Multiphase magnetizations: Problems and implications

Physics o f the Earth and Planetary Interiors, 16 (1978) 20-37 © Elsevier Scientific Publishing Company, Amsterdam.- Printed in The Netherlands

20

MULTIPHASE MAGNETIZATIONS: P R O B L E M S A N D I M P L I C A T I O N S i J.L. R O Y and P.L. L A P O I N T E

Geomagnetic Laboratory, Earth Physics Branch, Ottawa, Ont. KIA 0 Y3 (Canada}

(Received January 21, 1977; revised and accepted March 25, 1977)

Roy, J.L. and Lapointe, P.L., 1978. Multiphase magnetizations: problems and implications. Phys. Earth Planet. Inter., 16: 20-37. Experiments combining different cleaning and analytical techniques indicate that multiphase magnetizations may be quite common. However, these may not be recognized because of limited experimental work. Alternating field (AF) cleaning is often the only treatment applied to igneous and metamorphic rocks; thermal and/or AF cleanings are usually the only treatments applied to sedimentary rocks. In many instances, AF and thermal treatments are limited to 100 mT and 550°C respectively. Investigations based on such limited cleaning often fail to detect some of the phases of magnetization contained in the rock. Failure to detect one or more phases means that valuable data are not recovered and the whole magnetic history of the rock has not been unfolded, Most importantly, the undetected phase might be the initial so that a secondary magnetization can easily be mistaken for the initial with an erroneous interpretation as a result. It is therefore imperative to recognize all phases of magnetizations and if possible to separate them. Procedures that have been used to recognize and unravel multiphase magnetizations are described. These procedures make use of chemical, thermal and AF cleaning treatments, two-stage demagnetization, vector analysis, slicing of specimens and vector diagrams. The combination used depends on the rock studied. For example, it is found that AF followed by thermal treatment can be very useful for the study of igneous rocks; chemical leaching is by far the most effective cleaning technique for the study of red beds. A three-phase model describing the magnetization process of certain red beds is proposed. The slicing of specimens is used to explain intermediate directions with respect to field reversals. It is shown that graphical representation by vector diagrams can greatly facilitate the interpretation of the results. The examples show that, although a statistically well defined magnetization may be obtained after limited cleaning, it cannot be assumed to be the initial. One must ascertain that another magnetization has not remained undetected. This necessitates detailed and extensive experimental work using and devising new analytical procedures in an attempt to unfold the whole magnetic history of the rock. It is noted that tentative polar paths constructed from results obtained after inadequate experimental work cannot be up-graded by adding more data points of doubtful or unproven quality. The evolutionary process of polar paths is dependent upon increasing the reliability of palaeomagnetic results. I. Introduction For a few decades, palaeomagnetic and archaeomagnetic results have been used for different applications: to establish the pole position relative to a certain area, to c o n s t r u c t apparent polar w a n d e r paths, to estimate the relative m o t i o n s o f plates or continents, to establish polarity scales, to correlate different r o c k units, to s t u d y secular variation, to d e t e r m i n e palaeo1 Earth Physics Branch Contribution No. 665.

intensities, etc. The basic ingredient o f palaeomagnetic studies is o f course the m a g n e t i z a t i o n c o n t a i n e d in the rock. Clearly, i f reliable results are to be obtained, it is imperative to unfold the whole magnetic history o f the r o c k studied. Rocks which have been f o r m e d millions or billions o f years ago usually contain more than one magnetization. During their history, rocks are o f t e n subjected to oxidation, mild or intense reheating, burial, metam o r p h i s m a n d / o r m e t a s o m a t i s m which m a y p r o d u c e a n o t h e r magnetization o f thermal or chemical origin.

21 The process or event may be such that the previous magnetization is partially or completely obliterated and replaced by a new magnetization (e.g., the result of a reheating event), or that a new magnetization is simply superimposed upon the existing magnetization (e.g., the formation of a new magnetic material). Since more than one of these (re-)magnetization processes (or events) may have affected the rock under investigation, the (multiphase or multicomponent) magnetization may be very complex and one phase (perhaps the initial one) could easily remain undetected unless detailed experimental work is performed.

2. Origin of magnetization Alternating field (AF) cleaning is often the only treatment applied to igneous or metamorphic rocks and it is sometimes assumed or implied that the magnetization remaining after cleaning in fields of 10-50 mT is a "primary" TRM (thermoremanent magnetization). This is a hazardous proposition since the magnetic carriers have not been identified. There are many examples in the palaeomagnetic literature (e.g., Hargraves and Burt, 1967; Irving et al., 1974; Buchan and Dunlop, 1976; Roy and Lapointe, 1976) where the magnetization revealed at 50 mT resists cleaning in fields exceeding I00 mT and often as high as 300 mT. Such high remanent coercive forces (RCF) are usually associated with hematite or hemoilmenite. This carrier could have been formed during metasomatism or metamorphism, for example, to produce a CRM (chemical remanent magnetization) of secondary origin. It could also have been formed during initial cooling to produce a T-CRM (thermochemical remanent magnetization or high-temperature CRM). Buddington and Lindsley (1964) have shown that, in many rocks, some phases in the FeO-Fe2Oa-TiO2 system do not reach equilibrium until cooling to temperatures as low as 650-550°C. Since this is the blocking temperature (Tb) range for titanomagnetite and hemoilmenite, the initial magnetization could easily be a T-CRM. Other investigators (e.g., Carmichael and Nicholls, 1967; Gromm6 et al., 1969) have also argued that T-CRM could be produced in the 500-600°C range near the Curie point of magnetite and titanomagnetite. After a more comprehensive review for evidence of chemical changes in igneous rocks, Merrill (1975) concludes

"that high-temperature chemical changes that affect the magnetic minerals should be common in igneous rocks". Of course, a TRM could also be acquired during initial cooling or later during a reheating event. Consequently, it is possible for a rock to carry one, two or more magnetizations of chemical or thermal origin and any of which could be the initial one. Although cleaning in fields of a few tens of milliteslas may remove a magnetization with low RCF's, it provides little or no information about the magnetic carriers. Cleaning in fields of a few hundreds of milliteslas will provide some information since the RCF of hematite is much higher than the RCF of magnetite. Even so, the magnetization remaining after cleaning in high fields (200-300 mT) could be composed of two CRM's with overlapping RCF spectra. Because of all these possibilities, thermal experiments are extremely useful. They will provide the Tb of the different carders and it is possible that two magnetizations that could not be separated by AF cleaning can be distinguishable under thermal treatment since the respective Tb spectra do not necessarily correspond to the respective RCF spectra. It is therefore very advantageous to make use of both treatm6nts. Sedimentary rocks are usually subjected to thermal cleaning. Different temperatures have been used for "blanket" cleaning ranging from 400-500°C (Baag and Helsley, 1974; Bingham and Evans, 1976) to 650°C (Roy and Park, 1969). In general, the interpretation is fairly simple: the magnetization remaining after cleaning is regarded as the "primary" magnetization and the magnetization removed as a "secondary" magnetization; little attention is usually given to the magnetic carriers and especially to the carrier of the assumed "primary" magnetization - its identification, its mode of acquisition and the timing of its genesis during the lengthy development of loose particles into a solid rock. Partly for this reason, different explanations have been given for intermediate directions, that is, directions which are different from the mean direction found in most samples: (1) they are aberrant directions (Farrell and May, 1969); (2) they indicate the direction of the magnetic field during a reversal (e.g., Vlasov and Kovalenko, 1963; Harrison and Somayajulu, 1966; Bingham and Evans, 1976); and (3) they represent the direction of the resultant vector of two (almost) oppositely directed magnetizations, one acquired before and one acquired after a field reversal

22 (Roy and Robertson, 1968). In explanation (2), it is supposed that the magnetization was acquired rapidly (on a geological time scale) and represents a point in time; in explanation (3) it is supposed that the magnetization was acquired sIowly over a period long enough to span field-reversal time scales: a diagenetic process was invoked. A decade ago, in a series of articles describing leaching of red bed samples in hydrochloric acid (HC1), Collinson (1965a, b, 1966a, b, 1967) showed that the magnetization was carried by two forms of hematite: specularite and red pigmentation. More importantly, he demonstrated that the technique could be used to separate the carriers of magnetization. Burek (1971) also showed that the DRM (detrital remanent magnetization) of Cambro-Ordovician red beds could be uncovered by means of HC1 leaching under pressure. Roy and Park (1972) used a chemical leaching immersion technique (Park, 1970) to separate successively three magnetizations in Carboniferous red beds. It was shown that all three magnetizations (one of which carried by the red pigment) were acquired within 35 m.y. of deposition but over a period of time long enough to span one or more field reversals. It was then suggested that the three magnetizations were parts (or phases) of a magnetization process that operated throughout the formation of the rock, that is, from its deposition to its solidification. The red pigment was considered not as a secondary magnetization but as a late phase of this initial process. In a study of Cainozoic sediments, Larson and Walker (1975) showed that new magnetic minerals were still being formed after 2 - 5 m.y. These observations indicate that multiphase CRM's are an important and, in some instances, the major constituent of magnetizations in sediments. It is therefore obvious that magnetizations acquired at field-reversal times can easily be inaccurate and unreliable for field direction determinations unless the different phases of magnetization can be separated by some experimental and analytical method. Since part if not all of the magnetization is of chemical origin, it seems logical to use chemical leaching as a cleaning technique. 3. Cleaning techniques The purpose of this article is to show that the recognition and separation of the components of mag-

netization contained in a rock, whether they are phases of the initial process of magnetization or the result of overprinting, often necessitate the use of more than one technique. The techniques used and described are chemical leaching, thermal demagnetization, AF demagnetization, two-stage demagnetization (AF followed by thermal), vector analysis and slicing of specimens. Through examples describing the results obtained from different techniques it is shown, for example, that because of the possible presence of CRM components in igneous, metamorphic and sedimentary rocks: (1) AF cleaning alone is inadequate for the study of igneous and metamorphic rocks; (2) thermal cleaning alone is inadequate for the study of sedimentary rocks; and (3) the end-point (As and Zijderveld, 1958; McElhinny and Gough, 1963) and the minimum dispersion (Irving et al., 1961) criteria do not necessarily indicate that a single magnetization has been isolated. Although these criteria were originally used to define a direction of magnetization, they often have been used in recent years to argue that a single (and in some instances "primary") magnetization has been obtained. Examples will be shown where these criteria are met and yet two magnetizations are clearly present. These examples are drawn from several rock units of Precambrian or Carboniferous age so that the complexity of the magnetizations described cannot be attributed to the peculiarities of one particular rock unit. All stereograms shown are equal-area polar projections. The different treatments performed are identified on each figure. The mean direction of the site, from which the example is taken, is also shown. The AF and thermal treatments were performed in residual fields <2 nT. Chemical treatment consisted of immersing in HCI (8 or 10 Nas indicated) for a given time, washing in water, storing in zero field for sometime (usually 2 - 5 days), measuring and re4mmersing in fresh acid. Different terminologies have been used to define the relative time of acquisition of the different magnetizations carried by a rock. Primary is often used to define the first magnetization acquired by the rock; sometimes, it is used to describe the magnetization with an intensity larger than that of other coexisting magnetizations. Secondary is generally used to describe any magnetization acquired after the first; sometimes

23 it is used to describe a magnetization with a lesser intensity than that of coexisting magnetizations• For the remainder of this article, we shall use the following terminology which we believe avoids any confusion. lnitial = first magnetization acquired by a rock. For an igneous rock, it would be during initial cooling; for a sedimentary rock it would be from deposition to solidification and can be described as the initial process of magnetization acquired during the physical formation of the rock. Predominant or dominant = the magnetization with the largest intensity. Secondary = any magnetization which is not part of the initial process and consequently acquired at a later stage.

F

4. Two-stage demagnetization For many years, one of the key reference poles in the construction of the North American apparent polar path for the period 2 . 2 - 1 . 8 b.y. ago has been a well-defined pole (mean: 272°E, 18°S) obtained from the Nipissing diabase by several workers (Symons, 1970, 1971 ; Patel, 1971 ; Patel and Palmer, 1974; Symons and Londry, 1975; Roy and Lapointe, 1976). In many reconstructiorLs, it is assumed to be the location of the pole at time of emplacement of the diabase ( - 2 1 5 0 m.y.; Van Schmus, 1965; Fairbaim et al., 1969). However, that pole is derived from a direction (~10 °, - 4 0 °) of magnetization obtained after AF cleaning only, usually in fields 50 mT, and, in some instances, up to 120 mT.

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Fig. 1• Results of a two-stage demagnetization applied to a Nipissing diabase specimen from Ontario, Canada. The AF cleaning shows a perfectly good end-point (?) between 20 and 50 mT which is the range usually used for cleaning of igneous rocks. Cleaning in a 150 mT field shows the presence of an underlying magnetization A whose direction is determined by sub-

sequent thermal cleaning. The magnetization remaining after cleaning in 20-50 mT is then composed of A and B. This example also shows that in some instances, vector analysis cannot be performed with any confidence. In this specimen, the respective Tb and RCF spectra of the two magnetizations are totally unknown except that they appear to overlap; this is indicated by the fact that the intensity changes between 20 mT and 625°C are irregular. In this instance, A and B cannot be separated vectorially. However, for the other specimens of this site 23 (see Roy and Lapointe, 1976, fig. 5) the intensity ratio of (~ + ~)/~ was very large (5-15) and the site mean directions of B obtained vectoriaUy and A obtained directly are shown.

24 Figs. 1 and 2 show examples of directions of magnetization found in the Nipissing diabase. These are from a study by Roy and Lapointe (1976) where more examples are given; they did not publish Fig. 1 and more information has'been added to their fig. 7 to become our Fig. 2. A large intensity magnetization with low RCF is first removed by cleaning in a field of 20 mT and the direction of magnetization then moves into the direction (~010 °, - 4 0 °) found by the previous workers where it remains even after cleaning in fields

Fig. 2. Cleaning of six specimens from the same site show (apparently) extremely good end-points; minimum dispersion was also experienced, the direction changes between 20 and 100 mT were minimal (<10 ° of arc). Nevertheless, this is not an isolated magnetization since the underlying magnetization A can be recovered in all six specimens by further cleaning at 150 mT and then 500, 550 and 600°C; the direction changes after each of these cleaning steps are shown for one specimen. For the other five specimens, only the direction after 600°C is given. For each specimen, the negatively inclined northward direction is then the direction of the resultant vector (A + B). The mean site direction for B obtained vectorially is 022° , -44 ° (the intensity ratio of (A + B)/A was >7). The dispersion of the A directions may be owing to one or more of the following causes: low intensity (<10 -2 A/m) of A, parasitic magnetizations of the demagnetized carriers or incomplete removal of the B magnetization in all specimens. The tight grouping of the southward direction obtained by vector analysis indicates that it could be a reverse B direction. It is therefore possible that a field reversal occurred during the acquisition of the B remanence.

of 100 mT indicating clearly that a magnetization (B) with fairly high RCF is contained in these rocks. Thermal experiments showed that this magnetization had Tb of 500-600°C; those experiments also showed that another magnetization (A) with higher Tb (550 -675°C) and positively inclined was present. Although magnetization A could be uncovered by thermal cleaning alone, it was found that its direction could be more accurately determined by first cleaning in 180 mT and then cleaning thermally. In order to explain the greater effectiveness of the two-stage over the one-stage technique, a model that is consistent with the results obtained from different sites and cleaning techniques is illustrated in Fig. 3. Magnetization A has the highest T b (and apparently the highest RCF) and the lowest intensity of all magnetizations found in the sampled Nipissing rocks: For simplification of the model all magnetizations exclud. ing A have been incorporated into a Z magnetization. Since A has some Tb higher than those of Z, it should normally be uncovered by thermal cleaning alone. However, because the Tb spectra of A and Z overlap (in some instances at least) and owing to the fact that the intensity ratio of Z/A is always quite large, the temperature range where A dominates is usually small. So, when using a stepwise demagnetization technique, the window (45°C in Fig. 3b) through which A can be seen could easily be missed. Cleaning at high fields (180 mT for these rocks) renders the intensity ratio smaller; the effect, apparently, is to widen the window and thereby to optimize the availability of A. The experiments show that these samples (Figs. 1 and 2) carry two very hard magnetizations: A and B. Two magnetic carriers were recognized: one is hematite in hemollmenite and the other is magnetite in titanomagnetite. The very high Tb (550-675°C) and the high RCF (200 mT) of magnetization A indicate that it is carried by hematite and is a CRM. Magnetization B with Tb of 500-600°C could be carried by magnetite and/or hematite. The fact that the Tb often exceed the Curie point of magnetite (578°C) and that the RCF are also high (often exceeding 150 mT; Fig. 2) indicates that at least part of the remanence is carried by hematite. It then appears that B is also of chemical origin. On the basis that A has higher Tb and higher RCF than B, it might be argued (as is often done) that A predates B. However, deductions of sequence of magnetization based solely on Tb and RCF

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are not without ambiguities since the magnetization with the highest Tb or RCF is not necessarily the oldest (Buchan and Dunlop, 1976). Indeed CRM's can be of initial (T-CRM) or secondary origin (see Section 2) and it is difficult to ascertain a sequence of magnetization without showing indisputable evidence that one

of the magnetizations is initial. In the Nipissing study, this is provided by two positive contact tests between the diabase and the Huronian sediments intruded by the diabase which clearly show that magnetization A was acquired at the time of cooling of the diabase, 2150 m.y. ago. It therefore predates B and is probably a T-CRM. Magnetization A yields a pole at 258%, 42°N, more than 60 ° away from the previously determined pole: 27"2°E, 18°S. Magnetization B yields a pole at 274°E, 15°S, in good agreement with the latter pole. It is thought that the younger (than A) magnetization B might have been acquired during the Hudsonian orogeny, some 300 m.y. later (see Roy and Lapointe, 1976). This exemplifies the difficulties that might be encountered when dealing with old rocks and the caution that one must exercise with the interpretation of the results. Very detailed and extensive experimental work was needed to recognize the initial magnetization A. Even so, its age (or proof that it was initial) could be established only with the aid of a contact test. Consequently, B which had previously been regarded as initial is secondary and younger than A; thorough cleaning indicates hhat it is probably a CRM; its exact age, however, cannot be determined with certainty and one can only tentatively suggest that its carriers were formed during the subsequent Hudsonian orogeny.

5. The necessity of chemical leaching The inadequacy of thermal cleaning for the study of red beds can be most eloquently demonstrated by comparing chemical leaching and thermal demagnetization results. The results of two specimens from the same core, one treated chemically and the other thermally are shown in Fig. 4. These specimens are from the collection of Roy and Park (1974). Under thermal cleaning the direction moves from 179 °, - 3 0 ° at 20°(] to 336 °, ~4 ° at 560°C where it remains up to at least 674°C, apparently showing a perfectly good end-point. However, this is not the direction of a single magnetization but that of theresulrant vector of two magnetizations as shown by the chemical leaching results. Under chemical leaching, the direction moves to within 5° of the (thermally determined) end-point

26

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27 after 367 h in 8 N HC1. However, upon further leaching the direction moves gradually to a southward position where it remains for 3,445 h in 10 N HC1; from 1,300 to 4,745 h, there is very little change in either direction (Fig. 4) or intensity (Fig. 4c). Thus, the specimen contains three types of magnetic carriers with very different solubilities in acid (or acid reaction rates); each carries a differently directed magnetization indicating that they were formed at different times. It has been shown (Roy and Park, 1974) that all three magnetizations were acquired prior to folding (within 35 m.y. of deposition) and it has been suggested that they are phases (DRM, CRMA and CRMB) of the initial process of magnetization. The DRM (detrital remanent magnetization) was formed by the alignment of magnetic particles along the ambient field during or shortly after deposition (mechanical phase); these particles are practically insoluble in acid (they resist to 4,745 h of leaching in 10 N HC1). Although both the CRMA and the CRM~ are probably of chemical origin, they can be differentiated. The most northerly direction (367 h) corresponds to the disappearance of the red coloration throughout the specimen (verified by slicing specimens similarly treated). The magnetization (CRMB) removed after 367 h is then carried by the red pigment and the magnetization (CRMA) gradually removed from 367 to 1,300 h is attributed to hematite growth during diagenesis. Roy and Park (1974) showed that the magnetization sequence is most probably DRM ~ CRMA -+ CRM B. This notation will be adopted for the remainder of this article to describe the early, intermediate and late phases of the magnetization process. The chemical leaching results clearly indicate that the magnetization remaining after 367 h is the sum of two unresolved magnetizations, that is, DRM + CRMA; the (367 h) direction cannot therefore be used directly to define the direction of a magnetizing field unless both magnetizations were acquired during a period when the direction of the field changed very little. Evidently this is not the case here since the directions at 367 and 4,745 h (DRM) are so much different. The 367 h and 674°C results are alike in all aspects, declination (Fig. 4b), inclination and intensity (Fig. 4c) showing that the magnetization remaining after the 674°C cleaning is also composed of DRM and CRMA. It must be concluded that the DRM and the

CRMA cannot be separated thermally even by cleaning at temperatures near the Curie point of hematite. It is therefore wrong to assume that a single magnetization has been isolated on the basis that an end-point has been obtained after cleaning at high temperature. For example, the use of the 674°C result for determination of the field direction and pole position at that time would be deplorable. The pole position in Maringouin Formation time was 117°E, 34°N (Roy and Robertson, 1968) in accordance with other Carboniferous results; the pole obtained from the 674°C result is 138°E, 15°N, in error by 26 °. In contrast, the site mean direction obtained from the DRM yields a pole at 117°E, 40°N, in very good agreement with the Maringouin pole.

6. The use o f vector analysis

Because remanence acquisition is a lengthy process, red beds contain a recording of ancient field changes. Each phase of magnetization carries a different portion of that recording. Therefore in order to retrieve the information, the phases should be separated whenever possible. This usually requires the use of some technical and/or analytical method. Failure to separate those phases could possibly lead to erroneous interpretation. Fig. 5 is given as an example. Let us first consider the results obtained by chemical leaching solely on the basis of the graphical representation of the directions (triangles) as shown in stereogram (Fig. 5a) and without the benefit of the vector diagrams (Fig. 5b and c). One could then possibly draw the conclusion that these direction changes represent an excursion of the field since the direction executes a movement from 180 °, +20 ° to ~240 °, +05 ° and then comes back to its point of departure. Vector analysis shows that this apparent excursion is really a reversal with the CRMA being anti-parallel to the DRM and CRMB. Vector analysis is, of course, very useful for separating different phases of magnetization. However, it is often difficult to apply properly because the transition point marking the predominance of one phase (say CRMr0 to another (CRMA) is not known. Vector diagrams are often a must for the unravelling of complex magnetizations. Fig. 5 shows another example of a method used by Roy and Park (1974) where more

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-°OWN Fig. 5. Stereogram vector diagrams showing the results obtained by chemical leaching (triangles) and the method used to determine a northerly directed vector (CRM~A). Leaching first removes a southerly directed vector (CR---~B) since the direction moves north until 540 h. The most northerly direction cannot be established from the stereogram (a) but is quite evident in the vector diagram (b) where triangle 540 is clearly to the north of all other triangles. The C ~ A can be best determined graphically by removing the contribution of the DRM. This is accomplished by subtracting the DRM (4,750 h) from the observed values (triangles). This yields a vector diagram (squares) composed of a southerly directed CRM B and a northerly directed CRM A. Since the CRM B is removed first, the direction (from 0 h) moves gradually north until the CRM B has been leached out. The most northerly directed square 540 in (b) indicates the horizontal component of the CRI~A. The total vector is shown in (c). The site mean directions of the C--~ A (6 directions) and the DRI~ (8 directions) are shown.

29 examples can be found. Three phases (DRM, CRaMA and CRMB) of magnetization are present and are clearly shown (triangles) on the vector diagram (Fig. 5b). The contribution of the DRM which has been definitely isolated by prolonged leaching is removed by subtracting the 4750 h vector from all observed vectors. This leaves the CRMA and CRM B (shown by the squares) and the transition point is indicated by the 540 h vector (square) which is considered a good approximation of the ~ A direction. These rocks are virtualiv~free of any viscous mamaeti-> zation sothat the NRM = DRM + CRMA + CRM B. The CRM B is then obtained by subtracting the DRM + CRMA which in this instance is represented by the 540 h observed vector (triangle). The directions obO O tained are: DRM = 190, +10 ; CRMa = 347 °, -12°; CRM B = 180 °, +10 °. It is noted that without the vector diagram and only on the basis of the stereogram, the 404 h (triangle) could have been taken as the transition point. This yields a westerly direction. For vector analysis of multiphase magnetizations, it is often necessary to make certain assumptions. It is evidently important to verify if these assumptions are justified and to investigate the possible consequences if they are not. For example, for the determination of the ~ A direction (Fig. 5), it has been assumed that the DRM is constant, that is, none of the DRM has been removed by leaching after 540 h. The fact that very little changes (in both direction and intensity) occur for the last 3,450 h (1300--4750) of leaching indicates that the DRM-carrying grains are extremely resistant to acid leaching and there is no reason to suppose that some of them were removed in the 540-1,300 h leaching period. Furthermore even if this had occurred, it would not invalidate the use of vector subtraction for the finding of the C----I--I--I--I--I-R~di/~ rection. The results already show that the ~ and the C---R-R~Aare nearly anti.parallel (within 13°). Therefore, if the intensity of the DRM at 540 h was larger than the intensity used (4,750 h), then the intensity of the CRMA was also larger than the value of 0.6 • 10 -3 Aim obtained; the determination of the direction of the C---I-I-I-I-I-I~[A would not, however, be greatly modified. In fact, the result would be to bring the directions of the two magnetizations closer to anti-parallelism. It can therefore be concluded that although the vector subtraction was performed under the worst circumstances, that is, by using the smallest possible

intensity of DRM, the two directions were nevertheless shown to be within 13 ° of anti-parallelism. A more detailed discussion on accuracy of vector deterruination can be found in Roy and Park (1974, p. 445). A combination of chemical and thermal treatments on paired specimens (1 and 2) can also be used very effectively for separation of all three phases. The DRM can be isolated by leaching (specimen 1). The CRMA can be obtained by vector subtraction of the chemically cleaned result (specimen 2) from the thermally cleaned result (specimen 1). The advantage of this method is that it is often easier to determine accurately the CRM B thermally than chemically. The obtention of the CRM B chemically is based on the deterruination of the transition point which might occur during a leaching period. Since only the CRM B is removed thermally, its contribution is established by simply subtracting, from the NRI~, the thermally cleaned vector (DRM + CRMA) which is often recognized over a wide range (end-point) below the Curie point of hematite (Fig. 4).

7. Slicing of specimens The direction of magnetization contained in a specimen of finite size can be a reliable estimate of the direction of the ambient field only if that magnetization was acquired in a field of single polarity, that is, no field reversal occurred during the period of acquisition of t ~ t magnetization. Except for a few quiet intervals (e.g., Permo-Upper Carboniferous), the polarit y of the field appears to have reversed quite frequently. Therefore, the probability that a field reversal occurred during the lengthy process of magnetization of red beds is high. Evidently a field reversal occurred during magnetization of the specimen shown in Fig. 5. The agreement of directions of the three phases (DRM, CRMA and CRMB) of magnetization indicates that the reversal occurred between and not during phases of magnetization. However, there is no reason for the timing of a reversal to be limited to between phases and reversals can and do occur during phases. When this takes place, the mean direction of magnetization observed in a specimen does not represent the direction of the field before or after the reversal but some other intermediate direction whose meaning has been

30 given different interpretations as discussed in Section 2. Evidence in support of the contention (Roy and Robertson, 1968) that most intermediate directions are the result of two oppositely directed magnetizations is accumulating. Numerous examples have been given by Roy and Park (1974); all the intermediate directions found in their Carboniferous red beds could be explained by that hypothesis. One of their exampies is shown in Fig. 6. Larson and Walker (1975) have shown that different parts o f some Cainozoic red bed samples carry magnetizations of opposite polarities. A detailed study of intermediate directions found in

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Fig. 6. Results of chemical leaching of a specimen from the Shepody Formation, New Brunswick, Canada. The site mean direction (both polarities) obtained from the CRMB is shown. With 250 h of leaching the direction moves to an intermediate direction and remains there until 880 h, apparently having reached an end-point. However, when the specimen is cut into halves along the bedding plane, it is shown that the direction of the lower half is reasonably near the northerly site mean direction while the upper half upon further leaching moves very close to the southerly site mean direction. The values in brackets indicating the intensities in 10-3 A/m before and after slicing show that the intensities of each half are greater than that of the whole specimen. These results indicate that a field reversal occurred during the acquisition of the DRM; the lower half being magnetized in a northerly directed field and the upper half in a southerly directed field. The intermediate direction of the whole specimen is the direction of the vector sum of the magnetizations of each half.

Riversdale sediments (Roy, 1977)shows that they represent the direction of the resultant vector of two magnetizations. Current studies by the authors of other red bed formations ranging from Precambrian to Carboniferous indicate that the hypothesis is holding true. In general, we observe that when a specimen (2.5 cm diameter, 2.2 cm height) yields an intermediate (mean) direction, the direction of magnetization is not uniformly distributed throughout the specimen. Non-uniformity of direction within a specimen is readily detected with a direct-read astatic magnetometer by placing the specimen in different attitudes under the instrument (Roy, 1971). For example, a difference between the readings obtained by placing the specimen in an upright and then an inverted position indicates that the upper and lower parts are magnetized along different directions. This is easily verified by slicing the specimen and measuring each part. Some examples taken from different formations are shown in Figs. 6 - 9 indicating that this explanation is not peculiar to a formation in particular but may be applicable to red beds in general. As the examples show, the specimens were sliced at different stages of the cleaning treatment. Decision to slice is taken usually because the degree of non-uniformity of direction as shown by the astatic magnetometer is high and sometimes because the intensity is very low (Fig. 8). Slicing of chemically treated specimens also permits one to verify the acid penetration. The mean direction (both polarities) for the site from which these samples were taken are shown (or given in the legend; Fig. 9). These examples show that a field reversal occurred during the acquisition of the remanence so that these specimens carry two magnetizations of opposite polarities. This is evidenced by the fact that, after slicing, the direction of one half is near the site mean direction. The direction of the other half may be near the site mean direction of opposite polarity or may remain in an intermediate direction depending on whether the polarity transition occurred near the slicing-line (Fig. 7), or in that half (Fig. 8) of the specimen. The vector sum of the two halves, although it does not differ by much, is not equal to the vector of the whole specimen. There are two reasons for this: (1) the slicing causes the loss of a 3 m m thickness of specimen; and (2) the pre-slicing measurement is the result of integration of all parts of the specimen as opposed to the measurements of two separate halves.

31

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Fig. 7. Results of chemical leaching of a specimen from the Jacobsville Formation (Precambrian), Michigan, U.S.A. A positively inclined CRMB is rapidly removed in 500 h of leaching and the direction moves to a low inclination as shown in (a) and (c). The specimen is then sliced and, as shown in (e), the intensity of the whole specimen (before slicing) 3 ' 10-3 A/m is the result of intensities 4.6 • 10-3 and 1.7 • 10-3 A/m carried by the lower and upper half, respectively. The fact that the magnetizations are oppositely directed after prolonged leaching indicates that the reversal occurred during the early phase (acquisition of the DRM) and near the slice. The site-mean directions are shown in (a). Besides establishing if intermediate directions are owing to a field reversal, slicing provides one with half specimens which might be used for field direction determinations. Furthermore, in some instances, it can be used to determine the timing of the reversal. In the chemically treated specimens (Figs. 6 - 8 ) the dimetions remain in the same position under prolonged leaching indicating that the reversal occurred during the early phase while the DRM was formed. In the thermally treated specimen (Fig. 9) the reversal occurred most probably during the early (DRM) or intermediate (CRMA) phase rather than during the latephase CRM B whose direction (164 °, +12 ° obtained by vector analysis) is close to the site mean direction; furthermore the degree of non-uniformity at 20 and IO0°C was low. 8. The red pigment magnetization The remanence (CRMB) carried by the pigment of red beds has often been neglected because it is usually

labelled "secondary" and therefore considered of little importance. Roy and Park (1974) have shown that the C---1--I~B obtained by vector analysis of NRM and cleaned results could be used for field direction determinations and other purposes such as field direction changes during the magnetization process. A recent study of Riversdale sediments (Roy, 1977) cleaned chemically, thermally or by AF showed that one or more reversals occurred during magnetization (Fig. 10). The chemical results show that in some specimens a reversal occurred during the early phase. The thermal results show that in other specimens a reversal occurred during the early and/or the intermediate phase. This is evidenced by the north, south and intermediate directions (Fig. 10a and b) found after leaching during 900 h or cleaning at 675°C. The obtention of the C--l~ B by vector analysis shows that a field reversal occurred during the late phase in eleven specimens. However, in the other 54 specimens the CRM B (or at least most of it) was acquired in a southerly directed field. This indicates that, for the majority of

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Fig. 8. Results of chemical leaching of a Jacobsville Formation specimen. A steeply inclined CRMB is removed after 500 li of leaching. The intensity Its then very low (0.3 • 10 - 3 A/m) as shown in (b) and (c). Slicing shows that it is composed of an easterly magnetization (1 = 2.6 • 10 - 3 A/m) and a westerly magnetization (1 = 2.4 • 10 - 3 A/m). The polarity change probably occurred during acquisition of the DRM in the upper half since its direction is more off (~30 °) the site mean direction than that of the lower half.

the specimens, the pigmentation took place after the field reversal. The result is that 83% of the CRM B directions can be applied to the determination of the southerly directed field while only 54% of the directions obtained after cleaning can be used for that purpose. The agreement between the CRM B mean direction and the mean direction obtained after cleaning indicates that the field direction changed very little during the magnetization process. An extensive study of the Jacobsville Formation

20*"

Fig. 9. Results of thermal demagnetization of a specimen from the Riversdale Formation (Carboniferous) of Nova Scotia, Canada. The site mean direction is 178°, +06° (358°, -06°). The direction moves to an intermediate position at 450 and 500°C. Because the degree of non-uniformityof direction is then high, it is sliced in halves. The intensity before slicing was low because the upper and lower halves were magnetized in opposite directions, each being near the site mean direction. The halves are not usually measured immediately after slicing (500°C in this instance) because they are exposed to the earth's field during slicing. The vector sum of the upper and lower half vectors is shown by c r o s s e s . These indicate the approximate (see text) vectors if the specimen had not been sliced; it is noted that the 675°C direction would have been about 40° away from the site mean direction. It cannot be determined ff the field reversal occurred during the early or intermediate phase since the DRM and CRMA have not been separated by this thermal treatment.

(Roy and Robertson, 1978) indicates that most of the magnetization was acquired in an E - W direction and that a field reversal occurred during the early phase of magnetization (Figs. 7 and 8). At thirteen of the 37 sites, a steeply inclined magnetization (32) is found. Its Tb and RCF are very high (Fig. 11). Chemical leaching indicates that most if not all o f J 2 is carded by the red pigment. Although other explanations are possible, it is believed that J2 is the CRM B. If so, it indicates that a large field direction change took

33

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Fig. 10. Directions of magnetization of 65 specimens or half specimens from Riversdale sediments (Roy, 1977), subjected to chemical (triangles), thermal (circles and dots), or AF (squares) treatments. (a) and (b) show the directions obtained at 900 h, 675°C or 290 roT. (a) shows two distinct groups nearly oppositely directed. The statistics are shown in lower right; N = number of sites, n = number of specimens and the bold characters indicate the unit weight. (b) shows the intermediate directions of the resultant vector of two magnetizations that could not be separated; many of them are the direction of one half of a specimen whose other half has a direction shown in (a). The southerly directed ~ B in (c) was obtained by vector subtraction of the cleaned results in (a) and (b) from the NRM; eleven specimens were excluded because in those the CRM B was acquired before or during the reversal (see Roy, 1977).

+

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Fig. 11. Results of chemical, thermal and AF treatments of some Jacobsville Formation sediments. The steeply incUned magnetization (J2) is found at thirteen sites and the grouping is fairly tight. As shown by the very small changes in direction, cleaning up to 650-675°C and up to 200 mT cannot eliminate this magnetization. Chemical leaching, however, can uncover the easterly (westerly) magnetization (see also Figs. 7 and 8) found at all 37 sampled sites except one. Although caeaning at 685°C and 290 mT cannot isolate the latter magnetization (the site mean direction is shown), it does indicate that it is present.

34 place before completion of the magnetization process of the Jacobsville. The meaning of J2 will be discussed more fully upon presentation of those results.

9. Discussion

As mentioned in Sections 1 and 2, different magnetic carriers can be formed during the history of the rock and it is probable that multiphase magnetizations~ are quite common. If so, one cannot assume that the "stable" magnetization (often so-called because of minimum within-site dispersion of directions, or became the direction has apparently reached an endpoint) obtained after minimum cleaning is the only (and initial?) magnetization. Attempts should at least be made to establish if other magnetizations are present, and if possible to separate them and identify their remanence carriers. This obviously is best accomplished by subjecting specimens of the same rock unit to different cleaning and analytical techniques. We have described by means of examples the combinations of techniques that we have used to separate phases of magnetization that would not even have been recognized if only one technique had been used. It has been shown that, because of the potential omnipresence of CRM, AF cleaning alone is insufficient for the study of any rock. For igneous and metamorphic rocks, both thermal and AF (in high fields) cleaning treatments should be performed. Thermal cleaning will at least provide one with the Tb of the different carriers and it might then be possible to gain some insight into the origin of the different phases of magnetization that could be present. It has been shown that thermal cleaning is insufficient for the study of sedimentary rocks since it cannot recognize all the phases that can be separated by chemical leaching. We have adopted the view that the magnetization of red beds is acquired over a long period of time. Roy and Park (1974) recognized three phases of magnetization based on the resistance to acid leaching of three magnetic carriers that they identified as DRM, CRM A and CRMB; they showed that the CRM B was carried by the red cement and that the DRM and CRM A could not be separated thermally. The results from other formations obtaiiaed from chemical and thermal experiments performed by Morris (1977) and by the authors are in accordance with those findings. We suggest that

this could be known as a three-phase model. We do not contend that this simple three-phase model fully describes the magnetization process of red beds. One has only to read Blatt et al. (1972, especially ch. 10) to realize all the complexities and ramifications that could be encountered in the development of red beds. Much more work is needed before we can comprehend the whole magnetization process which may be composed of many more phases or sub-phases. Nevertheless, imperfect as it may be, the threephase model can explain the data much more satisfactorily than other models where it is blindly assumed that the magnetization remaining after cleaning at 500°C was acquired rapidly at and/or shortly after deposition (Baag and Helsley, 1974). The results of Roy and Park (1974) clearly show that thermal treatment at such intermediate temperatures cannot even demagnetize all of the CRMa; this is particularly well illustrated in their fig. 11 (see also Figs. 4 and 9 in this article). So, with part of the CRM B remaining and both the CRM A and the DRM still intact, it is difficult to draw any conclusions about the behaviour of the field during reversals. It should be noted that chemical leaching avoids many of the complications that may arise with thermal demagnetization. Chemical leaching physically removes the remanence-carrying grains while thermal treatment does not destroy but simply demagnetizes them. The demagnetized grains could possibly acquire a viscous magnetization. Furthermore, Dunlop (1971) has shown that the behaviour of hematite (which is the major constituent of red beds) is complex and not very well understood. Alteration, hardening and/or annealing could occur during heating. This might be the reason why the CRM a and the DRM cannot be separated thermally. In fact, Roy and Park (1974) found that after demagnetization at 674°C, the CRM A could not be removed chemically while it could be in the paired (thermally) untreated specimen. With chemical leaching, destruction of the magnetic carriers eliminates any possibility of remagnetization or alteration. This is evidenced by the fact that when left in the earth's field thermally demagnetized specimens acquire viscous magnetizations, often very large, while frequently chemically treated specimens have very little or no susceptibility to those magnetizations. Although chemical leaching is more effective than thermal or AF, it is preferable to experiment with the

35 three techniques since each can provide some additional information. Furthermore, the results can often be used to extract the CRM B by vector analysis. Some serious problems remain to be solved. Owing to the low permeability of some rocks (e.g., argillites), the acid penetration rate is too slow to affect the successive removal of the phases. With a very slow rate of penetration, the carriers of two or more phases are destroyed in the outer zone of the specimen before any carrier in the inner zone can be affected (Roy and Lapointe, 1976). We have argued that most intermediate directions are the result of two magnetizations acquired before and after a field reversal. This is based on the fact that even when the DRM has been isolated, the directions of the half-specimens obtained by slicing usually pull apart, away from the direction of the whole specimen and towards the direction of the field before or after reversal. This does not necessarily mean that directions of the field during reversal cannot be found. For example, it is possible that some of the directions (Fig. 10b) represent the field direction. Several authors (some of them referenced previously) have suggested that their intermediate directions represent the field direction during transition. However, it must be pointed out that all suggestions advanced so far have been based on NRM or thermally cleaned results where two or three phases of magnetization are still present. Therefore depending on the timing of the reversal and the relative acquisition rate of each remanence phase, these directions could indicate the direction of the field during the acquisition of one of the phases, or could be the resultant of integration of magnetization acquired over one or more phases.

10. Conclusions The examples presented here show that much experimental work is needed to unfold the magnetic history of most rocks. Some of the techniques that can be used to do so have been described. We do not claim that these techniques are sufficient to unfold the whole magnetic history. It is probable that other techniques capable of retrieving more or other information will be devised. Nevertheless, these examples are sufficient to demonstrate that results obtained with no or very limited cleaning work reveal only the more acces-

sible data which may or may not represent the entire magnetic recording contained in the rock and consequently the earlier part of that recording might be missing. Yet, in many instances, directions of magnetization so obtained have been associated (not necessarily by the authors themselves) with the field at the time of formation of the rock. Often, in hurried attempts to construct polar paths for geological periods where good quality data are scarce or lacking, this same result will then be used with the sole support of data of doubtful or unproven quality (sometimes unpublished, or taken from abstracts, or listed as preliminary by the author) to propose a tentative polar path. Frequently, this path will subsequently be used in a circular argument to justify a result also obtained after limited experimental work. Gradually, the path may pass from the tentative to the accepted stage not on the quality but on the quantity of its constituents and especially on its repeated use. This scenario is often followed because it would appear that a polar-path syndrome is developing, that is, a result will be more readily accepted if it yields a pole on an existing path. In our view, a portion of polar path pre-conceived on scanty evidence cannot be up-graded by adding results of equal quality. A tentative polar path should remain so until it is either confirmed or negated by data of higher quality. The construction of polar paths is an evolutionary process. To claim that the existing polar path is the final product is preposterous. Except for the last few hundred million years, it is quite possible that the paths as we know them today will change drastically. Data for the Precambrian for example are not numerous considering the time span involved and the extent of secondary geologic processes possible. Most importantly, most of the available data have been obtained with no or limited cleaning. The examples shown here indicate that in many studies much of the information might have remained undetected and consequently the results might have been misinterpreted because the whole magnetic history of the rock was not uncovered. That is not to say that results obtained after limited cleaning are of no value. A well defined magnetization whether it is initial or secondary can be used to define the trend of the path although in the latter case it cannot be used for its calibration. It might be the initial because it was the only one or because it was the more easily accessible. For example, the NRM of a

36 red bed acquired in a field of single polarity may yield the mean direction of the field during its formation. However, these results will remain uncertain until it has been shown that they really represent the initial magnetization. This can be accomplished only by more detailed laboratory experiments. In order to increase the reliability of the results, rock units should be investigated under the assumption that the magnetization is very complex until proven otherwise. One should therefore make use of all the techniques available and possibly devise new ones to ascertain that the whole magnetic history has been unfolded.

Acknowledgement We wish to thank W.A. Morris for many discus. sions and critical reading of the manuscript.

References As, J.A. and Zijderveld, J.D.A., 1958. Magnetic cleaning of rocks in paleomagnetic research. Geophys. J., 1: 308-319. Baag, C.G. and Helsley, C.E., 1974. Evidence for penecontemporaneous magnetization of the Moenkopi Formation. J. Geophys. Res., 79: 3308-3321. Bingham, D.K. and Evans, M.E., 1976. Paleomagnetism of the Great Slave Supergroup, Northwest Territories, Canada: the Stark Formation. Can. J. Earth Sci., 13: 563-579. Blatt, H., Middleton, G. and Murray, R., 1972. Origin of Sedimentary Rocks. Prentice Hall, Englewood Cliffs, N.J., 634 pp. Buchan, K.L. and Dunlop, D.J., 1976. Paleomagnetism of the Haliburton intrusions: Superimposed magnetizations, metamorphism, and tectonics in the late Precambrian. J. Geophys. Res., 81: 2951-2990. Buddington, A.F. and Lindsley, D.H., 1964. Iron-titanium oxide minerals and synthetic equivalents. J. Petrol., 5: 310-357. Burek, P.J., 1971. An advanced device for chemical demagnetization of red beds. Z. Geophys. 37: 493-498. Carmichael, I.S.E. and NichoUs, J., 1967. Iron-titanium oxides and oxygen fugacities in volcanic rocks. J. Geophys. Res., 72: 4665-4687. Collinson, D.W., 1965a. Origin of remanent magnetization and initial susceptibility of certain red sandstones. Geophys. J. R. Astron. Soc., 9: 203-217. Collinson, D.W., 1965b. Depositional remanent magnetization in sediments. J. Geophys. Res., 70: 4663-4668. Collinson, D.W., 1966a. Magnetic properties of the Taiguati Formation, Bolivia. Geophys. J. R. Astron. Soc., 11: 337-347.

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