Meteorite magnetism and the early solar system magnetic field

324

Physics o f the Earth and Planetarv Interiors, 20 (1979) 324-341 © Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands

METEORITE MAGNETISM AND THE EARLY SOLAR SYSTEM MAGNETIC FIELD TAKESI NAGATA National Institute o f Polar Research, Tokyo 1 73 (Japan)

(Accepted for publication in revised form April 11, 1979)

Nagata, T., 1979. Meteorite magnetism and the early solar system magnetic field. Phys. Earth Planet. Inter., 20: 324-341. The ferromagnetism of irons, stony-irons, E-, H-, L- and LL-chondrites and achondrites is due to a metallic phase comprising mostly Fe and Ni and small amounts of Co and P. The ferromagnetic constituent in non-metamorphosed C-chondrites is magnetite, but some metamorphosed C-chondrites contain FeNi metallic grains too. Among the stony meteorites, the content of metals as determined by their saturation magnetization (Is) sharply decreases in the order E --, H --, L ~ LL ~ achondrites, whereas the 1s value for magnetite and additional metals in C-chondrites ranges from the I s value of achondrites to that of L-chondrites. With an increase of Ni-content in the metallic phase in chondrites of the order E ~ H ~ L ~ LL --, C, the relative amount of Ni-poor kamacite magnetization, Is(a), in the total I s decreases in the same order, from Is(,*)[1s ~ 1 for E-chondrites to Is(a)/1 s ~ 0 for C-chondrites. Thus, E-, H-, L-, LL- and C-chondrites and achondrites axe well separated in a diagram of Is(cOils versus I, which could be called a magnetic classification diagram for stony meteorites. As the surface skin layer of all meteorites is anomalously magnetized, it must be removed and the natural remanent magnetization (NRM) of the unaltered interior only must be examined for the paleomagnetic study. The NMR of C-chondrites is highly stable and that of achondrites is reasonably stable against AF-demagnetizarion, whereas the NMR of E-chondrites and ordinary chondrites as well as stony-iron meteorites is not very stable in most cases. Although the NRM of iron meteorites is reasonably stable, it is not attributable to the extraterrestrial magnetic field. The paleointensity for Allende C3-chondrite is estimated to be about 1.00e assuming that its NRM is of TRM origin. The paleointensity for other reasonably reliable C-chondrites (Orgueil, Mighei, Leoville and Karoonda) is also around 1 0 e . The paleointensity for two achondrites has been determined to be about 0.10e. The NRM Of other aehondrites also suggests that their paleointensity is roughly 0.10e. The NRM of ordinary chondrites is less stable than that of C-chondrites and achondrites so that the estimated paleointensity for ordinary chondrites is less reliable. The paleointensity for comparatively reliable ordinary chondrites ranges from 0.1 to 0.40e. The paleointensity values of 1 0 e for C-chondrites and 0 . 1 0 e for achondrites may represent the early solar nebula magnetic field about 4.5 × 109 years ago. A possibility that the paleomagnetic field for achondrltes was a magnetic field attributable to a dynamo within a metallic core of their parent planet may also not be rejected.

1. Introduction Magnetic properties o f meteorites have been studied specifically in relation t o the early solar system magnetic field. Early studies o f magnetic properties o f m e t e o r i t e s were therefore c o n c e r n e d with the natural r e m a n e n t magnetization ( N R M ) o f various m e t e o r i t e s in addition to the origin o f their ferromagnetic char-

acteristics (e.g., Stacey et al., 1961; Weaving, 1962; Gus'kova, 1963; Banerjee et al., 1972; Butler, 1972; Gus'kova, 1972; Brecher and Arrhenius, 1974). Since the time w h e n magnetic studies on lunar materials and the M o o n itself were extensively p r o m o t e d in c o n j u n c t i o n w i t h petrographical and chemical studies o f the same lunar materials, magnetic studies on m e t e o r i t e s have also b e e n m u c h developed in regard to

325 precise experimental techniques and sample material preparations. A meteorite-shower fall of Allende carbonaceous chondrite in 1969 gave a stimulating opportunity for synthetic scientific studies on carefully preserved samples of this chondrite, including its magnetic studies, in relation to formation and evolution of the solar system. Recent efforts to recover a large number of meteorites from Antarctica (Nagata, 1975; Cassidy et al., 1978) have already resulted in the collection of about 1,600 new samples of meteorite in Antarctica. Since these Antarctic meteorites have been treated with extreme care in regard to avoiding possible chemical and magnetic contaminations in the course of their collection, transportation, storage and subdivision, these meteoritic samples can be considered potentially usable for future paleomagnetic studies. At present we are engaged in a systematic paleomagnetic study of the Antarctic meteorites. Even the Antarctic meteorites, which are fresh in most ways, have been subjected to some natural effects such as formation of fusion crust on entry into the Earth's atmosphere and weathering effects during the long period spent in and on the Antarctic ice sheet after their fall. Since, however, the thickness of affected surface skin layer is less than 1 nun in general for stony meteorites, the unaltered interior only can be examined by experimental procedures to exclude the surface skin layer. The main scope of this summary report is to find the most plausible value of an ambient magnetic field for various kinds of meteorite when they were formed about 4.5 X 109 years ago within the early solar system, on the basis of the paleomagnetic data of these meteorite samples. However, background knowledge of the basic magnetic properties of meteorites is of prime importance for estimating a reliable paleomagnetic field from NRM characteristics. As far as possible in this report, therefore, attempts are made to understand the basic magnetic properties of meteorites on the level of our present understanding of the magnetic properties of terrestial and lunar rocks. Experimental procedures and measuring instruments used in the author's own measurements of various magnetic characteristics of meteorites described here are essentially the same as those reported on the magnetic properties of lunar rocks (e.g., Nagata et al., 1970, 1972, 1973). In addition to the author's experimental results, most available data of basic magnetic properties and particularly paleointensity data of

meteorites, obtained by other investigators, are summarized for comparison as well as general discussions.

2. General magnetic properties and a magnetic classification of meteorites The main ferromagnetic constituents in meteorites have been indentified as native irons which comprise Fe and Ni and small amounts of Co and P, except in most carbonaceous chondrites which contain magnetites or substituted magnetities (e.g., Nagata and Sugiura, 1976). Iron meteorites are composed almost entirely of native iron so that they are strongly ferromagnetic, while stony-iron meteorites are approximately 50:50 mixtures of native iron and silicates. Stony meteorites have been chemically classified into six groups. Firstly they are classified tnto two groups, i.e. chondrites which contain characteristic chondrule grains and achondrites which do not contain chondrules. Then chondrites are further classified into five chemically classified groups, i.e. E-, H-, L-, LL- and carbonaceous (C-) chondrites. In regard to chemical and mineralogical characteristics of these six groups of stony meteorites, the following three major features have been well established. (a) Urey-Craig law. Among five groups of chondrites, the total amount of Fe atoms is nearly constant. The content of metallic iron (Fe °) and Fe in FeS being noted by CFe, there is an inequality relation represented by CFe(E) > CFe(H) > CFe(L) > CFe(LL) > CF,(C)

(1)

Then, denoting the- content of FeO in silicate minerals by CFeO CFeo(E) < CFeo(H) <(CFeO(L) CFeo(LL) <

CFeO(C) (la)

(b) Achondrites contain only a very small amount of metallic iron and no magnetite so that they are magnetically very weak: CFeO(achondrite) < < CFeO(LL)

(2)

(c) Prior rule. The ratio of Ni-content to Fe-content in metallic components, noted by RNi/F¢, among four groups of chondrites (excluding C-chondrites) is characterized by RNi/Fe(E)
326 CFe value in each meteorite can be well represented by its saturation magnetization (Is). Experimental results concerning a relation between I s and the content of Fe° for a wide range of Fe° in meteoritic materials (from achondrites to iron meteorites) have led to an empirical relationship represented by 1s = (210 --- 10)CFeo emu g-1

(at 20°C)

(4)

where Cwo denotes the weight content of Fe ° (Nagata, 1978). It is certain that CFeo is a little different from Cve because Cr'e is the sum of CFeOand the content of Fe in troilite (FeS). However, the content of antiferromagnetic FeS is approximately constant in all kinds of chondrites, being several weight per cent. Therefore, (1) can be replaced by CFeo(E) > Cveo(H) > Cwo(L) > Cwo(LL) > Cwo(C) (4a) The chemical characteristics of stony meteorites, given by (4a) and (2), can be approximately represented by I s values, as in (4). Namely Is(E) > Is(H) > I s ( L ) > Is(LL) >Is(achondrites)

(5)

In Table I,_the observed values of 1s for various stony meteorites are summarized together with their magnetic coercive force (He). This table demonstrates that the inequality relationship represented by (5) holds for E-, H-, L- and LL-chondrites and achondrites. Although the content of native iron is small in C-chondrites, they contain magnetite in most cases so that their I s values range from less than 1 emu g-i to 10 emu g-t or a little more. Gus'kova and Pochtarev (1969) presented an inequality relationship given by (4a) with the magnetic susceptibilities for E-, H-, Land LL-chondrites. However, the intrinsic magnetic parameter, Is, more exactly represents CFeo than does magnetic susceptibility which, in addition, is dependent on material structures. The prior rule, given by (3), is mainly concerned with the Ni-content in a binary Fe-Ni alloy system. In such a system, the two equilibrium phases are an a-phase of Ni-poor composition and bcc crystal structure (kamaeite) and a "},-phaseof Ni-rich composition and fee crystal structure (taenite). When the Ni-content is less than about 6 wt%, only the a-phase exists. When the Ni-content becomes larger than 6 wt%, however, both the Ni-poor a-phase and Ni-rich ?-phase are present. Since the diffusion coefficient of Ni atoms is

very small particularly through the "}-phase crystal lattice, the metallic grains in natural meteorites of such a chemical con~position contain an unequilibrium mixture of very fme grains of a- and "/-phases ((a + ?)phase; plessite) in addition to the stable a-phase component. This (a + -/)-phase is generally unstable during heating. With subsequent heating above the temperature at which the transformation from the (a + -/)phase to the -/-phase occurs, this phase converts to the "}-phase after cooling below the critical transformation temperature on the laboratory time scale. For even larger amount of Ni, the metal system consists of a-, (a + -/)- and -/-phases. With a further increase in Ni-content, the metal comprises (a + ?)and -/-phases, and finally becomes -/-phase only for a large Ni-content beyond a critical value. From a set of heating and cooling thermomagnetic curves of meteorite samples, the saturation magnetization intensities of a-, (a + ,/)- and -/-phases, i.e. Is(a), Is(a + ,/) and Is(?), respectively, can be separately evaluated (Nagata, 1979a), Is(a)/Is, Is(a + ,/)/I s and Is(?)/I s being approximately determined from individual meteorite thermomagnetic curves, where Is(a) + &(a + -/) + Is('/) = Is

for E-, H-, L- and LL-chondrites and aehondrite. In Table I results of the thermomagnetic analyses of E-, H-, L- and LL-chondrites and achondrites to separate Is(a), Is(a + ,/) and Is(,/) from one another are summarized. With an increase in Ni-content in the metallic phase in the order E -~ H -~ L -~ LL, the relative content of the Ni-poor a-phase decreases while that of the Ni-rich (a + 7)-phase or -/-phase increases. We may thus conclude that

which is regarded as a magnetic representation of the Prior rule expressed by (3). The total content of Ni in the metallic phase in chondrites, estimated from the magnetically evaluated values of the Ni-eontent in each phase, is in approximate agreement with the Ni-content in the metallic phase determined by chemical analysis, as illustrated in Fig. 1 where a plot of zero in both magnetically and chemically estimated values of Ni-content represents a typical carbonaceous chondrite. Assuming then that the magnetic data of Is(a ), Is(a + 77) and Is(77) can

327 TABLE I Magnetic parameters of stony meteorites Meteorite

E-chondrite Yamato-(a) H-chondrite Seminole Kessen Yonozu Yamato-74371 Yamato-74647 Mt. Boulder-2 Mr. Brown Yamato-(d) L -chondrite Yamato-74191 Dalgety Down Bjarb61e Barratta Yamato-(m) Fukutomi Homestead Yamato-74362 Allan Hill number 9 Mino LL-chondrite Yamato-74442 Yamato-74646 St. Severin C-chondrite Orgueil Ivuna Mokoia Yamato-74662 Yamato-(c) Allende Leoville Karoonda Achondrite Yamato-(b) Yamato-74013 Yamato-74037 Yamato-74097 Yamato-74648 Yamato-75032 Yamato-7307 Yamato-74159 Yamato-74450

Classification

Is (emu g-l)

Is(~)/I s (%)

E4, s

48.0

97

0

3

0

12

5.6

(1)

H4 H4 H4, s Hs Hs H6 H6 H

24.3 34.4 24.2 33.5 27.9 27.4 40.0 32.3

94 95 87 95 94 88 90 94

6 5 13 5 6 10 5 . 6

0 0 0 0 0 2 5 0

0 0 0 0 0 0 0 0

18 8 42 10 14 10 23

8.2 6.5 10.2 7.7 7.1 9.5 9.9 6.8

(1) (1) (1) (1) (1) (1) (2) (1)

L4 L4 L4 L4 Ls L$ Ls L6 L6 L

6.8 9.7 13.0 12.0 16.6 22.9 10.0 8.1 8.4 11.0

79 85 85 80 90 82 80 81 65 80

21 14 10 15 0 18 15 19 35 20

0 0 0 3 10 0 5 0 0 0

0 0 0 0 0 0 0 0 0 0

30 117 4 20 38 160 3

11.6 10.5 10.8 11.6 10.2 10.2 12.3 11.4 15.7 12.1

(1) (1) (2) (2) (1) (1) (2) (1) (0) (1)

6.0 3.2 4.7

45 19 45

35 7 55

20 74 0

0 0 0

85 20 500

22.6 31.5 20.5

(0) (0) (1)

11.9 11.2 8.0 0.81 10.8 0.61 10.3 7.8

0 0 0 5 0 0 6 0

0 0 0 0 0 0 0 0

0 0 0 0 0 95 0 0

100 100 100 95 100 5 94 100

107 149 157 143 34 155

(2) (0) (1) (1) (1) (1)

81 66 100 100 100 100 100 100 100

0 0 0 0 0 0 0 0 0

19 34 0 0 0 0 0 0 0

0 0 0 0 0 0 0 0 0

42 10 13 85 93 13 265 58

(1) (1) (0) (0) (0) (0) (1) (0) (0)"

LL4 LLs,6 LL6 CI C1 C2 C2 Ca Ca C3 C4 Diogenite Diogenite Diogenite Diogenite Diogenite Diogenite Howardite Eucrite Euerite

0.19 0.17 0.32 0.32 0.20 0.042 0.53 0.061 0.22

/s(tX+ 7)/1 s (%)

ls(.r)[1s (%)

ls(Mt)/I s (%)

Hc (Oe)

Ni 0 Fe ° + Ni0 (wt%)

Ref. *

(3,4)

(3,4)

* (0) This paper. (1) Nagata and Sugiura (1976). (2) Stacey et al. (1961). (3) Larson et al. (1974). (4) Breeher and Arrhenius (1974).

328

wt% 30

~u 2o

§ lo

// 0

10

/.

/.

20 30 Ni CONTENT (CHEMICAL)

40 wt°l*

Fig. 1. Comparison of the Ni-contentin the metallic phase determined magneticallywith that chemicallyanalysed.

approximately represent the Ni distribution in the metallic phase, the magnetically estimated value of Ni in the FeNi metallic phase, Ni°/(Fe ° + Ni°), for E-, H-, L- and LL-chondrites are given in Table I. These values of Ni°/(Fe ° + Ni °) for various chondrites may more directly represent the Prior rule. In the case of C-chondrites, 1s is mostly due to the saturation magnetization of magnetite, Is(Mt), and consequently Is(a) + Is( rv + 3') + Is(3') + Is(Mt) = Is

where Is(Mt) > Is(Or) + Is(or + 3') + Is(3,)

for most C-chondrites. It has been shown, for example, that Orgueil (CO and Mighei (Ca) chondrites contain only magnetite as the ferromagnetic constituent, and Ornans (C2) contains both magnetite and FeNi metallic phase (Anders, 1964). Yamato-(c) Cs-chondrite contains magnetite as the major opaque mineral and a very small amount of metals in addition (Okada, 1975). However, the major ferromagnetic constituent in Allende Ca-chondrite was identified as taenite of composition 31.4% Fe, 67% Ni, 1.6% Co (Clark et al., 1970), and this result has been confirmed by Banerjee and Hargraves (1972) and Butler (1972). On the other hand, Larson et al. (1974), Watson et al. (1975) and Herndon et al. (1976) carried out thermomagnetic

analyses of 5 C l-chondrites, 18 C2-chondrites, 13 Cschondrites and 2 C4-chondrites. Their results indicated that 5 C3-chondrites and a C4-chondrite among the samples examined contain a considerable amount of FeNi metals of the t~-phase in addition to magnetite, though the ferromagnetic constituent is magnetite only in all C 1-chondrites. It thus seems that the ferromagnetic constituents in carbonaceous chondrites are sensitively dependent on their degree of metamorphism. Since 3,-phase FeNi metals of about 65 wt% Ni have their Curie point at about 600°C and their thermomagnetic curves are reversible with temperature, it is rather difficult to distinguish such a 3,-phase FeNi alloy from magnetite magnetically. One possible way is to check a characteristic phase transition of magnetite magnetically between an orthorhombic structure below and a cubic structure above a critical temperature, which is -153°C for pure magnetite (Nagata, 1965). This kind of lowtemperature test was carried out for AUende by Wasilewski (private communication), confirming that Allende does not contain a detectable amount of magnetite. Summarizing these experimental data, it may be provisionally concluded that the ferromagnetic constituents are mainly magnetite in C~ and C2 chondrites, whereas there is a small amount of FeNi metal of either the ct- or 3'-phase in addition to magnetite in some C-chondrites (Ca and C4). It seems that the metallic composition of achondrites has a considerably wide dispersion, depending on the chemical and mineralogical compositions, as shown in Table I. However, the total content of metallic component is always very small in any achondrite, as represented by the I s values of achondrites. In summary, the combination of a modified UreyCraig law represented by (1') and (3) and a modified Prior rule represented by (6) may reasonably well classify all stony meteorites into the chemically classified six groups, though I s values of some C-chondrites overlap with those of achondrites while I s values of some other C-chondrites overlap with those of L- and LL-chondrites, and ls(OO[ls values of achondrites have a wide range to cover those of E-chondrites and ordinary chondrites. Figure 2 iUustrates a magnetic classification diagram on the basis o f I s values on the abscissa and Is(OO/Is values on the ordinate for all stony meteorites given in Table I. In this diagram consisting of

329 % 100

o

°o

~

' '

2o

3o

5ATklRATION MAGNETIZATION

40

t.matgm

50

(Is)

Fig. 2. ls(a)[1 s versus 1 s d i a g r a m t o classify s t o n y m e t e o r i t e s magnetically.

two intrinsic magnetic parameters, I s and Is(et), E-, H-, L-, LL- and C-chondrites and achondrites are well separated, and chemical interpretations of the abscissa and ordinate values of individual stony meteorites are reasonably well defined. In Table I, the magnetic coercive force (He) of individual stony meteorites also are summarized as far as observed data are available. Other structure-sensitive magnetic parameters such as the initial magnetic susceptibility, the saturation remanent magnetization and the remanence coercive force have been measured for the majority of the listed stony meteorites. The magnetic coercive force, however, will be the best representative parameter of structure-sensitive magnetic properties. It may be pointed out for Table I that He values of C-chondrites are considerably larger in general than those of the other stony meteorites, though some stony meteorites belonging to L- and LL-chondrites and achondrites have H e values larger than 100 Oe. A large value of H e, which is largely due to the singledomain structure and/or the shape anisotropy of ferromagnetic grains, is an important necessary condition for keeping the remanent magnetization stable.

3. Natural remanent magnetization (NRM) of meteorites Iron, stony-iron and stony meteorites in general all possess natural remanent magnetization (NRM). The

intensity and direction of the NRM of carbonaceous chondrites are generally the most stable against AFdemagnetization, and those of achondrites are reasonably stable; the NRM of many enstatite chondrites and ordinary chondrites (H-, L- and LL-chondrites) are, however, unstable. Some ordinary chondrites have a reasonably stable NRM. Carbonaceous chondrites, which still contain a fair abundance of volatile elements, are believed to have been formed at the very beginning of the primordial solar system and thermally metamorphosed only very little since their formation time. Although the formation mechanism for carbonaceous chondrites has not yet been clarified in detail, it is most likely that these chondrites formed as planetesimals (less than 10 km in linear scale) or parts of planetesimals and have never been parts of primordial planets (larger than 102 km in linear scale). Therefore, the origin of the observed stable NRM of C-chondrites may not be attributable to a planetary dynamo magnetic field. The C-chondrite NRM may represent solar nebula magnetic fields at the very active protosun stage, or a magnetic field within a nebula surrounding the protosun (e.g., Alfv6n and Arrhenius, 1976). The observed reasonably stable NRM of achondrites may also be of interest from another viewpoint. It is almost certain that achondrites are the products of intense metamorphism of primordial planets, and they may represent a silicate layer after melting and differentiation of each primordial planet into a metal or iron-stony core and silicate layers, If so, the achondrite NRM could represent dynamo magnetic fields of primordial planets. Reasonably stable NRM of iron meteorites, however, may have to be interpreted very carefully. Brecher and Albright (1977)have experimentally demonstrated, for instance, that the direction of the TRM of octahedrites was subject to the direction of spontaneous magnetization of kamacite-phase domains whose t~ (110) plates nucleated and grew on the octahedral ~, (111) planes of the taenite phase. Consequently, even the cooling procedure of an octahedrite in a nonmagnetic space can result in stable remanent magnetization of intensity equivalent to that of the TRM acquired in a magnetic field of several Oe. These investigators, therefore, have concluded that no reliable information regarding the presence, strength and source of ancient solar system magnetic fields can be

330 retrieved from iron meteorites. On the other hand, Gorshkov (private communication) has studied the NRMs of 385 individual pieces of Sikhoto-Alin iron meteorite shower (Autumn, 1947). He has shown that the average intensity of the NRM (In) decreases with an increase in the average radius (R) of individual pieces, suggesting that the major source of observed NRM is mainly due to the NRM of an uppermost surface layer of about 1 cm in thickness. This result may suggest that the NRM of iron meteorites is largely due to the TRM of their surface acquired in the geomagnetic field on entry into the Earth's atmosphere. It can be suggested from these two experimental results that the NRM of iron meteorites is not reliable as an indicator of an ambient magnetic field at the time when they were formed in the extraterrestrial space. The stability of the NRM of other chondrites and stony-iron meteorites against AF-demagnetization is largely different for individual samples (Larson et al., 1973; Nagata, 1979b). In general, the NRMs of E-, H-, L- and LL-chondrites and stony-iron meteorites are much less stable compared with those of C-chondrites and achondrites. It has been experimentally demonstrated, however, that some selected ordinary chondrites (i.e. H-, L- and LL-chondrites) have considerably more stable NRM against AF-demagnetization. Since these chondrites were considerably metamorphosed during their formation processes, it seems that the identification of the NRM acquisition mechanism for these chondrites is more complicated than that for C-chondrites and achondrites. A problem regarding the NRM of meteorites is the fusion-crust magnetization which is attributable to TRM and/or CRM in the skin layer of meteorites on entry into the Earth's atmosphere. An anomalously high intensity of the NRM of the fusion-crust layer of stony meteorites in comparison with the NRM of their interior was first found by Weaving (1962). Butler (1972) also examined the NRM of the fusion crust of Allende C-chondrite in comparison with a comparatively uniform NRM of its interior part. In this case, the intensity of the fusion crust is 1.4 X 10 -3 emu g-1 while that of the interior is 3.0 X 10 -4 emu g-l, where the thickness of the strongly magnetized layer is about 1 mm or less. Nagata and Sugiura (1977) have examined the fusion-crust NRM of a howardite in comparison with its interior NRM. In their result, the NRM of a fusion crust of about 0.5 mm thickness is represented

YAMATO - 74450

x 10"Semulsample

6 >. i--

~4 z

== z

2 ]1

FUSION CRUST 0

0.5

Am 1.0

d

1.5mm

Fig. 3. Example of changes of NRM direction (top) and intensity (bottom) caused by continuously scraping off the surface skin of an achondrite. Am = Weight of scraped-off surface skin, d = thickness of scraped-off surface skin. Numerals in units of g in the direction diagram represent Am values.

by I n = 7.6 X 10 -s emu g-I in average intensity, while the interior NRM intensity is 6.3 X 10 -6 emu g-l. Similar studies on the comparison of fusion-crust NRM with interior NRM has been made for two other chondrites and four achondrites in the present work. Figure 3 illustrates an example of a change of NRM intensity and direction caused by gradually scraping off the surface skin of a stony meteorite from the surface. Figure 4 shows three examples of the distribution of fusion-crust NRM intensity with depth thus experimentally obtained. As shown in these results, the surface skin layer of a stony meteorite with the fusion crust is strongly magnetized in comparison with the interior NRM, but the thickness of the anomalously magnetized layer is always less than 1 mm and the interior NRM is fairly uniform. In the case of iron

331 NRM OF FUSION CRUST OF ACHONDRITE I 61 x lO'4ernulgrn

Y M - 75032 DIOGENITE

/

NRM must be completely removed and the uniform interior NRM only must be examined in discussing the paleomagnetic field in extraterrestrial space on the basis of the NRM of meteorites.

4 4. NRM of carbonaceous

In general, carbonaceous chondrites possess a highly stable NRM which, in view of their high magnetic coercive force, should be maintained for a long time. For these reasons, the paleomagnetism of carbonaceous chondrites may be of particular interest in connection with the magnetic field in the early solar system.

o i-

YM- 74/,50 tO I-. Z

chondrites

EUCRITE

Z

o

4.1. Allende (Ca)

-0

0-5 DEPTH FROM SURFACE

1.0 turn

Fig. 4. Examples of anomalous surface-skin magnetization as a function of depth from the surface for achondrites. meteorites, Gorshkov (private communication) has shown that the strongly magnetized skin layer is about 20 mm thick. Since the fusion-crust NRM is certainly due to heating of the surface-skin layer on entry of the meteorite into the Earth's atmosphere, this part of the

The NRM of Allende (C3) has been studied by several investigators. Table II summarizes Is, Hc, Curie point (Hcpt) , the intensity of the initial NRM (In(0)) and the ratio of the remaining NRM after AF-demagnetizion up to a 500.Oe peak (In(500)) to In(0) of this C3-chondrite, measured independently by four research groups. Observed values Of/s, He and Hcpt for different pieces of this meteorite obtained independently by different research groups are in reasonable agreement with one another. Herndon et al. (1976) also estimated I s = 0.66 emu g-l, and Wasilewski (private communication) has obtained H e = 170 Oe for this meteorite. The intensity of the NRM of four different pieces

TABLE II Magnetic properties and NRM of Allende * Parameter

This paper

Is Hc

In(0) /n(5OO)/In(O)

Hcpt

Unit

Reference

0.61 143 2.7 × 10-4 0.83

~576 ~620

* Pb - Pb age = 4.533 ± 0.004 X 109 years.

Banerjee and Hargraves (1972) -

(1.9-2.5) × 10-4 0.87

620

Butler (1972)

-

3.4 × 10-4 --

610

Brecher and Arrhenius (1974) 0.48-0.73 138 + 3 (2.2-3.4) X 10-4 ~0.63 _

emu g-1 Oe emu g-I °C

332 ALLENOE x 10"~ emu/gm 30 .0

50

® 170

®170

°,70

® L~IO*C

o 200°C

Z

( F.= 1.080e.)

( F . = 1.31 Oe.I o

ds

,:o

o 2(30"C

,s

o's

(F,,=0.93Oe.)

ds

lb

TRM ( H = 0.59 Oe. ) ( after BUTLER, 1972 )

x, O~tm'~Igm

¢o

1.5

Fig. 5. KiSnigisberger-Thellierdiagram to derive the paleointensity for Allende (after Butler, 1972).

of this meteorite ranges from 2 X 10 -4 to 3.4 X 10 -4 emu g-a, which suggests that the NRM intensity is fairly uniform. As indicated by the three observed values ofln(5OO)/In(O)in Table II, the NRM of Allende C3-chondrite is extremely stable against AF-demagnetization. Buffer (1972) has also shown that 60% o f the NRM remains after AF-demagnetization up to 400 Oe. The paleointensity studies on this meteorite have been made with several different methods by different research groups. The paleointensity of AUende was examined with the K6nigisberger-Thellier method independently by Banerjee and Hargraves (1972) and Buffer (1972) assuming that the acquisition process of NRM is thermoremanence. In the study of Banerjee

and Hargraves, a linear relationship between NRMlost and TRM-gained holds fora temperature range lower than 130°C, inconsistent data being obtained on heating above 130°C. In Buffer's work, the upper limit temperature (To) for the linear relationship is 150°C, as shown in Fig. 5. The paleointensity (Fo) for Allende thus estimated for a temperature range below To is given in Table III, where agreement ofF0 and To between the two independent measurements is satisfactory. However, the paleointensity value represents a magnetic field, in which this chondrite was cooled down from 130°C to 150°C, acquiring TRM during the cooling process, whereas NRM corresponding to this cooling temperature range is only about 1/3 of the total NRM in Butler's experiment (see Fig. 5) and less than 1/3 of the total NRM in the experiment of Banerjee and Hargraves. In the experimental process, the mineralogical structure of the ferromagnetic constituents in this C-chondfite is changed on heating above 130-150°C so that the acquisition mechanism for the major parts of the NRM cannot be identified by the K6nigisberger-Thellier method. An alternative method for determining the paleointensity in such a case is the so-called NRM-ARM method (e.g. Stephenson and Collinson, 1974). The results of application of this technique to Allende by Nagata, together with Sugiura, as illustrated in Fig. 6, shows that a linear relationship between NRM-lost by the AF-demagnetization and ARM-gained holds for an alternating magnetic field larger than 250-Oe peak (up to 800-Oe peak in this experiment). The range of linear relationship covers the majority of the total NRM (i.e. 7n(0) = 2.4 X 10-4 emu g-a in this case).

TABLE Ill Paleointensity of Allende carbonaceouschondrite Paleofield

Method

Max. appl. temp.

intensity (Oe)

1.09 ± 0.08 1.11 ± 0 . 1 4 0.73 1.00 ± 0 . 1 1

(K.-T.)-TRM I (K.-T.)-TRM NRM-ARM

0.95

pTRM-NRM

0.25

(S.-M.)-ARM

K6nigisberger-Thelliermethod. 2 Shashkanov-Metallovamethod. z

Ref.

( T o ) (°C)

2

130 150 _

150

Banerjee and Hargraves(1972) Butler ( 1 9 7 2 ) This paper Gus'kova (personal communication, 1978) Brecher (personal communication, 1978)

333 3 "X lU t, m u l ~ m

ALLENDE 2

!

!, 2

4

6

ARM - gained ( h f l . B O e l

xlO'%mu/~

Fig. 6. NRM-lost versus ARM-gained diagram to derive the paleointensity for Allende by the ARM-NMR method.

paleointensity is ambiguous if no unique linear relationshiF holds between NRM-lost and the pTRM lost by the AF-demagnetization. Assuming however that the pTRM acquired in a low temperature range below 150°C represents the NRM parts of magnetically lower coercive domains less than 500 Oe in microcoercivity, Fo = 0.95 Oe might be accepted as the more reasonable value of paleointensity in this case. Summing up these experimental results on NRM of Allende in Table III, we may conclude that the very stable NRM of this C3-chondrite was acquired in a magnetic field of about 1 0 e by the TRM mechanism or a similar acquisition mechanism of remanence such as CRM, because the observed high stability of NRM against the AF-demagnetization is a characteristic of TRM (including pTRM) or CRM only.

4.2. Orgueil f C1) To evaluate the paleointensity for the NRM of TRM origin by the NRM-ARM method, a transformation factor ( f ) defined by

I(TRM)/I(ARM)= f h ( T R M ) / h ( A R M ) plays an essential role, where h(TRM) and h(ARM) are the magnetic fields applied for the acquisitions of TRM and ARM, respectively. Stephenson and Collison (1974) obtained f = 1.34 from experimental data on metallic grain assemblages and a lunar sample. A similar experiment to determine f for three synthesized samples containing iron grains of different sizes has also been carried out in the present study. The mean value of f thus experimentally obtained is 1.3. The paleointensity for Allende on the TRM-origin assumption is then estimated to be 0.73 Oe, as given in Table III. Gus'kova (private communication) has recently examined the NN_NRMof Allende by the ShashkanovMetallova method (Shashkanov and Metallova, 1977), which is essentially based on ARM characteristics in comparison with the NRM microcoercivity spectrum. Her result, F o = 1.00 + 0.11 Oe, is given in Table III. Brecher (1977) also estimated the paleointensity for Allende with the aid of the so-called pTRM-NRM method. In comparison with the AF-demagnetization curve of pTRM acquired by cooling from 150°C with that of NRM, she has obtained Fo = 0.95 Oe for H < 500 Oe, and Fo = 0.25 Oe for H > 5 0 0 Oe, where H denotes AF-.demagnetizing field intensity. In the case of the pTRM-NRM method, the determination of

The NRM of this Cl-chondrite is less stable than that of Allende against AF-demagnetization, but still its In(5OO)/In(O) value amounts to about 1/10 (Banerjee and Hargraves, 1972) and the H c value is 107 Oe, as shown in Table 1. By the K6nigisberger-Thellier method, the paleointensity for this Cl-chondrite has been estimated to be Fo = 0.67 Oe (Banerjee and Hargraves, 1972) for temperatures below 70°C. Namely, a linear relationship between NRM-lost and TRMgained holds only for temperatures below 70°C, inconsistent data being obtained on heating above 70°C. It is considered that the paleointensity for Orgueil is less reliable than that of AUende. However, Gus'kova (private communication) has recently examined the paleointensity for three pieces of Orgueil by the Shashkanov-Metallova method, and s h e has obtained F0 = 1.23-1.50 Oe, the average value being Fo = 1.36 + 0.09 Oe. Therefore, the paleointensity for Orguefl can also be estimated to be about 1 Oe.

4. 3. Mighei ( C2) The NRM of this C2-chondrite is a little less stable than that of Orgueil against AF-demagnetization, as indicated by the In(3OO)/In(O) value of about 0.1. However, the large coercive force observed, He = 141 Oe (Brecher and Arrhenius, 1974), suggests that the NRM of this C2-chondrite is still worthy to be examined in detail. With the aid of the Shashkanov-Metallova

334

diagram holds for temperatures below 300°C, and the paleointensity for this C3-chondrite is estimated to be Fo = 0.97 Oe from the linear relationship.

LEOVILLE (C4)

i.o 7~4°

\

•x ,Ii

a." 7

Hp = 0.97 Oe

4.5. Karoonda (C4)

2~a.

-- 0-!

er

" ' , e - ................

o

o's

pTRM (H

~ 4 701C

...............................

):o

):5

= 0.50e)

z'.o x IO-3emu

Fig. 7. K6nigisberger-Thellier diagram to derive the paleointensity for Leoville.

method, Gus'kova (private communication) has recently examined six pieces of Mighei. The paleointensity of this meteorite thus evaluate d ranges from 0.87 to 1.78 Oe, the average value being Fo = 1.24 + 0.36 Oe.

4.4. Leoville (Ca) The NRM of Leoville C3-chondrite is even less stable against AF-demagnetization as indicated by In(200)/ In(0) = 0.1 a n d H c = 34 Oe. As shown in Fig. 7, however, the thermal demagnetization experiment on this Ca- chondrite has shown that its NRM smoothly decreases with increasing temperature, with a large gradient up to 300°C and then a smaller gradient above 300°C suggesting that the majority of the NRM of Leoville can be attributed to TRM acquired in a low temperature range. As shown in the figure, a linear relationship between NRM-lost and TRM-gained in a K6nigisberger-TheUier

The Hc value of Karoonda C4-chondrite is 155 Oe, but the stability of its NRM against AF-demagnetization is smaller than that of Leoville, as indicated by 1n(200)/1n(O) = 0.08. Sugiura and Nagata have recently examined its pTRM characteristics in some detail, and have found from results of the pTRM-NRM experiment that the NRM of this C4-chondrite is atrributabl¢ to TRM acquired by cooling from 300°C with 0.89 Of in paleointensity.

4.6. Summary of NRMs of Orgueil, Mighei, Leoville and Karoonda The paleointensity and magnetic coercive force (Hc) of the four carbonaceous chondrites are summarized in Table IV, where the maximum applicable temperature (To) for the paleointensity determination also is given for the case where a heating procedure is involved in the experiment. Although the stability of the NRM of these four C-chondrites against AF-demagnetization is considerably less that that of Allende, their NRM is still reasonably stable as indicated by ln(2OO)[In(O) = 0.26, 0.171 0.10, and 0.08, respectively, for Orgueil, Mighei, Leoville and Karoonda. As summarized in Table IV, the paleointensity for these C-chondrites is around 1 0 e . Taking into consideration the fact that the paleointensity values for Allende determined for its different pieces by various investigators with different experimental methods

TABLE IV Paleointensity of four carbonaceous chondrites

Paleofield intensity (Oe)

Method

Mighei (C2) Leoville(C3)

0.67 1.36 ± 0.09 1.24 ± 0.39 0.97

Karoonda (C4)

0.89

(K.-T.)-TRM (S.-M.)-ARM (S.-M)-ARM (K.-T.)-TRM pTRM-NRM

Sample Orgueil (C1) "

Max. appl. temp.

He

(To) (°C)

(Oe)

70 300 300

107 141 34 155

Ref. Banerjee and Hargraves (1972) Gus'kova (personal communication, 1~78) Gus'kova (personal communication, 1978) Nagata and Sugiura (this paper) Nagata and Sugiura (this paper)

335 TABLE V Paleointensity of other carbonaceous chondrites Sample

Paleofield intensity (Oe)

Ivuna (C 1)

0.5 0.7 ((0.33-1.35) 0.18

Murray ( C 2 ) Murchison (C2) Renazzo (C2) Mokoia (C2) Ornans (Ca) Yamato-C (Ca) Yamato-74662 (C3)

0.4

~3

2.3 ~0 1.03e 0.06 0.64 0.93

Method

Max. appl. temp. (To) (°C)

Hc (Oe)

Ref. Brecher (1979)

pTRM-NRM

250

(K.-T-.)-TRM

90

350-760

Banerjee and Hargraves(1972) Brecher (1979)

pTRM-NRM

250

pTRM-NRM (K.-T.)-TRM

250

178

(S.-M.)-ARM NRM-ARM

-

(K.-T.)-TRM

100

157 143

range from 0.7 to 1.10e, as shown in Table Iii, it can be provisionally concluded that the most plausible value of paleointensity for these C-chondrites is also about 1 0 e .

Brecher (1979)

Brecher (1979) Stacey et al. (1961) Gus'kova (personal communication, 1978 Nagata (this paper) Nagata (this paper)

magnetic constituents in this Ca-chondrite are complicated, consisting of both magnetites and FeNi metals. Hence the paleointensity data of the three C-chondrites are also simply listed without comment.

4. 7. Other carbonaceous chondrites 5. N R M o f a c h o n d r i t e s

The paleointensity values of several other carbonaceous chondrites have been reported by Stacey et al. (1961), Banerjee and Hargraves (1972) and Brecher (1977). In addition, provisional data of the paleointensity of two C3-chondrites have been obtained in the author's laboratory and Gus'kova (private communication) has recently estimated the paleointensity of Ornans C3-chondrite. These paleointensity data for C-chondrites are summarized in Table V. As the investigators themselves did not put much reliance on Murchison (Banerjee and Hargraves, 1972) and Makoia (Stacey et al., 1961), and no description of experimental procedures to justify the reliability o f the data obtained is available for Ivuna, Murray, Murchison or Renazzo, the Final data of paleointensity for these C-chondrites are simply summarized in Table V without comment. In regard to the paleointensity study of Yamato-(c) and Yamato-74662 C a-chondrites, there remains a little ambiguity on the reproducibility of experimental data. The paleointensity data for Ornans Ca-chondrite may be reasonably reliable, but as already described in section 2, the ferro-

The content of ferromagnetic native iron is generally small in achondrites so that their NRM intensity is also small in general. Since achondrites are believed to represent the silicate layer surrounding a metallic or stony-metallic core in primordial planets, the paleointensity study of achondrites will be of special interest in association with a possible dynamo field of the primordial planet.

5.1. Yamato- 74013 (diogenite) This diogenite was retrieved from the East Antarctic ice-sheet surface with extreme care to avoid chemical and magnetic contaminations. Minerological studies of this diogenite in detail by Takeda et al. (1978) have shown that the opaque minerals in this achondrite are troilite, Co-rich Ni-poor metallic iron and chromite. The metallic phase comprises 98% Fe, 0.3% Ni and 0.7% Co, while the chromite phase consists of 60% Cr2Os, 24% FeO, 6% MgO and 7% A1203. A thermomagnetic analysis of this diogenite has indicated that

336 3 , X 104emu/gm

I

l

YAMATO - 74013

1

37p/~50

~o~

The NRM of this howardite is In(0 ) = 6.3 × 10 -6 emu g-S, and it is reasonably stable against AF-demagnetization with respect to the direction and intensity, as indicated by ln(200)/In(O ) = 0.57 (Nagata, 1979b). In the KSnigisberger-Thellier experiment on this sample, the paleointensity is estimated as Fo = 0.07 Oe and To = 240°C.

300~

•

5.3. Other achondrites 0

1

2

ARM-gained

[ h = 1.80e. )

x 10-Semu/gm

Fig. 8. NRMqost versus ARM-gained diagram to derive the paleointensity for Yamato-740t 3 diogenite by the ARMNRM method.

the Co-rich Ni-poor metallic phase is identified as a major ferromagnetic phase of Curie Point 7920C, whereas a minor ferromagnetic phase having a Curie point at 570°C is unidentified. Observed values o f I s and He for this sample are given in Table I. The NRM of this diogenite ranges from 2.7 X 10 -6 to 3.4 X 10 -6 emu g-1 in intensity, and it is very stable against AF-demagnetization with respect to both intensity and direction, as represented by In(200)/ In(0) = 0.87 and ln(4OO)]In(O) = 0.33. The paleointensity of Yamato-74013 diogenite has been determined by the standard NRM-ARM method, the result being Fo = 0,093 Oe, as shown in Fig. 8.

5.2. Yamato- 7307 (howardite) This howardite was also retrieved from Antarctica with extreme care to minimize various possible contaminations. Observed values of 1s and H e for this sample is given in Table I. Chemical and mineralogical compositions of Ya_mato-7307 howardite have been analyzed in fair detail by Yagi et al. (1978) and Miyamote et al. (1978), and the basic magnetic properties have been studied by Nagata et al. (1976). Although some metallic grains in this achondrite are taenite of 54 wt% Ni, most metallic phases are Ni-poor kamacites as indicated by the bulk chemical composition of the metallic phase given by 0.39 wt% Fe, 0.0115 wt% Ni and 0.007 wt% Co.

The NRMs of several other achondrites have been measured. In addition to Yamato-74013 diogenite and Yamato-7307 howardite, the NRMs of six diogenites, two eucrites and one howardite retrieved from Antarctica have been measured and paleointensity studies are underway at present. The NRMs of one aubrites, four diogenites, three howardites and five eucrites have also been examined by Brecher et al. (1979). Although paleointensity estimates for these achondrites have not yet been obtained, a comparison of stable NRM intensity with the saturation remanent magnetization, I R (which is frequently called saturated IRM), can indicate the order of magnitude of the paleointensity, as demonstrated by Fuller (1974) for lunar materials. In Table VI, observed values of In(0), In(100), 1R and H e are summarized for four diogenites and two eucrites together with those values for Yamato-74013 diogenite and Yamato-7307 howardite whose paleointensities have been estimated. As suggested by comparison of In(100) with In(0), the NRMs Of these eight achondrites are reasonably stable towards AF-demagnetization; most of the NRM of these achondrites is held by ferromagnetic grains of microcoercivity larger than 100 Oe. Assuming then the ratio ln(O)[I R is approximately proportional to the paleointensity and calibrating the linear relation coefficient from the data of Yamato-74013 and -7307, the paleointensity of the four diogenites and the two eucrites can be roughly estimated, as given in parentheses in Table VI. If the original FuUer's diagram, which was calibrated with lunar sample data, is adopted for estimating the paleointensity from observed values of In(0) and IR, the paleointensity of these achondrites is estimated to be about two-thirds of the values listed. However, the paleointensity of achondrites thus estimated probably indicates only the order of magnitude.

337 TABLE VI Paleointensity and comparison o f I n with I R for achondrites Sample

In(0) (emu g-I )

ln(100) (emu g-I )

IR (emu g-l)

In(O)[IR

He (Oe)

Paleointensity (Oe)

Yamato-74013 (diogenite) Yamato-7307 (howardite)

3.4 × 10 4

3.2 X 10 "~

1.2 X 10-3

2.8 X 10-a

10

6.3 X 10 4

5.9 X 10 --6

2.7 X 10-3

2.3 X 10-3

13

Yamato-(b) (diogenite) Yamato-74097 (diogertite) Yamato-74648 (diogenite) Yamato-75032 (diogenite) Yamato-74159 (eucrite) Yamato-74450 (eucrite)

15.4 X 10 -~

23.5 X 104.6

3.5 X 10-3

4.4 X 10 -a

42

(0.15)

4.0 X 10 -~

3.2 X 10 -6

4.0 X 10-a

1.0 X 10-a

13

(0.033)

7.5 X 10 -a

4.9 X 10 -a

85

(0.16)

' 36.5 X 10 -6

32.5 X 104

0.093 (NRM -ARM) 0.07 ((K.-T.)-TRM)

4.2 X 10 4

3.8 X 10 "~

6.5 X 10 -3

0.65 X 10 -a

93

(0.022)

22.6 X 10 -~

23.2 X 10 -~

4.0 X 10 -a

5.7 X 10-a

265

(0.19)

1.3 X 10 -~

0.83 X 10 -~

4.4 X 10 -4

0.30 X 10 -3

58

(0.010)

5.4. Paleointensity o f achondrites As far as the experimental data summarized in Table VI are concerned, the paleointensity for achondrites may be provisionally represented by Fo = 0.1 Oe in order o f magnitude.

6. NRM o f ordinary chondrites As mentioned in section 3, a number o f observed NRMs o f E-chondrites and ordinary chondrites (i.e. H-, L- and LL-chondrites) are not very stable towards AF-demagnetization, In(100)/In(0) becoming smaller than 0.1. However, some ordinary chondrites still have a very stable NRM.

6.1. Comparatively stable NRMs of ordinary chondrites Yamato-74191 L4-chondrite possesses a stable NRM as indicated by ln(lOO)/ln(O) = 0.77 and In(300)] In(0) = 0.52. Sugiura (private communication) has examined the NRM of this L4-chondrite to evaluate the paleointensity by means o f the standard N R M -

ARM method, obtaining Fo = 0.13 Oe. Weaving (1962) examined a stable NRM o f Brewster L6-chondrite, which is represented by In(100)/In(0) = 0.93 and ln(300)/ln(O ) = 0.40. By comparing TRM with NRM, paleointensity for this L6-chondrite is estimated to be about 0 . 1 0 e . NRMs o f F u k u t o m i Ls-chondrite (In(lOO)[In(O) = 1.03) and Yonozu 1-14.s-chondrite (In(100)/In(0) = 1.00) are also reasonably stable. By the KiSnigisbergerThellier method, Nagata and Sugiura (1976) estimated the paleointensity as F0 = 0.10 Oe with To = 400°C for F u k u t o m i a n d F o = 0.18 Oe with To = 470°C for Yonozu. These estimated values o f paleointensity for ordinary chondrites are summarized in Table VII.

6.2. Other chondrites In addition to the four ordinary chondrites possessing a reasonably stable NRM, the paleointensity values o f a number of ordinary chondrites and an E-chondrite have been reported. An estimate o f the reliability o f these paleointensity values is neither simple nor easy, because some chondrites examined have rather unstable NRM as indicated by In(100)/In(0 ) < 0.1 and no experimental guarantee for the thermal stability of

338 TABLE VII Paleointensity for E-chondrites and ordinary chondrites Meteorite

Classification

Paleointensity (Oe)

Remarks

Ref. *

Yamato-74191 Fukutomi

L4 Ls

0.13 0.10

(7) (3)

Brewster Yonozu

L6 H4, S

0.1 0.18

Farmington Mt. Brown

Ls H6

0.18 0.25

NRM-ARM (K.-T.)-TRM (To = 400°C) NRM-TRM (K.-T.)-TRM (TO = 470°C) NRM-TRM NRM-TRM

Dalgety Down

L4

0.68

(3)

Seminole

H4

0.39

Rakovka Mordvinovka Okhask Pultusk

L6 L H4 H4

0.4 0.4 0.3 0.21

(K.-T.)-TRM (To = 470°C) (K.-T.)-TRM (To = 350°c) NRM-TRM NRM-TRM NRM-TRM NRM-TRM

(4) (4) (4) (4)

Mezo Madaras Flenovka Parvomayski Poselok Tarbagartay Orlovka

L3 L L L H

0.13 0.19 0.12 0.27 0.25

NRM-TRM NRM-TRM NRM-TRM NRM-TRM NRM-TRM

(5) (5) (5) (5) (5)

Jelica Vavilovka Bald Mountain Andover Aumale Rose City Abee

LL6 LL6 L4 L6 L6 H6 E4

0.01 0.08 0.01 0.01 0.02 0.01 0.33

NRM-TRM NRM-TRM NRM-TRM NRM-TRM NRM-TRM NRM-TRM NRM-TRM

(6) (6) (6) (6) (6) (6) (6)

(2) (3) (1) (1)

(3)

* (1) Stacey et al. (1961). (2) Weaving (1962). (3) Nagata and Sugiura (1976). (4) Gus'kova (1963). (5) Gus'kova and Pochtarev (1969). (6) Brecher and Ranganayaki (1975). (7) Sugiura (private communication).

ferromagnetic constituents has been given for the other chonddtes whose paleointensity has been evaluated by comparison of NRM with TRM in various proposed ways. Therefore the paleointensities for these chondrites are simply reported from the respective investigator's reports in Table VII without comment. As seen in this table, the paleointensity for L- and H-chondrites is most dominant in a magnetic field range o f 0 . 1 - 0 . 4 Oe, except for the paleointensity data given by Brecher and Ranganayaki 0 9 7 5 ) . We may provisionally conclude therefore that the most plausible paleointensity for ordinary chondrites is around 0 . 2 0 e and ranges from 0.1 to 0 . 4 0 e . However, it has not yet been clarified how these

ordinary chondrites could acquire NRM in the early solar system space, because, although considerably metamorphosed, they show no evidence for being part of parent primordial planets.

7. Some discussions on the origin of meteoritic paleomagnetic field In this summarizing report, stony meteorites are classified into three groups from the viewpoints of stability of their NRM and their metamorphism history. These are: (a) carbonaceous chonddtes, which are considered in general to be the most primordial

339 meteorite possessing a stable NRM; (b) achondrites, which are considered to be the most differentiated stony meteorites with a reasonably stable NRM; and (c) an intermediate group consisting of E-, H-, L- and LL-chondrites which are well metamorphosed but not strongly differentiated and have weakly stable NRM in most cases. A genetic relationship among the principal groups of stony meteorites suggests that C l-chondrites represent first products formed by accretion of cosmic dusts as planetesimals in the primordial solar nebula, while ordinary chondrites are intermediate products formed by thermalmetamorphism, and achondrites are final products formed by differentiation within meteorite parent planets (Ringwood, 1961). It has however been shown that all kinds of meteorite ranging from the most primordial carboneous chondrites to the most differentiated achondrites were formed during a period of less than 2 X 107 years about 4.5 X 109 years ago (Podosek, 1970). Paleointensities of about 1 . 0 0 e for carbonaceous chondrites, about 0 . 1 0 e for achondrites, and intermediate values for other chondrites hence may represent ambient magnetic fields at places where and times when the respective meteorites were formed in the solar nebula about 4.5 X 109 years ago. The most plausible source of the magnetic field for achondrites is the magnetic field of a self-exciting dynamo within the metallic core of a primordial planet, in the surface part of which the achondrites were finally formed. If a model of a primordial planet consisting of a eucrite crust, a diogenite mantle and a pallasite core, such as proposed by Mason (1967), is accepted, however, it may be difficult to presume the presence of an effective dynamo within the pallasite core. Since the protosun of a T-Tauri phase which may have continued during the.period of meteorite formation is believed to be very active in emitting solar wind and in producing convective mbtion in the interior to form an intense magnetic field hydrodynamically (Eger and Cameron, 1965), the solar magnetic field at this stage might be extended mostly along the solar ecliptic plane by the solar wind. If the solar magnetic field is of a dipole-like configuration, the primordial planetesimals of planets making a Kepler motion around the protosun would be continuously subjected to a magnetic field approximately perpendicular to the ecliptic plane. This solar wind magnetic field can also

be the paleomagnetic field for achondrites. Such a possibility for the presence of a magnetic field in the early solar system has already been suggested by several investigators (Sonett et al., 1970; Sonett, 1971; Brecher, 1973; Alfv6n and Arrhenius, 1976). An estimate of the magnitude of the primordial solar wind magnetic field has indicated that it was about 0 . 5 0 e at 1 A.U. and 0.05 Oe at 3 A.U. from the protosun (e.g., Brecher, 1973). If achondrites were finally formed in primordial planets at a distance of around 2 A.U. from the protosun, the paleointensity is estimated to be about 0 . 1 0 e . It seems that an interpretation of the observed paleointensity for carbonaceous chondrites is somewhat difficult. As carbonaceous chondrites still contain a considerable amount of volatile substance, these carbonaceous chondrites may never have been heated to a high enough temperature to lose volatile substances since the time of their formation. The temperature of the primordial solar nebula after reaching its equilibrium state has been estimated as 225 K at 1 A.U., 130 K at 2.8 A.U. and 97 K at 5.2 A.U. from the protosun (Kusaka et al., 1970). Before reaching the equilibrium temperature, however, the solar nebula experienced much higher temperatures, such as 1500-2000 K at 1 A.U. and 600-1000 K at 5 A.U. If carbonaceous chondrites were formed by the accretion of cosmic dusts in the thermally equilibrated solar nebula, their temperature may have reached only several hundred °C in their parent planetesimal, owing mostly to the radioactive heat source. Then, an acquisition of TRM in a lower-temperature range, probably lower than 300°C as suggested by the observed To value of Leoville and Karoonda, in the presence of an ambient magnetic field, may be considered probable. If we assume further that carbonaceous chondrites were formed at a distance less than 1 A.U. from the protosun, the primordial solar wind magnetic field there could be about 1 0 e in intensity. It is generally believed however that carbonaceous chondrites were formed at 3.5 A.U. or more in distance from the protosun (Clayton et al., 1976, 1977). If so, the primordial solar wind magnetic field at this position would be much less than 0 . 1 0 e even at the most active T-Tauri phase. Recently, Srnka and Bibhas (1978)have pointed out that the magnetic dipole moment attributable to the spin of electrically charged, elongated, non-

340

magnetic intersteller grains o f size 0 . 0 1 - 1 . 0 / ~ n in kinetic equilibrium with their surroundings can exceed 10 -2 emu crn -3 and the corresponding internal magnetic field can approach 1 0 e for grains of size 0.01 pro. The average magnetic field caused by a large number o f aligned grains of this kind in the primordial solar nebula may not be sufficiently large to produce the paleointensity for carbonaceous chondrite or achondrites, because the density o f cosmic dusts is less than l 0 -7 gm cm -3 even on the ecliptic plane (Kusaka et al., 1970). It is possible, however, that magnetically aligned small grains statistically coagulate tO form a small planetesimal in which a stable NRM is established by the DRM mechanism (e.g., Nagata, 1961). As for the paleointensity for ordinary chondrites, nothing certain can be proposed, because the possible processes for theri formation and magnetization are open to so many hypothetical possibilities.

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