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Lunar paleointensity from three Apollo 15 crystalline rocks using an A.R.M. method
Earth and Planetary Science Letters, 23 (1974) 185 - 188 © North-Holland Publishing Company, Amsterdam - Printed in The Netherlands
LUNAR PALEOINTENSITY
FROM THREE APOLLO 1 5 CRYSTALLINE USING AN A.R.M. METHOD
ROCKS
S.K. BANERJEE Department of Geology and Geophysics, University o f Minnesota Minneapolis, Minn. (USA) and J.P. MELLEMA Department of Physics, University of Minnesota Minneapolis, Minn. (USA} Original manuscript received August 13, 1973 To authors for revision December 11, 1973 Revision received June 27, 1974
Three Apollo 15 crystalline rocks were used for determining lunar paleointensity at 3.3 AE using a new ARMmethod of paleointensity determination. The values were found to be 4900 % 2200 7, and 7600 7. Thus an average lunar paleointensity of 4900 3' is concluded for this period.
1. Introduction The natural remanent magnetization (NRM) carried by the lunar samples has been the subject of extensive studies. One o f the objects of these studies is to determine the magnitude of the ancient magnetic field (paleointensity) which existed at the times o f formation o f these rocks on the lunar surface. Lunar paleointensity and its possible variations with time have important implications when fomulating a theory as to the formation of the moon and its subsequent evolution. For example, NRM in returned lunar samples, the magnetic anomalies observed by the magnetometer aboard the Apollo 15 subsatellite, and the lunar station magnetometer measurements all point to the conclusion that there was once a source of magnetic field in the m o o n which was responsible for the NRM o f the lunar samples whose ages range from 3.9 to 3.2 AE. Sonnett and Runcorn [I] and Strangway et al. [2] have argued very persuasively that such a source is most likely a central lunar core dynamo which is no longer active. If a relatively reliable magnetic history of a lunar dipolar magnetic field can be deducted from
samples form the various missions, one may be able to draw conclusions as to the origin, duration and magnitude of this lunar dynameo, and hence, shed some light on lunar origin and evolution. At the present time there is neither extensive measurements nor a general agreement on the magnitude of lunar paleointensity even for a few rocks o f the same age. Fig. 1 shows a histogram of lunar paleointensities taken from the literature (see Murthy and Banerjee [3] for sources) and displayed in terms of two different age groupings. As can be seen, there is not even a remote agreement for either age group. One of the contributory reasons for this disagreement lies in the character of the conventional Thellier and Thellier [4] method o f paleointensity determination. Basically, this technique consits of giving a rock a sequence of partial thermal demagnetizations in zero field and partial thermoremanent magnetizations in a known field. It is well known, however, that lunar rocks contain ultrafine iron grains (sizes ranging from 0.01/2m to 20 or 100/am) which acquired their NRM on the lunar surface at an ambient oxygen fugacity 5 to 10 orders of magnitude lower than the 10 -6 or 10 -7 value which can be obtained in a lab-
186
S.K. BANERJEE AND J.P. MELLEMA 2. Theory
Q
Of)
A completely general relation predicted ([6], also see our companion paper) for the ratio of normalized ARM (/)ARM)and normalized TRM (PTRM)for singledomain grains is:
~2
PaRM_ (Msb~2
Apollo 12 and 15 Age 3 . l - 3 . 3 AE
_~4 O.
.ol E zo
i
0
20
,I
, 40
,
i
60
,I
i
i
80
Apollo II and 14 Age 3.7--:3.9 AE
0.
O r)
O.
zo 0
II
20
b.
I
40
60
(a)'~everal tenths HM(IO00~') of on Oersted"
Xb +kTblMsb~ Hd
,
I00
H M (1000)")
_~4
(r__'l'12
I
I00 (b)1-170 K~"
80
Fig.1. Histograms of lunar paleointensities reported in the literature for rocks of age ranges: (a) 3.1-3.3 AE, and (b) 3.73.9 AE.
Here M s is the saturation moment per grain, T is the temperature at which ARM is given (room temperature for our case), k is the interaction parameter and k is Boltzmann's constant. The subscript b denotes the value at the blocking temperature (i.e. the temperature at which the thermoremanence was acquired). H d and H are the d.c. inducing fields for ARM and TRM, respectively. In a paleointensity experiment, H is the unknown parameter to be determined. Certain approximations can, however, be made to eq.1 without detracting too much from the desired accuracy as was shown for single domain CrO 2 powders in our companion paper. If, for example, the single-domain grain moment M s is large (that is, either M s per unit volume is large or the actual single domain volume is large, or both) kT/M s and kTb/Msb will be negligible compared to X and Xb which are determined from ARM experiments. This was actually observed to be the case for CrO 2. Then eq.1 may be modified to: PARM - -
oratory vacuum system and is customarily used for lunar paleointensity determination. Therefore, the repeated heatings in such a comparatively high oxygen fugacity have inevitably degraded sample integrity leaving one dissatisfied with the conventional method. In a companion paper [5] we have shown, using welldocumented samples, that an ARM-method which requires mostly room temperature measurements and only one heating run is a preferable alternative. Moreover, it is accurate to + 15 % which is as good as the better examples of the Thellier and Thellier method. In this short note, we would like to present our paleointensity observations using this ARM-method on three crystalline rocks of age 3.3 AE from the Apollo 15 mission.
(Msbl2 -
PT.M 'Ms'
( T 11/2
'Z-b'
~-
(2)
We further showed for CrO 2 that X is nearly temperature-independent except when T is in the vicinity of T b and that even making the approximation Xb = ), produces an error of only 15 % in the deduced H (paleointepsity). For the lunar samples, the situation is slightly different in that the saturation magnetization per unit volume is large (= 1700 emu) while the single-domain size is very small (close to an effective diameter of 200 A, see Gose et al. [71). As a result kT/M s was calculated to be 6.8 while the actually measured X was 20. Thus, we have again used the same approximations as for CrO 2 (i.e. kT/M s < X and X = Xb) which lead to the following expression for paleointensity//: (Msb~2
( T ~1/2 ( PTRM ~ Hd
(3)
LUNAR PALEOINTENSITY FROM THREE APOLLO 15 CRYSTALLINE ROCKS NRM D:341 ° I =-39 5 ° 140 20 D 110
10
/ ~
- To
E Q; L~ 0
187
behavior of intensity and clustering of directions; these demagnetized intensities were then substituted lbr PTRM in eq.3 above. The blocking temperatures T b were determined by thermally demagnetizing an isothermal remanent magnetization (IRM) in a vacuum of 10 -6 torr. This procedure yields the highest value of T b which is the one required for substitution into eq.2. T b was found to be 740°C. The ratio Ms/Msbwas obtained by dividing the saturation magnetization at 25°C with that at 740°C. ARM was given at 25°C in a known field H a. ARM divided by saturation IRM (at 15 kOe) yielded the term PARM in eq.3.
4. Results and discussion
0.5 e,Z
0
i
i
10
20
HAF (Oe) Fig.2. An example of changes in intensity and direction (inset) of the NRM-vector of a lunar rock due to AF-demagnetization.
3. Experimental approach The rocks used were igneous crystalline samples numbered 15058,52; 15495,46 and 15535,28. The "soft" part of the NRM was demagnetized using increasingly stronger alternating fields until the intensity was nearly constant and the directions of magnetization became clustered. An example is given in Fig.2. As is well known, the distinction between "hard" and "soft" components of the NRM can sometimes be a rather arbitrary distinction but we were guided by this respect by the work o f Pearce and Strangway [8]. These authors demagnetized a rock from the Apollo 15 mission to as high an A F as 1000 d e and then remeasured the rock after taking it on the Apollo 16 mission. They showed that a new "soft" component was acquired which could be easily demagnetized with a peak A F of 20 d e . We have used peak A F fields between 20 and 50 d e to separate out the stable component o f NRM of our rocks as determined by a flat
Table 1 shows the paleointensities obtained from the three Apollo 15 crystalline rocks used. The values are not as widely different from one another as has been found previously when Thellier and Thellier's method of paleointensities was applied to rocks o f the same age. This lack of spread in paleointensities is an encouraging result and we intend to apply the same method to a larger number of rock samples taken from other missions. The success with lunar rocks probably is due to the presence o f true single-domain grains in lunar rocks [9]. A parallel goal that is being pursued in our laboratory is to find out by trial and error what oxygen fugacity (using a H2/CO 2 gas-buffered furnace) is required for heating these same lunar rocks to 770 °C so that chemical degradation can be avoided during a conventional Thellier and Thellier determination. Hopefully, we will be able to compare values from the two different methods and see if they agree. At the moment our confidence in the ARM-method is justified TABLE 1 Paleointensity of Apollo 15 rocks by ARM-method Sample No.
Paleointensity (T)
15058,52 15495A6 15535,28
4900 2200 7600
The above values are correct to within ± 15 % as determined from a test of this method using a sample of CrO2 powder [5] to which ARM and TRM were imparted in the laboratory in 1 d e
188
S.K. BANERJEE AND J.P. MELLEMA
by its obvious success in predicting the correct paleointensity for the single domain CrO 2 case [5, this issue]. It may be inquired as to what would be the case if k T / M s is not negligible compated to k. In this case, the average paleointensity given in Table 1 will be the upper limit while a lower limit was also applied and yielded an average lower limit of 613 3` for the same rocks. The conclusion is inescapable: these three Apollo 15 rocks did acquire their NRM in a finite-sized magnetic field ranging between 613 3` and 4900 3,, which is two or three orders of magnitude greater than the present ambient interplanetary field at the moon.
Acknowledgements This work was supported by NASA grant NGR 24-
005-248 and NSF grant GA-31299. We thank K.A. Hoffman for reviewing the manuscript.
References [1] C.P. Sonnett and S.K. Runcorn, Comments Astrophys. Space Phys. 3 (1971) 149. [2] D.W. Strangway et al., Proc 18th Ann. Conf. Magnetism and Magnetic Materials (1973). [3] V.R. Murthy and S.K. Banerjee, The Moon 7 (1973) 149. [4] E. Thellier and O. Thellier, Ann. Geophys. 15 (1959) 285. [5] S.K. Banerjee and J.P. Mellema, Earth Planet. Sci. Eett. 23 (1974) 177, this issue. [6] W.F. Jaep, J. Appl. Phys. 42 (1971) 2790. [7] W. Gose et al., Proc. Third Lunar Sci. Conf., vol. 3 (1972) 2387. [8] G.W. Pearce and D.W. Strangway, Apollo. 16 Preliminary Science Report - NASA Document (1972). [9] F.-D. Tsay, Nature Phys. Sci. 246 (1973) 76.
LUNAR PALEOINTENSITY
FROM THREE APOLLO 1 5 CRYSTALLINE USING AN A.R.M. METHOD
ROCKS
S.K. BANERJEE Department of Geology and Geophysics, University o f Minnesota Minneapolis, Minn. (USA) and J.P. MELLEMA Department of Physics, University of Minnesota Minneapolis, Minn. (USA} Original manuscript received August 13, 1973 To authors for revision December 11, 1973 Revision received June 27, 1974
Three Apollo 15 crystalline rocks were used for determining lunar paleointensity at 3.3 AE using a new ARMmethod of paleointensity determination. The values were found to be 4900 % 2200 7, and 7600 7. Thus an average lunar paleointensity of 4900 3' is concluded for this period.
1. Introduction The natural remanent magnetization (NRM) carried by the lunar samples has been the subject of extensive studies. One o f the objects of these studies is to determine the magnitude of the ancient magnetic field (paleointensity) which existed at the times o f formation o f these rocks on the lunar surface. Lunar paleointensity and its possible variations with time have important implications when fomulating a theory as to the formation of the moon and its subsequent evolution. For example, NRM in returned lunar samples, the magnetic anomalies observed by the magnetometer aboard the Apollo 15 subsatellite, and the lunar station magnetometer measurements all point to the conclusion that there was once a source of magnetic field in the m o o n which was responsible for the NRM o f the lunar samples whose ages range from 3.9 to 3.2 AE. Sonnett and Runcorn [I] and Strangway et al. [2] have argued very persuasively that such a source is most likely a central lunar core dynamo which is no longer active. If a relatively reliable magnetic history of a lunar dipolar magnetic field can be deducted from
samples form the various missions, one may be able to draw conclusions as to the origin, duration and magnitude of this lunar dynameo, and hence, shed some light on lunar origin and evolution. At the present time there is neither extensive measurements nor a general agreement on the magnitude of lunar paleointensity even for a few rocks o f the same age. Fig. 1 shows a histogram of lunar paleointensities taken from the literature (see Murthy and Banerjee [3] for sources) and displayed in terms of two different age groupings. As can be seen, there is not even a remote agreement for either age group. One of the contributory reasons for this disagreement lies in the character of the conventional Thellier and Thellier [4] method o f paleointensity determination. Basically, this technique consits of giving a rock a sequence of partial thermal demagnetizations in zero field and partial thermoremanent magnetizations in a known field. It is well known, however, that lunar rocks contain ultrafine iron grains (sizes ranging from 0.01/2m to 20 or 100/am) which acquired their NRM on the lunar surface at an ambient oxygen fugacity 5 to 10 orders of magnitude lower than the 10 -6 or 10 -7 value which can be obtained in a lab-
186
S.K. BANERJEE AND J.P. MELLEMA 2. Theory
Q
Of)
A completely general relation predicted ([6], also see our companion paper) for the ratio of normalized ARM (/)ARM)and normalized TRM (PTRM)for singledomain grains is:
~2
PaRM_ (Msb~2
Apollo 12 and 15 Age 3 . l - 3 . 3 AE
_~4 O.
.ol E zo
i
0
20
,I
, 40
,
i
60
,I
i
i
80
Apollo II and 14 Age 3.7--:3.9 AE
0.
O r)
O.
zo 0
II
20
b.
I
40
60
(a)'~everal tenths HM(IO00~') of on Oersted"
Xb +kTblMsb~ Hd
,
I00
H M (1000)")
_~4
(r__'l'12
I
I00 (b)1-170 K~"
80
Fig.1. Histograms of lunar paleointensities reported in the literature for rocks of age ranges: (a) 3.1-3.3 AE, and (b) 3.73.9 AE.
Here M s is the saturation moment per grain, T is the temperature at which ARM is given (room temperature for our case), k is the interaction parameter and k is Boltzmann's constant. The subscript b denotes the value at the blocking temperature (i.e. the temperature at which the thermoremanence was acquired). H d and H are the d.c. inducing fields for ARM and TRM, respectively. In a paleointensity experiment, H is the unknown parameter to be determined. Certain approximations can, however, be made to eq.1 without detracting too much from the desired accuracy as was shown for single domain CrO 2 powders in our companion paper. If, for example, the single-domain grain moment M s is large (that is, either M s per unit volume is large or the actual single domain volume is large, or both) kT/M s and kTb/Msb will be negligible compared to X and Xb which are determined from ARM experiments. This was actually observed to be the case for CrO 2. Then eq.1 may be modified to: PARM - -
oratory vacuum system and is customarily used for lunar paleointensity determination. Therefore, the repeated heatings in such a comparatively high oxygen fugacity have inevitably degraded sample integrity leaving one dissatisfied with the conventional method. In a companion paper [5] we have shown, using welldocumented samples, that an ARM-method which requires mostly room temperature measurements and only one heating run is a preferable alternative. Moreover, it is accurate to + 15 % which is as good as the better examples of the Thellier and Thellier method. In this short note, we would like to present our paleointensity observations using this ARM-method on three crystalline rocks of age 3.3 AE from the Apollo 15 mission.
(Msbl2 -
PT.M 'Ms'
( T 11/2
'Z-b'
~-
(2)
We further showed for CrO 2 that X is nearly temperature-independent except when T is in the vicinity of T b and that even making the approximation Xb = ), produces an error of only 15 % in the deduced H (paleointepsity). For the lunar samples, the situation is slightly different in that the saturation magnetization per unit volume is large (= 1700 emu) while the single-domain size is very small (close to an effective diameter of 200 A, see Gose et al. [71). As a result kT/M s was calculated to be 6.8 while the actually measured X was 20. Thus, we have again used the same approximations as for CrO 2 (i.e. kT/M s < X and X = Xb) which lead to the following expression for paleointensity//: (Msb~2
( T ~1/2 ( PTRM ~ Hd
(3)
LUNAR PALEOINTENSITY FROM THREE APOLLO 15 CRYSTALLINE ROCKS NRM D:341 ° I =-39 5 ° 140 20 D 110
10
/ ~
- To
E Q; L~ 0
187
behavior of intensity and clustering of directions; these demagnetized intensities were then substituted lbr PTRM in eq.3 above. The blocking temperatures T b were determined by thermally demagnetizing an isothermal remanent magnetization (IRM) in a vacuum of 10 -6 torr. This procedure yields the highest value of T b which is the one required for substitution into eq.2. T b was found to be 740°C. The ratio Ms/Msbwas obtained by dividing the saturation magnetization at 25°C with that at 740°C. ARM was given at 25°C in a known field H a. ARM divided by saturation IRM (at 15 kOe) yielded the term PARM in eq.3.
4. Results and discussion
0.5 e,Z
0
i
i
10
20
HAF (Oe) Fig.2. An example of changes in intensity and direction (inset) of the NRM-vector of a lunar rock due to AF-demagnetization.
3. Experimental approach The rocks used were igneous crystalline samples numbered 15058,52; 15495,46 and 15535,28. The "soft" part of the NRM was demagnetized using increasingly stronger alternating fields until the intensity was nearly constant and the directions of magnetization became clustered. An example is given in Fig.2. As is well known, the distinction between "hard" and "soft" components of the NRM can sometimes be a rather arbitrary distinction but we were guided by this respect by the work o f Pearce and Strangway [8]. These authors demagnetized a rock from the Apollo 15 mission to as high an A F as 1000 d e and then remeasured the rock after taking it on the Apollo 16 mission. They showed that a new "soft" component was acquired which could be easily demagnetized with a peak A F of 20 d e . We have used peak A F fields between 20 and 50 d e to separate out the stable component o f NRM of our rocks as determined by a flat
Table 1 shows the paleointensities obtained from the three Apollo 15 crystalline rocks used. The values are not as widely different from one another as has been found previously when Thellier and Thellier's method of paleointensities was applied to rocks o f the same age. This lack of spread in paleointensities is an encouraging result and we intend to apply the same method to a larger number of rock samples taken from other missions. The success with lunar rocks probably is due to the presence o f true single-domain grains in lunar rocks [9]. A parallel goal that is being pursued in our laboratory is to find out by trial and error what oxygen fugacity (using a H2/CO 2 gas-buffered furnace) is required for heating these same lunar rocks to 770 °C so that chemical degradation can be avoided during a conventional Thellier and Thellier determination. Hopefully, we will be able to compare values from the two different methods and see if they agree. At the moment our confidence in the ARM-method is justified TABLE 1 Paleointensity of Apollo 15 rocks by ARM-method Sample No.
Paleointensity (T)
15058,52 15495A6 15535,28
4900 2200 7600
The above values are correct to within ± 15 % as determined from a test of this method using a sample of CrO2 powder [5] to which ARM and TRM were imparted in the laboratory in 1 d e
188
S.K. BANERJEE AND J.P. MELLEMA
by its obvious success in predicting the correct paleointensity for the single domain CrO 2 case [5, this issue]. It may be inquired as to what would be the case if k T / M s is not negligible compated to k. In this case, the average paleointensity given in Table 1 will be the upper limit while a lower limit was also applied and yielded an average lower limit of 613 3` for the same rocks. The conclusion is inescapable: these three Apollo 15 rocks did acquire their NRM in a finite-sized magnetic field ranging between 613 3` and 4900 3,, which is two or three orders of magnitude greater than the present ambient interplanetary field at the moon.
Acknowledgements This work was supported by NASA grant NGR 24-
005-248 and NSF grant GA-31299. We thank K.A. Hoffman for reviewing the manuscript.
References [1] C.P. Sonnett and S.K. Runcorn, Comments Astrophys. Space Phys. 3 (1971) 149. [2] D.W. Strangway et al., Proc 18th Ann. Conf. Magnetism and Magnetic Materials (1973). [3] V.R. Murthy and S.K. Banerjee, The Moon 7 (1973) 149. [4] E. Thellier and O. Thellier, Ann. Geophys. 15 (1959) 285. [5] S.K. Banerjee and J.P. Mellema, Earth Planet. Sci. Eett. 23 (1974) 177, this issue. [6] W.F. Jaep, J. Appl. Phys. 42 (1971) 2790. [7] W. Gose et al., Proc. Third Lunar Sci. Conf., vol. 3 (1972) 2387. [8] G.W. Pearce and D.W. Strangway, Apollo. 16 Preliminary Science Report - NASA Document (1972). [9] F.-D. Tsay, Nature Phys. Sci. 246 (1973) 76.