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Equilibration temperatures in enstatite chondrites
Gleoahfmfca etcaanochtmfcahata.1974.Vol. 88,pp. 471to 177. PergamonPress. PrintedIn NorthernIreland
Equilibration temperatures in en&We chondrites* JOHNW. LAEIWCR and PETER R. BUSEC~ Depsxtmtmts of Geology end Chemistry end Center for Meteorite Studies, Arkon8 state University, Tempe, Arizone 86281, U.&A. (Reeked 6 Novetnber 1972; acceptedin revekedform4 June 1973) A~~~~q~b~tion temper&urea for enstatite chondrites are Ctkkd8ted using 8 method LAXMER (1968). The temperstures mnge from 640’ to 840%. The method yields suggested by ~KRper8talreS which, in principle, are co& on 8 relative SC& but the 8bsolute error m8y be 8 1~ as 160’. There is 8 good correlation between the calculated temperatures and petrologio type ss well as other miuereiogio charactenstice and bulk composition. Partial pre~ux~ of eulfur and oxygen at the time of equilibration were: pS, N 10-8-10-‘a 10-8’.
atm aud p02 N 10-2*-
INTRODUCTION STUXHESon the thermal history of chondritic meteorites were hampered for many years by lack of suitable geothermometers. Recently, however, the picture has brightened somewhat, at least for ordinary chondritee. A significant development was the reco~tion that most chondrites have been metamo~ho~d sometime after accumulating (VAN SCHMUSand WOOD, 1967). Minimum metamorphic temperatures for the most highly recrystallized meteorites, petrologic type 6, have been estimated to be about 800 -f 60°C based on the clino-orthopyroxene thermometer (VAN SCHMVS and KOFFMAN, 1967) and the structural state of the feldspar8 (I’AN SCHMUS and RIBBE, 1968). Both methods have inherent uncertainties and may be in error by as much as 100’ but the inferred temperatures seem plausible. Estimates based on oxygen isotope ratios from seven recrystallized ordinary chondrites center on 950 & lOO”C, lending support to the vahdity of the mineralogical thermometers (Osnm et al., 1972). The enstatite chondrites present special problems owing to their unusual mineralogy. They consist almost entirely of enstatite, metal and troilite, and the aecessory minerals are mostly rare sulfides, many unique to these meteorites. SKIXNER and LUCE (1971) have reported and applied experimental data on the system Unfortunately, the interpretation is not straightforward CaS-MgS-Mns-FeS. when applied to the meteorites. Two of their potential geothermometers yield rather low temperatures of less than 300 and 4OO”C,suggesting that these systems continued to re-equilibrate as the meteorites cooIed from their peak metamorphic temperatures. fn two other cases, more realistic ‘metamorphic’ temperatures of 70~9OO’C are indicated, but the inferred cooling rates exceed O*l”/miu, an extraordinarily high value compared to measured cooling rates in the order of a few degrees per million years for ordinary chondrites {WOOD, 1967). Skinner and Lute suggest that these temperatures and cooling rates may have been achieved during the impact event which ejected the meteorites from their parent bodies. Their data on MnS solubility in CaS yield temperatures of 750-900°C for six highly recrystallized ohondrites, which seems plausible. However, the solubility of MgS in * Contribution No. 82 from the Center for 3feteorite Studies, Arizona State University. 471
472
JOEN W. LARIBCERand PETER
R. BUSECK
CaS points to similar temperatures for five other meteorites which appear to have suffered the least metamorp~sm. We therefore decided to apply a geothermometer suggested by LARIMER(1968) for enststite chondrites. In 1968, the analytical data required to use the method were available for only one meteorite, Jajh deh Kot Lalu. Since then data have become available on 14 of the 15 remaining enstatite chondrites. This method also has inherent uncertainties amo~nt~g to -&GO0 or so but, in principle, the inferred temperatures should be reliable on a relative scale. EQUILIBRATION TEMPERATURES
Like other mineralogic thermome~rs, the calculated temperatures are those at which the phases were last in equilibrium and, therefore, represent the minimum temperatures of heating. The temperature determination is made by solving three equations involving three unknowns. Each equation is based on thermodynamic data for a chemical reaction. The actual number of unknowns is four, total pressure (P), temperature (T) and the partial pressures of sulfur @S,) and oxygen (PO,). But the effects of total pressure are negligible, reducing the variables to three. The greatest uncertainty arises from lack of experimental data on the activity coefficients for some of the components. The three chemical reactions are : Si + 0, = SiO,
ZCaSiO, + S, = 2CaS + 2SiOz + 0, 2Fe + S, = 2FeS.
(4 (B) (C-9
Equilibrium constants for the reactions may be written: 6
=
aSiO,l(aSi
.IiB =
4aS
Kc
GdG
=
. pO,)
* u&o, * ~O~~~~~~~iO * af3tL) - Fs8).
(1) (2)
(3)
Each equilibrium constant is a function of P and T according to the relation: log R = A/T - B - C(P - 1)/T,
(4)
where P = total pressure, A = M/2-303R, B = A5/2*303R and C = AVJ2*303R. (The change in volume involves only the condensed species in the reactions if the gas phase is assumed to be ideal.) The constants for the three chemical reactions are presented in Table 1. Note that updated thermod~amic data have been used resulting in values whkh differ slightly from those originally presented by LARINER (1968). We may consider the components in the chemical reactions to be distributed between five phases : metal, oldhamite, free SiO,, enstatite and troilite. Oldhamite, and troilite are essentially pure phases (KEXL, 1968) ; their activities must be close
to unity thereby greatly simplifying the algebra. The activity of Fe in the metal must be close to unity, certainly not less than about O-5. This leaves two
Equilibration temperatures in enstatite chondrites
473
Table 1. Data for equilibrium constants with total pressurecorrection terms* 1ogK =
A/T -B
Solid phases
-C(P - 1)/T
A
Si-SiO, CaS-SiO,-CaSiO, Fe-FeS
47,216 f 100 -20,876 f 450 16,426 f 100
B
0
9.05 f 0.01 0.36 f 0.10 6.22 * 0.02
0.056 0.18 0.18
*The thermodynamic data from ROBIE and W&DBAUM (1968) were used in all cases, except CaS (LARIMER,1968),to calculate the constants A and B. The molar volume data from which the constant C was calculated are discussedby LARIIVIER (1968).
components where activities should be known in order to make a precise temperature determination, Si in metal and CaSiO, in enstatite. Activity coefficients for Si in molten Fe have been determined (SCHWERDFEGER and ENQEL, 1964), but there are no data on the solid phases. We therefore assume that the solid solutions can be treated as supercooled liquids and simply correct for the change in standard states from liquid to solid. Schwerdfeger and Engle present four equations based on four independent experimental studies relating the activity coefficient of Si in Fe (at low Si concentrations) to temperature. Extrapolating these results to 700-9OO’C and correcting for the change in state yields activity coefficients of between 1O-3 and lo-* in three cases and lo-* in the fourth. Since this latter extrapolated value was obtained from data which covered the smallest temperature range, it is considered the least reliable. We therefore adopt a value of 10-S’5*0’5. The activity coefficient of CaSiO, in MgSiO, can be estimated from the solubility data on the system MgSiO,-CaMgSi,O, (BOYD and SCHAIRER, 1964). WILLIAMS (1970) has determined the relationships and from his data an activity coefficient of about 10 (for the CaSiO, concentrations of interest) is calculated for temperatures of 800-900°C. It is difficult to estimate the error involved but it is unlikely to affect our calculations significantly. The three equations may now be put into a more useful form by substituting the three equilibrium constants into equation (4) : logp0, h4ps,
47,216
= - 7
= logp0,
logpS, = -
+ 9.05 +
20,876
+ -jy--
0.056 (P - 1)
- 0.36 +
T
0.18 (P T
- log %i
1) -
2 log %,sio,
16,426 + 6.22 + 0.13 (P - I) - 2 log are. 7 T
(6)
(7)
Total pressure can be considered negligible because the two terms containing P from equations (6) and (7) cancel one another when the equations are combined, leaving only the smaller term from equation (5). Even at 10 kb, corresponding to the pressure at the center of a 1000 km object with a density of 3.5 g/cm3, a correction of only 5” (at 727’C) is indicated. This term, therefore, is ignored in the calculations.
474
JOHNW. mm
and
PETERR. BUSEOK
The following substitutions are then made : aFe = O-75 f O-25, a,, = 10’-3’5*0’6’Nsi and acasio = 10 Noasio where Nx is the measured mole fraction. The three equations may how be combmed and rearranged to solve for T. T = 9914 ~ 650/[3.71 (f0.75)
- 2
IOg
NCaSiO,
-
log
Nsi].
(8)
The errors in the activity coefficients and thermodynamic data lead to an uncertainty of &150° in the calculated temperatures. However, there must be some ‘true’ values for the thermodynamic constants and the activity coefficients cannot be expected to vary much over the observed concentration ranges. On a relative scale, therefore, the calculated temperatures should be precise to -&20” or so. RESULTSANDDISCUSSION LARIMER(1968) calculated an equilibration temperature using this method for the Jajh deh Kot Lalu meteorite, the only enstatite chondrite for which accurate analytical data existed. Since then, KEIL (1968) has reported data on 13 additional enstatite chondrites, and BUSECKand HOLDSWORTH (1972) have reported similar data on a fourteenth, Yilmia. These data are presented in Table 2 along with the calculated temperatures. The calculated equilibration temperatures fall into an interesting pattern. The temperatures increase from 640 to 840°C and correlate with increasing metamorphic grade (VAN SCHMUSand WOOD, 1967). The only exceptions are the first three meteorites listed, Adhi Kot, Indarch and Kota-Kota. However, the temperatures inferred for these meteorites may not be meaningful since there are indications that the mineral assemblages in these meteorites have not equilibrated. This is suggested by the presence of glass, zoned pyroxenes and free SiO, next to olivine Table 2. Analytical
Meteorite* Adhi Kot Indarch Kota-Kota Abee St. Sauveur St. Marks Yil??%ia Ufana
Atlanta Jajh Deh Kot Lalu Khairpur Dsniels Kuil Hvittis Pallistfer
Blithfield
data (mole%)
TYpe
E3 E4 E4 E4 E4 E5 E6 E6 E6 E6 E6 E6 E6 E6 E6
* Italicized meteorites are finds.
and calculated equilibrium temperatures
CaO in Enstatite
Fe0 in Enstatite
Si in Metal
Temperature
0.18
1.12 1.18 1.24 0.75 0.55 0.52 0.13 0.38 0.32 0.22 0.27 0.26 0.17 0.45 0.04
6.82 6.78 5.12 6.44 5.62 6.92 1.83 3.38 2.38 2.57 2.37 2.40 2.18 2.78 3.16
680 780 780 640 650 660 790 810 820 820 820 820 820 820 840
0.56 0.58 0.11
0.13
0.11 1.16 1.08 1.33 1.29 1.38 1.38 1.46 1.28 1.46
(“C)
475
Equilibration temperatnreein em&We chondritea
(Bms,
1967; Kxrn, 1968). Thus, by analogy with ordinary chondrites, the lower grades may not have equilibrated. Besides the correlation with petrologic types, there are some sign&ant groupings. St. Sauveur and St, Marks, frequently lumped together in the past ae an Intermediate Type (AND=, 1964; Km, 1968) on the baa& of trace element content and mineralogic features, have temperatures about 170’ lower than moet petrologia type 6’s (previously lumped together as Type II). Yilmia, which has tentatively been claas&xl E6 by BusEc~and~o~DswoR~ (1972), has a temperature about 30” lower than the remaining members of this group. Buseck and Hol~wo~h have pointid out that its mineralogy, for example, the presenceof zincian daubreelito, common to types 4 and 6 but absent in type 6, suggests that it should be cla&.%d somewhere between the type 6 and type 6 groups. The remaining type 6 meteorites cluster around a temperature of 820 & PC except for Blith.6eld which has a temperature of 84O”C, the highest of all. Blithfield stands out in other respects; it has the lowest Fe/Si and Si/Mg ratios of any meteorite listed here, Among enstatite chondritee these major element ratio6 decrease with increasing metamorphic grade (WON, 1966; UEE, 1968) which would suggest that Blithfield should be the most highly me~rno~ho~d of the group, The ~mperat~s determined by SUER and LUGS (1971) from their study of the system CaS-MgS-WFeS are compared to those obtained here (Table 3). There ie good agreement between the estimates based on the solubility of MnS in oldhamite and our method. But in most other casea the agreement ie poor aa expected since each mineral assemblage recorde only that temperature at which it ceased to equilibrate. Thus, as Skinner and Lute point out, the low temperatures ( < 300 and 400°C) inferred for highly metamorphosed chondrites simply reflect the ability of these sul6de ay&ema to remain in equilibrium down to rather low temperatures. Also, their suggestion that the high temperatures and rapid cooling rates
metamorphic
Table
3. Comparison of minimum equi~b~tion temperat-
from various thermometers
(SWNNEBand LUCE, 1911) This work Type
Adhi Kot Indarch Kota-Rota Abee St. Sauveur St. Marks Yihia Ufane, Atlanta Jajh Deh Kot LaIu Khairpur Daniela Kuil Hvittie Pillistfer Blithfield
3 4 4 4 4 5 6 6 6 6 6 6 6 6 6
PC)
680 780 780 640 650 650 790 810 820 820 820 820 820 820 840
MIlSiXl oldhamite
MgS in oldhamite
CaS in alabandite
810 800
770 700 <600 870 820 t600
830 820 740 890 750 750 t600 780 770 790
CaS in niningerite
<400 <400 <400
Ala-Nin solid solution 690 650 <300 780 700 <300 (300 <300 c300 t300 <300 t300 <300 t300 t300
476
Jorin W. L~RIxERand PETERR. BUSECK
deduced for the least metamorphosed chondrites are due to impact events seems plausible. In the phase assemblage which we have considered, high equilibration temperatures are expected owing to the notoriously sluggish sub-solidus reaction rates between metal and silicates. Moreover, it seems probable that the phases would be unable to re-equilibrate during the brief thermal pulse associated with an impact event. We therefore think that the temperatures reached during metamorphism must have been at least as high as those calculated in this study (with the exceptions noted above) and those based on MnS solubility in CaS, perhaps because it is so refractory. LARIXER (1968) also pointed out that the mole fraction of FeSiO, in enstatite might provide a check on the estimated temperatures and partial pressure of oxygen at the time of equilibration. However, considerable uncertainty has been introduced by KEIL’S (1968) observation that the Fe content of the silicates, even in the most highly metamorphosed chondrites, varies by factors of 5 or more. (Though the Fe contents are quite low, Keil states, p. 6354, that the variations are probably real because the analytical error should be less than -&20 per cent of the Fe content.) This implies that the reaction : Fe + SiO, + $0, G.?FeSiO, did not reach eq~librium, a curious but not entirely unexpected observation. There is a systematic decrease in the average Fe content of the silicates in going from the least to the most recrystallized chondrites, suggesting that equilibrium was approached but probably never achieved. The cause can probably be traced to the immobility of oxygen in the system. Oxygen, of course, must be a mobile species if phases such as this are to equilibrate. Normally, oxygen is mobile as gaseous 02, H,O, CO or CO,. However, in the mineral assemblage characteristic of enstatite chondrites, the oxygen partial pressure would be only 10-20-10-60 atm at temperatures of 1200 to 500°C {as inferred from the Xi-SiO, reactions. Fu~hermore, during metamo~hism, C would be present as graphite, an unusually stable phase under these conditions, and there is no evidence that H was present; in fact it seems likely that the parent body was essentially free of hydrogen. If this explanation is correct, it naturally raises the question of how the reaction Si + 0,~ SiOa managed to equilibrate, which evidently it did as the Si content of the metal normally varies by less than &20 per cent. We have no simple explanation but it is worth noting that the vapor pressures of Si and Fe are lOlo-102* (JANAF TabZes, 1968) times as high as oxygen.
Calculated equilibration temperatures for a major mineral assemblage in enstatite chondrites range from 640 to 840°C. These temperatures are quite similar to those estimated for ordinary chondrites from other mineralogical thermometers. But since the errors involved in all cases are probably greater than &lOO”C, little significance can be attached to this result at present. Once the temperature has been calculated for enstatite chondrites, it is possible to substitute it into the appropriate equations to calculate the partial pressures of S, and OZ. The pS, is fixed on the phase boundary between Fe and FeS. At 640°
Eq~bration
~mperat~
in enstatite chondrites
477
and S&O’, pS, values of 1O-11’6 and 10-8’a are calculated. Similar calculations at the same temperatures yield ~0, values of 10-37’*and 10-2*.o. These (PO,values are about 20 orders of magnitude less than those calculated for terrestrial and lunar rocks and about 15 orders of magnitude less than ordinary chondrites (at similar temperatures). These data emphasize the fact enstatite chondrites evolved in uniquely reducing environments within the solar system. Ack~owZedgemer&s---This work was supported in part by NASA Grant NGL-03-001-001 and National Science Foundation Grants GA-32297 and GA-26791. We acknowledge Dr. R. 5. West helpful comments on an earlier version of the mamrscript and JOYCE STEINER for help in preparing the manuscript. REFERENCES ANDERS El.(1964) Origin, age and composition of meteorites. Space So& Rev. 3,683-714. BINNS R. A. (1967) Olivine in enstatite chondrites. Amer. MineraE.5$$1549-1654. BOYD F. R. and SCHMRER J. F. (1964) The system MgSiOs-CaMgSizO,. J. PetTOl. 5, 275-309. BUSECK P. R. and HOLDSWORTH E. (1972) Mineralogy and petrology of the Yilmia enstatite chondrite. Meteoritica 7,429-448. JANAP ThermochemicalTables (1968) Third Addendum. Compiled by the Thermal Research Lab., Dow Chemical Company, Midland, Michigan. KEIL K. (1968) ~ner~o~cal and chemical relation~ips among en&at&e chondrites. J. #eophya. Rec. 73,694~6976. L-R J. W. (1968) An experimental investigation of oldhamite, CaS; and the petrologic sign&ance of oldhamite in meteorites. GLeochim. Coenwchim. Acta 32;, 965-982. &ON B. (1966) The enstatite chondrites. aeochim. Coemochim. Acta 30,23-39. O~wra N., CLAYTON R. N. and MAYEDA T. K. (1972) Oxygen isotope temperatures of “equilibrated” ordiuary ohondrites. Geochim. Coemoohim. Acta 36, 157-168. ROBIE R. A. and WALDBATJM D. R. (1968) Thermodynamic properties of minerals and related substances at 29&Hi°K (25%) aud 1 atmosphere (1.013 bars) pressureand at higher temperatures. U.S. Gr,ol.Surv. BuEE.l2!@, 266 pp. SCHSVERDEEOER K. and ENGEL H. J, (1964) Die freie Bild~~enth~pie von Silizium und die Aktivit&en von Silizium in fXissigenEisen und Kobalt. ATch E~n~~~~~. 35,533-540. SKINBER B. J. and LWCE F. D. (1971)Solid solutionsof the type (Ca, Mg, Mn, Fe)S and their use as geothermometersfor the enstatite chondrites. AmeT. MineTd. 66,126Q-1296. VAN SCHNU~ W. R. and Korrm D. M. (1967) Equilibrium temperaturesof iron and magnesium in chondritiometeorites. Science166, lOOQ-1011. VAN SC-S W. R. and WOOD J. A. (1967) A chemical-petrologicalclassificationfor the chondritic meteorites. cfeochim.Coemochim. Aeta 31, 747-766. VAN SCIXMWS W. R. and RIBBE P. H. (1968) The composition and structural state of feldspar from chondritiometeorites. Qkochim.Coemochim. Acta 32,1327-1342. W~A.MS R. J. (1970) Reaction constants in the system Fe-MgO-SiO, between 1300’ and 900” at one atmosphere: theory, experiment and application. Ph.D. Thesis, Johns Hopkins University. WOOD J. A. (1967)Chon~~: their metallic minerals, thermal histories and parent planets. Icc&?Y&?, 6, l-49.
Equilibration temperatures in en&We chondrites* JOHNW. LAEIWCR and PETER R. BUSEC~ Depsxtmtmts of Geology end Chemistry end Center for Meteorite Studies, Arkon8 state University, Tempe, Arizone 86281, U.&A. (Reeked 6 Novetnber 1972; acceptedin revekedform4 June 1973) A~~~~q~b~tion temper&urea for enstatite chondrites are Ctkkd8ted using 8 method LAXMER (1968). The temperstures mnge from 640’ to 840%. The method yields suggested by ~KRper8talreS which, in principle, are co& on 8 relative SC& but the 8bsolute error m8y be 8 1~ as 160’. There is 8 good correlation between the calculated temperatures and petrologio type ss well as other miuereiogio charactenstice and bulk composition. Partial pre~ux~ of eulfur and oxygen at the time of equilibration were: pS, N 10-8-10-‘a 10-8’.
atm aud p02 N 10-2*-
INTRODUCTION STUXHESon the thermal history of chondritic meteorites were hampered for many years by lack of suitable geothermometers. Recently, however, the picture has brightened somewhat, at least for ordinary chondritee. A significant development was the reco~tion that most chondrites have been metamo~ho~d sometime after accumulating (VAN SCHMUSand WOOD, 1967). Minimum metamorphic temperatures for the most highly recrystallized meteorites, petrologic type 6, have been estimated to be about 800 -f 60°C based on the clino-orthopyroxene thermometer (VAN SCHMVS and KOFFMAN, 1967) and the structural state of the feldspar8 (I’AN SCHMUS and RIBBE, 1968). Both methods have inherent uncertainties and may be in error by as much as 100’ but the inferred temperatures seem plausible. Estimates based on oxygen isotope ratios from seven recrystallized ordinary chondrites center on 950 & lOO”C, lending support to the vahdity of the mineralogical thermometers (Osnm et al., 1972). The enstatite chondrites present special problems owing to their unusual mineralogy. They consist almost entirely of enstatite, metal and troilite, and the aecessory minerals are mostly rare sulfides, many unique to these meteorites. SKIXNER and LUCE (1971) have reported and applied experimental data on the system Unfortunately, the interpretation is not straightforward CaS-MgS-Mns-FeS. when applied to the meteorites. Two of their potential geothermometers yield rather low temperatures of less than 300 and 4OO”C,suggesting that these systems continued to re-equilibrate as the meteorites cooIed from their peak metamorphic temperatures. fn two other cases, more realistic ‘metamorphic’ temperatures of 70~9OO’C are indicated, but the inferred cooling rates exceed O*l”/miu, an extraordinarily high value compared to measured cooling rates in the order of a few degrees per million years for ordinary chondrites {WOOD, 1967). Skinner and Lute suggest that these temperatures and cooling rates may have been achieved during the impact event which ejected the meteorites from their parent bodies. Their data on MnS solubility in CaS yield temperatures of 750-900°C for six highly recrystallized ohondrites, which seems plausible. However, the solubility of MgS in * Contribution No. 82 from the Center for 3feteorite Studies, Arizona State University. 471
472
JOEN W. LARIBCERand PETER
R. BUSECK
CaS points to similar temperatures for five other meteorites which appear to have suffered the least metamorp~sm. We therefore decided to apply a geothermometer suggested by LARIMER(1968) for enststite chondrites. In 1968, the analytical data required to use the method were available for only one meteorite, Jajh deh Kot Lalu. Since then data have become available on 14 of the 15 remaining enstatite chondrites. This method also has inherent uncertainties amo~nt~g to -&GO0 or so but, in principle, the inferred temperatures should be reliable on a relative scale. EQUILIBRATION TEMPERATURES
Like other mineralogic thermome~rs, the calculated temperatures are those at which the phases were last in equilibrium and, therefore, represent the minimum temperatures of heating. The temperature determination is made by solving three equations involving three unknowns. Each equation is based on thermodynamic data for a chemical reaction. The actual number of unknowns is four, total pressure (P), temperature (T) and the partial pressures of sulfur @S,) and oxygen (PO,). But the effects of total pressure are negligible, reducing the variables to three. The greatest uncertainty arises from lack of experimental data on the activity coefficients for some of the components. The three chemical reactions are : Si + 0, = SiO,
ZCaSiO, + S, = 2CaS + 2SiOz + 0, 2Fe + S, = 2FeS.
(4 (B) (C-9
Equilibrium constants for the reactions may be written: 6
=
aSiO,l(aSi
.IiB =
4aS
Kc
GdG
=
. pO,)
* u&o, * ~O~~~~~~~iO * af3tL) - Fs8).
(1) (2)
(3)
Each equilibrium constant is a function of P and T according to the relation: log R = A/T - B - C(P - 1)/T,
(4)
where P = total pressure, A = M/2-303R, B = A5/2*303R and C = AVJ2*303R. (The change in volume involves only the condensed species in the reactions if the gas phase is assumed to be ideal.) The constants for the three chemical reactions are presented in Table 1. Note that updated thermod~amic data have been used resulting in values whkh differ slightly from those originally presented by LARINER (1968). We may consider the components in the chemical reactions to be distributed between five phases : metal, oldhamite, free SiO,, enstatite and troilite. Oldhamite, and troilite are essentially pure phases (KEXL, 1968) ; their activities must be close
to unity thereby greatly simplifying the algebra. The activity of Fe in the metal must be close to unity, certainly not less than about O-5. This leaves two
Equilibration temperatures in enstatite chondrites
473
Table 1. Data for equilibrium constants with total pressurecorrection terms* 1ogK =
A/T -B
Solid phases
-C(P - 1)/T
A
Si-SiO, CaS-SiO,-CaSiO, Fe-FeS
47,216 f 100 -20,876 f 450 16,426 f 100
B
0
9.05 f 0.01 0.36 f 0.10 6.22 * 0.02
0.056 0.18 0.18
*The thermodynamic data from ROBIE and W&DBAUM (1968) were used in all cases, except CaS (LARIMER,1968),to calculate the constants A and B. The molar volume data from which the constant C was calculated are discussedby LARIIVIER (1968).
components where activities should be known in order to make a precise temperature determination, Si in metal and CaSiO, in enstatite. Activity coefficients for Si in molten Fe have been determined (SCHWERDFEGER and ENQEL, 1964), but there are no data on the solid phases. We therefore assume that the solid solutions can be treated as supercooled liquids and simply correct for the change in standard states from liquid to solid. Schwerdfeger and Engle present four equations based on four independent experimental studies relating the activity coefficient of Si in Fe (at low Si concentrations) to temperature. Extrapolating these results to 700-9OO’C and correcting for the change in state yields activity coefficients of between 1O-3 and lo-* in three cases and lo-* in the fourth. Since this latter extrapolated value was obtained from data which covered the smallest temperature range, it is considered the least reliable. We therefore adopt a value of 10-S’5*0’5. The activity coefficient of CaSiO, in MgSiO, can be estimated from the solubility data on the system MgSiO,-CaMgSi,O, (BOYD and SCHAIRER, 1964). WILLIAMS (1970) has determined the relationships and from his data an activity coefficient of about 10 (for the CaSiO, concentrations of interest) is calculated for temperatures of 800-900°C. It is difficult to estimate the error involved but it is unlikely to affect our calculations significantly. The three equations may now be put into a more useful form by substituting the three equilibrium constants into equation (4) : logp0, h4ps,
47,216
= - 7
= logp0,
logpS, = -
+ 9.05 +
20,876
+ -jy--
0.056 (P - 1)
- 0.36 +
T
0.18 (P T
- log %i
1) -
2 log %,sio,
16,426 + 6.22 + 0.13 (P - I) - 2 log are. 7 T
(6)
(7)
Total pressure can be considered negligible because the two terms containing P from equations (6) and (7) cancel one another when the equations are combined, leaving only the smaller term from equation (5). Even at 10 kb, corresponding to the pressure at the center of a 1000 km object with a density of 3.5 g/cm3, a correction of only 5” (at 727’C) is indicated. This term, therefore, is ignored in the calculations.
474
JOHNW. mm
and
PETERR. BUSEOK
The following substitutions are then made : aFe = O-75 f O-25, a,, = 10’-3’5*0’6’Nsi and acasio = 10 Noasio where Nx is the measured mole fraction. The three equations may how be combmed and rearranged to solve for T. T = 9914 ~ 650/[3.71 (f0.75)
- 2
IOg
NCaSiO,
-
log
Nsi].
(8)
The errors in the activity coefficients and thermodynamic data lead to an uncertainty of &150° in the calculated temperatures. However, there must be some ‘true’ values for the thermodynamic constants and the activity coefficients cannot be expected to vary much over the observed concentration ranges. On a relative scale, therefore, the calculated temperatures should be precise to -&20” or so. RESULTSANDDISCUSSION LARIMER(1968) calculated an equilibration temperature using this method for the Jajh deh Kot Lalu meteorite, the only enstatite chondrite for which accurate analytical data existed. Since then, KEIL (1968) has reported data on 13 additional enstatite chondrites, and BUSECKand HOLDSWORTH (1972) have reported similar data on a fourteenth, Yilmia. These data are presented in Table 2 along with the calculated temperatures. The calculated equilibration temperatures fall into an interesting pattern. The temperatures increase from 640 to 840°C and correlate with increasing metamorphic grade (VAN SCHMUSand WOOD, 1967). The only exceptions are the first three meteorites listed, Adhi Kot, Indarch and Kota-Kota. However, the temperatures inferred for these meteorites may not be meaningful since there are indications that the mineral assemblages in these meteorites have not equilibrated. This is suggested by the presence of glass, zoned pyroxenes and free SiO, next to olivine Table 2. Analytical
Meteorite* Adhi Kot Indarch Kota-Kota Abee St. Sauveur St. Marks Yil??%ia Ufana
Atlanta Jajh Deh Kot Lalu Khairpur Dsniels Kuil Hvittis Pallistfer
Blithfield
data (mole%)
TYpe
E3 E4 E4 E4 E4 E5 E6 E6 E6 E6 E6 E6 E6 E6 E6
* Italicized meteorites are finds.
and calculated equilibrium temperatures
CaO in Enstatite
Fe0 in Enstatite
Si in Metal
Temperature
0.18
1.12 1.18 1.24 0.75 0.55 0.52 0.13 0.38 0.32 0.22 0.27 0.26 0.17 0.45 0.04
6.82 6.78 5.12 6.44 5.62 6.92 1.83 3.38 2.38 2.57 2.37 2.40 2.18 2.78 3.16
680 780 780 640 650 660 790 810 820 820 820 820 820 820 840
0.56 0.58 0.11
0.13
0.11 1.16 1.08 1.33 1.29 1.38 1.38 1.46 1.28 1.46
(“C)
475
Equilibration temperatnreein em&We chondritea
(Bms,
1967; Kxrn, 1968). Thus, by analogy with ordinary chondrites, the lower grades may not have equilibrated. Besides the correlation with petrologic types, there are some sign&ant groupings. St. Sauveur and St, Marks, frequently lumped together in the past ae an Intermediate Type (AND=, 1964; Km, 1968) on the baa& of trace element content and mineralogic features, have temperatures about 170’ lower than moet petrologia type 6’s (previously lumped together as Type II). Yilmia, which has tentatively been claas&xl E6 by BusEc~and~o~DswoR~ (1972), has a temperature about 30” lower than the remaining members of this group. Buseck and Hol~wo~h have pointid out that its mineralogy, for example, the presenceof zincian daubreelito, common to types 4 and 6 but absent in type 6, suggests that it should be cla&.%d somewhere between the type 6 and type 6 groups. The remaining type 6 meteorites cluster around a temperature of 820 & PC except for Blith.6eld which has a temperature of 84O”C, the highest of all. Blithfield stands out in other respects; it has the lowest Fe/Si and Si/Mg ratios of any meteorite listed here, Among enstatite chondritee these major element ratio6 decrease with increasing metamorphic grade (WON, 1966; UEE, 1968) which would suggest that Blithfield should be the most highly me~rno~ho~d of the group, The ~mperat~s determined by SUER and LUGS (1971) from their study of the system CaS-MgS-WFeS are compared to those obtained here (Table 3). There ie good agreement between the estimates based on the solubility of MnS in oldhamite and our method. But in most other casea the agreement ie poor aa expected since each mineral assemblage recorde only that temperature at which it ceased to equilibrate. Thus, as Skinner and Lute point out, the low temperatures ( < 300 and 400°C) inferred for highly metamorphosed chondrites simply reflect the ability of these sul6de ay&ema to remain in equilibrium down to rather low temperatures. Also, their suggestion that the high temperatures and rapid cooling rates
metamorphic
Table
3. Comparison of minimum equi~b~tion temperat-
from various thermometers
(SWNNEBand LUCE, 1911) This work Type
Adhi Kot Indarch Kota-Rota Abee St. Sauveur St. Marks Yihia Ufane, Atlanta Jajh Deh Kot LaIu Khairpur Daniela Kuil Hvittie Pillistfer Blithfield
3 4 4 4 4 5 6 6 6 6 6 6 6 6 6
PC)
680 780 780 640 650 650 790 810 820 820 820 820 820 820 840
MIlSiXl oldhamite
MgS in oldhamite
CaS in alabandite
810 800
770 700 <600 870 820 t600
830 820 740 890 750 750 t600 780 770 790
CaS in niningerite
<400 <400 <400
Ala-Nin solid solution 690 650 <300 780 700 <300 (300 <300 c300 t300 <300 t300 <300 t300 t300
476
Jorin W. L~RIxERand PETERR. BUSECK
deduced for the least metamorphosed chondrites are due to impact events seems plausible. In the phase assemblage which we have considered, high equilibration temperatures are expected owing to the notoriously sluggish sub-solidus reaction rates between metal and silicates. Moreover, it seems probable that the phases would be unable to re-equilibrate during the brief thermal pulse associated with an impact event. We therefore think that the temperatures reached during metamorphism must have been at least as high as those calculated in this study (with the exceptions noted above) and those based on MnS solubility in CaS, perhaps because it is so refractory. LARIXER (1968) also pointed out that the mole fraction of FeSiO, in enstatite might provide a check on the estimated temperatures and partial pressure of oxygen at the time of equilibration. However, considerable uncertainty has been introduced by KEIL’S (1968) observation that the Fe content of the silicates, even in the most highly metamorphosed chondrites, varies by factors of 5 or more. (Though the Fe contents are quite low, Keil states, p. 6354, that the variations are probably real because the analytical error should be less than -&20 per cent of the Fe content.) This implies that the reaction : Fe + SiO, + $0, G.?FeSiO, did not reach eq~librium, a curious but not entirely unexpected observation. There is a systematic decrease in the average Fe content of the silicates in going from the least to the most recrystallized chondrites, suggesting that equilibrium was approached but probably never achieved. The cause can probably be traced to the immobility of oxygen in the system. Oxygen, of course, must be a mobile species if phases such as this are to equilibrate. Normally, oxygen is mobile as gaseous 02, H,O, CO or CO,. However, in the mineral assemblage characteristic of enstatite chondrites, the oxygen partial pressure would be only 10-20-10-60 atm at temperatures of 1200 to 500°C {as inferred from the Xi-SiO, reactions. Fu~hermore, during metamo~hism, C would be present as graphite, an unusually stable phase under these conditions, and there is no evidence that H was present; in fact it seems likely that the parent body was essentially free of hydrogen. If this explanation is correct, it naturally raises the question of how the reaction Si + 0,~ SiOa managed to equilibrate, which evidently it did as the Si content of the metal normally varies by less than &20 per cent. We have no simple explanation but it is worth noting that the vapor pressures of Si and Fe are lOlo-102* (JANAF TabZes, 1968) times as high as oxygen.
Calculated equilibration temperatures for a major mineral assemblage in enstatite chondrites range from 640 to 840°C. These temperatures are quite similar to those estimated for ordinary chondrites from other mineralogical thermometers. But since the errors involved in all cases are probably greater than &lOO”C, little significance can be attached to this result at present. Once the temperature has been calculated for enstatite chondrites, it is possible to substitute it into the appropriate equations to calculate the partial pressures of S, and OZ. The pS, is fixed on the phase boundary between Fe and FeS. At 640°
Eq~bration
~mperat~
in enstatite chondrites
477
and S&O’, pS, values of 1O-11’6 and 10-8’a are calculated. Similar calculations at the same temperatures yield ~0, values of 10-37’*and 10-2*.o. These (PO,values are about 20 orders of magnitude less than those calculated for terrestrial and lunar rocks and about 15 orders of magnitude less than ordinary chondrites (at similar temperatures). These data emphasize the fact enstatite chondrites evolved in uniquely reducing environments within the solar system. Ack~owZedgemer&s---This work was supported in part by NASA Grant NGL-03-001-001 and National Science Foundation Grants GA-32297 and GA-26791. We acknowledge Dr. R. 5. West helpful comments on an earlier version of the mamrscript and JOYCE STEINER for help in preparing the manuscript. REFERENCES ANDERS El.(1964) Origin, age and composition of meteorites. Space So& Rev. 3,683-714. BINNS R. A. (1967) Olivine in enstatite chondrites. Amer. MineraE.5$$1549-1654. BOYD F. R. and SCHMRER J. F. (1964) The system MgSiOs-CaMgSizO,. J. PetTOl. 5, 275-309. BUSECK P. R. and HOLDSWORTH E. (1972) Mineralogy and petrology of the Yilmia enstatite chondrite. Meteoritica 7,429-448. JANAP ThermochemicalTables (1968) Third Addendum. Compiled by the Thermal Research Lab., Dow Chemical Company, Midland, Michigan. KEIL K. (1968) ~ner~o~cal and chemical relation~ips among en&at&e chondrites. J. #eophya. Rec. 73,694~6976. L-R J. W. (1968) An experimental investigation of oldhamite, CaS; and the petrologic sign&ance of oldhamite in meteorites. GLeochim. Coenwchim. Acta 32;, 965-982. &ON B. (1966) The enstatite chondrites. aeochim. Coemochim. Acta 30,23-39. O~wra N., CLAYTON R. N. and MAYEDA T. K. (1972) Oxygen isotope temperatures of “equilibrated” ordiuary ohondrites. Geochim. Coemoohim. Acta 36, 157-168. ROBIE R. A. and WALDBATJM D. R. (1968) Thermodynamic properties of minerals and related substances at 29&Hi°K (25%) aud 1 atmosphere (1.013 bars) pressureand at higher temperatures. U.S. Gr,ol.Surv. BuEE.l2!@, 266 pp. SCHSVERDEEOER K. and ENGEL H. J, (1964) Die freie Bild~~enth~pie von Silizium und die Aktivit&en von Silizium in fXissigenEisen und Kobalt. ATch E~n~~~~~. 35,533-540. SKINBER B. J. and LWCE F. D. (1971)Solid solutionsof the type (Ca, Mg, Mn, Fe)S and their use as geothermometersfor the enstatite chondrites. AmeT. MineTd. 66,126Q-1296. VAN SCHNU~ W. R. and Korrm D. M. (1967) Equilibrium temperaturesof iron and magnesium in chondritiometeorites. Science166, lOOQ-1011. VAN SC-S W. R. and WOOD J. A. (1967) A chemical-petrologicalclassificationfor the chondritic meteorites. cfeochim.Coemochim. Aeta 31, 747-766. VAN SCIXMWS W. R. and RIBBE P. H. (1968) The composition and structural state of feldspar from chondritiometeorites. Qkochim.Coemochim. Acta 32,1327-1342. W~A.MS R. J. (1970) Reaction constants in the system Fe-MgO-SiO, between 1300’ and 900” at one atmosphere: theory, experiment and application. Ph.D. Thesis, Johns Hopkins University. WOOD J. A. (1967)Chon~~: their metallic minerals, thermal histories and parent planets. Icc&?Y&?, 6, l-49.