A direct comparison of the properties of CRM and VRM in the low-temperature oxidation of magnetite

Transcription

1 Geophys. J. lnt. (1991) 105, A direct comparison of the properties of CRM and VRM in the lowtemperature oxidation of magnetite A. K. Gapeev,'S. K. Gribov,' D. J. Dunlop,2 0. 0zdemir2 and V. P. Shcherbakov' ' Borok Geophysical Observatory, 0. Yu. Schmidt Institute of Physics of the Earth, USSR Academy of Sciences, Nekouzski Region, Yaroslavskaya Oblast, USSR Geophysics Laboratory, Department of Physics, University of Toronto, Toronto, Canada M5S 1A7 Accepted 1990 November 26. Received 1990 November 26; in original form 1990 May 25 SUMMARY We report results of a collaborative investigation of the experimental properties of chemical remanent magnetization (CRM) acquired when nearly singledomain size magnetite undergoes lowtemperature oxidation to maghemite and of viscous remanent magnetization (VRM) acquired by the daughter phase maghemite. Magnetite grains nm in diameter were oxidized for varying lengths of time at a temperature of 220 "C. The acquisition of CRM in fields of 0.2,0.5 and 1 Oe was monitored at frequent intervals during heating runs lasting 30 hr. Zerofield decay of this remanence was then monitored for periods of hr. Subsequently VRM acquisition and decay were measured for similar time periods for the daughter phase maghemite (z = 0.96). The principal results are as follows. (1) CRM and VRM have similar quasilog t time dependences, in acquisition as well as decay of remanence. (2) After a time 39 times the acquisition time, there is no further decay and a true CRM residual is isolated. (3) Residual (timeindependent) CRM forms about 45 per cent and viscous CRM forms about 55 per cent of the total CRM, which is appropriately called a chemicoviscous remanent magnetization (CVRM). (4) The alternating field (AF) stability of residual CRM is significantly greater than that of either VRM or total CVRM. However, the coercivity spectra are sufficiently similar that AF cleaning would not be an effective means of separating CRM formed in the direction of the primary remanence of parent magnetite from VRM formed in the applied field direction. Key words: chemical remanent magnetization (CRM), lowtemperature oxidation, maghemite, magnetite, remagnetization, viscous remanent magnetization (VRM). 1 INTRODUCTION Ozdemir & Dunlop (1989) observed that chemical remanent magnetization (CRM) produced by oxidizing singledomain (SD) size magnetite (Fe,O,) to maghemite (yfe,03) in a magnetic field Ho has many similarities to viscous remanent magnetization (VRM) produced by exposing the same magnetite to Ho for similar lengths of time under nonoxidizing conditions. In hightemperature experiments, CRM and VRM had almost identical intensities, which reached similar Hopkinson peaks just below the Curie point. The responses of CRM and VRM produced at the same temperature to alternating field (AF) demagnetization at room temperature were almost indistinguishable. These similarities suggested that hightemperature CRM has a mechanism different from conventional growth CRM (Kobayashi 1961), which is blocked when daughterphase crystallites reach the critical size for stable SD behaviour, and should properly be termed chemicoviscous remanent magnetization (CVRM). In experiments at nearambient temperatures, on the other hand, CRM was about twice as intense as VRM produced under the same time and temperature conditions, although AF stabilities remained similar. Ozdemir & 407

2 408 A. K. Gapeev et al. Dunlop (1989) concluded that there might be separate timedependent (VRMlike) and timeindependent ('true' or growth CRM) contributions to chemical remanence, the timedependent component becoming dominant at high temperatures. The timedependent part would continue to grow viscously in the presence of an applied field and would gradually decay away if the field were removed. To test this idea, one needs to measure the time dependence of chemical remanence as it is being acquired. All measurements in Ozdemir & Dunlop (1989) had to be made at room temperature, after cooling the sample at the conclusion of a run. In the present study, by making measurements at high temperature with a specially designed spinner magnetometer, the timedependent acquisition and decay of CRM could be monitored continuously over periods of several days. Oxidation experiments were carried out at a single temperature (220 "C) and time (30 hr) which were known to result in complete conversion to the single daughter phase maghemite (Gapeev & Gribov 1989). The design of the present experiments extends previous work in several other ways. (1) The AF demagnetization of remanences which had been cooled to room temperature immediately after they were acquired (i.e. CRM+VRM) was compared to the demagnetization of remanences that were allowed to decay for long times in zero field before cooling, thus isolating the residual or 'true' growth CRM without VRM. In this way we could compare directly, the AF stability of the timedependent and timeindependent parts of the chemical remanence. (2) As a further comparison, VRM was produced in the daughter phase maghemite after all CRM experiments were complete. The time dependence and AF stability of VRM were compared with the corresponding properties of CRM immediately after acquisition and of residual CRM. (3) Magnetite concentration was varied in replicate acquisition and decay experiments. (4) The dependence of CRM intensity on applied field was also tested. 2 SAMPLES AND EXPERIMENTAL METHODS The starting material is a synthetic magnetite precipitated from a solution of ferrous and ferric chloride by adding sodium hydroxide. A particle size in the range nm ( pm) was achieved by aging the solution at moderate temperatures. The grains are rounded in form. Their sizes and shapes were observed using a scanning electron microscope. The Xray unit cell edge determined from the (751) and (840) diffraction lines of the starting material, using CoKa radiation was f A. This value compares well with the ASTM standard value of A for magnetite. Hysteresis properties of the starting material, measured with a vibratingsample magnetometer on a 1 per cent by weight dispersion of magnetite in a maximum field of 0.5 T, were saturation magnetization M, = 92.5 f 1.0 A m2 kg', reduced saturation remanence M,IM, = and coercive force H, = 136 Oe (p,hc = 13.6 mt). These values are characteristic of pm grains of hydrothermally grown magnetite (Dunlop 1986). Experimental samples were made by dispersing magnetite as uniformly as possible in a kaolin matrix and forming the mixture into 10 mm cubes. Excess water was carefully baked out before beginning the experiments. There were three sets of samples: series A contained 1 per cent by weight of magnetite; series B contained 3 per cent magnetite; and series C contained 10 per cent magnetite. In the primary experiments, samples were oxidized in air at a constant temperature T =22O"C which could be maintained within fl"c for periods of a week or more. Before heating, each sample was AF demagnetized along three orthogonal axes to a peak field of 2 koe (200 mt). Remanences were measured periodically at 220 "C by rotating the sample in a hightemperature spinner magnetometer (Burakov 1977), whose fluxgate sensor (a permalloy ring with carefully balanced secondary halfwindings) surrounded the water jacket of the furnace. The heater current and the CRM magnetizing field (supplied by a single winding on the water jacket) were suppressed momentarily during measurements. The entire unit was enclosed in a twolayer permalloy shield, which reduced stray fields at the sample location to <5nT. The noise threshold of the magnetometer (3 X A m') is several orders of magnitude less than the typical signal we were measuring and the longterm drift was <0.5 per cent per hour and nonsystematic. It was this excellent stability that made very long remanence acquisition and decay runs feasible. Hysteresis measurements and AF demagnetization of the remanences were carried out at room temperature, after cooling the samples in zero field to prevent acquisition of partial thennoremanent magnetization (TRM). Hysteresis parameters measured on an Aseries sample after oxidation for 30hr at 220 C were Ms=82.3f 1.0 A mz kg', M,,/M, = 0.166, H, = 148 Oe. Xray diffraction carried out on sample C1 after a similar heating in air revealed a shift of the three principal spinel peaks to slightly higher angles. Superstructure reflections from the (210) and (211) planes were also evident. The angles for all lines matched standard ASTM values for maghemite. The unit cell edge a, = , corresponds to an oxidation parameter z = 0.96 (Gapeev & Gribov 1989). Virtually no haematite can have formed, since after oxidation no rhombohedra1 Xray lines were seen and Ms remained 90 per cent of M, before oxidation. We conclude that the starting magnetite has undergone singlephase oxidation to maghemite. 3 EXPERIMENTAL RESULTS FOR SERIES A SAMPLES 3.1. CRM and VRM acquisitionhe dependence and applied field In the first set of experiments, samples Al, A2 and A3 (each containing 1 per cent by weight magnetite) were oxidized at 220 C for 30 hr in applied fields H,=O.2, 0.5 and 1.0 Oe (20, 50 and 100 pt) respectively. The acquisition of remanence as a function of time t after heating to 220 "C and switching on Ho is illustrated in Fig. 1. Chemical remanence is acquired gradually, not abruptly. The time dependence is close to log t. In fact it is closer to being logarithmic than the time dependence of VRM (also shown

3 CRM and VRM in lowtemperatureoxidized magnetite 409 IOr nm Fe, 0,, oxidized at 220 OC fa samples, 1% by wt j Fwe 1. Acquisition of chemicoviscous remanence (CRM + VRM) during oxidation of magnetite in three different applied..ids, and the comparative acquisition of VRM by the oxidized phase maghemite, over a time span of 120 s to 30 hr at 220 "C. in Fig, l), which was acquired in a later experiment after the. CRM of sample A3 had been AF demagnetized and the daughter product maghemite was again heated to 220 "C and exposed to a 100pT field. Notice that the pure VRM is about 50 per cent of the CRM acquired over the same time interval, after subtracting the initial (t = 120 s) remanences in each case. Equivalently, the CRM acquisition rate (i.e., am/a log t) is about twice the VRM acquisition rate. For any time interval, the CRM acquired is proportional to the field strength, at least over the pt range. The remaining experiments used a 100 pt field to maximize remanence intensities CRM and VRM decayhe dependence After 30 hr of oxidation at 220 "C the field was switched off and decay of the CRM of sample A3 in zero field was measured at intervals during the next 162 hr (about 7 days). As noted earlier, the temperature and duration of the acquisition experiment were carefully chosen to achieve essentially total (96 per cent) conversion to maghemite. We therefore assumed that further chemical changes during the additional 162 hr at 220 "C were minimal, so that the decay observed can be ascribed entirely to viscous relaxation. The results are given in Fig. 2 (curve 1). /OO 200 nm Fe3 0,, oxidzed at 220OC fsamp/e A3,I% by wil \ $ 04 u, F ,i1 1 L I I I,,,,I lo Time, f (sj Figure 2. Zerofield decay of CVRM and VRM in sample A3, measured at 220 "C over a time span of 150 s to 162 hr. Curves 1 and 2 continue curves 1 and 2 of Fig. 1. CVRM and VRM decay logarithmically with time at almost the same rate and abruptly reach a residual level of no further decay after hr or days (about 3 times the duration of each acquisition experiment). Our interpretation is that the decaying part of CVRM is physically equivalent to the maghemite VRM, while the residual part of CVRM represents 'true' growth CRM. The two parts of CVRM have approximately equal intensities. I

4 410 A. K. Gapeeu et al. About 13 per cent of the CRM was lost in the first 150s after the field was turned off. This part of the remanence, affecting grains with relaxation times r < 150 s, is slightly less than the shortterm (t < 120 s) remanence acquired when the field was switched on (Fig. 1, top curve), which amounted to about 18 per cent of the remanence acquired in 30 hr. The difference (4 per cent) represents nonviscous isothermal remanent magnetization (IRM), unrelated to either chemical change or long times, which must be subtracted from the total remanence to arrive at estimates of VRM and CRM components. The decay of the chemical remanence during the period 150s to =3 x 105s (about 3.5 days) was almost exactly logarithmic with time. Thereafter there was no measurable decay for the final 3.25 days. We interpret this to mean that the chemical remanence consists of two components: VRM with time constants extending up to 3 times the original acquisition time of 30hr; and residual CRM, blocked by volume growth and not susceptible to viscous decay, which is isolated after about 3.5 days in zero field. If only relaxation times >150 s are considered (since for practical purposes this is about the minimum laboratory measurement time), VRM represents =45 per cent and residual CRM =37 per cent of chemical remanence acquired under these experimental conditions. In view of the intricate geometry of parent and daughter phases, and the exchange (and possibly also magnetostatic) coupling between them, blocking residual CRM in an oxidizing crystal must be a more complicated process than blocking CRM by growth of an isolated crystal with no magnetic parent through a critical blocking volume, as envisaged by Kobayashi (1961). In both cases, nontheless, CRM results when the magnetization of the newly formed mineral is immobilized in the face of thermal excitations, and this is basically controlled by the excitation volume of the new crystals or crystallites. We will continue to use growth CRM as a shorthand for the probably complex volume blocking process in twophase systems. In support of the CRM + VRM interpretation of CVRM, we next consider the VRM decay results for the same sample (Fig. 2, curve 2). This.VRM was acquired by the daughter phase maghemite after the sample had experienced the following magnetic history: initial AF demagnetization of the magnetite parent phase in 200 mt; 30 hr of oxidation in a 100pT field at 220 C; 162hr in zero field at 220 C; zerofield cooling to room temperature (RT); AF demagnetization of the maghemite daughter phase in 100mT (these results will be described in the next section). From this initial demagnetized state, sample A3 was heated to 220 C and exposed to a 100 pt field for 30 hr, the VRM acquired being measured at intervals (Fig. 1 curve 2). Then the field was switched off and the decay of VRM was measured at 220 C for the next 162 hr. Finally the sample was cooled in zero field to RT and AF demagnetized. In other words, every effort was made to carry out exactly parallel experiments on the virgin magnetite and the oxidized maghemite. The only difference is that a phase change from Fe,O,+ yfe,o, was occurring during the first acquisition experiment, so that the daughter maghemite acquired a chemical remanence, whereas chemical changes had reached completion before the second cycle of experiments, so that the maghemite then acquired a pure VRM. The VRM decays by an amount equivalent to 40 per cent of the total chemical remanence in the first cycle of experiments over the time span 150 s to about 4 x lo5 s, then levels off at a residual remanence of 9 per cent. The residual remanence in this case presumably represents timeindependent initial IRM, since only very minor chemical changes took place during the VRM experiment. The corresponding figures for the first cycle of experiments were timedependent remanence decay: 45 per cent; and initial IRM: 5 per cent. The numerical agreement is reasonably close when one recalls that the initial IRMs are not equivalent in the two cases. In the first cycle, the initial phase was virgin magnetite; in the second cycle, it was maghemite with a 192 hr heating history and harder (higher M,,IM, and H,) hysteresis parameters. We conclude that about YO per cent of the timedependent remanence that decayed in the first cycle of experiments is physically equivalent to the VRM that decayed in the second experimental cycle. The residual remanence isolated after 3.5 days of viscous decay in the first cycle must represent true CRM blocked by volume growth of maghemite crystallites. Returning to the acquisition results (Fig. 1, curve l), we are led to the conclusion that not only the VRM component but also the CRM component of the overall remanence must grow in proportion to log t, since the total remanence has a log t time dependence. Equivalently, the difference between the VRM and chemical remanence acquisition curves for A3 in Fig. 1 at any given time equals the amount of true CRM acquired, and this difference increases approximately as log t. There is no a priori reason to expect growth CRM to increase as log t (in fact it is rather inconvenient, since it complicates the separation of the CRM and VRM components), but there seems to be no other interpretation of the overall log t increase in the total remanence. Gallagher, Feitknecht & Mannweiler (1968) found that the degree of oxidation was proportional to t, as expected for oxygen diffusion, at least in the initial stages of oxidation for grain sizes of tens to hundreds of nm. In Haneda & Morrish s (1977) experiments, ultrafine magnetites (7.7, 11.9 and 22nm) oxidized in air according to a thirdorder rate process over periods of 140 hr, depending on temperature. Brown & O Reilly (1987) showed that iron removal rather than oxygen addition was the mechanism of aqueous oxidation of a synthetic =200 nm magnetite. About onehalf conversion to maghemite occurred in 60 days at 100 C although the kinetics were not monitored at very frequent intervals. The key to our observations may lie in the strong grainsize dependence of oxidation rate (Colombo et al. 1968; Haneda & Morrish 1977). Haneda & Morrish emphasized that the size dependence is so marked that oxidation is probably an either/or process, grains less than a critical size for given (T, t) conditions being totally oxidized while larger grains are essentially unoxidized. This situation is obviously akin to Neel s (1949) blocking model of VRM and TRM, in which SD grains with magnetic relaxation times T equal to or slightly less than observation time t tend to be remagnetized viscously, slightly larger grains with larger 7 s are unaffected, and exposure to a field over a long time span leads to remagnetization in a window that moves at a uniform rate across the log t specturm (see, e.g.,

5 CRM and VRM in lowtemperatureoxidized magnetite 411 Dunlop 1973). Since maghemitization of fine particles is a thermal activation process with a frequency factor similar to that for SD remagnetization (Haneda & Morrish 1977, table l), a distribution of particle sizes will lead to a distribution of critical oxidation times and a log t dependence of maghemite (and CRM) production, whatever the kinetic law for narrowly sized particles may be. We are grateful to one of the referees for suggesting this approach. The log t growth of CRM calls into question our earlier assumption that all chemical change ceased after 30 hr at 220 C. Obviously this is an idealization and in fact the oxidation parameter was 96 per cent, not 100 per cent, after this time. A final 4 per cent oxidation, in zero field during the succeeding 162 hr, might account for the small difference between the CRM + VRM and pure VRM decay curves in Fig CRM and VRMAF demagnetization characteristics Roomtemperature AF demagnetization in steps to 100 mt was carried out on four different remanences, in the following order: (1) residual CRM after the first cycle of experiments; (2) residual VRM after the second cycle of experiments; (3) undecayed VRM produced by reheating sample A3 to 220 C after completion of the second cycle, exposing it to Ho = 100 pt for 30 hr, and immediately cooling in zero field to RT; and (4) undecayed CRh4 produced by heating a fresh sample to 220 "C, exposing it to 100 pt for 30 hr, and immediately cooling in zero field to RT. Results of the experiments are shown in Fig. 3. All of the demagnetization curves are relatively hard and have the inflected form characteristic of SD magnetite or maghemite. They are quite similar to the demagnetization curve of total TRM in the unoxidized SD magnetite that Ozdemir & Dunlop (1989) used as their starting material (Fig. 3, dashed curve). Since the CRMs in the present experiments were produced at a relatively low temperature, we had not anticipated such high coercivities. There is a distinct difference in hardness between the undecayed remanences ('acquisition only' curves in Fig. 3) and the residual remanences ('acquisition + decay' curves). The residual remanences are harder, with a difference in median destructive field (MDF) relative to the undecayed remanence amounting to 68mT. This difference is understandable in the case of the chemical remanence, since the reversible (VRM) component which has decayed away is likely to be softer than the irreversible (true CRM) component that is isolated as a residual. In the case of pure VRM, the residual was interpreted to be an initial IRM and there is no obvious reason, for an applied field as weak as 100yT, why IRM should be so much harder than VRM. It is possible that some of the residual is undemagnetized CRM from the first cycle of experiments, with coercivities >lo0 mt. It is also surprising that the demagnetization curves for both undecayed remanences are so similar. If our interpretation is correct, chemical remanence contains about nm Fe, 0, ~ oxidized of 220 *C [sample A3,lX by wt/ vrm facquisirian +zero fmd decoy/ $ o.2/,,,;; 0. I.. OO Peak alternating field (mtl : Figure 3. Roomtemperature (RT) AF demagnetization curves of residual CVRM and VRM after 162 hr decay at 220 "C compared with AF demagnetization curves of similar CVRM and VRM that were cooled quickly to RT after the 30 hr acquisition experiment. Residual CVRM is harder than undecayed CVRM, showing that the VRM component of CVRM is slightly softer than growth CRM. All the demagnetization curves are similar in form and hardness to the demagnetization curve of total TRM in the magnetite sample used by Ozdemir & Dunlop (1989) (dashed). The numbers on the curves indicate the sequence of experimental cycles [each cycle consisting of heating, acquisition, (decay), cooling, AF demagnetization]. Cycles 13 were performed on a single sample, A3, whereas cycle 4 used a fresh A series sample.

6 412 A. K. Gapeev et al. equal fractions of VRM and CRM, and its demagnetization curve might be expected to lie between the residual CRM and undecayed VRM curves. Instead it is slightly softer than the VRM curve. 4 EXPERIMENTAL RESULTS FOR SERIES B AND C SAMPLES 4.1. CRM and VRM acquisitiondependence on magnetite concentration In order to test the effect of particle interaction on VRM and CRM acquisition, sample B1 (3 per cent by weight magnetite) was carried through two experimental cycles identical to those described for sample A3 except that the CRM decay was monitored at 220 "C for 426 hr (about 2.5 weeks) and VRM decay lasted for 471 hr (about 2.8 weeks). In addition, sample Cl (10 per cent magnetite) was exposed to a 50 pt field at 220 "C for 30 hr and CRM acquisition was monitored. The reduced field (50 instead of loopt) kept the remanence signal in approximately the same range as for the B series experiments. Clumping is inevitable in synthetic particle dispersions, and makes the effect of interactions difficult to quantify. Qualitatively, it is safe to assume that interactions are more important in series C samples than in series A samples, although probably not 10 times more important. The CRM and VRM acquisition characteristics (Fig. 4) are strongly curved compared to the corresponding results for sample A3 (Fig. l), when the data are plotted on a log t scale. Walton & Dunlop (1985) predicted from a mean random field theory that particle interactions should result in nonlog t behaviour, but their acquisition curves were convex downward (as were the data they were attempting to explain), whereas the present acquisition curves are convex upward. The remanence acquiring capacity per unit mass of magnetite decreased as the concentration increased. Sample A3 (1 per cent magnetite) acquired a CRM of 7.7 X A m2 kg' and a VRM of 4.9 x lo' A mz kg' over the time span 120 s to 30 hr (Fig. 1). For sample B1, the initial acquisition rates were quite similar but decreased with time, so that after 30 hr, MCRM was only 5.2 X lo' A mz kg' and MVRM was 2.7 x A mz kg' (Fig. 4). For sample C1, assuming that MCRM is proportional to Ho for 10 per cent concentration as it was for 1 per cent concentration (Fig. l), the corresponding value MCRM would be 3.6 X lo' Am2kg'. This decrease in the efficiency of acquiring remanence is presumably due to particle interaction CRM and VRM decaytime dependence The CRM decay results for sample B1 are quantitatively very similar to the CRM decay data for sample A3, with one difference. The remanence levels immediately after switching off Ho and after very long times are virtually indistinguishable in the two samples (about 87 and 42 per cent respectively of the CRM intensity after 30hr oxidation), but the residual CRM level is reached only after =106s decay (about 11.5 days or 9 times the acquisition time), compared with about 3.5 days decay or 3 times the acquisition time in sample A3. VRM decay in the second experimental cycle also did not level off until =106s had elapsed (Fig. 5, curve 2). In this case, the residual VRM was similar to that for sample A3 (10 per cent), but the initial VRM after the field was switched off was 43 per cent, about 6 per cent lower than for A3. As a result, the VRM decay rate is noticeably less than the CRM decay rate in sample B3, and the CRM + VRM model of chemical remanence is less convincingly demonstrated. In order to check the VRM results, a third and fourth cycle of experiments were carried out. The third cycle was identical to the third cycle for sample A3. VRM was produced during 30 hr at 220 "C, after which the sample was immediately cooled in zero field to RT and AF demagnetized to 1OOmT (these results will be discussed in the next section). The fourth cycle replicated the second cycle, but the VRM decay was monitored for only 258 hr nm Fe3 0,, oxidized ot 220 O C f I1 Samp/e C/ 1/0 % by wtj Figure 4. Acquisition of CVRM and VRM by B and C series samples, under conditions similar to the A series samples (Fig. 1). Sample B1 was carried through four cycles: 1, CVRM acquisition+decay; 2, VRM acquisition+decay; 3, VRM acquisition only; 4, VRM acquisition + decay (replicate). Results for cycles 1,2 and 4 are shown; the cycle 3 acquisition curve is very similar to the cycle 2 and 4 curves. For 3 and 10 per cent magnetite concentrations, the acquisition curves deviate from log t form, and the efficiency of CVRM acquisition per unit weight of oxide decreases.

7 * IO $09 B 8 08 e 06 F 2 05 p 04 P b 02 E ZOOnm Fe3 0,, ox/dized at 220 "C fsampe BI,3% by wl/ because the residual level (again around 10 per cent of initial CRM intensity) was reached after about 5 X lo6 s (=6 days or 4.5 times the acquisition time). In this case, the decay rate was identical to that in the second cycle (Fig. 5, curve 4), but the initial VRM intensity was 3 per cent lower, with the result that the residual level was reached earlier. It is remarkable that the residual CRMs were identical in the A3 and B1 experiments: about 40 per cent of the initial CRMs after 30 hr of oxidation. Likewise, the residual VRMs in three independent experiments were about 10 per cent of initial CRMs in companion experiments. The decay times required to reach these base levels were quite variable from 3 to 9 times the 30 hr VRM or CRM acquisition times. They seemed to depend on the amount of remanence demagnetized more or less instantaneously after suppressing the field as well as on the viscous decay rate. The lower the starting remanence, the more quickly the residual level was reached (Fig. 5) CRM and VRMAF demagnetization characteristics Figure 6 compares AF demagnetization curves of residual CRM and VRM in sample B1 after cycles 1 and 2 with that nm FeJ 0,, oxidzed at 22OOC fsomple Bl, 3% by wi/ $ /acquisition I I I I I I I I I Peak olfernotinq field H" fmrl Figure 6. RT AF demagnetization of residual CVRM and VRM, and of undecayed VRM, following cycles 1, 2 and 3 respectively, for sample B1. Residual remanences again have similar demagnetization curves (cf. Fig. 3), which are harder than demagnetization curves of undecayed remanences. Undecayed CVRM (for companion sample B2) was marginally softer than undecayed VRM, as for the A series samples (Fig. 3).

8 414 A. K. Gapeev et al. of undecayed VRM after cycle 3. Results after cycle 4 are identical with those after cycle 2 and are not plotted. The curves are very similar in their shapes and relative positions to the corresponding results for sample A3 (Fig. 3). An AF demagnetization curve of undecayed CRM measured for a companion sample B2 (Fig. 6; see also Fig. 9, curve 2) is similar to, but slightly softer than, the curve for undecayed VRM, repeating the pattern seen with the A series samples (Fig. 3). 5 EXPERIMENTAL RESULTS COMPARING PARENT AND DAUGHTER PHASES 5.1. VRM acquisition before and after oxidation A final suite of experiments was undertaken on two fresh samples, B2 and B3, each containing 3 per cent magnetite. Our main purpose was to test the assumed similarity between VRMs acquired by parent phase magnetite and daughter phase maghemite (Ozdemir & Dunlop 1989). Ozdemir & Dunlop's experiments spanned the 20"600 "C temperature range and mixtures of phases (magnetite + maghemite, maghemite + haematite) were present after many runs. For consistency, the VRM acquired by the unoxidized parent magnetite at each temperature was used as a standard. Our runs, on the other hand, resulted in almost complete conversion to the single daughter phase maghemite, and so we were able to make a direct comparison of the VRM of this daughter with the VRM acquired by its parent before oxidation. In the first experimental cycle, B2 was sealed in an evacuated quartz capsule, heated at 220 "C in a 100 pt field for 30 hr, immediately cooled in zero field to RT, removed from its capsule, and AF demagnetized to 100mT. In the second cycle, B2 was heated in air, exposed to the same field for 30 hr, cooled to RT, and AF demagnetized. The third cycle was identical to the second cycle. Sample B2 thus successively acquired three undecayed remanences: (1) VRM of unoxidized magnetite; (2) CRM+VRM during oxidation from magnetite to maghemite; and (3) VRM of maghemite after oxidation. The acquisition characteristics of these remanences are compared in Fig. 7. The initial (120s) and final (30 hr) remanence levels in CRM acquisition were very similar to those measured for sample B1 under similar conditions, although the acquisition curve had a somewhat different curvature (curve 2, Fig. 7 and curve 1, Fig. 4). VRM acquisition characteristics were also quite similar for the maghemite (curve 3, Fig. 7 and curve 2, Fig. 4), although sample B2 acquired more VRM than sample B1. However, the VRM acquisition curve for the unoxidized magnetite is unusual in form, at first resembling the CRM acquisition curve, then levelling out and reaching the same ultimate level as the maghemite VRM acquisition curve after 30 hr. The departure from log I behaviour is striking. We do not understand the results, but they were reproducible CRM and VRM decay In the final experiments, a fresh sample B3 was sealed in an evacuated quartz capsule, heated at 220 C in 100pT for 30 hr, maintained at 220 "C in zero field for 473 hr, then cooled to RT, where the capsule was opened and the residual magnetite VRM was AF demagnetized. The second cycle repeated the first cycle, except that all heatings were in air and viscous decay was monitored for 285 hr. The CRM decay results for B3 (Fig. 8, curve 5) are similar to those for B1 (Fig. 5, curve 1) in starting level and decay rate. However, a residual CRM was not isolated after 285 hr of decay, when the experiment was discontinued. The VRM of unoxidized magnetite (Fig. 8, curve 4) decays in quite a similar way to the VRM of oxidized maghemite in sample B1 (curves 2 and 4, Fig. 5), despite the contrast between the acquisition characteristics of the two remanences. The main difference is that both the starting level and the residual VRM after =106s decay are higher for magnetite than for maghemite. c\l F P 6? s OOnm Fe, O4, 220 C f sample BZ, 3 % by wi.)

9 IO OOnm Fe, 0,,220 "C fsomple 83,3% by wf. I 'L D Fo 5 E 04! 03 b,,202 E O I =: 0 ' I 1 1 j I 03 lo4 lo5 lo AF demagnetization characteristics Figure 9 compares AF demagnetization curves for the three undecayed remanences acquired by B2 and for the residdal VRM of B3. Numbers on the curves correspond to the numbers in Figs 7 and 8. Curves 2 and 3 (undecayed CRM and VRM respectively) closely resemble previous data (e.g., curves 3 and 4, Fig. 3). The demagnetization of residual VRM carried by magnetite (curve 4) is similar to the demagnetization of residual VRM in maghemite (curve 2, Figs 3 and 6), except that it is somewhat more resistant at high demagnetizing fields. The undecayed VRM of magnetite (curve 1) at first demagnetizes in a similar way to the other undecayed remanences, but is considerably more resistant at high cleaning fields. These results did not give the clearcut answer we had hoped for to the question of how similar VRMs are in the parent and daughter phases. There are general resemblances, but there are also a number of unanticipated differences, particularly in acquisition characteristics \ foa 200 nm Fe, 0,,220 OC f samples 82 B 83,3 % by wf) $ > 0.5 i 0.4 VRM, unoxidized Fe3 0, /acquisition +zero field decay) /acquisition only) 0. O.*I I L 10 I I I I I I, I ~. _ Peak affernoiing fiefd fmti FIpe 9. RT AF demagnetization of CVRM and VRMs following runs 1, 2, 3 and 4 of Figs 7 and 8. Residual VRM of unoxidized magnetite is harder than undecayed VRM of the same material, which in turn is somewhat harder than undecayed VRM or CVRM after oxidation of the starting material to maghemite.

10 416 A. K. Gapeev et al. 6 DISCUSSION AND INTERPRETATION OF RESULTS It was our intention to singlephase oxidize Fe304 completely to yfe,o, during the first 30hr run of each series of experiments. Subsequent decay (Ho = 0) and acquisition experiments on the same sample would then involve purely viscous processes. We seem to have been successful in this aim. No haematite was detected in the oxidation products by Xray diffraction, and after 30 hr at 220 C, conversion to maghemite was 96 per cent complete and the rate of CRM acquisition was decreasing logarithmically. An increase in CRM acquisition time, in order to come closer to 100 per cent oxidation, was undesirable because decay runs would have had to be prohibitively long. (Even in the present runs, decay had to be monitored for up to 3 weeks to isolate residual CRM and VRM plateaus.) A higher temperature would have accelerated oxidation and shortened acquisition and decay times, but we preferred as low a temperature as possible to minimize physical changes in the fine grains. In this we also seem to have been moderately successful. The bulk coercive force changed from 13.6mT for the parent magnetite to 14.8 mt in the daughter maghemite, and the corresponding change in M,,IM, was from to The first important observation in our experiments is that CRM in the Fe304+ yfe,o, oxidation reaction is acquired, and subsequently decays in zero field, approximately in proportion to log t. The time dependence is actually closer to log t for CRM than it is for VRM acquired by the same samples after oxidation. A logarithmic time decay of CRM is not too surprising, since we anticipated that 'CRM' might consist of independent VRM and true CRM parts. Only the VRM or reversible part decays; the true CRM, once produced, is irreversible and forms a timeindependent residual. The log t acquisition rate of this irreversible CRM is at first sight surprising, but may be due to a broad spectrum of oxidation times in magnetite grains of different sizes. The total remanence can be separated into four parts: IRM acquired almost instantaneously after Ho is switched on; VRM acquired in a time <150S; VRM acquired on a timescale of 150s to 30hr; and growth CRM acquired during the same time interval. In the first series of experiments, with sample A3, these amounted to about 5, 13, 45 and 37 per cent respectively of the total remanence (Figs 1 and 2). If we restrict our attention to remanence acquired in times > 150s, the VRM and true CRM contributions to total remanence are about 55 and 45 per cent respectively. The relative contributions are the same for sample B1 (Figs 4 and 5). These two parts of the total CRM, viscous and nonviscous, or reversible and irreversible, can be separated in two ways. The first method is to compare the rate of acquisition of CRM during oxidation (curve 1 in Fig. 1 or Fig. 4) with the rate of acquisition of VRM by the oxidized sample in a later experiment (curve 2 in the same figures). Such a comparison assumes that the CRM has a viscous part whose mechanism is identical to VRM in the daughter product. The second method makes no a priori assumption about the nature of the CRM. By monitoring the timedependent decay of the CRM for long times, one observes that the decay is logarithmic, as for VRM, but ends abruptly after a time 39 times the acquisition time of 30 hr. Given these observations, it is natural to assume that the total remanence consists of a VRM with relaxation times t as much as 39 times the duration of the acquisition experiment plus a nondecaying residual CRM. Whichever method is used, the inferred proportions of CRM and VRM in the overall CVRM are the same. Our starting magnetite had a much larger average grain size than the SD magnetite studied by Ozdemir & Dunlop (1989) but it nevertheless proved to be very viscous. For each series of experiments, the CRM acquired was approximately twice as intense as the VRM produced in the daughter phase in a later experiment. In this respect, our results are very similar to those of Ozdemir & Dunlop, although 100nm grains (approximately the lower limit of sizes observed) should be well above critical blocking size at 220 C. Possibly our starting material has a fraction of ultrafine grains not obvious in SEM photographs, and these grains are responsible for both VRM and the viscous part of CRM. Since the virgin magnetite is even more viscous than the oxidized maghemite (Fig. 7), it is clear that originally homogeneous grains are not subdivided into numerous crystallites after oxidation, although such subdivision during oxidation is quite possible. In the one test carried out, the intensity of CRM produced by applied fields of different strengths H, for the same exposure time t was proportional to Ho for weak fields (H, < 100 yt or 1 Oe). When the concentration of magnetite was increased from 1 to 3 to 10 per cent, the CRM intensity per unit weight of oxide decreased by about a factor 2 (Figs 1 and 4). Increased interaction among the SD or nearly SD particles decreases the efficiency of the CRM process. CRMs and VRMs which were cooled quickly to RT immediately after acquisition had AF demagnetization curves that were comparatively hard and SDlike (Figs 3, 6 and 9). MDFs were about 25mT for either remanence. Residual CRMs and VRMs which were allowed to decay viscously at 220 "C for long times before being cooled to RT had significantly harder AF demagnetization curves, with MDFs of 3035 mt. The CRM + VRM model of chemical remanence suggests that residual CRM should be harder than undecayed VRM, as observed, with undecayed CRM having intermediate hardness, contrary to observation. Thus our model that chemicoviscous remanence consists of independent VRM and 'true' or growth CRM components is too simplistic. In most other respects, however, this simple CVRM model gives a reasonable firstorder picture of the experimental behaviour of chemical remanence acquisition in the oxidation of fineparticle magnetite. 7 IMPLICATIONS FOR PALAEOMAGNETISM AND MAGNETIC ANOMALY MODELLING Maghemitization of magnetite or titanomagnetite is probably the commonest chemical alteration with which palaeomagnetists have to deal. It is characteristic of surface or nearsurface oxidation, particularly in the presence of groundwater or seawater, and of lowtemperature metamorphic environments. The oxidation temperature of 220 "C in

11 CRM and VRM in lowtemperatureoxidized magnetite 417 our experiments would correspond to the laumontite (zeolite) zone in the Icelandic lava pile, with an original burial depth of 1.52km (AdeHall, Palmer & Hubbard 1971), or to similar depths in the seafloor near active spreading centres. Direct sampling of the oceanic crust (Smith & Banerjee 1986) and of analogous crustal sections in Iceland (Hall 1985) and the Troodos ophiolite (Hall & Fisher 1987) shows that at these depths, hightemperature alteration of primary magnetite and production of secondary magnetite are the dominant magnetic reactions. Maghemitization is complete at depths of km (Hall 1985; Smith & Banerjee 1986), with temperatures 5100 C. However, because of the difference between the timescale of hours to weeks for our laboratory experiments and the 12Myr timescale for maghemitization in the oceanic crust (Ozima 1971), the kinetics of our 220 C laboratory simulations are equivalent to those for oxidation in nature at =loo C temperatures. Both achieve essentially 100 per cent oxidation to a singlephase spinel, without the appearance of rhombohedra1 phases. Ozdemir & Dunlop (1985) and Heider & Dunlop (1987) have shown that when nearsdsize titanomagnetite or magnetite with a prior TRM is maghemitized, CRM forms parallel to the TRM and not in the direction of the field applied during oxidation. In the present experiments, the parent magnetite was initially demagnetized; as a result, both CRM and VRM components of CVRM were parallel to the applied field. In nature, on the other hand, the parent magnetite normally has a remanence. In this case, residual CRM would presumably couple to the parent remanence, while the VRM, since it is acquired by nearly superparamagnetic grains or regions of grains, would be decoupled and follow the applied field. This hypothesis remains to be tested in future experiments, but if it is true, CVRM would be a bivectorial remanence with a resultant direction intermediate between the parent remanence and applied field directions. Such an intermediate direction would have no palaeomagnetic or palaeotectonic significance, but might be mistakenly thought to be significant if the composite nature of the remanence was not recognized. With kinetics and field strength equivalent to those in the present oxidation experiments (T = 220 C, t = 30 hr, H,, = 1 Oe), which resulted in VRM and residual CRM with approximately equal intensities, the resultant CVRM (MCRM+MVRM) would have a direction about midway between M,,, and MVRM. For example, if primary magnetization occurred before, and CVRM acquisition after, a field reversal, MCRM + MVRM might have an anomalously shallow inclination that is entirely spurious. What is more, because the CRM and VRM components have similar AF demagnetization characteristics (Figs 3, 6, 9), the bivectorial nature of M,,,, would not be obvious from AF cleaning. A gradual directional swing in MCVRM in the course of viscous decay experiments (cf. Figs 2, 5 and 8) would be expected, but long storage tests are not routinely carried out in most palaeomagnetic laboratories. Laboratory CRMs with apparently stable intermediate directions (Bailey & Hale 1981; Ozdemir & Dunlop 1985; Heider & Dunlop 1987) have usually been ascribed to magnetostatic interactions giving a net field in an intermediate direction, but it is possible they could in some cases be due to superimposed VRM and CRM. The NRMs of basalts drilled during DSDP and ODP legs frequently have anomalously shallow inclinations. These could also in some cases be unrecognized composites of normal and reversed remanences with similar coercivity spectra. A short storage test at aboveambient temperatures (e.g. Dunlop & Hale 1977) might be more informative than the usual AF cleaning experiment. It is common practice for those modelling oceanic magnetic anomalies (e.g. Raymond & LaBrecque 1987; Verhoef & ArkaniHamed 1990) to lump together as TRM all remanence parallel to the primary field direction, most of which is actually CRM (Hall 1977), and to use CRM to mean secondary remanence in a different (usually reversed) direction, however formed. This terminology is undoubtedly much influenced by the early ideas of Marshall & Cox (1971, 1972), but in the case of SD or nearly SD magnetite and titanomagnetite, the TRM is mostly maghemite CRM that has inherited the primary TRM direction (Ozdemir & Dunlop 1985; BeskeDiehl 1989), while CRM judging by the results of our present study, is largely VRM. While the physical mechanisms of remanence have been somewhat clouded by this terminology, the basic idea of composite remanences of different ages and origins is sound. What has perhaps not been appreciated is that CRMs and VRMs produced at relatively modest temperatures have surprisingly high resistance to AF cleaning (Figs 3, 6, 9; see also Dunlop & Ozdemir 1990) and that their coercivity spectra are so similar that AF demagnetization is not a good way of resolving the composite remanence. Zerofield storage or thermal demagnetization are likely to be more successful. 8 CONCLUSIONS Our main conclusions from these experiments are the following. (1) CRM and VRM had similar properties in oxidized magnetites nm in size (larger than RT equilibrium SD size). The similarity between CRM and VRM is not confined to grains only slightly larger than critical blocking size. (2) In oxidation at 220 C, CRM intensity increased as log t for exposure times up to 1 day at least. (3) CRM intensity decayed in zero field in proportion to log t, in exactly the same way and at almost the identical rate as VRM acquired by the oxidized daughter maghemite in a later experiment. (4) A VRM of similar intensity was acquired by the parent phase magnetite, but the acquisition and decay were nonlogarithmic with time and more complicated than those of the daughter phase. (5) After a time 3 times the acquisition time in some experiments, but 4.59 times the acquisition time in other cases, there was no further viscous decay of CRM and a true CRM residual was isolated. (6) Residual CRM and VRM each form approximately 50 per cent of the total remanence, which is appropriately called a CVRM (Ozdemir & Dunlop 1989). (7) The AF stability of residual CRM was similar to, but slightly greater than, that of either VRM or CVRM which were quenched and not allowed to decay viscously. Thus the

12 418 A. K. Gapeeu et al. viscous part of CVRM is slightly softer to AF demagnetization than the residual CRM. (8) The intensity of CVRM was proportional to applied field, for weak fields, just like VRM or growth CRM (Kobayashi 1962; Stokking & Tauxe 1990). (9) The efficiency of the CVRM process, as measured by remanence intensity per unit mass of maghemite, decreased by about a factor 2 when the magnetic oxide concentration was increased from 1 to 10 per cent by weight. (10) CVRM produced by oxidizing nearsdsize magnetite with an initial remanence could be a composite remanence with residual CRM in the initial remanence direction and VRM in the field direction. Such bivectorial remanences with spurious intermediate directions might be difficult to decompose by AF cleaning. They are a likely contaminant of the oceanic magnetic anomaly record. ACKNOWLEDGMENTS The comments of the reviewers, Drs Guy M. Smith aod Subir K. Banerjee, about the effect of oxidation kinetics on the time dependence of CRM and palaeomagnetic implications of the experimental results were very useful. This research was supported by Energy Mines and Resources (EMR) Canada and the Natural Sciences and Engineering Research Council (NSERC) of Canada through EMR Research Agreements and NSERC Operating Grant A7709 to DJD. DJD and 00 are grateful to the USSR Academy of Sciences for sponsoring a visit to the Borok Observatory, during which this work was begun. REFERENCES AdeHall, J. M., Palmer, H. C. & Hubbard, T. P., The magnetic and opaque petrological response of basalts to regional hydrothermal alteration, Geophys. J. R. astr. Soc., 24, Bailey, M. E. & Hale, C. J., Anomalous magnetic directions recorded by laboratoryinduced chemical remanent magnetization, Nature, 294, BeskeDiehl, S. J., Comments on Magnetization of the oceanic crust: thermoremanent magnetization or chemical remanent magnetization? by C. A. Raymond & J. L. 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