Other
Scientific paper
Sep 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30q.498c&link_type=abstract
Meteoritics, vol. 30, no. 5, page 498
Other
1
Meteorites, Allan Hills 77257, Allan Hills 82106, Allan Hills 82130, Lewis Cliff 85440, Lewis Cliff 88201, Lewis Cliff 88774, Meteorite Hills 78008, Yamato 74130, Yamato 74659, Yamato 791538, Planetesimal, Pyroxene, Ureilites
Scientific paper
LEW88774 is a Cr and Ca-rich ureilite that includes pyroxene with coarse exsolution lamellae, and minor Cr-rich spinel, and Fe-Cr carbide, which have not been found in other ureilites. [1] indicated that it requires slow cooling to form such coarse exsolution lamellae. [2] suggested that the equilibration temperature is near-solidus (~1300 degrees C), as usual for ureilites. They preferred a model of original crystallization deep in the mantle of the ureilite parent body, because of the presence of graphite. Since equilibration at high temperature and the subsequent rapid cooling by a possible breaking-up of the parent body are important aspects of the thermal history of ureilites, we reinvestigated the equilibration temperatures of the coexsisting pyroxene polymorphs. We reanalyzed the coarse exsolution lamellae of LEW88774. The composition of the exsolved augite and the host low-Ca pyroxene are Ca33.7Mg52.8Fe13.5 and Ca4.4Mg75Fe20.6, respectively. These imply an equilibration temperature of about 1170 degrees C by the two pyroxene geothemometer [5]. Chikami et al. [4] estimated that exsolution lamellae of LEW88774 were produced at 1280 degrees C and cooled at 0.01 degrees C/year until 1170 degrees C, based on a method developed by Miyamoto and Takeda [3]. Y74130 is also a Ca-rich ureilite, and consists of olivine, augite, enclosed pigeonite in augite, and low-Ca pyroxene [6]. From these assemblages, equilibration temperatures of 1200 to 1300 degrees C were deduced [5]. Pyroxenes of MET78008 are also augite, pigeonite, and orthopyroxene [7], and they yielded the temperature of 1250 degrees C. On the other hand, ALH82106 and ALH82130 are paired meteorites and include coexisting pigeonite-augite. This assemblage gave equilibration temperature of 1250 degrees C. LEW85440 consists of olivine, orthopyroxene, and minor augite [9]. The temperature from the orthopyroxene-augite pair is about 1220 degrees C. In Y791538 coexisting pigeonite-orthopyroxene pair is found [9]. The temperature is 1280 degrees C according to a modified phase diagram of [10]. LEW88201 is an orthopyroxene-bearing ureilite with one grain of coexisitng pigeonite-augite pair (personal comm., T. Baba), which gives temperature of 1240 degrees C according to the Ishii's pigeonite eutectoid reaction line [11]. The pyroxenes in Y74659 are mainly pigeonite with a few crystal of orthopyroxene. The temperature is about 1340 degrees C [10, 12]. ALH77257 also contained a few crystal of orthopyroxene. The equilibration temperature of 1275 degrees C is estimated from diffusion calculation of MnO and Fa zoning [12]. Thus the temperature of LEW88774 calculated from Ca diffusion profile is found to be consistent with the equilibration temperature in other ureilites and it is as high as those of other ureilites. Judging from these results, the development of coarse exsolution lamellae of pyroxene are consistent with origin of ureilites through a step-wise cooling history, with slow development of the coarse, equant, homogenous mafic silicate core crystals, followed by an abrupt transition to rapid cooling [2,6,7,8,9]. Rapid cooling from such high equilibration temperature was possibly caused by a collision of planetesimals and then the pyroxenes were quenched in a small fragment ejected from the parent body. Acknowledgments: We thank the MWG of the USA and NIPR for the samples and Prof. P. H. Warren and Dr. M. Prinz, T. Baba, and T. Ishii for data and discussion. References: [1] Prinz M. et al. (1994) LPS XXV, 1107-1108. [2] Warren P. and Kallemeyn G. (1994) LPS XXV, 1465-1466. [3] Miyamoto M. and Takeda H. (1994) JGR, 99, 5669-5677. [4] Chikami J. et al. (1995) 20th NIPR Symp. Antarc. Meteorites, 43-46, [5] Lindsley D. (1983) Am. Mineral., 68, 477-493. [6] Takeda H. (1987) EPSL, 81, 358-370. [7] Takeda H. et al. (1989) Meteoritics, 24, 73-81. [8] Takeda H. et al. (1986) LPS XXVII, 863-864. [9] Takeda H. (1989) EPSL, 93, 181-194. [10] Longhi J. and Boudreau A. E. (1980) Am. Mineral., 65, 563-573. [11] Ishii T. (1975) Mineral. J., 8, 48-57. [12]Miyamoto M. et al. (1993) JGR, 98, 5301-5307.
Chikami Jun
Mikouchi Takashi
Miyamoto Manabu
Takeda Hidenori
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