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Ni, Ga, Ge and Ir in the metal of iron-meteorites-with-silicate-inclusions
Qeochimica et Cosmochimics Acts, 1970,Vol. 34. pp. 957 to 964. Pergamon Preslr.Printedin Northern Ireland
JOEN T. Institute
WASSON
of Geophysics and Planetary Physics and Department of Chemistry University of California, Los Angeles, California 90024
A~ct-neutron-&~tiv~tion determined concentrations of Ni, Ga, Ge and Ir in the metal of 19 iron-meteorites-with-silicate-inclusionsare in good agreement with the &s&lo&ion of BUNCH, KJZIL and OLSEN, which is baaedmainly on the mineralogy and texture of the inclusions. Odessa-type and Copiapo-type irons-with-silicate-inoluaions are found to be very closely related to each other, and to other members of chemical group I of the iron meteorites. Kendall County is probably related to these irons. Elga is & new Weekeroo Station-type iron-with-silicateinclusion, and Netsohaevo is closely relsted to these objects. Tucson is unique. Odessa-type and Copiapo-type meteoriteaappear to have formed non-igneousfy, whereas shock mixing and associated differentistion of silicates appears to have been a late step in the history of the Weekeroo Station-type irons. INTRODUCTION THE NAME, iron-meteorite-with-silicate-inclusions* is commonly applied to those iron meteorites with silicate ~on~nts between about 0.1 and 40 vol. */& &SON (1967) WLS the first to summarize the properties of the IMSI, and listed L? meteorites which fall into this category. BUNCHet al. (1970) have carried out a detailed electron-microprobe investigation of the phases of 18 of these objects, including the 17 studied by Mason. The presence of silicates in the IMSI makes it possible to apply the Rb-Sr age-determination method, and WASSERBURGI, BURNETT and coworkers have reported a number of accurate ages for these objects (e.g. WASSERBURGI and BURNITT, 1969). They have aIso determined K-Ar ages on the silicate material, and these are in good agreement with the Rb-Sr ages. The Rb-Sr age of one IMSI, Kodiakanal, is 3.8 Gy (gigayears) (BURNETT and W~LSSERBTJR~, 1967) and is the only meteorite with an Rb-Sr age which is distinctly different from the 4.7 Gy age of the solar system. No Rb-Sr ages greater than 4.7 Gy have been found in the eight IMSI studied by these techniques, in contrast to reported ages of up to 10 Gy determined in earlier K-Ar determinations based on the analysis of the metal of iron meteorites, including some IMSI. The high K-Ar ages appear to be erroneous. The 18 IMSI studied by MASON (196’7) and Bu~ca: et al. (1970) are chemically and petrologically different from the two major classes of stony-irons, the pallasites and mesosiderites. MASONproposed three major groups of INSI based mainly on petrologioal evidence. These have been confirmed but with considerable refinement in the more detailed study of BUNCH and coworkers. The 18 meteorites studied by the latter authors are assigned as follows: Odessa-type (four members); Copiapo-type (eight members); Weekeroo Station-type (three members); and three objects of unique type, Enon, Kendall County and Netachaevo. In this study I have incIuded all objects included in the BUNCH et al. study except Enon, in
* Both the singular and plural forms of this compound are abbreviated IMSI throughout this paper. 957
958
J. T. WASSQN
which the metal is present as very fine (typically about Z-mm) grains of uniform distribution (see Fig. 7 of BUNCH et al., 1970). Enon is structurally much more similar to a mesosiderite or a chondrite than to an iron meteorite, although chemically distinct from members of the former classes. I have also included two ad~tional IMSI (Tucson and Elga). Not included in this paper (although data are presented in WASSON, 1970) are several group-1 or group-I-related irons which contain minor amounts of silicates, which are probably of either the Odessa-type or the closely related Copiapo-type. Among these are Wichita County (OLSEN 1967), Zenda (READ, 1963) and Canyon Diablo a,nd Poundegin (OLSEN and Fucus, 1968). EXPERIMENTAL Nickel was determined by atomic absorption spectroscopy, and Ga, Ge and Ir by radiochemical neutron activation. Details of these procedures are given by WASSONand K.IMBEBLIN (1967) for the former three elements, and by KIMBERLIINet nt. (1968) for Ir. All concentrations are the means of two or more dete~i~atio~. The error limits (at 95 *A confidence) are estimated to be 2 oA (Ni), 4 % (Ga and Ge) and 10 “/0 (Ir) of the reported mean values. Comparisons of our data with those of other investigators are given in the above papers, and show no evidence of systematic errors in our results. A dental drill was used to remove all visible silicate inclusions from the samples. Any undissolved residue was weighed, and the analyzed mass corrected accordingly. With rare exoeptions, these corrections were less than 0.5 per cent. All samples weighed about 1 g, and consisted of l-3 sawed solid pieces, except Elga, which consisted of turnings. The source museums and catalog numbers of the analyzed specimens are listed in Table 1. RESULTS ARD Discussion
Mean Ni, Ga, Ge and Ir concentrations in the 19 meteorites are listed in Table 1. They are listed roughly in order of decreasing Go concentration, except that all meteorites belonging to one of the three types defined by BUNCHet aE. (1970) are listed together. The types as assigned in the latter study are listed in the fourth column of the table, and the compositions of the orthopyroxene from that study (for Tucson, from BUNCRand FUCHS, 1969) are listed in the last column. Replicate as well as mean data for a number of these meteorites have previously been presented by WASSON (1970) and KIMBERLIN (1967). Some of the values have been revised slightly since bhose publications, the result of additional determinations or corrections of errors involved in the previous data reduction. CZassification
With the exception of Tacubaya, the classi~eation of the IMSI by BUNCH et al. (1970) is confirmed by my study of the metal of these meteorites. This is best seen when the data are plotted, and compared with fields defined by the nine resolved chemical groups of iron meteorites. The four Odessa-type IMSI are members of chemical group I (WASSO~;~, 1970). Group I is defined as those coarse and medium octahe~ites which plot within the uppermost fields (extending down to about 190 ppm Ge) on both the Ge vs. Ga and Ge vs. Ni portions of Fig. 1. Only one Copiapo-type IMSI (Udei Station) falls directly into both group-I fields, but all others fall in or near the fields, or an extrapolation of the group-1 field down to 78 ppm Ge. Although with the exception of Udei Station our analyses are capable of
969
Ni, Ga, Ge and Ir in the metal of iron-meteorites-with-silicate-inclusions Table 1. Mean ISi, Gs, Ge and Ir in metal and ferrosilite content of orthopyroxene of iron-meteorites-with-silicate-inclusions Ni Meteorite
Some’
Cat. No.
Kendall County
FMNH
Camp0 de1 Cielo
UPit
Linwood
H&U UCLA AML FMNH MHNP LEWU AMNH ASU SI ST SI MCNM KMAN HmU FMNH SI FMNH
Odessa Toluoa Taoubaya Copiapo Pine River Udei Station Four Corners Woodbine Pitt5 Persimmon Creek Colomera Elga Weekwoo Station Kodiakanal Netsch&vo Tucson
Qa (ppm)
ae
Ir
100 Fa
@pm)
Fe -I- En
Typet
(%)
MB 1022
KC
6.43
70.9
I-7
0.7
393
Od
6.62
90.0
392
3.2
6.2
632
Od
6.4
90.4
374
2.7
7.2
420
Od
7.20
74.7
285
2.2
6.6 6.6
(PPm)
Od
7.98
69.0
250
1.9
COPS
8.13
64.3
249
I.9
7.0
615
cop
7.01
69.8
252
2.5
6.7
-
128.415 2149
Cop
7.40
76.9
234
2.6
3.6
3946 166a
COP
8.83
60.4
204
0.51
8.3
Cop
8.90
48.7
179
2.0
6.5
2169
Cop
10.6
36.7
114
1.4
7.7
1378
Cop
12.80
33.0
94.2
0.86
6.9
318 -
COP WS
14.45
34.1
78.3
0.65
7.86
28.4
74.6
7.7
2315 672
WSS ws
8.03
23.3
72.3
3.2
7.61
28.2
67.0
2.3
21.2
Me574
WS
8.22
20.8
65-6
8.2
16.6
1096
No
8.638
24-o
65.7
1.8
13.8
Me 69
Tuf
9.46
2.1
0.4
0,941
o+Ms
7.2 23-5 -
* The 80-e abbreviationa have the following interpretations: AML, Americau Meteorite Laboratory; AMNH, American Mueeum of Natural History; ASU, ArizonaState University; FMNH, Field Museum of Natural History; HerU. Harvard University; KMAN, Committee on Meteorites, Acedemy of Sciences, USSR; LawU, Lawrence University: MCNM, Muse0 National de Ciencsg Naturals8 de Madrid; MIXNP, Mu&urn d’Hiatoire Nahmlie de Pa&; 81.l?&ioual Mueeuxu of Natural History, Smit~oni~ Institution; UCLA, University of California, Loi, Angeles: UPit, University of Pittsburgh. t After BUNOHet al. (1970), except as noted. The distinction between the Odessa aud Copiapo types is minor. See text for details. $ See text for a discussion of the olaesiflcation of these meteorites. $ One value of 7.1 ‘A not included in mean.
distinguishing the Odessa-type IMSI from the Copiapo-type, the differences are very small, and, as discussed in WASSON (1970), the data are more in keeping with a common origin for these objeots. BUNCHet al. (1970) also note that the Odessa-type and Copiapo-type IMSI are very similar. As shown by the last column in Table 1, the Fs contents of their orthopyroxenes indicate similar oxidation atates, providing that their coexisting olivine, orthopyroxene and metal are roughly equilibrium assemblages. BUNCH et al.distinguish the two types mainly on the basis of relative abundances of different minerals, and on the basis of the texture of the nodules. For example, Odessa-type inclusions tend to have more abundant graphite and sulfide minerals, and to be rounded rather than angular. However, numerous exceptions are noted: e.g. Pine River (Copiapo-type) has abundant sulfides, and Campo de1 Cielo has large, angular fragments. The only quantitative means of distinguishing the two types found by BUNCEet al. was the di~erences in the detailed composition of the troilite. With the exception of Tacubaya, all Mn, Zn and Ti contents of Copiapotype troilite were less than O-02, O-02 and 0.03 per cent, respectively, whereas the lowest 2
J. T. WASSON
960
IID
30 20
30
40
50
Gallium (ppm)
60
\ , , , 9i$ I
6
8
, /,.
IO
I
Nickel (%I
Fig. 1, Plots of Ge vs. Ga and Ni for iron-meteori~s-with-s~ic&te-in~l~io~. Odessa-type IMSI frailin group-1 fields, which are shown by the closed lines in the upper portion of each plot. Copiapo-type IMSI fall in or near the group-1 fields, or an extrapolation of this field (shown broken) down to 78 ppm Ge. Weekeroo Station-type IMSI form a loose cluster on the Ge-Ga plot, a tight cluster on the Go-Ni plot. The locations of other chemical groups of iron meteorites are shown by shading, and are identified on the Ge-Ge plot. The abbreviations of the meteorite names can be readily understood if it is noted that they &re listed in order of decreasingGe in Table 1.
values of these three elements in Odessa-type troilite was O-15, 0.28 and O-08 per Tacubaya has a Mn content of O-06 T, and is otherwise similar cent, respectively. to the other Copiapo-type objects. My data on the metal of Tacubaya showed that it was very similar to Toluca, * the chief difference being a 7 per cent lower Ga Thus, although differing in their detailed properties, it content in Tacubaya. appears that the Odessa-type and Copiapo-type IMSI have formed by the same processes operating on the same batch (or very similar batches) of parent material. * NININ~ERand NININGER(1950) suggested that Tacubaya was a re-heated specimen of Tolues. Although the evidence for reheating seem minimal, our data are consistentwith such an assignment. However, the apperent differencesin Ga content and in the type of inclusions indicate that, for the time being, they should be consideredseparate falls.
Ni, Ga, Ge and
Ir in the metal of in-met~rit~-~it~-s~ic~~-inelusio~
961
Kendall County is an interesting IMSI, which plots just to the left of the group- 1 Ge-Ga and Ge-Ni fields in Fig. 1. However, it differs from group-l (Odessa-type or Copiapo-type) IMSI in two important ways: its silicates are highly reduced compared to these objects, and its Ni content of 5.4 per cent is 15 per cent lower than the lowest values (about 6.4%) found in group-I. The low ferrosilite content of the orthopyroxene may be the result of cosmic reheating in an open system. Like the Odessa-type IMSl (and other group-I irons) Kendall County contains abundant graphite. Thermodynamic calculations show that the reaction Fe0 + C = Fe + CO has a negative AC at temperatures above about 1000°C. Thus, if any of these objects where heated in a situation where CO could escape, the Fe0 would be irreversibly reduced. Although such a process would also reduce the Ni content of the metal (by dilution), the amounts of Fs, orthopyroxene which need to be reduced in order to decrease the Ni content by 15 per cent are rather large, and this explanation of the low Ni content is rather unsatisfactory. WASSON (1970) has listed other objects (Ballinger, Zacatecas~ which plot well to the low-Ni side of the group-I Ge-Ni field, and which, like Kendall County, have Ir contents very similar to those of the group-I irons. For the present time I prefer to consider it likely that all these objects are related to the group-I irons (and Odessa-type and Copiapo-type IMSI), despite my inability to understand the details of their variant compositions. The Weekeroo Station-type IMSI form a rather loose cluster on the Ge-Ga portion of Fig. 1. The 30 per cent range in Ga concentrations is larger than that in resolved groups of iron meteorites having such small ranges of Ge (about 13 per cent). This may indicate that an appreciable fraction of the Ga is present in the inclusions of these objects. The cluster is very compact on the GeNi plot, and plots well away from an extrapolation of the Copiapo field. We have classified Elga as Weekeroo Station-type on the basis of the composition of its metal. It was not studied by BUNCHet al. (1970), but PLYASHKEVICH (1962) has shown that the silicate inclusions are rich in alkali feldspar, which is in keeping with the properties of other Weekeroo Station-type IMSI. I follow BUNCHet al. (19’70) in assigning Kodiakanal to this group, despite its different Rb-Sr age and metal structure (BENCEand BURNETT,1969). Netsohaevo also plots together with the Weekeroo Station-type objects. This curious object is a mixture of angular silicate fragments (containing distinguishable chon~ules~ embedded in large areas (linear dimensions exceeding 5 cm in some cases) of clear metal with the Widmanst~tten pattern of a medium octahe~i~ (BUCHWALD,1967). BUCHWALDhas also noted that Netschaevo was heated to temperatures of about 1100°C in the forge. However, the forging has not caused melting of the metal in the specimens which he studied, since taenite ribbons are still discernible. The forging probably lasted for several hours. It seems unlikely that this heat treatment brought about changes in the bulk compositions of phases (such as ferromagnesian silicates) with melting points considerably higher than 1100%. BUNCH.et al. (1970) report mineral compositions in Netschaevo which are similar to those in Weekeroo Station-type IMSI, but some phases present in these, such as clinopyroxene, K-feldspar, glass and rutile, were not observed in
J. T.
962
%vASSON
Netschaevo. The compositional evidence clearly favors the conclusion that Netschaevo may be closely related to the Weekeroo Station-type IMSI. Possible relationships between these objects will be discussed in more detail below. There are a few other iron meteorites which have Ni, Ga, Ge and Ir contents similar to those of Weekeroo Station-type IMSI. The two which are most similar are Barranca Blanca (data given by WASSONand KIMBERLIN,1967) and Arlington.
2001 ioo50 202 IO3 5E .‘5 E 2:, $ 0 Io.5020.10.05- i’x,
pI%A
-Ttl
Gallium
(ppm)
Nickel
(%I
Fig. 2. Plots of Ge vs. Ge and Ni for iron-m&eorites-with-silicate-inclusions. This illustration is very similar to Fig. 1 except that the Ge and Ga axes have been extended much lower to show the location of the Tucson IMSI.
Tucson is an unique IMSI which has a very low Ge content. It is plotted in Fig. 2, which also shows the locations of other IMSI and of the resolved chemical groups of iron meteorites. It is highly reduced, and contains O.8o/0 Si dissolved in the metal (War and WASSON,1969). Its mineralogy has been studied by BUNCHand Fucns (1969), who report finding a new mineral, brezinaite (C&S,), in it. Tucson appears unrelated to the chemical groups, and the other IMSI. As pointed out by War and WASSON (X970), it may be related to Si-containing Nedagolla or to Santiago Papasquiero. The association of very low Ge contents in these irons with other evidence of formation in a highly reduced environment is enigmatic, and deserving of further study. Origin In this section I will confine myself to a discussion of the formation conditions of the three large groups, the closely related Odessa-type and ~opiapo-ty~ IMSI, and the Weekeroo Station-type objects. I have previously argued that the group-I
Ni, Ga, Ge md Ir in the metal of iron-meteoritcte-with-silicate-incIusions 963
irons and associated IMSI have formed non-igneously (WASSON, 1970). A chief basis for this belief is the rapidity with which immiscible metal and silicate would gravitationally separate from a melt, even near the center of a very small parent body (FISH et al., 1960). Furthermore, the silicates which have been analyzed are of chondritic composition (analyses by Jarosewich of Woodbine (MASON, 1967) and Campo de1 C?ielo (BUNCH et al., 1970), and thus include minerals which melt (and solidify) over a rather wide temperature range. Shock mixing of the metal and silicate would probably tend to fractionate the silicates; if molten metal intruded silicates (BUXCH et ak., 1970), the low-melting silicates would tend to be carried along by the melt, whereas the high-melting particles would Iag behind. The 4*7Gy Rb-Sr age of Campo de1 Cielo and similar ages for other Odessa-type and Copiapo-type irons (see WASSERBURCJ and BURNETT, 1969) indicate that these objects cooled to below about 1000°C very early in the history of the solar system. Although such evidence does not rule out an igneous origin, it puts an upper limit of about 50 km on the radius of the parent body in which such a process could have occurred. I favor the idea that these meteorites have not been molten since accretion, and that the high metal-to-silicate ratios in them resulted from metalsilicate ~actionation during the agglomeration of the solar nebula andfor during the accretion of the parent body. The large (about 30-cm) crystals of y-iron necessary to account for the Widmanstatten pattern were formed by solid-state growth at subsolidus temperatures. The Weekeroo Station irons appear to have had a somewhat different history. One of them, Kodiakanal, has an Rb-Sr age of 3.8 Gy, much lower than that of any other meteorite (BURNETTand WASSERBURU,1967). The initial Srs7/i!V6ratios of this object and of Weekeroo Station (WASSERBTJE~and BURNETT, 1969) clearly indioate that the silicates have formed from a parent material having a lower RbfSr ratio. The silicate inclusions are globular, and show evidence of plastic flow (see the photo~aphs of Weekeroo Station and RIga shown by WASSERBURUand BURNETT (1969) and VINOC+RADOV (1965), respectively). The silicates are rather Fe- and alkali-rich, and would melt at relatively low temperatures. BENCE and BURNETT (1969) have pointed out that there are distinct mineralogical differences between different inclusions of Kodiakanal. It seems likely that the Weekeroo Station-type IMSI are shock-mixed material, and that the silicates are a predominantly low-melting fraction resulting from partial melting of a parent material having a much lower Rb/Sr ratio. The Netschaevo silicates may be samples of this parent material. They are lower in Fe0 and alkali content than the Weekeroo Station-type silicates, and contain relict chondrules. Large metal veins are available adjacent to these silicates. This is an answer which poses another question, however; how did the present spatial arrangement of metal and silicate in Netschalivo originate? Perhaps the answer is selective agglomeration and accretion, as I have proposed for Odessa-type and Copiapo-type IMSI. It would be very interesting to obtain other evidence (e.g. bulk-chemical analysis, Rb-Sr age data) bearing on the origin of this object. Ackmowledgments-I am greatly indebtedto T. E. BUNCH for numerous discussions of the L. DENNIS,S. DORRANCE, D. CONDAR, mineralogy of these objects. J. KAUFMAN,J. KFMBERLIN, samples S. MOISSIDIS,C. PAWLAKzmd R. SCRAXJDY provided experiment& ~ist~Rce. ii%eteorite
964
J. T. WASSON
were supplied by T. E. BUNCH, D. S. BURNETT, R. S. CLARKE, JR., C. FRONDEL, E. L. KRINOV, L. KVASHA, V. M_AIQ~ON, C. B. MOORE, E. OLSEN, P. PELLAS and W. F. READ. Neutron irradiations were provided by J. HORNOR, J. HORNBUCKLE and their associates at the UCLA Nuclear Reactor. This research was supported in part by NSF grant GA 1347 and NASA contract NAS g-8096. REFERENCES BENCE A. E. and BURNETT D. S. (1969) Chemistry and mineralogy of the silicates and metal of the Kodiakanal meteorite. &o&m. Cosmochim. Acta t33,387407. BUCHWALDV. F. (1967) Studies of six iron meteorites. Analecta Beologica No. 2, 75 pp. BUNCH T. E. and FUCHSL. H. (1969) A new mineral brezinaite Cr9S4and the Tucson meteorite. Amer. Mineral. 54, 1509-1518. BUNCH T. E., KEIL K. and OLSEN E. (1970) Mineralogy and petrology of silicate inclusions in iron meteorites. Contrib. Mineral. Petrol. 25, 297-340. BURNETT D. S. and WASSERBURGG. J. (1967) Evidence for the formation of an iron meteorite at 3.8 x 10’ years. Earth Planet. Sci. Lett. 2, 137-147. FISH R. A., GOIZS G. G. and ANDERS E. (1960) The record in the meteorites. III. On the development of meteorites in asteroidal bodies. Astrophys. J. 132, 243-258. KIMBERLIN J., CHAROONRATANAC. and WASSON J. T. (1968) Neutron activation determination of Ir in meteorites. Radiochim. Acta 10, 69-75. MASON B. (1967) The Woodbine meteorite, with notes on silicates in iron meteorites. Mineral. Mag. 36, 120-126. NININGER H. H. and NININGER A. D. (1950) The Nininger Collection of Meteorites, p. 139. American Meteorite Museum, Winslow, Arizona. OLSEN E. (1967) Amphibole: First occurrence in a meteorite. Science 156, 61-62. OLSEN E. and FUCHS L. (1968) Krinovite, NaMgzCrSi,Olo: A new meteorite mineral. Science
161,786-787. PLYASHKEVICH L. N. (1962) Some data regarding
the composition and structure of the iron meteorite Elga (in Russian). Meteoritika 22, 61-60. Trans. Wis. Acad. Sci., Arts Lett. 52, 153-158. READ W. F. (1963) The Zenda meteorite. VINOGRADOV A. P. (1965) The composition of meteorites. Pure Appl. Chem. 10, 459-493. WAI C. M. and WASSON J. T. (1969) Silicon concentrations in the metal of iron meteorites. Geochim. Coamochim. Acta 32, 1465-1471. WAI C. M. and WASSON J. T. (1970) Silicon in the Nedagolla ataxite and the relationship between Si and Cr in reduced iron meteorites. Geochim. Cosmochim. Acta 34, 408-410.
WASSERBURG G. J. and BURNETT D. S. (1969) The status of isotopic age determinations on iron and stone meteorites. In Meteorite Research, (editor P. M. Millman), pp. 467-479. D. Reidel. WASSON J. T. (1970) The chemical classification of iron meteorites-IV. Irons with Ge concentrations greater than 190 ppm and other meteorites associated with group 1. Icarus in press. WASSON J. T. and KIMBERLIN J. (1967) The chemical classification of iron meteorites--II. Irons and pallaaites with germanium concentrations between 8 and 100 ppm. Geochim. Cosmochim. Acta 31, 2065-2093.
JOEN T. Institute
WASSON
of Geophysics and Planetary Physics and Department of Chemistry University of California, Los Angeles, California 90024
A~ct-neutron-&~tiv~tion determined concentrations of Ni, Ga, Ge and Ir in the metal of 19 iron-meteorites-with-silicate-inclusionsare in good agreement with the &s&lo&ion of BUNCH, KJZIL and OLSEN, which is baaedmainly on the mineralogy and texture of the inclusions. Odessa-type and Copiapo-type irons-with-silicate-inoluaions are found to be very closely related to each other, and to other members of chemical group I of the iron meteorites. Kendall County is probably related to these irons. Elga is & new Weekeroo Station-type iron-with-silicateinclusion, and Netsohaevo is closely relsted to these objects. Tucson is unique. Odessa-type and Copiapo-type meteoriteaappear to have formed non-igneousfy, whereas shock mixing and associated differentistion of silicates appears to have been a late step in the history of the Weekeroo Station-type irons. INTRODUCTION THE NAME, iron-meteorite-with-silicate-inclusions* is commonly applied to those iron meteorites with silicate ~on~nts between about 0.1 and 40 vol. */& &SON (1967) WLS the first to summarize the properties of the IMSI, and listed L? meteorites which fall into this category. BUNCHet al. (1970) have carried out a detailed electron-microprobe investigation of the phases of 18 of these objects, including the 17 studied by Mason. The presence of silicates in the IMSI makes it possible to apply the Rb-Sr age-determination method, and WASSERBURGI, BURNETT and coworkers have reported a number of accurate ages for these objects (e.g. WASSERBURGI and BURNITT, 1969). They have aIso determined K-Ar ages on the silicate material, and these are in good agreement with the Rb-Sr ages. The Rb-Sr age of one IMSI, Kodiakanal, is 3.8 Gy (gigayears) (BURNETT and W~LSSERBTJR~, 1967) and is the only meteorite with an Rb-Sr age which is distinctly different from the 4.7 Gy age of the solar system. No Rb-Sr ages greater than 4.7 Gy have been found in the eight IMSI studied by these techniques, in contrast to reported ages of up to 10 Gy determined in earlier K-Ar determinations based on the analysis of the metal of iron meteorites, including some IMSI. The high K-Ar ages appear to be erroneous. The 18 IMSI studied by MASON (196’7) and Bu~ca: et al. (1970) are chemically and petrologically different from the two major classes of stony-irons, the pallasites and mesosiderites. MASONproposed three major groups of INSI based mainly on petrologioal evidence. These have been confirmed but with considerable refinement in the more detailed study of BUNCH and coworkers. The 18 meteorites studied by the latter authors are assigned as follows: Odessa-type (four members); Copiapo-type (eight members); Weekeroo Station-type (three members); and three objects of unique type, Enon, Kendall County and Netachaevo. In this study I have incIuded all objects included in the BUNCH et al. study except Enon, in
* Both the singular and plural forms of this compound are abbreviated IMSI throughout this paper. 957
958
J. T. WASSQN
which the metal is present as very fine (typically about Z-mm) grains of uniform distribution (see Fig. 7 of BUNCH et al., 1970). Enon is structurally much more similar to a mesosiderite or a chondrite than to an iron meteorite, although chemically distinct from members of the former classes. I have also included two ad~tional IMSI (Tucson and Elga). Not included in this paper (although data are presented in WASSON, 1970) are several group-1 or group-I-related irons which contain minor amounts of silicates, which are probably of either the Odessa-type or the closely related Copiapo-type. Among these are Wichita County (OLSEN 1967), Zenda (READ, 1963) and Canyon Diablo a,nd Poundegin (OLSEN and Fucus, 1968). EXPERIMENTAL Nickel was determined by atomic absorption spectroscopy, and Ga, Ge and Ir by radiochemical neutron activation. Details of these procedures are given by WASSONand K.IMBEBLIN (1967) for the former three elements, and by KIMBERLIINet nt. (1968) for Ir. All concentrations are the means of two or more dete~i~atio~. The error limits (at 95 *A confidence) are estimated to be 2 oA (Ni), 4 % (Ga and Ge) and 10 “/0 (Ir) of the reported mean values. Comparisons of our data with those of other investigators are given in the above papers, and show no evidence of systematic errors in our results. A dental drill was used to remove all visible silicate inclusions from the samples. Any undissolved residue was weighed, and the analyzed mass corrected accordingly. With rare exoeptions, these corrections were less than 0.5 per cent. All samples weighed about 1 g, and consisted of l-3 sawed solid pieces, except Elga, which consisted of turnings. The source museums and catalog numbers of the analyzed specimens are listed in Table 1. RESULTS ARD Discussion
Mean Ni, Ga, Ge and Ir concentrations in the 19 meteorites are listed in Table 1. They are listed roughly in order of decreasing Go concentration, except that all meteorites belonging to one of the three types defined by BUNCHet aE. (1970) are listed together. The types as assigned in the latter study are listed in the fourth column of the table, and the compositions of the orthopyroxene from that study (for Tucson, from BUNCRand FUCHS, 1969) are listed in the last column. Replicate as well as mean data for a number of these meteorites have previously been presented by WASSON (1970) and KIMBERLIN (1967). Some of the values have been revised slightly since bhose publications, the result of additional determinations or corrections of errors involved in the previous data reduction. CZassification
With the exception of Tacubaya, the classi~eation of the IMSI by BUNCH et al. (1970) is confirmed by my study of the metal of these meteorites. This is best seen when the data are plotted, and compared with fields defined by the nine resolved chemical groups of iron meteorites. The four Odessa-type IMSI are members of chemical group I (WASSO~;~, 1970). Group I is defined as those coarse and medium octahe~ites which plot within the uppermost fields (extending down to about 190 ppm Ge) on both the Ge vs. Ga and Ge vs. Ni portions of Fig. 1. Only one Copiapo-type IMSI (Udei Station) falls directly into both group-I fields, but all others fall in or near the fields, or an extrapolation of the group-1 field down to 78 ppm Ge. Although with the exception of Udei Station our analyses are capable of
969
Ni, Ga, Ge and Ir in the metal of iron-meteorites-with-silicate-inclusions Table 1. Mean ISi, Gs, Ge and Ir in metal and ferrosilite content of orthopyroxene of iron-meteorites-with-silicate-inclusions Ni Meteorite
Some’
Cat. No.
Kendall County
FMNH
Camp0 de1 Cielo
UPit
Linwood
H&U UCLA AML FMNH MHNP LEWU AMNH ASU SI ST SI MCNM KMAN HmU FMNH SI FMNH
Odessa Toluoa Taoubaya Copiapo Pine River Udei Station Four Corners Woodbine Pitt5 Persimmon Creek Colomera Elga Weekwoo Station Kodiakanal Netsch&vo Tucson
Qa (ppm)
ae
Ir
100 Fa
@pm)
Fe -I- En
Typet
(%)
MB 1022
KC
6.43
70.9
I-7
0.7
393
Od
6.62
90.0
392
3.2
6.2
632
Od
6.4
90.4
374
2.7
7.2
420
Od
7.20
74.7
285
2.2
6.6 6.6
(PPm)
Od
7.98
69.0
250
1.9
COPS
8.13
64.3
249
I.9
7.0
615
cop
7.01
69.8
252
2.5
6.7
-
128.415 2149
Cop
7.40
76.9
234
2.6
3.6
3946 166a
COP
8.83
60.4
204
0.51
8.3
Cop
8.90
48.7
179
2.0
6.5
2169
Cop
10.6
36.7
114
1.4
7.7
1378
Cop
12.80
33.0
94.2
0.86
6.9
318 -
COP WS
14.45
34.1
78.3
0.65
7.86
28.4
74.6
7.7
2315 672
WSS ws
8.03
23.3
72.3
3.2
7.61
28.2
67.0
2.3
21.2
Me574
WS
8.22
20.8
65-6
8.2
16.6
1096
No
8.638
24-o
65.7
1.8
13.8
Me 69
Tuf
9.46
2.1
0.4
0,941
o+Ms
7.2 23-5 -
* The 80-e abbreviationa have the following interpretations: AML, Americau Meteorite Laboratory; AMNH, American Mueeum of Natural History; ASU, ArizonaState University; FMNH, Field Museum of Natural History; HerU. Harvard University; KMAN, Committee on Meteorites, Acedemy of Sciences, USSR; LawU, Lawrence University: MCNM, Muse0 National de Ciencsg Naturals8 de Madrid; MIXNP, Mu&urn d’Hiatoire Nahmlie de Pa&; 81.l?&ioual Mueeuxu of Natural History, Smit~oni~ Institution; UCLA, University of California, Loi, Angeles: UPit, University of Pittsburgh. t After BUNOHet al. (1970), except as noted. The distinction between the Odessa aud Copiapo types is minor. See text for details. $ See text for a discussion of the olaesiflcation of these meteorites. $ One value of 7.1 ‘A not included in mean.
distinguishing the Odessa-type IMSI from the Copiapo-type, the differences are very small, and, as discussed in WASSON (1970), the data are more in keeping with a common origin for these objeots. BUNCHet al. (1970) also note that the Odessa-type and Copiapo-type IMSI are very similar. As shown by the last column in Table 1, the Fs contents of their orthopyroxenes indicate similar oxidation atates, providing that their coexisting olivine, orthopyroxene and metal are roughly equilibrium assemblages. BUNCH et al.distinguish the two types mainly on the basis of relative abundances of different minerals, and on the basis of the texture of the nodules. For example, Odessa-type inclusions tend to have more abundant graphite and sulfide minerals, and to be rounded rather than angular. However, numerous exceptions are noted: e.g. Pine River (Copiapo-type) has abundant sulfides, and Campo de1 Cielo has large, angular fragments. The only quantitative means of distinguishing the two types found by BUNCEet al. was the di~erences in the detailed composition of the troilite. With the exception of Tacubaya, all Mn, Zn and Ti contents of Copiapotype troilite were less than O-02, O-02 and 0.03 per cent, respectively, whereas the lowest 2
J. T. WASSON
960
IID
30 20
30
40
50
Gallium (ppm)
60
\ , , , 9i$ I
6
8
, /,.
IO
I
Nickel (%I
Fig. 1, Plots of Ge vs. Ga and Ni for iron-meteori~s-with-s~ic&te-in~l~io~. Odessa-type IMSI frailin group-1 fields, which are shown by the closed lines in the upper portion of each plot. Copiapo-type IMSI fall in or near the group-1 fields, or an extrapolation of this field (shown broken) down to 78 ppm Ge. Weekeroo Station-type IMSI form a loose cluster on the Ge-Ga plot, a tight cluster on the Go-Ni plot. The locations of other chemical groups of iron meteorites are shown by shading, and are identified on the Ge-Ge plot. The abbreviations of the meteorite names can be readily understood if it is noted that they &re listed in order of decreasingGe in Table 1.
values of these three elements in Odessa-type troilite was O-15, 0.28 and O-08 per Tacubaya has a Mn content of O-06 T, and is otherwise similar cent, respectively. to the other Copiapo-type objects. My data on the metal of Tacubaya showed that it was very similar to Toluca, * the chief difference being a 7 per cent lower Ga Thus, although differing in their detailed properties, it content in Tacubaya. appears that the Odessa-type and Copiapo-type IMSI have formed by the same processes operating on the same batch (or very similar batches) of parent material. * NININ~ERand NININGER(1950) suggested that Tacubaya was a re-heated specimen of Tolues. Although the evidence for reheating seem minimal, our data are consistentwith such an assignment. However, the apperent differencesin Ga content and in the type of inclusions indicate that, for the time being, they should be consideredseparate falls.
Ni, Ga, Ge and
Ir in the metal of in-met~rit~-~it~-s~ic~~-inelusio~
961
Kendall County is an interesting IMSI, which plots just to the left of the group- 1 Ge-Ga and Ge-Ni fields in Fig. 1. However, it differs from group-l (Odessa-type or Copiapo-type) IMSI in two important ways: its silicates are highly reduced compared to these objects, and its Ni content of 5.4 per cent is 15 per cent lower than the lowest values (about 6.4%) found in group-I. The low ferrosilite content of the orthopyroxene may be the result of cosmic reheating in an open system. Like the Odessa-type IMSl (and other group-I irons) Kendall County contains abundant graphite. Thermodynamic calculations show that the reaction Fe0 + C = Fe + CO has a negative AC at temperatures above about 1000°C. Thus, if any of these objects where heated in a situation where CO could escape, the Fe0 would be irreversibly reduced. Although such a process would also reduce the Ni content of the metal (by dilution), the amounts of Fs, orthopyroxene which need to be reduced in order to decrease the Ni content by 15 per cent are rather large, and this explanation of the low Ni content is rather unsatisfactory. WASSON (1970) has listed other objects (Ballinger, Zacatecas~ which plot well to the low-Ni side of the group-I Ge-Ni field, and which, like Kendall County, have Ir contents very similar to those of the group-I irons. For the present time I prefer to consider it likely that all these objects are related to the group-I irons (and Odessa-type and Copiapo-type IMSI), despite my inability to understand the details of their variant compositions. The Weekeroo Station-type IMSI form a rather loose cluster on the Ge-Ga portion of Fig. 1. The 30 per cent range in Ga concentrations is larger than that in resolved groups of iron meteorites having such small ranges of Ge (about 13 per cent). This may indicate that an appreciable fraction of the Ga is present in the inclusions of these objects. The cluster is very compact on the GeNi plot, and plots well away from an extrapolation of the Copiapo field. We have classified Elga as Weekeroo Station-type on the basis of the composition of its metal. It was not studied by BUNCHet al. (1970), but PLYASHKEVICH (1962) has shown that the silicate inclusions are rich in alkali feldspar, which is in keeping with the properties of other Weekeroo Station-type IMSI. I follow BUNCHet al. (19’70) in assigning Kodiakanal to this group, despite its different Rb-Sr age and metal structure (BENCEand BURNETT,1969). Netsohaevo also plots together with the Weekeroo Station-type objects. This curious object is a mixture of angular silicate fragments (containing distinguishable chon~ules~ embedded in large areas (linear dimensions exceeding 5 cm in some cases) of clear metal with the Widmanst~tten pattern of a medium octahe~i~ (BUCHWALD,1967). BUCHWALDhas also noted that Netschaevo was heated to temperatures of about 1100°C in the forge. However, the forging has not caused melting of the metal in the specimens which he studied, since taenite ribbons are still discernible. The forging probably lasted for several hours. It seems unlikely that this heat treatment brought about changes in the bulk compositions of phases (such as ferromagnesian silicates) with melting points considerably higher than 1100%. BUNCH.et al. (1970) report mineral compositions in Netschaevo which are similar to those in Weekeroo Station-type IMSI, but some phases present in these, such as clinopyroxene, K-feldspar, glass and rutile, were not observed in
J. T.
962
%vASSON
Netschaevo. The compositional evidence clearly favors the conclusion that Netschaevo may be closely related to the Weekeroo Station-type IMSI. Possible relationships between these objects will be discussed in more detail below. There are a few other iron meteorites which have Ni, Ga, Ge and Ir contents similar to those of Weekeroo Station-type IMSI. The two which are most similar are Barranca Blanca (data given by WASSONand KIMBERLIN,1967) and Arlington.
2001 ioo50 202 IO3 5E .‘5 E 2:, $ 0 Io.5020.10.05- i’x,
pI%A
-Ttl
Gallium
(ppm)
Nickel
(%I
Fig. 2. Plots of Ge vs. Ge and Ni for iron-m&eorites-with-silicate-inclusions. This illustration is very similar to Fig. 1 except that the Ge and Ga axes have been extended much lower to show the location of the Tucson IMSI.
Tucson is an unique IMSI which has a very low Ge content. It is plotted in Fig. 2, which also shows the locations of other IMSI and of the resolved chemical groups of iron meteorites. It is highly reduced, and contains O.8o/0 Si dissolved in the metal (War and WASSON,1969). Its mineralogy has been studied by BUNCHand Fucns (1969), who report finding a new mineral, brezinaite (C&S,), in it. Tucson appears unrelated to the chemical groups, and the other IMSI. As pointed out by War and WASSON (X970), it may be related to Si-containing Nedagolla or to Santiago Papasquiero. The association of very low Ge contents in these irons with other evidence of formation in a highly reduced environment is enigmatic, and deserving of further study. Origin In this section I will confine myself to a discussion of the formation conditions of the three large groups, the closely related Odessa-type and ~opiapo-ty~ IMSI, and the Weekeroo Station-type objects. I have previously argued that the group-I
Ni, Ga, Ge md Ir in the metal of iron-meteoritcte-with-silicate-incIusions 963
irons and associated IMSI have formed non-igneously (WASSON, 1970). A chief basis for this belief is the rapidity with which immiscible metal and silicate would gravitationally separate from a melt, even near the center of a very small parent body (FISH et al., 1960). Furthermore, the silicates which have been analyzed are of chondritic composition (analyses by Jarosewich of Woodbine (MASON, 1967) and Campo de1 C?ielo (BUNCH et al., 1970), and thus include minerals which melt (and solidify) over a rather wide temperature range. Shock mixing of the metal and silicate would probably tend to fractionate the silicates; if molten metal intruded silicates (BUXCH et ak., 1970), the low-melting silicates would tend to be carried along by the melt, whereas the high-melting particles would Iag behind. The 4*7Gy Rb-Sr age of Campo de1 Cielo and similar ages for other Odessa-type and Copiapo-type irons (see WASSERBURCJ and BURNETT, 1969) indicate that these objects cooled to below about 1000°C very early in the history of the solar system. Although such evidence does not rule out an igneous origin, it puts an upper limit of about 50 km on the radius of the parent body in which such a process could have occurred. I favor the idea that these meteorites have not been molten since accretion, and that the high metal-to-silicate ratios in them resulted from metalsilicate ~actionation during the agglomeration of the solar nebula andfor during the accretion of the parent body. The large (about 30-cm) crystals of y-iron necessary to account for the Widmanstatten pattern were formed by solid-state growth at subsolidus temperatures. The Weekeroo Station irons appear to have had a somewhat different history. One of them, Kodiakanal, has an Rb-Sr age of 3.8 Gy, much lower than that of any other meteorite (BURNETTand WASSERBURU,1967). The initial Srs7/i!V6ratios of this object and of Weekeroo Station (WASSERBTJE~and BURNETT, 1969) clearly indioate that the silicates have formed from a parent material having a lower RbfSr ratio. The silicate inclusions are globular, and show evidence of plastic flow (see the photo~aphs of Weekeroo Station and RIga shown by WASSERBURUand BURNETT (1969) and VINOC+RADOV (1965), respectively). The silicates are rather Fe- and alkali-rich, and would melt at relatively low temperatures. BENCE and BURNETT (1969) have pointed out that there are distinct mineralogical differences between different inclusions of Kodiakanal. It seems likely that the Weekeroo Station-type IMSI are shock-mixed material, and that the silicates are a predominantly low-melting fraction resulting from partial melting of a parent material having a much lower Rb/Sr ratio. The Netschaevo silicates may be samples of this parent material. They are lower in Fe0 and alkali content than the Weekeroo Station-type silicates, and contain relict chondrules. Large metal veins are available adjacent to these silicates. This is an answer which poses another question, however; how did the present spatial arrangement of metal and silicate in Netschalivo originate? Perhaps the answer is selective agglomeration and accretion, as I have proposed for Odessa-type and Copiapo-type IMSI. It would be very interesting to obtain other evidence (e.g. bulk-chemical analysis, Rb-Sr age data) bearing on the origin of this object. Ackmowledgments-I am greatly indebtedto T. E. BUNCH for numerous discussions of the L. DENNIS,S. DORRANCE, D. CONDAR, mineralogy of these objects. J. KAUFMAN,J. KFMBERLIN, samples S. MOISSIDIS,C. PAWLAKzmd R. SCRAXJDY provided experiment& ~ist~Rce. ii%eteorite
964
J. T. WASSON
were supplied by T. E. BUNCH, D. S. BURNETT, R. S. CLARKE, JR., C. FRONDEL, E. L. KRINOV, L. KVASHA, V. M_AIQ~ON, C. B. MOORE, E. OLSEN, P. PELLAS and W. F. READ. Neutron irradiations were provided by J. HORNOR, J. HORNBUCKLE and their associates at the UCLA Nuclear Reactor. This research was supported in part by NSF grant GA 1347 and NASA contract NAS g-8096. REFERENCES BENCE A. E. and BURNETT D. S. (1969) Chemistry and mineralogy of the silicates and metal of the Kodiakanal meteorite. &o&m. Cosmochim. Acta t33,387407. BUCHWALDV. F. (1967) Studies of six iron meteorites. Analecta Beologica No. 2, 75 pp. BUNCH T. E. and FUCHSL. H. (1969) A new mineral brezinaite Cr9S4and the Tucson meteorite. Amer. Mineral. 54, 1509-1518. BUNCH T. E., KEIL K. and OLSEN E. (1970) Mineralogy and petrology of silicate inclusions in iron meteorites. Contrib. Mineral. Petrol. 25, 297-340. BURNETT D. S. and WASSERBURGG. J. (1967) Evidence for the formation of an iron meteorite at 3.8 x 10’ years. Earth Planet. Sci. Lett. 2, 137-147. FISH R. A., GOIZS G. G. and ANDERS E. (1960) The record in the meteorites. III. On the development of meteorites in asteroidal bodies. Astrophys. J. 132, 243-258. KIMBERLIN J., CHAROONRATANAC. and WASSON J. T. (1968) Neutron activation determination of Ir in meteorites. Radiochim. Acta 10, 69-75. MASON B. (1967) The Woodbine meteorite, with notes on silicates in iron meteorites. Mineral. Mag. 36, 120-126. NININGER H. H. and NININGER A. D. (1950) The Nininger Collection of Meteorites, p. 139. American Meteorite Museum, Winslow, Arizona. OLSEN E. (1967) Amphibole: First occurrence in a meteorite. Science 156, 61-62. OLSEN E. and FUCHS L. (1968) Krinovite, NaMgzCrSi,Olo: A new meteorite mineral. Science
161,786-787. PLYASHKEVICH L. N. (1962) Some data regarding
the composition and structure of the iron meteorite Elga (in Russian). Meteoritika 22, 61-60. Trans. Wis. Acad. Sci., Arts Lett. 52, 153-158. READ W. F. (1963) The Zenda meteorite. VINOGRADOV A. P. (1965) The composition of meteorites. Pure Appl. Chem. 10, 459-493. WAI C. M. and WASSON J. T. (1969) Silicon concentrations in the metal of iron meteorites. Geochim. Coamochim. Acta 32, 1465-1471. WAI C. M. and WASSON J. T. (1970) Silicon in the Nedagolla ataxite and the relationship between Si and Cr in reduced iron meteorites. Geochim. Cosmochim. Acta 34, 408-410.
WASSERBURG G. J. and BURNETT D. S. (1969) The status of isotopic age determinations on iron and stone meteorites. In Meteorite Research, (editor P. M. Millman), pp. 467-479. D. Reidel. WASSON J. T. (1970) The chemical classification of iron meteorites-IV. Irons with Ge concentrations greater than 190 ppm and other meteorites associated with group 1. Icarus in press. WASSON J. T. and KIMBERLIN J. (1967) The chemical classification of iron meteorites--II. Irons and pallaaites with germanium concentrations between 8 and 100 ppm. Geochim. Cosmochim. Acta 31, 2065-2093.