Showing papers on "Phenocryst published in 1994"

Petrology and Geochemistry of Boninites from the North Termination of the Tonga Trench: Constraints on the Generation Conditions of Primary High-Ca Boninite Magmas

Abstract: We report here a detailed mineralogical, geochemical, and experimental study of a high-Ca boninite suite from the northern termination of the Tonga trench. Most samples are strongly olivine porphyritic and show a significant range of phenocryst compositions including a very refractory olivine-spinel assemblage Fo94-CrN = 87. They are also characterized by a wide range of incompatible-element contents, e.g., (La/Yb)y varies from 05 to 16, whereas compatible major-element concentrations (A12O3, FeO, CaO, SiO2, and MgO) remain essentially the same. Primary melt compositions for the suite were established on the basis of an experimental study of melt inclusions in phenocrysts and numerical modelling of the reverse of fractional crystallization. Tongan primary melts are characterized by high MgO contents (22-24 wt.%) and originated in the mantle wedge at pressures of 20-25 kbar and temperatures of 1450-1550 °C. H2O contents in primary melts were estimated from direct measurements of melt inclusions by ion probe, and range from 2-0 to 10 wt.%, and a strong correlation exists between H2O and other incompatible element contents. The primary melts crystallized in the presence of an H2O-rich fluid in the temperature range 1390-1150 °C and pressures of 1-7-O15 kbar. Continuous degassing of melts took place during crystallization. Trace-element concentrations in primary melts were estimated using proton- and ion-probe analyses of melt inclusions in olivine, and whole-rock analyses. Our data suggest that three independent components (D, El, and E2) were involved. Component D was a refractory mantle depleted in incompatible elements, likely to be hot 'dry' lherzolite produced by previous melting within a mantle plume. Component El was an H2O-rich fluid containing LILE and Th, and had an H2O/K2O value of 20; it was probably produced by dehydration of the subducted slab. Component E2 is thought to have been an incompatible- element-enriched silicate melt of plume origin. Formation of high-Ca boninites requires interaction of hot 'dry' residual mantle, associated with plumes, with a subduction-rel ated H2O-bearing component.

Mineralogy, Chemistry, and Genesis of the Boninite Series Volcanics, Chichijima, Bonin Islands, Japan

Abstract: The Bonin archipelago represents an uplifted fore-arc terrain which exposes the products of Eocene supra-subduction zone magmatism. Chichijima, at the centre of the chain, represents the type locality for the high-Mg andesitic lava termed boninite. The range of extrusives which constitute the boninite series volcanics are present on Chichijima, and are disposed in the sequence boninite-andesite-dacite with increasing height in the volcano-stratigraphy. Progression to evolved compositions within the Chichijima boninite series is controlled by crystal fractionation from a boninite parental magma containing ? 15% MgO. Olivine and clinoenstatite are the initial liquidus phases, but extraction of enstatitic orthopyroxene, followed by clinopyroxene and plagioclase, is responsible for the general evolution from boninite, through andesite, to dacite. Some andesites within the overlying Mikazukiyama Formation are petrographically distinct from the main boninite series in containing magnetite phenocrysts and a high proportion of plagioclase. As such, these andesites have affinities with the calc-alkaline series. Major and trace element data for 74 boninitic series rocks from Chichijima are presented. Although major element variation is dominantly controlled by high-level crystal fractionation, the large variations in incompatiable trace element concentrations at high MgO compositions cannot be explained by this mechanism. Nd, Pb, and Sr isotopic data indicate the following: (1) a strong overprint on 87Sr/86Sr by seawater alteration; (2) Pb isotopes lie above the northern hemisphere reference line (NHRL) and are thus similar to the <30-Ma are and basin lavas of the Izu—Bonin system, and (3) ?Nd(40 Ma) ranges between 2.8 and 6.8 within the boninite series volcanics. Differences in rare-earth elements (REE), Zr, Ti, and 143Nd/144Nd at similar degrees of fractionation can be explained by the addition of a component of fixed composition from the down-going oceanic crustal slab to a variably depleted source region within the overlying wedge. Data presented for Sm/Zr and Ti/Zr indicate that boninite series volcanics are characterized by low values for both of these ratios. In particular, boninites appear to have uniquely low Sm/Zr ratios. These characteristics may be the result of slab melting in the presence of residual amphibole; the resultant melt could combine with typical slab dehydration fluids and infiltrate the overlying mantle wedge. Such a fluid—melt component could mix either with shallow mantle or directly with primitive melts from depleted mantle. Trace elements, REE, and isotope data thus point to a model for boninite genesis which requires tightly constrained pressure—temperature conditions in the slab combined with melting of a variably depleted source in the overlying wedge. Such constraints are rarely met except during the subduction of juvenile oceanic crust beneath a young, hot overriding plate.

Vesiculation of May 18, 1980, Mount St. Helens magma

Abstract: The May 18, 1980, eruption of Mount St. Helens, Washington, produced both white and gray pumice, similar in composition but varying in phenocryst, microlite, and vesicle content. The white pumice has fewer phenocrysts, no microlites, and higher vesicularity, and is thus less dense than the gray. In addition, vesicles in the white pumice are larger and more interconnected than those in the gray. Both white and gray have effective (crystal-free) vesicularities (85.7% and 72.2%, respectively), close to the traditionally accepted "fragmentation vesicularity" of 75%-77%. Given that the two pumice types were erupted concurrently for several phases of the eruption and therefore probably had similar eruption paths, we suggest that differences in vesicularity and vesicle size distributions result from the presence or absence of microlites. The presence of microlites in the gray pumice not only increased the magma viscosity and effective vesicularity, but appears to have aided bubble nucleation and hindered bubble expansion and coalescence. Thus, magmas with microlites may fragment at a lower bulk vesicularity than those without microlites. Fragmented microlite-bearing clasts are also likely to expand less after fragmentation and therefore more closely preserve the bubble distribution and structure at the time of magma fragmentation.

Magmatic vapor source for sulfur dioxide released during volcanic eruptions: evidence from mount pinatubo.

Abstract: Sulfur dioxide (SO(2)) released by the explosive eruption of Mount Pinatubo on 15 June 1991 had an impact on climate and stratospheric ozone. The total mass of SO(2) released was much greater than the amount dissolved in the magma before the eruption, and thus an additional source for the excess SO(2) is required. Infrared spectroscopic analyses of dissolved water and carbon dioxide in glass inclusions from quartz phenocrysts demonstrate that before eruption the magma contained a separate, SO(2)-bearing vapor phase. Data for gas emissions from other volcanoes in subduction-related arcs suggest that preeruptive magmatic vapor is a major source of the SO(2) that is released during many volcanic eruptions.

Role of olivine cumulates in destabilizing the flanks of Hawaiian volcanoes

Abstract: The south flank of Kilauea Volcano is unstable and has the structure of a huge landslide; it is one of at least 17 enormous catastrophic landslides shed from the Hawaiian Islands. Mechanisms previously proposed for movement of the south flank invoke slip of the volcanic pile over seafloor sediments. Slip on a low friction decollement alone cannot explain why the thickest and widest sector of the flank moves more rapidly than the rest, or why this section contains a 300 km3 aseismic volume above the seismically defined decollement. It is proposed that this aseismic volume, adjacent to the caldera in the direction of flank slip, consists of olivine cumulates that creep outward, pushing the south flank seawards. Average primary Kilauea tholeiitic magma contains about 16.5 wt.% MgO compared with an average 10 wt.% MgO for erupted subaerial and submarine basalts. This difference requires fractionation of 17 wt.% (14 vol.%) olivine phenocrysts that accumulate near the base of the magma reservoir where they form cumulates. Submarine-erupted Kilauea lavas contain abundant deformed olivine xenocrysts derived from these cumulates. Deformed dunite formed during the tholeiitic shield stage is also erupted as xenoliths in subsequent alkalic lavas. The deformation structures in olivine xenocrysts suggest that the cumulus olivine was densely packed, probably with as little as 5–10 vol.% intercumulus liquid, before entrainment of the xenocrysts. The olivine cumulates were at magmatic temperatures (>1100°C) when the xenocrysts were entrained. Olivine at 1100°C has a rheology similar to ice, and the olivine cumulates should flow down and away from the summit of the volcano. Flow of the olivine cumulates places constant pressure on the unbuttressed seaward flank, leading to an extensional region that localizes deep intrusions behind the flank; these intrusions add to the seaward push. This mechanism ties the source of gravitational instability to the caldera complex and deep rift systems and, therefore, limits catastrophic sector failure of Hawaiian volcanoes to their active growth phase, when the core of olivine cumulates is still hot enough to flow.

Effusive silicic volcanism in the Central Andes: The Chao dacite and other young lavas of the Altiplano‐Puna Volcanic Complex

Abstract: The largest known Quaternary silicic lava body in the world is Cerro Chao in north Chile, a 14-km-long coulee with a volume of at least 26 cu km. It is the largest of a group of several closely similar dacitic lavas erupted during a recent (less than 100,000 year old) magmatic episode in the Altiplano-Puna Volcanic Complex (APVC; 21-24 deg S) of the Centra; Andean Volcanic Zone. The eruption of Chao proceeded in three phases. Phase 1 was explosive and produced approximately 1 cu km of coarse, nonwelded dacitic pumice deposits and later block and ash flows that form an apron in front of the main lava body. Phase 2 was dominantly effusive and erupted approximately 22.5 cu km of magma in the form of a composite coulee covering approximately 53 sq km with a 400-m-high flow front and a small cone of poorly expanded pumice around the vent. The lava is homogeneous with rare flow banding and vesicular tops and selvages. Ogives (flow ridges) reaching heights of 30 m form prominent features on its surface. Phase 3 produced a 6-km-long, 3-km-wide flow that emanated from a collapsed dome. Ogives are subdued, and the lava is glassier than that produced in previous phases. All the Chao products are crystal-rich high-K dacites and rhyodacites with phenocrysts of plagioclase, quartz, hornblende, biotite, sphene, rare snidine, and oxides. Phenocryst contents reach 40-60 vol % (vesicle free) in the main phase 2 lavas but are lower in the phase 1 (20-25%) and phase 3 (approximately 40%) lavas. Ovoid andesitic inclusions with vesicular interiors and chilled margins up to 10 cm are found in the later stages of phase 2 and compose up to 5% of the phase 3 lava. There is little evidence for preeruptive zonation of the magma body in composition, temperature (approximately 840 C), fO2 (19(exp -11), or water content, so we propose that eruption of the Chao complex was driven by intrusion of fresh, hot andesitic magma into a crystallizing and largely homogeneous body of dacitic magma. Morphological measurements suggest that the Chao lavas had internal plastic viscosities of 10(exp 10) to 10(exp 12) Pa s, apparent viscosities of 10(exp 9) Pa s, surface viscosities of 10(exp 15) to 10(exp 24) Pa s, and a yield strength of 8 x 10(exp 5) Pa. These estimates indicate that Chao would have exhibited largely similar rheological properties to other silicic lava extrusions, notwithstanding its high phenocryst content. We suggest that Chao's anomalous size is a function of both the relatively steep local slope (20 deg to 3 deg) and the available volume of magma. The eruption duration for Chao's emplacement is thought to have been about 100 to 150 years, with maximum effusion rates of about 25 cu m/s for short periods. Four other lavas in the vicinity with volumes of approximately 5 cu km closely resemble Chao and are probably comagnetic. The suite as a whole shares a petrologic and chemical similarity with the voluminous regional Tertiary to Pleistocene ignimbrites of the APVC and may be derived from a zone of silicic magmatism that is thought to have been active since the late Tertiary. Chao and the other young lavas may represent either the waning of this system or a new episode fueled by intrusions of mafic magma.

Dissolved volatile concentrations in an ore-forming magma

Abstract: Infrared spectroscopic measurements of glass inclusions within quartz phenocrysts from the Plinian fallout of the 22 Ma tuff of Pine Grove (southwestern Utah) show that the trapped silicate melt contained high concentrations of H 2 O (6-8 wt%) and CO 2 (60-960 ppm). Inclusion compositions are consistent with either open- or closed-system degassing of this high-silica rhyolite during ascent from 16 km (430 MPa) to 9 km (250 MPa) depth, prior to eruption. Intrusive porphyries from the Pine Grove system are nearly identical in age, composition, and mineralogy to the tephra, and some contain high-grade Mo mineralization. Assuming that the porphyry magmas originally contained similar abundances of volatile components as the erupted rocks, they would have been saturated with fluid at pressures far greater than those at which the porphyries were emplaced and mineralized. Even if no initial exsolved fluid was present when the magma began to ascend, it would have contained 19 to 39 vol% bubbles at the ∼3 km depth (80 MPa) of emplacement of many porphyries. The decrease in magma density and increase in porosity would have facilitated magma convection as well as advection of magmatic aqueous fluid through an interconnected network of bubbles. The data are consistent with formation of Climax-type Mo porphyry deposits by prolonged fluid flux from a large volume (>20 km 3 ) of relatively Mo-poor (1-5 ppm) magma.

Cenozoic alkali basaltic magmas of western Germany and their products of differentiation

Abstract: Analytical data on major elements and 31 trace elements in olivine nephelinites, nepheline basanites, basanitic alkali olivine basalts and their differentiates (tephrites, hawaiites, mugearites, benmoreites, latites, phonolites and trachytes) from Hegau, Kaiserstuhl, Rhon, Hessian Depression, Vogelsberg, Westerwald, Siebengebirge, E Eifel and Hocheifel are evaluated. They were based on 400 samples with new or unpublished data on about one third of the rocks. The Sr−Nd isotopic compositions for 78 rocks are included. The alkali basaltic volcanism is caused by adiabatic decompression of asthenospheric mantle updomed to a minimum depth of 50 km in connection with the Alpine continent collision. The chemical compositions of the primary basaltic melts from the different areas are similar containing about one hundred-fold enrichment of highly incompatible elements relative to the primitive mantle from partial melting of depleted and secondarily enriched peridotite. The elements Cs, K, Pb and Ti are specifically depleted in the basalts partly because of phlogopite being residual at partial melting. The Tertiary alkali basalts range in Nd-isotopic composition from 0.51288 to 0.51273 and in Sr-isotopic ratios from 0.7032 to 0.7042. These ranges indicate mixtures of HIMU, depleted and enriched mantle components in the metasomatically altered peridotite source which resembles that of certain ocean islands. The Nd-Sr-isotopic compositions of the Quaternary E Eifel are close to bulk Earth ratios. East and W Eifel plots differ distinctly from the Tertiary Hocheifel which is geographically intermediate. This isotopic difference, beside specific K/Na ratios, is probably caused by separate metasomatic pulses that immediately preceded the respective periods of volcanism. The metasomatically altered mantle had partly primitive mantle signatures (Nb/Ta, Zr/Sm and Th/U ratios) and partly ocean island (or MORB) source properties (Rb/Cs). A MORB source can be excluded because of the low K/Rb and high Th/U ratios. A correlation of δD with 87Sr/86Sr in amphibole and phlogopite and a slightly larger δ18O than in MORB is conformable with a seawater and crustal impact on the source of alkali basalts. Slightly higher than average water concentrations in the source of certain primary basaltic melts (indicated by amphibole phenocrysts in their basalts) are required for differentiation of these basalts in magma chambers of the upper crust. Model calculations are presented to explain compositions of differentiates which range from about 60% to about 20% residual melt. The latter are represented by phonolites and trachytes. The Nd- and Sr-isotopic signatures of the majority of differentiates indicate contamination by a granitic partial melt from the wall rocks of magma chambers. Olivine nephelinite magma was the common source of contaminated differentiates.

Petrology of Volcán Tequila, Jalisco, Mexico: disequilibrium phenocryst assemblages and evolution of the subvolcanic magma system

Abstract: Volcan Tequila is an extinct stratovolcano in the western Mexican Volcanic Belt that has erupted lavas ranging from andesite to rhyolite during the last 0.9 Ma. Following an early period of rhyolitic volcanism, the main edifice of the volcano was constructed by central vent eruptions that produced ∼ 25 km3 of pyroxene-andesite. At about 0.2 Ma central activity ceased and numerous flows of hornblende-bearing andesite, dacite, and rhyodacite erupted from vents located around the flanks of the volcano. Bimodal plagioclase phenocryst rim compositions in lavas from both the main edifice and the flanks indicate that magma mixing commonly occurred shortly prior to or during eruption. Compositions of endmember magmas involved in mixing, as constrained by whole-rock major and trace element abundances, phenocryst compositions, and mineral-melt exchange equilibria, are similar to those of some lavas erupted from the central vent and on the flanks of the volcano. Estimated pre-eruptive temperatures for hornblende-bearing lavas (970°–830°C) are systematically lower than for lavas that lack hornblende (1045°–970°C), whereas magmatic H2O contents are systematically higher for hornblende-bearing lavas. In addition to stabilizing hornblende, high magmatic water contents promoted crystallization of calcic plagioclase (An70–82). Frequent injections of magma into the base of the subvolcanic plumbing system followed by eruption of mixed magma probably prevented formation of large volumes of silicic magma, which have caused paroxysmal, caldera-forming eruptions at other stratovolcanoes in western Mexico. The later stages of volcanic activity, represented by the flank lavas, indicate a change from a large magma storage reservoir to numerous small ones that developed along a NW-trending zone parallel to regional fault trends. Sr and Nd isotopic data for lavas from the Tequila region and other volcanoes in western Mexico demonstrate that differentiated calc-alkaline magmas are formed primarily through crystal fractionation of mantle-derived calc-alkaline basalt coupled with assimilation of crustal material.

Volcanological and petrological evolution of Nyiragongo volcano, Virunga volcanic field, Zaire

Abstract: Three major phases are distinguished during the growth of Nyiragongo, an active volcano at the western limit of the Virunga Range, Zaire Lavas erupted during phase 1 are strongly undersaturated melilitites characterized by the presence of kalsilite phenocrysts, perovskite, and the abundance of calcite in the matrix Such lavas crop out mainly on the inner crater wall and progressively evolve toward more aphyric melilite nephelinites well represented on the flanks of the volcano Adventive vents lying at the base of the cone developed along radial fracture systems and erupted olivine and/or clinopyroxene – rich melilitites or nephelinites Stage 2 lavas are melilite-free nephelinites Clinopyroxene is the main phenocryst and feldspathoids are abundant in the lavas exposed on the crater wall These flows result from periodic overflowing of a magma column from an open crater Extensive fissure flows which erupted from the base of the cone at the end of this stage are related to widespread draining out of magma which in turn induces the formation of the summit pit crater Magmas erupted during stage 3 are relatively aphyric melilite nephelinites and the main volcanological characteristic is the permanent lava lake observed into the pit crater until the 1977 eruption Fluctuations of the level of the lava lake was responsible for the development of the inner terraces Periodic overflowing of the lava lake from the central pit formed the nepheline aggregate lava flows Petrography and major element geochemistry allow the determination of the principal petrogenetic processes Melilitites and nephelinites erupted from the summit crater are lavas derived, via clinopyroxene fractionation, from a more primitive melt The abundance of feldspathoids in these lavas is in keeping with nepheline flotation Aphyric melilite nephelinites covering the flanks and the extensive fissure flows have a homogeneous chemical composition; rocks from the historical lava lake are slightly more evolved All these lavas differentiated in a shallow reservoir Lavas erupted from the parasitic vents are mainly olivine and/or clinopyroxene-phyric rocks Rushayite and picrites from Muja cone are peculiar high-magnesium lavas resulting from the addition of olivine xenocrysts to melilitic or nephelinitic melts Fluid and melt inclusions in olivine and clinopyroxene phenocrysts indicate a crystallization depth of 10–14 km A model involving two reservoirs located at different depths and periodically connected is proposed to explain the petrography of the lavas; this hypothesis is in accordance with geophysical data

On the relationship between dike width and magma viscosity

Abstract: On the basis of measurements of bulk rock and phenocryst chemical compositions, phenocryst content, and estimation of water content and magma temperature, the magma viscosity of 44 dikes from Oki-Dozen and Tango, southwest Japan, and Ocros, Peruvian Andes, was estimated to examine a relationship between viscosity and dike width. The results show that magma viscosity increases with dike width: mafic magmas with viscosities of 101–102 Pa s form dikes l m wide, while felsic magmas with viscosities of 106–107 Pa s form dikes 100 m wide. This indicates that magma viscosity as well as magma driving pressure and host rock stiffness are important factors in explaining the variation of dike width. Because magma viscosity is a measure of internal friction, the magmatic force required to widen the fracture during hydrofracturing is proportional to viscosity. On the basis of the range of chemical composition of the studied samples, dike widths formed by highly viscous felsic magma should be of the order of 100 m maximum. Furthermore, the observed basaltic dike width variations from various tectonic settings indicate that the effective factor on driving pressure of dike magma may be not magma pressure but compressive stress. On the other hand, dikes derived from low-viscosity basaltic magma and exceeding 100 m width, may be formed by extremely high magma pressure associated with flood basalt volcanism. This is supported by (1) the observation that a rifting event with decreased compressive stress was not always associated with the volcanism and (2) the inferred effusion rates for this type of volcanism are anomalously high, suggesting increased magma pressure. In addition, the active magma generation and the decreased host rock stiffness by locally anomalous thermal conditions like a hotspot phenomenon may assist flood basalt volcanism or emplacement of anomalously wide mafic dikes.

Petrology of impact-melt rocks at the Chicxulub multiring basin, Yucatán, Mexico

Abstract: Compositions and textures of melt rocks from the upper part of the Chicxulub structure are typical of melt rocks at other large terrestrial impact structures. Apart from variably elevated iridium concentrations (less than 1.5 to 13.5 +/- 0.9 ppb) indicating nonuniform dissemination of a meteoritic component, bulk rock and phenocryst compositions imply that these melt rocks were derived exclusively from continental crust and platform-sediment target lithologies. Modest differences in bulk chemistry among samples from wells located approximately 40 km apart suggest minor variations in relative contributions of these target lithologies to the melts. Subtle variations in the compositions of early-formed pyroxene and plagioclase also support minor primary differences in chemistry between the melts. Evidence for pervasive hydrothermal alteration of the porous mesostasis includes albite, K-feldspar, quartz, epidote, chlorite, and other phyllosilicates, as well as siderophile element-enriched sulfides, suggesting the possibility that Chicxulub, like Sudbury, may host important ore deposits.

32. experimental phase relations of basaltic andesite from hole 839b under hydrous and anhydrous conditions1

Abstract: Experimental phase relations were used to assess the role of volatiles and crustal level fractional crystallization in the petrogenesis of lavas from Hole 839B in the central Lau Basin. Melting experiments were performed on Sample 135-839B-15R-2, 63-67 cm, at 1 atm, anhydrous, and 2 kbar, H2O-saturated (~6 wt% H2O in the melt) to determine the influence of variable pressure and H2O content on phase appearances, mineral chemistry, and liquid line of descent followed during crystallization. The effects of H2O are to depress the liquidus by ~100°C, and to suppress crystallization of Plagioclase and orthopyroxene relative to olivine and high-Ca clinopyroxene. At 1 atm, anhydrous, olivine and Plagioclase coexist near the liquidus, whereas orthopyroxene and then clinopyroxene appear with decreasing temperature. Crystallization of 50 wt% produces a residual liquid that is rich in FeO* (10.8 wt%) and poor in A12O3 (13.6 wt%). At 2 kbar, H7O-saturated, the liquidus phases are olivine and chromian spinel, with high-Ca clinopyroxene appearing after -10% crystallization. Plagioclase saturation is suppressed until -20% crystallization has occurred. The residual liquid from 35 wt% crystallization is rich in AI2O3 (17.4 wt%), and poor in MgO (4.82 wt%); it contains moderate FeO* (8.2 wt%), and resembles the low-MgO andesites recovered from Hole 839B. On the basis of these experiments we conclude that the primitive lavas recovered from Hole 839B have experienced crystallization along the 01 + Cpx saturation boundary, under hydrous conditions (an ankaramitic liquid line of descent), and variable amounts of olivine and chromian spinel accumulation. The low-MgO andesites from Hole 839B are the products of hydrous fractional crystallization, at crustal pressures, of a parent magma similar to basaltic andesite Sample 135-839B- 15R-2, 63-67 cm.

Nd, Pb and Sr isotopic data from the Napak carbonatite-nephelinite centre, eastern Uganda: an example of open-system crystal fractionation

Abstract: Nd, Pb and Sr isotopic data from nephelinite lavas from the Tertiary nephelinite-carbonatite complex of Napak, eastern Uganda, show large isotopic variations that can only be attributed to open-system behaviour. Possible explanations of the data include mixing between nephelinitic melts derived from an isotopically heterogeneous mantle, or interaction between a HIMU melt and mafic granulites. In both models crystal fractionation, involving olivine and clinopyroxene, played an important role. Major element chemistry, textural evidence and isotopic data from clinopyroxene phenocrysts from the olivine-bearing nephelinites, suggest that the pyroxenes did not crystallize from their host liquids. The isotopic data from the clinopyroxene phenocrysts support an interpretation of crystal fractionation in an open magma system that was undergoing continuous isotopic change. This study emphasises the importance of using combined isotopic data from both whole rock and mineral phases to interpret the evolutionary history of a single eruptive centre.

Extreme halogen abundances in tin-rich magma of the Taylor Creek Rhyolite, New Mexico

Abstract: Crystal-bearing glass inclusions are hosted by quartz phenocrysts in eruptive units of the tin-mineralized Taylor Creek Rhyolite, New Mexico. Inclusions from four units were refused at 2 to 4 kbars and temperatures > or = 890 degrees C and analyzed for major and minor elements by electron microprobe and for H, Li, Be, B, Rb, St, Y, Nb, Mo, Sn, Cs, Ce, Th, and U by ion microprobe. Average glass inclusion compositions are chemically representative of Taylor Creek melt, and comparison of inclusion compositions with their whole-rock equivalents suggests that Li, F, Cl, Ca, and H 2 O degassed from cooling lava and/or magma after inclusion entrapment.Taylor Creek magma was strongly and variably enriched in F and Cl prior to eruption; average F concentrations of melt apparently ranged from 0.15 to 3.9 wt percent. Eruptive units containing the most F-enriched glass inclusions are characterized by comparatively high average molar (Na 2 O/Na 2 O + K 2 O) ratios, low Li concentrations, and relatively low phenocryst contents which are compatible with high preeruptive concentrations of F in magma. Average C1 concentrations were more uniform, ranging from 0.23 to 0.37 wt percent. High concentrations of F and Cl are in accord with the style and extent of alteration in vapor-phase altered parts of Taylor Creek Rhyolite and are consistent with alkali chloride-rich fluid inclusions in vapor-phase minerals of altered rhyolites. Preeruptive enrichments in other volatiles were not as significant; B in melt was <100 ppm and the apparent H 2 O content of melt was <2 wt percent.Geochemical studies of other tin-topaz rhyolites and molybdenite-bearing, Climax-type, porphyritic granites imply that their magmatic F concentrations were highly variable and that some fractions of magma were strongly enriched in halogens. We suggest that such high levels of halogen enrichment may be generally representative of other highly evolved granitic magmas genetically associated with lithophile mineralization.