Primitive ultrafine matrix in ordinary chondrites

Earth and Planetao' Science Letters, 56 ( 1981 ) 107-126 Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands

107

ill

Primitive ultrafine matrix in ordinary chondrites E . R . R a m b a l d i i B.J. F r e d r i k s s o n 2 a n d K. F r e d r i k s s o n 2 I Institute of Geophysics and Planeta O, Pl~vsics, Universi O' of California, Los Angeles. CA ~)0024 (U.S.A.) " Department of Mtneral Sciences, Smithsonian Institution, Washington, DC 20560 (U.S.A.)

Received March 17, 198 I Revised version accepted August 13, 1981

Ultrafine matrix material has been concentrated by sieving and filtering disaggregated samples of six ordinary chondrites of different classes. This component(s), "Holy Smoke" (HS), is enriched in both volatile, e.g. Na, K, Zn, Sb, and Pb, as well as refractory elements, e.g. W and REE: however, the element ratios vary greatly among the different chondrites. SEM studies show that HS contains fragile crystals, differing in composition, and apparently in gross disequilibrium not only among themselves but also with the major mineral phases and consequently thermodynamic equilibration did not occur. Thus HS must have originated from impacting bodies a n d / o r was inherent in the "primitive" regolith. Subsequent impact brecciation and reheating appears to have altered, to varying degrees, the original composition of this ultrafine matrix material. Recent "cosmic dust" studies may indicate that HS still exists in the solar system. Survival of such delicate material must be considered in all theories for the origin of chondrites.

I. Introduction

It has long been believed that the matrix between chondrules in ordinary chondrites consists mostly of broken and pulverized chondrules, and thus has essentially the same mineralogy and chemistry as the chondrules themselves. However, in the early 1960's several workers (e.g. [1]) pointed out that at least some C-type chondrites contain phases with vastly different thermal histories, i.e. at least one high-temperature (olivine and pyroxene) and one low-temperature (layer lattice silicates with oxidized nickel) component. As great enrichments of rare gases were being found, not only in carbonaceous chondrites, but also in some ordinary chondrites (see Reynolds [2] for reviews and references), it appeared likely that they, too, had a gas-bearing phase(s) which probably was of relatively low-temperature origin. In order to better characterize this component Fredriksson and Nelen [3] attempted to mechanically concentrate it by crushing and sieving or by drilling ordinary chondrites. They found a tenfold enrichment of

carbon, relative to bulk, in their finest, < 75 ~m, fractions constituting <<5 wt.%. It was suggested that this fine-grained material contained a "submatrix" (Holy Smoke, John O'Keefe, personal communication) which might be related to the main constituents of carbonaceous chondrites, and thus conceivably could have a similar trace element distribution. In 1972 one of us (B.J.F.), through the courtesy of Dr. S.R. Taylor, had an opportunity to use the MS-7 spark source mass spectrometer at the Australian National University, Canberra, to analyze similarly separated samples of so-called equilibrated chondrites as well as the unequilibrated LL3 Semarkona for a number of trace elements. The composition of a small fraction, < 100 mesh, obtained by gentle crushing or drilling was compared to that of the bulk material. The results as summarized in Table 1 show consistent, although variable trends, notably enrichments of some volatile as well as of some more refractory elements, such as REE. However, it was decided to delay publication until independent confirmation could be obtained because: (1)

0012-821X/81/0000-0000/$02.75 ~' 1981 Elsevier Scientific Publishing Company

108

TABLE 1 T r a c e e l e m e n t c o n t e n t in bulk, " m a t r i x ' - e n r i c h e d a n d v a r i o u s g r a i n size r a c t i o n s of o r d i n a ~ c h o n d r i t e s d e t e r m i n e d by s p a r k s o u r c e m a s s s p e c t r o m e t e r t e c h n i q u e ( c o n c e n t r a t i o n s in p p m ) Pb

Sn

Bi

Pr

Sm

Gd

Dy

Er

Yb

Semarkona (LL3) Bulk silicatcs

0.96

0.31

-

0.12

0.19

0.31

0.25

0.24

0.23

Drilled matrix

2.1

2.0

-

0.51

0.31

(I.43

0.41

0.27

0.20

b u l k silicates

0.65

-

-

-

0.30

0.30

0.18

0.17

drilled m a t r i x

5.6

-

1.2

4.4

3.0

2.8

0.73

0,26

0.22

0.2

2.2

1.55

1.4

0.44

-

0.18

-

-

-

-

0.52

-

Rupota (L4) Light inclusion

D a r k inclusion drilled m a t r i x

12.9

Forest Vale ( H4) > 100 m e s h

2.0

< 100 m e s h

3.0

20.0

0.41

0.28

0.26

-

0.67

0.51

0.51

Bjurb61e (L4) Bulk silicates

0.31

0.08

-

0.05

0.10

0.12

0.11

0.08

0.08

< 100 m e s h

0.76

0.5

-

0.21

0.25

0.28

0.33

0.25

0.28

Drilled matrix

8.5

0.5

-

0.15

0.33

0.28

0.20

-

A llegan (tt5) > 100 m e s h

0.12

-

-

< 100 m e s h

0.4

0.21

-

0.17

0.45 *

0.21

0.26

0.29

0.18

0.16

0.26

0.33

0.5

0.27

0.25

0.30

0.36

0.44

0.30

0.28

* C h e m i c a l a n a l y s i s by V i r g i n i a O v e r s b y .

the small samples and possibilities for contamination combined with questionable standards and a relatively novel technique [4] made the accuracies uncertain, and (2) the elemental distribution trends strongly contradicted then popular theories on the genetic relationships between different types of chondrites especially with regard to the Van Schmus-Wood [5] classification scheme. Thus when a project to separate and analyze chondrules from different chondrites [6] was started, the remaining "matrix" samples were further separated according to grain size and the fractions analyzed by INAA. The preliminary results [7,8] were qualitatively in agreement with the above-mentioned work (thus ruling out contamination problems). These results were striking because of the extreme enrichment of rather incompatible elements, such as alkalies, W, and REE, and encouraged the more rigorous study reported here, which includes direct SEM observations and X-ray spectroscopic analyses of the ultrafine particles constituting part of the analyzed matrix fractions.

2. Analytical techniques 2.1. Sample disaggregation The material to be disaggregated was placed in a 50-ml Ni crucible and covered with double distilled water (filtered through 0.08 /~m Nuclepore and freshly boiled to remove air). A small amount of an organic anti-oxidant was added. The crucible was placed in a bell jar under vacuum to facilitate water penetration and subsequently immersed in liquid nitrogen, slightly above the water level inside, for 5-10 minutes after the water had frozen. The crucible was then removed to a moderately heated hot plate and, before all ice was molten (to avoid temperatures above 0°C), transferred to an ultrasonic bath for - 2 minutes and then returned to the vacuum jar and back for another freeze. This procedure was repeated 10 to 20 times for each sample. Expansion of the freezing water as well as thermal strain between the phases facilitate the disaggregation. After removal of some chondrules and resistant

109 large fragments, the disaggregated samples were wet sieved through 200 mesh ( - 7 5 /~m) a n d / o r 400 mesh ( - 4 0 / ~ m ) and then through finer sieves or, more often, Nuclepore filters. The size fractions then represent deliberately biased samples (not the bulk) of each chondrite. It must also be understood that clear-cut size fractions were in no case obtained. Although few grains larger than a specified filter size might be expected in a smaller size fraction, it is certain that material contained on a certain sieve or filter will contain grains much smaller than the nominal pore size. Consequently the mean grain size of material retained on or passing through a sieve or filter (which also have greatly different efficiencies) will be much less than the nominal pore size. Thus the "mesh" and "/tm" designations, which only roughly indicate relative mean grain size, have been retained in both tables and illustrations. A few samples were disaggregated in alcohol using only an ultrasonic probe in order to check for possible contamination; none was found. Hamlet was treated this way only.

irradiated, and they appeared to contain only trace amounts of Br. However in some cases no Br was found in small samples irradiated with filters, e.g. Allegan >0.08 /~m; thus we conclude that no detectable Br contamination has occurred.

2.2. Instrumental neutron activation technique

3. Analytical results

The samples were irradiated for 6hours at a flux of 7 × 10 ~l n / c m 2 s and during a cooling period of four weeks the activities of 24Na, 42K, 46Sc, 51Cr, 56Mn, 58C0 (from Ni), 59Fe, 6°C0, 65Zn, 75Se' 76As' 82Br, I I°Ag, 122Sb, 140La' 152Eu' 153Sm' ~9yb, 177Lu, 187W, 19lOs, t921r, 197Hg, 198Au, were counted with a large-volume (Ge(Li) detector for the determination of these elements. The precision (95%) confidence limit) varied considerably for a given element as a function of the sample weight and of the concentration. Generally, for Na, Mn, Sc, Cr, Sm, Fe, Co, Ni, Au and Ir the precision was 5% or better of the amount present. For La, Eu, Yb, Lu, Os, W, As, Sb, Se, Zn and Br the precision varied from 5 to 25% in the various samples, and for Ag and Hg it was in some cases as low as 40-50%. Because it proved impossible to efficiently recover small amounts of material (~< 5 mg) from the Nuclepore filters, some samples were irradiated with the filters. In order to check for possible contamination, blank Nuclepore filters were also

The analytical results for trace and minor elements in the grain size fractions of the six meteorites studied are presented in Table2 together with bulk data on four of them determined on 500-mg aliquots. The weight of each fraction analyzed is indicated in the second column, which in all cases except Allegan represents the total amount of material recovered. In all samples this amount decreases with the grain size.

2.3. Electron microprobe technique Major elements were determined on small aliquots of each size fractions; all > 8 # m were ground to < 5 # m after coarser metal had been removed. The technique was the same as used for individual chondrules [6], i.e. the fine-grained material, usually a few milligrams, was pressed into pellets and analyzed with the electron microprobe using a "broad beam", similarly prepared standards and conventional correction procedures. The detailed results will be published elsewhere together with the chondrule analyses (Fredriksson et al., in preparation) because only Na (K is too low for reliable analysis) and A1 seem relevant to this study.

3.1. Siderophile elements The amount of metal present in any grain size fraction generally can be estimated from its Ni and Co content. Compared to the Ni and Co contents of bulk chondrites (Table2), the finegrained material appears to be depleted in metal. In Rupota and Hamlet the Co content remains constant, whereas Ni decreases with decreasing grain size; this variation cannot be attributed to analytical error or low precision in the determination. With Co concentrated in kamacite and Ni in

I10 TABLE 2

Trace element content of Cdifferent grain size fractions * of six ordinary chondrites determined by instrumental neutron activation technique (note that size fractions are biased and do not together represent the bulk: furthermore, since metal has been removed before preparing the grain size fractions, the content of siderophile elements in the bulk is outsidc the range covered by the size fractions) Meteorite

Sample

Na (ppm)

K (ppm)

Mn (ppm)

Sc (ppm)

Cr (ppm)

La (ppb)

7172 10,200 10,200 5013

327 540 514 444

2438 2673 2165 987

8.4 8.7 7.3 3.2

3356 3990 3467 1553

358 860 860 574

194 225 227 135

91 97 -

6460 6310 6140

820 777 822

2610 2520 2550

8.5 8.1 8.2

4570 5110 4940

617 868 1030

256 266 320

6280 7046 8998 12,000 6155 13,285

696 666 1100 1440 793 2294

2414 3010 2924 2580 1210 1476

8.4 8.4 8.4 8.3 3.8 5.1

3340 4124 5420 7225 4200 4700

327 683 772 1040 828 2540

7.61 2.77 3.62

4210 7380 9670

527 1005 1210

3095 2990 2620

8.2 9.1 9.6

3010 4100 3880

333 7.16

7806 5620 19,800

830 813 6430

2675 3058 2050

9.1 8.5 9.9

6670 5860 8520 9550

910 760 1290 1790

2600 2830 2800 2450

9.0 8.7 8.8 8.5

weight

Sm (ppb)

Eu (ppb)

Yb (ppb)

Lu (ppb)

Fe (%)

232 246

37 -

24.1 21.9 23.1 25.7

73 77 78

249 323 333

30 50 53

16.9 19.1 16.6

190 363 436 547 303 700

92 85 140 148 202 1140

227 390 532 575

47 44 49 -

26.6 12.2 12.0 12.5 8.3 12.2

268 2660 2930

205 616 713

58 108 264

162 330 402

31 60 94

18.5 16.4 14.6

3520 3058 3210

400 457 2720

250 123 467

87 56 360

250 192 567

37 50 166

18.6 18.4 11.9

3995 3660 4770 5285

390 360 657 968

217 185 276 370

54 78 133 255

246 210 248 267

27 39 28 89

20.6 17.9 16.6 14.7

(mg)

Ties~itz(H3) Bulk >400 >8 >0. 0 8

98.6 19.9 10.6

Weston (H4) >400 >8 >1

86.2 20.3 7.86

Allegan (H5) Bulk >400 > 1 5 ** >7 >1 >0. 0 8

95.9 51.2 113.6 5.79 0.374

Rupota (L4) >400 >8 >1

BjurbO&(L4) Bulk >200 >8

Ham&t(LL4) Bulk >400 >8 >1

79.4 107.9 25.3

* Grain size fractions, in mesh (200, 400) and /~m (8, 1, 0.08), only indicate decreasing relative mean grain size (see text p. 109). ** 1 5 ~ m s i e v e .

taenite, the results for Rupota and Hamlet seem to indicate that while the kamacite content remains approximately constant taenite decreases with decreasing grain size, which implies a decrease in total metal content. In Weston the finest, > 1 ~m, fraction has Ni and Co contents that are respectively 10 and 49% (Ni) and 14 and 39% (Co) lower than the coarser and intermediate fractions, which again indicates that the lowest metal content is in the finest fraction. In Bjurb6le Co and Ni display an opposite trend, apparently indicative of a lower

taenite but higher kamacite content in the finer, > 8 /zm, fraction, Rambaldi et al. [9] have observed that the Co content in the metal phase of BjurbiSle is quite variable. This observation, associated with the very low Fe content of the fine fraction, suggests that the three-fold increase in kamacite required by the high Co may not be real, but due to an exceptionally high Co content in the metal. Tieschitz and Allegan are enriched in Ni and Co in the > 0.08-/zm fractions relative to the coarser ones. In Tieschitz this increase is paralleled

Co

Ni

Ir

Os

W

As

Au

Sb

Se

Zn

Ag

Br

Hg

(ppm)

(ppm)

(ppb)

(ppb)

(ppb)

(ppm)

(ppb)

(ppb)

(ppm)

(ppm)

(ppm)

(ppm)

(ppm)

814 220

15,600 5740

709 310

44 250

285

4850

283

-

817

10,900

195

-

263

11,700

490

560

193

320

15,800

763

912

1800

230

10,600

516

620

1160

900 39 23 35 73 140

17,100 2160 1320 2230 4970 7070

754 314 344 560 533 1050

865 355 448 596 -

170 -

150 162

7130 5560

322 280

351 -

148

5470

205

-

266 56 186

9193 4150 3360

377 345 714

728 114 97 116

12,750 4725 2310 1620

500 225 267 370

800 339

166 856

1.94 1.20

194 171

80 145

7.7 8.7

1160

1.30

394

262

5.5

960

2630

0.84

303

245

5.3

3070

1.02

148

237

10.1

1.43

300

318

10.8

218

1.37

544

900

10.0

296

1.98

206 35 55 160 355 4240

90 106 1140 2970

8.6 8.0 6.6 8.7 6.5 -

0.57 1.08

82 97

139 366

-

1.63

272

464 443 -

89 279 -

0.73 0.33 1.05

565 320 340 474

394 740

1.60 0.38 0.57 0.86

1570 1380 6300

-

1.36 1.40

-

3.40

1.98

24.3

1.40

15.6

14.6

1.30

40

2.20

47

3.80

2.50

44

45 110 79 118 210 820

2.54 -

-

7.3 4.0 7.5 19.6 16.3 86.0

8.1 5.2

53 318

-

0.55 1.55

19

500

6.6

480

2.80

3.36

43

98 51 1210

70 39 454

11.5 10.4 2.6

41 107 365

-

0.48 0.28 4.2

1.50 21.7

146 56 178 338

96 279 8350 22,400

10.4 9.8 5.7 2.7

49 128 125 160

-

0.79 1.9 5.9 -

by an increase in total Fe (greater than in the bulk sample) which might indicate the presence of magnetite, which was identified in the matrix of this meteorite by Wood [10]. As no chemical data are available for magnetite in Tieschitz, it is impossible to ascertain if the observed high Ni content is due to the presence of Ni-rich magnetite or, more likely, to a Ni-rich metal or Ni in a carbonaceous type matrix. The abundances of Au, Ir, W and Sb (relative to Ni) in the size fractions of Tieschitz, Bjurb~le

62

2.75

and Allegan are shown in Fig, 1, together with the ratios in the magnetic and non-magnetic portions taken from Rambaldi [11-13] and Rambaldi and Cendales [14,15]. The values are normalized to the ratios in the bulk. The results for Hamlet, Rupota and Weston are presented in Fig. 2. Iridium is a refractory element and its concentration increases, relative to Ni, with decreasing grain size in Bjurb6le and Hamlet but not in the other chondrites (Figs. 1 and 2). In ordinary chondrites from 10 to 24% of the total Ir content is present in the non-magnetic

112

10z

<> 0 A0

lO

zx

A

10

0<> <>Z 0

<)

[]

•

4>

u)

"E 1 "13 [-O

W/Ni

~- 10-~

I

-~ ~ []

I__.L_.L

l

I

O 102 0

.L.

bulk metal Tieschitz A ~, Bjurb61e [] []

(> [] 0

A

Allegan

0

Ir/Ni

non

mag. • •

I

i

L

0

I

I

I

]

!

I

I

[]

(>

10

A

0

A

<>Lx

¢

A

~---/.z m ti i

.08 I

<>

Au/Ni

I Sb/Ni

.1

A

t

I

bulk

j~j

L

7 8

t5

mesh I ~-metal4

J . _ _ L - L-

400 200 200 100-100 200

decreasing grain size

~-/.Lm~

I bulk - - ~

I

L

081

L

L

I

I

•

[] ~ I~ ~I~A --mesh - - ~ ~-metal~ I

t

I

,

I

7 8 15

400 200 200 100-I00 200 decreasing grain size

Fig. I, Siderophile element ratios in bulk fractions and separated metal and non-magneticportions of Tieschitz, Bjurb~le and Allegan. Ratios have been normalized to the values in the whole chondrite. Enrichments (relative to Ni) in Sb, W, and to a minor extent Au and Ir are present in the fine bulk fractions. Grain sizes given in mesh (100 to 400) and p,m (15 to 0.08) only indicate decreasing relative mean grain size (see text p. 109).

portion; this has been attributed to the presence of a fine-grained, refractory-rich component, possibly metallic [9,16]. The fine-grained samples of Tieschitz, Bjurb01e and Allegan (Fig. 1) have lower I r / N i ratios than the non-magnetic portions which indicates that the Ir-rich material is included within chondrules or coarse-grained silicates and not dispersed throughout the fine-grained matrix. Osmium behaves as Ir. Tungsten is the only refractory metal which displays a lithophile character in chondrites [14]. In type 3 chondrites more than 80% of the W is in non-magnetic minerals, and its concentration in

the metal increases with petrographic type within each chondrite group [14]. Our W data are not complete, but there appears to be an increase in the W content with decreasing grain size apparently with the exception of Weston (Table2). The largest W enrichment is observed in the > 0.08-ffm fraction of Allegan, which has a W / N i ratio almost 90 times greater than the bulk. With the exclusion of Tieschitz, the ratios of As, Au and Sb to Ni increase with decreasing grain size (Figs. 1 and 2) and, in the case of Weston, Rupota, BjurbOle and Hamlet this trend is paralleled by a decrease in the apparent metal content.

113

• Hamlet qD Rupota 0 Weston

103 g

Au/Ni

e

I

I

o

8

,io2t- ¢

0

t~

W/Ni

o

o

o

I

I

Sb/Ni I

•

4 ID

o o

Ir/Ni

As/Ni

~

Fig. 2. Siderophile element ratios in the bulk fractions of Hamlet, Rupota and Weston. The ratios have been normalized to those of the chemical groups represented. With decreasing grain size Sb, W and Au are enriched relative to Ni; iridium only in Hamlet, and As in Hamlet and Rupota. Grain sizes in mesh (400) and # m (8 to 1) only indicate decreasing relative mean grain size (see text p. 109).

The highest Au enrichments are found in the finest fractions of Allegan and Hamlet, with factors of 50 and 18 relative to the bulk samples, while for As the highest enrichment, a factor of 4, was found in the > 1-#m fraction of Hamlet. The A u / N i ratios in the fine-grained fractions of Tieschitz, Bjurb/Me and Allegan are substantially higher than in the non-magnetic portions (Fig. 1) indicating that Au, unlike Ir, is enriched in the "matrix-rich" fractions. The siderophile element most strongly enriched in the fine-grained material is Sb. It is the most volatile among the siderophile elements considered here. The greatest enrichment is found in Hamlet, where the > l-~m fraction contains 22.4 ppm Sb, which corresponds to a S b / N i ratio - 1800 times higher than in the whole meteorite. From these results it is apparent that the degree of enrichment of the analyzed siderophile elements in the finest fraction varies greatly among the chondrites studied.

3.2. Alkalies, aluminum, and REE In four of the chondrites analyzed by INAA (Allegan, Rupota, BjurbOle and Hamlet) there is an increase in the Na and K contents with decreasing sample grain size (Table2). Electron probe analyses show the same trends and the absolute values for Na also agree within < 20%. Also, as illustrated in Table3 where A1/Si, N a / S i and A1/Na ratios are presented, the much more refractory A1 increases with decreasing grain size in Allegan, BjurbOle and Tieschitz similarly as Na. However, for Rupota the A1/Si and N a / S i ratios decrease slightly in the > 1-/zm fraction relative to the intermediate, but are still considerably higher than those in the coarse fraction. A1/Si and N a / S i ratios for Weston remain essentially constant. All samples have almost unvarying A I / N a ratios which correspond to those in the whole meteorites [1720]. In Fig. 3a the (Na + K)/Feto t ratios have been plotted as a function of the grain size. In Tieschitz, the low value of this ratio in the > 0.8-#m fraction is probably related to the presence of magnetite [9], and the high Fe content (Table 2) dilutes the concentration of most elements, alkalies included; in Weston the alkali content does not vary with grain size. Sodium and K are major constituents of chondritic feldspar, and our results imply an increase in the abundance of "feldspathic" material in the fine-grained fractions. In "equilibrated" (grades 4-6) chondrites feldspar is generally interstitial between larger olivine and pyroxene crystals, but in Type 3 chondrites, which have alkali contents similar to other chondrites (including Type 6), feldspar cannot be identified, because the alkalies are either finely dispersed throughout the meteorites [5,21] or located in glass-bearing chondrules. Substantial enrichments of alkalies in chondrule rims have been recently reported by Allen et al. [22] and King and King [23]. Our results indicate also an increase in the K / ( N a + K) ratio with decreasing grain size for Tieschitz, BjurbOle, Hamlet and Allegan, but not for the gas-rich meteorites Rupota and Weston (Fig. 3b). Recently Fredriksson et al. [24], in a study of ultrathin polished thin sections of Bjurbtle, identified a feldspathic material, consisting of a mixture of albite (An 1-5) and Ca-plagioclase (An 85), which

114 TABLE 3 Ratios of Na, A1, Si in size fractions (electron probe data, wt.%) and bulk (literature data recalculated to silicates only, wt.%) for five chondrites Grain size *

Allegan

BjurNSle

>200 AI2Os/SiOa (× 100) Na20/SiO 2 (X 100) AI203/Na20

>400

5.1 2.4 2.1

Grain size *

>8

8.0 3.3 2.4

>0.05

8.0 3.6 2.2

9.4 4.1 2.3

Rupota

A1203/SIO 2 (× 100) NazO/SiO 2 (X 100) AI20/Na20

bulk [171

>200

5.3 2.3 2.3

5.4 2.5 2.2

>400

>8

> 1

bulk

[18] 4,1 2,0 2,1

Tieschitz

7.3 3.4 2.1

9.5 4.3 2.2

4.8 2.3 2.1

Weston

>400

>8

> I

bulk [I 7]

>400

>8

> 1

bulk [19]

>400

>8

> 1

bulk [20]

4.1 1.7 2.5

7.2 3.1 2.3

6.6 2.9 2.3

5.4 2.4 2.3

7.5 3.8 2.0

8.4 4.4 1.9

8.5 5.1 1.7

6.5 2.6 2.5

6.0 2.6 2.3

6.1 2.8 2.2

6.1 2.6 2.3

6.1 2.6 2.3

* Grain sizes, in mesh (200, 400) and p,m (8, I, 0.05), only indicate relative mean grain size (see text p. 109).

a.

is interstitial between chondrules and lithic fragments and often appears to permeate chondrules. They also identified K-feldspar-rich areas. Rare earths are also enriched in the fine-grained

10-t 0

~

No+K

~

0

Uetot4~

I I

"'~

I

I

t

b.

~ K Na+K

" "U]

~zx

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-.-~

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<~ ~ ID

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~ ^.

~

Allegan

m a t e r i a l r e l a t i v e to the w h o l e m e t e o r i t e s ( T a b l e s 1 a n d 2). T h e e n r i c h m e n t f a c t o r s g e n e r a l l y d e c r e a s e f r o m light to h e a v i e r R E E . T h e M S - 7 a n d I N A A d a t a are r e m a r k a b l y c o n s i s t e n t c o n s i d e r i n g the t o t a l l y d i f f e r e n t s a m p l i n g a n d a n a l y t i c a l techn i q u e s . W e also d e t e r m i n e d Sc, a r e f r a c t o r y lithop h i l e e l e m e n t like the R E E , in all o u r s a m p l e s b u t n o e n r i c h m e n t was o b s e r v e d . H o w e v e r , in t w o cases, T i e s c h i t z a n d A l l e g a n , a d e p l e t i o n of Sc w i t h d e c r e a s i n g g r a i n size is o b s e r v e d ( T a b l e 2). T h i s is also i n d i c a t e d in Fig. 3c w h i c h s h o w s the v a r i a t i o n o f t h e L a / S c r a t i o as a f u n c t i o n of g r a i n size. T h e R E E f r a c t i o n a t i o n p a t t e r n ( r e l a t i v e to a v e r a g e c h o n d r i t e a b u n d a n c e s ) is s h o w n in Fig. 4 for H a m l e t , R u p o t a a n d W e s t o n . W h i l e in the c o a r s e f r a c t i o n s ( > 4 0 0 m e s h ) the R E E are n e i t h e r en-

fO-I La/Sc /J.m I

I

i I I

I

f5 "----decreasing grain size 08

f

78

wmesh-~ I

I

400

200

Fig. 3. (a) Alkalies are enriched in the fine bulk separates of four chondrites, not in Tieschitz and Weston, and in some cases (b) an enrichment of K over Na is also observed. Enrichment of La (and the other REE) with decreasing sample grain size is uncorrelated with other refractory lithophile elements, e.g. Sc (c).

115

IO

~'~~"~~~i> .~ c-

I

3000

Hamlet

~ I

I

I

I

~:1

I

E 0-

2000

O"w"

Rupota

0

<>

I ~m 1" >400mesh

I~1Bj(K=6450 ppm)

1000 -

O • ......

~1 0 ~E

OOmesh 1

I

I

~-

I

I

0

1

I

I

I

1

2

3

4

Weston

x..,.--I

La

I

I

Sm Eu

x_~>4OOmesh I

I

Yb Lu

Fig. 4. REE fractionation patterns in coarse (>400 mesh) and fine (>8 to I /.*m) fractions of Hamlet, Rupota and Weston. REE are enriched in the fine fractions compared to the coarse. The enrichment of La is substantially greater than for the other REE. Note a positive Eu anomaly in Hamlet. REE fractionation pattern observed in a volatile-rich clast from the H6 chondrite Supuhee [38] is'also shown.

riched n o r fractionated, the > 1-tim fractions show s t r o n g e n r i c h m e n t a n d fractionation. The Eu a n o m a l y is c o r r e l a t e d with alkali a b u n d a n c e s . N o Eu e n r i c h m e n t is o b s e r v e d in W e s t o n (Fig. 4) where the alkali c o n t e n t is similar in the coarse a n d fine fractions ( T a b l e 2 ) ; in R u p o t a a n d H a m l e t , an e n r i c h m e n t of alkalies in the > 1-/~m fraction (Fig. 4) is a c c o m p a n i e d b y a positive Eu a n o m a l y . This is also illustrated in Fig. 5 where the S m / E u ratios in the fine fractions of each m e t e o r i t e are p l o t t e d versus the K content. W i t h the exclusion of BjurbOle, which has an e x c e p t i o n a l l y high K content (we suspect c o n t a m i n a t i o n ) in the o t h e r four s a m p l e s the S m / E u ratio is negatively c o r r e l a t e d

5

Sm/Eu Fig. 5. With the exclusion of Bjurb6le, the Sm/Eu ratios in the finest separates of each chondrite are negatively correlated with K content. Allegan and Hamlet fractions have high K (and Na) concentrations and low Sm/Eu ratios as a consequence of a strong Eu enrichment relative to Sm (and for Hamlet heavy REE) when compared to the coarser fractions and bulk chondrite values (Table 2 and Fig. 4). In Weston there is no alkali or Eu enrichment. Symbols as in Fig. 3.

with K a b u n d a n c e s . These d a t a also indicate an e n r i c h m e n t of a feldspathic c o m p o n e n t in the ultrafine fractions o f the higher-grade chondrites,

3.3. Volatile elements T h e c o n c e n t r a t i o n of the volatile elements Pb a n d Sn (Table 1), a n d Zn, Br, Sb, Hg, A g ( T a b l e 2), g e n e r a l l y increases with decreasing grain size. The o n l y exception is Se, which is present in the fine fractions at c o n c e n t r a t i o n at c o n c e n t r a t i o n levels lower than in the coarser s a m p l e s or the bulk. Selenium is chalcophile in c h o n d r i t e s a n d is present in troilite at c o n c e n t r a t i o n levels of 100-150 p p m ( R a m b a l d i , unpublished). T h e low Se a b u n d a n c e in the fine-grained samples indicates a decrease in the sulfide content, which parallels the t r e n d previously o b s e r v e d for the metal. The elec-

116

tron probe da~a also show a consistent decrease of S, except possibly for Weston, where it is nearly as constant as is Se. Zinc is generally considered chalcophile in chondrites, but its low concentration in troilite (up to 100 ppm; Rambaldi, unpublished), coupled with the low sulfide abundance in the fine-grained samples, indicates that the observed increase in Zn content must be attributed to some phase other than troilite. Ferromagnesian silicates (olivine, pyroxenes) contain only 10-50 ppm Zn [25], but chromite has high Zn concentrations (up to 4000 ppm). With the exclusion of Hamlet, the Cr content does not correlate with Zn in our samples; the most extreme case Tieschitz where Cr decreases by a factor of 2.6 while Zn increases by a factor of 12 in the >0.08-/~m fraction relative to the coarse ( > 400 mesh) fraction. The data for Br, Ag and Hg are not complete and, as already discussed, are of low accuracy. However, this is not the cause of the sharp increase in Br and Hg content observed in most of the fine-grained samples. A problem in the case of Br is that a substantial amount of Br in chondrites ( 3 / 4 of the total) resides in a fraction leachable in hot water [26]. We have no way of estimating the amount of Br loss in our samples, but when comparing the results for BjurNSle (which was disaggregated with distilled 0°C water) with those for Rupota or Hamlet (disaggregated with alcohol) in Table2, there seems to be no indication that Br loss occurred. Mercury (?) and Ag are both siderophile in condrites [27,28]. Unlike other highly volatile elements, Hg is surprisingly overabundant (terrestrial contamination has often been suggested) in chondrites, and its concentration is highly variable, even in different samples of the same chondrite [29]. It is also highly variable in CI chondrites; the values determined in Orgueil range from 2.4 to 213 ppm [27]. The abundances, relative to CI, of 18 elements (Os and rare earths excluded) in the finest grain size fraction of the six chondrites studied are shown in Fig. 6. For Tieschitz the data for both the > 8 and > 0.08-/~m fractions have been included. The CI values used to calculate the enrichment factors in Fig. 6 were taken from Ganapathy and Larimer

TABLE 4 Condensation temperatures of elements analyzed in fine-grained chondrite fractions, compare Fig. 6 for order of listing Element

Condensation temperature

Reference

(°K) W Sc Ir Fe Co Ni Cr Au Mn As K Ag Na Sb Se Zn Br

1798 1644 1555 1336 1348 1349 1277 1225 I 190 1157 1080 993 982 9t2 684 684 357

[331 [33] [33] [28] [34] [34] [34] [35] [28] [35] [33] [28] [28] [35] [28] [281 [361

[30] except for W, Mn and K for which the following values were used (in ppm): 0.088 [31], 1950 [32], and 545 [32], respectively. The predicted condensation temperatures of the elements are given in Table4 and also shown in Fig. 6. The nonvolatile elements (W to Cr) are "conventionally" plotted in order of decreasing "condensation temperature". Ganapathy and Larimer [30] observed in volatile-rich material from Abee a factor of 30 depletion for these elements relative to CI. The relatively high concentrations (W excluded) in our samples are probably related to the presence of abundant material, produced by the break-up of coarse grains (chondrules, large silicates), which, in most cases, must represent a substantial portion of our fine-grained fractions. The most depleted nonvolatile elements in our samples are Co and Ni, which is consistent with their low metal content relative to the bulk chondrites. Significantly, however, most non-volatile elements fall within 0.5 to 2 times the CI values. The volatile elements in Fig. 6 are plotted approximately in the order of increasing enrichment relative to CI as observed in Abee [30]. They are strongly fractionated and, with the exclusion of

117

102

A Tieschifz A Tieschitz o Weston qDRupota [] BjurbSle • Hamlet 0 Allegan

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(>8/xm) (>.08 Fm) (>1 Fm) (>1 /xm) (>8Fm) (>I/xm) (>.08Fm)

2000

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Temperature "'-,

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I

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O0

Ni

Or

Mn

$e

As

I

I

Na

K

I

I

I

I

I

I

Au Br Hg Zn Ag Sb

2OO

Fig. 6. Element abundances (relative to CI) in the finest grain size fractions. For Tieschitz the data for both > 8 - and >0.08-p.m fractions have been plotted. The scatter is large, but strong enrichment of W and some volatile elements evident. The volatile enrichment trend in material separated from Abee [28] is also shown as well as condensation temperatures of the elements (see Table 4).

Na, K (which was not analyzed in Abee), and Au all the data plot below the Abee enrichment trend, represented by a heavy line. The general trend in Fig. 6 indicates an increasing abundance, relative to CI, from Se to Sb, with Br and Zn "strongly depleted" in ordinary chondrites [37,38] and falling below the trend defined by the other elements. However, the degree of enrichment or depletion varies greatly for each element in the different chondrites. The largest variation is shown by Sb, where enrichment factors range from 1.6 for Tieschitz to 160 for Hamlet. The other elements display variations by less than a factor of 15. Part of the scattering observed in Fig. 6 is prob-

ably related to the different grain size ranges of material analyzed ( > 4 0 0 mesh and > 8 # m for BjurbOle, << 1 /~m to > 0.08/Lm for Tieschitz and Allegan) and therefore, to different degrees of dilution by fragments of coarser silicate material. Most likely, however, the scattering is due to original compositional differences or alterations of the HS in the various samples. In Fig. 7 the volatile element abundances have been normalized to the bulk chondrite values in Table2. When data were not available, as for Rupota and Weston, our values were normalized to the mean composition of the chemical group to which the meteorite belongs. For Rupota we aver-

118

0

@ 0

I

I

@AA

IAdA 0

.I- Se As Na K

0

0 Weston I

lo

1:

:

0 Rupota I I I

I

.

A

A

Kesen, by Dreibus et al. [39]. The Hg content of H and L chondrites was obtained using the average ordinary chondrite abundance of 1.1 atoms/ 1O6 Si given by Larimer and Anders [37]. The results for the volatile elements alone (Fig. 7) indicate that at least three (apparently) different volatile enrichment patterns are present. For Bjurbole and Allegan the enrichment factor relative to the bulk chondrite values shows a tenfold increase from Se to K, then remains constant, defining a plateau at a value of about 9 for Bjurbole (K to Sb) and 20 for Allegan (Au to Sb). In Weston and Rupota the first four elements (Se to K) are not enriched (the factors cluster around l), then the factor increases from 2 (Au) to about 9 (Br, Zn, Sb), while Hg and Ag stand out with an disenrichment factor of - 35. As previously cussed, the rather low precision of our Br, Hg and Ag determinations and the lack of bulk values for these elements in Weston and Rupota might have altered to some unknown extent the trends shown in Fig. 7 for these two meteorites. Tieschitz and Hamlet have unique volatile enrichment patterns. In Tieschitz eight out of the ten elements analyzed have abundances relative to the bulk values ranging from 0.4 (As) to 3 (Sb), but Zn and Ag are strongly enriched by factors of 60 and 160, respectively. In Hamlet the volatile element abundance ratios increase from Se (0.25) to Sb (200) in a pattern which very closely resembles that found in fragments from the Abee enstatite chondrite [30].

A 3.4. SEM studies A Tieschitz l Hamlet

Au Br Hg Zn Ag Sb

Fig. 7. Volatile element abundances, in the finest grain size fractions. patterns are discussed in the text.

relative to the bulk rock, The different enrichment

aged the Br content of Hamlet (0.79 ppm) and Bjurbole (0.48 ppm) and used a value of 0.63 ppm Br, which is not too different from the Br content of 0.550 ppm found in the > 400 mesh fraction (Table2). For Weston we chose the Br content (0.36 ppm) determined in another H4 chondrite,

In order to better understand the nature of the fine-grained “matrix-enriched” material, a SEM study of particles from the > l-pm and > 0.08~pm fractions of several samples but especially Bjurbole and Allegan was carried out. These meteorites are of particular interest in this study because, while they are compositionally similar to other chondrites of the same petrographic type and chemical group, they are also very friable and thus easy to disaggregate. They are also “equilibrated”, in the conventional sense of having constant Fe/Mg ratios in olivines and pyroxenes, i.e. in those which are large enough for ordinary electron probe analysis. We believed that a study of the texture and com-

Fig. X. SEM images of matrix particles from the Allegan chondrite. A. Cluster of intergrown hexagonal, extremely delicate crystals only -0.7 pm wide and -0. I pm thick. Scale bar, 0.1 pm. B. Thin flake of apparent layer lattice silicate. analyses of points 1 and .? in Table 5. Scale bar. I pm. C. Spherulc of Al(Ca) silicate and olivine (?) splinter. Analyses in Table 5. Scale bar, 1 pm. D. Anhedral small olivine (‘?) grain. I, with approximately normal Fe/Mg ratio for Al&an on top of larger subhedral (- 7 pm) iron-rich olivine (‘?) grain, .?. Analyses in Table 5. Scale bar, 1 pm.

120 TABLE 5 C o m p o s i t i o n of m a t r i x grains (see Fig. 8) from A l l e g a n d e t e r m i n e d b y energy-dispersive X-ray analysis (values in wt.%) Fig. 8B

SiO 2 A1203 FeO MgO CaO K20 Na20

Fig. 8C

Fig. 8D

1

2

1

2

1

2

27 36 6 17 2 13 -

26 46 3 10 2 12

51 42 ~0 ~0 6 1 ~ 1

31 ~ 0 22 47 --0 --0 -

39 6 21 33 < 1 t0

23 7 59 8 2 --0 -

position of the fine-grained particles in these chondrites might provide important information with regard to their origin. Very small amounts of material retained on the Nuclepore filters were transferred to discs of pyrolitic graphite and coated with carbon or gold, the latter coating allowing better images but yielding poorer energy dispersive X-ray analyses (especially interfering with Na-K,). In some cases when only minute amounts of material were available a small wedge of the Nuclepore filter was mounted on the graphite disc. Images in the magnification range 10K to 50K and X-ray spectra were obtained of selected particles. These studies are still in progress and will be reported elsewhere. However, a few grains from Allegan are shown in Fig. 8 and semiquantitative analyses given in Table 5. These values were obtained by calculating the ratios of all elements detected to Si and correcting these ratios according to data obtained on a known mineral standard crushed to the same size as the unknowns. Assuming a total of 100% the approximate composition could be obtained. Some of the crystals are euhedral and extremely delicate, e.g. the particle shown in Fig. 8A. In this case no analysis could be made, but other similar grains (e.g. Fig. 8B) indicate the possibility of an Orgueil type [40, fig. 1] layer lattice silicate, or perhaps material similar to the "chlorite-like" grains once reported from BjurbNe by Fredriksson [41] but later disregarded as possible weathering products (e.g. [42]). Phases rich in K, Ca or AI and Si like those in Fig. 8B, C and Table 4 apparently have

Ca-rich hexagonal flake (no photo) 2 16 < I 5 76 -

not been previously reported from ordinary chondrites, although Fredriksson et al. [24] reported co-existing albite, anorthite, and a K-rich phase also BjurbNe. This material might well be responsible for the concentration of geochemically different elements in our "matrix". In Fig. 8D, two apparent olivines with widely different F e / M g ratios (Table 5) are shown. This also strongly indicates gross disequilibrium between matrix and chondrules (see also Fredriksson et al. [6]) as well as between phases in the matrix.

4. Discussion

Chemical differences between CI and ordinary chondrites have often been interpreted in terms of nebular fractionation processes. Some of these processes may have depleted volatile elements in ordinary chondrites relative to CI. The depletion factors vary considerably, from 0.2-0.4 × CI for the "normally depleted" elements (As, Sb, Ga, Se and others) to factors as low as 10-3 × CI for the "strongly depleted" elements in In and T1 [37]. The depletion of the first group of elements is believed to have been produced by loss of nebular gas from the ordinary chondrite formation region during condensation [28,43] or to have resulted from the mixing in approximately fixed proportions (1 to 3) of two materials, one (generally identified with the fine-grained, oxidized matrix of carbonaceous or unequilibrated chondrites) containing CI abundances of volatile elements, and

121 one (chondrules and large metal grains) which lost its complement of volatiles during a degassing process [37,44]. The very low abundance of In, TI, Bi, and Cd in ordinary chondrites is taken by some authors as an indication that at the time of accretion of chondritic matter, these elements were only partially condensed upon the fine-grained, volatile-rich matrix [37,38]. The discovery of clasts in the brecciated H6 chondrite Supuhee containing "strongly depleted" elements in concentrations even greater than those found in CI chondrites, has suggested the presence in chondrites of a volatile-rich component, sometimes called "mysterite", which is believed to represent a late nebular condensate, formed near the end of the accretion process, by collection of volatile materials still present in the nebular gas [45]. Enrichments of volatile elements have also been found in some Type 3 chondrites, e.g. Krymka (LL3) and Mez6-Madaras (L3) [45], and in gas-rich meteorites [46]. As inferred from the volatile element patterns observed, different types or alterations of this material must exist. At least two volatile components were postulated by Higuchi et al. [45] to explain their results for Krymka, Mez6 Madaras and Supuhee: a Tl-rich and a Tl-poor (Bi, Ag-rich) variety. A third type (In-rich) may be present in unequilibrated and gas-rich chondrites [47]. The volatile-rich clast of the Supuhee chondrite studied by Higuchi et al. [45] and Davis et al. [48] is enriched in Na and REE relative to CI. The REE pattern is strongly fractionated, the enrichment factor decreasing from 4 (La) to 2 (Lu), and there is an indication of a negative Yb anomaly. Although this trend is indicative of igneous differentiation, these authors point out that similar fractionation patterns have been found in Allende matrix and whole rock samples, and they believe that the REE fractionation observed in the Supuhee clast may have been produced during condensation. Similar fractionation trends are found in the fine-grained fractions of our six chondrites. A comparison between the REE enrichment pattern in these fractions and that of the Supuhee clast (Fig. 4) indicates that these fractionated materials are probably genetically related. Recently Ganapathy and Larimer [30] have iso-

lated a material from the E4 chondrite Abee which is enriched in Sb, Ag, Hg, Zn and Br by factors ranging from 10.4 (Br) to 574 (Sb) relative to CI. Other elements (Au, As, Na) are also enriched in this material, which the authors believe might represent a primary late condensate from the solar nebula. As indicated in Figs. 6 and 7, there appears to be a close similarity between the pattern of element enrichment in Abee and in the fine fractions of the six chondrites studied, particularly in the case of Hamlet. This further supports our view that various amounts of volatile-rich materials of this nature are present in ordinary chondrites, independent of petrographic type. Large differences in relative volatile element abundances are present among the fine fractions of the various meteorites and there appears to be no correlation between element enrichment and condensation temperature. These elements differ considerably in geochemical properties, some are siderophile (Sb, Ag, Hg, Au, and As), others lithophile (Br, Na and K) or chalcophile (Zn and Se). As already discussed by Higuchi et al. [45], the availability of a specific substrate might affect the efficiency with which volatile elements of different geochemical character could have been collected. In this case various groups of volatiles displaying similar geochemical affinities may have been more efficiently collected upon one specific substrate, e.g. possibly metallic for siderophile elements, or a sulfide for the chalcophile, and subsequently introduced or retained in the chondrites. Recent studies on the volatile element content of coexisting dark and light portions of gas-rich chondrites [46] and of clasts in the gas-rich chondrite Leighton [49] indicate a non-uniform enrichment in volatile elements, inconsistent with prior suggestions of admixture of CI or C2 chondritic matter [50]. In the Leighton chondrite each clast displays its own volatile element trend, and mixing models with end members of fixed composition do not explain the observed differences [49]. Rambaldi and Cendales [15], observed that, unlike other siderophile elements, Cu and Sb are not fractionated among chondrite groups. They suggested that these elements did not enter into solid solution in metallic Ni-Fe until after the metal-

122

silicate fractionation process which resulted in the H, L and LL chondrites. Fine-grained Cu and Sb-rich particles might have been preserved until accretion of the parent body, and entered the metal at a later stage, possibly together with other volatile siderophiles. Some "ad hoc" assumptions are required in order to explain the enrichment of W (Table2), which is considered a strongly refractory metal. In ordinary chondrites W, unlike the other refractory metals, is partitioned between metal and silicates, and its partition coefficient increases with petrographic type within each ordinary chondrite group, indicating that in Type 3 chondrites most W is in oxidized form [14]. If Types 3 to 6 chondrites were to be considered a "metamorphic sequence", then it must be assumed that W was mostly present in oxidized form in the "unmetamorphosed precursors" of Types 5 and 6 chondrites. In this case the reduction process required to transfer W from the silicates into the metal must have occurred on the parent body during the supposed process of recrystallization of the silicates. During this process high W concentrations are probably expected to be achieved at first in the fine-grained metal. More recently Rambaldi et al. [51] have suggested that chondrites of different petrographic type might accrete materials with different W m e t a l / W oxide ratios, high in T y p e 6 and decreasing towards Type 3. Oxidized W would be produced by reaction of fine-grained metal with nebular oxygen, while coarse-grained metals remain substantially unaffected by this oxidation process. If so, residual fine-grained metal would be initially depleted and not enriched in tungsten. W~nke et al. [52] performed laboratory experiments on the mobility of several siderophile elements, including W, Ir, Au, Ni, and Cu, at different temperatures, and the results indicate that, unlike Ir, upon heating at temperatures of 800-1000°C in vacuum, W does not behave as a refractory element and is easily transported as gaseous WO 3. These results seem to indicate that volatile W oxides may form at temperatures well below the predicted condensation temperature of this element as a refractory metal. Perhaps it is possible that by a similar process gaseous compounds of W may form in reheated chondrites and recondense at a later stage.

An important problem concerning the great variation in the volatile element compositions observed in chondrites is that of assessing the extent to which post-accretional processes on the surface or in the interior of the parent body (impacts, volcanism, metamorphism) may have altered the original volatile element trends. It is widely recognized that gas-rich meteorites were formed as breccias in the regolith of chondritic parent bodies, and that the large content of volatiles was introduced at this stage by admixture of volatile-rich material [50,53]. Yet some authors believe that the enrichment of some elements may have originated as emanations from the metamorphism in the interior of the parent body, which found their way towards the surface where they were subsequently deposited [54]. Takahashi et al. [55] have suggested that the volatile element patterns of chondrites may have been established by nebular condensation and remained substantially unchanged during the postaccretional processes. This view is also shared by Ikramuddin et al. [56] who observed that the volatile loss pattern of artificially heated Tieschitz and Krymka did not match those of H 3 - 6 and L3-6 chondrites. They concluded that metamorphism, if any, of H and L chondrites did not take place in an open system. In a study of noble-gas distribution patterns in a suite of LL chondrites (including Hamlet), Alaerts et al. [57] analyzed a minor phase (called "q" soluble in H N O 3, which is enriched in primordial noble gases relative to the bulk meteorites. The percentage of bulk gas in "phase q" decreases with increasing petrographic type (3 to 6), which might be suggestive of losses during metamorphism. But the elemental ratios of the noble gases in this material also varies with petrographic type in a fashion indicative of condensation from a cooling solar nebula, rather than of metamorphic losses. These authors conclude that the relationship between volatile content and degree of equilibration observed in chondrites does not seem to be the product of outgassing during metamorphism, but to have been established in the solar nebula. As supported by these studies, it is not unreasonable to suggest that the volatile element enrich-

123 ment observed in our "matrix-enriched" samples also may have survived from the nebular condensation and the early accretion. Similarly the differences observed do not seem to be consistent with relatively uniform metamorphism in the interior of a parent body but rather with erratic events affecting different elements in different meteorite matrices to varying extents. In this case it would be very important, in order to better characterize the type of materials present in our samples, to know the concentration of other volatile elements, e.g. T1, Bi, Cd, In, Cs, and rare gases, and to establish if they were introduced or redistributed during later brecciation event(s) on the surface of a parent body, as for Supuhee and the gas-rich meteorites. This last possibility is supported by enrichments of elements from relatively brittle material such as feldspar and phosphate and depletion of elements contained within relatively ductile (i.e. resistant to fragmentation) material such as metal and sulfides in the fine fractions of the chondrites studied (J. Kerridge, private communication). Two of the meteorites studied here (Weston and Rupota) are gas-rich and therefore probably acquired their complement of volatiles by a process similar to that invoked for the other gas-rich meteorites [50,53]. Tieschitz is a Type 3 chondrite, and like the volatile-rich Krymka, MezO Madaras, and Sharps contains inclusions of foreign chondritic material, some of them carbonaceous [58,59]. In all these cases, admixture of surviving, more "primitive" volatile material may have occurred during the brecciation events. A major problem arises with the Type 4 and 5 BjurbOle, Hamlet and Allegan, which have "equilibrated" olivine and pyroxene and for which the presence of inclusions of different composition has not been reported except for aggregates of troilite and chloritic material in Bjurb61e [42]. If these meteorites acquired the volatile-rich material during accretion, and not during a later brecciation event on the surface of a body, then its composition appears to be incompatible with that of major mineral phases and in gross disequilibrium with it. Even if thermal recrystallization occurred in a closed system without appreciable loss of volatiles, a considerable redistribution of elements must have

taken place among the various minerals present. Strongly siderophile elements like Sb, Au, As and Ag would, upon such recrystallization, easily diffuse into the abundant metal present, and other trends (REE and K / N a fractionations) would probably be erased by the strong reheating required to equilibrate the silicates. Our SEM data indicate the presence of mineral phases, including iron-magnesium silicates with different F e / M g ratios, with compositions inconsistent with the observed mineralogy of "equilibrated" chondrites. The possibility still remains that mixtures of small amounts of surviving volatile-rich materials were added to these chondrites during multiple fragmentation and brecciation events on the surface of a parent body. The lack of (reported) foreign inclusions in these chondrites does not necessarily exclude these processes because many chondrites are "monomict" breccias. In this case the observed volatile-rich material could have been introduced well after (or during, if by impact [60]) the process responsible for the equilibration of the silicate minerals. Very recent analyses of "cosmic dust" (or so-called Brownlee particles), by Ganapathy and Brownlee [61] and Hudson et al. [62] suggest that it may be the carrier of at least some of the volatile elements and related to HS. Again it must be emphasized that we have not analyzed pure "Holy Smoke" but only samples enriched in such material, and we do not exactly know the location of the different elements. Thus, in not knowing what we have analyzed, we are in the same predicament as other investigators of mysterious, e.g. "q", etc., components. However there are reasons to suspect that the ultrafine matrix also contains volatile elements not yet analyzed, e.g. Cd, Bi, T1, and perhaps rare gases, and that some isotopic anomalies may exist. 5. Conclusions

(1) Ordinary chondrites, including the so-called equilibrated types, independent of chemical class and petrographic grade contain a matrix component of apparently low-temperature origin which is akin both to carbonaceous chondrites and to unusual carbonaceous fragments found in some

124

ordinary and enstatite chondrites. (2) Both volatile and refractory elements, such as Na and A1, and trace elements, such as the volatiles Sb, Ag, Zn, and the refractory W and REE, are enriched in the matrix relative to the bulk material, sometimes exceeding the CI level, but in different meteorites in grossly varying ratios. Apparently different types of carriers must account for the presence of these elements in the matrix component. (3) Chemical composition and mineralogy of the ultrafine material are clearly inconsistent with thermodynamic equilibration because it is in gross disequilibrium internally as well as with the major mineral phases. (4) The Sb, data presented here and the Cu data published previously [15] seem to make it necessary to postulate at least two generations of chondritic metal: first, Sb and Cu-poor metal, formed during condensation and primary accretion [15], and was partly removed by metal-silicate fractionation; in a later process, involving melting, reduction and probably chondrule formation, a metal phase cogenetic with chondrule silicates was formed at higher temperatures and together with the preexisting metal, it incorporated available Sb and Cu. In this case, the matrix with high Sb (and probably high Cu) content may represent a residue from the first fractionation, partly surviving the later process. (5) The ultrafine matrix material may have been present in the primitive regolith (or "fluffy body") and redistributed and altered during impact events which resulted in brecciation, reheating, and possibly formation of chondrules and of a major part of the cominuted matrix. The impacting matter may also have been a source for the enriched elements, and, if of differing compositions, the primary cause for the observed variations of enrichment factors. If the impacting material had constant composition the variations would have to be explained similarly as if the enriched elements were in the regolith.

Acknowledgements The authors wish to thank H. Suess, H. W~inke, J.T. Wasson, F. Fudali, J. Kerridge for comments

and suggestions, and S.R. Taylor for advice and use of the Ms-7 at A.N.U., Canberra. We are also grateful to our editor, F. Begemann, who corrected some rather serious ambiguities. Ms. P. Brenner made the separations and, together with J. Nelen, most electronprobe analyses. Ms. E. Flenteye (U.C.S.D.) assisted with the SEM studies. The samples were activated in the TRIGA Research Reactor, Institut ft~r Kernchemie, JohannesGutenberg Universit~t, Mainz; the INAA work was done in the Abteilung Kosmochemie, MaxPlanck-Institut, Mainz. This research was supported in part by the Deutsche Forschungsgemeinschaft and NASA grant 05-007-329. One of us (K.F.) thanks S. Dillon Ripley for support from the Smithsonian Fluid Research Fund.

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