Petrochemical study of the Yamato-691 enstatite chondrite (E3) IV: Descriptions and mineral chemistry of opaque-mineral nodules

Opaque-mineral nodules in Y-691 consist mainly of kamacite, perryite, schreibersite, troilite, daubreelite, niningerite, oldhamite, sphalerite, djerfisherite, caswellsilverite, graphite, roedderite, a silica mineral, albite, enstatite, and forsterite. The Ni and Si contents of kamacite are 2-3.5wt% and about 2wt%, respectively, being controlled by exsolution of perryite and schreibersite. Troilite shows a wide range of the Cr contents from 2.4 to 0.05wt%, and is classified into two types, massive and porous. The massive troilite is high in the Cr contents more than 0.6wt% and contains sometimes thin daubreelite lamellae, although the porous troilite is low in the Cr contents less than 1.2wt% and nearly free from daubreelite lamellae. The porous texture seems to be due to tiny inclusions of FeCl_2 in the troilite. Niningerite in Y-691 is the lowest in Mn and Fe contents in comparison to those in other enstatite chondrites. Djerfisherite in Y-691 is richer in Cu, Ni and Na in comparison to those in Qingzhen. Roedderite in Y-691 is similar in chemical composition to that in Indarch. Enstatite in Y-691 nodules shows a wide range of MgO/(MgO+FeO) molar ratio from 0.91 to 0.99,and sometimes forsterite occurs with a silica mineral in a nodule, suggesting that the silicates in the opaque-mineral nodules were not in equilibrium. Opaque-mineral nodules were classified into two main types based on the texture; concentric and massive, which are further divided into subtypes based on the modal volume of the main-minerals. The constituent minerals in these nodules seem to have polygenesis, and the genesis of nodules is discussed in detail. Any thermal metamorphic effects after accretion was not petrographycally detected in Y-691 nodules

Unusual inclusions within olivine megacrysts in the Dar al Gani 735 shergottite

Unusual inclusions occur within olivine megacrysts in the DaG735 olivine-phyric shergottite. They consist of pyroxene and chromite in a ratio of about 2:1, and show strange textures. They are grouped into two types, symplectic and non-symplectic. The pyroxenes in unusual inclusions are similar in composition to those in the main lithology of DaG735, but the chromites differ in TiO_2 contents which are lower by half than those of chromites in the main lithology. The symplectic textures indicate exsolution origin of the unusual inclusions within olivine megacrysts, and the reaction to produce the unusual inclusions suggests that the original olivine megacrysts may have contained some amounts of Cr^(+2) in addition to Cr^(+3), and the Cr^(+2) was oxidized to Cr^(+3) during the formation of the unusual inclusions

Petrography and petrology of the ALH-77005 shergottite

The ALH-77005 shergottite is a cumulate gabbroic rock consisting of brown olivine, low-and high-Ca pyroxene, plagioclase glass, Ti-poor and-rich chromite, ilmenite, whitlockite, and sulfides. Chromites in ALH-77005 show four types of chemical zoning, and these types suggest multiple magma mixing. Some plagioclase glass has plagioclase rims, and the rims were produced from plagioclase melts by rapid crystallization. Ubiquitous occurrences of plagioclase glass and shock-melt pockets that were produced by in-situ melting indicate that ALH-77005 has experienced shock pressures ranging from 50 to 80 GPa

Petrology and mineralogy of the Y-793605 martian meteorite

Yamato-793605 is a lherzolitic shergottite and very similar in texture and mineralogy to other two lherzolitic shergottites, ALHA77005 and LEW88516,but is not a pair with the latter two. It consists mainly of olivine, pyroxene, maskelynite, chromite, ilmenite, and pyrrhotite. Small silicate inclusions occur in olivine and pyroxene. Olivine has a wide compositional range from Fo_ to Fo_, that included in pyroxene is more magnesian, and that in contact with maskelynite is more ferroan. Chromite shows remarkable compositional zoning, and the ranges change according to the occurrence of chromite; chromite included in olivine is Cr-Ti-poor, that in pyroxene is Cr-rich and Ti-poor, and that in contact with maskelynite is Ti-rich. There is a compositional gap between chromites in olivine and pyroxene. Silicate inclusions in olivine consist mainly of aluminous pyroxene, fine-grained intergrowth of plagioclase and silica, and rhyolitic glass. Plagioclase of the intergrowth is more calcic with An_ than maskelynite of the host lithology (An_). The rhyolitic glass saturates with silica components and is peraluminous, whereas the host lithology does not include any silica mineral and is not peraluminous. These differences between the silicate inclusions and the host lithology, as well as the compositional gap between chromites in olivine and pyroxene, suggest that magma mixing took place in the Y-793605 magma reservoir between olivine and pyroxene crystallization stages. Y-793605 experienced secondary alteration of olivine, nonrhyolitic glass, chromite, ilmenite, pyrrhotite, and phosphate by introduction of Si and K, and loss of Ti, Cr, Mg, Ca, and Na probably at Antarctica

Petrology of the Yamato 980459 shergottite

Y980459 is a new type of shergottite, consisting mainly of olivine, orthopyroxene with augite mantles, and glassy groundmass. The constituent minerals crystallized under a rapid cooling condition. The cores of olivine phenocrysts are Fo85-73, and Fo85 is the most magnesian composition among all martian meteorites. The magnesian olivine may be mostly phenocrysts which crystallized from the parent magma having the Y980459 bulk composition. Many magmatic inclusions occur in olivine phenocrysts around Fo74. They consist mainly of glass, and the trapped melt composition was estimated from the inclusion glass by addition of the wall olivine. The trapped melt represents the early- to middle-stage melt of the Y980459 crystallization sequence. The cores of pyroxene are orthopyroxene, which are surrounded by augite mantles showing remarkable normal zoning. The glassy groundmass contains quenched crystals of ferroan olivine and clinopyroxenes set in a glassy groundmass. Shock-melt pockets occur in Y980459, and the shock degree, which was originally defined for ordinary chondrites (D. Stoffler et al.; Geochim. Cosmochim. Acta, 55, 3845, 1991), corresponds to S3 for Y980459. A new stage classification for lithological types of the shergottite group is proposed, and the Y980459 meteorite is classified as an olivine-bearing glassy basaltic type

Petrochemical study of the Yamato-691 enstatite chondrite (EH3) II: Descriptions and mineral compositions of unusual silicate-inclusions

The Yamato (Y)-691 chondrite includes unusual silicate-inclusions besides chondrules. An inclusion containing high CaO and Al_2O_3 contents and other opaque massive inclusions containing high FeO content are similar in bulk chemical compositions to fine-grained CAI\u27s and amoeboid olivine inclusions in carbonaceous chondrites, respectively. They are concluded to be melted finegrained CAI\u27s and melted amoeboid olivine inclusions. Most unusual inclusions containing high FeO content were originally high-temperature condensates under oxidized conditions and followed by melting events. After the consolidation, they were brought into extremely reduced conditions, where they accreted with magnesian chondrules on a parent body of the Y-691 enstatite chondrite

Petrochemical study of the Yamato-691 enstatite chondrite (EH3) I: Major element chemical compositions of chondrules and inclusions

The major element chemical compositions of more than 120 chondrules and inclusions in Yamato (Y)-691 were obtained using a defocussed beam of an electron-probe micro-analyzer. Most chondrules in Y-691 are depleted in Fe, Mn, Cr, and Ca in comparison to the Y-691 whole rock composition, indicating that the depleted elements reside in sulfides and/or metals occurring outside chondrules. Compared with the solar system abundances, radial-Px, porphyritic and granular chondrules are depleted in all elements studied except for Si, but the Sinormalized contents of Al, Ti, Mg, and alkalies are similar to the Y-691 whole rock composition. On the other hand, barred-Ol-Px chondrules are enriched in refractory and moderately-refractory lithophiles (Al, Ti, Ca, and Mg) and poor in moderately-volatile elements (Mn, K, and Na). Some inclusions show chemical compositions similar to fine-grained CAI\u27s and amoeboid olivine inclusions in carbonaceous chondrites

An overview of the research consortium,"Antarctic carbonaceouschondrites with CI affinities, Yamato-86720, Yamato-82162, andBelgica-7904

Three Antarctic meteorites, Yamato (Y)-82162,Y-86720,and Belgica (B)-7904,are unusual carbonaceous chondrites having characteristics of both CI and CM chondrite groups. Therefore, a research consortium has been carried out in 1987-1990,and all the results are summarized here. Remarkable results are; (1) Several minerals that are rare or new in carbonaceous chondrites were found in the consortium chondrites. They are sodian talc, FeO-bearing periclase or magnesiowustite, an unknown Fe-Ca-O-S mineral, rhodochrosite, chalcopyrite, eskolaite, schreibersite, Co-rich metal, Ni-Co-free kamacite, and so on. (2) Phyllosilicate veins were firstly found in Y-82162,suggesting that the hydration took place in the parent body. (3) Detailed mineralogy and petrology of altered chondrules and clasts suggest that most of the phyllosilicates in chondrules and clasts were produced from anhydrous precursors in the solar nebula. (4) At the final stage of the chondrite formation, the three chondrites underwent intense impact-shock or metamorphic heating after the hydrous alteration on their parent body. (5) Mineralogical, chemical, and oxygen isotopic data indicate that the three chondrites have some characteristics of both CI and CM groups, and the genetical relationships between the two groups and the consortium chondrites have been suggested. In addition, a new carbonaceous chondrite group CY is proposed for the three unusual chondrites

Petrochemical study of the Yamato-691 enstatite chondrite (E3) V: Comparison of major element chemistries of chondrules and inclusions in Y-691 with those in ordinary and carbonaceous chondrites

Major element compositions of chondrules and inclusions in Y-691 are different on the whole from those in ordinary and carbonaceous chondrites; the (CaO+AlO_)/(CaO+AlO_+SiO_2) and MnO/MgO ratios of chondrules in Y-691 are lower than those in ordinary and carbonaceous chondrites, but the SiO_2/(SiO_2+MgO) and MgO/(MgO+FeO) ratios are higher. These differences suggest that the chondrule precursors of Y-691 were produced by intense fractional condensation in comparison with those of ordinary and carbonaceous chondrites. For Y-691,the Ca/Al ratios of chondrules showing high Na/Al are lower than those of chondrules showing low Na/Al. This indicates that a fraction of Ca component was expelled from the chondrule precursors by reactions with a nebular gas, which also introduced alkalis into the chondrule precursors. In addition to magnesian chondrules, Y-691 includes unusual inclusions and ferroan chondrules. One unusual glassy inclusion, as well as others, are similar in chemical composition to fine-grained CAI\u27s and amoeboid olivine inclusions in carbonaceous chondrites, respectively. The precursors of the unusual inclusions and ferroan barred-Ol-Px chondrules seem to have formed under oxidized conditions, whereas the precursors of magnesian chondrules were produced from a reduced nebular gas. The oxidizing conditions may have been produced by evaporation of chondritic or cometary materials in the solar nebula

Petrochemical study of the Yamato-691 enstatite chondrite (E3) III: Descriptions and mineral compositions of chondrules

The Yamato-691 enstatite chondrite includes abundant chondrules of various textural types. Among them radial-Px type is the most predominant and the largest. Barred-Ol-Px type and porphyritic to granular types are common. Cryptocrystalline chondrules are small and transparent-SiO_2 chondrules are rare. The constituents of these chondrules are forsterite, enstatite, bronzite, diopside, spinel, plagioclase, silica mineral and/or glass with small amounts of metals and sulfides. Generalized crystallization sequence of chondrules is olivine, low-Ca pyroxene, high-Ca pyroxene, spinel, plagioclase, silica minerals, and glass, in the order. MgO/(MgO+FeO) molar ratios of olivine in barred-Ol-Px, prophyritic to granular, and radial-Px chondrules are 0.92-0.99,0.98-0.99,and 0.96-0.99,respectively. Ferroan olivines in barred-Ol-Px chondrules experienced various degrees of reduction after solidification, indicating that they were formed under oxidizing conditions and later they were reduced by reaction with a reducing gas. On the other hand, FeO-bearing olivine in radial-Px chondrules seems to be relict and reacted with the chondrule melts at high-temperatures to produce dusty olivine. MgO/(MgO+FeO) molar ratios of pyroxenes are 0.81-0.99 for barred-Ol-Px chondrules, 0.96-1.00 for porphyritic to granular chondrules, and 0.74-1.00 for radial-Px chondrules. Al_2O_3 of high-Ca pyroxene ranges widely from 2 up to 18wt% and is more than that of coexisting low-Ca pyroxene. The Al_2O_3 content of pyroxene increases with decreasing temperature during crystallization. Low-Ca pyroxene in barred-Ol-Px chondrules is orthopyroxene and richer in Al_2O_3 (1-14wt%) than that in porphyritic to granular and radial-Px chondrules (less than 1wt%), which crystallized originally as protoenstatite. Pure albite in peripheral portions of some chondrules may have been formed from aluminous diopside by introduction of alkalies from the ambient reducing gas. Magnesian nature of spinel in some chondrules is due to diffusional exchange of Mg and Fe in a reducing gas. Silica minerals which were precursors of transparent-SiO_2 chondrules might have been produced by oxidation of metallic silicon in Fe-metals and/or fractional condensation of a reducing nebular gas