Magnetic Stability of Oceanic Gabbros from ODP Hole 735B

H.-U. Worm

Magnon International, Am Burgberg 5, 37586 Dassel *

 

Abstract

Ocean Drilling Program (ODP) Hole 735B was drilled to a depth of 1.5 km in a tectonic window of gabbroic lower oceanic crust created at the Southwest Indian Ridge. The gabbros have a very stable natural remanent magnetization (NRM) of reversed polarity with most unblocking temperatures slightly below the Curie temperature of magnetite. The NRM includes a drilling induced overprint but its intensity decays strongly towards the interior of the drill core. The demagnetization data yield no or only very small secondary magnetization component acquired during the present Brunhes chron or an earlier normal chron, suggesting cooling through most of the blocking temperature range during chron C5r and a strong resistance against the acquisition of thermoviscous magnetization. A novel furnace has been designed to measure magnetizations and their time dependences at high temperatures (up to 580C) inside a commercial SQUID magnetometer. Magnetic viscosity experiments have been conducted on the gabbros at temperatures up to 550C to determine the time and temperature stability of remanent magnetization. Viscosities are generally small and increase little with temperature below the main blocking temperature, where the increase becomes almost an order of magnitude. Extrapolations to geological times infer viscous acquisitions that would be 5 25% of a thermoremanence in 100 k.y. and at temperatures of 200 500C. At ocean bottom temperature the predicted magnetization of one sample acquired in the present Brunhes chron should be 10% of the NRM. However, this is not recognized during NRM demagnetization and pTRM acquisitions at 250C are also much smaller than predicted. It thus appears that the NRMs are generally magnetically harder than magnetizations acquired after heating to 570C in the laboratory. Susceptibility changes during heating are small (< 5%) indicating a seemingly stable magneto-mineralogy, but conspicuous minima occur after heating to 520C. Also, quasi paleointensity experiments reveal characterestic patterns in the NRM/pTRM ratios and also large increases in pTRM capacity after heating to 570C. Moreover, ARM acquisition in the low field range ( 10 mT) is strongly enhanced after heating by factors up to three. The alteration of the magneto-mineralogy is interpreted to result from the annealing of defects in magnetite that originate from tectonically induced strain. The oceanic gabbros of Hole 735B are thus ideal source layer material for marine magnetic anomalies, and secondary thermoviscous acquisition, as a possible cause for anomalous skewness, is essentially absent.

 

Keywords: Rock Magnetism, Magnetite, Magnetic Viscosity, Oceanic Crust, Crustal Magnetization.

1.      Introduction

The vertical structure of the sources of lineated marine magnetic anomalies have remained poorly known ever since the recognition, more than 30 yr ago, that the ocean crust records reversals of the geomagnetic field. Inferences on the magnetization of lower crustal rocks from studies of dredged rocks [1,2] are ambiguous because these surficial samples have been subjected to varying degrees of seawater alteration that may have significantly affected the magnetic properties. The first long in situ section of gabbros was recovered in 1987 during ODP Leg 118. During this leg, Hole 735B was drilled to 505 meters below sea floor (mbsf) in a tectonic window at the Southwest Indian Ridge where lower crustal rocks are exposed [3]. Hole 735B was reoccupied in 1997 during Leg 176 and deepened to 1508 mbsf [4, 5].

During the site survey for Leg 118 [6], sea surface magnetic anomalies were mapped over large regions of the rift mountains of the Southwest Indian Ridge adjacent to the transform fault. Extensive dredging of these regions, including Atlantis Bank, recovered largely gabbro and peridotite, suggesting that these lithologies must be responsible for the anomalies [6], a possibility first raised by the laboratory work of Fox and Opdyke [1] and Kent et al. [2]. The hypothesis that gabbro could be a major contributor to the magnetic anomaly over Site 735 was confirmed by direct measurements on cores [7-9]. With the addition of the Leg 176 cores, however, it now appears that the 1.5-km Hole 735B gabbro section is the principal source of the lineated magnetic anomaly over the site, to the extent that this section is representative of the crust in three dimensions.

This study focusses on the thermoviscous properties of the gabbros, i.e. the magnetic stability of remanent magnetization with time and temperature. This is of importance with regard to the maximum possible temperature and depth of the sources of marine magnetic anomalies and their anomalous skewness [10, 11]. The more general paleomagnetic results obtained for Leg 176 samples are published in a separate paper [12].

 

2.      Rock Magnetism

Pure or nearly pure magnetite has been found as the sole magnetic carrier of remanence in Hole 735B rocks [3, 4, 9]. Thermomagnetic curves on more than 30 Leg 176 samples have been measured in the laboratory Grubenhagen (GGA, Germany), with a Curie balance (in vaccum and a field of 0.55 T). All curves show Curie temperatures very close to that of pure magnetite (Tc = 577C) and nearly all curves are reversible to within 5% of the initial magnetization, indicating the absence of significant maghemitization, the low temperature oxidation of magnetite.

Hysteresis loops have been measured on 40 samples at the Institute of Rock Magnetism at the University of Minnesota with a vibrating sample magnetometer in applied fields of up to 1 Tesla. Samples were taken as bulk samples with volumes of typically 6 cm, because subsamples may not be represantative for the generally coarse-grained gabbros. The hysteresis parameters coercivity, Hc(average) = 13.9 4.7 mT, coercivity of remanence, Hcr(average) = 29.9 6.9 mT, and the ratio of saturation remanence over saturation magnetization, Mrs/Ms(average) = 0.22 0.07, indicate pseudo-single domain behavior for practically all samples despite a significant range in magnetite grain sizes [4, 5].

The hysteresis parameters confirm that the gabbros are well capable of preserving a paleomagnetic signal.

The frequency-dependence of susceptibility Xfd has been determined for all samples described in sections 3 & 4 with a Bartington instruments in order to determine whether the magnetite grain size range extends into the superparamagnetic (SP) regime. However, for all samples is Xfd < 0.5% within the resolution limit of the instrument. Therefore it can be concluded that grains with sizes around 10 20 nm are essentially absent [13].

Susceptibility variations during thermal demagnetization have routinely been measured after each heating step of the later described quasi-paleointensity measurements in order to monitor possible changes in the magneto-mineralogy. Changes are generally less than 3% and at first, this was considered to indicate very stable magnetic phases. There are, however, small but very distinct changes with a minimum at 520 - 540C and a sharp subsequent increase (Fig. 1), suggesting a common mineralogical cause.

The acquisition of anhysteretic remanent magnetization (ARM) has been measured twice on several samples, the first following alternating field demagnetization of NRM and the second following a subsequent heating to 580C, because during the evaluation of the viscosity experiments the suspicion arose that remanence acquisition becomes enhanced after heating. The results indeed show always a signifiantly increased ARM acquisition after heating in the low field range ( 10 mT) with factors up to three at 2.5 mT, while the final ARMs show little changes. It was also noted that susceptibilities always increased from before the first ARM acquisition (which was > 6 months after NRM af demagnetization) to values being 2 4 % higher after acquisition. Further on, susceptibilities were increased by 0 4 % by heating to 580C, but decreased subsequently by 3 5 % after the final ARM.

 

3.      Paleomagnetism

Paleomagnetic analyses of Leg 176 gabbros on the basis of alternating field and thermal demagnetizations have revealed a radially directed drilling induced magnetic remanence (DIRM) of the drill core in addition to the primary paleomagnetic reversed component magnetization [4]. While the drill core is azimuthally unoriented the samples coordinates refer to the split core plane in the sense that north points horizontally outward. The standard inch-sized samples studied in Grubenhagen were cut in half to give an inner and outer approx. 1.2 cm long cylinder. For the outer specimens the DIRM is mostly strong and noticable as a northerly component and requires 20 to >30 mT alternating fields for its demagnetization (Fig. 3a). However, the radial overprint decreases strongly from the rim towards the center of the drill core. The samples selected for thermal experiments in this study are the inner halves, and they possess no or only minor secondary components (Fig. 3b). For all shown samples, except 147R7-60 cm, each demagnetization was followed by a pTRM acquisition (next section) at the same temperature. And if the pTRMs were not completely erased at the next higher demagnetization temperature, then the depicted curves do not represent solely the NRM demagnetization. Sample 147R7-60cm has been continuously thermally demagnetized. The normalized intensity decay curves (Fig. 3c) show that for most samples 80% of the NRM is blocked up to 500C. Sample 136R3-119 shows 50% unblocking at 500C but this sample carries also a significant northerly DIRM component and the measured unblocking spectrum is thus not equal to that of an undisturbed NRM. For most samples the NRM intensity remaining after 540C demagnetization is still > 50% of the initial NRM.

 

4.      Remanence Acquisition and Intensity

The gabbros contain primary magmatic magnetite which is expected to carry a primary thermoremanent magnetization (TRM) [4, 9]. However, secondary magnetite formed during the alteration of olivine and pyroxenes at temperatures above and possibly also below the Curie temperature of magnetite [7, 12]. A fraction of the NRM may thus constitute a chemical remanent magnetization (CRM) that could record a different field direction than the TRM if acquired at a later stage.

Eleven samples were selected for partial thermoremanent magnetizations (pTRM) experiments. Eight of the samples are pairs or triples, respectively, from long unbroken core pieces. pTRMs were imparted following each demagnetization level, as in Thellier-like paleointensity determinations. The purpose was to see if the pTRM measurements yield estimates of the same field intensities for each of the chosen temperature intervals among samples, or different intensities for subsequent intervals. Also, a CRM may be recognized by very different NRM/pTRM ratios above and below the mineral growth temperature.

For pTRM acquisition the samples NRM direction was aligned with the ovens field direction as well as possible, to 5 20. This was important because of the mostly large magnetic anisotropy. The ratio of a TRM parallel kmax to the TRM parallel kmin was > 2 for some samples.

Results are displayed as Arai plots in figure 4, where each point represents a temperature and the gained pTRM intensity is plotted versus the demagnetized NRM. If a sample had cooled over the whole temperature range in the same field intensity and no alteration occured, then all points would lie on a straight line. The slope would be given by the ratio of paleofield to laboratory field (40 T) intensities (to be corrected for different paleo- and laboratory cooling rates).

A different view of the same data as in figure 4 is shown in figure 5, where the ratios of pTRMs gained to NRMs lost in distinct temperature intervals are depicted. The symbols at 250C represent the ratios of pTRMs gained in the interval 25 - 250C to the NRM demagnetized in the same interval, both normalized to the initial NRM.

A general feature is the observation that the curves can be separated into two segments, one up to ~500C where the intensity of the pTRM gained is mostly smaller than the NRM fraction lost, and the interval above where the gained pTRM is always much larger than the demagnetized NRM fraction. Up to 250C the pTRM/NRM ratios are typically < 10% (Fig. 5). At 350C typically 10 20% of the NRM gets demagnetized but only 2-3% of the NRM intensity is gained during pTRM acquisition at the same temperature (with the exception of 153R6) (Fig. 4). There are conspicuous minima in the pTRM/NRM ratios for several samples at 500C, that are sometimes even negative. Negative values are caused by a decreased pTRM intensities despite an increased acquisition temperature, being most notable for the 153R6 samples. For the 350-450 interval neighboring samples (136R3, 160R6) have apparently similar pTRM/NRM ratios (Fig. 5), but even here the scatter is much larger than expected for a reliable paleointensity determination; the ratio ranges from 0.35 to 0.63 for the 136R3 samples.

The pTRM checks (570 → 500) (Fig. 4) are evidence for large increases in pTRM capacity presumably due to magneto-mineralogical changes, which are also indicated by the conspicuous susceptibility changes at and above 500C (Fig. 1).

 

5.      Thermoviscous Properties

5.1 Background and Theory

One of the important and still open questions regarding the sources of marine magnetic anomalies is that for the stability of magnetic remanence with time and temperature. So far the evidence is mostly indirect. In this case we know that the gabbros of Hole 735B preserved a stable remanence of reversed polarity for more than 11 million years. Little is known, however, about the cooling history of the drilled section, and the question for the rate of viscous acquisition of magnetization at a given temperature can only be answered by experiments.

In nature, the gabbros magnetization was in equilibrium with the paleomagnetic field upon cooling below the Curie temperature. Then, at a later stage associated with an unknown lower temperature the field reversed its polarity and the magnetization attempted to re-equilibrate with the external field, expressed as thermoviscous changes in magnetization. Since we havent observed clear evidence for opposite remanence components, it may be assumed that the temperature at the time of the next field reversal had dropped to a value where viscous changes are so small that the reversed component was not recorded.

The purpose of the following experiments is to determine the rate of viscous magnetization acquisition and its temperature dependence.

Ideally, measurements should take place in a reversed field at a constant temperature after a primary pTRM has been acquired during cooling from Tc . However, measuring viscosity in a field is disadvantadgeous for two main reasons:

1)      The SQUID Sensors measure the magnetic flux originating from the stray field of the samples magnetic moment as well as the applied field itself, and because it is practically impossible to stabilize the dc field to << 10-4, the small viscous changes may be undetectable due to dc fluctuations.

2)      The magnetic mineralogy undergoes alterations, subtly even after multiple heatings, and newly formed phases will acquire a CRM in an applied field. The CRM would obscure the purely viscous changes towards seemingly larger values.

It is argued that viscous changes following a pTRM acquisition by cooling from Tc to To in a field of intensity +H and a field reversal from +H to H are very similar, if not equal, to viscous decay in zero field following a pTRM(Tc → To) acquisition in +2H (Fig. 6).

Viscous magnetization is the approach from a non-equilibrium towards the equilibrium magnetization. Based on Nels SD theory viscosity can be expressed as [e.g. 14, 15]:

M(t) = [Meq (Meq Mo) e -t/t ] N(t) dt (1)

M(t) is the time dependent magnetization, Meq the equilibrium magnetization, Mo the initial magnetization, t a relaxation time, and N(t) its distribution function.

In the case of figure 6a it is assumed that the sample has been cooled from above Tc in a field of intensity 1 to a constant temperature to acquire a magnetization (TRM) of intensiy 1, which is Mo in equation 1. Upon the field reversal at time to the magnetization attempts re-equilibration towards Meq ( - Mo).

The difference in the case of figure 6b is that the initial TRM is double in intensity (acquired in a field of intensity 2) and that the new equilibrium magnetization is Meq = 0.

Equation 1 thus becomes:

M(t) = Mo [-1 + 2 e -t/t ] N(t) dt (2a)

or :

M(t) = Mo [2 e -t/t ] N(t) dt (2b)

which predict that initial and final intensities are different but the rates of change to be identical. The latter is not strictly true because the relaxation time t itself is field dependent [15], but for external fields H << Hc, the coercivity, differences are negligible.

 

5.2 Experimental Setup

The employed magnetometer is a commercial SQUID magnetometer (SRM 755 by 2G) located in the laboratory Grubenhagen, modified and equipped with an electric furnace that holds a sample of 1 inch diameter and allows heating while measuring up to a temperature of 580C while the temperature is constant to < 0.5C. Achieving this goal was not simple and to our knowledge it has not been accomplished in any other paleomagnetic lab before.

The main problem to circumvent is produced by the current of the heating wires because currents generate magnetic fields easily much larger in amplitude than the stray field of the samples under study. The solution has been achieved by employing high frequency currents whose secondary magnetic fields were shielded by an aluminum tube. The furnace itself is a quartz tube on which platinum wire is wound non-inductively. Measurements are performed without moving the sample out of the furnace and without switching it off.

The measuring scheme has been the following: The sample (25 mm diameter, ~12 mm length) was demagnetized by heating inside the magnetometer and in zero field (i.e. < 20 nT) to 580C. Then, the sample was cooled in a field (typically 25 to 70 T) produced by a coil located in the front part of the magnetometer to a temperature of 550C and thermal equilibrium was awaited for 15 min.. The field was switched off and the viscous magnetization decay was measured for periods up to 1200 s. For the next viscosity measurement the sample was reheated to 580C, cooled to 525C in field, equilibrated, and measured as before. For lower temperatures the sample was only reheated to the next higher previous measurement temperature.

 

5.3 Results

For the three samples under study viscous changes are generally linear on a log time scale. Results are shown in figure 7. Hence, viscosity can conveniently be characterized by a viscosity coefficient S:

VRM = Mo - S log t (3)

For comparing viscosities at different temperatures, in order to predict times in which the initial magnetization has decreased to a certain percentage, the viscosity coefficient S is normalized by the magnetization reached 10 s after field removal Mo(10 s). The temperature dependence of S is displayed in figure 8 and tabulated in Table 1.

 

6.      Discussion and Conclusions

The gabbros possess very stable NRMs of reversed polarity with no clear normal component during thermal demagnetization for any temperature interval. The single component NRM suggests that the whole gabbroic section cooled through the blocking temperatures from ~570C to near ambient in a single geomagnetic polarity interval, the reversed chron between C5An.1n and C5r.2n [13].

The viscosity experiments have been conducted in an attempt to quantify viscous overprints and their temperature dependences. Viscosities increases generally little with increasing temperature up to 500C (Fig. 8), but strongly above 520C towards the main unblocking temperature. For one sample (Fig.7d) viscous changes deviate from the otherwise observed linear log(t) behavior with a kink in the curve. This may be due to the proximity of the main blocking temperature. From the viscosity coefficients of table 1 it is not immediately apparent how much viscous overprint can be acquired in time intervals typical for the duration of polarity chrons and provided the extrapolation from laboratory times is justifyable. Assuming the latter, figure 9 shows how the experimental results are extrapolated up to > 100 k.y. for two temperatures. For sample 103R2-119cm, the sample with the highest Q-factor, the changes are the smallest among the three samples. Still, the viscous component acquired during the present Bruhnes chron should amount to around 5% of the TRM, while assuming that the viscosity at ambient temperatures is similar to the viscosity at 200C, as is true for 147R7-60cm (Table 1). The viscosity of 147R7-60cm is only slightly higher, but more pronounced for 153R6-64cm. Still, even at 500C and in time periods up to 1 m.y., viscous changes would not overwrite the primary TRM polarity.

As long as the cooling history of the gabbroic lower crustal section is poorly known, the thermoviscous components of earlier polarity chrons cannot be infered, but for the Bruhnes chron the extrapolated results at 20C can be compared to the NRM demag components. The extrapolated viscosity results predict that 5 to ~20% of the NRM should be parallel to the present normal polarity field. However this is not the case. For the three samples (Fig. 3b), and for almost all studied 735B samples [13], there is no recognizable recent field component. While it can be argued that a drilling induced magnetization may hide the soft ambient component for 103R2-119cm, the other two samples behave nearly uni-vectorial.

Hence the question arises whether it is illegetimate to extrapolate viscosity results from laboratory to geological time scales. While this cannot be ruled out, there is also evidence for severe magneto-mineralogical alteration associated with heating of the gabbros above ~500C. The subtle change in susceptibilities (Fig. 1) may at first not appear as very significant, however the distinct minima at 520C suggest a common cause. A re-examination of previous susceptibility changes during shipboard thermal demagnetizations shows that for most samples a similar minimum at 520C occured, but it is sometimes not apparent due to larger overall changes. The pTRM checks (Fig. 4) of the quasi-paleointensity experiments indicate huge changes in pTRM capacity following heating to 570C. Moreover, the distinct pattern of pTRM/NRM ratios (Fig. 5) with minima at 500C for some samples and large increases above 500 further supports the notion of a common mineralogical cause.

The negative pTRM/NRM ratios at 500C (Fig. 5) are hard to comprehend as the decreased pTRM intensity despite an increased acquisition temperature is against common pTRM models. Without further examination it can only be speculated that magnetostatic interactions between grains with different blocking temperatures are responsible for this phenomenon.

On the basis of large increases of the pTRM/NRM ratios above 500C alone, it could be suspected that a magnetite formation temperature Tf well below the Curie temperature is responsible for this increase, because magnetite formed below Tc carries a chemical remanence similar to a pTRM(Tf) and lower in intensity than a TRM. However, this would imply that the majority of magnetite grains in all samples formed at rather low temperatures, an implication that appears incompatible with petrographic studies [4, 9, 16], albeit the studies state that some of the secondary magnetite may have formed below Tc.

The ARM acquisition measurements prior to and after heating, respectively, show that remanence acquisition in small fields is suppressed in the unheated state compared to after heating (Fig. 2). This is interpreted to be due to defect-pinned domain walls of multidomain grains in the initial state, while the domain walls become more mobile by heating due to annealing of the defects. Similarly, the isothermal increase of susceptibility caused by the initial ARM acquisition may also be regarded as evidence for an effect of defects.

It is speculated here that the cause for the changes in magnetic properties is the defect structure of magnetite which presumably formed by strain. In their petrographic study on Hole 735B gabbros Pariso & Johnson [9] describe strained ilmenite but were unable to confirm the finding for magnetite because of its isotropic optical properties. However, in an SEM study on Leg 176 samples Trimby [17] observed defect structures in magnetite attributed to strain. All viscosity experiments have been performed after multiple heatings to Tc and presumably annealing of the defects, thus softening the magnetic properties. Viscous acquisition in nature during the past 800 k.y. may thus have been much harder than during the experiments in the lab.

The viscosities extrapolated to geological times can be compared to the NRM fraction demagnetized at 250C but also to the pTRM gained at this temperature, because the time-temperature relationship of Nels SD theory predicts that aproximately equal intensities are acquired at 10 - 20C in 800 k.y. and at 250C in 10 minutes [15]. NRM demagnetization and pTRM acquisition (Fig. 3 5) are consistent in the sense that no discernable Brunhes component is recognized and hardly any pTRM is gained by 250C. The reason the viscosity experiments predict much larger recent components most likely results from the alteration of the magnetic properties preceding the viscosity measurements.

The magneto-mineralogical alteration occuring during heating above ~500C limits possible paleointensity determinations to the data gained below 500C. However, the dissimilar pTRM/NRM ratios even for neighboring samples in the 250 - 430C interval indicates that no reliable paleointensity information can be gained from these gabbros.

Viscosity experiments on gabbros from the upper 500 m of Hole 735B have also been conducted in a study by Bowles & Johnson [18]. The main differences between their and this experimental setups are: (i) the thermally demagnetized state of their samples before viscous acquisition opposed to TRM here, (ii) in-field (1.5 Oe) opposed to zero field measurements, and (iii) fluxgate versus SQUID magnetization sensors. The Bowles & Johnson results appear to be in disagreement with our experiments in several aspects. First, while we observe viscous changes that behave linear on a log time scale, their magnetization curves are concave up on a log-t scale. Secondly, while we find slight increases of viscosity up to 500C and a large increase above, Bowles & Johnson report a maximum viscosity for T = 250C, and nearly indistinguishable viscous acquisitions at 350, 450 and 525C. This holds true when absolute magnetization values are compared, but when these are normalized to saturation magnetization at the respective temperatures viscosity of the Bowles & Johnson experiments is also largest at the highest temperature. Hence, as far as the temperature dependence is concerned our results are not that different from Bowles & Johnsons.

The non-linear behavior on a log time scale prohibits extrapolations to geological times because it would result in values exceeding by far the NRM intensity. Therefore, Bowles & Johnson [18] receded to using a hyperbolic tangent data fit that yields an asymptotic maximum VRM which is around 25% of the NRM intensity. However, 82 -99% of this hypothetical maximum VRM would already be reached within only one month and at 250C.

In contrast, the shapes of the viscosity curves and the rates of acquisition of this study are fundamentally different. While we predict viscous components to reach 5 to 20% (depending on sample) of the TRM in > 105 years and at 200C, the Bowles & Johnson paper infers 20% acquisition in only one month and very little changes thereafter. If true, the latter VRM would act like an induced magnetization because of its nearly instantaneous acquisition on a geological time scale.

The contradicting results can be compared to the measured NRMs and its VRM components in particular. As stated earlier, secondary field components, aside from DIRMs, are generally absent. Viscous components are thus even much smaller than predicted by extrapolations of this study and much more in disagreement with the Bowles & Johnson inference.

We attribute this behavior to magnetically harder NRMs than laboratory TRMs. Strain-induced defects in the magnetite crystals are presumably responsible for the hard NRM resisting VRM acquisition in nature. Upon heating above ~500C in the laboratory the defects have been annealed and viscosity is more easily acquired than in nature.

The detection of a strain-hardened magnetization reopens the question for the timing of the remanence acquisition. Whether the gabbros did indeed cool from the Curie temperature to near ambient within a single polarity chron because that is what the single component of reversed polarity suggests, or if the introduced strain froze the initially acquired remanence at some temperature below 500C (as the upper limit) with little additional remanence acquisition upon further cooling.

In any case it must be concluded that the gabbros magnetizations are extremely stable and that they preserved only the primary field direction - aside from the drilling induced component. Even after reheating and annealing of the defects viscous acquisition at temperatures up to 500C and in time periods up to > 100 k.y. accounts for less than 5 25% of a thermoremanence. The gabbros thus constitute ideal sources for marine magnetic anomalies.

 

Acknowledgements. This research used samples provided by the Ocean Drilling Program (ODP). ODP is sponsored by the U.S. National Science Foundation (NSF) and participating countries under management of Joint Oceanographic Institutions (JOI), Inc. Funding for this research was provided by the Deutsche Forschungsgemeinschaft (DFG). Comments by Jeff Gee and the reviewers Paul Kelso and Bruce Moskowitz led to significant improvements of the manuscript.

References

[1] P.J. Fox and N.D. Opdyke, Geology of the oceanic crust: magnetic properties of oceanic rocks, J. Geophys. Res., 78, 51395154, 1973.

[2] D.V. Kent, B.M. Honnorez, N.D. Opdyke and P.J. Fox, Magnetic properties of dredged oceanic gabbros and source of marine magnetic anomalies, Geophys. J. R. Astron. Soc., 55, 513537, 1978.

[3] P.T. Robinson, R. Von Herzen, et al., Proceedings of the Ocean Drilling Program, Initial Reports Leg 118, 826 pp., Ocean Drilling Program, College Station, TX, 1989.

[4] H.J.B. Dick, J.H. Natland, D.J. Miller et al., Proceedings of the Ocean Drilling Program, Initial Reports [CD-ROM] 176, 1999.

[5] H.J.B. Dick et al., A long in situ section of the lower oceanic crust: Results of ODP Leg 176 Drilling at the Southwest Indian Ridge, Earth Planet. Sci. Lett., 2000.

[6] Dick, H.J.B., Schouten, H., Meyer, P.S., Gallo, D.G., Bergh, H., Tyce, R., Patriat, P., Johnson, K.T.M., Snow, J., and Fisher, A., Tectonic evolution of the Atlantis II Fracture Zone. In Von Herzen, R.P., Robinson, P.T., et al., Proc. ODP, Sci. Results, 118: College Station, TX (Ocean Drilling Program), 359398, 1991.

[7] E. Kikawa and J.E. Pariso, Magnetic properties of gabbros from Ocean Drilling Progran Hole 735B at the Southwest Indian Ridge, Proceedings of the Ocean Drilling Program, Sci. Results, Volume 148, College Station, TX, 285-307, 1991.

[8] E. Kikawa and K. Ozawa, Contribution of oceanic gabbros to sea-floor spreading magnetic anomalies, Science, 258, 796-799, 1992.

[9] J.E. Pariso and H.P. Johnson, Do lower crustal rocks record reversals of the Earth's magnetic field? Magnetic petrology of gabbros from Ocean Drilling Program Hole 735B. J. Geophys. Res., 98,1601316032, 1993.

[10] S.C. Cande & D.V. Kent, Constraints imposed by the shape of marine magnetic anomalies on the magnetic source, J. Geophys. Res., 81, 4157-4162, 1976.

[11] J. Dyment and J. Arkani-Hamed, Spreading-rate dependent magnetization of the oceanic lithosphere inferred from the anomalous skewness of marine magnetic anomalies, Geophys. J. Int., 121, 789-804, 1995.

[12] J. Gee and H.-U. Worm, Paleomagnetism of ODP Leg 176 gabbros, in preparation.

[13] H.-U. Worm, On the superparamagnetic-stable single domain transition for magnetite, and frequency dependence of susceptibility, Geophys. J. Int., 133, 201-206, 1998.

[14] H.-U. Worm, M. Jackson, P. Kelso and S.K. Banerjee, Thermal demagnetization of partial thermoremanent magnetization, J. Geophys. Res., 93, 12196-12204, 1988.

[15] D.J. Dunlop and . zdemir, Rock Magnetism, Cambridge University Press, 1997.

[16] D. Stakes, C. Mevel, M. Cannat and T. Chaput, Metamorphic stratigraphy of Hole 735B, . In Von Herzen, R.P., Robinson, P.T., et al., Proc. ODP, Sci. Results, 118: College Station, TX (Ocean Drilling Program), 153180, 1991.

[17] P. Trimby, personal communication, Leg 176 post-cruise meeting, 1999.

[18] J.A. Bowles and H.P. Johnson, Behavior of crustal magnetization at high temperatures: Viscous magnetization and the marine magnetic source layer, Geophys. Res. Lett., 26, 2279-2282, 1999.


Table 1: Viscosity coefficients at various temperatures of three samples from Hole 735B.

Sample

103R2, 119 cm

147R7, 60 cm

153R6, 64 cm

NRM [A/m]

3.86

8.16

5.08

Q-Factor

22.8

6.0

5.4

S (550C) / M(10 s)

0.01, 0.1

0.018

0.018 (50 T)

0.020 (25 T)

S (525C) / M(10 s)

0.014

-

0.012

S (500C) / M(10 s)

0.0063

0.0044

0.0056

S (450C) / M(10 s)

-

0.0030

-

S (400C) / M(10 s)

0.0023

0.0038

0.0029

S (300C) / M(10 s)

0.0017

0.0051

0.0034

S (200C) / M(10 s)

0.0014

0.0019

0.0040

S (120C) / M(10 s)

-

0.0020

-

S (25C) / M(10 s)

 

0.0013

 

 

Natural remanent magnetization NRM, Q-factor = ratio of remanent to induced magnetization, and viscosity coefficients S at temperatures between 20 and 550C normalized by initial magnetization at 10 s after field removal M(10 s).


Figures

Fig. 1: Susceptibility (κ) changes during stepwise thermal demagnetization measured at room temperature and normalized to initial value (κo). Heating times were 10 minutes for each temperature.

Fig. 2: Acquisition of anhysteretic remanent magnetization (ARM) following more than 6 months after alternating field demagnetization of NRM () and after heating to 580C for 10 min. (▲). Low field ARMs are always enhanced after heating.

 

 

 

c)

Fig. 3: Alternating field (a) and thermal demagnetization of NRMs (b), displayed as orthogonal vector plots (a, b), where H is the horizontal component and V the vertical. Normalized intensity decay of thermal samples is shown in c). Samples a are from the rim and carry a mostly strong northerly, drilling induced overprint. Only the inner samples b were thermally demagnetized and here secondary components are much smaller.Sample 147R7-60cm was continuously thermally demagnetized.

Fig. 4: Quasi paleointensity determinations by NRM demagnetization versus pTRM acquisition. Samples were stepwise thermally demagnetized and each demagnetization was followed by a pTRM acquisition. Each point represents the NRM lost versus the pTRM gained for a certain temperature, both normalized to the initial NRM value. Triangles represent pTRM checks at 500C following demagnetization at 570C.

Fig. 5: The ratios of pTRM gained to NRM lost for subsequent temperature intervals. The same data as in figure 4. A symbol at 250C depicts the interval 25 - 250, at 350 the interval 250 - 350, and so on. Negative ratios result from decreased pTRM intensities at increased acquisition temperatures.

Fig. 6: Model for viscous magnetization changes (solid line) following a field change (dashed line) from +1 to 1 where the magnetization is a TRM acquired in a field of intensity 1 (a), compared to changes following a field removal from +2 to 0 where the magnetization is a TRM acquired in a field of intensity 2 (b).

Fig. 7: Viscous changes with time (t) of the magnetization (M) following the field removal in which the samples acquired a TRM by cooling from the Curie temperature.

Fig. 8: Viscosity coefficients S (eq. 3 & figure 7) normalized by initial magnetization at 10 s for samples 103R2-119cm (n), 147R7-60cm (l) and 153R6-64cm (u).

Fig. 9: Extrapolation of viscous magnetization from the laboratory experiments (Fig. 7) to geological time scales.

 

 



* Measurements were performed while at the Bundesanstalt fr Geowissenschaften & Rohstoffe, Hannover, Germany; accepted for publication in Earth Planet. Sci. Lett., September 20, 2001.