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Magnetic properties of Antarctic shergottite meteorites EETA 79001 and ALHA 77005: possible relevance to a Martian magnetic field
Earth and P1anetary 5c1ence Letter5, 77 (1986) 159-164
159
E15ev1er 5c1ence Pu6115her5 8.V., Am5terdam - Pr1nted 1n 7 h e Nether1and5
[4]
Ma9net1c pr0pert1e5 0f Antarct1c 5her90tt1te mete0r1te5 E E 7 A 79001 and A L H A 77005: p055161e 1 e1evance t0 a Mart1an ma9net1c f1e1d D.W. C0111n50n 1n5t1tute 0f Lunar and P1anetary 5c1ence, 5ch001 0f Phy51c5, 7he Un1ver51ty, Newca5t1e up0n 7yne NE1 7R U (En91and) Rece1ved 5eptem6er 18, 1985; rev15ed ver510n accepted January 6, 1986 7 h e p05516111ty that the parent 60dy 0f the 5NC mete0r1te5 15 Mar5 1mp11e5 that the ma9net1c pr0pert1e5 0f the5e mete0r1te5 may pr0v1de ev1dence c0ncern1n9 anc1ent Mart1an ma9net1c f1e1d5. E E 7 A 79001 p055e55e5 a weak, very 5ta61e pr1mary ma9net12at10n, the pr0pert1e5 0f wh1ch are c0n515tent w1th 1t5 ac4u151t10n 1n an am61ent ma9net1c f1e1d e1ther dur1n9 the mete0r1te•5 f0rmat10n 0r dur1n9 the 5evere 5h0ck event 1ater 1n 1t5 h15t0ry. 7 h e 5amp1e5 0f A L H A 77005 5tud1ed p055e55ed n0 mea5ura61e pr1mary ma9net12at10n: the 065erved remanence appear5 t0 6e a v15c0u5 ma9net12at10n ac4u1red 1n 10ca1 1a60rat0ry f1e1d5.7he ma9net1c carr1er5 1n the mete0r1te5 are f1ne-9ra1ned ma9net1te and a 10wer Cur1e p01nt m1nera1, pr06a61y t1tan0ma9net1te 0r pyrrh0t1te, pre5ent t0 the extent 0f 1e55 than 0.1% 6y we19ht. E5t1mate5 0f the 5tren9th 0f the ma9net121n9 f1e1d f0r EE7A 79001 are 1n the ran9e 1-10 ,tt7.
1. 1ntr0duct10n
0 n e 0f the m05t 1ntere5t1n9 recent deve10pment5 1n mete0r1te 5tud1e5 ha5 6een the 5tudy 0f the 5her90tt1te, nakh11te and cha5519n1te (5NC) ach0ndr1te mete0r1te5 and the emer9ence 0f ev1dence that the1r parent 60dy may 6e the p1anet Mar5.7h15 ev1dence 1nc1ude5 the c105e 51m11ar1ty0f the 5her90tty and EE7A 79001 maj0r e1ement c0mp051t10n w1th that 0f the Mart1an 5urface mater1a1 a5 mea5ured 6y the V1k1n9 5pacecraft [1,2], h19h 150t0p1c rat105 1n trapped n061e 9a5e5 (e.9. 4°Ar/36Ar, 129Xe/132Xe) and 1n n1tr09en (15 N/14N) 1n EE7A 79001, c0mpara61e w1th th05e f0und 1n the Mart1an atm05phere [3], and the1r an0ma10u51y y0un9 a9e5 0f - 1 . 3 6.y. 0r 1e55 [2,15]. 7here 15 5t111 c0n51dera61e d15cu5510n a60ut the 519n1f1cance 0f the data, wh1ch 15 d1ff1cu1t f0r n0n-5pec1a115t5 t0 a55e55, 6ut 1f the mete0r1te5 are 1ndeed 0f Mart1an 0r191n, the1r 9e01091ca1, m1nera1091ca1 and phy51ca1 pr0pert1e5 c1ear1y 0ffer the 0pp0rtun1ty 0f der1v1n9 further ev1dence f0r Mart1an h15t0ry and ev01ut10nary pr0ce55e5. 7he 5tudy 0f the weak permanent ma9net12at10n (natura1 remanent ma9net12at10n, NRM) p055e55ed 6y terre5tr1a1 and extraterre5tr1a1 r0ck5 15 a p0werfu1 t001 f0r pr0v1d1n9 ev1dence 0f the ex15tence 0f ma9net1c f1e1d5 at the t1me 0f 0r191n 0f 0012-821X/86/$03.50
• 1986 E15ev1er 5c1ence Pu6115her5 8.V.
the r0ck5 and a150 dur1n9 519n1f1cant event5 1n the1r h15t0ry, 5uch a5 5evere metam0rph15m and 5h0ck. 1n part1cu1ar, the pa1ae0ma9net15m 0f mete0r1te5 may 1n pr1nc1pa1 pr0v1de 1nf0rmat10n 0n ma9net1c f1e1d5 1n the ear1y 501ar ne6u1a (v1a 5tud1e5 0f the N R M 0f ch0ndru1e5) and 0f ma9net1c f1e1d5 a550c1ated w1th mete0r1te parent 60d1e5.7hu5 1n th15 re5pect the 5NC mete0r1te5 are 0f c0n51dera61e 1ntere5t 1n 0ffer1n9 the p05516111ty 0f 06ta1n1n9 ev1dence 0f ma9net1c f1e1d5 at 50me per10d 0f Mart1an h15t0ry. 7he t1me 0f ac4u151t10n 0f any N R M 15 11ke1y t0 6e that 0f the f1na1 c0011n9 0f the mete0r1t1c mater1a1 6e10w the Cur1e temperature 0f the c0n5t1tuent ma9net1c m1nera15, 0r that 0f the 5h0ck event re5p0n5161e f0r the eject10n 0f the mete0r1te fr0m the Mart1an 5urface 1nt0 an eventua1 Earth 0r61t-cr0551n9 traject0ry. 7he 519n1f1cance 0f p1anetary ma9net1c f1e1d5 15, 0f c0ur5e, 1n the1r p055161e 0r191n thr0u9h a dynam0 pr0ce55 1n a m01ten, e1ectr1ca11y-c0nduct1n9 c0re, a5 0ccur5 1n the Earth and pr06a61y dur1n9 the pa5t 1n the M00n (a1th0u9h th15 15 current1y 50mewhat c0ntr0ver51a1 [4]). 7he pre5ent Mart1an ma9net1c f1e1d 15 very weak and 15 n0t we11 character12ed [5], and any ev1dence 0f a pa5t Mart1an f1e1d 15 p0tent1a11y 0f 1ntere5t and re1evant t0 the p1anet•5 h15t0ry and 5tructure. 1n the pre5ent 5tudy the ma9net1c pr0pert1e5 0f
160
fra9ment5 0f tw0 5her90tt1te5 fr0m the Antarct1c have 6een 1nve5t19ated, EE7A 79001 (fr0m E1ephant M0ra1ne) and A L H A 77005 (fr0m A1an H1115). 7hree adjacent fra9ment5, taken fr0m 51te5 w1th1n the mete0r1te rem0te fr0m any fu510n cru5t, were ava11a61e fr0m each mete0r1te. 7here 15 n0 ev1dence 0f 519n1f1cant terre5tr1a1 weather1n9 0f the mete0r1te5.
- N~M (x10-6Am 2 k9 1 )
EE7A
7900%
1
- ~ ,167 •
~,~.
......~;
~.... A......................
2. Remanent ma9net15m •
A
•
20
•
~
F1ELD { m 7 )
2.1. E E 7 A 79001
•
8•0
•
~06~
,60
~
~
7EMP (0C)
F19. 1. A1ternat1n9 f1e1d dema9net12at10n 0f ch1p5 167 and 169 fr0m EE7A 79001, and therma1 dema9net12at10n 0f 167.
7hree d15t1nct 11th01091e5 are pre5ent 1n th15 mete0r1te, the fra9ment5 5tud1ed 1n th15 paper c0m1n9 fr0m 11th0109y A (ma1n1y pyr0xene and 011v1ne) wh1ch c0n5t1tute5 the 6u1k 0f the r0ck [8]. 7he three fra9ment5 are un1f0rm 1n appearance, and are dark 9rey 1n c010ur w1th a few 5ma11 ( - 1 mm) 61ack15h c1a5t5. 7he N R M 1nten51ty and 1n1t1a1 5u5cept16111ty 0f the fra9ment5 are 5h0wn 1n 7a61e 1. W1th1n the 11m1t 5et 6y err0r5 1n mutua1 0r1entat10n 0f the fra9ment5 the N R M 1n them 15 un1f0rm 1n d1rect10n. 5amp1e5 167 and 169 were dema9net12ed 1n an a1ternat1n9 f1e1d up t0 99 m7, and the re5u1t5 are 5h0wn 1n F195. 1 and 2. 7he N R M 15 5ta61e 1n d1rect10n 1n 60th 5amp1e5 and a hard c0mp0nent 0f a60ut 0ne-th1rd 0f the 1n1t1a1 1nten51ty rema1n5 1n each after 99 m 7 dema9net12at10n. 7he N R M rema1n1n9 1n 167 after dema9net12at10n wa5 therma11y dema9net12ed (1n a1r). 7he max1mum 610ck1n9 temperature 15 - 4 0 0 ° C (F19. 1), and n0 further chan9e 1n N R M d1rect10n 15 apparent up t0 th15 temperature (F19. 2). 7hu5,
the5e fra9ment5 0f EE7A 79001 appear t0 p055e55 a 51n91e, h19h1y 5ta61e c0mp0nent 0f NRM. 8ef0re therma1 dema9net12at10n 0f 167, v15c0u5 ma9net12at10n (VRM) ac4u151t10n wa5 te5ted 1n 167 and 169, 1n an am61ent f1e1d 0f 100 /~7. Ac4u151t10n wa5 109ar1thm1c w1th t1me w1th an avera9e c0eff1c1ent 0f - 1.0 × 1 0 - 6 A m 2 k9 1 per decade. F19. 3 5h0w5 the ac4u151t10n 0f anhy5teret1c ma9net12at10n (ARM) 1n 167 and 169 1n a 61a51n9 f1e1d 0f 180 ~7. 7he 61a51n9 f1e1d wa5
7A8LE 1 Ma9net1c pr0pert1e5 0f EE7A 79001 and ALHA 77005 5amp1e
Ma55 (9)
NRM ( x 10 6 A m2 k9 1)
1n1t1a1 5u5cept16111ty ( × 1 0 6 m.~ k9 t)
EE7A 79001, 167 168 169
1.41 1.05 0.80
32.3 30.7 26.0
0.52 0.47 0.43
0.83 0.19 0.35
25.5 7.8 34.1
3.8 2.4 4.2
ALHA 77005, 65 66.1 66.2
F19. 2. D1rect10na1 chan9e5 dur1n9 a1ternat1n9 f1e1d ( a ) and therma1 dema9net12at10n (6) 0f 167 and a1ternat1n9 f1e1d dema9net12at10n 0f 169. 7he ch1p5 were n0t mutua11y 0r1ented f0r the5e te5t5. 7he chan9e 1n the d1rect10n 0f NRM 0f 167 pr10r t0 therma1 dema9net12at10n wa5 cau5ed 6y d1ff1cu1ty 1n accurate re-0r1entat10n 0f the ch1p 1n a d1fferent h01der.
161 19C
~0~e
167
0/0/0~
A~ M
(x10-6Arn2
k9-1 )
/
169
0~
// / 20
~0
6;
86
1;0
F1ELD ( r n 7 )
F19. 3. Ac4u151t10n 0f anhy5tere11c remanent ma9net12at10n (ARM) 1n 167 and 169 1n a 61a51n9 f1e1d 0f 180 F7.
fra9ment5 are 5h0wn 1n 7a61e 1. 0n1y tw0 0f the fra9ment5, 65 and 66.2, c0u1d 6e mutua11y 0r1ented and the1r N R M d1rect10n5 were 5eparated 6y appr0x1mate1y 50 °. F19. 4a 5h0w5 a.f. dema9net12at10n 0f 66.2 dur1n9 wh1ch there wa5 n0 519n1f1cant chan9e 1n N R M d1rect10n, and a150 a decay curve 0f an0ther 5amp1e 0f 77005 mea5ured 6y Na9ata [6]. 7h15 1atter fra9ment c1ear1y p055e55e5 a harder c0mp0nent 0f N R M than 66.2. VRM ac4u151t10n 6y 66.2 15 rap1d ( - 2 0 × 10 6 A m 2 k9 -1 per decade, F19. 46) and 1t 15 11ke1y that the d0m1nant c0mp0nent 0f N R M 1n th15 5amp1e 15 a VRM 0f 5h0rt t1me c0n5tant ac4u1red 1n 10ca1 1a60rat0ry f1e1d5. 5amp1e 66.1 ha5 a 51m11ar1y h19h v15c051ty c0eff1c1ent.
3. R0ck ma9net15m 3.1. EE7A 79001
pr0v1ded 6y an array 0f 5ma11 ma9net5 1n the wa11 0f the 5amp1e h01der, wh1ch wa5 tum61ed dur1n9 A R M ac4u151t10n. 7he 5hape 0f the A R M curve5 5u99e5t5 a c0n51dera61e pr0p0rt10n 0f 51n91e-d0ma1n part1c1e5 am0n9 the ma9net1c carr1er5, f0r wh1ch there 15 a150 ev1dence fr0m 0ther mea5urement5 (5ee 5ect10n 2).
2.2. ALHA 77005 7he 5amp1e5 0f th15 mete0r1te are dark 9rey61ack 1n c010ur, f1ne-9ra1ned and 0f un1f0rm 11th0109y. N R M and 1n1t1a1 5u5cept16111ty 0f the three
ALHA •
77005, 66.2
7herma1 dema9net12at10n 0f the N R M 0f 167 1nd1cated the pre5ence 0f a ma9net1c carr1er w1th a max1mum 610ck1n9 temperature 0f 350 400°C, and th15 15 c0nf1rmed 6y mea5urement5 0f 1nduced ma9net12at10n (J~) and 5aturat10n remanent ma9net12at10n (Jr~) a9a1n5t temperature (F19. 5). 7he re1at1ve1y 5ma11 departure 0f the J~-7 curve (F19. 5a) fr0m that expected f0r a parama9net1c carr1er ref1ect5 the 5ma11 pr0p0rt10n 0f ••ferr0ma9net1c•• mater1a1 pre5ent, 6ut tw0 Cur1e temperature5 at 350°C and 600°C are apparent. 7he5e are a150 5een 1n the Jr~-7 decay curve (F19. 56). 80th the5e curve5 are 06ta1ned 6y heat1n9 the 5amp1e 1n a1r. 7he 1nduced ma9net12at10n curve (J~-8) a150 re-
vRM
(x10-6Am2
k9 -1 ) £E7A
79001
K. . . . -
~,~1...1~.
~
ParQrr~9~t,c decay
Jr5 - 7
8 = 390 m7
: 20
%.~. c61
• F1ELD ( m 7 )
71ME (H175)
F19. 4. A1ternat1n9 f1e1d dema9net12at10n and VRM ac4u151t10n 1n 100 #7 f0r 77005, 66.1.
2~0
•
,~0
•
6~0 ~ 7EMPERA7URE
•
2~
•
,~0
(0C)•
F19. 5. 7herma1 decay 0f 1nduced ma9net12at10n (J1 and 5aturated remanent ma9net12at10n (Jr~) f0r EE7A 79001.
162
f1ect5 the 5ma11 am0unt 0f ferr0ma9net1c mater1a1 pre5ent, 5h0w1n9 0n1y a 5ma11 departure fr0m the 5tra19ht 11ne expected fr0m a parama9net1c mater1a1. 7he 1n1t1a1 5u5cept16111ty 0f the 5amp1e5 (7a61e 1) can rea50na61y 6e acc0unted f0r 6y the parama9net1c 1r0n c0ntent. 1t can 6e 5h0wn [7] that each 1% 6y we19ht 0f Fe 2+ and Fe 3+, expre55ed a5 wt.% 0f F e 0 and Fe20 3, c0ntr16ute5 2.07 × 10 ~ and 2.26 x 10 8 m 5 k9 1 re5pect1ve1y t0 1n1t1a1 5u5cept16111ty. 7ak1n9 an appr0x1mate mean va1ue 0f 2.1 x 10 5 m 3 k 9 - L the 065erved 5u5cept16111ty 0f a60ut 0.45 x 10 --6 m 3 k9 -1 0f the three 79001 5amp1e5 (7a61e 1) c0rre5p0nd5 t0 a we19ht c0ntent 0f ( F e 0 + Fe203) 0f a60ut 21%. 7h15 c0mpare5 w1th an avera9e c0ntent 0f a60ut 20% F e 0 4u0ted 6y 5tee1e and 5m1th [8]. 7he 510pe 0f the upper (5tra19ht) part 0f the J1-8 curve f0r ch1p 169 c0rre5p0nd5 t0 a 5u5cept16111ty 0f 0.44 x 10- 6 m 5 k9 1, c0mpared w1th an 1n1t1a1 5u5cept16111ty 0f 0 . 4 3 x 1 0 6 m 5 k9 1, a re5u1t wh1ch 15 a150 c0n515tent w1th parama9net1c 1r0n 6e1n9 the d0m1nant c0ntr16ut10n t0 5u5cept16111ty 1n the5e 5amp1e5.
3.2. ALHA 77005 7he n0n-parama9net1c c0ntent 0f the 5amp1e5 0f th15 mete0r1te 15 a9a1n very 5ma11, a5 1nd1cated
.
66.1 8 = 480m7
77005,
80
8
x10-3Am2.k9-1) 98 1L Jr5
77005, 661
7
9
3
e~e~0~0~e ~
2 1 0
1
°~.0~0 2~0
•
4~0
•
7EMPERA7URE(°C)
~6~0
F19. 7. Decay 0f 5atura1ed 1RM 0f 77005. 66.1 w1th temperature.
6y the a1m05t parama9net1c decay 0f 1nduced ma9net12at10n w1th temperature (F19. 6) and a near11near ma9net12at10n curve. 7here 15 n0 ev1dence 0f a 10w-temperature Cur1e p01nt 1n the J~-7 curve, and the pre5ence 0f a 51n91e ma9net1c carr1er, pr06a61y ma9net1te 15 c0nf1rmed 6y the decay w1th temperature 0f 5aturated remanence (F19. 7). A1th0u9h the ma9net1c pr0pert1e5 0f 77005 are d0m1nant1y parama9net1c, there 15 a marked d15crepancy 6etween 1n1t1a1 5u5cept16111ty 0f ch1p 66.1, 2.4 x 10 6 m 3 k9 1, and the h19h-f1e1d 5u5cept16111ty der1ved fr0m the 510pe 0f the upper part 0f the J1-8 Curve 0f 0.44 × 10 6 m 3 k9-1. 7h15 re1at1ve1y h19h 1n1t1a1 5u5cept16111ty 5u99e5t5 the pre5ence 0f 5uperparama9net1c mater1a1 1n the5e 5amp1e5, and the h19h ma9net1c v15c051ty 065erved 15 c0n515tent w1th a 5u65tant1a1 pr0p0rt10n 0f the ma9net1c 9ra1n5 6e1n9 0n the 5uperparama9net1c-51n91e d0ma1n 60undary.
20
4. D15cu5510n
0
1
~;0
1 ~;0 • ~0 7EMPERA7URE(°C)
1 #00
F19. 6. 7herma1 decay 0f 1nduced ma9net12at10n 1n A L H A 77005, 66. 7 h e 6r0ken 11ne repre5ent5 parama9net1c decay 0f the 1n1t1a1 va1ue 0f J1- 7he 5119ht r15e 1n •]1 at - 6 0 0 ° C 15 pr06a61y 519n1f1cant.
7here 15 5tr0n9 ev1dence 0f a 51n91e, 5ta61e c0mp0nent 0f N R M 1n the 5amp1e5 0f EE7A 79001. 1t5 m05t 11ke1y 0r191n 15 thr0u9h an am61ent ma9net1c f1e1d at the t1me 0f f0rmat10n 0f the mete0r1te mater1a1 0n the parent 60dy, 0r p055161y at a 1ater t1me when the mete0r1te exper1enced the
163 we11-d0cumented 5h0ck event [9]. At th15 5ta9e 0f the 1nve5t19at10n5 1t 15 n0t p055161e t0 c1ear1y d15t1n9u15h 6etween the tw0 p055161e 0r191n5, 6ut the 1mp0rtant fact0r 15 that an am61ent ma9net1c f1e1d 15 re4u1red whether the pr0ce55 6e therm0remanent 0r 5h0ck ma9net12at10n. (7he p05516111ty 0f ma9net12at10n 1n a tran51ent ma9net1c f1e1d 9enerated dur1n9 a 5h0ck event, a pr0ce55 wh1ch ha5 6een pr0p05ed f0r the 0r191n 0f the N R M 0f 50me 1unar r0ck5, 15 5t111 5pecu1at1ve and 15 n0t c0n51dered further here.) A55um1n9 that the mete0r1te fe11 at a t1me 0f the 0rder 0f 104-105 year5 a90, 1t 15 p055161e that appr0x1mate1y 0ne-th1rd 0f the N R M 1nten51ty c0u1d ar15e fr0m a v15c0u5 ma9net12at10n ac4u1red 1n the earth•5 ma9net1c f1e1d. H0wever, th15 15 un11ke1y t0 6e the extreme1y hard c0mp0nent rema1n1n9 after a1ternat1n9 f1e1d (a.f.) dema9net12at10n. A150, 1t 15 un11ke1y that the VRM and pr1mary N R M w0u1d 6e 1n the 5ame d1rect10n (n0 519n1f1cant chan9e 1n N R M d1rect10n 15 065erved dur1n9 a.f. 0r therma1 dema9net12at10n) and 1t 5eem5 pr06a61e that the mete0r1te w0u1d under90 50me rand0m var1at10n 1n 1t5 0r1entat10n w1th re5pect t0 the ma9net1c f1e1d dur1n9 1t5 6ur1a1 and m19rat10n 1n 5n0w and 1ce and 5u65e4uent re-emer9ence 0n the 5urface, tend1n9 t0 rand0m12e the d1rect10n 0f VRM and reduce 1t5 1nten51ty. 1t ha5 0n1y 6een p055161e t0 make an appr0x1mate e5t1mate 0f the 5tren9ht 0f the anc1ent ma9net121n9 f1e1d. Dur1n9 an attempted 7he111er-7he111er pa1ae01nten51ty determ1nat10n 0n EE7A 79001, 168 a pr061em ar05e 1n that a P 7 R M ac4u1red 1n c0011n9 fr0m a certa1n temperature wa5 apparent1y n0t c0mp1ete1y rem0ved 0n re-heat1n9 t0 the next h19her temperature. Further te5t5 0f th15 techn14ue are env15a9ed. 8ecau5e 0f the d1ff1cu1ty 0f the per515tence 0f a 5u65tant1a1 pr0p0rt10n 0f the N R M a9a1n5t a.f. dema9net12at10n, the A R M pa1ae01nten51ty techn14ue [10] cann0t 6e u5ed re11a61y 0n the5e 5amp1e5. 7he techn14ue 15 a150 unpr0ven f0r r0ck5 1n wh1ch ma9net1te (0r tatan0ma9net1te) 15 carry1n9 the remanence, and exper1menta1 te5t5 have n0t 6een enc0ura91n9 (A. 5tephen50n, pr1vate c0mmun1cat10n and [11]). 7he rat10 0f N R M t0 5aturated 1RM, the 1atter 4uant1ty 6e1n9 u5ed a5 a ••n0rma112at10n•• fact0r pr0p0rt10na1 t0 ma9net1c carr1er c0ntent, 15 appr0x1mate1y 2.0 x 10 ~, wh1ch 15 10w. C0mpared w1th 51m11ar rat105 ca1cu1ated f0r
many 1unar 5amp1e5, th15 va1ue c0rre5p0nd5 t0 an upper 11m1t 0f the ma9net121n9 f1e1d 0f the 0rder 0f 1-10 1~7 [12]. A 51m11ar N R M / 1 R M rat10 ( - 1.0 x 10 -5) wa5 f0und 1n an0ther E E 7 A 79001 5amp1e 6y C150w5k1 [13]~ A1th0u9h 0n1y e5t1mate5 are p055161e, th15 meth0d, the rat10 0f 5aturated A R M t0 N R M and 50me 10w temperature data fr0m the 7he111er-7he111er techn14ue a11 5u99e5t a pa1ae01nten51ty 0f - 1 0 /~7 0r 1e55 f0r EE7A 79001. N0 pa1ae01nten51ty data are der1va61e fr0m the N R M 0f the 5amp1e5 0f A L H A 77005 5tud1ed here. Na9ata [6] 5h0w5 an apparent1y va11d pa1ae01nten51ty re5u1t fr0m an0ther ch1p 0f 77005, u51n9 an A R M meth0d, 1nd1cat1n9 a ma9net121n9 f1e1d 0f - 1 /~7, 6ut f0r the rea50n5 5tated a60ve th15 5h0u1d pr06a61y a9a1n 6e re9arded a5 at 6e5t 0n1y an e5t1mate. 7he Cur1e p01nt near 600°C 5h0wn 6y 60th mete0r1te5 15 m05t pr06a61y due t0 ma9net1te. 7he 10wer Cur1e p01nt 0f 350-400°C f0und 1n th15 5tudy and 0ne at 325 ° 6y C150w5k1, 0r p055161y pyrrh0t1te (Fe51 + ,, where 0.10 ~< x ~< 0.14), w1th a Cur1e temperature u5ua11y 4u0ted at 300-320°C f0r x = 0.14 [14]. 0 n c0011n9, the 10w-temperature Cur1e p01nt wa5 much 1e55 marked, 5u99e5t1n9 p055161e 0x1dat10n 0r reh0m09en12at10n 0f t1tan0ma9net1te (5. C150w5k1, pr1vate c0mmun1cat10n) 0r a1terat10n 0f pyrrh0t1te. 50me recent mea5urement5 6y the auth0r 0n natura1 pyrrh0t1te 5h0w that 50me therma1 a1terat10n appear5 t0 0ccur 0n heat1n9 t0 600°C 1n a1r, re5u1t1n9 1n a p00r1y def1ned Cur1e p01nt 0n c0011n9. 1r0n 5u1ph1de (0f un5pec1f1ed c0mp051t10n) 15 rep0rted a5 6e1n9 pre5ent 1n E E 7 A 79001 6y 5tee1e and 5m1th [8] and W00d and A5hwa1 [15]. Muen0w and 600d1n9 [16] 4u0te the 5u1phur c0ntent 0f 11th0109Y A 0f 79001 a5 0.16%, and 5tate that 1t 15 • und0u6ted1y 1n the f0rm 0f a 5u1ph1de••. Var10u5 11ne5 0f ev1dence (ma9n1tude 0f 1n1t1a1 5u5cept16111ty and 5aturat10n 1nduced ma9net12at10n, J 1 - 7 curve5) 1nd1cate that the c0ntent 0f ma9net1c m1nera15 15 <0.1% 1n the5e 5amp1e5 0f EE7A 79001, and thu5 the 5u1phur c0ntent 15 5uff1c1ent f0r an appr0pr1ate am0unt 0f pyrrh0t1te t0 6e pre5ent. Fr0m the therma1 dema9net12at10n decay cure the 6u1k 0f the N R M 0f EE7A 79001 appear5 t0 6e carr1ed 6y the 10w Cur1e p01nt m1nera1. 7h15 may 1nd1cate that a temperature n0t 9reater than
164 4 0 0 ° C ex15ted dur1n9 the ma9net121n9 event, 6ut c0nf1rmat10n 0f the a 6 0 v e re5u1t 1n 0 t h e r 5amp1e5 15 de51ra61e. 7 h e ma9net1c pr0pert1e5 0f a n 0 t h e r ch1p f r 0 m 11th0109y A 0f E E 7 A 79001 h a v e 6 e e n 5tud1ed 6 y C150w5k1 [13,141. 7 h e pr0pert1e5 0f the N R M 1nc1ud1n9 h19h c0erc1v1ty, are e55ent1a11y 51m11ar t0 th05e 065erved 1n the pre5ent 5tudy, a1th0u9h C150w5k1 rep0rt5 50me m 0 v e m e n t 0f the r e m a n e n c e v e c t 0 r a 6 0 v e 30 m 7 dema9net121n9 f1e1d, w1th m 0 r e rap1d d e c a y 0f 1nten51ty.
5. C0nc1u510n5 1t 15 c1ear that there 15 50me var1a6111ty 0f ma9net1c pr0pert1e5 a m 0 n 9 d1fferent 5amp1e5 0f e a c h 0f the5e tw0 mete0r1te5, part1cu1ar1y 1n A L H A 7 7 0 0 5 . 7 h e ne9at1ve re5u1t5 06ta1ned 1n the pre5ent 5tudy 0f th15 5her90tt1te d0 n0t ru1e 0ut the p05516111ty 0f the ex15tence 0f a ma9net121n9 f1e1d at 50me 5ta9e dur1n9 1t5 h15t0ry, a5 5u99e5ted 6y the re5u1t5 0f N a 9 a t a [6] 06ta1ned f r 0 m a n 0 t h e r ch1p 0f 77005. 1n that 5amp1e pre5uma61y a 1ar9er pr0p0rt10n 0f ma9net1c 9ra1n5 are a 6 0 v e the 11m1t1n9 5 u p e r p a r a m a 9 n e t 1 c 512e a n d a61e t0 reta1n a 5ta61e r e m a n e n c e . 7 h e character15t1c5 0f the N R M 0f the ch1p5 f r 0 m E E 7 A 79001 5tr0n91y 5u99e5t the p r e 5 e n c e 0f a w e a k am61ent ma9net1c f1e1d dur1n9 the 0r191na1 c0011n9 0f the mete0r1te 0r dur1n9 the 1ater (180 m.y.) 5h0ck e v e n t [9]. A1th0u9h pa1ae0f1e1d ev1d e n c e f r 0 m ma9net1c 5tud1e5 0f 5 h e r 9 0 t t y 5amp1e5 15 at pre5ent am619u0u5 [16], there 15 e n 0 u 9 h ev1dence n0w accumu1at1n9 fr0m 0ther 5NC mete0r1te5, 1nc1ud1n9 2a9am1 [17], t0 e n c 0 u r a 9 e f u r t h e r ma9net1c 5tud1e5 w1th a v1ew t0 e5ta6115h1n9 the pre5ence 0r 0therw15e 0f a ma9net121n9 f1e1d 0n the p a r e n t 6 0 d y . 1f the p a r e n t 6 0 d y 15 1ndeed c0nf1rmed a5 Mar5, t h e n the 0 u t c 0 m e 0f the 5tud1e5 c0u1d 6e part1cu1ar1y reward1n9.
Ackn0w1ed9ement5 7 h e a u t h 0 r 15 1nde6ted t0 the Mete0r1te W 0 r k 1n9 6 r 0 u p , J 0 h n 5 0 n 5 p a c e C e n t r e , f0r acce55 t0 the Antarct1c mete0r1te 5amp1e5. 7 h a n k 5 are d u e t0 A. 5 t e p h e n 5 0 n f0r u5efu1 d15cu5510n, a n d t0 a n 0 n y m 0 u 5 referee5 f0r 5u99e5t10n5 f0r 1mpr0v1n9 the paper.
Reference5 1 A.K. 8a1rd and 8.C. C1ark, 0n the 0r191na1 19ne0u5 50urce 0f Mart1an f1ne5,1caru545, 113 123, 1981. 2 J.C. Lau1, 7he 5her90tty c0n50rt1um: an 0verv1ew, 5u6m1tted t0 6e0ch1m. C05m0ch1m. Acta, 1985. 3 D.D. 809ard, E.E. Ny4u15t and P. J0hn50n, N061e 9a5 c0ntent5 0f 5her90n1te5 and 1mp11cat10n5 f0r the Mart1an 0r191n 0f 5NC mete0r1te5, 6e0ch1m. C05m0ch1m. Acta 48, 1723-1739, 1984. 4 D.W. C0111n50n, Pa5t and pre5ent ma9net15m 0f the M00n, 6e0phy5. 5urv. 7. 57-74, 1984. 5 C.7. Ru55e11, P1anetary ma9net15m, Rev. 6e0phy5. 5pace. Phy5. 17. 295 301, 1979. 6 7. Na9ata, Pa1ae0ma9net15m 0f Antarct1c ach0ndr1te5, Mem. Nat1. 1n5t. P01ar Re5., 5pec. 155ue N0. 17, 233 242, 1980. 7 D.W. C0111n50n, An e5t1mate 0f the haemat1te c0ntent 0f 5ed1ment5 6y ma9net1c ana1y515, Earth P1anet. 5cL Lett. 4, 417 420, 1969. 8 1.M. 5tee1e and J.V. 5m1th, M1nera109y 0f E1ephant M0ra1ne EE7A 79001 tw0-c0mp0nent ach0ndr1te w1th re5em61ance5 t0 5her90tty. 1n: Lunar and P1anetary 5c1ence X111, pp. 764-765, 1982. 9 J. W00den, C.-Y, 5h1h, L. Ny4u15t, 8. 8an5e1, H. W1e5mann and 6. McKay, R6-5r and 5m-Nd 150t0p1c c0n5tra1nt5 0n the 0r191n 0f [•;E7A 79001: a 5ec0nd Antarct1c 5her90tt1te. 1n: Lunar and P1anetary 5c1ence X111, pp. 879 880, 1982. 10 A. 5tephen50n and D.W. C0111n50n, Lunar ma9net1c f1e1d pa1ae01nten51t1e5 determ1ned 6y an anhy5teret1c remanent ma9net12at10n meth0d, Earth P1anet. 5c1. ken. 23, 220 228, 1974. 11 D.J. Dun10p. M.E. 8a11ey and M.F. We5tc0tt-Eew15, Lunar pa1ae01nten51ty determ1nat10n u51n9 anhy5teret1c remanence (ARM): a cr1t14ue, Pr0c. 6th Lunar 5c1. C0nf. 3, pp. 3063-3069. 1975. 12 5.M. C150w5k1, D.W. C0111n50n, A. 5tephen50n. 5.K. Runc0rn and M. Fu11er, A rev1ew 0f 1unar pa1ae01nten51ty data and 1mp11cat10n5 f0r the 0r191n 0f 1unar ma9net15m, Pr0c. 13th Lunar P1anet. 5c1. C0nf., J. 6e0phy5. Re5. 88, 691-704, 1983. 13 5.M. C150w5k1. Ma9net1c pr0pert1e5 and remanence 0f Antarct1c 5her90tt1te A79001, 1n: Lunar and P1anetary 5c1ence X111, p. 106, 1982. 14 F.J. 5chwar2, Ma9net1c pr0pert1e5 0f pyyrh0t1te and thc1r u5e m 9e0109y and 9e0phy51c5, 6e01. 5urv. Can., Pap. 74-59, pp. 1-23, 1974. 15 C.A. W00d and L.D. A5hwa1, 5NC mete0r1te5:19ne0u5 r0ck5 fr0m Mar5•, Pr0c. 12th Lunar P1anet. 5c1. C0nf., pp. 1359-1375, 1981. 16 D.W. Muen0w and J.L. 600d1n9, "•Mart1an•" v01at11e5 1n 5her90tt1te EE7A 79001: p055161e 519n1f1cance 0f 5ec0ndary m1nera15, 1n: Lunar and P1anetary 5c1ence XV1, pp. 593 594, 1985. 17 5.M. C150w5k1, Ma9net15m 0f 5her90tt1te mete0r1te5, 1n: Lunar and P1anetary 5c1ence XV1, 5upp1. A, pp. 9 10. 1985.
159
E15ev1er 5c1ence Pu6115her5 8.V., Am5terdam - Pr1nted 1n 7 h e Nether1and5
[4]
Ma9net1c pr0pert1e5 0f Antarct1c 5her90tt1te mete0r1te5 E E 7 A 79001 and A L H A 77005: p055161e 1 e1evance t0 a Mart1an ma9net1c f1e1d D.W. C0111n50n 1n5t1tute 0f Lunar and P1anetary 5c1ence, 5ch001 0f Phy51c5, 7he Un1ver51ty, Newca5t1e up0n 7yne NE1 7R U (En91and) Rece1ved 5eptem6er 18, 1985; rev15ed ver510n accepted January 6, 1986 7 h e p05516111ty that the parent 60dy 0f the 5NC mete0r1te5 15 Mar5 1mp11e5 that the ma9net1c pr0pert1e5 0f the5e mete0r1te5 may pr0v1de ev1dence c0ncern1n9 anc1ent Mart1an ma9net1c f1e1d5. E E 7 A 79001 p055e55e5 a weak, very 5ta61e pr1mary ma9net12at10n, the pr0pert1e5 0f wh1ch are c0n515tent w1th 1t5 ac4u151t10n 1n an am61ent ma9net1c f1e1d e1ther dur1n9 the mete0r1te•5 f0rmat10n 0r dur1n9 the 5evere 5h0ck event 1ater 1n 1t5 h15t0ry. 7 h e 5amp1e5 0f A L H A 77005 5tud1ed p055e55ed n0 mea5ura61e pr1mary ma9net12at10n: the 065erved remanence appear5 t0 6e a v15c0u5 ma9net12at10n ac4u1red 1n 10ca1 1a60rat0ry f1e1d5.7he ma9net1c carr1er5 1n the mete0r1te5 are f1ne-9ra1ned ma9net1te and a 10wer Cur1e p01nt m1nera1, pr06a61y t1tan0ma9net1te 0r pyrrh0t1te, pre5ent t0 the extent 0f 1e55 than 0.1% 6y we19ht. E5t1mate5 0f the 5tren9th 0f the ma9net121n9 f1e1d f0r EE7A 79001 are 1n the ran9e 1-10 ,tt7.
1. 1ntr0duct10n
0 n e 0f the m05t 1ntere5t1n9 recent deve10pment5 1n mete0r1te 5tud1e5 ha5 6een the 5tudy 0f the 5her90tt1te, nakh11te and cha5519n1te (5NC) ach0ndr1te mete0r1te5 and the emer9ence 0f ev1dence that the1r parent 60dy may 6e the p1anet Mar5.7h15 ev1dence 1nc1ude5 the c105e 51m11ar1ty0f the 5her90tty and EE7A 79001 maj0r e1ement c0mp051t10n w1th that 0f the Mart1an 5urface mater1a1 a5 mea5ured 6y the V1k1n9 5pacecraft [1,2], h19h 150t0p1c rat105 1n trapped n061e 9a5e5 (e.9. 4°Ar/36Ar, 129Xe/132Xe) and 1n n1tr09en (15 N/14N) 1n EE7A 79001, c0mpara61e w1th th05e f0und 1n the Mart1an atm05phere [3], and the1r an0ma10u51y y0un9 a9e5 0f - 1 . 3 6.y. 0r 1e55 [2,15]. 7here 15 5t111 c0n51dera61e d15cu5510n a60ut the 519n1f1cance 0f the data, wh1ch 15 d1ff1cu1t f0r n0n-5pec1a115t5 t0 a55e55, 6ut 1f the mete0r1te5 are 1ndeed 0f Mart1an 0r191n, the1r 9e01091ca1, m1nera1091ca1 and phy51ca1 pr0pert1e5 c1ear1y 0ffer the 0pp0rtun1ty 0f der1v1n9 further ev1dence f0r Mart1an h15t0ry and ev01ut10nary pr0ce55e5. 7he 5tudy 0f the weak permanent ma9net12at10n (natura1 remanent ma9net12at10n, NRM) p055e55ed 6y terre5tr1a1 and extraterre5tr1a1 r0ck5 15 a p0werfu1 t001 f0r pr0v1d1n9 ev1dence 0f the ex15tence 0f ma9net1c f1e1d5 at the t1me 0f 0r191n 0f 0012-821X/86/$03.50
• 1986 E15ev1er 5c1ence Pu6115her5 8.V.
the r0ck5 and a150 dur1n9 519n1f1cant event5 1n the1r h15t0ry, 5uch a5 5evere metam0rph15m and 5h0ck. 1n part1cu1ar, the pa1ae0ma9net15m 0f mete0r1te5 may 1n pr1nc1pa1 pr0v1de 1nf0rmat10n 0n ma9net1c f1e1d5 1n the ear1y 501ar ne6u1a (v1a 5tud1e5 0f the N R M 0f ch0ndru1e5) and 0f ma9net1c f1e1d5 a550c1ated w1th mete0r1te parent 60d1e5.7hu5 1n th15 re5pect the 5NC mete0r1te5 are 0f c0n51dera61e 1ntere5t 1n 0ffer1n9 the p05516111ty 0f 06ta1n1n9 ev1dence 0f ma9net1c f1e1d5 at 50me per10d 0f Mart1an h15t0ry. 7he t1me 0f ac4u151t10n 0f any N R M 15 11ke1y t0 6e that 0f the f1na1 c0011n9 0f the mete0r1t1c mater1a1 6e10w the Cur1e temperature 0f the c0n5t1tuent ma9net1c m1nera15, 0r that 0f the 5h0ck event re5p0n5161e f0r the eject10n 0f the mete0r1te fr0m the Mart1an 5urface 1nt0 an eventua1 Earth 0r61t-cr0551n9 traject0ry. 7he 519n1f1cance 0f p1anetary ma9net1c f1e1d5 15, 0f c0ur5e, 1n the1r p055161e 0r191n thr0u9h a dynam0 pr0ce55 1n a m01ten, e1ectr1ca11y-c0nduct1n9 c0re, a5 0ccur5 1n the Earth and pr06a61y dur1n9 the pa5t 1n the M00n (a1th0u9h th15 15 current1y 50mewhat c0ntr0ver51a1 [4]). 7he pre5ent Mart1an ma9net1c f1e1d 15 very weak and 15 n0t we11 character12ed [5], and any ev1dence 0f a pa5t Mart1an f1e1d 15 p0tent1a11y 0f 1ntere5t and re1evant t0 the p1anet•5 h15t0ry and 5tructure. 1n the pre5ent 5tudy the ma9net1c pr0pert1e5 0f
160
fra9ment5 0f tw0 5her90tt1te5 fr0m the Antarct1c have 6een 1nve5t19ated, EE7A 79001 (fr0m E1ephant M0ra1ne) and A L H A 77005 (fr0m A1an H1115). 7hree adjacent fra9ment5, taken fr0m 51te5 w1th1n the mete0r1te rem0te fr0m any fu510n cru5t, were ava11a61e fr0m each mete0r1te. 7here 15 n0 ev1dence 0f 519n1f1cant terre5tr1a1 weather1n9 0f the mete0r1te5.
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F19. 1. A1ternat1n9 f1e1d dema9net12at10n 0f ch1p5 167 and 169 fr0m EE7A 79001, and therma1 dema9net12at10n 0f 167.
7hree d15t1nct 11th01091e5 are pre5ent 1n th15 mete0r1te, the fra9ment5 5tud1ed 1n th15 paper c0m1n9 fr0m 11th0109y A (ma1n1y pyr0xene and 011v1ne) wh1ch c0n5t1tute5 the 6u1k 0f the r0ck [8]. 7he three fra9ment5 are un1f0rm 1n appearance, and are dark 9rey 1n c010ur w1th a few 5ma11 ( - 1 mm) 61ack15h c1a5t5. 7he N R M 1nten51ty and 1n1t1a1 5u5cept16111ty 0f the fra9ment5 are 5h0wn 1n 7a61e 1. W1th1n the 11m1t 5et 6y err0r5 1n mutua1 0r1entat10n 0f the fra9ment5 the N R M 1n them 15 un1f0rm 1n d1rect10n. 5amp1e5 167 and 169 were dema9net12ed 1n an a1ternat1n9 f1e1d up t0 99 m7, and the re5u1t5 are 5h0wn 1n F195. 1 and 2. 7he N R M 15 5ta61e 1n d1rect10n 1n 60th 5amp1e5 and a hard c0mp0nent 0f a60ut 0ne-th1rd 0f the 1n1t1a1 1nten51ty rema1n5 1n each after 99 m 7 dema9net12at10n. 7he N R M rema1n1n9 1n 167 after dema9net12at10n wa5 therma11y dema9net12ed (1n a1r). 7he max1mum 610ck1n9 temperature 15 - 4 0 0 ° C (F19. 1), and n0 further chan9e 1n N R M d1rect10n 15 apparent up t0 th15 temperature (F19. 2). 7hu5,
the5e fra9ment5 0f EE7A 79001 appear t0 p055e55 a 51n91e, h19h1y 5ta61e c0mp0nent 0f NRM. 8ef0re therma1 dema9net12at10n 0f 167, v15c0u5 ma9net12at10n (VRM) ac4u151t10n wa5 te5ted 1n 167 and 169, 1n an am61ent f1e1d 0f 100 /~7. Ac4u151t10n wa5 109ar1thm1c w1th t1me w1th an avera9e c0eff1c1ent 0f - 1.0 × 1 0 - 6 A m 2 k9 1 per decade. F19. 3 5h0w5 the ac4u151t10n 0f anhy5teret1c ma9net12at10n (ARM) 1n 167 and 169 1n a 61a51n9 f1e1d 0f 180 ~7. 7he 61a51n9 f1e1d wa5
7A8LE 1 Ma9net1c pr0pert1e5 0f EE7A 79001 and ALHA 77005 5amp1e
Ma55 (9)
NRM ( x 10 6 A m2 k9 1)
1n1t1a1 5u5cept16111ty ( × 1 0 6 m.~ k9 t)
EE7A 79001, 167 168 169
1.41 1.05 0.80
32.3 30.7 26.0
0.52 0.47 0.43
0.83 0.19 0.35
25.5 7.8 34.1
3.8 2.4 4.2
ALHA 77005, 65 66.1 66.2
F19. 2. D1rect10na1 chan9e5 dur1n9 a1ternat1n9 f1e1d ( a ) and therma1 dema9net12at10n (6) 0f 167 and a1ternat1n9 f1e1d dema9net12at10n 0f 169. 7he ch1p5 were n0t mutua11y 0r1ented f0r the5e te5t5. 7he chan9e 1n the d1rect10n 0f NRM 0f 167 pr10r t0 therma1 dema9net12at10n wa5 cau5ed 6y d1ff1cu1ty 1n accurate re-0r1entat10n 0f the ch1p 1n a d1fferent h01der.
161 19C
~0~e
167
0/0/0~
A~ M
(x10-6Arn2
k9-1 )
/
169
0~
// / 20
~0
6;
86
1;0
F1ELD ( r n 7 )
F19. 3. Ac4u151t10n 0f anhy5tere11c remanent ma9net12at10n (ARM) 1n 167 and 169 1n a 61a51n9 f1e1d 0f 180 F7.
fra9ment5 are 5h0wn 1n 7a61e 1. 0n1y tw0 0f the fra9ment5, 65 and 66.2, c0u1d 6e mutua11y 0r1ented and the1r N R M d1rect10n5 were 5eparated 6y appr0x1mate1y 50 °. F19. 4a 5h0w5 a.f. dema9net12at10n 0f 66.2 dur1n9 wh1ch there wa5 n0 519n1f1cant chan9e 1n N R M d1rect10n, and a150 a decay curve 0f an0ther 5amp1e 0f 77005 mea5ured 6y Na9ata [6]. 7h15 1atter fra9ment c1ear1y p055e55e5 a harder c0mp0nent 0f N R M than 66.2. VRM ac4u151t10n 6y 66.2 15 rap1d ( - 2 0 × 10 6 A m 2 k9 -1 per decade, F19. 46) and 1t 15 11ke1y that the d0m1nant c0mp0nent 0f N R M 1n th15 5amp1e 15 a VRM 0f 5h0rt t1me c0n5tant ac4u1red 1n 10ca1 1a60rat0ry f1e1d5. 5amp1e 66.1 ha5 a 51m11ar1y h19h v15c051ty c0eff1c1ent.
3. R0ck ma9net15m 3.1. EE7A 79001
pr0v1ded 6y an array 0f 5ma11 ma9net5 1n the wa11 0f the 5amp1e h01der, wh1ch wa5 tum61ed dur1n9 A R M ac4u151t10n. 7he 5hape 0f the A R M curve5 5u99e5t5 a c0n51dera61e pr0p0rt10n 0f 51n91e-d0ma1n part1c1e5 am0n9 the ma9net1c carr1er5, f0r wh1ch there 15 a150 ev1dence fr0m 0ther mea5urement5 (5ee 5ect10n 2).
2.2. ALHA 77005 7he 5amp1e5 0f th15 mete0r1te are dark 9rey61ack 1n c010ur, f1ne-9ra1ned and 0f un1f0rm 11th0109y. N R M and 1n1t1a1 5u5cept16111ty 0f the three
ALHA •
77005, 66.2
7herma1 dema9net12at10n 0f the N R M 0f 167 1nd1cated the pre5ence 0f a ma9net1c carr1er w1th a max1mum 610ck1n9 temperature 0f 350 400°C, and th15 15 c0nf1rmed 6y mea5urement5 0f 1nduced ma9net12at10n (J~) and 5aturat10n remanent ma9net12at10n (Jr~) a9a1n5t temperature (F19. 5). 7he re1at1ve1y 5ma11 departure 0f the J~-7 curve (F19. 5a) fr0m that expected f0r a parama9net1c carr1er ref1ect5 the 5ma11 pr0p0rt10n 0f ••ferr0ma9net1c•• mater1a1 pre5ent, 6ut tw0 Cur1e temperature5 at 350°C and 600°C are apparent. 7he5e are a150 5een 1n the Jr~-7 decay curve (F19. 56). 80th the5e curve5 are 06ta1ned 6y heat1n9 the 5amp1e 1n a1r. 7he 1nduced ma9net12at10n curve (J~-8) a150 re-
vRM
(x10-6Am2
k9 -1 ) £E7A
79001
K. . . . -
~,~1...1~.
~
ParQrr~9~t,c decay
Jr5 - 7
8 = 390 m7
: 20
%.~. c61
• F1ELD ( m 7 )
71ME (H175)
F19. 4. A1ternat1n9 f1e1d dema9net12at10n and VRM ac4u151t10n 1n 100 #7 f0r 77005, 66.1.
2~0
•
,~0
•
6~0 ~ 7EMPERA7URE
•
2~
•
,~0
(0C)•
F19. 5. 7herma1 decay 0f 1nduced ma9net12at10n (J1 and 5aturated remanent ma9net12at10n (Jr~) f0r EE7A 79001.
162
f1ect5 the 5ma11 am0unt 0f ferr0ma9net1c mater1a1 pre5ent, 5h0w1n9 0n1y a 5ma11 departure fr0m the 5tra19ht 11ne expected fr0m a parama9net1c mater1a1. 7he 1n1t1a1 5u5cept16111ty 0f the 5amp1e5 (7a61e 1) can rea50na61y 6e acc0unted f0r 6y the parama9net1c 1r0n c0ntent. 1t can 6e 5h0wn [7] that each 1% 6y we19ht 0f Fe 2+ and Fe 3+, expre55ed a5 wt.% 0f F e 0 and Fe20 3, c0ntr16ute5 2.07 × 10 ~ and 2.26 x 10 8 m 5 k9 1 re5pect1ve1y t0 1n1t1a1 5u5cept16111ty. 7ak1n9 an appr0x1mate mean va1ue 0f 2.1 x 10 5 m 3 k 9 - L the 065erved 5u5cept16111ty 0f a60ut 0.45 x 10 --6 m 3 k9 -1 0f the three 79001 5amp1e5 (7a61e 1) c0rre5p0nd5 t0 a we19ht c0ntent 0f ( F e 0 + Fe203) 0f a60ut 21%. 7h15 c0mpare5 w1th an avera9e c0ntent 0f a60ut 20% F e 0 4u0ted 6y 5tee1e and 5m1th [8]. 7he 510pe 0f the upper (5tra19ht) part 0f the J1-8 curve f0r ch1p 169 c0rre5p0nd5 t0 a 5u5cept16111ty 0f 0.44 x 10- 6 m 5 k9 1, c0mpared w1th an 1n1t1a1 5u5cept16111ty 0f 0 . 4 3 x 1 0 6 m 5 k9 1, a re5u1t wh1ch 15 a150 c0n515tent w1th parama9net1c 1r0n 6e1n9 the d0m1nant c0ntr16ut10n t0 5u5cept16111ty 1n the5e 5amp1e5.
3.2. ALHA 77005 7he n0n-parama9net1c c0ntent 0f the 5amp1e5 0f th15 mete0r1te 15 a9a1n very 5ma11, a5 1nd1cated
.
66.1 8 = 480m7
77005,
80
8
x10-3Am2.k9-1) 98 1L Jr5
77005, 661
7
9
3
e~e~0~0~e ~
2 1 0
1
°~.0~0 2~0
•
4~0
•
7EMPERA7URE(°C)
~6~0
F19. 7. Decay 0f 5atura1ed 1RM 0f 77005. 66.1 w1th temperature.
6y the a1m05t parama9net1c decay 0f 1nduced ma9net12at10n w1th temperature (F19. 6) and a near11near ma9net12at10n curve. 7here 15 n0 ev1dence 0f a 10w-temperature Cur1e p01nt 1n the J~-7 curve, and the pre5ence 0f a 51n91e ma9net1c carr1er, pr06a61y ma9net1te 15 c0nf1rmed 6y the decay w1th temperature 0f 5aturated remanence (F19. 7). A1th0u9h the ma9net1c pr0pert1e5 0f 77005 are d0m1nant1y parama9net1c, there 15 a marked d15crepancy 6etween 1n1t1a1 5u5cept16111ty 0f ch1p 66.1, 2.4 x 10 6 m 3 k9 1, and the h19h-f1e1d 5u5cept16111ty der1ved fr0m the 510pe 0f the upper part 0f the J1-8 Curve 0f 0.44 × 10 6 m 3 k9-1. 7h15 re1at1ve1y h19h 1n1t1a1 5u5cept16111ty 5u99e5t5 the pre5ence 0f 5uperparama9net1c mater1a1 1n the5e 5amp1e5, and the h19h ma9net1c v15c051ty 065erved 15 c0n515tent w1th a 5u65tant1a1 pr0p0rt10n 0f the ma9net1c 9ra1n5 6e1n9 0n the 5uperparama9net1c-51n91e d0ma1n 60undary.
20
4. D15cu5510n
0
1
~;0
1 ~;0 • ~0 7EMPERA7URE(°C)
1 #00
F19. 6. 7herma1 decay 0f 1nduced ma9net12at10n 1n A L H A 77005, 66. 7 h e 6r0ken 11ne repre5ent5 parama9net1c decay 0f the 1n1t1a1 va1ue 0f J1- 7he 5119ht r15e 1n •]1 at - 6 0 0 ° C 15 pr06a61y 519n1f1cant.
7here 15 5tr0n9 ev1dence 0f a 51n91e, 5ta61e c0mp0nent 0f N R M 1n the 5amp1e5 0f EE7A 79001. 1t5 m05t 11ke1y 0r191n 15 thr0u9h an am61ent ma9net1c f1e1d at the t1me 0f f0rmat10n 0f the mete0r1te mater1a1 0n the parent 60dy, 0r p055161y at a 1ater t1me when the mete0r1te exper1enced the
163 we11-d0cumented 5h0ck event [9]. At th15 5ta9e 0f the 1nve5t19at10n5 1t 15 n0t p055161e t0 c1ear1y d15t1n9u15h 6etween the tw0 p055161e 0r191n5, 6ut the 1mp0rtant fact0r 15 that an am61ent ma9net1c f1e1d 15 re4u1red whether the pr0ce55 6e therm0remanent 0r 5h0ck ma9net12at10n. (7he p05516111ty 0f ma9net12at10n 1n a tran51ent ma9net1c f1e1d 9enerated dur1n9 a 5h0ck event, a pr0ce55 wh1ch ha5 6een pr0p05ed f0r the 0r191n 0f the N R M 0f 50me 1unar r0ck5, 15 5t111 5pecu1at1ve and 15 n0t c0n51dered further here.) A55um1n9 that the mete0r1te fe11 at a t1me 0f the 0rder 0f 104-105 year5 a90, 1t 15 p055161e that appr0x1mate1y 0ne-th1rd 0f the N R M 1nten51ty c0u1d ar15e fr0m a v15c0u5 ma9net12at10n ac4u1red 1n the earth•5 ma9net1c f1e1d. H0wever, th15 15 un11ke1y t0 6e the extreme1y hard c0mp0nent rema1n1n9 after a1ternat1n9 f1e1d (a.f.) dema9net12at10n. A150, 1t 15 un11ke1y that the VRM and pr1mary N R M w0u1d 6e 1n the 5ame d1rect10n (n0 519n1f1cant chan9e 1n N R M d1rect10n 15 065erved dur1n9 a.f. 0r therma1 dema9net12at10n) and 1t 5eem5 pr06a61e that the mete0r1te w0u1d under90 50me rand0m var1at10n 1n 1t5 0r1entat10n w1th re5pect t0 the ma9net1c f1e1d dur1n9 1t5 6ur1a1 and m19rat10n 1n 5n0w and 1ce and 5u65e4uent re-emer9ence 0n the 5urface, tend1n9 t0 rand0m12e the d1rect10n 0f VRM and reduce 1t5 1nten51ty. 1t ha5 0n1y 6een p055161e t0 make an appr0x1mate e5t1mate 0f the 5tren9ht 0f the anc1ent ma9net121n9 f1e1d. Dur1n9 an attempted 7he111er-7he111er pa1ae01nten51ty determ1nat10n 0n EE7A 79001, 168 a pr061em ar05e 1n that a P 7 R M ac4u1red 1n c0011n9 fr0m a certa1n temperature wa5 apparent1y n0t c0mp1ete1y rem0ved 0n re-heat1n9 t0 the next h19her temperature. Further te5t5 0f th15 techn14ue are env15a9ed. 8ecau5e 0f the d1ff1cu1ty 0f the per515tence 0f a 5u65tant1a1 pr0p0rt10n 0f the N R M a9a1n5t a.f. dema9net12at10n, the A R M pa1ae01nten51ty techn14ue [10] cann0t 6e u5ed re11a61y 0n the5e 5amp1e5. 7he techn14ue 15 a150 unpr0ven f0r r0ck5 1n wh1ch ma9net1te (0r tatan0ma9net1te) 15 carry1n9 the remanence, and exper1menta1 te5t5 have n0t 6een enc0ura91n9 (A. 5tephen50n, pr1vate c0mmun1cat10n and [11]). 7he rat10 0f N R M t0 5aturated 1RM, the 1atter 4uant1ty 6e1n9 u5ed a5 a ••n0rma112at10n•• fact0r pr0p0rt10na1 t0 ma9net1c carr1er c0ntent, 15 appr0x1mate1y 2.0 x 10 ~, wh1ch 15 10w. C0mpared w1th 51m11ar rat105 ca1cu1ated f0r
many 1unar 5amp1e5, th15 va1ue c0rre5p0nd5 t0 an upper 11m1t 0f the ma9net121n9 f1e1d 0f the 0rder 0f 1-10 1~7 [12]. A 51m11ar N R M / 1 R M rat10 ( - 1.0 x 10 -5) wa5 f0und 1n an0ther E E 7 A 79001 5amp1e 6y C150w5k1 [13]~ A1th0u9h 0n1y e5t1mate5 are p055161e, th15 meth0d, the rat10 0f 5aturated A R M t0 N R M and 50me 10w temperature data fr0m the 7he111er-7he111er techn14ue a11 5u99e5t a pa1ae01nten51ty 0f - 1 0 /~7 0r 1e55 f0r EE7A 79001. N0 pa1ae01nten51ty data are der1va61e fr0m the N R M 0f the 5amp1e5 0f A L H A 77005 5tud1ed here. Na9ata [6] 5h0w5 an apparent1y va11d pa1ae01nten51ty re5u1t fr0m an0ther ch1p 0f 77005, u51n9 an A R M meth0d, 1nd1cat1n9 a ma9net121n9 f1e1d 0f - 1 /~7, 6ut f0r the rea50n5 5tated a60ve th15 5h0u1d pr06a61y a9a1n 6e re9arded a5 at 6e5t 0n1y an e5t1mate. 7he Cur1e p01nt near 600°C 5h0wn 6y 60th mete0r1te5 15 m05t pr06a61y due t0 ma9net1te. 7he 10wer Cur1e p01nt 0f 350-400°C f0und 1n th15 5tudy and 0ne at 325 ° 6y C150w5k1, 0r p055161y pyrrh0t1te (Fe51 + ,, where 0.10 ~< x ~< 0.14), w1th a Cur1e temperature u5ua11y 4u0ted at 300-320°C f0r x = 0.14 [14]. 0 n c0011n9, the 10w-temperature Cur1e p01nt wa5 much 1e55 marked, 5u99e5t1n9 p055161e 0x1dat10n 0r reh0m09en12at10n 0f t1tan0ma9net1te (5. C150w5k1, pr1vate c0mmun1cat10n) 0r a1terat10n 0f pyrrh0t1te. 50me recent mea5urement5 6y the auth0r 0n natura1 pyrrh0t1te 5h0w that 50me therma1 a1terat10n appear5 t0 0ccur 0n heat1n9 t0 600°C 1n a1r, re5u1t1n9 1n a p00r1y def1ned Cur1e p01nt 0n c0011n9. 1r0n 5u1ph1de (0f un5pec1f1ed c0mp051t10n) 15 rep0rted a5 6e1n9 pre5ent 1n E E 7 A 79001 6y 5tee1e and 5m1th [8] and W00d and A5hwa1 [15]. Muen0w and 600d1n9 [16] 4u0te the 5u1phur c0ntent 0f 11th0109Y A 0f 79001 a5 0.16%, and 5tate that 1t 15 • und0u6ted1y 1n the f0rm 0f a 5u1ph1de••. Var10u5 11ne5 0f ev1dence (ma9n1tude 0f 1n1t1a1 5u5cept16111ty and 5aturat10n 1nduced ma9net12at10n, J 1 - 7 curve5) 1nd1cate that the c0ntent 0f ma9net1c m1nera15 15 <0.1% 1n the5e 5amp1e5 0f EE7A 79001, and thu5 the 5u1phur c0ntent 15 5uff1c1ent f0r an appr0pr1ate am0unt 0f pyrrh0t1te t0 6e pre5ent. Fr0m the therma1 dema9net12at10n decay cure the 6u1k 0f the N R M 0f EE7A 79001 appear5 t0 6e carr1ed 6y the 10w Cur1e p01nt m1nera1. 7h15 may 1nd1cate that a temperature n0t 9reater than
164 4 0 0 ° C ex15ted dur1n9 the ma9net121n9 event, 6ut c0nf1rmat10n 0f the a 6 0 v e re5u1t 1n 0 t h e r 5amp1e5 15 de51ra61e. 7 h e ma9net1c pr0pert1e5 0f a n 0 t h e r ch1p f r 0 m 11th0109y A 0f E E 7 A 79001 h a v e 6 e e n 5tud1ed 6 y C150w5k1 [13,141. 7 h e pr0pert1e5 0f the N R M 1nc1ud1n9 h19h c0erc1v1ty, are e55ent1a11y 51m11ar t0 th05e 065erved 1n the pre5ent 5tudy, a1th0u9h C150w5k1 rep0rt5 50me m 0 v e m e n t 0f the r e m a n e n c e v e c t 0 r a 6 0 v e 30 m 7 dema9net121n9 f1e1d, w1th m 0 r e rap1d d e c a y 0f 1nten51ty.
5. C0nc1u510n5 1t 15 c1ear that there 15 50me var1a6111ty 0f ma9net1c pr0pert1e5 a m 0 n 9 d1fferent 5amp1e5 0f e a c h 0f the5e tw0 mete0r1te5, part1cu1ar1y 1n A L H A 7 7 0 0 5 . 7 h e ne9at1ve re5u1t5 06ta1ned 1n the pre5ent 5tudy 0f th15 5her90tt1te d0 n0t ru1e 0ut the p05516111ty 0f the ex15tence 0f a ma9net121n9 f1e1d at 50me 5ta9e dur1n9 1t5 h15t0ry, a5 5u99e5ted 6y the re5u1t5 0f N a 9 a t a [6] 06ta1ned f r 0 m a n 0 t h e r ch1p 0f 77005. 1n that 5amp1e pre5uma61y a 1ar9er pr0p0rt10n 0f ma9net1c 9ra1n5 are a 6 0 v e the 11m1t1n9 5 u p e r p a r a m a 9 n e t 1 c 512e a n d a61e t0 reta1n a 5ta61e r e m a n e n c e . 7 h e character15t1c5 0f the N R M 0f the ch1p5 f r 0 m E E 7 A 79001 5tr0n91y 5u99e5t the p r e 5 e n c e 0f a w e a k am61ent ma9net1c f1e1d dur1n9 the 0r191na1 c0011n9 0f the mete0r1te 0r dur1n9 the 1ater (180 m.y.) 5h0ck e v e n t [9]. A1th0u9h pa1ae0f1e1d ev1d e n c e f r 0 m ma9net1c 5tud1e5 0f 5 h e r 9 0 t t y 5amp1e5 15 at pre5ent am619u0u5 [16], there 15 e n 0 u 9 h ev1dence n0w accumu1at1n9 fr0m 0ther 5NC mete0r1te5, 1nc1ud1n9 2a9am1 [17], t0 e n c 0 u r a 9 e f u r t h e r ma9net1c 5tud1e5 w1th a v1ew t0 e5ta6115h1n9 the pre5ence 0r 0therw15e 0f a ma9net121n9 f1e1d 0n the p a r e n t 6 0 d y . 1f the p a r e n t 6 0 d y 15 1ndeed c0nf1rmed a5 Mar5, t h e n the 0 u t c 0 m e 0f the 5tud1e5 c0u1d 6e part1cu1ar1y reward1n9.
Ackn0w1ed9ement5 7 h e a u t h 0 r 15 1nde6ted t0 the Mete0r1te W 0 r k 1n9 6 r 0 u p , J 0 h n 5 0 n 5 p a c e C e n t r e , f0r acce55 t0 the Antarct1c mete0r1te 5amp1e5. 7 h a n k 5 are d u e t0 A. 5 t e p h e n 5 0 n f0r u5efu1 d15cu5510n, a n d t0 a n 0 n y m 0 u 5 referee5 f0r 5u99e5t10n5 f0r 1mpr0v1n9 the paper.
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