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Rare-earth elements in matrix, inclusions, and chondrules of the Allende meteorite
ICARUS
19,
523-530
(1973)
Rare-Earth Elements Chondrules of
in Matrix, the Allende
TSUYOSHI Geological
Survey
of Japan,
Inclusions, Meteorite
and
TANAKA
Htiamoto,
Takatsu-ku,
Kawasaki,
Japan
AND
AKIMASA
MASUDA
Department of Chemistry, Science Kagurazaka, Shinjuku-ku, Received
February
21,
1973;
University of Tokyo, Tokyo, Japan revised
March
26,
1973
Rare-earth elements in a whole-rock sample and in major components of the Allende meteorite were investigated; for a few samples, abundances of Ba, Sr, Ca, and Al were also determined. Of the materials investigated in the present work, C&Al-rich inclusions G and 0 seem to be of the greatest significance. In spite of the minor difference in mineralogy between them, the apparent chondritenormalized RE pattern is much different between these two inclusions. (Yb and Eu in inclusion G appear exceptionally irregular). This observation is inferred to reflect a rather subtle difference in condition of condensation. It is also worthwhile to note that, while two portions (pink and white) of the inclusion G show similar aspects in the abundances of lithophile trace elements investigated, they show a remarkable difference at the same time. The white portion (G,) of inclusion G can be considered to be a mixture of chondritic material and highly fractionated material like the faintly pink portion (G,) picked from the same inclusion. This would suggest the possibility that the G,-like material was produced from chondritic dust. The “matrix” separated from Allende was found to be fractionated with respect to the R,E abundances relative to representative chondrite. It has also a very high value for the Ba abundance.
1.
INTRODUCTION
In the early morning of 8 February 1969, thousands of individual meteoritic stones rained down over a strewn field of about 300km* of northern Mexico, near the city of Hidalgo de1 Parral in the southcentral part of the state of Chihuahua ; one weighing 15kg fell in the town of Pueblito de Allende. A detailed report on this Type III carbonaceous chondrite was published by Clarke et al. (1971). They described this meteorite in terms of four major components, i.e., matrix, chondrules, irregular aggregates (white or pink), and dark inclusions. Point-counting on one section gave the following volume constitution : Copyright 0 1973 by Academic Press, Inc. All rights of reproduction in any form reserved. Printed in Great Britain
523
chondrules 34, aggregates 9, and matrix 57%. These authors also emphasized the significance of the contrasts between matrix, chondrules, and aggregates. The greatest interest has been a’roused by the white aggregates, composed largely of such minerals as anorthite, gehlenite, and spinel, the first to condense from a gas of cosmic composition (Lord, 1965; Larimer, 1967; Marvin et al., 1970; Grossman, 1972). Marvin et al. (1970) observed the presence of Ca-Al-rich glass associated with spinels and other crystalline phases in Allende. Since the studies by Schmitt et al. (1963, 1964) on rare-earth (RE) elements in chondritic meteorites, the relative abundances of those elements in chondrites
TANAKAANDMASUDA
524
have been commonly believed to be constant. The recent study by Masuda et al. (1973), however, revealed that this is not always the case in a strict sense, although the extent of fractionation ranges from virtually negligible to comparatively small in most cases. In addition, it has also been found that the relative abundances of those tightly associated lithophile elements are considerably fractionated in minor cases (Masuda et al., 1973 ; Nakamura and Masuda, unpublished). Thus, the accurate determination of RE abundances in major components of an unusually heterogeneous meteorite like Allende might be expected to reveal new meteoritic information. For RE elements in Allende, however, there are heretofore only studies on whole-rock samples by Wakita and Schmitt (1970) and by Masuda et al. (1973), and on a Ca-rich inclusion by Gast et al. (1970). 2. SAMPLE
Samples studied in the present work are whole rock, matrix, olivine chondrules, Ca-Al-rich chondrule, and two Ca-Al-rich inclusions. In the preparation of olivine chondrule samples, three chondrules each weighing about 6Omg were crushed and mixed. “Matrix” was obtained by crushing a sample, sieving it through a 200-mesh sieve, and centrifuging the 200-mesh fraction (grain size less than 74pm) in methylene iodide (density 3.32). This sample with density greater than 3.32 should be relatively free of material from the chondrules (cf. Table 3 of Clarke et al., 1971).
A Ca-Al-rich chondrule was taken from USNM 3529 (analytical data on another similar chondrule from 3529 are presented by Clarke et al., 1971); this material was rather friable. A Ca-Al-rich inclusion (inclusion 0) was also picked out from the same stone. (There is some question about whether this material should be classified as “inclusion” or “chondrule.“) Ca-Alrich aggregate (inclusion G) was taken from USNM 3598. Two portions, one white and another faintly pink, of this inclusion were analyzed separately; the former will be referred to as inclusion G white (G,) and
the latter as inclusion G pink (G,). (The naming of inclusions is by Dr. Brian Mason who supplied them.) Inclusion G is an aggregate of spinel, pyroxene (fassaite), and anorthite, with minor amounts of sodalite and nepheline. The Ca-Al-rich chondrule as well as inclusion 0 consists of spine& pyroxene (fassaite), melilite, and anorthite, and lack sodalite and nepheline. 3. EXPERIMENTAL
RE elements (Masuda et al., 1973) plus Ba and Sr were determined by the stable isotope dilution technique. Impurity corrections were made for the chemical reagents used. Al and Ca were determined by the atomic absorption technique for the solution from which RE elements had been separated. According to our examination, recovery of Al and Ca in this eluate is nearly 100%. 4. RESULTS Results of our analyses are presented in Table I and in Figs. 1 and 2. As regards the RE abundances, the samples analysed might be roughly divided into three groups : whole-rock sample, matrix, and olivine chondrule have the RE abundances corresponding to chondrite (Leedey)-normalized values of 1.1 to 2.4, whereas the Ca-AIrich chondrule and inclusion 0 have RE abundances a factor of 13-22 higher than Leedey (see Fig. 1). A RE pattern for a Ca-rich inclusion investigated by Gast et al. (1970) is somewhat similar to that for our Ca-Al-rich chondrule. As shown in Fig. 2, the chondrite-normalized RE pattern for inclusion G is quite unique and appears most interesting. Ba abundances in matrix and wholerock sample investigated by us are surprisingly high compared with the normal abundance (about 4 ppm) in chondrites. King (1969) and Clarke et al. (1970) obtained 10 and 5 ppm, respectively, for Allende. Although one might suspect contamination, we regard these extremely high values as present in the samples investigated. Moore and Brown (1963)
Amount taken
Ba Sr Al Ca
Ii1
La Cf3 Nd Sm Eu Gd DY Er Yh
Specimen
(mg)
29.6
8.03 18.4 14.9 4.00 0.217 1.93 1.38 0.130 1.01 0.00606 13.7 4.39 18.6 9.45
Inclusion pink
G
ABUNDANCES
97.9
12.3 7.66
5.01 11.0 9.07 2.46 0.276 1.47 0.945 0.216 0.680 0.0301
Inclusion white
OF RE, G
Ba,
49.3
7.40 19.2 14.9 5.00 1.22 6.34 7.71 4.98 3.79 0.738 42.5 118 16.2 16.2
Inclusion
Sr (ppm),
0
Ca AND
92.5
4.95 12.7 9.64 3.15 1.48 4.36 5.38 3.76 3.73 0.583 8.11 18.4
I in WHOLE
Ca-Al rich chondrule
Al (%)
TABLE
179.9
1.56 1.94
0.648 1.64 1.18 0.379 0.140 0.500 0.618 0.398 0.406 0.0600 -
Olivine chondrule
ROCK
AND
1029
0.876 2.12 1.19 0.334 0.115 0.397 0.462 0.303 0.331 0.0514 165 -
Matrix
ITS COMPONENTS
1074
0.663 1.69 1.03 0.313 0.110 0.384 0.451 0.301 0.330 0.0495 108
Whole rock
OF ALLENDE
-
0.249 0.0387 4.21 -
0.0866 0.311 0.390 0.255
0.716 0.230
0.378 0.976
Leedey chondrite
k is
8
2 3
8 z 8
E
526
TANAKA
AND
MASUDA
~a-A,
rich
chondrula
\\ \I \I
k
\I \\
i\ \I
f,,IA,III,III!IfIIII Y
Al Ca Sr Ba FIG.
triangles), (divided
Lace
Nd
SmEuGd
Dy
Er
Yb Lu
1. Chondrito-normalized patterns of RE and other elements for matrix (half-solid circles), olivine chondrule (solid circles), open circles), and Ca-Al-rich inclusion 0 (open triangles).
showed that the distribution of Ba in the Holbrook chondrite is heterogeneous. In normalization of RE and Ba abundances, those in the Leedey chondrite are
whole-rock Ca-Al-rich
sample (solid chondrule
employed in our work. This is based on our judgment that the Leedey chondrite sample analysed by Masuda et al. (1973) is relatively typical in relative abundances
IO
Al Co Sr Ea
2. Chondrite-normalized center-dotted circles refer FIG.
to
La Ce
Nd
SmEuGd
Dy
patterns of RE and other elements pink portion (G,) and open circles
Er
Yb Lu
for two to white
portions portion
ofinclusion (G,.,).
G;
REINCOMPONENTS
of RE elements. In the discussion to follow, our main interest will be focused on the chondrite-normalized RE pattern.
5. DISCUSSION
Figures 1 and 2 show that the chondritenormalized RE patterns of components of Allende are subject to rather wide variations. So far as the RE abundances relative to a representative chondrite are concerned, olivine chondrule, Ca-Al-rich chondrule, and inclusion 0 are least fractionated. However, the Ca-Al-rich chondrule shows some positive Eu anomaly and appears to have a small negative aberration for Dy ; and inclusion 0 has negative anomalies for both Eu and Yb. Accordingly, it turns out that, so far as RE abundances are concerned, the olivine chondrule is the least fractionated material. Although the absolute abundances in matrix and wholerock samples are at levels similar to olivine chondrule, the whole RE pattern is not horizontal, and the individual RE elements are fractionated relative to each other. This may suggest that the “matrix” contains a very minute amount of material highly fractionated in RE elements. It may appear to be contradictory that the absolute abundances of RE elements in the wholerock sample are lower than in any components. This may be because the RE distribution in individual stones is not homogenized and the whole-rock sample studied in the present work happened to be impoverished in RE elements. Alternatively, this could imply that there should be other components that are considerably impoverished in RE abundances and have not been investigated in the present work. (Data which are considered to be consistent with this inference will be presented elsewhere.) In general, the chondrules in each class have a chemical composition similar to that of the matrix (Larimer and Anders, 1970). These authors have also pointed out that chondrule formation does not seem to have been causally related to either of the lithophile element or siderophile element fractionations. Moreover, they suggested
OFALLENDE
527
that it may have occurred at -700°K during or after metal-silicate fractionation, presumably by partial remelting of the antecedent dust. Even if the distribution observed in the olivine chondrule and “matrix” of Allende is a special case, it would be of some significance in consideration of chondrule formation and of the sequence of chondrule formation and elemental fractionation, It may be conceivable that chondrule formation took place over some time interval and by several different processes. Based on studies on xenon isotopes, Fireman et al. (1970) suggested that some chondrules of Allende may predate other chondrules by approximately 5m.y. Here let us turn our attention to RE patterns of Ca-Al-rich aggregates (see Figs. 1, 2). It is intriguing that, in spite of minor difference in mineralogy as mentioned above, the chondrite-normalized RE patterns for aggregate G and inclusion 0 are markedly different. Inasmuch as we have a reason to believe that the pink portion G, is much closer to “end-member” material than the white portion G, is, more attention will be paid to Gp. (A relationship between G, and G, will be discussed later.) As seen in Fig. 2, the inclusion G appears to have negative anomalies for Ce and Eu and a positive anomaly for Yb ; in particular, anomalies for Eu and Yb are prominent. Even apart from these anomalies, the RE pattern designated by other RE elements in inclusion G seems quite strange to us. While the decreasing slope from La to Sm is gentle, the drop from around Sm down to Lu is rapid. It appears that the degree of abundance enrichment is saturated for the lightest RE elements. This saturated level for G, is similar to the level of the approximately horizontal RE pattern of inclusion 0. The fact that inclusions G, and 0 are similar in their major mineral assemblages means that they are roughly similar in major chemical composition as well. Note that the Lu abundance in inclusion 0 is nearly 120 times higher than in G,. It is also worth mentioning that the Lu abundance in G, is as low as a sixth of the chondritic abundance.
528
TANAKA
AND
A process of vaporization, transportation, and recondensation seems to account for the present results on the Ca-Al-rich inclusions. It is assumed that dust was “almost completely vaporized at a certain locale in the solar nebula, and was transported to another locale to recondense. For the sake of simplicity, the vaporized material is assumed to have been transported in one direction. It is conceivable that the distance between the vaporization locale and the recondensation locale varies, depending on the volatility of the elements or their compounds. (Such a distance need not be overly long for refractory elements.) Thus, the concentrations of trace elements in the condensate could be va,riable as a function of distance from the original locale. Of course, the cooling rate (and the changing pressure) would affect seriously the concentrations of those elements in individual recondensates. In most cases, since the difference in volatility of RE metals or their oxides is rather small, RE elements would all behave similarly and a nonselective leveling of absolute abundances would result for RE elements. Under certain circumstances, however, a selective recondensation could arise even for RE elements, perhaps reflecting the rather small but generally systematic difference in lattice energy of relevant RE compounds. Inclusion 0 is considered to correspond to nonselective recondensation and inclusion G to fractional recondensation. Also it should be noted that the extent of fractionation between the RE group and the alkaline-earth group is not so selective for inclusion 0 and the fractionation within alkaline earths therein is almost nonselective, in contrast to the features seen for G,. Fractionation could take place in evaporation, too, However, the temperature range favorable for fractional vaporization would be extremely narrow, if it is to occur. Thus we tend to favor fractional condensation. The idea presented here is quite similar to the model advanced by Wood (1963).
As mentioned above, it appears that the anomaly for Eu is negative (too low) and the anomaly for Yb is positive (too high),
MASUDA
relative to adjacent RE elements (cf. Fig. 2), for inclusion G. One might pay great attention to the apparent fact that the direction of this anomaly is opposite for Eu and Yb. However, it may be of more essential significance that the chondritenormalized values are rather similar for Eu and Yb and their values are comparable with that of Ba. [Note that (1) both Eu and Yb could be divalent in liquid and solid under highly reducing conditions and (2) when their oxides are vaporized, their states in the gaseous phase are predominantly monoatomic, especially for Yb (Samsonov, 1969 ; Nguyen and de SaintSimon, 1972).] Thus, the results for inclusion G imply that Eu and Yb should be dealt with separately from other RE elements and might rather be associated with Ba (and other alkaline-earth metals). This interpretation is in keeping with the results for inclusion 0 (see Fig. 1) ; note that the chondrite-normalized values for Eu and Yb for inclusion 0 are similar to each other, and are rather comparable with those for alkaline earth elements. Despite the great difference in apparent RE pattern between inclusions G, and 0, they can be discussed along the same line. Aspects for the Ca-Al-rich chondrule (divided open circles in Fig. 1) are somewhat different from inclusion 0; while Yb remains normal in this material, Eu deviates in the positive direction. This would suggest that the Ca-Al-rich chondrule investigated was produced under less reducing conditions than inclusions G and 0. When our attention is drawn to the chondrite-normalized pattern shown by RE elements other than Ce, Eu, and Yb in Fig. 2, we notice a less prominent irregularity in the curve at Dy. It is worth pointing out that the function curves of such properties of RE elements as heat of sublimation, boiling point and transition energy from 4f”-’ 5d6s2 to 4f”6s2 (Nguyen and de Saint-Simon, 1972) show a small irregularity or minimum at Dy. The abundances of light and middle RE elements in G, are lower than in G,, whereas those of Er and Lu in the former are higher than in the latter. As a first
RE
IN
COMPONENTS
approximation, the inclusion G, is understood in terms of mixture of two extremes, i.e., G, material and chondritic material. Since our present concern is directed mainly to RE elements, let us consider the olivine chondrule studied by us as a chondritic material. According to our estimates, the G, material can be approximated as a mixture of 55% G,-like material and 45% chondritic material represented by the olivine chondrule. (If the RE abundances in Leedey are taken in place of those in the olivine chondrule mentioned above, the mixing percentages of G,-like material and chondritic become 66 and 34%, respectively). The calculated values are represented by crosses in Fig. 3. Comparison of the calculated values with those observed in G, (open circles) shows that the agreement between the two sets of values is rather good, except only for Eu. The disagreement for Eu may be ascribed to the relatively great variability or the mobility of Eu in meteoritic matter (Masuda et al., 1973). The general good agreement here is of importance, because it suggests that dust of chondritic composition was present in the neighborhood of the fractionated dust which coalesced into G, material; it follows that the G, material might be derived from the chondritic dust. (Naturally, we cannot rule out the possibility that the G, material itself comprises several percent chondritic material.)
Fig. 3. Comparison from
a combination
of the observed of G, (center-dotted
values
(open circles)
OF
529
ALLENDE
It has been inferred above that the apparently great difference between inclusions G, and 0 reflects a subtle difference in circumstance of condensation ; in particular, pertaining to temperature conditions (evaporation temperature, temperature gradient of the ambient medium, and cooling rate). A difference such as this could not come about by slow cooling; it would also indicate that the event in question was a more or less localized one. Marvin et aE. (1970) synthesized a spine1 plus glass assemblage similar to those in Allende by melting a mixture of MgO, Al,O,, CaO, and SiO,. To do this, they reduced the heat “gradually” (15 see). Although they did not have to quench in their experiment, their cooling rate can be said to be rapid for a phenomenon occurring in nature. The interpretation which has been presented to account for the formation of white aggregate in Allende is quite similar to the model presented by Wood (1963) to explain chondrule formation. Although our results may be open to other interpretations, we think that the above interpretation of the Allende inclusions is a tenable explanation. But we do not necessarily intend to regard it as generalizable to other meteorites or inclusions, and do not intend to assert that the chondrules were formed in the same way as the Allende inclusions investigated by us.
circies) for and olivine
0, with chondrule
the
values (crosses) (solid circles) (cf.
calculated text).
530
TANAKA
AND
Wood’s (1963) model is not favored by all as an explanation of chondrule formation (Wasson, 1972). Nevertheless, his discussion seems instructive in considering the formation of Ca-Al-rich inclusions in Allende in light of vaporization and recondensation. He considers that dust particles were revolatilized by shock waves propagated by T Tauri-type eruptions. Certainly this is one of the conceivable causes for volatilization of dust. According to Hayashi’s (1972) recent work, during the stage of disk formation of solar system, a shock front arises at density and temperature discontinuities, quite apart from T Tauri-type eruptions. His calculation also reveals the possibility of vaporization of solid particles. ACKNOWLEDGMENT The samples investigated in this work were supplied through the courtesy of Dr. Brian Mason, Smithsonian Institution. We are also grateful for his valuable information and comments. REFERENCES CLARKE, R. S., JR., JAROSEWICR, E., MASON, B., NELEN,J.,G~MEZ,M.,ANDHYDE,J.R.(~~~~). The Allende, Mexico, meteorite shower. Smithsonian Contributions to the Earth Sciences, No. 5. FIREMAN, E. L., DEFELICE, J., AND NORTON, E. (1970). Ages of the Allende meteorite. Geochim. Cosmochim. Acta 34, 873-881. GAST, P. W., HUBBARD, N. J., AND WIESMANN, H. (1970). Chemical composition and petrogenesis of basalts from Tranquillity Base. Geochim. Cosmochim. Acta Suppl. 1, 2, 11431163, Pergamon. GROSSMAN, L. (1972). Condensation in the primitive solar nebula. Geochim. Cosmochim. Acta 36, 597-619. HAYASHI, C. (1972). Abstract, Symposium on the Moon and the Planets, University of Tokyo, July 29-30, 1972. KING, E. A., JR. (1969). Petrography and
MASUDA chemistry of the Pueblito de Allende meteorite. EOS Trans. Amer. Geophys. Union 50, 459. LARIMER, J. W. (1967). Chemical fractionations in meteorites-I : Condensation of the elements. Geochim. Cosmochim. Acta 31, 1215-1238. LARIMER, J. W., AND ANDERS, E. (1970). Chemical fractionations in meteorites-III : Major element fractionations in chondrites. Geochim. Cosmochim. A& 34, 367-387. LORD, H. C., III (1965). Molecular equilibria and condensation in a solar nebula and cool stellar atmospheres. Icarus 4, 279-288. MARVIN, U. B., WOOD, J. A., AND DICKEY, J. S., JR. (1970). Ca-Al rich phases in the Allende meteorite. Earth Planet. Sci. Lett. 7, 346-350. MASUDA, A., NAKAMVRA, N., AND TANAKA, T. (1973). Fine structures of mutually normalized rare-earth patterns of chondrites. Geochim. Cosmochim. Actu (in press). MOORE, C. B., AND BROWN, H. (1963). Barium in stony meteorites. J. Geophys. Rea. 68, 4293-4296. NGUYEN, L.-D., AND DE SAINT-SIMON, M. (1972). Systematic study of the decomposition of very small quantities of rare earth oxides by mass spectrometry. I&. J. Mass Spectr. Ion Physics 9, 299-314. SAMSONOV, G. V. (1969). Physico-chemical properties of oxides-“Metallurgy” (handbook, Japanese translation). Publishing Office, Moscow. SCHMITT, R. A., SMITH, R. H., LASCH, J. E., MOSEN, A. W., OLEHY, D. A., AND VASILEVSKIS, J. (1963). Abundances of the fourteen rare earth elements, scandium and yttrium in meteoritic and terrestrial matter. Geochim. Cosmochim. Acta 27, 577-622. SCHMITT, R. A., SMITH, R. H., AND OLEHY, D. A. (1964). Rare-earth, yttrium and scandium abundances in meteoritic and terrestrial matter-II. Geochim. Cosmochim. Acta 28,
67-86. WAKITA, H., AND SCHMITT, R. A. (1970). Rare earth and other elemental abundances in the Allende meteorite. Nature (London) 227, 478-479. WASSON, J. T. (1972). Formation of ordinary chondrites. Rev. Geophys. Space Phys. 10, 711-759. WOOD, J. A. (1963). On the origin of chondrules and chondrites. Icarus 2, 152-180.
19,
523-530
(1973)
Rare-Earth Elements Chondrules of
in Matrix, the Allende
TSUYOSHI Geological
Survey
of Japan,
Inclusions, Meteorite
and
TANAKA
Htiamoto,
Takatsu-ku,
Kawasaki,
Japan
AND
AKIMASA
MASUDA
Department of Chemistry, Science Kagurazaka, Shinjuku-ku, Received
February
21,
1973;
University of Tokyo, Tokyo, Japan revised
March
26,
1973
Rare-earth elements in a whole-rock sample and in major components of the Allende meteorite were investigated; for a few samples, abundances of Ba, Sr, Ca, and Al were also determined. Of the materials investigated in the present work, C&Al-rich inclusions G and 0 seem to be of the greatest significance. In spite of the minor difference in mineralogy between them, the apparent chondritenormalized RE pattern is much different between these two inclusions. (Yb and Eu in inclusion G appear exceptionally irregular). This observation is inferred to reflect a rather subtle difference in condition of condensation. It is also worthwhile to note that, while two portions (pink and white) of the inclusion G show similar aspects in the abundances of lithophile trace elements investigated, they show a remarkable difference at the same time. The white portion (G,) of inclusion G can be considered to be a mixture of chondritic material and highly fractionated material like the faintly pink portion (G,) picked from the same inclusion. This would suggest the possibility that the G,-like material was produced from chondritic dust. The “matrix” separated from Allende was found to be fractionated with respect to the R,E abundances relative to representative chondrite. It has also a very high value for the Ba abundance.
1.
INTRODUCTION
In the early morning of 8 February 1969, thousands of individual meteoritic stones rained down over a strewn field of about 300km* of northern Mexico, near the city of Hidalgo de1 Parral in the southcentral part of the state of Chihuahua ; one weighing 15kg fell in the town of Pueblito de Allende. A detailed report on this Type III carbonaceous chondrite was published by Clarke et al. (1971). They described this meteorite in terms of four major components, i.e., matrix, chondrules, irregular aggregates (white or pink), and dark inclusions. Point-counting on one section gave the following volume constitution : Copyright 0 1973 by Academic Press, Inc. All rights of reproduction in any form reserved. Printed in Great Britain
523
chondrules 34, aggregates 9, and matrix 57%. These authors also emphasized the significance of the contrasts between matrix, chondrules, and aggregates. The greatest interest has been a’roused by the white aggregates, composed largely of such minerals as anorthite, gehlenite, and spinel, the first to condense from a gas of cosmic composition (Lord, 1965; Larimer, 1967; Marvin et al., 1970; Grossman, 1972). Marvin et al. (1970) observed the presence of Ca-Al-rich glass associated with spinels and other crystalline phases in Allende. Since the studies by Schmitt et al. (1963, 1964) on rare-earth (RE) elements in chondritic meteorites, the relative abundances of those elements in chondrites
TANAKAANDMASUDA
524
have been commonly believed to be constant. The recent study by Masuda et al. (1973), however, revealed that this is not always the case in a strict sense, although the extent of fractionation ranges from virtually negligible to comparatively small in most cases. In addition, it has also been found that the relative abundances of those tightly associated lithophile elements are considerably fractionated in minor cases (Masuda et al., 1973 ; Nakamura and Masuda, unpublished). Thus, the accurate determination of RE abundances in major components of an unusually heterogeneous meteorite like Allende might be expected to reveal new meteoritic information. For RE elements in Allende, however, there are heretofore only studies on whole-rock samples by Wakita and Schmitt (1970) and by Masuda et al. (1973), and on a Ca-rich inclusion by Gast et al. (1970). 2. SAMPLE
Samples studied in the present work are whole rock, matrix, olivine chondrules, Ca-Al-rich chondrule, and two Ca-Al-rich inclusions. In the preparation of olivine chondrule samples, three chondrules each weighing about 6Omg were crushed and mixed. “Matrix” was obtained by crushing a sample, sieving it through a 200-mesh sieve, and centrifuging the 200-mesh fraction (grain size less than 74pm) in methylene iodide (density 3.32). This sample with density greater than 3.32 should be relatively free of material from the chondrules (cf. Table 3 of Clarke et al., 1971).
A Ca-Al-rich chondrule was taken from USNM 3529 (analytical data on another similar chondrule from 3529 are presented by Clarke et al., 1971); this material was rather friable. A Ca-Al-rich inclusion (inclusion 0) was also picked out from the same stone. (There is some question about whether this material should be classified as “inclusion” or “chondrule.“) Ca-Alrich aggregate (inclusion G) was taken from USNM 3598. Two portions, one white and another faintly pink, of this inclusion were analyzed separately; the former will be referred to as inclusion G white (G,) and
the latter as inclusion G pink (G,). (The naming of inclusions is by Dr. Brian Mason who supplied them.) Inclusion G is an aggregate of spinel, pyroxene (fassaite), and anorthite, with minor amounts of sodalite and nepheline. The Ca-Al-rich chondrule as well as inclusion 0 consists of spine& pyroxene (fassaite), melilite, and anorthite, and lack sodalite and nepheline. 3. EXPERIMENTAL
RE elements (Masuda et al., 1973) plus Ba and Sr were determined by the stable isotope dilution technique. Impurity corrections were made for the chemical reagents used. Al and Ca were determined by the atomic absorption technique for the solution from which RE elements had been separated. According to our examination, recovery of Al and Ca in this eluate is nearly 100%. 4. RESULTS Results of our analyses are presented in Table I and in Figs. 1 and 2. As regards the RE abundances, the samples analysed might be roughly divided into three groups : whole-rock sample, matrix, and olivine chondrule have the RE abundances corresponding to chondrite (Leedey)-normalized values of 1.1 to 2.4, whereas the Ca-AIrich chondrule and inclusion 0 have RE abundances a factor of 13-22 higher than Leedey (see Fig. 1). A RE pattern for a Ca-rich inclusion investigated by Gast et al. (1970) is somewhat similar to that for our Ca-Al-rich chondrule. As shown in Fig. 2, the chondrite-normalized RE pattern for inclusion G is quite unique and appears most interesting. Ba abundances in matrix and wholerock sample investigated by us are surprisingly high compared with the normal abundance (about 4 ppm) in chondrites. King (1969) and Clarke et al. (1970) obtained 10 and 5 ppm, respectively, for Allende. Although one might suspect contamination, we regard these extremely high values as present in the samples investigated. Moore and Brown (1963)
Amount taken
Ba Sr Al Ca
Ii1
La Cf3 Nd Sm Eu Gd DY Er Yh
Specimen
(mg)
29.6
8.03 18.4 14.9 4.00 0.217 1.93 1.38 0.130 1.01 0.00606 13.7 4.39 18.6 9.45
Inclusion pink
G
ABUNDANCES
97.9
12.3 7.66
5.01 11.0 9.07 2.46 0.276 1.47 0.945 0.216 0.680 0.0301
Inclusion white
OF RE, G
Ba,
49.3
7.40 19.2 14.9 5.00 1.22 6.34 7.71 4.98 3.79 0.738 42.5 118 16.2 16.2
Inclusion
Sr (ppm),
0
Ca AND
92.5
4.95 12.7 9.64 3.15 1.48 4.36 5.38 3.76 3.73 0.583 8.11 18.4
I in WHOLE
Ca-Al rich chondrule
Al (%)
TABLE
179.9
1.56 1.94
0.648 1.64 1.18 0.379 0.140 0.500 0.618 0.398 0.406 0.0600 -
Olivine chondrule
ROCK
AND
1029
0.876 2.12 1.19 0.334 0.115 0.397 0.462 0.303 0.331 0.0514 165 -
Matrix
ITS COMPONENTS
1074
0.663 1.69 1.03 0.313 0.110 0.384 0.451 0.301 0.330 0.0495 108
Whole rock
OF ALLENDE
-
0.249 0.0387 4.21 -
0.0866 0.311 0.390 0.255
0.716 0.230
0.378 0.976
Leedey chondrite
k is
8
2 3
8 z 8
E
526
TANAKA
AND
MASUDA
~a-A,
rich
chondrula
\\ \I \I
k
\I \\
i\ \I
f,,IA,III,III!IfIIII Y
Al Ca Sr Ba FIG.
triangles), (divided
Lace
Nd
SmEuGd
Dy
Er
Yb Lu
1. Chondrito-normalized patterns of RE and other elements for matrix (half-solid circles), olivine chondrule (solid circles), open circles), and Ca-Al-rich inclusion 0 (open triangles).
showed that the distribution of Ba in the Holbrook chondrite is heterogeneous. In normalization of RE and Ba abundances, those in the Leedey chondrite are
whole-rock Ca-Al-rich
sample (solid chondrule
employed in our work. This is based on our judgment that the Leedey chondrite sample analysed by Masuda et al. (1973) is relatively typical in relative abundances
IO
Al Co Sr Ea
2. Chondrite-normalized center-dotted circles refer FIG.
to
La Ce
Nd
SmEuGd
Dy
patterns of RE and other elements pink portion (G,) and open circles
Er
Yb Lu
for two to white
portions portion
ofinclusion (G,.,).
G;
REINCOMPONENTS
of RE elements. In the discussion to follow, our main interest will be focused on the chondrite-normalized RE pattern.
5. DISCUSSION
Figures 1 and 2 show that the chondritenormalized RE patterns of components of Allende are subject to rather wide variations. So far as the RE abundances relative to a representative chondrite are concerned, olivine chondrule, Ca-Al-rich chondrule, and inclusion 0 are least fractionated. However, the Ca-Al-rich chondrule shows some positive Eu anomaly and appears to have a small negative aberration for Dy ; and inclusion 0 has negative anomalies for both Eu and Yb. Accordingly, it turns out that, so far as RE abundances are concerned, the olivine chondrule is the least fractionated material. Although the absolute abundances in matrix and wholerock samples are at levels similar to olivine chondrule, the whole RE pattern is not horizontal, and the individual RE elements are fractionated relative to each other. This may suggest that the “matrix” contains a very minute amount of material highly fractionated in RE elements. It may appear to be contradictory that the absolute abundances of RE elements in the wholerock sample are lower than in any components. This may be because the RE distribution in individual stones is not homogenized and the whole-rock sample studied in the present work happened to be impoverished in RE elements. Alternatively, this could imply that there should be other components that are considerably impoverished in RE abundances and have not been investigated in the present work. (Data which are considered to be consistent with this inference will be presented elsewhere.) In general, the chondrules in each class have a chemical composition similar to that of the matrix (Larimer and Anders, 1970). These authors have also pointed out that chondrule formation does not seem to have been causally related to either of the lithophile element or siderophile element fractionations. Moreover, they suggested
OFALLENDE
527
that it may have occurred at -700°K during or after metal-silicate fractionation, presumably by partial remelting of the antecedent dust. Even if the distribution observed in the olivine chondrule and “matrix” of Allende is a special case, it would be of some significance in consideration of chondrule formation and of the sequence of chondrule formation and elemental fractionation, It may be conceivable that chondrule formation took place over some time interval and by several different processes. Based on studies on xenon isotopes, Fireman et al. (1970) suggested that some chondrules of Allende may predate other chondrules by approximately 5m.y. Here let us turn our attention to RE patterns of Ca-Al-rich aggregates (see Figs. 1, 2). It is intriguing that, in spite of minor difference in mineralogy as mentioned above, the chondrite-normalized RE patterns for aggregate G and inclusion 0 are markedly different. Inasmuch as we have a reason to believe that the pink portion G, is much closer to “end-member” material than the white portion G, is, more attention will be paid to Gp. (A relationship between G, and G, will be discussed later.) As seen in Fig. 2, the inclusion G appears to have negative anomalies for Ce and Eu and a positive anomaly for Yb ; in particular, anomalies for Eu and Yb are prominent. Even apart from these anomalies, the RE pattern designated by other RE elements in inclusion G seems quite strange to us. While the decreasing slope from La to Sm is gentle, the drop from around Sm down to Lu is rapid. It appears that the degree of abundance enrichment is saturated for the lightest RE elements. This saturated level for G, is similar to the level of the approximately horizontal RE pattern of inclusion 0. The fact that inclusions G, and 0 are similar in their major mineral assemblages means that they are roughly similar in major chemical composition as well. Note that the Lu abundance in inclusion 0 is nearly 120 times higher than in G,. It is also worth mentioning that the Lu abundance in G, is as low as a sixth of the chondritic abundance.
528
TANAKA
AND
A process of vaporization, transportation, and recondensation seems to account for the present results on the Ca-Al-rich inclusions. It is assumed that dust was “almost completely vaporized at a certain locale in the solar nebula, and was transported to another locale to recondense. For the sake of simplicity, the vaporized material is assumed to have been transported in one direction. It is conceivable that the distance between the vaporization locale and the recondensation locale varies, depending on the volatility of the elements or their compounds. (Such a distance need not be overly long for refractory elements.) Thus, the concentrations of trace elements in the condensate could be va,riable as a function of distance from the original locale. Of course, the cooling rate (and the changing pressure) would affect seriously the concentrations of those elements in individual recondensates. In most cases, since the difference in volatility of RE metals or their oxides is rather small, RE elements would all behave similarly and a nonselective leveling of absolute abundances would result for RE elements. Under certain circumstances, however, a selective recondensation could arise even for RE elements, perhaps reflecting the rather small but generally systematic difference in lattice energy of relevant RE compounds. Inclusion 0 is considered to correspond to nonselective recondensation and inclusion G to fractional recondensation. Also it should be noted that the extent of fractionation between the RE group and the alkaline-earth group is not so selective for inclusion 0 and the fractionation within alkaline earths therein is almost nonselective, in contrast to the features seen for G,. Fractionation could take place in evaporation, too, However, the temperature range favorable for fractional vaporization would be extremely narrow, if it is to occur. Thus we tend to favor fractional condensation. The idea presented here is quite similar to the model advanced by Wood (1963).
As mentioned above, it appears that the anomaly for Eu is negative (too low) and the anomaly for Yb is positive (too high),
MASUDA
relative to adjacent RE elements (cf. Fig. 2), for inclusion G. One might pay great attention to the apparent fact that the direction of this anomaly is opposite for Eu and Yb. However, it may be of more essential significance that the chondritenormalized values are rather similar for Eu and Yb and their values are comparable with that of Ba. [Note that (1) both Eu and Yb could be divalent in liquid and solid under highly reducing conditions and (2) when their oxides are vaporized, their states in the gaseous phase are predominantly monoatomic, especially for Yb (Samsonov, 1969 ; Nguyen and de SaintSimon, 1972).] Thus, the results for inclusion G imply that Eu and Yb should be dealt with separately from other RE elements and might rather be associated with Ba (and other alkaline-earth metals). This interpretation is in keeping with the results for inclusion 0 (see Fig. 1) ; note that the chondrite-normalized values for Eu and Yb for inclusion 0 are similar to each other, and are rather comparable with those for alkaline earth elements. Despite the great difference in apparent RE pattern between inclusions G, and 0, they can be discussed along the same line. Aspects for the Ca-Al-rich chondrule (divided open circles in Fig. 1) are somewhat different from inclusion 0; while Yb remains normal in this material, Eu deviates in the positive direction. This would suggest that the Ca-Al-rich chondrule investigated was produced under less reducing conditions than inclusions G and 0. When our attention is drawn to the chondrite-normalized pattern shown by RE elements other than Ce, Eu, and Yb in Fig. 2, we notice a less prominent irregularity in the curve at Dy. It is worth pointing out that the function curves of such properties of RE elements as heat of sublimation, boiling point and transition energy from 4f”-’ 5d6s2 to 4f”6s2 (Nguyen and de Saint-Simon, 1972) show a small irregularity or minimum at Dy. The abundances of light and middle RE elements in G, are lower than in G,, whereas those of Er and Lu in the former are higher than in the latter. As a first
RE
IN
COMPONENTS
approximation, the inclusion G, is understood in terms of mixture of two extremes, i.e., G, material and chondritic material. Since our present concern is directed mainly to RE elements, let us consider the olivine chondrule studied by us as a chondritic material. According to our estimates, the G, material can be approximated as a mixture of 55% G,-like material and 45% chondritic material represented by the olivine chondrule. (If the RE abundances in Leedey are taken in place of those in the olivine chondrule mentioned above, the mixing percentages of G,-like material and chondritic become 66 and 34%, respectively). The calculated values are represented by crosses in Fig. 3. Comparison of the calculated values with those observed in G, (open circles) shows that the agreement between the two sets of values is rather good, except only for Eu. The disagreement for Eu may be ascribed to the relatively great variability or the mobility of Eu in meteoritic matter (Masuda et al., 1973). The general good agreement here is of importance, because it suggests that dust of chondritic composition was present in the neighborhood of the fractionated dust which coalesced into G, material; it follows that the G, material might be derived from the chondritic dust. (Naturally, we cannot rule out the possibility that the G, material itself comprises several percent chondritic material.)
Fig. 3. Comparison from
a combination
of the observed of G, (center-dotted
values
(open circles)
OF
529
ALLENDE
It has been inferred above that the apparently great difference between inclusions G, and 0 reflects a subtle difference in circumstance of condensation ; in particular, pertaining to temperature conditions (evaporation temperature, temperature gradient of the ambient medium, and cooling rate). A difference such as this could not come about by slow cooling; it would also indicate that the event in question was a more or less localized one. Marvin et aE. (1970) synthesized a spine1 plus glass assemblage similar to those in Allende by melting a mixture of MgO, Al,O,, CaO, and SiO,. To do this, they reduced the heat “gradually” (15 see). Although they did not have to quench in their experiment, their cooling rate can be said to be rapid for a phenomenon occurring in nature. The interpretation which has been presented to account for the formation of white aggregate in Allende is quite similar to the model presented by Wood (1963) to explain chondrule formation. Although our results may be open to other interpretations, we think that the above interpretation of the Allende inclusions is a tenable explanation. But we do not necessarily intend to regard it as generalizable to other meteorites or inclusions, and do not intend to assert that the chondrules were formed in the same way as the Allende inclusions investigated by us.
circies) for and olivine
0, with chondrule
the
values (crosses) (solid circles) (cf.
calculated text).
530
TANAKA
AND
Wood’s (1963) model is not favored by all as an explanation of chondrule formation (Wasson, 1972). Nevertheless, his discussion seems instructive in considering the formation of Ca-Al-rich inclusions in Allende in light of vaporization and recondensation. He considers that dust particles were revolatilized by shock waves propagated by T Tauri-type eruptions. Certainly this is one of the conceivable causes for volatilization of dust. According to Hayashi’s (1972) recent work, during the stage of disk formation of solar system, a shock front arises at density and temperature discontinuities, quite apart from T Tauri-type eruptions. His calculation also reveals the possibility of vaporization of solid particles. ACKNOWLEDGMENT The samples investigated in this work were supplied through the courtesy of Dr. Brian Mason, Smithsonian Institution. We are also grateful for his valuable information and comments. REFERENCES CLARKE, R. S., JR., JAROSEWICR, E., MASON, B., NELEN,J.,G~MEZ,M.,ANDHYDE,J.R.(~~~~). The Allende, Mexico, meteorite shower. Smithsonian Contributions to the Earth Sciences, No. 5. FIREMAN, E. L., DEFELICE, J., AND NORTON, E. (1970). Ages of the Allende meteorite. Geochim. Cosmochim. Acta 34, 873-881. GAST, P. W., HUBBARD, N. J., AND WIESMANN, H. (1970). Chemical composition and petrogenesis of basalts from Tranquillity Base. Geochim. Cosmochim. Acta Suppl. 1, 2, 11431163, Pergamon. GROSSMAN, L. (1972). Condensation in the primitive solar nebula. Geochim. Cosmochim. Acta 36, 597-619. HAYASHI, C. (1972). Abstract, Symposium on the Moon and the Planets, University of Tokyo, July 29-30, 1972. KING, E. A., JR. (1969). Petrography and
MASUDA chemistry of the Pueblito de Allende meteorite. EOS Trans. Amer. Geophys. Union 50, 459. LARIMER, J. W. (1967). Chemical fractionations in meteorites-I : Condensation of the elements. Geochim. Cosmochim. Acta 31, 1215-1238. LARIMER, J. W., AND ANDERS, E. (1970). Chemical fractionations in meteorites-III : Major element fractionations in chondrites. Geochim. Cosmochim. A& 34, 367-387. LORD, H. C., III (1965). Molecular equilibria and condensation in a solar nebula and cool stellar atmospheres. Icarus 4, 279-288. MARVIN, U. B., WOOD, J. A., AND DICKEY, J. S., JR. (1970). Ca-Al rich phases in the Allende meteorite. Earth Planet. Sci. Lett. 7, 346-350. MASUDA, A., NAKAMVRA, N., AND TANAKA, T. (1973). Fine structures of mutually normalized rare-earth patterns of chondrites. Geochim. Cosmochim. Actu (in press). MOORE, C. B., AND BROWN, H. (1963). Barium in stony meteorites. J. Geophys. Rea. 68, 4293-4296. NGUYEN, L.-D., AND DE SAINT-SIMON, M. (1972). Systematic study of the decomposition of very small quantities of rare earth oxides by mass spectrometry. I&. J. Mass Spectr. Ion Physics 9, 299-314. SAMSONOV, G. V. (1969). Physico-chemical properties of oxides-“Metallurgy” (handbook, Japanese translation). Publishing Office, Moscow. SCHMITT, R. A., SMITH, R. H., LASCH, J. E., MOSEN, A. W., OLEHY, D. A., AND VASILEVSKIS, J. (1963). Abundances of the fourteen rare earth elements, scandium and yttrium in meteoritic and terrestrial matter. Geochim. Cosmochim. Acta 27, 577-622. SCHMITT, R. A., SMITH, R. H., AND OLEHY, D. A. (1964). Rare-earth, yttrium and scandium abundances in meteoritic and terrestrial matter-II. Geochim. Cosmochim. Acta 28,
67-86. WAKITA, H., AND SCHMITT, R. A. (1970). Rare earth and other elemental abundances in the Allende meteorite. Nature (London) 227, 478-479. WASSON, J. T. (1972). Formation of ordinary chondrites. Rev. Geophys. Space Phys. 10, 711-759. WOOD, J. A. (1963). On the origin of chondrules and chondrites. Icarus 2, 152-180.