Ferrous ion order-disorder in meteoritic pyroxenes and the metamorphic history of chondrites

EARTH AND PLANETARY SCIENCE LETTERS 2 (1967) 372-376. NORTH-HOLLAND PUBL. COMP., AMSTERDAM

FERROUS ION ORDER-DISORDER IN METEORITIC PYROXENES AND THE METAMORPHIC HISTORY OF CHONDRITES R o b e r t W. DUNDON Research Institute f o r Natural Sciences, Woodstock College, Woodstock, Maryland 21163, USA and L o u i s S. W A L T E R Goddard Space Flight Center, National Aeronautics and Space Administration, Greenbelt, Maryland 20771, USA

Received 21 April 1967 Revised 2 June 1967

The 57Fe MSssbauer absorption was used to investigate f e r r o u s ion o r d e r - d i s o r d e r in pyroxenes separated f r o m 25 stony m e t e o r i t e s . Equilibrated chondrites have o r d e r e d FeII in t h e i r pyroxenes, with the exception of Farmington. The examined unequilibrated chondrites, with the exception of MezbMadaras and Clovis No. 1, have considerable f e r r o u s ion d i s o r d e r . These r e s u l t s a r e in a g r e e m e n t with the theory of J. A. Wood on the origin of m e t e o r i t e s . Bjurbole pyroxene probably r e t a i n s d i s o r d e r e d f e r r o u s ion, which is i n t e r p r e t e d in t e r m s of m e t a m o r p h i s m at higher t e m p e r a t u r e s than M e z b - M a d a r a s and Clovis No. 1. The unusually high degree of d i s o r d e r in the Farmington pyroxene is m o r e likely the r e s u l t of r e h e a t i n g than the r e s u l t of shock.

I. ~TRODUCTION T h e u s e f u l n e s s of t h e M 5 s s b a u e r e f f e c t f o r t h e d e t e c t i o n of f e r r o u s i o n d i s o r d e r i n f e r r o magnesian pyroxenes has been discovered by M a r z o l f , D e h n a n d S a l m o n [1] i n t h i s l a b o r a t o r y , b y B a n c r o f t , B u r n s a n d H o w i e [2] a t C a m b r i d g e , a n d b y E v a n s , G h o s e a n d H a f n e r [3] a t t h e U n i v e r s i t y of C h i c a g o . T h e i r w o r k t a k e n t o g e t h e r w i t h t h e X - r a y s t u d y of G h o s e [4] h a s s h o w n t h a t the ferrous ion prefers the M 2 sites in these m i n e r a l s a n d t h a t t h e q u a d r u p o l e s p l i t t i n g of f e r r o u s i o n on t h e M2 s i t e i s e n o u g h l e s s t h a n o n t h e M 1 s i t e to m a k e p o s s i b l e a t l e a s t a s e m i q u a n t i t a t i v e e s t i m a t e of t h e r e l a t i v e f e r r o u s i o n order-disorder in pyroxenes separated from stony meteorites.

2. M A T E R I A L S A N D M E T H O D S The separated meteoritic pyroxenes supplied b y B r i a n M a s o n of t h e U.S. N a t i o n a l M u s e u m w e r e b o u n d w i t h a n a c e t o n e s o l u t i o n of a c e t a t e c e m e n t a n d c e m e n t e d to f i l t e r p a p e r . S a m p l e s

having notable ferrous ion disorder were checked f o r s i g n i f i c a n t a m o u n t s of o l i v i n e i m p u r i t y u n d e r a polarizing microscope. Chromite is another possible iron-bearing impurity, but its Mbssb a u e r a b s o r p t i o n d o e s n o t i n t e r f e r e w i t h t h a t of pyroxene. The Mbssbauer spectrometer used for this study is a modified Nuclear Science and Engin e e r i n g C o m p a n y M o d e l B, c o n s t a n t v e l o c i t y s p e c t r o m e t e r [1]. T h e 57Co i n p a l l a d i u m s o u r c e h a s a c h e m i c a l i s o m e r s h i f t of + 0 . 4 4 m m / s e c r e l a t i v e to r e a g e n t g r a d e s o d i u m n i t r o p r u s s i d e . Positive velocities indicate that the absorber is moving toward the source. The peak positions, heights and widths reported here are the results of a l e a s t - s q u a r e s f i t to L o r e n t z i a n p e a k s , b e g i n n i n g f r o m i n i t i a l e s t i m a t e s of t h e s e q u a n t i t i e s from the raw data. The pyroxene peaks are lab e l e d A1, B1, B 2 a n d A 2 i n o r d e r of i n c r e a s i n g v e l o c i t y a s s h o w n i n fig. 1. T h e A p e a k s a r e d u e to F e H o n M 1. T h e B p e a k s a r e due to F e H o n M 2. A s a s e m i - q u a n t i t a t i v e m e a s u r e of f e r r o u s i o n d i s o r d e r we h a v e c h o s e n t h e m e a n r a t i o of t h e p e a k h e i g h t s of t h e o u t e r two p e a k s to t h e h e i g h t s of t h e i n n e r two p e a k s . T h u s w e d e f i n e

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Fig. 1. T y p i c a l s p e c t r a . The d o t s r e p r e s e n t e x p e r i m e n t a l p o i n t s and t h e s o l i d l i n e s t h e L o r e n t z i a n p e a k s fitted to t h e p o i n t s by a l e a s t - s q u a r e s p r o c e d u r e . T h e A p e a k s a r e due to FeII on s i t e M 1 in the c r y s t a l s t r u c t u r e . T h e B p e a k s a r e due to FeII on s i t e M 2.

374

R.W. DUNDON and L. S. WALTER Table 1 T e r r e s t r i a l pyroxenes [5]. Peak B 1

DOM 1 * 7666 EB 130 EB 13

Peak B 2

Pos a

Ht b

Wc

Pos

Ht

W

-0.07 -0.09 -0.10 -0.09

11.8 14.6 9.3 8.8

0.39 0.32 0.39 0.31

2.03 2.03

9.1 13.2 9.8 7.5

0.42 0.32 0.39 0.33

2.01 2.03

10-5 counts d

10-5 GFP e

1.9 2.4 2.5 1.4

8.5 2.6 6.9 2.7

a) b) c) d)

Pos indicates the Doppler velocity in m m / s e c of the maximum absorption. Ht is the percent decrease in counts from the base line value. W is the full width at half height of the absorption line in m m / s e c . Total counts in fitted base line. / N e) GFP is a measure of the goodness of fit. G F P = ( ~_~ d~.)/N, where N is the number of data points (velocities) and

di is

$--l'--

the difference for data point i between the counts measured and the counts calculated for the Lorentzian peak with the parameters given in the previous columns. * F r o m a rapidly annealed andesite, Dominican Islands. This sample was run in incompletely purified form. GFP is high mainly due to points off the pyroxene peaks from iron bearing impurities in the sample. the p e a k h e i g h t r a t i o (PHR) a s the m e a n of (height of A 1 / h e i g h t of B1) and (height of A 2 / h e i g h t of B2). 3. RESULTSAND DISCUSSION

Terrestrial pyroxenes are generally quite well ordered. Ghose found by a careful X-ray diffraction study of an orthopyroxene Mg0.93Fe1.07Si206 t h a t 90% ± 10°/o of the M2 sites were occupied by Fell [4]. The most highly disordered terrestrial pyroxene of the three examined by Marzolf, Dehn and Salmon [1] had a PHR of only 0.12. For comparison with meteoritic pyroxenes we have examinedthree terrestrial pyroxenes from rocks annealed slowly in nature and one from a rapidly cooled andesite. These minerals were supplied by Harold Hess [5] of Princeton University. The Mbssbauer absorption patterns of these samples are summarized in table 1. All of the samples have completely ordered ferrous ion, within the limits of detection (about 5% disordered FeII is a reasonable limit on the amount that could be present and escape detection). Ghose and Hafner [6] have examined several volcanic pyroxenes, includingthe sample DOM I of table 1. They found some disorder in this sample, but less than in the two pyroxenes from Japanese volcanic rocks and considerably less than that found in pyroxenes from metamorphic rocks heated to 1000oc for seven days. The Mbssbauer absorption patterns of most of the meteoritic pyroxenes studied are summarized in table 2. The equilibrated meteorites with the exception of Farmington are ordered.

M e z S - M a d a r a s and C l o v i s No. 1, w h i c h h a v e o r d e r e d Fell, a r e u n e q u i l i b r a t e d , i . e . , t h e y h a v e c o n s i d e r a b l e g r a i n - t o - g r a i n v a r i a t i o n in i r o n and m a g n e s i u m c o n c e n t r a t i o n s [7]. T h e o t h e r u n e q u i l i b r a t e d m e t e o r i t e s show c o n s i d e r a b l e a b s o r p t i o n at the A p e a k p o s i t i o n s a s m e a s u r e d by the P H R v a l u e s g i v e n in p a r e n t h e s e s : B a r r a t t a (0.20), B i s h u n p u r (0.25"), B j u r b o l e (0.46"), B r e m e r v i i r d e (0.20"), Carraweena (0.22), Chainpur (0.26"), H a l l i n g e b e r g (0.26), N g a w i (0.36"), P r a i r i e Dog C r e e k (0.22"), S e m a r k o n a (0.48"), S h a r p s (0.45), S i n d h r i (0.26"), and T i e s c h i t z (0.50*). T h e P H R v a l u e s m a r k e d w i t h a s t e r i s k s are upper limits, since these samples retain m o r e than an e s t i m a t e d 2 o r 3% o l i v i n e , w h i c h a b s o r b s quite n e a r to the A p e a k s in the p y r o x e n e s p e c t r a . M o r e r e l i a b l e P H R v a l u e s w i l l be r e ported for these meteorites after purer separates are prepared. E v a n s , G h o s e and H a f n e r [3] h a v e shown that the d e g r e e of F e d i s o r d e r in p y r o x e n e s is n o t i c e a b l y i n c r e a s e d by h e a t i n g at 1 0 0 0 o c f o r s e v e n days. The d i s o r d e r i s f u r t h e r i n c r e a s e d by h e a t ing f o r 13 h r at l l 0 0 o c . T h u s , the o r d e r i n g of i r o n in the p y r o x e n e s of m o s t e q u i l i b r a t e d c h o n d r i t e s r e p r e s e n t s e q u i l i b r a t i o n at t e m p e r a t u r e s substantially below these. S e v e r a l a u t h o r s h a v e d i s c u s s e d the q u e s t i o n of the e q u i l i b r a t i o n t e m p e r a t u r e of c h o n d r i t e s . It s h o u l d be e m p h a s i z e d that t h i s e q u i l i b r a t i o n h a s b e e n the c o m p o s i t i o n a l e q u i l i b r i u m b e t w e e n p h a s e p a i r s . T h e p r o b l e m at hand is to r e l a t e the k i n e t i c s of the o r d e r i n g p r o c e s s to the p r o c e s s of c o m p o s i t i o n a l e q u i l i b r a t i o n and at p r e s e n t s u f f i c i e n t d a t a a r e not a v a i l a b l e .

FERROUS ION ORDER-DISORDER

375

Table 2 A1 Pos

B1

Ht I W

Pos I Ht

] W

B2

A2

10 -5 x

10-5 x

P o s I Ht ] W

P o s [ Ht ] W

counts

GFP

2.20

3.3

0.79

1.9 1.7 1.8 1.1 1.6

2.4 2.5 2.0 1.3 2.8

2.41 2.38 2.41 2.43

1.8 1.4 1.8 6.3

0.51 0.50 0.40 0.39

2.7 2.6 3.1 2.9 1.9 1.5 1.5

3.1 2.5 3.0 3.6 2.1 1.4 1.9

1.5

2.0

1.8 1.0 1.9

3.2 1.6 4.6a

PHR

m

H group chondrites

Sharps Clovis No. 1 Coolidge Richardton Miller

-0.52

1.9

0.29

1.6

0.29

0.9 1.5 6.4

0.26 0.35 0.36

-0.10 -0.07 -0.09 -0.08 -0.07

7.8 12.4 6.4 14.9 9.1

0.39 0.34 0.36 0.31 0.28

2.04 2.01 2.02 2.03 2.01

5.0 11.6 5.9 14.2 8.7

0.23 0.33 0.37 0.33 0.30

-0.10 -0.10 -0.11 -0.09 -0.08 -0.08 -0.08

5.8 7.1 5.9 8.2 7.3 7.3 13.9

0.35 0.33 0.40 0.32 0,30 0.33 0.34

2.05 2.02 2.03 2.02 2.02 2.02 2.02

5.1 6.3 4.7 8.0 6.6 7.0 15.2

0.35 0.31 0.41 0.34 0.32 0.33 0.35

-0.08

13.5

0,31

2.01113.0

0.33]

-0.08 -0.03 -0.11

18.4 14.0 7.8

0.33 0.27 0.39

2.01 1.98 2.05

0.36 0.37 0.39

0.45

L group chondrites

MeziS-Madaras Hallingeberg Carraweena Barratta Farmington Holbrook Shaw

-0.53 -0.48 -0.45 -0.49

0.26

0.22 0.20

0.90

LL group chondrite

Appley Bridge

I -

A chondrites

Johnstown Haraiya Goalpara

-0.23

6.3

0.34

17.4 14.3 7.6

2.23

4.7

0.27

0.39

a) Adding an A 1 peak to the computed s p e c t r u m does not improve the fit. Adding an A2 peak reduces the GFP to 3.6 x 105. In a r e l a t i v e way, h o w e v e r , o n e c a n n o t e t h a t the unequilibrated chondrites show marked diso r d e r w h i l e , in g e n e r a l , e q u i l i b r a t e d c h o n d r i t e s s h o w o r d e r i n g of i r o n i n t h e p y r o x e n e . T h i s i s c o n s i s t e n t w i t h t h e t h e o r y of W o o d on t h e o r i g i n of c h o n d r i t e s . He d e r i v e s t h e e q u i l i b r a t e d c h o n d r i t e s by t h e low t e m p e r a t u r e m e t a m o r p h i s m of u n e q u i l i b r a t e d c h o n d r i t e s [8]. O r d e r i n g of t h e i r o n w o u l d b e a n a t u r a l p r o d u c t of t h i s p r o c e s s ( a s i t i s i n t h e c a s e of A1-Si o r d e r i n g in p l a g i o c l a s e i n m e t a m o r p h i c r o c k s ) . A n a l t e r n a t i v e to low t e m p e r a t u r e compositional equilibration is that this process took place at high temperature but that Fe ordering is a much more rapid process and that this ordering was achieved as the material cooled. The unequilibrated chondrites w o u l d t h e n h a v e to b e q u e n c h e d m u c h m o r e r a p idly. T h i s , h o w e v e r , i s i n o p p o s i t i o n to t h e s u g g e s t i o n of K e i l a n d F r e d r i k s s o n [7] t h a t c o m p o sitional homogeneity is the result of r a p i d quenching while heterogeneity results from slow cooling. In t h e c a s e of B j u r b o l e , w e c a n a s s u m e f o r the present that some ferrous ion disorder will be evident after the remaining olivine is removed f r o m t h e p y r o x e n e . T h e a b s e n c e of c o m p o s i t i o n a l

variations within the Bjurbole pyroxene and the p r e s e n c e of s o m e f e r r o u s i o n d i s o r d e r i n d i c a t e metamorphism at higher temperature than that undergone by MezS-Madaras and Clovis. T h e d i s t r i b u t i o n of F e I I o v e r b o t h s i t e s i n H a r a i y a p y r o x e n e i s e x p e c t e d on t h e b a s i s of stoichiometry. The Ca:Fe:Mg atomic ratios are a p p r o x i m a t e l y 1 0 : 5 5 : 3 5 [9]. A s s u m i n g t h a t a l m o s t a l l t h e C a l l i s on t h e M 2 s i t e a n d a l l t h e MgII on t h e M 1 s i t e , 27% of t h e i r o n w i l l b e on M 1 a n d 73% o n M 2. If w e a s s u m e t h a t P H R i s e q u a l to t h e r a t i o of t h e s e two p e r c e n t a g e s a s a n a p p r o x i m a t i o n , w e e s t i m a t e a P H R of 0.37, i n g o o d a g r e e m e n t w i t h t h e m e a s u r e d v a l u e of 0.39. The Farmington pyroxene separate is the m o s t h i g h l y d i s o r d e r e d h y p e r s t h e n e o b s e r v e d to d a t e . T h i s s e p a r a t e i s q u i t e f r e e of o l i v i n e * The meteorite has an unusual texture and microstructure which suggest that it has been shocked [10]. H o w e v e r , t h e a b s e n c e of d i s o r d e r e d p y r o x ene in Goalpara argues against a simple correl a t i o n of s h o c k w i t h F e d i s o r d e r . G o a l p a r a b e a r s * Point counts of two portions of the powdered sample gave 1 grain of olivine to 99 of pyroxene and 3 grains of olivine to 103 of pyroxene.

376

R.W. DUNDON and L. S. WALTER

both cubic and hexagonal diamonds which are p r o b a b l y f o r m e d b y s h o c k [11], b u t h a s no m e a s urable Fe disorder in the pyroxene phase. T h e r e c e n t w o r k of B i n n s [12] s u p p o r t s r e h e a t i n g a s t h e c a u s e of b l a c k e n i n g a n d of c a v i t i e s lined with troilite crystals in Farmington. By h e a t i n g m a t e r i a l f r o m P a r n a l l e e to 1 0 5 0 o c f o r 20 d a y s h e w a s a b l e to f o r m c a v i t i e s l i n e d w i t h t r o i l i t e a n d to b l a c k e n t h e m a t e r i a l . S u c h r e h e a t i n g w o u l d a l s o c a u s e a n i n c r e a s e in F e d i s o r d e r . T h e d a t a of E v a n s , G h o s e a n d H a f n e r [3] m a y b e u s e d to c a l c u l a t e P H R v a l u e s of 0.37 a n d 0.41 f o r t h e i r p y r o x e n e h e a t e d to 1 0 0 0 o c a n d l l 0 0 o c , r e spectively. Assuming that their samples were h e a t e d l o n g e n o u g h to a t t a i n e q u i l i b r i u m , o u r m e a s u r e d P H R of 0 . 9 0 w o u l d c o r r e s p o n d to s o m e temperature greater than ll00oc for Farmington pyroxene. S t u d i e s of p y r o x e n e s f r o m b l a c k c h o n d r i t e s besides those reported here (Farmington, Carr a w e e n a a n d B a r r a t t a ) a r e u n d e r way. E x p e r i m e n t s to d e t e r m i n e t h e e f f e c t of h e a t i n g a n d a n nealing pyroxenes are planned in our laboratory a n d a r e a l s o b e i n g c a r r i e d out b y H a f n e r a n d c o w o r k e r s a t t h e U n i v e r s i t y of C h i c a g o .

ACKNOWLEDGEMENTS W e a r e i n d e b t e d to D r . B r i a n M a s o n f o r s u p plying the pyroxene separated from meteorites a n d to D r . H a r o l d H e s s f o r t h e t e r r e s t r i a l p y -

roxenes. Dr. John Marzolf has helped with the instrumentation and programming. This work was supported by the National Aeronautics and S p a c e A d m i n i s t r a t i o n u n d e r C o n t r a c t No. N s G 670. REFERENCES [1] J. Marzolf, J. Dehn and J. Salmon, in: The MSssb a u e r effect and its applications in c h e m i s t r y , ed. R. Gould (American Chemical Society, 1967). [2] G. M. Bancroft, R.G. Burns and R.A. Howie, Nature 213 (1967) 1221. [3] B. J. Evans, S. Ghose and S. Hafner, J. Geol. (1967), in p r e s s . [4] S. Ghose, Z. Krist. 122 (1965) 6. [5] H . H . H e s s , Am. J. Sci., Bowen Volume, p. 185. [6] S. Ghose and S. Hafner, Z. Krist. (1967), in p r e s s . [7] K.Keil and K. F r e d r i k s s o n , J. Geophys. Res. 69 (1964) 3487; R . T . D o d d Jr., W.R. Van Schmus and D.M.Koffman, Geochim. Cosmochim. Acta (1967), in p r e s s . The l a t t e r p a p e r has an extensive survey of unequilibrated chondrites. [8] See the reviews of E . A n d e r s , Space Sci. Rev. 3 (1964) 583, for the theory of Wood and a review of the supporting evidence. [9] B r i a n Mason, private communication. [10] P . R . Buseck, B. Mason and H. B. Wiik, Geochim. Cosmochim. Acta 30 (1966) 1; D.Heymann, Icarus 6 (1967) 189. [11] R . E . H a n n e m a n and H.M. Strong, Science 155 (1966) 995. This a r t i c l e has r e f e r e n c e s to the p r e vious discussion of the origin of diamonds in m e teorites. [12] R. A. Binns, Science 156 (1967) 1226.