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Volatilization of sodium from silicate melt spheres and its application to the formation of chondrules
Volatilization of sodium from silicate melt spheres and its application to the formation of ~hondrules AURA
TSUCHIYAMA.
HIKO~CONAGAHARA and IKUO KUSHIRO
Geological Institute, Um\ersity
of Tokyo,
Hongo.
Tokyo.
113.Japan
Abstrart -The rates of volatilization of Na from liquid spheres of chondrule compositions have been determined as functions of time, temperature. partial pressure of oxygen. and sizes of the spheres. The Na,O content in the sphere is uniform in each run. but It decreases with time of the run, indicating that the rate of diffusion of Na in the liquid is greater than that of volatilization. and that the latter is the rate-controlling process. The rate of sodium volatilization becomes greater with increasing temperature and with decreasing PO2 and size of the spheres. The relation of the Na,O content in the liquid sphere with time and its &e indicate that the amount of Na,O volatilized from the liquid spheres within unit time is proportional to the surface area of the spheres and the concentration of Na,O in the liquid. From these relations, the rate of volatilization of sodium can be obtained at constant temperature and poZ, The rate of volatilization of sodium satisfies the Arrhenius relation within the temperature range from about 14SO--1600 C at 10 -92 atm po,; the activation energy for the sodium volatilization is approximately 100kcal mole -I. The rate is also approximately proportional to ~0~’ ’ within the range of po2 from IO Ii’ ’ to lo- ’ ’ atm at about 1500 C. Based on the present results and the NazO contents m chondrules. it is suggested that they experienced an instant healing with maximum temperature of 1400~~2200 C followed-by an immediate c&o&g.
INTRODUCTION IT IS KNOWN that alkalis voiatitize
easily from silicate
(e.g. LOVERING. 1960; et rd., 1975). It is likely therefore that alkalis were partially volatilized from chondrules, because the chondrules are believed to have been formed by crystallization from liquid at high temperatures (e.g. WOOQ 1963; KIEFFER, 1975). NOTSU iif Q/. (1978) also showed that Na in melts of the Allende meteorite were rapidly volatilized into air by d.c. arc heating. TS~~H~YAMA et a[. (1980b)also found significant loss of Na in some of the residual glasses of experimentally reproduced chondrules (TSUCHIYAMA rt tri., 198Oa). However, the Na contents in chondrules in chondrites are not so depleted compared with Cl-chondrites or the cosmic abundance (OSRORN er (11.. 1973; GCMIDING rt Al.. 1980). In the present study, systematic experiments were conducted on the volatilization of Na in order to determine the rates of volatiIization of Na from liquids with chondrule compositions and to understand the thermal histories of chondrules based on the rate of volatilization of Na. Volatilization studies of molten lunar basalts (NAIJGHTON et (11.. 1971) indicate that the volatilized sodium species are Na atoms with lesser amounts of Na, and Na,O molecules. DONALDSON (1979) studied volatilization of Na and K from an alkali olivine basalt as functions of time, temperature and partial pressure of oxygen, and showed that with increasing temperature or decreasing po,, the rate of volatilization of Na increases, which is consistent with the reaction: melts
at high
temperatures
WALTER and C~KROY, 1964; DOXALVS~N
Na,O(liy)
+ 2Najgas) + i Oz(gas).
Many Na volatilization experiments on melts of cosmochemical interest have been conducted under vacuum conditions. (e.g. GIBSON and HABBARD, 1972; C&DING
and MUENOW, 1977). Heating
experiments
in I-atm gas-mixing furnace using the wire-loop method (DONALIXOK et al., 1975) are good simulations for the Na ~~olatiiizatio~t from chondrules. because the spherical charges used in the experiments are similar to those of chondrules and because po2 can be readily controlled in the experiments. In the wireloop method it is known that the Na volatilization depends on the size of a charge (CORREAK and GIBB, 1979). In the present experiments, volatilization of Na was determined with two chondrule compositions as functions of time. temperature, po, and size. From reaction (1). the Na volatilization is also expected to be dependent on the partial pressure of Na, pN,, However, it is very difficult to control I)~~, and it was not controlled in our experiments.
(1) 1357
EXPERIMENTAL The wire-loop (TSUCHIYAMA
periments.
method
TECHNIQUE
used in the previous
experiments
et rd.. 1980a) was applied to the present ex-
In this study about
40 mg
samples
(about
3.2 mm in diameter) of the chondrule compositions were melted for various durations in the temperature range I45&16OO~C and in the po, range 10-‘0-Z~10-5~0atm in a constant H&IO, gas stream (total flow rate is 420cm3.min‘). Although the charges are greater m size than the majority of chondrules (< I mm and ~2 mg). the size effect can be calibrated by a functional relationship between the volatilization rate of Na and the size as discussed later. After the charges were melted under the given conditions, they were quenched into water. All the charges were clear glass without quench crystals. The glasses were analyzed with the JEOL-5 electron probe micro-analyzer
ing materials (for example. SiO, = 59.4’,,. A1,03 = 4.87”,, and MgO = 22.9”,, for the 12X min run at I505 C and 10m9 ’ atm poZ), At constant temperature and po,. the Na,O content in the glasses decreases with time as shown in Figs 2 and 3. The curves for the variation of Na,O content are stgmoidal in Figs 2 and 3. where the horizontal scales arc log time. whereas the Na,O content decreases exponentially with time. The results for the Fe loss are described in detail in the Ap~&is. Possible contamination of Na during the runs is ruled out because the Na,O content for the blank runs did not increase with time (Table 2). Temperature dependence of the Na volatilization at IO ‘L’ atm po, for sample 1 is shown in Fig. 2. The rate of the Na volatilization becomes greater with increasing temperature: the time required for the decrease of Na,O content to one half of the initial value. f, 2. is about 120. 50, 20 and IOmin at about 1450. 1500, 1550 and 1600 C. respectively. The results for the sample 2 at 1586 C and lOmy ’ atm po, ate also shown in Fig. 2. The volatilization of Na for the sample 2 is faster than that for sample I ; f, 2 for the Na,O content is about 5 min. Figure 3 shows po, dependence of the Na volatilization at about 1500 C for sample I. With decreasing po, the volatilization of Na becomes greater: the time required for the decrease of Na,O content to one half of the initial value is about 800, 200. 50 and 20 min at IO_ 5 0. IO-- “. 10 ‘).’ and IO “‘.’ atm pol. respectively. As shown in Table2. the size of the charge also affects the Na volatilization: at about 1500 C and IO-“’ atm PO,. the Na,O content of the charges during 32 min runs decreases from I.75 to 0.57 WV),, with decreasing the weight of the charges from 89.1 to 4.1 mg. The Na,O content of the run 208 (the weight of the charge is about 40mg) is also consistent with the above results.
Table 1. Chemical compositions (wt%) of the starting materials Sample
1
Sample
53.52 4.61
2
K20
12.14 2.31 22.08 1.32 2.45 0.04
43.55 3.10 0.14 18.52 0.27 29.59 2.30 1.08 0.11
Total
98.48
98.66
SiO? A1203
0.01
TiO? Fe0 MnO Moo C‘?lO Na20
Glasses prepared by meltinq the startinq materials in qraphite capsules at high pressure (10 kb) were analyzed by the JEOL-5 electron probe micro-analyzer. with 15 KV accelerating voltage and 0.02~tA sample current, The electron beam was moved during counting periods to avoid loss of Na. Correction was made after the method of NAKAMUKA and KUSHIRO (19701. Table 1 shows the chemical compositions of the starting materials; sample 1 is a mixture of bronztte (70”,,), albite (20”,,) and olivtne (IO”,,) and sample 2 is the Yamato 74115 (H5) chondrite from which metallic iron has been removed. The stze effects were examined using sample I of various different wetghts (4.lM9.1 mg) for 32 min runs at about 1500 C and lOmy * atm po,. Blank runs were also conducted usmg the melts of composition wtth small amount of diopsidej, anorthite,,) alkalis as tmpurities (Na,O = 0.1 I”,,), in order to examine the possibtlity of contamination by Na during the runs. RESULTS Na,O content of all the charges was uniform throughout the charge within the analytical errors. Examples of the scanning profiles of the charges for Na are given in Fig. I. On the other hand. concentration gradients of Fe were detected near the Pt wires because of diffusion of Fe into the Pt wires during the runs. The results of the experiments are summarized in Table 2. where average bulk compositions of Na,O and Fe0 are given. Because of the losses of Na and Fe, the contents of the other elements such as Si, Al and Mg increase compared with those of the start-
DISCUSSION
Rute qf rolatilizution
of’ No
The present results on the variation of Na,O contents with temperature and po, are consistent with the
Na,O
1-1
01 a
Fig. I. Scanning
4
In,”
I
”
b
32 mnn
c 128
ml”
profiles of Na for the runs at 1505-C and loo”-’ atm po, for different 201 (4 min): (b) run 208 (32 min); (c) run 210 (128 min).
durattona
(a) run
1350
Volatilization of sodium from silicate melt spheres Table Sample
2.
Run
Run conditions Weight (mg)
and results
Temperature (“Cl
Log PO:, (atm)-
; 1 1 1 1 1 1 1 1 1
230 231 232 233 234 276* 239 238 237 236 240 205 204 203 202 201 206 207 208 209 210 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 257 25% 259 260 256 261 262 263 264 265
40 40 40 40 40 40 40 40 40 40 40 40 40 40 40 40 40 40 40 40 40 40 40 40 40 40 40 40 40 40 40 40 40 40 40 40 40 40 40 40 40 89.1 67.4 33.7 15.4 4.1
1607 1607 1607 1607 1.607 1609 1555 1555 1555 1555 1555 1505 1505 1505 1505 1505 I.505 1505 1505 1505 1505 1456 1456 1456 1456 145& I.507 1507 1507 1507 1507 1508 1508 1508 150% 1508 1509 1509 1509 1509 1509 1507 1507 1507 1507 1507
2 2 2 2 2
271 272* 273” 274* 275*
40 40 40 40 40
1586 1586 1586 1586 1586
-9.2 -9.2 -9.2 -9.2 -9.2
277 27% 279
40 40 40
I.506 1506 1506
-9.2 -9.2 -9.2
1
1 1 1 1 1 1 1 1 1 1 1 I 1 1 1 1 1 1 I 1 1 1 1 1 1 1 1 1 : 1 1 1 1
Di50An50 (Blank)
* 0.3 mm@ Pt wires
were
used
of the experiments
-9.2
-9.2 -9.2 -9.2 -9.2 -9.2 -9.2 -9.2 -9.2 -9.2 -9.2 -9.2 -9.2 -9.2 -9.2 -9.2 -9.2 -9.2 -9.2 -9.2 -9.2 -9.2 -9.2 -9.2 -9.2 -9.2 -10.2 -10.2 -10.2 -10.2 -10.2 -7.0 -7.0
-7.0 -7.0 -7.0 -5.0 -5.0 -5.0 -5.0 -5.0 -9.2 -9.2 -9.2 -9.2 -9.2
(0.1 mm&~ Pt wires
results of DONALDSON (1979) for the basattic melts, and the results on the size effect are similar to those of C~RRICAN and GIBB (1979) for basaltic melts. To obtain the rate of volatilization of Na, the following assumptions are made; the amount of Na volatilized from the liquid within unit time, J, is
Duration (min) 1 4 8
Na20 (wt%)
Fe0
(wt%)
1.74 1.35 1.52 1.19 0.62 2.24 2.17 1.83 1.44 0.98 2.29 2.28 2.33 2.32 2.22 2.05 1.91 1.41 0.98 0.59 2.29 2.12 1.98 1.64 0.98 2.15 1.82 1.45 1.02 0.60 2.36 2.13 1.87 1.56 1.06 2.40 2.35 2.18 1.77 1.0" 1.75 1.53 1.37 1.05 0.57
11.60 10.86 9.93 10.98 10.20 8.94 11.84 11.46 11.03 10.02 9.60 12.34 12.30 12.18 12.22 1.1.93 11.56 11.14 10.02 9.22 9.19 11.88 11.52 11.24 Q.%5 9.65 11.75 10.99 10.56 10.01 3.63 11.81 11.71 11.45 10.38 10.91 12.22 12.24 12.06 12.05 12.11 11.78 10.37 10.33 8.85 4.65
1 2 4 8 16
0.91 0.78 0.41 0.24 0.09
18.92 15.72 11.35 10.37 9.20
32 64 128
0.12 0.15 0.14
2.29
9.7 11 20 2 4 8 16 32 0.25 0.5 1 2 4 8 16 32 64 128 4 16 32 64 180 4 8 16 32 64 16 32 64 130 256 32 64 128 330 900 32 32 32 32 32
were
used
in the oter
runs).
assumed to be proportion~~l to the surface area of the liquid sphere, A, and to the concentration of Na,O in the liquid, C. Based on the above assumptions, we obtain J = kAC‘.
(21
1360
Fig. 2. The variation
of the I%,0 content of the charges wirh time at various temperatures at 10e9 ‘atm po,. Symbols: open square. at 1607-1609 C; solid square, at 1555 C’; open circle, at 1505 C and solid circle, at 1456’C for sample 1; and solid triangle, at 1586 C for sample 2. Curves in this and next figures are drawn by visual best-fit.
where k- is the rate of the first order reaction or the rate of volatilization of Na which is considered to be a function of both temperature and poL. Variations of the Na,O concentrations in the liquid with time, C(t), can be calculated by solving the following massbalance equation: &!c= dt with the initial condition
C = f,
_J
(3)
: at
t = 0.
(4)
where I; is the volume of the liquid. Since P* = 4w3/3 and A = 4nrz, eqn (3) is solved with eqns (2) and (4) as follows: In(C/Co) = - 3& Y’ where r is the radius of sphere.
(5)
In order to determine whether or not the present results can be explained by the above model. In (f/Co) is plotted against t (Figs 4 and 5). The plots show straight lines for each set of temperature and po,. The rate of volatilization of Na. k, can be obtained from the slopes of the lines in Figs 4 and 5 by linear regression and is summarized in Table 3. It is noted that the experimental results of DONALDSON (1979) are also consistent with eqn (5). The size effect can be also explained by eqn (5). Because the weight of the charge. W, is proportional to ?. In(C’C,) is plotted against \I’-’ ’ in Fig. 6. These plots give an approximately straight line. (The scatter in Fig. 6 is considered to be mainly due to errors of the proportionality of w‘.-.’ ’ to r; the charges used in the experiments are not perfect spheres but ellipsoids.)
Fig. 3. The variation of the Sal0 content in the charges u-lth time at various i>oj for the sample 1 at about 15OO’C. Symbols: open square. at 10-‘“,2 atm: solid square, at 10 -‘* atm: open circle. at 1tK’ ’ atm and solid circle. at lo-” ” atm.
Volatilization
of sodium
from silicate
.
In C/Co
I
1361
melt spheres
I
1456
0
1505
.
1555
c
1607
“C
100
(4 In C/Co
0
-1
-2 ,
-
_fl-
0
10
5
f m,n
15
(b)
Fig. 4. Plots of In (C/C,,) against time, r, at various temperatures: (a) for sample 1 at 1O-9 ’ atm po,, and (b) for sample 2 at 1586’ C and 10-9.2 atm po,, Symbols are the same as those in Fig. 2. Lines in this and next figures are drawn by a least-square method.
nius plots of k for sample 1. From Fig. 7 the temperature dependence of k can be described as follows: k = k,exp ( - E,,,/R T),
(6)
where E,,, is the activation energy of the Na volatilization, R is the gas constant and k, is considered to be a function of po,. Within the temperature range from 1450 to 16oo”C, E,,, is calculated as about 100 kcal mmole- ’ and k, is about 10” cm.min-’
for the sample 1. Activation energy of Na self diffusion, Ediffr is about 20 kcal.mol- ’ in both obsidian (MAGARITZ and HOFMANN, 1978) and felspar glasses (JAMBONand CARRON, 1976). If the the value of Ediff in the melts of chondrule compositions in the temperature range from 1450 to 1600°C is similar to those in the obsidian or felspar glass, the Na volatilization process is not considered to be controlled by the Na diffusion in the melts. The rate of volatiliza-
A.
1362
TSUCHIYAMA
L’I ul.
r
*
. -1.6 200
400
600
t ml”
Fig. 5. Plots of In (C;C,) against time, t, at various po2 for sample I at about
-----! 1300
800
1500 C. Symbols
arc the
same as those in Fig. 3.
tion of Na for the sample 2 (also plotted in Fig. 7) is greater than the values for the sample 1. This suggests that k in eqn (6) is dependent on the bulk chemical compositions. The sample 2 has a higher MO!Si ratio (M = Mg, Fe and Ca) than the sample 1 (Table 1) and Na might be more easily volatilized from this less polymerized melt. .Efict of po, on k. Figure 8 shows the effect of po, on the rate of volatilization of Na, k. at about 1500-C for the sample 1. From Fig. 8, we obtain the relation that k is approximately proportional to ~0,“~: k, = k&,1:4.
(7)
This result might be consistent with reaction rather than reaction (1):
the following
NaOl Z(liq) $ Na(gas) + 1,;40*(gas), for which the equilibrium
constant
K, = .Lf~:l~~.,o,
@a)
K, is as follows;
the glasses. The loss of Na,O from the melt would involvetwo processes: (i) Na is transported in the liquids to the surface of liquid spheres by diflusion and/‘or convection, and (ii) Na is volatilized at the surface into the gas phase. The fact that the level of Na,O content in the glass decreases with time at constant temperature and po, (e.g. Fig. I) suggests that process (i) is faster than process (ii). Specific time of the Na,O loss, T, for which the Na,O contents in the liquids are decreased to l;‘e of the initial contents. can be estimated for the two processes as follows. If the diffusion is rate limiting, b> solving diffusion equation in spherical coordinate5 (CRANK, 1975) we obtain T,,~,,= r’/n’D. (9) where D is the diffusion coefficient for Na. while if the volatilization is rate limiting, it is evident from eqn (5) that T\Oi~ - r/31\.
(8b)
2.
Homogeneity of‘ Nu20 in the glass. As already mentioned, the Na,O contents are uniform throughout
Figure 9 shows the relation of the specific time with the radius of the liquid sphere. r. estimated from
The rate of volatilization of Na, k, and Table 3. the specific time of the Na volatilization, ~~~1, obtained in the experiments Sample No. 1
1 1 1 1 1 1 2
Temperature ("C) 1607 1555 1505 1456 1507 1508 1509 1586
(101
log
(atm) -9.2 -9.2 -9.2 -9.2 -10.2 -7.0 -5.0 -9.2
7
k
pCj2 (cm.min
-1
3 .5x1o-3 1.5x10-3 6.1~10-~ 2.6~10-~ 1.2x1o-3 1.7x10-4 4.9x1o-5 8.8x1o-3
)
vol (min)
15 35 88 210 46 310 1100 6.1
Volatilization
of sodium
from silicate
1363
melt spheres I
-
InC.Co0
F1 \ \
-.2
\ \
1
\ \
-.4
l
\ \. I
.6
‘;.
t
\ \
-.a
‘\ \ \
1
A -1.9
L
\ \ \ \
1 2
\ \
t 1.4
\ \
r
4 1.6
c I 1
0
2
3
4 W
Fig. 6. Plots of In(C‘:C,)
against
(weight)-
’ 3 for
sample
this figure is drawn
eqns (9) and (IO). The solid and dashed lines show the rates of loss of Na,O controlled by diffusion and volatilization respectively. The specific time of the present results (circle in Fig. S), which corresponds to TV”,. suggests that D in the liquid used in the present experiments must be greater than about 10-‘cm2~sec~’ at about 145s.1600 C, if a mechanical mixing such as convection would not be present in the liquid spheres. LOFGREN (1977) reported con1650
1600
5
(@l/3)
1 at 1505-1507
6
7
C and lo-‘*
atm pal. A line in
by visual best-fit.
vectional patterns of microllites in glass spheres of synthetic basalts. In the present experiments, however, convection is not considered to be an effective process of mass-transportation, because the concentration gradients of Fe near the Pt wires still existed after the runs. In fact. sdif., values, estimated from the extrapolated data of the Na-self-diffusion coefficients in obsidian glass (MAGARITZ and HOFMANN, 1978) (triangles in Fig. 8) are less than the values of t,,, of the present
1550
I
-l/3
1500
1
1450
1
C
I
-2 A
\ log
0
k \
0
-3
\
0
\
0 \
I
-4 5.2
5.4
5.6
Fig 7. The Arrhenius plot of the rate of volatilization of Na,O, (triangle). Lines in this and next figures are drawn
1/T
5.6 x,O-~
K-’
for sample 1 (circles) and for sample by a least-square method.
2
1364
-2-
-6
-12
-10
-6
-6
-4
log 4, Fig. 8. Plots of log k against log po, for sample 1 at about results. (Because the diffusion coefficients in the chondrule melts are considered to be greater than those in the obsidian melt, ~~~~~ for the chondrule melts should be much less than T,,, .) Application of’ the rates of c’olutilization c~fNN to chondrules Models for the formation main types: (i) chondrules
of chondrules are directly
are of two condensed
1500 C.
from the solar nebula (e.g. W~IJJD. 1963: BLANDERand KATZ, 1967). and (ii) chondrules are the products of remelting of preexisting materials (e.g. KIEFFER. 1975). BLAKDER and KATZ (1967) proposed that metastable liquid droplets (chondrules) were directly condensed from the solar nebula. If this is the case, Na,O in the chondrule melts were in equilibrium with Na and O2 in the solar nebula and the Na,O contents of
1000
Fig. 9. The relation between the specific time and the radius of the liquid sphere. The solid and dashed lines show *diff and T”“, respectively. The experimental results of T,~, (cn-cles) and the estimated values of rd,,, (triangles, see text) are also given.
I365
Volatilization of sodium from silicate melt spheres
TO
/.I/
L
In 1.
1
\ -
T
_I-
> l/T,..
l/T
Fig. IO. A schematic diagram showing relations of 5di,r and T,,, with l/T(K-‘). T,,, > rdifl at temperatures less than T,,,, because E,,, > E,,,,-.
the chondrules should be dependent on reaction @a). The equilibrium constant for this reaction is given by eqn (8b). Although the value of K, is not known at present, it seems likely that K 1 is significantly large at high temperature because of high-vapour pressure of Na; in consequence, most of the Na in the solar nebeula was not condensed into the liquid droplets at high temperature. If chondrules are formed by a local heating of preexisting materials, the Na,O contents in the chondrules can be discussed using the above model with the rate of volatilization of Na, k, combined with the variation of temperature of chondrules and po, of the solar nebula (dependence of k on temperature and po, is shown by eqns (6) and (7) respectively). Although pNa also affects the volatilization of Na, as already mentioned, pNd in the solar nebula at this stage is considered to have been nearly equal to zero because most of Na in the solar nebula had been already condensed to albite at 70@9Oo”C (GROSSMANand LARIMER, 1977); that is, the effect of Na can be ignored. In the above model, the homogeneity of Na,O is assumed. The homogeneity in the liquid chondrules can be explained by comparing the two specific terms of the Na diffusion and Na volatilization, rdiff and 5“0,’ given in eqns (9) and (10) respectively. They are rewritten as follows;
ti-
t
Fig. 11. A Schematicaly drawn thermal history of chondrules.
An instant
heating followed by an immediate ing is assumed.
cool-
From eqn (13) T,,, is approximately 2300’ C (1. = lo- ’ cm, k, = 10’Ocm~min-’ and D = 1 cm’. set-‘); that is, the liquid chondrules are estimated to be homogeneous at temperatures less than about 2300°C. We assume that the chondrules are formed by an instant heating at time t = 0 and then cooled according to the Stefan-Boltzman relation: dT/dt = -b(T4
- T;Z,,),
(14)
where T and Tnehare the absolute temperatures of the chondrule and of the solar nebula respectively and h is the proportional constant. The thermal history assumed above is illustrated in Fig. 11. Variation of Na,O contents in the liquid chondrules with time or temperature can be obtained by solving the massbalance eqn (3) and eqn (14) with the initial conditions : C = Co and T = To at t = 0,
(15)
together with eqns (2) and (6) assuming that po, in that solar nebula is constant (that is, k, in eqn (6) is constant). Temperature of the nebula at the time is considered to be sufficiently low (Tnch << 700 C). Because k c 0 for 7’ < 700°C and T4 x Tze,, for T > 700 C, Tneb can be effectively assumed to be as -273-C (OK) in this calculation, so that the solution is ln(C/Co) = (3k,:hr)(RIE,,,)CF(T)
(11)
- UT,)],
(16)
where F(T) and F(T,) are defined as follows;
and r,,~ = (ri3k0)exp(R,,,!RT),
(12)
where the Arrhenius relation is assumed for D and k. is greater than Because E,,, (about 100 kcal.mole-‘) Edirf (about 20 kcal.mole-I), 7vol is greater than rdifr (the liquid chondrules are homogeneous) in a lower temperature region (Fig. 10). Putting 7vol = 7diff, then we obtain the maximum temperature, T,,,, below which TV,,,> ~~,rr: Trnll .I. = (EL,.,, I, - E,,,,,)lRln(3rk,/n’D,). _...
(13)
F(x) = [(l/x
+ R/&J2
+ (R/Ev0J2]exp(
- f&,/RX):
x = Tor To.
(17)
From eqn (16), C(t- co) or C(T= Tnuh),which corresponds to the final concentration of Na,O in the chondrule, Cc,,, can be obtained as functions of Co and To: ln(C,,lC,) where
= -(3k,lbr)(RIE,,,,)~(T,),
7;lcb = - 273°C
(OK) is also
assumed.
(18) The
1366
with the maximum temperature of the heating. r,. estimated from eqn (18)for from IO’” to 10Zh C3. At temperatures
various k,, hr in the range greater than about 2300 C. the effect of the diffusion in the liquid should be also conbidered.
relation of the ratio of the final and initial NazO concentrations of the chondrules, C,,/‘C,, with the maximum temperature of the heating process, 7& are given in Fig. 12 for various k,,/hr values (E,,, = l~kcal.mole-~). Crystallization of chondrules affects Na volatilization; crystals covering the surface of chondrules prevent Na volatilization, and Na is enriched in residual !iquids due to fractional crystallization. Because these two effects compensate each other, crystallization of chondrules is not taken into consideration in the following estimation. The value of b is approximately 10~‘l”C-3,min~l because the cooling rate of the chondrule is estimated to be about 100-C’min-’ at about 15OO‘C by experimentally-produced barred olivine chondrule (TSUCHIYAMArt ul., 1980a). This cooling rate may be a maximum value because recent experimental studies reveal that radial pyroxene chondrules are produced with cooling rates of about I’ C-min.- i (HEWINSet cd.,1981; NAGAHARAand TsuCHIYAMA,in preparation). The value of k, depends on p,,, in the solar nebula and on the bulk chemical compositions of chondrules. Value of po, in the solar nebula probably ranged from lo-” atm (at 1500 C) to 10e2’ atm (at 700°C) as estimated from the equilibrium experiments in the olivine-pyroxene~metallic iron system (WILLIAMS, 1971). Though the effect of the bulk chemical composition on k, is not well known, the variation of k, seems to be within the order of unity based on Fig. 7 and on the assumption that E,,, is not affected by the buik chemical composition. If & is 10” cm.min-‘. as obtained in the present experiments at 10~g.2 atm po, (for the sample 1, and the
radius of the chondrule. r, is 10 ’ cm. then ii,,& 13 approximately 102’ C3. k,:br is increased by one order of magnitude on decreasing po, by four orders of magnitude, because I\,, is proportional to 170~’’ (Fig. 8). The calculated value of C,,,, C, from Fig. 12 for T = 1700 C is consistent with the values of C,,,/C,, about 0.551.0, estimated from the Na ratio of the ordinary chondrites and C-chondrites (OSROKN c’r Q/., 1973; &ODIh.G et a/., 1980). It is also noted in Fig. 12 that c‘,,:C, abruptly changes from zero to unity with decreasing 7;). This suggests that the maximum temperature of the chondrule, 70. is not extremely high; that is less than about 2200 C. if we assume ko,!hr = 10z2’* C”. On the other hand, many chondrules were considered to be completely melted. In such a case. 7;, must be greater than the liquidus temperature of chondrules (within the range from 1400 to 1700 C). Therefore, the maximum temperature of the heating process must be restricted to thz range from about 140@-2200 ‘C. if the chondrules were formed by local heating of the preexisting materials. il~hlfOll.lCd~~e))lPnf,s~~We are grateful to Profeswr N. OW:MA of Tsukuba Universit), Profesor H. NAGASAWA of Gakushuin Unwersity, Dr S. KAHTO of the Unwersity of Tokyo and Dr E. TAKAHASM of the Geophysical Laborntory, Carnegie Institution, for critical reading the manuscript. Thanks are also due to Professor T. NAGATA of the National Institute of Polar Research. Japan. for providing Yamato chondrite used in the present experiments. REFERENCES
BLA~GDER M. mordial
and KATZ J. L. (1967) Condensation of pridust. Gwclzim. Cowoci~irtr. A<,!cf 31, 1025 1034.
Volatilization
of sodium
CORRIGAV G. and GIBB G. F. (1979) The loss of Fe and Na
from a basaltic melt during experiments using wire-loop method. Minrrul. Mug. 43, 121~126. CRANK _I.(1975) Tllr Mathrntutics $Dif(usion. 414pp. Clarendon Press. DoE;At.r)soN C. H.. WILLIAMS R. J. and LOFGREN G. E. (1975) A sample holding technique for study of crystal growth in silicate melts. 4nl. Minrraf. 60, 324326. DONALIXON C. H. (1979) Composition changes in basalt melt contained in a wire loop of Pt,,RhZO: effect of temperature. time and oxygen fugasity. ,Mincrtr/. Msg. 43, 115~119. GIBSON E. K. and HUBBARII N. .I. (1972) Thermal volatilezation studies on lunar samples. Proc. 3rd Luncv Plunc,t. %I. ClIr$. pp. 2003 1014. GooI,I~(; J. L. and MUENOW D. W. (1977) Experimental vaporlration of the Holdrook chondrite. Meteoritics 12, 401~408. Go~I>IN~; J. L.. KEIL K.. F~IUJ~KA T. and SCHMITT R. A. (1980) Elemental abundances in chondrules from unequilibrated chondrites: evidence for chondrule origin by melting of pre-existing materials. Eurth Plunrr. Sci. Lelt. 50, 171~180. GK~SSMAN L. and LARIMER J. W. (1977) Early chemical history of the solar system. Rrr. Geopln\. Spuce Phys. 12, 71 101. H~wrus R. H.. KLEIN L. C. and FASAKO B. V. (1981) Conditions of formation of pyroxene excentroradlal chondrules (abstract). In Lunur Pkmrr. Sci. XII. JAMBOY A. and CARRON J. P. (1976) Diffusion of Na, K. Rb and c’s In glasses of alblte and orthoclase composition, Geoc~hini. Co.smochitn. ACfU 40, 897-903. KIE~~ER S. W. (1975) Droplet chondrules. Science 189, 333 -330. LOF(;RE~ G. E. (1977) Dynamic crystallization experiments bearing on the origin of textures in impact generated liquid\. Proc. Srh Lanur Planr~. Sd. Cor$, pp. 2079-2095. Lovt RI&C;T. S. (1960) High temperature fusion of possible parent materials for tektites. Nuture 186, 1028~1031. MA<,~RITZ M. and HOFLIANU A. W. (1978) Diffusion of Sr, Ba and Na in obsidian. G~chirn. Covnochim. Ac.ru 42, 595m605. NAK*MI.R\ Y. and KUSHIRO 1. (1970) Compositional relations of coexisting orthopyroxene. pigeonite and aguite In a tholeiitlc andesite from Hakone Volcano. Conrrlh. Minrrul. Prrrol. 26, 265-~275. NAUC~H-IONJ. J., DEKHV J. V. and LEWIS V. A. (1971) VolatihLatlon from heated lunar samples and the investigation of lunar eroSIon by volatihzed alkalis. Proc. 2nd LMtrr Plunrt. sci. Con/., pp. 449-457. N01sll K.. O~UMA N.. NISHIDA N. and NAC;ASAWA H. (1978) High temperature heating of the Allende meteorite. Gcv&in~. Covnwhim. Acta 42, 903-907. OSBOR~ T. W.. S~ITFI R. H. and SCHMITT R. A. (1973) Elemental composition of individual chondrules from ordmary chondrites. Geoc~hirn. Cosmochim. ,A(tu 37, 1909 1942. TSL’(‘HIYAMAA., NA(;AHARA H. and KUSHIRO I. (198Oa) Ex-
from silicate
1367
melt spheres
perimental reproduction of textures of chondrules. Eurth Plunrt. Sci. Left. 48, 155~-165. TSUC‘HIYAMAA., NA(;AHARA H. and KCJSHIKO I. (1980b) Investigations on the experimentally produced chondrules: chemical compositions of ohvine and glass. and formation of radial pyroxene chondrules. ,2letn. Not. In.st. Polar Res. Sprc. I~.vrc~17, 83-~94. WALTER L. S. and CARRON M. K. (1964) Vapor pressure and vapor fractionation of silicate melts of tektite composition. Geoclnn~. Co\tnochirn. Acta 2X, 937 951. WlLLiAhfs R. J. (1971) Equilibrium temperatures. pressures. and oxygen fugacities of the equilibrated chondrites. Gwchim. Co\ornockinl. Acitr 35, 407- 4 I I Wooo J. A. (1963) On the origin of chondrules and chondrites. Iaurns 2, 151 1X0. APPENDIX
Pt wires of 0.1 mmcb were used in the present experiments except for the runs 272-275 and 279 (0.3 mm4). Concentration gradients of Fe were detected near the Pt wires. Average values of the Fe0 content m the charges are given in Table 2. At constant temperature and po,. the Fe0 content in the charges decreases with time. which is similar to the Fe loss from a basaltic melt in PtsORhzO wire loops (DONALDSON. 1979) and in Ag,,Pd,” wire loops (CORRI~;AN and GIBB. 1979). The Fe loss increases with increasing temperature In a basaltic melt (DOUALUSON. 1979). Similar results are obtained m the present experiments: times required for 20”,, loss of Fe0 are about 100. 40, 20 and 10 mln at about 1450. 1500. 1550 and 1600 C respectively at IO-‘.’ atmpo,. DONALI)SO’. (1979) also investigated the effect of po, on the loss of Fe and concluded that the Fe loss increased with decreasing poZ. Similar results are obtained in the present experiments: times required for the 20”,, loss of Fe0 are about 1000. 50 and 20min at lo--“. 10 9.2 and IO-‘” ’ atm pol respectively at about 1500 C for sample 1. .4t lo- ‘.” atm po,. no significant loss of Fe0 was detected within 1000 min. The size of the charges also affects the Fe loss. Fe loss increases with decreasing sire; for 32 min runs for the sample 1. no significant loss of Fe was detected for 89.1 mg charge whereas about 60”,, loss of Fe0 was detected for 4.1 mg charge. This is because the ratio of the contact area of the Pt wires with the melts and volume of the melts increases with decreasing the size of the charge. On the other hand. there is no significant difference in Fe loss between the Pt wires of different diameter for runs at about 1600 C and 10~’ ’ atm po, for the sample 1 and for runs at 1586 C and 10m9’ atm poi for the sample 2. The rate of the Fe loss for the sample 2 IS greater than that for the sample I; It takes only a few mmutes for the 20”,, loss of Fe0 for the sample 2 but a few tens of minutes for the sample 1. This may be due to greater diffusion coefficient of Fe in the less polymerized melt of the sample 2.
TSUCHIYAMA.
HIKO~CONAGAHARA and IKUO KUSHIRO
Geological Institute, Um\ersity
of Tokyo,
Hongo.
Tokyo.
113.Japan
Abstrart -The rates of volatilization of Na from liquid spheres of chondrule compositions have been determined as functions of time, temperature. partial pressure of oxygen. and sizes of the spheres. The Na,O content in the sphere is uniform in each run. but It decreases with time of the run, indicating that the rate of diffusion of Na in the liquid is greater than that of volatilization. and that the latter is the rate-controlling process. The rate of sodium volatilization becomes greater with increasing temperature and with decreasing PO2 and size of the spheres. The relation of the Na,O content in the liquid sphere with time and its &e indicate that the amount of Na,O volatilized from the liquid spheres within unit time is proportional to the surface area of the spheres and the concentration of Na,O in the liquid. From these relations, the rate of volatilization of sodium can be obtained at constant temperature and poZ, The rate of volatilization of sodium satisfies the Arrhenius relation within the temperature range from about 14SO--1600 C at 10 -92 atm po,; the activation energy for the sodium volatilization is approximately 100kcal mole -I. The rate is also approximately proportional to ~0~’ ’ within the range of po2 from IO Ii’ ’ to lo- ’ ’ atm at about 1500 C. Based on the present results and the NazO contents m chondrules. it is suggested that they experienced an instant healing with maximum temperature of 1400~~2200 C followed-by an immediate c&o&g.
INTRODUCTION IT IS KNOWN that alkalis voiatitize
easily from silicate
(e.g. LOVERING. 1960; et rd., 1975). It is likely therefore that alkalis were partially volatilized from chondrules, because the chondrules are believed to have been formed by crystallization from liquid at high temperatures (e.g. WOOQ 1963; KIEFFER, 1975). NOTSU iif Q/. (1978) also showed that Na in melts of the Allende meteorite were rapidly volatilized into air by d.c. arc heating. TS~~H~YAMA et a[. (1980b)also found significant loss of Na in some of the residual glasses of experimentally reproduced chondrules (TSUCHIYAMA rt tri., 198Oa). However, the Na contents in chondrules in chondrites are not so depleted compared with Cl-chondrites or the cosmic abundance (OSRORN er (11.. 1973; GCMIDING rt Al.. 1980). In the present study, systematic experiments were conducted on the volatilization of Na in order to determine the rates of volatiIization of Na from liquids with chondrule compositions and to understand the thermal histories of chondrules based on the rate of volatilization of Na. Volatilization studies of molten lunar basalts (NAIJGHTON et (11.. 1971) indicate that the volatilized sodium species are Na atoms with lesser amounts of Na, and Na,O molecules. DONALDSON (1979) studied volatilization of Na and K from an alkali olivine basalt as functions of time, temperature and partial pressure of oxygen, and showed that with increasing temperature or decreasing po,, the rate of volatilization of Na increases, which is consistent with the reaction: melts
at high
temperatures
WALTER and C~KROY, 1964; DOXALVS~N
Na,O(liy)
+ 2Najgas) + i Oz(gas).
Many Na volatilization experiments on melts of cosmochemical interest have been conducted under vacuum conditions. (e.g. GIBSON and HABBARD, 1972; C&DING
and MUENOW, 1977). Heating
experiments
in I-atm gas-mixing furnace using the wire-loop method (DONALIXOK et al., 1975) are good simulations for the Na ~~olatiiizatio~t from chondrules. because the spherical charges used in the experiments are similar to those of chondrules and because po2 can be readily controlled in the experiments. In the wireloop method it is known that the Na volatilization depends on the size of a charge (CORREAK and GIBB, 1979). In the present experiments, volatilization of Na was determined with two chondrule compositions as functions of time. temperature, po, and size. From reaction (1). the Na volatilization is also expected to be dependent on the partial pressure of Na, pN,, However, it is very difficult to control I)~~, and it was not controlled in our experiments.
(1) 1357
EXPERIMENTAL The wire-loop (TSUCHIYAMA
periments.
method
TECHNIQUE
used in the previous
experiments
et rd.. 1980a) was applied to the present ex-
In this study about
40 mg
samples
(about
3.2 mm in diameter) of the chondrule compositions were melted for various durations in the temperature range I45&16OO~C and in the po, range 10-‘0-Z~10-5~0atm in a constant H&IO, gas stream (total flow rate is 420cm3.min‘). Although the charges are greater m size than the majority of chondrules (< I mm and ~2 mg). the size effect can be calibrated by a functional relationship between the volatilization rate of Na and the size as discussed later. After the charges were melted under the given conditions, they were quenched into water. All the charges were clear glass without quench crystals. The glasses were analyzed with the JEOL-5 electron probe micro-analyzer
ing materials (for example. SiO, = 59.4’,,. A1,03 = 4.87”,, and MgO = 22.9”,, for the 12X min run at I505 C and 10m9 ’ atm poZ), At constant temperature and po,. the Na,O content in the glasses decreases with time as shown in Figs 2 and 3. The curves for the variation of Na,O content are stgmoidal in Figs 2 and 3. where the horizontal scales arc log time. whereas the Na,O content decreases exponentially with time. The results for the Fe loss are described in detail in the Ap~&is. Possible contamination of Na during the runs is ruled out because the Na,O content for the blank runs did not increase with time (Table 2). Temperature dependence of the Na volatilization at IO ‘L’ atm po, for sample 1 is shown in Fig. 2. The rate of the Na volatilization becomes greater with increasing temperature: the time required for the decrease of Na,O content to one half of the initial value. f, 2. is about 120. 50, 20 and IOmin at about 1450. 1500, 1550 and 1600 C. respectively. The results for the sample 2 at 1586 C and lOmy ’ atm po, ate also shown in Fig. 2. The volatilization of Na for the sample 2 is faster than that for sample I ; f, 2 for the Na,O content is about 5 min. Figure 3 shows po, dependence of the Na volatilization at about 1500 C for sample I. With decreasing po, the volatilization of Na becomes greater: the time required for the decrease of Na,O content to one half of the initial value is about 800, 200. 50 and 20 min at IO_ 5 0. IO-- “. 10 ‘).’ and IO “‘.’ atm pol. respectively. As shown in Table2. the size of the charge also affects the Na volatilization: at about 1500 C and IO-“’ atm PO,. the Na,O content of the charges during 32 min runs decreases from I.75 to 0.57 WV),, with decreasing the weight of the charges from 89.1 to 4.1 mg. The Na,O content of the run 208 (the weight of the charge is about 40mg) is also consistent with the above results.
Table 1. Chemical compositions (wt%) of the starting materials Sample
1
Sample
53.52 4.61
2
K20
12.14 2.31 22.08 1.32 2.45 0.04
43.55 3.10 0.14 18.52 0.27 29.59 2.30 1.08 0.11
Total
98.48
98.66
SiO? A1203
0.01
TiO? Fe0 MnO Moo C‘?lO Na20
Glasses prepared by meltinq the startinq materials in qraphite capsules at high pressure (10 kb) were analyzed by the JEOL-5 electron probe micro-analyzer. with 15 KV accelerating voltage and 0.02~tA sample current, The electron beam was moved during counting periods to avoid loss of Na. Correction was made after the method of NAKAMUKA and KUSHIRO (19701. Table 1 shows the chemical compositions of the starting materials; sample 1 is a mixture of bronztte (70”,,), albite (20”,,) and olivtne (IO”,,) and sample 2 is the Yamato 74115 (H5) chondrite from which metallic iron has been removed. The stze effects were examined using sample I of various different wetghts (4.lM9.1 mg) for 32 min runs at about 1500 C and lOmy * atm po,. Blank runs were also conducted usmg the melts of composition wtth small amount of diopsidej, anorthite,,) alkalis as tmpurities (Na,O = 0.1 I”,,), in order to examine the possibtlity of contamination by Na during the runs. RESULTS Na,O content of all the charges was uniform throughout the charge within the analytical errors. Examples of the scanning profiles of the charges for Na are given in Fig. I. On the other hand. concentration gradients of Fe were detected near the Pt wires because of diffusion of Fe into the Pt wires during the runs. The results of the experiments are summarized in Table 2. where average bulk compositions of Na,O and Fe0 are given. Because of the losses of Na and Fe, the contents of the other elements such as Si, Al and Mg increase compared with those of the start-
DISCUSSION
Rute qf rolatilizution
of’ No
The present results on the variation of Na,O contents with temperature and po, are consistent with the
Na,O
1-1
01 a
Fig. I. Scanning
4
In,”
I
”
b
32 mnn
c 128
ml”
profiles of Na for the runs at 1505-C and loo”-’ atm po, for different 201 (4 min): (b) run 208 (32 min); (c) run 210 (128 min).
durattona
(a) run
1350
Volatilization of sodium from silicate melt spheres Table Sample
2.
Run
Run conditions Weight (mg)
and results
Temperature (“Cl
Log PO:, (atm)-
; 1 1 1 1 1 1 1 1 1
230 231 232 233 234 276* 239 238 237 236 240 205 204 203 202 201 206 207 208 209 210 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 257 25% 259 260 256 261 262 263 264 265
40 40 40 40 40 40 40 40 40 40 40 40 40 40 40 40 40 40 40 40 40 40 40 40 40 40 40 40 40 40 40 40 40 40 40 40 40 40 40 40 40 89.1 67.4 33.7 15.4 4.1
1607 1607 1607 1607 1.607 1609 1555 1555 1555 1555 1555 1505 1505 1505 1505 1505 I.505 1505 1505 1505 1505 1456 1456 1456 1456 145& I.507 1507 1507 1507 1507 1508 1508 1508 150% 1508 1509 1509 1509 1509 1509 1507 1507 1507 1507 1507
2 2 2 2 2
271 272* 273” 274* 275*
40 40 40 40 40
1586 1586 1586 1586 1586
-9.2 -9.2 -9.2 -9.2 -9.2
277 27% 279
40 40 40
I.506 1506 1506
-9.2 -9.2 -9.2
1
1 1 1 1 1 1 1 1 1 1 1 I 1 1 1 1 1 1 I 1 1 1 1 1 1 1 1 1 : 1 1 1 1
Di50An50 (Blank)
* 0.3 mm@ Pt wires
were
used
of the experiments
-9.2
-9.2 -9.2 -9.2 -9.2 -9.2 -9.2 -9.2 -9.2 -9.2 -9.2 -9.2 -9.2 -9.2 -9.2 -9.2 -9.2 -9.2 -9.2 -9.2 -9.2 -9.2 -9.2 -9.2 -9.2 -9.2 -10.2 -10.2 -10.2 -10.2 -10.2 -7.0 -7.0
-7.0 -7.0 -7.0 -5.0 -5.0 -5.0 -5.0 -5.0 -9.2 -9.2 -9.2 -9.2 -9.2
(0.1 mm&~ Pt wires
results of DONALDSON (1979) for the basattic melts, and the results on the size effect are similar to those of C~RRICAN and GIBB (1979) for basaltic melts. To obtain the rate of volatilization of Na, the following assumptions are made; the amount of Na volatilized from the liquid within unit time, J, is
Duration (min) 1 4 8
Na20 (wt%)
Fe0
(wt%)
1.74 1.35 1.52 1.19 0.62 2.24 2.17 1.83 1.44 0.98 2.29 2.28 2.33 2.32 2.22 2.05 1.91 1.41 0.98 0.59 2.29 2.12 1.98 1.64 0.98 2.15 1.82 1.45 1.02 0.60 2.36 2.13 1.87 1.56 1.06 2.40 2.35 2.18 1.77 1.0" 1.75 1.53 1.37 1.05 0.57
11.60 10.86 9.93 10.98 10.20 8.94 11.84 11.46 11.03 10.02 9.60 12.34 12.30 12.18 12.22 1.1.93 11.56 11.14 10.02 9.22 9.19 11.88 11.52 11.24 Q.%5 9.65 11.75 10.99 10.56 10.01 3.63 11.81 11.71 11.45 10.38 10.91 12.22 12.24 12.06 12.05 12.11 11.78 10.37 10.33 8.85 4.65
1 2 4 8 16
0.91 0.78 0.41 0.24 0.09
18.92 15.72 11.35 10.37 9.20
32 64 128
0.12 0.15 0.14
2.29
9.7 11 20 2 4 8 16 32 0.25 0.5 1 2 4 8 16 32 64 128 4 16 32 64 180 4 8 16 32 64 16 32 64 130 256 32 64 128 330 900 32 32 32 32 32
were
used
in the oter
runs).
assumed to be proportion~~l to the surface area of the liquid sphere, A, and to the concentration of Na,O in the liquid, C. Based on the above assumptions, we obtain J = kAC‘.
(21
1360
Fig. 2. The variation
of the I%,0 content of the charges wirh time at various temperatures at 10e9 ‘atm po,. Symbols: open square. at 1607-1609 C; solid square, at 1555 C’; open circle, at 1505 C and solid circle, at 1456’C for sample 1; and solid triangle, at 1586 C for sample 2. Curves in this and next figures are drawn by visual best-fit.
where k- is the rate of the first order reaction or the rate of volatilization of Na which is considered to be a function of both temperature and poL. Variations of the Na,O concentrations in the liquid with time, C(t), can be calculated by solving the following massbalance equation: &!c= dt with the initial condition
C = f,
_J
(3)
: at
t = 0.
(4)
where I; is the volume of the liquid. Since P* = 4w3/3 and A = 4nrz, eqn (3) is solved with eqns (2) and (4) as follows: In(C/Co) = - 3& Y’ where r is the radius of sphere.
(5)
In order to determine whether or not the present results can be explained by the above model. In (f/Co) is plotted against t (Figs 4 and 5). The plots show straight lines for each set of temperature and po,. The rate of volatilization of Na. k, can be obtained from the slopes of the lines in Figs 4 and 5 by linear regression and is summarized in Table 3. It is noted that the experimental results of DONALDSON (1979) are also consistent with eqn (5). The size effect can be also explained by eqn (5). Because the weight of the charge. W, is proportional to ?. In(C’C,) is plotted against \I’-’ ’ in Fig. 6. These plots give an approximately straight line. (The scatter in Fig. 6 is considered to be mainly due to errors of the proportionality of w‘.-.’ ’ to r; the charges used in the experiments are not perfect spheres but ellipsoids.)
Fig. 3. The variation of the Sal0 content in the charges u-lth time at various i>oj for the sample 1 at about 15OO’C. Symbols: open square. at 10-‘“,2 atm: solid square, at 10 -‘* atm: open circle. at 1tK’ ’ atm and solid circle. at lo-” ” atm.
Volatilization
of sodium
from silicate
.
In C/Co
I
1361
melt spheres
I
1456
0
1505
.
1555
c
1607
“C
100
(4 In C/Co
0
-1
-2 ,
-
_fl-
0
10
5
f m,n
15
(b)
Fig. 4. Plots of In (C/C,,) against time, r, at various temperatures: (a) for sample 1 at 1O-9 ’ atm po,, and (b) for sample 2 at 1586’ C and 10-9.2 atm po,, Symbols are the same as those in Fig. 2. Lines in this and next figures are drawn by a least-square method.
nius plots of k for sample 1. From Fig. 7 the temperature dependence of k can be described as follows: k = k,exp ( - E,,,/R T),
(6)
where E,,, is the activation energy of the Na volatilization, R is the gas constant and k, is considered to be a function of po,. Within the temperature range from 1450 to 16oo”C, E,,, is calculated as about 100 kcal mmole- ’ and k, is about 10” cm.min-’
for the sample 1. Activation energy of Na self diffusion, Ediffr is about 20 kcal.mol- ’ in both obsidian (MAGARITZ and HOFMANN, 1978) and felspar glasses (JAMBONand CARRON, 1976). If the the value of Ediff in the melts of chondrule compositions in the temperature range from 1450 to 1600°C is similar to those in the obsidian or felspar glass, the Na volatilization process is not considered to be controlled by the Na diffusion in the melts. The rate of volatiliza-
A.
1362
TSUCHIYAMA
L’I ul.
r
*
. -1.6 200
400
600
t ml”
Fig. 5. Plots of In (C;C,) against time, t, at various po2 for sample I at about
-----! 1300
800
1500 C. Symbols
arc the
same as those in Fig. 3.
tion of Na for the sample 2 (also plotted in Fig. 7) is greater than the values for the sample 1. This suggests that k in eqn (6) is dependent on the bulk chemical compositions. The sample 2 has a higher MO!Si ratio (M = Mg, Fe and Ca) than the sample 1 (Table 1) and Na might be more easily volatilized from this less polymerized melt. .Efict of po, on k. Figure 8 shows the effect of po, on the rate of volatilization of Na, k. at about 1500-C for the sample 1. From Fig. 8, we obtain the relation that k is approximately proportional to ~0,“~: k, = k&,1:4.
(7)
This result might be consistent with reaction rather than reaction (1):
the following
NaOl Z(liq) $ Na(gas) + 1,;40*(gas), for which the equilibrium
constant
K, = .Lf~:l~~.,o,
@a)
K, is as follows;
the glasses. The loss of Na,O from the melt would involvetwo processes: (i) Na is transported in the liquids to the surface of liquid spheres by diflusion and/‘or convection, and (ii) Na is volatilized at the surface into the gas phase. The fact that the level of Na,O content in the glass decreases with time at constant temperature and po, (e.g. Fig. I) suggests that process (i) is faster than process (ii). Specific time of the Na,O loss, T, for which the Na,O contents in the liquids are decreased to l;‘e of the initial contents. can be estimated for the two processes as follows. If the diffusion is rate limiting, b> solving diffusion equation in spherical coordinate5 (CRANK, 1975) we obtain T,,~,,= r’/n’D. (9) where D is the diffusion coefficient for Na. while if the volatilization is rate limiting, it is evident from eqn (5) that T\Oi~ - r/31\.
(8b)
2.
Homogeneity of‘ Nu20 in the glass. As already mentioned, the Na,O contents are uniform throughout
Figure 9 shows the relation of the specific time with the radius of the liquid sphere. r. estimated from
The rate of volatilization of Na, k, and Table 3. the specific time of the Na volatilization, ~~~1, obtained in the experiments Sample No. 1
1 1 1 1 1 1 2
Temperature ("C) 1607 1555 1505 1456 1507 1508 1509 1586
(101
log
(atm) -9.2 -9.2 -9.2 -9.2 -10.2 -7.0 -5.0 -9.2
7
k
pCj2 (cm.min
-1
3 .5x1o-3 1.5x10-3 6.1~10-~ 2.6~10-~ 1.2x1o-3 1.7x10-4 4.9x1o-5 8.8x1o-3
)
vol (min)
15 35 88 210 46 310 1100 6.1
Volatilization
of sodium
from silicate
1363
melt spheres I
-
InC.Co0
F1 \ \
-.2
\ \
1
\ \
-.4
l
\ \. I
.6
‘;.
t
\ \
-.a
‘\ \ \
1
A -1.9
L
\ \ \ \
1 2
\ \
t 1.4
\ \
r
4 1.6
c I 1
0
2
3
4 W
Fig. 6. Plots of In(C‘:C,)
against
(weight)-
’ 3 for
sample
this figure is drawn
eqns (9) and (IO). The solid and dashed lines show the rates of loss of Na,O controlled by diffusion and volatilization respectively. The specific time of the present results (circle in Fig. S), which corresponds to TV”,. suggests that D in the liquid used in the present experiments must be greater than about 10-‘cm2~sec~’ at about 145s.1600 C, if a mechanical mixing such as convection would not be present in the liquid spheres. LOFGREN (1977) reported con1650
1600
5
(@l/3)
1 at 1505-1507
6
7
C and lo-‘*
atm pal. A line in
by visual best-fit.
vectional patterns of microllites in glass spheres of synthetic basalts. In the present experiments, however, convection is not considered to be an effective process of mass-transportation, because the concentration gradients of Fe near the Pt wires still existed after the runs. In fact. sdif., values, estimated from the extrapolated data of the Na-self-diffusion coefficients in obsidian glass (MAGARITZ and HOFMANN, 1978) (triangles in Fig. 8) are less than the values of t,,, of the present
1550
I
-l/3
1500
1
1450
1
C
I
-2 A
\ log
0
k \
0
-3
\
0
\
0 \
I
-4 5.2
5.4
5.6
Fig 7. The Arrhenius plot of the rate of volatilization of Na,O, (triangle). Lines in this and next figures are drawn
1/T
5.6 x,O-~
K-’
for sample 1 (circles) and for sample by a least-square method.
2
1364
-2-
-6
-12
-10
-6
-6
-4
log 4, Fig. 8. Plots of log k against log po, for sample 1 at about results. (Because the diffusion coefficients in the chondrule melts are considered to be greater than those in the obsidian melt, ~~~~~ for the chondrule melts should be much less than T,,, .) Application of’ the rates of c’olutilization c~fNN to chondrules Models for the formation main types: (i) chondrules
of chondrules are directly
are of two condensed
1500 C.
from the solar nebula (e.g. W~IJJD. 1963: BLANDERand KATZ, 1967). and (ii) chondrules are the products of remelting of preexisting materials (e.g. KIEFFER. 1975). BLAKDER and KATZ (1967) proposed that metastable liquid droplets (chondrules) were directly condensed from the solar nebula. If this is the case, Na,O in the chondrule melts were in equilibrium with Na and O2 in the solar nebula and the Na,O contents of
1000
Fig. 9. The relation between the specific time and the radius of the liquid sphere. The solid and dashed lines show *diff and T”“, respectively. The experimental results of T,~, (cn-cles) and the estimated values of rd,,, (triangles, see text) are also given.
I365
Volatilization of sodium from silicate melt spheres
TO
/.I/
L
In 1.
1
\ -
T
_I-
> l/T,..
l/T
Fig. IO. A schematic diagram showing relations of 5di,r and T,,, with l/T(K-‘). T,,, > rdifl at temperatures less than T,,,, because E,,, > E,,,,-.
the chondrules should be dependent on reaction @a). The equilibrium constant for this reaction is given by eqn (8b). Although the value of K, is not known at present, it seems likely that K 1 is significantly large at high temperature because of high-vapour pressure of Na; in consequence, most of the Na in the solar nebeula was not condensed into the liquid droplets at high temperature. If chondrules are formed by a local heating of preexisting materials, the Na,O contents in the chondrules can be discussed using the above model with the rate of volatilization of Na, k, combined with the variation of temperature of chondrules and po, of the solar nebula (dependence of k on temperature and po, is shown by eqns (6) and (7) respectively). Although pNa also affects the volatilization of Na, as already mentioned, pNd in the solar nebula at this stage is considered to have been nearly equal to zero because most of Na in the solar nebula had been already condensed to albite at 70@9Oo”C (GROSSMANand LARIMER, 1977); that is, the effect of Na can be ignored. In the above model, the homogeneity of Na,O is assumed. The homogeneity in the liquid chondrules can be explained by comparing the two specific terms of the Na diffusion and Na volatilization, rdiff and 5“0,’ given in eqns (9) and (10) respectively. They are rewritten as follows;
ti-
t
Fig. 11. A Schematicaly drawn thermal history of chondrules.
An instant
heating followed by an immediate ing is assumed.
cool-
From eqn (13) T,,, is approximately 2300’ C (1. = lo- ’ cm, k, = 10’Ocm~min-’ and D = 1 cm’. set-‘); that is, the liquid chondrules are estimated to be homogeneous at temperatures less than about 2300°C. We assume that the chondrules are formed by an instant heating at time t = 0 and then cooled according to the Stefan-Boltzman relation: dT/dt = -b(T4
- T;Z,,),
(14)
where T and Tnehare the absolute temperatures of the chondrule and of the solar nebula respectively and h is the proportional constant. The thermal history assumed above is illustrated in Fig. 11. Variation of Na,O contents in the liquid chondrules with time or temperature can be obtained by solving the massbalance eqn (3) and eqn (14) with the initial conditions : C = Co and T = To at t = 0,
(15)
together with eqns (2) and (6) assuming that po, in that solar nebula is constant (that is, k, in eqn (6) is constant). Temperature of the nebula at the time is considered to be sufficiently low (Tnch << 700 C). Because k c 0 for 7’ < 700°C and T4 x Tze,, for T > 700 C, Tneb can be effectively assumed to be as -273-C (OK) in this calculation, so that the solution is ln(C/Co) = (3k,:hr)(RIE,,,)CF(T)
(11)
- UT,)],
(16)
where F(T) and F(T,) are defined as follows;
and r,,~ = (ri3k0)exp(R,,,!RT),
(12)
where the Arrhenius relation is assumed for D and k. is greater than Because E,,, (about 100 kcal.mole-‘) Edirf (about 20 kcal.mole-I), 7vol is greater than rdifr (the liquid chondrules are homogeneous) in a lower temperature region (Fig. 10). Putting 7vol = 7diff, then we obtain the maximum temperature, T,,,, below which TV,,,> ~~,rr: Trnll .I. = (EL,.,, I, - E,,,,,)lRln(3rk,/n’D,). _...
(13)
F(x) = [(l/x
+ R/&J2
+ (R/Ev0J2]exp(
- f&,/RX):
x = Tor To.
(17)
From eqn (16), C(t- co) or C(T= Tnuh),which corresponds to the final concentration of Na,O in the chondrule, Cc,,, can be obtained as functions of Co and To: ln(C,,lC,) where
= -(3k,lbr)(RIE,,,,)~(T,),
7;lcb = - 273°C
(OK) is also
assumed.
(18) The
1366
with the maximum temperature of the heating. r,. estimated from eqn (18)for from IO’” to 10Zh C3. At temperatures
various k,, hr in the range greater than about 2300 C. the effect of the diffusion in the liquid should be also conbidered.
relation of the ratio of the final and initial NazO concentrations of the chondrules, C,,/‘C,, with the maximum temperature of the heating process, 7& are given in Fig. 12 for various k,,/hr values (E,,, = l~kcal.mole-~). Crystallization of chondrules affects Na volatilization; crystals covering the surface of chondrules prevent Na volatilization, and Na is enriched in residual !iquids due to fractional crystallization. Because these two effects compensate each other, crystallization of chondrules is not taken into consideration in the following estimation. The value of b is approximately 10~‘l”C-3,min~l because the cooling rate of the chondrule is estimated to be about 100-C’min-’ at about 15OO‘C by experimentally-produced barred olivine chondrule (TSUCHIYAMArt ul., 1980a). This cooling rate may be a maximum value because recent experimental studies reveal that radial pyroxene chondrules are produced with cooling rates of about I’ C-min.- i (HEWINSet cd.,1981; NAGAHARAand TsuCHIYAMA,in preparation). The value of k, depends on p,,, in the solar nebula and on the bulk chemical compositions of chondrules. Value of po, in the solar nebula probably ranged from lo-” atm (at 1500 C) to 10e2’ atm (at 700°C) as estimated from the equilibrium experiments in the olivine-pyroxene~metallic iron system (WILLIAMS, 1971). Though the effect of the bulk chemical composition on k, is not well known, the variation of k, seems to be within the order of unity based on Fig. 7 and on the assumption that E,,, is not affected by the buik chemical composition. If & is 10” cm.min-‘. as obtained in the present experiments at 10~g.2 atm po, (for the sample 1, and the
radius of the chondrule. r, is 10 ’ cm. then ii,,& 13 approximately 102’ C3. k,:br is increased by one order of magnitude on decreasing po, by four orders of magnitude, because I\,, is proportional to 170~’’ (Fig. 8). The calculated value of C,,,, C, from Fig. 12 for T = 1700 C is consistent with the values of C,,,/C,, about 0.551.0, estimated from the Na ratio of the ordinary chondrites and C-chondrites (OSROKN c’r Q/., 1973; &ODIh.G et a/., 1980). It is also noted in Fig. 12 that c‘,,:C, abruptly changes from zero to unity with decreasing 7;). This suggests that the maximum temperature of the chondrule, 70. is not extremely high; that is less than about 2200 C. if we assume ko,!hr = 10z2’* C”. On the other hand, many chondrules were considered to be completely melted. In such a case. 7;, must be greater than the liquidus temperature of chondrules (within the range from 1400 to 1700 C). Therefore, the maximum temperature of the heating process must be restricted to thz range from about 140@-2200 ‘C. if the chondrules were formed by local heating of the preexisting materials. il~hlfOll.lCd~~e))lPnf,s~~We are grateful to Profeswr N. OW:MA of Tsukuba Universit), Profesor H. NAGASAWA of Gakushuin Unwersity, Dr S. KAHTO of the Unwersity of Tokyo and Dr E. TAKAHASM of the Geophysical Laborntory, Carnegie Institution, for critical reading the manuscript. Thanks are also due to Professor T. NAGATA of the National Institute of Polar Research. Japan. for providing Yamato chondrite used in the present experiments. REFERENCES
BLA~GDER M. mordial
and KATZ J. L. (1967) Condensation of pridust. Gwclzim. Cowoci~irtr. A<,!cf 31, 1025 1034.
Volatilization
of sodium
CORRIGAV G. and GIBB G. F. (1979) The loss of Fe and Na
from a basaltic melt during experiments using wire-loop method. Minrrul. Mug. 43, 121~126. CRANK _I.(1975) Tllr Mathrntutics $Dif(usion. 414pp. Clarendon Press. DoE;At.r)soN C. H.. WILLIAMS R. J. and LOFGREN G. E. (1975) A sample holding technique for study of crystal growth in silicate melts. 4nl. Minrraf. 60, 324326. DONALIXON C. H. (1979) Composition changes in basalt melt contained in a wire loop of Pt,,RhZO: effect of temperature. time and oxygen fugasity. ,Mincrtr/. Msg. 43, 115~119. GIBSON E. K. and HUBBARII N. .I. (1972) Thermal volatilezation studies on lunar samples. Proc. 3rd Luncv Plunc,t. %I. ClIr$. pp. 2003 1014. GooI,I~(; J. L. and MUENOW D. W. (1977) Experimental vaporlration of the Holdrook chondrite. Meteoritics 12, 401~408. Go~I>IN~; J. L.. KEIL K.. F~IUJ~KA T. and SCHMITT R. A. (1980) Elemental abundances in chondrules from unequilibrated chondrites: evidence for chondrule origin by melting of pre-existing materials. Eurth Plunrr. Sci. Lelt. 50, 171~180. GK~SSMAN L. and LARIMER J. W. (1977) Early chemical history of the solar system. Rrr. Geopln\. Spuce Phys. 12, 71 101. H~wrus R. H.. KLEIN L. C. and FASAKO B. V. (1981) Conditions of formation of pyroxene excentroradlal chondrules (abstract). In Lunur Pkmrr. Sci. XII. JAMBOY A. and CARRON J. P. (1976) Diffusion of Na, K. Rb and c’s In glasses of alblte and orthoclase composition, Geoc~hini. Co.smochitn. ACfU 40, 897-903. KIE~~ER S. W. (1975) Droplet chondrules. Science 189, 333 -330. LOF(;RE~ G. E. (1977) Dynamic crystallization experiments bearing on the origin of textures in impact generated liquid\. Proc. Srh Lanur Planr~. Sd. Cor$, pp. 2079-2095. Lovt RI&C;T. S. (1960) High temperature fusion of possible parent materials for tektites. Nuture 186, 1028~1031. MA<,~RITZ M. and HOFLIANU A. W. (1978) Diffusion of Sr, Ba and Na in obsidian. G~chirn. Covnochim. Ac.ru 42, 595m605. NAK*MI.R\ Y. and KUSHIRO 1. (1970) Compositional relations of coexisting orthopyroxene. pigeonite and aguite In a tholeiitlc andesite from Hakone Volcano. Conrrlh. Minrrul. Prrrol. 26, 265-~275. NAUC~H-IONJ. J., DEKHV J. V. and LEWIS V. A. (1971) VolatihLatlon from heated lunar samples and the investigation of lunar eroSIon by volatihzed alkalis. Proc. 2nd LMtrr Plunrt. sci. Con/., pp. 449-457. N01sll K.. O~UMA N.. NISHIDA N. and NAC;ASAWA H. (1978) High temperature heating of the Allende meteorite. Gcv&in~. Covnwhim. Acta 42, 903-907. OSBOR~ T. W.. S~ITFI R. H. and SCHMITT R. A. (1973) Elemental composition of individual chondrules from ordmary chondrites. Geoc~hirn. Cosmochim. ,A(tu 37, 1909 1942. TSL’(‘HIYAMAA., NA(;AHARA H. and KUSHIRO I. (198Oa) Ex-
from silicate
1367
melt spheres
perimental reproduction of textures of chondrules. Eurth Plunrt. Sci. Left. 48, 155~-165. TSUC‘HIYAMAA., NA(;AHARA H. and KCJSHIKO I. (1980b) Investigations on the experimentally produced chondrules: chemical compositions of ohvine and glass. and formation of radial pyroxene chondrules. ,2letn. Not. In.st. Polar Res. Sprc. I~.vrc~17, 83-~94. WALTER L. S. and CARRON M. K. (1964) Vapor pressure and vapor fractionation of silicate melts of tektite composition. Geoclnn~. Co\tnochirn. Acta 2X, 937 951. WlLLiAhfs R. J. (1971) Equilibrium temperatures. pressures. and oxygen fugacities of the equilibrated chondrites. Gwchim. Co\ornockinl. Acitr 35, 407- 4 I I Wooo J. A. (1963) On the origin of chondrules and chondrites. Iaurns 2, 151 1X0. APPENDIX
Pt wires of 0.1 mmcb were used in the present experiments except for the runs 272-275 and 279 (0.3 mm4). Concentration gradients of Fe were detected near the Pt wires. Average values of the Fe0 content m the charges are given in Table 2. At constant temperature and po,. the Fe0 content in the charges decreases with time. which is similar to the Fe loss from a basaltic melt in PtsORhzO wire loops (DONALDSON. 1979) and in Ag,,Pd,” wire loops (CORRI~;AN and GIBB. 1979). The Fe loss increases with increasing temperature In a basaltic melt (DOUALUSON. 1979). Similar results are obtained m the present experiments: times required for 20”,, loss of Fe0 are about 100. 40, 20 and 10 mln at about 1450. 1500. 1550 and 1600 C respectively at IO-‘.’ atmpo,. DONALI)SO’. (1979) also investigated the effect of po, on the loss of Fe and concluded that the Fe loss increased with decreasing poZ. Similar results are obtained in the present experiments: times required for the 20”,, loss of Fe0 are about 1000. 50 and 20min at lo--“. 10 9.2 and IO-‘” ’ atm pol respectively at about 1500 C for sample 1. .4t lo- ‘.” atm po,. no significant loss of Fe0 was detected within 1000 min. The size of the charges also affects the Fe loss. Fe loss increases with decreasing sire; for 32 min runs for the sample 1. no significant loss of Fe was detected for 89.1 mg charge whereas about 60”,, loss of Fe0 was detected for 4.1 mg charge. This is because the ratio of the contact area of the Pt wires with the melts and volume of the melts increases with decreasing the size of the charge. On the other hand. there is no significant difference in Fe loss between the Pt wires of different diameter for runs at about 1600 C and 10~’ ’ atm po, for the sample 1 and for runs at 1586 C and 10m9’ atm poi for the sample 2. The rate of the Fe loss for the sample 2 IS greater than that for the sample I; It takes only a few mmutes for the 20”,, loss of Fe0 for the sample 2 but a few tens of minutes for the sample 1. This may be due to greater diffusion coefficient of Fe in the less polymerized melt of the sample 2.