Showing papers on "Phenocryst published in 1991"

The redox states of basic and silicic magmas: a reflection of their source regions?

Abstract: At present the best estimates of the oxygen fugacity of spinel-lherzolites that could be the source material of basic magmas is about five log units below the Ni−NiO buffer to one above it. However partially glassy basic lavas, ranging from MORBs to minettes, all with olivine on their liquidus, cover a wider range, and may have oxygen fugacities that extend to four log units above NNO. Surprisingly the range of oxygen fugacities observed in silicic lavas and ashflows with quartz phenocrysts is smaller, despite a crustal dominated evolution. The peralkaline silicic lava type pantellerite is the most reduced, equivalent to MORBs, whereas the large volume ashflows with phenocrysts of hornblende and/or sphene are the most oxidised. As the concentration of water in the basic lavas is correlated with increase in their redox state, it would seem that water could be the agent of this increase. That this is unlikely is seen in the behavior of silicic ashflows and lavas, particularly those of Yellowstone. Here the silicic magmas of the last 2Ma contain about 2 wt% FeO(total), and typically phenocrysts of fayalite, quartz and Fe−Ti oxides. Despite extensive exchange of the 18O of the magma with meteoric water after caldera collapse (Hildreth et al. 1984), there is no displacement of the redox equilibria. Thus the thermal dissociation of molecular H2O to H2, and its subsequent diffusive loss to cause oxidation must have been minimal. This could only be so if the activity of water was small, as it would be if H2O reacted with the silicate liquid to form OH groups (Stolper 1982). The conclusion is that silicic magmas with small amounts of iron and large amounts of water do not have their redox states reset, which in turn presumably reflect their generation. By analogy basic magmas with large amounts of iron and far less water are even less likely to have their redox equilibria disturbed, so that their oxygen fugacities will also reflect their source regions. The effect of pressure on the ferric-ferrous equilibrium in basic magmas can be calculated from experimental measurements of the partial molar volumes of FeO and Fe2O3, and their pressure derivatives ϖV/ϖP, in silicate liquids. The effect of pressure is found to be about the same on the liquid as it is for various solid oxygen buffers. Accordingly there should be mantle source regions covering the same range of oxygen fugacity as that found in basic lavas, but so far samples of spinel-lherzolite of equivalent oxygen fugacity to minettes or other potassic lavas have not been found. Whether or not the redox state of phlogopite-pyroxenites is equivalent to these potassic lavas cannot be established without experiment.

The evolution of Mauna Kea Volcano, Hawaii: Petrogenesis of tholeiitic and alkalic basalts

Abstract: Mauna Kea Volcano has three exposed rock units. Submarine shield-building tholeiites form the oldest unit. Subaerial, interbedded tholeiitic and alkalic basalts form the intermediate age unit (70–240 Ka), and they are partially covered by evolved alkalic lavas, hawaiites and mugearites (4–66 Ka). In contrast to other Hawaiian volcanoes, such as Haleakala and Kauai, lavas from Mauna Kea do not define systematic temporal variations in Pb, Sr or Nd isotopic ratios. However with decreasing age the tholeiitic basalts are increasingly enriched in incompatible elements; therefore the shield and postshield tholeiites were derived from compositionally distinct parental magmas. Submarine shield lavas from the east rift contain forsterite-rich olivine (up to Fo90.5) providing evidence for MgO-rich (14.4 to 17%) magmas. Postshield tholeiitic and alkalic basalts with similar isotopic ratios may have been derived from the same source composition by different degrees of partial melting. If a compositionally and isotopically homogeneous source and a batch melting model are assumed, inversion of incompatible element abundance data for the postshield basalts requires low degrees (<2%) of melting of a garnet Iherzolite source which had near-chondritic abundances of heavy rare-earth elements (REE) but less than chondritic abundances of highly incompatible elements such as Ba, Nb and light REE. As the volcano migrated away from the hotspot, eruption rates decreased enabling high Fe-Ti basalts to form by fractional crystallization in shallow crustal magma chambers. The associated phenocryst-rich, high-MgO postshield lavas (picrites and ankaramites) are products of phenocryst accumulation. Eventually basaltic eruptions ceased, and the youngest Mauna Kea lavas are exclusively hawaiites and mugearites which formed from alkalic basalt parental magmas by clinopyroxene-dominated fractionation at lower crustal pressures.

Reequilibration of chromite within Kilauea Iki lava lake, Hawaii

Abstract: Chromite mainly occurs as tiny inclusions within or at the edges of olivine phenocrysts in the 1959 Kilauea Iki lava lake. Liquilus chromite compositions are only preserved in scoria that was rapidly quenched from eruption temperatures. Analyses of drill core taken from the lava lake in 1960, 1961, 1975, 1979, and 1981 show that chromite becomes richer in Fe+2, Fe+3, Ti and poorer in Mg, Al, Cr than the liquidus chromite. The amount of compositional change depends on the time elapsed since eruption, the cooling history of the sample, the extent of differentiation of the interstitial melt, and the position of the chromite inclusion within the olivine phenocryst. Compositional changes of the chromite inclusions are thought to be a result of reequilibration with the residual melt by cationic diffusion (Mg, Al, Cr outwards and Fe+2, Fe+3, Ti inwards) through olivine. The changing chemical potential gradients produced as the residual melt cools, crystallizes and differentiates drives the reequilibration process. Major and minor element zoning profiles in olivine phenocrysts suggest that volume diffusion through olivine may have been the major mechanism of cationic transport through olivine. The dramatic compositional changes observed in chromite over the 22 years between eruption and 1981 has major implications for othe molten bodies.

Picritic glasses from Hawaii

Abstract: ESTIMATES of the MgO content of primary Hawaiian tholeiitic melts range from 8wt% to as high as 25wt% In general, these estimates are derived from analysis of the whole-rock composition of lavas, coupled with the compositions of the most magnesian olivine phenocrysts observed But the best estimate of magma composition comes from volcanic glass, as it represents the liquid composition at the time of quenching; minimal changes occur during the quenching process Here we report the discovery of tholeiitic basalt glasses, recovered offshore of Kilauea volcano, that contain up to 150 wt% MgO To our knowledge, these are the most magnesian glasses, and have the highest eruption temperatures (~ 1,316°C), yet found The existence of these picritic (high-MgO) liquids provides constraints on the temperature structure of the upper mantle, magma transport and the material and thermal budgets of the Hawaiian volcanoes Furthermore, picritic melts are affected little by magma-reservoir processes, and it is therefore relatively straightforward to extrapolate back to the composition of the primary melt and its volatile contents

New data on magmatic H 2 O contents of pantellerites, with implications for petrogenesis and eruptive dynamics at Pantelleria

Abstract: Infrared spectroscopic analyses of melt inclusions in quartz phenocrysts from pantellerites erupted at Pantelleria, Italy, show that the magmas contained moderate pre-eruptive H2O contents, ranging from 1.4 to 2.1 wt.%. Melt H2O concentrations increase linearly with incompatible elements, demonstrating that H2O contents were not buffered significantly during fractionation by any crystalline or vapor phase. The relatively low H2O contents of pantellerites are consistent with an origin by partial melting of alkali gabbros rather than fractional crystallization of basalt. Preeruptive H2O concentrations do not correlate with the volume or explosivity of pantellerite eruptions; decompression history is critical in determining the style of pantellerite (and other) eruptions.

Nephelinite-carbonatite liquid immiscibility at Shombole volcano, East Africa: Petrographic and experimental evidence

Abstract: We summarize the evidence for silicate-carbonate liquid immiscibility in two nephelinite lavas from Shombole volcano, East Africa, and discuss its significance for carbonatite petrogenesis. The nephelinite lavas contain spherical to irregular globules ≤ 0.5 cm containing low-Sr calcite, Sr-Ca and K-Ba zeolites, fluorite, aegirine, strontianite, and fluorapatite. The globules are interpreted to be magmatic in origin, and represent quenched immiscible carbonate liquid. Most phases in the globules form an interlocking mosaic of euhedral crystals, however, rare blebby intergrowths of calcite and strontianite indicate eutectic crystallization from a melt. The phase assemblages and respective compositions of minerals in the globules and silicate groundmass are nearly identical, indicating that the samples were quenched when two liquids were in near-equilibrium. Experiments with the samples at 200–500 MPa and 975–925 °C have reproduced the natural assemblages (phenocrysts + 2 liquids) exactly and the compositions of experimentally generated solid phases closely match the original phenocrysts. The natural and experimentally produced carbonatites are both sovitic (calcite carbonatite) in composition.

Development of the Long Valley, California, magma chamber recorded in precaldera rhyolite lavas of Glass Mountain

Abstract: Glass Mountain, California, consists of >50 km3 of high-silica rhyolite lavas and associated pyroclastic deposits that erupted over a period of >1 my preceding explosive eruption of the Bishop Tuff and formation of the Long Valley caldera at 0.73 Ma. These “minimum-melt” rhyolites yield Fe-Ti-oxide temperatures of 695–718°C and contain sparse phenocrysts of plagioclase+quartz+magnetite+apatite±sanidine, biotite, ilmenite, allanite, and zircon. Incompatible trace elements show similar or larger ranges within the Glass Mountain suite than within the Bishop Tuff, despite a much smaller range of major-element concentrations, largely due to variability among the older lavas (erupted between 2.1 and 1.2 Ma). Ratios of the most incompatible elements have larger ranges in the older lavas than in the younger lavas (1.2–0.79 Ma), and concentrations of incompatible elements span wide ranges at nearly constant Ce/Yb, suggesting that the highest concentrations of these elements are not the result of extensive fractional crystallization alone; rather, they are inherited from parental magmas with a larger proportion of crustal partial melt. Evidence for the nature of this crustal component comes from the presence of scarce, tiny xenocrysts derived from granitic and greenschist-grade metamorphic rocks. The wider range of chemical and isotopic compositions in the older lavas, the larger range in phenocryst modes, the eruption of magmas with different compositions at nearly the same time in different parts of the field, and the smaller volume of individual lavas suggest either that more than one magma body was tapped during eruption of the older lavas or that a single chamber tapped by all lavas was small enough that the composition of its upper reaches easily affected by new additions of crustal melts. We interpret the relative chemical, mineralogical, and isotopic homogeneity of the younger Glass Mountain lavas as reflecting eruptions from a large, integrated magma chamber. The small number of cruptions between 1.4 and 1.2 ma may have allowed time for a large magma body to coalesce, and, as the chamber grew, its upper reaches became less affected by new inputs of crustal melts, so that trace-element trends in magmas erupted after 1.2 Ma are largely controlled by fractional crystallization. The extremely low Sr concentrations of Glass Mountain lavas imply extensive crystallization in chambers at least hundreds of cubic kilometers in volume. The close similarity in Sr, Nd, and Pb isotopic ratios between the younger Glass Mountain lavas and unaltered Bishop Tuff indicates that they tapped the same body of magma, which had become isotopically homogenous by 1.2 Ma but continued to differentiate after that time. From 1.2 to 0.79 Ma, volumetric eruptive rates may have exceeded rates of differentiation, as younger Glass Mountain lavas become slightly less evolved with time. Early-erupted Bishop Tuff is more evolved than the youngest of the Glass Mountain lavas and is characterized by slightly different trace element ratios. This suggests that although magma had been present for 0.5 my, the composiional gradient exhibited by the Bishop Tuff had not been a long-term, steady-state condition in the Long Valley magma chamber, but developed at least in part during the 0.06-my hiatus between extrusion of the last Glass Mountain lava and the climactic eruption.

The 1783 Lakagigar eruption in Iceland: geochemistry, CO 2 and sulfur degassing

Abstract: About 12.3 km3 of basaltic magma were erupted from the Lakagigar fissure in Iceland in 1783, which may have been derived from the high-level reservoir of Grimsvotn central volcano, by lateral flow within the rifted crust. We have studied the petrology of quenched, glassy tephra from sections through pyroclastic cones along the fissure. The chemical composition of matrix glass of the 1783 tephra is heterogeneous and ranges from olivine tholeiite to Fe−Ti rich basalt, but the most common magma erupted is quartz tholeiite (Mg#43.6 to 37.2). The tephra are characterized by low crystal content (5 to 9 vol%). Glass inclusions trapped in plagioclase and Fo86 to Fo75 olivine phenocrysts show a large range of compositions, from primitive olivine tholeiite (Mg#64.3), quartz tholeiite (Mg#43–37), to Fe−Ti basalts (Mg#33.5) which represent the most differentiated liquids and are trapped as rare melt inclusions in clinopyroxene. Both matrix glass and melt inclusion data indicate a chemically heterogeneous magma reservoir, with quartz tholeiite dominant. LREE-depleted olivine-tholeiite melt-inclusions in Mg-rich olivine and anorthitic-plagioclase phenocrysts may represent primitive magma batches ascending into the reservoir at the time of the eruption. Vesicularity of matrix glasses correlates with differentiation, ranging from 10 to 60 vol.% in evolved quartz-tholeiite glasses, whereas olivine-tholeiite glasses contain less than 10 vol.% vesicles. FTIR analyses of olivine-tholeiite melt-inclusions indicate concentrations of 0.47 wt% H2O and 430 to 510 ppm for CO2. Chlorine in glass inclusions and matrix glasses increases from 50 ppm in primitive tholeiite to 230 ppm in Fe−Ti basalts, without clear evidence of degassing. Melt inclusion analyses show that sulfur varies from 915 ppm to 1970 ppm, as total FeO* increases from 9 to 13.5 wt%. Sulfur degassing correlates both with vesicularity and magma composition. Thus sulfur in matrix glasses decreases from 1490 ppm to 500 ppm, as Mg # decreases from 47 to 37 and vesicularity of the magma strongly increases. These results indicate loss of at least 75% of sulfur during the eruption. The correlation of low sulfur content in matrix glasses with high vesicularity is regarded as evidence of the control of a major exsolving volatile phase on the degassing efficiency of the magma. Our model is consistent a quasi-permanent CO2 flux through the shallow-level magmatic reservoir of Grimsvotn. Following magma withdrawal from the reservoir and during eruption from the Lakagigar fissure, sulfur degassing was controlled by inherent CO2-induced vesicularity of the magma.

Hawaiian basalt and Icelandic rhyolite: Indicators of differentiation and partial melting

Abstract: In spite of the voluminous basaltic volcanism on the island of Hawaii, rhyolite is not produced Iceland, on the other hand, exhibits common rhyolitic volcanism amounting to some 10–12% of its surface rocks This contrast is investigated using the fundamental igneous processes exhibited by sheet-like Hawaiian lava lakes and Shonkin Sag laccolith in Montana Highly differentiated, residual melts normally reside within inwardly advancing solidification fronts and are generally inaccessible to eruptive processes Only when a large initial phenocryst population is present, from which a thick basal cumulate can rapidly form, is it possible to supply highly differentiated melt into the active (ie, eruptable) portion of the magma chamber Although there is protracted control of differentiation at Hawaii by settling of olivine, further differentiation occurs within the solidification fronts Only by repeated transport and holding is it possible to differentiate beyond the critical composition of the leading edge of the solidification front (∼ 7% MgO and 515% SiO2) Crystal size distributions (CSDs) for Hawaii and Shonkin Sag are used to demonstrate the inferred physical and chemical processes of solidification, including the kinetics of crystallization

Intrusion of basaltic magma into a crystallizing granitic magma chamber: The Cordillera del Paine pluton in southern Chile

Abstract: The Cordillera del Paine pluton in the southernmost Andes of Chile represents a deeply dissected magma chamber where mafic magma intruded into crystallizing granitic magma. Throughout much of the 10x15 km pluton, there is a sharp and continuous boundary at a remarkably constant elevation of 1,100 m that separates granitic rocks (Cordillera del Paine or CP granite: 69–77% SiO2) which make up the upper levels of the pluton from mafic and comingled rocks (Paine Mafic Complex or PMC: 45–60% SiO2) which dominate the lower exposures of the pluton. Chilled, crenulate, disrupted contacts of mafic rock against granite demonstrate that partly crystallized granite was intruded by mafic magma which solidified prior to complete crystallization of the granitic magma. The boundary at 1,100 m was a large and stable density contrast between the denser, hotter mafic magma and cooler granitic magma. The granitic magma was more solidified near the margins of the chamber when mafic intrusion occurred, and the PMC is less disrupted by granites there. Near the pluton margins, the PMC grades upward irregularly from cumulate gabbros to monzodiorites. Mafic magma differentiated largely by fractional crystallization as indicated by the presence of cumulate rocks and by the low levels of compatible elements in most PMC rocks. The compositional gap between the PMC and CP granite indicates that mixing (blending) of granitic magma into the mafic magma was less important, although it is apparent from mineral assemblages in mafic rocks. Granitic magma may have incorporated small amounts of mafic liquid that had evolved to >60% SiO2 by crystallization. Mixing was inhibited by the extent of crystallization of the granite, and by the thermal contrast and the stable density contrast between the magmas. PMC gabbros display disequilibrium mineral assemblages including early formed zoned olivine (with orthopyroxene coronas), clinopyroxene, calcic plagioclase and paragasite and later-formed amphibole, sodic plagioclase, mica and quartz. The early formed gabbroic minerals (and their coronas) are very similar to phenocrysts in late basaltic dikes that cut the upper levels of the CP granite. The inferred parental magmas of both dikes and gabbros were very similar to subalkaline basalts of the Patagonian Plateau that erupted at about the same time, 35 km to the east. Mafic and silicic magmas at Cordillera del Paine are consanguineous, as demonstrated by alkalinity and trace-element ratios. However, the contemporaneity of mafic and silicic magmas precludes a parent-daughter relationship. The granitic magma most likely was derived by differentiation of mafic magmas that were similar to those that later intruded it. Or, the granitic magma may have been contaminated by mafic magmas similar to the PMC magmas before its shallow emplacement. Mixing would be favored at deeper levels when the cooling rate was lower and the granitic magma was less solidified.

Olivine xenocrysts in picritic magmas

Abstract: Experimental partial melting of plagioclase lherzolite at 0.7 MPa confining pressure has produced euhedral olivine crystals by corrosion and overgrowth during cooling. The particular conditions of this experiment allow observation of both processes recorded in the crystal shape: corrosion boundaries are rounded, or straight when parallel to [001] intersection of {110} planes; (010) and {110} facets are developed by fast overgrowth during quenching. These observations support the contention that phenocrysts in basaltic or picritic magmas are, in part, xenocrysts. The possible mantle origin of olivine crystals in two natural occurrences of ultramafic magmas; the picritic pillow lavas of the Troodos, and a wehrlitic intrusion of the oman ophiolite, is investigated. In both cases discriminant characteristics are deduced from detailed microstructural study. The mantle origin of olivine megacrysts in the investigated picrites raises the question about the existence of picritic magmas in the mantle.

Styles of zoning in central Andean ignimbrites - Insights into magma chamber processes

Abstract: Data are presented showing that calc-alkaline high-K ignimbrites from the Altiplano-Puna Volcanic Complex of the Central Volcanic Zone of the Andes, showing a variety of compositional zonations. The characteristics of the juvenile material from the zoned and heterogenous ignimbrites suggest that crystallization of the observed phenocrysts occurred in prezoned magma chambers consisting of two or more layers. It is suggested that the width/height ratio of a magma chamber plays a critical role in the control of the style of zonation that may develop in a closed magma chamber.

Argon isotope and halogen chemistry of phlogopite from South African kimberlites: a combined step-heating, laser probe, electron microprobe and TEM study

Abstract: Argon isotopic analyses were undertaken on phlogopite from the Swartruggens kimberlite dyke and a Premier kimberlite (1200 Ma) peridotite xenolith. Groundmass phlogopite from Swartruggens yields a plateau age of 145.0± 0.4 Ma, consistent with previous age determinations. Phlogopite phenocrysts from Swartruggens and macrocrysts from the Premier xenolith yield complex age spectra, with anomalously old ages, attributed to incorporation of excess radiogenic argon. Laser probe analyses on single phlogopite grains reveal systematic zonations in excess Ar and CI concentrations across (001) cleavage surfaces. One Premier macrocryst exhibits ages in excess of 2.3 Ga and CI levels of 1600 ppm at its centre. These values decrease systematically to 1.2 Ga and 1300 ppm Cl along grain margins. Similar results were obtained from a single Swartruggens phenocryst, which exhibits a range in values of 340—800 Ma and 300—1300 ppm Cl. A second Swartruggens phenocryst is characterised by smaller variations in age (140–230 Ma) and CI content (390–470 ppm ). Fluorine concentrations, determined by electron microprobe, are relatively constant or increase slightly towards grain edges. The laser probe profiles cannot be reconciled with the step-heating results, probably due to phlogopite degradation during invacuo furnace heating. Transport of Ar and CI in kimberlitic phlogopite appears to be dominated by radial diffusion (cylindrical geometry). The variety of laser probe profiles obtained suggests that Ar and Cl diffusion is governed by factors such as lattice diffusion, diffusion anisotropy, and structural defects, which reduce the effective radii of diffusion and may impart a component of pipe diffusion. It is suggested that the xenolith phlogopite entrapped excess 40Ar and halogens in the mantle lithosphere, in response to elevated Ar and halogen fluid pressures. Swartruggens phenocrysts appear to have crystallised from a volatile-rich kimberlite melt. Subsequent magma devolatilisation prior to emplacement reduced Ar partial pressure and CI content. Possible reasons for enhanced F levels after devolatilisation include increased F solubility in the kimberlite melt, extraction of F from infiltrating hydrothermal fluids and local heterogeneities in fluid composition. The final distributions of Ar, Cl and F in kimberlitic phlogopite are variably dependent on several parameters, including local fluid composition, timing of melt devolatilisation, diffusion/ exchange mechanisms, and mineral composition.

Eruptive pegmatite magma: Rhyolite of the Honeycomb Hills, Utah

Abstract: The Honeycomb Hills rhyolite represents differentiation in a highly evolved magma. A pyroclastic sequence 12.5 m thick and a dome of -0.2 km3 occur in western Utah in a region populated with several Tertiary topaz rhyolites. Phenocrysts consist of quartz, sanidine, and albite (10-500/o total) in a glassy or fine-grained groundmass. Primary phenocrysts and megacrysts of topaz and fluorsiderophyllite (lolo total) and accessory phases usually associated with rare-element pegmatites occur: fergusonite, ishikawaite, columbite, fluocerite, thorite, monazite, and zircon. Whole-rock composition (SiO, : 73.3o/o, TiO, : 0.01, AlrO3 : 14.0, Fe,O, : 0.28, FeO : 0.55, MnO : 0.07, MgO < 0.01, CaO:0.42, NarO : 4.59, &O : 4.44, PrO, < 0.01, F: 0.61, Cl : 0.10, and maximum values Rb : 1960 ppm, Cs : 78,Li: 344, Sn : 33, Be : 80, and Y : 156) is peraluminous, highly evolved, and comparable to rare element pegmatites. Elevated F contents of up to 2.30/o in glass account for low silica and high alumina contents because the granite minimum shifts toward the Ab apex of the Q-Ab-Or ternary with increasing F. Mineralogy and distribution of trace elements with order of eruption indicate evacuation of a cool (570610 'C), chemically stratified magma chamber. Chemical variation within the erupted volume can be modeled by Rayleigh fractionation of 7 5o/o f the phenocryst phases. Spatial variation of some elements, notably Li, Be, B, F, and Cs, may be due to volatile transfer. Enrichment of HrO and F in interstitial melt during crystallization reduced viscosity enough to allow eruption of the highly crystalline lava of the dome.

Mineralogy and geothermobarometry of magmatic epidote-bearing dikes, Front Range, Colorado

Abstract: Epidote phenocrysts (Ps 19-24 ) in Laramide-age, porphyritic dacite dikes in the Front Range of Colorado prove that epidote can crystallize directly from a magma. The dikes are high-K, calc-alkaline dacites and rhyodacites (SiO 2 = 64%-70%). Phenocryst assemblages consist of combinations of epidote, plagioclase (An 53-19 ), biotite [Fe 3+ / Fe total = 0.25; Mg/(Mg + Fe 2+ ) = 0.42-0.55], quartz, aluminous amphibole (Al total = 2.4-3.0 per formula unit), igneous garnet (And 2-6 Gros 13-27 Spess 2-13 Pyr 7-26 Alm 51-60 ), igneous muscovite, and sanidine. Holocrystalline, aphanitic groundmasses consisting mainly of quartz, plagioclase, and alkali feldspar comprise 60%-75% of the dike rocks and require that epidote grew early in the crystallization sequence. Petrographic and microprobe data distinguish three types of magmatic epidote phenocrysts, with different petrogenetic connotations implied by each type. Type I epidote occurs as euhedral phenocrysts, up to 8 mm long, with low content of allanite component ( Thermobarometric calculations provide empirical evidence that epidote is stable in silicic, calc-alkaline magmas at high total pressure, high water content, and high oxygen fugacity. The dacitic magmas originated by partial melting at temperatures above 800 °C. Temperatures (±50 °C) ranged from 800 to 880 °C during early phenocryst growth down to 620 to 670 °C during groundmass crystallization. Phenocrysts formed at pressures between 7.2 ± 1.0 and 12 ± 2.0 kbar. Oxygen fugacity was within the magnetite stability field at two log units (±1.0) below the hematite-magnetite buffer. H 2 O fugacity calculations give values ranging from 3.4 to 16 kbar, depending on how annite activity in biotite is modeled. The occurrences of types I and III epidote combined with geobarometry give a minimum pressure of epidote stability in the dike magmas of 8.0 ± 1.0 kbar. These results agree with experimental evidence and with conditions inferred for the crystallization of magmatic epidote in large, Cordilleran plutons. Phenocrysts in the dikes were preserved in conditions outside their stability limits by rapid emplacement and quenching in shallow-level dikes.

Precaldera lavas of the southeast San Juan Volcanic Field: Parent magmas and crustal interactions

Abstract: Early intermediate composition volcanic rocks of the Oligocene (circa 34–29 Ma) southeast San Juan volcanic field, southern Colorado, comprise the Conejos Formation. Conejos lavas include both high-K calc-alkaline and alkaline magma series (54–69% SiO2) ranging in composition from basaltic andesite (basaltic trachyandesite) to dacite (trachydacite). The subsequent Platoro caldera complex (29–27 Ma) was superimposed on a cluster of broadly precursory Conejos stratocones. Precaldera volcanism occurred in three pulses corresponding to three time-stratigraphic members: (1) the Horseshoe Mountain member, (2) the Rock Creek member, and (3) the Willow Mountain member. Each member exhibits distinctive phenocryst modes and incompatible trace element contents. Horseshoe Mountain lavas (hornblende-phyric) have relatively low alkali and incompatible element abundances, Rock Creek lavas (anhydrous phenocrysts) and ash-flow tuffs have the highest abundances, and Willow Mountain lavas (diverse mineralogy) are intermediate. All Conejos lavas exhibit low ratios of lead (206Pb/204Pb = 17.5 to 18.2) and neodymium (eNd = −8 to −4) isotopes and high 87Sr/86Sr (0.7045 to 0.7056) compared to depleted asthenospheric mantle. These values lie between those of likely mantle compositions and the isotopic composition of Proterozoic crust of the southern Rocky Mountains. Mafic lavas of the Horseshoe Mountain member have the lowest Pb and Nd isotope ratios among Conejos members but trend toward higher isotopic values with increasing degrees of differentiation. Compositions within the Rock Creek series trend toward higher Pb and lower Nd isotope ratios with increasing SiO2. Willow mountain volcanic sequences define diverse chemical-isotopic correlations. We interpret the chemical and isotopic differences observed between mafic lavas of each member to reflect derivation from compositionally distinct mantle derived parent magmas that have experienced extensive deep level crustal contamination. Chemical and isotope correlations within each member indicate Conejos magmas differentiated via staged polybaric, multiprocess evolutionary paths. Stage I occurred near the base of the crust as mantle-derived basalt evolved to basaltic andesite by the MASH mechanism. Stage II followed ascent to shallower crustal levels where basaltic andesite differentiated to more evolved compositions by combined fractionation and assimilation of heterogeneous crust.

Nickel-copper sulfides from the 1959 eruption of Kilauea Volcano, Hawaii: Contrasting compositions and phase relations in eruption pumice and Kilauea Iki lava lake

Abstract: Iron-nickel-copper sulfides in picritic eruption pumice from the 1959 eruption of Kilauea Volcano, Hawaii, and associated Kilauea Iki lava lake have been studied in detail. The sulfides occur as micrometer-sized subspherical blebs in glass inclusions within olivine phenocrysts and in glass within quenched groundmass. The proportion of blebs present within the volume of a thin section and within individual olivine grains is widely variable. Sulfide minerals present in the eruption pumice include monosulfide solid solution (mss), isocubanite, pentlandite, bornite, and an unidentified metal-excess iron-rich sulfide. Bulk sulfides in the eruption pumice are enriched in Ni relative to Cu. These minerals are also present in Kilauea Iki lava lake, along with chalcopyrite, but the bulk sulfides are markedly enriched in Cu relative to Ni. In the eruption pumice, the high-temperature history of the sulfide liquid precursor to the blebs involved crystallization entirely to mss and subsequent exsolution of isocubanite on cooling or crystallization of mss and formation of residual Cu-rich sulfide liquid. In Kilauea Iki lava lake, the high-temperature history of the sulfide liquid precursor to the olivine-hosted blebs involved crystallization and subsequent unmixing of isocubanite, whereas that of the sulfide liquid precursor to groundmass glass-hosted blebs involved exsolution of bornite solid solution from cubic chalcopyrite + isocubanite and chalcopyrite + isocubanite from bornite solid solution. The presence of pentlandite + chalcopyrite and pentlandite + bornite solid solution suggests that the blebs have equilibrated internally below approximately 600 degrees C. The Ni-rich bulk sulfide composition in the pumice reflects the higher temperature and rate of quenching. The Cu-rich bulk sulfide composition in Kilauea Iki reflects a lower temperature of quenching and more extensive annealing and crystallization history of the lava. The wide variability in bulk sulfide composition in the samples and lack of equilibration with coexisting olivine are inconsistent with an origin solely by early magmatic, sulfide liquid immiscibility. Instead, bulk sulfide compositions reflect late magmatic processes. Blebs in the eruption pumice and Kilauea Iki lava largely represent cooling- and quench-induced segregation of sulfides with metals scavenged from the silicate liquid.

Mantle contribution to the evolution of Middle Tertiary silicic magmatism during early stages of extension: the Egan Range volcanic complex, east-central Nevada

Abstract: The Egan Range volcanic complex lies 30 km northwest of Ely, on the edge of a highly extended domain in east-central Nevada. It consists mainly of lavas with subordinate tuffs and sedimentary rocks. The rocks are divided into three stratigraphic and lithologic groups that correlated with widespread middle Tertiary volcanic rocks associated with early stages of extension in the region. Volcanic rocks of the early group are predominantly two-pyroxene dacite and andesite lavas, all of which contain quenched, mafic inclusions and have compositions indicating they were derived by mixing between a contaminated mantle melt and a rhyodacitic crustal component. Rocks of the middle group are relatively homogeneous biotite, hornblende dacite and rhyodacite lavas. Elevated compatible and incompatible element concentrations and straight-line correlations of compositional data in the early and middle groups support a simple mixing model. Minor fractionation of clinopyroxene is required to explain some low Cr concentrations. Major element variations of the late group can be successfully modeled by crystal fractionation of observed phenocrysts accompanied by moderate assimilation of a crustal component to account for elevated Rb, Th, U, and light rare earth element concentrations. Rocks of all three groups appear to be related to a common primary magma type, the composition of which can be modeled from the mafic inclusions in the early group. Low Ni and Mg contents in the inclusions indicate that olivine was fractionated prior to their participation in mixing of early group magmas. Based on estimated volumes of volcanic rocks in the Egan Range volcanic complex and in the region, and on the petrologic models for each group, a significant amount of basalt must have been added to the crust during this middle Tertiary magmatic episode.

Pyroxene chemistry and evolution of alkali basaltic rocks from Burgenland and Styria, Austria

Abstract: The mineral chemistry of several Pliocene alkali basaltic rocks from Burgenland and Styria (Eastern Austria) have been investigated in order to determine the evolution path of the basalt magmas prior to eruption. With their wide range of substitutions, clinopyroxenes provide the best records of the evolution history of rocks. Pyroxene phenocrysts of the investigated basalts show both concentric and sector zoning. The investigation of sector zoned crystals shows, that not only Ti, Al and Fe contents are different in different sectors but there can be significant differences also in their Cr content. This fact apparently suggests that the distribution of Cr between clinopyroxene and melt could be influenced by crystallization kinetics. The depth of crystallization and differentiation of the basalts can be estimated from Ti and Al contents of clinopyroxene phenocrysts. From a combination of data on clinopyroxene composition, compatible trace element contents and mg-values of the rocks, it is concluded, that the alkali basalts of Pauliberg and Steinberg underwent slight olivine and clinopyroxene fractionation in shallow magma chambers prior to eruption, while the nephelinite of Stradnerkogel evolved mainly through clinopyroxene fractionation under high pressure conditions, probably in the upper mantle.

Petrochemical evidence for coupled magma chambers beneath the Sakurajima volcano, Kyushu, Japan

Abstract: We report systematic petrochemical changes of historical lava flows from the Sakurajima volcano. The lavas are all calc-alkalic in composition. They change in composition from dacitic to andesitic at an increasing rate with time, and show a straight-linear correlation in chemical composition. Plagioclase phenocrysts show a bimodal compositional distribution with peaks at about An58 and An85, regardless of variation in bulk rock composition. The intensity of An85 peak increases, while that of the An58 peak decreases with eruption stage. Orthopyroxene phenocrysts also show a bimodal compositional distribution. K2O content of the lavas progressively drops at almost the same rate with every gigantic eruption. These chemical and petrological aspects with other lines of evidence support binary mixing of dacitic and basaltic magmas, and a progressive increase in proportion of basaltic magma in the mixed magmas. The mixing has progressively proceeded for 475 years at least. The magma chamber model proposed in this paper consists of upper and lower chambers and a set of a huge cylinder and a mobile plug between the chambers. Both the cylinder and the plug have same frustum form. The plug subsiding into the lower chamber drives the basaltic magma upward through an opening between the cylinder-wall and the plug, and mixes it with dacitic magma in the upper chamber. Continuous influx of volatile-rich basaltic magma into the upper chamber may result in repetitive volcanic eruptions, and causes a continuous change in magma composition in the upper chamber, which erupts as lavas with discrete compositions.

The dykes and sills of the Early Tertiary Faeroe Island basalt plateau

Abstract: The Early Tertiary basalt plateau of the Faeroe Islands is cut by dykes and sills. Chemical analyses show a two-fold division of the intrusive rocks into a group of low-TiO2 (0·73–1·93%), MORB-type tholeiitic basalts and a group of high-TiO2 (2·09–3·90%) tholeiitic basalts. The low-TiO2 group comprises about 15% picrites and olivine-phyric basalts and 85% plagioclase-phyric basalts, and shows a chemical range largely explicable in terms of low-pressure fractional crystallisation of olivine ± plagioclase ± clinopyroxene. The high-TiO2 group is strongly dominated by plagioclase-phyric basalts with only few olivine-phyric compositions. The chemical trends are less regular than those formed by the low-TiO2 basalt dykes and a number of subgroups may be identified on the basis of bulk rock chemistry. Dykes belonging to a specific subgroup were probably fed from the same magma chamber.Petrographically and chemically the dykes and sills are clearly related to the upper 2·5 km of the lava sequence. Field evidence suggests that some of the dykes were contemporaneous with the exposed lavas, while other dykes and the sills were intruded in response to a slight doming of the plateau during the final stages of volcanic activity. Our investigations demonstrate that high-TiO2 and low-TiO2 magmas were both emplaced until the very end of magmatism, with the latter being mainly concentrated in the northern part of the archipelago. We briefly sketch a possible relationship between the supposed NE-Atlantic mantle plume, the distribution of the various magma types and the location of the Early Tertiary continental splitting zone north of the islands.

Textural evidence for calcite carbonatite magmas, Dicker Willem, southwest Namibia

Abstract: Carbonatites from a complex in southwestern Namibia retain features such as flow-aligned phenocrysts of calcite or dolomite enclosed in a granular or spinifex-textured calcite groundmass, comb layering, and gravity layering. All textures are compatible with Ca-rich carbonatite magmas carrying liquidus calcite or dolomite quenching on emplacement into a subvolcanic environment. Oxygen isotope fractionation between silicate and oxide minerals in sovite cumulates indicate high temperatures (600-900 °C), compatible with those determined experimentally for crystallization of liquidus calcite or dolomite. We propose that a Ca-rich carbonatite magma (proto-alvikite) separated immiscibly from a carbonate-rich ijolite and evolved by fractional crystallization of magnetite, acmitic pyroxene, and calcite to a dolomite-phyric alvikite.

Morphology and chemistry of olivine phenocrysts of Mangamate Tephra, Tongariro Volcanic Centre, New Zealand

Abstract: Four members of Mangamate Tephra [MT], erupted c. 10,000 years bp from Mt Tongariro, are identifiable in tephra sequences of both the Mt Tongariro and Mt Ruapehu ring plains, and are morphologically and mineralogically distinct from underlying andesitic tephras derived from Mt Ruapehu. Mangamate Tephra comprises dominantly grey angular lithic lapilli units, contrasting strongly with older and younger, distinctly pumiceous lapilli units from Mt Ruapehu. Three members of MT contain forsteritic olivine (fo75–88). Two olivine habits are identified. Type [I] non-skeletal (granular or polyhedral) olivines are subhedral equant to tabular crystals in thin section and occur only in Waihohonu Lapilli Member. Type [II] skeletal olivines are euhedral elongate, incomplete crystals with hollow interiors and hopper and H-shaped chain morphologies. They are found in Poutu Lapilli, Waihohonu Lapilli and Oturere Lapilli members. Type [II] skeletal olivines, not previously reported in Central North Island tephras, ...

Mineralogical relations and magma mixing in calc-alkaline andesites from lake Atitlán, Guatemala

Abstract: Quaternary calc-alkaline andesites erupted form three neighboring volcanoes along the Guatemalan volcanic front have mineralogic compositions and textures which show varying degrees of disequilibrium. Basaltic andesites and andesites (SiO2 % = 50−59), erupted from Atitlan volcano located nearer to the trench, have the lowest degree of disequilibrium. These lavas contain an anhydrous phenocryst assemblage of mildly bimodal plagioclase, olivine, augite opx, and magnetite. Orthopyroxene occurs at the expense of olivine with increasing whole rock SiO2. Most pyroxene phenocrysts show a trend of Fe enrichment. Andesites from Toliman (SiO2% = 53−62) and San Pedro (Si02% = 54−67) volcanoes, located further away from the trench, show comparatively high and moderate degrees of disequilibrium, respectively. Toliman andesites have bimodal plagioclase compositions and textures. Olivine persists with increasing whole rock Si02 and lacks clear modal relations with coexisting orthopyroxene and hornblende phenocrysts. When compared to Atitlan andesites, Toliman olivines are more forsteritic and pyroxenes contain higher proportions of Mg-rich rims, though normal zoned phenocrysts occur within the same rock. Toliman andesite also have lower proportions of phenocrysts to microphenocrysts, more calcic plagioclase groundmass compositions, and higher modal phenocrystic magnetite. San Pedro andesites have disequilibrium assemblages similar to Toliman andesites but are not as striking. Magma mixing is proposed as the dominant cause for observed disequilibrium. Disequilibrium features are preserved best in Toliman and San Pedro andesites because inferred durations between mixing and eruption are shortest, and consequently, these mixed andesites more clearly record mafic and silicic endmember compositions. The mafic component is a relatively high temperature, high-Al basalt containing phenocrysts of Mg-rich olivine (Fo = 78−80), calcic plagioclase (An 70−80), augite and titanomagnetite. The silicic component contains quartz, sodic plagioclase (An 40−50), Fe-rich orthopyroxene and titanomagnetite. Short durations between mixing and eruption produce petrographic features which, in part, mimic the effects of increasing PH20 in a fractionating magma. Inferred mixing durations for Atitlann andesites are longer and involve a less-silicic composition. The intervolcano disequilibrium relations suggest that as Si02 in a silicic endmember increases, the duration and efficiency of mixing decreases.