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Paleomagnetic systematics of ordinary chondrites
Earth and Planetary Science Letters, 25 (1975) 57-67 © North-Holland Publishing Company, Amsterdam - Printed in The Netherlands
L_Zd
PALEOMAGNETIC SYSTEMATICS OF O R D I N A R Y CHONDRITES* AVIVA BRECHER and RAMBABU P. R A N G A N A Y A K I Department of Earth and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Mass. (U.S.A.)
Original manuscript received August 27, 1974 To authors for revision September 24, 1974 Revised version received October 31, 1974
A pilot systematic study of the remanent magnetic behavior of ordinary chondrites reveals several correlations with their chemical-petrological classification. The intensity of natural remanent magnetization (NRM) decreases in the sequence E ~ H ~ L ~ LL-chondrites. Its relative stability and degree of directional coherence under progressive AF demagnetization increase in the same order. These trends parallel a decrease in average metal content (from ~20% in E to 2% in LL) and a corresponding Ni enrichment of metal grains (from ~6% Ni in E to ~35% in LL). Within each chemical group, the stability of NRM is inversely related to petrologic subtype. Shocked meteorites display a distinctive demagnetization curve, an unusually large soft component of NRM, and pronounced directional scatter of NRM components. These characteristics can be reproduced reversibly by brief laboratory heating (to ~800°C) of unshocked chondrites. Comparative studies of laboratory (saturation isothermal, IRMs, and thermal, TRM) remanence acquisition and behavior, suggest that other magnetic parameters, such as the ratios NRM/TRM and IRMs/NRM, may be useful for classification. Patterns of magnetic behavior are explained in terms of metallography and thermal history. In contrast to the carbonaceous chondrites, very few ordinary chondrites preserve a useful paleomagnetic record. The LL-chondrites hold the best promise in this regard. Paleofield estimates obtained for eight representative chondrites range from 0.01 to 0.3 Oe. 1. Introduction The discovery that some chondritic meteorites possess a stable extra-terrestrial natural remanence (NRM) [ 1 - 3 ] aroused immediate interest in its nature and origin [ 4 - 6 ] . An extensive survey o f natural remanent magnetization (NRM) intensity and bulk magnetic susceptibility in a large number o f meteorites [7,8] indicated that their values cluster for chemical groups (H, L, LL) as well as intragroup (for H, L) by petrologic subtypes. Various attempts were made to infer or model in the laboratory the NRM acquisition and to estimate the paleofields required to imprint it [3,4,6,9].The NRM was generally interpreted as a thermal (or thermo-chemical) remanence, acquired during cooling (with simultaneous compositional changes) o f the N i - F e metal grains, in parent-body magnetic fields o f 0 . 1 - 0 . 9 0 e [3,4,10]. Recent investigations [ 1 1 ] have established that a * Contribution No. 20 of the M.I.T. Laboratory for Paleomagnetism.
magnetic record is preserved intact in most carbona. ceous chondrites, requiring primordial fields o f ~<10e. Thus far, research on meteoritic paleomagnetism has provided few conclusive answers, but many tantalizing leads, regarding the physical conditions prevailing during and after formation o f small bodies in the early solar system [11,12]. The present study attempts to: (1) search for systematic variations of remanent properties with chemical class and metamorphic subtypes; (2) identify the stable component o f NRM and infer its mode o f acquisition; and (3) correlate the magnetic properties o f meteorites with their composition and thermal histories. A magnetic characterization of chondrites, compared with the existing classification schemes [ 13 ] and integrated with other evidence [14] may: (1) further constrain models o f origin; (2) resolve the current controversy regarding the nature o f " m e t a m o r p h i s m " and attendant chemical fractionation and re-equilibration processes; and (3) confirm cogenetic groups and relative place o f origin in their parent body.
58 2. Experimental procedures Magnetic moments were measured on a Schonstedt SSM- 1A spinner magnetometer (sensitivity 10 -7 emu). NRM values (Table 1) are in the range 10 -1 to 10 -5 emu and are accurate to 1%. The meteorite samples weighed from 0.3 to 4 g. The NRM was cleaned by conventional alternating field (AF) demagnetization. A Permali AF demagnetizer with a three-axis tumbler was used, caiaable of providing 2,000-Oe peak fields. Earth's field was nulled, to <100 3' at the sample, by 3 pairs of Helmoholtz coils. Samples from each chemical group were selected for detailed laboratory remanence studies and for paleointensity determination. A single heating variant was chosen for estimating the paleofield intensity [17] to prevent chemical alteration and metal-phase diffusional re-equilibration. Samples were heated to ~<800°C in about 2 hours, held there for ~30 minutes, and allowed to cool to room temperature in a 0.17-Oe field. The heating level selected was ~<800°C, not only because this exceeds the Curie temperature of the kamacite, the principal ferromagnetic mineral in chondrites [3,5,6,9, 12], but also because it corresponds to peak "metamorphic" temperatures estimated for chondrites [13b, 14]. Heating and cooling were performed in flowing argon to remove any reactive gases evolved, and minimize chemical changes. The TRM acquired in 0.17 Oe was then progressively AF demagnetized, and its stability and degree of directional coherence relative to NRM were assessed. To monitor the effect of heating on the magnetic grains, the saturation remanence (IRMs) imprinted in 12-kOe fields and its demagnetization behavior were measured before and after heating. The method employed here for estimating paleointensities can resolve between thermal-like and nonthermal NRM components [ 17], as was illustrated for carbonaceous chondrites [11 a, e]. Thermal (TRM), thermochemical (TCRM), or chemical (CRM) remanence cannot be distinguished from AF demagnetization curves alone. The fractional loss of NRM is compared to that of TRM for each demagnetization step. If a linear plot which extrapolates to the origin results, the assumption that the NRM is entirely a TRM, or a primary TCRM [29], is validated and the slope yields the paleofield strength. When several linear segments result, several thermal-like components of NRM are
AVIVA BRECHERAND RAMBABUP. RANGANAYAKI indicated, necessitating either different field-strengths or different efficiencies of TRM acquisition by discrete size-ranges of magnetic grains for a given field strength. For linear plots with nonzero intercepts, a secondary chemical or thermochemical remanence is usually invoked [17,1 la,f, 12], but the paleointensity may still be estimated. Nonlinear plots preclude paleofield determinations. 3. Experimental results 3.1. Intensity o f NRM The intensity and stability of NRM for 22 ordinary chondrites are indicated in Table 1. The specific intensity ranges for each chemical group are: 3.9 X 10 - 3 2.8 X 10 -1 for E;9.1 × 10-4-2.3 XlO-2 for H; 3.5 X 10-4-6.5 X 10 -3 for L; and 7.9 X 10 - 5 1.1 × 10 -3 for LL. 3.2. NRM stability Samples were AF demagnetized stepwise in peak fields of 15, 30, 60, 120, 200, 300, and 500 Oe. The resulting NRM demagnetization curves (Figs. 1-4) represent the microcoercivity spectrum of the magnetic grains. All the meteorites possess at least 2 NRM components, the larger of which is usually soft [5,6,9]. This unstable component is dearly removed by cleaning in HAFt<60 Oe and resides in large multidomain grains. The relative magnitude of the soft component varies systematically with chemical class (Table 1). The demasnetization behavior of E-chondrites studied is shown in Fig. la: their NRM is very soft. The NRM of H-chondrites (Fig. 2) is also soft, only slightly harder on the average than in E-chondrites. The NRM!s of L-chondrites (Fig. 3), though still soft, are considerably harder than those of E- and H-chondrites. The demagnetization behavior of the LL-chondrites is shown in Fig. 4. As a class, they have the largest stable residual of NRM. Within each group, the NRM of lower metamorphic type chondrites is more resistant to AF demagnetization (Figs. 1-4). 3.3. NRM directional behavior The NRM demagnetization spectra alone do not indicate whether NRM is directionaUy stable. Only
59 TABLE 1 Characteristics of natural remanent magnetization Meteorite
Type
NRM (emu/g) X 10+3
AF range for directionaUy stable NRM component*
Fraction of NRM lost 60 Oe
200 Oe
intermediate to high field none none
0.84 0.98 0.97
0.99 0.98 0.99
intermediate field none none none high field
0.97 0.98 0.92 0.99 0.97
0.97 0.97 O.90 0.98 0.99
Abee Hvittis Khairpur
E4 E6 E6
276.0 10.6 3.9
Ochansk Tysnes Island Quenggouk Lancon Rose City
H4 H4 H4 H6 H6
7.2 0.9 1.5 22,9 19.5
Bald Mountain Bjurbole Tadjera Andover Aumale Bushof Cabezo de Mayo Utrecht Zavid
L4 L4 L5 L6 L6 L6 L6 L6 L6
6.5 0.4 1.5 4.7 4.5 0.8 0.7 0.9 2.2
entire field intermediate to high field none intermediate to high field intermediate to high field entire field none intermediate field low field (up to 120 Oe)
0.94 0.49 0.99 0.35 0.98 0.94 0.82 0.78 0.97
0.99 0.37 0.98 0.98 0.98 0.98 0.93 0.76 0.98
Sokobanja Jelica St. Severin Vavilovka
LL4 LL6 LL6 LL6
0.1 1.1 0.5 0.2
intermediate field intermediate field high field intermediate to high field
0.17 0.52 0.43 0.32
0.07 0.88 0.60 0.41
* Low field: up to 60 Oe cleaning field; a soft component is usually cleaned in 15 Oe. Intermediate field: 60-200 Oe cleaning field. High field: 200-500 Oe cleaning field.
samples with b o t h a stable direction and intensity of magnetization are considered useful for paleointensity determination. To ascertain the presence o f a directionally stable NRM component, we projected the change o f the NRM vector under progressive cleaning onto two orthogonal sets o f axes [15]. Constancy o f direction, independent o f intensity loss, denotes a stable, possibly primordial, remanence [ 11 a, e]. In such diagrams (Figs. 5 - 8 ) the number o f NRM components present and their relative stability are evident. The range o f directional coherence encountered is illustrated by the well-behaved L4 Bald Mtn. in Fig. 5b at one extreme and b y the shocked L5 Tadjera (Fig. 6c) at the other. Similar diagrams indicate that only Abee (E4) among E-chondrites examined possesses an NRM component with a stable direction, in the intermediate to high field microcoercivity range ( 1 2 0 - 5 0 0 Oe), representing 10% o f the total moment. Superimposed, is a large soft component.
Among the heavily shocked H-chondrites [ 19], Rose City (H6), has one directionally stable component in the high field ( 2 0 0 - 5 0 0 Oe) range, which comprises only ~1% o f the initial moment. Ochansk (H4) has at least one component directionally stable in the intermediate ( 6 0 - 2 0 0 Oe) field microcoercivity range. No other H-chondrites have directionally stable remanence. Of the L-chondrites, Aumale (L6) (Fig. 7c), Andover (L6), and Bjurb61e (L4), each have a stable component in the intermediate to high field region. Bald Mtn. (L4) is directionally the stablest (Fig. 5b). For these meteorites, the stable NRM fraction is sizable: 2 0 - 5 0 % for L4's (Figs. 2, 3). All the LL-chondrites have unidirectional components o f NRM: Sokobanja (LL4) and Jelica (LL6) o f intermediate (up to 200 Oe), St. Severin and Vavilovka (LL6) (Fig. 8) o f high coercivity.
60 NRM= E
CHONDRITES
Ca)
IOo
3.4. Acquisition and relative stability of laboratory remanence
~, ABEE ( E 4 ) c~ KHAIRPUR ( E 6 ) 0 HVITTIS (E6)
Eight chondrites were selected for modeling studies of NRM (Table 2). All except Tadjera possessed at least one stable component o f NRM, useful for paleo-
i0-1
(a)
NRM = L4 AND L5 CHONDRITES
~ =0"~,
I.C D.S
BJURROLE (L4) 0 BALD MOUNTAIN (I,41 O TADJERA (L5)
0.8
~ 0.7 I0 -3
IE 0.s
(b)
r\
"~
~
o
0.9 O.S 0.7
500
~0.5
~0.4
15 30 60
120 200 H , ~ (oe) •
0.3
o
. 15 30 60
, 120
300
16'
~
200 300 HAF (De) - ~
'6° 15 3O ~go
500
' 120 HAF(Oe)
IO'~
~ O 0 •
It=,,
-1 tO - ( b )
~[ ~'A 0
.
ZI O 0 • •
BUSHOF (L6) ZAVID (L6) CABEZO DE MAYO (L6) UTRECHT (L6) ANOOVER (L6)
TYSNES ISLAND (H4) OCHANSK (N4) LANCON (H6) ROSE CITY IN6)
• OUeNG~OUR (.) "
3
~
O.sl
,o
O.S
~=
xZO. 7 z
i
O
-
4
'
'
"_o.6
~
~o'II I I is so so
o.s 0.4
0
(C)
~t'b (:~ I~m-- m
~O. 7
~
IO'ZI~
(O)
O.S
=E0.8
~"
NRM , L6 CHONDRITES
Fig. 1. The absolute (a) and normalized (b) AF demagnetization behavior of E-chondrites. Lower metamorphic type Abee (E4) has a harder NRM than Hvittis (E6) and Khairpur (E6). NRM: H CHONDRITES
' ZOO
L
m
I ~zo HAF
(De)
I zoo
300
6
(d) 0.4
'6511 I / 15 50 60
0.3
I 120 HA~ ~oe)
I 200
0,2
I 300
0,1
0,2
15 30 60
0.1 15 30 60
120
200 HAF (Oe)
300
500
Fig. 2. The AF demagnetization behavior of H-chondrites, on an absolute scale (a) and normalized to the initial NRM (b).
120
2OO HAF (De)
300 •
i
SOD
Fig. 3. The absolute (a) and normalized (b) AF demagnetiza= tion behavior of L4 and L5-chondrites; (c,d) NRM demagnetization curves for L6 chondrites. Their NRM is, on the average, significantly harder than that of both E and H-chondrites.
61
RM,
LL CHONDRITES
BALD MOUNTAIN
(O)
(L4)
(O)
1.0 0.9
SOKOBANJA (LL4) ST. S E V E R I N ( L L 6 )
0.~
5 ~
0,8 0.6
0.7
0.4
0.6
I~0-5
0.2
ZO.4
,5
0.3
30 6 o
,~o
2~o
3~o
5 0~"0
HAF (Oil) 0.2 3
0.1
I
t
~5 30 6o
I
I
I
I
~2o
2oo
300
5oo
(b)
2
HAF (011}
(c)
I O.~
i0-2 Y
t
* 3
(b)
t
%
=EO.(
-I
q
-2 0.2
-3 I
3
o
o
It IO'~l ( e m u / g )
Z I0"
I I 15 30 6 0
I 120 HAFIOe)
I 200
I 300
Fig. 4. NRM demagnetization curves (a, b) of LL-chondrites: these have the hardest NRM among E, H, L and LL groups. Sokobanja (LL4) has the stablest NRM within the LL-group.
1
t
I0 "3
i0 -5
~ (OI
I
~
~
~
{ $onl
02
I I [15} Oo)
I O}
o.,
0.6
TRM
(60)
0.8
Fig. 5. (a) Normalized comparative demagnetization spectra of NRM, thermal (TRM), and saturation isothermal remanence before (IRM0) and after (IRMs1)` heating. The fact that IRM °'1 curves are identical, denotes the lack of alteration in heating. (b) The changes of the NRM vector in progressive AF cleaning, projected on two orthogonal planes. Good directional coherence is seen for the entire NRM. (c) The fractional NRM vs. TRM losses in AF cleaning yield 3 linear segments. Only the slope of the high stability (He > 60 Oe) component is used to derive the paleofield intensity: NRM/TRM ~ Ho/Hlab, where Hla b ~ 0.17 Oe leads to H o ~ 0.01 Oe. The lower coercivity (Hc < 60 Oe) segment yields H o ~ 0.2 oe.
TABLE 2 Natural and laboratory remanence behavior and paleofield estimates for selected chondrites
Meteorite
Type
Specific moment (emu/g)
NRM
TRM
IRMs°
H0 (Oe)
Stability to AF demagnetization
NRM NRM NRM NRM NRM NRM NRM NRM
IRM]
Abee
E4
2.8 X 10-1
2.5 X 10-3
2.3 X 10-1
1.3 X 10-1
0.33
Rose City
H6
1.9 X 10-2
3.0 X 10-3
4.7 X 10-2
5.9 X 10-2 3.0 X 10-2 3.5 X 10-2
0.01
1.5 X 10--2 6.1 X 10-2 2.9 X 10-2 1.7 X 10-2
0.01 0.02
Bald Mountain
L4
6.5 X 10-3
4.4 X 10-4
4.5 X 10-2
Tadjera Andover
L5 L6
1.5 X 10-3 4.7 × 10-3
5.0 X 10-4 4.7 × 10-4
2.7 X 10-2 2.4 × 10-1
Aumale Jelica Vavilovka
L6 LL6 LL6
4.5 X 10-3 1.1 X 10-a 2.2 X 10-4
4.9 X 10-4 3.4 X 10-4 2.2 X 10-4
1.0 X 10-1 1.2 X 10-1 2.1 X 10-1
,.o
0.01 ?
0.01 0.08
< < < < < < < <
TRM < IRMs0 < TRM < IRMs° < TRM < IRMs0 • TRM < IRM ° < TRM < IRMs0 < TRM < IRM~ < IRMs0 < TRM ~ TRM < IRMs1 <
IRM~ IRM 1 IRM 1 IRM~ IRM~ IRMs0 IRMs1 IRMs°
(L51
TADJERA 1.0 0.8
(0)
VAVILOVKA
,~ NRM
I.O
o IRM o 0 TRM I • IRMI
0.8
0.2 I 5 30 60
120
200 300 HAF(oe )
500
I'\ 0.6-
(b)
O,z
(oI ii
o.e
/I \\
/ /
=E 0.6
I
0,4 / ( , / 0.6 TRM
(b)
1.0
'~
I 1.0
,L I -0.4 -0.2
I I () 0.2 0.4 x 10"4 ( emu/g)
I 0.6
AUMALE( L 6 )
(a)
1.01 0.8 0.6 0.4 0,2 200 300 HAF(Oe) 0.4
(b)
0.2
o
?~} (3)~
1°)-0.2
,
0.4
1120) e(I5) -0.6 J(~ 0.2
0.4 0.6 TRM
0.8
1.0
-0.6
'
I
~.~/ / / ~/
I
"i
s
I
3//..s3
;#/'
0.4 -
'~ (c)
0.~
I
3
/ 0.2
7
Tadjera; (a) NRM and TRM spectra are manifestly different, and IRMs°'j shows that some new higher coereivity magnetic material has been generated in heating; (b) indicates that no linear segment can be recovered from the NRM vs. TRM plot; and (c) the complete NRM directional scatter denotes the absence of a paleomagneticaUy useful component.
0.2
T(o~
4
113°1
~^4 / .
My-Mz I
(5OOl t
-O.E
120
(c)
O.S
'~
~
Fig. 6. Same diagrams as in Fig. 5, for the shocked meteorite
15 30 60
500
~-,~, 0.6 1.0 1.4 x I O'4(emulg)
0.2 0.
0.4
06 TRM
08
1.0
-0.'
I
0.8
l
I 300
xO, E
~ . ~
3I c
I 200 HAF 1.0
iso~
•ii \~, ,,.
i (ZOQ ~):~o~ (,2o, ~ ) l 0.2 0,4
I 120
co)
1-~
O. "~ 2 My-Mz~2 4
/ I
I I 15 30.60
\
/
~ 0.~
IRM~
0.4
0.2
I£
IRM 0 0 TRM I
0.6
0.4
0,2
NRM
c:
0.6
LO
(0.)
(LL6)
I
-0.4 -0.2 0.2 x 10-3(emu/Q)
Fig. 7. TRM is much stabler than NRM (a), although IRMs spectra show a thermally-induced softening of the microcoercivity spectrum. Aumale shows good directional coherence, denoting a stable NRM, for H c > 60 oe (e). The comparison of NRM with TRM (b) yields H o ~ 0.02 Oe.
Fig. 8. The same set of diagrams as Figs. 5 - 7 for Vavilovka, show (a) similarly hard demagnetization spectra for NRM, TRM, and IRMs, and only minor thermally produced changes; (b) directions of NRM stabilize after cleaning in HAF > 6 0 O e ; (c) the linear fit to this stable NRM vs. TRM fraction yields a value of H o ~ 0 . 8 0 e . The nonzero intercept indicates the presence of a nonthermal remanence.
intensity determinations. Tadjera was included to see whether it would yield fictitious values for paisefield strength. Table 2 lists the values of NRM; TRM acquired by cooling from ~'800°C in a laboratory field of 0.17 Oe; and saturation remanence before (IRMs0) and after (IRM 1) heating. Relative stabilities are also schematically indicated. Sets of demagnetization curves are shown only for Bald Mtn., Tadjera, Aumale, and Vavilovka in Figs. 5 - 8 . The apparent dissimilarity in the demagnetization curves of NRM and TRM is due mostly to the relatively large soft component of NRM, whose nature is controversial [3-6,9,12]. However, our results support the conclusion of previous investigators that the stable NRM in chondrites is thermal (TRM) or thermochemical (TCRM). Comparing IRMs0,1 moments and stabilities, most samples, with the exception of Bald Mtn. (Fig. 5); appear altered to some degree by heating to 800°C, due to either chemical or metallurgical phase changes (section 4), or changes in the sizes of magnetic grains. Nonetheless, the TRM can be used to estimate paisefield strength, since the fractional loss of TRM in AF cleaning varies linearly with that of NRM (section 3.5). Note that the remanence ratios IRMs/NRM and NRM/ TRM show systematic inter- and intra-group trends (Table 3). IRMs/NRM is an index of the degree of re-
63 TABLE 3 Correlative trends between the remanent magnetic properties of ordinary chondrites and their chemistry and metallography Chondlite class: Average | metal / content composition
Magnetic remanent properties
E
~
Fe°/Fetot 0.8 Fe-Ni (wt.%) ~27 Niin metal (%) ~ 5
H
~
0.63 17 8
3.5. PaleofieM intensity estimates Paleointensity determinations for the chondrites in Table 2 are shown graphically in Fig. 5 - 8 . Only the
~
0.33 6 12
NRM-intensity, 4.10003-3.1001 1.10-3-2.100 2 (emu/g) NRM-stable < 5 < 5 fraction (%) IRMs-intensity >1001 ~ 5.10-2 (emu/g) IRMs-relative (see Table 2, Figs. 5-8) stability IRMs/NRM ~ 1 ~ 1 NRM/TRM (0.15 Oe)~102 ~10
manence saturation [ 18 ]. It is typically ~ 1 0 - 1 0 0 for multidomain (MD) TRM-carrying terrestrial rocks, ~ 2 0 0 - 5 0 0 for carbonaceous chondrites [1 la, e], and was used as an empirical means for estimating lunar paleofield intensities [20]. Among the chondrites of Table 2, IRMs/NRM varies from >~1 in E and H, to 1 0 - 5 0 in L, and 1 0 2 - 1 0 3 in LL-chondrites. Within the L-group, IRMs/NRM increases from IA to L6. NRM/TRM varies in the opposite sense, diminishing from ~ 1 0 0 in E to ~ 1 0 in H, and 1 in LL chondrites; and from IA (~20) to L6 (~3). If NRM is a weak-field T(C)RM carried by MD grains [ 18], the latter ratio reflects the relative efficiency of TRM acquisition. However at present, both ratios are only heuristic indices of relative variation in magnetic behavior since the large soft NRM component may obscure the stable, TRM-like fraction. In examining the relative directional coherence of NRM and of weak-field TRM, it is noteworthy that in general the TRM exhibits greater scatter than NRM, except for the shocked chondrite Tadjera. Also, the directionally stable TRM and NRM component reside in different grain fractions, with distinct ranges of microcoercivities.
L
3.10-4_6.100 3 5-10 <10°1
LL
Trend direction
0.08 2 35 8.10-s-1.100 3 <50-60
?
>1001
10-50 (L4 ~L6) 102-103 3-20 (L4 ~'L6) ~ 1
higher coercivity, i.e. the paleomagnetically stable portion of NRM was used for paleointensity determination. The corresponding line extrapolates close to the origin for Abee (E4), Rose City (H6), and Bald Mtn. (L4), thus confirming its thermal character. For Andover (L6), Aumale (L6), Jelica and Vavilovka (LL6), nonzero intercepts of the higher-coercivity linear segments (Figs. 7,8) were obtained. Paleointensities are within the range 0 . 0 1 - 0 . 3 0 e (Table 2). For the severely shocked Tadjera, which lacks a stable NRM component, the scatter of points was so great that no reliable estimate could be made (Fig. 6). For comparison, paleointensities previously reported for 14 other ordinary chondrites, range from "~0.1 to 0 . 9 0 e [3,4,6,9].* The present values are lower by factors of 1 0 - 1 0 0 than those (~<10e) obtained by a variety of methods for several carbonaceous chondrites [ 11 ].
3. 6. Shock history and magnetic behavior Among the chondrites whose remanent behavior was studied, Rose City (H6) and Tadjera (L5) are * They are as follows: Ochansk (H4) 0.1-0.30e; Pulutsk (H5) 0.2-0.25 Oe; R'akovka (L6) and Mordvinovka 0.4 Oe; Zhovtvyevi Khutor 0.15-0.20e [6]; Mezo-Madaras (L3) 0.13 Oe; Elenovka (L) 0.19 Oe; Orlovka (H) 0.25 Oe; Pervomayski Poselok (L) 0.12 Oe; Tarbagatay (L) 0.27 Oe [9]; Farmington (L5) 0.18 Oe; Homestead (L5) 0.90e, and Mt.Browne 0.25 Oe [3]; and Brewster (L6) 0.16 Oe [4].
64 known to be severely shocked; Ochansk (H4) and BjurbiSle (IA) were mildly reheated, and Sokobanja (LIA) appears brecciated [13,14,19,21-25]. We suggest a number of criteria for recognizing shocked or reheated chondrites by their magnetic behavior. The normalized NRM demagnetization curves of chondrites previously recognized as shocked or reheated [19] show a characteristic two-humped profile, indicating a bimodal distribution of remanence. A much larger soft fraction of NRM than is found in "normal" chondrites and greater scatter of directions between NRM components of different coercivity are also evident in these cases. Corroborating evidence may be drawn from an earlier study [26]: the AF demagnetization behavior for Bruderheim and Potter, both shocked L6 chondrites; for Achilles, a lightly reheated H chondrite; and for Goalpara and Novo Urei, shocked ureilites, conforms to our criteria for shock recognition. The NRM of other shocked or reheated chondrites [19], Farmi'ngton (L5), Ness County (L6), and Kelly (LIA), were insufficiently AF cleaned [26] for the purpose' of our test. However, a thermal demagnetization curve for Farmington also exhibits a bimodally distributed NRM coercivity spectrum [3]. Generalizations regarding the validity of these criteria are still premature. Several chondrites among those in Figs. 1 - 4 which are not known to be shocked display bimodal demagnetization curves. So do Melrose and Lalande (L5), but they do not satisfy the NRM directional scatter criterion since their NRM is directionally stable up to 400 Oe [26]. A high-coercivity directionally stable component of NRM was found in the shocked Rose City (H6), but not in the unshocked Cabezo de Mayo (L6). When "normal" chondrites are heated and cooled, their smoothly distributed NRM microcoercivity spectrum changes to a bimodally distributed TRM (Figs. 5,7,8). Conversely, similar heat-treatment restores (Fig. 6) a "normal" appearance of the TRM demagnetization curve to the shocked chondrite, Tadjera. This provides further support for our contention that a bimodal AF demagnetization curve is diagnostic of shockreheating. We note that IRMs demagnetization spectra harden upon heating in most cases, including the shocked Tadjera and Rose City (Table 2, Figs. 5-8). This is contrary to the model of shock-induced stable remanence (SRM) recently proposed for chondrites and for
lunar sample magnetism [27,28], according to which IRM s should soften in heating as shock-introduced' defects and dislocations, which pin the NRM, anneal and metal grains coarsen.
4. Discussion The conveniently systematized information available on chondrites [ 13,14,19,21-25] enables us to interpret variation trends of the magnetic properties in terms of metal chemistry and microstructure (Table 3). The average Ni-Fe metal contents vary among the four chemical groups of ordinary chondrites, from approximately 25-30% wt. in E and ~17% wt. in H, to "6% wt. in L and ~2% wt. in LL [22,23]. The average Ni content of metal varies in the opposite sense [22] ,from "6% for E- and 8% for H-, to ~12% for L- and 35% for LL-chondrites. A direct correspondence was demonstrated, for a large number of chondrites [5,7,8], between the degree of reduction and average abundance of metal, which define the chemical groups, and the average NRM intensity. Our results confirm this rough trend. A deft. nite correlation also exists between demagnetization behavior and chemical type, the hardness increasing as the overall metal content decreases. The metal composition provides the key to explaining the inverse correlation trend observed for the stability of NRM (Table 3). According to the binary Ni-Fe phase equilibrium diagram and the accepted thermal history of chondrites [21 ], enrichment of the metal in Ni from E to LL's implies progressively finer grain size of strongly ferromagnetic kamacite down to plessitic (a + 3') microstructure. But finer ferromagnetic grains, subgrains, or inclusions correspond to higher coercivities and therefore to higher stability of remanence [12, 16]. The microstructure of metal phases must be invoked to explain trends in NRM stabi~ty behavior, because intergroup variations in the average size of metal grains ("0.22 mm) in E, H, and L and (~0.17 mm) in LL chondrites [22,23] are too slight to account for the observed range of magnetic effects. At least within the H and L groups, NRM intensity increases with metamorphic subtype [8]. Our data suggest an opposite trend for NRM stability: the lower subtypes display the higher stability to AF cleaning. This corresponds again to the Ni enrichment of metal
65 in chondrites of lower metamorphic subtype within each chemical group [22]. For E-chondrites, it was shown that finer metal grains contain more Ni [23], so that a grain-size effect may parallel or compound the intragroup compositional trend leading to the high NRM stability in lower subtypes. Furthermore, a grain-shape effect may also be important. In type 3 chondrites, metal grains are ovoid or spheroidal, whereas in recrystallized subtypes, coarser amoeboid intergrowths are typical [13,22]. Explanations similar to the above, may be advanced to explain E ~ L L intergroup and intragroup L4~L6 variation trends of remancnce ratios (Table 3). Since the degree of re-equilibration of mineral phases increases with metamorphic type, the contribution from secondary TCRM to NRM should increase correspondingly. The nonzero intercepts of TRM vs. NRM plots for Andover (L6), Aumale (L6), Jelica (LL6), and Vavilovka (LL6) (Figs. 7,8) are in accord with this. The relative stabilities of NRM, TRM, and IRM s (Figs. 5-8) and the values of IRMs/NRM and NRM/ TRM ratios indicate that most carriers of remanence are multidomain (MD) grains, if criteria developed for characterizing thermo-remanence carriers in terrestrial rocks [18] are applicable to chondrites. A fair fraction of single-domain (SD)-like grains, carrying high-coercivity NRM, is present in LL-chondrites. This agrees with microscopic metallographic observations on chondrites [21-25]. Detailed mineralogical and metallographic information is available for most chondrites studied [19,21-25]. Metal phases commonly occurring, kamacite (a) and taenite (3') grains with plessitic (a + 3') fine-grained centers, are all ferromagnetic, with a range of Curie points (TC <~750°C) determined by the Ni content [ 12,16]. Ni-rich martensite (a2) and polycrystalline kamacite also occur, primarily in shocked or reheated chondrites [ 19,21 ]. Their presence suggests the explanation for the effects of laboratory heating on the demagnetization spectra of normal and shocked chondrites since their formation originally involved rapid massive (i.e. diffusionless) phase transformations of 3'-Ni-Fe during post-shock quench cooling. By heating "normal" chondrites in the laboratory to temperatures above the a + 3' transitions (~800°C), and cooling rapidly, polycrystaUine kamacite and metastable Ni-rich martensite form. The martensite further decomposes, in part,
to plessite [21 ], thus generating new high-coercivity carriers of TRM (Figs. 5,7). The reverse transformation (plessite~3,-field-*martensite) takes place upon heating the shocked meteorites (e.g. Tadjera, Fig. 7). Therefore, shock-metamorphism events can be simulated in the laboratory and their cumulative effects on metallurgy and magnetics can be assessed. Paleointensity estimates for shocked or reheated chondrites, such as Farmington, Ochansk [3,6] or Rose City, are problematic. Shock events [14,19] probably introduce magnetic effects [27,28], but it is difficult to separate the variables involved (pressure-temperature experienced, magnetic field, metal-phase transitions with some diffusional re-equilibration). These could partly or completely erase the original magnetic record and compound it as a result of shockmetamorphism [28]. Ideally, to resolve between "magnetic temperatures", indicating the level of metamorphic reheating, and Curie points of intermixed magnetic phases, thermomagnetic analysis, thermal and AF demagnetization should all be undertaken in modeling studies of remanence for shocked meteorites [2-4, 121. To account for discrepancies between previous paleointensity estimates [3,4,6,9] and ours (Table 2), we conjecture that most of the earlier values are overestimates, because TRM was matched to a larger and softer component of NRM, rather than to the small demonstrably stable component. Typically, as in Bald Mtn. (Fig. 5c), the slope of the lower coercivity (Hc 60 Oe) segments yields a value H 0 >~0.20e, comparable to earlier values, However, in Bald Mtn. the soft and hard components are unidirectional (Fig. 5b) and only one magnetizing event and field must account for the entire NRM. Either two discrete (compositionally or by size) grain fractions acquired TRM in the same field with different efficiencies; or the soft fraction was remagnetized later by the locally enhanced effective field of the primary, high-coercivity remanence. Until this situation is clarified, the paleofield estimates listed in Table 2 must be viewed cautiously as lower limits to magnetizing fields. Yet another caveat in inferring the nature of NRM in chondrites and interpreting paleointensities derives from the observation that the original NRM cannot be duplicated in the laboratory. First, some thermally induced changes in amounts and sizes of carrier grains usually occur, as seen by comparing IRMs before and
66 after heating. Second, not all magnetic grains capable of carrying TRM can be mobilized efficiently on laboratory time-scales, which correspond to rapid quench by comparison to estimated cooling rates of 0.1-10°C/ m.y. inside chondrite parent bodies [21 ]. Various lines of evidence favor acquisition of primary thermo-chemical remanence (TCRM) [3,6,9,12] by Ni-Fe metal grains of ordinary chondrites either inside asteroidal parent bodies [21], or during "metamorphism" in a planetesimal stage [ 14]. Shock-reheating or later "metamorphism" in the parent body may have modified (secondary TCRM) the initial magnetization. We favor external extended source fields for imprinting the NRM, possibly enhanced by induction mechanisms [30], since dynamo-generation of magnetic fields in small asteroidal parent bodies is unlikely [12,14] and an origin in a single, planetary-sized body [10] is unacceptable. The electrical conductivity evolution with increasing differentiation from primitive, carbonaceous chondrite composition to basaltic achondrites is under study [31 ]. Results to date indicate that the efficiency of solar wind as a source of heating and magnetizing fields [12] may have been much higher in the early solar system. Connections between inferred cooling rates [21 ], the nature and degree of "metamorphism" [13], shock history [19], ages and fractionation patterns [ 14] on the one hand, and magnetic evidence on the other hand, are still unclear. The anisotropy of magnetic susceptibility [1, 3, 4b, 1 le, 12], which can index the magnetic fabric corresponding to the petrographic texture observed in chondrites [32] may cast light on both the ambient magnetic fields and on the kinetics of grains aggregation in the early solar system, since anisotropy ratios (1.08 to 2.1) reported for chondrites appear to reflect primary textural alignment of grains, rather than degree of metamorphism [32].
5. Conclusions The main conclusions of this study on the magnetic systematics of ordinary chondrites are: (1) The decrease in intensity and increase in stability of NRM to AF demagnetization roughly parallel the decrease in Ni-Fe metal content and the corresponding enrichment of the metal phases in Ni. (2) The stability of NRM within a chemical type
appears to decrease with increasing degree of metamorphism, a feature interpretable in terms of composition, size, and shape of metal grains. (3) The LL-chondrites are most likely to have preserved a usefule magnetic record and are best suited for paleointensity determinations, diaplaying remarkable NRM stability and directional coherence. (4) Paleofield intensities required to imprint the NRM as a T(C)RM are in the range 0 . 0 1 - 0 . 3 0 e . These results await further verification.
Acknowledgments This research has been supported, in part, by NASA Grant NGL-22-009-187. It includes part of the M.I.T. General Examination project of Rambabu P. Ranganayaki. We thank Drs. R.T. Dodd and E.R.D. Scott for their interest and helpful discussions. Dr. M. Gaffey of M.I.T. is thanked for the generous loan of specimens from his personal collection of meteorites. The revised manuscript benefited from helpful editorial comments from the reviewers and Ms. V. M. Burns. We thank Ms. R. Regan for typing the manuscript.
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19 a D. Heymann, The origin of hypersthene chondrites: ages and shock effects of black chondrites, Icarus 6 (1967) 189. 19 b N.L. Carter, C.B. Raleigh and P.S. De Carli, Deformation of olivine in stony meteorites, J. Geophys. Res. 73 (1968) 5439. 19 c G.J. Taylor and D. Heymann, Shock reheating and the gas retention ages of chondrites, Earth Planet. Sci. Lett. 7 (1969) 151. 20 M. Fuller, Lunar magnetism, Rev. Geophys. Space Phys. 12 (1974) 23. 21 J.A. Wood, Chondrites:their metallic minerals, thermal histories, and parent planets, Icarus 6 (1967) 1. 22 R.T. Dodd, The metal phase in unequilibrated ordinary chondrites and its implications for calculate accretion temperatures, Geochim. Cosmochim. Acta 38 (1974) 485. 23 J.T. Wasson and C.M. Wai, Composition of the metal, schreibersite and perryite of enstatite chondrites and achondrites, Geochim. Cosmochim. Acta 34 (1970) 169. 24 H.C. Urey and T. Mayeda, The metallic particles of some chondrites, Geochim. Cosmochim. Acta 17 (1959) 113. 25 P. Ramdohr, The Opaque Minerals in Stony Meteorites (Elsevier, Amsterdam, 1973). 26 E.E. Larson, D.E. Watson, J.M. Herndon and M.W. Rowe, Partial AF demagnetization studies of 40 meteorites, J. Geomag.Geoelec. 25 (1973) 331. 27 A.J. Meadows, Remanent magnetization in meteorites, Nature 237 (1972) 274. 28 a S. Cisowsky, M. Fuller, M.F. Rose and P.J. Wasilewski, Magnetic effects of experimental shocking of lunar soil, Geochim. Cosmochim. Acta, Suppl. 4, 3 (1973) 3003. 28 b P.J. Wasilewski, Shock remagnetization associated with meteorite impact at planetary surfaces, The Moon 6 (1973) 264. 28 c P.J. Wasilewski and A.S. Dean, Remanent magnetization and structural effects due to shock in natural and manmade iron-nickel alloys, in: Metallurgical Effects at High Strain Rates (Plenum Press, 1974) 225. 28 d M.W. Rowe, Constraints on the magnetic field which magnetized the Farmington meteorite parent body, Abstract 44, 37th Meeting of the Meteoritical Society ( 7 - 9 Aug. 1974), and preprint, Meteoritics (1974). 29 K. Kellogg, E.E. Larson and D.E. Watson, Thermochemical remanent magnetization and thermal remanent magnetization comparison in a basalt, Science 170 (1970) 628. 30 C.P. Sonett, The relationship of meteoritic parent body thermal histories and electromagnetic heating by a premain sequence T-Tauri sun, in: Physical Studies of Minor Planets, ed. T. Gehrels, NASA SP-267 (1971) 239. 31 A. Brecher, The electrical conductivity of carbonaceous meteorites, Meteoritics 8 (1973) 330. 32 R.T. Dodd, Preferred orientation of chondrules in chondrites, Icarus 4 (1965) 308.
L_Zd
PALEOMAGNETIC SYSTEMATICS OF O R D I N A R Y CHONDRITES* AVIVA BRECHER and RAMBABU P. R A N G A N A Y A K I Department of Earth and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Mass. (U.S.A.)
Original manuscript received August 27, 1974 To authors for revision September 24, 1974 Revised version received October 31, 1974
A pilot systematic study of the remanent magnetic behavior of ordinary chondrites reveals several correlations with their chemical-petrological classification. The intensity of natural remanent magnetization (NRM) decreases in the sequence E ~ H ~ L ~ LL-chondrites. Its relative stability and degree of directional coherence under progressive AF demagnetization increase in the same order. These trends parallel a decrease in average metal content (from ~20% in E to 2% in LL) and a corresponding Ni enrichment of metal grains (from ~6% Ni in E to ~35% in LL). Within each chemical group, the stability of NRM is inversely related to petrologic subtype. Shocked meteorites display a distinctive demagnetization curve, an unusually large soft component of NRM, and pronounced directional scatter of NRM components. These characteristics can be reproduced reversibly by brief laboratory heating (to ~800°C) of unshocked chondrites. Comparative studies of laboratory (saturation isothermal, IRMs, and thermal, TRM) remanence acquisition and behavior, suggest that other magnetic parameters, such as the ratios NRM/TRM and IRMs/NRM, may be useful for classification. Patterns of magnetic behavior are explained in terms of metallography and thermal history. In contrast to the carbonaceous chondrites, very few ordinary chondrites preserve a useful paleomagnetic record. The LL-chondrites hold the best promise in this regard. Paleofield estimates obtained for eight representative chondrites range from 0.01 to 0.3 Oe. 1. Introduction The discovery that some chondritic meteorites possess a stable extra-terrestrial natural remanence (NRM) [ 1 - 3 ] aroused immediate interest in its nature and origin [ 4 - 6 ] . An extensive survey o f natural remanent magnetization (NRM) intensity and bulk magnetic susceptibility in a large number o f meteorites [7,8] indicated that their values cluster for chemical groups (H, L, LL) as well as intragroup (for H, L) by petrologic subtypes. Various attempts were made to infer or model in the laboratory the NRM acquisition and to estimate the paleofields required to imprint it [3,4,6,9].The NRM was generally interpreted as a thermal (or thermo-chemical) remanence, acquired during cooling (with simultaneous compositional changes) o f the N i - F e metal grains, in parent-body magnetic fields o f 0 . 1 - 0 . 9 0 e [3,4,10]. Recent investigations [ 1 1 ] have established that a * Contribution No. 20 of the M.I.T. Laboratory for Paleomagnetism.
magnetic record is preserved intact in most carbona. ceous chondrites, requiring primordial fields o f ~<10e. Thus far, research on meteoritic paleomagnetism has provided few conclusive answers, but many tantalizing leads, regarding the physical conditions prevailing during and after formation o f small bodies in the early solar system [11,12]. The present study attempts to: (1) search for systematic variations of remanent properties with chemical class and metamorphic subtypes; (2) identify the stable component o f NRM and infer its mode o f acquisition; and (3) correlate the magnetic properties o f meteorites with their composition and thermal histories. A magnetic characterization of chondrites, compared with the existing classification schemes [ 13 ] and integrated with other evidence [14] may: (1) further constrain models o f origin; (2) resolve the current controversy regarding the nature o f " m e t a m o r p h i s m " and attendant chemical fractionation and re-equilibration processes; and (3) confirm cogenetic groups and relative place o f origin in their parent body.
58 2. Experimental procedures Magnetic moments were measured on a Schonstedt SSM- 1A spinner magnetometer (sensitivity 10 -7 emu). NRM values (Table 1) are in the range 10 -1 to 10 -5 emu and are accurate to 1%. The meteorite samples weighed from 0.3 to 4 g. The NRM was cleaned by conventional alternating field (AF) demagnetization. A Permali AF demagnetizer with a three-axis tumbler was used, caiaable of providing 2,000-Oe peak fields. Earth's field was nulled, to <100 3' at the sample, by 3 pairs of Helmoholtz coils. Samples from each chemical group were selected for detailed laboratory remanence studies and for paleointensity determination. A single heating variant was chosen for estimating the paleofield intensity [17] to prevent chemical alteration and metal-phase diffusional re-equilibration. Samples were heated to ~<800°C in about 2 hours, held there for ~30 minutes, and allowed to cool to room temperature in a 0.17-Oe field. The heating level selected was ~<800°C, not only because this exceeds the Curie temperature of the kamacite, the principal ferromagnetic mineral in chondrites [3,5,6,9, 12], but also because it corresponds to peak "metamorphic" temperatures estimated for chondrites [13b, 14]. Heating and cooling were performed in flowing argon to remove any reactive gases evolved, and minimize chemical changes. The TRM acquired in 0.17 Oe was then progressively AF demagnetized, and its stability and degree of directional coherence relative to NRM were assessed. To monitor the effect of heating on the magnetic grains, the saturation remanence (IRMs) imprinted in 12-kOe fields and its demagnetization behavior were measured before and after heating. The method employed here for estimating paleointensities can resolve between thermal-like and nonthermal NRM components [ 17], as was illustrated for carbonaceous chondrites [11 a, e]. Thermal (TRM), thermochemical (TCRM), or chemical (CRM) remanence cannot be distinguished from AF demagnetization curves alone. The fractional loss of NRM is compared to that of TRM for each demagnetization step. If a linear plot which extrapolates to the origin results, the assumption that the NRM is entirely a TRM, or a primary TCRM [29], is validated and the slope yields the paleofield strength. When several linear segments result, several thermal-like components of NRM are
AVIVA BRECHERAND RAMBABUP. RANGANAYAKI indicated, necessitating either different field-strengths or different efficiencies of TRM acquisition by discrete size-ranges of magnetic grains for a given field strength. For linear plots with nonzero intercepts, a secondary chemical or thermochemical remanence is usually invoked [17,1 la,f, 12], but the paleointensity may still be estimated. Nonlinear plots preclude paleofield determinations. 3. Experimental results 3.1. Intensity o f NRM The intensity and stability of NRM for 22 ordinary chondrites are indicated in Table 1. The specific intensity ranges for each chemical group are: 3.9 X 10 - 3 2.8 X 10 -1 for E;9.1 × 10-4-2.3 XlO-2 for H; 3.5 X 10-4-6.5 X 10 -3 for L; and 7.9 X 10 - 5 1.1 × 10 -3 for LL. 3.2. NRM stability Samples were AF demagnetized stepwise in peak fields of 15, 30, 60, 120, 200, 300, and 500 Oe. The resulting NRM demagnetization curves (Figs. 1-4) represent the microcoercivity spectrum of the magnetic grains. All the meteorites possess at least 2 NRM components, the larger of which is usually soft [5,6,9]. This unstable component is dearly removed by cleaning in HAFt<60 Oe and resides in large multidomain grains. The relative magnitude of the soft component varies systematically with chemical class (Table 1). The demasnetization behavior of E-chondrites studied is shown in Fig. la: their NRM is very soft. The NRM of H-chondrites (Fig. 2) is also soft, only slightly harder on the average than in E-chondrites. The NRM!s of L-chondrites (Fig. 3), though still soft, are considerably harder than those of E- and H-chondrites. The demagnetization behavior of the LL-chondrites is shown in Fig. 4. As a class, they have the largest stable residual of NRM. Within each group, the NRM of lower metamorphic type chondrites is more resistant to AF demagnetization (Figs. 1-4). 3.3. NRM directional behavior The NRM demagnetization spectra alone do not indicate whether NRM is directionaUy stable. Only
59 TABLE 1 Characteristics of natural remanent magnetization Meteorite
Type
NRM (emu/g) X 10+3
AF range for directionaUy stable NRM component*
Fraction of NRM lost 60 Oe
200 Oe
intermediate to high field none none
0.84 0.98 0.97
0.99 0.98 0.99
intermediate field none none none high field
0.97 0.98 0.92 0.99 0.97
0.97 0.97 O.90 0.98 0.99
Abee Hvittis Khairpur
E4 E6 E6
276.0 10.6 3.9
Ochansk Tysnes Island Quenggouk Lancon Rose City
H4 H4 H4 H6 H6
7.2 0.9 1.5 22,9 19.5
Bald Mountain Bjurbole Tadjera Andover Aumale Bushof Cabezo de Mayo Utrecht Zavid
L4 L4 L5 L6 L6 L6 L6 L6 L6
6.5 0.4 1.5 4.7 4.5 0.8 0.7 0.9 2.2
entire field intermediate to high field none intermediate to high field intermediate to high field entire field none intermediate field low field (up to 120 Oe)
0.94 0.49 0.99 0.35 0.98 0.94 0.82 0.78 0.97
0.99 0.37 0.98 0.98 0.98 0.98 0.93 0.76 0.98
Sokobanja Jelica St. Severin Vavilovka
LL4 LL6 LL6 LL6
0.1 1.1 0.5 0.2
intermediate field intermediate field high field intermediate to high field
0.17 0.52 0.43 0.32
0.07 0.88 0.60 0.41
* Low field: up to 60 Oe cleaning field; a soft component is usually cleaned in 15 Oe. Intermediate field: 60-200 Oe cleaning field. High field: 200-500 Oe cleaning field.
samples with b o t h a stable direction and intensity of magnetization are considered useful for paleointensity determination. To ascertain the presence o f a directionally stable NRM component, we projected the change o f the NRM vector under progressive cleaning onto two orthogonal sets o f axes [15]. Constancy o f direction, independent o f intensity loss, denotes a stable, possibly primordial, remanence [ 11 a, e]. In such diagrams (Figs. 5 - 8 ) the number o f NRM components present and their relative stability are evident. The range o f directional coherence encountered is illustrated by the well-behaved L4 Bald Mtn. in Fig. 5b at one extreme and b y the shocked L5 Tadjera (Fig. 6c) at the other. Similar diagrams indicate that only Abee (E4) among E-chondrites examined possesses an NRM component with a stable direction, in the intermediate to high field microcoercivity range ( 1 2 0 - 5 0 0 Oe), representing 10% o f the total moment. Superimposed, is a large soft component.
Among the heavily shocked H-chondrites [ 19], Rose City (H6), has one directionally stable component in the high field ( 2 0 0 - 5 0 0 Oe) range, which comprises only ~1% o f the initial moment. Ochansk (H4) has at least one component directionally stable in the intermediate ( 6 0 - 2 0 0 Oe) field microcoercivity range. No other H-chondrites have directionally stable remanence. Of the L-chondrites, Aumale (L6) (Fig. 7c), Andover (L6), and Bjurb61e (L4), each have a stable component in the intermediate to high field region. Bald Mtn. (L4) is directionally the stablest (Fig. 5b). For these meteorites, the stable NRM fraction is sizable: 2 0 - 5 0 % for L4's (Figs. 2, 3). All the LL-chondrites have unidirectional components o f NRM: Sokobanja (LL4) and Jelica (LL6) o f intermediate (up to 200 Oe), St. Severin and Vavilovka (LL6) (Fig. 8) o f high coercivity.
60 NRM= E
CHONDRITES
Ca)
IOo
3.4. Acquisition and relative stability of laboratory remanence
~, ABEE ( E 4 ) c~ KHAIRPUR ( E 6 ) 0 HVITTIS (E6)
Eight chondrites were selected for modeling studies of NRM (Table 2). All except Tadjera possessed at least one stable component o f NRM, useful for paleo-
i0-1
(a)
NRM = L4 AND L5 CHONDRITES
~ =0"~,
I.C D.S
BJURROLE (L4) 0 BALD MOUNTAIN (I,41 O TADJERA (L5)
0.8
~ 0.7 I0 -3
IE 0.s
(b)
r\
"~
~
o
0.9 O.S 0.7
500
~0.5
~0.4
15 30 60
120 200 H , ~ (oe) •
0.3
o
. 15 30 60
, 120
300
16'
~
200 300 HAF (De) - ~
'6° 15 3O ~go
500
' 120 HAF(Oe)
IO'~
~ O 0 •
It=,,
-1 tO - ( b )
~[ ~'A 0
.
ZI O 0 • •
BUSHOF (L6) ZAVID (L6) CABEZO DE MAYO (L6) UTRECHT (L6) ANOOVER (L6)
TYSNES ISLAND (H4) OCHANSK (N4) LANCON (H6) ROSE CITY IN6)
• OUeNG~OUR (.) "
3
~
O.sl
,o
O.S
~=
xZO. 7 z
i
O
-
4
'
'
"_o.6
~
~o'II I I is so so
o.s 0.4
0
(C)
~t'b (:~ I~m-- m
~O. 7
~
IO'ZI~
(O)
O.S
=E0.8
~"
NRM , L6 CHONDRITES
Fig. 1. The absolute (a) and normalized (b) AF demagnetization behavior of E-chondrites. Lower metamorphic type Abee (E4) has a harder NRM than Hvittis (E6) and Khairpur (E6). NRM: H CHONDRITES
' ZOO
L
m
I ~zo HAF
(De)
I zoo
300
6
(d) 0.4
'6511 I / 15 50 60
0.3
I 120 HA~ ~oe)
I 200
0,2
I 300
0,1
0,2
15 30 60
0.1 15 30 60
120
200 HAF (Oe)
300
500
Fig. 2. The AF demagnetization behavior of H-chondrites, on an absolute scale (a) and normalized to the initial NRM (b).
120
2OO HAF (De)
300 •
i
SOD
Fig. 3. The absolute (a) and normalized (b) AF demagnetiza= tion behavior of L4 and L5-chondrites; (c,d) NRM demagnetization curves for L6 chondrites. Their NRM is, on the average, significantly harder than that of both E and H-chondrites.
61
RM,
LL CHONDRITES
BALD MOUNTAIN
(O)
(L4)
(O)
1.0 0.9
SOKOBANJA (LL4) ST. S E V E R I N ( L L 6 )
0.~
5 ~
0,8 0.6
0.7
0.4
0.6
I~0-5
0.2
ZO.4
,5
0.3
30 6 o
,~o
2~o
3~o
5 0~"0
HAF (Oil) 0.2 3
0.1
I
t
~5 30 6o
I
I
I
I
~2o
2oo
300
5oo
(b)
2
HAF (011}
(c)
I O.~
i0-2 Y
t
* 3
(b)
t
%
=EO.(
-I
q
-2 0.2
-3 I
3
o
o
It IO'~l ( e m u / g )
Z I0"
I I 15 30 6 0
I 120 HAFIOe)
I 200
I 300
Fig. 4. NRM demagnetization curves (a, b) of LL-chondrites: these have the hardest NRM among E, H, L and LL groups. Sokobanja (LL4) has the stablest NRM within the LL-group.
1
t
I0 "3
i0 -5
~ (OI
I
~
~
~
{ $onl
02
I I [15} Oo)
I O}
o.,
0.6
TRM
(60)
0.8
Fig. 5. (a) Normalized comparative demagnetization spectra of NRM, thermal (TRM), and saturation isothermal remanence before (IRM0) and after (IRMs1)` heating. The fact that IRM °'1 curves are identical, denotes the lack of alteration in heating. (b) The changes of the NRM vector in progressive AF cleaning, projected on two orthogonal planes. Good directional coherence is seen for the entire NRM. (c) The fractional NRM vs. TRM losses in AF cleaning yield 3 linear segments. Only the slope of the high stability (He > 60 Oe) component is used to derive the paleofield intensity: NRM/TRM ~ Ho/Hlab, where Hla b ~ 0.17 Oe leads to H o ~ 0.01 Oe. The lower coercivity (Hc < 60 Oe) segment yields H o ~ 0.2 oe.
TABLE 2 Natural and laboratory remanence behavior and paleofield estimates for selected chondrites
Meteorite
Type
Specific moment (emu/g)
NRM
TRM
IRMs°
H0 (Oe)
Stability to AF demagnetization
NRM NRM NRM NRM NRM NRM NRM NRM
IRM]
Abee
E4
2.8 X 10-1
2.5 X 10-3
2.3 X 10-1
1.3 X 10-1
0.33
Rose City
H6
1.9 X 10-2
3.0 X 10-3
4.7 X 10-2
5.9 X 10-2 3.0 X 10-2 3.5 X 10-2
0.01
1.5 X 10--2 6.1 X 10-2 2.9 X 10-2 1.7 X 10-2
0.01 0.02
Bald Mountain
L4
6.5 X 10-3
4.4 X 10-4
4.5 X 10-2
Tadjera Andover
L5 L6
1.5 X 10-3 4.7 × 10-3
5.0 X 10-4 4.7 × 10-4
2.7 X 10-2 2.4 × 10-1
Aumale Jelica Vavilovka
L6 LL6 LL6
4.5 X 10-3 1.1 X 10-a 2.2 X 10-4
4.9 X 10-4 3.4 X 10-4 2.2 X 10-4
1.0 X 10-1 1.2 X 10-1 2.1 X 10-1
,.o
0.01 ?
0.01 0.08
< < < < < < < <
TRM < IRMs0 < TRM < IRMs° < TRM < IRMs0 • TRM < IRM ° < TRM < IRMs0 < TRM < IRM~ < IRMs0 < TRM ~ TRM < IRMs1 <
IRM~ IRM 1 IRM 1 IRM~ IRM~ IRMs0 IRMs1 IRMs°
(L51
TADJERA 1.0 0.8
(0)
VAVILOVKA
,~ NRM
I.O
o IRM o 0 TRM I • IRMI
0.8
0.2 I 5 30 60
120
200 300 HAF(oe )
500
I'\ 0.6-
(b)
O,z
(oI ii
o.e
/I \\
/ /
=E 0.6
I
0,4 / ( , / 0.6 TRM
(b)
1.0
'~
I 1.0
,L I -0.4 -0.2
I I () 0.2 0.4 x 10"4 ( emu/g)
I 0.6
AUMALE( L 6 )
(a)
1.01 0.8 0.6 0.4 0,2 200 300 HAF(Oe) 0.4
(b)
0.2
o
?~} (3)~
1°)-0.2
,
0.4
1120) e(I5) -0.6 J(~ 0.2
0.4 0.6 TRM
0.8
1.0
-0.6
'
I
~.~/ / / ~/
I
"i
s
I
3//..s3
;#/'
0.4 -
'~ (c)
0.~
I
3
/ 0.2
7
Tadjera; (a) NRM and TRM spectra are manifestly different, and IRMs°'j shows that some new higher coereivity magnetic material has been generated in heating; (b) indicates that no linear segment can be recovered from the NRM vs. TRM plot; and (c) the complete NRM directional scatter denotes the absence of a paleomagneticaUy useful component.
0.2
T(o~
4
113°1
~^4 / .
My-Mz I
(5OOl t
-O.E
120
(c)
O.S
'~
~
Fig. 6. Same diagrams as in Fig. 5, for the shocked meteorite
15 30 60
500
~-,~, 0.6 1.0 1.4 x I O'4(emulg)
0.2 0.
0.4
06 TRM
08
1.0
-0.'
I
0.8
l
I 300
xO, E
~ . ~
3I c
I 200 HAF 1.0
iso~
•ii \~, ,,.
i (ZOQ ~):~o~ (,2o, ~ ) l 0.2 0,4
I 120
co)
1-~
O. "~ 2 My-Mz~2 4
/ I
I I 15 30.60
\
/
~ 0.~
IRM~
0.4
0.2
I£
IRM 0 0 TRM I
0.6
0.4
0,2
NRM
c:
0.6
LO
(0.)
(LL6)
I
-0.4 -0.2 0.2 x 10-3(emu/Q)
Fig. 7. TRM is much stabler than NRM (a), although IRMs spectra show a thermally-induced softening of the microcoercivity spectrum. Aumale shows good directional coherence, denoting a stable NRM, for H c > 60 oe (e). The comparison of NRM with TRM (b) yields H o ~ 0.02 Oe.
Fig. 8. The same set of diagrams as Figs. 5 - 7 for Vavilovka, show (a) similarly hard demagnetization spectra for NRM, TRM, and IRMs, and only minor thermally produced changes; (b) directions of NRM stabilize after cleaning in HAF > 6 0 O e ; (c) the linear fit to this stable NRM vs. TRM fraction yields a value of H o ~ 0 . 8 0 e . The nonzero intercept indicates the presence of a nonthermal remanence.
intensity determinations. Tadjera was included to see whether it would yield fictitious values for paisefield strength. Table 2 lists the values of NRM; TRM acquired by cooling from ~'800°C in a laboratory field of 0.17 Oe; and saturation remanence before (IRMs0) and after (IRM 1) heating. Relative stabilities are also schematically indicated. Sets of demagnetization curves are shown only for Bald Mtn., Tadjera, Aumale, and Vavilovka in Figs. 5 - 8 . The apparent dissimilarity in the demagnetization curves of NRM and TRM is due mostly to the relatively large soft component of NRM, whose nature is controversial [3-6,9,12]. However, our results support the conclusion of previous investigators that the stable NRM in chondrites is thermal (TRM) or thermochemical (TCRM). Comparing IRMs0,1 moments and stabilities, most samples, with the exception of Bald Mtn. (Fig. 5); appear altered to some degree by heating to 800°C, due to either chemical or metallurgical phase changes (section 4), or changes in the sizes of magnetic grains. Nonetheless, the TRM can be used to estimate paisefield strength, since the fractional loss of TRM in AF cleaning varies linearly with that of NRM (section 3.5). Note that the remanence ratios IRMs/NRM and NRM/ TRM show systematic inter- and intra-group trends (Table 3). IRMs/NRM is an index of the degree of re-
63 TABLE 3 Correlative trends between the remanent magnetic properties of ordinary chondrites and their chemistry and metallography Chondlite class: Average | metal / content composition
Magnetic remanent properties
E
~
Fe°/Fetot 0.8 Fe-Ni (wt.%) ~27 Niin metal (%) ~ 5
H
~
0.63 17 8
3.5. PaleofieM intensity estimates Paleointensity determinations for the chondrites in Table 2 are shown graphically in Fig. 5 - 8 . Only the
~
0.33 6 12
NRM-intensity, 4.10003-3.1001 1.10-3-2.100 2 (emu/g) NRM-stable < 5 < 5 fraction (%) IRMs-intensity >1001 ~ 5.10-2 (emu/g) IRMs-relative (see Table 2, Figs. 5-8) stability IRMs/NRM ~ 1 ~ 1 NRM/TRM (0.15 Oe)~102 ~10
manence saturation [ 18 ]. It is typically ~ 1 0 - 1 0 0 for multidomain (MD) TRM-carrying terrestrial rocks, ~ 2 0 0 - 5 0 0 for carbonaceous chondrites [1 la, e], and was used as an empirical means for estimating lunar paleofield intensities [20]. Among the chondrites of Table 2, IRMs/NRM varies from >~1 in E and H, to 1 0 - 5 0 in L, and 1 0 2 - 1 0 3 in LL-chondrites. Within the L-group, IRMs/NRM increases from IA to L6. NRM/TRM varies in the opposite sense, diminishing from ~ 1 0 0 in E to ~ 1 0 in H, and 1 in LL chondrites; and from IA (~20) to L6 (~3). If NRM is a weak-field T(C)RM carried by MD grains [ 18], the latter ratio reflects the relative efficiency of TRM acquisition. However at present, both ratios are only heuristic indices of relative variation in magnetic behavior since the large soft NRM component may obscure the stable, TRM-like fraction. In examining the relative directional coherence of NRM and of weak-field TRM, it is noteworthy that in general the TRM exhibits greater scatter than NRM, except for the shocked chondrite Tadjera. Also, the directionally stable TRM and NRM component reside in different grain fractions, with distinct ranges of microcoercivities.
L
3.10-4_6.100 3 5-10 <10°1
LL
Trend direction
0.08 2 35 8.10-s-1.100 3 <50-60
?
>1001
10-50 (L4 ~L6) 102-103 3-20 (L4 ~'L6) ~ 1
higher coercivity, i.e. the paleomagnetically stable portion of NRM was used for paleointensity determination. The corresponding line extrapolates close to the origin for Abee (E4), Rose City (H6), and Bald Mtn. (L4), thus confirming its thermal character. For Andover (L6), Aumale (L6), Jelica and Vavilovka (LL6), nonzero intercepts of the higher-coercivity linear segments (Figs. 7,8) were obtained. Paleointensities are within the range 0 . 0 1 - 0 . 3 0 e (Table 2). For the severely shocked Tadjera, which lacks a stable NRM component, the scatter of points was so great that no reliable estimate could be made (Fig. 6). For comparison, paleointensities previously reported for 14 other ordinary chondrites, range from "~0.1 to 0 . 9 0 e [3,4,6,9].* The present values are lower by factors of 1 0 - 1 0 0 than those (~<10e) obtained by a variety of methods for several carbonaceous chondrites [ 11 ].
3. 6. Shock history and magnetic behavior Among the chondrites whose remanent behavior was studied, Rose City (H6) and Tadjera (L5) are * They are as follows: Ochansk (H4) 0.1-0.30e; Pulutsk (H5) 0.2-0.25 Oe; R'akovka (L6) and Mordvinovka 0.4 Oe; Zhovtvyevi Khutor 0.15-0.20e [6]; Mezo-Madaras (L3) 0.13 Oe; Elenovka (L) 0.19 Oe; Orlovka (H) 0.25 Oe; Pervomayski Poselok (L) 0.12 Oe; Tarbagatay (L) 0.27 Oe [9]; Farmington (L5) 0.18 Oe; Homestead (L5) 0.90e, and Mt.Browne 0.25 Oe [3]; and Brewster (L6) 0.16 Oe [4].
64 known to be severely shocked; Ochansk (H4) and BjurbiSle (IA) were mildly reheated, and Sokobanja (LIA) appears brecciated [13,14,19,21-25]. We suggest a number of criteria for recognizing shocked or reheated chondrites by their magnetic behavior. The normalized NRM demagnetization curves of chondrites previously recognized as shocked or reheated [19] show a characteristic two-humped profile, indicating a bimodal distribution of remanence. A much larger soft fraction of NRM than is found in "normal" chondrites and greater scatter of directions between NRM components of different coercivity are also evident in these cases. Corroborating evidence may be drawn from an earlier study [26]: the AF demagnetization behavior for Bruderheim and Potter, both shocked L6 chondrites; for Achilles, a lightly reheated H chondrite; and for Goalpara and Novo Urei, shocked ureilites, conforms to our criteria for shock recognition. The NRM of other shocked or reheated chondrites [19], Farmi'ngton (L5), Ness County (L6), and Kelly (LIA), were insufficiently AF cleaned [26] for the purpose' of our test. However, a thermal demagnetization curve for Farmington also exhibits a bimodally distributed NRM coercivity spectrum [3]. Generalizations regarding the validity of these criteria are still premature. Several chondrites among those in Figs. 1 - 4 which are not known to be shocked display bimodal demagnetization curves. So do Melrose and Lalande (L5), but they do not satisfy the NRM directional scatter criterion since their NRM is directionally stable up to 400 Oe [26]. A high-coercivity directionally stable component of NRM was found in the shocked Rose City (H6), but not in the unshocked Cabezo de Mayo (L6). When "normal" chondrites are heated and cooled, their smoothly distributed NRM microcoercivity spectrum changes to a bimodally distributed TRM (Figs. 5,7,8). Conversely, similar heat-treatment restores (Fig. 6) a "normal" appearance of the TRM demagnetization curve to the shocked chondrite, Tadjera. This provides further support for our contention that a bimodal AF demagnetization curve is diagnostic of shockreheating. We note that IRMs demagnetization spectra harden upon heating in most cases, including the shocked Tadjera and Rose City (Table 2, Figs. 5-8). This is contrary to the model of shock-induced stable remanence (SRM) recently proposed for chondrites and for
lunar sample magnetism [27,28], according to which IRM s should soften in heating as shock-introduced' defects and dislocations, which pin the NRM, anneal and metal grains coarsen.
4. Discussion The conveniently systematized information available on chondrites [ 13,14,19,21-25] enables us to interpret variation trends of the magnetic properties in terms of metal chemistry and microstructure (Table 3). The average Ni-Fe metal contents vary among the four chemical groups of ordinary chondrites, from approximately 25-30% wt. in E and ~17% wt. in H, to "6% wt. in L and ~2% wt. in LL [22,23]. The average Ni content of metal varies in the opposite sense [22] ,from "6% for E- and 8% for H-, to ~12% for L- and 35% for LL-chondrites. A direct correspondence was demonstrated, for a large number of chondrites [5,7,8], between the degree of reduction and average abundance of metal, which define the chemical groups, and the average NRM intensity. Our results confirm this rough trend. A deft. nite correlation also exists between demagnetization behavior and chemical type, the hardness increasing as the overall metal content decreases. The metal composition provides the key to explaining the inverse correlation trend observed for the stability of NRM (Table 3). According to the binary Ni-Fe phase equilibrium diagram and the accepted thermal history of chondrites [21 ], enrichment of the metal in Ni from E to LL's implies progressively finer grain size of strongly ferromagnetic kamacite down to plessitic (a + 3') microstructure. But finer ferromagnetic grains, subgrains, or inclusions correspond to higher coercivities and therefore to higher stability of remanence [12, 16]. The microstructure of metal phases must be invoked to explain trends in NRM stabi~ty behavior, because intergroup variations in the average size of metal grains ("0.22 mm) in E, H, and L and (~0.17 mm) in LL chondrites [22,23] are too slight to account for the observed range of magnetic effects. At least within the H and L groups, NRM intensity increases with metamorphic subtype [8]. Our data suggest an opposite trend for NRM stability: the lower subtypes display the higher stability to AF cleaning. This corresponds again to the Ni enrichment of metal
65 in chondrites of lower metamorphic subtype within each chemical group [22]. For E-chondrites, it was shown that finer metal grains contain more Ni [23], so that a grain-size effect may parallel or compound the intragroup compositional trend leading to the high NRM stability in lower subtypes. Furthermore, a grain-shape effect may also be important. In type 3 chondrites, metal grains are ovoid or spheroidal, whereas in recrystallized subtypes, coarser amoeboid intergrowths are typical [13,22]. Explanations similar to the above, may be advanced to explain E ~ L L intergroup and intragroup L4~L6 variation trends of remancnce ratios (Table 3). Since the degree of re-equilibration of mineral phases increases with metamorphic type, the contribution from secondary TCRM to NRM should increase correspondingly. The nonzero intercepts of TRM vs. NRM plots for Andover (L6), Aumale (L6), Jelica (LL6), and Vavilovka (LL6) (Figs. 7,8) are in accord with this. The relative stabilities of NRM, TRM, and IRM s (Figs. 5-8) and the values of IRMs/NRM and NRM/ TRM ratios indicate that most carriers of remanence are multidomain (MD) grains, if criteria developed for characterizing thermo-remanence carriers in terrestrial rocks [18] are applicable to chondrites. A fair fraction of single-domain (SD)-like grains, carrying high-coercivity NRM, is present in LL-chondrites. This agrees with microscopic metallographic observations on chondrites [21-25]. Detailed mineralogical and metallographic information is available for most chondrites studied [19,21-25]. Metal phases commonly occurring, kamacite (a) and taenite (3') grains with plessitic (a + 3') fine-grained centers, are all ferromagnetic, with a range of Curie points (TC <~750°C) determined by the Ni content [ 12,16]. Ni-rich martensite (a2) and polycrystalline kamacite also occur, primarily in shocked or reheated chondrites [ 19,21 ]. Their presence suggests the explanation for the effects of laboratory heating on the demagnetization spectra of normal and shocked chondrites since their formation originally involved rapid massive (i.e. diffusionless) phase transformations of 3'-Ni-Fe during post-shock quench cooling. By heating "normal" chondrites in the laboratory to temperatures above the a + 3' transitions (~800°C), and cooling rapidly, polycrystaUine kamacite and metastable Ni-rich martensite form. The martensite further decomposes, in part,
to plessite [21 ], thus generating new high-coercivity carriers of TRM (Figs. 5,7). The reverse transformation (plessite~3,-field-*martensite) takes place upon heating the shocked meteorites (e.g. Tadjera, Fig. 7). Therefore, shock-metamorphism events can be simulated in the laboratory and their cumulative effects on metallurgy and magnetics can be assessed. Paleointensity estimates for shocked or reheated chondrites, such as Farmington, Ochansk [3,6] or Rose City, are problematic. Shock events [14,19] probably introduce magnetic effects [27,28], but it is difficult to separate the variables involved (pressure-temperature experienced, magnetic field, metal-phase transitions with some diffusional re-equilibration). These could partly or completely erase the original magnetic record and compound it as a result of shockmetamorphism [28]. Ideally, to resolve between "magnetic temperatures", indicating the level of metamorphic reheating, and Curie points of intermixed magnetic phases, thermomagnetic analysis, thermal and AF demagnetization should all be undertaken in modeling studies of remanence for shocked meteorites [2-4, 121. To account for discrepancies between previous paleointensity estimates [3,4,6,9] and ours (Table 2), we conjecture that most of the earlier values are overestimates, because TRM was matched to a larger and softer component of NRM, rather than to the small demonstrably stable component. Typically, as in Bald Mtn. (Fig. 5c), the slope of the lower coercivity (Hc 60 Oe) segments yields a value H 0 >~0.20e, comparable to earlier values, However, in Bald Mtn. the soft and hard components are unidirectional (Fig. 5b) and only one magnetizing event and field must account for the entire NRM. Either two discrete (compositionally or by size) grain fractions acquired TRM in the same field with different efficiencies; or the soft fraction was remagnetized later by the locally enhanced effective field of the primary, high-coercivity remanence. Until this situation is clarified, the paleofield estimates listed in Table 2 must be viewed cautiously as lower limits to magnetizing fields. Yet another caveat in inferring the nature of NRM in chondrites and interpreting paleointensities derives from the observation that the original NRM cannot be duplicated in the laboratory. First, some thermally induced changes in amounts and sizes of carrier grains usually occur, as seen by comparing IRMs before and
66 after heating. Second, not all magnetic grains capable of carrying TRM can be mobilized efficiently on laboratory time-scales, which correspond to rapid quench by comparison to estimated cooling rates of 0.1-10°C/ m.y. inside chondrite parent bodies [21 ]. Various lines of evidence favor acquisition of primary thermo-chemical remanence (TCRM) [3,6,9,12] by Ni-Fe metal grains of ordinary chondrites either inside asteroidal parent bodies [21], or during "metamorphism" in a planetesimal stage [ 14]. Shock-reheating or later "metamorphism" in the parent body may have modified (secondary TCRM) the initial magnetization. We favor external extended source fields for imprinting the NRM, possibly enhanced by induction mechanisms [30], since dynamo-generation of magnetic fields in small asteroidal parent bodies is unlikely [12,14] and an origin in a single, planetary-sized body [10] is unacceptable. The electrical conductivity evolution with increasing differentiation from primitive, carbonaceous chondrite composition to basaltic achondrites is under study [31 ]. Results to date indicate that the efficiency of solar wind as a source of heating and magnetizing fields [12] may have been much higher in the early solar system. Connections between inferred cooling rates [21 ], the nature and degree of "metamorphism" [13], shock history [19], ages and fractionation patterns [ 14] on the one hand, and magnetic evidence on the other hand, are still unclear. The anisotropy of magnetic susceptibility [1, 3, 4b, 1 le, 12], which can index the magnetic fabric corresponding to the petrographic texture observed in chondrites [32] may cast light on both the ambient magnetic fields and on the kinetics of grains aggregation in the early solar system, since anisotropy ratios (1.08 to 2.1) reported for chondrites appear to reflect primary textural alignment of grains, rather than degree of metamorphism [32].
5. Conclusions The main conclusions of this study on the magnetic systematics of ordinary chondrites are: (1) The decrease in intensity and increase in stability of NRM to AF demagnetization roughly parallel the decrease in Ni-Fe metal content and the corresponding enrichment of the metal phases in Ni. (2) The stability of NRM within a chemical type
appears to decrease with increasing degree of metamorphism, a feature interpretable in terms of composition, size, and shape of metal grains. (3) The LL-chondrites are most likely to have preserved a usefule magnetic record and are best suited for paleointensity determinations, diaplaying remarkable NRM stability and directional coherence. (4) Paleofield intensities required to imprint the NRM as a T(C)RM are in the range 0 . 0 1 - 0 . 3 0 e . These results await further verification.
Acknowledgments This research has been supported, in part, by NASA Grant NGL-22-009-187. It includes part of the M.I.T. General Examination project of Rambabu P. Ranganayaki. We thank Drs. R.T. Dodd and E.R.D. Scott for their interest and helpful discussions. Dr. M. Gaffey of M.I.T. is thanked for the generous loan of specimens from his personal collection of meteorites. The revised manuscript benefited from helpful editorial comments from the reviewers and Ms. V. M. Burns. We thank Ms. R. Regan for typing the manuscript.
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