Silicate inclusions in group IAB irons and a relation to the anomalous stones Winona and Mt Morris (Wis)

Geochimm

et Cosmochmlca

Acta, 1977, Vol. 41, pp. 1439 to 1456

Pergmon

Press. Prmted

m Great Britam

Silicate inclusions in group IAB irons and a relation to the anomalous stones Winona and Mt Morris (Wis) RICHARDW. BILD* Department of Chemistry and Institute of Geophysics and Planetary Physics, University of California, Los Angeles, CA 90024, U.S.A. (Received 14 December 1976; accepted in revised form 6 June 1977) Abstract-Silicates are found in many group IAB irons; in some cases as abundant angular cm-sized inclusions and in other cases as smaller fragments or single grains in troilite or graphite nodules. The mineralogy of the silicates is chondritic--olivine, pyroxene, albitic plagioclase-as is the bulk composition. The degree of oxidation of the olivine and pyroxene is intermediate between E and H chondrites (Fa 1-8, Fs 4-9). IAB inclusions have ages of about 4.5 Gyr, I’2g-Xe’2g formation intervals in the ranges of chondrites and contain planetary-type rare gases. Samples of San Cristobal, Campo de1 Cielo, Mundrabilla and Woodbine were examined by microprobe and bulk inclusions from Campo de1 Cielo, Copiapo, Landes and Woodbine were analyzed by instrumental and radiochemical neutron activation analysis. Nonvolatile lithophilic and siderophilic elements in Copiapo, Landes and Woodbine have approximately chondritic abundances. The chondritic level of lithophiles indicates the inclusions have not undergone igneous differentiation while the chondritic levels of siderophiles is evidence the metal is native to the inclusions and not matrix metal injected into the silicates. The two Campo de1 Cielo inclusions analyzed have roughly chondritic abundances of lithophiles but have fractionated rare earth patterns and widely varying amounts and abundances (relative to Ni) of siderophiles. These inclusions appear to have experienced some partial melting. Siderophile ratios for the inclusions have some differences when compared to matrix metal. One Camp0 de1 Cielo inclusion contains kamacite (5.5% Ni) with over lOOOpg/g Ge. Three-isotope 0 analyses by Clayton and coworkers of parts of the same or neighboring inclusions to those analyzed chemically place the inclusions slightly below the terrestrial fractionation line of CLAYTON et al. (1976) and rule out the possibility of the inclusions being trapped fragments of one of the ordinary chondrite groups. The IAB silicates formed probably in a similar manner as chondrite groups but in a different region of the nebula and they record the O2 fugacity and 0 isotopic composition of that location. They later became trapped in the metal-rich matrix probably as the result of collisions producing the breccialike texture. The relationship of the silicates to the kamacite-taenite structure of the metal requires that the metal-silicate mix have been heated to over lOOOKfor an extended period. Two anomalous stony meteorites, Winona and Mt. Morris (Wis), are similar to IAB inclusions in mineralogy, bulk composition, FeO/(FeO + Mg) ratio of the silicates, and chromite composition and are possibly related to the IAB silicates. Winona also has an age of 4.6 Gyr and contains planetary-type rare gases. Microprobe data are reported for the major minerals of these anomalous meteorites. Although attempts to detect IAB levels of Ge in the metal phases were not successful, the weight of the evidence favors a relationship between these meteorites and IAB.

INTRODUCTION THE LARGEST of the groups of iron meteorites containing silicate inclusions is group IAB. General properties of the group are reviewed by SCOTTand WASSCIN (1975). Silicates are reported in about half of the 90+ members of this group (e.g. BUCHWALD,1975 and tabulations in WASSON,1974) and microprobe analyses of major phases from about 20 are available in the literature (BUNCHet al., 1970, 1972; RAMBALDI et al., 1974; SCOTI and BILD, 1974). Abundances of silicates vary widely from plentiful, angular cm-sized nodules (e.g. Fig. la) to rare individual grains. Bulk chemical analyses are published for inclusions from Woodbine (JAROSEWICH,1967), Landes (KRACHEX, 1974) and Campo de1 Cielo (WLOTZKAand JAROS~V* Present address: Dept. Planetary Sciences, University of Arizona, Tucson, Arizona, U.S.A.

ICH, 1977) and these and other authors point out that the major elements occur in the inclusions at the same levels as in chondrites. The mineralogy of the inclusions is also very similar to that of ordinary chondrites-olivine, orthopyroxene and albitic plagioclase with varying amounts of metal, troilite, schreibersite, chromite, ‘clinopyroxene and other accessories. Unlike ordinary chondrites, graphite is found in most IAB silicate inclusions. Other published data also indicate a relationship between the IAB inclusions and chondrites. Inclusions from six IAB irons are dated and all fall in the range 4.4-4.8 Gyr (BURNETT and WASSERBURG,1967; BCGARD et al., 1968; WASSERBURGand BURNER, 1969; PODOSEK, 1971). ALEXANDWet al. (1969) report an I’2g/Xe’2g formation interval for a Toluca inclusion that falls in the same range as chondritic meteorites and PODCSEK(1970) reports a similar result for an inclusion from Camp0 de1 Cielo. Rare gases in

1439

1440

R.

w

IAB inclusions are planetary-type and occur at chondritic levels (ALEXANDER and MANUEL,1968: HINTENBERGERet uI., 1969; BOGARDet al., 1971). Trace element data on the inclusions are almost completely lacking. In a number of isotopic investigations (e.g. BURNETTand WASSERBURG, 1967 and BOGARDet al.. 1968) it was assumed that the metal and silicate formed together and that the age determined for the inclusion applied to the entire meteorite. MAXJN (1967) proposed formation by injecting molten metal into a mass of chondritic silicates and suggested the meteorites might represent a core-mantle boundary zone of one or more asteroids. The structures have also been interpreted as old silicates trapped mechanically in quickly cooled younger Ni-Fe melt (BUNCH et al.. 1970), and as silicates trapped when partially melted metal in a parent body refroze before it could completely segregate from the silicates (KELLYand LARIMER.1977). Winona is classified as an anomalous stony meteorite (WASSON,1974) though published data show many similarities with IAB silicate inclusions in mineralogy. bulk chemical composition, rare gas content and age (MASON and JAROSEWICH.1967; RAMDOHR,1973). Mason and Jarosewich also note that another anomalous stone, Mt. Morris (Wis). is very similar to Winona. Chondrites are considered primitive meteorites because they contain nonvolatile elements in ratios only slightly altered from those in the sun and. presumably, in the early solar nebula. They are thus the best available probes of chemical and physical conditions in the regions of the early solar nebula where

BILD

they formed. If other types of chondritic meteorites are recognized, probes of conditions in other regions of the early solar nebula become available. The purpose of this work is to further investigate the history of IAB silicate inclusions and their relationship to chondrites. Bulk silicate inclusions from Woodbine. Copiapo, Landes and Campo de1 Cielo were analyzed for a suite of major, minor and trace elements by radiochemical and instrumental neutron activation analysis. Sections containing inclusions from Camp0 de1 Cielo, San Cristobal and Mundrabilla were analyzed by electron microprobe. The relationship of the Winona and Mt. Morris (Wis) to IAB silicates was investigated further both optically and by electron probe.

SAMPLE SOURCES AND PREPARATION OF SAMPLES FOR NEUTRON ACTIVATION ANALYSIS The samples examined m this study and their sources are shown in Table 1. Bulk sdicate inclusions, either cut or broken from matrix metal and/or troilite were used for neutron activation analysis. The Campo de1 Cielo inclusions were received in this form. The inclusions from Copiapo, Landes and Woodbine were obtained by sawing slices of the meteorite l-4 mm thick and breaking the melusions from the matrix. An Al,O, wheel with water coolant was used for cutting Landes and for preliminary cuttmg of Copiapo. The sawing of Woodbine and final sawing of Copiapo were done with a wire saw using Sic-glycerol slurry. Cut slices were washed with distilled water to remove cutting compounds and with acetone to remove water. The freed silicate inclusions were then broken into approximately 0.2 g chips for neutron activation analysis. Surfaces of the inclusions that were not fresh breaks were cleaned by light grinding with a Dremel Moto-Tool using

Table I. Sources of samples Sample

Form Received

Source* and Catalog Number

Campo de1

Cielo

NMNH 5615 Location IIB

Silicate matrix

inclusion metal.

free

of

26/3

Campo de1

Cielo

NMNH 5615+ Location IIB

Silicate matrix

inclusion metal.

free

Of

30/13-14

Campo de1

Cielo

IJCLA 43

Metal

Copiapo

UCLA 845

Slab containing silicate inclusi

Landes

AML H91.104

Slab contalnlng cm-sized silicate inclusions.

San

AIUV (‘low UCLA 868)

Microprobe

NMNH 2169

Slab with mm to cm sized silicate inclusions.

NMNfl 1198

Microprobe

thin

sectIon.

NMNH 854 UCLA 395

Microprobe Microprobe

thin thick

section. section.

Crlstobal

Woodb one Mt.

Morris

Winona

(Wis)

slab,

slllcate

thick

free.

cm-sized .ons.

section.

*Key to source symbols: AIUV-Analytlsches Institut, Univ. Vienna; AML-~ American Meteorite Laboratory; NMNH-National Museum of Natural Hlstory. Washmgton, D.C.; UCLA-University of California at Los Angeles. + Via J. H. REYNOLDS.

Silicate inclusions in group IAB irons Table 2. Elemental concentrations

camp de1 Mel.3 IIB 2613

405 294

14.2

127.

determined

by INAA in silicate inclusions from four group IAB iron meteorites

14.3 mean* 14.2

169. 145.

5.3 5.5 5.4

321. 402. 355.

3.32 1.50 2.55

130. 126. 128.

Camp de1 Cielo 163 IIB 30113-14 142

9.9 15.2 mean* 12.4

15.6 17.0 16.3

7.8 5.7 6.8 --

46.6 55.6 50.8

1.59 1.32 1.46

34.4 33.8 34.1

copiapo

266 215

12.6 13.8 mean* 13.2 --

208. 241. 224.

21. 15. 18.

606 492. 550. --

2.45 1.73 2.09

394

14.0 13.9 meen* 14.0

270. 300. 280.

11.1 23. 16.

730. 890. 795.

1.73 2.05 1.86

15.0 13.3 mean* 14.2

316. 127. 225.

20. 15. 18.

776. 0.939 218. 546. 0.970 153. 665. -0.954 187.

tan&S

269

Woodbine

218 202

1441

490. 740. 600.

750. -750.

157. 159. 158.

2.36 2.30 2.33

9.3 8.8 9.1

4.05 4.48 4.23

5.6 5.9 ,.a

<9.4

32. 29. 31.

220. 182. 200.

0.18 -0.18

15.1 21.8 16.2

546. 216. 674. 199. 606. -208.

2.85 2.65 2.76

5.58 6.71 7.04

0.636 0.849 0.733

9.8 6.6 8.3

0.4 0.7 0.5

43. 32. 38.

167. 177. 182.

172. 147. 160.

0.53 -0.53

393. 487. 439.

776. 688. 733.

157. 181. 169.

1.98 8.07 11.7 2.19 8.21 14.2 2.08 -8.14 12.9

13.6 11.0 12.3

0.7 1.5 G

64. 58. 61.

416. 162. 291.

220. 250. 232.

--0.35 0.35

700. 850. 760.

---

153. 142. 149. --

2.19 1.99 2.10

9.3 16.3 12.1

0.9 0.5 0.l

48. 68. 56.

185. 220. 200.

327. 134. 234.

-----

146. 182. 163.

1.18 9.36 27.1 1.42 8.00 9.71 1.30 -8.51 18.7

13.0 10.6 11.8

0.75 0.6 0.7

24. 20. 22.

198. 139. 170.

9.5, 12.0 8.64 14.4 9.20 13.0

*Weighted by sample weights.

either Al,O, or Sic bits. The ground samples were washed again with acetone. The cleaned samples were crushed in a steel percussion mortar. To avoid contamination by the mortar just one or occasionally two blows to the piston were used. The crushed samples were weighed into polyethylene vials for irradiation. Camp0 de1 Cielo metal samples were hacksawed from a silicate free slab, lightly etched with 4N HN03 to remove surface contamination and washed with distilled water and acetone. Metal samples were not crushed.

EXPERIMENTAL

PROCEDURE

Instrumental neutron activation analysis (INNA) Samples were irradiated four different times during the course of analysis. All samples were run in replicate. The first irradiation was for 1 min at a flux of 1.5 x 1012ncm-2s-’ via a pneumatic rabbit system in the UCLA reactor. Samples were counted for 200s and for another 200 s a few minutes later. A high resolution (1.9 keV FWHM at 1.33 MeV) Ge(Li) detector coupled to a 4096 channel multichannel analyzer was used. The ele-

Table 3. Concentrations of five rare earth elements in silicate inclusions from four group IAB iron meteorites

Campo de1 Cielo IIB 26/3 Mean

90

20

86

140

14

50

16

90

110

20

-88*

130+ -

E+

70*

Is+

Campo de1 Cielo

71

76

80

211

38

IIR 30/13-14

44

18

120

130

26

Mean Copiapo

Mean Landes

Mean Woodbine

Mean

97+

170+

g+

240

290

100

310

57

260

225

89

280

41

250*

260*

95+ -

300*

-49+

150

180

110

190

32

230

400

130

410

47

180+

270'

120+

280'

38*

620

520

130

510

45

460

570

120

300

46

-550+

550*

120+

410+

-45*

-58'

494 -

lo uncertainty based on counting statistics (%). * l&14. + 15-19. + 2(r29. 0 2 30. Others < 10%.

R. W.

1442 Table 4. Concentrations Replicate Number

MeteOrlte

Campo IIB

de1 Cielo 26/3

1 3 * Mean

Campo de1 Clelo IIB 30/13-14

:

of nine elements determined by RNAA in sihcate mclusions from four group IAB iron meteorites Mass

0.249 0.354 0.362

Au

92. 31 133. 86.

Il.155 0.1411

11.9 13.9 12.9 __-

Mean*

cop1apo

1 2 Mean*

U.125 fl.211

161). 200. 185.

Lades

1 2 Mean*

n.337 0.264

240. .lOll 310.

1

0.211 0.199

290. 120 21”.

lvoodblne

BILD

Mea”*

lh.1 12.t4 Lost 13.7

6.64 4.52 8.56 6.58

17.7 18.7 18.2

3.2: 4.68 3.94

hlfl. 57.3 a 115. s4n <115. 99.8 4i.l 14‘.1

n.53 0.96 1.62 i11p

24.5 11.0 5,l.l 30.7 ‘.84 4.14 3.46 _-

230. 16”. 5R0. ?Jo.

rl.84 0.64 0.74

ll.h LO.: 15.? ~___

2.34 1.20 4.19 *

II d n.d. 0.94 0.94

258. 235. 283. ?59.

ii.574 0.:60 0 663

n.d. n.d.

195. 1x0. 188. 438. 150.

12.9 16.1 149

28.1 34.: _e 32.2

134. : 9

3911 500 fiflll;_

11. Ii 15.3 12.

n.d. n.d.

23 1 26.9 24 R

R4.9 98.1 90.7

39.9 157. .30 Q

780. 99”. 87ii.

111 9 15.8 15.1

1.55 ’ 60 ::no

4: 4 196. s47.4

111.8 4.69 m

19.8 11.3 Is_ ?

28.9 4 2

33”. 140. 240.

24 3 9 17.1

n.d n.d.

184. 140. 163. __

g

4

a

*Weighted by sample weights. n.d. = not determined. ments Al, Ca, Mg, V, Ti and Mn were determined. A third longer count several hours later was made specifically for Mn. Samples of pure SiOs, Al foil and MgO were irradiated to permit corrections to be made for interferences with Al, Mg and Na due to (n,p) reactions on Si, Al and Mg respectively. Standards consisting of USGS standard rock BCR-1 and artificial standards prepared from high purity chemicals were run in the same manner as the samples. After the rabbit runs were completed samples and standards were reirradiated in the UCLA reactor for 3 hr. The samples and standards were counted for Na and Mn activities after about 20 hr of decay and recounted several times during the next 34 weeks to yield data on longer lived elements. Rabbit data were reduced by hand; other INAA data were reduced using the SPECTRA computer program originally written by BAEDECKER (1976). Elemental concentrations determined by INAA are shown m Tables 2 and 3.

Radiochemical

neutron actwation

analysis (RNNA)

After INAA, samples were transferred to quartz vials and irradiated along with appropriate flux monitors at the University of Missouri reactor for 10 to 50 hr at a flux of 1 x 1014ncm-2 s -i After several days of decay samples were returned to UCLA, and reirradiated for 3 hr to reactivate the short lived elements In, Ni and Ge. The elements Au, Cd, Ga, Ge, In, Ir, Ni, Zn and in some cases Ru were isolated and counted. Detailed procedures are given by BAEDECKER et al. (1974) and references cited therein and by SUNDBERGand BOYNTON(1976). Results of the RNAA work are shown in Table 4. Accuracy

and precision

Accuracy and precision of the RNAA and INAA data are discussed by BAEDECKER et al. (1974) and BOYNTON et al. (1975). These two papers also contain comparisons of data from our lab with those obtained by other workers for a variety of USGS standard rocks and other materials.

Table 5. Estimates of lu errors in a single determination of an element by INAA based on counting statistics. Elements shown in Table 2 but not listed here have uncertainties less than 10% Meteorite

Replicate IO-14

Campo de1 Cielo IIB 26/30

3

K

4

ELI

lo Uncertainty (%) 20-29 Higher 15-19 La,Eu

Sm,Lu

Ca,Sm,Yb Lu

La

Lu,Ca,Eu

La

Hf(31%),Ti(58%) La(39%),Ti(62%)

Campo de1 Cielo

1

Yb,K

IIB 30/13-14

2

Eu,Sm Lu,K

Yb

Ca

1

La,Lu Yb,K

EIJ

Ti

2

Ca,La,K

Ti,Yb,K Lu,Eu

Copiapo

Landes

Woodbine

1

La

Yb(37%)

EU

2

La,Sm Yb,Lu

EU

1

Yb,K

LU

2

Ca,K,Lu

Yb,Eu

Yb

T1(35%) Ti(65%)

Eu,Ti Ti(33%)

Silicate inclusions in group IAB irons The 95% confidence limits of the mean of two determinations for RNAA are estimated to be: Cd *9”/, Ga and Ge *4x, In, Ir and Au 5 lo%, Ni *6x, Zn +5x. There is some indication Au values may be about 10% systematically high and the uncertainty in the Ru values has not yet been ascertained (SUNDBERG and BOYNTON, 1977). Estimates of lu uncertainties based on counting statistics for INAA data are given in Table 5. Sample inhomogeneities could easily account for as much as a 5% difference between replicates and in some cases much more. Because the rare earth elements (REE) in the samples included in this study occur at much lower levelsthan in the lunar samples discussedby BOYNTONet al. (1975) and BAEDECKETR et al. (1974), the computer program did not always recognize the REE peaks. Data for peaks of the 5 REE most precisely determined by INAA were plotTable 6. Compositions

Meteorite

1443

ted and summed by hand. The la uncertainties based on counting statistics are quite large in some cases (Table 5). ELECTRON

Electron microprobe procedures and standards for analysis of silicates, chromite and Ni in metal are as discussed in BILD(1974) and BILDand WASSON(1976). Results of analyses of olivine and pyroxene from San Cristobal (IB), Woodbine, Mundrabilla, Winona and Mt. Morris (Wis) are given in Table 6, feldspars from Mundrabilla, Winona and Mt. Morris (Wis) in Table 7, chromite from Winona in Table 8, and an unidentified mineral in Mt. Morris (Wis) in Table 9. The values in Tables 68 are averages of all grains of each mineral analyzed. .Analyses of individual grains are reported in BILD (1976). Micro-

of silicates in San Cristobal, Mundrabilla, (Wis) and Winona

No. of Grains

SiO2 Fe0 MgO

MICROBE

CaO

Woodbine, Mt. Morris

Weight % MnO A1203 Cr203

Na20

Ti02

Sum

Olivine

San Cristobal Mundrabilla Woodbine Mt. Morris (Wis) Winona

: 4 4 6

42.5 42.5 40.5 42.7 42.1

3.0 2.7 6.3 1.3 4.6

54.3 0.04 54.6 0.03 52.2<0.02 55.5 0.04 52.4 0.04

0.08 0.20 0.28 0.15 0.36

CO.04 <0.04 CO.03 CO.04 CO.04

Orthopyroxene San Cristobal 4 Mundrabilla 4 Woodbine 4 Mt. Morris (Wis) 6 Mt. Morris (Wis)*l Winona 6

58.9 58.3 57.8 58.8 59.7 58.3

4.3 4.4 5.2 2.8 1.3 4.6

35.5 34.9 35.0 36.6 39.2 34.8

0.29 0.44 0.27 0.24 -0.37

0.25 0.18 0.53 0.25 0.40 0.31

0.12 0.06 0.33 0.06 0.15 0.16 0.15 <0.03 -- <0.03 0.28 0.04

0.31 0.29 0.37 0.06 0.06 0.23

100.5 99.9 100.5 99.8 100.8 100.0

54.1 2.2 17.9 21.1 0.31 55.3 1.8 18.0 22.0 0.18

0.86 0.67

1.32 0.87

0.81 0.56

0.71 0.48

99.3 99.9

Al

Cr

Na

Ti

Sum

------

------

------

------

2.992 2.98@ 2.987 2.992 2.993

0.81 1.04 0.98 0.88 0.14 1.03

CO.02
CO.03 <0.03 CO.05 CO.03 CO.03

X0.03 99.9 CO.03 100.1 CO.03 99.3 X0.03 99.7 CO.03 99.5

Clinopyroxene Mundrabilla Woodbine

1 2

Si

Fe

Molar Compositions Ca Mn Mg

Olivine - cations per 4 oxygens San Cristobal Mundrabilla Woodbine Mt. Morris (Wis) Winona Orthopyroxene

1.009 1.009 1.013 1.010 1.011

Clinopyroxene

1.921 1.920 1.862 1.952 1.879

0.001 0.001 0.001 0.001 0.001

0.002 0.004 0.005 0.003 0.008

- cations per 6 oxygens

San Cristobal 2.003 Mundrabilla 1.997 2.009 ,Woodbine Mt. Morris (Wis) 2.000 Mt. Morris (Wis)*1.999 Winona 1.999

Mundrabilla Woodbine

0.059 0.054 0.108 0.026 0.094

0.121 0.126 0.129 0.080 0.035 0.133

1.798 1.787 1.792 1.853 1.930 1.777

0.030 0.039 0.032 0.032 0.005 0.038

0.008 0.013 0.007 0.007 -0.011

0.003 0.004 0.008 3.986 0.009 0.004 0.008 3.990 -- 0.005 3.984 0.002 0.004 -- 0.002 3.986 -- 0.002 3.986 -0.007 0.002 0.006 3.986

- cations per 6 oxygens 1.975 0.067 0.970 0.821 0.009 0.037 0.038 0.057 0.019 3.993 1.995 0.054 0.959 0.841 0.005 0.028 0.025 0.039 0.013 3.959 Olivine+ Orthopyroxen2 Fs En Wo Fa Fo

San Cristobal 3.0 Mundrabilla 2.7 Woodbine 5.5 Mt. Morris (Wis) 1.3 Mt. Morris (Wis)* Winona 4.8

97.0 97.3 94.5 98.7 95.2

6.2 6.5 6.6 4.1 1.8 6.8

92.3 91.5 92. 94. 98. 91.

1.5 2.0 1.6 1.6 0.3 2.0

Clinopyroxene

Fs En Wo 3.6 2.9

* A single grain 70 x 30 pm with this composition

+ Fe x lOO/(Fe + Mg) and Mg x lOO/(Fe + Mg). *Fe, Mg and Ca x lOO/(Fe + Mg + Ca).

O.C.A. 41/l&C

0.011 0.007 0.007 0.010 0.015 0.013

52. 44. 52. 45.

was located.

1444

R. W. BILD Table

7. CornposItIons

Meteorite

of feldspars

No.

in Mundrabilla,

Wmona

and Mt. Morris

(WIS)

of

Mundrabilla*

3

Winona

6

64.5 22.5

9.47 3.34 0.68 0.36

100.9

Mt. Morris (Wis)

5

63.0 23.5

8.81 4.72 0.48 0.79

101.3

-

9.74 2.79 0.64

--

Cations per 8 oxygens Si Mundrabilla

Al

Na

Ca

K

Fe

Sum

Ab

__

Winona

An

Or

X4.4 11.4 4.2

2.831 1.163 0.805 0.157 0.038 0.012 5.006 80.5 15.7 3.8

Mt. Florris (Wis) 2.765 1.216 3.750 0.222 0.027 0.026 5.006 75.0 22.2 2.7 *Only three elements were analyzed. Values for 9 to make Bence-Albee corrections for matrix effects.

MINERALOGY

OF IAB INCLUSIONS

Table

8. Microprobe

analysis

Fe Weight % oxides 12.3 Cations per 32 oxygens

Mg

2.872

assumed

of Winona

2.5

chromite.

Average

of seven grains

Zn

CT

Al

Ti

V

Sum

1.75

71.3

0.27

0.43

0.19

98.9

of umdentified

mmeral

m Mt. Morris

(WIS)

SiO,

TiO,

Al,O,

Cr,O,

MgO

Fe0

CaO

MnO

P

41.4

1.73

11.7

0.11

26.0

2.3

o.ns

fl.02 9.4

I

41.4

1.38

11.3

--

26.3

4.2

0.116 --

9 ,1 n

41.4

1.56

11.5

0.11

26.3

3.3

0.08

9.4

Si

Ti

Al

Average

m order

4.264 0.592 0.360 15.728 0.088 0.088 0.040 24.032

Table 9. Analysis Grain

of slli(1972).

Mn

10.2

Al were

RAMBALDI et ul. (1974). SCOTT and BILD (1974) and RAMDOHR et al. (1975) reported microprobe analyses of silicates from Landes. Bitburg, San Cristobal and Mundrabilla. All IAB inclusions contain olivine. orthopyroxene and albitic plagioclase with variable amounts of diopside, troilite. metal. schreibersite, chromate and phosphates. Graphite 1s characteristic of the IAB group though in Mundrabilla it 1s found only in the matrix metal of the meteorite and not m the mclusions (A. EL GORESY. private communication). Textures are granoblastic with grain sizes typically 100-500 pm across. The relatively large grain size, abundant 120” triple junctions and rounded grain boundaries are evidence of extensive solid-state recrystallization. No structures resembling chondrules have been found. Troilite and metal grains are distributed through the inclusions. (Fe x lOO)/(Fe + Mg) values range from 1 to 8 for IAB

probe analyses of slhcates from Woodbine and Mundrabilla are also reported m BUNCH ef ul (1970) and RAMD~HR et al. (1975). respectively. Microprobe determinations of Ge were made on metal from a Campo de1 Cielo inclusion, Winona and Mt. Morris (Wis). Iron meteorites of known Ge content were used as standards (Table IO). Nickel was determined simultaneously with Ge and was used to distinguish between kamacite and taenite in those samples containing both phases. GOLDSTEIN (1966, 1967) has shown Ge concentrates in taenite with Ni. The experimental details for these analyses are in BILD (1976).

BUNCH et ul. (1970) reported microprobe analyses cates from I2 different IAB Irons while BUNCH et al

and

9.02

h,O

Na,O

II,0

0.011 7.3 11

5.3

Il.10 6.6

Sum 100.1 ion.1 100.2

Cations per 24 oxygens Cr

Fe

Mg

P

5.599 0.177 1.868 0.011 5.232 0.264 0.007

I

5.728 0.144 1.836

Average

--

Mn

Ca

5.431 0.488 O.nfl9

:

1.00

:

H20 as OH- Sum

1.660 il.03fl2.748 18.074 1.641 0.026 3.035 18.098

(Sl+Tl):(Al+Cr):(Fe+Mg+Ca):(Y+Na) 3.13

Na

1.621 0.022 3.321 18.121

5.664 0.161 1.852 0.011 5.332 0.376 0.008 Catlon ratios:

K

3.07

n.9n

Fig. 1. [a) Sawed surlace of Copiapo IAB iron showing angular silicate inclusions and their relatio nship to the host metal. Light colored metal grains can also be seen inside the inclusions. Sample is from Max-F ‘lanck-Institut fiir Kernphysik, Heidelberg. Scale is in cm. (b) Photomicrograph of a thin se:ction of Wil lona showing recrystallized texture Striated grain below center is pfagioclase. Specimen NPvlNI-3 854, tr.ansmitted light, crossed polars, scale bar = 5Opm. (cl Photom~~ro~raph of Mt. Morris (:wis1. White grains are metal and troilite, dark gray are silicates, lighter gray textured areas are terre :strial weathcsing products and black areas are voids in the section. The triangular grains in the c:en ter are graphite. Specimen NMNH 1198, reflected light, scale bar = 100 ,~m.

Silicate inclusions in group IAB irons Table 10. Standards data for determination Meteorite

of Ge in metal by electron microprobe

27 VIII

counts/400 26 VIII

1700

168.3

164.3

Campo de1 Cielo (matrix metal)

392

130.8

130.5

Camp Verde

322

127.3

Nieder Finow

257

123.4

Avce

182

125.2

Four Corners

179

Puripica

174

Butler

GeX (WE) (kamacite)

Ternera

0.056

1447

set

(Thousands)+ 19 VIII 14 VIII 166.7

126.0

123.7

123.5

123.3 123.1

126.7

124.3

121.5

119.8

119.2

. 121.2

122.2

* All from WASSON(1974) except Butler (GOLDSTEIN.1966). + Normalized to 27 VIII counting rates.

olivine and from 4 to 9 for IAB orthopyroxene. These values are lower than ordinary chondrites (MASON.1963; KEEL and FREDERIKSSON, 1964; BILD and WASSON,1977) and higher than enstatite chondrites (KEIL, 1968). Reported plagioclase compositions range from Ab 87 to Ab 76 and laths of orthoclase in an antiperthite structure have been reported in Odessa (EL Goamy, 1967), San Cristobal (SCOTTand BILD, 1974) and Mundrabilla (RAMDOHRet al., 1975). The mineral compositions given in Tables 6 and 7 for San Cristobal, Mundrabilla and Woodbine inclusions are typical of IAB members. Diopside is much less abundant than orthopyroxene in Mundrabilla and Woodbine and was not found at all in San Cristobal. In the latter case the abundant occurrence of the Ca phosphate brianite (BILD, 1974) probably accounts for the lack of the Ca-rich pyroxene. Troilite grains in San Cristobal and Mundrabilla are single crystals while troilite in Woodbine is highly recrystallized on a very fine scale.

Copiapo, Woodbine and Landes

Elemental abundances of non-volatile, ~thophile elements (Na through I-If in Fig. 2) in Copiapo, Landes, and Woodbine are generally within a factor of two of CI levels. Abundances of non-volatile elements similar to those in CI chondrites is the property which classifies a meteorite as chondritic (WASSON, 1974). These same abundances vary by more than a factor of ten for non-chondritic (i.e. differen-

LITHOPHILES The means shown in Tables 2 to_ 4 for neutron activation data are averages of the replicates weighted by sample weights. For some meteorites there are systematic differences between replicates (e.g. Landes rare earths, replicate 1 lower; Copiapo siderophileq replicate 1 lower; Woodbine siderophiles, replicate 2 lower) indicating sample inhomogeneity at the approx 0.2 g level sampled. These elemental data were normalized to Mg and converted to atomic abundance ratios relative to CI chondrites. Figures 2 to 5 are plots of these ratios. CI chondrites are thought to be the best available estimate of the composition of the early solar nebula for all except the most volatile elements and thus are the best estimate of the starting material from which all meteorites (and other solid solar system bodies) formed. For this reason they are a useful reference composition to which other meteorite types can be compared. The elements in Figs. 2 and 3 are roughly grouped by chemical affinity: highly siderophilic elements, Ga and Ge, lithophiles found in feldspars, other lithophiles, and volatiles. Figures 4 and 5 show chondrite normalized rare earth abundances in order of atomic number.

Fig. 2. Elemental abundance ratios relative to CI chondrites normalized to Mg for silicate inclusions from three group IAB iron meteorites. The patterns in all cases are chondritic. High values for Zn, Cd, and In are probably con~mination (see text). la error estimates based on counting statistics are shown where they exceed 10%.

Fig. 3. Elemental abund~ac~ ratios relative to C‘I ckond&es normalized to Mg for two different silicate inclusions from Campo de1 Cielo (IA) iron meteorite. Lithophiles are basically ckondritic though Ca is sligktfy depleted. The results indicate inclusion IXB30jt3-14 contains slightly less feldspar and muck less metal than inclusion II3 X/11. la error estimates based on counting statistics are shown where they exceed lo%, tinted] meteorites such as a~ond~t~s. The abundances of Cr, Mn, Zn and V are depleted in Copiapo, Landes and Woodbine silicates (Fig. 2). Since all four of these elements are important ~onstit~ents of chromite; chromite may be depleted in the sampies analyzed. Replicates on the volatile elements Zn, Cd and In show a great deal of scatter, ~rti~u~rl~ for In. Prep aration of these three samples involved sawing with a SiC wheel and/or wire saw and this could have contaminated the samples, Samples of the wheel and wire have not been analyzed for these elements, but the two Campa de1 Cielo samples and a sample of silicate inclusion from Netscha&o igILD and WASSON,

L&

Sm Eli

Yb Lu

Fig. 4. Abundance ratios for 8% rare-ear& elements in silicate inclusions from tkreu group IAB iron meteorites. Values are relative to ordinary ckondrite rare earth proportions but bave been adjusted to C1: levels and normalized to Mg. Dashed lines between Eu and Yb were drawn by assuming a smooth curve connecting all BEE except Eu. 1~ error astima%es based on courting statistics are shown where they exceed IO?&

-safu

%Itu

Rg. 5. Abun~nc~ ratios for gve rare earth elements in two different inclusions from the Campo de1 Cieio (IA) iron meteorite. Normalizations are the same as for Fig. 4. The replicates for inclusion IIB 30/13-14 showed a large

variation, especiafly for Sm. so both re&ates are plotted, Ia error estimates based on counting statistics are shown where they exceed 10%. 1977) had no contact with either saw. Replicate agreement for these elements is good in these cases. Abundances ratios for both replicates are plotted in Fig. 2 for Cd and In in Copiapo, Landes and Woodbine and for 2% in Copiapo and Landes. The line connecting the points is drawn to the lower of the two rep% cates but even this should be considered an upper limit. The rare earth patterns (Frg. 4) are also close to chondritic levels. Wayne does have a slight negative ELI anomaly and an e~richm~t of all REE bq^ a factor of I.5 to 2. MASUDA(1969) reprted a virtually identical pattern for another Woodbine inclusion. The cause of the apparent La depletion is not clear. Perhaps during a heating event light rare earths were fractionated into a mineral or possibly a partial melt which was separated from the main body of the inclusions.

The inclusions from Campo de1 Cielo also have lithophile abundances at about chondritic levels (Fig. 3). The silicate rn~~ralo~ of inclusion IIB 26/3, as determined by mieropro~ point courting, is 57% orthopytoxene, 27% olivine, 16% plagioclase and 0.39,; diupside. The low Ca abundances in Fig. 3 reAect the lack of diopside and the different levels of Na, R and Al in the two inclusions imply digering amounts of plagioclase. WLOTZKA and JAROSEWICH(1977) provide a debihd mineralogical and bulk chemica1 description of several Campo de1 Cielo inclusions and report chemical differences between inclusions, These they

Silicate inclusions in group IAB irons

1449

SIDEROPHILES interpret as being the result of addition and subtraction of various amounts of diopside and/or plagioSiderophileMg ratios are also roughly at chondrite clase during partial melting. These two minerals have levels in all the inclusions analyzed (except Campo the lowest melting points of the LAB silicates. They de1 Cielo IIB 30/13-14) implying that the metal in report normative compositions for three inclusions the inclusions is native to the inclusions and not which vary by factors of three in clinopyroxene and matrix metal that has been mixed with the silicates. two in plagioclase, olivine and orthopyroxene. The Veins of metal that cross some inclusions may be mineralogy of inclusion IIB 2613 from this work is exceptions. Siderophile abundances relative to CI essentially identical to their inclusion 4. abundances are illustrated in Figs. 2 and 3. One interThere are several significant differences between the esting fact is that Fe abundances are always near 0.6, Camp0 de1 Cielo inclusions and the other IAB inclua rather low value comparable to those in LL chonsions analyzed. One is the depletion of siderophiles, drites. especially in inclusion IIB 30/13-14, and the differFigure 6 is a comparison of siderophile element/Ni ence in siderophile levels between the two inclusions ratios in bulk inclusions to siderophile elementpi (Fig. 3). Another is the rare earth pattern shown in ratios in the matrix metal. Matrix metal data are Fig. 5. The main features here are La and Sm delisted in Table 13 along with the sources of the data. pletions (usually Sm more depleted than La) and large Copiapo, Landes and Woodbine inclusions have positive Eu anomalies. Since both diopside and ratios very similar to siderophiles in the matrix metal. plagioclase are important carriers of REE the variable This indicates that either inclusion metal and matrix proportions of these minerals in different inclusions metal formed under similar conditions before agglomcould account for most of the features in Fig. 5. A eration or that they have equilibrated with each other detailed explanation of these patterns will require in situ (or both). The low Ge and Ir abundances relaknowledge of the REE contents of the individual tive to CI for Woodbine are consistent with the Ge Campo de1 Cielo minerals and will probably turn out and Ir trends at the high Ni end of group IB (SCOTT to be much more involved. A futher indication of the and BILD, 1974; Scold and WASSON,1975). complexity is that two inclusions analyzed by HINGallium is enriched relative to other siderophiles TENBERGER and BERGHOF(1970) have La depletions because of its lithophile tendencies (cf CHOU and similar to those in this work but negative Eu anomCOHEN, 1973; CHOU et al., 1973). This is especially alies of similar size to the positive anomalies in Fig. evident in the Campo de1 Cielo inclusion IIB 5. 30/13-14.which is depleted in metal. If plots of the In addition to evidence for partial melting of diopsiderophiles Au, Ni, Ge and Ir vs Ga are made for side and plagioclase (WLOTZKA and JAROSEWICH, the five replicates from the two Campo de1 Cielo in1977) and fractionation of REE there are other indicaclusions (data from Table 4) straight line relationtions Camp0 de1 Cielo silicates have been more ships are observed which extrapolate to about strongly heated than the other LAB silicates studied. Highly volatile elements Cd and In (this work) and Xe13’ (Camp0 de1 Cielo-HINTENBERGER et al., 1969; other IAB silicates--BoGARD et al., 1971 and ALMANDERand MANUEL, 1968) have lower abundances in Campo de1 Cielo silicates than other IAB silicates and POWSEK (1971) found evidence for a reheating event 3.4 Gyr ago that was strong enough to release significant amounts of low temperature Ar. Partial melting could have occurred at this time or earlier. WL~TZKA and JAROSEWICH (1977) report Fe0 zoning of olivine in Campo de1 Cielo inclusions. Centers of grains contain about 1.1 times more Fe0 than rims. Similar but more pronounced zoning of olivine occurs in ureilites (e.g. WLOTZKA,1972) and in Lodx Lsndes 0 Woodbine ran (BILD and WASSON,1976). In analogy to the ureilites, Wlotzka and Jarosewich attribute the zoning to reduction of Fe by graphite. WASSONet al. (1976) have 0.7 pointed out that the reaction of Fe0 and C proceeds only at high temperatures and low partial pressure of CO, i.e. the system must be open to CO escape. Fig. 6. Ratios of siderophile elements in bulk silicate incluThe small degree of reduction seen in Campo de1 sions to siderophiles in matrix metal, normalized to Ni, Cielo indicates these conditions did not prevail for for two Campo de1 Cielo inclusions and inclusions from three other group IAB irons. Differences between the a long period. Since there is independent evidence matrix metal and inclusion siderophile ratios are seen in for high temperatures, CO pressure was probably the all cases but are most pronounced in Campo de1 Cielo factor limiting the degree of reduction. inclusion IIB 26/3.

1450

R. w

BILD

3.5pg/g Ga at 0 pg/g Au, Ni, Ge or Ir (i.e. 0 metal content). This is similar to values for silicate fractions of ordinary chondrites reported by CHOU and COHEN (1973) and CHOU et al. (1973). Using this value of 3.5 pg/g for the Ga content of the silicate, the microprobe determined Ni content for inclusion IIB 26/3 metal (5.25x, average of 4 grains from Table 12), and assuming all Ni is contained in the metal leads to a calculated Ga content of the inclusion metal of 65 pg/g. This is approaching the matrix metal value of 91.1 pg/g. The Ga metal/silicate concentration ratio for this inclusion is 65/3.5 or about 19. If the volume of troilite and graphite in the inclusion 1s considered. this value is slightly higher. CHOU et al. (1973) report Ga metal/silicate concentration ratios of 0266.0 for a suite of mainly H group chondrites with values for most equilibrated H group members being in the range 3.0-3.9. Thermodynamic data reviewed by CHOU and COHEN(1973) indicate higher Ga metal/silicate partition coefficients at higher temperatures. However the lower oxygen fugacity for formation of the Campo de1 Cielo inclusions [as indicated by the lower FeO/(FeO + MgO) ratio of the olivine and pyroxene] also leads to a higher partition coefficient. Thus qualitatively the value of 19 for the Campo del Cielo inclusion (if it is an equilibrium value) is not unreasonable. Siderophile/Ni ratios in Copiapo, Woodbine and Landes are the same for inclusions and matrix to within a factor of 1.5. This is more variation than experimental scatter should allow but in most cases the matrix metal sample was taken from a different specimen than the inclusion so some of the variation could be due to inhomogeneities through the large meteorites. Some of these differences are also probably real differences between inclusion and matrix showing that complete equilibration has not been attained. The Co/Ni ratios in the inclusions are lower than m the matrix metal (Fig. 6). BUNCH rt al.(1970) and BUNCH et al. (1972) have reported different Co contents in inclusion and matrix metal from several IAB

irons. These differences could be due to the smaller grain size of the metal in the inclusions allowing equilibration with other phases to lower temperatures or to disequilibrium between matrix and inclusion metal. The lower ratio for the bulk inclusion could also be due to different kamacite-taenite proportions in the inclusion and metallic matrix. Siderophile/‘Ni ratios are quite different for the two Campo de1 Cielo inclusions. Inclusion IIB 30/13-14 seems to have siderophiles well equilibrated with the metal except for Ir (depleted) and Ga (enriched) (Fig. 6). This inclusion is highly depleted in siderophiles (Fig. 3). If partial melting caused the depletion of metal, the more severe depletion in inclusion IIB 30/13-14 indicates a longer exposure to higher temperatures which could have resulted in more complete equilibration of inclusion and matrix metal. Iridium is depleted relative to other siderophiles and matrix metal in IIB 30/13-14 and enriched in IIB 26/3 relative to matrix metal and to Ni, Au and Ga (Fig. 6). This wide variation indicates that Ir in the inclusions may be concentrated in some phase other than the metal. Relative to CI abundances (Fig. 3) inclusion ITB 26/3 is highly depleted in Ni and (to a lesser extent) Au and enriched in Ge relative to other siderophlles. The Ge and Ni values measured by microprobe for four metal grains in Campo de1 Clelo inclusion IIB 26/3 are shown in Tables 11 and 12. All grains have typical kamacite Ni contents (4.91;;--5.51$ but Ge contents are high& 1.5 to 2.6 times higher than the host metal value of 392 pg/g (WASSON,1974). GOLDSTEIN(1967) measured Ge in the metal of ten iron meteorites (four in group IAB) and found that Ge is distributed between kamacite and taenite in a manner analogous to Ni----i.e. Ge concentrations are greater in taenite than kamacite and form an “M” profile across a taenite body. The highest Ge concentrations, 2-3 times higher than that in average kamacite, is found in taenite at the interface with kamacite. The high Ge content in kamacite in Campo de1 Cielo inclusion IIB 26/3 is another indication that the metal

Table 11. Ge counting rates (thousands of counts/400sec) and Ge concentrations from standards curve (Fig. IV-I). Concentrations from 27 VIII 75 are of better quality than value from other dates. Other values are listed to show the reproducibility of Ge values in the various Campo de1 Cielo metal grains

Meteorite Winona Mt. Morris Campo

(Wis)

de1 Cielo

A*

27 VIII G Count

26 VIII c count

19 VIII count

rate

rate

rate

h&

&~PI

119.9

29+20

120.5

SO*?0

01+20

121.2

74i?O

149.1

104Oi2ll

Campo

de1 Cielo

B*

138.0

655?20

139.9

Campo

de1 Cielo

C*

135.6

572i20

133.9

513??fl

Campo

de1 Cielo

D*

grams

m skate

GJ

121."

72Oi20

* Metal

c

136.8

mclusion

IIB ‘hii

h13i 211

14 VIII Count G rate

&

122.1

lOSi20

149.9

1066+20

1451

Silicate inclusions in group IAB irons

sions are positively correlated with the Ge content .of the host metal (linear correlation coefficient r = 0.91, 7 meteorites). Campo de1 Cielo silicates contain only 1@30% of the Xe13* needed to place them on the trend of the other data. Partial loss of Xe during the stronger heating already inferred for Campo de1 Cielo silicates could explain this depletion. The link between these two parameters is not clear. It could be formation of host metal and inclusions at monotonically changing distances from the sun, depth of burial in the parent body, intensity of heating or something else. Until more is learned about this correlation it is probably best to put it in the category of “interesting curiosity”.

Table 12. Nickel contents of metal determined by microprobe simultaneously with the Ge data in Tables 10 and 11 Meteorite

?li%*

Ni% This Work

(Lit1

Campo de1 Cielo A

5.5

Campo de1 Cielo B

4.9

Campo de1 Cielo C

5.2

Campo de1 Cielo D+

5.4

_.-

Campo de1 Cielo (matrix metal)

7.05

6.62

Winona

___

13.9-6.2

Mt. Morris (Wis)

7.04

__.

AVCE?

5.4

5.49

Puripica

5.6

5.58

Camp Verde

7.02

7.06

Nieder Finow

7.04

8.26

Butler

6.2-41.2

15.2

Ternera

7.8-21.6

18.3

RELATIONSHIP OF IAB SILICATES TO MATRIX METAL AND OTHER CLUES TO THEIR FORMATION

* From WAS~~N(1974).

+ Data from 19 VIII normalized to 27 VIII count-

ing rates. in the inclusions formed independently of the matrix metal. Two possibilities are that the inclusion metal initially condensed with a high Ge/Ni ratio and this composition was preserved or that after condensation of the metal some process such as partial melting selectively removed Ni from (or added Ge to) the metal grains. Another possible explanation of the siderophile pattern in inclusion IIB 30/13-14 is that the inclusion was completely depleted of metal by partial melting. The metal now in the inclusion is a low temperature partial melt of matrix metal which is depleted in the refractory Ir. Xe13* has been measured in inclusions from eight group IAB irons (ALEXANDER and MANUEL, 1968; HINTENBERGERet al., 1969; PODOSEK, 1970; BCKXRD et al., 1971; BEGEMANN et al., 1976). With the excep

tion of Campo de1 Cielo, Xe13’ contents of the inclu-

As noted by BUNCHet al. (1970) the angularity of the silicate inclusions in IAB strongly suggests formation as a breccia (Fig. la). WASSON (1970) and Scorr (1972) have argued that IAB metal has never been molten and that chemical fractionations observed occurred during condensation and/or agglomeration. Various mechanisms of agglomerating metal-rich bodies have been proposed (e.g. ductility and cold welding--ORow.&N, 1969; magnetic propertiesWOOD, 1962); and OROWAN (1969), TUREIUANand CLARK(1969) and CLARKet al. (1972) have proposed formation of metal-rich bodies by inhomogeneous accumulation. Fragments of chondritic material could become trapped or embedded in the metal-rich bodies as they grew. Another mechanism would be to accrete metal-rich bodies onto a larger body with a chondritic surface. In either case angular silicate fragments could be produced and later burial and heating would recrystallize silicates and anneal metal to the large (2 50 cm) taenite grains required to produce the Widmanstltten patterns seen in some IAB irons. As observed for major elements by earlier workers, abundances of non-volatile elements in the inclusions are approximately chondritic. The roughly chondritic siderophile levels (except in Campo de1 Cielo) indicate that the metal in the inclusions is native to the inclu-

Table 13. Concentration of minor and trace elements in matrix metal of five iron meteorites as determined by INAA and RNAA

Campo de1 c1e1o

:

---------6.62

90. 94. 91. 90. 91.

Coplap

7.01

69.8*

2521

2.5*

1020.

_..

. ..

___

.__

.._

___

4.0

Landes

6.31

88.7*

4141

3.4*+

1410:

---

---

---

8.9

1.62

0.3

4.1+

Woodbine

10.6

37.3*

1142

1.4*

1730:

---

---

---

19.6

0.52

0.19

4.9+

3 4 Average

1.050 0.878 1.016 0.936

353. 355. .-.

2.8 2.4 __.

354.2.6

* WASSON,1974. Ni by atomic absorption. + E. R. D. SCOTT,unpublished data.

1310. 105t 1530 1490. -1440.

lost 7.57 _._ _.. 7.57

3.66 3.50 ... _._ 3.59

lost 17.5 _.. ___ 17.5

----10.1 10.8 10.4

----1.32 1.40 1.36

_---0.27 0.37 0.32

----4.1 4.3 s

1452

R. W. Bno

sions; random addition of matrix metal to silicate would not be expected to consistently result in chondritic levels of siderophiles. The high Ge content of kamacite in the Camp0 de1 Cielo inclusions and the variability of Co contents between matrix and inclusion metal are also evidence for independent formation for matrix and inclusion metal. In the inclusions studied (except Campo de1 Cielo) Na, K and Ca which concentrate in a low melting fraction during igneous differentiation and Mg which concentrates in the high melting fraction are present at about chond~ti~ levels indicating the inclusions have not undergone igneous differentiation. This means the inclusions were probably never heated to the melting point of albitic plagioclase, the lowest melting major silicate (m.p. _ f450 K). Melting the feldspar with no loss of melt from the inclusion would be evident. In even a very weak gravitational field high density phases such as metal grains would settle to one side of the inclusions in a relatively short trme. The feldspar liquid would tend to coalesce rather than remain as individual grain-sized masses and upon solidification feldspar should completely surround some grains of other silicates. Campo de1 Cielo inclusions were probably heated to or above 1450K. This work shows evidence of metal depletion and both this work and WLOTZKA and JARO~EWICH (1977) find evidence for some partial melting of silicates. The relationship of the metal structure to the silicates also tells something of the history of the material. In some cases the Widmanstatten pattern is continuous around the inclusions, in other cases the inclusions are swathed by kamacite. These both indicate the silicates were in place before kamacite started to precipitate and that the temperature was high enough to have the Ni-Fe in the gamma field (> 1040K at 6% Ni, >9# K at 12% Ni) for an extended period. This temperature approaches the melting point of the Fe-FeS eutectic (1261 K) and reaching the eutectic temperature would allow some mobility of a S-rich fluid. A period at high temperature would also account for the highly recrystallized textures of the silicates and the lack of visible chondrules. A slightly greater degree of heating at the Camp0 de1 Cielo location could lead to the mild partial melting inferred for its inclusions. Three-isotope oxygen analysis has proven to be a valuable tool for classifying meteorites (CLAYTONd al., 1976). Oxygen isotopes in silicates from Campo de1 Cielo, Landes and Woodbine were measured by the Chicago group. The four samples came from the same or neighbouring inclusions as those analyzed chemi~aily here. The results place the inclusions just below the terrestrial fractionation line at 6Oi8 about 4.5-5.0x, relative to SMOW (R. CLAYTON,personal communication ; CLAYTOEet al., 1976). These values are enriched in 016 relative to H chondrites by about the same amount as H chondrites are enriched over the L and LL groups. The oxygen isotopes indicate

the inclusions are not trapped fragments of another known chondrite group which have been reduced, but are chondrites of a new group. Several authors have proposed that the oxidation state of the chondrite groups as measured by FeO/(FeO + MgO) ratio of the silicates is correlated with heliocentric distance of formation. the more reduced groups forming closest to the sun (e.g. ANDERS,1971 : WASSON.1975). The IAB silicates fall between the H and E chondrites in oxidation state and thus presumably in heliocentric distance of formation.

PROPOSED MODEL FOR THE FORMATION OF IAB StLICATE INCLUSIONS The inclusion material first condensed and agglomerated to at least cm-sized bodies in the same manner and time and with a similar structure as ordinary chondrites. The bodies had chondritic ratios of nonvolatile elements but reflected the physical and chemical conditions cfOz. oxygen isotope ratios, nebular pressure, thermal history etc.) at their own formation location. The chondritic material was mixed with iron meteorite host material either by collision on one or more parent bodies or by agglomeration of cm-sized chondritic silicate masses with similar sized metal masses. Once the inclusions were trapped in the host metal the mixture was heated to at least 1000 K which allowed formation of the precursor taenite required by the Wi~anst~tten structure of the host and solid state r~rys~llization of the silicates.

WINONA AbID MT. MORRIS (WIS) MAWN and JAROSEWICH(1967) report that the Winona meteorite contains olivine, orthopyroxene and plagioclase (Fa,, Fs,. An,,, based on refractive indices}; diopside, a Ca-phosphate, kamacite, taenite. troilite, graphite and chromite. RAMDOHR(1973) also lists schreibersite, daubreelite, alabandiate and ilmenite. Mason and Jarosewich note a similarity of the bulk composition to that of ordinary chondrites (except for metallic Fe which has been terrestrially oxidized) and provide a normative mineralogy. Rare gas data of T. Kirsten (reported in MACON and JAROSEWICH, 1967) show planetary-type gases at levels typical of ordinary chondrites. These yield an exposure age of about 50 Myr, a K-Ar age of 4.6 f 0.2 Gyr, a He4-U age of about 2.7 Gyr and a large amount of excess Xe’29. The exposure age is long compared to ordinary chondrites but shorter than most irons. Winona sections examined in this study (Fig. I b) have an average grain size of about 0.1 mm but with occasional silicate grains up to five times larger. The texture is highly equiiibrated with abundant 120” triple junctions. Iron oxide is very abundant, on the

Silicate inclusions in group IAB irons

1453

(1.41 wt % Zn) of the Winona chromite also is in the same range as Zn in IAB chromite. Among the chromite compositions reported by BUNCH et al. (1970), SNETSINGER et nl. (1967) and BUNCHand KEIL (1971) IAB is the only group in which chromite is consistently Zn-rich. Some IIE irons also have Zn-rich chromite (BUNCHet al., 1970) as does Lodran (BILD and WASSON,1976). MACONand JAROSEWICH (1967) point out the close similarity in composition and structure of the Mt. Morris (Wis) meteorite and Winona. The Mt. Morris (Wis) stone is even more severely weathered than Winona. The section examined (Fig. lc) consists of angular grains of orthopyroxene (up to 1 mm long), plagioclase with albite twinning and a few large laths of olivine up to 4mm long. The olivine is clear in large areas but shows much evidence of weathering at the edges and along kracks. Several areas l-2 mm across consist of a mass of finer grained (typically around 0.02 mm) pyroxene. These are possibly larger grains that have recrystallized. Kamacite was the only metallic phase observed optically. Troilite is abundant as grains up to 1 mm across and entire grains go to extinction uniformly. Edges of the troilite grains are partially altered-to pentlandite (RAMDOHR,1973). Daubreelite exsolution is seen in many troilite grains. Schreibersite is also abundant; one grain is 3 mm long. Graphite is observed as inclusions in both phosphide and troilite. RAMDOHR(1973) lists the above phases and chalcopyrrhotite and chromite as occurring in Mt. Morris (Wis). He also failed to find taenite. Analyses of pyroxene, plagioclase and olivine from Mt. Morris (Wis) are given in Tables 6 and 7. Pyroxene are equilibrated (average compositions Fs,,,En,,Wo,,,) except for grain J which is Fs,,sEn,,Wo,,,. This grain fluoresces blue under the probe beam while the other pyroxene and olivine grains do not fluoresce visibly. The exposed section of the grain is about 70 x 30~. DODD et al. (197.5) found two orthopyroxenes, one low Ca (Wo, 2) and one high Ca (Wo.& in the highly metamorphosed Shaw chondrite and interpreted the low Ca phase as inverted protobronzite. The Shaw low Ca pyroxene is also Al-poor while the Mt. Morris low Ca pyroxene is Al-rich compared to the higher Ca pyroxene. Olivine (Fa,,J) appears equilibrated. Olivine and pyroxene with similar compositions (Fa,, Fs,,,) are found in inclusions in the Pine River (IAB) iron meteorite and these are about the equilibrium values expected from the trend of coexisting Fa and Fs values for other equilibrated meteorites (see Fig. 2 of BILD and WASSON,1976). Grain J is out of equilibrium with Fe-Ti Zn+Mn-w the other Mt. Morris (Wis) silicates. Table 9 shows the composition of two othei silicate Fig. 7. Segment of a triangular plot of divalent cations in meteoritic chromite. Note that chromite from inclusions grains whose identity is not certain. These grains in IAB irons and from Winona plot together in a unique fluoresced yellowish-green under the probe beam. region of the diagram. Winona data from this work, other Grain P (about 25 pm in diameter) is the larger of chromite data from SNETSINGER et al. (1967). BUNCH et al. (1970), BUNCHand KEIL (1971), BUNCHet al. (1972), the two. It was possible to keep the probe beam on BILD(1976),BILDand W~sso~ (1976). these small grains by watching for the characteristic

order of 10% of the section by volume, and unoxidized metal is very rare. Troilite is plentiful, grains are as large as 1 mm and show uniform extinction. Plagioclase makes up about 5% of the sample and shows albite twinning. Daubreelite occurs as lamellae in troilite and graphite occurs both as feathery deposits in troilite and metal and as triangular plates. Chromite forms subhedral to euhedral grains up to 0.2mm long and is generally associated with veins of iron oxide that presumably were once metal. Microprobe analyses of pyroxene, olivine, plagioclase and chromite from Winona are shown in Tables 6-9. The pyroxene and olivine compositions (Fs,,, and Fa,,) are similar to values reported by MASON and JAROSEWICH (1967) as is the Cr,03 value of the pyroxene (0.28 wt % this work, 0.2 wt % Mason and Jarosewich). MACON and GRAHAM (1970) reported 0.12% Ti and 0.2% Mn in orthopyroxene; my values are 0.14% Ti and 0.24% Mn. I find the plagioclase (Abs,,SAn,,Or,) to be more anorthitic than the value of An,, reported by Mason and Jarosewich. The chromite composition (Table 8) is plotted on Fig. 7. Winona chromite has an atomic Fe/(Fe + Mg) ratio that falls in the range shown by IAB chromite (data of BUNCH et al., 1970; BUNCH et al., 1972) and with is consistent the relationship between Fe/(Fe + Mg) ratio of chromite and Fe/(Fe + Mg) ratio of silicates observed by SNET~INGER et al. (1967) and BUNCH et al. (1970). The high Zn content

1454

R. W. B&U

Auorescence, The composition shown in the Table has water added to bring the total to 10CCOand is very sun&r to phfogopite. Ail oxide ~on~e~tra~ons are in the phjogopite ranges shown in ihER @5 ai. (1962) and the cation ratios shown in Tabie 9 are close to phlogopite ratios (ideal phlogopite is K*(Mg.Fe “k, (Si,Ai,O,,)(OH.F).+. There are also significant differences between the calculated formuh For the meteoritic mineral and ph~og~pite, par~~u~a~~~in the amount of OH, but the small grain size, possibihty of inclusions of other minerals and problems of voIatile alkali loss during repeated analysis of over~appin~ areas may be the cause. Fh~o~opite occurs terr~tr~a~l~ as a metamorphic mineral, usually associated with carbonates and as a primary mineral in some leucite-rich and dtramafic rocks. It does not occur as a weathering product of divine, pyroxene of plagiochise. The mineral grains in Mt. Morris are included tn the fabric of the meteorite and give no evidence of having been added during the terrestrial htstory of the .sampkz.In reflected light there is DOobvious r~~tion relation of these grains with other minerals. Observation in transmitted light 1s not possible because the parts of the se&on containing the grains are opaque due to the abundant iron oxide. A positive identification of the mineral was not possible.

variatron between kamacite and taenite in Butler was clearly detectable. The un~ertaintl~ reported in Table I1 for Ge m Winona and Mt. Morris (Wis) are tcr ranges based on mounting statistics. Because no Ge counting rate variation with Ni content was observed in Winona, it is unlikely that Ge was detected at all in this sample. The value determined for Mt. Morris (Wis) of 90 & 20&g seems genuine but smce there was no high Ni metal in which to check for higher Ge counting rates it is not completely certain that Ge was detected.

~vely.

at the Universltv of Missouri and UCLA reactors were

as 32% Ni. There was no discernible variation in Ge counting rate with varying Ni content in Winona. The

EAR 74-22495

In the following discussion I assume that Winona and Mt. Morris (Wis) are closciy related. The available data suggest tbaz they are, but the data on Mt. Morris (Wis) are less complete than those for Winona. These two meteorites are very similar to the silicate inclusions of IAB irons m bulk composition, mmeralogy, mineral com~sit~on, rare gas content and age. The combination of abundant graphite, oiivine and pyroxene oxidation state. albitic plagioclase and .&-rich. chromite makes the mineralogical similarity very strong. Mt. Morris silicates are more reduced Ge CONTENT OF METAL FROM WINCX%A than Winona and most IAB siiicates, but except for ANIl lVl37. RIfoRRIS (WKf pyroxene grain J. Fe0 contents do not fall below the Since the silicate mineralogy of Winona and Mt. values reported in Pine River (IA) silicates (Bu~cct Morris (Wis) has many srmilarities to group IAB inet ut., f970). Textures and degree of re~rystaiIi~atio~ ch&m, I attempted to measure the Ge content of are also very smrilar though the highly weathered the metai to see if it rtxembies IAB metal. Matrix state of Winona and Mt. Morris (Wis) may obscure metals from IAB irons have bulk Ge contents that some features. Oxygen isotope ratios have been vary inversely with Ni content from about 5oOpg/g measured for WitIons and are in~stiIlguish~bie from to about 200 nr;ig in IA down to 25 iilgii: for San Cristhe XAB silicates that have been analyzed (R. tinytobat the most &rich IB (WzSoN. 1974). Wi~o~a rol\j. private ~~~rnuflic~~tio~~ metal is roughly equally divided between kamacite The apparent low Ge content of the metals is the and taenite indicating the bulk Ni content is probabty only known dlss~n~ilari~y. but its importani;e is accenin the same range as IB irons. The data of HEWMAN tuated by the fact that metais in the Campo det Cielo and BRADY(1929) indicate metal ~utain~ng about inclusian have greater Ge contents than the matrtx 16% Ni. If Winona follows the IAB Ge-Ni trend, metal. However, an balance the evidence favors the there should be about 75 ,ug/g Ge in its metal, a level conclusion that Winona and probably Mt. Morm that might be detected by car&I rn~c~#p~~~ {Wis) are e!osety related to the silicates found in IAB rne~~~~~t~. The d&& of ~LIXTEPJ {19&i, L%i71 irons. show that Ce levels should be several times higher in the Ni-rich taenite rims than in the bulk metal. -2rknol~ir~~urtlrttts--Samples crammed m this study were Mt. Morris (Wisf metal is al1 kamacite (about 7 tn, suppbtd bq‘ R. ??. CLARLF. w. FiiEst. J. R5X%Xi?s, R Ni was determined in this work] so if it fottows the SfHAarnv and the UCLA met.exx%ecolfection. Dtxussrons IAB GeNi trends it could contain severa! hundred wttb J. T. WATSON, W V. BOYN’SON and E. R. D. SCOTT are gratefully acknowledged as is permission from E. R /of/g Ge. Nickel values from the 26 VIII run are grven tn D. SCOTT to use unpublished data I thank P. A. BAEIXCKER, w V. BOY&W,t.-t. &OU. 8-i. J, ~HIA-4.M Table 12 along with bulk Ni values from WAWN GJF.RDE, R GLIMP” R. E. &3?4ES,s. .b-iES~ S. KAss, 3. Knu(1974). The highest and lowest Ni points encountered BERLIN, C. Pr;s~v~,r, K. t. ROBINSON, L. L. SUNDRERG and P WARREN fOF technical assistance. Reactor irradiations in Winona in this run were 13.9 and 6.2”;, Ni respectthough no effort was made to seek either extreme. Other runs on Winona gave values as hrgh

conducred by & Cum. T Z.&NE and then associates. Mreroprobe faeitities were provided by the Dept of C&ology” UCLA. Thts research was supported by NSF grant

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