Augite Grains Research Articles

Properties of the Guin ungrouped iron meteorite: the origin of Guin and of group-IIE irons

Guin is an ungrouped iron meteorite containing 45 μg/g Ga and 186 μg/g Ge. It is a coarse octahedrite with an unusually high Ni content (92.5 mg/g). Its metal composition is generally similar to irons in groups IAB and IIICD, but its closest relative is the Cruz del Aire ungrouped iron. Guin contains 6 ± 3 vol.% silicate inclusions that are petrologically and compositionally similar to those in IIE irons. The largest inclusion (2 × 4 cm) consists primarily of a shock-melted plagioclase-rich matrix surrounding large (1–15 mm), partly-melted augite grains. The oxygen isotopic composition of the inclusion ( δ 18O = 5.29‰; δ 17O = 3.87‰ ) is near the LL chondrite field. The Guin inclusion is similar in bulk composition to selected melt pocket glasses in ordinary chondrites produced in situ by preferential melting of plagioclase by shock compression. We suggest that the Guin assemblage formed by impact-melting on a chondritic parent body. Silicate inclusions in IIE irons share many petrologic and compositional (but not oxygen isotopic) characteristics with the Guin inclusions, suggesting that IIE irons also formed by impact-melting of chondritic materials. Individual IIE irons were probably derived from separate cratering events.

Petrofabrics of laminated gabbros from the Centre 3 igneous complex, Ardnamurchan, Scotland

SummaryPetrofabric analysis of five specimens of laminated gabbro from the Centre 3 igneous complex, Ardnamurchan, was carried out by measuring orientations of grains of plagioclase, olivine, and augite with a Universal-stage. Linear alignments of primary precipitate mineral grains within the laminar fabrics were revealed by plots of data for coexisting plagioclase and olivine or augite. The plagioclase grains involved have tabular forms elongated parallel to their a-axes (rather than their c-axes). The olivine grains have flattened ovoid forms and the augite grains have sub-ophitic forms developed about prismatic primocrysts. In all specimens plagioclase displayed alignment of long grain-axes directed approximately down the dip of the lamination, save for one case in which a weak oblique alignment was observed. These lineations were reproduced by accompanying olivine and augite. The alignments are tentatively ascribed to flowage of a viscous boundary zone to the magma body in which crystal growth was taking place.

Iron-rich basaltic komatiites in the early Precambrian Vermilion District: Discussion

In their paper, Green and Schulz (1977) describe and present chemical analyses of moderately magnesian, pillowed lavas and chilled margins of sills from the Vermilion District, Minnesota. They point out that the analyses are similar but not identical to those of certain komatiitic lavas, and on this basis call their rocks iron-rich basaltic komatiites. They then propose liberalization of established numerical criteria of komatiite compositions (Brooks and Hart 1974) to accomodate their samples. The paper vividly illustrates the present chaotic state of komatiite nomenclature. The rocks that Green and Schulz call komatiites are almost identical to certain rocks (aphanitic pyroxenites) in the upper part of a thick, layered flow in Munro Township, Ontario, which was identified as tholeiitic by Arndt (1976) and Arndt et al. (1977). Evidence for the similarity between the rocks from the two areas is striking. Green and Schulz suggest that their samples may represent a seawater-chilled carapace of a very thick flow; samples from both localities are texturally similar, being composed largely of prismatic augite grains mantled by green amphibole; and their representative chemical analyses are almost identical (compare analyses E-151a and E-149, Table 1, Green and Schulz (1977) with analyses 8 and 9, Table 3, Arndt (1977)). These rocks also share many features with other rocks that traditionally and consistently have been described as tholeiitic. Examples include layered sills in Archean greenstone belts (MacRae 1969; Williams and Hallberg 1973; Viljoen and Viljoen 1970), basalts in greenstone belts (Pearce and Birkett 1974; Naldrett and Goodwin 1977) and the picritic and basaltic lavas of Baffin Bay (Clarke 1970). In Munro Township the lavas were specifically distinguished from the mafic to ultramafic komatiitic lavas of that area. The komatiites were shown to be genetically related to spinifextextured ultramafic lavas that almost certainly have crystallized directly from ultrabasic liquids (a criterion that must form part of any definition of

A study of subsolidus relations of the Skaergaard pyroxenes by analytical electron microscopy

Within augite and pigeonite grains of the Skaergaard ferrogabbro 4430, the Ca-poor phases contain only three mole percent of CaSiO3, and the Mg-Fe partition coefficients between the Ca-poor and Ca-rich phases are extremely small with 0.46 for augite and 0.51 for pigeonite grains. These values indicate existence of diffusion within each grain (intragranular diffusion) at considerably low temperatures.

Some petrological aspects of imbrium stratigraphy

Petrochemical studies of clasts in breccias from Fra Mauro and Apennine Front provide insights into different structural levels of pre-Imbrium terra crust. Most of the Fra Mauro breccias and included clasts are ‘kreep’ basalts showing both igneous and cataclastic textures and are interpreted as the surface veneer of the terra crust. Similar samples were collected from the Apennine Front near Spur Crater, but they are mixed in with clasts of coarse, plutonic texture. One clast type is spinel pyroxenite whose mineralogy and petrochemistry are consistent with the original rock type being garnet pyroxenite. Such a mineral assemblage could only be stable at greater than 300 km depth, well within the lunar upper mantle. It is suggested that these clasts represent garnet pyroxenite xenoliths, emplaced within the lunar crust during early volcanism, that underwent a phase transition to spinel pyroxenite. Another clast type is plutonic norite, in which coarsely exsolved inverted pigeomte is associated with anorthitic plagioclase. Similar terrestrial rocks are found m Stillwater-type layered intrusions, and it is suggested that the lunar norites were parts of terra crustal layered complexes developed beneath a veneer of impact brecciated crust. Examples of the latter are illustrated by poikiloblastic-textured gabbroic anorthosite, with inverted pigeonite oikocrysts surrounding grains of plagioclase, augite, pigeomte, orthopyroxene, olivine and ilmenite. Application of mineral geothermometers indicates crystallization of these rocks below 1100 °C, temperatures that are well below the liquidus. Hence these textures probably developed largely by solid state recrystallization during impact-metamorphism.

Phase Relations and Crystallization Sequence in a Contact-Metamorphosed Rock from the Gunflint Iron Formation, Minnesota

Portions of the Gunflint Iron Formation, originally a ferruginous sediment, were metamorphosed by the intrusion of the Duluth Complex to assemblages containing: pigeonite (Wo10En24Fs66)+olivine (Fo13Fa37)+Fe-Ti oxide (Mt62Usp34Hc4)+plagioclase (An94Ab6)+ vapor+augite (Wo40En20Fs40) or cummingtonite Fe/(Fe+Mg) ˜ 0.69; quartz was present but probably was not in equilibrium with olivine. Comparison with synthetic phase-equilibrium studies indicate conditions of initial recrystallization of T≥ 800 °C, Ptotal ≥ 2kb, fo2 slightly below that of the pure fayalite-magnetite-quartz assemblage, and PH2O < Ptotal. During the slow cooling process following initial recrystallization, the phases present underwent a complex series of exsolution, inversion, oxidation, and hydration reactions. Pigeonite initially exsolved augite along (001), then inverted to orthopyroxene, which then exsolved augite along (100). The augite exsolved only pigeonite on (001) during its cooling history. The Fe-Ti oxide for the most part oxidized to an intergrowth of magnetite and ilmenite, although unoxidized portions later exsolved ulvöspinel. Cummingtonite exsolved actinolite, forming irregular patches of the latter. Olivine, orthopyroxene, and augite reacted with plagioclase to form retrograde amphiboles. Orthopyroxene had difficulty nucleating during this slow cooling process, forming only at widely spaced points in mosaics of primary pigeonite grains, and never nucleating within primary augite grains. The resulting orthopyroxene grains are much larger than the original pigeonite grains.

GEOLOGY AND PETROLOGY OF ESAN VOLCANO, HOKKAIDO, JAPAN

Esan, a volcano of the Nasu volcanic zone, is situated at the eastern end of the Kameda Peninsula, southwestern Hokkaido (Figs. 1 & 2; Table 1). In the Pleistocene time, the activity of Esan volcano begun with eruption of pyroclastic flows (Fig. 3), followed by extrusion of two lava domes, Kaiko-zan and Todo-yama. Then, a viscous lava erupted again, forming a semicircular ridge which has been called as “somma.” After a long quiescence, probably in the Holocene age, the final activity occurred, erupting nuee ardentes, pumice flows, lava flows and dome lavas (Fig. 4). Esan (618m) is the latest lava dome. During the whole history, the center of eruption have migrated from northwest to southeast. Esan volcano is composed of the calc-alkalic rocks which are divided into two types (Table 2). One is augite-hypersthene andesite of the early to middle stages, and the other is quartz-augite-hypersthene andesite of the final stage, in which quartz phenocrysts, bipyramidal or resorbed in form, are included as much as 2-8% by volume. As shown in Table 3, Esan lavas are intermediate in composition, ranging from 57.75 to 62.69% in silica content. They are similar in chemistry to other calc-alkalic rocks from the Nasu volcanic zone in Hokkaido (Figs. 5, 6 & 7). Mode of occurrence of phenocrystic quartz and experimental results by Wyllie (1971) (Fig. 8) suggest that the quartz phenocrysts in Esan lavas have not derived from the basement rocks, but crystallized from an intermediate magma together with large plagioclase, hypersthene, augite and hornblende prior to the crystallization of ordinary phenocrystic minerals under a certain condition of several kb, 1000-1100°C and 1-2% in water contents. Subsequently, the quartz phenocrysts suffered magmatic resorption by decreasing of pressure due to ascent of the magma. Simultaneously, hornblende phenocrysts resolved into the aggregates of minute grains of plagioclase, hypersthene, augite and magnetite (Table 4).

Metamorphosed Mafic Dikes from the Southern Bighorn Mountains, Wyoming

In the southern Bighorn Mountains two groups of mafic dikes occur within a terrain dominated by quartzofeldspathic gneiss. Un-metamorphosed dolerite dikes possess ophitic or subophitic texture and consist primarily of plagioclase (average composition, Ans 60 and augite (average Ca:Mg:Fe = 38:44:18). Metadolerite dikes in the same area consist of two distinctive facies. In each dike a granoblastic margin facies of clear plagioclase (average composition, An 38 ) and hornblende grades into an interior facies possessing relict ophitic or subophitic texture and consisting of clouded plagioclase (average composition, An 49 ) and granular augite (average Ca:Mg:Fe = 42:45:13). Many of the equant augite grains are peripherally replaced by hornblende. The metadolerites, similar in bulk chemical composition to the dolerites of the area, formed by recrystallization of comparable dolerites during the last regional metamorphism (low amphibolite facies) which affected the gneiss country rock. The dikes were metamorphosed under water-poor conditions which allowed only their margins to be completely recrystallized. Dike interiors underwent partial recrystallization accompanied by diffusion of iron from augite to relict plagioclase, causing clouding, and diffusion of calcium from plagioclase to form slightly more calcic augite and hornblende.

Petrology of a Fine-Grained Igneous Rock from the Sea of Tranquillity

All phases in a thin section of sample 10022 have been analyzed by electron microprobe. Augite grains show strong iron enrichment in the outer 15 to 20 microns. Pigeonite cores occur within augite grains. The plagioclase has an anorthite content of between 73 and 81 mole percent and is high in Si and low in Al compared to stoichiometric feldspar. Residual phases include microcrystalline Fe-rich "pyroxene," plagioclase, K-rich alkali feldspar, silica, and rare areas rich in P and Zr with concentrations of Ba, Y, and rare earth elements. The density, viscosity, and crystallization history of the lava of sample 10022 are discussed.

A study of the heavy minerals of the pre-Cambrian and Paleozoic rocks of Baraboo Range, Wisconsin

ABSTRACT The heavy minerals of the Huronian quartzite of the Baraboo Range consist mostly of zircon and rutile. There are occasional grains of biotite and apatite and rare grains of amphibole, augite, and magnetite. Secondary minerals are hematite and chlorite. No tourmaline was found in the quartzite, and only a single grain of garnet was seen. The heavy minerals in the quartzites are not different from those in the underlying igneous rocks. Zircon and tourmaline are common heavy minerals in the Paleozoic rocks. These rocks contain an abundance of garnet outside the Range, but this mineral is rare within the Range. The extreme rareness of garnet and the apparent absence of tourmaline in the Huronian quartzite show that the materials for the Paleozoic sandstones were largely derived from sources other than the rocks of the Range and the rareness of garnet in the sandstones within the Range indicates that only limited quantities of sands were introduced from the outside.

On an Analcite-basalt from Rathjordan, Co. Limerick

Amongst the rock-specimens from Co. Limerick belonging to the Allport Collection in the British Museum, a. specimen of basalt from Rathjordan, Co. Limerick, attracted attention by the fact that, in thin slices under the microscope, it showed small, round sections of isotropic material containing central and octagonally arranged, marginal inclusions, and thus resembling leucite. The rock from Rathjordan has been described by Hull and by Allport. According to Allport the rock is a columnar, fine-grained, black basalt which occurs interbedded with ashes, and consists of ‘innumerable small grains of augite and magnetite set in an amorphous glass, the whole forming a matrix in which serpentinous pseudomorphs of olivine and felspar are porphyritically imbedded’.