Showing papers on "Phenocryst published in 1988"

The May 18, 1980, eruption of Mount St. Helens: 3. Stability and chemistry of amphibole in the magma chamber

Abstract: Additional experiments have been done with the May 18, 1980, Mount St. Helens dacite and a more mafic, October 1980 sample to resolve questions concerning amphibole stability and dissolved volatiles in the magma chamber prior to the May 18 eruption. The experiments were done at 920°C, at fluid pressures of 220 or 320 MPa, and, in contrast to previous work, at an ƒO2 between the NNO and MnO-Mn3O4 oxygen buffers. Fe-Ti oxides are present in the melt under these conditions, and amphibole is stable when XH2O in the fluid is greater than 0.67. The An content of plagioclase in equilibrium with melt decreases with decreasing XH2O in the fluid and, in the amphibole-bearing experiments, reaches the natural plagioclase rim compositions (An49) at an XH2O of 0.67. Under these H2O-undersaturated conditions the experimentally produced amphibole, low-Ca pyrpxene, and Ca-rich pyroxene are compositionally equivalent to phenocrysts in the May 18 white pumice. The melts (glasses) in amphibole-bearing experiments range from the average plagioclase melt inclusion composition [Rutherford et al., 1985] to slightly less evolved compositions as XH2O approaches 1.0. Melt inclusions in natural amphiboles were analyzed, and the compositions were plotted on SiO2 variation diagrams along with experimental glass analyses. The amphibole melt inclusions define a liquid line of descent for the magina which extends from relatively primitive compositions (68 wt % SiO2, anhydrous basis) to the more evolved average plagioclase melt inclusion composition (73 wt % SiO2). The volatile content of the amphibole melt inclusions (difference method) reaches 5.0±0.5 wt %, which compares favorably with the volatile content of the amphibole-bearing experimental melts produced at XH2O = 0.67.

Crystal capture, sorting, and retention in convecting magma

Abstract: The mystery of producing strong compositional diversity among suites of comagmatic igneous rocks is investigated by considering the dynamic evolution of basaltic magma in a sheet-like chamber. A central conclusion is that inward-progressing crystallization produces strong viscosity and temperature gradients that promote convection only near the leading edge of the upper thermal boundary layer. Convection is apparently confined to an essentially isoviscous, isothermal region that hugs the downward-growing roof zone. Strong changes in viscosity with crystallization divide the upper and lower thermal boundary layers into regions of decreasing viscosity and crystallinity (N) called "rigid crust" (N ≥ 0.5), "mush" (0.5 ≥ N ≥ 0.25), and "suspension" (N ≤ 0.25). The strong increase in viscosity near the mush-suspension interface acts as a capture front that overtakes and traps slowly settling crystals. Initial phenocrysts mostly escape capture, but crystals nucleated and grown in the suspension zone can escape only if the capture front slows to a critical rate attainable only in bodies thicker than about 100 m. Escaping crystals are redistributed and sorted by convection driven by the advance of the capture front itself. Crystal-laden plumes traverse the central, hot core of the body and deposit partially resorbed and sorted crystals within the lower suspension zone. Convection is never vigorous but is part of an overall intimate balance between roofward heat loss, rigid-crust growth, crystallization kinetics, and transport and sorting of sinking escaped crystals. There is a strong similarity between these processes and those producing both varves and saline pan deposits. It is clear that lavas, lava lakes, and sills are indeed examples of true magma chambers strongly exhibiting certain aspects of this over-all process. These aspects commonly also characterize the large mafic magmatic bodies. Because strong compositional changes in the residual melt occur largely outward (that is, at lower temperatures and higher crystallinities) of the capture front, which is immobile and mostly within rigid crust, the possible range in comagmatic compositions available for eruption anywhere within the active magma is very limited. This is in broad agreement with the compositional range observed in basaltic lava lakes, sills, and plutons like Skaergaard. The tuning of convection, crystallization kinetics, and phase equilibria in chambers of this type can produce a variety of textures and layering but not a diversity of compositions.

Crystallization of Mount St. Helens 1980–1986 dacite: A quantitative textural approach

Abstract: Quantitative measurements of crystal size distributions (CSDs) have been used to obtain kinetic information on crystallization of industrial compounds (Randolph and Larson 1971) and more recently on Hawaiian basalts (Cashman and Marsh 1988). The technique is based on a population balance resulting in a differential equation relating the population densityn of crystals to crystal sizeL, i.e., at steady staten =no exp(−L/itGτ), whereno is nucleation density,G is the average crystal growth rate,τ is the average growth time, and the nucleation rateJ =noG. CSD (Inn vsL) plots of plagioclase phenocrysts in 12 samples of Mount St. Helens “blast” dacite and 14 samples of dacite from the 1980–1986 Mount St. Helens dome are similar and averageGτ = 9.6 (± 1.1) × 10−3 cm andno = 1−2 × 106 cm−4. Reproducibility of the measurements was tested by measuring CSDs of 12 sections cut from a single sample in three mutually perpendicular directions; precision of the size distributions is good in terms of relative, but not necessarily absolute values (± 10%). Growth and nucleation rates for plagioclase have been calculated from these measurements using time brackets ofτ = 30–150 years; growth ratesG are 3−10 × 10−12cm/s, and nucleation ratesJ are 5−21 × 10−6/cm3 s.G andJ for Fe-Ti oxides calculated from CSD data areG = 2−13 ± 10−13 cm/sec andJ = 7−33 × 10−5/cm3 s, respectively. The higher nucleation rate and lower growth rate of oxides resulted in a smaller average crystal size than for plagioclase. Sizes of plagioclase microlites (<0.01 mm) in the blast dacite groundmass have been measured from backscatter SEM photographs. Nucleation of these microlites was probably triggered by intrusion of material into the cone of Mount St. Helens in spring 1980. This residence time of 52 days gives minimum crystallization estimates ofG = 1−3 × 10−11 cm/s andJ = 9−16 × 1O3/cm3 s. The skeletal form of the microlites provides evidence for nucleation and growth at high values of undercooling (ΔT) relative to the phenocrysts. A comparison of nucleation and growth rates for the two crystal populations (phenocrysts vs microlites) suggests that while growth rate seems to be only slightly affected by changes inΔT, nucleation rate is a very strong function of undercooling. A comparison of plagioclase nucleation and growth rates measured in the Mount St. Helens dacite and in basalt from Makaopuhi lava lake in Hawaii suggests that plagioclase nucleation rates are not as dependent on composition. Groundmass textures suggest that plagioclase microphenocrysts crystallized at depth rather than in the conduit, in the dome, or after extrusion onto the surface. Most of this crystallization appears to be in the form of crystal growth (coarsening) of groundmass microphenocrysts at small degrees of undercooling rather than extensive nucleation of new crystals. This continuous crystallization in a shallow magmatic reservoir may provide the “overpressurization” needed to drive the continuing periodic domebuilding extrusions, which have been the pattern of activity at Mount St. Helens since December 1980.

Plio-Quaternary volcanism in Ecuador

Abstract: Extensive sampling, major element chemistry on over 300 samples and K-Ar radiometric dating have been carried out on the Ecuadorian Upper Tertiary–Quaternary volcanoes. The results show important space–time variations of the volcanic activity, between Late Miocene time and the present. In Late Miocene time a continuous volcanic belt, located approximately along the present volcanic front (VF), affected the whole country from the Cuenca basin to the south, up to Colombia to the north. Major changes occurred at about 5 Ma: volcanic activity stopped south of the Guayaquil fault belt and never resumed; to the north the active volcanic axis shifted eastward to the Cordillera Real (CR) area with a simultaneous relative decrease in intensity. Since Early Quaternary time the volcanic belt widened westward to the Western Cordillera where the volcanism resumed at about 1.5–1.0 Ma, giving rise to the very wide active volcanic zone of Ecuador.The Plio-Quaternary products show significant longitudinal and latitudinal chemical and mineralogical changes. Volcanics of the VF and Interandean Depression contain amphibole and define a calc-alkaline trend with a K2O content lower than that of the CR products, which are characterized by a mostly anhydrous phenocryst assemblage. In both areas andesites dominate, but extreme compositions (basaltic andesites and rhyolites) are more diffuse in the CR than the VF. No significant transverse zoning has been detected in the northern region (north of the Chota-Mira transverse tectonic line). The observed temporal and spatial variations are interpreted as a result of the subduction of the Carnegie Ridge anomalous oceanic crust, underthrusting of which began approximately 6 Ma ago.

Cyclic development of igneous features and their relationship to high-temperature hydrothermal features in the Henderson porphyry molybdenum deposit, Colorado

Abstract: The Henderson porphyry molybdenum deposit was formed by the superposition of coupled alteration and mineralization events, of varying intensity and size, that were associated with each of at least 11 intrusions. Deposition of molybdenite was accompanied by time-equivalent silicic and potassic alteration. High-temperature alteration and mineralization are spatially and temporally linked to the crystallization of compositionally zoned magma in the apex of stocks. Differences in hydrothermal features associated with each intrusion (e.g., mass of ore, orientation and type of veins, density of veins, and intensity of alteration) correlate with differences in primary igneous features (e.g., composition, texture, morphology, and size). The systematic relations between hydrothermal and magmatic features suggest that primary magma compositions, including volatile contents, largely control the geometry, volume, level of emplacement, and mechanisms of crystallization of stocks. These elements in turn govern the orientations and densities of fractures, which ultimately determine the distribution patterns of hydrothermal alteration and mineralization.Based on primary igneous features, intrusions are grouped into four types, 1 to 4. The first three types define a potassic series of compositions (normarive orthoclase/albite greater than 1) and form a continuum from texturally and compositionally simple intrusions (type 1) to complex intrusions (type 3). Intensity and volume of high-temperature alteration and mineralization associated with a given stock increase from type 1 (2% of total ore) to type 3 (77% of total ore). Complex and strongly mineralizing intrusions (type 3) are separated from one another in time by relatively simple and weakly mineralizing intrusions (type 1). Stocks of type 4, the youngest intrusions, define a sodic series of compositions (normative orthoclase/albite less than 1) and are associated with the waning stages of mineralization.A number of textural features indicate that highly mineralizing type 3 stocks contained high primary concentrations of volatile components, e.g., the presence of unidirectional solidification textures, aplitic rather than aphanitic groundmass, extreme local textural variability, low percentages of phenocrysts in apices, brecciation of apices, and low volumes of dikes. From apex to root zone, textural units within the Seriate stock (type 3), the most productive stock in the deposit, include: (1) a Brain Rock unit with abundant quartz + or - fluorite crenulate layers, (2) a Border unit of generally low but variable phenocryst content (0-25%), (3) a transition zone, 5 to 10 m wide, of unidirectional solidification textures, (4) an Intermediate unit of moderate phenocryst content (25%), (5) another transition zone, 5 to 10 m wide, of unidirectional solidification textures, (6) a Porphyry unit of high phenocryst contents (40%), and (7) a Granite Porphyry or Granite unit. Crystals in layers of unidirectional textures always project toward the interior of the stock and indicate progressive inward solidification of magma along the walls of a chamber. Variations in texture are accompanied by variations in primary composition: the apex of the Seriate stock (the region above the deepest transition zone of unidirectional textures) is enriched in K 2 O and SiO 2 and depleted in Na 2 O, F, Nb, and Y relative to deeper units.Mineralization in the deposit is controlled by moderately outward-dipping concentric veins of both replacement and open-space origin, and by steeply dipping radial veins of replacement origin. Both sets of veins were formed nearly simultaneously about a stock and are focused in a systematic manner on the apex. A classic stockwork of veins occurs only in regions where vein sets associated with spatially separated intrusions overlap. Fluids that filled outward-dipping concentric structures flowed downward and outward from a stock. Fluid pressures in excess of lithostatic pressures, generated during crystallization of magma, were required to open and fill concentric structures. As magma solidified progressively inward, the zone of fluid evolution also migrated downward and inward. With time, the volume of evolved fluids decreased: vein densities decrease from greater than 200/m in the Seriate Brain Rock unit to less than 0.1/m immediately below the last transition zone separating the Seriate Intermediate unit from Seriate Porphyry unit. Crosscutting relations between veins and high-level dikes associated with the Seriate stock establish that most of the ore related to the Seriate stock was deposited prior to solidification of its deepest transition zone of unidirectional solidification textures.The absence of high-temperature veins and associated hydrothermal alteration in deep cores of stocks, the distribution of ore about the high levels of stocks, the orientation of veins about the apex of stocks, and the crosscutting relations between veins and dikes of the same stock are suggestive of initially high concentrations of molybdenum and volatiles in the apex of a stock immediately prior to the onset of significant crystallization. Additional accumulation of ore components, if any, ceased after solidification of the apex. Assignment of molybdenum in the ore shell about the Seriate stock to the volume of solid occupied by the apex of the Seriate stock yields concentration levels in the apical magma of approximately 13,000 ppm Mo.

Geochemistry of the Hawi lavas, Kohala Volcano, Hawaii

Abstract: Hawi lavas form the late stage alkalic cap on Kohala Volcano and range in composition from hawaiite to trachyte. New, detailed field mapping of Kohala and reinterpretation of previously published age data suggest that there was no significant eruption hiatus between the Hawi and underlying Pololu shield lavas as was previously suggested. Mineral and whole-rock chemical data are consistent with a crystal fractionation origin for the hawaiite to trachyte compositional variation observed within the Hawi lavas. Plagioclase, clinopyroxene, Ti-magnetite, olivine and apatite fractionation are needed to explain this variation. The clinopyroxene fractionation may have occurred at moderate pressure because it is virtually absent in these lavas and is not a near liquidus phase at pressures of less than 8 Kb. Plagioclase would be buoyant in the Hawi hawaiite magmas so a mechanism like dynamic flow crystallization is needed for its fractionation and to account for the virtual absence of phenocrysts in the lavas. Hawi lavas are distinct in Sr and Nd isotopic ratios and/or incompatible element ratios from the Pololu lavas. Thus they were derived from compositionally distinct sources. Compared to other suites of Hawaiian alkalic cap lavas, Hawi lavas have anomalously high concentrations of phosphorus and rare earth elements. These differences could be due to greater apatite content in the source for the Hawi lavas.

The geochemistry and petrogenesis of K-rich alkaline volcanics from the Batu Tara volcano, eastern Sunda arc

Abstract: Mineralogical, major and trace element, and isotopic data are presented for leucite basanite and leucite tephrite eruptives and dykes from the Batu Tara volcano, eastern Sunda arc. In general, the eruptives are markedly porphyritic with phenocrysts of clinopyroxene, olivine, leucite ±plagioclase±biotite set in similar groundmass assemblages. These K-rich alkaline volcanics have high concentrations of large-ion-lithophile (LIL), light rare earth (LRE) and most incompatible trace elements, and are characterized by high 87Sr/86Sr (0.70571–0.70706) and low 143Nd/ 144Nd (0.512609–0.512450) compared with less alkaline volcanics from the Sunda arc. They also display the relative depletion of Ti and Nb in chondrite-normalized plots which is a feature of subalkaline volcanics from the eastern Sunda arc and arc volcanics in general. Chemical and mineralogical data for the Batu Tara K-rich rocks indicate that they were formed by the accumulation of variable amounts of phenocrysts in several melts with different major and trace element compositions. The compositions of one of these melts estimated from glass inclusions in phenocrysts is relatively Fe-rich (100 Mg/(Mg + Fe2+)=48–51) and is inferred to have been derived from a more primitive magma by low-pressure crystal fractionation involving olivine, clinopyroxene and spinel. Mg-rich (mg ∼90) and Cr-rich (up to 1.7 wt. % Cr2O3) zones in complex oscillatory-zoned clinopyroxene phenocrysts probably also crystallized from such a magma. The marked oscillatory zoning in the clinopyroxene phenocrysts is considered to be the result of limited mixing of relatively ‘evolved’ with more primitive magmas, together with their phenocrysts, along interfaces between discrete convecting magma bodies.

The petrology of some picrites from Mauna Loa and Kilauea volcanoes, Hawaii

Abstract: The mineralogy and chemistry of picrites from Mauna Loa and Kilauea have been investigated to evaluate, for Hawaiian tholeiitic picrites, the contrasting genetic models which have been proposed for these Mg-rich volcanics, namely products of direct crystallization of high-Mg melts (20–25% MgO) or the result of accumulation of olivine phenocrysts into less Mg-rich melts. Genetic interpretations rely heavily on Mg-Fe partitioning relations between olivines and their picrite hosts. Although the 100 Mg/(Mg + Fe2+) ratios (M) of picrites are wide-ranging (M=73.6–82.9 for Fe2O3/FeO=0.15), with MgO as high as 27.8%, the average 100 Mg/(Mg+Fe) ratios (mg) of the cores of olivine phenocrysts (megacrysts) show only restricted compositional variation (mg=87.2–89.0). Successive olivine generations are progressively more Fe-rich. Olivine/liquid Mg-Fe partitioning data and the Mn and Ni abundances in olivine phenocrysts collectively indicate that they were precipitated by Mg-rich basaltic melts with 12–14% MgO. Spinel compositions range from liquidus magnesiochromites, occurring mainly as inclusions in olivine phenocrysts, to groundmass titanomagnetites which crystallized at nearsolidus temperatures. The Cr2O3 contents and M values of liquidus magnesiochromites suggest that their parent melts were neither Mg-rich picritic (MgO>20%) nor relatively Mg-poor basaltic types. On MgO variation diagrams (extending from approximately 7% to more than 25% MgO), Mauna Loa and Kilauea picrites and their respective microcrystalline/glassy groundmasses (the major component of quickly-cooled picrites) plot on linear regression lines (‘olivine control lines’). At a given MgO content, Kilauean picrites and tholeiites (M<75) generally contain more TiO2 FeO t , CaO, K2O and P2O5, and less SiO2 and Na2O than Mauna Loan types. The compositions of the groundmasses in picrites and Mg-rich ol-tholeiites equate closely with those of the Mg-poor tholeiites (7–9% MgO) which dominate the petrology of each shield. Low-pressure closed system differentiation of Hawaiian tholeiitic magmas (10–15% MgO) can yield picritic derivatives which differ, however, from the extrusive picrites by virtue of distinctly higher FeO t contents and correspondingly more Fe-rich olivines and Cr-spinels. The calculated Mg-Fe olivine megacryst-‘liquid’ partition coefficient K D for individual picrites indicate that lowpressure equilibria (K D =0.30–0.34) are defined only by melts with approximately 12–14% MgO (M∼ 71–74). Assessed in conjunction with Ni-MgO modeling, these data indicate that the more Mg-rich picrites (MgO> 14–15%) are indeed olivine-enriched and do not represent melt compositions. Olivine enrichment resulted from post-eruptive mechanical (flow) differentiation of extruded ‘mushes’ of intratelluric cognate olivine phenocrysts (mg∼88) and tholeiitic melts (M∼60), which are ‘residua’ of the parental magmas (12–14% MgO), following the crystallization of the olivine phenocrysts. The ‘parental’ magmas of both picrite suites were generated by 35–40% melting of relatively Fe-rich spinel lherzolites (mg∼84) containing kaersutitic amphibole as a major primary constituent.

K-metasomatism and detachment-related mineralization, Harcuvar Mountains, Arizona

Abstract: The Bullard detachment fault, a regional low-angle normal fault exposed in the Harcuvar Mountains of west-central Arizona, separates lower-plate mylonitic rocks and chloritic breccia from upper-plate volcanic and sedimentary rocks. Areally extensive K-metasomatism has converted upper-plate mafic flows and felsic ash-flow tuffs into rocks with 8 to 12 wt. % K 2 O, 2 O, and a simple K-feldspar-hematite-quartz mineralogy. The secondary K-feldspar is very pure (Or 95 to 99.5), monoclinic, and structurally similar to orthoclase. Differences in δ 18 O values between secondary K-feldspar replacing sanidine phenocrysts (9‰ to 11‰) and K-feldspar replacing groundmass (11‰ to 14‰) in the tuffs imply differential O-exchange with migrating fluids. Whole-rock δ 18 O values for tuffs (10‰ to 14‰) and mafic flows (6‰ to 9‰) do not, therefore, represent primary igneous values. The rocks apparently became K-metasomatized and 18 O-enriched while interacting with low- to moderate-temperature, neutral to alkaline, oxidizing brines that accumulated in an extensional basin above the detachment fault. Cu-Au-Ag mineralization is concentrated in faults and fissures in metasomatized mafic flows above the detachment fault. Fluid-inclusion studies show that mineralizing fluids had minimum temperatures of 290 to 330 °C along the detachment fault, 240 to 290 °C in mafic flows above the detachment fault, and 100 to 130 °C in barite-calcite-Mn-oxide veins farther from the detachment fault. The predominant mineralizing fluids near the detachment fault and in the mafic flows were saline brines with 13 to 17 equivalent wt.% NaCI More dilute brines with 6 to 12 equivalent wt. % NaCI formed the barite-calcite-Mn-oxide veins. Inferred δ 18 O values of the mineralizing fluids range from +3‰ for high-temperature quartz-sulfide veins to -5‰ for lower-temperature barite-calcite-Mn-oxide veins. The high salinities, oxygen isotope compositions, and geologic setting indicate that the mineralizing fluids were basinal brines. The mineralizing fluids apparently evolved from early deep-level, reducing basin brines to a later stage marked by the influx of higher-level, oxidizing basin brines. Relatively minor amounts of less evolved, lower- 18 O meteoric water entered the system during the very late stages of mineralization. Paragenetic relations and geochemical and isotopic data indicate that mineralization was superimposed on previously K-metasomatized rocks. Mineralization and K-metasomatism may be indirectly linked, however, because both occurred during detachment faulting and both involved basinal brines. Specifically, K-metasomatism of upper-plate units liberated elements, such as Cu, Pb, Zn, Mn, Sr, and Na, that were incorporated into the mineralizing basin brines. Multiple fluid regimes during detachment faulting are indicated, because basin-brine-dominated mineralization overprinted lower-plate mylonitic rocks and breccia that had probably previously equilibrated with igneous or metamorphic fluids at deeper levels of the detachment system.

Petrological and geochemical relationships between pyroxene megacrysts and associated alkali-basalts from Massif Central (France)

Abstract: Major, trace element, and REE analyses, as well as Sr isotopic ratios, have been obtained on twelve clinopyene megacrysts and phenocrysts and their alkali-basalt hosts from the French Massif Central. Equilibrium between crystals and host was examined based on petrographic and geochemical data. Two types of pyroxenes are recognized: the acmite-bearing clinopyroxenes, rich in incompatible elements and the salitic clinopyroxenes, poor in incompatible elements. 87Sr/ 86Sr isotopic data reveal no significant difference between clinopyroxenes and host lavas: they are in apparent isotopic equilibrium. The Sr isotopic ratios of the two types of pyroxenes are also quite similar. However pyroxene crystals from the first group are not in equilibrium with their host; they have crystallized at high-pressure from differentiated alkali-lavas and have been incorporated in a more primitive magma. Pyroxene crystals from the second group are in apparent equilibrium with their host lava; they have crystallized at various pressures. For the latter, distribution coefficients are proposed for compatible elements, trace elements and REE.

Complex zoning and resorption of phenocrysts in mixed potassic mafic magmas of the Highwood Mountains, Montana

Abstract: Disequilibrium phenocryst assemblages and complex compositional zoning in clinopyroxene, mica, and olivine phenocrysts provide a detailed record of multiple mixing, fractional crystallization, and degassing events during the high-level evolution of potassic mafic magmas in the Eocene Highwood Mountains province. The contrasting phenocryst assemblages of minettes (diopside + phlogopite + olivine) and mafic phonolites-shonkinites (salite + leucite + olivine) permit unambiguous documentation of mixing between these two magma types and also between more primitive and more evolved members of each type. The varied behavior of crystals mixed into disequilibrium liquids, deduced from detailed textural and microprobe analyses, is consistent with the results of experimental studies of plagioclase dissolution. Phlogopite and diopside xenocrysts within more evolved mafic phonolite liquids initially underwent peripheral resorption. Further dissolution of diopside produced a network ofinterior cavities that were subsequently plated by salite crystallized from the host magma. In contrast, salite xenocrysts within more primitive mafic phonolite liquids underwent partial dissolution at their margins, yet remained euhedral. In some cases dissolution was restricted to a single, compositionally distinct sector. Halos of Fe-Al-rich salite subsequently formed around the melt-filled cavities by diffusion. Complexly zoned salite and biotite crystals containing one or more sharply bounded, epitaxial bands of diopside and phlogopite, respectively, are common in the mafic phonolites and shonkinites. On the basis of features such as the different salite compositions on either side of diopside bands and their lack of leucite inclusions and oscillatory zoning, it is argued that each band in a crystal (rarely more than six) is a record of a mixing event with minette magma. We infer that the contrasting phenocryst assemblages of the compositionally similar minettes and mafic phonolites largely reflect differences in HrO activity during their crystallization, and we propose that degassing of ascending minette magmas was an important petrogenetic process. We conclude that the high-level evolution of the Highwood magmatic system involved repeated mixing in multiple reservoirs among batches of variably fractionated and degassed mafic phonolite magma and periodic influxes of more primitive, undegassed minette magma.

Samples from the crystallising boundary layer of a zoned magma chamber

Abstract: The spatial and chemical relationships between the melt occupying the reservoir and the mineral assemblages crystallising at the margins are reconstructed for the magma chamber which produced the 11000 yrBP tephra deposit of Laacher See Volcano The melt showed vertical chemical zonation immediately prior to eruption, and throughout most of the magma volume only a small fraction of crystals were present The eruption also ejected crystal-rich “nodules”, ranging from mafic to felsic in composition, which are samples of the materials crystallising at the boundaries of the chamber New data on nodule petrography and chemical compositions of whole-rocks, minerals and interstitial glasses are presented Volume fraction of interstitial glass is not systematically related to mineral assemblage and varies typically between 1 and 20 vol%, ie the crystals interlock One exception is a group of mafic nodules with glass volume fractions between 25 and 40 vol% Bulk compositions of mafic nodules show strong enrichments or depletions in all major elements relative to the mafic phonolite interstitial melt Felsic nodules show much less pronounced differences with their interstitial melt Felsic nodules contain interstitial glasses with a range of compositions similar to that in the zoned bulk of the chamber and were probably derived from different heights on the walls Mafic nodules have glass compositions similar to those at the base of the zoned liquid column and were probably derived from the floor Modal mineralogy, glass composition and mineral composition are systematically related in the nodules whereas in individual pumices samples derived from the main body of the chamber, a broader range of mineral compositions are often found Mineral assemblages were especially diverse in the upper part of the chamber It is deduced that the whole of the essentially liquid part of the chamber was emptied by the eruption, that strongly contrasting mineral assemblages were forming simultaneously on the walls and floor, that the gradient in crystal content in the crystallisation boundary layer was more gradual at the floor than at the walls, and that the pumice mineralogy is not a simple phenocryst assemblage but is a mixture of crystals which grew from melts separated in space and/or time

Magma–cumulate mixing identified by U–Th disequilibrium dating

Abstract: Magma mixing is believed to be important in magmagenesis1, particularly in volcanic arcs2, where its influence is revealed by heterogeneous phenocryst populations3 and quenched inclusions4. Nevertheless isotope studies, particularly of uranium-series dis-equilibria, have concentrated on the analysis of bulk rock samples5–9; comprehensive U–Th mineral-separate data from arc volcanics are scarce10–14. Here we present the first U–Th isotopic evidence for mixing between a crystal mush and a magma in an andesite from the island of Santorini, in the Aegean arc. The lava contains crystal populations from two sources of distinct thorium isotope composition: one from a basic cumulate; the other phenocrysts from a dacite magma. The crystal populations have statistically indistinguishable crystallization ages of 79+14−12kyr and 93+29−22kyr, and are thought to have physically intermingled just before eruption. The crystals did not re-equilibrate and hence retain the isotopic compositions and ages of the parental materials.

Nature and cause of compositional variation among the alkalic cap lavas of Mauna Kea Volcano, Hawaii

Abstract: Most Hawaiian basaltic shield volcanoes are capped by moderately to strongly evolved alkalic lavas (MgO<4.5 wt.%). On Mauna Kea Volcano the cap is dominantly composed of hawaiite with minor mugearite. Although these lavas contain dunite and gabbroic xenoliths, they are nearly aphyric with rare olivine and plagioclase phenocrysts and xenocrysts. The hawaiites are nearly homogeneous in radiogenic isotope ratios (Sr, Nd, Pb) and they define coherent major and trace element abundance trends. These compositional trends are consistent with segregation of a plagioclase-rich cumulate containing significant clinopyroxene and Fe-Ti oxides plus minor olivine. Elements which are usually highly incompatible, e.g., Rb, Ba, Nb, are only moderately incompatible within the hawaiite suite because these elements are incorporated into feldspar (Rb, Ba) and oxides (Nb). However, in the most evolved lavas abundances of the most incompatible elements (P, La, Ce, Th) exceed (by ∼5–10%) the maximum enrichments expected from models based on major elements. Apparently, the crystal fractionation process was more complex than simple, closed system fractionation. The large amounts of clinopyroxene in the fractionating assemblage and the presence of dense dunite xenoliths with CO2 inclusions formed at minimum pressures of 2 kb are consistent with fractionation occurring at moderate depths. Crystal segregation along conduit or magma chamber walls is a possible mechanism for explaining compositional variations within these alkalic cap lavas.

Geologic evidence for a magma chamber beneath Newberry Volcano, Oregon

Abstract: At Newberry Volcano, central Oregon, more than 0.5 m.y. of magmatic activity, including caldera collapse and renewed caldera-filling volcanism, has created a structural and thermal chimney that channels magma ascent. Holocene rhyolitic eruptions (1) have been confined mainly within the caldera in an area 5 km in diameter, (2) have been very similar in chemical composition, phenocryst mineralogy, and eruptive style, and (3) have occurred as recently as 1300 years ago, with repose periods of 2000--3000 years between eruptions. Holocene basaltic andesite eruptions are widespread on the flanks but are excluded from the area of rhyolitic volcanism. Basaltic andesite in fissures at the edge of the rhyolite area has silicic inclusions and shows mixed basalt-rhyolite magma relations. These geologic relations and the high geothermal gradient that characterizes the lower part of a drill hole in the caldera (U.S. Geological Survey Newberry 2) indicate that a rhyolitic magma chamber has existed beneath the caldera throughout the Holocene. Its longevity probably is a result of intermittent underplating by basaltic magma.

Relationships between mineralogical, chemical, and isotopic properties of some North American kimberlites

Abstract: Late Precambrian to Cretaceous kimberlites from North America have initial Sr and Nd isotopic compositions (eSr0 = −4 to 37, eNd0 = −4 to 6) which form a trend falling roughly within the mantle array. Two groups can be defined on the basis of mineralogical and geochemical characteristics: group A encompasses most of the northeast American kimberlites whose rare earth element (REE) concentrations are moderately fractionated, relative to chondrites. Nd isotopic ratios (eNd0 = 3 to 6) fall within the field of common ocean island basalts. They are also distinctive by the presence of phlogopite phenocrysts and high-temperature garnet peridotite xenocrysts. Group B includes kimberlites from the Colorado-Wyoming, Kansas and British Columbia areas and one sample from Pennsylvania. They lack the above quoted peridotite suite and abundant phlogopite. Their light REE concentrations show a strong enrichment, relative to chondrites, correlated with relatively unradiogenic Nd (eNd0 = −4 to 2). Kimberlites of both groups have radiogenic Pb (206Pb/204Pb = 18.17–19.41) and a wide range in 207Pb/204Pb. In both cases, melts were probably generated in the sublithospheric mantle, but group B melts interacted with an enriched component, resulting mainly from magmatic processes, and probably located within the deep lithosphere. By contrast, the Prairie Creek lamproite (Arkansas), as other ultrapotassic rocks, received its distinctive geochemical properties from a crustal component present in the mantle source.

Olivine: a monitor of magma evolutionary paths in kimberlites and olivine melilitites

Abstract: Petrographic and chemical criteria indicate that the overwhelming majority of olivines in kimberlites are probably cognate phenocrysts. The implied low volume of xenocryst olivines requires that primitive kimberlite magmas are highly ultrabasic liquids. Two chemically distinctive olivine populations are present in all of the kimberlites studied. The dominant olivine population, which includes large rounded olivines and smaller euhedral crystals, is Mg-rich relative to late-stage rim compositions. It is characterized by a range in 100 Mg/(Mg + Fe) and uniform Ni concentration, reflecting Rayleigh-type crystallization during magma evolution. The most Mg-rich of these olivines are considered to be similiar to those in the mantle source rocks. The second compositional population, generally very subordinate, though markedly more abundant in the megacrystrich Monastery kimberlite, is Fe-rich relative to rim compositions. This group of olivines crystallized from evolved liquids in equilibrium with iron-rich megacrysts, both entrained by the kimberlite magma during ascent. Differences between the chemical fields of Fe-rich olivines in Group I and Group II kimberlites point to relatively deeper derivation of the latter suite. Olivine chemistry can be used to characterize kimberlite magma sub-types, and may prove to be a useful tool for evaluating the diamond potential of kimberlites.

Petrology and geochemistry of the Loch Ba ring-dyke, Mull (N.W. Scotland): an example of the extreme differentiation of tholeiitic magmas

Abstract: The Loch Ba ring-dyke in the Tertiary igneous central complex of Mull, N.W. Scotland is composed predominantly of a banded rhyolitic welded tuff. The rhyolite contains numerous inclusions of dark aphanitic rock. The textural relationships between the different rocks indicate rapid, violent and intimate mixing during emplacement of the dyke. The dark glassy component varies continuously from basaltic andesite to andesite, dacite and rhyolite. These glasses are enriched in FeO and depleted in MgO at a given SiO2 content in comparison to other tholeiitic highly differentiated volcanic rocks. The rhyolite contains an average of 4% phenocrysts and is associated with the mineral assemblage plagioclase (An32 to An21)-sanidine(Or50–60)-hedenbergite-fayalite-magnetite-ilmenite-apatite-zircon. Mineral aggregates involving either plagioclase-hedenbergite-ilmenite or plagioclase-fayalite-magnetite are common, but aggregates containing fayalite and hedenbergite together are scarce. The dark glassy components are either phenocryst free or contain less than 0.2% phenocrysts. The main phenocrysts associated with the dark glasses are plagioclase (An65-An30), high calcium clinopyroxene ranging continuously from augite to pure hedenbergite, pigeonite, magnetite, ilmenite and rare apatite. Zoning in minerals is generally weak or absent. The plagioclase feldspar, high calcium clinopyroxenes and pigeonites have similar compositional ranges to the minerals observed in the Middle and Upper Zones of the Skaergaard Intrusion. The mineral compositions are systematically related to SiO2 content and Mg number of the glasses. The data demonstrate that mineral compositions and assemblages similar to the Skaergaard form from silica-rich andesitic to rhyolitic liquids. The various mafic glasses are interpreted to have been derived from a zoned magma chamber underlying an upper layer of rhyolitic magma. Differentiation is attributed to fractional crystallization of the observed mineral assemblages causing SiO2 enrichment and FeO depletion. However, glasses with less than 57% SiO2 have unusual compositions with very low MgO and P2O5 as well as variable Al2O3 and TiO2. Their peculiarities could be explained by andesitic magmas assimilating cumulate mineral aggregates precipitated from more differentiated dacite and rhyolite magmas. The bulk compositions of these cumulates have high FeO, low SiO2 and negligible MgO and P2O5. It is suggested that the high density of the mineral aggregates containing fayalite-hedenbergite-magnetite and ilmenite caused them to settle through the zoned chamber to be assimilated by high temperature, less differentiated magmas.

Origin of metaluminous and alkaline volcanic rocks of the Latir volcanic field, northern Rio Grande rift, New Mexico

Abstract: Volcanic rocks of the Latir volcanic field evolved in an open system by crystal fractionation, magma mixing, and crustal assimilation. Early high-SiO2 rhyolites (28.5 Ma) fractionated from intermediate compositionmagmas that did not reach the surface. Most precaldera lavas have intermediate-compositions, from olivine basaltic-andesite (53% SiO2) to quartz latite (67% SiO2). The precaldera intermediate-composition lavas have anomalously high Ni and MgO contents and reversely zoned hornblende and augite phenocrysts, indicating mixing between primitive basalts and fractionated magmas. Isotopic data indicate that all of the intermediate-composition rocks studied contain large crustal components, although xenocrysts are found only in one unit. Inception of alkaline magmatism (alkalic dacite to high-SiO2 peralkaline rhyolite) correlates with, initiation of regional extension approximately 26 Ma ago. The Questa caldera formed 26.5 Ma ago upon eruption of the >500 km3 high-SiO2 peralkaline Amalia Tuff. Phenocryst compositions preserved in the cogenetic peralkaline granite suggest that the Amalia Tuff magma initially formed from a trace element-enriched, high-alkali metaluminous magma; isotopic data suggest that the parental magmas contain a large crustal component. Degassing of water- and halogen-rich alkali basalts may have provided sufficient volatile transport of alkalis and other elements into the overlying silicic magma chamber to drive the Amalia Tuff magma to peralkaline compositions. Trace element variations within the Amalia Tuff itself may be explained solely by 75% crystal fractionation of the observed phenocrysts. Crystal settling, however, is inconsistent with mineralogical variations in the tuff, and crystallization is thought to have occurred at a level below that tapped by the eruption. Spatially associated Miocene (15-11 Ma) lavas did not assimilate large amounts of crust or mix with primitive basaltic magmas. Both mixing and crustal assimilation processes appear to require development of relatively large magma chambers in the crust that are sustained by large basalt fluxes from the mantle. The lack of extensive crustal contamination and mixing in the Miocene lavas may be related to a decreased basalt flux or initiation of blockfaulting that prevented pooling of basaltic magma in the crust.

The fluid dynamics of crustal melting by injection of basaltic sills

Abstract: When basaltic magma is emplaced into continental crust, melting and generation of granitic magma can occur. We present experimental and theoretical investigations of the fluid dynamical and heat transfer processes at the roof and floor of a basaltic sill in which the wall rocks melt. At the floor, relatively low density crustal melt rises and mixes into the overlying magma, which would form hybrid andesitic magma. Below the roof the low-density melt forms a stable layer with negligible mixing between it and the underlying hotter, denser magma. Our calculations applied to basaltic sills in hot crust predict that sills from 10-1500 m thick require only 2-200 years to solidify, during which time large volumes of overlying layers of convecting silicic magma are formed. These time scales are very short compared with the lifetimes of large silicic magma systems of around 106 years, and also with the time scale of 107 years for thermal relaxation of the continental crust. An important feature of the process is that crystallisation and melting occur simultaneously, though in different spots of the source region. The granitic magmas formed are thus a mixture of igneous phenocrysts and lesser amounts of restite crystals. Several features of either plutonic or volcanic silicic systems can be explained without requiring large, high-level, long-lived magma chambers.

Picritic primary magma and its source mantle for Oshima-Ōshima and back-arc side volcanoes, Northeast Japan arc

Abstract: Oshima-Ōshima volcano is an endmember of a geochemical variation which is characterized by a low FeO content toward the back-arc side across the NE Japan arc. Analyses of the basalts show primitive characteristics. Variation trends of the chemical compositions indicate initial olivine control then olivine+clinopyroxene control from a picritic to a differentiated basalt. The more magnesian basalts have the more magnesian olivine phenocrysts. The most magnesian (MgO 15%) of all rock samples, contains olivine phenocrysts with a composition of Fo 93.7 as a liquidus phase and is considered a product of a mantle-derived magma. The possible range in FeO and MgO content of source mantle for the Oshima-Ōshima magma can be demonstrated. Ichinomegata lherzolite inclusions, also from the back-arc side of NE Japan, is unlikely to be a candidate for the source mantle for high FeO. The upper mantle beneath the back-arc side is considered to be compositionally zoned; a Fe-rich mantle (Ichinomegata lherzolite) at shallower place and a Fe-poor mantle (the source mantle for back-arc side volcanoes).

North Mountain basalt from Digby, Nova Scotia: models for a fissure eruption from stratigraphy and petrochemistry

Abstract: The early Mesozoic, quartz normative, North Mountain basalts in southwestern Nova Scotia (Digby area) form three units: a coarse massive lower flow (~190 m) bearing minor lenses of mafic pegmatite, a middle unit of thin amygdaloidal basaltic flows (~50 m), and an upper flow unit of massive phenocryst-rich basalt (~160 m). The two thick units show phenocrysts of orthopyroxene (bronzite) and (or) pigeonite, augite, and zoned plagioclase in a granular matrix of augite, pigeonite, and plagioclase. Variation diagrams and chondrite-normalized rare-earth-element patterns relate all chemical diversity between and within flows to removal and (or) accumulation of plagioclase and pyroxene phenocrysts (~1:1). High K, Rb, and Ba, appear related to assimilation of continental crust. Constancy of fractionation-independent element ratios and variations in phenocryst content vertically and along the 200 km strike of the basalts suggest (1) crystal settling and accumulation together with assimilation and mixing in a lower ...

High-MgO lavas from the Keweenawan midcontinent rift near Mamainse Point, Ontario

Abstract: Detailed study of the basal 500 m of the Keweenawan Mamainse Point volcanics in Ontario, Canada, has revealed that the section is dominated by basaltic to picritic lavas with up to 23 wt% MgO. Two stratigraphic suites with distinct chemical characteristics are separated by four flows of high-TiO 2 ferrobasalt containing large and abundant plagioclase spherulites. The focus of this study is on the lower suite that contains the most MgO-rich compositions and that generally has lower TiO 2 content. Although most of the flows are olivine-phyric, the olivine phenocrysts are skeletal, suggesting in situ crystallization from magmas with high normative-olivine content. Furthermore, the compositional variation of the lavas does not fall along an olivine-only control line. Taken together, the data indicate that the compositional variation of the lavas represents fractionation of high-MgO liquids rather than accumulation of olivine in basaltic liquids. Most of the fractionation apparently involved crystallization of olivine, clinopyroxene, and Cr-spinel, but later fractionation involved olivine, clinopyroxene, and plagioclase. The occurrence of picritic lavas low in the overall volcanic section is similar to relations in other flood-basalt provinces, and should eventually yield important tectonic and magmatic constraints for the development of the Keweenawan midcontinent rift of North America.

High-fluorine rhyolite: An eruptive pegmatite magma at the Honeycomb Hills, Utah

Abstract: The Honeycomb Hills rhyolite dome in western Utah displays chemical and mineralogical features characteristic of a rare-element pegmatite magma. The lavas show extreme enrichments in such trace elements as Rb (≤1960 ppm), Cs (≤78), Li (≤344), Sn (≤33), Be (≤270), and Y (≤156). Phenocrysts (10%-50% by volume) include sanidine (Or 66-70 ), plagioclase (Ab 83-92 ), quartz, biotite approaching fluorsiderophyllite, and fluortopaz, as well as accessory phases common to highly differentiated granites and pegmatites, including zircon, thorite, fluocerite, columbite, fergusonite, and samarskite. Low temperatures (600 to 640 °C), coupled with high phenocryst and silica content, might normally preclude eruption due to the extremely high viscosity of the melt. However, high concentrations of fluorine (2%-3%) could domal lavas significantly reduce viscosity and allow eruption of domal lavas even after dewatering of the mama during the initial pyroclastic phase of the eruptive cycle. Fractionation of phenocrysts and accessory phases, for which partition coefficients have been measured, is sufficient to account for most compositional gradients inferred in the preeruptive magma body, although transport by a fluid phase formed a may have caused upward enrichments in Li, Be, and Cs. If the Honeycomb Hills magma had crystallized at depth, it would have formed a rare-element pegmatite.