Antimony in iron meteorites

Earth and Planetary Science Letters, 53 (1981) 1 - 10

1

Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands [11

ANTIMONY IN IRON METEORITES JOHN WILLIS * Department o f Chemistry and Institute o f Geophysics and Planetary Physics, University o f California, Los Angeles, CA 90024 (U.S.A.)

Received July 30, 1980 Revised version received December 17, 1980

Antimony concentrations determined by radiochemical neutron activation analysis in 60 iron meteorites range from 0.2 ng/g to 36 t~g/g. The meteorites with the highest Sb concentrations are those of the non-magmatic groups lAB and IIICD, while the lowest Sb concentrations are found in groups IVA and IVB, the groups with the lowest concentrations of the other most volatile siderophiles Ge and Ga. In all groups Sb is positively correlated with Ni. In each of the magmatic groups slopes on log Sb vs. log Ni plots decrease with increasing Ni. This decrease may reflect an increasing tendency to avoid schteibersite during the analysis of high-Ni meteorites because Sb partitions strongly into schreibersite. Schreibersite from New Westville is enriched in Cr, Ni, Ge, As, Sb and Au and depleted in Fe, Co and Ir; the content of Sb in schreibersite is 540 × higher than the bulk metal value. The Sb abundances of the iron meteorite groups are as expected from volatility trends with the exception of lAB and IIAB in which abundances appear depleted. The most likely explanation for this and the decreasing slope in the magmatic groups is that one or more Sb-rich phases were not sampled during metal analyses.

1. Introduction The chemical classification of iron meteorites (see Kracher et al. [1]) now includes 13 groups, principally defined by their Ga, Ge and Ni contents. Eleven o f these are believed to have formed by the fractional crystallization o f planetary cores. Evidence for this comes from their chemical fractionation trends, the absence o f chondritic silicates, and the large size of the precursor taenite grains [2]. These groups were earlier called "igneous" and now are referred to as the "magmatic" groups. The other two groups, IAB and IIICD, may have formed in a b o d y o f chondritic composition as individual pools of shock melt varying systematicaUy in their relative amounts of metal and sulfides [3]. The moderately volatile elements, Ge, Ga, Sb, Cu, and As, have been found to be depleted in the ordi* Present address: Department of Metallurgy and Materials Science, Lehigh University Bethlehem, PA 18015, U.S.A.

nary chondrites in a manner related to their volatility [4,5] and this trend is mirrored in the iron meteorites [6] (Fig. 1). This depletion is attributed to the inability o f the later condensing elements to be assimilated into existing metal grains due to kinetic factors [5,7]. The degree o f the depletion serves as an indicator o f the conditions existing in the solar nebula during the condensation o f each element. Gallium and germanium are generally the most useful elements for estimating condensation conditions because: (1) they have the lowest condensation temperatures [7] and show the lowest CI-chondrite normalized abundances, (2) they undergo only minor (Ge) or moderate (Ga) fractionation under planetary conditions and so it is relatively easy to obtain the bulk parent b o d y abundances, and (3) data exist for these elements on nearly every iron meteorite. Antimony is comparable to Ga in volatility [7] and shows similar levels o f depletion relative to chondritic values. But it has not been used because of a paucity o f data, contradictory data for Sb values < 1 0 ng/g, and some uncertainty

0012-821X/81]0000-0000]$02.50 © 1981 Elsevier Scientific Publishing Company

r H"

l 0 H Chondrites o L Chondrites A I I I A B Irons

L

1.0

~Z

~

0.5

~

0.2 ..--.L

Au

.I

L

As

Cu

m

Ca

Sb

_2_*

to sample structural types prior to the development of the iron meteorite classification system. Consequently, while we have good coverage for groups IIAB and IIIAB, other important groups, such as IVA and IVB, were inadequately sampled and some groups (liD, IIF) not sampled at all. To remedy this lack of data, we modified the Sb procedure developed by Elzie [10] and Witter [11 ] and now routinely determine Sb together with Ga, Ge and Ir in our RNAA studies of iron meteorites.

Ge

Fig. 1. The abundance ratios of moderately volatile elements relative to CI chondrites in ordinary chondrites and group IIIAB iron meteorites. The elements are arranged in order of decreasing condensation temperature for a nebular pressure of 1 0 -6 atm. The observed depletion is attributed to the inability of the later condensing elements to be assimilated into large existing gains d u e to kinetic constraints.

regarding trends within groups. Only one analytical team [8,9] has determined Sb in a large suite of irons (77) but the suite was chosen

2. E x p e r i m e n t a l The Sb procedure used in this work was incorporated into the radiochemical scheme for determining Ga, Ge and Ir in a single sample [12,13]. High Sb concentrations can be determined by INAA but below 0.5 #g/g the precision is much less than that obtained by the RNAA procedure and the accuracy is compromised by our inability to adequately resolve the 564.1-keV Sb peak from the 562.8-keV As line.

TABLE 1 Concentration of Sb in iron meteorites as determined by INAA Meteorite

Group

Source a

Catalogue No.

Ni (mg/g) b

Britstown Kofa Lime Creek

ungr ungr ungr

BM FMNH YalU

1927,81 1993 P15

195 182.7 291

Oktibbeha County

IB

AMNH

62

Santa Catharina

ungr

HarU

Twin City

ungr

SI

Sb (~g/g) 2.24 0.64 2.63

585

37.7 37.7 mean 37.7

-

336.2

1770

300.6

2.66 2.94 mean

1.99 2.68 mean

Udei Station

IA

AMNH

Yamato 75031

ungr

NIPR

3946 -

88.3

2.34 0.68

142

0.92 1.07 mean

a Source abbreviations are the same as in Table 2. b Ni content from Kracher et al. [1], Wasson [26] and Scott and Wasson [27].

2.80

1.00

Concentrations of Sb determined by INAA are listed in Table 1. Samples were mostly blocks sawed in roughly cubical shape and having masses of ~0.4 g, for meteorites with coarse structures, the masses were increased to ~0.8 g. Samples were etched lightly both before and after irradiations and were wrapped in A1 foil during irradiations. Irradiations were carried out in the core of the UCLA reactor for 3 hours at a flux of 2 X 1012 neutrons cm -2 s -1. In addition, 13 lowSb irons were irradiated at the University of Missouri reactor for 25 hours at a flux of 1 × 1014 neutrons cm -2 s -1. The latter irradiation conditions increased the sensitivity by an order of magnitude. These samples are designated with an asterisk in Table 2. No Sb replicates were determined for some irons because the Sb procedure was developed subsequent to the RNAA study of the first sample. Samples were dissolved in a solution 3N in HNO3 and 1.4N in HC1 and containing Ga, Ge, Sb and Ir carriers. Germanium was extracted from the solution with CC14, then Ga and Sb were extracted with isopropyl ether. The Ga was separated from the Sb by back extraction with H20. The Sb was reduced to the 3+ state with SnC12 and back extracted from the ether with 7N HC1. The HC1 concentration was reduced to 3N and Sb2S 3 precipitated with H2S, which was then dissolved in 7N HC1 for counting. If the gamma-ray spectrum was not free of interfering peaks, the solution was oxidized to give Sb s÷ and the ether extraction and sulfide precipitation steps were repeated. All counting was performed on liquid samples. A 7.5 × 7.5-cm well-type NaI(T1) detector was used to count the 564-keV gamma of 122Sb. In order to measure the recovery of Sb carrier, an aliquot of the sample solution was evaporated onto high-purity A1 foil, and Sb determined by INAA. Typical values for the chemical yield were 30-40%. Flux monitors containing about 10/ag Sb were prepared by evaporating a standard solution onto high-purity A1 foil. The standard solution was prepared by dissolving Sb203 in 4N HC1 to obtain a solution containing 1.227 mg Sb per gram solution. Although the precision for the RNAA procedure was not quantitatively assessed, we estimate from the agreement of duplicates that it is about +8% at 95% confidence limits on the mean of duplicate deter-

minations. The uncertainty increases below 100 ng/g and should be doubled at Sb concentrations below ~10 ng/g unless otherwise indicated. For INAA 95% confidence limits are of the order of-+12% for concentrations above ~500 ng/g. To separate phosphides, one sample, New Westville, was dissolved in 4N HC1 [14] containing Ge and Sb carriers, following irradiation. After 24 hours with gentle heating, a small amount (~0.1%) of insoluble residue remained. This residue was separated and weighed. An aliquot of the solution and the residue were each analyzed by INAA for As, Au, Co, Cr, Fe, Ir, and Ni and our high Ni/Fe concentration ratio confirmed that our residue was phosphide. The phosphide residue was dissolved in a solution 3N in HNO3 and 1.4N in HC1 and containing carrier solutions of Ge and Sb. The metal solution was treated with C12 to oxidize Sb to the 5+ state and then both solutions were processed in a manner similar to that used for the other samples.

3. Results The concentrations of Sb in 60 iron meteorites, along with their Ni contents and chemical classifications, are listed in Table 2. A comparison of our results with those of other investigators is shown in Table 3. Only Smales et al. [8,9] and Seitner et al. [15] analyzed enough irons to allow an adequate comparison. The data of Seitner et al. for Coahuila and Bohumilitz are excessively high. Not shown in Table 3 is their datum for Carlton which is five times higher than Smales' value and a datum for Braunau, a typical IIA iron, which is almost an order of magnitude higher than IIA irons having similar Ir contents. The results of Smales et al. [9] are in good agreement with ours with one exception: Both RNAA and INAA values for Santa Catharina (Tables 1 and 2) are twice theirs. The value for Duchesne of Smales et al. [8] is also in disagreement with our datum. Smales et al. [8] data are generally much too high for irons having low Ge contents. They report values of 460 rig/g, and 30 ng/g for Bristol and Altonah, at least 3 X higher than would be inferred from the trends through more recent data. The data obtained on the separated phases of New Westville are reported in Table 4. No flux monitors

TABLE 2 Concentration of antimony in iron meteorites Meteorite

Group

Source a

Catalogue No.

Ni b (mg/g)

Sb (ng/g) replicates

Allan Hills A76002 Alt Bela Babb's Mill (Blake's) c Barbacena Bendeg6 Bischtiibe Bohumilitz Britstown Cabin Creek Canyon Diablo Coahuila Dehesa Duchesne c DunganviUe Gibeon c Harlowton Henburg Hill City c ltapuranga Itutinga Jamestown c Jaralito Jerslev Kokomo c Linville Losttown MiUarville Morasko Mount Magnet Mount Sir Charles Nantan Nenntmannsdorf New WestviUe (metal) c New Westville (schreibersite) c N'Goureyma c Oktibbeha County Paracutu Patos de Minas (Hex) Petropavlovsk Pooposo Prambanan Quesa Rancho Gomelia Rembang e Rica Aventura Sanclerl~ndia Santa Catharina Santa Clara Santa Luzia

IA liD ungI ungr IC IA IA ungr IliA IA IIA ungr IVA IIIA IVA IA IIIA IVA IA IliA IVA IIICD liB IVB ungI liD IVA IA ungr IVA IIICD liB IVA

FMNH NMW NMW UNM UNM FMNH FMNH BM NMW AML UCLA MHNP UCLA CamU UCLA ASU OSU SI USP UNM HarU UCLA MMUC HarU NMW SI UCIg PAC WAM SI MPIH YalU SI

ungr IB IA IIA ungr IA ungr ungr IIIB IVA IVA IliA ungr IVB lib

FMNH AMNH MPIM SI KMAN NMW FMNH NMW ASU ANUC FMNH UNM HarU ASU UNM

-

70.0 100.4

-

114.0

I38.1 I39.2 Me932 Me898 1927,81 F6342 34.5092 378 144

109 63.9 78.8 73.7 195 82 _+4 69.8 54.9 118 93.2 77.8 78.2 88.2

808 1045.9 -

1436 140.1 558 1026 977,540 269 1071 -

12375e 5669

74.7

90.9 67 _+3 72 74.5 65.2 56.6 158.8 158.0 100 95.7

P66 1412

65.6 145.6 83.0 68 61.8 93.6

Me597 62 15208 Mell60 H443 1044.6

92.6 585 75.4 53.6 81.0 69.8 101.4 110.4 97 _q-6

-

249 86; 85 3.5 48 63 510; 370 250; 380 2100 81 290; 260; 300 31 650; 460 8.0 49 8.6 400 27 4.4 250; 270 4.3 d; 28.0 1.5 310; 280 62; 56 1.5 720 93 ~<8.0

mean 249 85 3.5 48 63 440 + 70 320 _+60 2100 81 280 31 560 _+ 100 8.0 49 8.6 400 27 4.4 260 28 1.5 300 59 1.5 720 93 ~<8.0

290; 250 270 450 450 2 2 310; 310 310 86 86 8.4 8.4 9800 9800 5.6 5.6 36,100; 34,300 35,200 370; 420 400 36 36 76 76 320; 340 330 14 14 330 330 136; 129; 156; 144 141

-

88.2

5.6

5.6

-

92.1

2.7; 8.0

5 + 3

35 ; 34 2700 1.6;4.3 100

34 2700 3 _+1 100

I44.1 1060 143

74.7 336.2 179 63

T A B L E 2 (continued) Meteorite

Group

Source a

Catalogue No.

Ni b (rag/g)

Sb (ng/g) replicates

Santiago Papasquiero c Sgo Juli~o de Moreira Tieraco Creek Tlacotepec c TocopiUa Verissimo Warburton Range c Winburg Yamato 75031 Yanhuitlan c Yardea Zaffra

ungr liB IIIB IVB IIA IIIA IVB IC-An ungr IVA IA IIICD-An

ASU FMNH TUD UCLA ASU MNB WAIT MMUC NIPR FMNH UMel AMNH

721.4 Me536 369 169.1 174,938 Me962 5673 2614

74.8 61 105 158.2 55.4 75.4 178.0 69.8 142 74.9 69.2 71.2

mean

0.5 110 170; 200 0.2 46; 50 26; 27 1.8 120 1000 3.6 280; 280 310; 310

0.5 110 180 0.2 48 26 1.8 120 1000 3.6 280 310

a Source abbreviations are as follows: AML = American Meteorite Laboratory; AMNH = American Museum o f Natural History; ANUC = Australian National University, Canberra; AS U = Arizona State University; BM = British Museum (Natural History); C a m U = Cambridge University; FMNH = Field M u s e u m o f Natural History, Chicago; HarU = Harvard University; KMAN = C o m m i t t e e on Meteorites, A c a d e m y of Sciences; U.S.S.R.; MHNP = Museum Histoire Naturelle, Paris; MMUC = Mineralogical M u s e u m o f the University, Copenhagen; MNB = Museum Nacional Brazil; MPIH = Max-Planck-Institut, Heidelberg; MPIM = Max-Planck-Institut, Mainz; NIPR = National Institute o f Polar Research, Japan; NMW = Naturhistorisches M u s e u m , Vienna; OSU = Oregon State University; PAC = Polish A c a d e m y of Sciences; SI = Smithsonian Institution, Washington D.C.; T U D = Technical University o f Denmark; UCLA = University of California, Los Angeles; UCIg = University o f Calgary; UMel = University o f Melbourne; UNM = University of New Mexico; USP = University of Sgo Paulo; WAIT = Western Australian Institute o f Technology; WAM = Western Australia Museum; YalU = Yale University. b Ni c o n t e n t from Kracher et al. [1], Wasson [26] and Scott and Wasson [27]. c Meteorites irradiated at the University of Missouri reactor. d This replicate n o t included in the mean. TABLE 3 Comparison o f Sb results ~ g / g ) with other published results on the same meteorites. Meteorite

Ours

Onishi and SandeU

Hamaguchi et al. [ 24 ]

Smales et al. [8 ]

Smales et al. [9 ]

[281 Bendeg6 Bischttibe Bohumilitz C a n y o n Diablo Coahuila Duchesne Gibeon

0.063 0.44 0.32 0.28 0.031 0.003 0.0086

Henbury Morasko M o u n t Magnet Santa Catharina Sgo Juligo de Moreira Tocopilla

0.027 0.27 0.45 2.7 0.11 0.048

0.8

0.5

Tanner and Ehmann

Seitner et al. [ 15 ]

Kiesl [ 17]

[221

0.35

0.31 0.58 0.04 0.01

0.45

0.09

0.061 0.48 0.35 0.33 0.043 0.011 0.007 0.031 0.27 0.47 1.56 0.10 0.11 0.055

0.92 0.29

0.51 0.48

0.42

6 TABLE 4 Concentration ratios for major and trace elements in the phosphides of New Westville normalized to the bulk values (Ge and Sb were determined by RNAA, P by microprobe analyses [29] and the rest by INAA) Element

P

Cr

Fe

Co

Ni

Ge

As

Sb

Ir

Au

Phosphide/bulk

110

2.2

0.58

0.23

4.5

6.5

3.7

540

0.62

16

were available for the INAA and only concentration ratios were obtained. Concentrations determined from RNAA were 145 ng/g Ge and 8.4 ng/g Sb in the metal and 950 ng/g Ge and 9800 ng/g Sb in the phosphide. The schreibersite/metal ratios obtained in this work are generally similar to those obtained by Jochum et al. [14]. Only the two elements most affected by phosphide size, Ge and Ir, were inconsistent with the results of Jochum et al. Schreibersite in New Westville generally occurs as narrow grain boundary precipitates or as small blebs in plessite [16] and is similar in width to the large rhabdites in group IIAB. While my Ge ratio is similar to the Ge ratio of the rhabdites analyzed by Jochum et al. [14], my Ir concentration ratio is similar to that of the fine schreibersite particles analyzed by them and is much less than those found for the rhabdites. The main surprise of this work is the high enrichment of Sb in phosphide. Jochum et al. did not analyze Sb and.Kiesl [17] found Sb to be depleted in the phosphide phases of Canyon Diablo and Odessa. Since the phosphides of Kiesl [17] were larger than ours, this may indicate an inverse correlation between size and Sb concentration, as Jochum et al. found for Ir. On the other hand, the Sb values that Kiesl reported for the metal and troilite phases in Canyon Diablo differ significantly from those reported by Smales et al. [8] and this discrepancy casts doubt on the validity of Kiesl's phosphide Sb values.

and IIIAB, the low-Ni portions of the trends are linear with slopes near 10 but the high-Ni portions have slopes near 1.5. In group IVA, the data are not yet adequate to define a trend. Wasson et al. [3] proposed that group IAB irons formed from small pools of impact melt on a body of

S00.Sb(ng/g)

My results and those of Smales et al. [9] for groups lAB, IIAB, IIIA, and IVA are shown in Fig. 2. Group lAB data lie on a straight line which extends through the high-Ni end of the group (San Cristobal, 250 mg/g Ni, and Oktibbeha County, 585 mg/g Ni, are not shown) and has a slope of 1.6. In groups IIAB

/

200.

~.

t,oo./oo 20.

+ 0 Ch

++

-! 0.

+

+ +

5.

+

+ ++ 2.

+

+

6~

7~

X IRB + IVR I[IRB o I [AB

a~ 9~ tC~0 ' ' Ni (mg/g) Fig. 2. A log-log plot of Sb against Ni for the groups lAB, IIAB, IIIAB, and IVA based on data of this work and that of Smales et al. [9]. Goose Lake, GL, and Chinautla, Ch, are anomalous members of groups IAB and IVA, respectively. San Cristobal and Oktibbeha County, the extreme members of group lAB at 250 mg/g Ni and 2.2 ~g/g Sb and at 585 mg/g Ni and 36.1 gg/g Sb, respectively, have been left off to avoid elongating the plot excessively. Regression lines have been drawn through the low Ni portions of IIAB and IIIAB as discussed in the text. so

4. Discussion

X )~,GL X

chondritic composition. These impact melts varied in temperature and degree of equilibration between melted and unmelted solids. At the lowest temperatures melts consisted primarily of Ni-rich sulfides and adjacent metal and the degree of equilibrium with unmelted solids was minimal. These melts produced the high-Ni group members. At the highest temperatures, the melt had a composition near the FeS-Fe eutectic or somewhat more metal rich and produced the lowNi irons of the group. Compositions from this end of the group are most representative of the siderophile element content of the parent body. The element-Ni trends of group lAB reflect the degree of equilibration between the elements' host phase and the melt pool. The positive Sb-Ni slope thus indicates that Sb resided in a low melting phase, Groups IIAB, IIIAB and IVA are magmatic groups and the element-Ni trends were established during crystallization [18]. The change in the Sb-Ni slopes of groups IIAB and II1AB is indicative of a change in the solid/liquid distribution coefficient for Sb, ksb, and/or kNi during crystallization and may indicate the crystallization of a second phase, such as a sulfide or phosphide, into which Sb partitions favorably. With the vast increase in the analytical data available on iron meteorites, it has become possible to calculate the bulk composition of the parent body prior to metal-silicate fractionation [19]. For the magmatic groups, this is done by estimating the composition of the first solid (that with the lowest Ni content) and employing the Rayleigh equation in conjunction with

element-element distributions to determine the composition of the core (see Willis [19] for a detailed discussion). A problem arises in determining the bulk Sb content of groups IIAB and IIIAB; the slopes in the lowNi portions of these groups are steep resulting in very low ksb values (~0.1) with large uncertainties. The uncertainty in a low ksb value is magnified in the calculated bulk content. For instance, an increase of 5% in the slope will increase the calculated bulk content by 50% and an increase of 10% in the slope will result in a 330% increase. It should be noted that these low values for ksb preclude the use of an arithmetic mean for these groups since the solid would not have the mean Sb value until 92% of the melt had solidified, and in most groups the last 10-20% appears to be missing. Table 5 lists the moderately volatile element abundances of the major groups along with values for some chondrite classes for reference purposes. Abundances for group IIIAB are plotted in Fig. 1, abundances for groups IAB, IIAB, and 1VA in Fig. 3. With the exception of Cu (3 × low), the trend in IIIAB follows that of the ordinary chondrites. This suggests that the mechanism(s) responsible for fractionating the moderately volatiles in the ordinary chondrites was also responsible for the fractionation trend in group IIIAB and that nebular conditions at the time of formation were similar for both groups. Estimates of the nebular pressure at the time of formation for these groups are in agreement: ~ 1 0 -6 atm for the chondrites [5] and ~5 × 10 -4 atm for IIIAB

TABLE 5 Concentration of moderately volatile elements in iron meteorite groups IAB, IIAB, IIIAB and IVA and H, L, and CI chondrites (data from this paper and Willis [19] unless otherwise indicated)

IAB IIAB IIIAB IVA Ha La CI b

Ni (mg/g)

Ge Ozg/g)

Sb (ng/g)

Ga (ug/g)

Cu (/~g/g)

As (~g/g)

Au (~g/g)

64.0 64.2 86.9 80.0 16.2 12.9 10.7

399 173 38.9 0.124 11.1 10.3 32.5

270 373 275 15 88 90 140

95 58 19.7 2.15 5.6 5.6 9.9

130 128 161 151 86 86 112

11.5 9.9 10.4 5.1 2.16 1.70 1.70

1.56 1.41 1.24 1.20 0.213 0.157 0.152

a From Rambaldi and Cendales [30] and Rambaldi et al. [31]. b Data compiled by J.T. Wasson (unpublished).

o IAB A II A B o IVA

1

i

T - - T " " ~

.I

0

Z

I0 c N • -"

0

--+--4

5

0

~ iO-i

¢ 2

t

~ IO"2 e--

"

5 2

Au

As

Cu

~

Sb

" Ge

Fig. 3. Abundance ratios, normalized to Ni, of the moderately volatile elements in iron meteorite groups IAB, IIAB, and IVA relative to CI chondrites arranged in order of decreasing condensation temperature.

[20]. Wasson and Wai [6] concluded that the mechanism responsible for the moderately volatile element fractionation of the ordinary chondrites also operated in the formation region of the group IVA irons, but more efficiently (producing lower abundances) at the lowest nebular temperatures. Examination of the trend in group IVA (Fig. 3) supports this idea. The low Sb abundance in groups lAB and IIAB cannot be explained by its nebular volatility because these groups have their full complement of Ga and Ge. Antimony, therefore, should have been fully condensed into the parent bodies of these groups. The factor of two depletion in group IIAB could be a result of inaccuracies in the slope as mentioned previously but this explanation cannot be used for group lAB as the Rayleigh equation has not been used to calculate the abundance of this non-magmatic group. One is forced to the conclusion that the Sb depletion for these groups is the result of not sampling some Sb-, Cu-, and P-bearing phase or phases. There are three candidates for the Sb-enriched phases: silicates, sulfides, and phosphides. Metalsilicate differentiation takes place earliest in the

history of the parent body and, for the magmatic groups, this separation may be essentially complete. A parent body having the composition of a reduced chondrite would yield about 75% silicate material and about 25% metal-sulfide material if nearly all Fe were present as Fe-Ni or FeS. For this separation to account for the observed Sb depletion, an Sb metal/ silicate distribution coefficient of 1 is required. Evidence for lithophilic tendencies of Sb in meteorites is lacking; the available data suggest strong siderophilic tendencies. Studies of ordinary chondrites [21 ] show the Sb to be concentrated in the metal phase by a factor of 16 relative to the non-magnetic phases. Analyses of achondrites [22] also indicate a depletion of Sb in these silicate materials. Our unpublished analyses of metal and silicate fractions from Udei Station give a distribution coefficient of 14, but metal/silicate distribution coefficients estimated from analyses of coexisting metal and silicate phases in lAB iron meteorites may not be reliable as the metal and silicate material may not have been in chemical equilibrium with each other [23]. Analyses of pallasitic metal and olivine [22,24,25] suggest a metal/silicate distribution coefficient of 6. Such high metal/silicate ratios can only account for a small (~25%) Sb depletion. Antimony has chalcophilic tendencies in a terrestrial environment and might be expected to concentrate in troilite in iron meteorites. Analyses by Smales et al. [8] and Tanner and Ehmann [22] indicate that the sulfide Sb content is between one and two times that of the metal. Avoidance of troilite can influence the Sb abundance in one of two ways. If troilite is a liquidus phase during most of the crystallization of the metal, the Sb can partition between three phases. To a first approximation, this depresses the Sb concentrations in the meteorites uniformly by a factor dependent on the relative amount of sulfide present and the sulfide/liquid metal distribution coefficient. On the other hand if, as expected, troilite remains in solution during the early part of the crystallization, the low-Ni portion of the Sb-Ni trend will still accurately reflect the partitioning of Sb between liquid and solid metal. The enrichment of Sb in the schreibersite of New Westville can be used to infer an enrichment in all meteoritic schreibersite, but because of the inverse relationship between phosphide size and degree of enrichment [14], it is probable that the coarse

10 of California Press, 1975) 1418 pp. 17 W. Kiesl, in: Activation Analysis in Geochemistry and Cosmochemistry, A.O. Brunfeldt and E. Steinnes, eds. (Universitetsforlaget, Oslo, 1971) 2 4 3 - 2 5 1 . 18 E.R.D. Scott, Chemical fractionation in iron meteorites and its interpretation, Geochim. Cosmochim. Acta 36 (1972) 1205-1236. 19 J. Willis, The bulk composition of iron meteorite parent bodies, Ph.D. Dissertation, University of California, Los Angeles, Calif. (1980). 20 D.W. Sears, Condensation and the composition of iron meteorites, Earth Planet. Sci. Lett. 41 (1978) 123-138. 21 K.F. Fouehd and A.A. Smales, The distribution of trace elements in chondritic meteorites, 2. Antimony, arsenic, gold, palladium and rhenium, Chem. Geol. 2 (1967) 105-134. 22 J.T. Tanner and W.D. Ehmann, The abundance of antimony in meteorites, tektites and rocks by neutron activation analysis, Geochim. Cosmochim. Acta 31 (1967) 2007-2026. 23 R.W. Bild, Silicate inclusions in group lAB irons and a relation to the anomalous stones Winona and Mt. Morris (Wis.), Geochim. Cosmochim. Acta 41 (1977) 1 4 3 9 1456.

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