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Cadmium isotope fractionation in fractions of two H3 chondrites
166
Earth and Planetary Science Letters, 48 (1980) 166-170 © Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
161 CADMIUM ISOTOPE FRACTIONATION IN FRACTIONS O F TWO H3 CHONDRITES K.J.R. R O S M A N , J.R. DE LAETER and M.P. GORTON 1
Department of Physics, WesternAustralian Institute of Technology. South Bentley, W.A. 6102 (Australia)
Received August 31, 1979 Revised version received February 11, 1980
Measurements of Cd isotope abundances reveal anomalies of up to 2.8% and 3.1% for the 116Cd/t 06Cd ratio in separates of the unequilibrated (H3) chondrites Tieschitz and Brownfield. Samples of chondrules, matrix and whole rock from Tieschitz yield a correlation between isotopic fractionation and Cd concentration, implying a two-component mixture of unfractionated and fractionated Cd. A correlation is also observed in mineral concentrates prepared from Brownfield, but it is not as definite. We favour the interpretation that Tieschitz has fractionated Cd uniformly distributed throughout the meteorite with a non-uniformly distributed unfractionated Cd component. The trend observed in the Brownfield data suggests a non-uniform distribution of the fractionated component, possibly brought about by terrestrial weathering processes.
I. Introduction The isotopic abundances of elements in meteorites are o f importance in reconstructing the events which gave rise to the solar system. Recently a number of new isotopic anomalies have been found in Allende inclusions which were already known to exhibit O and Mg anomalies: Ca [1], Sr [2], Ba and Nd [3] and Sm [4,5]. Most of these anomalies appear to be specific to certain isotopes and their occurrence has been explained by arguing that isotopic heterogeneities existed in the early solar system, resulting from incomplete mixing of nuclides formed by different nucleosynthetic processes. In 1976 we presented data which demonstrated the existence of isotopically fractionated Cd in two H3 chondrites - Brownfield and Tieschitz [6]. The meteoritic Cd was isotopically fractionated with an enrichment in the heavier isotopes. The magnitude of the fractionation was established by the doublespiking technique to be 2.3%o and 1.5%~ per mass unit for whole rock samples o f Brownfield and Tieschitz respectively. Subsequently we presented a
survey o f Cd isotopic abundances in 28 meteorites and three terrestrial samples, in which only Brownfield and Tieschitz showed a significant fractionation with respect to the laboratory standard [7]. These data also show a range o f fractionation in Tieschitz extending from 0.6%~ to 1.5%c per mass unit. This paper investigates the distribution o f Cd in these two meteorites in order to account for these isotopic results, and also to throw fresh light on the process of fractionation giving rise to these effects.
2. Methods and results The experimental procedures used for the separation and measurement of Cd are the same as those described by Rosman and de Laeter [7]. All fractionations and concentrations presented in this paper were obtained using the double-spiking technique. In calculating the mass fractionation it has been assumed that the Cd in these samples is linearly fractionated with respect to the laboratory reagent Cd as shown previously [6]. All errors are 95% confidence limits.
1 Present address: Department of Geology, University of
Toronto, Toronto, Ont. M5S 1A1, Canada.
Tieschitz. Two pieces of Tieschitz were crushed to
167 allow chondrules to be hand separated. Chondrules f r o m one piece were sorted according to size, and identified as large and small chondrules. No a t t e m p t was made to sort the chondrules separated from the o t h e r piece. Samples o f the chondrules and the m a t r i x (the residue after hand-picking for chondrules) were analysed, and the results are given in Table 1. In Fig. 1 the mass fractionation for each sample has been plotted against the inverse o f the Cd conc e n t r a t i o n ; the resulting linear trend is quite striking and suggests a t w o - c o m p o n e n t mixture for this meteorite. A least squares line through the points has a slope o f 1 17 + 19%0 a m u - I ppb and an intercept on the fractionation axis o f 0.12 +- 0.19%o. B r o w n f i e M . Earlier w o r k on Brownfield d e m o n -
TIESCHITZ A :E ,<
,
x"
l
z 0 < z Iu <
~" CHONDRULES O MATRIX M21
•
W2
• oos
-o~o
I
..~
i
.o~o
CONCENTRATION
WHOLE ROCK
I
.o.
.o;o
.o~o
( P P B ) ] ;1
Fig. 1. Correlation of isotopic fractionation with concentration for Cd in Tieschitz (see Table 1 for symbol identification).
strated that the magnitude o f the fractionation effect was remarkably u n i f o r m [6] though high. Even so, an
TABLE 1 Analytical results a for Cd in Tieschitz Sample
Mass (mg)
Fractionation b Concenrelative to tration reagent Cd (ppb) (%c per ainu)
520 510 291
1.6 ± 0.4 0.620.3 1.3 ± 0.5
75 ± 4 202± 4 143 ± 7
216 207 211
3.1 ± 1.0 2.6 ± 0.8 1.9 ± 0.8
38 ± 9 51 ± 10 69 ~ 10
265 78
1.0 ± 1.0 0.7 ± 0.7
104 ± 8 256 ± 26
Whole rock c
Wl d W2 d W3 Chondrule$
CL e CS e CA f
a t t e m p t was made to see if a distinct fractionated c o m p o n e n t comparable to that o f Tieschitz, could be identified. Brownfield is a " f i n d " and is a heavily weathered m e t e o r i t e with the chondrules and matrix c e m e n t e d together to such an extent that it was not possible to effect a separation o f these c o m p o n e n t s . Instead mineral concentrates were prepared, despite the obvious limitations o f this technique in terms o f potential c o n t a m i n a t i o n . F o u r fractions were prepared and labelled as magnetite, intermediates, heavies and fines. The " m a g n e t i t e " fraction was separated magnetically and is estimated to be 95% pure. The " f i n e s " were obtained from the dust pro-
Matrix
MI e M2 f
a All errors shown are 95% confidence limits. b The fractionation has been corrected for a blank of approximately 3 ng per total sample. c Isotopic data for these 3 samples are also given in Rosman and de Laeter [71. d Wl and W2 are separate whole rock samples from the same piece of meteorite. W3 is a separate piece. e A piece of the meteorite was roughly crushed, then separated into matrix (MI) and small (CS) and large (CL) chondrules. f A whole rock sample was roughly separated into chondrules of assorted sizes (CA) and matrix (M2).
BROWNFIELD
~ONC| NTRATK)N (PPB)~4
Fig. 2. Correlation of isotopic fractionation with concentration for Cd in Brownfield (see Table 2 for symbol identification).
168 TABLE 2 Analytical results a for Cd in Brownfield Sample
Fraction by weight (%)
Mass (mg)
Concentration (ppb)
Mass fractionation (%0 per mass unit)
Whole rock 1 b (R1) Whole rock 2 b (R2) Magnetite (Ma) Intermediates (1) Heavies (1I) Fines Distilled water soluble 6M HCI soluble (F6) 6M HCI insoluble 1M HCI soluble (light etch)~ 1 M HCI insoluble ) (F 1)
100 100 20.5 17.0 35.6 26.9
1196 1215 516 460 5 15
425 ± 2 386 ± 2 441 ± 6 244 ± 5 385 ± 4
2.3~0.4 2.3±0.3 2.6±0.3 1.8±0.9 2.1±0.3
426 543
( (5 { (172 t (23 (56 I~(201
± 2) c * 2) c ± 2) c ÷ 2) c - 2) c ±
2.3±0.5 1.6±0.4 2.8~0.6
a All errors shown are 95% confidence limits. b Isotopic data for these two samples are also given in Rosman and de Laeter [7]. c These data represent the quantity of Cd, in ng, leached from the fines. duced during crushing and were recovered by decanting a suspension in acetone. The remaining fractions were separated by flotation using bromoform and methylene iodide. The "intermediate" fraction consisted of material with specific gravity between 2.9 and 3.3, whereas the "heavies" had a specific gravity greater than 3.3. The latter probably consisted mainly of bronzites and sulphides with some magnetite and olivine. Data for these fractions and two whole rock samples are presented in Table 2, and are plotted in Fig. 2. The nature of the relationship is not as clear in this case. A straight line fitted through the points has a slope of - 4 9 8 ± 232%c amu -~ ppb with an intercept at 3.6 -+ 0.6%0 ainu -1 . Although the "intermediates" contain less Cd with perhaps a slightly lower fractionation, the concentrates display relatively little spread in either concentration or mass fractionation. This corroborates the consistent values for mass fractionation displayed by unspiked analyses reported previously [6].
3. Discussion
3. I. Distribution of the fractionated Cd component The isotopic abundance data presented in this and previous studies [6,7] clearly demonstrate the exis-
tence of isotopically fractionated Cd in meteorites. The observed effects are large, with anomalies in the 1 16/106 ratio reaching 2.8% in Brownfield and 3.1% in Tieschitz. The distinct linear trend revealed in the Tieschitz data (Fig. 1) suggests a two-component mixture of highly fractionated with unfractionated (-~ terrestrial) Cd. A linear relationship may also exist in Brownfield (Fig. 2), but the trend is not clearly defined. The Tieschitz data could be interpreted as indicating a mixture in each sample of chondrules with a low concentration of fractionated Cd and matrix with a relatively high concentration of unfractionated Cd. Because the meteorite is a "fall" the unfractionated component is unlikely to have a terrestrial origin. Also, the non-zero fractionations observed in matrix samples may be attributed to unseparated chondrule fragments. The larger proportion of fractionated Cd found in the larger chondrules may be accounted for by a relatively smaller surface area for contamination by the unfractionated matrix component. Analyses of the matrix show a wide range in concentration, suggesting that the Cd may be contained in a minor heterogeneously distributed matrix mineral such as a sulphide. However, under this interpretation the mathematics of the mixing relationship place some very demanding constraints on the distribution of Cd. One possible consequence of this interpretation is
169 a constant product of the proportion (by weight) of chondrules present in any sample with the concentration of fractionated Cd in the chondrules. If Cd has the same concentration in all chondrule materials, the proportion of chondrule present in any piece of meteorite must therefore be the same. The alternative is that the two quantities are inversely related. Both of these constraints are sufficiently demanding to cast considerable doubt on this interpretation. A preferred alternative interpretation of the Tieschitz data is that fractionated Cd of uniform fractionation is homogeneously distributed in the meteorite and is present at the same concentration in both chondrules and matrix, while the unfractionated Cd is heterogeneously distributed. The unfractionated component is seen as having a secondary origin. It may have been deposited onto grains containing fractionated Cd prior to accretion, or it may have been deposited directly into the accreted body. Because Tieschitz is a "fall" it is unlikely that this component has a terrestrial origin but this possibility cannot be ruled out. The linear trend of negative slope suggested by the Brownfield data (Fig. 2) is consistent with the existence of a uniformly distributed component of unfractionated Cd in the presence of varying amounts of fractionated Cd. Calculations based on this interpretation yield a fractionation of 3.6%o per mass unit for the fractionated component and a uniform distribution of unfractionated Cd at 139 ppb. However, because of the narrow spread in concentration these figures have high uncertainties. Because Brownfield is a heavily weathered "find" it is possible that the unfractionated component has a terrestrial origin. It is also possible that the relatively lower fractionated Cd content of some mineral concentrates may be due to preferential leaching in the terrestrial environment. In fact a uniform distribution of fractionated Cd similar to that observed in Tieschitz may once have existed.
3.2. Origin of the fractionated Cd The relative rarity of meteorites containing fractionated Cd suggests that both Tieschitz and Brownfield may have accreted from material which condensed in the same region of the solar system. The origin of the fractionated Cd is not known.
Isotopic fractionation arises in equilibrium processes due to the mass dependence of the free energy of the different phases. Non-equilibrium processes such as diffusion and distillation can also separate isotopic mixtures by a kinetic effect. Isotopic fractionation is most prominent among the lighter elements where the relative mass difference of the isotopes is greatest and it is surprising to find measurable effects in elements as heavy as Cd. Mechanisms for generating isotopic fractionation in the solar system have been discussed by Arrhenius and Alfv6n [8], Russell et al. [9] and Clayton [10], while reviews indicating the experimental evidence have been presented recently by Podosek [11] and Clayton [12]. It is also possible that the fractionated Cd we observe represents a primordial heterogeneity which has been preserved duringthe formation of the solar system. Cameron and Truran [13] have proposed that anomalous components may have been injected into the proto-solar cloud by a nearby supernova, which also triggered its collapse. If prior to the supernova the star contained isotopically fractionated elements [14,15], these would be injected into the proto-solar cloud. Hence, if the anomalous component is preserved during the formation of the solar system, both isotope fractionation and abundance variations attributable to nucleosynthesis may be present.
4. Conclusion
Further measurements on Cd from two unequilibrated chondrites confirm the existence of isotopically fractionated Cd in the solar system. Analyses of separates from these meteorites show trends consistent with a two-component mixture of fractionated and unfractionated Cd. The preferred interpretation is that fractionated Cd is homogeneously distributed in Tieschitz and heterogeneously distributed in Brownfield, with unfractionated Cd heterogeneously distributed in Tieschitz and homogeneously distributed in Brownfield. Because of the weathered nature of Brownfield it is possible that the unfractionated Cd in this meteorite has a terrestrial origin, although this seems unlikely in the case of Tieschitz which is a "fall". The non-uniform distribution of the fractionated Cd component in Brownfield may be due to terrestrial weathering processes.
170
The origin of the fractionated Cd is not known. Although isotopic fractionation is generally considered to originate within the solar system a primordial origin should not be overlooked.
Acknowledgements
5
6 7
We thank Dr. I. Fletcher and the reviewers for valuable criticism of the paper. The sample of Tieschitz was supplied by Dr. G. Kurat. Miss C. Jackson assisted in the preparation of the manuscript. This research was supported by the Australian Research Grants Committee.
8 9
10
References 1 T. Lee, D.A. Papanastassiou and G.J. Wasserburg, Calcium isotopic anomalies in the Allende meteorite, Ap. J. 220 (1978) 121. 2 D.A. Papanastassiou and G.J. Wasserburg, Strontium isotopic anomalies in the Allende meteorite, Geophys. Res. Lett. 5 (1978) 595. 3 M.T. McCulloch and G.J. Wasserburg, Barium and neodymium isotopic anom',dies in the Allende meteorite, Ap. J. 220 (1978) L15. 4 G.W. Lugmair, K. Marti and N.B. Scheinin, Incomplete mixing of products from r-, p- and s-process nucleosyn-
11 12 13
14
15
thesis: Sm-Nd systematics in Alende inclusions EK 1-4-1, in: Lunar Science IX (Lunar Science Institute, Houston, Texas, 1978) 672. M.T. McCulloch and G.J. Wasserburg, More anomalies from the Allende meteorite: samarium, Geophys. Res. Left. 5 (1978) 599. K.J.R. Rosman and J.R. de Laeter, Isotopic fractionation in meteoritic cadmium, Nature 261 (1976) 216. K.J.R. Rosman and J.R. de Laeter, A survey of cadmium isotope abundances, J. Geophys. Res. 83 (1978) 1279. G. Arrhenius and H. Alfv6n, Fractionation and condensation in space, Earth Planet. Sci. Lett. 10 (1971) 253. W.A. Russell, D.A. Papanastassiou and T.A. Tombrello, Ca isotope fractionation on the Earth and other solar system materials, Geochim. Cosmochim. Acta 42 (1978) 1075. D.D. Clayton, Origin of Ca Al-rich inclusions, II. Sputtering and collisions in the three-phase interstellar medium, Ap. J. (in press). F.A. Podosek, Isotopic structures in solar system materials, Ann. Rev. Astron. Astrophys. 16 (1978) 293. R.N. Clayton, Isotopic anomalies in the early solar system, Ann. Rev. Nucl. Sci. 28 (1978) 501. A.G.W. Cameron and J.W. Truran, The supernova trigger for formation of the solar system, Icarus 30 (1977) 447. R.E. White, A.H. Vaughan, Jr., G.W. Preston and J.P. Swings, Isotopic abundances of Hg in mercury stars inferred from Hg II ?,3984, Ap. J. 204 (1976) 131. G. Michaud, J. Reeves and Y. Charland, Diffusion and isotope anomalies of Hg in Ap stars, Astron. Astrophys. 37 (1974) 313.
Earth and Planetary Science Letters, 48 (1980) 166-170 © Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
161 CADMIUM ISOTOPE FRACTIONATION IN FRACTIONS O F TWO H3 CHONDRITES K.J.R. R O S M A N , J.R. DE LAETER and M.P. GORTON 1
Department of Physics, WesternAustralian Institute of Technology. South Bentley, W.A. 6102 (Australia)
Received August 31, 1979 Revised version received February 11, 1980
Measurements of Cd isotope abundances reveal anomalies of up to 2.8% and 3.1% for the 116Cd/t 06Cd ratio in separates of the unequilibrated (H3) chondrites Tieschitz and Brownfield. Samples of chondrules, matrix and whole rock from Tieschitz yield a correlation between isotopic fractionation and Cd concentration, implying a two-component mixture of unfractionated and fractionated Cd. A correlation is also observed in mineral concentrates prepared from Brownfield, but it is not as definite. We favour the interpretation that Tieschitz has fractionated Cd uniformly distributed throughout the meteorite with a non-uniformly distributed unfractionated Cd component. The trend observed in the Brownfield data suggests a non-uniform distribution of the fractionated component, possibly brought about by terrestrial weathering processes.
I. Introduction The isotopic abundances of elements in meteorites are o f importance in reconstructing the events which gave rise to the solar system. Recently a number of new isotopic anomalies have been found in Allende inclusions which were already known to exhibit O and Mg anomalies: Ca [1], Sr [2], Ba and Nd [3] and Sm [4,5]. Most of these anomalies appear to be specific to certain isotopes and their occurrence has been explained by arguing that isotopic heterogeneities existed in the early solar system, resulting from incomplete mixing of nuclides formed by different nucleosynthetic processes. In 1976 we presented data which demonstrated the existence of isotopically fractionated Cd in two H3 chondrites - Brownfield and Tieschitz [6]. The meteoritic Cd was isotopically fractionated with an enrichment in the heavier isotopes. The magnitude of the fractionation was established by the doublespiking technique to be 2.3%o and 1.5%~ per mass unit for whole rock samples o f Brownfield and Tieschitz respectively. Subsequently we presented a
survey o f Cd isotopic abundances in 28 meteorites and three terrestrial samples, in which only Brownfield and Tieschitz showed a significant fractionation with respect to the laboratory standard [7]. These data also show a range o f fractionation in Tieschitz extending from 0.6%~ to 1.5%c per mass unit. This paper investigates the distribution o f Cd in these two meteorites in order to account for these isotopic results, and also to throw fresh light on the process of fractionation giving rise to these effects.
2. Methods and results The experimental procedures used for the separation and measurement of Cd are the same as those described by Rosman and de Laeter [7]. All fractionations and concentrations presented in this paper were obtained using the double-spiking technique. In calculating the mass fractionation it has been assumed that the Cd in these samples is linearly fractionated with respect to the laboratory reagent Cd as shown previously [6]. All errors are 95% confidence limits.
1 Present address: Department of Geology, University of
Toronto, Toronto, Ont. M5S 1A1, Canada.
Tieschitz. Two pieces of Tieschitz were crushed to
167 allow chondrules to be hand separated. Chondrules f r o m one piece were sorted according to size, and identified as large and small chondrules. No a t t e m p t was made to sort the chondrules separated from the o t h e r piece. Samples o f the chondrules and the m a t r i x (the residue after hand-picking for chondrules) were analysed, and the results are given in Table 1. In Fig. 1 the mass fractionation for each sample has been plotted against the inverse o f the Cd conc e n t r a t i o n ; the resulting linear trend is quite striking and suggests a t w o - c o m p o n e n t mixture for this meteorite. A least squares line through the points has a slope o f 1 17 + 19%0 a m u - I ppb and an intercept on the fractionation axis o f 0.12 +- 0.19%o. B r o w n f i e M . Earlier w o r k on Brownfield d e m o n -
TIESCHITZ A :E ,<
,
x"
l
z 0 < z Iu <
~" CHONDRULES O MATRIX M21
•
W2
• oos
-o~o
I
..~
i
.o~o
CONCENTRATION
WHOLE ROCK
I
.o.
.o;o
.o~o
( P P B ) ] ;1
Fig. 1. Correlation of isotopic fractionation with concentration for Cd in Tieschitz (see Table 1 for symbol identification).
strated that the magnitude o f the fractionation effect was remarkably u n i f o r m [6] though high. Even so, an
TABLE 1 Analytical results a for Cd in Tieschitz Sample
Mass (mg)
Fractionation b Concenrelative to tration reagent Cd (ppb) (%c per ainu)
520 510 291
1.6 ± 0.4 0.620.3 1.3 ± 0.5
75 ± 4 202± 4 143 ± 7
216 207 211
3.1 ± 1.0 2.6 ± 0.8 1.9 ± 0.8
38 ± 9 51 ± 10 69 ~ 10
265 78
1.0 ± 1.0 0.7 ± 0.7
104 ± 8 256 ± 26
Whole rock c
Wl d W2 d W3 Chondrule$
CL e CS e CA f
a t t e m p t was made to see if a distinct fractionated c o m p o n e n t comparable to that o f Tieschitz, could be identified. Brownfield is a " f i n d " and is a heavily weathered m e t e o r i t e with the chondrules and matrix c e m e n t e d together to such an extent that it was not possible to effect a separation o f these c o m p o n e n t s . Instead mineral concentrates were prepared, despite the obvious limitations o f this technique in terms o f potential c o n t a m i n a t i o n . F o u r fractions were prepared and labelled as magnetite, intermediates, heavies and fines. The " m a g n e t i t e " fraction was separated magnetically and is estimated to be 95% pure. The " f i n e s " were obtained from the dust pro-
Matrix
MI e M2 f
a All errors shown are 95% confidence limits. b The fractionation has been corrected for a blank of approximately 3 ng per total sample. c Isotopic data for these 3 samples are also given in Rosman and de Laeter [71. d Wl and W2 are separate whole rock samples from the same piece of meteorite. W3 is a separate piece. e A piece of the meteorite was roughly crushed, then separated into matrix (MI) and small (CS) and large (CL) chondrules. f A whole rock sample was roughly separated into chondrules of assorted sizes (CA) and matrix (M2).
BROWNFIELD
~ONC| NTRATK)N (PPB)~4
Fig. 2. Correlation of isotopic fractionation with concentration for Cd in Brownfield (see Table 2 for symbol identification).
168 TABLE 2 Analytical results a for Cd in Brownfield Sample
Fraction by weight (%)
Mass (mg)
Concentration (ppb)
Mass fractionation (%0 per mass unit)
Whole rock 1 b (R1) Whole rock 2 b (R2) Magnetite (Ma) Intermediates (1) Heavies (1I) Fines Distilled water soluble 6M HCI soluble (F6) 6M HCI insoluble 1M HCI soluble (light etch)~ 1 M HCI insoluble ) (F 1)
100 100 20.5 17.0 35.6 26.9
1196 1215 516 460 5 15
425 ± 2 386 ± 2 441 ± 6 244 ± 5 385 ± 4
2.3~0.4 2.3±0.3 2.6±0.3 1.8±0.9 2.1±0.3
426 543
( (5 { (172 t (23 (56 I~(201
± 2) c * 2) c ± 2) c ÷ 2) c - 2) c ±
2.3±0.5 1.6±0.4 2.8~0.6
a All errors shown are 95% confidence limits. b Isotopic data for these two samples are also given in Rosman and de Laeter [7]. c These data represent the quantity of Cd, in ng, leached from the fines. duced during crushing and were recovered by decanting a suspension in acetone. The remaining fractions were separated by flotation using bromoform and methylene iodide. The "intermediate" fraction consisted of material with specific gravity between 2.9 and 3.3, whereas the "heavies" had a specific gravity greater than 3.3. The latter probably consisted mainly of bronzites and sulphides with some magnetite and olivine. Data for these fractions and two whole rock samples are presented in Table 2, and are plotted in Fig. 2. The nature of the relationship is not as clear in this case. A straight line fitted through the points has a slope of - 4 9 8 ± 232%c amu -~ ppb with an intercept at 3.6 -+ 0.6%0 ainu -1 . Although the "intermediates" contain less Cd with perhaps a slightly lower fractionation, the concentrates display relatively little spread in either concentration or mass fractionation. This corroborates the consistent values for mass fractionation displayed by unspiked analyses reported previously [6].
3. Discussion
3. I. Distribution of the fractionated Cd component The isotopic abundance data presented in this and previous studies [6,7] clearly demonstrate the exis-
tence of isotopically fractionated Cd in meteorites. The observed effects are large, with anomalies in the 1 16/106 ratio reaching 2.8% in Brownfield and 3.1% in Tieschitz. The distinct linear trend revealed in the Tieschitz data (Fig. 1) suggests a two-component mixture of highly fractionated with unfractionated (-~ terrestrial) Cd. A linear relationship may also exist in Brownfield (Fig. 2), but the trend is not clearly defined. The Tieschitz data could be interpreted as indicating a mixture in each sample of chondrules with a low concentration of fractionated Cd and matrix with a relatively high concentration of unfractionated Cd. Because the meteorite is a "fall" the unfractionated component is unlikely to have a terrestrial origin. Also, the non-zero fractionations observed in matrix samples may be attributed to unseparated chondrule fragments. The larger proportion of fractionated Cd found in the larger chondrules may be accounted for by a relatively smaller surface area for contamination by the unfractionated matrix component. Analyses of the matrix show a wide range in concentration, suggesting that the Cd may be contained in a minor heterogeneously distributed matrix mineral such as a sulphide. However, under this interpretation the mathematics of the mixing relationship place some very demanding constraints on the distribution of Cd. One possible consequence of this interpretation is
169 a constant product of the proportion (by weight) of chondrules present in any sample with the concentration of fractionated Cd in the chondrules. If Cd has the same concentration in all chondrule materials, the proportion of chondrule present in any piece of meteorite must therefore be the same. The alternative is that the two quantities are inversely related. Both of these constraints are sufficiently demanding to cast considerable doubt on this interpretation. A preferred alternative interpretation of the Tieschitz data is that fractionated Cd of uniform fractionation is homogeneously distributed in the meteorite and is present at the same concentration in both chondrules and matrix, while the unfractionated Cd is heterogeneously distributed. The unfractionated component is seen as having a secondary origin. It may have been deposited onto grains containing fractionated Cd prior to accretion, or it may have been deposited directly into the accreted body. Because Tieschitz is a "fall" it is unlikely that this component has a terrestrial origin but this possibility cannot be ruled out. The linear trend of negative slope suggested by the Brownfield data (Fig. 2) is consistent with the existence of a uniformly distributed component of unfractionated Cd in the presence of varying amounts of fractionated Cd. Calculations based on this interpretation yield a fractionation of 3.6%o per mass unit for the fractionated component and a uniform distribution of unfractionated Cd at 139 ppb. However, because of the narrow spread in concentration these figures have high uncertainties. Because Brownfield is a heavily weathered "find" it is possible that the unfractionated component has a terrestrial origin. It is also possible that the relatively lower fractionated Cd content of some mineral concentrates may be due to preferential leaching in the terrestrial environment. In fact a uniform distribution of fractionated Cd similar to that observed in Tieschitz may once have existed.
3.2. Origin of the fractionated Cd The relative rarity of meteorites containing fractionated Cd suggests that both Tieschitz and Brownfield may have accreted from material which condensed in the same region of the solar system. The origin of the fractionated Cd is not known.
Isotopic fractionation arises in equilibrium processes due to the mass dependence of the free energy of the different phases. Non-equilibrium processes such as diffusion and distillation can also separate isotopic mixtures by a kinetic effect. Isotopic fractionation is most prominent among the lighter elements where the relative mass difference of the isotopes is greatest and it is surprising to find measurable effects in elements as heavy as Cd. Mechanisms for generating isotopic fractionation in the solar system have been discussed by Arrhenius and Alfv6n [8], Russell et al. [9] and Clayton [10], while reviews indicating the experimental evidence have been presented recently by Podosek [11] and Clayton [12]. It is also possible that the fractionated Cd we observe represents a primordial heterogeneity which has been preserved duringthe formation of the solar system. Cameron and Truran [13] have proposed that anomalous components may have been injected into the proto-solar cloud by a nearby supernova, which also triggered its collapse. If prior to the supernova the star contained isotopically fractionated elements [14,15], these would be injected into the proto-solar cloud. Hence, if the anomalous component is preserved during the formation of the solar system, both isotope fractionation and abundance variations attributable to nucleosynthesis may be present.
4. Conclusion
Further measurements on Cd from two unequilibrated chondrites confirm the existence of isotopically fractionated Cd in the solar system. Analyses of separates from these meteorites show trends consistent with a two-component mixture of fractionated and unfractionated Cd. The preferred interpretation is that fractionated Cd is homogeneously distributed in Tieschitz and heterogeneously distributed in Brownfield, with unfractionated Cd heterogeneously distributed in Tieschitz and homogeneously distributed in Brownfield. Because of the weathered nature of Brownfield it is possible that the unfractionated Cd in this meteorite has a terrestrial origin, although this seems unlikely in the case of Tieschitz which is a "fall". The non-uniform distribution of the fractionated Cd component in Brownfield may be due to terrestrial weathering processes.
170
The origin of the fractionated Cd is not known. Although isotopic fractionation is generally considered to originate within the solar system a primordial origin should not be overlooked.
Acknowledgements
5
6 7
We thank Dr. I. Fletcher and the reviewers for valuable criticism of the paper. The sample of Tieschitz was supplied by Dr. G. Kurat. Miss C. Jackson assisted in the preparation of the manuscript. This research was supported by the Australian Research Grants Committee.
8 9
10
References 1 T. Lee, D.A. Papanastassiou and G.J. Wasserburg, Calcium isotopic anomalies in the Allende meteorite, Ap. J. 220 (1978) 121. 2 D.A. Papanastassiou and G.J. Wasserburg, Strontium isotopic anomalies in the Allende meteorite, Geophys. Res. Lett. 5 (1978) 595. 3 M.T. McCulloch and G.J. Wasserburg, Barium and neodymium isotopic anom',dies in the Allende meteorite, Ap. J. 220 (1978) L15. 4 G.W. Lugmair, K. Marti and N.B. Scheinin, Incomplete mixing of products from r-, p- and s-process nucleosyn-
11 12 13
14
15
thesis: Sm-Nd systematics in Alende inclusions EK 1-4-1, in: Lunar Science IX (Lunar Science Institute, Houston, Texas, 1978) 672. M.T. McCulloch and G.J. Wasserburg, More anomalies from the Allende meteorite: samarium, Geophys. Res. Left. 5 (1978) 599. K.J.R. Rosman and J.R. de Laeter, Isotopic fractionation in meteoritic cadmium, Nature 261 (1976) 216. K.J.R. Rosman and J.R. de Laeter, A survey of cadmium isotope abundances, J. Geophys. Res. 83 (1978) 1279. G. Arrhenius and H. Alfv6n, Fractionation and condensation in space, Earth Planet. Sci. Lett. 10 (1971) 253. W.A. Russell, D.A. Papanastassiou and T.A. Tombrello, Ca isotope fractionation on the Earth and other solar system materials, Geochim. Cosmochim. Acta 42 (1978) 1075. D.D. Clayton, Origin of Ca Al-rich inclusions, II. Sputtering and collisions in the three-phase interstellar medium, Ap. J. (in press). F.A. Podosek, Isotopic structures in solar system materials, Ann. Rev. Astron. Astrophys. 16 (1978) 293. R.N. Clayton, Isotopic anomalies in the early solar system, Ann. Rev. Nucl. Sci. 28 (1978) 501. A.G.W. Cameron and J.W. Truran, The supernova trigger for formation of the solar system, Icarus 30 (1977) 447. R.E. White, A.H. Vaughan, Jr., G.W. Preston and J.P. Swings, Isotopic abundances of Hg in mercury stars inferred from Hg II ?,3984, Ap. J. 204 (1976) 131. G. Michaud, J. Reeves and Y. Charland, Diffusion and isotope anomalies of Hg in Ap stars, Astron. Astrophys. 37 (1974) 313.