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[PPS22-26] A mineralogical and chemical study of primitive achondrite NWA 6704
Keywords:Primitive achondrite, NWA6704
Results & Discussion: Seven polished thin sections (TS-1~7; 10×18mm; 10×20mm; 9×10mm; 10×12mm; 10×11mm; 15×28mm; 19×28mm) have been investigated. The texture is best characterized by the existence of many orthopyroxene (Opx) megacrysts up to 1.56 cm in length (Fs40-42En53-57Wo3-4). Olivine (Fa50-53) typically occur as interstices associated with feldspar and have subhedral shapes, but locally occur as the vermicular olivine within the Opx. The vermicular olivine share the same optical extinction position under crossed-nicols and occurs only in one place near the center of each Opx crystal if present. Feldspar (Ab91-93An5-6Or2-30) occurs in the Opx and the interstices as anhedral cuspate grains with extremely low dihedral angles and sharing the same optical extinxtion position over up to 1cm, which implies that it’s pseudomorph of a melt-filled interstitial pore. Other phases include chromite (Cr# = 0.90-0.96) and awaruite (78-81 wt% Ni) also occur associated with feldspar. We estimated fO2 of FMQ -2.6 using the Olv-Opx-Spl oxygen geobarometer (4).
The texture is properly represented by aggregate of large Opx hollow (skeletal) megacrysts with finer interstices. Other phases such as olivine are present in the hollow cores, and some of them are isolated from the interstices. The contiguity of feldspar in the Opx megacrysts with or without olivine in three dimensions is clearly demonstrated by the fact that the same optical extinction position are shared by feldspar isolated in the megacrysts and those present in the interstices. The distribution of the Opx hollow crystal and the vermicular olivine are highly heterogeneous. In TS-1 and TS-6, Opx <6.2 mm across are dominant and more than five of them include vermicular olivine; in TS-2~5 and TS-7, Opx megacrysts up to 1.56cm across are common and only a few of them contain vermicular olivine. The vermicular olivine may be a decomposition product of the precursor pyroxene through the abrupt heating (>1400℃). There is a negative correlation between size of Opx and the number of vermicular-olivine bearing Opx. Given this fact, a new scenario can be derived: this vermicular olivine and its host acted as nuclei for crystallization of hollow Opx crystals, and the number of nuclei limits the size of Opx. Thus, more nuclei in a given area result in smaller grain size as seen in TS-1,6. The abnormally large size of Opx and its hollow morphology indicates that the initial crystallization occurred under rapid cooling. The SEM images, however, show that some pigeonite contain sub-micrometer-size augite (Fs17En45Wo39) exsolution lamellae. We estimated equilibrium temperatures of 1050℃ using two-pyroxene geothermometry (3) and of 773℃ using Olv-Spl FeMg-1 exchange thermometry (4). The cooling rate between 1100-950℃ was estimated to be ~0.02℃/hr from the thickness and wavelength of multiple exsolution lamellae (5), indicating significant decrease in cooling rate at a later crystallization stage. The inferred thermal history suggests high internal temperature of the parent planetesimal due to 26Al decay, which was abruptly heated by impact followed by rapid cooling up to the ambient temperature to sustain slower cooling.
References: (1) Iizuka et al. (2013), (2) Warren et al. (2012), (3) Lindsley & Andersen (1983), (4) Ballhaus et al. (1991), (5) Jackson (1961)