CONDENSATION AND STABILITY OF CHEMICALLY HETEROGENEOUS SUBSTANCES R. Wagner and J. W. Larimer, Dept. of Geology, Arizona State Univ., Tempe, Ariz. 85281

A more rigorous computational method has been developed to estimate the stability of chemically heterogeneous condensates in equilibrium with a cos- mic gas. The potential significance of the study can perhaps best be under- stood by recalling a basic principle in predicting thermodynamic behavior in heterogeneous systems: the addition of components tends to increase boiling points and decrease melting points. In other words, since virtually all con- densates from a cosmic gas contain impurities all previously estimated con- densation temperatures are too low, some more so than others.

The computational method is a modified and greatly expanded version of the one used by Palme and Wlotska (1). A vapor pressure equation is written for each component (n) and these simultaneous equations involving n unknowns (the concentrations of each component) are solved using a successive approxi- mation method at a given P and T. Two condensates were studied: a) metal, which is relatively straightforward and (b) molten oxides, which is quite complex but potentially the most significant system that can now be examined.

Metal The major components of the metal phase are Fe and Ni. Grossman (2) made the first attempt to compute the Ni content of the initial conden- sate by first estimating the condensation temperature of pure Fe, and assum- ing this to be the condensation temperature, computed the Ni content from the vapor pressure curve for Ni. Kelly and Larimer (3) did the same, but ex- panded the number of components. The discovery of a metal particle, rich in refractories yet still containing appreciable Fe and Ni, prompted the develop- ment of the more rigorous computational method (1). Obviously adding Ni to Fe will raise the boiling point (or drop the vapor pressure curve) over that of pure Fe. Our computations indicate a modest increase in the temperature at which the bulk of the Fe and Ni begin to condense, about 10 to 25O depend- ing on the composition and the compositional-activity coefficient relations which are reasonably well known.

Molten oxides It has become almost axiomatic that liquids are unstable in cosmic systems at the low pressures (< 1 atm) generally contemplated. The reasoning, however, contains an unstated or overlooked assumption. With only a few exceptions, the reasoning is based on a comparison of pure solid to pure liquid stabilities. By considering a heterogeneous liquid, rather than say pure Mg2Si04, the stability field is expanded to both higher (lower vapor pressure) and lower temperatures (lower melting point). Essentially pure Mg2Si04 is well known to coexist with liquids down to temperatures sev- eral hundred degrees below the melting point of pure Mg2Si04, yet no attempt has been made to estimate the stability of these liquids.

In our computations a simple model for liquid oxides was assumed (4); other more sophisticated models will eventually have to be examined. This model, as is the case for several others, avoids the problem of incomplete activity coefficient data by redefining the components in such a way that the liquid is ideal. Only the major components have been considered to date: A1203, CaO, MgO, Si02 and Ti02. Additional components, even minor ones, would of course increase even further the stability fields which we have es- timated.

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CONDENSATION AND STABILITY

R. Wagner et al.

In lieu of phase diagrams or other means of representation, here we will discuss the highlights of our results. The most stable liquids match a group of refractory rich inclusions in chondrites which are all nearly spherical in shape and whose bulk composition deviates considerably from that predicted for solid condensates (5). The liquid, which starts off rich in A1203 and Ti02, acquires appreciable MgO and Si02 at temperatures well above the con- densation temperature of Mg2Si04 and MgSi03. If effect, these calculations reopen the possibility that chondrules and some refractory-rich inclusions may have been molten or partially molten yet still in equilibrium with a cos- mic gas, even at low pressures. However, it should be emphasized that even though these calculations represent a significant improvement over previous ones, there are still large uncertainties in the data and the model.

References

(1) Palme, H. and Wlotska, P. (1977). Earth Planet Sic. Lett. 33, p. 45-60. (2) Grossman, L. (1972). Geochim. Cosmochim. Acta. 39, p. 1119-1140. (3) Kelly, W. R. and Larimer, J. W. (1977). ~eochim~~osmochim. Acta. 41,

p. 92-111. (4) Burnham, C. W. (1975). Geochim. Cosmochim. Acta. 39, p. 1077-1084. (5) McSween, H. Y. (1978). Geochim. Cosmochim. Acta (In press).

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NEBULAR CONDENSATION OF MODEMTELY VOLATILE ELEMENTS, THEIR BUNDANCES I N IRON METEORITES, AND THE QUANTIZATION OF GE AND GA ABUNDANCES. C.M. Wai, J .T . Wasson, J. Willis and A. Kracher, Univ. of C a l i f o r n i a , Los Angeles, CA.

Wasson and Wai (1976) noted t h a t CI-normalized abundances of 4 moderately v o l a t i l e elements i n IIIAB, I V A and I V B i r o n s decrease wi th decreas ing conden- s a t i o n temperature. Wai and Wasson (1977) c a l c u l a t e d condensation temperatures f o r 1 8 moderately v o l a t i l e elements, and found t h a t t h e i r CI-normalized abun- dances i n o rd inary and CM chondr i t es a l s o decrease wi th decreas ing temperature. The c o r r e l a t i o n was s i g n i f i c a n t l y improved by inc lud ing es t imated a c t i v i t y co- e f f i c i e n t s i n t h e c a l c u l a t i o n s . We here r e p o r t pre l iminary r e s u l t s of a s tudy of t h e r e l a t i o n s h i p between abundance r a t i o s and condensation temperatures of moderately v o l a t i l e elements i n i r o n m e t e o r i t e groups (Wasson, 1974; S c o t t and Wasson, 1975).

I n Table 1 a r e l i s t e d condensat ion temperatures f o r 1 3 elements a t nebular p ressures of 10-4 and 10-6 atm. With t h e except ion of S t h e s e appear t o con- dense a s o r i n Fe-Ni g r a i n s . Dominant gaseous s p e c i e s , t h e h o s t phase and t h e a c t i v i t y c o e f f i c i e n t a r e l i s t e d . Condensation temperatures of elements o t h e r t h a t N i , Co, Pd and C r ag ree wi th l i t e r a t u r e va lues . The Wai-Wasson condensa- t i o n temperatures of G a have been reduced by 170" a s a r e s u l t of r e c e n t d a t a on gaseous GaOH ( B a t t a t e t a l . , 1974) showing t h a t GaOH i s t h e dominant Ga gaseous s p e c i e s a t temperatures i n t h e condensation range. The new va lues a r e lower than t h e condensation temperatures of Sb, and t h e second lowest (next t o Ge) among s i d e r o p h i l e elements.

Fig. 1 i l l u s t r a t e s t h e r e l a t i o n s h i p between meteor i te /CI abundance r a t i o s and condensation temperature f o r t h r e e i r o n m e t e o r i t e groups and H group chon- d r i t e s . The method of determining mean composit ions of t h e groups w i l l be discussed below. Abundances of Cu and Sb were increased by a f a c t o r of 3.5 t o a l low f o r t h e i r d e p l e t i o n by post-nebular processes by roughly t h i s amount i n t h e m e t a l l i c p o r t i o n s of t h e i r o n s .

Abundance r a t i o s f o r H group c h o n d r i t e s and IIIAB i r o n s a r e t h e same wi th in 20%. C l e a r l y , t h e s e groups formed by very s i m i l a r nebula processes , and probably a t nebula l o c a t i o n s near one another . A s d iscussed by Wai and Wasson (1977), t h e i r p a t t e r n s seem w e l l expla ined by t h e gradual l o s s of agglomerable nebular mat te r wi th time. It was suggested t h e r e t h a t wi th decreas ing nebular temperature a n i n c r e a s i n g l y l a r g e f r a c t i o n of each element cannot d i f f u s e i n t o Fe-Ni g r a i n i n t e r i o r s , but r a t h e r condenses homogeneously a s f i n e a e r o s o l s t h a t f a i l e d t o s e t t l e t o t h e nebula median plane and agglomerate.

Abundances of moderately v o l a t i l e s i n I V A a r e s i m i l a r t o those i n H and I I I A B f o r t h e elements Co-Cu, but a r e much lower f o r t h e t h r e e most v o l a t i l e elements Sb, G a and Ge. This sugges t s t h a t t h e nebular coo l ing r a t e a t temper- a t u r e s below 950 K was lower a t t h e I V A l o c a t i o n than a t t h e IIIAB l o c a t i o n . The low P concen t ra t ion i n group I V A is a s c r i b e d t o p l a n e t a r y d i f f e r e n t i a t i o n r a t h e r than condensation. Perhaps t h e I V A body was more ox id ized , and perhaps core format ion occurred a t lower temperatures , both favor ing format ion of phos- phates r a t h e r than phosphides. A l t e r n a t i v e l y , t h e r e may have been k i n e t i c reasons f o r t h e P remaining wi th t h e s i l i c a t e s dur ing c o r e formation.

Group IVB abundances show a s t r o n g , r e l a t i v e l y uniform decrease wi th in - c reas ing temperatures , i n d i c a t i n g r a p i d coo l ing of t h e nebula according t o t h e Wasson-Wai model. Kel ly and Larimer (1977) suggested t h a t t h e mean I V B compo-

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MODERATELY VOLATILE ELEMENTS IN IRON METEORITES

Wai C.M. et al.

Fract~on of Iron Condensed distribution coefficient kX is obtained from the 09909 0 5 0 2 01 005 001 0001 regression line on a log X:log Ni plot (slope =

(kX-l)/(kNi-1)), Xi, the concentration of X in the initial solid to crystallize is determined from the line and an empirically estimated Nii concentration, , and the mean core concentration determined from the relationship xi=kX.R. We have initiated a com- puterized study of the compositions of the fractiok ally cryatallized iron meteorite groups; Fig. 3 -shows preliminary results for group IVA. All data

"are from the literature; where more than one source !of data were available mean compositions were esti- mated after allowance for interlaboratory biases. Data on the high-Ni anomalous IVA members (circled) were not included in the regression line calcula- tions.

Fig. 3 suggests that the k values for Ga, Ge

Co W P Au As Cu Sb GI Ge sit ion resulted from "quenching in" equilibrium compo-

I I and Au vary during crystallization. In this case

i 0 - 7 r 1 1 1 1 1 1 1 1 1 1 1 10 I

2 15 210 I '25k can no longer be obtained by linear regression,

Nickel (wt. %) rather a model must be generated to predict the variation in k. In group IIIAB (Fig. 4) k values for Ga and Ge change from <1 at low Ni contents (positive correlation with Ni) to >1 (negative correlation). Greenland (1970) suggested that k depended linearly on g, the fraction crystas lized, but the two dashed curves in Fig. 4 show that this relationship doesnot allow the simultaneous matching of both the low and high Ni distributions. In contrast, a good fit was obtained by a linear relationship of k with 1/Ni.

The narrow, "quantized" ranges of Ga and Ge within groups combined with their large ranges (by factors of 2000 and 40000, respectively) among all groups make them excellent taxanomic parameters. The reason for this quanti- zation has never been fully explained. Scott (1972) correctly surmized that

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sitions at high nebula temperatures. Fig. 2 shows abundances of moderately volatiles as a function of Ni content at a nebular pressure of 10-6 atm. At the nebular temperature yielding the observed IVB Ni con- tent of 17%, equilibrium concentrations of most moder- ately volatiles are 1-2 orders of magnitudes lower than observed. Particularly striking is the CI nor- malized SbIGa ratio which is about 0.01 in equilibrium metal, but Q1.7 in IVB. In contrast with the situa- tion near their 50% condensation temperatures Sb is more volatile than Ga at high temperatures.

The mean compositions of iron meteorite groups needed for diagrams such as Fig. 1 can in principle be obtained by averaging available data. However, in 'groups that formed by fractional crystallization some elements show large ranges (e.g., Ir a factor of 5000

"; U,

C 2 10': r

H U 0 - % +

; 0 - e - z

g lo-=: - .

1200 1100 loo0 900 800 900 in IIAB). 1 53% condenvll~on temperature ( K)

For such groups Scott's (1977) suggestion allows a much more precise estimate: the solidlliquid

g &Id i ! 9

- ' P e : 0 -

- A

A i - :

- A -

! - A -

- : :

- p~2=10-~otm A

+ H . 8 nIAB - o IVA o i - A I V B :

-

, , , , , , , , .A;

MODERATELY VOLATILE ELEMENTS I N IRON METEORITES

Wai C.Pl. e t a l .

f e r e n c e s between groups , b u t could n o t e x p l a i n why G a and G e behaved d i f f e r e n t l y from o t h e r v o l a t i l e s such as As and Sb. The new condensa t ion tempera- t u r e f o r Ga now makes t h e matter c l e a r : G a and Ge

w i t h i n groups r e f l e c t k v a l u e s n e a r u n i t y . The -- combination of t h e s e two f a c t o r s produces t h e

q u a n t i z a t i o n .

71 75 80 8.5 90 95 10.0 105 REFERENCES: B a t t a t D. e t a l . (1974) JCS Faraday I 4 Ni (%I 70, 2280. ; Greenland L.P. (1970) Amer. Minera l . 55,

455.; K e l l y W.R. and ~ a r i m e r 7 . w . (1977) GCA 41, 93. ; S c o t t E.R.D. (1972) a 36, 1205.; S c o t t E.R.D. (1977) Minera l . Mag. 9, 265.; S c o t t E.R.D. and Wasson - J . T . (1975) Rev. Geophys. Space Phys. 13, 527.; Wai C.M. and Wasson J .T. (1977) EPSL 36, 1.; Wasson J .T . (1974) M e t e o r i t e s , S p r i n g e r . ; Wasson J.T. and Wai C.M. (1976) Nature 261, 114.

Tab le 1. Equ i l ib r ium 50% condensa t ion t empera tu re s of N i , Co, Fe and 1 0 mod- e r a t e l y v o l a t i l e e lements having a p p r e c i a b l e a f f i n i t i e s f o r Fe-Ni.

Major Condensed Host A c t i v i t y Condensat ion temp. (K) Element g a s s p e c i e s phase c o e f f . 10-4 a t m 10-6 a t m

N i N i N i Fe-Ni 1 1349 1197 Co Co Co Fe-Ni 1 1348 1195 Fe Fe Fe Fe-Ni 1 1336 1185 Pd Pd Pd Fe-Ni 0.5 1334 1175 Cr C r C r Fe-Ni 2 1277 1137 P PN P Fe-Ni 10-5 1290 1120 Au Au Au Fe-Ni 5 1230 1085 A s As As Fe-Ni 10-I 1135 1000 Cu Cu Cu Fe-Ni 5 1118 990 Sb Sb Sb Fe-Ni 1 910 805 G a GaOH G a Fe-Ni 10-1 905 763 G e GeO G e Fe-Ni 1 812 684 S H2S FeS FeS 1 648 648

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