Distribution of major and minor constituents between mica and host ultrabasic rocks,and between zoned mica and zoned spinel

Ultrabasic rocks with Niggli values for si between 26 and 100, and k between 0.6 and 1 have been chosen for the study of the distribution of 23 major and minor constituents between phlogopite and the host rock. The rocks include kimberlites, carbonatites and lamprophyres which contain abundant zoned micas and a few zoned spinels. Chemical variations in zoned mica and spinel were determined by electron probe microanalysis, and distribution coefficients for Al, Ti, Fe, Cr, Mg, Mn, and Ni were calculated between mica and spinel for the average composition of the mica and spinel, and for chemically-related and adjacent zones. The spinel changes from chromite in the centre to chromian ulvöspinel in the rim, and the mica from chromian phlogopite, through titaniferous biotite to chromium-poor phlogopite. Distribution coefficients for Cr and Fe between adjacent zones in spinel (0.8 and 0.02 for Cr; 1.1 and 3.1 for Fe) and in mica (0.4 and 0.1 for Cr; 2 and 0.3 for Fe) show more variation than the distribution coefficients between the cogenetic phlogopite and chromite (0.04 for Cr, 0.2 for Fe), and biotite and ulvöspinel (0.08 for Cr; 0.2 for Fe). It is concluded that distribution coefficients for major and minor constituents between related zones are more meaningful than those calculated from the average chemical composition of zoned minerals.The results indicate that phlogopite is the principal carrier of potassium, rubidium, aluminium, fluorine and primary water in kimberlite and carbonatite. Minor elements, such as chlorine, lithium, barium, nickel, chromium, titanium and zinc are present mainly in the mica, whereas sodium, strontium, calcium, carbon and manganese are more abundant in other minerals.     

An analytical electron microscopic study of a pyroxene-amphibole intergrowth

Samples of a garnet granulite from the mafic border units of the Lake Chatuge, Georgia alpine peridotite body were found to contain lamellar intergrowths of a pargastic amphibole in augite having the typical appearance of an exsolution feature. Single crystal X-ray diffraction, optical, electron microprobe and conventional and analytical electron microscopic studies have provided data limiting the compositions and structures of the coexisting phases. Individual lamellae of both materials are from 0.5 to 2.0 m in width with the lamellar interface parallel to {0 1 0}. The formulae of the minerals, as determined by a combination of electron microprobe and analytical electron microscopy, are (Na_0.1Ca_1.0Mg_0.6Fe³⁺ _0.3)(Si_1.8Al_0.2)O₆ for the pyroxene and Na_0.7Ca_1.9(Mg_2.1Fe²⁺ _1.4Fe³⁺ _0.5Ti_0.1Cr_0.1Al_0.8)(Si_5.9Al_2.1) O₂₂(OH)₂ for the amphibole. Several other studies have described intergrowths similar to those observed in this work, in general favoring exsolution as the formation mechanism for the intergrowths. In the Lake Chatuge samples however, replacement of pyroxene by amphibole is in part indicated by continuous gradation of amphibole lamellae into amphiboles rimming the clinopyroxenes.Contribution No. 368 from the Mineralogical Laboratory, Department of Geological Sciences, The University of Michigan, Ann Arbor, Michigan     

Lawsonite zone blueschists and a sodic amphibole producing reaction in the Tavşanli Region,Northwest Turkey

The petrology and mineralogy of lawsonite zone metabasites have been studied northeast of town of Tavanli, NW Turkey. In the field the metabasites are characteristically green and lack foliation; the essential mineral assemblage being sodic pyroxene+ lawsonite+chlorite+quartz±sodic amphibole. Sodic pyroxene of aegirine-jadeite composition occurs as pseudomorphs after magmatic augite. Lawsonite and chlorite are the other two dominant minerals. Sodic amphibole forms progressively from a reaction between sodic pyroxene, chlorite and quartz, and an isograd representing the first abundant occurrence of sodic amphibole in basic rocks has been mapped. The widespread occurrence of sodic pyroxene pseudomorphs in other blueschist terrains indicates that the inferred sodic amphibole producing reaction is of general significance for blueschist metabasites.The conversion of greenstones with the assemblage albite+chlorite+actinolite directly into glaucophane-lawsonite blueschists without any intervening lawsonite zone illustrates the influence of the initial mineral assemblage on the reaction path.     

The influence of amphibole fractionation on the evolution of calc-alkaline andesite and dacite tephra from the central Aleutians,Alaska

Petrologic studies of tephra from Kanaga, Adak, and Great Sitkin Islands indicate that amphibole fractionation and magma mixing are important processes controlling the composition of calc-alkaline andesite and dacite magmas in the central Aleutians. Amphibole is ubiquitous in tephra from Kanaga and Adak Islands, whereas it is present only in a basaltic-andesite pumice from Great Sitkin. Dacitic tephra from Great Sitkin do not contain amphibole. Hornblende dacite tephra contain HB+PLAG+OX±OPX±CPX phenocrysts with simple zoning patterns, suggesting that the dacites evolved in isolated magma chambers. Andesitic tephra from Adak contain two pyroxene and hornbelende populations, and reversely zoned plagioclase, indicating a more complex history involving mixing and fractional crystallization. Mass balance calculations suggest that the andesitic tephra may represent the complements of amphibole-bearing cumulate xenoliths, both formed during the evolution of high-Al basalts. The presence of amphibole in andesitic and dacitic tephra implies that Aleutian cale-alkaline magmas evolve in the mid to lower crust under hydrous (>4 wt.% H₂O) and oxidizing (Ni–NiO) conditions. Amphibole-bearing andesites and pyroxene-bearing dacites from Great Sitkin indicates fractionation at several levels within the arc crust. Despite its absence in many calc-alkaline andesite and dacite lavas, open system behavior involving amphibole fractionation can explain the trace element characteristies of lavas found on Adak Island. Neither open nor closed system fractionation involving a pyroxene-bearing assemblage is capable of explaining the trace element concentrations or ratios found in the Adak suite. We envision a scenario where amphibole was initially a liquidus phase in many calc-alkaline magmas, but was later replaced by pyroxenes as the magmas rose to shallow levels within the crust. The mineral assemblage in these evolved lavas reflects shallow level equilibration of the magma, whereas the trace element chemistry provides evidence for a earlier, amphibole-bearing, mineral assemblage.     

Sodic amphiboles and pyroxenes from fenites in East Africa

During alkali metasomatism of the country-rock associated with ijolite-carbonatite complexes the development of sodic amphibole and/or pyroxene is characteristic. In this paper, some new chemical analyses of these minerals, together with published analyses from fenites of Kenya, Uganda and Tanzania, include those of co-existing pairs of amphibole and pyroxene. The common amphiboles of the fenites are magnesioarfvedsonites with 100 Mg: Mg+Fe+Mn ranging from 67 to 36. They co-exist with aegirines having 0.75 to 0.89 ions Fe⁺³. Most of these minerals are poor in Ca; co-existing pairs tend to show corresponding increases in Ca and in Fe⁺². In the syenitic fenites of Tororo and Budeda, considered to have formed at higher temperatures, the stable mineral is aegirine-augite. New analyses of richterite, magnesioarfvedsonite and aegirine from carbonate-rich rocks are also presented, and the relation between fenites and carbonatites is discussed.     

The nakhlite meteorites: Augite-rich igneous rocks from Mars

The seven nakhlite meteorites are augite-rich igneous rocks that formed in flows or shallow intrusions of basaltic magma on Mars. They consist of euhedral to subhedral crystals of augite and olivine (to 1 cm long) in fine-grained mesostases. The augite crystals have homogeneous cores of Mg′=63% and rims that are normally zoned to iron enrichment. The core-rim zoning is cut by iron-enriched zones along fractures and is replaced locally by ferroan low-Ca pyroxene. The core compositions of the olivines vary inversely with the steepness of their rim zoning - sharp rim zoning goes with the most magnesian cores (Mg′=42%), homogeneous olivines are the most ferroan. The olivine and augite crystals contain multiphase inclusions representing trapped magma. Among the olivine and augite crystals is mesostasis, composed principally of plagioclase and/or glass, with euhedra of titanomagnetite and many minor minerals. Olivine and mesostasis glass are partially replaced by veinlets and patches of iddingsite, a mixture of smectite clays, iron oxy-hydroxides and carbonate minerals. In the mesostasis are rare patches of a salt alteration assemblage: halite, siderite, and anhydrite/gypsum. The nakhlites are little shocked, but have been affected chemically and biologically by their residence on Earth.Differences among the chemical compositions of the nakhlites can be ascribed mostly to different proportions of augite, olivine, and mesostasis. Compared to common basalts, they are rich in Ca, strongly depleted in Al, and enriched in magmaphile (incompatible) elements, including the LREE. Nakhlites contain little pre-terrestrial organic matter. Oxygen isotope ratios are not terrestrial, and are different in anhydrous silicates and in iddingsite. The alteration assemblages all have heavy oxygen and heavy carbon, while D/H values are extreme and scattered. Igneous sulfur had a solar-system isotopic ratio, but in most minerals was altered to higher and lower values. High precision analyses show mass-independent fractionations of S isotopes. Nitrogen and noble gases are complex and represent three components: two mantle sources (Chas-E and Chas-S), and fractionated Martian atmosphere.The nakhlites are igneous cumulate rocks, formed from basaltic magma at ∼1.3 Ga, containing excess crystals over what would form from pure magma. After accumulation of their augite and olivine crystals, they were affected (to various degrees) by crystallization of the magma, element diffusion among minerals and magma, chemical reactions among minerals and magma, magma movement among the crystals, and post-igneous chemical equilibration. The extent of these modifications varies, from least to greatest, in the order: MIL03346, NWA817, Y000593, Nakhla=Governador Valadares, Lafayette, and NWA998.Chemical, isotopic, and chronologic data confirm that the nakhlites formed on Mars, most likely in thick lava flows or shallow intrusions. Their crystallization ages, referenced to crater count chronologies for Mars, suggest that the nakhlites formed on the large volcanic constructs of Tharsis, Elysium, or Syrtis Major. The nakhlites were suffused with liquid water, probably at ∼620 ma. This water dissolved olivine and mesostasis glass, and deposited iddingsite and salt minerals in their places. The nakhlites were ejected from Mars at ∼10.75 Ma by an asteroid impact and fell to Earth within the last 10,000 years.Although the nakhlites are enriched in incompatible elements, their source mantle was strongly depleted. This depletion event was ancient, as the nakhlites’ source mantle was fractionated while short-lived radionuclides (e.g., ) were still active. This differentiation event may have been core formation coupled with a magma ocean, as is inferred for the moon.     

Alteration of basalts from site 396 B,DSDP: Petrographic and mineralogic studies

Samples of crystalline basalt from Site 396 B are all more or less altered, usually in strongly zoned patterns. Evidence has been found for several related or independent alteration stages, including (1) minor localized deuteric (amphibole and mixed clay minerals in miarolitic voids); (2) minor widespread nonoxidizing (pyrite on walls of vugs and cracks); (3) localized diffusion-controlled vug filling ( glauconite in black halos); (4) pervasive low level oxidizing (transformation of titanomagnetite to cation-deficient titanomaghemite); (5) localized diffusion-controlled strongly oxidizing (breakdown of olivine and titanomaghemite in brown zones). Plagioclase and pyroxene are essentially unaltered. Detailed analyses of gray and brown zones in pillow basalts show that low temperature oxidation has proceeded in a step-wise fashion, with the relative stabilities of the igneous minerals controlling the steps. Secondary minerals that crystallized from pore fluids on to the walls of vugs may or may not be related to local alteration of primary phases.During the most intense stage of alteration, brown oxidation zones grew into basalt fragments behind diffusion controlled fronts. The specific reactions and products of this stage differ among the lithologic units at the site. A model is proposed whereby efficient seawater circulation through the pillow units maintains the pH and the concentrations of Mg²⁺ and SiO₂ dissolved at low levels in pore fluids, so that olivine is replaced by hydrous ferric oxides, and Mg and SiO₂ are removed from the system. In the massive basalt unit, circulation is somewhat less effective and Mg and SiO₂ are retained in smectites.Deposition of authigenic minerals in the sequence saponite/Fe-Mn oxides/phillipsite/calcite in vugs and cracks may reflect the gradual closing of the systems and probably signals the end of localized oxidation in parts of the core. Mineral compositions indicate that most of these deposits formed from seawater at very low temperature.     

Ultracataclasis, sintering, and frictional melting in pseudotachylytes from East Greenland

Large volumes of pseudotachylyte (an intrusive, fault-related rock interpreted to form by a combination of cataclasis and melting) occur in Tertiary normal faults and accommodation zones along 400 km of the East Greenland volcanic rifted margin. Analysis of representative pseudotachylyte samples reveals a wide range of mesoscopic and microscopic textures, mineralogies, and chemistries in the aphanitic pseudotachylyte matrix. Three distinct types of pseudotachylyte (referred to as angular, rounded and glassy) are identified based on these characteristics. Angular pseudotachylyte (found primarily in dike-like reservoir zones) is characterized by angular grains visible on all scales, with micron-scale fragments of mica and amphibole. Its matrix is enriched in Fe₂O₃, MgO, and TiO₂ relative to the host rock, with minor increases in CaO, K₂O, and small decreases in Na₂O. Rounded pseudotachylyte is found in reservoir zones, injection veins (pseudotachylyte-filled extension fractures), and fault veins (small faults with pseudotachylyte along their surfaces). It is characterized by smooth-surfaced, compacted grains on microscopic scales, and encloses rounded, interpenetrative lithic clasts on outcrop scale. Its matrix is enriched in Fe₂O₃, MgO, TiO₂, and Al₂O₃ relative to the host rock, with minor depletion in Na₂O and K₂O. Glassy pseudotachylyte is found primarily along fault surfaces. Its matrix is characterized by isotropic, conchoidally fractured material containing microscopic, strain-free amphibole phenocrysts, and is enriched in TiO₂, Al₂O₃, K₂O, Fe₂O₃, MgO, CaO, and Na₂O relative to the host rock. These observations suggest that angular pseudotachylyte was produced by cataclasis, with enrichment in metallic oxides resulting from preferential crushing of mechanically weak amphibole and mica minerals found in the gneissic host rock. Cataclasis and concomitant frictional heating resulted in the textural and chemical modification of angular pseudotachylyte by sintering or melting, producing rounded and glassy pseudotachylyte, respectively. Compositional and textural observations constrain the temperatures reached during frictional heating (700–900°C) which in turn delimit the amount of frictional heat imparted to the pseudotachylytes during slip. Our results suggest that the East Greenland pseudotachylytes formed during small seismic events along faults at shallow crustal levels. Consistent relative ages and widespread occurrence of pseudotachylyte-bearing faults in East Greenland suggest that widespread microseismicity accompanied the early development of this volcanic rifted margin.     

The MARID (mica-amphibole-rutile-ilmenite-diopside) suite of xenoliths in kimberlite

Within the ‘glimmerite’ nodules occurring within kimberlite pipes we recognize the MARID suite consisting of varying proportions of mica, amphibole, rutile, ilmenite and diopside. Banding of some specimens is interpreted as cumulate layering. All specimens were deformed either before incorporation into the host kimberlite or during intrusion. Compared with minerals in peridotite xenoliths, the MARID ones are lower in Al₂O₃ and Cr₂O₃, but richer in total iron. The MARID micas, amphiboles, diopsides, ilmenites and probably rutiles contain substantial Fe₂O₃ indicative of oxidizing conditions. The amphibole is potassic richterite. Micas of the megacryst suite in kimberlite have less total iron and Fe₂O₃ than micas of the MARID suite. We suggest that the rocks of the MARID suite crystallized under oxidizing conditions from a magma, chemically similar to kimberlite, within the higher parts of the upper mantle: the presence of amphibole restricts the depth to less than ~ 100 km. A xenolith containing olivine and orthopyroxene as well as minerals similar to but not the same compositionally as MARID-types is interpreted as a metasomite, possibly representing wall-rock of a magma body from which MARID-suite rocks crystallized.     

Petrology of Ocellar Lamprophyres in South Delhi Fold Belt,Danva, Sirohi District,Rajasthan, India

Ocellar lamprophyres are reported from Danva, in Sirohi district, Rajasthan which lies in the Pindwara-Watera sector of the South Delhi Fold Belt (SDFB). They intrude mafic metavolcanics and garnetiferous biotite schist of the Ajabgarh Group of the Delhi Supergroup. These lamprophyres are unaltered, show porphyritic and panidiomorphic textures with olivine, clinopyroxene, amphibole, biotite and spinels constituting the phenocryst phase in a ground mass of clinopyroxene, amphibole, mica and analcime. Petrochemically, these lamprophyres can be classified as analcime monchiquite (alkaline lamprophyre), though they have some affinity towards ultramafic lamprophyres and lamproites. Chemistry of the minerals suggests that the clinopyroxenes have a compositional range of diopside and salite, amphiboles are typically kaersutites, biotites are titanium rich and spinels are Al-rich ulvospinels. These minerals plot in discriminant fields of minerals from alkaline and ultramafic lamprophyres.The Danva lamprophyres are characterized by a variety of ocellar features that include (i) porphyritic ocellus with microphenocrysts of kaersutites and biotite in ground mass of analcime, (ii) zoned ocellus with concentric zones of carbonates and analcime and (iii) composite ocellus in which type (i) ocellus enclose the type (ii) ocellus. These ocellar features are interpreted to represent late stage magmatic segregation and magmatic crystallization involving two immiscible magmatic liquids.The Danva lamprophyres are in strike continuity of the Pipela lamprophyres and therefore confirm alkaline magmatism for over 20 kms in SDFB during end-Neoproterozoic tectono-thermal event. The close spatial association of the lamprophyres with Cu-Zn-Au deposits of Danva and Pipela area and the first report of monchiquite with affinity towards ultramafic lamprophyres and lamproites may be significant in gold (and possibly diamond) exploration in the SDFB.     

Sub-solidus dehydration of amphiboles in an andesitic magma

Several petrologic experiments have demonstrated that in igneous and metamorphic reactions amphibole minerals can break down by a subsolidus dehydration reaction, but evidence for the reaction in natural rocks has been lacking. Evidence for the breakdown of an edenite-pargasite amphibole by a subsolidus dehydration reaction has now been found in an andesite flow from Garner Mountain, southern Cascase Range. The andesite contains one modal percent of crystal clots formed of crystallites of opx, cpx, plag, K-spar, opaque and quartz. The crystal clots retain the original amphibole morphology and intra-clot pyroxenes are aligned with crystallographic c parallel to c in the amphibole precursor; these conditions would not be duplicated by a melting reaction.Microprobe analyses of the bulk clot and the intra-clot minerals suggest the solid-state reaction: 100 amph+10 SiO₂=>55 cpx+33 plag+22 opx+ 1 opq+1 ksparPyroxene thermometry of the andesite groundmass pyroxenes and the intra-clot pyroxenes demonstrates that the amphibole dehydration reaction occurred in the xenocrystic amphiboles as a result of heating by the near-solidus andesite magma.     

Paragenetic Relationships in an Amphibolitic Tectonic Block in the Franciscan Terrain, Panoche Pass, California

Two tectonic inclusions within the Franciscan mélangeof California display a relict epidoteamphibolitic assemblagethat has been modified by rather continuous degrees of retrogressivereplacement to blueschist facies minerals. The relict assemblageconsists of plagioclase +hornblende+epidote+sphene±rutile±calcicpyroxene±garnet; superimposed during later blueschistmetamorphism is the asemblage sodic amphibole+lawsonite±sodicpyroxene±pumpellyite±phengitic mica ±albite+sphene±Ca-carbonate.In an attempt to evaluate chemical redistribution accompanyingblueschist metamorphism, electron microprobe analyses have beendetermined for all major minerals from both assemblages. Hornblende commonly exhibits replacement by crossite aroundgrain boundaries and replacement patches of more riebeckiticamphibole adjacent to mafic grains or inclusions; actinoliticamphibole occurs in veins cross-cutting hornblende and in somecases these veins are zoned to include a central core of sodicamphibole. Omphacitic pyroxene occurs in veinlets and replacescalcic pyroxene in some cases; a more aluminous omphacite varietyis commonly associated with the breakdown of epidote. Garnetis highly retrogressed to chlorite. In general, relict Ca-Alsilicates are in various stages of breakdown, being largelyconverted to lawsonite and pumpellyite; concurrently Na-Al silicatesformed by replacement where early calcic minerals participatedin blueschist reactions. Reactants and products may be separatedby phases not participating in the reactions. Whole rock compositionspoint strongly to a closed system with respect to all majorcations, and the principal source for Na participating in replacementreactions appears to be from the breakdown of albite. The mechanismfor replacement of the older assemblage is primarily attributedto a contemporaneous series of coupled reactions among nearlyall the early minerals and a fluid phase. Sea-floor spreading models may explain the tectonic emplacementof the amphibolite from a relatively high temperature-high pressureregime into a comparatively low temperature-high pressure environmentleading to the observed mineralogical re-adjustments.     

An electron microscopic study of amphibole lamellae in augite

Ion-thinned samples of augite from four plutonic igneous rocks have been examined in the electron microscope at 100 kV. Lamellae less than 0.08 m thick were observed parallel to (010) in all four samples. Electron diffraction shows that the lamellae consist of clino-amphibole, space group I2/m, with crystallographic axes parallel to those of the augite. Analytical electron microscopy of the lamellae in one specimen shows that they are a hornblende. The amphibole lamellae have nucleated at the interface between the augite host and exsolution lamellae of orthopyroxene parallel to (100) of the augite. The interface between the amphibole lamellae and the augite is coherent.It is thought that the amphibole lamellae have formed by exsolution from the augite, implying the existence of finite solid solution between members of the pyroxene and amphibole groups of minerals.     

The mineralogy of the glaucophane schists and associated rocks from Île de Groix,Brittany, France

Ninety-seven mineral phases consisting of ten chloritoids, fifteen epidotes, sixteen garnets, four sphenes, seven rutiles, seven pyroxenes, thirteen blue amphiboles, two green amphiboles, eleven phengites, two paragonites, a mariposite, seven chlorites, and two specimens of albite were obtained from the metamorphic rocks of Île de Groix, and their chemical, physical, optical and X-ray properties determined. The chloritoids are all optically positive, monoclinic polymorphs with large 2V, moderate refractive indices and characterized by high densities. Their fluorine contents have been used to propose a new upper limit for OHF substitution in the chloritoid structure, suggesting that partial pressure of fluorine might modify the stability of chloritoids from that determined in pure H₂O. The epidotes belong to the Al-Fe epidote series and are epidote sensu stricto. The almandine-rich garnets and the chloromelanites are metastable relics in the glaucophane schists. The grossular contents of the calcareous schist garnets are believed to have become depressed under high CO₂ pressure and the low Tschermak's contents of the pyroxenes are to be explained by equilibria involving epidote at high and low temperature when the Tschermak's components will break down to epidote group minerals. The sphenes contain appreciable amounts of combined water, fluorine substituting for oxygen and aluminium substituting for silicon and titanium. The presence of H₃O⁺ is suspected in a specimen of blue amphibole. The barroisite has a composition between glaucophane and hornblende. On account of its high Fe³⁺ content it is believed to have formed under higher P _{O 2} than the blue amphiboles. The paragonites which occur in the ohloritoid veins are unstable in the potassium-rich aluminous schists. The phengites show a tendency towards sericitic composition due to post-glaucophanisation readjustments under the lower pressure conditions of the greenschist facies. Some of the Fe³⁺ contents of the chlorites are interpreted as due to oxidation of ferrous iron, e.g. 2 [Fe(OH)₂]2FeOOH + H₂. The minerals show strong chemical control of the host rock and their Mn contents are directly related to those of the minerals from which they have evolved through retrogression.Chloritoids and epidotes that are not associated with garnets contain higher amounts of manganese; similarly, the two blue amphiboles with the highest FeMg ratios were obtained from rocks in which garnet has not appeared. It is therefore believed that ottrelite and piemontite would be stable only at the lowest subfacies of the greenschist facies. Also, the ironrich amphiboles must have evolved from low-grade iron-aluminium chlorites, since on the appearance of garnet in a schist iron-aluminium chlorites react with quartz to give almandine and Mg-rich chlorites. The Fe²⁺Mg ratios of the blue amphiboles therefore reflect the grade of the original schist in which the minerals formed.     

Petrology and Mineral Chemistry of Plutonic Igneous Xenoliths from the Carbonatite Volcano, Oldoinyo Lengai, Tanzania

Magmatic plutonic rocks in the Oldoinyo Lengai pyroclasticsare jacupirangite, pyroxenite, ijolite, nepheline syenite andwollastonitite. Mainly cumulates, they are combinations of nepheline,clinopyroxene, Ti-andradite, spinel (sensu lato), apatite, perovskite,titanite, wollastonite, sulphides, mica, glass and K-rich feldspar,most of which are strongly zoned. Low analytical sums for glasses,vesiculation of intergranular glasses, and the generally explosivenature of the volcanicity point to significant concentrationof dissolved volatiles in the parent magma; the absence of hydrousphases suggests that the dominant volatile is CO₂. Cumulatetextures, widely variable modes, veining and variation in specimenconsolidation and metasomatism all indicate derivation froma structurally complicated and multiply injected sub-volcaniccomplex. Complex zoning of phases and mineral disequilibrium is attributedto convective percolation of fluids through permeable cumulates,possibly complicated by magma replenishment during crystallizationof individual magma batches. Olivine, mica and pyroxene megacrystsin some ijolites indicate polybaric crystallization; mixingof potassic and sodic magmas may be the cause of these megacryst-bearingijolites, but the main parent is highly evolved, of carbonatedijolitic (nephelinitic) composition and with Nd and Sr isotopecharacteristics slightly more depleted than Bulk Earth. KEY WORDS: xenoliths; ijolite; jacupirangite; nepheline syenite ^*Corresponding author. Telephone 031 650 4837. Fax: 031 668 3184. e-mail: jbdawson{at}glg.ed.ac.uk     

Petrology and mineralogy of dredged gabbro from Gettysburg Bank,eastern Atlantic

Gabbros have been dredged from Gettysburg Bank, 110 km west of Portugal on the Azores/Gibraltar fracture zone. Primary minerals in olivine, pyroxene and brown hornblende gabbros are partially replaced by metamorphic minerals. Igneous textures are inhomogeneously overprinted by a granular polyhedral deformation and a cataclastic deformation. Amphiboles show characteristics which indicate a transition from crystallisation in a magma chamber to formation of amphibole in solid gabbro under metasomatic conditions. Of the amphiboles analysed, chlorine was present in the green amphiboles but below 0.05% in the brown suggesting the penetration of sea water after the formation of the brown amphibole but during the formation of the green.     

Editorial

The Rattlesnake Tuff of eastern Oregon comprises >99% ofhigh-silica rhyolite glass shards and pumices representing 280km³ of magma. Glassy, crystal-poor, high-silica rhyolite pumicesand glass shards cluster in five chemical groups that rangein color from white to dark gray with increasing Fe concentration.Compositional clusters are defined by Fe, Ti, LREE, Ba, Eu,Rb, Zr, Hf, Ta, and Th. Progressive changes with increasingdegree of evolution of the magma occur in modal mineralogy,mineral composition, and partition coefficients. Partition coefficientsare reported for alkali feldspar, clinopyroxene, and titanomagnetite.Models of modal crystal fractionation, assimilation, successivepartial melting, and mixing of end members cannot account forthe chemical variations among rhyolite compositions. On theother hand, 50% fractionation of observed phenocryst compositionsin non-modal proportions agrees with chemical variations amongrhyolite compositions. Such non-modal fractionation might occuralong the roof and margins of a magma chamber and would yieldcompositions of removed solids ranging from syenitic to granitic.A differentiation sequence is proposed by which each more evolvedcomposition is derived from the previous, less evolved liquidby fractionation and accumulation, occurring mainly along theroof of a slab-like magma chamber. As a layer of derivativemagma reaches a critical thickness, a new layer is formed, generatinga compositionally and density stratified magma chamber. KEY WORDS: high-silica rhyolite; partition coefficients; differentiation; zoned ash-flow tuff; layered convection     

Geochemical and mineralogical evidence for the occurrence of at least three distinct magma types in the ‘famous’ region

Bulk rock major and trace element variations in selected basalts from the Famous area, in conjunction with a detailed study of the chemical compositions of phenocryst minerals and associated melt inclusions are used to place constraints on the genetic relationship among the various lava types. The distribution of NiO in olivine and Cr-spinel phenocrysts distinguishes the picritic basalts, plagioclase phyric basalts and plagioclase-pyroxene basalts from the olivine basalts. For a given Mg/Mg+Fe²⁺ atomic ratio of the mineral, the NiO content of these phenocrysts in the former three basalt types is low relative to that in the phenocrysts in the olivine basalts. The Zr/Nb ratio of the lavas similarly distinguishes the olivine basalts from the plagioclase phyric and plagioclase pyroxene basalts and, in addition, distinguishes the picritic basalts from the other basalt types. These differences indicate that the different magma groups could not have been processed through the same magma chamber, and preclude any direct inter-relationship via open or closed system fractional crystallization.The Fe-Mg partitioning between olivine and host rock suggests that the picritic basalts represent olivine (±Cr-spinel) enriched magmas, derived from a less MgO rich parental magma. The partitioning of Fe and Mg between olivine, Cr-spinel and coexisting liquid is used to predict a primary magma composition parental to the picritic basalts. This magma is characterized by relatively high MgO (12.3%) and CaO (12.6%) and low FeO^* (7.96%) and TiO₂ (0.63%).Least squares calculations indicate that the plagioclase phyric basalts are related to the plagioclase-pyroxene basalts by plagioclase and minor clinopyroxene and olivine accumulation. The compositional variations within the olivine basalts can be accounted for by fractionation of plagioclase, clinopyroxene and olivine in an open system, steady state, magma chamber in the average proportions 453223. It is suggested that the most primitive olivine basalts can be derived from a pristine mantle composition by approximately 17% equilibrium partial melting. Although distinguished by its higher Zr/Nb ratio and lower NiO content of phenocryst phases, the magma parental to the picritic basalts can be derived from a similar source composition by approximately 27% equilibrium partial melting. It is suggested that the parental magma to the plagioclase-pyroxene and plagioclase phyric basalts might have been derived from greater depth resulting in the fractionation of the Zr/Nb ratio by equilibration with residual garnet.C.O.B. Contribution No. 722     

Anionic and cationic variations in zoned phlogopite

Complete chemical analyses of the inner and outer portions of zoned phlogopite indicate differences in concentrations of 15 major and minor constituents. Concentrations of aluminium, sodium and zinc increase while those of other oxides decrease in the rim. Distribution coefficients of major and minor oxides between the rim and the core (KDoxr–c) indicate a slight decrease for Si, Fe²⁺, Mg, K and Sr (0.99–0.9); a greater decrease for Rb and water (0.86–0.83), and a significant decrease for Ti, Fe³⁺, Mn, Ni, and F (0.50–0.71).Physical and structural properties of the phlogopite portions from the core and the rim are similar, but phlogopite in the core contains exsolved rutile needles and fine-grained mica specks (0.5%) which cause white appearance of the core in reflected light.The zoned phlogopite provides an example of considerable anionic and cationic variations within a single crystal. It is of mineralogical and petrological interest because, unlike in most silicates, the concentrations of its iron and magnesium exhibit the same trend and an opposite trend to that of aluminium. Aluminium increases from 14% to 18%, and replaces Si, Ti, Mg, and Fe thus indicating its geochemical importance during late stages of crystallization.