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1.
Seismic, sidescan sonar, bathymetric multibeam and ODP (Ocean Drilling Program) data obtained in the submarine channel between the volcanic islands of Gran Canaria and Tenerife allow to identify constructive features and destructive events during the evolution of both islands. The most prominent constructive features are the submarine island flanks being the acoustic basement of the seismic images. The build-up of Tenerife started following the submarine stage of Gran Canaria because the submarine island flank of Tenerife onlaps the steeper flank of Gran Canaria. The overlying sediments in the channel between Gran Canaria and Tenerife are chaotic, consisting of slumps, debris flow deposits, syn-ignimbrite turbidites, ash layers, and other volcaniclastic rocks generated by eruptions, erosion, and flank collapse of the volcanoes. Volcanic cones on the submarine island flanks reflect ongoing submarine volcanic activity. The construction of the islands is interrupted by large destructive events, especially by flank collapses resulting in giant landslides. Several Miocene flank collapses (e.g., the formation of the Horgazales basin) were identified by combining seismic and drilling data whereas young giant landslides (e.g., the Güimar debris avalanche) are documented by sidescan, bathymetric and drilling data. Sediments are also transported through numerous submarine canyons from the islands into the volcaniclastic apron. Seismic profiles across the channel do not show a major offset of reflectors. The existence of a repeatedly postulated major NE-SW-trending fault zone between Gran Canaria and Tenerife is thus in doubt. The sporadic earthquake activity in this area may be related to the regional stress field or the submarine volcanic activity in this area. Seismic reflectors cannot be correlated through the channel between the sedimentary basins north and south of Gran Canaria because the channel acts as sediment barrier. The sedimentary basins to the north and south evolved differently following the submarine growth of Gran Canaria and Tenerife in the Miocene.  相似文献   

2.
Glasses from Mauna Loa pillow basalts, recent subaerial vents, and inclusions in olivine were analyzed for S, Cl, F, and major elements by electron microprobe. Select submarine glasses were also analyzed for H2O and CO2 by infrared spectroscopy. The compositional variation of these tholeiitic glasses is dominantly controlled by crystal fractionation and they indicate quenching temperatures of 1,115-1,196 °C. Submarine rift zone glasses have higher volatile abundances (except F) than nearly all other submarine and subaerial glasses with the maximum concentrations increasing with water depth. The overwhelming dominance of degassed glasses on the submarine flanks of Mauna Loa implies that much of volcano's recent submarine growth involved subaerially erupted lava that reached great water depths (up to 3.1 km) via lava tubes. Anomalously high F and Cl in some submarine glasses and glass inclusions indicate contamination possibly by fumarolic deposits in ephemeral rift zone magma chambers. The relatively high CO2 but variable H2O/K2O and S/K2O in some submarine rift zone glasses indicates pre-eruptive mixing between degassed and undegassed magma within Mauna Loa's rift system. Volatile compositions for Mauna Loa magmas are similar to other active Hawaiian volcanoes in S and F, but are less Cl-rich than Ll'ihi glasses. However, Cl/K2O ratios are similar. Mauna Loa and Ll'ihi magmas have comparable, but lower H2O than those from Kilauea. Thus, Kilauea's source may be more H2O-rich. The dissimilar volatile distribution in glasses from active Hawaiian volcanoes is inconsistent with predictions for a simple, concentrically zoned plume model.  相似文献   

3.
The main goal of this investigation is estimating volume of volatile emission, atmospheric and climatic impact of the Kurile Lake caldera-forming eruption, one of the Earth’s largest Holocene explosive eruptions. The volatile content of magma before the eruption was estimated by comparing H2O, S, Cl and F contents in natural quenched glassy melt inclusions trapped by plagioclase phenocrysts. The volatile content of igneous rocks after eruption was estimated by comparing concentrations of degassed matrix glasses. As a result of KO-eruption not more than (3.7–4.2) × 1012 kg of water, (4.3–4.9) × 1010 kg of chlorine, (8.6–9.8) × 109 kg of fluorine and (2.6–2.9) × 1010 kg of sulphur were injected into the atmosphere. This eruption had to produce an important climatic impact.  相似文献   

4.
Trace element concentrations of altered basaltic glass shards (layer silicates) and zeolites in volcaniclastic sediments drilled in the volcanic apron northeast of Gran Canaria during Ocean Drilling Program (ODP) leg 157 document variable element mobilities during low-temperature alteration processes in a marine environment. Clay minerals (saponite, montmorillonite, smectite) replacing volcanic glass particles are enriched in transition metals and rare earth elements (REE). The degree of retention of REE within the alteration products of the basaltic glass is correlated with the field strength of the cations. The high field-strength elements are preferentially retained or enriched in the alteration products by sorption through clay minerals. Most trace elements are enriched in a boundary layer close to the interface mineral-altered glass. This boundary layer has a key function for the physico-chemical conditions of the subsequent alteration process by providing a large reactive surface and by lowering the fluid permeability. The release of most elements is buffered by incorporation into secondary precipitates (sodium-rich zeolites, phillipsite, Fe- and Mn-oxides) as shown by calculated distribution coefficients between altered glasses and authigenic minerals. Chemical fluxes change from an open to a closed system behavior during prograde low-temperature alteration of volcaniclastic sediments with no significant trace metal flux from the sediment to the water column.  相似文献   

5.
F, Cl, S and P were determined, using electron microprobe, in magmatic inclusions trapped within minerals and glass mesostasis from Wudalianchi volcanic rocks. The initial volcanic magma from Wudalianchi corresponds to the basanitic magma crystallized near the surface ( pressure < 91 Mpa ). The potential H2O content of this magma is in the range 2 — 4 wt. %. The initial composition of volcanic magmas varies regularly from early to late volcanic events. From the Middle Pleistocene to the recent eruptions (1719 – 1721 yr.), the basicity of volcanic magma tends to increase, as reflected by an increase in MgO and CaO contents and by a progressive decrease in SiO2 and K2O contents. Meanwhile. from early (Q2 ) to late (Q3) episodic eruptions of the Middle Pleistocene, the initial concentrations of chlorine in volcanic magmas range from 1430 – 1930 ppm to 1700 ppm and decrease to 700 — 970 ppm for the first episodic eruption during the Holocene (Q 4 1 ). The chlorine concentrations of volcanic magmas of recent eruption (Q 4 2 ) are increased again to 2600 – 2870 ppm. A parallel evolution trend for phosphorus and chlorine concentrations in magmas has been certified: 1500 – 5970 ppm (Q2)→ 3500 – 4210 ppm (Q3)→ 1100– 3500 ppm (Q 4 1 )→ 6800– 7900 ppm (Q 4 2 ). The fluorine contents of volcanic magmas, from early to late volcanic events, show the same trend: 770 – 2470 ppm → 200–700 ppm → 700 – 800 ppm. During the crystallization-evolution of volcanic magmas, fluorine and phosphorus tend to be enriched in residual magmas as a result of crystal-melt differentiation. for example. the fluorine contents reach 5000– 6800 ppm and the phosphorus contents, 2.93wt.% in residual magmas. An appreciable amount of chlorine may be lost from water rich volcanic magmas prior to eruption as a result of degassing. Apparently, water serves as a gas carrier for the chlorine. The chlorine contents of residual magmas may decrease to 100 – 300 ppm. The volcanic magmas from Wudalianchi are poor in sulfur, normally ranging from 200 to 400ppm. On account of the behavior of sulfur in magmas and the strontium and oxygen isotopic analyses ((87Sr /86Sr)i=0.70503– 0.70589; δ18O = + 5.50 – + 6.89 ‰ ), it can be considered that the basanitic magmas in the Wudalianchi volcanic area came from the upper mantle and have not yet been contaminated probably by continental crust materials.  相似文献   

6.
Submarine volcanic rocks dredged during RV Meteor cruise M43-1 comprise alkali basalts, basanites, nephelinites and their differentiates representing both basement-shield and young post-shield volcanics of Gran Canaria, Tenerife, La Palma and El Hierro. The primitive lavas vary widely in trace element composition (e.g., Zr/Y=6.6-11.7, (La/Sm)N=2.3-5.4, and Ba/Yb=71-311), and they are characterized by steep, rare-earth element patterns with mean (La/Yb)N=16, and by pronounced, positive primitive mantle-normalized Nb and Ta and negative K anomalies similar to HIMU-type basalts. Rocks from the submarine flanks west and north of Gran Canaria are isotopically and geochemically identical to rocks of the subaerial Miocene shield stage, but they are distinct from rocks of the post-shield stages (Zr/Nb=6.3-8.9, 87Sr/86Sr=0.70327-0.70332, 143Nd/144Nd=0.51289-0.51293, 206Pb/204Pb=19.55-19.88). Most rocks dredged from the submarine flanks of Tenerife are isotopically and geochemically similar to rocks of the adjacent subaerial shield remnants, but a few resemble rocks of the subaerial post-shield stages (total range in Zr/Nb=4.6-6.1, 87Sr/86Sr=0.70300-0.70329, 143Nd/144Nd=0.51281-0.51292, 206Pb/204Pb=19.51-19.96). Rocks from the southern submarine ridge of La Palma cover the entire compositional range of the subaerial rocks of that ridge. Additionally, they comprise a high Zr/Nb group which resembles rocks of the ca. 1-Ma-old Taburiente shield of northern La Palma (total range in Zr/Nb=3.0-6.4, 87Sr/86Sr=0.70297-0.70314, 143Nd/144Nd=0.51288-0.51296, 206Pb/204Pb=19.21-19.79). Rocks from the southern submarine ridge of El Hierro compositionally resemble subaerial rocks of the island (Zr/Nb=4.1-6.2, 87Sr/86Sr=0.70296-0.70314, 143Nd/144Nd=0.51291-0.51297, 206Pb/204Pb=19.25-19.91). The degree of melting in the subcanarian mantle is interpreted to decrease from east to west across the archipelago whereas the proportion of depleted mantle component in the melting anomaly increases, as illustrated by Sr, Nd and Pb isotopes. The isotopic characteristics of the mantle source beneath the Canary Islands represents a mixture of HIMU, DMM and EM I. The overall isotopic signature is intermediate between that of Madeira to the north, which trends towards more depleted compositions, and that of the Cape Verde Islands to the south which shows a pronounced trend towards enriched mantle compositions. A clear trend towards the EM II component is only evident in more evolved rocks dredged from a seamount between Tenerife and Gran Canaria, some of which contain terrigenous sedimentary xenoliths. We propose a genetic model which relates similar mantle source signatures of volcanic archipelagos off West Africa to a common, large-scale lower mantle upwelling which, according to geophysical data, becomes more diffuse in the upper mantle. Narrow plumes or blobs feeding the volcanic centers along the passive margin may rise from this thermal anomaly due to upwelling in small, continent-parallel upper-mantle convection cells.  相似文献   

7.
Cooling rates have been determined for basaltic glasses from different depths of the submarine section of the drill core recovered in the 1999 phase of Hawaii Scientific Drilling Project (HSDP2). The glasses include degassed blocky hyaloclastite clasts and undegassed pillow rims. The degassed glassy clasts were generated in subaerial or shallow submarine environments, during explosive interactions between lava and seawater, before eventual deposition under water. The volatile contents of the glassy pillow rims are consistent with eruption and quenching in water several hundred metres deep. The cooling rates have been calculated from the calorimetric properties of the glass across the glass transition. The heat capacity (cp) of each sample was measured during several cycles of heating from room temperature to temperatures above their glass transition using a differential scanning calorimeter (DSC). Their compositions did not change during the thermal treatment, a prerequisite for successful cp measurements, although the glasses with higher H2O contents became more opaque and their mid-IR spectra changed. Each cp-T path exhibits the now classic features of the glass transition; glassy and liquid states separated by a hysteresis marking the transition. After experiencing the same experimental thermal history the glass transition occurs at lower temperatures in glasses with higher H2O contents. Except for one sample, the cp-T path measured on initial heating also releases energy stored during the natural quench, which is not recovered during subsequent experimental cooling. The energy stored in the HSDP2 glasses is much less than that observed in hyperquenched natural and synthetic glasses. Even so, the Tool-Narayanaswamy enthalpy relaxation geospeedometer, usually used to determine the cooling rates in volcanic glasses, is unable to deal with this energy release. For those samples that exhibit this feature an alternative method, developed for hyperquenched glasses, is applied. This uses the energy released to calculate Tf, from which the cooling rate is calculated. The degassed blocky hyaloclastite clasts exhibit cooling rates 0.1-72.2 K s−1, while the undegassed pillow rims span 0.2-46.4 K s−1. The fastest cooling rates are consistent with the cooling of lava bodies in seawater. The wide variation for both types of glass could reflect quenching at different distances from the basalt-seawater interface. However, for the degassed hyaloclastite clasts the range could indicate that the clasts were generated by different processes operating during the explosive interaction between lava and seawater in the littoral zone. In the undegassed pillow lavas, glassy rims may have been reheated, giving rise to more complex, slower, thermal histories, as a result of latent heat released during the crystallisation of pillow interiors, or flow replenishment. Both types of glass may also have experienced reheating from succeeding flows or deposits. Compared to deep-sea limu o Pele hyaloclastite fragments, whose hyperquench rates indicate simultaneous cooling and fragmentation, the shallow blocky hyaloclastite clasts may have formed during post-cooling brittle fragmentation.  相似文献   

8.
During its 1800-year-long persistent activity the Stromboli volcano has erupted a highly porphyritic (HP) volatile-poor scoriaceous magma and a low porphyritic (LP) volatile-rich pumiceous magma. The HP magma is erupted during normal Strombolian explosions and lava effusions, while the LP one is related to more energetic paroxysms. During the March–April 2003 explosive activity, Stromboli ejected two typologies of juvenile glassy ashes, namely highly vesicular LP shards and volatile-poor HP shards. Their textural and in situ chemical characteristics are used to unravel mutual relationships between HP and LP magmas, as well as magma dynamics within the shallow plumbing system. The mantle-normalized trace element patterns of both ash types show the typical arc-lava pattern; however, HP glasses possess incompatible element concentrations higher than LP glasses, along with Sr and Eu negative anomalies. HP shards are generally characterized by higher Li contents (to ~20 ppm) and lower δ7Li values (+1.2 to −3.8‰) with respect to LP shards (Li contents of 7–14 ppm and δ7Li ranging between +4.6 and +0.9‰). Fractional crystallization models based on major and trace element compositions, combined with a degassing model based on open-system Rayleigh distillation and on the assumption that melt/fluidDLi > 1, show that abundant (~30%) plagioclase precipitation and variable degrees of degassing can lead the more primitive LP magma to evolve toward a differentiated (isotopically lighter) HP magma ponding in the upper conduit and undergoing slow continuous degassing-induced crystallization. This study also evidences that in March 2003 Stromboli volcano poured out a small early volume of LP magma that traveled slower within the conduit with respect to later and larger volumes of fast ascending LP magma erupted during the April 5 paroxysm. The different ascent rates and cooling rates of the two LP magma batches (i.e., pre- and post-paroxysm) resulted in small, but detectable, differences in their chemical signatures. Finally, this study highlights the high potential of in situ investigations of juvenile glassy ashes in petrologic and geochemical monitoring the volcanic activity and of Li isotopes as tracers of degassing processes within the shallow plumbing system.  相似文献   

9.
Previous published data, combined with our results of 13 new radiocarbon ages and extensive geological fieldwork, indicate that during the past 11 ka 24 monogenetic basaltic eruptions occurred in the north sector of Gran Canaria. These eruptions can be grouped into three periods of eruptive activity: 1900–3200 14C a BP; 5700–6000 14C a BP; and an older period represented by only one eruption, El Draguillo, dated at 10 610 ± 190 14C a BP. Archaeological studies have shown that the more recent eruptions affected prehistoric human settlements on the island. Field studies demonstrate that the eruptions typically built strombolian cones (30–250 m in height) and associated relatively long lava flows (100–10 350 m in length); a few eruptions also produced tephra fall deposits. The total erupted volume of these eruptions is about 0.388 km3 (46.1% as tephra fall, 41.8% as cinder cone deposits and 12.1% as lava flows). The relatively low eruption rate (～0.04 km3 ka?1) during the past 11 ka is consistent with Gran Canaria's stage of evolution in the regional volcano‐tectonic setting of the Canary Archipelago. The results of our study were used to construct a volcanic hazards map that clearly delimits two sectors in the NE sector of Gran Canaria, where potential future eruptions would pose a substantial risk for densely populated areas. Copyright © 2009 John Wiley & Sons, Ltd.  相似文献   

10.
A review of published and newly measured densities for 40 hydrous silicate glasses indicates that the room-temperature partial molar volume of water is 12.0 ± 0.5 cm3/mol. This value holds for simple or mineral compositions as well as for complex natural glasses, from rhyolite to tephrite compositions, prepared up to 10–20 kbar pressures and containing up to 7 wt% H2O. This volume does not vary either with the molar volume of the water-free silicate phase, with its degree of polymerization or with water speciation. Over a wide range of compositions, this constant value implies that the volume change for the reaction between hydroxyl ions and molecular water is zero and that, at least in glasses, speciation does not depend on pressure. Consistent with data from Ochs and Lange (1997, 1999), systematics in volume expansion for SiO2–M2O systems (M=H, Li, Na, K) suggests that the partial molar thermal expansion coefficient of H2O is about 4 × 10−5 K−1 in silicate glasses. Received: 30 June 1999 / Accepted: 5 November 1999  相似文献   

11.
 Eruptions from Ruapehu Volcano on 11 and 14 October 1995 and 17 June 1996 distributed at least 36×106 m3 of sulphur(S)-rich tephra over the central and eastern North Island of New Zealand. The tephras added between 30–1500 kg ha–1 S to at least 25 000 km2 of land in primary production. Smaller but beneficial amounts of selenium (Se) and in some areas potassium and magnesium were also supplied. Addition of S to the soils in the form of sulphate and elemental S resulted in a drop in soil pH and an increase in pasture S contents within seven weeks of the eruptions. The soils affected by the tephra are naturally low in S and Se, but following the eruptions S was not required in fertilizer applications in many areas. The strongest and longest lasting effects of S and Se deposition were in high anion-retention soils particularly Hapludands (moist, moderately weathered soils, derived from volcanic ash). Soluble fluorine concentrations within the tephras were low compared to historic Icelandic and Chilean examples. However, pastoral livestock deaths were apparently caused by fluorosis in addition to starvation when tephra covered feed. The Ruapehu tephra contained very low concentrations of other soluble toxic elements. Received: 17 January 1997 · Accepted: 31 March 1997  相似文献   

12.
The Bandas del Sur Formation preserves a Quaternary extra-calderarecord of central phonolitic explosive volcanism of the LasCañadas volcano at Tenerife. Volcanic rocks are bimodalin composition, being predominantly phonolitic pyroclastic deposits,several eruptions of which resulted in summit caldera collapse,alkali basaltic lavas erupted from many fissures around theflanks. For the pyroclastic deposits, there is a broad rangeof pumice glass compositions from phonotephrite to phonolite.The phonolite pyroclastic deposits are also characterized bya diverse, 7–8-phase phenocryst assemblage (alkali feldspar+ biotite + sodian diopside + titanomagnetite + ilmenite + nosean–haüyne+ titanite + apatite) with alkali feldspar dominant, in contrastto interbedded phonolite lavas that typically have lower phenocrystcontents and lack hydrous phases. Petrological and geochemicaldata are consistent with fractional crystallization (involvingthe observed phenocryst assemblages) as the dominant processin the development of phonolite magmas. New stratigraphicallyconstrained data indicate that petrological and geochemicaldifferences exist between pyroclastic deposits of the last twoexplosive cycles of phonolitic volcanism. Cycle 2 (0·85–0·57Ma) pyroclastic fall deposits commonly show a cryptic compositionalzonation indicating that several eruptions tapped chemically,and probably thermally stratified magma systems. Evidence formagma mixing is most widespread in the pyroclastic depositsof Cycle 3 (0·37–0·17 Ma), which includesthe presence of reversely and normally zoned phenocrysts, quenchedmafic glass blebs in pumice, banded pumice, and bimodal to polymodalphenocryst compositional populations. Syn-eruptive mixing eventsinvolved mostly phonolite and tephriphonolite magmas, whereasa pre-eruptive mixing event involving basaltic magma is recordedin several banded pumice-bearing ignimbrites of Cycle 3. Theperiodic addition and mixing of basaltic magma ultimately mayhave triggered several eruptions. Recharge and underplatingby basaltic magma is interpreted to have elevated sulphur contents(occurring as an exsolved gas phase) in the capping phonoliticmagma reservoir. This promoted nosean–haüyne crystallizationover nepheline, elevated SO3 contents in apatite, and possiblyresulted in large, climatologically important SO2 emissions. KEY WORDS: Tenerife; phonolite; crystal fractionation; magma mixing; sulphur-rich explosive eruptions  相似文献   

13.
王岚  李文厚  林潼  梁浩  苟红光 《地质科学》2010,45(4):1088-1097
火山岩油气藏的研究一般以陆上喷发沉积的火山岩体为主,事实上全世界四分之三的火山活动是在水下发生的,只是水下喷发沉积的火山岩在各个方面的研究还比较少,在勘探中尚未引起注意。本文以三塘湖盆地石炭系火山岩油藏为例,对比分析了陆上与水下喷发沉积形成的火山岩在岩性、颜色、结构构造以及储集空间特征和地震反射特征等方面的差异,指出了三塘湖盆地石炭系哈尔加乌组火山岩为水下喷发环境形成。通过对研究区两种类型的火山岩油藏系统的研究,总结出了水下喷发沉积的火山岩储层的形成机理以及其形成规模油气藏的条件,为今后的火山岩油气藏的勘探指出了新的方向。  相似文献   

14.
The strata-bound Cu−Pb−Zn polymetallic sulfide deposits occur in metamorphic rocks of greenschist phase of the middle-upper Proterozoic Langshan Group in central Inner Mongolia. δ34S values for sulfides range from −3.1‰ to +37.3‰, and an apparent difference is noticed between vein sulfides and those in bedded rocks. For example, δ34S values for bedded pyrite range from +10.6‰ to +20.0‰, while those for vein pyrite vary from −3.1‰ to +14.1‰. δ34S of bedded pyrrhotite is in the range +7.9‰–+23.5‰ in comparison with +6.5‰–+17.1‰ for vein pyrrhotite. The wide scatter of δ34S and the enrichment of heavier sulfur indicate that sulfur may have been derived from H2S as a result of bacterial reduction of sulfates in the sea water. Sulfur isotopic composition also differs from deposit to deposit in this area because of the difference in environment in which they were formed. The mobilization of bedded sulfides in response to regional metamorphism and magmatic intrusion led to the formation of vein sulfides. δ18O and δ13C of ore-bearing rocks and wall rocks are within the range typical of ordinary marine facies, with the exception of lower values for ore-bearing marble at Huogeqi probably due to diopsidization and tremalitization of carbonate rocks. Pb isotopic composition is relatively stable and characterized by lower radio-genetic lead. The age of basement rocks was calculated to be about 23.9 Ma and ore-forming age 7.8 Ma.207Pb/204Pb−206Pb/204Pb and208Pb/204Pb−206Pb/204Pb plots indicate that Pb may probably be derived from the lower crust or upper mantle. It is believed that the deposits in this region are related to submarine volcanic exhalation superimposed by later regional metamorphism and magmatic intrusion.  相似文献   

15.
 Time-resolved luminescence spectra of natural and synthetic hydrous volcanic glasses with different colors and different Fe, Mn, and H2O content were measured, and the implications for the glass structure are discussed. Three luminescence ranges are observed at about 380–460, 500–560, and 700–760 nm. The very short-living (lifetimes less than 40 ns) blue band (380–460 nm) is most probably due to the 4T2(4D) →6A1(6S) and 4A1(4G) →6A1(6S) ligand field transitions of Fe3+. The green luminescence (500–560 nm) arises from the Mn2+ transition 4T1(4G) →6A1(6S). It shows weak vibronic structure, short lifetimes less than 250 μs, and indicates that Mn2+ is tetrahedrally coordinated, occupying sites with similar distortions and ion–oxygen interactions in all samples studied. The red luminescence (700–760 nm) arising from the 4T1(4G) →6A1(6S) transition of Fe3+ has much longer lifetimes of the order of several ms, and indicates that ferric iron is also mainly tetrahedrally coordinated. Increasing the total water content of the glasses leads to quenching of the red luminescence and decrease of the distortions of the Fe3+ polyhedra. Received: 30 July 2001 / Accepted: 15 November 2001  相似文献   

16.
The Beer–Lambert law is traditionally used to determine water and carbon concentrations in glasses from their infrared (IR) spectra. In practice, this method requires estimation of the thickness and density of the glass as well as the calibration of the molecular absorptivities of the species concerned. All of these parameters can be sources of practical difficulties and analytical uncertainty. These weaknesses in the application of the Beer–Lambert law have been overcome by an empirical analysis of the infrared spectra. Using a set of 292 spectra obtained on 113 natural and experimental tholeiitic glasses (SiO2 48.5–51 wt%; water contents 0–4000 ppm H2O), it can be shown that the thickness–density (ρ d) product of a glass sample can be directly and reliably inferred from its IR spectrum. This allows the Beer–Lambert law to be rewritten. The new form no longer requires thickness or density estimations to determine volatile contents. Moreover, if needed, the thickness of the glass slab can also be accurately determined from the IR spectra. This new method is developed for quantitative determination of water concentrations in MORB glasses but can also be applied to any minor species (carbon, sulfur, etc.) provided it is active in the IR domain and that a suitable independent frequency of IR absorption can be identified. Precision is about 60 ppm H2O on O–H− contents. This method, tested on natural and experimental MORB-type glasses, can be applied to any chemical composition provided a set of reference spectra is available. Received: 16 September 1999 / Accepted: 18 February 2000  相似文献   

17.
The basic and ultrabasic alkaline rocks of western Makhtesh Ramon, Israel crop out in numerous lava flows and subvolcanic bodies. The rock suite is composed of tephrite, basanite, basanitic nephelinite, analcimite, olivine nephelinite, and melilite-olivine nephelinite and in many outcrops is represented by glass-bearing varieties. Melt and fluid inclusions have been studied in olivine, clinopyroxene, and plagioclase phenocrysts. The EP, SIMS and microthermometry methods were used for inclusion study. The geochemical data obtained on glasses of melt inclusions (major, REE, trace elements, volatiles) are compared with the data on whole-rock and groundmass glass compositions. The compositions of melt inclusions reflect the different stages of rock crystallization: the initial products of crystallization are similar to whole-rock compositions whereas final portions of melts are usually enriched in SiO2, Al2O3, and alkalis, and depleted in mafic components. The data on contemporaneous melt and CO2 inclusions were used for the evaluation of the P–T conditions of rock generation. The following parameters were obtained: tephrite: P = 6.3–7.7 kbar and T = 1,150–1,250°C; basanite: P = 6.6–9.2 kbar and T = 1,150–1,250°C; olivine and analcime-olivine nephelinite: P = 5.6–8.2 kbar and T = 1,150–1,250°C; melilite-olivine nephelinite: 4.0–5.4 kbar and T mainly between 1,150 and 1,200°C. Magma genesis was restricted to P–T conditions of spinel- and plagioclase-lherzolite fields. These data suggest the shallowest depth of magma genesis occurred in Makhtesh Ramon compared to other occurrences of Early Cretaceous magmatism at the Middle East. Differences in the degree of batch partial melting of the same source rocks best explain the diversity of the igneous suite in western Makhtesh Ramon.  相似文献   

18.
We have identified an ash layer in association with Australasian microtektites of ∼0.77 Ma old in two sediment cores which are ∼450 km apart in the central Indian Ocean Basin (CIOB). Morphology and chemical composition of glass shards and associated microtektites have been used to trace their provenance. In ODP site 758 from Ninetyeast Ridge, ash layer-D (13 cm thick, 0.73–0.75 Ma) and layer-E (5 cm thick, 0.77–0.78 Ma) were previously correlated to the oldest Toba Tuff (OTT) eruptions of the Toba caldera, Sumatra. In this investigation, we found tephra ∼3100 km to the southwest of Toba caldera that is chemically identical to layer D of ODP site 758 and ash in the South China Sea correlated to the OTT. Layer E is not present in the CIOB or other ocean basins. The occurrence of tephra correlating to layer D suggests a widespread distribution of OTT tephra (∼3.6 × 107 km2), an ash volume of at least ∼1800 km3, a total OTT volume of 2300 km3, and classification of the OTT eruption as a super-eruption.  相似文献   

19.
The composition of S-rich apatite, of volatile-rich glass inclusions in apatite, and of interstitial glasses in alkaline xenoliths from the 1949 basanite eruption in La Palma has been investigated to constrain the partitioning of volatiles between apatite and alkali-rich melts. The xenoliths are interpreted as cumulates from alkaline La Palma magmas. Apatite contains up to 0.89 wt% SO3 (3560 ppm S), 0.31 wt% Cl, and 0.66 wt% Ce2O3. Sulfur is incorporated in apatite via several independent exchange reactions involving (P5+, Ca2+) vs. (S6+, Si4+, Na+, and Ce3+). The concentration of halogens in phonolitic to trachytic glasses ranges from 0.15 to 0.44 wt% for Cl and from <0.07 to 0.65 wt% for F. The sulfur concentration in the glasses ranges from 0.06 to 0.23 wt% SO3 (sulfate-saturated systems). The chlorine partition coefficients (DClapatite/glass) range from 0.4 to 1.3 (average DClapatite/glass = 0.8), in good agreement with the results of experimental data in mafic and rhyolitic system with low Cl concentrations. With increasing F in glass inclusions DFapatite/glass decreases from 35 to 3. However, most of our data display a high partition coefficient (~30) close to DFapatite/glass determined experimentally in felsic rock. DSapatite/glass decreases from 9.1 to 2.9 with increasing SO3 in glass inclusions. The combination of natural and experimental data reveals that the S partition coefficient tends toward a value of 2 for high S content in the glass (>0.2 wt% SO3). DSapatite/glass is only slightly dependent on the melt composition and can be expressed as: SO3 apatite (wt%) = 0.157 * ln SO3 glass (wt%) + 0.9834. The phonolitic compositions of glass inclusions in amphibole and haüyne are very similar to evolved melts erupted on La Palma. The lower sulfur content and the higher Cl content in the phonolitic melt compared to basaltic magmas erupted in La Palma suggest that during magma evolution the crystallization of haüyne and pyrrhotite probably buffered the sulfur content of the melt, whereas the evolution of Cl concentration reflects an incompatible behavior. Trachytic compositions similar to those of the (water-rich) glass inclusions analyzed in apatite and clinopyroxene are not found as erupted products. These compositions are interpreted to be formed by the reaction between water-rich phonolitic melt and peridotite wall-rock.  相似文献   

20.
We studied the systematics of Cl, F and H2O in Izu arc front volcanic rocks using basaltic through rhyolitic glass shards and melt inclusions (Izu glasses) from Oligocene to Quaternary distal fallout tephra. These glasses are low-K basalts to rhyolites that are equivalent to the Quaternary lavas of the Izu arc front (Izu VF). Most of the Izu glasses have Cl ∼400-4000 ppm and F ∼70-400 ppm (normal-group glasses). Rare andesitic melt inclusions (halogen-rich andesites; HRA) have very high abundances of Cl (∼6600-8600 ppm) and F (∼780-910 ppm), but their contents of incompatible large ion lithophile elements (LILE) are similar to the normal-group glasses. The preeruptive H2O of basalt to andesite melt inclusions in plagioclase is estimated to range from ∼2 to ∼10 wt% H2O. The Izu magmas should be undersaturated in H2O and the halogens at their preferred levels of crystallization in the middle to lower crust (∼3 to ∼11 kbar, ∼820° to ∼1200°C). A substantial portion of the original H2O is lost due to degassing during the final ascent to surface. By contrast, halogen loss is minor, except for loss of Cl from siliceous dacitic and rhyolitic compositions. The behavior of Cl, F and H2O in undegassed melts resembles the fluid mobile LILE (e.g.; K, Rb, Cs, Ba, U, Pb, Li). Most of the Cl (>99%), H2O (>95%) and F (>53%) in the Izu VF melts appear to originate from the subducting slab. At arc front depths, the slab fluid contains Cl = 0.94 ± 0.25 wt%, F = 990 ± 270 ppm and H2O = 25 ± 7 wt%. If the subducting sediment and the altered basaltic crust were the only slab sources, then the subducted Cl appears to be almost entirely recycled at the Izu arc (∼77-129%). Conversely, H2O (∼13-22% recycled at arc) and F (∼4-6% recycled) must be either lost during shallow subduction or retained in the slab to greater depths. If a seawater-impregnated serpentinite layer below the basaltic crust were an additional source of Cl and H2O, the calculated percentage of Cl and H2O recycled at arc would be lower. Extrapolating the Izu data to the total length of global arcs (∼37,000 km), the global arc outflux of fluid-recycled Cl and H2O at subduction zones amounts to Cl ∼2.9-3.8 × 1012 g/yr and H2O ∼0.7-1.0 × 1014 g/yr, respectively—comparable to previous estimates. Further, we obtain a first estimate of global arc outflux of fluid-recycled F of ∼0.3-0.4 × 1012g/yr. Despite the inherent uncertainties, our results support models suggesting that the slab becomes strongly depleted in Cl and H2O in subduction zones. In contrast, much of the subducted F appears to be returned to the deep mantle, implying efficient fractionation of Cl and H2O from F during the subduction process. However, if slab devolatilization produces slab fluids with high Cl/F (∼9.5), slab melting will still produce components with low Cl/F ratios (∼0.9), similar to those characteristic of the upper continental crust (Cl/F ∼0.3-0.9).  相似文献   

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