Iron-silicate fractionation within ordinary chondrite groups

Earth and Planetary Sctence Letters, 28 (1976) 479-484 © Elsevier Seientihc Pubhshlng Company, Amsterdam Printed in The Netherlands

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IRON-SILICATE FRACTIONATION WITHIN ORDINARY CHONDRITE GROUPS R T DODD Department of fiarth and Space Scwnees, State Umverslty of New York at Stony Brook, N Y (USA)

Received September 10, 1975 Revised version received October 27, 1975

A comparison of recent bulk chemical analyses of fresh, well-classified ordinary chondrltes reveals that the uneqmhbrated 1t-3 and LL-3 chondrltes tend to be iron-poor relative to equlhbrated It- and LL-group chondrltes (types 4-6) A more complex relatmnshlp m the L-group suggests that It consists of two chemical subgroups, m each of which iron is deficient in the lower petrologic types The available data suggest that the chondrlte parent bodies accreted lnhomogeneously

1 lntroductmn Recent analyses of ordinary chondrltes (see, e g , [1 ], and following papers) suggest that the three groups of ordinary chondrltes are remarkably homogeneous with regard to major, non-volatile elements Though there is evidence for some fractlonatlon of hthophfle elements between high- and low-Iron chondrItes [2], the effect is far subtler than the slderophIle element fractmnatlon which sets these two major groups apart The possibility of saderophile element fractlonatlon within each chemical group has not yet been explored, largely for want of suitable data X-ray fluorescence analyses by the Capetown group [3], though numerous and highly precise, concentrate heavily on meteorltes of the higher petrologic types (L-6, LL-6, H-5) On the other hand, the number of modern wet cheimcal analyses of fresh, well-classified ordinary chondrates is small fewer than 70 such analyses are scattered unevenly over the 12 petrologic types of the H-, L-, and LL-groups Though the data are few, there is some evidence that the abundances of slderophtle elements vary somewhat in the H-group For example, Dodd [4] noted that both the total iron content and the ratio of metalhc to total iron are lower in H-3 than in H-4 to 6 chondrItes Chou et al [5] showed that the H-3 chondrIte Bremervorde is markedly deficient in germanium and nickel

The present study was undertaken to determine whether variation of iron content with petrologic type is characteristic of all three groups of ordinary chondrItes The data used for this purpose are wet chemical analyses, all prepared since 1950 and most by two analysts H B Wuk and E JarosewIch Because weathering can affect total iron, only analyses of falls are used Finally, all meteorites included have been classifted by petrologic type [6] or can be so classified on the basis of published petrographic data Sixty-three analyses meet all of these cmerIa

2 Results and dlscussmn The total Iron contents of 69 ordinary chondrttes are summarized in Fig 1 The six bracketed analyses in Fig 1 were made before 1950 and accepted by Urey and Craig [7] as superior They are included because of a dearth of more recent analyses of type H-6 chondrites The other analyses meet all of the criteria discussed above 2 1 Htgh-lron (H-group) chondntes

With several exceptions, the H-group data show a general tendency toward iron-enrichment in the high petrologic types This tendency persists, though with more scatter, where total Fe is plotted against S102

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Total Fe (Wt %) I lg 1 Distribution of total Iron contents by petrologic type lor 69 ordinary chondntes Brackets mark pro-1950 analyses winch were regarded as supermr by Urey and Craig [7] Other analyses meet all selectmn cnterm d]s~.ussed m the text Marked analyses (L-group) may belong to rather the L- or LL-group Sources of data refs 7, 8, 1 0 a n d 2 1 - 3 9

(Fig 2), It clearly represents a true lron-sxlxcate fractmnatmn and ts not an artifact produced by loss of oxygen through reduction of FeO to metal during metamorplusm of the H-group chondntes [4] Fig 1 suggests that the H-6 chondntes vary widely in iron content It is not clear how much of this variation ts s~gmficant and how much ~s due to inadequate samphng of mhomogeneous materml In the single case (Rose City) where a modern analysis confirms an earlier report of extremely high total iron (36%), the lneteonte is clearly breccxated and its metal is ~rregularly distributed [8]

2 2 Low-tron (L- and LL-group} chondrttes Before we consider the data for low-iron chondntes, we must acknowledge and try to deal with an ambxgmty m the classlflcatton of unequlhbrated low-iron chondrltes L-3 and LL-3 c h o n d m e s cannot be resolved

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St02 (Wt%) I lg 2 S]O 2 total Fe relatmnsh]ps for ]l- and L-group chondrltes Diagonal bars m II diagram mark pre-1950 analyse Knyahmya (K) and Cynthaana (C) are noted Shapes of symbols denote petrologic types (triangle - 3, square = 4, star = 5, circle = 6) Arrows qgmfy speculattve evolutmnary tracks for high- and low-Fe L-subgroups The trends reflect addmon ot L-3 metal Data sources as for t ]g 1

solely on the basts ot iron content, for thmr iron contents overlap broadly Nor can they be classified on the basis of mineral composition [9], for they lack the homogeneous silicates f\)und in chondrxtes of the higher petrologic types Dodd et al [10] and Van Schmus and Wood [6] used the raUos of metalhc to total Iron and metalhc iron to nickel to classify type L- and EL-3 chondntes, for the two groups appear to be well-resolved on these grounds (Fig 3) each falls m a dlstmctwe range for each ratio, each shows characterlsUc varlaUons w]th the degree of lnhomogenelty ot ohvme Two of the equilibrated chondntes m Fig 3 are of uncertain classification Van Schmus and Wood [6] assigned Cynthlana to the L-group (L-4), but its Iron content (19 28 wt %) is the lowest yet reported for an L-group d l o n d n t e , and it falls near the LL-group

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types LL-4 and LL-5 are clearly needed It is more than usually Important that these be accompanied by detailed petrologic descriptions as many LL-group chondrltes are breccias and difficult to assign to one petrologic type [6,12], the risk of analyzing atypical material is high

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The pattern of iron variation among L-group chondrltes is complex and intriguing The data In Fig 1 fall in two clusters Each includes four type 3 chondrltes, and in each, these contain less iron than the assomated equilibrated chondntes Though it is possible that the low-iron L-3 chondntes are mlsclasslfied LLgroup meteorites, Fig 3 makes this seem quite unlikely Moreover, this explanation does not account for the associated type 4 and type 5 chondrltes, whose olivine compositions and metalhc to total iron and Iron to mckel ratios are appropriate to the L-group

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t ig 3 Ratios of metallic to total iron and Fe°/NI in relatmn to petrologm type m low-Fe chondrltes Symbols as for Fig 2, open figures denote L- and halt-filled figures LL-group chondrltes P M D data (percent mean deviation of iron in ohvme) from [10] Cynthmna (C) and Knyahlnya (K) are noted Data sources as for Fig 1

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The few modern analyses of LL-group chondrltes (Fig 1) refer almost wholly to types 3 and 6 Though the available data suggest that total iron varies directly with petrologic type, more analyses of

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Total Fe (Wt %) Fig 4 Gas-retention ages and iron contents of L-group ordinary chondrltes Type symbols conform with Fig 2 Open symbols mark modern wet chemical analyses used in Figs 1-3, filled symbols refer to XRF analyses by the Capetown group [ 1], half-filled symbols mark analyses quoted as superior by Urey and Craig [7] Sources for ages refs 13, 14 and 40-43

482 The data in Fig 1 suggest, rather, that the L-group consists of two subgroups, with somewhat different ranges of ~ron content, and within each of which total Iron Increases with petrologic type That Iron-rich and -poor L-3 chondntes show small but consistent textural differences [19] supports this interpretation So too does the practical restriction of low gas-retenuon ages [ 13,14] to relatively Iron-poor L-group chondrltes (Fig 4), though few such ages refer to well-analyzed, well-classified chondrites It is not clear where to draw the line between the two inferred subgroups Fig 1 shows a lnlnImum at 2 1 - 2 1 5% Fe Though this seems a logical boundary, a plot of total iron against silica (Fag 2) shows no clear division If the subgroups are real, they appear to overlap somewhat in Fe and S102, though they may differ in the pattern of variation of these components (see arrows in Fig 2) It is evident that confirmation of the subgroups and estabhshment of their chemical boundaries must await more, and more precise, bulk chenucal analyses ot L-group chondrltes Minor elements The chemical distinction among H-, L-, and LE-group chondrites involves sMerophfle elements other than Iron It IS unportant to establish

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whether this is also true of the Intla-group variations reported here The sunplest element to examine should be nickel, the most abundant of the siderophile minor elements If the intra-gloup u on variations reflect metal-silicate fractlonatlon, nickel and iron should covary It is not evident that they do (Fig 5) Data for both the H- and L-groups (EL-group data, which are few, are omitted) scatter widely and show no clear pattern Though be and N] appear to covary in the L-gloup as a whole, there are no convincing trends in the inferred subgroups The meaning of these data IS unclear As nickel IS of low abundance an these meteolltes (1 2%), an Fe-N1 trend may be present but obscured by analytical uncertainties, which are likely to be on the order of 5~ relative [15] At piesent, there is no COlnpelhng evidence either lot or against covarlahon of NI and Fe within each group of ordinary chondntes It is even more difficult to estabhsh intra-group trends for the sldelophlle trace elements, for the observed variations of iron content (on the order of, or less than, 10% relative) are near the limit of precision for analyses in the ppm range Though Ge, and Ir appear to be defiment in [t-3 chondrites [5], a plot of these elements against m m showed no systematic variation from type 4 to type 6 This is also true of Ge in the L-group [16] It is again unclear whether such variation is absent or masked by analytical uncertaint]es Recogmtlon of iron variation within each group should stimulate a careful search for parallel variations in the other slderophde elements

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3 Conclusions and implications ®

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Each chemical group of ordinary chondrltes shows small but distract variations o f total Iron with petrologic type Iron increases more or less regularly from type 3 to type 6 In the H- and EL-groups, suggesting that each group represents an evolutionary sequence within a single parent body The L-group may consist of two subgroups which have slightly different ranges of Iron content and different trends of F e - S I variation It ls not yet clear whether NI and the other slderophlle elements show similar trends, 1 e , whether the pattern reflects merely Iron-silicate or metal-sIhcate fractIonatlon These obselvatlons have important implications for

483 the i n t e r p r e t a t i o n o f o r d i n a r y c h o n d n t e s T h e presence o f t w o s u b g r o u p s In the L-group m a y e x p l a i n the writer's o b s e r v a t m n t h a t the c o m p o s i t i o n s o f olivine a n d o r t h o p y r o x e n e m H- and LL-group c h o n d r l t e s vary m o r e or less s y s t e m a t i c a l l y w i t h p e t r o l o g i c type, b u t t h a t those m L-group c h o n d r l t e s do n o t [12,17] S y s t e m a t i c v a r l a t t o n o f l i o n c o n t e n t w i t h i n each chemical g r o u p m a y also explain a p p a r e n t overlaps b e t w e e n groups, e g in the cases o f C y n t h l a n a and K n y a h l n y a It also n m s t be t a k e n i n t o a c c o u n t m a t t e m p t s to infer the m e t a m o r p h i c h i s t o r y o f c h o n d r l t e s f r o m mineralogical data The p a t t e r n s described here also bear o n the chemical e v o l u t i o n o f c h o n d r l t e p a r e n t b o d i e s and, b y extension, larger b o d i e s in the solar s y s t e m It appears t h a t the m e t e o r i t e p a r e n t b o d i e s a c c r e t e d l n h o m o geneously eather t h e y f o r m e d at a t u n e or place w h e r e the l i o n / s i l i c a t e l a t l o was changing, or t h e y a c c e p t e d vartable p r o p o m o n s o f ~ron a n d silicate f r o m a ~eservolr o f c o n s t a n t c o m p o s i t i o n As the sizes o f m e t a l a n d silicate p a r t M e s m t y p e 3 o r d i n a r y c h o n d r l t e s vary s y s t e m a t i c a l l y w i t h the iron c o n t e n t s o f these m e t e o r ltes [ 1 8 , 1 9 ] , the writer prefers the l a t t e r e x p l a n a t i o n and a process o f a e r o d y n a m i c sorting as suggested b y Whipple [20] A detailed s t a t e m e n t o f this m o d e l Is in p r e p a r a t m n

Acknowledgements The writer gratefully a c k n o w l e d g e s valuable discussions o f this p a p e r w i t h J Delano, J V Heyse, a n d J W S n e l l e n b u r g Dr Susan Kesson a n d P r o f W R V a n S c h m u s kindly reviewed a selnl-final d r a f t o f the m a n u s c r i p t The s u p p o r t o f the N a t m n a l Science F o u n d a t i o n ( G A - 2 5 3 1 5 A ) is gratefully a c k n o w l e d g e d

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