Other
Scientific paper
Jul 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993metic..28..404m&link_type=abstract
Meteoritics, vol. 28, no. 3, volume 28, page 404
Other
4
Achondrites, Cooling Rates, Diffusion, Olivine, Pallasites
Scientific paper
We estimated cooling rates of the Yamato (Y) 8451 pallasite by applying the diffusion calculation similar to that used in our previous studies [1] to measure chemical zoning of its olivines with an electron microprobe. Because of textural similarity [2], these results were compared with those of the Y 74357 and MAC 88177 primitive achondrites to obtain some constraints on the formation and structure of their primitive crusts. The Y 8451 pallasite is described by Yanai and Kojima [3]. Olivine (Ol) in Y 8451 shows the reverse zoning of Mg-Fe toward the rim from Fa(sub)10.4 to Fa(sub)9.8 within a few hundred micrometers (Fig. 1). The MnO content in Ol of Y 8451 gradually increases toward the rim from about 0.35 to 0.45 wt%. The CaO content in Ol of Y 8451 gradually decreases toward the rim within a few hundred micrometers from about 0.1 to 0.01 wt%. The Cr(sub)2O(sub)3 content in Ol also decreases toward the rim from about 0.1 to 0.04 wt% within a few hundred micrometers. The Al(sub)2O(sub)3 content slightly decreases toward the rim, although it is very low (0.01 wt%). The preservation of chemical zonings in Ol suggests rapid cooling of the Y 8451 pallasite. Diffusion calculations show that a cooling rate of about 500 degrees C/yr from 1100 degrees C [2] to 600 degrees C gives the best fit for the observed Mg-Fe profile at the rim of Ol in the Y 8451 pallasite (Fig. 1). This cooling rate corresponds to a burial depth of about 5 m under solid rock. Y 74357 orthopyroxene (Opx) shows the reverse zoning of Mg-Fe at the rim within a few tens of micrometers, probably due to reduction [4]. In contrast to this, Fa(sub)8 of Ol in Y 74357, which is a similar value to the outermost rim composition of Opx, is almost constant throughout the grain. This result has been interpreted as Y 74357 cooling slow enough to reduce the whole grain of Ol, but too fast to reduce the entire Opx. The MnO content in Opx slightly increases at the rim, probably due to reduction. The CaO content in Opx gradually decreases toward the rim from about 1.3 to 0.7 wt%. The Cr(sub)2O(sub)3, Al(sub)2O(sub)3, TiO(sub)2, and Na(sub)2O contents in Opx of Y 74357 and MAC 88177 and in augite of Y 74357 gradually decrease at the rim within a few tens of micrometers. This result suggests that liquid rich in these elements was present and was extracted at the late stage of formation of this meteorite, because these elements tend to concentrate in Opx compared with liquid. These results may be attributed to the growth process and not to subsolidus cooling episodes. The CaO content in Y 74357 Ol gradually decreases from core to rim from 0.05 to 0.02 wt%. The other elements in Ol show no clear zoning profiles. In contrast to the Y 8451 pallasite, a cooling rate of 1.5 degrees C/yr (70 m in depth) gives the best fit for the observed Mg-Fe profile at the rim of Opx and the CaO profile at the rim of Ol in the Y 74357 primitive achondrite [4]. These results suggest that the Y 8451 pallasite records relatively rapid cooling compared with the primitive achondrites. The cooling rate of the Y 78451 pallasite is slower than that (a few degrees C/hr) calculated for ureilites [5]. References: [1] Miyamoto M. et al. (1986) JGR, 91, 12804-12816. [2] Hiroi T. et al. (1992) 17th Symp. Antarct. Meteorites, 53-56. [3] Yanai K. and Kojima H. (1987) Photographic Catalog of Antarctic Meteorites. [4] Miyamoto M. and Takeda H. (1991) Meteoritics, 26, 374. [5] Miyamoto M. et al. (1985) Proc. LPSC 16th, in JGR, 90, D116-D122. Fig. 1, which appears here in the hard copy, shows the chemical zoning profile of an olivine in the Y 8451 pallasite. Open circles indicate the Fa [=100x Fe/(Mg + Fe)] component. Curves show calculated diffusion profiles. Numbers on curves show cooling rates in degrees C/yr. Data are at 5-micrometer intervals.
Miyamoto Manabu
Takeda Hidenori
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