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OPAQUE-RICH CHONDRULES: NOT DUE TO THE CANONICAL NEBULAR GAS?
B. Cohen
1
, Y. Yu
1
,
R. H. Hewins
1
and B. Zanda
1,2
.
1
Department of Geological Sciences, Rutgers University, Piscataway, NJ 08855,
USA.
2
Muséum National d'Histoire Naturelle, 61 rue Buffon, 75005 Paris, France. (bosmat@eden.rutgers.edu)
Introduction:
Chondrules are enigmatic bead-
shaped objects embedded in chondritic meteorites.
Their igneous textures were recognized on their
discovery, but the exact mechanism of their formation
is still not completely understood. It is widely
believed that chondrule precursors were melted in the
solar nebula, under canonical low pressures of
hydrogen gas, at temperatures close to their liquidus.
Chondrules exhibit a wide range of chemical
compositions, varying in their content of FeO, FeS,
Fe-metal and moderately volatile elements. We here
present the results of melting/crystallization (and
evaporation/reduction) experiments conducted at
various hydrogen pressures. According to several
heating models chondrule precursors were heated for
very short times though others argue for longer
heating-cooling cycles. We have done experiments
lasting between 60 sec and 18 hrs to examine the
influence of different H
2
pressures on the distribution
of FeS and Fe metal in chondrules, and find that
canonical H
2
pressure does not explain the Fe.
Experiments:
Mineral mixtures equivalent to CI
at 400K [1,2] were pressed into pellets, inserted into a
Deltech vertical muffle-tube furnace at 1580
°
C for
different lengths of time and then quenched. The
pressure in the furnace was first lowered to 1x10
-8
atm. Then hydrogen gas was bled into the furnace to a
total pressure of 1.4x10
-6
atm and the experiments
were performed. Isothermal control experiments were
done in H
2
-CO
2
at 1 atm pressure, at an oxygen
fugacity of -3 below the iron wüstite buffer and
1565
°
C.
Results:
In experiments conducted under
"nebular" conditions we found that evaporation
produced objects resembling type I chondrules [3] but
without the metallic iron and dusty olivines (Fig.1a),
common in many natural type I chondrules [4]. None
of the residues contained any metallic iron or iron
sulfide, not even when flash heated for 60 sec. When
the temperature was lowered to 1350
°
C, some sulfide
was found in a charge that was flash heated. And
metallic iron survived for up to about 30 min.
At 1 atm pressure, metallic iron was present in all
charges heated for 6 hours or less. Iron metal appears
in large beads 1mm in diameter and/or as smaller
round grains of several microns (Fig. 1b). Dusty
olivines existed in all charges even those heated for
12 hours. Sulfur did not appear in any of the charges
not even in those that were flash heated.
Fig. 1. CI starting material was heated for 1 hour
under different conditions. (a) This charge was
formed at "canonical" hydrogen pressures and
exhibits homogenous olivine (Fo
82
) and no metallic
iron. (b) This charge, exhibiting both metallic iron
and dusty olivine (Fo
93
), was formed at 1atm, oxygen
fugacity of IW-3 and 1565°C.
a
b
Lunar and Planetary Science XXXI
1212.pdf
OPAQUE-RICH CHONDRULES: NOT DUE TO CANONICAL NEBULA
B. Cohen et al.
Discussion:
Silicate melts are not stable at
canonical pressures, and the survival of chondrules is
generally attributed either to kinetic effects or to dust-
enriched compositions [5]. Despite open-system
behavior, type I chondrule (silicate) compositions can
be produced with heating for at least 18 hours at near-
liquidus temperatures in vacuum [3]. We observe
extensive S loss, as predicted thermodynamically [2],
with a greater effect for higher H
2
pressure, as [11].
With 60 sec heating, 1350°C and low pressure, some
S is preserved. Such conditions might be appropriate
for dark-zoned chondrules, which contain primary
troilite [6]. However, as formation in the canonical
nebula appears ruled out for Na-rich type II
chondrules [7], higher T and t might also be possible
for higher ambient H
2
S pressure.
At low hydrogen pressures, charges resembling
opaque-rich type I chondrules [4] were generally not
produced. Loss of Fe from Fe liquid was almost as
fast as loss of S, and much faster than evaporation of
Fe from the silicate melt. In Semarkona type I
chondrules, a similar mechanism can be inferred from
the correlation between reduced Fe and Fa (Fig. 2).
The finest (least melted) chondrules are Fe- and
Fig. 2. Type I chondrules in Semarkona from [6] and,
triangles [10]. The vertical trend reflects evaporation
of reduced Fe derived from FeS and the horizontal
trend reduction of Fe in silicate followed by
evaporation or mechanical loss. In low pressure
experiments, the vertical trend is captured only with
60 sec heating at 1350°C, and the horizontal trend
requires 12 hours.
FeO-rich and the coarsest ones Fe- and FeO-poor. S
is lost from the chondrules along this trend even
faster than Fe [6]. Within the first stages of the
melting, the metallic iron decreases dramatically from
the combined effect of Fe-vapor loss and immiscible
liquid loss, while little or no change of the Fa content
of the chondrules takes place. It is only for a more
intense heating pulse that the silicates coarsen and the
Fa contents of the olivine decrease.
Our charges do not reflect gas redox state at low
pressure until ~12 hours, because of survival of FeO
in silicates. Therefore we cannot infer nebular
environment from chondrule mineral compositions
(especially if they were flash-heated).
If chondrules were actually formed at ~1600°C
under the harsh conditions of the canonical nebula,
how was metallic iron able to survive? Under higher
H
2
pressures metallic iron was retained in charges
quite efficiently for up to 6 hours and dusty olivines
were preserved for as long as 12 hours. Conceivably
chondrules were heated during passage of a shock
wave which boosted H
2
pressure so as to stabilize Fe
metal, though pressures approaching 1 atm are
unlikely [8]. Carbon-bearing precursors might explain
the occurrence of Fe at canonical H
2
pressures, as Fe
is readily produced by heating silicate with a reducing
agent like C [9].
However, the C in the form of
kerogen in the present low P runs was not particularly
effective and extensive reduction required heating for
12 hours.
Conclusions:
Chondrule minerals do not indicate
the oxygen fugacity of ambient gas, for canonical
nebular pressures. Chondrules containing metallic
iron blebs and dusty olivines require either abundant
solid C or high H
2
pressure. Chondrules with primary
troilite need some combination of short heating time,
low temperature and moderate ambient S pressures to
allow partial survival of FeS [6]. Relatively long
heating in "dust-enriched" microenvironments [2,5]
where the ambient pressure of lithophile elements
was high seems plausible.
References:
[1] Anders E. and Grevesse N.
(1989) Geochim. Cosmochim. Acta 53, 197-214 [2]
Wood J. A. and Hashimoto A. (1993) Geochim.
Cosmochim. Acta 57, 2377-2388 [3] Cohen et al.,
(1999) Meteoritics and Planet. Sci. 34 A27 [4]
McSween H. Y. (1977) Geochim. Cosmochim. Acta
41
, 1843-1860 [5] Ebel D. S. and Grossman L. (1997)
Lunar Planet. Sci. 28 317-318 [6] Hewins, R.H., Yu,
Y., Zanda, B. and Bourot-Denise, M. (1997) Antarct.
Met. Research 10, 294-317 [7] Yu Y. and Hewins R.
H. (1998) Geochim. Cosmochim. Acta 62, 159-172
[8] Hood L. L. and Horanyi M. (1993) Icarus 106
179-189 [9] Connolly H.C. Jr. et al. (1994) Nature
371
136-139. [10] Jones R.H. and Scott E.R.D.
(1989) Proc. 19th Lunar Planet. Sci. Conf. 523-536,
LPI, Houston. [11] Nagahara H. and Ozawa K.
(1994) Meteoritics 29, 508-509.
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Lunar and Planetary Science XXXI
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