【摘要】 利用透射电镜研究了陨石和地幔岩中斜方辉石与斜顽辉石的多型转变:吉林陨石和清镇陨石顽火辉石中的斜顽辉石多型晶域宽度既有奇数倍也有偶数倍9A,双晶现象发育,出现频率高,是原顽火辉石转变而成。尖晶石和石榴子石二辉橄榄岩斜方辉石中的斜顽辉石仅具偶数倍9晶域宽度,未见双晶现象,出现频率低,是剪切应力作用下由斜方辉石转变而成。测量证实OREN向CLEN的相变按13.3°机制进行。在相应高温条件下,相变时应变率ε可能大于10^-6sec^-1。更多还原

【Abstract】 The polymorphic transformation of ortho-enstatite （OREN） and clinoenstatite （CLEN） in five natural samples has been investigated by HREM. The observed pyroxenes were selected from two meteorites-Jilin chondrite and Qingzhen enstatite chondrite, two mantle-derived rocks in basalts—spinel-and garnet-lherzolite and one Alpine ultramafic massif in Dazuka, Tibet.1. Meteorite.CLEN in enstatite possesses the field of any multiple of 9A wide, twinning and high concentration. Compared with the criteria of CLEN formation suggested by Buseck P.R. （1980）, the CLEN in Jilin and Qingzhen chondrites were produced by conversion of protoenstatite. Jilin chondrite with dominant OREN would be formed at a slower cooling rate than Qingzhen chondrite which possesses 30—50% CLEN with OREN and CLEN alternating frequently. As experiment shows that the concentration of OREN could rise slowly in expense of CLEN in the range 655-975℃ （Smyth,1974）,Qinhzhen chondrite preserving primary features of PEN to CLEN conversion is thought not to suffer obviously later thermometamorphism. This proves that the meteorite containing considerable amounts of CLEN produced by conversion of PEN would undergo light or no metamorphism during its forming history.2. CLEN in the mantle-derived rocks. The characteristics of CLEN in enstatite from spinel-and garnet-lherzolite fall in the field of 2n×9A only （2×4×,6×and 8×9A）without twinning. These features imply that CLEN was produced by shear deformation of OREN. Two mechanisms of OREN to CLEN transformation due to shearing have been proposed by former researchers:one is a macroscopic shear of 18.3° on（100） parallel to [001], the other is a shear of 13.3°, In this work, the calculated value of Burgers vector is 0.8C₀, this is wellmatched in 0.83C-0, i.e. the angle of shear is 13.3°.The spinel-and garnet-lherzolite are formed under the upper mantle at 1000℃, 15--22 Kb and 1100℃, ～40Kb respectively. These temperatures were not lower than 1000℃ after they involved in basalts. It means that OREN to CLEN transformation in the mantle-derived rocks took place under high temperature, instead of moderate to low temperatures as suggested by Buseck P.R. （1980）.Under this temperature/pressure condition of spinel-lherzolite, the formation of CLEN from OREN requires 2--3 kilobars of shear strength （Rieeker R. E., 1967）. As having tabular granuloblastie texture, it is assumed that OREN to CLEN transformation in garnet-lherzolite might be produed in higher shear stress.According to experimental results of Raleigh, C. B. （1971）,OREN in mantlederived rocks should transfer to CLEN only at a strain rate of>10^-6 sec^-1 in the range 1000°/1200℃. That is much higher than a reasonable geologic strain rate of 10^-13 to 10^-15sec^-1, but close to the strain rate for volcanism and seismieity.3. OREN to CLEN transformation in Alpine ultrabasie rock. CLEN transformed from OREN in harzburgite, Dazuka massif is different from that in meteorite in having no （2n+1）9 field width and lower concentrations, and also different from that in the mantle-derived in twinning appearance. Because many features show that the massif was under constant action of tectonic compression during its forming history, CLEN in the Dazuka massif should be produced by shearing,On the assumption of a reasonable geologic strain rate of 10^-12 to 10^-15 sec^-1, OREN to CLEN transformation only occurred at low temperature in the Dazuka massif （<600℃）. It is likely that the polymorphie transformation took place during emplacement of the massif, while it was basically consolidated.更多还原