| Impact is one of the most common processes on the celestial bodies in our solar system.A wide distribution of craters have been observed on the surface of Vesta,suggesting that Vesta has experienced a strong impact.Howardite-Eucrite-Diogenite(HED)meteorites are considered to be derived from Vesta.However,only few high-pressure minerals have been described in the HED meteorites.The purpose of our research is to conduct detailed mineralogical studies in the HED meteorites to constrain the temperature and pressure conditions of the impact events that HED meteorites have experienced,to further understand impact characteristics and history in HED meteorites.In our study,we selected eucrite meteorites as the research objects,including Northwest Africa(NWA)2650 and Dhofar 007.SEM and TEM were used to conduct petrographic observations.The structures were characterized by electron backscatter diffraction,transmission electron microscopy(selected area electron diffraction),and laser micro-Raman spectrometer.The chemical compositions were analyzed by Electron Probe Micro-Analyzer and TEM-EDS.The following results were obtained:(1)The impact metamorphism strength of two Eucrite meteorite is weaker than others;(2)More high-pressure minerals in two Eucrite were identified.Such as vacancy-rich clinopyroxene,super-silicic garnet,coesite,stishovite,tissintite,maskelynite,reidite,grossular+kyanite+silica glass,of which reidite,grossular,and kyanite were discovered in Eucrite meteorites firstly;(3)Plagioclase glass has various occurrences,which are related to impact melting closely,and the formation mechanism of the veins like plagioclase glass is same with shock melt veins;(4)The transformation of plagioclase to grossular+kyanite+silica glass has been observed,and the high pressure phase transformation of plagioclase has been carried forward.NWA 2650 is a brecciated eucrite meteorite.NWA 2650 can be divided into a host rock and shock-induced melt region.The host-rock portion of this meteorite contains two regions with different mineral grain sizes.Based EPMA data of pyroxene and plagioclase,the coarse-grained region may has the same source as the fine-grained region.The major minerals in the host-rock porition are pyroxene and plagioclase,and the minor minerals are quartz,ilmenite,chromite,troilite,zircon,apatite,and merrillite.In the shock-induced melt region and adjacent region,we observed many mineralogical records of shock metamorphism,including shock-induced melting veins,plagioclase glassy veins and high pressure minerals.The width of the impact melt vein varies widely,ranging from several micrometers to two millimeters.The mineral assemblages vary between different shock-induced melt veins.Some of them are composed of fine-grained vacancy-rich clinopyroxene,some of them are composed of super-silicic garnet.And some of the melt veins have obvious zonation in mineral assemblage.In the shock-induced melt veins with a zonation texture,the edge zone is usually composed of fine-grained vacancy-rich clinopyroxene,and the center of the vein is composed of super-silicic garnet,clinopyroxene,and coesite.Nine high-pressure phases have been observed in NWA 2650,including fine-grained vacancy-rich clinopyroxene,super-silicic garnet,coesite,stishovite,reidite,tissintite,maskelynite,grossular,and kyanite.Among these high pressure minerals,reidite and kyanite are for the first time reported in the HED meteorites.The fine-grained vacancy-rich clinopyroxene,super-silicic garnet,and coesite indicate that they are products of rapid crystallization from the melt.The lamellae-shaped occurrence of stishovite and reidite suggests that their formation mechanism may be solid state transformation.We observed three different occurrences of tissintite,with the locations including the inside of the impact melting vein,the edge of the impact melting vein and the interior of the plagioclase glassy vein.All of them are fine-grained aggregates of tissintite,which we believe they are products of rapid crystallization.We have also observed three types of plagioclase glass.The first plagioclase glass is near shock-induced melt veins and could be the product of high temperature melting and quenching.The second type is plagioclase glassy veins,which may be product of shear melting.The third type of plagiocled glass is close to shock-induced melt veins and could be maskelynite.Formation mechanism of the third type of plagioclase glass is solid state transformation.In addition,we have observed another phase behavior of plagioclase for the first time,grossular+kyanite+silica glass crystallized from the plagioclase melt.Based on the compositions of orthopyroxene and augite,the host rock of NWA 2650 should have experienced an equilibrium temperature of 814±33?.The presence of maskelynite and reidite may indicate a minimum peak impact pressure of up to 20 GPa,while other high pressure phases indicate that the impact pressure during impact melt vein crystallization may range from>8 GPa to>2 GPa,and the temperature distribution has non-uniform characteristics.The impact melt veins penetrated through the coarse-grained region and fine-grained region,indicating that the impact metamorphic event occurred after the fracturing and brecciation.Dhofar 007 is an accumulate eucrite meteorite,and its impact metamorphic mineralogical features are similar to those of NWA 2650.There are also abundant shock-induced melt veins and high-pressure minerals in the sample.The width of the impact melt veins varies widely,from one micron to a few millimeters.The minerals in the molten vein are mainly fine-grained super-silicic garnet or vacancy-rich clinopyroxene.The wider impact melt veins exhibit a banding phenomenon about mineral.The edge zone is usually fine-grained vacancy-rich clinopyroxene,and the center of the vein is composed of super-silicic garnet with silica(coesite and stishovite).Eight high pressure phases have been observed in Dhofar 007,including vacancy-rich clinopyroxene,super-silicic garnet,coesite,stishovite,tissintite,maskelynite,grossular,and kyanite.The assemblage of grossular+kyanite minerals in the Dhofar 007 is quite similar to that in NWA 2650.The fine-grained clinopyroxene,super-silicic garnet,and coesite imply that they are products of rapid crystallization from the melt.The lamellae-shaped stishovite indicate that it may be formed by solid state transformation.We observed two different tissintite aggregates in Dhofar 007,which locate in the impact melt vein and the edge of the impact melt vein.Both tissintite are fine-grained aggregates,and we believe that both are products of rapid crystallization.We have also observed two types of plagioclase glass.One is close to the impact melt vein,which is the product of high temperature melting and quenching.Another type of plagioclase glass is maskelynite and close to the region near the impact melt vein,which is formed by solid state transformation.In addition,we have observed grossular+kyanite assemblage in the Dhofar 007 meteorite,which is crystallized from the plagioclase melt.Based on the compositions of orthopyroxene,the host rock of Dhofar 007 should have experienced an equilibrium temperature of 850?.The presence of maskelynite may indicate a minimum peak impact pressure of up to 20 GPa,while other high pressure minerals indicate that the impact pressure may range from>8 GPa to>2 GPa during impact melt vein crystallization. |
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