| The Dong Ujimqin Qi mesosiderite is the first recorded fall of a stony-iron meteorite in China. It contains about 50 vol.% silicates and 50 vol.% metal. Metal occurs as irregular large masses and matrix fine grains. Metallic portion contains kamacite, taenite and troilite. Schreibersite is an accessory phase. Silicate phases exhibit a porphyritic texture in which pyroxene and olivine phenocrysts are embedded in a fine-grained groundmass. Silicates in the matrix are mainly composed of pyroxene and plagioclase with minor olivine, chromite, silica, cordierite, merrillite, and stanfieldite. Orthopyroxene phenocryst exhibits two different appearances on their BSE images. One has well developed overgrowth. The other is characterized by fine inclusions of plagioclase, without overgrowth. Olivine phenocrysts occur as subhedral mineral clasts, and most are surrounded by coronas. Plagioclase normally appears as laths, but sometimes can be found embedded in pyroxene crystals. Silica occurs interstitially between silicates, or as small inclusions within pyroxenes. Merrillite normally connects to pyroxene. Stanfieldite appears either beside or inside metal. Cordierite occurs along metal boundaries. Chromite occurs as isolated grains in the matrix, sometimes as inclusions inside pyroxene.Olivine (Fo60.9-70.1) in Dong Ujimqin Qi is within the range typical of mesosiderite olivine grains of Fo55-90. Molar Fe/Mn ratios of pyroxenes are consistent with mesosiderite pyroxenes (16-35). Plagioclase grains ranging from An80.9 to An87.7, is only slightly lower than the range of plagioclase compositions in mesosiderites (An88-95). All of the petrographic and chemical data are consistent with classification of Dong Ujimqin Qi as a mesosiderite. Modal abundances of silicate phases are consist with those of class B mesosiderite. Abundant extremely fine-grained silicates and the sharp boundaries existing between silicate minerals in the matrix indicate little metamorphic recrystallization of the matrix silicates. These features suggest that Dong Ujimqin Qi mesosiderite is a member of subgroup 1B.Dong Ujimqin Qi contains abundant silica and phosphate, which are due to redox reactions between P-rich metal and silicates. The symplectic aggregates of silica and troilite in pyroxene could be a result of reduction of Fe from the pyroxene due to sulphur metasomatism. Reduction occurred after silicate were mixed with metal. Corona around olivine and rim on orthopyroxene were produced as a result of metamorphism. Dong Ujimqin Qi seems to have experienced the following thermal history:1) initial crystallization and cooling of silicates.2) metal-silicate mixing. During or after the metal-silicate mixing, reduction of silicates occurred. The reducing agent is phosphor and sulfur.3) thermal metamorphism that produced corona on olivine and rim on orthopyroxene.GRV 020175 is an Antarctic mesosiderite, containing about 43 vol.% silicates and 57 vol. % metal. Metal occurs in a variety of textures from irregular large masses, to veins through silicates, and to matrix fine grains. Metallic portion contains kamacite, troilite and minor taenite. Terrestrial alteration is evident as partial replacement of the metal and troilite veins by Fe oxides. Silicate phases exhibit a porphyritic texture and pyroxene, plagioclase, minor silica and rare olivine phenocrysts are embedded in a fine-grained groundmass. The matrix is ophitic and consists mainly of pyroxene and plagioclase grains. Some orthopyroxene phenocrysts occur as euhedral crystals with chemical zoning from a magnesian core to a ferroan overgrowth, others are characterized by many fine inclusions of plagioclase composition. Pigeonite almost was inverted to orthopyroxene host with augite lamellae, enclosed by more magnesian rims. Olivine occurs as subhedral crystal, surrounded by a necklace of tiny chromite grains (about 2-3μm). Plagioclase has a heterogeneous composition. However, zoning could not be found. Silica occurs interstitially to silicates, or as small inclusions within pyroxenes, or as coarse grains (up to 200-300μm). Coarse-grained silica probably represents material crystallized from a trapped liquid.Molar Fe/Mn ratios (19-32) of pyroxenes are consistent with mesosiderite pyroxenes (16-35). And most plagioclase compositions (An87.5-96.6) are within the range of mesosiderite plagioclase grains (An88-95). Olivine (Fo≈53.8) is only slight lower than the range of olivine compositions in mesosiderites (Fo55-90). All petrographic characteristics and chemical compositions of GRV 020175 are consistent with those of mesosiderite. Along with comparison of class A and B mesosiderites, GRV 020175 is a plagioclase rich class A mesosiderite. That silicate grains of all sizes show interlocking boundaries, rims of large pyroxene grains have ophitic textures indistinguishable from the matrix, olivine is rare and extensively replaced by pyroxene, and most pigeonite inverted to orthopyroxene with one or two sets of augite exsolution lamellae, is characteristic of subgroup 3 mesosiderites. So GRV 020175 is further classified as a type 3A mesosiderite.Pyroxene geothermometry of GRV 020175 gives a peak metamorphic temperature (～1000℃) and a closure temperature (-875℃). It means that following silicates crystallized and cooled rapidly to a relatively low temperature, silicates were reheated up to 1000℃and subsequently cooled to 875℃. It's worth noting that pigeonite not inverted to orthopyroxene suggests a relatively rapid cooling after reheating. Correlated Fe/Mn and Fe/Mg ratios in inverted pigeonite and close association of troilite (±metal) and silica in pyroxenes could be a result of reduction of Fe from the pyroxene due to sulphur metasomatism. Corona on olivine, rim on inverted pigeonite and overgrowth on orthopyroxene, and homogenized matrix pyroxenes are produced by metamorphism. The hosts of inverted pigeonite were altered by redox reactions and the rims of inverted pigeonite were formed by metamorphism. It suggests that FeO reduction occurred prior to the metamorphism. GRV 020175 experienced the following thermal history:1) Rapid crystallization from a magma in a lava flow on the surface or as a shallow intrusion.2) Following primary igneous crystallization, the silicate underwent varying degrees of reheating, followed by rapid cooling.3) Metal mixed with silicate. During or after the metal-silicate mixing, reduction of FeO out of silicates occurred. Correlated Fe/Mn and Fe/Mg ratios in pyroxenes and close association of troilite (±metal) and silica in pyroxenes are result from redox reactions. The reducing agent is sulfur.4) Following redox reaction, it underwent thermal metamorphism, which produced corona on olivine, rims on inverted pigeonite phenocrysts and overgrowths on orthopyroxene phenocrysts, and homogenized matrix pyroxenes. Combined detail studies of Dong Ujimqin Qi and GRV 020175 have allowed us to make the following inferences regarding formation history of mesosiderite:1) Mesosiderite have experienced a complex history involving a range of processes including initial melting, fractional crystallization, crustal remelting and crystallization, metal-silicate mixing, redox and metamorphism.2) Silicates of GRV 020175 are very similar to those of HED in petrographic and geochemical characteristics. the HED body were thermal metamorphosed at temperatures of 800-1000℃. This metamorphism resulted from the eruption and subsequent burial of lava flows to depths of 20 km or more and that deeply buried basalts were reheated to their melting points. We suggests that the same process may have caused crustal remelting on the mesosiderite body. The mesosiderite body underwent varying degrees of crustal remelting. GRV 020175 was near the heat source, while Dong Ujimqin Qi was far from the heat source.3) Some of small curvilinear metal trails pass through orthopyroxene grains in GRV 020175. FeO is high near metal trails, indicating the Fe from metal entered into the orthopyroxene. These trails could be formed by the injection of metal from the metallic portion of mesosiderite and may reflect the molten state of the metal during metal-silicate mixing. Moreover, the metallic portion of Dong Ujimqin Qi Show widmanstatten pattern suggests that the metal is molten, during metal-silicate mixing.4) The features that abundant silica and phosphate, correlated Fe/Mn and Fe/Mg ratios in pyroxenes, and close association of troilite (±metal) and silica in pyroxenes, appear to result from redox reactions. Reducing agent phosphorus and sulfur are present in molten metal. Reducing agent is not identical in different mesosiderites. The metal of GRV 020175 is lacke of phosphorus, while metal of Dong Ujimqin Qi contains abundant phosphorus. It means metal of the two mesosiderite came from different region, or experienced different evolving process.5) Metamorphism on Dong Ujimqin Qi and GRV 020175 produced corona on olivine and rims on pyroxene, and meanwhile metamorphism on GRV 020175 also homogenized the Mg# of matrix pyroxenes. That means the metamorphism on GRV 020175 was more intensive than that on Dong Ujimqin Qi. It coincides with their metamorphic grade that Dong Ujimqin Qi is grade 1, while GRV 020175 is grade 3. It is notable that matrix plagioclase and core of pyroxene phenocrysts compositions remain variable. It suggests that the combination of metamorphic temperature and time was not sufficient to homogenize plagioclase and pyroxene phenocrysts. The high degree of disequilibrium showed by the silicate minerals in mesosiderites shows that the metamorphism was not pervasive enough to reequilibrate whole materials.
|
PDF Full Text Request