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1.
Studies of the geology, geochemistry of thermal waters, and of one exploratory geothermal well show that two related hot spring systems discharge in Canõn de San Diego at Soda Dam (48°C) and Jemez Springs (72°C). The hot springs discharge from separate strands of the Jemez fault zone which trends northeastward towards the center of Valles Caldera. Exploration drilling to Precambrian basement beneath Jemez Springs encountered a hot aquifer (68°C) at the top of Paleozoic limestone of appropriate temperature and composition to be the local source of the fluids in the surface hot springs at Jemez Springs. Comparisons of the soluble elements Na, Li, Cl, and B, arguments based on isotopic evidence, and chemical geothermometry indicate that the hot spring fluids are derivatives of the deep geothermal fluid within Valles Caldera. No hot aquifer was discovered in or on top of Precambrian basement. It appears that low- to moderate-temperature geothermal reservoirs (< 100°C) of small volume are localized along the Jemez fault zone between Jemez Springs and the margin of Valles Caldera.  相似文献   

2.
Over the last 42 ka, volcanic activity at Lipari Island (Aeolian Arc, Italy) produced lava domes, flows and pyroclastic deposits with rhyolitic composition, showing in many cases evidence of magma mixing such as latitic enclaves and banding. In this same period, on nearby Vulcano Island, similar rhyolitic lava domes, pyroclastic products and lava flows, ranging in composition from shoshonite to rhyolite, were erupted. As a whole, the post-42 ka products of Lipari and Vulcano show geochemical variations with time, which are well correlated between the two islands and may correspond to a modification of the primary magmas. The rhyolitic products are similar to each other in their major elements composition, but differ in their trace element abundances (e.g. La ranging from 40 to 78 ppm for SiO2 close to 75 wt%). Their isotopic composition is variable, too. The 87Sr/86Sr (0.704723–0.705992) and 143Nd/144Nd (0.512575–0.512526) ranges partially overlap those of the more mafic products (latites), having 87Sr/86Sr from 0.7044 to 0.7047 and 143Nd/144Nd from 0.512672 to 0.512615. 206Pb/204Pb is 19.390–19.450 in latites and 19.350–19.380 in rhyolites. Crystal fractionation and crustal assimilation processes of andesitic to latitic melts, showing an increasing content in incompatible elements in time, may explain the genesis of the different rhyolitic magmas. The rocks of the local crustal basement assimilated may correspond to lithotypes present in the Calabrian Arc. Mixing and mingling processes between latitic and rhyolitic magmas that are not genetically related occur during most of the eruptions. The alignment of vents related to the volcanic activity of the last 40 ka corresponds to the NNW–SSE Tindari–Letojanni strike-slip fault and to the correlated N–S extensional fault system. The mafic magmas erupted along these different directions display evidence of an evolution at different PH2O conditions. This suggests that the Tindari–Letojanni fault played a relevant role in the ascent, storage and diversification of magmas during the recent volcanic activity.  相似文献   

3.
The electron spin resonance (ESR) dating method was employed on quartz phenocrysts separated from pumice of the El Cajete and Battleship Rock Members of the Valles Rhyolite in the Valles caldera, New Mexico. The results of heating experiments indicate that Ti impurity centers have two components; a thermally stable one and a less stable, temperature sensitive one. ESR dates using the stable Ti center yield eruption ages of 59 ± 6 ka for the Battleship Rock Member and 53 ± 6 ka for the El Cajete Member while recent 14C dates (S. Reneau and J. Gardner, unpub. data) from carbonized logs in the El Cajete pumice indicate that its age is older than 50 ka. Our results indicate that volcanism in the Valles caldera is much younger than previously thought (≥ 130 ka) and that recent revisions to the post-0.5 Ma stratigraphy of Valles caldera are probably in error. The results suggest that ESR dating of quartz may be a useful method for obtaining ages of units in other Quaternary volcanic areas.  相似文献   

4.
We investigated the distribution of Cl, F, Li, and Be in pumices, obsidians, and crystallized dome rocks at Chaitén volcano in 2008?C2009 in order to explore the behavior of these elements during explosive and effusive volcanic activity. Electron and ion microprobe analyses of matrix and inclusion glasses from pumice, obsidian, and microlite-rich dome rock indicate that Cl and other elements were lost primarily during crystallization of the rhyolitic dome after it had approached the surface. Glass in pumice and microlite-free obsidian has 888?±?121?ppm Cl, whereas residual glass in evolved microlite-rich dome rock generally retains less Cl (as low as <100?ppm). Estimated Cl losses were likely >0.7?Mt Cl, with a potential maximum of 1.8?Mt for the entire 0.8-km3 dome. Elemental variations reflect an integrated bulk distribution ratio for Cl?>?1.7 (1.7 times more Cl was degassed or incorporated into crystals than remained in the melt). Because Cl is lost dominantly as the very last H2O is degassed, and Cl is minimally (if at all) partitioned into microlites, the integrated vapor/melt distribution ratio for Cl exceeds 200 (200 times more Cl in the evolved vapor than in the melt). Cl is likely lost as HCl, which is readily partitioned into magmatic vapor at low pressure. Cl loss is accelerated by the change in the composition of the residual melt due to microlite growth. Cl loss also may be affected by open-system gas fluxing. Integrated vapor-melt distribution ratios for Li, F, and Be all exceed 1,000. On degassing, an unknown fraction of these volatiles could be immediately dissolved in rainwater.  相似文献   

5.
After the March–April 1986 explosive eruption a comprehensive gas study at Augustine was undertaken in the summers of 1986 and 1987. Airborne COSPEC measurements indicate that passive SO2 emission rates declined exponentially during this period from 380±45 metric tons/day (T/D) on 7/24/86 to 27±6 T/D on 8/24/87. These data are consistent with the hypothesis that the Augustine magma reservoir has become more degassed as volcanic activity decreased after the spring 1986 eruption. Gas samples collected in 1987 from an 870°C fumarole on the andesitic lava dome show various degrees of disequilibrium due to oxidation of reduced gas species and condensation (and loss) of H2O in the intake tube of the sampling apparatus. Thermochemical restoration of the data permits removal of these effects to infer an equilibrium composition of the gases. Although not conclusive, this restoration is consistent with the idea that the gases were in equilibrium at 870°C with an oxygen fugacity near the Ni–NiO buffer. These restored gas compositions show that, relative to other convergent plate volcanoes, the Augustine gases are very HCl rich (5.3–6.0 mol% HCl), S rich (7.1 mol% total S), and H2O poor (83.9–84.8 mol% H2O). Values of D and 18O suggest that the H2O in the dome gases is a mixture of primary magmatic water (PMW) and local seawater. Part of the Cl in the Augustine volcanic gases probably comes from this shallow seawater source. Additional Cl may come from subducted oceanic crust because data by Johnston (1978) show that Cl-rich glass inclusions in olivine crystals contain hornblende, which is evidence for a deep source (>25km) for part of the Cl. Gas samples collected in 1986 from 390°–642°C fumaroles on a ramp surrounding the inner summit crater have been oxidized so severely that restoration to an equilibrium composition is not possible. H and O isotope data suggest that these gases are variable mixtures of seawater, FMW, and meteoric steam. These samples are much more H2O-rich (92%–97% H2O) than the dome gases, possibly due to a larger meteoric steam component. The 1986 samples also have higher Cl/S, S/C, and F/Cl ratios, which imply that the magmatic component in these gases is from the more degassed 1976 magma. Thus, the 1987 samples from the lava dome are better indicators than the 1986 samples of degassing within the Augustine magma reservoir, even though they were collected a year later and contain a significant seawater component. Future gas studies at Augustine should emphasize fumaroles on active lava domes. Condensates collected from the same lava-dome fumarole have enrichments ot 107–102 in Cl, Br, F, B, Cd, As, S, Bi, Pb, Sb, Mo, Zn, Cu, K, Li, Na, Si, and Ni. Lower-temperature (200°–650°C) fumaroles around the volcano are generally less enriched in highly volatile elements. However, these lower-termperature fumaroles have higher concentration of rock-forming elements, probably derived from the wall rock.  相似文献   

6.
Recent drilling and sampling of hydrothermal fluids from Long Valley permit an accurate characterization of chemical concentrations and equilibrium conditions in the hydrothermal reservoir. Hydrothermal fluids are thermodynamically saturated with secondary quartz, calcite, and pyrite but are in disequilibrium with respect to aqueous sulfide-sulfate speciation. Hydrothermal fluids are enriched in 18O by approximately 1‰ relative to recharge waters. 18O and Cl concentrations in well cuttings and core from high-temperature zones of the reservoir are extensively depleted relative to fresh rhyolitic tuff compositions. Approximately 80% of the Li and 50% of the B are retained in the altered reservoir rock. Cl mass balance and open-system 18O fractionation models produce similar water-rock ratios of between 1.0 and 2.5 kg kg−1. These water-rock ratios coupled with estimates of reservoir porosity and density produce a minimum fluid residence time of 1.3 ka. The low fluid Cl concentrations in Long Valley correlate with corresponding low rock concentrations. Mass balance calculations indicate that leaching of these reservoir rocks accounts for Cl losses during hydrothermal activity over the last 40 ka.  相似文献   

7.
Cosmogenic exposure dating of moraines during the last two decades has vastly improved knowledge on the timing of glaciation worldwide. Due to a variety of geologic complications, such as moraine degradation, snow cover, bedrock erosion and isotopic inheritance, samples from multiple large boulders (>1–2 m) often lead to the most accurate moraine age assignments. However, in many cases, large boulders are not available on moraines of interest. Here, I test the suitability of pebble collections from moraine crest surfaces as a sample type for exposure dating. Twenty-two 10Be ages from two Pleistocene lateral moraine crests in Pine Creek valley in the upper Arkansas River basin, Colorado, were calculated from both pebble and boulder samples. Ten 10Be ages from a single-crested Bull Lake lateral moraine range between 3 and 72 ka, with no statistical difference between pebble (n = 5) and boulder (n = 5) ages. The lack of a cluster of 10Be ages suggests that moraine degradation has led to anomalously young exposure ages. Twelve 10Be ages from a single-crested Pinedale lateral moraine have a bimodal age distribution; one mode is 22.0 ± 1.4 ka (three boulders, two pebble collections), the other is 15.2 ± 0.9 ka (two boulders, five pebble collections). The interpretation of the two age modes is that two glacier maxima of similar extent were attained during the late Pleistocene. Regardless of moraine age interpretations, that 10Be ages from pebble collections and boulders are indistinguishable on moraines of two different ages, and in two different age modes of the Pinedale moraine, suggests that pebble collections from moraine crests may serve as a suitable sample type in some settings.  相似文献   

8.
The Kula volcanic field in Western Turkey comprises about 80 cinder cones and associated basaltic lava flows of Quaternary age. Based on geomorphological criteria and K-Ar dating, three eruption phases, β2–β4, were distinguished in previous studies. Human footprints in ash deposits document that the early inhabitants of Anatolia were affected by the volcanic eruptions, but the age of the footprints has been poorly constrained. Here we use 3He and 10Be exposure dating of olivine phenocrysts and quartz-bearing xenoliths to determine the age of the youngest lava flows and cinder cones. In the western part of the volcanic field, two basalt samples from a 15-km-long block lava flow yielded 3He ages of 1.5 ± 0.3 ka and 2.5 ± 0.4 ka, respectively, with the latter being in good agreement with a 10Be age of 2.4 ± 0.3 ka for an augen gneiss xenolith from the same flow. A few kilometers farther north, a metasedimentary xenolith from the top of the cinder cone Çakallar Tepe gave a 10Be age of 11.2 ± 1.1 ka, which dates the last eruption of this cone and also the human footprints in the related ash deposits. In the center of the volcanic field, a basalt sample and a metasedimentary xenolith from another cinder cone gave consistent 3He and 10Be ages of 2.6 ± 0.4 ka and 2.6 ± 0.3 ka, respectively. Two β4 lava flows in the central and eastern part of the volcanic province yielded 3He ages of 3.3 ± 0.4 ka and 0.9 ± 0.2 ka, respectively. Finally, a relatively well-preserved β3 flow gave a 3He age of ∼13 ka. Taken together, our results demonstrate that the penultimate eruption phase β3 in the Kula volcanic field continued until ∼11 ka, whereas the youngest phase β4 started less than four thousand years ago and may continue in the future.  相似文献   

9.
We present an inventory of B, Cl and Li concentrations in (a) key minerals from a set of ultramafic samples featuring the main evolutionary stages encountered by the subducted oceanic mantle, and in (b) fluid inclusions produced during high-pressure breakdown of antigorite serpentinite. Samples correspond to (i) nonsubducted serpentinites (Northern Apennine and Alpine ophiolites), (ii) high-pressure olivine-bearing antigorite serpentinites (Western Alps and Betic Cordillera), (iii) high-pressure olivine-orthopyroxene rocks recording the subduction breakdown of antigorite serpentinites (Betic Cordillera). Two main dehydration episodes are recorded by the sample suite: partial serpentinite dewatering during formation of metamorphic olivine, followed by full breakdown of antigorite serpentine to olivine+orthopyroxene+fluid. Ion probe and laser ablation ICP-MS (LA ICP-MS) analyses of Cl, B and Li in the rock-forming minerals indicate that the hydrous mantle is an important carrier of light elements. The estimated bulk-rock B and Cl concentrations progressively decrease from oceanic serpentinites (46.7 ppm B and 729 ppm Cl) to antigorite serpentinites (20 ppm B and 221 ppm Cl) to olivine-orthopyroxene rocks (9.4 ppm B and 45 ppm Cl). This suggests release of oceanic Cl and B in subduction fluids, apparently without inputs from external sources. Lithium is less abundant in oceanic serpentinites (1.3 ppm) and the initial concentrations are still preserved in high-pressure antigorite serpentinites. Higher Li contents in olivine, Ti-clinohumite of the olivine-orthopyroxene rocks (4.9 ppm bulk rock Li), as well as in the coexisting fluid inclusions, suggest that their budget may not be uniquely related to recycling of oceanic Li, but may require input from external sources.Laser ablation ICP-MS analyses of fluid inclusions in the olivine-orthopyroxene rocks enabled an estimate of the Li and B concentrations in the antigorite breakdown fluid. The inclusion compositions were quantified using a range of salinity values (0.4-2 wt.% NaClequiv) as internal standards, yielding maximum average fluid/rockDB∼5 and fluid/rockDLi∼3.5. We also performed model calculations to estimate the B and Cl loss during the two dehydration episodes of serpentinite subduction. The first event is characterized by high fluid/rock partition coefficients for Cl (∼100) and B (∼60) and by formation of a fluid with salinity of 4-8 wt.% NaClequiv. The antigorite breakdown produces less saline fluids (0.4-2 wt.% NaClequiv) and is characterized by lower partition coefficients for Cl (25-60) and B (12-30). Our calculations indicate that the salinity of the subduction fluids decreases with increasing depths. fluid/rockDB/fluid/rockDCl<1 (∼0.5) indicates that Cl preferentially partitions into the evolved fluids relative to B and that the B/Cl of fluids progressively increases with increasing depths and temperatures.Despite light element release in fluids, appreciable B, Cl and Li are still retained in chlorite, olivine and Ti-clinohumite beyond the antigorite stability field. This permits a bulk storage of about 10 ppm B, 45 ppm Cl and 5 ppm Li, i.e., concentrations much higher than in mantle reservoirs. Chlorite is the Cl repository and its stability controls the Cl and H2O budget beyond the antigorite stability; B and Li are bound in olivine and clinohumite. The subducted oceanic mantle thus retains light elements beyond the depths of arc magma sources, potentially introducing anomalies in the upper mantle.  相似文献   

10.
Noncondensible gases from hot springs, fumaroles, and deep wells within the Valles caldera geothermal system (210–300°C) consist of roughly 98.5 mol% CO2, 0.5 mol% H2S, and 1 mol% other components. 3He/4He ratios indicate a deep magmatic source (R/Ra up to 6) whereas δ13C–CO2 values (−3 to −5‰) do not discriminate between a mantle/magmatic source and a source from subjacent, hydrothermally altered Paleozoic carbonate rocks. Regional gases from sites within a 50-km radius beyond Valles caldera are relatively enriched in CO2 and He, but depleted in H2S compared to Valles gases. Regional gases have R/Ra values ≤1.2 due to more interaction with the crust and/or less contribution from the mantle. Carbon sources for regional CO2 are varied. During 1982–1998, repeat analyses of gases from intracaldera sites at Sulphur Springs showed relatively constant CH4, H2, and H2S contents. The only exception was gas from Footbath Spring (1987–1993), which experienced increases in these three components during drilling and testing of scientific wells VC-2a and VC-2b. Present-day Valles gases contain substantially less N2 than fluid inclusion gases trapped in deep, early-stage, post-caldera vein minerals. This suggests that the long-lived Valles hydrothermal system (ca. 1 Myr) has depleted subsurface Paleozoic sedimentary rocks of nitrogen. When compared with gases from many other geothermal systems, Valles caldera gases are relatively enriched in He but depleted in CH4, N2 and Ar. In this respect, Valles gases resemble end-member hydrothermal and magmatic gases discharged at hot spots (Galapagos, Kilauea, and Yellowstone).  相似文献   

11.
The Grande Découverte Volcanic Complex (GDVC), active since at least 0.2 Ma, is the most recent volcanic complex of the Basse-Terre Island (Guadeloupe, Lesser Antilles Arc). A detailed geochronological study using the K–Ar Cassignol–Gillot technique has been undertaken in order to reconstruct the history of effusive activity of this long-lived volcanic system. Twenty new ages permit to suggest that the GDVC experienced at least six main effusive stages, from 200 ka to present time. To the north of the GDVC, the GDS (Grande Découverte–Soufrière volcano) has been active since at least 200 ka, and to the south, the TRMF (Trois-Rivières–Madeleine Field), started to be emplaced 100 ka. Morphological investigations suggest that the whole TRMF volcanism was emitted from vents distinct from the GDS, most probably a large E–W fissure network linked to the Marie-Galante rift. The mean age of 62 ± 5 ka, obtained for the E–W Madeleine–Le Palmiste alignment suggests that a fissure-opening event occurred at that time. However, whole-rock major and trace element signatures are similar for both systems, suggesting that a common complex magma-plumbing system has fed the overall GDVC. We report very young ages for lava flows from the TRMF, which implies that < 10 ka volcanic activity is now identified for both massifs. Although hazards associated with such effusive volcanism are much lower than those associated with potential flank-collapse of the Soufrière lava dome or a magmatic dome eruption with explosive phases within the GDS, the emplacement of relatively large Holocene age lava flows (3–1 × 108 m3) suggests that a revised integrated volcanic hazard assessment for Southern Basse-Terre should now consider the potential for renewed future activity from two Holocene volcanic centers including the TRMF.  相似文献   

12.
The recent finding of mafic enclaves in the Rocche Rosse (RR) lava flow, the last magmatic product on Lipari (Aeolian Islands, Italy) (AD 1230 ± 40), opens the possibility to investigate in detail the most recent magmatic system of the island, an important issue for the volcanic hazard assessment of the area. The RR lava flow is an aphyric rhyolitic coulée consisting of grey and black pumice and black and grey obsidian. Enclaves have ellipsoidal to spheroidal shape and vary from mm-sized in the central portion of the flow, to cm-sized, at the top and in the flow front, where they are also more abundant. Enclaves are shoshonitic-latitic (group A) and trachytic (group B) in composition. The mineralogy of group A consists of dominant clinopyroxene crystals with minor abundance of feldspar (plagioclase > K-feldspar), olivine and biotite, while group B is composed of feldspar (K-feldspar > plagioclase) with minor clinopyroxene, olivine and biotite. Geochemical modeling suggests that the host rhyolitic rocks could be the product of AFC (Assimilation plus Fractional Crystallization) of a magma compositionally similar to the associated shoshonitic-latitic enclaves, which, in turn, could be obtained, through an AFC process, from the primitive melts erupted as olivine hosted melt inclusions during the last 15 ka at Vulcano. The already-known last 42 ka relationship between Lipari and Vulcano Islands is here reinforced until historical time, especially for the last 1 ka. The geochemical and petrological overlap between Lipari and Vulcano is interpreted to reflect the existence of a similar magmatic system underneath the two islands. The nearly aphyric RR rhyolites are interpreted to be the products of a superheated (temperature far above the liquidus) and initially water-undersaturated magma that underwent degassing close to the surface inhibiting microlite crystallization.  相似文献   

13.
The influx of10Be into a globigerinid ooze core (CH72-02) from the eastern North Atlantic has been studied. This core contains a depositional record of the first 11 δ18O stages covering the last 423 ka. It is shown that the marine deposition of10Be is strongly influenced by the sedimentation of clays. Clay particles appear 10 times more efficient than the carbonate component as a carrier in bringing10Be to the bottom sediments. In core CH72-02, the deposition rates of10Be averaged over each oxygen-isotope stage for the past 11 stages show a scatter of ±40% about the mean value of 6.6 × 108 atoms cm−2 ka−1. However, after correction for changes in lithology, the data show that the production rate of10Be over the same period has varied no more than ±25%, and the variations are not systematic in that high or low10Be production appear to be associated with either cold or warm climates. On the time scale of this investigation (intervals of ca. 50 ka over the last 420 ka, with resolutions as fine as 10 ka for portions of the record), it is unlikely that the shielding effect of the solar wind has deviated by more than ±25% or the geomagnetic field intensity has deviated by more than a factor of 1.6 from their long-term averages.  相似文献   

14.
Trachytic lavas of Rishiri Volcano, northern Japan, show a peculiar geochemical variation across lava flow units. Samples collected systematically in a vertical cross section from a lava flow unit with a thickness of about 20 m are nearly homogeneous in major element compositions. However, some trace elements, including Li, B and Cs, are considerably depleted in samples collected from the main part of the flow unit, compared to those obtained from the surface of the lava flow (clinker layer). In particular, Cs content of the main flow unit is as low as ∼30% of the clinker layer. 11B / 10B ratios of samples from the main flow unit are also slightly lower than those of the clinker samples, and the isotope compositions positively correlate with boron concentrations. These geochemical variations cannot be explained by magmatic processes in magma chambers, post-eruptive weathering, or alteration process. Rather, we infer these systematics resulted from escape of these elements from the lava flow unit during post-eruptive degassing. Vapor phases in which Li, B and Cs dissolved are suggested to have been transported through veins formed in the main flow unit as fractures due to slight shearing along the flow planes after lava emplacement. In the Tanetomi lava, only rocks of the clinker layer preserve original composition of magmas, although they are porous and brownish due to extensive oxidization. On the other hand, rocks of the main flow unit do not retain original magma compositions, although they are dense and grayish, and seem to be much fresher compared to the clinkers. A similar geochemical modification of lavas can occur in other volcanic systems, especially for lavas consisting of relatively thick flow units.  相似文献   

15.
Fumarole discharges (95–560°C) collected from the dacite dome inside Mount St. Helens crater show temporal changes in their isotopic and chemical compositions. A δD vs. δ18O plot shows that condensed waters from the gases are mixtures of meteoric and magmatic components, but that the apparent magmatic end-member in 1994 was depleted by about 7‰ in δD relative to the apparent end-member in 1980. Based on δD modeling, approximately 63% of shallow, post-1980 magma has yet to degas. Surprisingly, Cl and F contents in the 1994 samples were only 0.47 and 3.8%, respectively, of the concentrations determined for end-member magmatic fluid in 1980. The data indicate that Cl (and F and B) is degassed from magma relatively quickly compared to water and/or that most of the Cl degassed in later years is dissolved into the shallow Mount St. Helens hydrothermal system. Because metals are often transported in magmatic and hydrothermal fluids as Cl complexes, rapid changes in surface volatile compositions may have implications for the timing and location of metals transport and deposition in some volcanoes.  相似文献   

16.
Products of the latest eruptions from the Valles caldera, New Mexico, consist of the El Cajete Pyroclastic Beds and Battleship Rock Ignimbrite, a sequence of pyroclastic fall and density current deposits erupted at ~ 55 ka, capped by the later Banco Bonito Flow erupted at ~ 40 ka, and collectively named the East Fork Member of the Valles Rhyolite. The stratigraphy of the East Fork Member has been the subject of conflicting interpretations in the past; a long-running investigation of short-lived exposures over a period of many years enables us to present a more complete event stratigraphy for these eruptions than has hitherto been possible. The volume of rhyolitic magma erupted during the 55 ka event may have been more than 10 km3, and for the 40 ka event can be estimated with rather more confidence at 4 km3. During the earlier event, plinian eruptions dispersed fallout pumice over much of the Valles caldera, the southern Jemez Mountains, and the Rio Grande rift. We infer a fallout thickness of several decimeters at the site of the city of Santa Fe, and significant ash fall in eastern New Mexico. In contrast, pyroclastic density currents were channeled within the caldera moat and southwestward into the head of Cañon de San Diego, the principal drainage from the caldera. Simultaneous (or rapidly alternating) pyroclastic fallout and density current activity characterized the ~ 55 ka event, with density currents becoming more frequent as the eruption progressed through two distinct stages separated by a brief hiatus. One early pyroclastic surge razed a forest in the southern caldera moat, in a similar manner to the initial blast of the May 18, 1980 eruption of Mt. St. Helens. Ignimbrite outflow from the caldera through the drainage notch may have been restricted in runout distance due to steep, rugged topography in this vicinity promoting mixing between flows and air, and the formation of phoenix clouds. Lavas erupted during both the ~ 55 and ~ 40 ka events were largely confined to the caldera moat. Any future rhyolitic eruptions of similar magnitude in the southern or western parts of the Valles caldera will likely affect similar areas.  相似文献   

17.
The eruptive history of the Tequila volcanic field (1600 km2) in the western Trans-Mexican Volcanic Belt is based on 40Ar/39Ar chronology and volume estimates for eruptive units younger than 1 Ma. Ages are reported for 49 volcanic units, including Volcán Tequila (an andesitic stratovolcano) and peripheral domes, flows, and scoria cones. Volumes of volcanic units 1 Ma were obtained with the aid of field mapping, ortho aerial photographs, digital elevation models (DEMs), and ArcGIS software. Between 1120 and 200 kyrs ago, a bimodal distribution of rhyolite (~35 km3) and high-Ti basalt (~39 km3) dominated the volcanic field. Between 685 and 225 kyrs ago, less than 3 km3 of andesite and dacite erupted from more than 15 isolated vents; these lavas are crystal-poor and show little evidence of storage in an upper crustal chamber. Approximately 200 kyr ago, ~31 km3 of andesite erupted to form the stratocone of Volcán Tequila. The phenocryst assemblage of these lavas suggests storage within a chamber at ~2–3 km depth. After a hiatus of ~110 kyrs, ~15 km3 of andesite erupted along the W and SE flanks of Volcán Tequila at ~90 ka, most likely from a second, discrete magma chamber located at ~5–6 km depth. The youngest volcanic feature (~60 ka) is the small andesitic volcano Cerro Tomasillo (~2 km3). Over the last 1 Myr, a total of 128±22 km3 of lava erupted in the Tequila volcanic field, leading to an average eruption rate of ~0.13 km3/kyr. This volume erupted over ~1600 km2, leading to an average lava accumulation rate of ~8 cm/kyr. The relative proportions of lava types are ~22–43% basalt, ~0.4–1% basaltic andesite, ~29–54% andesite, ~2–3% dacite, and ~18–40% rhyolite. On the basis of eruptive sequence, proportions of lava types, phenocryst assemblages, textures, and chemical composition, the lavas do not reflect the differentiation of a single (or only a few) parental liquids in a long-lived magma chamber. The rhyolites are geochemically diverse and were likely formed by episodic partial melting of upper crustal rocks in response to emplacement of basalts. There are no examples of mingled rhyolitic and basaltic magmas. Whatever mechanism is invoked to explain the generation of andesite at the Tequila volcanic field, it must be consistent with a dominantly bimodal distribution of high-Ti basalt and rhyolite for an 800 kyr interval beginning ~1 Ma, which abruptly switched to punctuated bursts of predominantly andesitic volcanism over the last 200 kyrs.Electronic Supplementary Material Supplementary material is available in the online version of this article at Editorial responsility: J. Donnelly-NolanThis revised version was published online in January 2005 with corrections to Tables 1 and 3.An erratum to this article can be found at  相似文献   

18.
Two groups of rhyolites have been recognized at San Vincenzo (Tuscany, Italy). Group A rhyolites are characterized by plagioclase, quartz, biotite, sanidine and cordierite mineral assemblages. They show constant MgO and variable CaO and Na2O contents. Initial87Sr/86Sr ratios in group A samples range between 0.71950 and 0.72535, whereas the Nd isotopic compositions are relatively constant (0.51215–0.51222). Group B rhyolites are characterized by orthopyroxene and clinopyroxene as additional minerals, and show textural, mineralogical and chemical evidence of interaction with more mafic magmas. The Sr and Nd isotopic ratios range between 0.71283–0.71542 and 0.51224–0.51227 respectively. Magmatic inclusions of variable size (1 mm to 10 cm) were found in groups B rhyolites. These inclusions consist mainly of diopsidic clinopyroxene and minor olivine and biotite. They are latitic in composition and represent blobs of hybrid intermediate magmas entrained in the rhyolitic melts. These magmatic inclusions have relatively high Sr contents (996–1529 ppm) and Sr and Nd isotope-ratios of 0.70807–0.70830 and 0.51245–0.51252 respectively.87Sr/87Sr data on minerals separated from both group A and B rhyolites and magmatic inclusions reveal strong isotopic disequilibria due to the presence of both restitic and newly crystallized phases in group A rhyolites and due to interaction of rhyolites with a mantle-de-rived magma in group B rhyolites. Isotopic data on whole rocks and minerals allow us to interpret the group A rhyolites as representative of different degrees of melting of an isotopically fairly homogeneous pelitic source; conversely, group B rhyolites underwent interactions with a mantle-derived magma. The crustal source as inferred from isotopic systematics would be characterized by87Sr/86Sr and143Nd/144Nd ratios close to 0.7194 and 0.51216 respectively. The sub-crustal magma would have Sr isotopic composition close to 0.7077 and a143Nd/144Nd ratio greater than or equal to 0.51252. These isotopic features are different from those reported for the parental magmas postulated for Vulsini and Alban Hills in the nearby Roman Magmatic Province, and are similar to those of the Vesuvius and Ischia magmas.  相似文献   

19.
A buried, old volcanic body (pre‐Komitake Volcano) was discovered during drilling into the northeastern flank of Mount Fuji. The pre‐Komitake Volcano is characterized by hornblende‐bearing andesite and dacite, in contrast to the porphyritic basaltic rocks of Komitake Volcano and to the olivine‐bearing basaltic rocks of Fuji Volcano. K‐Ar age determinations and geological analysis of drilling cores suggest that the pre‐Komitake Volcano began with effusion of basaltic lava flows around 260 ka and ended with explosive eruptions of basaltic andesite and dacite magma around 160 ka. After deposition of a thin soil layer on the pre‐Komitake volcanic rocks, successive effusions of lava flows occurred at Komitake Volcano until 100 ka. Explosive eruptions of Fuji Volcano followed shortly after the activity of Komitake. The long‐term eruption rate of about 3 km3/ka or more for Fuji Volcano is much higher than that estimated for pre‐Komitake and Komitake. The chemical variation within Fuji Volcano, represented by an increase in incompatible elements at nearly constant SiO2, differs from that within pre‐Komitake and other volcanoes in the northern Izu‐Bonin arc, where incompatible elements increase with increasing SiO2. These changes in the volcanism in Mount Fuji may have occurred due to a change in regional tectonics around 150 ka, although this remains unproven.  相似文献   

20.
At Cotopaxi volcano, Ecuador, rhyolitic and andesitic bimodal magmatism has occurred periodically during the past 0.5 Ma. The sequential eruption of rhyolitic (70–75% SiO2) and andesitic (56–62% SiO2) magmas from the same volcanic vent over short time spans and without significant intermingling is characteristic of Cotopaxi’s Holocene behavior. This study documents the eruptive history of Cotopaxi volcano, presenting its stratigraphy and geologic field relations, along with the relevant mineralogical and chemical nature of the eruptive products, in order to determine the temporal and spatial relations of this bimodal alternation. Cotopaxi’s history begins with the Barrancas rhyolite series, dominated by pumiceous ash flows and regional ash falls between 0.4 and 0.5 Ma, which was followed by occasional andesitic activity, the most important being the ample andesitic lava flows (∼4.1 km3) that descended the N and NW sides of the edifice. Following a ∼400 ka long repose without silicic activity, Cotopaxi began a new eruptive phase about 13 ka ago that consisted of seven rhyolitic episodes belonging to the Holocene F and Colorado Canyon series; the onset of each episode occurred at intervals of 300–3,600 years and each produced ash flows and regional tephra falls with DRE volumes of 0.2–3.6 km3. Andesitic tephras and lavas are interbedded in the rhyolite sequence. The Colorado Canyon episode (4,500 years BP) also witnessed dome and sector collapses on Cotopaxi’s NE flank which, with associated ash flows, generated one of the largest cohesive debris flows on record, the Chillos Valley lahar. A thin pumice lapilli fall represents the final rhyolitic outburst which occurred at 2,100 years BP. The pumices of these Holocene rhyolitic eruptions are chemically similar to those of older rhyolites of the Barrancas series, with the exception of the initial eruptive products of the Colorado Canyon series whose chemistry is similar to that of the 211 ka ignimbrite of neighboring Chalupas volcano. Since the Colorado Canyon episode, andesitic magmatism has dominated Cotopaxi’s last 4,400 years, characterized by scoria bomb and lithic-rich pyroclastic flows, infrequent lava flows that reached the base of the cone, andesitic lapilli and ash falls that were carried chiefly to the W, and large debris flows. Andesitic magma emission rates are estimated at 1.65 km3 (DRE)/ka for the period from 4,200 to 2,100 years BP and 1.85 km3 (DRE)/ka for the past 2,100 years, resulting in the present large stratocone.  相似文献   

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