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1.
To investigate the origin of compositional zonation in the Bishop Tuff magma body, we have analyzed trace elements in the matrix glass of pumice clasts and in quartz-hosted melt inclusions. Our results show contrasting patterns for quartz in different parts of the Bishop Tuff. In all samples from the early part of the eruption, trace element compositions of matrix glasses are similar to but slightly more evolved than quartz-hosted melt inclusions. This indicates a cogenetic relationship between quartz crystals and their surrounding matrix glass, consistent with in situ crystallization. The range of incompatible element concentrations in melt inclusions and matrix glass from single pumice clasts requires 16–20 wt% in situ crystallization. This is greater than the actual crystal content of the pumices (<15 % crystals). In contrast to the pattern for the early pumices, pyroclastic flow samples from the middle part of the eruption show contrasting trends: In some clasts, the matrix is more evolved than the inclusions, whereas in other clasts, the matrix is less evolved. In the late Bishop Tuff, all crystal-rich samples have matrix glasses that are less evolved than the melt inclusions. Trace element abundances indicate that the cores of quartz in the late Bishop Tuff crystallized from more differentiated rhyolitic magma that was similar in many ways, yet distinct from the early-erupted Bishop Tuff. Our results are compatible with a model of secular incremental zoning (Hildreth and Wilson in Compositional zoning of the Bishop Tuff. J Petrol 48(5):951–999, 2007), in which melt batches from underlying crystal mush rise to various levels in a growing magma body according to their buoyancy. Early- and middle-erupted quartz crystallized from highly evolved rhyolitic melt, but then some parts of the middle-erupted magma were invaded by less differentiated rhyolite such that the matrix melt at the time of eruption was less evolved than the melt inclusions. A similar process occurred but to a greater extent in magma that erupted to form the late Bishop Tuff. In addition, there was a final, major magma mixing event in the late magma that formed Ti-rich rims on quartz and Ba-rich rims on sanidine, trapped less evolved rhyolitic melt inclusions, and resulted in dark and swirly crystal-poor pumice that is a rare type throughout much of the Bishop Tuff.  相似文献   

2.
Constraining the pressure of crystallization of magmas is an important but elusive task. In this work, we present a method to derive crystallization pressures for rocks that preserve glass compositions (either glass inclusions or matrix glass) representative of equilibration between melt, quartz, and 1 or 2 feldspars. The method relies on the well-known shift of the quartz–feldspar saturation surface toward higher normative quartz melt compositions with decreasing pressure. The critical realization for development of the method is the fact that melt, quartz and feldspars need to be in equilibrium at the liquidus for the melt composition. The method thus consists of calculating the saturation surfaces for quartz and feldspars using rhyolite-MELTS over a range of pressures, and searching for the pressure at which the expected assemblage (quartz+1 feldspar or quartz+2 feldspars) is found at the liquidus. We evaluate errors resulting from uncertainties in glass composition using a series of Monte Carlo simulations for a quartz-hosted glass inclusion composition from the Bishop Tuff, which reveal errors on the order of 20–45 MPa for the quartz+2 feldspars constraint and on the order of 25–100 MPa for the quartz+1 feldspar constraint; we suggest actual errors are closer to the lower bounds of these ranges. We investigate the effect of fluid saturation in two ways: (1) By applying our procedure over a range of water contents for three glass compositions; we show that the effect of fluid saturation is more important at higher pressures (~300 MPa) than at lower pressures (~100 MPa), but reasonable pressure estimates can be derived irrespective of fluid saturation for geologically relevant H2O concentrations >3 wt% and (2) by performing the same type of pressure determinations with a preliminary version of rhyolite-MELTS that includes a H2O–CO2 mixed fluid phase; we use a range of H2O and CO2 concentrations for two compositions characteristic of early-erupted and late-erupted Bishop Tuff glass inclusions and demonstrate that calculated pressures are largely independent of CO2 concentration (for CO2 <1,000 ppm), at least for relatively high H2O contents, as expected in most natural magmas, such that CO2 concentration can be effectively neglected for application of our method. Finally, we demonstrate that pressures calculated using the rhyolite-MELTS geobarometer compare well with those resulting from H2O–CO2 glass inclusion barometry and Al-in-hornblende barometry for an array of natural systems for which data have been compiled from the literature; the agreement is best for quartz-hosted glass inclusions, while matrix glass yields systematically lower rhyolite-MELTS pressures, suggestive of melt evolution during eruptive decompression.  相似文献   

3.
The evolution of large bodies of silicic magma is an importantaspect of planetary differentiation. Melt and mineral inclusionsin phenocrysts and zoned phenocrysts can help reveal the processesof differentiation such as magma mixing and crystal settling,because they record a history of changing environmental conditions.Similar major element compositions and unusually low concentrationsof compatible elements (e.g. 0·45–4·6 ppmBa) in early-erupted melt inclusions, matrix glasses and bulkpumice from the Bishop Tuff, California, USA, suggest eutectoidfractional crystallization. On the other hand, late-eruptedsanidine phenocrysts have rims rich in Ba, and late-eruptedquartz phenocrysts have CO2-rich melt inclusions closest tocrystal rims. Both features are the reverse of in situ crystallizationdifferentiation, and they might be explained by magma mixingor crystal sinking. Log(Ba/Rb) correlates linearly with log(Sr/Rb)in melt inclusions, and this is inconsistent with magma mixing.Melt inclusion gas-saturation pressure increases with CO2 fromphenocryst core to rim and suggests crystal sinking. Some inclusionsof magnetite in late-erupted quartz are similar to early-eruptedmagnetite phenocrysts, and this too is consistent with crystalsinking. We argue that some large phenocrysts of late-eruptedquartz and sanidine continued to crystallize as they sank severalkilometers through progressively less differentiated melts.Probable diffusive modification of Sr in sanidine phenocrystsand the duration of crystal sinking are consistent with an evolutionaryinterval of some 100 ky or more. Crystal sinking enhanced thedegree of differentiation of the early-erupted magma and pointsto the importance of H2O (to diminish viscosity and enhancethe rate of crystal sinking) in the evolution of silicic magmas. KEY WORDS: crystal settling; differentiation; melt inclusions; rhyolite; trace elements  相似文献   

4.
Melt inclusions in quartz phenocrysts from a single clast of pumice near the base of the plinian pumice fall of the Bishop Tuff were studied to test ideas concerning separation of melt and crystals in silicic magmas. Ten analyzed inclusions from the pumice clast are of high silica rhyolite composition with very low contents of the highly compatible elements Ba, Sr, and Eu, consistent with extensive fractionation. The concentrations of U, La, Ce, Mg, and Ca of these ten melt inclusions vary considerably as determined by ion microprobe. Petrologic considerations indicate that uranium is an incompatible element with a maximum bulk partition coefficient D of about 0.2 and that the evolution of the uranium content of the melt was controlled by crystallization of the magma. A minimum of 33 wt% perfect fractional crystallization is required to explain the observed range in uranium. However, only 17 wt% crystals occurred in the pumice clast. The greater calculated fraction of crystals requires significant separation of crystals and melt before the eruption of the plinian pumice fall in spite of the fact that crystal mixing (settling, etc.) did not occur in the Bishop magma.  相似文献   

5.
Thermodynamic models are vital tools to evaluate magma crystallization and storage conditions. Before their results can be used independently, however, they must be verified with controlled experimental data. Here, we use a set of hydrothermal experiments on the Late-erupted Bishop Tuff (LBT) magma to evaluate the rhyolite-MELTS thermodynamic model, a modified calibration of the original MELTS model optimized for crystallization of silicic magmas. Experimental results that are well captured by rhyolite-MELTS include a relatively narrow temperature range separating the crystallization of the first felsic mineral and the onset of the ternary minimum (quartz plus two feldspars), and extensive crystallization over a narrow temperature range once the ternary minimum is reached. The model overestimates temperatures by ~40 °C, a known limitation of rhyolite-MELTS. At pressures below 110 MPa, model and experiments differ in the first felsic phase, suggesting that caution should be exercised when applying the model to very low pressures. Our results indicate that for quartz, sanidine, plagioclase, magnetite, and ilmenite to crystallize in equilibrium from LBT magma, magma must have been stored at ≤740 °C, even when a substantial amount of CO2 occurs in the coexisting fluid. Such temperatures are in conflict with the hotter temperatures retrieved from magnetite–ilmenite compositions (~785 °C for the sample used in the experiments). Consistent with other recent studies, we suggest that the Fe–Ti oxide phases in the Late Bishop Tuff magma body are not in equilibrium with the other minerals and thus the retrieved temperature and oxygen fugacity do not reflect pre-eruptive storage conditions.  相似文献   

6.
Large pyroclastic rhyolites are snapshots of evolving magma bodies, and preserved in their eruptive pyroclasts is a record of evolution up to the time of eruption. Here we focus on the conditions and processes in the Oruanui magma that erupted at 26.5 ka from Taupo Volcano, New Zealand. The 530 km3 (void-free) of material erupted in the Oruanui event is comparable in size to the Bishop Tuff in California, but differs in that rhyolitic pumice and glass compositions, although variable, did not change systematically with eruption order. We measured the concentrations of H2O, CO2 and major and trace elements in zoned phenocrysts and melt inclusions from individual pumice clasts covering the range from early to late erupted units. We also used cathodoluminescence imaging to infer growth histories of quartz phenocrysts. For quartz-hosted inclusions, we studied both fully enclosed melt inclusions and reentrants (connecting to host melt through a small opening). The textures and compositions of inclusions and phenocrysts reflect complex pre-eruptive processes of incomplete assimilation/partial melting, crystallization differentiation, magma mixing and gas saturation. ‘Restitic’ quartz occurs in seven of eight pumice clasts studied. Variations in dissolved H2O and CO2 in quartz-hosted melt inclusions reflect gas saturation in the Oruanui magma and crystallization depths of ∼3.5–7 km. Based on variations of dissolved H2O and CO2 in reentrants, the amount of exsolved gas at the beginning of eruption increased with depth, corresponding to decreasing density with depth. Pre-eruptive mixing of magma with varying gas content implies variations in magma bulk density that would have driven convective mixing. Electronic Supplementary Material Supplementary material is available for this article at and is accessible for authorized users.  相似文献   

7.
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.  相似文献   

8.
Peralkaline magma evolution and the tephra record in the Ethiopian Rift   总被引：3，自引：3，他引：0  
The 3.119 ± 0.010 Ma Chefe Donsa phreatomagmatic deposits on the shoulder of the Ethiopian Rift mark the northern termination of the Silti-Debre Zeyit Fault Zone, a linear zone of focused extension within the modern Ethiopian Rift. These peralkaline pumice fragments and glass shards span a wide range of glass compositions but have a restricted phenocryst assemblage dominated by unzoned sanidine. Glass shards found within the ash occupy a far more limited compositional range (75–76 wt% SiO2) in comparison with the pumice (64–75 wt% SiO2), which is rarely mingled. Thermodynamic modeling shows that liquids broadly similar to the least evolved glass composition can be achieved with 50–60 % fractionation of moderately crustally contaminated basalt. Inconsistencies between modeled solutions and the observed values of CaO and P2O5 highlight the important role of fluorine in stabilizing fluor-apatite and the limitations of current thermodynamic models largely resulting from the scarce experimental data available for the role of fluorine in igneous phase stability. On the basis of limited feldspar heterogeneity and crystal content of pumice at Chefe Donsa, and the difficulties of extracting small volumes of Si-rich melt in classical fractional crystallization models, we suggest a two-step polybaric process: (1) basaltic magma ponds at mid-upper-crustal depths and fractionates to form a crystal/magma mush. Once this mush has reached 50–60 % crystallinity, the interstitial liquid may be extracted from the rigid crystal framework. The trachytic magma extracted at this step is equivalent to the most primitive pumice analyzed at Chefe Donsa. (2) The extracted trachytic liquid will rise and continue to crystallize, generating a second mush zone from which rhyolite liquids may be extracted. Some of the compositional range observed in the Chefe Donsa deposits may result from the fresh intrusion of trachyte magma, which may also provide an eruption trigger. This model may have wider application in understanding the origin of the Daly Gap in Ethiopian magmas—intermediate liquids may not be extracted from crystal-liquid mushes due to insufficient crystallization to yield a rigid framework. The wide range of glass compositions characteristic of the proximal Chefe Donsa deposits is not recorded in temporally equivalent tephra deposits located in regional depocenters. Our results show that glass shards, which represent the material most likely transported to distal depocenters, occupy a limited compositional range at high SiO2 values and overlap some distal tephra deposits. These results suggest that distal tephra deposits may not faithfully record the potentially wide range in magma compositions present in a magmatic system just prior to eruption and that robust distal–proximal tephra correlations must include a careful analysis of the full range of materials in the proximal deposit.  相似文献   

9.
The Youngest Toba Tuff contains five distinct glass populations, identified from Ba, Sr and Y compositions, termed PI (lowest Ba) – PV (highest Ba), representing five compositionally distinct pre-eruptive magma batches that fed the eruption. The PI–PV compositions display systematic changes, with higher FeO, CaO, MgO, TiO2 and lower incompatible element concentrations in the low-SiO2 PIV/PV, than the high-SiO2 PI–PIII compositions. Glass shard abundances indicate PIV and PV were the least voluminous magma batches, and PI and PIII the most voluminous. Pressure estimates using rhyolite-MELTS indicate PV magma equilibrated at ~6 km, and PI magma at ~3.8 km. Glass population proportions in distal tephra and proximal (caldera-wall) material describe an eruption which commenced by emptying the deepest PIV and PV reservoirs, this being preferentially deposited in a narrow band across southern India (possibly due to jet-stream and/or plinian eruption transport), and as abundant pumice clasts in the lowermost proximal ignimbrites. Later, shallower magma reservoirs erupted, with PI being the most abundant as the eruption ended, sourcing the majority of distal ash from co-ignimbrite clouds (PI- and PIII-dominant), where associated ignimbrites isolated earlier (PIV- and PV-rich) deposits. This study shows how analysis of tephra glass compositional data can yield pre-eruption magma volume estimates, and enable aspects of magma storage conditions and eruption dynamics to be described.  相似文献   

10.
Volcán Popocatépetl has explosively erupted in Plinian style at least five times in the last 23,000 years. Extreme deviations in composition and the occurrence of dissolution features in plagioclase and pyroxene, and the occasional presence of xenocrysts of Cr-rich Fe–Ti oxides and Mg-rich olivines and pyroxenes indicate that magma mixing has been a major process affecting the magmatic system. The nearly invariant composition of the erupted products (andesitic–dacitic) suggests, however, that mixing is not acting alone and must be balanced by assimilation and/or crystallization. To investigate the magmatic processes that have modified the Plinian magmas, textural and compositional variations and growth rates in plagioclase were used to approximate the frequency of mixing events affecting each magma. Systematic analysis of Sr, Nd, and Pb isotopes was carried out on plagioclase, pyroxene, and pumice matrix glass to constrain the extent of assimilation of upper crustal rocks. Additionally, a series of phase equilibrium experiments were carried out to constrain the depth where such mixing and assimilation occurred. We find that magma was stored at one of two different depths beneath Popocatépetl with magma mixing acting in both reservoirs. Mixing frequency and the relative impact on mineral compositions and textures has varied with time. Assimilation of calcareous rocks underneath Popocatépetl has not been pervasive and does not contribute significantly to the evolution of the Plinian magmas. The similar compositions of magmas with diverse mixing histories suggest that fractional crystallization, and possibly assimilation of deep crust, takes place at depth and that intermediate magmas ascend into the upper crust already differentiated.  相似文献   

11.
Melt inclusions in olivine Fo83–72 from tephras of 1867, 1971 and 1992 eruptions of Cerro Negro volcano represent a series of basaltic to andesitic melts of narrow range of MgO (5.6–8 wt %) formed by ~46 wt % fractional crystallization of olivine (~6 wt %), plagioclase (~27 wt %), pyroxene (~13 wt %) and magnetite (<1 wt %) from primitive basaltic melt (average SiO2 = 49 wt %, MgO = 7.6 wt %, H2O = 6 wt %) as it ascended to the surface from the depth of about 14 km. The crystallization occurred at increasing liquidus temperature from 1,050 to 1,090 °C in the pressure range from 400 to 50 MPa and was induced by release of mixed H2O–CO2 fluid from the melt at decreasing pressure. Matrix glass compositions fall at the high-Si end of the melt inclusion trend and represent the final stage of melt crystallization during and after eruption. The bulk compositions of erupted Cerro Negro magmas (tephras and lavas) range from high- to low-MgO (3–10 wt %) basalts, which form a compositional array crossing the trend of melt inclusions so that virtually no rock from Cerro Negro has composition akin to true melt represented by the inclusions. The variations of the bulk magma (rocks) and melt (melt inclusions) compositions can be generated in a dyke connecting a deep primitive magma reservoir with the Cerro Negro edifice. While the melt inclusions represent the compositional trend of instantaneous melts along the magma pathway at decreasing pressure and H2O content, occurrence of low-Mg to high-Mg basalts reflects the process of phenocryst re-distribution in progressively evolving melt. The crystallization scenario is anticipated to operate everywhere in dykes feeding basaltic volcanoes and can explain the predominance of plagioclase-rich high-Al basalts in island arc as well as typical compositional variations of magmas during single eruptions.  相似文献   

12.
Zircon has the outstanding capacity to record chronological, thermal, and chemical information, including the storage history of zoned silicic magma reservoirs like the one responsible for the Bishop Tuff of eastern California, USA. Our novel ion microprobe approach reveals that Bishop zircon rims with diverse chemical characteristics surround intermediate domains with broadly similar compositions. The highest Y, REE, U, and Th concentrations tend to accompany the largest excesses in Y + REE3+:P beyond what can be explained by xenotime substitution in zircon. Apparent Ti-in-zircon temperatures of <720°C for zircon rims are distinctly lower than most of the range in eruption temperatures, as estimated from FeTi-oxide equilibria and zircon solubility at quench. While permissive of crystallization of zircon at near-solidus conditions, the low Ti-in-zircon temperatures are probably better explained by sources of inaccuracy in the temperature estimates. After apparently nucleating from different melts, zircons from across the Bishop Tuff compositional spectrum may have evolved to broadly similar chemical and thermal conditions and therefore it is possible that there was no significant thermal gradient in the magma reservoir at some stage in its evolution. There is also no compelling evidence for punctuated heat ± chemical influxes during the intermediate stages of zircon growth. Judging by the zircon record, the main volume of the erupted magma evolved normally by secular cooling but the latest erupted portion is characterized by a reversal in chemistry that appears to indicate perfusion of the magma reservoir by—or zircon entrainment in—a less evolved melt from the one in which the zircons had previously resided.  相似文献   

13.
Compositional Zoning of the Bishop Tuff   总被引：14，自引：0，他引：14  
Compositional data for >400 pumice clasts, organized accordingto eruptive sequence, crystal content, and texture, providenew perspectives on eruption and pre-eruptive evolution of the>600 km3 of zoned rhyolitic magma ejected as the Bishop Tuffduring formation of Long Valley caldera. Proportions and compositionsof different pumice types are given for each ignimbrite packageand for the intercalated plinian pumice-fall layers that eruptedsynchronously. Although withdrawal of the zoned magma was lesssystematic than previously realized, the overall sequence displaystrends toward greater proportions of less evolved pumice, morecrystals (0·5–24 wt %), and higher FeTi-oxide temperatures(714–818°C). No significant hiatus took place duringthe 6 day eruption of the Bishop Tuff, nearly all of which issuedfrom an integrated, zoned, unitary reservoir. Shortly beforeeruption, however, the zoned melt-dominant portion of the chamberwas invaded by batches of disparate lower-silica rhyolite magma,poorer in crystals than most of the resident magma but slightlyhotter and richer in Ba, Sr, and Ti. Interaction with residentmagma at the deepest levels tapped promoted growth of Ti-richrims on quartz, Ba-rich rims on sanidine, and entrapment ofnear-rim melt inclusions relatively enriched in Ba and CO2.Varied amounts of mingling, even in higher parts of the chamber,led to the dark gray and swirly crystal-poor pumices sparselypresent in all ash-flow packages. As shown by FeTi-oxide geothermometry,the zoned rhyolitic chamber was hottest where crystal-richest,rendering any model of solidification fronts at the walls orroof unlikely. The main compositional gradient (75–195ppm Rb; 0·8–2·2 ppm Ta; 71–154 ppmZr; 0·40–1·73% FeO*) existed in the melt,prior to crystallization of the phenocryst suite observed, whichincluded zircon as much as 100 kyr older than the eruption.The compositions of crystals, though themselves largely unzoned,generally reflect magma temperature and the bulk compositionalgradient, implying both that few crystals settled or were transportedfar and that the observed crystals contributed little to establishingthat gradient. Upward increases in aqueous gas and dissolvedwater, combined with the adiabatic gradient (for the 5 km depthrange tapped) and the roofward decline in liquidus temperatureof the zoned melt, prevented significant crystallization againstthe roof, consistent with dominance of crystal-poor magma earlyin the eruption and lack of any roof-rind fragments among theBishop ejecta, before or after onset of caldera collapse. Amodel of secular incremental zoning is advanced wherein numerousbatches of crystal-poor melt were released from a mush zone(many kilometers thick) that floored the accumulating rhyoliticmelt-rich body. Each batch rose to its own appropriate levelin the melt-buoyancy gradient, which was self-sustaining againstwholesale convective re-homogenization, while the thick mushzone below buffered it against disruption by the deeper (non-rhyolitic)recharge that augmented the mush zone and thermally sustainedthe whole magma chamber. Crystal–melt fractionation wasthe dominant zoning process, but it took place not principallyin the shallow melt-rich body but mostly in the pluton-scalemush zone before and during batchwise melt extraction. KEY WORDS: Bishop Tuff; ignimbrite; magma zonation; mush model; rhyolite  相似文献   

14.
The 3.7 ka year-old Averno 2 eruption is one of the rare eruptions to have occurred in the northwest sector of the Phlegraean Fields caldera (PFc) over the past 5 ka. We focus here on the fallout deposits of the pyroclastic succession emplaced during this eruption. We present major and trace element data on the bulk pumices, along with major and volatile element data on clinopyroxene-hosted melt inclusions, in order to assess the conditions of storage, ascent, and eruption of the feeding trachytic magma. Crystal fractionation accounts for the evolution from trachyte to alkali-trachyte magmas; these were intimately mingled (at the micrometer scale) during the climactic phase of the eruption. The Averno 2 alkali trachyte represents one of the most evolved magmas erupted within the Phlegraean Fields area and belongs to the series of differentiated trachytic magmas erupted at different locations 5 ka ago. Melt inclusions record significant variations in H2O (from 0.4 to 5 wt%), S (from 0.01 to 0.06 wt%), Cl (from 0.75 up to 1 wt%), and F (from 0.20 to >0.50 wt%) during both magma crystallization and degassing. Unlike the eruptions occurring in the central part of the PFc, deep-derived input(s) of gas and/or magma are not required to explain the composition of melt inclusions and the mineralogy of Averno 2 pumices. Compositional data on bulk pumices, glassy matrices, and melt inclusions suggest that the Averno 2 eruption mainly resulted from successive extrusions of independent magma batches probably emplaced at depths of 2–4 km along regional fractures bordering the Neapolitan Yellow Tuff caldera.  相似文献   

15.
The crustal history of volcanic rocks can be inferred from the mineralogy and compositions of their phenocrysts which record episodes of magma mixing as well as the pressures and temperatures when magmas cooled. Submarine lavas erupted on the Hilo Ridge, a rift zone directly east of Mauna Kea volcano, contain olivine, plagioclase, augite ±orthopyroxene phenocrysts. The compositions of these phenocryst phases provide constraints on the magmatic processes beneath Hawaiian rift zones. In these samples, olivine phenocrysts are normally zoned with homogeneous cores ranging from ∼ Fo81 to Fo91. In contrast, plagioclase, augite and orthopyroxene phenocrysts display more than one episode of reverse zoning. Within each sample, plagioclase, augite and orthopyroxene phenocrysts have similar zoning profiles. However, there are significant differences between samples. In three samples these phases exhibit large compositional contrasts, e.g., Mg# [100 × Mg/(Mg+Fe+2)] of augite varies from 71 in cores to 82 in rims. Some submarine lavas from the Puna Ridge (Kilauea volcano) contain phenocrysts with similar reverse zonation. The compositional variations of these phenocrysts can be explained by mixing of a multiphase (plagioclase, augite and orthopyroxene) saturated, evolved magma with more mafic magma saturated only with olivine. The differences in the compositional ranges of plagioclase, augite and orthopyroxene crystals between samples indicate that these samples were derived from isolated magma chambers which had undergone distinct fractionation and mixing histories. The samples containing plagioclase and pyroxene with small compositional variations reflect magmas that were buffered near the olivine + melt ⇒Low-Ca pyroxene + augite + plagioclase reaction point by frequent intrusions of mafic olivine-bearing magmas. Samples containing plagioclase and pyroxene phenocrysts with large compositional ranges reflect magmas that evolved beyond this reaction point when there was no replenishment with olivine-saturated magma. Two of these samples contain augite cores with Mg# of ∼71, corresponding to Mg# of 36–40 in equilibrium melts, and augite in another sample has Mg# of 63–65 which is in equilibrium with a very evolved melt with a Mg# of ∼30. Such highly evolved magmas also exist beneath the Puna Ridge of Kilauea volcano. They are rarely erupted during the shield building stage, but may commonly form in ephemeral magma pockets in the rift zones. The compositions of clinopyroxene phenocryst rims and associated glass rinds indicate that most of the samples were last equilibrated at 2–3 kbar and 1130–1160 °C. However, in one sample, augite and glass rind compositions reflect crystallization at higher pressures (4–5 kbar). This sample provides evidence for magma mixing at relatively high pressures and perhaps transport of magma from the summit conduits to the rift zone along the oceanic crust-mantle boundary. Received: 8 July 1998 / Accepted: 2 January 1999  相似文献   

16.
The Island Park-Mount Jackson series in the Yellowstone volcanic field, Wyoming (USA), is a suite of rhyolitic domes and lavas that erupted between the caldera-forming eruptions of the Mesa Fall Tuff (1.3 Ma) and the Lava Creek Tuff (0.6 Ma). Combined zircon U/Pb geochronology, Raman spectroscopy, oxygen isotopic and trace elemental compositions document storage conditions of these magmas between consecutive supereruptions. Based on comparison with co-erupted melt compositions and textural criteria, four zircon compositional groups are identified that record different stages along a continuous magmatic evolution from trace element-poor rhyolite at high temperatures to extremely fractionated rhyolite where zircon trace elements are highly enriched (e.g., >?1000 ppm U). These latter zircon domains are dark in cathodoluminescence images and show broadened Raman peaks relative to near-endmember zircon, indicating that substitution of non-stoichiometric trace elements into zircon leads to distortion of the crystal lattice. Some of these zircon domains contain inclusions of U-Th-REE-phases, likely originating from coupled dissolution–reprecipitation of metastable trace element-rich zircon in the presence of a fluid phase. Rhyolite-MELTS simulations indicate that at the conditions required to produce the observed enrichment in trace elements, a fluid phase is likely present. These findings illustrate that zircons can be assembled from a variety of co-existing magmatic environments in the same magma reservoir, including near-solidus volatile-rich melts close to the magmatic–hydrothermal transition.  相似文献   

17.
We present two-feldspar thermometry and diffusion chronometry from sanidine, orthopyroxene and quartz from multiple samples of the Bishop Tuff, California, to constrain the temperature stratification within the pre-eruptive magma body and the timescales of magma mixing prior to its evacuation. Two-feldspar thermometry yields estimates that agree well with previous Fe–Ti oxide thermometry and gives a ~80 °C temperature difference between the earlier- and later-erupted regions of the magma chamber. Using the thermometry results, we model diffusion of Ti in quartz, and Ba and Sr in sanidine as well as Fe–Mg interdiffusion in orthopyroxene to yield timescales for the formation of overgrowth rims on these crystal phases. Diffusion profiles of Ti in quartz and Fe–Mg in orthopyroxene both yield timescales of <150 years for the formation of overgrowth rims. In contrast, both Ba and Sr diffusion in sanidine yield nominal timescales 1–2 orders of magnitude longer than these two methods. The main cause for this discrepancy is inferred to be an incorrect assumption for the initial profile shape for Ba and Sr diffusion modelling (i.e. growth zoning exists). Utilising the divergent diffusion behaviour of Ba and Sr, we place constraints on the initial width of the interface and can refine our initial conditions considerably, bringing Ba and Sr data into alignment, and yielding timescales closer to 500 years, the majority of which are then within uncertainty of timescales modelled from Ti diffusion in quartz. Care must be thus taken when using Ba in sanidine geospeedometry in evolved magmatic systems where no other phases or elements are available for comparative diffusion profiling. Our diffusion modelling reveals piecemeal rejuvenation of the lower parts of the Bishop Tuff magma chamber at least 500 years prior to eruption. Timescales from our mineral profiling imply either that diffusion coefficients currently used are uncertain by 1–2 orders of magnitude, or that the minerals concerned did not experience a common history, despite being extracted from the same single pumice clasts. Introduction of the magma initiating crystallisation of the contrasting rims on sanidine, quartz, orthopyroxene and zircon was prolonged, and may be a marker of other processes that initiated the Bishop Tuff eruption rather than the trigger itself.  相似文献   

18.
The chemical compositions of melt inclusions in a primitive and an evolved basalt recovered from the mid-Atlantic ridge south of the Kane Fracture Zone (23°–24°N) are determined. The melt inclusions are primitive in composition (0.633–0.747 molar Mg/(Mg+Fe2+), 1.01–0.68 wt% TiO2) and are comparable to other proposed parental magmas except in having higher Al2O3 and lower CaO. The primitive melt inclusion compositions indicate that the most primitive magmas erupted in this region are not near primary magma compositions. Olivine and plagioclase microphenocrysts are close to exchange equilibrium with their respective basalt glasses, whose compositions are displaced toward olivine from 1 atm three phase saturation. The most primitive melt inclusion compositions are close to exchange equilibrium with the anorthitic cores of zoned plagioclases (An78.3-An83.1; the hosts for the melt inclusions in plagioclase) and with olivines more forsteritic (Fo89-Fo91) than the olivine microphenocrysts (the hosts for the melt inclusions in olivine). Xenocrystic olivine analyzed is Fo89 but contains no melt inclusions. These observations indicate that olivines have exchanged components with the melt after melt inclusion entrapment, whereas plagioclase compositions have remained the same since melt inclusion entrapment. Common denominator element ratio diagrams and oxide versus oxide variation diagrams show that the melt inclusion compositions, which represent liquids higher along the liquid line of descent, are related to the glass compositions by the fractionation of olivine, plagioclase and clinopyroxene (absent from the mincral assemblage), probably occurring at elevated pressures. A model is proposed whereby clinopyroxene segregates from the melt at elevated pressures (to account for its absence in the erupted lavas that have the chemical imprint of clinopyroxene fractionation). Zoned plagioclases in the erupted lavas are thought to be survivors of decompressional melting during magma ascent. Since similar primitive melt inclusions occur in olivine microphenocrysts and in the cores of zoned plagioclases, any model must account for all phases present.  相似文献   

19.
The Aeolian Islands are an arcuate chain of submarine seamounts and volcanic islands, lying just north of Sicily in southern Italy. The second largest of the islands, Salina, exhibits a wide range of compositional variation in its erupted products, from basaltic lavas to rhyolitic pumice. The Monte dei Porri eruptions occurred between 60 ka and 30 ka, following a period of approximately 60,000 years of repose. The bulk rock composition of the Monte dei Porri products range from basaltic-andesite scoria to andesitic pumice in the Grey Porri Tuff (GPT), with the Monte dei Porri lavas having basaltic-andesite compositions. The typical mineral assemblage of the GPT is calcic plagioclase, clinopyroxene (augite), olivine (Fo72?84) and orthopyroxene (enstatite) ± amphibole and Ti-Fe oxides. The lava units show a similar mineral assemblage, but contain lower Fo olivines (Fo57?78). The lava units also contain numerous glomerocrysts, including an unusual variety that contains quartz, K-feldspar and mica. Melt inclusions (MI) are ubiquitous in all mineral phases from all units of the Monte dei Porri eruptions; however, only data from olivine-hosted MI in the GPT are reported here. Compositions of MI in the GPT are typically basaltic (average SiO2 of 49.8 wt %) in the pumices and basaltic-andesite (average SiO2 of 55.6 wt %) in the scoriae and show a bimodal distribution in most compositional discrimination plots. The compositions of most of the MI in the scoriae overlap with bulk rock compositions of the lavas. Petrological and geochemical evidence suggest that mixing of one or more magmas and/or crustal assimilation played a role in the evolution of the Monte dei Porri magmatic system, especially the GPT. Analyses of the more evolved mineral phases are required to better constrain the evolution of the magma.  相似文献   

20.
We use comprehensive geochemical and petrological records from whole-rock samples, crystals, matrix glasses and melt inclusions to derive an integrated picture of the generation, accumulation and evacuation of 530 km3 of crystal-poor rhyolite in the 25.4 ka Oruanui supereruption (New Zealand). New data from plagioclase, orthopyroxene, amphibole, quartz, Fe–Ti oxides, matrix glasses, and plagioclase- and quartz-hosted melt inclusions, in samples spanning different phases of the eruption, are integrated with existing data to build a history of the magma system prior to and during eruption. A thermally and compositionally zoned, parental crystal-rich (mush) body was developed during two periods of intensive crystallisation, 70 and 10–15 kyr before the eruption. The mush top was quartz-bearing and as shallow as ~3.5 km deep, and the roots quartz-free and extending to >10 km depth. Less than 600 year prior to the eruption, extraction of large volumes of ~840 °C low-silica rhyolite melt with some crystal cargo (between 1 and 10%), began from this mush to form a melt-dominant (eruptible) body that eventually extended from 3.5 to 6 km depth. Crystals from all levels of the mush were entrained into the eruptible magma, as seen in mineral zonation and amphibole model pressures. Rapid translation of crystals from the mush to the eruptible magma is reflected in textural and compositional diversity in crystal cores and melt inclusion compositions, versus uniformity in the outermost rims. Prior to eruption the assembled eruptible magma body was not thermally or compositionally zoned and at temperatures of ~790 °C, reflecting rapid cooling from the ~840 °C low-silica rhyolite feedstock magma. A subordinate but significant volume (3–5 km3) of contrasting tholeiitic and calc-alkaline mafic material was co-erupted with the dominant rhyolite. These mafic clasts host crystals with compositions which demonstrate that there was some limited pre-eruptive physical interaction of mafic magmas with the mush and melt-dominant body. However, the mafic magmas do not appear to have triggered the eruption or controlled magmatic temperatures in the erupted rhyolite. Integration of textural and compositional data from all available crystal types, across all dominant and subordinate magmatic components, allow the history of the Oruanui magma body to be reconstructed over a wide range of temporal scales using multiple techniques. This history spans the tens of millennia required to grow the parental magma system (U–Th disequilibrium dating in zircon), through the centuries and decades required to assemble the eruptible magma body (textural and diffusion modelling in orthopyroxene), to the months, days, hours and minutes over which individual phases of the eruption occurred, identified through field observations tied to diffusion modelling in magnetite, olivine, quartz and feldspar. Tectonic processes, rather than any inherent characteristics of the magmatic system, were a principal factor acting to drive the rapid accumulation of magma and control its release episodically during the eruption. This work highlights the richness of information that can be gained by integrating multiple lines of petrologic evidence into a holistic timeline of field-verifiable processes.  相似文献   

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