Phenocryst Compositions Research Articles

Chemical evolution of a pleistocene rhyolitic center: Sierra La Primavera, Jalisco, M�xico

The late Pleistocene caldera complex of the Sierra La Primavera, Jalisco, Mexico, contains well-exposed lava flows and domes, ash-flow tuff, air-fall pumice, and caldera-lake sediments. All eruptive units are high-silica rhyolites, but systematic chemical differences correlate with age and eruptive mode. The caldera-producing unit, the 45-km3 Tala Tuff, is zoned from a mildly peralkaline first-erupted portion enriched in Na, Rb, Cs, Cl, F, Zn, Y, Zr, Hf, Ta, Nb, Sb, HREE, Pb, Th, and U to a metaluminous last-erupted part enriched in K, LREE, Sc, and Ti; Al, Ca, Mg, Mn, Fe, and Eu are constant within analytical errors. The earliest post-caldera lava, the south-central dome, is nearly identical to the last-erupted portion of the Tala Tuff, whereas the slightly younger north-central dome is chemically transitional from the south-central dome to later, moremafic, ring domes. This sequence of ash-flow tuff and domes represents the tapping of progressively deeper levels of a zoned magma chamber 95,000 ± 5,000 years ago. Since that time, the lavas that erupted 75,000, 60,000, and 30,000 years ago have become decreasingly peralkaline and progressively enriched only in Si, Rb, Cs, and possibly U. They represent successive eruption of the uppermost magma in the post-95,000-year magma chamber. Eruptive units of La Primavera are either aphyric or contain up to 15% phenocrysts of sodic sanidine ≧quartz >ferrohedenbergite >fayalite>ilmenite±titanomagnetite. Whereas major-element compositions of sanidine, clinopyroxene, and fayalite phenocrysts changed only slightly between eruptive groups, concentrations of many trace elements changed by factors of 5 to 10, resulting in crystal/glass partition coefficients that differ by factors of up to 20 between successively erupted units. The extreme variations in partitioning behavior are attributed to small changes in bulk composition of the melt because major-element compositions of the phenocrysts and temperature, pressure, and oxygen fugacity of the magma all remained essentially constant. Crystal settling and incremental partial melting by themselves appear incapable of producing either the chemical gradients within the Tala Tuff magma chamber or the trends with time in the post-caldera lavas. Transport of trace metals as volatile complexes within the thermal and gravitational gradient in volatilerich but water-undersaturated magma is considered the dominant process responsible for compositional zonation in the Tala Tuff. The evolution of the post-caldera lavas with time is thought to involve the diffusive emigration of trace elements from a relatively dry magma as a decreasing proportion of network modifiers and/or a decreasing concentration of complexing ligands progressively reduced trace-metal-site availability in the silicate melt.

Oxygen isotope thermometry of basic lavas and mantle nodules

Measurements have been made of the oxygen isotope and chemical composition of glass and phenocrysts in lavas and coexisting minerals in mantle nodules. Temperatures of formation of these assemblages have been estimated from various chemical thermometers and range from 855° to 1,300° C. The permil fractionations between coexisting orthopyroxene and clinopyroxene in the lavas and nodules are all near zero. The fractionations between pyroxene and olivine vary from +1.2 to −1.4 and are a smooth function of temperature over the entire range. This function is given by T(° C)=1151-173Δ (px-d)-68Δ2(px-d) and has an uncertainty of ±60° (2σ). At temperatures above 1,150° C, olivine in the nodules becomes more18O-rich than coexisting clinopyroxene, orthopyroxene, and plagioclase. In combination with the experimental work of Muehlenbachs and Kushiro (1974), the olivine-pyroxene fractionations indicate that olivine also becomes substantially more18O-rich than basaltic liquids above 1,200° C. Geothermometers based on the oxygen isotope equilibration of basaltic liquid with olivine, pyroxene, and plagioclase are presented.

The Macquarie Island ophiolite complex: Mid-Tertiary oceanic lithosphere from a major ocean basin

The rocks of Macquarie Island are part of the mid-Tertiary oceanic lithosphere from a major ocean basin. They were probably created at the Indian—Australian—Pacific spreading ridge. The basalts and dolerites are usually porphyritic, carry plagioclase (An 87-80) as a dominant phenocryst phase with less abundant olivine (Fo 89-85), chrome spinel and rare clinopyroxene (Ca 45Mg 50Fe 5|Ca 38Mg 50Fe 12) phenocrysts. Normatively the rocks range from ne- to Q-bearing, with most falling near the critical plane of normative silica undersaturation. Dykes tend to be more Fe-rich than lavas, and to include the more di-poor rocks. The rocks also range compositionally from typical ocean floor basalts through to varieties relatively enriched in some incompatible trace elements, particularly Nb (20–60 ppm), that otherwise retain ocean-floor basalt phenocryst assemblages, major-element compositions and Ti, Ni, Cr and Zr contents. This enrichment, also characteristic of ocean-floor basalts from the “abnormal” ridge segments near 45° N and 36° N (FAMOUS area) on the Mid-Atlantic Ridge, causes the rocks to plot away from the ocean-floor basalt fields on popular trace-element diagrams intended to identify tectonic affinities of basalts. The upper parts of the Macquarie Island oceanic lithosphere section can be thought of as a vertical slice through a magma column, differentiating at shallow levels. The layered and massive gabbros that underlie the basalts and dolerites are composed essentially of olivine, plagioclase and clinopyroxene. Olivine and plagioclase are cumulate phases in the layered rocks, clinopyroxene is postcumulus. Mineral compositions of the gabbros, particularly those of the layered rocks, are closely resembled by phenocryst compositions in the basalts and dolerites. Plagiogranites and trondheimites are unknown from the island, and norites very rare. Thus, Macquarie Island basalts, dolerites and gabbros form a distinctive igneous association that ought to make Macquarie Island-type ophiolite complexes from major ocean basins an easily recognized ophiolite type in continental orogenic terranes, even when dismembered.

A regular solution model for met-aluminous silicate liquids: Applications to geothermometry, immiscibility, and the source regions of basic magmas

Adopting a set of multioxide components and using published compositional data on olivineand plagioclase-liquid equilibria we have developed a 17 component regular solution model for met-aluminous silicate liquids. The partial molar excess free energies predicted from this model can be used together with phenocryst compositions as an effective geothermometer, with an approximate error of 20 °C (30 °C for olivine, 12 °C for plagioclase). The regular solution formulation is also successful in predicting liquid immiscibility at (1) high mole fractions of silica commonly observed in phase diagrams, and at (2) lower temperatures in lunar basalts and intermediate lavas. The model yields activities of silica which are consistent with those obtained from solid-liquid silica buffers in rocks which contain olivine and enstatite or quartz. From predicted activities of KAlSi3O8 in liquids coexisting with plagioclase a value is obtained for the limiting Henry's law activity coefficient of KAlSi3O8 in the solid. This coefficient agrees well with that inferred from plagioclase-sanidine equilibrium phenocryst assemblages in rhyolites. The activities of silica obtained from this model are used to place constraints on the pressure-temperature regions where various types of basic magmas are generated. In conjunction with plagioclase geothermometry an application is given where the pressure, temperature, and water content of an olivine andesite is predicted from the activity of silica.

Chemical compositions of phenocrysts in volcanic rock from a central cone of Aso volcano (1)(Abstracts of Papers Submitted for the Meeting of the Society)

Chemical compositions of phenocrysts in volcanic rock from a central cone of Aso volcano (1)(Abstracts of Papers Submitted for the Meeting of the Society)

Petrology of the Rogue and Galice Formations, Klamath Mountains, Oregon: Identification of a Jurassic Island Arc Sequence

Petrographic, field, and geochemical evidence from the Rogue and Galice formations in the Klamath Mountains of Oregon indicate that the rocks formed in a calc-alkaline volcanic arc of Jurassic age. The volcanic rocks range from basalt to rhyolite and are predominantly (> 95 %) fragmental in character. They are interlayered with a sequence of volcaniclastic rocks 5-8 km thick. The Rogue and Galice formations are part of a Late Jurassic-Early Cretaceous complex that includes a paired metamorphic belt of lawsonite-bearing schists along the Oregon coast and garnet-bearing amphibolite to the east, faulted against the Rogue and Galice formations. Major-element analyses, plotted on AFM, FeO*/MgO, and alkalis versus silica diagrams, yield ambiguous results when used to infer the magmatic affinities of these weakly metamorphosed rocks. However, abundances of Ti, Cr, Zr, and REE, and clinopyroxene phenocryst compositions in volcanic rocks from the Rogue and Galice Formations are identical to those in volcanic rocks from Holocene calc-alkaline volcanic arcs.

Criteria for the identification of ancient volcanic arcs

Characteristic features of recent volcanic arcs must be preserved in the rock record to be useful in determining the magmatic affinities of metavolcanic rocks. This paper reviews criteria suggested by others, and proposes new criteria for the recognition of ancient volcanic arc complexes. Major element abundances, which discriminate magma types in recent volcanic rocks, are very susceptible to modification during low-grade metamorphism, and therefore are of limited value for determining magmatic affinities of altered volcanic rocks. Ti and Zr, Cr and the rare-earth elements, are only slightly affected by low-grade metamorphism. These elements show distinctive trends that allow ocean-floor basalts to be discriminated from most volcanic arc basalts. Clinopyroxene phenocrysts are commonly the only unaltered remnant phase present in metavolcanic rocks. Compositions of clinopyroxene phenocrysts from a suite of fractionated volcanic rocks can be employed as a petrogenetic indicator, because each magma series displays a distinctive Ca Fe Mg trend during differentiation. The much greater abundance of pyroclastic volcanic rocks versus flows in modern volcanic are sequences is a preservable criterion for identifying ancient volcanic arcs. Interbedded with the pyroclastic volcanic rocks are thick deposits of graywackes and mudstones. The volcanic arc deposits are overprinted by high-temperature/low-pressure metamorphism. Parallel to and on the seaward side of the volcanic arc metamorphic belt is a belt of low-temperature/high-pressure metamorphic rocks. These two metamorphic belts comprise a paired metamorphic belt that is diagnostic of Pacific-type convergent plate margins. These criteria together distinguish volcanic arc deposits from other volcanic—sedimentary deposits.

Petrology and regional significance of the Roxboro Metagranite, North Carolina

The pluton located at Roxboro, North Carolina, is predominantly a light gray to medium gray, microphaneritic metagranite. Phenocrysts of plagioclase, quartz, and perthite are accompanied by porphyroblasts of epidote. Relict igneous textural relationships suggest two possible fractional crystallization models, in both of which the order of crystallization was plagioclase, quartz, and then K-rich feldspar. A crude approximation of the composition of the original plagioclase phenocrysts is An 25 to An 40 . Based on the composition of locally present granophyre, the pluton was emplaced under almost dry conditions with a P total of about 350 bars and a temperature in the vicinity of 950 °C. The shallow depth of emplacement suggests that the Roxboro Metagranite represents the root of a volcanic sequence that once unconformably overlay the presently exposed sequence. During the middle Paleozoic, this granitic intrusive was metamorphosed at a minimum pressure of about 3 kb and a temperature of approximately 400 °C. A foliation, as shown by stringers of mainly biotite and epidote, was produced by the deformational phase accompanying regional metamorphism. All K-rich feldspar is now nearly pure microcline (∼Or 97 ), and all plagioclase is now nearly pure low albite (Ab 97–99 ). Such feldspar compositions accompanied by late growth of ferristilpnomelane indicate a re-equilibration under lower grade conditions than those realized during the peak of the major regional metamorphic event.

Evolution of oceanic crust: 3. Petrology and chemistry of basalts from the East Pacific Rise and the Siqueiros Transform Fault

Basalt samples obtained from the Siqueiros transform fault/fracture zone and the adjacent East Pacific Rise are mostly very fresh oceanic tholeiite and fractionated oceanic tholeiite with Fe+3/ Fe+2 ∼ 0.25; however, alkali basalts occur in the area as well. The rocks of the tholeiitic suite are ol + pl phyric and ol + pl + cpx phyric basalts, while the alkali basalts are ol and ol + pl phyric. Microprobe analyses of the tholeiitic suite phenocrysts indicate that they are Fo68–Fo86, An58–An75, and augite (Ca34Mg50Fe16). The range of olivine and plagioclase compositions represents the chemical variation of the phenocryst compositions with fractionation. The phenocyrsts in the alkali basalts are Fo81 and An69. The suite of tholeiites comprises a fractionation series characterized by relative enrichment of Fe, Ti, Mn, V, Na, K, and P and depletion of Ca, Al, Mg, Ni, and Cr. The fractionated tholeiites occur on the median ridge (which is a sliver of normal oceanic crust) of the double Siqueiros transform fault, on the western Siqueiros fracture zone, and on the adjoining East Pacific Rise, while the two transform fault troughs contain mostly unfractionated or only slightly fractionated tholeiite. We suggest that the fractionated tholeiites are produced by fractional crystallization of more ‘primitive’ tholeiitic liquid in a crustal magma chamber below the crest of the East Pacific Rise. This magma chamber may be disrupted by the transform fault troughs, thus explaining the paucity of fractionated tholeiites in the troughs. The alkali basalts are found only on the flanks of a topographic high near the intersection of the northern transform trough with the East Pacific Rise.

Interaction of Meteoric Water with Magma: An Oxygen-Isotope Study of Ash-Flow Sheets from Southern Nevada

Glassy rocks from compositionally zoned ash-flow sheets and cogenetic lava flows, erupted from the late Miocene Timber Mountain–Oasis Valley caldera complex, show systematic oxygen-isotope variations (90 analyses) among all major phenocryst phases: quartz, sanidine, plagioclase, biotite, augite, and magnetite. Oxygen-isotope compositions of each phenocryst phase become lighter in O18 with decreasing age and are interpreted as indicating major interaction between meteoric ground water and batholithic-sized bodies of silicic magma prior to eruption. Each of the ash-flow sheets is compositionally zoned from silicic and phenocryst-poor at the base of more mafic and phenocryst-rich at the top; this pattern represents in inverse order the original compositional zonation in the magma chamber. The changing isotopic fractionations between phenocrysts also reflect crystallization temperatures that varied vertically over several kilometers in the differentiated magma. Posteruption oxidation and cooling effects have not significantly modified oxygen-isotope compositions of phenocrysts from glassy rocks.

Petrology of the Berlins Porphyry: A study of the crystallisation of granitic magma

Abstract The Berlins Porphyry, exposed in the lower Buller valley, South Island, New Zealand consists of many small plutons of dacite-granodiorite composition as well as a few surface flows. Stratigraphic evidence indicates emplacement and final solidification of the sub-surface plutons at shallow depths, certainly less than two kilometres. Mineralogy, phenocryst abundances, and total rock chemistry all show little variation and indicate that the plutons at present exposed are probably satellites from a larger sub-surface batholith. Many small intrusions and the margins of most of the larger plutons are composed of black glassy dacite (hyalodacite), which is inferred to represent the original magma chilled at the time of emplacement. Calculations using phenocryst and groundmass compositions suggest that T = 790°C, PH2O = 1.5 kbar, and fO2 = 1013.5 when the magma was crystallising at depth. The inner portion of most larger plutons is composed of microgranodiorite grading into granodiorite which completed i...

The delaney clause

Northwest Africa (NWA) 12379 is a new metal-rich chondrite with unique characteristics distinguishing it from all previously described meteorites. It contains high Fe,Ni-metal content (∼ 70 vol.%) and completely lacks interchondrule matrix; these characteristics are typical only for metal-rich carbonaceous (CH and CB) and G chondrites. However, chondrule sizes (60 to 1200 μm; mean = 370 μm), their predominantly porphyritic textures, nearly equilibrated chemical compositions of chondrule olivines (Fa18.1–28.3, average Fa24.9±3.2, PMD = 12.8; Cr2O3 = 0.03 ± 0.02 wt.%; FeO/MnO = 53.2 ± 6.5 (wt.-ratio); n = 28), less equilibrated compositions of low-Ca pyroxenes (Fs3.2–18.7Wo0.2–4.5; average Fs14.7±3.7Wo1.4±1.3; n = 20), oxygen-isotope compositions of chondrule olivine phenocrysts (Δ17O ∼ 0.2–1.4‰, average ∼ 0.8‰), and the presence of coarse-grained Ti-bearing chromite, Cl-apatite, and merrillite, all indicate affinity of NWA 12379 to unequilibrated (type 3.8) ordinary chondrites (OCs). Like most OCs, NWA 12379 experienced fluid-assisted thermal metamorphism that resulted in formation of secondary ferroan olivine (Fa27) that replaces low-Ca pyroxene grains in chondrules and in inclusions in Fe,Ni-metal grains. Δ17O of the ferroan olivine (∼ 4‰) is similar to those of aqueously-formed fayalite in type 3 OCs, but its δ18O is significantly higher (15–19‰, average = 17‰ vs. 3―12‰, average = 8‰, respectively). We suggest classifying NWA 12379 as the ungrouped metal-rich chondrite with affinities of its non-metal fraction to unequilibrated OCs and speculate that it may have formed by a collision between an OC-like body and a metal-rich body and subsequently experienced fluid-assisted thermal metamorphism. Trace siderophile element abundances and isotopic compositions (e.g., Mo, Ni, Fe) of the NWA 12379 metal could help to constrain its origin.

10. Chemical compositions of phenocrysts of Miocene Miyakeyama rhyolite(Abstracts of Papers Submitted for the Meetings of the Society)

10. Chemical compositions of phenocrysts of Miocene Miyakeyama rhyolite(Abstracts of Papers Submitted for the Meetings of the Society)

Feldspar Fractionation and Anomalous Sr87/Sr86 Ratios in a Suite of Peralkaline Silicic Rocks

The Quaternary Ethiopian volcano Fantale is a central volcano with a summit caldera. The most recent flows are pantellerites characterized by high Rb/Sr ratios. These cover earlier weakly peralkaline rhyolites and trachytes. Results of Sr 87 /Sr 86 analyses on selected samples from the volcano are presented. The whole-rock results on a basalt, two trachytes, and nine more silicic rocks show a correlation of the Sr 87 /Sr 86 ratio with Rb/Sr. Eleven of the rocks fall on an isochron giving an apparent age of 2.6 m.y. However, this does not correspond to the age of the volcano. Data from separated feldspar phenocrysts and host residual glassy silicic matrix show that the feldspars are enriched in radiogenic strontium relative to the matrix. Chemical analyses of the lavas suggest that the composition of the upper part of the sub-volcanic magma chamber at Fantale changed progressively from that of a trachyte to that of a weakly peralkaline rhyolite, finally becoming pantelleritic. It is suggested that the isotopic composition also changed with increasing enrichment in Rb/Sr and Sr 87 /Sr 86 , thus accounting in a general way for the isochron relation noted above. Data from the Fantale tuff, a 2 km 3 welded ash flow, indicates that the Fantale magma chamber was chemically zoned. The contrasting isotopic compositions of the feldspar phenocrysts and matrix support the concept of feldspar fractionation in this zoned magma chamber. It is suggested that the phenocrysts crystallized at a high level in the magma chamber and then fell into a less radiogenic, less fractionated environment.

Chemical Composition of Magma and Feldspar Phenocrysts of the Granite Porphyry Complex (So-called Kumano Acidic Rocks) in Southeastern Kii Peninsula, Japan

Chemical Composition of Magma and Feldspar Phenocrysts of the Granite Porphyry Complex (So-called Kumano Acidic Rocks) in Southeastern Kii Peninsula, Japan

Raja richardsoni from the continental slope off south-west England : Forster G.R., 1965. J. mar. biol. Ass., U.K., 45 (3): 773–777.

Magmas forming large igneous provinces (LIP) on continents are generated by extensive melting in the deep crust and underlying mantle and associated with break-up of ancient supercontinents, followed by formation of a new basaltic crust in the mid-oceanic rifts. A lack of the unifying model in understanding the sources of LIP magmatism is justified by lithological and geochemical complexity of erupted magmas on local (e.g. a cross-section) and regional (a single and different LIP) scales. Moreover, the majority of LIP rocks do not fit general criteria for recognizing primary/primitive melts (i.e. < 8 wt% MgO and absence of high-Fo olivine phenocrysts).This study presents the mineralogical (olivine, Cr-spinel, orthopyroxene), geochemical (trace elements and Sr-Nd-Hf-Pb isotopes) and olivine-hosted melt inclusion compositional characteristics of a single primitive (16 wt% MgO), high-Ti (2.5 wt% TiO2) picrite with high-Mg olivine (up to 91 mol% Fo) from the Letaba Formation in the ~ 180 Ma Karoo LIP (south Africa). The olivine compositions (unusually high δ18O (6.17‰), high NiO (0.36–0.56 wt%) and low MnO and CaO (0.12–0.20 and 0.12–0.22 wt%, respectively)) are used to argue for a non-peridotitic mantle source. This is supported by the enrichment of the rock and melts in most incompatible trace elements and depletion in heavy rare earth elements (e.g. high Gd/Yb) that reflects residual garnet in the source of melting. The radiogenic isotopes resemble those of the model enriched mantle (EM-1) and further argue for a long-term enrichment of the source in incompatible trace elements.The enriched high-Ti compositions, strongly fractionated incompatible trace elements, presence of primitive olivine and high-Cr spinel in the Letaba picrites are closely matched by olivine-phyric rocks from the ~ 260 Ma Emeishan (Yongsheng area, SW China) and ~ 250 Ma Siberian (Maimecha-Kotuy region, N Siberia) LIPs. However, many other compositional parameters (e.g. trace element and δ18O compositions of olivine phenocrysts, Fe2 +/Fe3 + in Cr-spinel, Sr-Nd-Hf isotope ratios) only partially overlap or even diverge. We thus imply that parental melts of enriched picritic rocks with forsteritic olivine from three major continental igneous provinces – Karoo, Emeishan and Siberia cannot be assigned to a common mantle source and similar melting conditions.The Karoo picrites also exhibit some mineralogical and geochemical similarities with rocks and glasses in the south Atlantic Ridge and adjacent fracture zones. The geodynamic reconstructions of the continental plate motions since break-up of the Gondwanaland in the Jurassic support the current position of the source of the Karoo magmatism in the southernmost Atlantic. Co-occurrence of modern and recent anomalous rocks with normal mid-ocean ridge basalts in this region can be related to blocks/rafts of the ancient lithosphere, stranded in the ambient upper mantle and occasionally sampled by rifting-related decompressional melting.

The iron-titanium oxides of salic volcanic rocks and their associated ferromagnesian silicates

The co-existing microphenocrysts of magnetite and ilmenite together with the ferromagnesian silicates in salic volcanic rocks have been analysed with the electron microprobe. The temperatures and oxygen fugacities of the oxide equilibration have been estimated from the curves of Buddington and Lindsley (1965). The co-existing ferromagnesian silicate phenocrysts are either iron-rich olivine, or orthopyroxene or biotite and amphibole; for each of these groups of phenocrysts, the oxide equilibration data are specific and fall on three distinct curves, parallel to experimental oxygen buffer curves. Many of the investigated rhyolites were quenched at temperatures near 900°C, which may represent liquidus temperatures for those with sparse phenocrysts, and also the intrusion temperature of water-undersaturated granites. The composition of the biotite phenocrysts, which are Al-poor and Ti-rich, taken in conjunction with the oxide data, suggest that two Lassen dacites precipitated biotite at a water fugacity of approximately 400 bars. The composition of the later crystallizing ferromagnesian silicates, particularly the pyroxenes which show a wide range in Fe/Mg ratio, is strongly influenced by the prior crystallization of the oxide phases. If the biotite phenocrysts are typical of acid liquids, then they are incapable of generating by fractionation a peraluminous residual liquid; rather they would tend to make a liquid peralkaline.

The Petrology of an Adamellite-Porphyrite from the New England Bathylith (New South Wales)

The paper presents an account of the occurrence, petrography, mineralogy, and geochemistry of several intrusions of adamellite-porphyrite, an early member of the New England Bathylith (N.S.W.). The rock is composed of phenocrysts of quartz, andesine, hornblende, biotite, augite (and minor orthopyroxene) which are set in a fine-grained quartzofeldspathic groundmass. Compositions of silicate phenocrysts (which reveal widespread evidence of cataclasis) accord with coexisting phases of an assemblage appropriate to intermediate rocks. Xenolith mineralogy is similar to equivalent minerals occurring as phenocrysts. The recalculated bulk composition of a rock represented by the modal andesine, hornblende, biotite, augite, and magnetite of the adamellite-porphyrite is similar to the composition of a biotite–diorite; the composition of the quartzo-feldspathic groundmass plots in the low-melting region of the NaAISi3O8-KAISi3O8-SiO2-H2O system. Evidence is presented to indicate a hybrid origin for the adamellite-porphyrite whereby disrupted biotite-diorite was invaded and mixed with low-melting silicic alkalic liquid.

Natural liquids and the phonolitic minimum

AbstractThe composition of the feldspar phenocrysts, and the enclosing glassy or finely crystalline ground‐masses, of three phonolites suggests that the addition of calcium or the addition of excess sodium (molecular ratio Na2O + Ka2O &gt; Al2O3) to compositions in the feldspar field of the system NaAlSiO4‐KAlSiO4‐SiO2 will change the composition of the minimum on the nepheline‐feldspar boundary when this is projected into this system.

A stained slice method for rapid determination of phenocryst composition of volcanic rocks

A stained slice method for rapid determination of phenocryst composition of volcanic rocks