Nebular condensation of moderately volatile elements and their abundances in ordinary chondrites

Earth and Planetary Science Letters, 36 (1977) 1 - 1 3

1

© Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands

[41

NEBULAR CONDENSATION OF MODERATELY VOLATILE ELEMENTS AND T H E I R A B U N D A N C E S IN O R D I N A R Y C H O N D R I T E S C H I E N M. W A I I a n d J O H N T. W A S S O N

Institute o f Geophysics and Planetary Physics, Department o f Chemistry, and Department o f Earth and Space Sciences, University of California, Los Angeles, Calif. 90024 (USA) Received July 13, 1976 Revised version received February 7, 1977

The condensation reactions of seventeen moderately volatile elements in a cooling solar nebula were investigated, and condensation temperatures reported for six previously unstudied elements (F, Li, As, Se, Sb and Te). Literature condensation temperatures were confirmed for seven elements (P, Na, S, Mn, Cu, Ga, and Au) and significant differences found for four elements (Zn, Ge, Ag and Sn). A strong correlation is observed between ordinary-chondrite/Clehondrite abundance ratios and ideal-solution condensation temperatures, and the relationship is strengthened when condensation temperatures for seven elements are recalculated to include estimated activity coefficients. These results can be understood in terms of a model in which volatiles are lost as gases prior to condensation or as finely divided solids that are incompletely agglomerated, and the condensation/agglomeration efficiency gradually decreases as a function of time. A strong correlation between CM/CI chondrite abundance ratios and condensation temperature is also observed, but the decrease by a factor of 2 in the abundance ratios is appreciably less than the factor of 9 observed in ordinary chondrites. If the volatile-loss model is correct, the fractionation process in the nebular region where the CM chondrites formed was less efficient than that at the ordinary chondrite formation location.

1. Introduction We define t h e m o d e r a t e l y volatile e l e m e n t s in o r d i n a r y c h o n d r i t e s to b e e l e m e n t s m o r e volatile than iron having ordinary-chondrite/CI-chondrite * ratios < 0 . 9 a n d h a v i n g t h e same a b u n d a n c e in t y p e 3 a n d t y p e 5-6 o r d i n a r y c h o n d r i t e s t o w i t h i n a factor o f 2; Fig. 1 s h o w s t h e o r d i n a r y - c h o n d r i t e / C I ratios for 19 e l e m e n t s . D a t a for e l e m e n t s o t h e r t h a n A u are f r o m Wasson a n d C h o u [2]. Whereas A n d e r s [3] a n d Larimer a n d A n d e r s [4] suggested t h a t o r d i n a r y c h o n d r i t e / C l a b u n d a n c e ratios o f a s u b s e t (Cu-Sn) o f t h e s e e l e m e n t s t e n d e d t o cluster n e a r 0 . 2 5 , Wasson 1 On sabbatical leave 1975-76 from the Department of Chemistry, University of Idaho, Moscow, Idaho 83843, U.S.A. • The chondrite class symbols proposed by Wasson [ 1 ] are used in this paper; CI is synonymous with C1 or type-I carbonaceous chondrite, CM refers to that portion of C2 or type-II carbonaceous chondrites having high H20 contents and small (<0.4 mm) chondrule diameters.

t.o~i% tt~,~ ~ .~. o.oo ~ o6ocA 0 . 5 0 ~ 04o ~_o o3o ~ 0.20~o ~

o ,~o ~

~ It

l • " o

H3

H5-6 L3 L5-6

I

< °°'~° K ' Ao' Rb' l~ M,IN* P

A As Cu Sb F

G(I Ag Se Ge S Sn "re Zn

Fig. 1. Averaged literature ordinary-chondrite/CI-chondrite abundance ratios for 19 moderately volatile elements. Values are plotted separately for type-3 (primitive) and type 5-6 (recrystallized) members of two large groups of ordinary chondrites. Elements are arranged in order of decreasing abundance ratio. Anders [4,5 ] argues that the abundance ratios of the 9 elements Cu-Sn cluster near 0.25, whereas it appears to us that these are simply the central segment of a continuously diminishing sequence.

and Chou [2] argued that these were but a small portion of a continuous variation from 0.9 to 0.1, and found no evidence for clustering near any particular value. Anders [3] attributed the apparent clustering of abundance ratios near 0.25 to the mixing of 1 part volatile-rich matrix with 3 parts volatile-poor chondrules. Wasson and Chou [2] proposed that the continuous variation in abundance of moderately volatile elements resulted from a gradual separation of nebular solids and gases during a period in which temperature was decreasing, and, as a result, that the observed abundance ratios were correlated with the condensation temperatures of the elements. Anders [5] attempted to show that the WassonChou model was incorrect on the basis of two arguments; (1) calculated condensation temperatures did not fall in the sequence predicted on the basis of abundance ratios; and (2) the ordinary chondrite data were taken from many sources, and the range in abundance ratios could result from random errors associated with these data. Anders' [5] second point is not well taken. Although the data of Wasson and Chou [2] shown in Fig. 1 are from many sources, there is no question that abundance ratios of some well-determined moderately volatile elements fall significantly below 0.25 (Zn, Te). These can only be explained in ad hoc ("complexities in condensation behavior") fashion in Anders' model. Although some details of Fig. 1 will change as more accurate analyses become available, it is as likely that evidence for a "plateau" near 0.25 will become weaker as that it will become stronger. In this paper we report the investigation of the more serious of Anders' [5] criticisms, viz., that condensation temperatures do not fall into the sequence predicted by Wasson and Chou. To this end we have investigated the nebular equilibria determining the condensation and distribution of 17 moderately volatile elements for which adequate thermodynamic data are available. These include 16 of the 18 elements listed by Wasson and Chou and Au, which has properties justifying its designation as a moderately volatile element. 2. Method Details regarding the method of calculation of nebular equilibrium processes will be described else-

where (Wai and Wasson, in preparation); here we give only a brief summary. The approach is similar to that of Larimer [6], but we have calculated equilibria from free energy data at fixed temperature intervals. Larimer fitted equilibria data to the equations of the sort log K = A / T + B. Our approach should be slightly more exact, but is not expected to lead to significant differences in calculated equilibrium relationships. Elemental abundances were taken from Cameron's [7] compilation; although recent CI data indicate a need for minor revisions in some of these values, the effect on our condensation calculations is negligible. The 50% condensation temperature of a trace element that condenses in solid solution is independent of the relative abundance of the element. The nebula region where the ordinary chondrites formed is assumed to have a total pressure -- pH 2 of 10 - 4 t o 10 - 6 atm, and to initially have had a solar composition. Observed fractionations in ordinary chondrites (e.g., Ir from Ni [8] appear to demand that meteoritic solids formed by condensation in a cooling nebula that was initially hot enough to vaporize essentially all protosolar solid matter. All elements discussed in this paper are more volatile than Fe. Condensation of Fe-Ni, forsterite (Mg2 SiO4) and pyroxene (MgSiO3) occurs at about the same temperature [9], ~1340 K at l 0 - 4 a t m and 1180 K at 10 -6 atm. Most trace elements condensing at lower temperatures are expected to form solid solutions with Fe-Ni or silicates. After troflite (FeS) starts to form at 685 K, chalcophile elements may exist as solid solutions in it. Following the condensation of magnesian silicates, the ratio ofpH20/pH 2 in the nebula is about 4.2 X 10 - 4 . This ratio was assumed to be constant throughout the temperature range during which moderately volatile trace elements condense. The pH2S/pH 2 ratio at temperatures above 685 K was taken to be the solar S/H 2 ratio, 3.1 × 10 -s. Listed in Table 1 are 50% condensation temperatures at pressures of 10 -6 and 10 -4 atm of Fe and 17 moderately volatile elements, the host mineral phases with which they form solid solutions and comparison values from the literature. Ideal solution behavior was assumed. In a later section we discuss corrections needed for some elements that do not form ideal solutions. Sources of thermodynamic data are given in Appendix 1.

TABLE 1 Equilibrium nebular 50% condensation temperatures of moderately volatile elements and Fe. Activity coefficients taken to be unity Element

Fe Au P Li Mn Cu As Ga Ag Na Sb F Ge Sn Se Te Zn S

Major gaseous form

Fe Au PN Li Mn Cu As Ga Ag Na Sb HF GeO SnS H2Se Te Zn H2S

Condensed form

Host phase

Condensation temperature (K) this work

Fe Au Fe3P Li 2 SiO 3 Mn2SiO 4 Cu As Ga Ag NaA1Si30 8 3 Sb Ca5(PO4)3F 4 Ge Sn FeSe Te ZnS FeS

Fe-Ni Fe-Ni Fe3P 2 MgSiO 3 Mg2SiO 4 Fe-Ni Fe-Ni Fe-Ni Fe-Ni CaAI2Si20 8 Fe-Ni phosphates Fe-Ni Fe-Ni FeS Fe-Ni FeS FeS

literature 1

10 - 6 atm

10 - 4 atm

10 - 4 atm

ref.

1185 1127 1125 1091 1078 1035 911 875 879 874 805 780 687 625 684 600 668 648

1336 1284 1267 1225 1190 1170 1050 997 993 982 910 855 812 720 684 680 684 648

1350 1290 1267 1215 1175 1000 860 1090 1130 1060 680 660

1 1 3 2 1 1 1 1 2 2 2 1

2

S

6 7

1 Literature references: (1) Grossman and Larimer [9]; (2) Larimer [60]; (3) Grossman and Olsen [13]. 2 The Fe3P will have an appreciable Ni content, but the effect on the condensation temperature should be minor; Grossman and Olsen [13] reported the P condensation temperature to be 1416 K at 10 - 3 arm which we have confirmed and recalculated to 10 - 4 atm. 3 We have assumed an NaAISi30 8 activity of 0.4 when 50% of the Na has condensed. 4 We have assumed a Cas(PO4)3F activity of 0.1 when 50% of the F has condensed. 5 The 10 - 4 atm Ge condensation temperature of 1270 K shown in fig. 2 of Grossman and Larimer [9] appears to be misplotted. 6 Larimer [60] reported a condensation temperature of 1080 K for Zn as Zn2SiO 4 which we cannot confirm. 7 Reported to be 700 K at start of condensation, and recalculated by us to be 660 K at 50% condensation. If Cameron's [ 10] data is used, FeS is stable at 700 K.

3. Condensation calculations

calculate t h e c o n d e n s a t i o n o f Rb. A l t h o u g h a d e q u a t e data a p p e a r t o b e available for Na a n d K, we believe t h a t o u r c a l c u l a t i o n s yield c o n d e n s a t i o n t e m p e r a tures 1 0 0 - 2 0 0 ° t o o low, a n d here r e p o r t o n l y o u r results for Na.

N e b u l a r c o n d e n s a t i o n o f s o m e m o d e r a t e l y volatile e l e m e n t s are d e s c r i b e d b e l o w for t h o s e e l e m e n t s for w h i c h o u r c a l c u l a t i o n s differ f r o m t h o s e in t h e literature or w h o s e c o n d e n s a t i o n b e h a v i o r n o t b e e n discussed previously. T h e r e m a i n i n g e l e m e n t s will b e

Iron and nickel. We find slightly l o w e r Fe c o n d e n s a -

discussed in a l a t e r review p a p e r . T h e e l e m e n t s are listed in o r d e r o f d e c r e a s i n g a b u n d a n c e in o r d i n a r y c h o n d r i t e s relative t o CI c h o n d r i t e s as given b y Wasson a n d C h o u [2]. Because o f t h e i r i m p o r t a n c e as h o s t phases, c o n d e n s a t i o n c a l c u l a t i o n s o f Fe a n d S are discussed first. O f t h e 19 e l e m e n t s i n c l u d e d in Fig. 1, adequate thermodynamic data could not be found to

t i o n t e m p e r a t u r e s t h a n G r o s s m a n a n d L a r i m e r [9] because o f o u r use o f t h e C a m e r o n [7] a b u n d a n c e s , w h i c h give a n F e / H r a t i o a b o u t 2 5 % l o w e r t h a n t h e C a m e r o n [10] t a b l e t h e y used. We also t o o k i n t o c o n s i d e r a t i o n t h e a l l o y i n g e f f e c t o f Ni, w h i c h raises t h e c o n d e n s a t i o n t e m p e r a t u r e 3 - 5 ° relative t o t h a t for p u r e Fe. Because o f t h e i r i m p o r t a n t r e f e r e n c e

value we give our detailed results: pH2 10 -3 10-4 arm atm

10-s atm

10-6 atm

10% condensation 50% condensation

1277 K 1256 K

1203 K 1185 K

1453 K 1426 K

1360 K 1336 K

Sulfur. Equilibrium condensation of S occurs by the reaction H2S(g ) + Fe(s) = FeS(s) + H2(g)

(1)

Condensation begins at 685 K and is independent of nebular pressure; 50% condensation is achieved at 648 K. The formation of troilite must occur on the surfaces of metal grains. A difficult question is whether this reaction will occur at an appreciable rate after the surfaces of metal grains have been coated with FeS [2,4,11]. As pointed out by Wasson and Wai [12], a trace metal that condenses by dissolution in a host phase will condense homogeneously at slightly lower temperatures if kinetic factors prevent its reaction with or diffusion into previously condensed solids, but if S does not react with condensed phases, it cannot condense until H2S(s) becomes a stable condensate at about 200-250 K. This could play a role in determining the relative amounts in a condensate of S and other elements that have similar equilibrium condensation temperatures. Wasson and Wai suggest that these factors have increased the S fibundance in the IVA iron meteorites, and there is some evidence that they have increased the S abundance in ordinary chondrites as well.

Phosphorus. The major gaseous species of P in the nebula is PN(g); PS(g), PO(g), PH2(g), P2(g), and P(g) are also present in significant amounts. Condensation of PN(g) as phosphate, as phosphide, and as an alloy with Fe-Ni have been investigated. Grossman and Olsen [13] reported that at 10-3atm P would condense as Fe 3P at 1416 K. Allowance for nonideality (see following section) shows that condensation of P in Fe-Ni takes place at higher temperatures by the reaction: PN(g) = P(s) + ½N2(g)

(2)

If equilibrium between gases and solids is maintained below 900 K at 10 -4 atm, P will leave the metal phase

to form phosphates, whitlockite Caa(PO4)2 and farringtonite Mga(PO4)2. Formation of FeaP also becomes spontaneous at 900 K if the metal contains 1.2 at.% of P.

Lithium. The major gaseous form of Li is Li(g). The abundances of LiO(g) and Li20(g ) are ( 1 0 -9 relative to Li(g) at 1200 K and 10 -4 arm total pressure. Since the ionic radius of Li+ (0.68 A) is close to Mg2+ (0.65 .&), substitution of Li for Mg in silicates is expected. Shima and Honda [14] showed that Li in ordinary chondrites is in a phase soluble in HCI, probably olivine, whereas 'the other alkali metals are in other phases. Thermodynamic data are available for Li2SiO3,but not Li4SiO4.Since at high temperature the Li/Mg ratio is probably about same in pyroxene and olivine, the condensation of Li was calculated from the reaction: Li(g) + MgSiOa(s ) + -~H20(g) = ½Li2SiOa(s )

(3)

+ ½Mg2SiO4(s) + ½H2(g)

Arsenic. At 1 atm As vapor consists primarily of As4(g ). At a nebular pressure of l0 -4 atm, the major gaseous species is As(g) above 1000 K. Other gaseous species As2, AsN, AsO, and AsH 3 are present only in small amounts (~<1%). Thermodynamic data for gaseous and solid sulfides and solid iron arsenides are not available. However, it seems safe to assume that the first As condensate is as a solid solution with Fe-Ni, i.e., the condensation process is As(g) = As(s). Silver. Thermodynamic data for gaseous oxides or sulfides of Ag are not available. Our calculation was based on the assumption that Ag(g) is the major gaseous species in the nebula and that Ag(s) condenses in solid solution with Fe-Ni metal. The condensation of Ag2S(s ) occurs below 685 K; if it forms ideal solution with FeS, an appreciable fraction of Ag may enter into the sulphide phase below this temperature. Larimer's [60] condensation temperature is about 130 ° lower than ours. Sodium. The condensation of Na was calculated according to the equilibrium: Na(g) + ½CaA12Si208(s ) + ~!MgSiOa(s) + ½H20(g) = NaA1SiaOa(s ) + ½CaMgSi206(s) + ~Mg2SiO4(s ) + ½H2(g)

(4)

We assumed an activity of 0.4 for albite, NaAlSi30 s, in plagioclase at the 50% condensation temperature based on solar abundances and Grossman's [16] estimate that about 58% of Ca is present as anorthite, CaA12Si20 s [16]. Similar calculations for K gave a 50% condensation temperature about 100 ° higher than Na. Our calculated 50% condensation temperatures for Na and K agree with the lower and upper limits of the temperature range (980-1080 K) for Na and K silicates reported by Larimer [60]. We cannot reproduce the higher-temperature range (1060-1100 K) for plagioclase condensation reported by Grossman and Larimer [9].

Antimony. Sb, As, and P belong to the same group in the periodic table. Sb vapor consists mainly of tetramer Sb4(g ) at 1 atm pressure, but above 800 K at 10 -4 atm Sb(g) is the major gaseous species. The abundances of SbN(g) and SbO(g) are <10 -6 relative to Sb(g) at 900 K and 10 -4 atm total pressure. As with As, thermodynamic data for a number of potentially important Sb compounds are not available. However, it is probably safe to assume that the equilibrium Sb(g) = Sb(s) is the correct nebular condensation reaction. Fluorine. The stable gaseous form of F in the nebula is HF(g). Van Schmus and Ribbe [17] found F in meteoritic chloroapatite. D. Burnett (private communication) finds a sizable enrichment in an Allende Ca-Al-rich inclusion, where it probably substitutes for C1 in sodalite. Our calculation is based on fluorapatite, since we had thermodynamic data for this phase. Condensation as sodalite may occur at slightly higher temperatures. The condensation of F was calculated from the equilibrium: HF(g) + 3Caa(PO4)2 + ½CaMgSi20 6 + ½Mg2SiO4

(5)

= Cas(PO4)aF + {MgSiO a + ½H20(g) According to our calculation, whitlockite Ca3(PO4)2 is a stable phase below 900 K under equilibrium conditions. The 50% condensation of pure fluorapatite takes place at 800 K at a total pressure of 10 -4 atm, and at 855 K if we assume fluorapatite forms a solid solution with chloroapatite or other minerals such that the activity is 0.2 when 50% condensation has occurred.

Germanium. Ge, like other Group IV elements C and Si, forms a highly stable monoxide gas. The pGeO/ pGe ratio is 1.0 × 106 at 1000 K and 5.3 X 10 a at 800 K. The gaseous sulfide GeS(g) also exists in the nebula, but its partial pressure is only about 15% that of GeO(g) in the temperature range where Ge condenses. The condensation of GeO(g) in solid solution with Fe-Ni metal was calculated according to the process: GeO(g) + H2(g) = Ge(s) + HzO(g)

(6)

assuming ideal solution. Larimer [6] assumed that Ge(g) was the main gaseous species and hence greatly over-estimated the Ge condensation temperature. Kelly and Larimer [18] also noted that GeO(g) is the most stable gaseous species and stated that this depresses the condensation temperature for Ge about 100 ° over that given by Grossman and Larimer ([9], their fig. 2 gives a 50% condensation temperature at 10 -4 atm of 1270 K that is very high and possibly misplotted; table 5 gives a 90% condensation temperature at 10 - s atm of ~1000 K). Since Mg2GeO 4 forms a solid solution with forsterite, Mg2Si04 [19], substitution of Ge for Si in silicates is expected. The condensation of GeO(g) as Mg2C-eO4 was calculated according to the equilibrium: GeO(g) + 2Mg2SiO4(s ) + H20(g ) = 2MgSiOa(s )

(7)

+ Mg2GeO4(s) + H2(g) assuming that Mg2Ge04 forms an ideal solution with Mg2SiO4. The 50% condensation temperature would be 696 K at 10 -4 atm if this were the first Ge condensation reaction, i.e., if Ge had not already condensed as the metal. If metal and silicate remain in equilibrium, the amount of Ge in the silicate becomes greater than that in the metal below 600 K [12].

Selenium. It appears that the predominant gaseous species of Se is H2Se. Our calculation was based on AG~ values of H2Se obtained from the high-temperature thermodynamic functions given by Kelley [20] and AH~f,298and S~98 data given by Wagman et al. [21]. The calculated values for H2Se agree with those reported by Rowling and Toguri [22] from the study of H2(g ) + Se(1) = H2Se(g) equilibrium. Although other literature sources gave much higher AG~ values that make Se(g) more abundant than H2Se(g), we

believe that the above sources are more reliable. The condensation of Se was calculated from the equilibrium:

H~Se(g) + Fe(s) = FeSe(s) + H2(g)

(8)

This process, like the condensation of S, is independent of total pressure. The 50% condensation temperature of Se was obtained based on the assumption that FeSe forms an ideal solution with FeS.

Tin. The relative abundances of Sn(g) and SnS(g) were evaluated from the equilibrium: Sn(g) + HzS(g) = SnS(g) + H2(g )

(9)

From the A ~ values of SnS(s) [23] and the vapor pressure data of SnS(s) [24], we obtained AG~ for SnS(g). The pSnS/pSn ratio is 1.7 X 104 at 1000 K and 3.7 × 10 6 at 800 K. Thermodynamic data for gaseous oxides of Sn are not available, but it seems safe to assume that SnS(g) is the dominant gaseous species. Condensation of SnS(g) as its pure sulfide and as metal in solid solution with Fe-Ni were calculated from reaction (10) and (I 1), respectively. SnS(g) = SnS(s)

(10)

SnS(g) + H2(g) = Sn(s) + H2S(g)

(11)

Assuming Sn forms ideal solution in the metal, the 50% condensation temperature of Sn at 10 -4 atm is 720 K. Larimer [60] reported a Sn condensation temperature of 1060 K, presumably because he assumed Sn(g) to be the predominant gaseous species. Under ideal solution conditions, the fraction of Sn in the sulfide phase increases with decreasing temperature, and most Sn is in this phase below 500 K.

Zinc. In the temperature range 998-1083 K the gaseous species in equilibrium with ZnS (sphalerite) consists mainly of Zn(g) and S2(g) indicating that ZnS(g) is not stable [25]. The nebular abundance of ZnO(g) is also negligible relative to Zn(g). The condensation of Zn(g) as sphalerite ZnS(sph) was calculated according to the equilibrium: Zn(g) + H2S(g ) = ZnS(sph) + H2(g)

(12)

Below 685 K, H2S(g) is converted to FeS(s) and the condensation of sphalerite may be expressed by: Zn(g) + FeS(s) = ZnS(sph) + Fe(s)

(13)

If we assume that ZnS forms an ideal solution with FeS, the 50% condensation temperature of Zn is 684 K a t 10 -4 atm and 668 K at 10 -6 atm. The condensation of pure sphalerite takes place at 660 K at 10 - 4 atm and at 605 K at 10 -6 atm. The wurtzite form of ZnS is unstable relative to sphalerite below 1293 K, and was of no importance for our calculations. Note that, whereas the equilibrium condensation of troilite FeS is independent of nebular pressure, the condensation of ZnS is pressure-dependent. At a higher nebula pressure,e.g. 10 -3 atm, condensation of pure sphalerite (about 710 K at 10 -3 atm) will take place before the formation of FeS. Thus, detailed studies of the distribution of Zn in primitive meteorites could be used to infer nebular pressures. Zn can also condense in silicates. The condensation of Zn(g) as Zn2SiO4(s) was calculated from the equilibrium: Zn(g) + MgSiO3(s ) + n20(g ) = ~Zn2SiO4(s)

(14)

+ ½Mg2SiO4(s) + H2(g ) If Zn2SiO 4 forms an ideal solution with Mg2SiO4 at 10 - 4 atm, about 4%Zn would have condensed in the silicate at 685 K. However, after FeS or ZnS form most Zn should move into these phases. Latimer [6,60] gives a very high solid-solution condensation temperature (1080 K at 10 -4 atm) for Zn2SiO4, indicating that Zn would condense in this form. As noted in Table 1, our ZnS calculations agree with those of Latimer and Anders.

Tellurium. Vapor in equilibrium with Te metal is mainly diatomic below 1261 K, the 1-atm boiling point, but under nebular conditions dissociation of Te2(g) to Te(g) occurs above 600 K. In the temperature range where Te condenses Te(g) is more abundant than H2Te(g). The ratio ofpH2Te/pTe at 700 K is about 8 × 10 -2. The condensation was calculated from the process Te(g) = Te(s) with the assumption that the condensed phase is in solid solution in Fe-Ni. 4. Effects of nonideality on condensation temperatures

Fig. 2 illustrates the relationship between abundances of moderately volatile elements in H5-6 and 1.5-6 chondrites relative to those in CI carbonaceous

I0 08

P I'LI(~

AS+

eMn

06 05 04

(~Na

~Cu

~Ag'~sb~F

$Go

0.3

'('o)

J #se

Ge

O.2

Sn,lzs~

H5-6

9 0.1 1.0 "= 0.8

n**Te

10.4 atm 'Au'4"

I i(~

, , ,C,No,

'

OMn

,

,

,

(b)

os 05

¢pCu

8 04

{s0{F

03

se~ s~

R <

0.2

0.1

I

I

I

I

I

I

1 4 0 0 1300 1200 50%

1.0 0.8

s°t

Ge{ L5-6 10-4 a t m t

I

[

I

I

I100 I 0 0 0 9 0 0

I

I

Te I

I

800

700

600

Condensation Temperature (K)

i °-

'

'

'

'

. . . .

'

(C)

0.6 0.5 0.4

ooi

0.3

"StlSn

"1o

0.2 H5-6

U o

_o OC ¢: o

0.1 1.0 0.8

zo{ / ÷

Io-e atm J

I

I,I

~

1

~

I

0.6 0.5 0.4

I

I

~

I

I

t

C,No

Ao ?Li ~o M. PC' Cu

TM

~ (d)

As{

chondrites and their 50% condensation temperatures at 10 - 4 and 10 - 6 atm. The abundance ratios were taken from Wasson and Chou [2], except Au CI data from Chou et al. [26] and ordinary chondrite data from our unpublished results. Abundance ratios are plotted on a logarithmic scale because this gives a more useful representation o f the relative uncertainties and also improves the visual resolution of the more volatile elements. The pattern is similar if abundances ratios are plotted on a linear scale. Fig. 2 shows that logarithmic abundance ratios and 50% condensation temperatures are correlated. Wasson and Chou [2] reviewed various models which could account for such a relationship and concluded that the most plausible model involved a continuous loss of nebular volatiles from the ordinary chondrite formation region during a period of falling temperatures when condensation of moderately elements was taking place. The correlation between abundance ratio and condensation temperature is not perfect. To facilitate the discussion we have arbitrarily drawn a least-squares reference line on the upper portion of Fig. 2c; all points were given equal weight except As and Ag 1/2 weight, Sn 1/10 weight and Na was discarded. That the line is straight is also arbitrary; essentially any curve having a continuously negative slope is consistent with the Wasson-Chou model. Although part o f the scatter could result from selective agglomeration of condensed phases, we will assume that this was a negligible effect. A more important factor appears to be our assumption that solutions are ideal, i.e., that activity coefficients can be set equal to unity. The activity coefficient of a trace element E in a solution is related to its partial molar free energy (AGE) by the equation AG E = R T l n XETE,where X E and "}'Eare the mole fraction and activity coefficient o f E, respectively. Trace elements forming stable compounds with their host often have ~'E

03 ~s

0.2 L5-6 OI

I0-e t

I

1300 1200 50°/o

t

ts °

G,~ zo{

atm I

i

I

I

I

J

I100 I000 900 Condensotion

[

I

I

~TE

I

I

I

800 700 600 500

Temperoture

(K)

Fig. 2. The ratios of abundances of moderately volatile elements in ordinary chondrites to those in CI chondrites decrease with decreasing condensation temperature. Although data are shown only for the well-equilibrated types 5 and 6, mean abundance ratios are not significantly different in types 3 and 4. The 50% condensation temperatures are calculated at nebular pressures = pH 2 of (a, b) 10 - 4 atm and (c,d) 10 - 6 atm.

values less than unity if their atomic radii are similar to those of the host element. For example, P forms very stable Fe-Ni phosphides and its activity coefficient in Fe metal is around 5 X 10 -6 at 1273 K in the concentration range 0.2-1.0% P [27]. The presence of Ni and other alloying elements will probably increase the activity coefficient, leading to a value near 10 - s (+ a factor of 10). Like its congener P, As also forms stable compounds with Fe and Ni; the arsenides FeAs2 and NiAs2 exist in nature. The atomic radius of As is 1.48 A, slightly larger than that of Fe, 1.27 ,~ [28]. It seems likely that the activity coefficient of As in Fe-Ni is less than unity, but not nearly as small as that of P. An estimate of 10 -1 at about 1000-1100 K should be good to within an order of magnitude. Sb forms compounds with Fe but its atomic radius is ~1.66 A; we will not attempt to estimate its activity coefficient. According to Hultgren et al. [29], at infinite dilution, the activity coefficient of Cu in Ni at 973 K is about 6. Although we have not found data for 3'cu in Fe, the Ni value should be applicable, since Cu and Ni both have face-centered cubic structures, and most of the Fe-Ni of ordinary chondrites has the same structure above about 900 K. A value of 5 is probably good to within a factor of 2. The activity coefficient of Au (also face-centered cubic) in Ni at infinite dilution at I 150 K is about 10 [29]. No data are available for Fe, but an estimate of 5 nehr 1200 K should be good to within a factor of 2. Ag is a congener of Cu and Au, and intermediate between them in atomic number. It seems reasonable to estimate an activity coefficient of Ag of 5 and expect it to be valid ~thin a factor of 3, even though the bulk of the Fe-Ni may be body-centered cubic when Ag condenses at a nebular pressure ~<10-6 atm. Both Ga and Ge form weakly stable compounds with Fe, and have atomic radii near 1.4 A. Hultgren et al. [29] report an activity coefficient for Ga in Ni at 1223 K and 0.37 mole fraction Ga of 3 X 10 -a, but give no values at lower Ga concentrations or for the Ga-Fe system. It seems certain that the Ga activity coefficient in Fe-Ni in the temperature range 9 5 0 1100 K is significantly less than unity. We have used 10 -1 which should be valid to within about an order of magnitude. It is probable that the activity coeffi-

cient of Ge is also below unity, but we have not found enough information to justify an estimation. The high abundance ratio of Na implies a condensation temperature ~150 ° higher than our estimate. At 50% condensation the plagioclase is 40% albite, and it is rather certain that the albite activity coefficient is unity to within +30%. Thus the discrepancy does not result from nonideal solution properties, but must be explained in some other fashion. The precise host phase for Na during condensation is not known. In ordinary chondrites Na is often found in high concentrations in the glassy portion of chondrules [30], but chondrules probably formed by remelting, and the distribution of Na may have been altered in this process. Blander and Fuchs [31 ] note that condensation of Na into an Al-rich liquid occurs 1 0 0 200 ° higher than condensation of albite; perhaps chondrule formation produced silicate liquids that served as the host phase for Na. Two elements, Zn and Te, have abundance ratios significantly below that of S. The atomic radius of Te is 1.6 .~ and it seems likely that the activity coefficient for Te dissolved in Fe-Ni is appreciably greater than unity, and that the condensation temperature is distinctly lower than that of S. However, there is too little information to allow an estimation of the activity coefficient. Thermodynamic data for Zn provide greater constraints. As noted earlier, whereas the equilibrium condensation of S is independent of temperature, the Zn condensation temperature decreases with decreasing nebular pressure. However, even at 10 -6 atm the condensation of ZnS in ideal solid solution with FeS occurs at the same temperature as FeS formation, whereas the Zn abundance ratio is only half that of S. Sphalerite is a common constituent of meteorites having high Zn contents (e.g., group lAB and IIICD iron meteorites [32]) and occurs as a common accessory in FeS nodules in lAB irons [33]. The wholerock Zn/S ratios in these meteorites are not known precisely, but are probably less than the CI ratio, 1.2 X 10 -3 atoms/atom. If we assume a CI Zn/S ratio and that half the Zn is present as sphalerite exsolved from FeS during cooling, the activity of ZnS is unity when the mole fraction is 6 X 10-4; thus at the final equilibration temperature that is not known, but surely >400 K, 7ZnS is estimated to be 1.7 X 103. It seems likely that 7ZnS is near 1000 (+ a factor of 10)

at about 600 K, in which case Zn will condense as sphalerite rather than in solid solution with FeS. In summary, with the exceptions o f Au, Ga and Ge, application o f the activity coefficients inferred from diverse sources tends to change the condensation temperatures in a direction which increases the degree of correlations in Fig. 1. We have presented data on laboratory or natural systems that allow reasonably precise estimates o f the activity coefficients of four elements, P, Au, Cu and Zn. It also appears safe to assume that the activity coefficient of Ag in Fe-Ni is intermediate between those o f Au and Cu, and that As and Ga have activity coefficients appreciably less than unity. The activity coefficients are estimated for the temperatures at which 50% condensation would occur at nebular pressures o f 10 - 4 to 10 - 6 atm; no attempt has been made to estimate their dependence on temperature, but the estimated errors are thought to be substantially greater than the variations with temperature. Table 2 also lists revised 10 -4- and 10-6-atm 50% condensation temperatures for these seven elements based on the estimated activity coefficients. We believe that these condensation temperatures are significantly more accurate than those given in Table 1, and we recommend their use in preference to the latter. Fig. 3 shows the relationship between logarithms

I.O

I

I"l~

0.8 0.6 0.5

Pe

M.~.

I

I

]

i

I

=

SNo

I

~t

F'r....

H 5 6

=.

'

/

lls, G

Gei"S~ i" ~ S~t J/

atrn I

/

I

I

i

J

I

I

I

I

i

1400 1300 1200 I100 I 0 0 0 9 0 0 1.0 0.8

I

/Te.

I0 I

o

I

/

l" "I~I'S~D 1

-4

0.1

(O)I

=

"'-...

#PGa

I 03 E 0.2

I

~ cu

0.4

g

i

AuT Lil~'~l. n As

I

t.Au~ I ". Li

""

0.6 0.5 0.4

I

'

'l

i,,,

I

i

800

II

/ ~Zn[ I i /

700

'l

i00

I(l~)

4, No

÷cu "'.. .

°

03 0.2 H5-6 0.1 1300

IO'e atm t 12/0IO

I

i

I

I

I

I100 I 0 0 0 9 0 0

I

I

800

t

I

700

l

Zn I

600

I

500

5 0 % Condensotion Temperoture (K)

Fig. 3. Correction of P, Au, As, Ga, Cu and Ag in Fe-Ni and ZnS in FeS for nonideality increases the degree of correlation between ordinary chondrite/CI abundance ratios and condensation temperatures. The 10-6-atm condensation temperatures give a better fit than those at 10 - 4 atm, suggesting that the nebular pressure at the ordinary chondrite formation location was nearer the former value.

TABLE 2 50% condensation temperatures of 6 elements for which activity coefficients can be estimated. These condensation temperatures are believed to be more accurate than those in Table 1 Element

P Au As Cu Ga A4g Zn

Solute

P Au As Cu Ga Ag ZnS

Host

Fe-Ni Fe-Ni Fe-Ni Fe-Ni Fe-Ni Fe-Ni FeS

Activity coefficient

10 - s 5 10-1 5 10 -1 5 103

50% condensation temperature 10 - 4 atm

10 - 6 atm

1290 1230 1135 1118 1075 952 660 **

1120 * 1085 1000 990 932 843 605 **

* At 10 - 6 atm 50% condensation of P in Fe-Ni occurs at 1120 K; although this is lower than the Fe3P condensation temperature given in Table 1, 1120 K is the preferred value. See text for details. ** As a result of the high activity coefficient of ZnS in FeS, ZnS condenses at higher temperatures as a pure phase than as a solute in FeS; the listed temperatures are for condensation of pure ZnS.

10 of H-group/C1 abundance ratios and condensation temperatures, including the six revised temperatures from Table 2. Least-squares lines are drawn on both the 10 -4- and 10-6-atm diagrams, and the improved quality of the fit relative to Fig. 2a and c is readily apparent. It is of interest to attempt to determine the nebular pressure at the ordinary chondrite location from our data. We will confine our attention to the H-group data, since these seem to have best preserved the nebular record for siderophilic elements. We will further assume that no fractionations result from differences in the condensation mechanisms of trace and major elements, and that the lines are reasonable approximations of the condensation trajectories in Fig. 3a and b. Detailed inspection shows that there is more deviation from these lifles in Fig. 3a than in Fig. 3b. Note especially the better fit of Ga, Ge and Zn in the 10-6-atm diagram. We infer that the nebular pressure at the formation location of the ordinary chondrites was nearer 10 - 6 atm than 10 - 4 atm. Larimer [34] inferred a formation pressure of ~ 1 0 - 4 atm for the H group. A curious feature of Fig. 3 is the factor of two difference in abundance between S and Zn despite the rather similar, precisely determined condensation temperatures. We suspect that the Wasson-Wai [12] mechanism has resulted in a high S abundance: whereas ZnS undergoes homogeneous condensation, and may form a fine aerosol that escaped agglomeration, S remains in the gas phase as H2S and can continue to react with Fe-Ni over an extended temperature range.

5. Nebular fractionation of moderately volatile elements Wasson and Chou [2] discussed the abundances of moderately volatile elements in ordinary ctaondrites and gave detailed reasons for favoring the volatile-loss model * over the Larimer-Anders [4] two-component model. According to the latter; ordinary chondrites can be understood as a mixture of a high-temperature component (chondrules) from which volatiles have been removed, and a low-temperature component * Wasson and Chou [2] called this a gas-loss model, but volatileloss is a more accurate description since some volatiles may be lost not as gaseous species but as unagglomerated aerosols.

(matrix) which contains significant amount of volafiles, but did not recondense the volatiles "removed" during chondrule formation. The chief arguments against the two component model are: (1) There is no particular tendency for abundance ratios of moderately volatile elements to cluster near 0.25 (Fig. 1), as expected from a mixture of 3 parts high-temperature chondrules with 1 part low-temperature matrix. (2) The accurately determined Zn abundance ratio of 0.106 is not explained by the two-component model. (3) It is kinetically difficult to thoroughly outgas chondrules during brief chondrule forming events. Although Anders [5] states that part of the negative slope in Fig. 1 may reflect "progressively less complete loss from chondrules of the less volatile elements", it is not clear that this is practical. If diffusion is the mode of transport to the chondrule surface, the rate o f diffusion will be controlled by ionic properties rather than volatility. (4) An ad hoc mechanism is needed to prevent the re condensation of volatiles expelled from higher-petrologic type chondrules during a period when equilibration of the matrix of lower-petrologic types with nebular gases is continuing. According to the volatile-loss model, as time passes the mean nebular temperature decreases and the fraction of the initial gas contributing condensed solids to the finally agglomerated chondritic matter also decreases. The two most likely mechanisms for bringing about the loss of volatiles are: (1) a gradual removal of gas from the surface of the nebula by momentum exchange with sotaf wind or with solar photons; or (2) a gradual increase in the fraction of an element that condenses by homogeneous nucleation as a result of a gradual settling of previously condensed matter toward the nebular plane. If, as seems likely, homogeneously condensed phases had very small particle sizes, turbulence could prevent them from settling to the nebular plane and agglomerating there [35,36]. The volatile-loss model predicts a correlation between ordinary-chondrite/CI abundance ratios and the condensation temperature. A single mechanism accounts for the partial loss of all moderately volatile elements. Volatile loss during chondrule formation is not required, but if it occurs, recondensation presents

11 no problems. The chief criticism o f this model is that given by Anders [5], who argued that the elemental ordering in Fig. 1 was not a volatility sequence, but Fig. 3 demonstrates quite clearly that it is. If the volatile-loss model accounts for the ordinary chondrite observations, one would expect evidence for the occurrence of the same processes at other nebular locations. In fact, there is evidence that the amount o f volatile loss at the CM-chondrite formation location is also related to the condensation temperatures. In Fig. 4 are plotted CM/CI abundance ratios for the same set of elements used in Figs. 2 and 3 except F, for which only one CM chondrite value was available. The condensation temperatures are taken from Table 2 for P, Cu, Zn, Ga, As, Ag and Au and Table 1 for the remaining elements. A distinct negative correlation is observed at 10 - 4 and 10 - 6 atm, significant at the >99.9% level. It appears that

J.O 0.8

i

I/

'~

]

I=

P,AulJ~Li ASCII.

I

T

i

I

NO+ g~

I

I

l

0.6 0.5 0.4

CM

I I I I l I i I I I J I I t I 1400 1300 1200 I100 I000 9 0 0 8 0 0 7'00 6 0 0 i

o

Acknowledgements

We are indebted to W.V. Boynton for numerous fruitful discussions, to H. Palme and E. Anders for careful reviews, and to H.J. Chun for her patience. This research was supported by NSF grant DES7422495.

Sn

I0 "4 arm

o

>.,

i

(O)

'~'.s

0.3 0.2

I

the difference in distribution o f these elements between ordinary and carbonaceous chondrites could be one o f degree, i.e., abundance ratios tend to fall monotonically as a function o f condensation temperature in each group, but the total change in the CM group is a factor of 2 whereas that in the ordinary chondrites is a factor of 9. These observations can be understood by the volatile loss model if a smaller fraction o f the CM starting materials escaped prior to Zn condensation or if differences in agglomeration efficiency of different types o f grains were less pronounced at the CM location. Although it seems likely the nebular pressures at the CM location were appreciably less than those at the ordinary chondrite location, no choice between 10 - 4 and 10 - 6 atm can be made because o f the uncertainties in the data. A more detailed discussion o f the relative abilities of the volatile-loss and two-component models to account for volatile fractionations among carbonaceous chondrite groups will be given in a later publication.

0.8 0.6 0.5 0.4

i

a. Mnqt

CuT

I

'

1

r

I

i

I

No Ag #tee4 ~ JtSb GeJtse

i

Appendix

1. Sources

of thermodynamic

data

(b) :

*tt, zt

03 0.2

,i

P

s. CM 10 -6 atm

O.I ~ I i I I I J I ~ I i I ~ I i 13OO 12OO IIOO IOOO 900 800 700 600 500 .50*/. Condensation Temperature (K)

Fig. 4. CM/CI abundance ratios are also correlated with condensation temperatures, but the total change is only a factor of 2 whereas that in H-group chondrites is a factor of 9. It appears that the volatileqoss model can also account for the CM volatile pattern, but that the efficiency of the process was greater at the H-group location. These data distributions offer no basis for choosing between 10 - 4 and 10 - 6 arm pressure.

Gibbs free energies were taken directly from the following sources. JANAF [37]: gaseous Cud, H20, H2S, Lid, Li20, P, PH, PH2, PH3, PN, PO, PS; solid Cud, Cu20, Li20, Li2SiO 3, Mg2SiO4, MgSiO3. Hultgren et al. [38]: gaseous Ag, As, As2, As4, Au, Cu, Fe, Ga, Ge, Li, Mn, Ni, $2, Sb, St)2, Sb4, Se, Se2, Ses, Sn, Te, Te2, Zn. Robie and Waldbaum [23]: solid MnO, SnO2, ZnO, Ag2S, Cu2S, FeS, MnS, SnS, ZnS, Mn2SiO4, MnSiO3, Zn2SiO4, Ca3(PO4)2, Mg3(PO4)2, CaMgSi206, CaA12SiO8, NaAISi308. Faktor and Carasso [39]: solid Gee 2. Frasch and Thurmond [40]: gaseous Ga20. Hildenbrand [41]:gaseous Gee. Jolly and Latimer [42]: gaseous Gee. Komarek [27]: solid Fe3P. Richards [24]: gaseous SnS. Navrotsky [43]: solid Mg2GeO4. Rowling and Toguri [22]: gaseous H2Se. Smith and Chatterji [44]: solid Ga203. For the following species free energies or free energy functions were obtained by combining data from two sources. Enthalpy (H°T-/~298) and entropy (S~r-S~98) data from Kelley [20] and ~/~f,298 and ~298 data from Wagman et al.

12 [21]: gaseous H2Se, H2Te , GeO, GeS, AsN, AsO, ASH3, SbN and SbO. Enthalpy and entropy from Kelley [20] and S~98 data of Parker et al. I45]: solid Cas(PO4)3F. The free energy function (G°T-H°o/T) of Gronvold and Westrum [46] for solid FeSe was extrapolated to higher temperatures, A/-Pf,298 and ~298 taken from Wagman et al. [21] and Gronvold and Westrum [47].

Appendix 2. Sources of CI and CM chondrite data Below are listed mean concentration data and their sources used to obtain the CM/CI abundance ratios shown in Fig. 4. Following the element symbol are the CI concentration, CM concentration, reference, and remarks if necessary; data are in ~g/g if no units listed: Li, 1.3, 1.5 [48]; F, inadequate CM data; Na, 5020, 4400 ([49,50] - two low Murchison values discarded); Si, 10.3, 13.1% [51]; P, 840,900 [52,53];S,5.90, 3.42% [54]; Mn, 1770, 1560 [49]; Cu, 127, 116 [55 ]; Zn, 310, 182 ([56,26], and unpublished data); Ga, I0.0, 8.4 ([57,26], and published data); Ge, 32, 24 ([56,26], and unpublished data); As, 1.8, 1.95 [58]; Se, 19.5, 11.8 [56]; Ag, 182,156 ng/g ([56]; value of 33 in Mighei discarded); Sn, 1.6, 0 0.85 [59]; Sb, 138,105 ng/g [56];Te, 3.04, 1.86 [56];Au, 160, 166 ng/g ( [ 56,26 ], and unpublished data). CI abundance data calculated from these values are within 6% of those given by Wasson and Chou [2] except Sb, which is now 10% higher.

References 1 J.T. Wasson, Meteorites - Classification and Properties (Springer, New York, N.Y., 1974) 316 pp. 2 J.T. Wasson and C.-L. Chou, Fractionation of moderately volatile elements in ordinary chondrites, Meteoritics 9 (1974) 69. 3 E. Anders, Origin, age and composition of meteorites, Space Sci. Rev. 3 (1964) 583. 4 J.W. Larimer and E. Anders, Chemical fractionations in meteorites, II. Abundance patterns and their interpretation, Geochim. Cosmoehim. Acta 31 (1967) 1239. 5 E. Anders, On the depletion of moderately volatile elements in ordinary chondrites, Meteoritics 10 (1975) 283. 6 J.W. Latimer, Chemical fractionations in meteorites, I. Condensation of the elements, Geochim. Cosmoehim. Acta 31 (1967) 1215. 7 A.G.W. Cameron, Abundances of the elements in the solar system, Space Sci. Rev. 15 (1973) 121. 8 0 . Mfiller, P.A. Baedecker and J.T. Wasson, Relationship between siderophilic-element content and oxidation state of ordinary chondrites, Geochim. Cosmochim. Acta 35 (1971) 1121. 9 L. Grossman and J.W. Larimer, Early chemical history of the solar system, Rev. Geophys. Space Phys. 12 (1974) 71.

10 A.G.W. Cameron, A new table of abundances of the elements in the solar system, in: Origin and Distribution of the Elements, L.H. Ahrens, ed. (Pergamon, Oxford, 1968) 125. 11 J.F. Kerridge, Formation of iron sulphide in solar nebula, Nature 259 (1976) 189. 12 J.T. Wasson and C.M. Wai, Explanation for the very low Ga and Ge concentrations in some iron meteorite groups, Nature 261 (1976) 114. 13 L. Grossman and E. Olsen, Origin of the high-temperature fraction of C2 chondrites, Geochim. Cosmochim. Acta 38 (1974) 173. 14 M. Shima and M. Honda, Distributions of alkali, alkaline earth, and rare earth elements in component minerals of chondrites, Geochim. Cosmochim. Acta 31 (1967) 1995. 15 E. Anders, Meteorites and the early solar system, Ann. Rev. Astron. Astrophys. 9 (1971) 1. 16 L. Grossman, Condensation in the primitive solar nebula, Geochim. Cosmochim. Acta 36 (1972) 597. 17 W.R. van Schmus and P.H. Ribbe, Composition of phosphate minerals in ordinary chondrites, Geochim. Cosmochim. Acta 33 (1969) 637. 18 W.R. Kelly and J.W. Larimer, Chemical fractionations in meteorites, VIII. Iron meteorites and the cosmochemical history of the metal phase, Geochim. Cosmochim. Acta 41 (1977) 93. 19 A.E. Ringwood, The constitution of the mantle, I. Thermodynamics of the olivine-spinel transition, Geochim. Cosmochim. Acta 13 (1957) 303. 20 K.K. Kelley, High-temperature heat-content, heat-capacity, and entropy data for the elements and inorganic compounds, U.S. Bur. Mines Bull. 584 (1960). 21 D.D. Wagman, W.H. Evans, V.B. Parker, I. Halow, S.M. Baily and R.H. Schumm, Selected values of chemical thermodynamic properties, NBS Tech. Note 270-3, 270-4 (1968). 22 J.R. Rowling and J.M. Toguri, The thermodynamic properties of hydrogen selenide, Can. J. Chem. 44 (1966) 451. 23 R.A. Robie and D.R. Waldbaum, Thermodynamic properties of minerals and related substances at 298.15 K and one atmosphere pressure and at higher temperatures, U.S. Geol. Surv. Bull. 1259 (1968) 256 pp. 24 A.W. Richards, The heat and free energy of formation and vaporization of stannous sulphide, Trans. Farad. Soc. 51 (1955) 1193. 25 Z.A. Munir and M.J. Mitchell, Studies on the sublimation of lIB-VIA compounds, 1. The sublimation coefficient and activation energy for sublimation of single crystalline zinc sulfide, High-Temp. Sci. 1 (1969) 381. 26 C.-L. Chou, P.A. Baedecker and J.T. Wasson, Allende inclusions: volatile-element distribution and evidence for incomplete volatilization of presolar solids, Geochim. Cosmochim. Acta 40 (1976) 85. 27 K.L. Komarek, Direct reduction of iron ores containing phosphorus, Trans. Metall. Soc. AIME 227 (1963) 136. 28 L. Pauling, The Nature of the Chemical Bond (Cornell Univ. Press, Ithaca, N.Y., 1960). 29 R. Hultgren, P.D. Desai, D.T. Hawkins, M. Gleiser and

13

30 31

32

33

34

35

36 37 38

39

40

41

42

43

44 45

K.K. Kelley, Selected Values of the Thermodynamic Properties of Binary Alloys (American Society for Metals, Metals Park, Ohio, 1973) 1435 pp. G. Kurat, Formation of chondrules, Geochim. Cosmochim. Acta 31 (1967) 491. M. Blander and L.H. Fuchs, Calcium-aluminium rich inclusions in the Allende meteorite: evidence for a liquid origin, Geochim. Cosmochim. Acta 39 (1975) 1605. E.R.D. Scott and J.T. Wasson, Classification and properties of iron meteorites, Rev. Geophys. Space Phys. 13 (1975) 527. A. El Goresy, Mineralbestand und Strukturen der Graphitund Sulfideinschliisse in Eisenmeteoriten, Geochim. Cosmochim. Acta 29 (1965) 1131. J.W. Larimer, Chemical fractionations in meteorites, VII. Cosmothermometry and cosmobarometry, Geochim. Cosmochim. Acta 37 (1973) 1603. V.S. Safronov, Evolution of the Protoplanetary Cloud and the Formation of the Earth and Planets (Nauka, Moscow, 1969)(in Russian; English translation as NASA TT F-677 NTIS, Springfield, Va., 1972). P. Goldreich and W.R. Ward, The formation of planetesimals, Astrophys. J. 183 (1973) 1051. JANAF Thermochemical Tables, U.S. Govt. Dec. No. NSRDS-NBS 37 (1971) 2nd ed., unpaginated. R. Hultgren, P.D. Desai, D.T. Hawkins, M. Gleiser, K.K. Kelley and D.D. Wagman, Selected Values of the Thermodynamic Properties of the Elements (American Society for Metals, Metals Park, Ohio, 1973) 636 pp. M.M. Faktor and J.I. Carasso, Tetragonal germanium dioxide and equilibria in the G e - O - H system, J. Electrochem. Soc. 112 (1965) 817. C.J. Frasch and C.D. Turmond, The pressure of Ga20 over G a - G a 2 0 3 mixtures, J. Phys. Chem. 66 (1962) 877. D.L. Hildenbrand, The gaseous equilibrium Ge+SiO = GeO+Si and the dissociation energy of SiO, High-Temp. Sci. 4 (1972) 244. W.L. Jolly and W.M. Latimer, The equilibrium Ge(s) + GeO2(s) = 2GeO(g). The heat of formation of germanium oxide, J. Am. Chem. See. 74 (1952) 5757. A. Navrotsky, Thermodynamics of formation of the silicates and germanates of some divalent transition metals and of magnesium, J. Inorg. Nucl. Chem. 33 (1971) 4035. J.V. Smith and D. Chatterji, EMF investigation of G a Ga203 equilibrium, J. Am. Ceram. See. 56 (1973) 288. V.B. Parker, D.D. Wagman and W.H. Evans, Selected values of chemical thermodynamic properties. Tables for the alkaline earth elements, NBS Tech. Note 270-6 (1971).

46 F. Gronvold and E.F. Westrum, Jr., Low-temperature heat capacities and thermodynamic properties of the iron selenides Fe 1.04Se, Fe7Se 8 and Fe3Se 4 from 5 to 350°K, Acta Chem. Scand. 13 (1959) 241. 47 F. Gronvold and E.F. Westrum, Jr., Heat capacities and thermodynamic functions of iron disulfide (pyrite), iron diselenide, and nickel diselenide from 5 to 350 ° K. The estimation of standard entropies of transition metal chalcogenides, Inorg. Chem. 1 (1962) 36. 48 W. Nichiporuk, Lithium (3)i in: Handbook of Elemental Abundances in Meteorites, B. Mason, ed. (Gordon and Breach, London, 1971) 67. 49 H.B. Wiik, On regular discontinuities in the composition of meteorites, Comm. Phys.-Math. (Helsinki) 34 (1969) 135. 50 W. Nichiporuk and C.B. Moore, Lithium, sodium and potassium abundances in carbonaceous chondrites, Geochim. Cosmochim. Acta 38 (1974) 1691. 51 C.B. Moore, Silicon (14), in: Handbook of Elemental Abundances in Meteorites, B. Mason, ed. (Gordon and Breach, London, 1971) 125. 52 L. Greenland and J.F. Levering, Minor and trace element abundances in chondritic meteorites, Geoehim. Cosmochim. Acta 29 0 9 6 5 ) 821. 53 H. yon Michaelis, L.H. Ahrens and J.P. Willis, The composition of stony meteorites, II. The analytical data and an assessment of their quality, Earth Planet. Sci. Lett. 5 (1969) 387. 54 C.B. Moore, Sulfur (16), in: Handbook of Elemental Abundances in Meteorites, B. Mason, ed. (Gordon and Breach, London, 1971) 137. 55 G.G. Goles, in: Handbook of Elemental Abundances in Meteorites, B. Mason, ed. (Gordon and Breach, London, 1971) 555 pp. 56 U. Kr~/henbiihl, J.W. Morgan, R. Ganapathy and E. Anders, Abundance of 17 trace elements in carbonaceous chondrites, Geochim. Cosmochim. Acta 37 (1973) 1353. 57 K.F. Fouch6 and A.A. Smales, The distribution of trace elements in chondritic meteorites, 1. Gallium, germanium and indium, Chem. Geol. 2 (1967) 5. 58 M.E. Lipschutz, Arsenic (33), in: Handbook Elemental Abundances in Meteorites, B. Mason, ed. (Gordon and Breach, London, 1971) 261. 59 P.R. Buseck, Tin (50), in: Handbook of Elemental Abundances in Meteorites, B. Mason, ed. (Gordon and Breach, London, 1971) 377. 60 J.W. Larimer, Chemistry of the solar nebula, Space Sci. Rev. 15 (1973) 103.