Although there appears to be general agreement that some coarse-grained Ca-Al-rich inclusions (CAIs) from Allende passed through a molten or partially molten stage in their evolution, there are several competing hypotheses to account for the formation of the liquid phase in CAIs (e.g., 1-4). Studies of the phase equilibria of CAI compositions can help distinguish between these mechanisms

for generating liquids in CAIs

Paque, J.

Rossman, G. R.

Stolper, E.

Caltech Authors - Main

772 
THE INFLUENCE OF OXYGEN FtX;ACITY AND CX>OLING RATE ON THE CRYSTALLIZATION OF Ca-Al-RIOl 
INCLUSIONS FROM ALLENDE. E. Stolper , J. Paque, and G.R. Rossman, Div. Geol. Planet. Sci., Caltech, 
Pasadena, CA 91125 
Although there appears to be general agreement that some coarse-grained Ca-Al-rich inclusions 
(CAia) from Allende passed through a molten or partially molten stage in their evolution, there are 
several competing hypotheses to account for the formation of the liquid phase in CAis (e.g., 1-4). 
Studies of the phase equilibria of CAI compositions can help distinguish between these mechanisms 
for generating liquids in CAis. For example, CAis formed by condensation of supercooled liquid 
droplets ( 4) would be expected to have textures that differ significantly from those of partially 
molten droplets formed by near-equilibrium condensation {3). Equilibrium condensates would be 
expected to have crystallized at oxygen fugacities compatible with a nebular gas, whereas inclusions 
processed in planetary environments or by kinet i cally-dominated mechanisms need not reflect such 
an equilibration. We report here preliminary resul ts of experiments that explore the effects of 
oxygen fugacity and cooling rate on the mineral chemistries, crystallization sequences, and textures 
of CAis and show how these can yield insights into CAI evolution. 
OXYGEN FUGACITY DURING CAI CRYSTALLIZATION. The presence of Ti3+ and Ti4+ in pyroxenes in CAis is 
well documented (5). Since the Ti3+/Ti4+ of pyroxene at a given temperature is expected to be 
a monotonic function of f02, it is possible to determine the f02 levels at which these pyroxenes 
equilibrated by finding the conditions necessary to duplicate the Ti3+JTi4+ of the natural 
pyroxenes. A glass similar in composition to a typical Allende fassaite (CaO 23.7, MgO 10.9, 
Al2 03 20. 1 , Si02 40.2, Ti02 5.1 wt %) was used as a starting material. Oxygen fugacities were 
controlled by H2- C02 gas mixtures in which samples were log
10
( Ti 3/Ti 4 +) 
suspended for 1- 5 days at 1230°C on Pt wire loops. A 
solid elect r olyte sensor, calibrated at the Fe/FeO buffer, 
was used to measure f02 values. One experiment was con-
ducted in air. The experimental results are unreveraed. 
Run products consist primarily of clinopyroxene, with 
glass , plagiocl ase , olivine, and spinel. Macroscopically, 
- 1 0 
-II r---.----T---,.----\ _ _j 
-13 
exper imentally 
produced pyroxenes 
they vary from white in air, to grey (with a pink-purple -15 
tinge) at the Ti305/Ti02 buffer, to black (with a blue- ~ 
purple tinge) near the Ti203/Ti305 buffer. In thin section, Q _17 
the reduced pyroxenes show weak pink to green pleichroism. g '1,-- --------,, 
' ' 
-19 ' 
' T = 123o•c 
-21 
' 
' 
' 
' 
' 
' :'--
Experimentally produced pyroxenes were analyzed by 
electron microprobe and the amount of Ti3+ was calculated 
assuming R20 3 stoichiometry. Such recalculations are to be 
i nterpreted with caution, both because of their precision 
(i.e. , they are ve r y sens i tive to small uncertai nties in 
the analyses) and their accuracy (i.e., the assumption of 
stoichiometry is questionable [6]). The results of these 
recalculat i ons are shown in Fig. 1, in which Ti3+/Ti4+ is Allende pyroxenes 
FIG. 1 
shown versus f02• These results suggest, assuming that Allende pyroxenes crystallized near 
123o•c (approximately the temperature at which pyroxene begins to crystallize from a melt of 
CAI composition) , that the f02 levels at which they crystallized were lower than the lowest levels 
ac~teve~+in our experiments. Extrapolating the experimentally determined trend to the range of 
Ti /Ti ratios of Type B Allende pyroxenes suggests log 10fo 2 values of about -17.5 to -22.5. Since almost the full range of Ti3+JTi4+ ratios can be observed in a single inclusion (7), it 
is unlikely that this range reflects a real variation in f02 level at a single temperature. 
~-ollende pyroxene 
I 
w 
FIG . 2 
Optical and near-infrared spectra were taken on experi-
mentally produced pyroxene crystals. The a polarizations of 
pyroxenes produced at different oxygen fugacities and for a 
natural Allende pyroxene (NMNH 3529G) are compared i n Fig. 2, 
adjusted to a constant thickness of 50 ~m. Aperture sizes 
ranged from 92-320 ~m. All of the pyroxenes figured have 
~ 8.4 \, 
\ 
3-6 weight % Ti. The peaks at 470 and 600 nm in natural 
pyroxene samples with higher Ti contents have been attributed 
to Ti3+ crystal field effects while the broad feature between 
700 and 900 nm has been attributed to a Ti3+-Ti4+ intervalence 
charge transfer (8). The growth in the spectra of the experi-
mentally produced pyroxenes of the 470 nm band and the devel-
opment of a shoulder at 600 nm with decreasing f02 and the 
presence in these spectra of a broad peak from 700-900 nm 
<( 
m 
fr: 
0 
(fJ 
m 
<( 
""\ experimenloll y produced 
\ py roxenes (T= 1230 •c) 
Alog10 f0 2 = - 16.5 
log 10 f0 2 = - 14.9 0.2 
, log 10 f02 = -13.7 
" 
-, 
-........ _:--... _ 
are not inconsistent with these assignments. These features 
are on the tail of what is presumably a charge transfer peak 
in the UV regions of the spectra. The spectra of the experi-
mentally produced pyroxenes begin to approach that of the 
natural pyroxene at the lowest f02 achieved but have not 
reached it by this point. This corroborates the conclusion 
baaed on stoichiometrically determined Ti3+JTi4+ ratios 
300 500 708 908 1100 1300 1500 1700 1900 that Allende pyroxenes formed at f02 levels lower than the 
lowest (1o-16.5 atm, 1230°C) achieved in our experiments. ~AVELENGTH , nm The o~§eB fugacity of the nebular gas (H2/H20 - 500-
2000) at this temperature would be 10-16 •9 to 10- • atm. Thus, the inferred f0 2 levels for the 
Crystallization of Ca-Al-rich inclusions 
Stolper, E. e t. al . 
formation of Allende pyroxenes are compatible with equilibration with the gaseous nebula at this 
temperature. Although not ruling out alternative modes of origin for CAis, the oxidation state of 
Allende pyroxenes is compatible with their formation by equilibrium condensation from a cooling 
nebular gas (e.g., 3, 9). CA IB crystallizat ion sequence 
Crystallization sequences of melts similar in composi- (after spinel / melil i te en t r y) 
tion to CAis have been studied in air, and to a first 
approximation the results appear applicable to actual CAis 
(10). However, since CAis crystallized under reducing con-
ditions at which substantial amounts of Ti3+ were present, 
it is necessary to determine whether crystallization se-
quences are different at these low fOz levels. Synthesis 
experiments on a composition previously studied in air (10) 
have been conducted in Hz-COz gas mixtures on Ir wire 
1260-
1250 - - - t---- -- - -- ~- - plag in 
1240-
loops at f Oz levels approximately 1.2 orders of magnitude 
more oxidizing than the Tiz03/Ti305 buffer. Based on 
1230 -Figs. 1 and 2, this is not quite as low as the actual 
conditions under which CAis crystallized, but substantial 
Ti3+ is present. Results are compared with synthesis 
exper iments conducted in air in Fig. 3. The temperature 
of pyroxene entry is raised by about 5°C, but the crystal-
lization sequence (sp + mel + plag + cpx) is the same as 
air low 10 2 
FIG. 3 
in air. These r esults confirm the validity of phase equilibria determined in air for modelling 
the crystallization sequences of CAis. 
COOLING RATE ST UDIES . The textures and crystallization sequences of those CAis formed by crystal-
lization of melts depend both on melt composition and the time-temperature history of the inclusion. 
Until now, only equilibrium crystallization sequences have been studied (e.g., 10). A series of 
controlled cooling rate experiments has been conducted on a melt similar in composition to an 
average Type B CAI. Experiments wer e conducted in air on the same composition used in (10). 
Samples were held at 25-30° C above the liquidus for 3 hours on Pt wire loops and then cooled at 
approximately linear cooling rates of 2-200°C/hour. By quenching samples at different temperatures, 
the crystallization sequence at each cooling rate and the development of textures could be observed. 
Spinel was the first phase to crystallize and melilite the second over this range of cooling rates. 
Melts were supercooled by 50°C or more before spinel began to crystallize, while melilite, which 
appears at 1400° C in t he equilibrium crystallization sequence, began to crystallize at 1150-1190°C 
at all cooling rates studied, Clinopy roxene was the third phase to crystallize at 2D-200°C/ hour, 
appearing at ~112 5 °C. This contrasts with the equili brium crystallization sequence, where plagio-
clase begins to crystallize before pyroxene (Fig. 3). In all cases, early formed spinels are 
euhedral, nucleating on the Pt wire or the melt-gas interface. At the slower cooling rates, 
spinel nuclei are rare, resulting in l arge (up to 1 mm) spinel crystals. In some of the faster 
cooled experiments, a second generation of ragged spinels grew along with the melilites in the 
interior of the charge. The textures of the run products are dominated by dendritic melilites 
that result in a herringbone pattern t exture (Fig. 4). Pyroxenes grow between the limbs of the 
melilite dendrites. 
Textures resembling those produced in these experi-
ments do not appear to have been observed in CAis. In 
addition, anorthite crystallization appears to have preceded 
or been nearly simultaneous with pyroxene crystallization 
in coarse-grained CAis (11). Although the cooling rates 
studied are slower than those expected for radiative cooling 
of molten droplets (12), the failure of these experiments 
to reproduce CAI crystallization sequences (i.e. , plag 
before or simultaneous with pyroxene) or textures (in 
contrast to the success of experiments on chondrules [12)), 
suggests that coarse-grained CAis were never totally molten , 
that the subcooled liquid model for their origin (4) does 
not apply, and that cooling rates were slower than 20°C/hour 
in order to account for the early appearance of plagioclase 
in the crystallization sequences. However, a larger 
FIG. 4 · 2 °C/hr •, field of range of experimental conditions will have to be explored 
view 1. 4 X 2 . 1 mm before these suggest:lons can be confirmed. 
References: (1) Clayton D.D. (1980) Astrophys. J. 239, L.37. (2) Armstrong J. et al. (1982) G.C.A. 
(in press) . (3) Clayton R.N. et al. (1977) EPSL 34, 209. (4) Blander M. and Fuchs L. (1975) 
G.C.A. 39, 1605. (5) Dowty E. and Clark, J. (1973) Am. Mineral 58, 230. (6) Andersen D. and 
Lindsley D. (1981) LPS XII, 19. (7) Haggerty S. (1978) Nature 276, 221. (8) Mao H.K. and Bell P. 
(1974) Geophys. Lab. Yrb. 73, 488. (9) Grossman L. (1972) G.C.A. 36, 597. (10) Stolper E. (1981) 
LPS XII, 1049. ( 11 ) MacPherson G. and Grossman, L. (1981) EPSL 52, 16. (12) Tsuchiyama A. et al. 
(1980) EPSL 48, 155. 
77 3 


The Influence of Oxygen Fugacity and Cooling Rate on the Crystallization of Ca-Al Inclusions from Allende

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The Influence of Oxygen Fugacity and Cooling Rate on the Crystallization of Ca-Al Inclusions from Allende

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