Petrogenesis Of HED Meteorites And Implications For Magmatic Evolution Of Parent Body

The Howardite—Eucrite—Diogenite?HED?clan of meteorites are ultramafic and mafic igneous rocks,which is the largest quantity of magmatic meteorites available on the earth,providing an unmatched significance for studying the early magmatic evolution processes of the asteroids.In this thesis,nine different types of HED meteorite samples,including orthopyroxenite,gabbro,diabase,basalt,breccia and other rock types were selected for systemic lithological observation and geochemistry analysis.Our studies are mainly concentrated on the structure and texture of rocks,major and trace elements geochemical characteristics in terms of whole rocks as well as minerals,for instance orthopyroxene,clinopyroxene,plagioclase,quartz,spinel,iron,troilite,ilmenite and so on,and isotopes features?e.g.oxygen and chromium isotopes?of whole rock.Based on these new data,we have made some significant achievements regarding to the HED clan of meteorites petrogenesis,implication of magmatic evolution,thermal metamorphism and meteoritic origin.Progresses achieved in this contribution are summarized as follows:a.The origin of the HED clan of meteorites is magmatic rocks.Crystallization sequence of the HED clan are accumulated pyroxenite,gabbro,diabase and basalt,which are characterized by homology and inheritance in magmatic evolution.These meteorites are formed from deep to shallow,corresponding to high temperature to low temperature as well as low thermal metamorphism to high thermal metamorphism.The thermal metamorphism of the superficial basalt can be caused by the heating of the multi—stage eruption of the basaltic magma.b.The HED clan of meteorites is mainly composed of pyroxene and plagioclase,whereas variations in mineral chemical compositions in different types of samples.Generally,pyroxenes in orthopyroxenite are bronzite—hypersthene with generalized compositions of Fs_19.2^<sub>30.6Wo_1.43^<sub>5.00,while in Cumulate gabbros mostly are hypersthenes with generalized compositions of Fs_34.1^<sub>37.9Wo_1.88^<sub>3.24 and minor augite—diopside exsolution lamella with generalized compositions of Fs_14.0^<sub>16.0Wo_41.3^<sub>46.5.The majority of pyroxenes in basalt are pigeonites with generalized compositions of Fs_44.1^<sub>65.3Wo_5.11^<sub>22.3,but slightly augite exsolution lamella with generalized compositions of Fs_26.1^<sub>47.7Wo_26.4^<sub>45.0 and ferro—hypersthenes with generalized compositions of Fs_57.8^<sub>62.9Wo_1.83^<sub>4.96.Mostly plagioclases found in the HED clan of meteorites are anorthites with generalized compositions of An_90.0^<sub>97.4Ab_2.20^<sub>9.73,and a small amount of bytownites and labradorites,with generalized composition of An87.7⁸9.7 Ab10.0¹1.3 and An73.2⁷7.9 Ab20.8²4.4,respectively.c.The Mg^#value of pyroxene has been used extensively in identifying rock type and elucidating evolution process of magmas of HED family meteorites.Pyroxenes from orthopyroxenite,cumulate gabbro and basalts have distinctive Mg^#values.More specifically,the enstenite from the orthopyroxenite,cumulate gabbro and basalts have decreasing Mg^#'s of72.8,63.5 and 36.6,respectively.Similarly,the clinopyroxene from these rocks yielded Mg^#'s of 71.4,73.9 and 44.7,respectively.d.Compared with the oceanic basalt,the HED clan of meteorites has higher MgO and FeO_t,but lower Al₂O₃ content.The whole—rock composition of the HED clan of meteorites indicates that the total amount of rare earth elements?REE?,the ratio of LREE/HREE,the lithophile and incompatible trace elements are increasing from orthopyroxene through to cumulate gabbro and then to basalt.On the other hand,based on those pyroxene REE and incompatible trace elements results,we inferred that the accumulated pyroxenite and gabbro are the products of magma crystallization differentiation.In contrast,the formation process of the basalt is much more complicated.Most of them are formed by the crystallization of residual magma,whilst a few of them derived from non—equilibrium crystallization of magma partial melting.e.According to the oxygen isotopes data,chromium isotopes noble gas and shock metamorphic effects of meteorites,we infer that GRV 13001 basaltic monolith likely originated from an asteroid fragment with diameter of several kilometers.The asteroid fragment was located closer to Mars than Vesta before entering Earth's orbit,and the debris may have originated in the Vesta or other asteroids which have similar charactristics with Vesta.