Exsolution Features Research Articles

Pyroxenes in Serra de Magé: cooling history in comparison with Moama and Moore County

Single-crystal X-ray, optical, and microprobe study of pyroxenes in the Serra de Magéfeldspar cumulate eucrite indicate complex exsolution features from a slow cooling history. Two pyroxenes now exist: “low” orthohypersthene ( P2 1ca) as host ( ∼82 vol.%) and augite ( C2/c) in four distinct habits. This pyroxene pair yields an apparent “equilibration” temperature of ∼900°. These relations are typical for orthopyroxene of both the Stillwater and Kintoki-San types, indicating an original pigeonite pyroxene with a bulk composition En 51Fs 39Wo 10. Variations in augite-hypersthene textural relationships suggest variable initial compositions from about Wo 8 to Wo 11. The bulk composition is intermediate to those of initial pigeonites in Moama and Moore County but the augite-hypersthene tie line is longer suggesting a slower cooling history. Our examinations of all three meteorites show that Serra de Magéaugite lamellae are as thick or thicker than those in the other meteorites, contrary to the measurement of Miyamoto and Takeda. The compositional data, textural relations, and existence of P2 1ca hypersthene suggest at least a comparable if not slower cooling history for Serra de Magé.

Exsolution in terrestrial and lunar plagioclases revealed by chemical etching

A new chemical etching technique reveals fine scale exsolution structures on polished surfaces of plagioclases in the bytownite and labradorite range. A very obvious blue and locally brown stain is produced (by the action of citric acid) in growth zones of a restricted range of compositions on etched surfaces. The blue-stained zones contain very fine grooves, corresponding to the traces of thin exsolution lamellae. Exsolution features have been revealed in this way in plagioclases in a terrestrial diabase, lunar basalts and an amphibolite. Compositions of the exsolved areas found by microprobe analysis represent averages of the matrix and exsolution lamellae. Analytical methods with higher spatial resolution are required to determine the composition of the lamellae. This etching method permits easy optical identification of compositional zoning and of fine scale exsolution over relatively large areas and is therefore complementary to other techniques.

Fluid Precipitates in Rocks from the Earth's Mantle

Xenoliths of rocks from the Earth9s mantle are commonly included in alkalic basalt and kimberlite diatremes. The xenoliths, many of them highly deformed by plastic flow, consist primarily of olivine and pyroxene and may be derived from depths as great as the seismic low-velocity zone. Large numbers of very small bubbles of CO 2 -rich fluid are within the major phases. The combined techniques of optical and high-voltage electron petrography demonstrate that the smallest bubbles, many of them below optical resolution, are attached to crystal defects induced by deformation and exsolution and to grain boundaries. It is concluded that formation of the smallest bubbles precedes incorporation of the xenoliths into the host magma and occurs by solid state precipitation on the deformation and exsolution features. The bubbles then collect on migrating grain boundaries during syntectonic recrystallization. The occurrence of bubble formation before, xenolith incorporation into the magma strengthens previous suggestions that the tectonite xenoliths may be the magma source rock or residuum and that the asthenosphere is not partially melted. Furthermore, since the presence of analogous “bubble structure” in ceramics and metals is known to weaken significantly the high-temperature creep resistance of such materials, it appears likely that the mode of flow in mantle rocks may be influenced similarly.

Les pyroxenolites a grenat du massif de lherzolite de Moncaup (Haute Garonne - France); Caracteres communs avec certaines enclaves des basaltes alcalins

Pyroxenites at Moncaup display high temperature - high pressure primary assemblages (clinopyroxene + garnet; clinopyroxene + orthopyroxene + spinel) with remarkable exsolution features. Chemical composition of rocks and minerals, and the mode of occurrence, suggest in situ crystallization of anatectic melt extracted from peridotite; these rocks invite comparison with zoned dykes in alpine-type peridotites resulting from dynamic fractional crystallization. The Moncaup pyroxenites may be compared texturally, mineralogically and chemically with xenoliths in basalts and nephelinites of the Honolulu series. The Moncaup rocks and Honolulu xenoliths could have the same origin and petrogenetic history.

The mineralogy of the Gosse pile ultramafic intrusion, central Australia. II. Pyroxenes

Abstract The orthopyroxenes of Gosse Pile, a layered ultramafic sequence within the Giles Complex of central Australia, show unusual exsolution features such as the development of ‘hour‐glass’ shapes by clinopyroxene lamellae, and also the exsolution of spinel and rutile. Both orthopyroxene and clinopyroxene have high R2O3 contents and their distribution coefficients for Mg and Fe are high (average KD = 0.88). Some, if not all, of these features are considered to indicate that the Gosse Pile rocks crystallized under high pressure. This interpretation also explains the predominance of orthopyroxene over olivine as an early cumulus phase. Estimated conditions during crystallization are 10–14 kilobars and between 1,200‐l,400°C.

Geology of the Binneringie Dyke, Western Australia

The geology of the Binneringie Dyke, one of the largest true dyke occurrences in the world — it extends for something like 200 miles (320 km.) from east to west, and has a maximum width at Cowan Hill of two miles (3.2 km.) — is described. The dyke is characterised by strongly chilled margins, but only meagre contact effects on the country rocks. The marginal zone is enriched in magnesium and calcium, due to an overprecipitation of early formed ferromagnesians. The marginal rocks are glomeroporphyritic and non-porphyritic bronzite, olivine-bronzite and olivine-bronzite-quartz gabbros, the bronzite having complex exsolution features reflecting inversion from magnesian pigeonite: the rocks towards the centre of the dyke have a progressively more ferroan character, augite-pigeonite gabbros taking the place of bronzite gabbros (the ferroan pigeonite being univerted). There are minor intermediate and acid phases throughout the length of the dyke: these take the form of irregular granogabbro segregation patches, in which there is an excessive development of residual “red rock”, of granophyric and devitrified acid glass composition. They also take the form of pegmatoid clots, associated with much epidote, amphibole, chlorite, sphene and prehnite. Much of the intermediate and acid material is, however, in the form of sharply bounded dykes and dykelets, within the gabbro: these bodies are chilled against the gabbro, never project through the chilled margin of the main dyke into the country rock and appear to represent infillings by late magmatic phases of shrinkage cracks in the gabbro. Another suite of internal dykes and dykelets has a basaltic character: dykes of basaltic dolerite of volcanic aspect are represented and these reveal idiomorphic olivine phenocrysts, trachytic textures, quartz palimpsests of chalcedony filled vesicles, and devitrified tachylite glass base material.

Le massif ultrabasique des Beni Bouchera (Rif Interne, Maroc): Etude des p�ridotites de haute temp�rature et de haute pression, et des pyrox�nolites, � grenat ou sans grenat, qui leur sont associ�es

The ultrabasic rocks which form the Beni Bouchera massif are schistose and folded isoclinally. They outcrop at the center of a late formed anticline under a series of granulite facies precambrian or lower paleozoic rocks. The ultrabasic rocks appear to have been in situ during the major structural development of the series and were emplaced at the time of metamorphism. The massif itself is essentially a peridotite, apparently layered, which contains pyroxenite bands (about 3% of the total volume of the massif). The peridotites are olivinerich containing two aluminous pyroxenes and spinel. The pyroxenites are however quite varied. They are frequently banded themselves and in several centimeters thickness can vary from enstatite rocks at the exterior of the layers to garnet clinopyroxenites at the center with websterite and garnet websterite as intermediate rock type. They are also present as monofacies bands of enstatite or garnet clinopyroxenite rock. Occasionally the minerals in these bands show exsolution features.