Ancient magnetic field determinations on selected chondritic meteorites

Physics o f the Earth and Planetary Interiors, 20 (1979) 361-378 © Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands

361

ANCIENT MAGNETIC F I E L D DETERMINATIONS ON SELECTED CHONDRITIC METEORITES AVIVA BRECHER I and LINDY LEUNG 2 , 1 Department o f Earth and Planetary Sciences, Massachusetts Institute o f Technology, Cambridge, MA 02139 (U.S.A.) 2 Department of Geology, Wellesley College, Wellesley, MA 02181 (U.S.A.)

(Received February 5, 1979; revised and accepted June 1979) Brecher, A. and Leung, L., 1979. Ancient magnetic field determinations on selected chondritic meteorites. Phys. Earth Planet. Inter., 20: 361-378. Paleofield intensity determinations involving a comparison of the stable natural remanence (NRM) component with a laboratory thermoremanence (TRM) were carried out on nine chondrites selected in Brecher and Fuhrman (1979a, this issue, hereafter called Paper I), as well as on two manifestly unsuitable controls. To judge their reliability: (1) heat-alteration was monitored by comparing saturation coercivity spectra before and after heating; and (2) the NRM and TRM intensity and stability were compared to those of residual magnetization following zerofield cooling (TRMo) from above the Curie point of kamacite (Ni-Fe). The latter criterion separates the role of an external magnetic field (of 0.43 Oe) at cooling from intrinsic contributions to magnetic grain alignments, due to accretionary, metamorphic or shock-oriented petrofabrics. In some chondrites (e.g., Brownfield, H3B; Holyoke, H4C; Farley, HSA), a surprisingly large (~10% NRM) and stable TRM o proved so similar to NRM and TRM, that sizeable spurious "paleofields" - comparable to paleointensities obtained - were derived by the standard method for zero-field cooling. In other chondrites, with negligible TRMo (~1% of NRM) and irregular AF demagnetization curves, more reliable paleofield strengths in the range 0.01-0.09 Oe were obtained (e.g., Cavour, H6C). These seem representative of magnetic fields at the end of metamorphism intervals (~107 years after accretion) and/or at post-shock cooling. Thus, field strengths obtained from ordinary chondrites are typically weaker (by factors of 10-100) than those reliably determined from carbonaceous chondrites and ureilites, suggesting temporal decay of nebular magnetic fields, from the end of accretion until the end of metamorphism and early eatastrophic-collisional stages. 1. Introduction In an earlier magnetic survey o f chondrites (Brecher and Ranganayaki, 1975), and in Paper I, some criteria for selecting meteorites suitable for paleofield intensity determinations were established: (1) stable NRM intensity in A F demagnetization; (2) good directional clustering o f NRM or its convergence with progressive A F cleaning (Figs. 6 - 1 3 ) ; as well as (3) a smooth, unidirectional single-component NRM in vector-projection (van Zijl) plots; or at least some trend o f linear convergence to the origin, indicative o f a stable NRM component (Figs. 1 - 3 ) . * Present address: Department of Geophysics, Stanford University, Stanford, CA 94305, U.S.A.

In the present paper, we investigate the remanent capability o f the subset o f H-chondrites selected from those in Paper I (Tables I - I I I ) , and estimate the paleofield strength required to imprint the stable NRM portion as a thermoremanence (TRM), on cooling through the kamacite Curie point (~<770°C) (Table III). The following questions are considered: Firstly, are these meteorites capable o f recording an ancient pre-terrestrial magnetic field, and if they can record faithfully, do they? Secondly, is their magnetization due to a pre-shock (primary) or post-shock (seeon. dary) cooling episode? If the magnetization predates the collisional shock event, could information on the prevailing ancient fields be obtained? If magnetization post-dates the shock, h o w do the associated heat,

362 TABLE I Meteorites selected for heating experiments Meteorite type *

Chicago field museum

History and location

no.

Bjurb61e Brownfield Holyoke Farley Barbotan Saline Gilgoin St.

(L4AI) (H3BI) (I-I4CII) (I-I5A) (H5B) (H5CI) (H5DII)

Me 1426 Me 2577 Me 2307 Me 2216 Me 1844 Me 565 Me 538

Burdett Cavour Ochansk Pulutsk

(HSDII) (H6CM) (H4BI) (H5CI)

Me 2603 Me 246 Me 1441 Me 1587

Fall, 1899, Finland Find, Texas, USA Find, Colorado, USA Find, New Mexico, U.S.A. Fall, 1790, France Fall, 1899, Kansas, USA Find, New South Wales, Australia Find, Kansas, USA Find, S. Dakota, USA Fall, 1887, Russia Fall, 1868, Poland

mechanical shock, chemical phase changes and metamorphic petrofabric affect or modify the initial magnetic remanence o f the meteorite? Similar aspects and related problems for carbonaceous chondrites (Brecher, 1972; 1977), achondrites and ureilites (Brecher and Fuhrman, 1979b), the brecciated LL-chondrites (Brecher et al., 1977) and the iron meteorites (Brecher and Albright, 1977) have already been discussed. In a pilot magnetic survey o f chondrites (Brecher and Ranganayaki, 1975), paleofield estimates for 10 meteorites o f various types o f typically ~<0.10e, but occasionally as high as 0 . 3 0 e were obtained. In the present study, the aim was to verify the reliability o f these results, as well as to develop more stringent criteria for estimating paleofield strengths in ordinary chondrites with a complex history, and, in turn, to constrain current models o f their formation and evolution.

* Type consists of (chemical group classification, petrologic subtype, shock class and metal reheating class). See Paper I. (a)

(b)

2 COMPONENT PLOT HBB BROWNF'IELD

2 COMPONENT PLOT H4C HOLYOKE

2.4

0.1

1.9

"

+E

+E 1.4

0.9. 1.5

/

/

0.0 2.0

2.5

3.0

3.5

4.0

0.0

4.5

0.1

0.2

0.3

0.4

0.5

0.6

0.7

-N

+V 2.4 0.1

1.9 +E 1.4 n

O.go.3 ~

0.0

o:6

o:~

~:2

0.0

0.1

O. 2

-I-N

Fig. 1. Sets of NRM vector projections on two (arbitrary) orthogonal planes for the chondrites (a) Brownfield and (b) Holyoke, Smooth, linear convergence to the origin with progressive AF demagnetization indicates a single-component, unidirectional remanence. (a) has a well-behaved, simple two-component NRM. (b) has been directionally disturbed by higher shock, having three directionally-distinctNRM components. The units here and in Figs. 2 and 3 are 10-3 emu.

363

2. Experimental procedure The basic experimental techniques and equipment have been described in previously cited publications on meteoritic magnetism. The sets of two orthogonal projections of directional changes in NRM vectors during AF cleaning, shown in Figs. 1-3, permit the number of discrete directional components of NRM, corresponding presumably to major magnetizing events, to be analyzed. The directionally uniform portion of NRM is isolated, and is assumed to be the paleomagnetically significant remanence, which is to be compared with a laboratory analog. All of the chondrites shown in Figs. 1-3 have a rather simple magnetic history, though shock disturbance is evident in some (e.g., Saline, H5C; Burdett, H5D).

The samples were heated in air to 800°C in a TSD-1 Sch6nstedt thermal demagnetizer, held there for about 10 minutes and allowed to cool to room temperatures. This thermal spike is near the peak metamorphic temperatures estimated for chondrites (e.g., Wasson, 1974) and coincides with the Curie point of ferromagnetic kamacite. The time-scale (hours) for heating and cooling is compatible with those possible in shock-reheating events during collisional breakup of meteorites. The external laboratory ,.t'ield at cooling was 0.43 Oe, and its direction is indicated by the full dot in Figs. 6b-13b. The resulting thermoremanence (TRMe) so acquired was also progressively AF demagnetized to allow for a comparison of stability with that of NRM to be made (Figs. 4, 5, 6a-13a). The effect of heating on the magnetic grains

(a) 2COMPONENTPLOTH5B BARBOTAN

(b) 2 COMPONENT PLOT HSC SALINE 0.4 !

2.0.

0.3

1.5 -E

+E

1.O

0.2f

O.5

0.1.

O.O 0.0

0.2

0.4

016

0.8

O.C 0.0

0.2

0.4

0.6

0.8

+N

-V

-E

J

2.0'

0.3

1.5

4-E~2

1.0 0.5

O0 o.o o.o25 0.'or~o.075 01100o.125 o'.150 -N

O.(

-o'.3

-o.'2

-o'.1

?

-o-o

-V

Fig. 2. As Fig. 1, for the chondrites Barbotan (a) and Saline (b). Smooth, nearly linear NRM directional convergenceto the origin holds for both meteorites, with only a small vector disturbance superimposed on a directionally stable, presumably primary,

NRM.

364 (b) 2COMPONI-NT PLOT HSD GILGOIN STATION

j/

0,5' 04' +E

0.3'

(a) 2 COMPONENT PLOT HSD BURDETT 0.5

-E

0.3

0.2' 0.1, 0.0 0.0

0.0

(~.2 C).4 0.6 0".8

1~0

1.2

0.0

1.4

0.5

1.0 --N

-V

0.5" 0.4+E

-E

0.3.

0.3

0.2. a

0.1, 0.0 O0

0.0

o.2

o1

°3

0.4

-o~o

-0:;

-0'.2

o11

+V

+N

(c) 2 COMPONENT PLOT H6C CAVOUR

+E

1.8"

1.8

1.6'

1.6

1.4.

1.4

1.2'

1.2

1.0

1.0

0.8

0.8

O.O

0.6

0.4

0.4

0.2 0.0 0.0

O. 0".5

1.0

115 4- N

2.0

2.s

3.o

0.00.0

0".,

0;2

0.3

0.4

0'.s

0.6

o.7

N

Fig. 3. As Figs. 1 and 2, for the chondrites Gilgoin Station (a), Burdett (b), and Cavour (c). (a) and (c) show a basically stable, single-component NRM, while some degree of planar directional pinning, probably due to shock, is seen in (b).

365 H5 CHONDRITES

1.0

1

k

MOM NRM

& HSA 0 H5B H ~0 H~) H5D

~

H5 CHONDRITES TRM E '" ~' HSA FARLEY ~H5B BARI~3TAN

NRM 1o0

FARLEY BARI~OTAN A ~A~bI~T GILGOIN ST

I

0.8

o HSD GILGOIN ST )

0.6 MOM

~

o4 t

0.4

t

0.2

O. 0

QO

100

200

300 400 500 Demag level(Oe)

0

600

,

100

.

200 300 400 500 Demag. level (0 e )

600

k15 CHONDRITES TRM 0 1.0{

~ H~

~AA~ILI~(~TAN

X H5D BUR(3D(~TNTsT

0.8" 0.6. MOM TRM

0.4"

(c)

0.2,

0

100

200 300 400 Demog. level ( O e )

500

600

Fig. 4. Comparisons of the magnetic coercivity spectra of petrologic subgroup H5 chondrites for (a) natural remanence (NRM), (b) thermoremanence grRMe) acquired at cooling in a laboratory magnetic field of 0.43 Oe and (c) for residual magnetization following zero-field cooling (TRM o) (see also Tables II and liD. No systematic variation in relative stability with increasing shock exposure is apparent. Note, however, the similar TRM coercive spectra (b), in contrast to the range of NRM hardness (a). Zerofield residual moments (c) are remarkably stable magnetically, typically harder than NRM and comparable to TRMe.

was assessed by resaturating the samples (IRMs2) and examining the changes in the microcoercivity spectrum of the initial saturation remanence (IRMs~) (Figs. 6a-13a). The justification for this experimental procedure of heating chondrites in air, rather than in a buffercontrolled or inert-gas atmosphere, lies in previous experiences with chondrites (Brecher and Ranganayaki, 1975; Brecher and Fuhrman, 1977). For example, no difference was seen in TRM and IRMs spectra following heating-cooling cycles in air or in argon (e.g. Dhurmsala, LL6). In effect, this gives an interhal check for heat-induced alteration, by comparing the coercivity spectrum of saturation remanence (IRMs) before and after each heating cycle. If sizeable changes occur in either the intensity or coerciv-

ity of IRMs, the paleointensity estimate is rejected. Unlike the lunar rocks which may contain an oxidation-prone, pure Fe metal component, meteorites contain exclusively Ni-Fe alloys. If any oxidation occurs, it is a thin and superficial layer of iron oxide inside which a protective Ni-rich front has formed, as shown by early studies of heat modification of metal grains in the chondrite Renazzo (Brecher, 1972). The van Zijl method (Coe and Gromm6, 1973) was employed for estimating paleointensity, because it involves a single heating-cooling experiment (to 800°C), thus reducing risks of thermal alteration of chondrites. The fractional loss of NRM is compared to that of TRM e for each demagnetization step, and paleofield strength is obtained from the slope of the

366

HB CHONDRITES NRM

1.0 I

1.0

4 H51~ I~RBOTON [] H3B BROWNFIELD

0.8"

0.6-

(a)

HC CHONDRITES NRM

0.8 ~b~l~ - ~ \ 0.6

MOM NRM 0.4"

- O.4

Q2"

0.2

O.q 0

~.

¢

.~

loo

200

3oo

480

~o

ebo

0.0

o

100

200

Demog. level(Oe)

' ~ ~ 1 ~

HE, CHONDRITES TRM

i

~H4C HOLYOKE <>H5C SALINE

-~

1.01

300 400 500 Demog. level (Oe) HC CHONDRITES TRM

TRMo TRME 6 A H4C ~) w' HSC E ~ " H6C

0.8"

' I ~ ~ I I N 08.l~Tli~k. "~

I

TRMo TRME A , HOB

\\

O'H3,,

600

~

~

,

,

,

~

~

o8

(d)

MOM NRM 0.~1 0

~ .

o

loo

.

.

0.2-

.

20o 3o0 4oo Demag. level (Oe)

~oo

8~

0.0 o.o

1(~o.o 26o

3bo

4bo

sbo

8bo

Demag. level (O e )

Fig. 5. Comparisons o f the magnetic stability of NRM, TRM e and TRM o for two shock levels (B and C), across the petrologic recrystallization spectrum (3 to 6). Lower m e t a m o r p h i c grade meteorites have usually harder NRM (a, b) and TRM e (¢,d). Again, the TRM e r e m a n e n t coercivity spectra are remarkably similar to the intrinsic coercivity o f residual m o m e n t s TRM 0 (c, d). This suggests that a hard magnetization m a y n o t necessarily be a remanence, which requires the presence o f an external field.

TABLE II Experimental results Meteorite

Type

NRM *

50 Oe

200 Oe

IRM 1 *

IRM 2 *

IRM 2/IRM 1

Stability to AF demagnetization

Brownfield

H3 B

54.18

0.988

0.763

5024

5223

1.04

Holyoke

H4 C

30.7

1.016

0.792

3956

4320

1.09

Farley

H5 A

26.2

0.657

0.296

2107

2919

1.39

Barbotan (a)

H5 B

38.45

0.019

0.012

974

1919

1.97

IRM l < IRM 2 ~ NRM ~ TRMo < TRM e IRM I < IRM 2 < TRM e ~ NRM <~ TRMo NRM < IRM 1 < < IRM 2 <~ TRM o ~ TRM e NRM < < IRM 1 - IRM 2 < TRM e ~ TRMo

(b)

H5 B

Saline

H5 C

248 325

0.364

0.031

1238

2037

1.65

Gflgoin St.

H5 D

55

0.682

0.728

6027

4263

0.71

Burdett

H5 D

154.4

0.682

0.084

988

2724

2.76

Cavour

H6 C

182.5

0.804

0.184

1352

2111

1.56

* NRM, IRM 1 , IRM2: all 10 -3 e m u cm -3.

NRM < < IRM 1 TRMo < < IRM 2 ~ TRM e IRM 1 < IRM 2 ~ TRM e <~ TRM o < NRM NRM < < !RM 1 <~ IRM 2 (TRM o is bad) NRM < IRM 1 < IRM 2 < TRM e

H3B BROWN FIELD

1.2

367

H3B BROWNFIELD

(o)

1.0 0.8

MOM NRM"

z~NRM rl TRM o OTRM E

A NRM oTRMo

~TRME

~

IRMI s

0.6 O4

0

0,2 O.

0

~

100

•

•

,

'

200 300 400 Demog. level ( O e )

500

)

~

600

H3B P,ROWN FIELD 1.0 (c) 0.8

M__O.E10.6 NRM 0.4 NRM/TRMo NRM/TRM£ • TRMo / T R M E

0.2 0.0 0.0

r~

o12

6.4

o16

o:a

1.6

1/2

MONVTRM Fig. 6. Normalized AF demagnetization curves of the various types of magnetizations in Brownfield (H3B). In (a), note the similarity of NRM and TRM o, and the hardening of saturation r e m a n e n c e following heating with respect to IRM 1 , preceding heating, which suggestssome thermal alteration. Directions are plotted in projection on a stereonet in (b), where the NRM directions are extremely well clustered in AF cleaning, and the TRM 0 behavesjust like the TRMe. (c) depicts the method for determining the paleofield strength using the NRM vs. TRM e regression curve with progressive AF cleaning. The plot is fairly linear. However, the similarity of NRMo vs. TRMe leads to comparable ancient field Han c and fictitious field Hfict values. Hence the ancient field estimate (0.24 Oe) is not judged to be reliable. TABLE III Paleofield determinations from remanence ratios Meteorite

Type

NRM a

Brownfield Holyoke Farley Barbotan d (a) Co) Saline Gilgoin St. Burdett Cavour

H3 B H4 C H5 A

54.18 30.7 26.2

H5 B

38.45 248 325 55 154.4 182.5

a b c d

H5 H5 H5 H6

D D D C

TRM0 a

TRMe a

TRMo b NRM

TRM0 b TRMe

Hanc (Oe)

HA F c (Oe)

Hfict (Oe)

HA F c (Oe)

7.43 5.66 13.38

106.5 76.9 78.9

0.14 0.18 0_51

0.07 0.07 0.17

0.24 0.48 0.12

100-500 100-500 200-500

0.27 0.63 0.42

100-500 100-300 200-500

39.5 34.0 150.6 35.3 32.5

0.16 0.02 0.02 0.04 0.0013 0.00047

0.15 0.15 0.01 0.01 0.0026

0.01 0.18 0.03 0.31 0.09 0.04

50-500 100-500 200-300 0-500 300-500 300-500

0.37 0.59 0.27 -

50-500 200-500 50-300 -

5.988 5.07 2.22 0.211 0.0865

10-a emu cm -3. Initial intensity compared. Range of demagnetization level used for comparison from paleofield estimates. Barbotan, sample very inhomogeneous: Co) is strong and directionally stable, while (a) is weak and directionally scattered. Heating experiments were done using (a), so as to preserve the better sample (b).

368 N

& NRM oTRMo • TRM~ E~IRMI s • I RM2 s MOM - NRM

H.4C HOLYOKE

oTRM o '

O

TRM E

(a)

0.4'

O.~ 0.0

o

lOb

2bo

36o

460

Demag. l e v e l ( O e

sbo

6bo

)

1.0

0.8. 0.6 MOM NRM

0.4.

~

(c)

0.2' 0.(3

o.o

-'~ NRM/TRMo rJ N R M / T R M . • TRM 0/TRN~ E

0"2

64

6B

o'8

~b

MOM/NRM

Fig. 7. As Fig. 6, shown for Holyoke, another low-grade, medium-shocked ordinary chondrite. Its behavior is similar in all respects to that of Brownfield (see text). The close similarity of NRM and TRM o precludes a reliable estimate of paleofield strength (Table III).

linear segment of the normalized NRM vs. TRM e curves (Figs. 6-13). The new experimental procedure adopted here was to subject the samples to a second thermal cycle. This was identical to the old method, except that cooling now took place in the zero-magnetic field chamber of the Sch6nstedt TSD-1 thermal demagnetizer (Hext < 50 % or weaker by at least a factor of 1000 than previously applied fields) (Brecher and Albright, 1977). This residual magnetization (TRM0) is due to spontaneous ferromagnetism of individual grains, compounded by grain interaction effects, and to some preferred orientation of metal grains, as a result of accretionary or metamorphic petrofabrics.

Analysis of TRMo can reveal: (a) the intrinsic microcoercivity spectrum of magnetic grains (Figs. 4 and 5); and (b) the relative importance of an external magnetizing field (of ~ 1 0 e ) to both the intensity and the stability of meteoritic magnetization. A comparison of this TRM o with both NRM and TRM e is in order (Figs. 4-13), to determine if the identification of NRM with a thermoremanence is justified, and if inferred paleointensities are reliable (Table III).

3. Results and analysis The AF demagnetization curves of chondrites in general indicate the presence of two NRM compo-

369

1.04 ~ 0.8. ~ k

\

MOM NRM

HSA FARLEY

H5A FARLEY

\

~ ~_

06 \ \L.(

~

~ NRM oTRMo OTRME r'l IRMIs

~ ~ - ~ ~,~ ~

N

~"Q~

0.4 0.2 O.C

0 1.0

10'0

280 380 Demog. level(Oe )

400

~)

¸

0.8

MOM NRM

0.6 0.4

(c)

0.2O0 0.0

&NRM/TRMo EINRNVTRME OTRMo/I"RME 0:2

0'.4

0~6 0'.6 MOM/NRM

1.0

Fig. 8. The magnetic behavior of Farley (H5A). Note in (a) that TRMo, TRMe, and IRM 2 show similar coercivity curves under AF clearing. In (b), the NRM direction is migrating somewhat and a similar scatter is seen in TRM o directions. In (c), the NRM vs. TRM e plot shows two linear segments, but only that of higher coercivity is chosen for a paleofield estimate (0.12 Oe). However, TRM 0 vs. TRM e is also fairly linear, casting doubt on the reliability of a paleofield strength.

nents of different coercivities. Among the selected samples, the relative magnitude of the soft component varies from meteorite to meteorite (Table II), with Barbotan and Saline having the softest (~ 10% at 100 Oe), and Brownfield and Gilgoin Station the hardest (~95% at 100 Oe) NRM (Figs. 4 and 5). However, directions of NRM are very well clustered (Figs. 6b-13b), showing that the "soft" magnetic component does not differ in direction of magnetization from the harder component, and therefore both were probably acquired simultaneously in the same magnetization event. Thus the customary identification of the harder NRM fraction, residing in freer metal grains, with a primary component, and

of the softer fraction with a secondary magnetization, superimposed at a later time, does not seem necessary for these chondrites. Whenever two discrete directional NRM components are resolved (Figs. 1 3), and two linear segments of NRM against TRM result in regression plots, the results obtained from the smaUer, harder fraction of natural remanence are more reliable (Figs. 6 c - 1 3 c ) . The intensity ofTRM e varies from 0.1 to 3 times that of the corresponding NRM (Table III). The directions are very well grouped and lie in the vicinity of the ambient field, but are not always of the same polarity. The demagnetization curves are smooth and hard: at least 50% TRM e remains at 200 Oe. The

370 H58 I%ARBOTAN

1.01

H5B

0.8"

BARE,OTAN ~NRM OTRM 0

~.~

~NRM rn TRM o ~>TRM E

0.6' MOM NRM

0.4"

0.2.

0.0 0

L.

~

.~

~.

1oo

20o

300

4oo

~;o

o8

1.0

Dernog. level (Oe)

MOM

0,6

0.4

0

0.0

.

0.2

0

0.4

~

0.6

MOM/NRM

Fig. 9. Barbotan (HSB), which shows very similar characteristics to Farley (Fig. 8) and Saline (Fig. 10). All three meteorites have among the softest NRM and hardest TRM e curves (Figs. 4 and 5) and yield fictitious fields of 0 . 4 - 0 . 6 0 e from hard zero-field moments (TRMo). However, the rather weak stable portion of NRM, and its directional scatter relative to TRMe, do not inspire confidence in the paleofield obtained (0.02 Oe).

TRM hardness is comparable to that of the free-field moment (TRMo), indicating that it is the intrinsic coercivity of magnetic carriers (due to grain sizes, magnetocrystaUine or shape anisotropy, etc.) which determines the stability of TRM, rather than the blocking temperature spectrum in a magnetic field. The chondrites are capable of recording a stable TRM in ~<10e fields when cooling on short time-scales (shorter but similar to time intervals attained in postshock cool-down). The similarity or contrast between TRMe and NRM is related to the thermal nature of natural remanent magnetism. The initial intensities of TRMo are typically smaller than the original NRM, ranging from 0.05% to 50% of

NRM (Table III). Their AF demagnetization curves are t~;pically irregular and generally much harder than those of NRM (Figs. 4 and 5). Directions are much more scattered and show characteristic reversals, indicating that some intrinsic property (e.g., accretionary or metamorphic planar fabric) has a large effect on the direction of this residual magnetization (Figs. 6b-13b). Directional changes in TRMo moments are slight only for Brownfield (Fig. 6), Holyoke (Fig. 7), Farley (Fig. 8), and Barbotan (Fig. 9). Their intensities of TRMo are also very stable, even above high cleaning fields ( H A F ) 300 Oe) and changes irregularly. Directions of TRM0 in Brownfield, Burdett, Gilgoin Sta-

371 1.0~

HSC SALINE

N

& NRM ~TRb4 0

k\ b MOM

~

I

t

HSC SALINE

b,

& NRM nTRM o

c:) IRM~

0.6'

NRM 0.4"

o.o

-

16o

o

~

~

300

0

4o0

so0

Dernog. level ( O e ) 1.0,

0.8.

MOM NRM

0.6

0.4

• 0.2-

~

I P

J,/X NRM/TRM o J 3 E1NRM/I"RM_ iT;Mo/'TRIV~E

0:0 0.0

0.2

0.4 MOM/NRM

0.6

0.8

1.0

Fig. 10. Saline (H5C). Note, by comparison with Figs. 6-9 that: in (a) TRMo becomes more jagged and TRMe is very similar to IRM~; in (b) TRMo directions ate more scattered than for Farley (H5A), showing the effects of increasing shock level; in (c) the NRM vs. TRMe plot shows two distinct linear segments, but the TRM0 vs. TRMo is irregular. Thus zero-field moments are not an important contribution relative to TRMe, and a comparison of NRM and TRMe is warranted (/-/anc = 0.03 Oe). tion and Cavour, under AF cleaning, outline a greatcircle path, suggesting that the magnetization might be confined to some textural or crystallographic preferred plane (Brother, 1976, 1977b). Such irregular intensity changes in AF demagnetization may also indicate the presence of several types of magnetic grains of different intrinsic coercivities, whose moments are spontaneously aligned or oriented in planes, and which cannot be easily randomized by tumbling in alternating fields. For example, Saline (Fig. 10) shows a distinct bimodal distribution, which implies that the magnetization might be acquired by two discretely-sized populations of metal grains. This is also consistent with its NRM coercivity spectrum (Fig. 10a).

Assuming that the proportionality relation NRM/ TRMe = Hanc/Hla b holds, the paleointensity (Hane) is obtained from the slope of the linear segment of the normalized NRM vs. TRM curves (Figs. 1-7c). Only the higher coercivity component of NRM was compared with the similar portion of the TRM coercivity spectrum. Paleointensities obtained were found to range from 0.01 to 0 . 5 0 e , within the range of previously reported values of 0 . 0 1 - 0 . 3 0 e (Brecher and Ranganayaki, 1975), compared with values of 0 . 1 5 0 . 9 0 e obtained by different methods (Stacey et al., 1961; Guskova, 1972). A sensible test is to derive fictitious field values (Hnct) for the case of zero-field cooling, by applying TRMo/TRM e = Hfict/Hla b. These values are found to be between 0.3 to 0.6 O¢,

372

H5D GILGOINSTATIO:NRM

1.01

N 0.8

~,.~i~,,x~ - x_,.~.x,.\

HSD GILGQIN ST.

<>TRM0 • TRM¢

NRM

[] TRMo 0 TRM E

. Jl RM~ MOIvl

0.6

~RM 0.4

0.2 )

0.0 0

~bo

~o

46o

500 6&

Demog, level ( O e ) H5D

GILGOIN STATION

1.0

0.8

MOM NRM

0.6 0.4

& NRM/TRM o

0.2

=

NRM/rRME

• TRMo/TRM E 0.0

0.0

0:2

0:4 0.'6 MOM/NRM

0~8

1:0

Fig. 11. Gilgoin Station (HSD). Note that in (a) the NRM is the hardest of all demagnetization curves. The TRM 0 curve is markedly irregular and its direction, plotted in (b), seems to move along a great circle, showing signs of probable planar e6nf'mement and control. In (e), NRM vs. TRMe regression is linear and the Han e value is relatively high ( 0 . 3 0 e ) , but apparently reliable and related to the severe shock event.

also within the range of reported values for ancient fields (Hanc). Points for the TRMo spectra of Burdett (Fig. 12) and Cavour (Fig. 13) were too scattered to yield any fictitious field values. By default, their paleointensities become more reliable. Paleointensity interpretation earl also be made more difficult if thermally-induced changes in the amounts and sizes of magnetic grains do take place, as can be seen from the comparison' of IRMs before and after heating (Figs. 6a-13a). The IRMs demagnetization spectra tend to harden to various degrees after heating, in agreement with earlier observations of Brecher and Ranganayaki (1975). This hardening

of IRMs cannot be attributed to annealing of metal grains, but probably to metaUographie phase changes (o~~ 7, 7 ~ ix2), which thus produce a harder laboratory TRM than that acquired during initial cooling in a parent body. Examination of the thermaUy-induced changes in IRMs spectra show that Burdett and Barbotan, which were suspected, from NRM behavior, of having a finer metal grain structure, show the greatest increase in intensity, since freer grains are most susceptible to diffusional phase changes. Holyoke, Brownfield, and Gflgoin Station, which have fewer fine-grain carriers, have small IRMls/IRM2s ratios (<~1, Table II). Based on

373 HSD E~JRDETT

H5D BURDETT

~ , ~

1.8.

1.6"

~" 1.4' 7~" f/

/ ~

/

N

~ v

~

z~ NRM 0 TRMo 41' TRME [] IRMIo

lO,12o.8

~ NRM

13TRMo 0 TRME

•

MOM NRM 0.6

~

~ ) . . . .". . . . . . ..

0.4 0.2

0.0

0

260

360

400

soo

Demog. level ( Oe ) 2.6' 2.4' 2.2' 2.0.

.MOM NRM

~

1

~

H5D BURDETT

1.8, 1,6' 1.4'

1.2'

lO

(c!

0.8'

0.6"

" 1 _ _ j

~

z~NRM/TRMo [] NRM/TRME .TRMo / TRME

0.4. ~ 0.2. 0.0 ~ ~ ~' 0.0 Q2 0.4 0.6 0.8 1.0 1.2 1.4 1.8 1.8 2.0 22 2.4 2.6 MOM/NRM

Fig. 12. Burdett (I-15D). Note the scatter of TRMo, both in intensity (a) and direction (b). TRM e is very hard (a), stable, and close to the direction of applied field (b). The NRM vs. TRM e plot (c) shows two linear segments, whereas the other plots are very irregular. The average paleointensity obtained (0.09 Oe) is deemed reliable by our criteria.

the above observation and the fact that their IRM~ are much harder and show no "soft" component, we propose that, on heating, some of the multidomain grains are destroyed by compositional subdomain formation and that finer, more Ni-rich metal grains may change phase and subdivide to (- + ?), or ol2, which are both more magnetic and also harder to demagnetize.

4. Reliability criteria and possa"olevariation trends with shock level and metamorphic grade In all cases where the intensity of TRMo is very small and its demagnetization spectrum is irregular, a

smaller paleointensity (0.01-0.09 Oe) is obtained (Table III). This is in agreement with the observed 0.01-0.08 Oe range of paleointensities estimated for some of the H, L, and LL chondrites in Brecher and Ranganayaki (1975) and Brecher et al., 1977. When TRMo is stronger and similar to both NRM and TRMe, as for Brownfield, Holyoke, Gilgoin Station (Figs. 6, 7, 11), the paleofield estimate is usually large and probably dubious. It appears that H, L, and LL meteorites of higher metamorphic grade or petrologic subtype (5, 6) yield more reliable paleofields, of the order <~0.1 Oe; while others, e.g. E, H3, H4, yield higher paleofields of >~0.3 Oe (Brecher and Ranganayaki, 1975; Brecher et al., 1977). Of the 8 H-samples examined here, roughly 3

374

~ 0

,2 t 1.0

H6C CAVOUR

H6C CAVOUR

A NRM <~TRM0 ~TRM~~IRMFs

I~ -~

[3TRMo 0 TRME

MO___~M NRM 0.4t 0.2 0.0 o

26o

1do

36o

46o

s6o

Demog. level ( O e )

H6C CAVOUR 1.2. 1.0 MOM

NRM

4NRM/TRMo ~NRM/TRME WTRMo/TRME

0.8 0,6 J

0.4 0.2 o.o o.o

0"2

0:4

o16

d8

MOM/NRM

1:o

1:2

Fig. 13. Cavour (H6C), which is magnetically similar to Burdett (H5D) (Fig. 12) (see text). Their NRM is softest and TRMe hardest among various demagnetization curves (a). Their TRM o is particularly weak~ jagged and directionally scattered. TRM e values are weaker, but comparably stable to the NRM, yielding paleofields of -0.04 Oe.

groups can be recognized according to overall magnetic behavior: (I) High-shock, high-grade (Burdett, HSD; and Cavour, H6C). Both NRM's are strong and have two or more directional components, but a hard and stable component is also dearly present. The NRM is the softest and the TRM e the hardest of all the demagnetization curves (Table II, Figs. 12 and 13). Both TRMo's are very weak,jagged and directionally unstable in AF demagnetization. TRMe values are close to the ambient field direction, but are fairly weak, only about 0.2 of NRM. This indicates that the large "soft" NRM component, possibly due to shock, dominates and that the high inclination portion of the NRM vs. TRM e plot must be discarded, unless it is

construed as an effect of shock-enhancement of a weaker ambient field. If only the 300-500 Oe portion of the coercivity spectrum is compared, paleofield estimates of 0.04 and 0.09 Oe are obtained for Cavour and Burdett respectively. (II) Transitional (Farley, H5A; Barbotan, H5B; and Saline, H5C). The NRM values of these samples are also the softest, and TRMe values the hardest among the set of demagnetization curves (Figs. 8 10), as in case I (Table II). However, TRM o is comparable to TRM e and fictitious field strengths of 0.40 . 6 0 e are obtained. The direction of TRMe is different to that of the laboratory applied field, showing some internal control on the acquisition of laboratory magnetic remanenee.

375

H4B OCHANSK OCHANSK H4 B

1.0~

~ , ~

t

~,~ ~NRM 13TRMo •TRM g

0.8 0.6 MOM NRM

0.4

ANRM 13TRMo 0 TRME

0.2.

(b)

0.0 0

1~o

26o ~2)o .bo ~o

~o

Demog, level (Oe) 1.O

ff A

0.8

MOM NRM

0.6 0.4 i 0.2. 0.0

0.0

(c)

012

0"4 0~6 MOM/NRM

0~8

110

Fig. 14. Ochansk (H4B), like Pulutsk (Fig. 15), is an example of a totally scrambled magnetic record. The NRM is both soft (a) and directionally chaotic (b), while TRM e and TRMo have very similar coercivities (a), though TRM o scatters directionally. TRMe vs. TRMo yields the only quasi-linear plot in (c). This meteorite is clearly unsuited for paleofield estimates, though Soviet workers reported paleofields of ~0.10e.

The effects of shock are evident in these three samples. With increasing shock level (A-C), TRMo becomes more jagged and softer with respect to TRM¢ and the TRMo intensity decreases, while its direction becomes increasingly scattered (Table II, Figs. 8-10). The TRMe intensity also decreases with increasing shock, indicating that shock might have disturbed the internal grain alignment, which produces the zero-field momer~ts. The NRM also becomes softer as the shock is more severe (A to C), but the NRM intensity increases at higher shock levels, suggesting that shock does introduce some soft magnetization (see above, group I). (III) Low-grade, medium shock (Brownfield, H3B; Holyoke, H4C). These TRMe values are among the strongest of the 8 samples (Table III). However,

zero.field moments (TRMo) are weak, <~20%of NRM, yet smooth and directionally stable. The NRM is very similar to TRMo (Figs. 6, 7) and so the fictitious field value is comparable to the ancient field estimate. Farley (H5A) probably represents a transition member between groups II and III, in that its NRM and TRMe values are more comparable to those characteristic of group III than of group II (Fig. 8). Note that Gilgoin Station (H5D, Fig. 11) behaves very anomalously and is very different from the only other representative of H5D studied, Burdett (Fig. 12). The former is the only sample whose NRM coercivity spectrum is harder than both saturation and thermal magnetizations, and with an IRMs/IRM s 2 1 ratio
376 H5C PULUTSK

1.0~

PULUTSK H5C

0.8

0 TRME

0.4. MOM NRM O.2

O.Q 0

NRM TRMo TRME

b) 100

200 300 400 500 Demag. level ( O e ) PULUTSK H 5C

1.0' O. 8,

600

b NRM/TRM,., n NRM/'rRM'~

/

I

0.6

MOM0"4 NRM

0"00.6

0[2

o:4

o:s

o~e

~.6

MOM/NRM

Fig. 15. Pulutsk (HSC) is also manifestly unsuited for paleointensity determinations, by the criteria of Fig, 14. Note the similarity of NRM to TRMo, which suggests that no true remanenee has survived. The NRM vs. TRM e curve has no linear portions.

mated for any other H5 and H6. It also has an unusually strong TRMe and a relatively weak NRM, showing possible shock demagnetization and hardening effects. These stand in contrast to the observations for group II meteorites. However, according to the stated reliability criteria, the high paleofield obtained (0.30e) must be judged to be trustworthy. It is probably related to the severe shock event which led to extensive reerystallization, and suggests tenfold shockenhancement of a weaker ambient magnetic field. It appears that meteorites of lower grade and low shockexposures, and with hard NRM comparable to TRM in both intensity and stability, are most likely to have preserved evidence for primordial fields. However, they cannot be relied upon because of the strong effects of a primary aecretional-oriented texture on the magnetization, as shown in the zero-field cool-

ing experiments (TRMo behavior). The same close similarity of the NRM or TRM to TRMo demagnetization curves can also be seen in Ochansk (H4B) and Pulutsk (H5C) (Figs. 14 and 15) for which a substantial amount of magnetic observational data have been reported (Guskova, 1972; Brecher and Ranganayaki, 1975). The paleofield strengths estimated by Soviet workers from total T R M to N R M ratios are 0.25 Oe and 0.10e respectively.According to our observations, these two meteorite samples are so disturbed by shock or brecciation that they cannot even be used for paleointensit),determinations because of the scattered N R M directions and the strikingsimilarity of N R M to T R M o spectra. This cases doubt on both the thermal nature and the primary origin of their magnetization. The question ariseswhether the natural magnetiza-

377 tion is merely a spontaneous moment or truly a residual remanence (Brecher and Albright, 1977). Moreover, the Soviet paleofield strengths are probably overestimates, due to the fact that the AF demagnetization process was only carried out up to 300 Oe; hence a large portion of the "soft" component, possibly associated with shock, was included in the paleofield calculation. Ochansk and Pultusk are good examples of a totally erased or scrambled magnetic record. Consequently, chondrites of higher grade and high shock-exposure, with typically small and irregular TRMo and stable NRM, seem to be quite reliable for paleofield estimates. This is not surprising, in view of the higher temperatures which they attained in the last thermal event, which reset (at least in part) their magnetic memory. These weak magnetic fields are possibly typical of the late stages of asteroid thermal evolution and collisional breakup. The general trend apparent in paleointensities (Hanc) estimated for chondrites as a function of chemical type (Brecher and Ranganayaki, 1975) is a decrease i n n a n c from 0.30e for Abee (an E4 chondrite) to minima of the order <0.01 Oe in LL chondrites (Brecher et al., 1977). This is consistent with the temporal decay of an ambient ancient magnetic field during the interval of metamorphism. Cooling times of parent asteroidal bodies are estimated to be ~ 5 0 - 1 0 0 my in the temperature interval 500-800°C. Nickel-poor enstatite metal (<5% NiFe) has a higher Curie point ("750°C) and therefore records the magnetic field earlier and at higher temperatures than the Ni-rich metal of LL-chondrites (~35% Ni-Fe). Though present as finer grains in the latter, the kamacite (~5% Ni, "2% Co) in Dhurmsala, LL6, nucleates at lower temperatures (<500°C) and is intergrown with taenite (30--40% Ni) with low Curie temperatures (150-400°C). In addition, the estimated peak metamorphic reheating temperatures, bracketed by 200--400°C for Type 3 and 700-900°C for Type 6 chondrites, determine the degree (partial or total) of thermal overprinting of any earlier magnetization. Shock.reheating or metamorphic.reheating may, therefore, simply thermally clean a pre-existing accretionary remanence and add only a low temperature TRM, at late stages of cooling. The carbonaceous chondrites (Brecher, 1972; 1977) and ureilites (Brecher, 1977; Brecher and

Fuhrman, 1979b) have, in contrast, reliably provided higher, " 1 0 e paleomagnetic field strengths. Hence such strong fields must have been present during the severe collisional-shock modification of the latter. The differences in types of magnetic carriers and NRM imprinting processes suggest that these magnetic records are coeval and regional, and that they predate the NRM in chondrites. 5. Conclusions

It appears that reliable ancient magnetic field strengths can be recovered from ordinary chondrites only if very stringent criteria are applied in the selection of chondrites which could have preserved a primary NRM. Cooling of the sample from above its Curie point in zero-field seems to be a new and good standard for judging the suitability of NRM for paleointensity experiments. It indicates whether the magnetic moment is the original paleoremanence, or a magnetization due to some internal mechanism, such as oriented petrographic structure (Brecher and Albright, 1977). In the case of the present study, a few of the meteorites (Brownfield, Holyoke, Pultusk) show stable, but somewhat weaker TRMo values that are practically indistinguishable from their NRM values, suggesting that a fossil metamorphic oriented fabric, rather than any ambient field, might have controlled the acquisition of magnetic remanence. Thus, magnetic evidence obtained from ordinary chondrites cannot be used with full confidence to derive values for the ancient extraterrestrial field or to interpret the nature of their NRM. In general, systematicaUy weaker fields (0.01-0.10e) are obtained from chondrites than from carbonaceous chondrites and ureilites ( ~ 1 0 e ) . References

Brecher, A., 1972. Memoryof early magneticfields in earbonaceotis chondrites. In: H. Reeves(Editor), On the Origin of the Solar System.CNRS, Paris, pp. 260-273. Brecher, A., 1976. Textural remanence: a new model of lunar rock magnetism.Earth Planet. Sci. Lett., 29: 131-145. Brecher, A., 1977a. Meteoriticmagnetism:implicationsfor parent bodies of origin. In: A.H. Delsemme(Editor), Comets, Asteroidsand Meteorites: Interrelations,Evolu-

378 tion and Origins. University of Toledo Press, pp. 4 1 5 427. Brecher, A., 1977b. Interrelationships between magnetization directions, magnetic fabric and oriented petrographic features in lunar rocks. Proc. Lunar Sci. Conf. 8th, pp. 703-723. Brecher, A. and Albright, L., 1977. The thermoremanence hypothesis and the origin of magnetization in iron meteorites. J. Geomagn. Geoelectr., 29: 379-400. Brecher, A. and Fuhrman, M., 1979a. Magnetism, shock and metamorphism in chondritic meteorites. Phys. Earth Planet. Inter., 20: 350-360. Brecher, A. and Fuhrman, M., 1979b. The magnetic effects of brecciation and shock in meteorites: II, the ureilites and evidence for strong nebular magnetic fields. Moon Planets, 18: 251-263. Brecher, A. and Ranganayaki, R.P., 1975. Paleomagnetic systematics of ordinary chondrites. Earth Planet. Sci. Lett., 25: 57-67. Brecher, A. and Leung, L., 1978. Can ancient magnetic fields

in space be determined from ordinary chondrites (Abstract). Meteoritics, 13: 396. Brecher, A., Stein, J. and Fuhrman, M., 1977. The magnetic effects of brecciation and shock in meteorites: I. The LL-chondrites. Moon, 17: 205-216. Coe, R.S. and Gromm~, C.S., 1973. A comparison of 3 methods of determining geomagnetic paleointensities. J. Geomagn. G¢oelectri., 25: 415-435. Guskova, Ye.G., 1972. The Magnetic Properties of Meteorites. Nauka, Leningrad, and N A S A - T T - F - 7 9 2 , 1 0 8 pp. Leung, L., 1978. In Search of Ancient Magnetic Field Traces in H-Chondrites. Nininger Competition Ist prize award essay, 25 pp. Stacey, F.D., Lovering, J.F. and Parry, L G., 1961. Thermomagnetic properties, natural magnetic moments and magnetic anisotropies of some chondritic meteorites. J. Geophys. Res., 66: 1523. Wasson, J.T., 1974. Meteorites - Classification and ProPerties, Spinger, New York-Heidelberg-Berlin, 316 pp.