Rare earth elements abundance in meteoritic chondrites determined by radiochemical neutron activation analysi

Olehy, D. A.

Schmitt, R. A.

Smith, R. H.

English

NASA Technical Reports Server

Rare Earth Abundance s in Meteoritic Chondrules" 7 
R. A. Schmitt*, R. H. Smith and D. A. Olehy** 
General Atomic Division of General Dynamics Corporation 
San Diego, California 
Department of Chemistry*, Oregon State University i Corvallis, Oregon 
and * 
T o  be published in the Proceedings of the International Association 
of Geochemistry and Cosmochemist4 Symposium on the Origin and 
Distribution of the Elements, Paris, May 8-1 1, 19b7. 
) This work was supported in part by NASA contract NASw- 
partly by private funds of General Dynamics Corporation. 
* Now at Department of Chemistry, Oregon State University, Corvallis , 
Oregon 
** Now a t  National Reactor Testing Center, Idaho Falls, Idaho. 
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Introduction 
The composition of individual chondrules is known for 
only a few elements . . _. . from a few meteorites. Recently, 
the abundancks of 
and Cu were measured by thermal-neutron activation analysis in 
? 
seven elements, N a ,  Sc, Cr, Mn, Fe , Go 
I 5 96 individual chondrules separated from 28 chondritic meteor- 
i tes  (Schmitt, et al, 1965; Schmitt and Smith, 1966). Abundances 
of Si have been determined in some 275 individual Chondrules from 
1 9  different chondrites via 14-Mev neutron activation (Schmitt , et 
al ,  19b7a). Microprobe, mineralogical, and isotopic analyses of 
of selected chondrules and their grains were carried out by Fredriksson 
(19b3), K e i l  and Fredriksson (1964), Wood (1963) and Merrihue (1963). 
_-- The general conclusion that may be drawn from the above - 
studies is that elemental abundances in chondrules agree fairly 
well with solar abundance values; however, since too few elements 
have been investigated, these conclusions must  necessarily be re- 
garded with reservations. 
Several hypotheses suggest that chondrules predate 
whole rock chondrites and that chondrules may  have condensed 
out of the primitive solar matter under high-temperature pressure 
conditions (Wood, 1963). Lightning discharges in the primeval 
solar nebula may have remelted dust grains in .solar nebular 
2 
matter with subsequent chondrule formation on cooling (Whipple 19 66). 
Larimer and Anders (1967) have discussed the implications of these 
hypotheses and others on origin of chondrules and chondrites. 
The elemental abundance distribution of the 14  REE {rare earth 
elements), Y included, have been determined by RNAA (radiochemical 
neutron activation analysis) in  20 whole rock chondritic meteor- 
ites (Schmitt et a1 1963, 1964, Haskin and Gehl 1962) spread over 
the three major chondritic groups, carbonaceous, ordinary and enstatite 
chondrites. Their data have been summarized by Haskin et a1 (1966) a 
The most striking result was that the 14 RE relative abundances have 
remained essentially unchanged over the chondritic spectrum. 
Upon appropriate normalization, Clayton et.-al (1 9 61) 
have calculated theoretical RE abundance s within 
their general framework of elemental nucleosynthesis . The agreement 
between the experimental chondritic RE abundances and the theoretical 
values was surprisingly good, considering the many assumptions in- 
volved in the complicated theoretical analysis. 
Since this long and continuous series of non-volatile RE 
elements offers an excellent test for some aspects of nucleosynthesis 
models, the RE abundances must be investigated in  any representative 
matter which has been seriously proposed in the development of the 
evolution of our solar system and for any matter that may have predated 
the chondrites per se . 
3 
Exp erime nta 1 
The 1 4  rare earth elements (REE) and Y were determined by 
radiochemical neutron activation analysis (RNAA) in 30 combined 
magnetic chondrules (0.079 g) from Mokoia (Type I11 carbonaceous 
I 9 
chondrite), in 30 combined chondrules (0.080 g) from Chainpur (Un- 
equilibrated LLgroup chondrite), in  35 combined chondrules (0.081 9) 
from Forest City (H-group chondrite), and in 50 combined chondrules 
(0.077 9) from Richardton (H-group chondrite). 
Specific observations on the separation and characterization 
of these chondrules have been given elsewhere by Schmitt et a1 (1965, 
196%). In general, the chondrules that constituted the different batches 
i. 
were randomly picked; therefore, any elemental abundances should be 
truly representative and not deliberately biased. 
The RNAA method was that previously described by Schmitt 
et a1 (1963) and Goldberg et a1 (1963). In short, two chondrule 
batches , in two polyethylene vials, were irradiated simultaneously with 
the 14  REE and Y standards in the rotating rack of the TRIGA reactor 
for two hours at a neutron f lux of 1 . 8 ~ 1 0 ~ ~  cmW2 sec”l After neutron 
irradiation, the chondrule batches were fused with appropriate RE 
carriers and N a 2 0 Z .  Af te r  radiochemical decontamination of the REE , 
the REE were . separated by an ion-exchange column. 
The activity level of the individual REE, La ,, Srn and Eu- were sufficient 
to permit y-ray scintillation counting. The remainder of the  REE and Y 
4 
were counted by a Sharp Lowbeta counter with a background of % 0.3 
cpm . Beta counting rates  at % 0 . 45 geometry varied from about 5 to 
588)bove  background. * 
Experimental Results and Discussion 
Absolute abundances of the REE and Y have been tabulated in 
Table 1 For comparison, the normalized abundances (Haskin et a1 1966) 
of the 1 4  REE and Y in 20 chondritic meteorites have been included in 
the last column of Table 1. The (3 values of the REE and Y for the 
chondrules represent the standard deviation principally due to counting 
statistics, For the last column in Table 1, the values represent the 
mean deviation of the normalized values relative to Yb a t  unity. 
In Figs. 1 and 2,  the ratios of the respective normalized values 
in the chondrules to  the average normalized values in 20 chondrites have 
been plotted versus the ionic radii of the REE. 
From Table 1 and Figs. 1 and 2,  the following observations are 
noted. 
1. T d d  RE&. abundances in Mokoia chondrules are roughly 
twice those found in the whole chondrite, i. e. , 12.5 ppm 
versus 6.9 ppm . For Chainpur, an  unequilibrated LL-group 
chondrite, the RE absolute abundances in the chondrules 
are approximately equal to those determined i n  other 
whoie-rock , ordinary L-group chondrites. 
5 
2. Total absolute abundances of the REE in Forest City and 
Richardton chondrules are less compared to the REE in 
the respective chondrites, i,e., 4.1 ppm REE in Forest 
P 
City chondrules compared to  5.9 ppm in the whole-rock 
chondrite, and 3 . 7  ppm REE in Richardton chondrules versus 
5.6 ppm in the corresponding whole-rock chondrite. For 
both batches of chondrules, about 30 percent less total 
BEE reside 114 &O&I.iiCis a ~ m p a ~ e l  to the W h O l e T C d C r  1% 
is not known if  the observed depletion of REE in chondrules 
from ordinary chondrites may be ascribed to a deficiency 
in chondrules caused by RE diffusion out of the chondrules 
during metamorphism or to specific minerals that occlude 
the REE, whatever the mechanism. 
For Mokoia chondrules, a linear RE fractionation may be 
present, amounting to  2 2 0 %  for La relative to Yb. Within 
3 .  
experimental error, no significant RE fractionation occurs 
in Chainpur chondrules. From the accurately determined 
Sm/Eu ratios, Eu abundances in Mokoia and Chainpur 
chondrules are unchanged with respect to the adjacent REE. 
A very significant linear fractionation of the REE has  been 4. 
observed for both Forest City and Richardton chondrules; 
for the former, La has been fractionated by X 54%; for the 
latter, s 45%. In both cases, the absolute abundance 
of Yb has decreased% 10% and *20%, respectively, 
6 
in Forest City and Richardton chondrules relative to  Yb 
in the respective whole-rock chondrites . 
For both Forest City and Richardton chondrules, Eu has 5. 
been significantly fractionated with respect to the adjacent 
REE . The average Sm/Eu ratio of 2.7 2 0.4 in whole-rock 
chondrites compares to 1 . 6  and 1.5 in chondrules from 
these two H-group chondrites. 
Some obvious interpretations of the above RE fractionations 
involve diffusion of the REE and Y out of chondrules during metamorphism, 
as exemplified by the Forest City and Richardton data. The RE distri- 
bution in Mokoia and Chainpur indicates ~ 2 0 %  fractionation for La, the 
lightest REE; in fact, for Chainpur, almost all the error bars overlap 
unity, indicating little fractionation. Since Mokoia and Chainpur 
chondrules are certainly more primitive and unequilibrated than Forest 
City and Richardton chondrules, in the sense that they have been sub- 
jected to  significantly less metamorphism than ordinary chondrules, 
and since the absolute RE abundances in Mokoia and Chainpur chondrules 
are equal to or greater than those found in the respective whole 
chondrites, the overall RE decrease in Forest City and Richardton chon- 
drules seems consistent with diffusion of REE out of the chondrules 
during metamorphism . 
Since the S C + ~  and L u + ~  radii are rather similar at 0,818 and 
0.858, respectively, the element Sc A which has  some chemical 
7 
properties similar t o  the REE and particularly to the heaviest RE, Lu- 
would be expected to diffuse out of chondrules less than Lu. It is 
implicitly assumed that the WE and Sc are occluded in the same 
6 
chondrule mineral, altnougn such may not be the case. It has been 
experimentally observed (Schmitt et a l ,  1967b) that the ratios of Sc 
in chondrules to  the whole-rock chondrites was 1 .19  2 0 -43, 1.14 
- + 004Z0 1 . 9 0 2 0 . 6 6  and 1 . 6 3 ~ 0 . 6 2  for Mokoia, Chainpur, Forest 
City and Richardton, respectively. Values @ cited at3ove largely in- 
dicate the population standard deviation spread of Sc in -20 individual 
chondrules determined for each chondrite. Other Sc chondrule abundances 
from the above investigations show a similar trend that chondrules from 
ordinary chondrites are richer in Sc compared to  chondrules from Type I11 
carbonaceous or unequilibrated chondrites . If any Sc diffusion has oc- 
curred during metamorphism, the Sc data suggests a diffusion net gain 
by the chondrules, which apparently is in an opposite direction to any RE 
diffusion,, 
The fact that abundances of Lu and Yb arer20% less in 
chondrules compared to their matrices does not force the conclusion 
that RE abundances were originally identical in chondrules and in the 
whole-rock chondrites . Formation of chondrules at high temperatures 
may have been a sufficiently complicated process to accrete variable 
quantities of REE into chondrules and into the matrix material. How- 
ever, at the inferred accretion temperatures for the formation of 
8 
the high temperature mat te r  (fraction B of Larimer and Anders (1967) 
and chondrules of Wood (1963): >1300°K and - 1200°K for Type I11 
carbonaceous and ordinary chondrites, respectively), the volatility 
properties of the REE and Sc are sufficiently varied to introduce 
some fractionation, but not in  the observed pattern. Following the 
method and assumptions of Larimer and Anders (1967), we  have 
calculated the condensation temperatures for a few critical REE 
(see Table 2) .  A total pressure of 1 and 6.6 x atmospheres 
and normal cosmic elemental abundances of the elements were 
assumed and the m o s t  recent RE thermodynamic data by Research 
Chemicals (1966) was used in the calculations. 
Table 2, Condensation Temperatures of Pure RE Meta ls  
PT = 1 atm.  PT = 6.6 x lom3 atm. 
Element Temp. OK Temp. OK 
57La 1300 1150 
5 9Pr 1120 980 
5 8Ce 1360 1200 
6ZSm 560 
63Eu 550 
7 On 59 0 
7 l L U  1250 
2 1sc 1250 
48 0 
49 0 
52 0 
1120 
1090 
9 
The fa st cooling mechanism for condensation of the pure REE 
can be ruled out insofar as  chondrule formation is associated with the 
inferred high temperatures, At a n  inferred >1 300°K temperature for 
chondrule formation in  Type I11 carbonaceous chondrites, Sh, 
Eu and Yb' would have been severely fractionated with respect to La, 
Ce, Lu and Sc for total pressures of 6.6 x 1 0-3 or 1 atmosphere, 
both of which are well within the predicted range of pressures. In 
addition, the observed smooth linear fractionation of the REE in chondrules 
from ordinary chondrites rules out condensation of pure RE metals at 
- 1200°K. Moreover, the observed fractionation is in the opposite 
direction to prediction; i.e., Sm, Eu, yb and Lu according to this pure- 
metal fast cooling condensation mechanism would have been progressively 
depleted in chondrules and the reverse is observed. In addition, there 
is no significant fractionation between the adjacent REE, Yb and Lu, 
with their large condensation difference of N 60'K. 
In the above discussion, it has  been assumed that the RE 
distribution in the matr ix  (Wood 1963) or fraction A matter of Anders 
(1 9 64) represent the primordial distribution. Fraction-A matter for 
Type I11 carbonaceous and ordinary chondrites presumably accreted 
at temperatures of ~ 4 0 0 ~  and 530-650°K (Larimer and Anders 1967). 
No significant RE fractionation would be anticipated at these tempera- 
tures, even for the metal condensation mechanism. It is highly 
probable that RE oxides and silicates were formed at high temperatures 
10 
0 of > lo00  K and were more involved in the condensation history than 
the pure metals. Thermodynamic data of the RE oxides and silicates 
are needed to  decide whether the observed RE fractionation in chondrules 
from ordinary chondrites occurred during or subsequent to  gccretion of 
chondrules. 
Vaporization data of some RE oxides at elevated temperatures 
by White et al, (1962) show that the volatilities of the RE sesquioxides 
decrease markedly with atomic number, For e rnple, at 2500"K, rate8 
of effusion of La203, Nd203, Sm203,  Gd203 and Y 2 0 3  from a tungsten 
Knudsen cell were 13, 7.0,  4 .7 ,  1.6 and 0.25 x 10'4gm cm'2sec'1, 
respectively. At 2550°K, the effusion rate of Lu203,  the last REE, is 
approximately 0.1 of the Gd 0 rate. Since the partial pressure of a 
gaseous species is directly proportional to  the effusion rate and only 
inversely a s  the square root of the molecular weight, the partial pressure 
of the REE will be almost directly related to  the effusion rate. For equal 
2 3  
abundances of the REE in a solar nebula-e .g., La,  Sm and Gd, are 
within a factor of two of each other-the heavy RE oxides will condense 
before the light RE oxides, which is in line with the observed fraction- 
ation pattern in chondrules from ordinary chondrite s. Thermodynamic 
calculations on RE oxides in equilibrium with abundances of H2, O2 
and H20 in the primeval solar nebula should be carried out on a quanti- 
tative basis in order to establish their precise condensation temperatures. 
11 
The nearly identical abundances of Eu in the chondrules and 
whole-rock chondrites of Forest City and Richardton (Fig. 2) strongly 
suggest that the preponderance of Eu exists or existed in the divalent 
oxidation state in  chondrules and t h e  chondritic matrix during metamor- 
phism. If Eu diffusion has  occurred, equilibration has  been reached, 
resulting in zero net transport for Eu across the chondrule surface. 
Other divalent cations such a s  Fe, Mg and 'Mn have been equilibrated 
between the chondrules and matrices (Mason 1963, Keil and F'redriksson 
1964, and Schmitt et al. 1965 and 1967b). 
11 
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9 
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12 
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summary 
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Rare Earth Abundances in Meteoritic Chondrules

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