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Some rock magnetic properties of mid-Cretaceous basalts from Israel and India (Rajmahal traps), and their bearing on palaeointensity experiments
Physics of the Earth and Planetary Interiors, 70 (1992) 237—242 Elsevier Science Publishers B.V., Amsterdam
237
Some rock magnetic properties of mid-Cretaceous basalts from Israel and India (Rajmahal traps), and their bearing on palaeointensity experiments Graham J. Sherwood Geomagnetism Laboratory, Department of Earth Sciences, University of Liverpool, P.O. Box 147, Liverpool L69 3BX, UK (Received 1 December 1990; revision accepted 29 May 1991)
ABSTRACT Sherwood, G.J., 1992. Some rock magnetic properties of mid-Cretaceous basalts from Israel and India (Rajmahal traps), and their bearing on palaeointensity experiments. Phys. Earth Planet. Inter., 70: 237—242. A wide range of rock magnetic properties have been determined from two collections of mid-Cretaceous basalts; one from Israel, the other from the Rajmahal traps in northeast India. Deuteric oxidation is rare in both collections, with titanium-rich titanomagnetite being the principal remanence carrier in most cases. There are a number of differences in rock magnetic properties between the two groups. Some of these seem to be primary, whereas others appear to be caused by hydrothermal alteration and weathering, which are more prevalent in the Indian rocks. These rocks are being used in palaeointensity experiments, from which it is hoped to determine the strength of the Earth’s magnetic field during the long period of normal polarity in the mid-Cretaceous. Thellier palaeointensity experiments have been performed on two samples from each site. The degree of agreement between the two results is highly variable. The low blocking temperatures and the presence of secondary viscous components in many samples make Thellier palaeointensity experiments very difficult. A further problem is that of thermal alteration, two main types of which are observed. The first manifests itself as a large and sudden increase in partial thermoremanent magnetization (pTRM) capacity, and the second as a steady decrease in the size of pTRM with increasing temperature.
1. Introduction In this paper we compare the magnetic prop-
erties of two collections of mid-Cretaceous basalts, one from the Rajmahal Hills in northeastern India, and the other from the Maktesh Ramon, Cannel and Hermon areas of Israel. Ar—Ar dates from the normally magnetized Rajmahal and Ramon lavas give ages of between 115 and 119 Ma (Baksi, 1986; B. Lang, personal cornmunication, 1989), suggesting that they formed at the beginning of the 35 m.y. period of stable nonnal polarity, the Cretaceous Quiet Zone (CQZ).
These samples were collected as part of a project to look at the relationship between the strength of the geomagnetic field and the frequency of its reversals (Shaw and Sherwood, 1991). It is important for palaeointensity experiments to have a detailed knowledge of the mag0031-9201/92/$05.0O © 1992
—
netic mineralogy and domain states of the carriers of the remanence, as it is essential that the rock is carrying a primary thermoremanence
(TRM). Samples from each of the 50 localities (27 from Israel and 23 from India) have been studied using
a variety of rock magnetic techniques: (1) J~—T (Curie temperature determination), (2) hysteresis parameters C15, ~rs and He), (3) low-temperature susceptibility behaviour (between 78 K and room temperature), and (4) resistance to alternating field demagnetization.
2. Results of rock magnetic experiments 2.1. Curie temperatures For both Israeli and Indian rocks the degree of deuteric oxidation is on average very low: the
Elsevier Science Publishers B.V. All rights reserved
238
G. SHERWOOD
majority of samples contain primary titanomagnetites with Curie temperatures between 160— 400 C, with the titanomagnetites in the Israeli rocks tending to have lower Curie temperatures than those in the Indian basalts. Some samples also contain magnetite or titanium-poor titanomagnetites (T~ between 500—580 °C), though these are usually only minor phases. A few samples show a higher degree of deuteric oxidation, either containing only magnetite or titanium-poor titanomagnetite, or in two cases titanohaematite. Thermomagnetic curves from the Israeli basalts containing titanium-rich titanomagnetites tend to be more reversible than those from the Rajmahal samples, which often alter to magnetite after heating. This suggests that low-temperature oxidation (maghematization) is more advanced in the Indian rocks, possibly due to hydrothermal alteration at some stage in the past, or recent weathering. The difference in Curie temperatures between the two areas could be due to two causes: a difference in the primary titanium content in the titanomagnetite, or different degrees of lowtemperature oxidation. °
2.2. Hysteresis properties Using a vibrating sample magnetometer (maximum field iT), the hysteresis parameters were determined for all but the two samples containing haematite. The magnetic content for the two groups of samples has two very different distributions: the J~values for the Israeli rocks range between 0.23—3.2 Am2 kgt, whereas for the Indian rocks the spread is between 0.02—0.59 2~2 kg’. This difference is interpreted as a primary feature, The Jrs/Js ratios fall mainly between 0.08—0.16, suggesting a predominance of multidomain (MD) grains, with perhaps a small amount of smaller grains with pseudo-single (PSD) or single-domain (SD) behaviour. Some samples, mainly from India, have higher ~rs/~s ratios, in the range 0.16— 0.38, indicating a higher proportion of PSD and SD grains. No sample had a ~rs/~s ratio close to the SD value of 0.5. The generally low ~rs/~s ratios are not surprising considering the generally unoxidized nature
of the titanomagnetites. Optical and SEM studies of some of the Israeli samples show that the titanomagnetite grains are often of the order of 10 /xm, which would make them MD. The values for the coercive force, H~,from the Indian rocks are on average about 40% higher than those from the Israeli basalts, most samples having a coercive force between 4.5—7 kA m’, though I—Ia values of up to 21 kA m’ were observed. The higher H~values for the Indian samples may be caused partly by low-temperature oxidation. 2.3. Low-temperature susceptibility behaviour The variation of susceptibility between the boiling point of nitrogen (78 K) and room ternperature may be divided into three main types of behaviour (Senanayake and McElhinny, 1981; Shaw et aL, 1991): group 1 where susceptibility increases with temperature; group 2 where susceptibility decreases with temperature; and group 3 where the susceptibility peaks at 125 K. The ratio RS is defined as We find clear differences in behaviour between the Israeli and Indian basalts (Fig. 1). For the Israeli rocks, group 1 behaviour dominates, and the RS values are generally low (<0.2). For the Rajmahal samples, we observe group 1 behaviour in most samples, though many of these curves show an initial decrease in susceptibility (group 2 contamination). The RS values, even for the uncontaminated samples, are higher than those of the Israeli rocks, being around 0.4—0.6. Two Israeli samples show behaviour other than group 1 (Fig. 1): sample CA2 (the only one of either collection) has a small peak in susceptibility at 150°Cindicating the presence of a small amount of MD magnetite; and sample CAl has a Hopkinson peak at about 250 K, which probably marks the unblocking of a low-blocking temperature SD titanomagnetite (Shaw et al., 1991). The group 2 behaviour observed in the Rajmahal samples is interpreted as paramagnetic susceptibility. Although this is two orders of magnitude weaker than the ferrimagnetic susceptibility, the low concentrations of magnetic minerals in these rocks, combined with high levels of clay —
239
MID-CRETACEOUS BASALTS
1:
India
~7IxO.5~
—200
—100
TIC)
0
Israel
—200
—100
TIC)
Fig. 1. Examples of low-temperature susceptibility behaviour—most Israeli samples show group 1 behaviour with low RS values and most Indian samples have group 1 or group 1 with group 2 paramagnetic contamination with RS values higher that the Israeli samples.
minerals in some more altered samples, allow paramagnetic susceptibility to make a significant contribution, and to dominate occasionally (Fig. 1, Ri17). Group 1 behaviour with low RS values is usually observed in titanium-rich titanomagnetites, with the RS value decreasing with titanium content (Radhakrishnamurty, 1985). Senanayake and McElhinny (1981) have shown that low-temperature oxidation does not significantly affect the value of RS, which suggests that the observed difference in RS is the result of the titanomagnetites in the Israeli rocks being more titaniumrich than those in the Rajmahal basalts.
thought to have been remagnetized by lightning strikes. Further stepwise demagnetization to higher alternating fields has shown that about half the Israeli, and the majority of the Indian samples still have a stable component remaining at 100 mT, held presumably by SD grains.
3. The use of these rocks in palaeointensity experiments The Thellier palaeointensity technique has been attempted on these lavas. Both collections RA14-7
Banc
-
+1- 0.6
13.8
2.4. Alternating field demagnetization The percentage of NRM remaining after AF demagnetization to 10 mT is very variable. Most Israeli rocks have median destruction fields well below 10 mT, but the Rajmahal rocks are more resistant to AF, with a mean median destruction field of the order of 10 mT. A couple of the Israeli rocks have very soft remanences which have less than 2% of the NRM remaining after demagnetization to 10 mT: these rocks are
NR M 100 0
2 00 1 50
350
.300 5
R
275
•
2 50
2 25
•
TAM Fig. 2. An example of a Thellier experiment on an Israeli . . . basalt. A massive increase intemperatures TRM capacity observed between 200—225°C.At higher theisTRM capacity gradually decreases.
ci. SHERWOOD
240
are characterized by a low level of deuteric hightemperature oxidation, so primary low Curie ternperature titanomagnetite is common. A present-
day overprint of viscous origin is found in a significant number of samples. This is often not removed until thermal demagnetization to 200 C.
a)
c)
Israel
All data
°
Rajmahal 1%
P12
/
/
/
P12 II
pT 1]
/
/
/
0
1jT
20
I
Ph
CI
b)
j.iT
Israel II
/
/
/t I
/ /~0~’
(accepted)
Phi
—~
P12
/ /n
20
I,,/”
I
~ Ii
10
20 P11
20
pT
Fig. 3. Two Thellier palaeointensity experiments have been performed on each sample site. The two palaeointensity estimates obtained are compared in these diagrams, identical estimates falling on the diagonal. (a) A number of Israeli samples are contaminated by secondary magnetizations that cannot be separated from the primary magnetizations by thermal demagnetizations; this causes incorrect palaeointensity estimates. (b) Thirteen out of 25 samples give two acceptable palaeointensity estimates. (c) The Indian data are on the whole quite repeatable.
241
MID-CRETACEOUS BASALTS
The combination of the low Curie temperatures and the viscous overprint means that the temperature interval which can be used in palaeointensity experiments is quite small. The rocks usually have MD/PSD characteristics, and are, therefore, not considered ideal for palaeointensity work, as the pTRM unbiocking temperatures for MD grains are higher (up to T~) than their blocking temperatures (Bolshakov and Shcherbakova, 1979). Single-domain characteristics tend to be more common in deuterically oxidized basalts, which are usually considered to be the most suitable for palaeointensity experiments (Prévot et al., 1990). However, work with recent lavas (Sherwood et al., 1988; Rolph, 1992) suggests that some higher oxidized basalts may give anomalously high palaeointensities. This is a problem which is not clearly understood, but makes the choice of an ideal palaeointensity sample even more difficult. The reason for the high palaeointensity estimates is thought to be the continuation of the deuteric oxidation below the Curie temperatures of the daughter products, resulting in a thermochemical remanence. Unoxidized samples will not have this problem, though they do have many others. A major problem with the Israeli and Indian rocks is that of alteration during the Thellier experiment. Partial TRM checks were regularly made to observe major changes in TRM capacity during heating. The types of changes observed fall into two main categories, both of which are shown by the sample in Fig. 2: first, a massive increase in TRM capacity which may either occur progressively, resulting in a concave Arai plot, or very suddenly (Fig. 2); and secondly, a gradual decrease in TRM capacity on heating (Fig. 2), which usually occurs above the Curie temperature of the titanomagnetite phase, though, in the case of samples which also contain a little magnetite, before the NRM is fully demagnetized. The reasons for the increase in TRM are not obvious, particularly as changes in TRM capacity are not clearly linked to variations in susceptibility. The use of the Shaw palaeointensity method also has problems, particularly for the Israeli rocks which are not very resistant to AF demag-
netization. However, it is hoped that some intensity determinations can be made, which may be compared with those obtained by the Thellier technique. Despite the difficulties in using these rocks for palaeointensity experiments, we have obtained a number of preliminary palaeointensity results for the beginning of the CQZ (Sherwood and Shaw, 1991). From each of the sites with orientated samples (25 in Israel and 23 in India), we have measured two samples using the modified Thelher technique. Figure 3 compares the palaeointensity estimates obtained in the two experiments: sites where the two palaeointensity values are identical will fall on the diagonal. Results from Israel are very scattered (Fig. 3(a)), and there are a number of sites where one or both of the palaeointensity estimates are very high (> 100 jxT) or very low (< 3 jxT). It appears that these anomalous results are caused by secondary overprints, such as lightning strikes and viscous magnetizations, which have similar blocking temperature spectra to the primary component, and thus the primary NRM cannot be separated during thermal demagnetization. Shaw experiments may prove more successful with some of these samples. From the 25 Israeli lavas, six were rejected completely, six had one out of the two palaeointensity estimates accepted, and 13 (Fig. 3(b)) had both deemed acceptable. The Indian results (Fig. 3(c)) were all accepted, though repeatability was poor in a number of sites. Although the mean palaeointensity from Israel is much lower than that from India, the difference in palaeolatitude (equatorial for Israel and mid-latitude in the Southern Hemisphere for India) means that the time-averaged virtual dipole moment of the Earth calculated for the early part of the CQZ from each region is very similar. It appears that the Earth’s field was about 75% of its present strength (Sherwood and Shaw, 1991).
Acknowledgements This work was funded by a Natural Environment Research Council grant to iohn Shaw. I thank him and also our coworkers: Gidi Baer,
242
Geological Survey of Israel, S. Basu Mallik, Jadavpur University Calcutta and C. Radhakrishnamurty TIFR Bombay. Valodir Tselmovich carried out SEM work whilst I was on a study visit to the USSR, funded by the Royal Society and the Institute of the Physics of the Earth, Moscow
References 40Ar—39Ar incremental heating study of Baksi, A.K., 1986. whole-rock samples from the Rajmahal and Bengal Traps, eastern India. Terra Cognita, 6: 161. Bolshakov, A.S. and Shcherbakova, V.V., 1979. Thermomagnetic test for determining the domain structure of fernmagnetics. Izv. Akad. Nauk SSSR, Solid Earth, 15: 38—47. Prévot, M., Derder, M.E., McWilliams, M. and Thompson, J., 1990. Intensity of the Earth’s magnetic field: evidence for a Mesozoic dipole low. Earth Planet. Sci. Lett., 97: 129— 139.
0. SHERWOOD
Radhakrishnamurty, C., 1985. Identification of titanomagnetites by simple magnetic techniques and application to basalt studies. J. Geol. Soc. India, 26: 640—65 1. . . Roiph, T.C., 1992. High field intensities from recent and historical lavas. Phys. Earth Planet. Inter., 70: 224—230. Senanayake, W.E. and McElhinny, M.W., 1981. Hysteresis and susceptibility characteristics of magnetite and titanomagnetites: interpretation of results from basaltic rocks. Phys. Earth Planet. Inter., 26: 47—55. Shaw, J. and Sherwood, G.J., 1991. Palaeointensity reversal frequency—are they related? Geophys. Astrophys. Fluid Dyn., 55: in press. Shaw, J., Sherwood, G.J., Mussett, A.E., Rolph, T.C., Subbarao, K.V. and Sharma, P.V., 1991. The strength of the geomagnetic field at the Cretaceous—Tertiary boundary: palaeointensity results from the Deccan Traps (India) and the Disko Lavas (Greenland). J. Geomagn. Geoelectr., 43: 395—408. Sherwood, G.J. and Shaw, J., 1991. The relationship between magnetic field strength and reversal frequency: preliminary palaeointensity results from the Cretaceous Quiet Zone. Stud. Geophys. Geod., in press. Sherwood, G.J., Rolph, T.C. and Shaw, J., 1988. Comment on “Alteration of the coercivity spectrum and palaeointensity determination”. Geophys. Res. Lett., 15: 629—630.
237
Some rock magnetic properties of mid-Cretaceous basalts from Israel and India (Rajmahal traps), and their bearing on palaeointensity experiments Graham J. Sherwood Geomagnetism Laboratory, Department of Earth Sciences, University of Liverpool, P.O. Box 147, Liverpool L69 3BX, UK (Received 1 December 1990; revision accepted 29 May 1991)
ABSTRACT Sherwood, G.J., 1992. Some rock magnetic properties of mid-Cretaceous basalts from Israel and India (Rajmahal traps), and their bearing on palaeointensity experiments. Phys. Earth Planet. Inter., 70: 237—242. A wide range of rock magnetic properties have been determined from two collections of mid-Cretaceous basalts; one from Israel, the other from the Rajmahal traps in northeast India. Deuteric oxidation is rare in both collections, with titanium-rich titanomagnetite being the principal remanence carrier in most cases. There are a number of differences in rock magnetic properties between the two groups. Some of these seem to be primary, whereas others appear to be caused by hydrothermal alteration and weathering, which are more prevalent in the Indian rocks. These rocks are being used in palaeointensity experiments, from which it is hoped to determine the strength of the Earth’s magnetic field during the long period of normal polarity in the mid-Cretaceous. Thellier palaeointensity experiments have been performed on two samples from each site. The degree of agreement between the two results is highly variable. The low blocking temperatures and the presence of secondary viscous components in many samples make Thellier palaeointensity experiments very difficult. A further problem is that of thermal alteration, two main types of which are observed. The first manifests itself as a large and sudden increase in partial thermoremanent magnetization (pTRM) capacity, and the second as a steady decrease in the size of pTRM with increasing temperature.
1. Introduction In this paper we compare the magnetic prop-
erties of two collections of mid-Cretaceous basalts, one from the Rajmahal Hills in northeastern India, and the other from the Maktesh Ramon, Cannel and Hermon areas of Israel. Ar—Ar dates from the normally magnetized Rajmahal and Ramon lavas give ages of between 115 and 119 Ma (Baksi, 1986; B. Lang, personal cornmunication, 1989), suggesting that they formed at the beginning of the 35 m.y. period of stable nonnal polarity, the Cretaceous Quiet Zone (CQZ).
These samples were collected as part of a project to look at the relationship between the strength of the geomagnetic field and the frequency of its reversals (Shaw and Sherwood, 1991). It is important for palaeointensity experiments to have a detailed knowledge of the mag0031-9201/92/$05.0O © 1992
—
netic mineralogy and domain states of the carriers of the remanence, as it is essential that the rock is carrying a primary thermoremanence
(TRM). Samples from each of the 50 localities (27 from Israel and 23 from India) have been studied using
a variety of rock magnetic techniques: (1) J~—T (Curie temperature determination), (2) hysteresis parameters C15, ~rs and He), (3) low-temperature susceptibility behaviour (between 78 K and room temperature), and (4) resistance to alternating field demagnetization.
2. Results of rock magnetic experiments 2.1. Curie temperatures For both Israeli and Indian rocks the degree of deuteric oxidation is on average very low: the
Elsevier Science Publishers B.V. All rights reserved
238
G. SHERWOOD
majority of samples contain primary titanomagnetites with Curie temperatures between 160— 400 C, with the titanomagnetites in the Israeli rocks tending to have lower Curie temperatures than those in the Indian basalts. Some samples also contain magnetite or titanium-poor titanomagnetites (T~ between 500—580 °C), though these are usually only minor phases. A few samples show a higher degree of deuteric oxidation, either containing only magnetite or titanium-poor titanomagnetite, or in two cases titanohaematite. Thermomagnetic curves from the Israeli basalts containing titanium-rich titanomagnetites tend to be more reversible than those from the Rajmahal samples, which often alter to magnetite after heating. This suggests that low-temperature oxidation (maghematization) is more advanced in the Indian rocks, possibly due to hydrothermal alteration at some stage in the past, or recent weathering. The difference in Curie temperatures between the two areas could be due to two causes: a difference in the primary titanium content in the titanomagnetite, or different degrees of lowtemperature oxidation. °
2.2. Hysteresis properties Using a vibrating sample magnetometer (maximum field iT), the hysteresis parameters were determined for all but the two samples containing haematite. The magnetic content for the two groups of samples has two very different distributions: the J~values for the Israeli rocks range between 0.23—3.2 Am2 kgt, whereas for the Indian rocks the spread is between 0.02—0.59 2~2 kg’. This difference is interpreted as a primary feature, The Jrs/Js ratios fall mainly between 0.08—0.16, suggesting a predominance of multidomain (MD) grains, with perhaps a small amount of smaller grains with pseudo-single (PSD) or single-domain (SD) behaviour. Some samples, mainly from India, have higher ~rs/~s ratios, in the range 0.16— 0.38, indicating a higher proportion of PSD and SD grains. No sample had a ~rs/~s ratio close to the SD value of 0.5. The generally low ~rs/~s ratios are not surprising considering the generally unoxidized nature
of the titanomagnetites. Optical and SEM studies of some of the Israeli samples show that the titanomagnetite grains are often of the order of 10 /xm, which would make them MD. The values for the coercive force, H~,from the Indian rocks are on average about 40% higher than those from the Israeli basalts, most samples having a coercive force between 4.5—7 kA m’, though I—Ia values of up to 21 kA m’ were observed. The higher H~values for the Indian samples may be caused partly by low-temperature oxidation. 2.3. Low-temperature susceptibility behaviour The variation of susceptibility between the boiling point of nitrogen (78 K) and room ternperature may be divided into three main types of behaviour (Senanayake and McElhinny, 1981; Shaw et aL, 1991): group 1 where susceptibility increases with temperature; group 2 where susceptibility decreases with temperature; and group 3 where the susceptibility peaks at 125 K. The ratio RS is defined as We find clear differences in behaviour between the Israeli and Indian basalts (Fig. 1). For the Israeli rocks, group 1 behaviour dominates, and the RS values are generally low (<0.2). For the Rajmahal samples, we observe group 1 behaviour in most samples, though many of these curves show an initial decrease in susceptibility (group 2 contamination). The RS values, even for the uncontaminated samples, are higher than those of the Israeli rocks, being around 0.4—0.6. Two Israeli samples show behaviour other than group 1 (Fig. 1): sample CA2 (the only one of either collection) has a small peak in susceptibility at 150°Cindicating the presence of a small amount of MD magnetite; and sample CAl has a Hopkinson peak at about 250 K, which probably marks the unblocking of a low-blocking temperature SD titanomagnetite (Shaw et al., 1991). The group 2 behaviour observed in the Rajmahal samples is interpreted as paramagnetic susceptibility. Although this is two orders of magnitude weaker than the ferrimagnetic susceptibility, the low concentrations of magnetic minerals in these rocks, combined with high levels of clay —
239
MID-CRETACEOUS BASALTS
1:
India
~7IxO.5~
—200
—100
TIC)
0
Israel
—200
—100
TIC)
Fig. 1. Examples of low-temperature susceptibility behaviour—most Israeli samples show group 1 behaviour with low RS values and most Indian samples have group 1 or group 1 with group 2 paramagnetic contamination with RS values higher that the Israeli samples.
minerals in some more altered samples, allow paramagnetic susceptibility to make a significant contribution, and to dominate occasionally (Fig. 1, Ri17). Group 1 behaviour with low RS values is usually observed in titanium-rich titanomagnetites, with the RS value decreasing with titanium content (Radhakrishnamurty, 1985). Senanayake and McElhinny (1981) have shown that low-temperature oxidation does not significantly affect the value of RS, which suggests that the observed difference in RS is the result of the titanomagnetites in the Israeli rocks being more titaniumrich than those in the Rajmahal basalts.
thought to have been remagnetized by lightning strikes. Further stepwise demagnetization to higher alternating fields has shown that about half the Israeli, and the majority of the Indian samples still have a stable component remaining at 100 mT, held presumably by SD grains.
3. The use of these rocks in palaeointensity experiments The Thellier palaeointensity technique has been attempted on these lavas. Both collections RA14-7
Banc
-
+1- 0.6
13.8
2.4. Alternating field demagnetization The percentage of NRM remaining after AF demagnetization to 10 mT is very variable. Most Israeli rocks have median destruction fields well below 10 mT, but the Rajmahal rocks are more resistant to AF, with a mean median destruction field of the order of 10 mT. A couple of the Israeli rocks have very soft remanences which have less than 2% of the NRM remaining after demagnetization to 10 mT: these rocks are
NR M 100 0
2 00 1 50
350
.300 5
R
275
•
2 50
2 25
•
TAM Fig. 2. An example of a Thellier experiment on an Israeli . . . basalt. A massive increase intemperatures TRM capacity observed between 200—225°C.At higher theisTRM capacity gradually decreases.
ci. SHERWOOD
240
are characterized by a low level of deuteric hightemperature oxidation, so primary low Curie ternperature titanomagnetite is common. A present-
day overprint of viscous origin is found in a significant number of samples. This is often not removed until thermal demagnetization to 200 C.
a)
c)
Israel
All data
°
Rajmahal 1%
P12
/
/
/
P12 II
pT 1]
/
/
/
0
1jT
20
I
Ph
CI
b)
j.iT
Israel II
/
/
/t I
/ /~0~’
(accepted)
Phi
—~
P12
/ /n
20
I,,/”
I
~ Ii
10
20 P11
20
pT
Fig. 3. Two Thellier palaeointensity experiments have been performed on each sample site. The two palaeointensity estimates obtained are compared in these diagrams, identical estimates falling on the diagonal. (a) A number of Israeli samples are contaminated by secondary magnetizations that cannot be separated from the primary magnetizations by thermal demagnetizations; this causes incorrect palaeointensity estimates. (b) Thirteen out of 25 samples give two acceptable palaeointensity estimates. (c) The Indian data are on the whole quite repeatable.
241
MID-CRETACEOUS BASALTS
The combination of the low Curie temperatures and the viscous overprint means that the temperature interval which can be used in palaeointensity experiments is quite small. The rocks usually have MD/PSD characteristics, and are, therefore, not considered ideal for palaeointensity work, as the pTRM unbiocking temperatures for MD grains are higher (up to T~) than their blocking temperatures (Bolshakov and Shcherbakova, 1979). Single-domain characteristics tend to be more common in deuterically oxidized basalts, which are usually considered to be the most suitable for palaeointensity experiments (Prévot et al., 1990). However, work with recent lavas (Sherwood et al., 1988; Rolph, 1992) suggests that some higher oxidized basalts may give anomalously high palaeointensities. This is a problem which is not clearly understood, but makes the choice of an ideal palaeointensity sample even more difficult. The reason for the high palaeointensity estimates is thought to be the continuation of the deuteric oxidation below the Curie temperatures of the daughter products, resulting in a thermochemical remanence. Unoxidized samples will not have this problem, though they do have many others. A major problem with the Israeli and Indian rocks is that of alteration during the Thellier experiment. Partial TRM checks were regularly made to observe major changes in TRM capacity during heating. The types of changes observed fall into two main categories, both of which are shown by the sample in Fig. 2: first, a massive increase in TRM capacity which may either occur progressively, resulting in a concave Arai plot, or very suddenly (Fig. 2); and secondly, a gradual decrease in TRM capacity on heating (Fig. 2), which usually occurs above the Curie temperature of the titanomagnetite phase, though, in the case of samples which also contain a little magnetite, before the NRM is fully demagnetized. The reasons for the increase in TRM are not obvious, particularly as changes in TRM capacity are not clearly linked to variations in susceptibility. The use of the Shaw palaeointensity method also has problems, particularly for the Israeli rocks which are not very resistant to AF demag-
netization. However, it is hoped that some intensity determinations can be made, which may be compared with those obtained by the Thellier technique. Despite the difficulties in using these rocks for palaeointensity experiments, we have obtained a number of preliminary palaeointensity results for the beginning of the CQZ (Sherwood and Shaw, 1991). From each of the sites with orientated samples (25 in Israel and 23 in India), we have measured two samples using the modified Thelher technique. Figure 3 compares the palaeointensity estimates obtained in the two experiments: sites where the two palaeointensity values are identical will fall on the diagonal. Results from Israel are very scattered (Fig. 3(a)), and there are a number of sites where one or both of the palaeointensity estimates are very high (> 100 jxT) or very low (< 3 jxT). It appears that these anomalous results are caused by secondary overprints, such as lightning strikes and viscous magnetizations, which have similar blocking temperature spectra to the primary component, and thus the primary NRM cannot be separated during thermal demagnetization. Shaw experiments may prove more successful with some of these samples. From the 25 Israeli lavas, six were rejected completely, six had one out of the two palaeointensity estimates accepted, and 13 (Fig. 3(b)) had both deemed acceptable. The Indian results (Fig. 3(c)) were all accepted, though repeatability was poor in a number of sites. Although the mean palaeointensity from Israel is much lower than that from India, the difference in palaeolatitude (equatorial for Israel and mid-latitude in the Southern Hemisphere for India) means that the time-averaged virtual dipole moment of the Earth calculated for the early part of the CQZ from each region is very similar. It appears that the Earth’s field was about 75% of its present strength (Sherwood and Shaw, 1991).
Acknowledgements This work was funded by a Natural Environment Research Council grant to iohn Shaw. I thank him and also our coworkers: Gidi Baer,
242
Geological Survey of Israel, S. Basu Mallik, Jadavpur University Calcutta and C. Radhakrishnamurty TIFR Bombay. Valodir Tselmovich carried out SEM work whilst I was on a study visit to the USSR, funded by the Royal Society and the Institute of the Physics of the Earth, Moscow
References 40Ar—39Ar incremental heating study of Baksi, A.K., 1986. whole-rock samples from the Rajmahal and Bengal Traps, eastern India. Terra Cognita, 6: 161. Bolshakov, A.S. and Shcherbakova, V.V., 1979. Thermomagnetic test for determining the domain structure of fernmagnetics. Izv. Akad. Nauk SSSR, Solid Earth, 15: 38—47. Prévot, M., Derder, M.E., McWilliams, M. and Thompson, J., 1990. Intensity of the Earth’s magnetic field: evidence for a Mesozoic dipole low. Earth Planet. Sci. Lett., 97: 129— 139.
0. SHERWOOD
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