18 0.32 1.27 1.73 0.08 [0.22] 0.017 < 0.070 0.180 < 0.050 0.200 [0.49]

25.0 15.0 9.8 30.5 10.0

0.60 N/A 0.13 N/A N/A

112

J.M. Hall et al. / Physics ~f the Earth and Planetao, Interiors 88 (1995) 101-110

three for multidomain magnetite. Again, an overestimate of growth rate, also of about a factor of three, may be involved in using the highest observed acquisition rate, rather than an even longer-term, lower rate, found for 12 of 50 samples tested by Lowrie (1974). Other sources of error can only be envisioned at the present state of knowledge. For example, if VM growth shows up to three separate segments o v e r 10 4 min, it is possible that even longer-delayed onset mechanisms may be active over the further seven to eight log cycles involved in the extrapolation. It is with these uncertainties in mind that the operation of extrapolation is intentionally described as an exercise. The results of the exercise are given in Table 4, together with other contributions to the total magnetization. VM over the Brunhes varies from 0.05 SI (A m -1) or less to 2.66 SI (Fig. 5(a)). Most (69%) of values are less than 0.5 SI with an additional small peak centered on the 1.0-1.5 SI interval. Values contributing to this higher peak are almost exactly (4:5) divided equally between the little-altered flows of MUI and the highly hydrothermally altered dikes of MUII. In terms of contribution to total magnetization, VM ranges from 1% to close to 40%. Again, low (less than 5%) values are the most important single group but, in contrast to the distribution of magnitudes of Fig. 5, a weakly separated higher peak, centered on the 10-15% interval, contains 18 or 62% of the values (Fig. 5(b)). The magnitude of VM varies irregularly with depth from negligible to relatively high values in Magnetic Units I and II (Fig. 6). Only low values occur in Magnetic Unit III. The contribution of VM to total magnetization also shows rapid variation with depth in MU! and MUII. The two apparently high values in MUIII are poorly determined. The values given are maxima, and small arrows are used to indicate that actual values are probably much smaller. If this supposition is correct, then all values in MUIII are small. This is an interesting result, as MUIII contains two highly serpentinized ultramafic samples, CY-4, 1938.59, and CY-4, 2250.65. These samples contain abundant fine-grained secondary magnetite after olivine. This high magnetite abundance presumably

10--

• el *el

.~

• on • el •e e l• l

10--

eel eeJ

~ ~

• °,

ZE

eel eel

Z

,e.. oe,, .Dee*

~i *ooooeoe.oo,o°.o°o

• el leF

o , o o , e eo,e*o

eel

e Q o e e e e* ~eo* **e*

! il

oooo oooo

eon *en __een

0

• on

I 0

~

ooeo ee**

I 211

I 4(~

( b ) E s t i m a t e d c o n t r i b u t i o n to t o t a l m a g n e t i z a t i o n o f V M a c q u i r e d o~er 7x10 ~ ) r .

0,ii

eQoeoo

.""°i;'°

0

I 3SI

( a ) E s t ii m a t e d m a g n i t u d e o f V M a c q u i r e d o v e r 7 x 1 0 5 yr.

Fig. 5. (a) Distribution of the values of the estimated magnitude of the VM acquired over a period of 7 × 10 5 years. (b) Distribution of the values of the estimated contribution to the total magnetization of VM acquired over 7× 10 5 years.

accounts for the high susceptibility of both sampies, comparable with that of dikes from the Sheeted Complex, and the high J0 of CY-4, 1938.59. However, it seems clear that a corresponding high VM does not occur. In view of the important implication that this result has for the magnetic structure of oceanic crust and, possibly, oceanic mantle, further experimentation is in hand using a larger set of previously undemagnetized samples. An overall impression of the importance of VM over a time period with the duration of the Brunhes is seen in Fig. 7. It is clear from this figure that, regardless of the magnetic unit being considered, VM generally will make only a minor contribution to overall magnetization. In an attempt to account for the range of contributions to total magnetization, correlations have been sought between values for individual samples and the sample characteristics listed in Tables 1 and 4. No simple, overall relationships occur with MDF,

J.M. Hall et al. / Physics of the Earth and Planetary Interiors 88 (1995) 101-116

$200, J0, K, Tc or Fe-Ti oxide characteristics. There is, however, an interesting relationship between Fe-Ti oxide characteristics and both viscous fraction and absolute level of VM for the sequence of altered dike samples from SC 25.1a to CY-4, 984.2, of Magnetic Unit II. These sampies all contain SM, usually with either or both substantial included material and martitic hematite lamellae. Although SM grain size ranges and average values overlap considerably within this sequence, it is observed that 219.16 and CY-4, 984.22, for which the viscous value are highest,

contain the largest areas of SM without inclusions. For this sequence, at least, overall grain size is not as important as the degree of annealing that has taken place within grains.

3.3. Decay behavior (Table 2, Fig. 8) The decay characteristics of VRM often but not always resemble the growth characteristics of VM. Thus, one- and two-component decays occur in similar numbers and are commonly associated with growth mechanisms with the same number

Natural remanent (o) and induced ('k') magnetization ( SI )

Predicted VM acquired over

7xi0Syr ( SI )

~ediment . Upper Pillow Lavas facies-flows strongly altered by drawn down

0~, '

I

Ar

----"-=TI

/

""10"25"'~-~dq.l~ e

sea water

Lower Pillow Lavas facies-flows sometimes with fresh glass-alteration minimunt subordinant dikes

it%

•

•

I1

113

•

Ratio predicted VM over 7xl0Svr to tntal magnetizati.n

2,0 I

0.2

I

I

0.4

I

I

Magnetic Unit I Remanent >Induced magnetization

"

tt

.................

...

......

. .....................

o_~. . . . . . . .

IP. . . . . . . . . .

Basal Group facies-flows and dikes, l Top of interval corresponds to the top of the epidote-chlorite- quartz alteration zone

Sheeted Complex

I

{

IO

--,.

•

I~ o

•

•

-:--

Magnetic Unit II Induced > Remanent magnetizatinn

Gabbros with dikes and

plagiogranites near top of interval

!

10!J

31~ -

_~t

-7-7-~7~ t

, \

Cumulate ultramafics

\

-o4g

/

\

/

•

/

I

Bottom of section - - ~

~t

Magnetic Unit III Remanenl>or< Induced magnetization

~/

I -o

m ~

--0 •

•

Fig. 6. Distribution w i t h depth in the s e c t i o n of remanent • and induced * magnetization, and the p r e d i c t e d magnitude • and contribution to total magnetization of VM acquired o v e r 7 × 10 5 years o.

114

J.M. Hall et al. / Physics Of the Earth and Planetary Interiors 88 (1995) 101 110 Predicted viscous magnetization a c q u i r e d o v e r 7xlOSyr.

/

\

/ / /

/

/

Viscous

dominated

',\ ,\ \\

I

p I

r

anence dominated

i I

•

I

I

• Induced dominated

OO

.

\

, \

\

\

Induced magnetization Intensity of natural remanent magnetization Fig. 7. The partition of magnetization between natural remanent, induced and viscous acquisition mechanisms for the sample set as a whole. B, samples from Magnetic Unit 1; o, samples from Magnetic Unit lI; *, samples from Magnetic Unit Ill.

of components. However, for four of the 14 samples for which comparison is possible, VM growth and VRM decay have different numbers of com-

-

-

A

.

O, 220.1

0.3

ponents. Where two components are present in both growth and decay curves, the time of onset of the second component is either identical or probably indistinguishable within the uncertainty in picking the time of onset. The ratio of growth to decay rates varies from 0.36 to 4.42, with 10 of 12 values in the range from 0.76 to 1.55. For both the larger and the more restricted groups, the average ratio is statistically undistinguishable from unity. This is particularly convincing for the more restricted group, for which the average value is 0.94 _+ 0.25 (1 SD, n = 10).

19.16

0.5

4. Summary and discussion

0.7

I lO ~

lilne

m.n~

I lO 3

I

Fig. 8. A range of decay curves of V R M for samples from greenschist facies dikes plotted on a log-time basis. Multistage decay is strongly present for 219.16 and 220.1, but only weakly so or is absent for 221.6.

The main result of this investigation is that, on the basis of conditions of growth at ambient temperatures for 103-104 min. VM is unlikely to make a major contribution to the overall magnetization of older oceanic crust. Earlier work had shown this conclusion to be generally the case for

J.M. Hall et al. / Physics of the Earth and Planetary Interiors 88 (1995) 101-116

little altered submarine extrusives and, using simple experimentation, for a variety of submarine intrusives (Dunlop and Pr6vot, 1982). The contribution of this present study is to extend this result in a systematic manner to the often secondary magnetite rich greenschist facies sheeted dikes and the serpentinized ultramafics of lower oceanic crust. A further test is suggested that would help to constrain the main conclusion of this study more quantitatively. This is to explore the temperature dependence of VM to at least approximately 100°C to test the assumption that the effect is small where magnetite with Curie temperatures of 540°C or higher is the magnetic carrier. The results of such a test for the fine-grained, magnetite-bearing serpentinized ultramafics would be a particular interest. This type of material, with its potentially high blocking temperatures (e.g. Heider et al., 1988), is likely to be the potential magnetic source of the lower oceanic crust and upper oceanic mantle. Demonstration, for example, of small VM acquisition, even at relatively elevated temperatures, would allow selection between the oceanic magnetic source models of Arkani-Hamed (1989) and would, in turn, bear on the origin of the skewness of some oceanic linear magnetic anomalies. Within the general result that VM is unlikely to make a major contribution to the magnetization of older oceanic crust, there is considerable range of VM growth magnitudes and a variety of growth styles. It is notable that there are no simple relationships between VM characteristics and either magnetic history or lithologic type and history. Thus similar ranges of relatively strong and weak VM growth and a similar variety of growth styles are found for little-altered sea-floor extrusives and for sheeted dikes that have experienced greenschist facies hydrothermal metamorphism. A relationship between VM magnitude and the internal state of the F e - T i oxides in a sequence of greenschist facies dikes suggests that samples will have to be viewed in new ways to understand the range of growth values. Lack of VM growth for two samples of highly serpentinized ultramafics is surprising, although Dunlop and Pr6vot (1982) made a similar suggestion based

115

on storage tests. Work is currently under way to test the general nature of this result. In terms of the magnetic nature of the sampies, the most interesting result is the widespread occurrence of multistage growth and decay curves, with the second stage always having a higher associated growth rate. The time of onset of the second mechanism, and the ratio of the magnitudes of the first and second growth rates, are confined to reasonably restricted intervals. No simple relationship occurs between single-stage or multistage growth and lithology or magnetic history. The last result differs from that of Lowrie and Kent (1976), who found previous a.f. demagnetization to be necessary for multistage VM growth to occur. Lowrie (1974) has suggested that the second-stage process is the result of the activation of fine particles between single-domain and superparamagnetic sizes. Development of superparamagnetic grain sizes is expected (Butler, 1973) and probably observed (Evans and Wayman, 1972) in oceanic submarine basalts. However, it is more difficult to envision the presence of much very fine-grained (about 5 × 10 -2 /zm) magnetite in the well-annealed, coarse SM bearing greenschist facies sheeted dikes. Continuous upwards concavity in VM log acquisition curves can be predicted theoretically by the setting r > - 2 in hypothetical power-law grain volume distributions of the form N(V)

= v r

where N ( V ) is the distribution parameter, V volume and r the power to which V is raised (e.g. Worm et al., 1988). However, the experimental data are usually better described by a series of linear segments than by a continuous, upwards concave function. Again, a continuous upward concave function cannot account for the three linear segment growth curves described by Kent and Lowrie (1977), in which the third segment marks a return to the growth rate of the first. Thus it seems likely that an alternative explanation is needed for multistage VM growth. The origin of this type of growth, and its significance in extrapolating laboratory VM growth to geological time intervals, are matters deserving further theoretical and experimental work.

116

J.M. Hall et aL / Physics ~f the Earth and Planetat3' Interiors' 88 (199.5) 101-116

Acknowledgments The Geological Survey Department, Cyprus, and its Director, Dr. George Constantinou, are thanked for continued strong support in fieldwork and for access to drill cores. We wish to thank Earl Davis for advice on oceanic heat flow, Darlene van de Rijt for typing the manuscript, and Graphic Services, Dalhousie University, for helping in preparation of the diagrams. The laboratory studies were supported by NSERC (Canada) Operating Grant OGP0007812 to J.M. Hall.

References Arkani-Hamed, J., 1989. Thermoviscous remanent magnetization of oceanic lithosphere inferred from its thermal evolution. J. Geophys. Res., 94: 17421-17436. Butler, R.F., 1973. Stable single domain to superparamagnetic transition during low temperature oxidation of oceanic basalts. J. Geophys. Res., 78: 6868-6876. Dunlop, D.J., 1983. Viscous magnetization of 0.04-100 #.m magnetics. Geophys. J.R. Astron. Soc., 74: 667-687. Dunlop, D.J. and Pr~vot, M., 1982. Magnetic properties and opaque mineralogy of drilled submarine intrusive rocks. Geophys. J.R. Astron. Soc., 69: 763-802. Evans, M.E. and Wayman, M.L., 1972. The mid-Atlantic ridge near 45°N: XIX. An electron microscope investigation of the magnetic minerals in basalt samples. Can. J. Earth Sci., 9: 672-678. Gibson, I.L., Malpas, J., Robinson, P.T. and Xenophontos, C. (Editors), 1989. Cyprus Crustal Study Project: Initial Report, Hole CY-4. Geol. Surv. Can. Pap., 88-9, 393 pp. Gibson, I.L., Malpas, J., Robinson, P.T. and Xenophontos, C. (Editors), 1991. Cyprus Crustal Study Project: Initial Report, Holes CY-1 and la. Geol. Surv. Can. Pap., 90-20, 283 Pp. Hall, J.M. and Fisher, B.E., 1987. The characteristics and significance of secondary magnetite in a profile through the dike component of the Troodos, Cyprus, ophiolite. Can. J. Earth Sci., 24: 2144-2159. Hall, J.M., Walls, C.C., Yang, J.S. and Hall., S.L., 1991. The magnetization of oceanic crust: contribution to knowledge

from the Troodos, Cyprus, ophiolite. Can. J. Earth Sci.. 28: 1812-1826. Heider, F. Halgedahl, S.L. and Dunlop, D.J.. 1988. Temperature dependence of magnetic domains in magnetic crystals. Geophys. Res. Lett., 15: 499-502. Kent, D.V. and Lowrie, W., 1977. VRM Studies of Leg 37 igneous rocks. Initial Reports of the Deep Sea Drilling Project, 37. US Government Printing Office, Washington. DC, pp. 525-529. Lowrie, W, 1973. Viscous remanent magnetization in oceanic basalts. Nature, 243: 27-30. Lowrie, W., 1974. Oceanic basalt magnetic properties and the Vine and Matthews hypothesis. Z. Geophys., 40: 513-536. Lowrie, W. and Kent, D.V., 1976. Viscous remanent magnetization in basalt samples. Initial Reports of the Deep Sea Drilling Project, 34. US Government Printing Office, Washington, DC, pp. 479-484. Lowrie, W., Lovlie, R. and Opdyke, N.D., 1973a. The magnetic properties of Deep Sea Drilling Project basalts from the Atlantic Ocean. Earth Planet. Sci. Lett., 17: 338-349. Lowrie, W., Lovlie, R. and Opdyke, N.D., 1973b. Magnetic properties of Deep-Sea Drilling Project basalts from the North Pacific Ocean. J. Geophys. Res., 78: 7647-7660. Moskowitz, B.M., 1985. Magnetic viscosity, diffusion after effect, and disaccommodation in natural and synthetic samples. Geophys. J.R. Astron. Soc., 82: 143-161. N~el, L., 1950. Th~orie du trainage magnetique des substances massives dans le domaine de Rayleigh. J. Phys. Radium, 11: 49-61. Ozdemir, O. and Banerjee, S.K., 1981. An experimental study of magnetic viscosity in synthetic monodomain titanomagnemites. Implications for the magnetization of the ocean crust. J. Geophys. Res., 86: 11864-11868. Plessard, C. and Pr~vot, M. 1977. Magnetic viscosity of submarine basalts, Deep Sea Drilling Project, Leg 37. Initial Reports of the Deep Sea Drilling Project, 37. US Government Printing Office, Washington, DC, pp. 503-506. Stacey, F.D., 1963. The physical theory of rock magnetism. Adv. Phys., 12: 45-133. Thellier. E., 1938. Sur l'aimantation des terres cuites et ses applications g~ophysiques. Ann. Inst. Phys. Globe Univ. Paris Bur. Cent. Magn. Terr., 16: 157-302. Tivey, M. and Johnson, H.P., 1981. Characterization of viscous remanent magnetization in single- and multi-domain magnetite grains. Geophys. Res. Lett., 8: 217-220. Vine, F.J. and Matthews, D.M., 1963. Magnetic anomalies over ocean ridges. Nature, 199: 947-949. Worm, H-U., Jackson, M., Kelso, P. and Banerjee, S.K., 1988. Thermal demagnetization of partial thermoremanent magnetization. J. Geophys. Res., 93: 12196-12204.