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Volume 9, Number 9 18 September 2008 Q09Y03, doi:10.1029/2008GC002054
AN ELECTRONIC JOURNAL OF THE EARTH SCIENCES Published by AGU and the Geochemical Society
ISSN: 1525-2027
Paleointensity determination using continuous thermal measurements by a high-temperature vibrating thermomagnetometer David J. Dunlop Geophysics, Physics Department, University of Toronto, Toronto, Ontario M5S 1A7, Canada (
[email protected])
[1] A vibrating thermomagnetometer which measures magnetization M continuously at high temperature is used to test a new method of paleointensity determination involving single rather than double heatings. Loss of natural remanent magnetization (NRM) is recorded by the average of 20–25 measurements at the peak temperature T achieved in a zero-field heating step. Partial thermoremanent magnetization (pTRM) is calculated from the average M after in-field cooling to room temperature T0. For comparison with data taken at T0, values of M measured at T are multiplied by Ms(T0)/Ms(T), with the aid of the thermomagnetic or Ms(T) curve measured on a chip of the sample. NRM versus pTRM results from 11 heating-cooling steps on a diabase containing both single-domain magnetite inclusions in plagioclase and coarser multidomain magnetite grains reproduce the features of Thellier double-heating paleointensity results for samples from the same site. The NRM – pTRM plot is nonlinear, with convex-down curvature. For a stringent validation of the single-heating method, a truly single-domain sample needs to be tested. Components: 4552 words, 5 figures. Keywords: paleointensity; thermomagnetometer; magnetite; thermal cycling. Index Terms: 1521 Geomagnetism and Paleomagnetism: Paleointensity; 1594 Geomagnetism and Paleomagnetism: Instruments and techniques; 1503 Geomagnetism and Paleomagnetism: Archeomagnetism. Received 9 April 2008; Revised 3 July 2008; Accepted 29 July 2008; Published 18 September 2008. Dunlop, D. J. (2008), Paleointensity determination using continuous thermal measurements by a high-temperature vibrating thermomagnetometer, Geochem. Geophys. Geosyst., 9, Q09Y03, doi:10.1029/2008GC002054. ————————————
Theme: Advances in Instrumentation for Paleomagnetism and Rock Magnetism Guest Editors: M. Fuller, B. Goodman, J. Kirschvink, and K. Verosub
1. Introduction [2] Paleomagnetic field intensity is usually determined by the Thellier method [Thellier and Thellier, 1959; Coe, 1967]. A sample is heated to increasing temperatures and its remanence is measured at room temperature after each step. The first heating/ Copyright 2008 by the American Geophysical Union
cooling, in zero field, erases part of the natural remanent magnetization (NRM). Heating again to the same temperature and cooling in a laboratory field HL produces a partial thermoremanent magnetization (TRM) whose value is determined by vector subtraction of the results of the first and second steps. In the ideal case where the NRM is a
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TRM acquired in nature in a field HA by noninteracting single-domain grains, an Arai plot [Nagata et al., 1963] of NRM lost versus partial TRM gained in heatings to different temperatures is linear with a slope of –HA/HL, yielding the paleointensity HA. [3] Many variations on the Thellier method have been proposed, e.g., the IZZI protocol [Tauxe and Staudigel, 2004] in which the zero-field and infield steps are interchanged at successive temperatures. All are laborious and the labor is doubled by the two heatings required to each temperature. Kono [1974] introduced a method that dispenses with the second heating-cooling step by applying HL perpendicular to the initial NRM vector. NRM loss and partial TRM gain should then be recorded by independent orthogonal components of the composite remanence. Orthogonality is hard to achieve, particularly if the initial NRM is not a pure TRM produced by HA but has been overprinted by later fields with different directions. [4] There is a simple method of avoiding double heatings, without the need for special sample orientation, while preserving the Thellier protocol. It uses a thermomagnetometer which measures magnetization continuously during heating and cooling. NRM loss is tracked during heating in zero field, while partial TRM gain is recorded in the course of cooling in HL. This approach is justified by Ne´el’s [1949] thermal activation theory. Single-domain grains demagnetize only during heating: no NRM unblocking occurs in the cooling half of the first heating-cooling cycle. Likewise, partial TRM is blocked only during cooling: the heating half of the second cycle is superfluous. [5] In the single-heating method, the last measurement during heating, at temperature T before HL is applied, should equal the conventional first-heating result, reduced by the reversible decrease in spontaneous magnetization Ms(T). The thermomagnetic curve Ms(T), quickly determined on a small chip of the sample, is used to calculate the corresponding room temperature values. The ultimate measurement at room temperature after zeroing HL matches the conventional secondheating result and needs no correction. [6] The purpose of this study was to test how well this procedure works in practice. It proved impossible to use a single-domain sample for the test, as explained below, and one with overall pseudosingle-domain (PSD) behavior was substituted. This has its own interest because Ne´el’s [1949,
1955] theories do not adequately describe TRM in grains above single-domain size. For example, McClelland and Sugiura [1987] and Muxworthy [2000] demonstrated that partial TRMs produced by field-cooling 7.5 mm and larger magnetite grains from the Curie point TC to a temperature T partially demagnetize during zero-field cooling below T. This does not in itself violate the basic assumptions above. If a previously produced TRM (the NRM) is reheated to T, further demagnetization during zero-field cooling from T is slight [Biggin and Bo¨hnel, 2003]. However, there could well be other violations of a simple blocking/ unblocking picture that would invalidate the single-heating approach in larger grains.
2. Previous High-Temperature Magnetometers and Measurements [7] Wilson [1961, 1962] designed an ingenious vector measuring system with a furnace located just below an astatic magnetometer. The quartz sample holder could be rotated about a vertical axis and the sample itself rotated about its horizontal axis by a quartz screwdriver inserted through a small hole in the furnace wall, allowing full vector measurements of sample moment up to 700°C. Two NRM components were separated in a laterite baked by a lava flow, one the hematite primary TRM, the other a thermoviscous overprint. Paleofield intensity was determined by comparing, over the range 420 –650°C, continuous thermal demagnetization curves of NRM and of laboratory TRMs produced by 0.13, 0.40, and 0.82 Oe fields. Specific comparisons at 420, 470, 520, 570, and 595°C verified that the intensity of surviving TRM at each temperature was proportional to H and narrowly bracketed the ancient field intensity HA. This is a remarkable achievement for an early study, although the procedure is cumbersome and slow by today’s standards. Recently Zheng and Zhao [2006] obtained good results by a similar comparison of NRM and TRM thermal demagnetization curves. [8] Nagata and Ozima [1956] measured how TRM develops during cooling using a ballistic magnetometer with pickup coils encircling a water-jacketed furnace. Even with negative feedback amplification [Dunlop and West, 1969], ballistic measurements lack the sensitivity needed for most paleointensity work. Parry [1975, 1979] measured TRM and pTRM development during cooling using an astatic system and Sugiura [1989] reviews similar studies using SQUID gradiometer and vi2 of 8
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brating-sample magnetometer (VSM) systems. These studies were not aimed at paleointensity determination, although the detectors do have the requisite sensitivity. [9] Heiniger and Heller [1976] and Schmidt and Clark [1985] designed spinner magnetometer systems for high-temperature paleomagnetic measurements. As with Wilson’s instrument, the challenge comes in changing the sample orientation at high temperature so that all three vector components can be measured. This problem has been overcome recently by Le Goff and Gallet [2004], whose VSM incorporates three sets of pickup coils detecting orthogonal components. [10] One question that directly affects the singleheating method proposed in the present paper is whether continuous thermal demagnetization data, when multiplied by Ms(T0)/Ms(T), accurately reproduce stepwise data measured at room temperature T0. Schmidt and Clark [1985] performed this test for the Milton Monzonite of Australia, a rock with typically single-domain or small PSD character [Dunlop et al., 1997]. Continuous thermal demagnetization data, corrected to T0, almost perfectly reproduced vector plots of two-component NRMs determined from stepwise thermal demagnetization data. [11] Various protocols have been used to determine paleointensities from high-temperature measurements. Le Goff and Gallet’s [2004] method, like Wilson’s, compares separately determined NRM and TRM continuous thermal demagnetization curves but it uses only data between predetermined limits T1 and T2, chosen to avoid both viscous overprinting and chemical/textural alteration. Increments DNRM and DTRM between T and T2 or between T1 and T are compared to infer HA/HL. An extra cooling-heating cycle between T2 and T1, akin to an Ms(T) correction, is required to compute the increments. The procedure is automated and works well for the pottery samples tested but lacks the Thellier interweaving of NRM decrements and partial TRM increments that permits viscous and alteration effects to be detected in individual samples as the run proceeds. [12] Interwoven NRM and TRM increments are central to other approaches. Walton [1984] compares zero-field and in-field viscous increments in NRM over 10 min intervals at a set of ascending temperatures. This makes use of a tiny fraction of the TRM blocking/unblocking spectrum and yields
correspondingly tiny moments that only a SQUID can measure. Boyd [1986] and Tanaka et al. [1995] use SQUIDs and conventional double-heating increments in NRM and TRM but the lower temperature where measurements are made increases with each step as well as the upper heating temperature. Ms(T) correction is not needed for individual steps because NRM and TRM are measured at the same T. Tanaka et al. correct for Ms(T) in order to incorporate results for all steps in Arai plots. Some of the advantage of measuring at high T is lost because of the double heatings. The methodology of using SQUIDs and furnaces in combination is dealt with by Day et al. [1977] and Fuller [1987].
3. Experimental Design [13] The vibrating thermomagnetometer (VTM) used in this work is a modified Russian-designed instrument described briefly by Biggin and Poidras [2006]. The basic pickup coil and mechanical design is that of a VSM but the electronics have been upgraded. The vibration amplitude is unusually large for a VSM (several millimeters). Although the sensitivity is improved, wear and tear on the sample is considerable (see below) and the acoustic noise is so high as to require an isolated environment while experiments are running. Another limitation is that only the magnetization component parallel to the vibration axis can be measured. This forced the present experiments to be simulations in which both NRM (a laboratory TRM) and partial TRMs were produced parallel to the axis of the cylindrical sample, rather than full vector paleointensity experiments. A better test could be made using three-axis pickup coils as in the VSM of Le Goff and Gallet [2004]. [14] The VTM permits a variety of heating rates and atmospheres. In the present work, heating/ cooling was in an Ar atmosphere at a rate of 10°C/min. Measurements were made at 8 s intervals (i.e., approximately every 1.3°C) while heating in zero field and cooling in 50 mT (0.5 Oe), and also at 8 s intervals during the 3 min dwell times at peak temperature T and room temperature T0. There were 11 heating-cooling cycles to T = 100, 150 . . . 550, 570°C. [15] The sample is a 0.5-cm minicore of a Matachewan Diabase sample (Dunlop et al. [2005], site TK49). TK49 plagioclase phenocrysts contain inclusions of single-domain magnetite. Unfortunately, the vigorous vibrations of the VTM lique3 of 8
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[17] The other drawback of the method as implemented here is the fact that only one axis of magnetization could be measured with the VTM. It is difficult to orient the laboratory field exactly parallel to the primary NRM, especially if later overprints are present. For routine use of the single-heating method proposed here, three-axis pickup coils are needed so that directional changes can be monitored.
Figure 1. Continuous measurements by a vibrating thermomagnetometer of magnetization M during four successive cycles of cooling from 600°C to room temperature T0 in a field H of 50 mT (0.5 Oe) applied along the sample (and vibration) axis, suppression of H at T0 (giving TRM), and heating of the TRM in zero field from T0 to 600°C. The heating curve in run 2 is omitted; it is indistinguishable from those of runs 1 and 3. The TRM stabilizes at a constant value after the first cooling from 600°C.
fied the CaF2 matrix of a TK49 plagioclase separate and a rock core had to be substituted. The whole rock has an overall PSD character because of the combination of single-domain plagioclase inclusions and coarser magnetite in the groundmass. Both types of magnetite are essentially pure (titanium-free) and stoichiometric. AC susceptibility k(T) measured in a weak field with an Agico Kappabridge dropped sharply around 570°C. This is just below the Curie point of 575°C determined from a strong-field thermomagnetic curve measured in Ar with a Princeton Instruments VSM. [16] Ms(T) from the same thermomagnetic curve was used to reduce high-temperature paleointensity measurements to their room temperature equivalents. The minicore used for paleointensity work did not exhibit obvious magnetomineralogical changes after its many heatings, apart from a small decrease in total TRM intensity (Figure 1), but to be on the safe side k(T) and Ms(T) were measured on fresh material. For rocks less homogeneous than TK49 it might be advisable to compare Ms(T) data from several fresh chips taken from the core before heating.
[18] The great advantage of the new method is speed. Although not done in the present study, pTRM checks could be added to the experimental routine by a second heating in zero field, followed by cooling in zero field to a lower temperature where field H is applied. Another advantage that could potentially be utilized is the continuous recording of magnetization during heating and cooling. For paleointensity determination, only the data at peak temperature and room temperature are used, but the continuous record of both demagnetization in heating and remagnetization in cooling might give insights into subtle magnetomineralogical changes and could be used to test Biggin and Bo¨hnel’s [2003] speculation that a small selfreversed pTRM is acquired close to the Curie point.
4. Results [19] In preliminary experiments, the sample was repeatedly given a 50-mT TRM, which was then demagnetized by zero-field heating to 600°C. Infield magnetization was continuously monitored during cooling and remanent magnetization was measured during heating (Figure 1). Both changed rapidly just below the magnetite Curie point. All chemical and/or microstructural changes were complete after the first cooling. The four TRMs are very similar and subsequent cooling and heating curves nearly coincide. [20] Figure 2 illustrates the results of the first six zero-field heating plus in-field cooling cycles. The NRM (1) is the final TRM (4) of Figure 1. NRM fractions surviving 100°C, 150°C . . .350°C heatings are the (time-averaged) values 2, 6. . . 22. Corresponding values at T0 were later calculated using Ms(T) from the thermomagnetic curve. H = 50 mT is switched on at T and off after cooling to T0, giving reconstituted TRMs (NRM remaining + pTRM gained) 5, 9 . . .21, 25. Ideally, each reconstituted TRM should equal the NRM since the NRM and all pTRMs were produced in H = 50 mT applied along the sample axis. 4 of 8
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Thellier double heatings on a sister sample from site TK49 (blue points).
5. Discussion and Conclusions
Figure 2. Results of the first six paleointensity simulation heating-cooling cycles. Starting with an NRM (point 1) equal to the final TRM in run 4 of Figure 1, the sample is heated in zero field to 100°C where the temperature stabilizes for 3 min (20 – 25 measurements, point 2). A field H = 50 mT is then applied (2 ! 3), the sample is cooled to T0 (point 4), the field is turned off (4 ! 5) and the temperature stabilizes for 3 min (point 5, reconstituted TRM, consisting of NRM remaining + pTRM gained in the 100°C cycle). Subsequent cycles to higher temperatures track along common heating and cooling curves, and result in essentially identical reconstituted TRMs (5, 9 . . . 21, 25).
[21] Nonideal behavior is evident in the final four cycles to 400, 450, 500, and 550°C (Figure 3). In spite of episodic instrumental noise, including an unexplained jump in M in the 400°C cycle, we can confidently determine that each reconstituted TRM (1, 5, 9, 13, 17) is systematically less than the preceding one. NRM loss is thus greater than pTRM gain, which is characteristic multidomain behavior. A final cycle to 570°C (not shown) erases any remaining NRM and produces a new total TRM equal to the initial NRM. [22] The Arai plot (Figure 4, red points) was calculated using averaged values of NRM remaining (values measured at T multiplied by Ms(T0)/ Ms(T)) and reconstituted TRM. The difference between the reconstituted TRM and the NRM remaining is the pTRM gained. The measurements are quite noisy for the weak field used but averaging 20–25 data points over the 3 min dwell times gave acceptable results. Only 10% of the initial TRM is lost in heatings up to 350°C. The Arai plot is very similar to that determined for conventional
[23] The present experiments demonstrate that a VTM is capable of delivering data that are sufficiently accurate for paleointensity determination in the course of rapid continuous thermal cycling, even in the presence of substantial instrumental noise. It remains only to discover the optimal methodology. The Arai plot has the same convexdown shape seen in NRM versus pTRM results for double-heating Thellier experiments on TK49 (Figure 4). To this extent, the single-heating method was an acceptable time-saving substitute. A truly single-domain sample needs to be tested to demonstrate linearity of the Arai plot. [24] The nonlinearity of the Arai plot is not as minor as it at first appears. Forty-five percent of the
Figure 3. Four higher-temperature heating-cooling cycles, to 400, 450, 500, and 550°C. The results differ from ideal single-domain behavior in that the cooling and especially the heating curves do not coincide and the values of reconstituted TRM are systematically lower after each successive run (points 1, 5, 9, 13, 17). This trend ends only with the terminal run to 570°C (not shown), which restores the total TRM. Note that the instrumental noise is not constant but episodic and that occasionally there is an unexplained jump in the measured M, most notably just below 100°C in the cooling curve from 400°C. Simple averaging of 20– 25 successive measurements at T or T0 gave sufficient smoothing to calculate a meaningful Arai plot (Figures 4 and 5). The jumps in the 400°C run were partially smoothed using the general trend of the cooling curves for all other runs. 5 of 8
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heating-cooling cycles. For example, Biggin and Poidras [2006] have shown that the curvature of the Arai plot when NRM demagnetization and pTRM acquisition are interleaved can be removed by simply performing separate stepwise NRM demagnetization and pTRM acquisition experiments. The improved linearity justifies doubling the length of the experiment.
Figure 4. Red points: Arai plot calculated from the data of Figures 2 and 3. Values of NRM (i.e., TRM) remaining are measurements made at the peak temperature T of each cycle before switching on H, corrected to room temperature by multiplying by Ms(T0)/Ms(T). Values of partial TRM gained are calculated differences between the reconstituted TRM measured at the end of each cycle and the corresponding TRM remaining value (corrected to T0). The overall shape of the Arai plot is convex down but points up to 550°C fall on an approximate straight line (see Figure 5). Blue points represent the Arai plot calculated from results of a conventional Thellier double-heating experiment on another TK49 core. Calibrated against Thellier results for plagioclase and dark minerals separates [Dunlop et al., 2005], the TK49 cores contain single-domain and multidomain TRM carriers in a ratio of 2:1.
[26] Dekkers and Bo¨hnel [2006] avoid the problem of sample magnetic history by using multiple specimens, each heated once only to an intermediate temperature T chosen to avoid both viscous and alteration effects. The method relies on the linearity between pTRM intensity and field H, making it in principle applicable to all domain states. H varies from run to run, from 0 to 90 or 100 mT, giving reconstituted TRMs that range between ±15% and ±50% above or below the NRM. The (interpolated) correct paleofield yields a reconstituted TRM exactly equal to the NRM. [27] Each specimen run in the method of Dekkers and Bo¨hnel [2006] is equivalent to one thermal cycle in the present experiments, except that heating and cooling are both in field H. Can we improve linearity of the Arai plots by carrying over this protocol to repeated cycles to ascending temperatures T? For exact symmetry of heating and cooling half-cycles, H should not be switched off at
TRM blocks between 550 and 570°C but only 35% unblocks over the same interval. If only the highest-temperature data were utilized in a paleointensity run on TK49, the paleofield would be underestimated by 20% (Figure 5). Low-temperature points would give a 16–30% overestimate. Such selections of data might be justified by the expectation that fractions with lower blocking temperatures had been viscously overprinted in a later field or that high-blocking-temperature fractions might have suffered alteration. New magnetic material produced during laboratory heating can give rise to spurious excess pTRM and a decrease in slope above 550°C as in Figures 4 and 5. [25] Single thermal cycles with continuous hightemperature measurements are a time-saving alternative to conventional double-heating paleointensity methods. However, in the case of nonideal (PSD and multidomain) samples other considerations may outweigh the time saved by single
Figure 5. Analysis of the linearity of segments of the Arai plot (single-heating data). Line fits up to 550°C are similar to the eye, but slopes decrease from 1.30 for the fit up to 450°C to 1.16 for the 20– 550°C fit. These lines all overestimate the paleointensity (ideal slope of 1). The final segment, from 550 to 570°C underestimates paleointensity by almost 20%. 6 of 8
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T to measure NRM loss directly. How then can we separate remanent and induced moments at T? [28] One way is to turn the sample 180° at T: the remanence rotates while the induced moment is fixed. Even this minor perturbation to magnetic history may be unacceptable at the domain level. Day et al. [1977] tried applying H first parallel, then antiparallel to the NRM (a laboratory TRM) in successive in-field cycles to the same T. Inferred remanence values at T = 248°C differed by 40% in the two cases. Thus prospects for a more symmetric variant of single-heating thermal cycling are not bright.
Acknowledgments [29] The paleointensity experiments were performed at the Universite´ de Montpellier II, France. I thank Michel Pre´vot for hosting my visit, Thierry Poidras for his help with the magnetometer and experimental design, and the university for financial support. Some sample characterization was done at the Institute for Rock Magnetism, which is funded by NSF’s Earth Sciences Division, the Keck Foundation, and the University of Minnesota. Thanks are also due to Mark Dekkers, associate editor Mike Fuller, and an anonymous referee for very helpful comments and suggestions that improved the paper. This research is part of a program funded by Natural Sciences and Engineering Research Council of Canada grant A7709.
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