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PHYSICOCHEMICAL CONDITIONS OF CLINOPYROXENE CRYSTALLIZATION IN THE PARENTAL
BODIES OF ORDINARY CHONDRITES
N.G.Zinovieva, P.Yu.Pletchov, N.P.Latyshev, and L.B.Granovsky, Department
of Petrology, Faculty of Geology, Moscow State University, Vorob'evy Gory,
Moscow 119992, Russia (zinov@geol.msu.ru).


Thermo- and barometry of ortho- and clinopyroxene in ordinary chondrites
indicate that these minerals crystallized from chondritic melts, first,
under progressively more reduced conditions and significant pressures in
large parental bodies and then continued to crystallize in much smaller
bodies, under a pressure of approximately 9 kbar. The radii of the parental
bodies of ordinary during the second (planetary proper) phase could be of
the order of 750-850 km.

In order to evaluate the physicochemical conditions under which the
parental bodies of ordinary chondrites were produced, we examined
chondrites of different chemical groups and petrological types: low-I (3.0-
3.7), intermediate-II(3.8-4), and high-III(5-7), exemplified by Yamato-
82133 I(3), Raguli II(3.8), Okhansk II(4) (group H); and Yamato 74417 I(3),
Saratov II(4), Fucbin III(6), and Berdyansk III(6) (group L), as well as
used literature data on other ordinary chondrites. To evaluate the
crystallization temperatures of chondrules, we utilized the compositions of
ortho- and clinopyroxene, which are the most widely spread minerals (along
with olivine) of ordinary chondrites. The crystallization temperatures of
the chondrules were calculated by four clinopyroxene-orthopyroxene
thermometers [14, 13, 3, 11] and indicate that the chondrules of ordinary
chondrites of types I and II have crystallized within broad temperature
ranges: 938-1466(б for Yamato-82133 HI(3), 1106-1306(б for Yamato-74417
LI(3), 874-1479(б for Raguli ЭII(3.8), and 1032-1490(б for Saratov LII(4).
Conversely, the chondrules of chondrites of types III have crystallized at
an almost constant (within the accuracy of the calculations [12])
temperature: 985-1000(б for Berdyansk LIII(6) and 992-1030(б Fucbin
LIII(6). These results are in good agreement with the widths of the
crystallization temperature ranges published in [9] for ordinary chondrites
of the different petrological types of group LL and calculated for the
clinopyroxene by the geothermometer [4] but differ from these data by the
absolute values of the temperatures. The crystallization temperatures
calculated for ordinary chondrites of petrological types 6 and 7 by four
two-pyroxene geothermometers [14, 13, 3, and 11] vary insignificantly
(within +48Кб), and the temperatures calculated by each of these
thermometers are practically constant for chondrites of type 6 (for
example, the temperatures calculated by [13] for clinopyroxene-
orthopyroxene pairs are 1011 + 19Кб for different chondrules in Fucbin L6
and 993 + 8Кб for Berdyansk L6) and vary insignificantly (within the
accuracies of the thermometers) when calculated by different thermometers
for Yamato-74160 LL7 of type 7 (1053 + 52Кб).
The crystallization pressure of chondrites was evaluated by model [7],
which makes use of a baric dependence for the unit-cell parameters of
clinopyroxene (the volume of the unit cell and of the M1 polyhedron). This
model is less than the previous models dependent on the chemistry of the
melt and the mineral assemblages.
The applicability of this model to estimates of clinopyroxene
crystallization pressure is justified in [12]. The average pressure values
yielded by the clinopyroxene geobarometer [10] for ordinary chondrites of
different petrological types (I, II, and III) and discrete chemical groups
vary from 2.4 to 8.4 kbar, with the pressure under which clinopyroxene
crystallized in equilibrium chondrites of type III varying within a
narrower range, from 3.05 to 8.33 kbar, than the pressures of
unequilibrated chondrites of types I and II (0-10.55 kbar). Moreover,
chondrites of types I and II were found out to contain single clinopyroxene
grains that crystallized under a much higher pressure, up to 14.5 kbar,
with such clinopyroxene occurring much more frequently in the most strongly
unequilibrated chondrites (type I), which bear silicate chondrules whose
composition varies within broader ranges. Our earlier petrological analysis
of chondrules with such clinopyroxene grains [8, 15] revealed that all of
them bear pyroxene and olivine grains that are complicatedly zoned, with a
reversed zoning in the silicates giving way to their normal zoning as a
reflection of two crystallization stages of the chondrites [6].
Petrologically, the early high-temperature crystallization stage of
ordinary chondrites follows from the occurrence of relict grains (such as
silica-oversaturated pyroxene, jadeite-ureyite clynopyroxene and other
[15, 16]) with exsolution textures comparable with those of terrestrial
high-pressure solid solutions.
The presence of relict grains of high-pressure minerals, the
crystallization of native silicon simultaneously with kamacite [8], and the
occurrence of diamond in the matrix of chondrites provide evidence that the
crystallization of the immiscible (exsolved) chondritic melts ("recorded"
in the textures of chondrites) took place after a time period when they
evolved under reduced conditions and much higher pressures. The
crystallization pressure of relict grains of jadeite-ureyite pyroxenes
(found in chondrites types II and III) was avaluated by the clinopyroxene
barometer as 63-71 kbar.
The second crystallization stage of chondritic melts proceeded under
progressively more oxidizing conditions [7, 8, 15] after the breakup of the
parental planets, within much smaller bodies, as also follows (in addition
of pressure estimates obtained for the crystallization of clinopyroxene:
the maximum pressure in chondrites of type III did not exceed 8.5 kbar)
from the presence of protopyroxene in chondrites of types I, II, and III,
with this mineral known to crystallize under pressures of no more than ~8
kbar [1, 2]. The fact that protopyroxenes are less typical of chondrites of
type III suggests that the pressures within their parental bodies could
have been slightly higher than 8 kbar.
Assuming that the pressure within the stage-II parental body could be
close to 8-9 kbar, using the dependences of the radius of a body on its
inner pressure obtained for ice satellites [5], and taking into account the
greater density of a silicate sphere (3000 kg/m3) that an ice sphere (1000
kg/m3) (thereby we ignored the compressibility of the spheres), we arrived
at the conclusion that the radii of the parental bodies of ordinary
chondrites during the second (planetary) stage of their evolution could be
as great as 750-850 km.
Hence, our earlier conclusion that chondritic melts were formed in two
stages, first, under increasingly more reducing conditions within large
parental bodies and, then, under more and more oxidizing conditions within
much smaller bodies [7, 8] receives further support from the
thermobarometry of pyroxenes contained in ordinary chondrites of different
petrological types and chemical groups (LL, L, and H).

Acknowledgments - This work was supported by the Russian Foundation for
Basic Research (grant 04-05-64880), the Program "Universities of Russia -
Basic Researches" (grant UR.09.02.052); the Program "Support of Scientific
Schools" (grants 1301.2003.5 and 1645.2003.5).

References: [1] Boyd et al. (1964) JGR 69, 2101-2109; [2] Gasparik (1990)
Am. Min. 75, 1080-1091; [3] Kretz (1982) GCA. 46, 411-421; [4] Lindsley
(1983) Am. Ml. 68, 477-493; [5] Lupo&Lewis (1979) Icarus 40, 157-170; [6]
Marakushev (1999) Origin of the Earth and the nature of its endogenic
activity, M.: Nauka, 255 p.; [7] Marakushev et al. (1992) Cosmic Petrology,
Moscow, MSU-press, 312 p.; [8] Marakushev et al. (2003) Cosmic Petrology,
Moscow, Nauka, 387 p.; [9] McSween&Patchen (1989) Meteoritics 24, 219-226;
[10] Nimis (1999) Contr. Min. Petr. 135, 62-74; [11] Perchuk (1977) Doklady
AN USSR 233, N 3, 456-459; [12] Pletchov et al. (2005) This volume; [13]
Wells (1977) Contr. Min. Petr. 62, 129-139; [14] Wood &Banno S. (1973)
Contr. Min. Petr. 1973. 42, 109-124; [15] Zinovieva (2001) Petrology of
ordinary chondrites, Moscow, 262 p; [16] Zinovieva et al. (2002) Antarct.
Meteor. 27, 183-185.