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1.
H. Paulick C. Ewen H. Blanchard L. Zöller 《International Journal of Earth Sciences》2009,98(8):1879-1899
The Rodderberg volcanic complex (RVC) is located within the city limits of Bonn (Germany) approximately 20 km to the north
of the Quaternary East Eifel Volcanic Field (EEVF). It is the product of intense phreatomagmatic volcanism forming a 90 m
deep maar crater and strombolian eruptions. Deposit features indicate that the location of the vent(s) shifted from N to S
during the strombolian phase. The erupted leucite-nephelinite magma (on the order of ca. 1 × 10−2 km3) was largely homogenous with minor, stratigraphically controlled, variation in olivine and clinopyroxene microphenocryst
content. Stratigraphic evidence and thermoluminescence dating indicate that the RVC erupted during the glacial MIS 8 at around
300 ka. During this time, the EEVF experienced a transitional stage between two major phases of volcanic activity involving
a change in magma sources. This is consistent with the RVC geochemical data which show affinities to both the older EEVF leucite-nephelinite
association (430–380 ka) and the younger basanite association (<215 ka). In the Eifel, magma ascent through the upper crust
is apparently linked to tectonic fractures. It may be speculated that a tectonically controlled diking event channeled magma
to the north of the main EEVF and that the RVC represents an exceptional surficial expression of a significantly larger subsurface
intrusion. This scenario would be consistent with recent observations of diking-related volcanism in the East African Rift
zone and previously inferred models for magma ascent in similar intraplate volcanic fields. 相似文献
2.
The Rotoiti (~120 km3) and Earthquake Flat (~10 km3) eruptions occurred in close succession from the Okataina Volcanic Centre at ~50 ka. While accessory mineral geochronology
points to long periods of crystallization prior to eruption (104–105 years) and separate thermal histories for the magmas, little was known about the rates and processes of the final melt production
and eruption. Crystal zoning patterns in plagioclase and quartz reveal the thermal and compositional history of the magmatic
system leading up to the eruption. The dominant modal phase, plagioclase, displays considerable within-crystal zonation: An37–74, ~40–227 ppm MgO, 45–227 ppm TiO2, 416–910 ppm Sr and 168–1164 ppm Ba. Resorption horizons in the crystals are marked by sharp increases (10–30%) in Sr, MgO
and XAn that reflect changes in melt composition and are consistent with open system processes. Melt inclusions display further evidence
for open system behaviour, some are depleted in Sr and Ba relative to accompanying matrix glass not consistent with crystallization
of modal assemblage. MI also display a wide range in XH2O that is consistent with volatile fluxing. Quartz CL images reveal zoning that is truncated by resorption, and accompanied
by abrupt increases in Ti concentration (30–80 ppm) that reflect temperature increases ~50–110°C. Diffusion across these resorption
horizons is restricted to zones of <20 μm, suggesting most crystallization within the magma occurred in <2000 years. These
episodes are brief compared to the longevity (104–105 year) of the crystal mush zones. All textural and compositional features observed within the quartz and plagioclase crystals
are best explained by periodic mafic intrusions repeatedly melting parts of a crystal-rich zone and recharging the system
with silicic melt. These periodic influxes of silicic melt would have accumulated to form the large volume of magma that fed
the caldera-forming Rotoiti eruption. 相似文献
3.
The bulk (post-eruptive) wt% FeO concentration in each of 11 phenocryst-poor (<5%) andesite and dacite (60–69 wt% SiO2) lavas from different monogenetic vents in the Mexican arc has been measured by titration, in duplicate. The results match,
within analytical error, the wt% FeO content of the magmas during phenocryst growth (pre-euptive), which were calculated on
the basis of oxygen fugacity and temperature results from Fe–Ti two-oxide oxygen barometry. The average deviation between
the pre- and post-eruptive FeO concentrations is ±0.15 wt%. Application of the plagioclase-liquid hygrometer shows that at
the time of phenocryst growth, these 11 magmas contained from ~3–8 wt% H2O, which was extensively degassed upon eruption. There is no evidence that degassing of ≤8 wt% H2O changed the oxidation state of these magmas. Calculations of pre-eruptive and post-eruptive oxygen fugacity values relative
to the Ni-NiO buffer (in terms of log10 units) for the 11 samples span a similar range; pre-eruptive ∆NNO = −0.9 to +0.7 and post-eruptive ∆NNO = −0.4 to +0.8. The
data further show that extensive groundmass (closed-system) crystallization had no affect on bulk Fe3+/Fe2+ ratios. Finally, there is no systematic variation in the range of pre-eruptive Fe3+/FeT values of the samples as a function of SiO2 concentration (i.e., differentiation). Therefore, the results of this study indicate that the elevated Fe3+/FeT ratios of arc andesites and dacites, compared with magmas erupted in other tectonic settings, cannot be attributed to the
effects of (1) degassing of H2O, (2) closed-system crystallization, and/or (3) differentiation effects, but instead must be inherited from their parental
source rocks (i.e., mantle-derived arc basalts). 相似文献
4.
Sarah E. Gelman Chad D. Deering Francisco J. Gutierrez Olivier Bachmann 《Contributions to Mineralogy and Petrology》2013,166(5):1355-1374
The 20 ka ~0.1 km3 Omega dacite, which erupted shortly after the 26.5 ka Oruanui super-eruption, compositionally stands out among Taupo Volcanic Zone (TVZ) magmas, which are overwhelmingly characterized by rhyolites (>90 % by volume). The previously reported presence of inherited zircons in this zircon-undersaturated magma has provided unequivocal evidence for the involvement of upper-crustal material in a 1–10 year timescale prior to the Omega eruption. However, whether this crustal involvement is characterized by wholesale, melting of preexisting crust or subordinate bulk assimilation into an already differentiated magma body remains unclear. To disentangle these processes, we describe the mineral chemistry of the major phases present in the Omega dacite and determine intensive parameters describing magma chamber conditions. Dominantly unimodal populations of plagioclase (An50–60), orthopyroxene (Mg# from 58 to 68), and clinopyroxene (Mg# from 65 to 73), along with coexisting equilibrium pairs of Fe–Ti oxides, constrain pre-eruptive temperatures to 850–950 °C, a pressure between ~3 and 7 kbars, and an oxygen fugacity of ~NNO. MELTS thermodynamic modeling suggests that this phase assemblage is in equilibrium with the bulk rock and glass compositions of the Omega dacite at these estimated P–T–fO2 pre-eruptive conditions. Combining these petrological observations with insights into conductive thermal models of magma–crust interactions, we argue that the Omega dacite more likely formed in the mid-to-lower crust via protracted processing through fractional crystallization coupled with some assimilation (AFC). Incorporation of crustal material is likely to have occurred at various stages, with the inherited zircons (and potentially parts of glomerocrysts) representing late and subordinate upper-crustal assimilants. This petrogenetic model is consistent with the presence of a differentiating crustal column, consisting of a polybaric fractional crystallization and assimilation history. On the basis of petrological, thermal, and geophysical considerations, upper-crustal reservoirs, which feed large-scale rhyolitic volcanism in the TVZ, most likely take the form of large, long-lived crystal mush zones. Following large eruptions, such as the Oruanui event, this mush is expected to crystallize significantly (up to 70–80 vol% crystals) due to syn-eruptive decompression. Hence, the Omega dacite, immediately post-dating the Oruanui event, potentially represents incoming deeper recharge of less-evolved magma that was able to penetrate the nearly solidified upper-crustal mush. Over the past 20,000 years, similar intermediate recharge magmas have incrementally reheated, reconstructed, and reactivated the upper-crustal mush zone, allowing a gradual return to rhyolitic volcanism at the Taupo Volcanic Center. 相似文献
5.
R. J. Brown L. Civetta I. Arienzo M. D’Antonio R. Moretti G. Orsi E. L. Tomlinson P. G. Albert M. A. Menzies 《Contributions to Mineralogy and Petrology》2014,168(3):1-23
New geochemical and isotopic data on volcanic rocks spanning the period ~75–50 ka BP on Ischia volcano, Italy, shed light on the evolution of the magmatic system before and after the catastrophic, caldera-forming Monte Epomeo Green Tuff (MEGT) eruption. Volcanic activity during this period was influenced by a large, composite and differentiating magmatic system, replenished several times with isotopically distinct magmas of deep provenance. Chemical and isotopic variations highlight that the pre-MEGT eruptions were fed by trachytic/phonolitic magmas from an isotopically zoned reservoir that were poorly enriched in radiogenic Sr and became progressively less radiogenic with time. Just prior to the MEGT eruption, the magmatic system was recharged by an isotopically distinct magma, relatively more enriched in radiogenic Sr with respect to the previously erupted magmas. This second magma initially fed several SubPlinian explosive eruptions and later supplied the climactic, phonolitic-to-trachytic MEGT eruption(s). Isotopic data, together with erupted volume estimations obtained for MEGT eruption(s), indicate that >5–10 km3 of this relatively enriched magma had accumulated in the Ischia plumbing system. Geochemical modelling indicates that it accumulated at shallow depths (4–6 km), over a period of ca. 20 ka. After the MEGT eruption, volcanic activity was fed by a new batch of less differentiated (trachyte-latite) magma that was slightly less enriched in radiogenic Sr. The geochemical and Sr–Nd-isotopic variations through time reflect the upward flux of isotopically distinct magma batches, variably contaminated by Hercynian crust at 8–12 km depth. The deep-sourced latitic to trachytic magmas stalled at shallow depths (4–6 km depth), differentiated to phonolite through crystal fractionation and assimilation of a feldspar-rich mush, or ascended directly to the surface and erupted. 相似文献
6.
Kathryn E. Watts Ilya N. Bindeman Axel K. Schmitt 《Contributions to Mineralogy and Petrology》2012,164(1):45-67
A voluminous (>600 km3) and long-lived (~520–75 ka) phase of rhyolitic eruptions followed collapse of the Yellowstone caldera 640 ka. Whether these eruptions represent a dying cycle, or the growth of a new magma chamber, remains an important question. We use new U–Th zircon ages and δ18O values determined by ion microprobe, and sanidine Pb isotope ratios determined by laser ablation, to investigate the genesis of voluminous post-caldera rhyolites. The oldest post-caldera rhyolites, erupted between ~520 and 470 ka, exhibit extreme age and oxygen isotopic heterogeneity, requiring derivation from individual parcels of low-δ18O melts. We find a progressive increase in zircon homogeneity for rhyolite eruptions from ~260 to 75 ka, with homogeneous low-δ18O zircon values of 2.7–2.8‰ that are in equilibrium with low-δ18O host melts for the majority of the youngest eruptions. New sanidine Pb isotope data define separate arrays for post-caldera rhyolites and the caldera-forming tuffs that preceded them, indicating that they were not sourced from a mushy Lava Creek Tuff batholith that remained after caldera collapse. Rather, our new age and isotopic data indicate that the post-caldera rhyolites were generated by remelting of a variety of intracaldera source rocks, consisting of pre-Lava Creek Tuff volcanic and plutonic rocks and earlier erupted post-Lava Creek Tuff rhyolites. Batch assembly of low-δ18O melts starting at ~260 ka resulted in progressive homogenization, followed by differentiation and cooling up until the last rhyolite eruption ~75 ka, a trend that we interpret to be characteristic of a dying magma reservoir beneath the Yellowstone caldera. 相似文献
7.
The behavior of chlorine and sulfur during differentiation of the Mt. Somma-Vesuvius magmatic system 总被引:4,自引:0,他引:4
Summary Reheated silicate melt inclusions in volcanic rock samples from Mt. Somma-Vesuvius, Italy, have been analyzed for 29 constituents
including H2O, S, Cl, F, B, and P2O5. This composite volcano consists of the older Mt. Somma caldera, formed between 14 and 3.55 ka before present, and the younger
Vesuvius cone. The melt inclusion compositions provide important constraints on pre-eruptive magma geochemistry, identify
relationships that relate to eruption behavior and magma evolution, and provide extensive evidence for magmatic fluid exsolution
well before eruption.
The melt inclusion data have been categorized by groups that reflect magma compositions, age, and style of eruptions. The
data show distinct differences in composition for eruptive products older than 14.0 ka (pre-caldera rocks) versus eruptive
products younger than 3.55 ka. Moreover, pre-caldera eruptions were associated with magmas relatively enriched in SiO2, whereas eruptions younger than 3.55 ka (i.e., the syn- and post-caldera magmas which generated the Somma caldera and the
Vesuvius cone) were derived from magmas comparatively enriched in S, Cl, CaO, MgO, P2O5, F, and many lithophile trace elements.
Melt inclusion data indicate that eruptive behavior at Vesuvius correlates with pre-eruptive volatile enrichments. Most magmas
associated with explosive plinian and subplinian events younger than 3.55 ka contained more H2O, contained significantly more S, and exhibited higher (S/Cl) ratios than syn- and post-caldera magmas which erupted during
relatively passive interplinian volcanic phenomena.
Received January 10, 2000 Revised version accepted July 17, 2000 相似文献
8.
Lucia Pappalardo Luisa Ottolini Giuseppe Mastrolorenzo 《Contributions to Mineralogy and Petrology》2008,156(1):1-26
More than ca 100 km3 of nearly homogeneous crystal-poor phonolite and ca 100 km3 of slightly zoned trachyte were erupted 39 ka during the Campanian Ignimbrite super eruption, the most powerful in the Neapolitan
area. Partition coefficient calculations, equilibrium mineral assemblages, glass compositions and texture were used to reconstruct
compositional, thermal and pressure gradients in the pre-eruptive reservoir as well as timing and mechanisms of evolution
towards magma chamber overpressure and eruption. Our petrologic data indicate that a wide sill-like trachytic magma chamber
was active under the Campanian Plain at 2.5 kbar before CI eruption. Thermal exchange between high liquidus (1199°C) trachytic
sill and cool country rocks caused intense undercooling, driving a catastrophic and fast (102 years) in situ fractional crystallization and crustal assimilation that produced a water oversaturated phonolitic cap and
an overpressure in the chamber that triggered the super eruption. This process culminated in an abrupt reservoir opening and
in a fast single-step high decompression. Sanidine phenocrysts crystal size distributions reveal high differentiation rate,
thus suggesting that such a sill-like magmatic system is capable of evolving in a very short time and erupting suddenly with
only short-term warning. 相似文献
9.
Silicic recharge of multiple rhyolite magmas by basaltic intrusion during the 22.6 ka Okareka Eruption Episode, New Zealand 总被引:1,自引:0,他引:1
Phil Shane Ian A. Nairn Victoria C. Smith Miles Darragh Kate Beggs Jim W. Cole 《Lithos》2008,103(3-4):527-549
Deposits of the 22.6 ka Okareka Eruption Episode from Tarawera Volcanic Complex record the sequential and simultaneous eruption of three discrete rhyolite magmas following a silicic recharge event related to basaltic intrusion. The episode started with basaltic eruption ( 0.01 km3 magma), and rapidly changed to a plinian eruption involving a moderate temperature (750 °C), cummingtonite-bearing rhyolite magma (T1) with a volume of 0.3 km3. Hybrid basalt/rhyolite clasts demonstrate direct basaltic intrusion that helped trigger the eruption. Crystals, shards and lapilli of two other rhyolite magmas then joined the eruption sequence. They comprise a cooler (720 °C) crystal-rich biotite–hornblende rhyolite magma (T2) ( 0.3 km3), and a hotter (780 °C), crystal-poor, pyroxene–hornblende rhyolite magma (T3) ( 4.5 km3). All mid to late-stage ash units contain various mixtures of T1, T2 and T3 components with a general increase in abundance of T3 and rapid decline of T1 with time. About 4 km3 of T3 magma was extruded as lavas at the end of the episode. Contrasts in melt composition, crystal and volatile contents, and temperatures influenced viscosity and miscibility, and thus limited pre-eruption mixing of the rhyolite magmas. The eruption sequence and the restricted direct basaltic intrusion into only one magma (T1) is consistent with the rhyolites occupying separate melt pods within a large crystal-mush zone. Melt–crystal equilibria and volatile contents in melt inclusions indicate temporary magma storage depths of < 8 km. Each of the magmas display quartz crystals containing melt inclusions that are compositionally highly evolved relative to the accompanying matrix glass, and thus point to a stage of more complete crystallisation. The matrix glass, enriched in Sr and Ti, represents a re-melting event of underlying the crystal pile induced by basaltic intrusion, presumably part of the same event that erupted scoria at the start of the eruption. This recharge rhyolite melt percolated upward and hybridised with the resident melts in each of the three magma pods. The Okareka episode rhyolites contrast with other well-documented rhyolites that are either continuously or discontinuously zoned, or have been homogenised during re-activation to a uniform composition. Rapid basalt dike intrusion to shallow levels appears to have (prematurely?) triggered the Okareka rhyolites into eruption, so that their early ponding in separate melt pods has been recorded before it could be masked by mixing or stratification had amalgamation into a larger body occurred. 相似文献
10.
Evolution of silicic magmas in the Kos-Nisyros volcanic center,Greece: a petrological cycle associated with caldera collapse 总被引:2,自引:1,他引:1
Multiple eruptions of silicic magma (dacite and rhyolites) occurred over the last ~3 My in the Kos-Nisyros volcanic center
(eastern Aegean sea). During this period, magmas have changed from hornblende-biotite-rich units with low eruption temperatures
(≤750–800°C; Kefalos and Kos dacites and rhyolites) to hotter, pyroxene-bearing units (>800–850°C; Nisyros rhyodacites) and
are transitioning back to cooler magmas (Yali rhyolites). New whole-rock compositions, mineral chemistry, and zircon Hf isotopes
show that these three types of silicic magmas followed the same differentiation trend: they all evolved by crystal fractionation
and minor crustal assimilation (AFC) from parents with intermediate compositions characterized by high Sr/Y and low Nb content,
following a wet, high oxygen fugacity liquid line of descent typical of subduction zones. As the transition between the Kos-Kefalos
and Nisyros-type magmas occurred immediately and abruptly after the major caldera collapse in the area (the 161 ka Kos Plateau
Tuff; KPT), we suggest that the efficient emptying of the magma chamber during the KPT drew out most of the eruptible, volatile-charged
magma and partly solidified the unerupted mush zone in the upper crust due to rapid unloading, decompression, and coincident
crystallization. Subsequently, the system reestablished a shallow silicic production zone from more mafic parents, recharged
from the mid to lower crust. The first silicic eruptions evolving from these parents after the caldera collapse (Nisyros units)
were hotter (up to >100°C) than the caldera-forming event and erupted from reservoirs characterized by different mineral proportions
(more plagioclase and less amphibole). We interpret such a change as a reflection of slightly drier conditions in the magmatic
column after the caldera collapse due to the decompression event. With time, the upper crustal intermediate mush progressively
transitioned into the cold-wet state that prevailed during the Kefalos-Kos stage. The recent eruptions of the high-SiO2 rhyolite on Yali Island, which are low temperature and hydrous phases (sanidine, quartz, biotite), suggest that another large,
potentially explosive magma chamber is presently building under the Kos-Nisyros volcanic center. 相似文献
11.
The Occurrence of Forsterite and Highly Oxidizing Conditions in Basaltic Lavas from Stromboli Volcano, Italy 总被引:1,自引:1,他引:0
CORTES JOAQUIN A.; WILSON MARJORIE; CONDLIFFE ERIC; FRANCALANCI LORELLA 《Journal of Petrology》2006,47(7):1345-1373
We report the occurrence of unusual, high-magnesium (Fo96) olivinephenocrysts in a basaltic lava and an ejected lithic block fromthe Upper Vancori period (13 ka) and the recent activity (20022003)of Stromboli volcano, Italy. The samples that contain this distinctivemineral chemistry are a shoshonitic basalt and a basaltic andesitewith anomalous bulk-rock chemical characteristics in which theiron is highly oxidized (68 wt % Fe2O3 and <1 wt %FeO). In other respects these samples are similar to the majorityof Stromboli basalts, characterized by the coexistence of olivine,clinopyroxene, plagioclase and FeTi oxides as phenocrysts,and clinopyroxene, plagioclase and FeTi oxides in thegroundmass. In the high-magnesium olivine samples, FeTioxides (pseudobrookite) typically occur as symplectitic intergrowthswith the olivine phenocrysts, indicating simultaneous growthof the two phases. We propose, as a paragenetic model, thatthe Fo96 olivine phenocrysts crystallized from a highly oxidizedbasaltic magma in which most of the iron was in the ferric state;hence, only magnesium was available to form olivine. The highlyoxidized state of the magma reflects sudden degassing of volatilephases associated with instantaneous, irreversible, transientdegassing of the magma chamber; this is postulated to occurduring periods of sudden decompression induced by fracturingof the volcanic edifice associated with paroxysmic activityand edifice collapse. KEY WORDS: Stromboli; Mg-rich olivine; oxygen fugacity; redox state of magmas; irreversible processes 相似文献
12.
B. L. A. Charlier C. J. N. Wilson J. P. Davidson 《Contributions to Mineralogy and Petrology》2008,156(6):799-813
Rhyolitic pumices in the 26.5 ka Oruanui eruption (Taupo volcano, New Zealand) contain an average of 10 wt% crystals. About
2 wt% of the crystal population is feldspar crystals that display bluish–grey cloudy cores, the colour being imparted by exsolved
needles of rutile. The volume of cloudy-cored feldspars thus amounts to ~1.0 km3 in a total magma volume of ~530 km3. The cored feldspars show great variability in detail, but in general have a rounded cloudy core bounded by a zone rich in
glass and mineral inclusions, that was then overgrown by a euhedral clear rim. Sr-isotopic variations in eight representative
crystals were measured on micromilled samples of selected growth zones in the cores and rims, and linked to feldspar compositions
through microprobe traverses. The cloudy cores range from 87Sr/86Sr = 0.70547 to 0.70657, with compositions of An43 to An78. The overgrowth rims display wider variations: inner parts show extreme ranges in composition (maxima 87Sr/86Sr = 0.70764 and An78), while outer parts in seven of eight crystals are zoned, with outward-decreasing Sr-isotopic and An values to figures that
are in accord with the bulk pumice and other, clear-feldspar values, respectively. The three parts of the crystals represent
distinct regimes. The cloudy cores are inherited from an intermediate plutonic protolith that has been subjected to melting.
The inner overgrowth rims were crystallised from a high temperature, relatively radiogenic melt derived from Mesozoic-Palaeozoic
metasedimentary rocks (“greywacke”). The outer euhedral rims reflect mixing into and continued growth within the growing Oruanui
magma body. The cloudy-cored feldspars also contain rare zircon inclusions. Twenty one zircons were recovered by HF digestion
of a bulk sample of cloudy feldspars and analysed by SHRIMP for U–Th isotopes with which to calculate model ages. Eighteen
of 21 crystals returned finite ages, the model-age spectrum of which is similar to the age spectra from free zircons in Oruanui
pumices. Assembly of the Oruanui magma body was not only rapid (over ~40 kyr, as shown by other data) but involved a wide
open system, with significant contributions from partly-melted intermediate-composition igneous intrusions (cloudy cores)
and greywacke melts (inner overgrowths) being introduced into the magma body up to the point of eruption. Such open system
behaviour contrasts with that proposed in models for comparably voluminous silicic magmas derived dominantly by fractionation
(such as the Bishop Tuff) where the magma and its crystal cargo were better insulated thermally and chemically from country-rock
interaction. 相似文献
13.
The Central Plateau Member rhyolites have been erupted between 173 and 70 ka and are the youngest Yellowstone intracaldera
rhyolites. They mostly comprise very voluminous lava flows totaling ~600 km3 in volume. Their eruptive vents define two NNW-trending lineaments which are aligned with regional faults. We present new
whole rock, glass, and mineral analyses and propose a petrogenetic and volcano-tectonic model for these rhyolites. At a caldera-wide
scale, there is a temporal enrichment in elements such as Nb, Y and HREE, and a depletion in Sr, Ba, and Ce/Yb. Simultaneously,
clinopyroxene becomes less magnesian while Ti contents in quartz decrease. By contrast, quartz in all rhyolites is rounded
and bears long glass re-entrants, suggesting heating. Based on these data and observations, we propose that the Central Plateau
Member rhyolites have been generated as follows. A hydrothermally altered low-δ18O rhyolitic protolith beneath the Mallard Lake Resurgent Dome in the southwestern part of the caldera started to melt at ~250 ka.
Repeated heating pulses caused the melting front to expand radially, and a large crystal mush formed beneath much of the caldera.
The mush was able to differentiate but not erupt due to its high crystallinity and viscosity. Further inputs of heat and silicic
magma in this mush increased the degree of melting, forming crystal-poor magma batches which erupted a few hundred to a few
thousand years later through regional faults to form the Central Plateau Member rhyolites. 相似文献
14.
Jared P. Marske Aaron J. Pietruszka Frank A. Trusdell Michael O. Garcia 《Contributions to Mineralogy and Petrology》2011,162(2):231-252
New major and trace element abundances, and Pb, Sr, and Nd isotopic ratios of Quaternary lavas from two adjacent volcanoes
(South Pagan and the Central Volcanic Region, or CVR) located on Pagan Island allow us to investigate the mantle source (i.e.,
slab components) and melting dynamics within the Mariana intra-oceanic arc. Geologic mapping reveals a pre-caldera (780–9.4 ka)
and post-caldera (<9.4 ka) eruptive stage for South Pagan, whereas the eruptive history of the older CVR is poorly constrained.
Crystal fractionation and magma mixing were important crustal processes for lavas from both volcanoes. Geochemical and isotopic
variations indicate that South Pagan and CVR lavas, and lavas from the northern volcano on the island, Mt. Pagan, originated
from compositionally distinct parental magmas due to variations in slab contributions (sediment and aqueous fluid) to the
mantle wedge and the extent of mantle partial melting. A mixing model based on Pb and Nd isotopic ratios suggests that the
average amount of sediment in the source of CVR (~2.1%) and South Pagan (~1.8%) lavas is slightly higher than Mt. Pagan (~1.4%)
lavas. These estimates span the range of sediment-poor Guguan (~1.3%) and sediment-rich Agrigan (~2.0%) lavas for the Mariana
arc. Melt modeling demonstrates that the saucer-shaped normalized rare earth element (REE) patterns observed in Pagan lavas
can arise from partial melting of a mixed source of depleted mantle and enriched sediment, and do not require amphibole interaction
or fractionation to depress the middle REE abundances of the lavas. The modeled degree of mantle partial melting for Agrigan
(2–5%), Pagan (3–7%), and Guguan (9–15%) lavas correlates with indicators of fluid addition (e.g., Ba/Th). This relationship
suggests that the fluid flux to the mantle wedge is the dominant control on the extent of partial melting beneath Mariana
arc volcanoes. A decrease in the amount of fluid addition (lower Ba/Th) and extent of melting (higher Sm/Yb), and an increase
in the sediment contribution (higher Th/Nb, La/Sm, and Pb isotopic ratios) from Mt. Pagan to South Pagan could reflect systematic
cross-arc or irregular along-arc melting variations. These observations indicate that the length scale of compositional heterogeneity
in the mantle wedge beneath Mariana arc volcanoes is small (~10 km). 相似文献
15.
Millennial timescale resolution of rhyolite magma recharge at Tarawera volcano: insights from quartz chemistry and melt inclusions 总被引:4,自引:4,他引:0
Phil Shane Victoria C. Smith Ian Nairn 《Contributions to Mineralogy and Petrology》2008,156(3):397-411
Most rhyolite eruption episodes of Tarawera volcano have emitted several physiochemically distinct magma batches (∼1–10 km3). These episodes were separated on a millennial timescale. The magma batches were relatively homogeneous in temperature and
composition at pumice scale (>4 cm), but experienced isolated crystallisation histories. At the sub-cm scale, matrix glasses
have trace element compositions (Sr, Ba, Rb) that vary by factors up to 2.5, indicating incomplete mixing of separate melts.
Some quartz-hosted melt inclusions are depleted in compatible trace elements (Sr, Ti, Ba) compared to enclosing matrix glasses.
This could reflect re-melting of felsic crystals deeper in the crystal pile. Individual quartz crystals display a variety
of cathodoluminescence brightness and Ti zoning patterns including rapid changes in melt chemistry and/or temperature (∼50–100°C),
and point to multi-cycle crystallisation histories. The Tarawera magma system consisted of a crystal-rich mass containing
waxing and waning melt pockets that were periodically recharged by silicic melts driven by basaltic intrusion.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. 相似文献
16.
The Pleistocene Incapillo Caldera and Dome Complex (5,570 m) marks the southernmost siliceous center of the Andean Central
Volcanic Zone (~28°S), where the steeply dipping (~30°) segment of the subducting Nazca plate transitions into the Chilean
“flatslab” to the south. The eruption of the Incapillo Caldera and Dome Complex began with a 3–1 Ma effusive phase characterized
by ~40 rhyodacitic dome eruptions. This effusive phase was terminated by an explosive “caldera-forming” event at 0.51 Ma that
produced the 14 km3 Incapillo ignimbrite. Distinctive and virtually identical chemical signatures of the domes and ignimbrites (SiO2 = 67–72 wt%; La/Yb = 37–56; Ba/La = 16–28; La/Ta = 30–50; 87Sr/86Sr = 0.70638–0.70669; ε
Nd = −4.2 to −4.6) indicate that all erupted lavas originated from the same magma chamber and that differentiation effects between
units were minor. The strong HREE depletion (Sm/Yb = 6–8) that distinguishes Incapillo magmas from most of the large ignimbrites
of the Altiplano–Puna plateau can be explained by the extent and degree of partial melting at lower crustal depths (>40 km)
in the presence of garnet. At upper crustal depths, this high-pressure residual geochemical signature, also common to adjacent
late Miocene/Pliocene Pircas Negras andesites, was partially overprinted by shallow-level assimilation and fractional crystallization
processes. Energy-constrained AFC modeling suggests that incorporation of anatectic upper crustal melts into a fractionated
“adakite-like” dacitic host best explains the petrogenesis of Incapillo magmas. The diminution of the sub-arc asthenospheric
wedge during Nazca plate shallowing left the Incapillo magma chamber unreplenished by both mafic mantle-derived and lower
crustal melts and thus stranded at shallow depths within the Andean crust. Based on its small size and distinctive high-pressure
chemical signature, the Incapillo Caldera and Dome Complex provides an endmember model for an Andean caldera erupting within
a waning magmatic arc over a shallowing subduction zone. 相似文献
17.
A. Freundt A. Hartmann S. Kutterolf W. Strauch 《International Journal of Earth Sciences》2010,99(6):1453-1470
The Tiscapa maar in the center of Managua city formed by a phreatomagmatic eruption <3 ka ago. The eruption excavated a crater
deep into the basement exposing a coherent Pleistocene to Holocene volcaniclastic succession that we have divided into four
formations. The lowermost, >60 ka old basaltic–andesitic formation F1 comprises mafic ignimbrites and phreatomagmatic tephras
derived from the Las Sierras volcanic complex south of Managua. Formation F2 contains the ~60 ka basaltic–andesitic Fontana
tephra erupted from the Las Nubes Caldera of the Las Sierras complex 15 km to the S, the 25 ka Upper Apoyo tephra from the
Apoyo Caldera 35 km to the SE, and the Lower (~17 ka) and Upper (12.4 ka) Apoyeque tephras from the Chiltepe volcanic complex
15 km to the NW. These tephras are separated by weathering horizons and paleosols indicating dry climatic conditions. Fluvial
deposits of a SSW-NNE running paleo-river system build formation F3. The fluvial sediments contain, from bottom to top, scoriae
from the ~6 ka basaltic San Antonio tephra, pumice lapilli from the Apoyo and Apoyeque tephras and the 6.1 ka Xiloà tephra,
and scoriae derived from the Fontana tephra. The fluvial sediment succession thus reflects progressively deeper carving erosion
in the southern highlands (where a large-amplitude regional erosional unconformity exists at the appropriate stratigraphic
level) that began after ~6 ka. This suggests that the mid-Holocene tropical high-precipitation climatic phase affected western
Nicaragua about a thousand years later than other circum-Caribbean regions. The end of the wet climate phase ~3 ka ago is
recorded by a deep weathering zone and paleosol atop formation F3 prior to the Tiscapa eruption. Formation F4 is the Tiscapa
tuffring composed of pyroclastic surge and fallout deposits that cover a minimum area of 1.2 km2. The 4 × 109 kg of erupted basaltic magma is compositionally and genetically related to the low-Ti basalts of the N–S striking Nejapa-Miraflores
volcanic–tectonic alignment 5 km to the West of Tiscapa. Ascent and eruption mode of the Tiscapa magma were controlled by
the Tiscapa fault that has a very active seismic history as it achieved 12 m displacement in about 3000 years. Managua city
is thus exposed to continued seismic and volcanic risks. 相似文献
18.
The Matahina Ignimbrite (~160 km3 rhyolite magma, 330 ka) was deposited during a caldera-forming eruption from the Okataina Volcanic Centre, Taupo Volcanic
Zone (TVZ), New Zealand. Juvenile clasts are divided into three groups: Group (1) the dominant crystal-poor rhyolite type,
Group (2) a minor coarse-grained, mingled/mixed intermediate type, and Group (3) a rare fine-grained basalt. The ignimbrite
consists of the Group 1 type and is divided into three members: a lower and middle member, which is high-silica, crystal-poor
(<10 vol.%) rhyolite, and the upper member, which is low-silica and slightly more crystal-rich (up to 21 vol.%). Cognate,
crystal-rich (up to 50 vol.%) basalt to intermediate pumice occurs on top of lag breccias and within lithic-rich pyroclastic
density current deposits along the caldera margin (Groups 2 and 3). Several lines of evidence indicate that the intermediate
clasts represent the cumulate complement to the melt-rich rhyolite: (1) continuity in the compositions of plagioclase, orthopyroxene,
hornblende, and oxides and normal zoning of individual phenocrysts; (2) the silicic glass from the intermediate magma (interstitial
melt) overlaps compositionally with the bulk rock rhyolite and glass; (3) high Zr and a slight positive Eu anomaly in the
intermediate magma relative to quenched enclaves from other intermediate TVZ eruptions indicates zircon and plagioclase accumulation,
respectively; (4) an increase in the Cl contents in glass from the least evolved to most evolved is consistent with the concentration
of volatiles during magma evolution. Most of the compositional variations in the low- to high-silica rhyolites can be accounted
for by continued Rayleigh fractionation (up to 15%), following melt extraction from the underlying mush, under varying fO2–fH2O conditions to form a slightly compositionally zoned rhyolitic cap. This link to the varying fO2–fH2O conditions is evidenced by the strong correlation between key geochemical parameters (e.g. Dy, Y), that qualitatively reflect
fH2O conditions (presence or absence of hornblende/biotite), and fO2 estimated from Fe–Ti oxide equilibrium. Magma mingling/mixing between the basalt–andesite and the main slightly compositionally
zoned rhyolitic magma occurred during caldera-collapse, modifying the least-evolved rhyolite at the lower portion of the reservoir
and effectively destroying any pre-eruptive gradients. 相似文献
19.
Mikbi intrusion(MI) is a part of the Neoproterozoic Nubian Shield located along the NE-SW trending major fracture zones prevailing southern Eastern Desert of Egypt. In this study, we present for the first time detailed mineralogical and bulk-rock geochemical data to infer some constraints on the parental magma genesis and to understand the tectonic processes contributed to MI formation. Lithologically, it is composed of fresh peridotite, clinopyroxenite, hornblendite, anorthosite, gabbronorite, pyroxene amphibole gabbro, amphibole gabbro and diorite. All rocks have low Th/La ratios(mostly <0.2) and lack positive Zr and Th anomalies excluding significant crustal contamination. They show very low concentrations of Nb, Ta, Zr and Hf together with sub-chondritic ratios of Nb/Ta(2-15) and Zr/Hf(19-35),suggesting that their mantle source was depleted by earlier melting extraction event. The oxygen fugacity(logfO_2) estimated from diorite biotite is around the nickel-nickel oxide buffer(NNO) indicating crystallization from a relatively oxidized magma. Amphiboles in the studied mafic-ultramafic rocks indicate relative oxygen fugacity(i.e. ΔNNO; nickel-nickel oxide) of 0.28-3 and were in equilibrium mostly with 3.77-8.24 wt.% H_2 Omelt(i.e. water content in the melt), consistent with the typical values of subduction-related magmas. Moreover, pressure estimates(0.53-6.79 kbar) indicate polybaric crystallization and suggest that the magma chamber(s) was located at relatively shallow crustal levels. The enrichment in LILE(e.g., Cs, Ba, K and Sr) and the depletion in HFSE(e.g., Th and Nb) relative to primitive mantle are consistent with island arc signature. The olivine, pyroxene and amphibole compositions also reflect arc affinity. These inferences suggest that their primary magma was derived from partial melting of a mantle source that formerly metasomatized in a subduction zone setting. Clinopyroxene and bulkrock data are consistent with orogenic tholeiitic affinity. Consequently, the mineral and bulk-rock chemistry strongly indicate crystallization from hydrous tholeiitic magma. Moreover, their trace element patterns are subparallel indicating that the various rock types possibly result from differentiation of the same primary magma. These petrological, mineralogical and geochemical characteristics show that the MI is a typical Alaskan-type complex. 相似文献
20.
Mt. Shasta andesite and dacite lavas contain high MgO (3.5–5 wt.%), very low FeO*/MgO (1–1.5) and 60–66 wt.% SiO2. The range of major and trace element compositions of the Shasta lavas can be explained through fractional crystallization (~50–60 wt.%) with subsequent magma mixing of a parent magma that had the major element composition of an H2O-rich primitive magnesian andesite (PMA). Isotopic and trace element characteristics of the Mt. Shasta stratocone lavas are highly variable and span the same range of compositions that is found in the parental basaltic andesite and PMA lavas. This variability is inherited from compositional variations in the input contributed from melting of mantle wedge peridotite that was fluxed by a slab-derived, fluid-rich component. Evidence preserved in phenocryst assemblages indicates mixing of magmas that experienced variable amounts of fractional crystallization over a range of crustal depths from ~25 to ~4 km beneath Mt. Shasta. Major and trace element evidence is also consistent with magma mixing. Pre-eruptive crystallization extended from shallow crustal levels under degassed conditions (~4 wt.% H2O) to lower crustal depths with magmatic H2O contents of ~10–15 wt.%. Oxygen fugacity varied over 2 log units from one above to one below the Nickel-Nickel Oxide buffer. The input of buoyant H2O-rich magmas containing 10–15 wt.% H2O may have triggered magma mixing and facilitated eruption. Alternatively, vesiculation of oversaturated H2O-rich melts could also play an important role in mixing and eruption. 相似文献