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Magnetic processes in oceanic ridge
EARTH AND PLANETARY SCIENCE LETTERS 13 (1971) 1-5. NORTH-HOLLAND PUBLISHING COMPANY
MAGNETIC
PROCESSES
IN OCEANIC RIDGE
Minoru OZIMA
Geophysical Institute, University of Tokyo, Japan Received 1 July 1971 Revised version received 5 October 1971 It is shown that maghemitization (low temperature oxidation of titano-magnetite to titanomaghemite) can account for the recently discovered characteristic features of natural remanent magnetization of dredge basalts from the Mid-Atlantic Ridge, that is, (i) more than an order of magnitude difference in the natural remanent magnetization intensity between the axial and flank basalts and (ii) the time required for the change in the NRM, which is about two million years.
1. Introduction The general concept of ocean floor spreading is now well accepted. However, there still remain several features of ocean floor spreading which are at present not clearly understood. One of these is the high magnetic anomaly observed on the axis of a ridge when compared with anomalies on the flank. The amplitude of the magnetic anomaly on the axis is generally twice as large as on the flank. Moreover, recent studies [ 1 - 3 ] , on rocks dredged from the Mid-Atlantic Ridge have revealed that the intensity of the natural remanent magnetization (NRM) from the axial part of the ridge is more than one order of magnitude larger than those from the flank (fig. 1). Although there seems to be general agreement that the observed high NRM in the axial basalts is due to the extremely fine grain size of the ferromagnetic minerals, no clear mechanism has been defined to explain more than one order of magnitude decrease in the NRM in flank basalts in spite of the similar total iron oxide content (FeO + Fe2 O3) both in axial and flank basalts. The intensity of NRM decreases with distance from the axis and falls to about one twentieth within 20 km from the axis. Taking a value of 1 cm/sec for spreading rate, it is seen that more than one order of magnitude change in the NRM takes place in 2 my. Hence, in addition of the twofold larger magnetic anomaly amplitude in the axis, the central high magnetic anomaly can be further specified by (i) more
0 to-e
/
009~I
~007•
distance from the axis[kin)
I
' I
Fig. 1. Natural remanent magnetization (NRM) intensity (c.g.s. per cm 3) versus distance from the ridge axis (after Irving [2] ). than one order of magnitude difference in NRM between the axial and flank basalts and (ii) the time required for the change in the NRM, which is about 2 my. Any attempt to explain the origin of the high axial magnetic anomaly must account for the above two main characteristics. Several explanations have been offered. These may be classified in the following two types. One is to assume that the geomagnetic field has been stronger in the Quaternary than in the Tertiary period [4, 5]. From the compilation of paleomagnetic intensity
2
N. Ozima, Magnetic processes in oceanic ridge
determinations on a number of volcanic rocks from various parts in the world, Kono [6] has concluded that the mean geomagnetic intensity in the later Quaternary (Brunhes normal period) is almost 50 percent larger than in the earlier Quaternary and Tertiary. It is, however, obvious that an increase this large in the geomagnetic field cannot account for more than an order of magnitude difference between the axial and flank NRMs. Another explanation is to invoke the decay of remanent magnetization with time [5]. However, a simple viscous decay of NRM cannot explain the observed magnetic change with distance or time, since the pattern of decrease of the NRM with time is too abrupt to follow either an exponential or a logarithmic function of time, which represent the general pattern of viscous decay of remanent magnetization [7]. In addition, as Irving found [2], the flank basalts have different H c spectrums as well as blocking temperatures from the axial basalts. This clearly indicates that the reduction in the NRM intensity is not due to a simple viscous decay, but requires a change in the physical and chemical conditions of the ferromagnetic minerals. Haggerty and Irving [1] suggested that the high axial remanent magnetization is due to unoxidized titanomagnetite, while the weak flank remanent magnetization is due to destruction of the original TRM during subsequent maghemitization (oxidation of titanomagnetite to titanomaghemite). Schaeffer and Schwarz [3] suggested that CRM produced during the above maghemitization process would explain the observed large difference in NRMs, as CRM is known to be generally weaker than TRM. Banerjee [8], however, opposes these suggestions ([1,3]) on the basis of reaction kinetics; the above oxidation process (maghemitization process) would take place at a rate much faster than geological time scale, if the activation energy of 0.5 eV estimated for cation migration in titanomagnetite by Creer et al. [9] is assigned to the above oxidation process. As will be discussed in a later section, the activation energy obtained by Creer et al. refers to multi-phase oxidation of titanomagnetite, but not to maghemitization. If the value of 1.2 eV obtained for maghemitization by Readman and O'Reilly [ 10] is used, the time constant of maghemitization is about a few million years, which is the same magnitude as the time required for the change in NRM in the Mid-Atlantic Ridge basalt.
In series of papres on submarine basalts, Ozima et al. [11, 12] have called attention to the fact that maghemitization is rather general in submarine basalts and they showed that except for recently erupted ones, most submarine basalts have titanomaghemite as a major ferromagnetic constituent. Their conclusion is in accordance with the model employed by Haggerty and Irving [ 1] and by Schaeffer and Schwarz [3] to explain a central high magnetic anomaly. The purpose of the present paper is to extend the suggestions by Haggerty and Irving [1] and by Schaeffer and Schwarz [3], and to show that concurrent magnetic process accompanying maghemitization can account for the above mentioned main characteristics of the high axial magnetic anomaly.
2. Magnetic model of ocean floor basalts Ozima and Ozima [12] found that most of submarine basalts dredged from seamounts and Mohole basalts show a characteristic irreversible increase of Js and Curie temperature on heating in air and in vacuum. Though the similar irreversible increase of Js and Curie temperature is seen in some of subareal basalts, in most of these cases the change is only seen when the experiment is carried out in air, but not in vacuum. From these experimental results, they concluded that the irreversible increase in magnetization in submarine basalts is due to the unmixing of titanomaghemite to form magnetite and a Ti-rich phase, whereas multi-phase oxidation of titanomagnetite is generally responsible for the irreversible change in subareal basalts. Ore microscopic observation and Xray and microprobe analyses also indicate that titanomaghemite occurs exclusively in submarine basalts [11]. It is also to be noted that titanomagnetite can oxidize in laboratory conditions to titanomaghemite at low temperature, if the grain size is small and water is present [13]. All these conditions are likely to be found in the water-rich axial zone where ridge basalt was formed [2]. All lines of evidences, then, strongly suggest that in the axial zone fresh basalts have primary homogeneous titanomagnetite, while flank basalts away from the axis have titanomaghemite, the latter being the secondary product of the primary titanomagnetite. It is, however, difficult to estimate to what depth mag-
N. Ozima, Magnetic processes in oceanic ridge
hemitization proceeds within the basaltic layer. The observation on the drilled Mohole basalts [14] shows that titanomaghemite develops from the top to the bottom (about 13 m below thw top of the flow) of the drilled core. Although the latter observation suggests that maghemitization is not limited to the surface but extends to considerable depth, titanomaghemite may not exist in deeper parts, say more than a few hundred meters, because of its unstable nature at higher temperature and under high hydrostatic pressure [15]. As will be discussed in a later section, it appears that maghemitization is limited to the upper several tens to several hundreds of meters of the basaltic flow.
3. Rate of maghemitization First we will show that maghemitization can account for the observed rate of the change of NRM, which is about a few million years. Recently Readman and O'Reilly [ 10] have estimated the activation energy for maghemitization of various compositions of titanomagnetites. Assuming a first order rate process, they have found that the activation energies are 1.5, 1.4 and 1.2 eV for titanomagnetites ((1 -x) Fe3 04 x F% TiO4 ) with x = 0, 0.2, 0.4. Submarine basalts have generally high Ti content and their x value ranges from 0.3 to 0.5. Hence, it would be reasonable to take the value of 1,2 eV for average submarine basalts. The experiments by Readman and O'Reilly [10] were made at 240°C and the major part of the change in the magnetization (or maghemitization) was observed to take place within one hour. Hence, we can take a time constant for the reaction as X = 1 hr -x at 240°C. With the aid of the expression for the reaction time constant E X=Xo.exp kT
'
it is easy to extrapolate X to room temperature and we obtain the value at room temperature ~kr. t -----
2.5 × 10-11hr -l
The inverse of the time constant gives a measure of time in which the reaction takes place, that is,
t---l~4x106yr
3
.
The time duration thus estimated is the same order of magnitude as that of the change in NRM from the axis to the flank, the latter being about 2 my (see fig. l).
4. Decrease in magnetization due to maghemitization Next, we will show that maghemitization can also account for the enormous decrease of NRM. Microscopic observation shows that titanomaghemite generally develops along grain boundaries or internal voids, forming fine irregular lamellae in the host titanomagnetite grains. Since the Curie point of titanomaghemite is well above 300°C for x = 0.4, titanomaghemite will acquire chemical remanent magnetization (CRM) at the expense of the remanent magnetization of part of the titanomagnetite phase. It is then important to show that most of the titanomaghemite phase will acquire CRM, whose direction is opposite to the original magnetization of the grain. For mathematical simplicity, we assume a model in which each titanomagnetite grain has numerous fine randomly oriented titanomaghemite lamellae. On average two thirds of such titanomaghemite lamellae have their long axis perpendicular to the direction of the remanent magnetization of the host titanomagnetite, while the remaining one third is parallel to the magnetization. Because of the large shape anisotropy, titanomaghemite lamellae perpendicular to the original magnetization can not keep remance. We therefore consider only those lameUae parallel to the original remanent magnetization. The effective magnetic field (Heft) acting on the newly developed titanomaghemite lameUae will be the algebraic sum of the geomagnetic field and the demagnetizing field due to the net magnetization of the grain, that is, Heft = Ho - (L - M ) J
(1)
in which J denotes the remanent magnetization of the grain,/4o the geomagnetic field and L, M the demagnetizing factors of the titanomagnetite grain and each titanomaghemite lamellae (p 130, in [ 15 ] ). Be-
4
N. Ozima, Magnetic processes in oceanic ridge
cause of the elongated shape of lamellae, we may neglect M in eq. (1). Assuming that each titanomagnetite grain is spherical, eq. (1) then becomes, H etf= H o - ~ n J
.
(2)
The net magnetization of the grain (J) can be expressed as an algebraic sum of magnetization of titanomagnetite (J1) and titanomaghemite phases (arE), that is, J=(1-p)J1
+ ~ pJ2
,
(3)
in which p denotes volume fraction of titanomaghemite phase. J2 is further expressed as J2 = X 'Heft,
(4)
in which × is the total susceptibility (Xreversible + Xirreversable)of titanomaghemite phase. Then combining (2), (3) and (4), we obtain s l - ( s , - 13 × Ho ) p
S =--
(5) 1 + ~9 rrXp
From experimental data on titanomagnetite ( X = 0.5) grains (pl 63, in [15] ), we take J1 = 0.5 emu/cm a . Although there is no available data for the total susceptibility of titanomaghemite, the value may not be much different from that of titanomagnetite with the same Fe/Ti, the latter being about 0.2 (pl 63, in [15] ) for Fe/Ti -- 5 (which is equal to x = 0.5). Inserting the value Ho = 0.5 oe for the geomagnetic field, we obtain j ~ 0.5 - 0.47 p 1 +0.3p
(6)
With development of titanomaghemite, or increase of p, the net magnetization will decrease as seen in eq. (6). For example, the initial magnetization will be reduced to about 40 percent when the half of the original titanomagnetite grains are oxidized to titanomaghemite. If the whole grain is oxidized to titanomaghemite, the magnetization will be about 5 percent of the initial NRM intensity. In the above calculation, the largest uncertainty is in the choice of the value for X for titanomaghemite. However, it is easy to see that even an order of magnitude change in the value of X in eq. (5) still leads to
a similar conclusion. Hence, the conclusion would be valid, if we consider the variation in the composition of titanomagnetite with x ranging from 0.3 to 0.5. The above calculation is obvioulsy too simplified, but does explain that the net remanent magnetization can reduce to one-twentieth or even less of the original TRM throughout the process of maghemitization of the original titanomagnetite grain. Recently, Marshall and Cox [16] found that the NRM of submarine basalts is not detroyed by oxidation, if the oxidation occurs below the Curie point. The experiment shows that CRM produced on the submarine basalt during the oxidation does not reduce the initial remanence. The conclusion reached by Marshall and Cox is then obviously at variance with the present "maghemitization model", the latter suggesting and effective destruction of the initial remanence by the secondary CRM. However, their experiments do not specify what reaction takes place during the oxidation and it is difficult to judge whether the CRM is carried by titanomaghemite, or by magnetite which is the more likely product of the oxidation when the experiment is carried out in air. Hence, it is not possible to apply the experimental result to test the validity of our model. On the contrary, a recent study by Johnson and Merrill [I 7] shows that CRM production results in the reduction of the initial TRM and ARM, in which the CRM was produced during oxidation of magnetite to maghemite below their Curie temperatures. The result does not contradict with the present 'maghemitization model'. However, it is obvious that more experiments are needed to test the validity of the 'maghemitization model'.
5. Discussion and conclusion The reversely magnetized CRM acquired during maghemitization can explain the two principal characteristics of the high axial magnetic anomaly, that is, (i) the time required for the change in NRM, which is about 2 my, and (ii) more than an order of magnitude difference in the NRM between the axial basalts and flank basalts. An interesting implication of the above mechanism is that remanent magnetization of oceanic basalts far away from the ridge (more than a few hundred kilo-
N. Ozima, Magnetic processes in oceanic ridge
meters) should be essentially secondary CRM, which was acquired during maghemitization. Since the rate of maghemitization is larger than the average geomagnetic polarity interval (about 0.5 my), CRM production can not catch up with the polarity change of the geomagnetic field. The conclusion reached above is then clearly inconsistent with the well established geomagnetic polarity pattern recorded on the ocean floor, if the maghemitized basaltic layer constitutes the whole ocean floor basalt. The apparent contradiction must be due to the fact that magnemitization is rather limited to the upper part of the oceanic basalt layer, in which conditions favorable for maghemitization such as the presence of water are more likely found. Magnetic anomalies observed at the surface must be primarily due to magnetization of deeper part of ocean floor basaltic layer which has not suffered maghemitization and kept the primary remanent magnetization. The latter conclusion is in accordance with the observation that in spite of more than an order of magnitude difference in NRM between the axial and flank basalts, the difference in the amplitude of the magnetic anomaly is about two fold.
Acknowledgement I am grateful to Drs. A. Cox, S. Uyeda, D. Strangway and M. Kono for reading the manuscript and giving helpful comments.
References [ 1] S. E. Haggerty and E. Irving, On the origin of the natural remanence of the Mid-Atlantic Ridge at 45°N, Trans. Am. Geophys. Union 51 (1970) 273. [2] E. Irving, The Mid-Atlantic Ridge at 45°N. XlV. Oxidation and magnetic properties of basalt: review and discussion, Can. J. Earth. Sci. 7 (1970) 1528.
5
[3] R. M. Schaeffer and E. J. Schwarz, The Mid-Atlantic Ridge near 45°N. IX. Thermomagnetics of dredged samples of igneous rocks, Can. J. Earth Sci. 7 (1970) 268. [4] C. G. A. Harrison, Formation of anomaly patterns by dyke injection, J. Geophy s. Res. 73 ( 1968) 2137. [5] C. Carmichael, The Mid-Atlantic Ridge near 45°N. VII. Magnetic properties and opaque mineralogy of dredge samples. Can. J. Earth Sci. 7 (1970) 239. [6] M. Kono, Intensity of the Earth's magnetic field in pliocene and plistocene in relation to the amplitude of Mid-Ocean Ridge magnetic anomalies, Earth Planet. Sci. Letters 11 (1971) 10. [7] R. Street and J. C. Woolley, A study of magnetic viscosity, Proc. Phys. Soc. A26 (1949) 562. [8] S. K. Banerjee, Decay of marine magnetic anomalies by ferrous ion diffusion, Nature, Physical Science 229 (1971) 181. [9] K. M. Creer, J. lbbetson and W. Drew, Activation energy of cation migration in titanomagnetite, Geophys, J. 19 (1970) 93. [10] P. W. Readman and W. O'Reilly, The synthesis and inversion of non-stoichiometric titanomagnetites, Phys. Earth Planet. Interiors 4 (1970) 121. [ 11 ] M. Ozima and E. E. Larson, Low and high temperature oxidation of titanomagnetite in relation to irreversible change in the magnetic properties of submarine basalts, J. Geophys, Res. 75 (1970) 1003. [12] M. Ozima and M. Ozima, Characteristic thermomagnetic curves in submarine basalts, J. Geophys. Res. 75 (1971) 2051. [13] N. Sakamoto and M. Ozima, Magnetic properties of synthesized titanomaghemite, J. Geophys. Res. (in press). [ 14] A. Cox and R. R. Doell, Magnetic properties of the basalts in hole EM 7, Mohole Project, J. Geophys. Res. 67 (1962) 3997. [15] T. Nagata, Rock magnetism (Maruzen Co. Ltd., Tokyo, 1961). [ 16] M. Marshall and A. Cox, Effect of oxidation on the natural remanent magnetization of titanomagnetite in suboceanic basalt, Nature 230 (1971) 28. [17] H. P. Johnson and R. T. Merrill, Magnetic and mineralogical change associated with low-temperature oxidation of magnetite, J. Geophys. Res. (in press).
MAGNETIC
PROCESSES
IN OCEANIC RIDGE
Minoru OZIMA
Geophysical Institute, University of Tokyo, Japan Received 1 July 1971 Revised version received 5 October 1971 It is shown that maghemitization (low temperature oxidation of titano-magnetite to titanomaghemite) can account for the recently discovered characteristic features of natural remanent magnetization of dredge basalts from the Mid-Atlantic Ridge, that is, (i) more than an order of magnitude difference in the natural remanent magnetization intensity between the axial and flank basalts and (ii) the time required for the change in the NRM, which is about two million years.
1. Introduction The general concept of ocean floor spreading is now well accepted. However, there still remain several features of ocean floor spreading which are at present not clearly understood. One of these is the high magnetic anomaly observed on the axis of a ridge when compared with anomalies on the flank. The amplitude of the magnetic anomaly on the axis is generally twice as large as on the flank. Moreover, recent studies [ 1 - 3 ] , on rocks dredged from the Mid-Atlantic Ridge have revealed that the intensity of the natural remanent magnetization (NRM) from the axial part of the ridge is more than one order of magnitude larger than those from the flank (fig. 1). Although there seems to be general agreement that the observed high NRM in the axial basalts is due to the extremely fine grain size of the ferromagnetic minerals, no clear mechanism has been defined to explain more than one order of magnitude decrease in the NRM in flank basalts in spite of the similar total iron oxide content (FeO + Fe2 O3) both in axial and flank basalts. The intensity of NRM decreases with distance from the axis and falls to about one twentieth within 20 km from the axis. Taking a value of 1 cm/sec for spreading rate, it is seen that more than one order of magnitude change in the NRM takes place in 2 my. Hence, in addition of the twofold larger magnetic anomaly amplitude in the axis, the central high magnetic anomaly can be further specified by (i) more
0 to-e
/
009~I
~007•
distance from the axis[kin)
I
' I
Fig. 1. Natural remanent magnetization (NRM) intensity (c.g.s. per cm 3) versus distance from the ridge axis (after Irving [2] ). than one order of magnitude difference in NRM between the axial and flank basalts and (ii) the time required for the change in the NRM, which is about 2 my. Any attempt to explain the origin of the high axial magnetic anomaly must account for the above two main characteristics. Several explanations have been offered. These may be classified in the following two types. One is to assume that the geomagnetic field has been stronger in the Quaternary than in the Tertiary period [4, 5]. From the compilation of paleomagnetic intensity
2
N. Ozima, Magnetic processes in oceanic ridge
determinations on a number of volcanic rocks from various parts in the world, Kono [6] has concluded that the mean geomagnetic intensity in the later Quaternary (Brunhes normal period) is almost 50 percent larger than in the earlier Quaternary and Tertiary. It is, however, obvious that an increase this large in the geomagnetic field cannot account for more than an order of magnitude difference between the axial and flank NRMs. Another explanation is to invoke the decay of remanent magnetization with time [5]. However, a simple viscous decay of NRM cannot explain the observed magnetic change with distance or time, since the pattern of decrease of the NRM with time is too abrupt to follow either an exponential or a logarithmic function of time, which represent the general pattern of viscous decay of remanent magnetization [7]. In addition, as Irving found [2], the flank basalts have different H c spectrums as well as blocking temperatures from the axial basalts. This clearly indicates that the reduction in the NRM intensity is not due to a simple viscous decay, but requires a change in the physical and chemical conditions of the ferromagnetic minerals. Haggerty and Irving [1] suggested that the high axial remanent magnetization is due to unoxidized titanomagnetite, while the weak flank remanent magnetization is due to destruction of the original TRM during subsequent maghemitization (oxidation of titanomagnetite to titanomaghemite). Schaeffer and Schwarz [3] suggested that CRM produced during the above maghemitization process would explain the observed large difference in NRMs, as CRM is known to be generally weaker than TRM. Banerjee [8], however, opposes these suggestions ([1,3]) on the basis of reaction kinetics; the above oxidation process (maghemitization process) would take place at a rate much faster than geological time scale, if the activation energy of 0.5 eV estimated for cation migration in titanomagnetite by Creer et al. [9] is assigned to the above oxidation process. As will be discussed in a later section, the activation energy obtained by Creer et al. refers to multi-phase oxidation of titanomagnetite, but not to maghemitization. If the value of 1.2 eV obtained for maghemitization by Readman and O'Reilly [ 10] is used, the time constant of maghemitization is about a few million years, which is the same magnitude as the time required for the change in NRM in the Mid-Atlantic Ridge basalt.
In series of papres on submarine basalts, Ozima et al. [11, 12] have called attention to the fact that maghemitization is rather general in submarine basalts and they showed that except for recently erupted ones, most submarine basalts have titanomaghemite as a major ferromagnetic constituent. Their conclusion is in accordance with the model employed by Haggerty and Irving [ 1] and by Schaeffer and Schwarz [3] to explain a central high magnetic anomaly. The purpose of the present paper is to extend the suggestions by Haggerty and Irving [1] and by Schaeffer and Schwarz [3], and to show that concurrent magnetic process accompanying maghemitization can account for the above mentioned main characteristics of the high axial magnetic anomaly.
2. Magnetic model of ocean floor basalts Ozima and Ozima [12] found that most of submarine basalts dredged from seamounts and Mohole basalts show a characteristic irreversible increase of Js and Curie temperature on heating in air and in vacuum. Though the similar irreversible increase of Js and Curie temperature is seen in some of subareal basalts, in most of these cases the change is only seen when the experiment is carried out in air, but not in vacuum. From these experimental results, they concluded that the irreversible increase in magnetization in submarine basalts is due to the unmixing of titanomaghemite to form magnetite and a Ti-rich phase, whereas multi-phase oxidation of titanomagnetite is generally responsible for the irreversible change in subareal basalts. Ore microscopic observation and Xray and microprobe analyses also indicate that titanomaghemite occurs exclusively in submarine basalts [11]. It is also to be noted that titanomagnetite can oxidize in laboratory conditions to titanomaghemite at low temperature, if the grain size is small and water is present [13]. All these conditions are likely to be found in the water-rich axial zone where ridge basalt was formed [2]. All lines of evidences, then, strongly suggest that in the axial zone fresh basalts have primary homogeneous titanomagnetite, while flank basalts away from the axis have titanomaghemite, the latter being the secondary product of the primary titanomagnetite. It is, however, difficult to estimate to what depth mag-
N. Ozima, Magnetic processes in oceanic ridge
hemitization proceeds within the basaltic layer. The observation on the drilled Mohole basalts [14] shows that titanomaghemite develops from the top to the bottom (about 13 m below thw top of the flow) of the drilled core. Although the latter observation suggests that maghemitization is not limited to the surface but extends to considerable depth, titanomaghemite may not exist in deeper parts, say more than a few hundred meters, because of its unstable nature at higher temperature and under high hydrostatic pressure [15]. As will be discussed in a later section, it appears that maghemitization is limited to the upper several tens to several hundreds of meters of the basaltic flow.
3. Rate of maghemitization First we will show that maghemitization can account for the observed rate of the change of NRM, which is about a few million years. Recently Readman and O'Reilly [ 10] have estimated the activation energy for maghemitization of various compositions of titanomagnetites. Assuming a first order rate process, they have found that the activation energies are 1.5, 1.4 and 1.2 eV for titanomagnetites ((1 -x) Fe3 04 x F% TiO4 ) with x = 0, 0.2, 0.4. Submarine basalts have generally high Ti content and their x value ranges from 0.3 to 0.5. Hence, it would be reasonable to take the value of 1,2 eV for average submarine basalts. The experiments by Readman and O'Reilly [10] were made at 240°C and the major part of the change in the magnetization (or maghemitization) was observed to take place within one hour. Hence, we can take a time constant for the reaction as X = 1 hr -x at 240°C. With the aid of the expression for the reaction time constant E X=Xo.exp kT
'
it is easy to extrapolate X to room temperature and we obtain the value at room temperature ~kr. t -----
2.5 × 10-11hr -l
The inverse of the time constant gives a measure of time in which the reaction takes place, that is,
t---l~4x106yr
3
.
The time duration thus estimated is the same order of magnitude as that of the change in NRM from the axis to the flank, the latter being about 2 my (see fig. l).
4. Decrease in magnetization due to maghemitization Next, we will show that maghemitization can also account for the enormous decrease of NRM. Microscopic observation shows that titanomaghemite generally develops along grain boundaries or internal voids, forming fine irregular lamellae in the host titanomagnetite grains. Since the Curie point of titanomaghemite is well above 300°C for x = 0.4, titanomaghemite will acquire chemical remanent magnetization (CRM) at the expense of the remanent magnetization of part of the titanomagnetite phase. It is then important to show that most of the titanomaghemite phase will acquire CRM, whose direction is opposite to the original magnetization of the grain. For mathematical simplicity, we assume a model in which each titanomagnetite grain has numerous fine randomly oriented titanomaghemite lamellae. On average two thirds of such titanomaghemite lamellae have their long axis perpendicular to the direction of the remanent magnetization of the host titanomagnetite, while the remaining one third is parallel to the magnetization. Because of the large shape anisotropy, titanomaghemite lamellae perpendicular to the original magnetization can not keep remance. We therefore consider only those lameUae parallel to the original remanent magnetization. The effective magnetic field (Heft) acting on the newly developed titanomaghemite lameUae will be the algebraic sum of the geomagnetic field and the demagnetizing field due to the net magnetization of the grain, that is, Heft = Ho - (L - M ) J
(1)
in which J denotes the remanent magnetization of the grain,/4o the geomagnetic field and L, M the demagnetizing factors of the titanomagnetite grain and each titanomaghemite lamellae (p 130, in [ 15 ] ). Be-
4
N. Ozima, Magnetic processes in oceanic ridge
cause of the elongated shape of lamellae, we may neglect M in eq. (1). Assuming that each titanomagnetite grain is spherical, eq. (1) then becomes, H etf= H o - ~ n J
.
(2)
The net magnetization of the grain (J) can be expressed as an algebraic sum of magnetization of titanomagnetite (J1) and titanomaghemite phases (arE), that is, J=(1-p)J1
+ ~ pJ2
,
(3)
in which p denotes volume fraction of titanomaghemite phase. J2 is further expressed as J2 = X 'Heft,
(4)
in which × is the total susceptibility (Xreversible + Xirreversable)of titanomaghemite phase. Then combining (2), (3) and (4), we obtain s l - ( s , - 13 × Ho ) p
S =--
(5) 1 + ~9 rrXp
From experimental data on titanomagnetite ( X = 0.5) grains (pl 63, in [15] ), we take J1 = 0.5 emu/cm a . Although there is no available data for the total susceptibility of titanomaghemite, the value may not be much different from that of titanomagnetite with the same Fe/Ti, the latter being about 0.2 (pl 63, in [15] ) for Fe/Ti -- 5 (which is equal to x = 0.5). Inserting the value Ho = 0.5 oe for the geomagnetic field, we obtain j ~ 0.5 - 0.47 p 1 +0.3p
(6)
With development of titanomaghemite, or increase of p, the net magnetization will decrease as seen in eq. (6). For example, the initial magnetization will be reduced to about 40 percent when the half of the original titanomagnetite grains are oxidized to titanomaghemite. If the whole grain is oxidized to titanomaghemite, the magnetization will be about 5 percent of the initial NRM intensity. In the above calculation, the largest uncertainty is in the choice of the value for X for titanomaghemite. However, it is easy to see that even an order of magnitude change in the value of X in eq. (5) still leads to
a similar conclusion. Hence, the conclusion would be valid, if we consider the variation in the composition of titanomagnetite with x ranging from 0.3 to 0.5. The above calculation is obvioulsy too simplified, but does explain that the net remanent magnetization can reduce to one-twentieth or even less of the original TRM throughout the process of maghemitization of the original titanomagnetite grain. Recently, Marshall and Cox [16] found that the NRM of submarine basalts is not detroyed by oxidation, if the oxidation occurs below the Curie point. The experiment shows that CRM produced on the submarine basalt during the oxidation does not reduce the initial remanence. The conclusion reached by Marshall and Cox is then obviously at variance with the present "maghemitization model", the latter suggesting and effective destruction of the initial remanence by the secondary CRM. However, their experiments do not specify what reaction takes place during the oxidation and it is difficult to judge whether the CRM is carried by titanomaghemite, or by magnetite which is the more likely product of the oxidation when the experiment is carried out in air. Hence, it is not possible to apply the experimental result to test the validity of our model. On the contrary, a recent study by Johnson and Merrill [I 7] shows that CRM production results in the reduction of the initial TRM and ARM, in which the CRM was produced during oxidation of magnetite to maghemite below their Curie temperatures. The result does not contradict with the present 'maghemitization model'. However, it is obvious that more experiments are needed to test the validity of the 'maghemitization model'.
5. Discussion and conclusion The reversely magnetized CRM acquired during maghemitization can explain the two principal characteristics of the high axial magnetic anomaly, that is, (i) the time required for the change in NRM, which is about 2 my, and (ii) more than an order of magnitude difference in the NRM between the axial basalts and flank basalts. An interesting implication of the above mechanism is that remanent magnetization of oceanic basalts far away from the ridge (more than a few hundred kilo-
N. Ozima, Magnetic processes in oceanic ridge
meters) should be essentially secondary CRM, which was acquired during maghemitization. Since the rate of maghemitization is larger than the average geomagnetic polarity interval (about 0.5 my), CRM production can not catch up with the polarity change of the geomagnetic field. The conclusion reached above is then clearly inconsistent with the well established geomagnetic polarity pattern recorded on the ocean floor, if the maghemitized basaltic layer constitutes the whole ocean floor basalt. The apparent contradiction must be due to the fact that magnemitization is rather limited to the upper part of the oceanic basalt layer, in which conditions favorable for maghemitization such as the presence of water are more likely found. Magnetic anomalies observed at the surface must be primarily due to magnetization of deeper part of ocean floor basaltic layer which has not suffered maghemitization and kept the primary remanent magnetization. The latter conclusion is in accordance with the observation that in spite of more than an order of magnitude difference in NRM between the axial and flank basalts, the difference in the amplitude of the magnetic anomaly is about two fold.
Acknowledgement I am grateful to Drs. A. Cox, S. Uyeda, D. Strangway and M. Kono for reading the manuscript and giving helpful comments.
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[3] R. M. Schaeffer and E. J. Schwarz, The Mid-Atlantic Ridge near 45°N. IX. Thermomagnetics of dredged samples of igneous rocks, Can. J. Earth Sci. 7 (1970) 268. [4] C. G. A. Harrison, Formation of anomaly patterns by dyke injection, J. Geophy s. Res. 73 ( 1968) 2137. [5] C. Carmichael, The Mid-Atlantic Ridge near 45°N. VII. Magnetic properties and opaque mineralogy of dredge samples. Can. J. Earth Sci. 7 (1970) 239. [6] M. Kono, Intensity of the Earth's magnetic field in pliocene and plistocene in relation to the amplitude of Mid-Ocean Ridge magnetic anomalies, Earth Planet. Sci. Letters 11 (1971) 10. [7] R. Street and J. C. Woolley, A study of magnetic viscosity, Proc. Phys. Soc. A26 (1949) 562. [8] S. K. Banerjee, Decay of marine magnetic anomalies by ferrous ion diffusion, Nature, Physical Science 229 (1971) 181. [9] K. M. Creer, J. lbbetson and W. Drew, Activation energy of cation migration in titanomagnetite, Geophys, J. 19 (1970) 93. [10] P. W. Readman and W. O'Reilly, The synthesis and inversion of non-stoichiometric titanomagnetites, Phys. Earth Planet. Interiors 4 (1970) 121. [ 11 ] M. Ozima and E. E. Larson, Low and high temperature oxidation of titanomagnetite in relation to irreversible change in the magnetic properties of submarine basalts, J. Geophys, Res. 75 (1970) 1003. [12] M. Ozima and M. Ozima, Characteristic thermomagnetic curves in submarine basalts, J. Geophys. Res. 75 (1971) 2051. [13] N. Sakamoto and M. Ozima, Magnetic properties of synthesized titanomaghemite, J. Geophys. Res. (in press). [ 14] A. Cox and R. R. Doell, Magnetic properties of the basalts in hole EM 7, Mohole Project, J. Geophys. Res. 67 (1962) 3997. [15] T. Nagata, Rock magnetism (Maruzen Co. Ltd., Tokyo, 1961). [ 16] M. Marshall and A. Cox, Effect of oxidation on the natural remanent magnetization of titanomagnetite in suboceanic basalt, Nature 230 (1971) 28. [17] H. P. Johnson and R. T. Merrill, Magnetic and mineralogical change associated with low-temperature oxidation of magnetite, J. Geophys. Res. (in press).