Problems in determining paleointensities from very old rocks

Physics of the Earth and Planetary Interiors, 13 (1977) 319—324 © Elsevier Scientific Publishing Company, Amsterdam — Printed in The Netherlands

PROBLEMS IN DETERMINING PALEOINTENSITIES FROM VERY OLD ROCKS

319

1

J.L. ROY Earth Physics Branch, Ottawa, Ont. (Canada)

(Accepted for publication January 19, 1976)

Roy, J.L., 1977. Problems in determining paleointensities from very old rocks. Phys. Earth Planet. Inter., 13: 3 19—324. Polyphase magnetizations are not uncommon in old rocks. To obtain reliable paleointensities, these magnetizations need to be recognized and separated so that the paleointensity determination can be derived from one of the phases of magnetization. Some of the techniques used to detect and separate (and sometimes isolate) the different phases are described by means of a few examples. Special attention is given to the chemical remanent magnetization which can be found in both sedimentary and igneous rocks. It appears that a sedimentary rock near an igneous contact is the preferferred specimen for reliable paleointensity determinations.

1. Introduction The method used by the Thelliers (e.g., Thellier, 1937; Thellier and Thellier, 1959) in their archeomagnetic work has proven to be very reliable for determining both the direction and the intensity of the field in the historical past (Thellier, 1966). This technique in its original or modified form is now widely used for the determination of paleointensities and the articles in this issue bear witness to the difficulties involved. This is partly owing to the fact that old rocks often contain polyphase magnetizations which may or may not be of different origins, and which may have been acquired at different times during their geological history. Sometimes, polyphase magnetizations may not be recognized because the experimental method used may not discriminate between the different magnetizations. For example, the thermal decay characteristics of TRM (thermoremanent magnetization), CRM (chemical remanent magnetization) and DRM (detrital remanent magnetization) may be comparable. Clearly, for accurate determination of paleointensity, these different magnetizations must be recognized and 1Earth Physics Branch Contribution No. 499.

separated, and a value of paleointensity determined for one or all of the phases present. Although one experimental method may not discriminate between the different phases, two or three analytical procedures [using thermal, chemical and/or alternating field (AF) cleaning treatments] often will do so and this is illustrated below. The results are discussed in terms of applicability to paleointensity determinations.

2. Sedimentary rocks Little is known about the magnetization processes of sedimentary rocks. However, it appears from recent studies (Roy and Park, 1974; Uirson and Walker, 1975) that, at least for red beds, the initial magnetization is acquired gradually over a period of the order of millions of years. It has been shown that, during the mitial process of magnetization of some Carboniferous red beds, one detrital (DRM) and two chemical (CRMA and CRMB) remanent magnetizations were produced (Roy and Park, 1974). The three magnetizations (acquired over time scales of field reversals) could be clearly separated by chemical leaching (Fig.

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Fig. 1. Thermal demagnetization and chemical leaching of two specimens from the same core. The equal-area polar-projection stereogram (a) shows the direction changes during the cleaning treatment. The end-point of the horizontal component of the magnetic vector is shown in (b); the distance from the origin indicates the intensity after the indicated treatment and the position gives the declination. The end-point of the total intensity vector is shown in (c) as a function of the horizontal and vertical components. The inclination is given by the angle between the vector and the horizontal axis. Chemical leaching shows that the southerly NRM is composed of three magnetizations represented by the resultant vector (DRM + CRMA + CRMB); the DRM and the CRMB being southerly directed and the CRMA northerly directed. The CRMB is removed by leaching for 367 h leaving the northerly directed resultant vector (DRM CRMA); upon further leaching, the CRMA is removed to isolate a single magnetization, that is, the DRM. Thermal demagnetization to slightly below the Curie point ofhematite removes the CRMB leaving the resultant vector (DRM + CRMA) found by leaching for 367 h; this last phase (CRMB) of magnetization and/or the isolated DRM are perhaps of some use for paleointensity work.

1). The CRMB is carried by the red cement which is leached away after 367 h in 8 N HC1; the CRMA is removed by leaching for about 1,000 h while the DRM

particles remain insoluble for at least 4,545 h in 10 N HC1. The DRM is most probably carried by hematite particles.

321 Neither the DRM nor the CRMA can be demagnetized by thermal treatment up to near the Curie point of hematite as shown by the similarity of the 367 h and the 560—674°Cvectors. However, the CRMB is effectively removed by thermal cleaning and its original intensity is obtained by vector subtraction of the NRM (natural remanent magnetization) and 674°C vectors (for the specimen shown in Fig. 1, the CRMB intensity is larger than the NRM intensity and equal to 7.2 106 e.m.u.). Since this phase of magnetization can be separated it could be used for paleointensity measurements. Chemical remanence like thermoremanence is field dependent, but comparison of CRM with TRM produced in a laboratory suggests that a correction for the relative intensities of CRM/TRM is needed. For hematite grains, it appears that a reasonable value for this ratio is ~0.4(Stacey and Banerjee, 1974). This means that a paleointensity obtained from the CRMB using a thermal method would require a correction of about 2.5. Although more work is required to determine more accurately the relationship between DRM and applied field intensities, it appears that detrital remanence is also field dependent (e.g., Johnson et al., 1948; Irving and Major, 1964). Since the DRM can be isolated by prolonged chemical leaching, it may become a valuable source of information about the strength of the paleofield. In conclusion, it appears that, owing to the lengthy process of magnetization, the untreated NRM of red beds (and possibly other sedimentary rocks) is inadequate for reliable paleomntensity determinations; however, different phases of magnetization can often be separated and can be used for paleointensity work. -

3. Igneous rocks Igneous rocks are usually thought to acquire a TRM during cooling. However, Buddington and Lindsley (1964) have shown that, in many rocks, some phases in the FeO—Fe 203—Ti02 system do not reach equilibrium until cooling to temperatures as low as 650—550°C.Since this is the range of blocking temperatures for titanomagnetite and hemoilmenite, the primary magnetization could easily be a CRM. Many other workers (e.g., Carmichael and Nicholls, 1967; Grommé et a!., 1969) have argued that the primary magnetization of some basalts is not neces-

sarily a TRM but could be a CRM produced in the 500—600°Crange near the Curie temperatures of magnetite and titanomagnetite, It is also possible that a CRM might be produced by deuteric alteration at low temperatures. Secondary magnetizations may occur long after emplacement of the igneous body: a partial TRM acquired during a mild heating event or burial or a CRM produced during a metasomatism event. Therefore, an igneous rock may carry more than one stable magnetization and either the primary or secondary magnetization could be a TRM or a CRM. As an example of complex magnetization, the results of rock samples of the Nipissing diabase (2,150 m.y. old; Van Schmus, 1965; Fairbairn et al., 1969) are shown in Fig. 2. Rock samples Co9l and Co94 contain three magnetizations. The intensity of one magnetization (~1.0 l03e.m.u.)is much larger than that of the other two; the blocking temperature is below 400°C(Co91) and the remanent coercive forces are less than 200 G (Co94). The age of this magnetization (probably a TRM) is unknown, but vector analysis indicates a direction of magnetization much different from that of the present earth’s field so that it is not a recently acquired magnetization. The other two magnetizations have remanent coercive forces above 1,500 G (Co94) and blocking temperatures above 550°C(Co9l and Co94). Both are thought to be carried by hematite in composite ilmenite—hematite intergrowth. They cannot be separated by AF treatment alone. However, the steeply and positively inclined magnetization (A) can be uncovered by thermal (Co91) or two-stage (AF followed by thermal) (Co94) cleaning treatment; the blocking temperature range is from 550°Cup to 675°C(Fig. 2b) and it is believed to be a CRM acquired shortly after emplacement (Roy and Lapointe, 1975). The other and negatively inclined (~010°, —40°)magnetization (B) * of much larger intensity (about 50 times larger than magnetization A) was probably acquired some 300 m.y. later at the time of the Hudsonian orogeny (~ 1,850 .

m.y. ago). A further complication for the obtention of reliable paleointensity determinations is the fact that *

The direction obtained at about 0100, —40°is the direction of the resultant vector (B +A). The direction of B can be obtained by vector subtraction. However, for the Fig. 2 examples, owing to the large intensity ratio B/A the difference between the direction of B and B + A is very small (<3°)and vector subtraction has been neglected here.

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Fig. 2. Thermal (triangles) and AF (circles) cleaning treatments of Nipissing diabase rocks. Equal-area polar-projection stereograms with closed (open) symbols indicating positive (negative) inclinations. The steeply and positively inclined magnetization is recovered by thermal or two-stage (AF + thermal) cleaning treatment (but not by AF alone); it is probably a CRM acquired shortly after solidification. A second CRM (2riOlO°,—40°)which is uncovered by AF or thermal treatment may have occurred during the Hudsonian orogeny (~300m.y. later). The age of the third magnetization (blocking temperatures <400°C) is unknown but is not attributable to the present earth’s field.

sometimes the blocking temperature and/or remanent coercive force spectra of two stable magnetizations overlap. This is illustrated with sample Co89. In this instance, most of the blocking temperatures of magnetization B (intensity > 1.0~i04 e.m.u.) are between 550 and 600°C while magnetization A has blocking temperatures up to 67 5°C(intensity = 6 lO6 e.m.u.). However, the low intensities (<2 10_6 e.m.u.) found .

at 600 and 650°Cindicate that a small part of magnetization B has blocking temperatures in that range and evidently these intensities represent the length of the resultant vector of the A and B magnetizations whose directions are 140°apart. This lowering of intensity is in accordance with the 600°and 650°C intermediate directions (Fig. 2b) showing the gradual transition from the B to the A direction of magnetization.

323 N

very old rocks may be complex. The information contained in such rocks is undoubtedly useful for the These examples show that the magnetic history of

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tracking of the paleofield. However, inpresent, order and to extract that information, it of is necessary to recognize separate magnetization establish the theirphases age sequence; otherwise, a stableand secondary magnetization could be taken as primary. This possibility is enhanced if the intensity ratio secondary! primary is large. It is noted that AF demagnetization which in this instance could not detect the primary magnetization is often the only treatment applied to igneous rocks. It is therefore possible that undetected CRM in igneous rocks may be of frequent occurrence

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The fo!lowing example suggests that possibly the most reliable paleointensity determinations are to be obtained from rocks near a contact between an intruded (or extruded) igneous body and the pre-existing rock. The results from three sites sampled near (<50 m) a contact between Huronian sediments

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rock units. decay curve of a typical sample of those sediments sampled For comparison, Nipissing diabase the (sitestars 19) show near contact the direction between (a) these and thermal two a few kilometres away from the contact; thermal, chemical and probably a CRM acquired during diagenesis. Results indicate AF treatments indicating that the initial magnetization was that, near the contact, both the sediments and the diabase acquired their magnetization at about the same time, that is during cooling after emplacement of the diabase. If so, the magnetization of the diabase is a primary TRM or CRM; the magnetization of the sediments is a secondary TRM which could yield a reliable determination of the intensity of the field 2,150 m.y. ago.

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(sites 12 and 13) and the intruded Nipissing diabase (site 19) are shown in Fig. 3. The agreement between the directions of magnetization of the sediments and the diabase is remarkable and suggests. that the magnetization of both sediments and diabase occurred at about the same time. Comparison of the magnetization at sites 12 and 13 with that of the initial magnetization of Huronian sediments of the same formation (Roy et al., 1975) indicates that the magnetization near the contact is not primary but secondary; the initial magnetization was probably a CRM acquired during diagenesis (~2,250m.y. ago) and a typical example is given (Fig. 3a, Co27). The differences between the magnetizations near the contact (sites 12 and 13) and that of Co27 are striking: near the contact, the NRM intensities are much larger (factors of 10 to 24) and the decay curve with blocking ternperatures distributed from 20 to 650°Cis in contrast with the square-shouldered curve of Co27 where the blocking temperatures are confined to a short interval below the Curie point of hematite; the NRM directions are quite different from that of the initial magnetization (Co27, 675°C)and remain so up to 675°Cii. cating that the sediments were heated above the Curie point of hematite. It is suggested that this is a TRM acquired during cooling after emplacement of the Nipissing diabase. The square-shouldered thermal decay curve at the adjacent site 19 indicates that a single magnetization is present; the agreement between its direction and that obtained at sites 12 and 13 suggests that it was acquired during cooling, and that it is the primary magnetization of the Nipissing diabase. In principle, both the diabase and the baked sediments could be used to determine the intensity of the field 2,150 m.y. ago. However, the origin of magnetization of the diabase at this site is not known; it may be a TRM or a CRM. The relatively low NRM intensity favours the latter. Furthermore, igneous rocks are often susceptible to chemical changes that may occur during the experiment to obtain a NRM—TRM curve for paleointensity determinations. This problem is less likely to arise with sedimentary rocks especially like those of sites 12 and 13 since they were heated once before by the Nipissing diabase. Therefore an NRM—TRM curve following the Thellier technique should yield a reliable determination of the field at the time of cooling following the intrusion of the Nipissing diabase, 2,150 rn.y. ago.

Acknowledgement I wis to thank E. Irving for critical reading of the manJ,~script,and P.L. Lapointe and J.K. Park for participation in the experimental work described by above exaiIiples.

References Buddington, A.F. and Lindsley, D.H., 1964. Iron—titanium oxide minerals and synthetic equivalents. J. Petrol., 5: 310—357. Carmichael, I.S.E. and Nicholls, J., 1967. Iron—titanium oxides and oxygen fugacities in volcanic rocks. J. Geophys. Res., 72: 4665—4687. Fairbairn, H.W., Hurley, P.M., Card, K.D. and Knight, C.J., 1969. Correlation of radiometric ages of Nipissing diabase and Huronian metasediments with Proterozoic orogenic events in Ontario. Can. J. Earth Sci., 11:437—471. Grommé, C.S., Wright, T.L. and Peck, D.L. 1969. Magnetic properties and oxidation of iron-titanium oxide minerals in Alae and Makaopuki lava lakes, Hawaii. J. Geophys. Res., 74: 5277—5293. Irving, E. and Major, A. 1964. Post-depositional detrital remanent magnetization in a synthetic sediment. Sedimentology, 3: 135—143. Johnson, E.A., Murthy, T. and Torreson, OW., 1948. Prehistory of the earth’s magnetic field. Ten. Magn. Atmos. Electr., 53: 349—372. Larson, E.E. and Walker, T.R., 1975. Development of chemical remanent magnetization during early stages of red-bed formation in Late Cenozoic sediments, Baja California. Geol. Soc. Am. Bull., 86: 639—650. Roy, J.L. and Lapointe, P.L., 1975. Paleomagnetic results of Geol.1974. Soc. Am., Abstr., 7: 846.process of Roy,Aphebian J.L. and rocks. Park, J.K., The magnetization certain red beds: vector analysis of chemical and thermal results. Can. J. Earth Sci., 11: 437—471. Roy, J.L., Lapointe, P.L. and Anderson, P., 1975. Paleomagnetism of the oldest red beds and the direction of the Late Aphebian polar wander relative to Laurentia. Geophys. Res. Lett., 2: 537—540. Stacey, F.D. and Baneijee, S.K. 1974. The physical principles of Rock Magnetism. Elsevier, Amsterdam, 195 pp. Thellier, E., 1937. Sur l’aimantation des terres cuites et ses applications géophysiques. Ann. Inst. Phys. Globe, Univ. Paris, 16: 157—302. Thellier, E., 1966. Le champ magnétique terrestre fossile. Nucleus, 7: 1—35. Thellier, E. and Thellier, 0., 1959. Sur l’intensité do champ magnétique terrestre dans le passé historique et géologique. Ann. Géophys., 15: 285—376. Van Schmus, R., 1965. The geochronology of the Blind River—Bruce Mines area, Ontario, Canada. J. Geol., 73: 755—780.