Chemical fractionations in meteorites—VI. Accretion temperatures of H-, LL- and E-chondrites, from abundance of volatile trace elements

Qeochimica et Conmochimica Acta.1973,Vol.8, pp.329to 367.Per@amon Press.PrintedIn Northern Ireland

Chemical fractionations in meteorites-VI. Accretion temperatures of H-, LL-, and E-chondrites, from abundance of volatile trace elements J. C. LAUL,*

R.

GANAPATHY,

EDWARD

ANDERS

and JOHN W. MORGAN

Enrico Fermi Institute and Department of Chemistry, University of Chicago, Chicago, Illinois 60637, U.S.A. (Received 11 February 1972; accepted in reoised form 23 August 1972) Abstract-Extending our earlier work on 11 L-chondrites, we have measured 9 volatile elements (Ag, Bi, Cs, In, Rb, Tl, Se, Cd, Zn) by neutron activation analysis in 11 LL- and 10 E-chondrites; the first 6 elements also in 22 H-chondrites. The observed fractionation patterns are consistent with theoretical condensation curves, and hence were apparently established during condensation from the solar nebula. Ordinary chondrites seem to have accreted between 420 and 500’K at P M 10msatm; enstatite chondrites, at 450 to Ei20’K and P m 5 x 10m4atm. The valuea for ordinary chondrites agree with 01*-based temperatures by 0~0~ et al. (1972), and with other characteristics such as Few content, presence of FeS, and absence of Fe,O,. A few detailed trends were noted. Seven of the 54 meteorites seem to contain small amounta of a material enriched in Ag, Bi, and especially Tl; possibly a late condensate from a region depleted in metal. Silver shows considerable scatter, which suggests inhomogeneous distribution in the meteorites. Xenon correlates with In approximately as expected for equilibrium solubility, with some differences (petrologic type 3; E-chondrites) attributable to mineralogical factors. Meteorites of higher petrologic types are slightly deficient in Xe, probably due to gas losses during metamorphism. Cesium also appears to have been redistributed during metamorphism. Various features of the two-component model are critically examined in the light of the latest evidence. Apparently this model still is an adequate approximation of reality. INTRODUCTION IN THE fourth paper of this series (KEAYS et al., 1971; hereafter referred to as IV), we reported the abundances of 14 mostly volatile trace elements in L-chondrites. Our principal conclusions were : (1) the abundance pattern is consistent with the two-component model of meteorite formation (WOOD, 1962; ANDERS, 1964; LARIMER and ANDERS, 1967; hereafter referred to as II). According to this model, chondrites are a mixture of two types of condensate from the solar nebula: a finegrained material (matrix) that collected volatiles from the cooling gas up to the time of accretion, and a coarse-grained material (chondrules, metal particles) that ceased to take up volatiles at a higher temperature. (2) The most volatile metals (Bi, Tl, In) can be used as ‘cosmothermometers’, to indicate the accretion temperature of the meteorites. All three thermometers give reasonably concordant results, falling between 460 and 660’K at an assumed nebular pressure of 1 x lo-’ atm. These results agreed with earlier, preliminary estimates (II; ANDERS, 1968). In two subsequent papers, we extended this cosmothermometry method to differentiated objects, e.g. the Earth, Moon and Ca-rich achondrites (ANDERS et al., 1971; LAUL et al., 1972). Results again fell into the range 450-600”K, and agreed 018/016 fractionation (ONUMB.et al., 1972) and with estimates by other methods:

* Present address: Radiation Center, Oregon State University, Corvdlis, U.S.A. 329

Oregon 97331,

330

J. C. LAUL, R. GANAPATHY,EDWARD ANDERS and JOHN W. MORUAN

Fe2+ content of silicates (LARIMER, 1968a,

1973;

LARIMER

and ANDERS, 1970;

GROSSMAN, 1972).

We now report data for 43 chondrites of 3 additional classes: H, LL and E. Because of the emphasis on thermometry, we have shortened the list of elements to nine. In addition to the three thermometric elements Bi, Tl and In, we have measured three to six others that show variable abundance: Ag, Rb, Cs, Zn, Cd and Se (the last three only in LL and E chondrites). Noble gases were determined in aliquots of 10 meteorites (GANAPATHY and ANDERS, 1973), for comparison with the trace element data. EXPERIMENTAL Samplea. All petrologic types were represented, though not evenly: Class

H LL E

Total

Type 3

4

6

6

3 2 1

4 3 2

5 2 2

10 4 5

22 11 10

A disproportionatelylarge number of H6 and H6’s was measured, because the first few of these meteorites showed unexpected trends. All meteorites in this study were observed falls except for the enstatite chondrites whose rarity necessitated inclusion of two finds (Atlanta, E6 and Blithfield, E6). Most samples ranged from 0.1 to O-2 g, though some were as small as 0.05 g. Judging from the results in IV, sampling errors for samples of this size are less than 60 per cent for most elements. ProcecEure.Our basic procedure is available from the sources listed in IV. Reproducibility. No replicates were measured, except for Supuhee, a peculiar H6 chondrite, and Charsonville (H6). The overall reproducibility of our procedure can be judged from replicate analyses of the geochemical standard BCR-1, a sample of which was included with every one of our irradiations, and our replicate analyses of L-chondrites and lunar samples (IV; LAUL et al., 1971, 1972). For 16 BCR-1 samples measured over a period of 4 years, the standard deviation of a single result (per cent) is: Ag (lo), Bi (7), Cs (2), In (S), Rb (3), Tl (lo), Se (8), Cd (ll), Zn (10). Cs and Rb were analysed in only 6 samples. RESULTS Our data are shown in Tables 1 and 2. Meteorites are arranged by class and by increasing petrologic type. Elements are listed according to their volatility in a solar gas. Comparison zG?h other work. Only 22 of our meteorites had been previously studied (Table 3). We shall comment only on discrepancies by more than a factor of two. Selenium. Our values for Abee and Pantar-L are higher than previous determinations, but fit better into the general trend for meteorites of these classes (Tables 1, 2). Silver. Chainpur and Khairpur are lower than GREENLAND’S(1967) values but Hvittis and Pillistfer are higher. In replicate analyses of L-chondrites, no such large discrepancieswere found (IV). However, three of the above meteorites are E-chondrites, and in view of their peculiar mineralogy, the possibility of a more heterogeneous distribution of trace elements cannot be excluded.

Class H3 H3 H3 H4 H4 H4 H4 H5 H6 H5 H6 H5 H6 H6 H6 H6 H6 H6 H6 H6 H6 H6 H6 H6 H6 H6 Bs& Bs%

source,* number F265 U640 Cl973 H F1394 F1443 Fl719 F498 Fl841 8603.3 W F1994 F1566 W W F1660 F1484 F526 F1626 F2100 F1723 Fl880 F1880 Fl880 F1880 F268

Rb

2.0 46 44

2.5 3.1 0.43 0.94 2.3 1.5 2.6 0.55 0.75 1.0 1.2 1.2 1.8 0.72 1.5 0.51 2.4 1.9 1.7 2.0 1.1 1.5 2.9 2.3 2.3

ppm

Se

0.08 0.09

-

7.9 9.0 -

3.6 -

7.6 -

8.1 7.2 11.0 8.4 7.2 9.1 8.9 9.9 11.6 77 -

8.6 -

ppm

198 24 25

47 100 98 33 98 93 170 22 71 33 66 61 240 166 49 38 67 50 194 32 53 151 76 101 81

Ag ppb

cs

39 910 930

188 130 79 92 89 107 22 10 34 52 6.3 79 8.4 7.8 9.2 42 22 34 7.3 31 22 110 77 38 68

ppb

Bi

38 46 44

6.45 100 100 l-34 3.16 4.16 16.0 0.31 1.01 0.49 0.17 1.38 4.26 12.3 1.76 2.57 3.90 2.29 7.89 I.13 10.3 76.8 3.56 29 12.5

ppb

Tl

1.84 289 300

1.21 220 63 0.62 1.76 1.14 5.64 1.09 0.85 0.11 0.41 2.64 0.74 1.39 0.52 0.74 0.82 0.75 0.77 0.21 2.04 361 28 210 90

ppb

In

1.16 80 96.7

1.4 103 30 0.14 0.59 0.28 2.3 0.20 0.10 0.17 0.16 0.25 0.078 0.31 0.157 0.20 0.076 0.126 0.183 0.054 0.25 1.1 0.295 0.312 0.279

ppb

1.0 0.8

1.8

1.5

0.5 3.8

22b 1.0 3.3 1.4 26b 0.8 0.6

7.6s

0.028

0.021 0.032

0.047

0.025

0.165” 0.022 0,031 0.022 0.063b 0.033 0.026 0.0058“ 0.006 0.029

0.13‘

X0132 Ar,ss (10-s ccSTP/g)t

* Meteorite samples were obtained from the following sources. We express our sincere gratitude to the donors. A = Arizona State University, Tempe, Arizona (Dr. C. B. Moore). C = Geological Survey of Canada (Dr. K. R. Dawson). F = Field Museum of Natural History, Chicago (Dr. E. J. Olsen). H = Harvard Collection (Dr. C. Frondel). U = U.S. Nations1 Museum (Dr. R. S. Clarke, Jr.). W = Ward’s Natural Science Establishment, Rochester, New York. t Data from GANAPATHY and ANDERS (1973) except as noted. a. HEYMA~~N and MAZOR (1968); b. EUQSTER etal.(1969); C.MERRIHUE et al. (1962).

Trenzano BCR-1 BCR-1

Sherps Tieschitz ( > 100 mesh) Bath Bielokrynitschie Ochansk Tysnes Island-Light Allegan ( > 200 mesh) Cangas de Onis Pantar-Light Pultusk Richardton Cape Girardeau Charsonville A Charsonville B Doroninsk Kernouve Lanpon Nanjemoy Queen’s Mercy Sallea-Light Supuhee-Bulk Supuhee-Light Supuhee-Intermediate Supuhee-Dark

Bremervorde

Meteorite

Table 1. Results for H-chondrites

N3993 C F P1456 F2608 F2289 F1979 F578 F1538 F279

Adhi-Kot Abee Indaroh St. Sauveur St. Marks Atlanta Blithfield Hvittis Khairpur Pillistfer B&s.

46

0.094

6.2 12.3 18.4 24.8

300

44 145 925

96

13 86 87 87 14.4 0.39 0.83 5.2 3.9 5.7

85 105 107 115 3.7 1.2 3.3 5.8 6.2 8.6

48 139 132 115 2.X 2.0 7.3 5.2 16.0 13.9 85 774 918 793 6 4 39 70 27 220 110 216 240 241 35 36 61 147 103 103

90 519 490 501 44 8.3 12 18 7.5 86 -

220 316 377 301 189 14.4 58 167 81 133

16.1 24.5 23-O 27.5 -

1.4 2.4 2.4 2.5 0.8 1.5 1.1 1.7 0.77 1.9

E3 E4 E4 E4 E5 E6 E6 E6 E6 E6

81 6.8 3.6 9.1 1.2 9.6 6-3 4.3 0.69 0.59 0.66

31 1.5 2.3 22.8 0.84 14.6 0.85 3.2 0.88 0.96 0.75

67 36.4 8.7 8.6 2.9 5.1 10.2 16.2 8.7 8.9 1.3

17 17 56 25 17 96 3 38 63 30 42

-

155 248 122 283 107 151 225 590 128 175 36

81 82 65 74 66 66 44 74 65 64 70

67 356 98 29 40 57 78 56 85 99 554

98 6.3 18.4 8.2 45 6.1 8.9 6.4 9.1 5.8

2.1 2.5 1.9 0.54 1.2 2.0 3.2 55 1.4 3.3 1.8

LL3 LL3 LL4 LL4 LL4 LL5 LL5 LL6 LL6 LL6 LL6

0.16 0.13 0.9 0.07

0.012 0.019 0,016 0.24 0.08 0.07 0.052 0,035 0.10

6-O 13.6 95 9.2

0.6 1.2 1.3 4.1 37 6 36 6.2 65

X6’= Aq)es (lo-8 ocSTP/g) t

* Meteorite samples were obtained by gift, exchange, or purchase from the following institutions. We express our sincere gratitude to the donors. A = Arizona St&e University, Tempe, Arizona (Dr. C. B. Moore). C = Geological Survey of Canadda(Dr. K. R. Dawson). CH = University of Chicago (Dr. A. Turkevich). F = Field Museum of Natural History, Chicago (Dr. E. J. Olsen). N = American Museum of Natural History, New York (Dr. D. V. Manson). P = Museum National d’Histoire Naturelle, Paris (Dr. I?. Pellas). W = Ward’s Natural Science Establishment, Rochester, New York, t Values from Z.&RINGER (19681.

BCR-I

A42441 F437 CH F2235 F1374 F2095 F1717-18 F1353 F1621 Ii1691 W

In ppb

Tl ppb

Bi ppb

Cd ppb

Cs ppb

Zn ppm

Ag pph

Se ppm

number Class

Rb ppm

source,*

Chainpur P&J!mGe Hamlet Kelly Soko-Banjo Olivenza Umbala Jelica Manbhoom Ottawa St. Mesmin-Light

Meteorite

Table 2. Remlta for LL- and E-chondrites

Chemical fraction&ions

in meteorites--VI

333

Table 3. Comparison with literature data

Meteorite

This work

Other

(ppm)

Rubidium

Abee Bremerv6rde Cheinpur Hvittis Khairpur Oohenak Olivenzs Parnallee Richrtrdton Soko-Bsnjtx St. Marks Selenium

Type

2.4 2.5 2.1 1.7 0.77 1.5 2.0 2.5 1.2 1.2 0.8

3.41r 3.341, 3.05e 3.23* 2*15r 2.341, 3.08e, 3.01e 2.541, 0.52e, 0.84

3.62’”

1.63w 2.19” 2.85w 0*58e, 4.88e

E4 H4 E6 E4 LL6 E6 H4 H5 H5 H5 LL4

24.5 7,2 12.3 23 8.9 18.4 8.4 9.9 11.6 7.7 4.5

48, 11s 6.8C 10s 16*1C, 34s 1Ok 2Y 7.6C 4.89 6.3C, lO.Qk 9*7c 6*6k

Silver (ppb) Abee Chainpur Hvittis Inderch Khairpur Prmtar-Light Pillistfer

TYPO

This work

E4 LL3 E6 E4 E6 H5 E6

316 67 167 377 81 33 133

6008 120s 74s 4109 23Oe 33% 31q 62g

Hvittis Indaroh Khairpur Pillistfer

E6 E4 E6 E6

70 918 27 220

1100 <109, 1209 829

E4 E6 H4 E6 H3 LL3 LL4 E6 E4 LL6 H4 H6 LL3 H3 E5

139 2.0 1.34 7.3 6.45 67 8.7 6.2 132 16.2 4.15 0.49 36.4 100 2.1

71t 1.3t Q.Ot, 15.2t 7.3t 24.9m 65’=, 43*lt 6.6m 6.5’, lO.lt 81t 17*2t, 12.4t 39.8t, 66.2t l.Gq, 39 Il.‘im 88.6”’ 4.2t, 16.2t

E4 H3 LL3 LL4 E4 LL3 H6 H3

106 1.21 31 2.3 107 1.6 2.64 220

15ob, 84~ 13* 28” 2.1” 120’ o.s5n 0.94b 15on

Abee

E4

Hvittis Khairpur Pillistftx

E6 E6 E6

619 18 7.6 86

7209, 280h, Iloi, 74op 20s. 29h, <5$ 16P 229, 26% 361 7.89, lOh, 241

Cesium (ppb) H3 H4 LL6 H6

188 107 161 79

192w 96w 74w 88W

s AKAIWA (1966). b ANDERS and STEVENS 119601. C DIJFRESNE(1960). ’ ’ d FOUCHBand &ALES (1967). e GOPALU and WETHEEILL (1969). * GOPALANand WETHERILL (1970). g GREENLAND(I 967). h GREENLANDand GOLES f 19661. i GREENLAND and LOVZR& (1’966). J KAUSHAL and WETHEBILL (1969) k KIESL et al. (1970).

1700”

Bismuth (ppb) Abee Atlanta Bath Blithfield BremervBrde Chainpur Hamlet Hvittis Indarch Jelice Oohansk Pantar-Light Parnallee Sharps St. Marks

SO’,

Thallium (ppb) Abee BremervBrde Chainpur Hamlet Indaroh Parnallee Richardton

Indium (ppb) Abee

E4

86

Atlanta BremervBrde Chainpur Hvittis Indrtrch Khairpur Ochansk Pan&r-Light Pultusk Richardton Tysnes IslandLight

E6 H3 LL3 E6 E4 EG H4 H6 H4 H5

0.39 1.4 81 6.2 87 3.9 0.28 0.17 0.16 0.25

Zinc (ppm)

Bremervijrde Ochansk Olivenze Richardton

Other

Cadmium (ppb) E4 H3 LL3 E6 E6 H4 LL6 LL3 H5 LL4 E5

(ppm)

Abee Bath Hvittis Indrtrch Jelice Kheirour Och&sk Pantar-Light Pultusk Richardton Soko-Bsnje

Meteorite

H4

2.3

136’, 566, 513”. 130” 0.22u 8.2d 74u 6.128, 2*Qu 76d, QOU 4.2d, 0.24~ 1.7d, 0.488 2.48 1.06 0.2” 1.4s

m LAUL et al. (1970~~). * LAUL et al. i197Obl Values for tvoes corrected by facto;of l.S’(see text). “& P NISHIYURA and SANDELL (1964). 9 REED (1963). r REED et al. (1960). s RIEDER and W~NKE (1969). t SANTOLIQUIDO snd EHMANN (1972). u SCHMITTsnd SMITH (1968). p &ZHMITT et al. (1963): ’ w SMALESet al. (1964).

3 and 4

J. C. LAUL, R. GANAPATHY, EDWAD

334

AHDERS and Jam

W. MORWN

Zinc. Previous data exist for only 4 meteorites, all E-chondrites, Eight of the 14 analyses disagree with ours, but our own data show so large a variation for E-chondrites (in contrast to the well-established constancy of Zn in ordinary chondrites) that none of these values can be rejected on objective grounds. Ce&~m. The few available values agree with ours, except for the LLEi chondrite Olivenza. Wowever, LL-chondrites are known to have variable contents of K, Rb, and Cs (SNA.LESet al., 1964; KAISER and UHRINGIER, 1968; KENPE and M-R, 1969). Cadmium. Again, data are available only for E-chondrites, and the agreement is poor. At least part of the discrepancy may be due to sample heterogeneity, bmause our Khairpur sample

Temperature

f’K1

Fig. 1. Condensation curves of Bi, In and Tl. Lower part represents condensation as solid solution in nickel-iron (Bi, Tl) or troilite (In). After solubility limit is reached (inflection point), elements condense as pnre metals or sulfides. is consistently lower in Ag, Zn, and Cd than Greenland’s, while our Pillistfer sample is consistently higher than his. Bismuth. Our values for Bath, Ochansk, Bremervorde, Pantar-L, and St. Marks are lower than those of other authors, while that for Parnallee is higher. In the case of Pantar and St. Marks a sample mix-up is definitely excluded. Pantar is a polymiot brecoia, while St. Marks contains sparsely distributed rare sulfides (alabandite, etc.) Compositional variations must therefore be expected for both. This may also be the ease for the two Type 3’s, Bremervfirde and Parnallee. Many type 3’s are polymict breccias, containing sizable xenoliths of higher petrologic types (VAN SCEXIJS, 1967; BINNS, 1967, 1968). Our Bremervorde sample may represent such a xenolith, ae it is consistently low in Bi, Tl, and In. For Bath and Ochansk, a sample mix-up is a serious possibility, though intrinsic heterogeneity oannot be excluded. Thallium. Again, Bremervorde is much lower than the literature value. The only other disoordant case is Richardton. (Originally, 4 other values, by LAUL et al., 1970b, differed from ours by more than a factor of 2, but this discrepancy was traced to an error in their calculation, requiring a correction by a factor of 1-Qfor ~equ~ibra~ and carbonaceous chondrites. This correction has been applied to their data in Table 3.) Im-Gum. Five meteorites show discrepancies: Bremerviirde, Khairpur, Ochansk, Pantar-L, and Pultusk. The first 4 had shown similar variations for other elements, and we are therefore inclined to attribute them to compositional nonuniformity or sample mixups rather than analytical error. DISCUSSION ~~~en~~~

cwwes

Equilibrium condensation curves for Bi, In and Tl are shown in Fig. 1. Each curve has an inflection point, corresponding to the solubility limit of the metal in

Chemical fraction&ions

in meteorites-VI

336

nickel-iron (or sulfide in FeS in the case of In). Above this limit, the metal or sulfide condenses in pure form and below this limit, as a solid solution. The corresponding condensation equations are (LARIMER, 1973): Bi, Tl, Ins:

log (1 -

a) =

-

$

+

B’

-

log P,

=

-

AS, + 2.303R Bi, Tl in Ni-Fe:

log +

a

A = - T

B + log ‘t

=

a2 A” logr=T-B+logP,= -a

-logP,-log+

(1)

ALi, - AH, 2_303RT

-sR InSinFeS:

w

2.303RT

AH, -

+logP,

+1ogg(2)

2M,

2~303RT

-~R+logP,+log-;;i-. 2M (3) Here a = fraction of the element condensed, T = absolute temperature, AH,, = heat of vaporization, AH, = heat of solution in nickel-iron, R = gas constant, M, = entropy of vaporization, P, = total pressure in nebula (relative to standard pressure of 1 atm), E, H, M = abundances of element E, hydrogen, and ‘available’ metal (Fe + Ni, of small enough grain size to permit attainment of solubility equilibrium). The peculiar form of the equation for In reflects the existence of a volatile sulfide In,S, which disproportionates to In and InS (LARIMER, 1967, 1973): 2 In(g) + H,S(g) z IGW

+ H2

In,S(g) 22 MS(s)

+ In(g).

In IV, we had neglected this complication, and attempted to fit the meteoritic to equation (2). The following numerical values were used for the various constants: Solid solution

Pure element

Element

A or A”

B

A'

Br

Tl Bi In

6180 6617 4319

10.40 IO.85 9.34

9590 10310 12400

17.12 17.56 22.08

data

AH, kcal/mole 15.6 16.2 18.5

The pure element values were taken from LARIMER (1973) without change. The solid solution values were Larimer’s estimates for the H-group, adjusted slightly in order to improve the fit to our abundance data. The adjustments in AH, were 0.3 to 0.6 kcal/mole, well within Larimer’s stated error of 51.5 kcal/mole. In order to simplify the treatment, we did not use separate constants for each chondrite group, although M and AH, apparently varied somewhat from one group to another

336

J. C. LAUL,

R. GANAPATHY, EDWARDANDERSand JOHN W. MORGAN

LARIMER, 1973). The new A terms differ slightly from our earlier values (IV), being based on newer vapor pressure data and more accurate estimates of heats of solution. The B terms depend on the total pressure and the metal or troilite abundance in the nebula, neither of which is known exactly. The pressure in the asteroid belt probably was 1O-4-1O-5 atm (CAMERON, 1963, 1966, 1972; ANDERS, 1972) and the abundance of ava&ZabZemetal (=N) must have been somewhat less than the cosmic abundance of Fe + Ni, because some of the iron was present as silicate, sulfide, or coarse-grained (= chondrule-associated) metal, at the time the B-T1 group began to condense. In order to cover the combined uncertainty in P, and M, we used three values of B, which correspond to nominal Pt’s = 10-5, 10-4, and 5 x 1O-4 atm, and to M = 9.36 x 105,the cosmic abundance of Fe + Ni (CAMERON, 1968). Since both P, and M enter B logarithmically, these values apply equally well to other combinations, with an n-fold decrease in M being equivalent to an n-fold increase in P,. [According to CAMERON and PINE’S (1972) model of the nebular disk, the pressure at the base of the disk decreased by only a factor of 4 between 2 and 4 AU, and by only a factor of ~3 between the base of the disk and the top of the convection zone. The variation in M is expected to be of the same order. In terms of the two-component model, the fine-grained metal comes largely from the matrix (25 per cent in ordinary chondrites, 30-70 per cent in Echondrites; II), with perhaps a small contribution from metal vaporized and recondensed in the is tied up in silicates chondrule-forming process. Some fraction of the iron, from
Convpos&on of solid condensate Equations (1) to (3) are based on vapor pressure equations and hence accurately predict the composition of the gas phase. If they are to be used to predict the composition of the solid condensate, three additional parameters must be specified. (1) /?,the ratio of gas to unaccreted dust, relative to the cosmic ratio. With Fe and H as reference elements for the dust and gas, respectively, we may define /?=H v08~mee~~~~~~~~H~~*~~~, Fe unit where Hgas, Feduseare the number of H9 Fe atoms per (2) f, the fraction of (unaccreted) dust fine enough to collect volatiles. In terms of the two-component model, the dust consisted of chondrules and matrix grains, of which only the latter were fine-grained enough to collect volatiles. (3) f’, the fraction of matrix in the accreted material. It need not be identical to f, owing to preferential accretion of chondrules (WHIPPLE, 1972). With a = degree of condensation, the abundance of element E in matrix is E, = d&/?/f, where E, is the cosmic abundance of E. The abundance of E in the bulk chondrite is E, = uE,/if ‘If. Th us all moderately volatile elements fully condensed prior to accretion ( a = 1) should be present at a constant fraction @‘If of their cosmic abundance. Empirically it is found that 8 such elements (Cu, F, Ga, Ge, S, Sb, Se and Sn) are depleted in ordinary chondrites by nearly constant factors averaging O-25 (ANDERS,

Chemical fractiontutionsin meteorites-VI

331

1964, 1968; II), which implies fif’/f = 0.25. Because none of these parameters are known with certainty, we have often expressed this trend by the chemically equivalent segment that ordinary chondrites contain 25 per cent matrix with “cosmic” amounts of these 8 elements. Many other equivalent combinations are consistent with the data, e.g. X2.5 per cent matrix with twice the cosmic abundance of these elements, etc. R. T. Dodd (private communication) has in fact suggested on the basis of planimetric measurements that the matrix content is as low as lo-15 per cent, but this was not confirmed by Larimer (private communication) who notes that the visual classi~cation of ‘matrix’ is rather subjective. Moreover, it is implausible that the interstitial volume should be as small as IO-15 per cent. Even close-packed spheres of uniform size have a void space of 26 per cent; a more typical value for non-close-packed spheres is 42 per cent. It is rather surprising, of course, that bulk chondrites show a net depletion of volatile elements. Under closed-system conditions, the volatiles complementary to ~hon~les would condense on matrix, and if ~hondrules and matrix accrete in the same proportions in which they occur in the nebula (i.e. f = f’), no net depletion will result. LAFSMER and ANDERS (1967,p. 1259) have discussed three possible solutions to this paradox. The most viable of these is that chondrules accreted preferentially (e.g. f/f = 0.25 for p = 1,etc.) WHIPPLE (1972)has recently shown that such preferential accretion is indeed expected on aerod~amic grounds. There is little direct information on the gas/dust ratio ,8. It may have been greater or less than 1,depending on whether the region in which ordinary chondrites formed had been depleted or enriched in dust by gravitational settling. Its variation with time would depend on the efficiency of accretion. If accretion was inefficient, being terminated by dissipation of the nebula rather than exhaustion of the dust (CAMERON, 1972),B would remain constant. But if accretion was efficient, p would rise during accretion, causing the last of the dust to pick up more than its share of volatiles (IV, pp. 354-355), even iff’/f remained constant. This situation will be signaled by /?f’/’ values greater than 0.25.

As in IV, we shall present the data on two-element correlation plots, and compare them with concordia lines calculated from equations (1) to (3). In order to test the assumption (questioned by M. Blander) that diffusional equilibrium was attained in the solid-solution region, we shall show both the true equilibrium lines and their metastable equivalents for conditions where diffusion is negligibly slow so that even in the region where solid soluelements condense in pure form [equation (l)], tions are stable. Only Tl-Bi and In-T1 plots will be used, because the near-coincidence of the Bi and In curves over much of their range (Fig. 1) produces a bias toward concordancy. Ordinary and enstatite chondrites will be considered separately. Ordinary chondrites: Tl-Bi. Data for these two elements are shown in Fig. 2a, along with two concordia curves for 1O-4and 1O-6atm and the~metastable extensions (dot-dashed lines). Temperatures are marked in 10” intervals. Replicate measurements are generally represented by averages, with error bars indicating the range. Only in one case (Supuhee) are individual results shown. Concordia curves have a

338

J. Cl. LAUL, R. GANAPATHY,EDWARD ANDERSand JOHN W. MORUAN

I

Bi(ppbl (4

IO

loo

0

20

40

Bifppb)

60

80

lb)

Fig. 2a. Thalliun-bismuth correlation. Thirty-one meteorites (filled symbols) follow the trend of the theoretical correlation curves: initial rise with a slope of ~1, a flat portion, and a final, steep rise to the limiting abundance for complete condensation(dashedlines). Condensationapparently involved formation of alloys with Ni, Fe, because the points do not follow the metsstable extension (dotdashed line) of the curve for condensation of pure elements. Six others (open symbols), notably Supuhee and Krymka near the top of the diagram, contain too much Tl. Most of these are brecciated. Linear correlationof 4 Supuheepoints (Fig. 2b) suggeststhat enrichmentis caused by admixture of a phase (‘mysterite’) rich in Tl, Bi and Ag, possibly a late condensateintroducedin a brecciationevent. Half-filled symbols: either Bi deficiency due to sampling, or Tl enrichment (contamination or mysterite). Error bars represent range of replicates. Fig. 2b. Supuhee samples lie along straight lines on linear, two-elements plots. This suggeststhat they are binary mixtures of an ordinaryH6 chondritewith small amounts of a Tl, Bi, A@;-richphase (‘mysterite’).

cutoff at 25 per cent of cosmic abundance (44 ppb Bi, 68 ppb Tl),* as expected for complete condensation of volatiles in an H-chondrite with @‘/f = O-25. The majority of points, represented by solid symbols, fall within a factor of two (in Tl/Bi ratio) of the concordia curves, while a minority (open and half-filled symbols) lie systematically higher. None fall near the metastable extensions, which suggests that these elements did not condense in pure form in the region where their solid solutions are more stable. By Occam’s Razor,t it would seem that the assumption of alloy formation and diffusional equilibration was not entirely unwarranted. * Cosmic abundances used in this paper were taken from C-ON (1968) except where they differed substantially from recent results from this laboratory: Ag O-45, Bi O-139, Cd 1.50, In 0.184 atoms/lOs Si atoms (KR.AHENB~HL et al., 1973). t “Entia non aunt multiplicandapraeter necessitatem.” (EncyclopediaBrito~iccl, 11th Ed., 20, 868, 19,965, 1910).

Chemical fraction&ions in meteorites-VI

339

Let us first consider the solid points, Though they scatter appreoiably, they are consistent with the general trend of the curves: an initial rise with a slope of ml, a nearly flat portion in which Tl is almostconstant (1-2 ppb) while Bi rises from 1 to 30 ppb, and a final, near-vertical portion in which Tl jumps from 2 to 60 ppb, whiIe Bi rises slowly from 30 to 100 ppb. The open symbols fall by more than a factor of two above the caloulated curves, suggesting enrichment in Tl, or depletion in Bi. The most glaring examples are nine points near the top of the diagram, four of which (open) belong to the brecciated H6 chondrite Supuhee. The Supuhee sampIes show enrichments in Ag, peralleliug those of Tl and Bi. Terrestrial contamination is not a likely cause. The last three samples WBICB taken from the interior of the meteorite, to avoid snrf~e contamination. And we are nat aware of any terrestrial material having this peculiar composition. Sigpificsntly, the four Supuhee samples f&l on straight lines on line= Tl-Bi and Ag-Bi plots {Fig. 2b), with only one of the Tl pointstY;ngoff the line. This suggests that they are binary mixtures of B volatile-poor and a volatile-rich materisl. All intermediate compositions in such a binary system must lie on a straight line connecting tho two end-members. The volatile-poor end-member seems to be an ordinary II5 or H6 chondrite, judging from the fact that at < 1 ppb Bi (a value appropriate to an H5-H6 chondrite), the lines extrapolate to -6 ppb Tl and 76 ppb Ag. Considering the large uncertainty in th8 Tl extrqolation, these vslnes ere within the range of H5-H6 chondrites (Table 1). The n&ure of the volatile-rich end-member is more mysterious, and we shall therefore refer to it se mysterite. Judging from the data on the 4 Supuhee samples (Table 1), mysterite is enriched only in Tl, Bi and AS, but not in Rb, Cs, Se, In, Ar, or Xe. It may be significant that the 3 enriched elements are siderophile in the nebula (L~IZKUB, 1967), whiie the 6 nn-enriched elements ere not. The mass ratios of Ag/Tl a;nd IlilT in mystsrite, calculated from the lines in Fig. 2b, &FB @f5 and 6=14,lower than tha Cl ratios ef l-26 and 0.76 (I%%M?ZEN36EL eba$., 1973). R&tive to Tl = 100per oent, my&e&e thus containa 12 and 18 per Cent, its cosmictcomplement of Ag and Bi. It seem8 1ikeIy that my&&e is a late condensate from a metal-depleted region of the nebula (IV, p. 355), incorporated into Supnhee in a brecciation event. The enrichment pattern Tl > Bi > Ag parallels the valatilities of the free metals, and qualitatively resembles the oomposition of the nebular gss at a late stage of oondensation. The high absolute abundances suggest condensation under co~~t~ons where the ratio of metallic dust to gas was very low (i.e. /3> lf, owing to loss of metal or conversion to other chemical states [II, p. 1259). And the fact that eon-siderophile elements (e.g. In) are not enriched in mysterite supports the notion that condensation of metals WWJthe key to the probIem.

Whatever the origin of this Tl-rich material, its presence in one meteorite raises the edibility that it is aho present in others. Prime ~n~da~s are 10 meteorites falling above the correI&ion lines in which Tl > Bi. Six of these (open symbols) also are anomalous on an In-T1 plot. Pollowing William of Occam, we assume that they are enriched in Tl from mysterite. Among H-chondrites, they are Supuhee (H6), Sharps (H3), and Tysnes Island (H4). Among L- and LL-chondrites, the outstanding case is Krymka (L3), whose high Tl content confounded Lrln~ et al. (~9~Ob) and KEATS et al. f1971). Two others are Kelly (LL4) and Olivenza (LL5). Like Supuhee, most of these meteorites are brecciated. They may> therefore, have been contaminated with material from surface regions of the parent body, where a late condensate would be expected to accrete.

340

J. C. LAUZ, R. GANAPATHY, EDWARD

ANDERS and JOHN W. MORGAN

Of the remaining 38 meteorites, all but 4 (Modoc, Allegan, P&tusk, and Richardton, all shown by half-filled symbols) fall within a factor of 2 of the condensation curves. Modoc is probably contaminated with Tl from terrestrial sources (IV), and this is also possible for Richardton, for which ANDERS and STEVENS (1960) found a lower Tl value, O-94 vs. 25 ppb, and Allegan, a sieve fraction that may have become contaminated during handling. Alternatively, these three H5’s may be deficient in Bi, owing to unrepresentative sampling (see below). A re-check of these meteorites is planned. The majority of ordinary chondrites tend to lie along the 1O-5 atm curve, though some points fall closer to the lOA curve. Let us consider some of the reasons for the scatter. Actual pressure differences may be responsible for part of the dispersion. Pressures in the solar nebula depended both on distance from the center (R) and from the median plane (h), and since the 3 classes of ordinary chondrites probably formed over a range of R and h, some variations must be expected. Some of the scatter may be caused by variations in ,5’f’/f, or in matrix content f’ alone, from the assumed norm of 0.25. Such variations would displace points along a 45’ line. This is especially critical at the high end, where the fit also depends strongly on the validity of the assumed cosmic abundances. The importance of the first factor can be judged from the variations in Cu, Ga, S and Se (elements used as indicators of matrix content; II, IV). It appears that intrinsic variations in matrix content of ordinary chondrites are generally less than 20 per cent, but that sampling at the 0.1 g level may contribute another 20-30 per cent to the variance. Errors in cosmic abundances will be considered in a later section of this paper. Sample heterogeneity may also be a problem, judging from the poor reproducibility of some of the Bi replicates. This is definitely not due to analytical error, as shown by numerous analyses of lunar and terrestrial samples. Though Bi and Tl both are siderophile in the solar nebula, their geochemical behavior in the meteorite parent body is uncertain and possibly divergent. Bi is much less lithophile than Tl, and may thus become separated from it in a minor phase during metamorphism. Some of the scatter may, of course, reflect shortcomings of the model. Such shortcomings will be discussed in a later section. Ordinary chondrites: In-Tl. Most meteorites (filled symbols) fall between the 1O-6 atm and low4 atm lines, somewhat closer to the former (Fig. 3). Some points deviate from this trend, falling too far to the right. All but one of these (open symbols) also were anomalous on the Tl-Bi plot (Fig. 2a). This supports our suggestion tha.t the anomaly is due mainly to Tl-enrichment by addition of mysterite, not to a more complex change involving all three elements. It is noteworthy that most points conform to the unique trend of the theoretical curves : a flat initial portion, a steep region where Tl remains constant while In rises by 2 orders of magnitude, and a second flat portion where Tl rises faster than In. Such a trend is not predicted by any of the other models that have attempted to account for volatile-element data (WASSON, 1972 and earlier references; BLANDER and ABDEL-GAWAD, 1969; DODD, 1969; ARRHENIUS and ALFVI?N, 1971). Also, the metastable pure-element curves (dot-dashed lines) do not give an especially close

Chemical frsctionations

in meteorites-VI

341

Fig. 3. Indium-thallium correlation. Most meteorites (filled symbols) follow the unique trend of the theoretical curves: a flat initial portion, a steep rise by two orders of magnitude, and a final, nearly flat portion. They do not follow the metaetable extension of the pure-element curves (dot-dashed). Of the meteorites deviating from this trend, most (open-symbols) were anomalous on the Tl-Bi plot as well, apparently due to Tl-enrichment from mysterite.

fit to the data. This would seem to support our contention that solid solutions did form during condensation of volatile metals, contrary to the assertions of Blander. The clear separation between H-chondrites and L-, LL-chondrites of types 4-6 is striking. All have nearly identical Tl contents (~0.8 ppb), but whereas most L- and LL-chondrites have In contents above 0.5 ppb, most H-chondrites fall below O-3 ppb. This trend may reflect nothing more than biased sampling, however. Meteorites of low In content generally belong to petrologie types 5 and 6. Our set included 15 H5-H6’s but only 4 L5-L6’s. Judging from TENDON and WASSON’S(1968) data, L5-L6’s often do have In contents below O-3 ppb, and so a more balanced sampling probably would have eliminated the apparent difference.

The In condensation curve (Fig. 1) is very steep in this region, and hence a large difference in abundance corresponds to only a small difference in temperature. If we take the In data at face value, most L- and LL-chondrites accreted near ~455”K, and most H-chondrites, near ~460°K. These values agree with the Bi temperatures from Fig. 2a, and wibh the trend of the O1*/Olatemperatures of ONUMA et al. (1972). The H - L, LL temperature difference might be slightly greater if the H-chondrites accreted at higher pressures, as suggested by LARIMER (1973). Enstatite &o&rites: Tl-Bi and In-Tl. It is well known that E-chondrites, unlike ordinary chondrites, have highly variable apparent matrix contents, as indicated by the abundance of ‘normally depleted’ elements (ANDERS, 1964; II). Values range from ~0.7 for E3, E4’s to ~0.3 for E6’s. We have chosen to recalculate the data to a nominal matrix content of 25 per cent, to permit comparison with the concordia curves. The recalculation was based on the Se content (or, where unavailable, the S content) on the nominal assumption that 8.4 ppm Se or 1.93 per cent S corresponds to 25 per cent matrix, as in H chondrites. Though this assumption

342

J. C. LAUL, R. GANAPATFIY, EDWARD

AXDERS

and JOHN W. MORQAN

neglects differences in bulk chemistry between H and E chondrites, it gives exact results for the quantity of interest: fractional degree of condensation of a highly volatile element (Bi, Tl, In) relative to a moderately volatile element (S, Se, Cu, Ga). This makes it possible to use concordia curves with the same cutoff as for H-chondrites. The resulting graphs are shown in Fig. 4a, b.

E3

E4

I Bi Ippby

E5

E3 A

E6

E4

E5

E6

AA~-? / I I Illll I I Il~LLlL t l/lllll loo

0.2

I

‘IO

-f lppb)

100

(4 fb) Figs. 48, 4b. Enstatite chordrite%also oonfom to theoretical Bi-Tl. Tl-In correlation curves. The beat fit is obtained at nominal pressures of 10e4to 6 x 1tP4 atm.

On the Tl-Bi plot (Fig. 4a), most points fall close to the 5 x lOA atm line. It is interesting that the points for E4’s lie near the theoretical limit for 100 per cent condensation. Adhi Kot (E3) has Tl > Bi, and thus may be suspected of containing mysterite. On the In-T1 plot (Fig. 4b), the points are about equi~tant from the lo-* and 5 x 10”’ lines. The E4 points again fall near the limit for 100 per cent condensation. Adhi Kot, the deviant point in Fig. 4a, indeed lies far to the right of the curves, which strengthens the suspicion that it contains excess Tl from mysterite. Accretion ~e~~e~~r~ Nominal accretion ~mperat~s based on abundance data from IV and this paper are listed in Table 4. For each meteorite the degree of condensation a wzw calculated relative to Se, using CAMERON'S (1968)cosmic abundances. (Se is a good indicator of matrix content, being essentially undepleted in matrix but strongly depleted in chondrules ; II.) Where the Se content was not known, we either used the S content (E-chon~tes) or assumed an Se content equivalent to 25 per cent matrix (8.4, 9.2, or 92 ppm for H-, L- and LL-chondrites). Where replicate values of Bi, Tl, In were available (mainly L-chondrites from IV), temperatures were calculated separately for each such value and then averaged.

Chemical fraction&ions in meteorites-VI

343

The following criteria were used to downgrade or exclude values deemed unreliable. (1) When CLwas greater than 043, the temperature corresponding to a = 0.8 was

entered as an upper limit. This was necessary because T becomes increasingly sensitive to a at high values of cc,and thus is greatly affected by small errors in abundance, matrix content, or cosmic abundance. (2) For the 7 meteorites thought to contain excess Tl from mysterite, Tl-based temperatures are given in italics and were not included in the average. (3) For aIn between 0.005 and 0.05, the In temperature is parenthesized and given only one-half weight in the computation of the average temperature. For aIn less than 0=005, no In temper~t~e was calculated at all. This was necessary because the In condensation curve is very flat in the solid-solution region. The location of the inflection point is somewhat uncertain and probably variable from meteorite to meteorite, owing to variations in metal and sulfur abundance, and in Ni content of the metal which affects AH,. Even a small uncertainty in the location of the inflection point (e.g. a factor of 2) translates into a large temperature error (f30”), which may be further magnified by sampling errors. Temperature thus is an insensitive and poorly known function of abundance at low In contents. Validity of accretion temperatures Conc~~~a~ce. All three thermometers in Table 4 are highly concordant. This does not necessarily imply that they are highly reliable, however. The pure-element portions of the condensation curves of Bi and In nearly coincide (Fig. I), and this tends to widen the range of abundance combinations giving concordant temperatures. The condensation curve of Tl lies 30” lower, however, and hence the concordancy of Bi, In temperatures with Tl temperatures (as shown in Figs. 2, 3, 4 and Table 4) is more meaningful. Further support for the validity of these temperatures comes from their agreement with 01s/Or6 temperatures based on the method of ONUMA et al. (1972). Values for 11 ordinary chondrites (including 5 measured by REUTERet al., 1965) range from 448 to 478”K, close to the Bi, Tl, In temperatures in Table 4. Lower temperatures are again associated with lower petrologic types,*

Concept of a singEea~~~et~o~~empeTatu~e These temperatures were all calculated on the assumption that the matrix of each chondrite equilibrated with the gas at a single temperature and pressure. This is an oversimplification, because meteorite parent bodies probably swept out an extended region of space in which P and 27varied. And diffusional equilibration may * These temperatures sre based on the 01*/016 ratio of the bulk chondrite. Xn terms of the two-component model, they therefore sre s weighted average of the (pm-chondrule) dust temperature at the time of chondrule formation, and the matrix temperature at the time of accretion. Wowever, measurements on separated chondrules and matrix from the H3 ehondrite Ties&&z indicate nearly identical temperatures of 453 and 448’K for the two components. This supports the notion that chondrule formation took place just prior to, or eonc~rently with accretion (LAIUMBZ and ANDERS, 1970; WEIPPLE, 1972). Accordingly, the O18/O16 snd trace element thermometers measure substantially the same phenomenon, even though they EM not in exact agreement. 11

Table

4. Accretion

Meteorite Bremervbrde Sharps Tieschitz Bath Bielokrynitsohie Oohansk Tysnes Island-Light Allegen Cangas de Onis Pantar-Light I’ultusk Richardton Cape Girardeau Charsonville Doroninsk

temperature of chondritos (P 8 = 1O-5 atm for H-, 5 x lo-* atm for Echo&rites)

ClasS

Bi

Temperature Tl

L-,

LL-chondrites;

(OK)* In

456 <443 x443 461 457 457 4.53 487 468 481 500 462 456 457 456 457 467 45% 466 455 456 446

4GO <420 (420 458 448 451 433 452 455 490 470 433 457 454 445 455 457 457 476 441 42s 444

(458) (446 (446

Lanpon Kanj emoy Queen’s Mercy &&es-Light Supuhee-Light Trenzano

II3 H3 H3 II4 H4 H-4 II4 H5 Il.5 H5 H5 Ii5 Htl H6 II6 H6 H6 H6 H6 H6 B6 H6

Khohar Krymka Mesh-Madams Barratta Fukutomi Goodland Tennasilm Farmington Homestead Modoc Bruderheim

L3 L3 213 L4 L4 L4 IA4 L5 LS LO L6

456 455 (446 456 456 458 444 463 457 486 488

440 < 420 433 454 458 454 433 455 454 450 494

455 454 460 456 457

Chainpur Parnalloo Hamlet Kelly Soko-Bauja Olivenza Umbala Jelica Manbhoom Ott&W& St. Mesmm-Light Adhi Kot Abee Indarch

LL3 LL3 LL3-4 LL4 LL4 LL5 LL5 LL6 LL6 LL6 LL6 E3 E4 E4 E4 E5 Es E6 E6 E6 E6

<443 <443 456 455 456 456 455 454 455 456 469 488 t478 t478 483 533 520 492 508 495 492

428 443 446 429 446 430 457 437 451 454 451 455 460 458 460 508 515 489 491 497 484

IL3ITlOUVt5

St. sauveur st. Marks Atlanta Blithfield Hvittis Khairpur Pillistfer

(458) (458) 457

(458) (458) (458)

(458) (458) (458)

457 (458) (458) (458)

<446 456 (458) 456 457 455 456 457 (458) (458) (458) 487 476 t475 479 487 (526) (489) 488 (488) 488

Average 454 1443 <420 460 454 455 455 470 462 486 485 450 456 456 452 456 457 456 471 450 457 448 450 454 439 456 457 456 445 458 456 480 491 428 447 453 456 453 456 456 449 454 456 456 488 468 468 474 619 490 496 494 488

* Italicized values are systematically too low, owing to presence of a Tl-rich phase (mysterite), Parenthesized values are not well-determin or to aont amination. ed, beoause they fall in steep They are therefore given only one-half weight. portion of In condensation curve.

Chemical fraction&ions

in meteorites-VI

345

have been marginal for the larger grains, causing them to reflect higher condensation temperatures. We can get some idea of the extent of such variations from the Tl-Bi-In co~elation plots in Figs. Za, 3 and 4. Any observed composition can, in principle, be matched by mixing two components of different composition. For example, the points at 2-8 ppb Bi, 0.8 ppb Tl which correspond to a temperature around 455°K can, in principle, be duplicated by mixing 450” and 460” material in the right proportions. The discordant Salles point at 10 ppb Bi, 2 ppb Tl (Fig. 2a) can be reproduced by mixing 90 per cent 456OK material with 10 per cent 432’K material, or 80 per cent 500°K material with 20 per cent 432OK material. The number of possibilities becomes limitless if more than two components are considered and if pressure, delayed nucleation of Fe vapor, and supercooling are added as further degrees of freedom (e.g. Constrained Equilibrium Theory of BLANDERand ABDELGAWAD, 1969; BLANDER,1971). If the matrix indeed had such a composite origin, then the nominal accretion temperature of the bulk matrix probably should be interpreted as a weighted mean. Set we shall see that the major part of the matrix apparently formed at temperatures close to this mean. We can probably rule out the most extreme possibility: that the bulk of the volatile metals came in with low-temperature (<400’K) material of cosmic or Cl chondrite composition. Such material would have a Bi/Tl ratio less than f , and hence could not account for the numerous meteorites having a ratio substantiaI~y greater than 1. It wouid also be far too rich in In (and probably Cs, Xe) to account for most II-chondrites. It is harder to rule out a mixture of 430°K material with higher-temperature, e.g. >450°K, material. This is feasible for meteorites lying near a 45’ line from the 430” point, but not for those lying on the ‘pure-element’ portions of the oondensation curves away from a 45Oline. Pa~i~ularly on the In-T1 plot (Fig. 3), most meteorites have compositions not attainable by a binary mixture containing 43O*K material. And when a11three elements are considered together, few meteorites turn out to have the right compositions consistent with such a mixing model. If the matrix of chondrites is a mixture, the end-members cannot have been too far apart in temperature, probably no more than 20-30°K. ~~par~~o~ ~~~ ~~~o~~~~c~~ ~~~~~~e~. C~ERO~ and PINE (1972) have developed a model of the solar nebula, based on the LARSON(1969) adiabat. InterestingIy, this adiabat predicts temperature-pressure combinations (at mid-plane) very similar to those inferred in this paper, e.g. 485’K and 1.7 x IOU6atm. Cameron and Pine show that convection cells were probably present in the asteroid belt and that they cycled material from warm regions in the median plane to cooler regions above and below it. For the case given above, they calculate T = 330”K, P = O-7 x 1O-5 atm at the top of the convection zone, O-4AU above the base of the disk. The small variation in pressure is of interest, and seems to justify our assumption of a single pressure for each meteorite class. The variation in temperature is much larger than that suggested by our data, 20-30°K. Either the convection cells were much smaller than 0.4 AU, or the grains re-equi~bra~d continuously while rising and falling with the convecting gas. This is not unreasonable, because the time for a complete convective cycle would have been on the order of several years, for a velocity of ~1 km/set. If accretion took place mainly in the median plane, where the dust density was

346

J. C. LAVL, R. GANAPATHY,E. ANDERSand J. W. MORGAN

highest (CAMERON and PINE, 1972), the composition of the grains at the time of accretion would have reflected conditions in that region, regardless of their previous history.

Other inter-element correlations Xenon-In&urn. Data are shown in Fig. 5, along with theoretical correlation curves for P, = 1O-4 and 1O-5 atm. The condensation of Xe was assumed to follow

Fig. 5. Xenon-indium correlation. Ordinary chondrites roughly follow predicted condensation curves for AH, = -20 kc&/mole, but meteorites of petrologic types 3, 4 are richer in xenon than those of types 5, 6. This may reflect gas losses during metamorphism as well as mineralogical differences. The low xenon content of E-chondrites, coupled with high Ar/Xe ratios, suggests that their principal minerals have a low solvent capacity for xenon.

equation (2), with AH, - AHH, = -20 kcal/mole. [This is close to our estimates in IV, -20 to -25 kcal/mole, which contrasted sharply with available measurements on melts. These gave small positive rather than large negative heats of solution. However, solubility measurements of Ar, Kr, Xe in one meteoritic mineral, magnetite (LANCET and ANDERS, 1973), give AH’s of the right sign and magnitude, -12 to -15 kcal/mole.] The vertical position of the curves is indeterminate and was adjusted for an approximate fit to the data. This corresponds to a xenon distribution coefficient of 4 x lo5 cc STP g-1 atm-l at 450°K, five orders of magnitude higher than the experimental value for Fe,O,. For this model to be valid, the solubility of xenon in meteoritic silicates thus would have to be 5 orders of magnitude higher than that in magnetite. This is not wholly unreasonable, in view of the fact that the trapping sites in silicates tend to be structural holes (DAMON and KULP, 1958) with an abundance of 1O21-1O22/g, whereas in magnetite they are anion vacancies or clislocations, with an abundance of 1015-1018/g (LANCET and ANDERS, 1973). But obviously this point needs to be checked experimentally.

Chemical fractionations in meteorites-VI

347

The fit to the theoretical curves is far from impressive. Yet the data do not conform to any other simple trend. TANDON and WASSON (1968) showed that an AP-In plot for L-chondrites had a slope of 1, and that the Xe132-In trend, though steeper, was not inconsistent with such a slope. Accepting a unit slope for both gases, they argued that this was unattainable under the two-component model : it would require “In and the rare gases [to] condense out in the same ratio, independent of condensation temperature.” However, when additional data on L-chondrites became available (Fig. 5 of IV), the linear trend was less pronounced and the slope of a Xe-In plot was closer to 3. Even lower slopes are shown by the remaining three classes in Fig. 5, and since a small slope is in fact predicted by the 2-component condensation model, it seems worthwhile to interpret the data in terms of this model. The model predicts a steep rise at very low In contents, and a slope of zero between In contents of ~0.2 and 20 ppb. Though data are sparse and scattered, it appears that enstatite chondrites and the higher petrologic types of ordinary chondrites do indeed show such a trend over the following range of In contents (ppb) : E( l-30), H5, 6(0*1-l), L5, 6(0*07-0*3), LL6(06-4). All fall below or near the 1O-5 atm line. Taken by themselves, the lower petrologic types show a similarly flat trend between 1 and 30 ppb, with only the L chondrites having a slope significantly greater than zero. But their Xe contents are systematically higher, falling between the 1O-4 and 1O-5 atm lines. Four explanations may be considered. (1) Pressure. Type 3’s formed at higher pressures. This is unlikely, because the nebula apparently dissipated during or after the accretion of 3’s; hence pressures were, if anything, lower during that stage. (2) Incomplete equilibration. Higher petrologic types did not equilibrate completely with the noble gases in the nebula, because of rapid accretion (and perhaps also coarser grain size ; II, p. 1249). According to this model, meteorites of low Xe content should have high Ar/Xe ratios, because Ar presumably diffuses faster and therefore becomes enriched in partially equilibrated grains. However, the available data (Fig. 6) show just the reverse trend: high Ar36/Xe132 ratios are found mainly among meteorites of high Xe132 content and/or low petrologic type (see also MARTI, 1967; Z~~HRIN~ER,1968). (3) Metamorphism. Higher petrologic types initially contained more gas but lost it during metamorphism (Z~HRINGIER, 1966, 1968; WOOD, 1967; HEYMANN and MAZOR, 1968). In the past this model has been criticized because it would lead to strong preferential depletion of lighter gases, contrary to observation (MARTI, 1967). But judging from Fig. 5, the overall loss need have been only a factor of ~5 in most cases, and if the gas loss was small, fractionation must also have been small. Indeed, the Ar36/Xe132 ratios in ordinary chondrites show the trend expected for this model: low ratios tend to predominate among petrologic types 5 and 6 (Fig. 6). The correlation of Xe132 with C (MARTI, 1967 ; OTTIN~ and Z~RINCJER, 1967) also suggests that the loss was small. Macromolecular forms of carbon, as initially present in the accreted material, would be lost as volatile species (CO, CO,; FRENCH, 1966; WOOD, 1967) during metamorphism in a wide-open system, and since Xe and C would be lost at different rates, the correlation would be destroyed. (4) Mineralogical differences. Gas content depends not only on temperature and pressure, but also on mineralogy of substrate. In terms of this explanation, petrologic types 4 and 3 contain progressively greater amounts of a mineral of high solvent

348

J. C. LAUL, R. GANA.PATHY, EDWARDANDERSand JOHN W. MORGAN

capacity for noble gases, e.g. a hydrated silicate (JEFFERY and ANDERS, 1970; CANALAS et al., 1968; LANCET and ANDERS, 1973). Probably some combination of (3), (4) and (l), in that order, is responsible for the overall trend in Fig. 5. The irregular scatter of 5’s and 6’s apparently requires metamorphism; the contrast between E3,4’s and (H, L, LL)3,4’s calls for a mineralogical explanation. And since the four chondrite classes presumably formed at different pressures, some differences in gas content must be due to this cause. If this interpretation is correct, then noble gas contents do not always bear a simple relation to accretion temperatures and pressures. Silver-bismuth. Data are plotted in Fig. 7. Since Ag is essentially 100 per cent condensed at and below 560”K, the theoretical correlation curve is a horizontal line at 25 per cent the cosmic abundance of Ag, or 81 ppb. The points scatter almost symmetrically about the line, with a slight tendency for high Ag content to be correlated with high Bi content and low petrologic type, and vice versa. Above 10 ppb Bi, most meteorites lie above the line; below 2 ppb Bi, they lie below the line. Two type 3’s show exceptionally high Ag contents, 4 to 12 times the cosmic value. [A solitary meteorite of higher petrologic type, St. MesminLight (LLS) also has a high Ag content. It seems that contamination would have to be ruled out before much significance is attached to this exceptional result.] Except for the two Type 3’s, the scatter is probably no larger than expected from variations in matrix content (~30 per cent) and sample heterogeneity (a factor of 1.5 to 2, judging from replicate measurements in this paper and IV). There seems to have been no net depletion in Ag, judging from the agreement of the mean Ag abundance (weighted by frequency of petrologic types*) with the expected value for

LJ

w

P

-5 100 %a. 2

60

0

.

0 ti

20 F

101’ ’

’’” I

,

4

56

r---l HA...

L0000 LLOOOO d & =lowAr40

(
I

I111111

2 46 IO Xe13’(lO“OccSTP/g)

I

I

20

I

I

II

40 60

Fig. 6. Ar3’3/Xe132ratios in ordinary chondrites tend to be lower in meteorites of high petrologic type and/or low Xe13acontent. This suggests slight gas losses during metamorphism, perhaps also presence of a gas-rich mineral (hydrated silicate?) in lower petrologic types. * We are making the tacit assumption that the proportion of different petrologic types among meteorite falls is similar to that in the meteorite parent body. This is probably a good assumption for the L-chondrites, whose parent body seems to have been completely disrupted (ANDERS, 1964, 1965; HEYMLANN, 1967), but not necessarily for the other chondrite clasees.

Chemical fraction&ions

iu meteoritea-VI

349

0

Bi(ppb)

IO

100

Fig. 7. Silver-bismuth correlation. The Ag values scatter rather badly, but the mean abundance agrees with the expected value for complete condensation on 25 per cent matrix. Judging from the poor reproducibility of replicates, the scatter may reflect inhomogeneousdistribution of Ag. Symbols as in Figs. 5-6. The four Supuheepoints are marked by vertical ticks. Error bars represent range of replicates.

25 per cent matrix of cosmic Ag content: 76 vs. 74 ppb for H’s and 98 vs. 81 ppb for L’s. Thus the scatter probably reflects mainly the heterogeneous distribution of Ag, established either during accretion or during metamorphism. Rubidiurn-cesium. As first noted by SMALESet al. (1964), these elements show different trends in each chondrite clsss, and hence pose a real challenge to any model (Fig. 8). Two limiting compositions are shown, The upper corresponds to cosmic Rb and Cs levels in the bulk meteorite, as expected if both chondrules and matrix haa retained these elements. The lower corresponds to an abundance only 0.25 the cosmic value, as expected if only the matrix had retained Rb snd Cs. Values for E-chondrites are again corrected to a nominal matrix content of 25 per cent. L-chondrites tend to contain somewhat less than 3.4 ppm Rb, whichsuggestsslight Rb loss from the chondrules. Cesium contents are highly variable, snd often low enough to imply loss from both chondrules and matrix. L5, 6’s have less Cs than expected for 25 per cent matrix of cosmic Cs content (81 ppb), while L3,4’s have more, up to 600 ppb. In IV we assumed that all L-chondrites initially contained 281 ppb Cs, but thst some of the Cs WIS volatilized during metamorphism, migrating from the hotter (L5,6) to the cooler (L3,4) regions of the body. (Such metamorphic redistribution of trace elements had previously been proposed by WOOD (1967) and DODD (1969). In IV, pp. 354-355, we explained why only Cs and Br but not the thermometric elements Bi, Tl and In might be affected by this redistribution.) Since L5, 6’s are much more common than L3, 4’s, the pre-metamorphic Cs content of L5, 6’s could be comfortably below the 81 ppb limit and still supply enough Cs for L3, 4’s. In fact, the average Cs content of L- and H-chondrites, weighted by frequency of petrologic types, is only 68 and 47 ppb.

350

J. C. LAUL, R. GANAPATHY, EDWARD ANDERS and JOHN W. MORUAN 5-

I , ,111,

I

1 Illrlll

I

III&_

Fig. 8. Rubidium-oesium correlation. ‘Cosmic’ lines correspond to complete Rb, Cs retention in both chondrules and matrix; ‘0.25 cosmic’ corresponds to retention in matrix only. Symbols as in Figs. 5-6. Finds are marked by vertical ticks. Rubidium loss from chondrules seems to have been small for L-chondrites, variable and often large for the other three classes. Cesium is usually present at 125 per cent its cosmic abundance, which suggests partial-to-complete retention in matrix only. Some meteorites, especially of lower petrologic types, contain greater amounts of Cs, however. This may imply redistribution during metamorphism in the parent body, but other explanations must also be considered.

By and large, our new data are consistent with this picture, though they show several new trends. Rubidium contents tend to be lower in LL-, E- and H-chondrites, as already noted by SMALES et al. (1964), GOPALAN and WETHERILL (1969), and KAUSHAL and WETHERILL (1969). This suggests more complete outgassing of chondrules, owing to some combination of higher peak temperatures, longer heating times, or smaller size.* In H-chondrites, at least, Cs contents still correlate with petrologic class. Types 5 and 6 lie mainly below 81 ppb, types 3 and 4, above 81 ppb. However, LL-chondrites show an irregular trend, and almost all are enriched to >81 ppb. If this enrichment is to be attributed to metamorphism, material balance requires that the deeper, Cs-depleted regions of the LL-chondrite parent body are underrepresented in our suite. The irregular trend of the LL-chondrites may be related to the well-known variations in K content (KAISER and Z;~HRIN~ER, 1965; GOPALAN and WETHERILL, 1969) and to the presence of K, Rb-rich inclusions in at least one meteorite of this class, Krghenberg (KEMPE and MULLER, 1969). Cadmium-bismuth. These two elements correlate only in E-chondrites, not, in L- or LL-chondrites (Fig. 9). The reason for this correlation is nof obvious. In a gas of cosmic H&J/H, ratio, Bi is siderophile at all temperatures, while Cd is chalcophile below 1000°K (LARIMER, 1967). Zinc, which is even more strongly chalcophile than Cd, shows a similar behavior, correlating with Cd and Bi in E-chondrites but not in ordinary chondrifes. According to LARIMER’S (1967, 1973) calculations, any Zn not previously

condensed

as &Cl,

or Zn,SiO,

should

condense

as ZnS after

* After this was written, Dr. Thomas W. Osborn kindly drew our attention to data in his thesis (OSBORN,1971), showing that LL-chondmles are indeed dewed in Rb.

351

Chemical fractionations in meteorites-VI

2 0.1

I I

A

I111111

I

I

I I111111

Bilppb)

IO

I

1 I1111

100

Fig. 9. Cadmium-bismuth correlation. None of the ordinary chondrites have condensed their cosmic complement of Cd, which suggests that the condensation temparature of pure CdS lies below the accretion range of chondrites, 43O-500% E-chondrites show a correlation, and attain higher Cd contents. This may reflect higher pressures and a higher HsS/HsO ratio. Symbols as in Figs. 5-6.

the onset of FeS formation at 680°K. Cadmium sulfide, on the other hand, should condense at lower temperatures. In any case, the Zn-Cd-Bi correlation in E-chondrites suggests a peculiarity in condensation/volatilization behavior, perhaps related to the more reduced character of E-chondrites. The non-correlation of Cd and Bi in L- and LL-chondrites is equally puzzling. Apparently it also extends to H-chondrites. Since we did not measure Cd in Hchondrites, we plotted 3 Cd values from the literature (SCHMITT et al., 1963; GREENLAND, 1967) against our own Bi values (Fig. 9). Two other Cd measurements (Miller,

120 ppb; Ehole, 15 ppb) could not be plotted, for lack of Bi values. But they lie in the same general range as the data for L- and LL-chondrites. It is interesting that not even the lowest petrologic types approach the cosmic value for 25 per cent matrix, 280 ppb Cd. The condensation temperature of pure CdS is about 415°K at 10-S atm (LARIMDR,1973), and since most ordinary chondrites accreted above 430”K, condensation of Cd would necessarily remain incomplete. The relatively flat trend in Fig. 9 is consistent with condensation as a solid solution. Re-examination

of two-component

model

In the original formulation of the two-component model (ANDERS,1964; LARIMER and ANDERS,1967) some features could not be specified unequivocally. Now that additional data have become available, it is possible to define the model in more detail, to identify problem areas, and to assess its general adequacy. A fundamental question that must be answered is whether the Manichean notion of two and only two components is appropriate to the real world (P. Pellas, private communication, 1971). Chondrules. In the most idealized form of the two-component model, chondrules are completely devoid of volatiles, having lost them during the chondrule-forming

35%

J. C. LAUL,R. GANAPATHY, EDWARDANDERSand JOHNW. MORUAN

process. In reality, the loss seems to have been neither quantitative nor uniform, at least for the less volatile elements (alkalis, Mn). Chondrules from C2 chondrites still contain ~10 per cent their complement of Na and Mn (SCHMITTet al., 1965). And chondrules from many meteorites are enriched in refractory elements such as Al, Ca, Ir, which suggests partial volatilization even of major elements such as Mg, Si and Fe (WARREN,OSBORNand SCHMITT,1971; KURAT, 1971). This is consistent with currently popular mechanisms of chondrule formation, most of which involve rapid heating on a local scale (collisions, electric discharges). Volatile loss in such local events would vary from place to place and even from chondrule to chondrule, depending on size, peak temperature, heating time, and cooling rate. Still, the overall tendency seems to have been for the retention of an element to be consistently near 100 per cent or below 10 per cent. This is shown by the rarity of elements with intermediate degrees of depletion, such as Rb (Fig. 8). We can look for such partial retention of volatiles among the 9 ‘normally depleted’ elements that are supposed to be completely lost from chondrules but fully condensed in matrix. Partial retention in chondrules would manifest itself by depletion factors greater than O-25, correlated with volatility. The latest data for ordinary chondrites and C2’s (Fig. 2 of AYNDERS, 1971) show no systematic trend of this sort. Only Au displays the slight overabundance expected for an incompletely volatilized element. The depletion factor of O-5 corresponds to an average retention of 5 of the Au from chondrules.* 2Mutrix. In the simplest form of the model, each chondrite contains its own kind of matrix that equilibrated with the gas at a single temperature and pressure. As we saw in the discussion of accretion temperatures, this may be an oversimplilication. Probably the matrix in a given chondrite came from a variety of environments. But the total range in conditions cannot have been great: perhaps ~20-30’ in T, and a factor of 55 in P and M. Formation of 80zia solutions. From the condensation curves computed in 1967, it seemed that the most volatile metals condensed mainly as pure elements rather than alloys, owing to low solubility in the substrate or sluggish diffusion. However, on the newer diagrams the data give a much better fit to the alloy curves than to the metastable portions of the pure-element curves (Figs. 2a, 3, 4a, 4b). It would seem that diffusion rates were fast enough for the formation of solid solutions. Overabundance and accretion e@ciency. In the simplest form of the two-component model, the matrix acquires exactly its cosmic complement of a volatile element upon complete condensation; no more, no less. This requires special assumptions about disposal of the volatiles complementary to chondrules (e.g. II, p. 1259). In the more general case described above, a chondrite accreting at a time when a fraction a of a volatile element has condensed, will contain this element in an abundance E = (aE,,@‘)/f. Thus, when a = 1, E/E, = pf’/j. W e noted that for ordinary chondrites the average value of /lf’/f is 0.25, and so if this factor remained constant throughout accretion, the limiting abundance of a volatile element should be 0.25 its cosmic abundance. We have already noted that this limiting composition can be overshot

* Noteaddedilzproof. KTJRIMOTO et al. additional

elements Sb and As.

(1972) have seen a

similar ‘partial depletion’ for two

Cbmiofbl fraction&ions

in meteorites-VI

353

for highly volatile elements, when the gas/dust ratio p rises above 1 toward the end of accretion (II, p. 1259; IV, p. 355). The residual dust will collect not only its own complement of volatiles, but also the complements of chondrules and earlyaccreted, volatile-poor matrix. LARIMER and ANDERS(1967) concluded from the apparent absence of such enrichments that the gas/dust ratio never rose significantly above the cosmic value. This required that only a small part of the potentially accretable material be accreted, so that the left-behind volatiles be distributed over a large mass of dust. For the same reason, only a small part of the dust was to be converted to chon~ules; the high abundance of chondrules in meteorites then implied their preferential accretion. A mechanism for such preferential accretion has been developed by WHIPPLE(1972). Recent data have changed the picture, however. Most of the highly volatile elements do in fact show occasional enrichments above cosmic abundances, in Type 3’s or in the dark portions of gas-rich chondrites [In: TANDONand WASSON(1968) ; RIEDER and WXNKE (1969); and this work; Hg: REED and JOVANOVIC (1967) ; Bi: LAUZet al. (1970a); and this work; Tl, Cs, Ag: IV; and this work]. Thus it seems that accretion was more efficient than previously estimated. At least in the final stages of accretion, the gas phase was appreciably depleted in dust. We do not know whether this depletion first became significant during accretion of Type 3’s or at a still later time, because it is not clear that the overabundant trace elements are truly indigenous to the Type 3’s. They may have been incorporated into the meteorites during accretion, or in later brecciation events involving admixture of the lastaccreted dust (mysterite?) from the surface of the parent body. En&at& chondrites. The strongly reduced character of E-chondrites calls for a highly reducing environment. LARIMER(1968b) has pointed out that this could occur in a region of the nebula where the C/O ratio had risen slightIy above 0.9, owing to a concentration of carbon-rich dust. Though ad hoc, this is not an implausible suggestion. The solar C/O ratio seems to be only slightly below the critical value of 0.9, and so even a modest gas-dust fractionation would raise the ratio above this limit. Rracwoon (1961) has proposed a different model, involving reduction by carbon in the meteorite parent body. WASSO~ and WAI (1970) have discussed the origin of E-chondrites in some detail without, however, accounting for their highly reduced state. BLANDER(1971) has attempted to explain the sulfides in enstatite chondrites in terms of his Constrained Equilibrium Theory. At least one of his predictions is not confirmed by the present data. Aocording to his model, Zn, Pb, Cd, Tl, Bi and In should have “relative abundances in enstatite chondrites which are smaller than their natural relative abundances.” From Table 2, the abundance ratios for E4’s (relative to Cl’s) are: Zn (l-66), Cd (1*30), Bi (1+17), Tl (O-75), In (1.09). With allowance for the differences in matrix and Si content, there is no significant depletion, except for Tl. A few trends observed in this work and IV were not predicted Remainingpu.zzles. by the model, mainly: (I) Enrichment of Cs and Br in some L, LL-chondrites to ~5 times their cosmic abundance.

354

J. C. LATJL, R.

GANAPATHY,EDWARD ANDERS and JOHN W. MOR~UN

(2) Difference in In contents between H- and L, LL-chondrites. (3) Systematic underabundance of Cd in all petrologic types. Though tentatively explained by ad hoc assumptions, these trends must be regarded as challenges to the model until the proposed explanations have been checked. It is very desirable that alternative models be developed and explored (SUESS, 1965 ; D0~~,1969; AxRHENIus~~~ALFv~~N,~~~~; BLANDER, 1971). However,areasonable requirement would seem to be that all cosmochemical models be developed to a quantitative stage, able to account in detail for the full range of evidence. They must also have few enough degrees of freedom to permit testable predictions. Acknowledgments-We are greatly indebted to 3. W, LARIMER for much valuable advice and permission to use unpublished data. SANDRA CROMARTIEassisted most capably in the calculation of condensation curves, preparation of graphs, and typing of the manuscript. This work was supported in part by NASA Grant NGL 14-001-167. Some of the equipment used had been supplied by the AEC under Contract AT(ll-lf-382.

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DAMONP. E. and KULP L. J. (1958) Excess helium and argon in beryl and other minerals. Amer. ilfineral. 43, 433-459. DODD R. T. (1969) Metamorphism of the ordinary chondrites: a review. Gwchim. ~o~~h~rn. Acta 83, 161-203. DUFRESNEA. (1960) Selenium and tellurium in meteorites. Ceochim.C~em~ch~rn.Acta 20, 141148. EU~STERO., EBERHARDT P. and GEISSJ. (1969) Isotopic analyses of krypton and xenon in 14 stone meteorites. J. Beophys. Res. 74, 38763896. FOUCHBK. F. and SXALESA. A. (1967) The distributionof trace elementsin chondriticmeteorites. 1. Gallium, germanium and indium. Chem. CTeoE. 2, 5-33. FRENCHB. M. (1966) Some geological implications of equ~ibrium between graphite and a C-H-O gas phase a high temperatures and pressures. new. Geophys. 4, 223-253. GANAPATHV R. and ANDERSE. (1973) Noble gases in eleven II-chondrites. Geochirn.CosmochLim. Acta 37, 359-362. GOPALANK and WETHERILLG. W. (1969) Rubidium-strontium age of amphoterite (LL) chondrites. J. Geophys. Re.s. 74, 4349-4358. GOP&AN K. and WET~RILL G. W. (1970) rubidium-strontium studies on enstatite ehondrites: whole meteorite and mineral isochrons. J. Gwphys. Rec. 75,3457-3467. GREENLANDL. (1967) The abundance of selenium, tellurium, silver, palladium, cadmium, and zinc in chondritic meteorites. Geochim. Cosmochim.Acta 31,849-860. GREENING L. and GOLES G. G. (1965) Copper and zinc abundances in chondritic meteorites. Geochim.Coemochim.Acta 29, 13851292. GREENL~KDL. and LOVERIN~J. F. (1965) Minor and trace element abundances in chondritic meteorites. ~w~h~rn. Co~moch~m. Acta 29, 821-858. GROSSMANL. (1972) Condensation in the primitive solar nebula. Geo~h~m. Co~moch~m. Acta, 36, 597-619. HEYMANND. (1967) On the origin of hypersthene chondrites: ages and shock-effects of black chondrites. Icarus 6, 189-221. HEYMANND. and MAZORE. (1968) Noble gases in unequilibratedordinary chondrites. Gwchim. Cosmochim.A&a 32, l-19. JEFFERYP. M. and &DERS E. (1970) Primordialnoble gases in separatedmeteoriticmineral-I. Geoe~~~rn. eo~oGhirn. Aeta 84, 1175-1198. KAISERW. and Z~~RINGERJ. (1965) Kalium-Analysen von Amphoterit-Chondritenund deren K-A-Alter. 2. Naturforsch. 20a, 963-965. KAUSH~ S. K. and WETH~R~LLG. W. (1969) Rbs7-Srs7 age of bronzite (H gro~xp) chondrites. J. Geophys. Res. 74, 2717-2726. KEAYS R. R., GANAPATHY R. and ANDERSE. (1971) Chemical fractionationsin meteorites-IV. Abundances of fourteen trace elements in L-chon~ites; implications for cosmothermometry. Geochim. ~o~rno~h~rn. Acta 35, 337-363. KEMPEW. and M%LER 0. (1969) The stony meteorite Krahenberg-its chemical composition and the Rb-Sr age of the light and dark portions. In Meteorite Research (ed. P. M. Millman), pp. 418-428. Reidel. KIESL W., GRASSF., B~~CKL R. and PONTA U. (1970) Cosmochemical abundances of trace elements in meteorites. I. Determination of Se, Te, Tl, Sr, Ba and Ta in ohondrites. J. Rad~oa~~. Chem. 6,447-452. KRAHENB~~HL U., MORGANJ. W., GANAPATHYR. and ANDERS E. (1973) Abundance of 17 trace elements in carbonaceouschondrites. Submitted to Geochim. Cosmochim. Acta. KURAT G. (1971) Die chemische Zusammensetzung von Gliisern und Chondrenmatrizes im Chondriten von Tieschitz. Chem. Erde 80, 235-249. KVRIMOTOIX., PELLY I. Z., LAUL J. C. and LIPSCHUTZ M. El. (1972) Inter-element relationships between trace elements in primitive carbonaceous and unequilibrated ordinary chond rites. Geoehim.~o~~o~h~rn. Acta 37, 209-224. LANCET M. S. (1972) Carbon-isotope fractionations in the Fischer-Tropseh reaction and noblegas solubilities in magnetite: implications for the origin of organic matter and primordial gases in meteorites. Ph.D. Thesis. University of Chicago.

356

J. C.

LAUL,

R. GANAPATHY, EDWARD ANDERS and JOHN W. MORQAN

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