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EVALUATION OF THE CRYSTALLIZATION TEMPERATURES AND PRESSURES FOR
CLINOPYROXENE IN THE PARENTAL BODIES OF ORDINARY CHONDRITES
P.Yu.Pletchov, N.G.Zinovieva, 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 clinopyroxene in ordinary chondrites indicates
that, regardless of the chemical groups and petrological types of
chondrites, they crystallized from chondritic melts at pressures of >0
kbar.

McSween & Patcheon [5] applied pyroxene thermometry to constrain the
formation temperatures of LL chondrites and evaluated these temperatures at
900-1150Кб for meteorites of different petrological types: LL3 and LL7. The
pressures under which meteorites were produced were evaluated in [5] by the
Al concentrations in clino- and orthopyroxene. In this paper, the
crystallization temperatures of chondrites were evaluated using a number of
geothermometers, and the pressures were estimated by the clinopyroxene
geobarometer [6] for the crystallization of the magmatic mineral
assemblages in meteorites of each type.
All two-pyroxene thermometers are underlain by the utilization of the Ca,
Mg, and Fe distribution between ortho- and clinopyroxene. We used a
selection of experimental data to test different published variants of the
two-pyroxene geothermometer and have obtained the best agreement with
experimental data ((50(C) for the thermometer models [10, 9, 1A, 7]. These
models of the thermometer were then tested using natural (terrestrial)
assemblages for agreement with the olivine-clinopyroxene geothermometer
[2]. The differences between the temperatures yielded by this olivine-
clinopyroxene geothermometer and the aforementioned variants of the two-
pyroxene geothermometers lie within (15(б. The values determined by the
models [10, 9, 1A, 7] for the same assemblages from meteorites are
scattered within (48(C. Hence, the accuracy of the geothermometers utilized
in our research can be assumed as (50(C.
Pressure evaluations by the model [6] are based on the baric dependences
of some unit-cell parameters of crystallizing clinopyroxene (the volume of
its unit cell and the M1 polyhedron) and are dependent not so crucially on
the melt composition and the mineral assemblage. Nevertheless, these are a
number of calibrations of this geobarometers proposed for melts having
different compositions and existing under different conditions. The
calibration most suitable for meteorites (of planetary proper stage) is
that developed for terrestrial tholeiitic basalts and characterized by
practically anhydrous (<0.1 wt.% H2O) reduced conditions. The experimental
data utilized in this model cover a pressure range of 0-18 kbar, and the
differences between the values yielded by this model and experimental data
are (1.13 kbar at the maximum deviation of 3.86 kbar. Thus, it can be
safely presumed that calculated pressures of >5 kbar are significant. The
differences between tholeiitic basalts and meteorites and temperature
uncertainties can introduce additional uncertainties into the pressure
estimates. As was demonstrated above, the temperatures are determined by
two-pyroxene thermometers accurate to (50(C. An increase in the temperature
inputted into the model by 100(C results in a decrease in the pressure
estimate by 2.5-3 kbar for pressures of <25 kbar and by 1-1.5 kbar for
pressures of >25 kbar. The temperatures determined by the two-pyroxene
thermometers correspond to the temperatures of the latest equilibrium
between the clino- and orthopyroxene and can be somewhat underestimated
with respect to the actual values if the clino- and orthopyroxene could
continue exchanging components after crystallization. Thus, the
uncertainties in the temperature estimates can lead to pressure
overestimates by 2.5-3 kbar. At the same time, some of the calculated
pressure values can lie far outside the range of the experimental data
against which the model [6] was calibrated. At P> 20 kbar, the calculated
pressure is systematically lower than the experimental values [6], so that
the values thus obtained can be underestimated. The effect of other factors
(such as high alkalinity or high water contents) that can significantly
shift the pressure estimates are negligibly small for meteorites.

|Ordinary |T, КC* |P, |
|chondrites | |kbar** |
|Yamato-74417 |1106-130|3.6-12.|
|LI(3) |6 |2 |
|av.# # |(10)# |(10) |
| |1210 |7,4 |
|Yamato-82133 |938-1476|0-14.1 |
|HI(3) | |(10) |
|av. |(10) |8.4 |
| |1218 | |
| |1032-120|0.1-4.4|
|Saratov |4 | |
|LII(4) |(3) |(3) |
|av. |1122 |2.4 |
| |890-1464|0-13.7 |
|Raguli | |(12) |
|HII(3.8) |(12) |5,3 |
|av. |1104 | |
|Raguli |1000 |63.7-81|
|HII(3.8) | |.6 |
|Ur-Jd CPx | |(4) |
|av. | |70.1 |
| |985-1000|3.8-5.1|
|Berdyansk | | |
|LIII(6) |(6) |(6) |
|av. |993 |4,5 |
|Berdyansk |1000 |66.9-72|
|LIII(6) | |.8 |
|Ur-Jd CPx | |(8) |
|av. | |69,6 |
| |992-1030|3.6-8.3|
|Fucbin | | |
|LIII(6) |(7) |(7) |
|av. |1011 |5,7 |
|Yamato-74160 |1000-100|3.1-5.0|
|LLIII(7)# # #|5 | |
| |(2) |(2) |
|av. |1053 |4,0 |

* - temperatures calculated by the two-pyroxene geo thermometer [9]
** - pressures calculated by the clinopyroxene geobarometer [6]
# - amount of analyzed grains
# # - average temperature, pressure
# # # - composition of pyroxene pairs from the Yamato-74160 LLIII(7)
chondrite were borrowed from [8]

The table lists the temperatures and pressures of clinopyroxene
crystallization in ordinary chondrites of different chemical groups and
petrological types: low-I(3.0-3.7), intermediate-II(3.8-4), and high-III(5-
7), as exemplified by Yamato-82133 I(3), Raguli II(3.8), and Okhansk II(4)
(group Э); Yamato-74417 I(3), Saratov II(4), Fucbin III(6), and Berdyansk
III(6), (group L). The table also presents the analogous values for
clinopyroxene from other ordinary chondrites [8].

Our data definitely indicate that clinopyroxene in all of the ordinary
chondrites crystallized under pressures of >0 kbar (whatever the chemical
group or the petrological type of the meteorite). The most typical pressure
range is 0-10 kbar. Some early chondrules that bear pyroxene and olivine
grains with compound zoning (normal zoning giving way to reversed one and
reflecting two stages of meteorite formation [3]) yield pressures of up to
14.5 kbar. Some ordinary chondrites contain relict clinopyroxene grains of
jadeite-ureyite composition [4, 11-13], for which the crystallization
pressure was evaluated (by the clinopyroxene barometer [6]) at 63-71 kbar.

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" (grant 1301.2003.5).

References: [1] Kretz (1982) GCA. 46, 411-421, (thermometer A); [2]
Loucks (1996) Contr. Min. Petr. 125, 140-150; [3] Marakushev (1999) Origin
of the Earth and the nature of its endogenic activity, M.: Nauka, 255 p.;
[4] Marakushev et al. (2003) Cosmic Petrology, Moscow, Nauka, 387 p.; [5]
McSween&Patchen (1989) Meteoritics 24, 219-226; [6] Nimis (1999) Contr.
Min. Petr. 135, 62-74; [7] Perchuk (1977) Doklady AN USSR 233, N 3, 456-
459; [8] Takeda et al., (1984) EPSL 71, 329-339; [9] Wells (1977) Contr.
Min. Petr. 62, 129-139; [10] Wood &Banno S. (1973) Contr. Min. Petr. 1973.
42, 109-124; [11] Zinovieva (2001) Petrology of ordinary chondrites,
Moscow, 262 p; [12] Zinovieva et al. (2002) Antarct. Meteor. 27, 183-185;
[13] Zinovieva et al. (2005) 36th LPC (CD-ROM) 1038#.