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1.
On 16 July 1945, the first atomic bomb was detonated at the Alamogordo Bombing range in New Mexico, USA. Swept up into the nuclear cloud was the surrounding desert sand, which melted to form a green glassy material called ‘trinitite’. Contained within the glass are melted bits of the first atomic bomb and the support structures and various radionuclides formed during the detonation. The glass itself is marvelously complex at the tens to hundreds of micrometre scale, and besides glasses of varying composition also contains unmelted quartz grains. Air transport of the melted material led to the formation of spheres and dumbbell shaped glass particles. Similar glasses are formed during all ground level nuclear detonations and contain forensic information that can be used to identify the atomic device.  相似文献   

2.
The regolith of the Apollo 16 lunar landing site is composed mainly of feldspathic lithologies but mafic lithologies are also present. A large proportion of the mafic material occurs as glass. We determined the major element composition of 280 mafic glasses (>10 wt% FeO) from six different Apollo 16 soil samples. A small proportion (5%) of the glasses are of volcanic origin with picritic compositions. Most, however, are of impact origin. Approximately half of the mafic impact glasses are of basaltic composition and half are of noritic composition with high concentrations of incompatible elements. A small fraction have compositions consistent with impact mixtures of mare material and material of the feldspathic highlands. On the basis of major-element chemistry, we identified six mafic glass groups: VLT picritic glass, low-Ti basaltic glass, high-Ti basaltic glass, high-Al basaltic glass, KREEPy glass, and basaltic-andesite glass. These glass groups encompass 60% of the total mafic glasses studied. Trace-element analyses by secondary ion mass spectroscopy for representative examples of each glass group (31 total analyses) support the major-element classifications and groupings. The lack of basaltic glass in Apollo 16 ancient regolith breccias, which provide snapshots of the Apollo 16 soil just after the infall of Imbrium ejecta, leads us to infer that most (if not all) of the basaltic glass was emplaced as ejecta from small- or moderate-sized impacts into the maria surrounding the Apollo 16 site after the Imbrium impact. The high-Ti basaltic glasses likely represent a new type of basalt from Mare Tranquillitatis, whereas the low-Ti and high-Al basaltic glasses possibly represent the composition of the basalts in Mare Nectaris. Both the low-Ti and high-Al basaltic glasses are enriched in light-REEs, which hints at the presence of a KREEP-bearing source region beneath Mare Nectaris. The basaltic andesite glasses have compositions that are siliceous, ferroan, alkali-rich, and moderately titaniferous; they are unlike any previously recognized lunar lithology or glass group. Their likely provenance is within the Procellarum KREEP Terrane, but they are not found within the Apollo 16 ancient regolith breccias and therefore were likely deposited at the Apollo 16 site post-Imbrium. The basaltic-andesite glasses are the most ferroan variety of KREEP yet discovered.  相似文献   

3.
Complexities in the nature of large-scale silicic eruptions and their magmatic systems can be discerned through micro-analytical geochemical studies. We present high-resolution, stratigraphically constrained compositional data on glassy matrix material and feldspar crystals from the initial fall deposits and earliest ignimbrite (base of member A) of the 2.08 Ma, ~?2500 km3 Huckleberry Ridge Tuff (HRT), Yellowstone. We use these data to document the nature of the magmatic system and compositional changes related to the transition from fall to widespread ignimbrite deposition, inferred to mark the onset of caldera collapse. Although major element glass compositions are relatively uniform, trace elements span a large range (e.g. Ba 10–900 ppm, Sr/Rb?=?0.005–0.09), with highly evolved glasses dominating in the fall deposits. Several trace elements (e.g. Ba and light rare earth elements) in the glass samples serve to define statistically significant compositional clustering in the fall deposits and basal ignimbrite. These clusters are inferred to reflect melt compositions controlled by fractional crystallisation processes and are interpreted to represent multiple, discrete melt-dominant domains that were tapped by multiple vents. The onset of widespread ignimbrite deposition is marked by an increase in the number of erupted melt compositional clusters from four in the fall deposits to eight, representing nine melt-dominant domains. There is an absence of geographical variation of glass compositions within the basal ignimbrite, with samples from proximal to distal localities north, west and south of the HRT caldera exhibiting similar variability. Pairing of glass analyses with sanidine major and minor element compositional data suggests that the nine melt compositional domains converged at depth into two compositionally distinct upper-crustal magmatic lineages that were both active during these early stages of the eruption. Our data collectively indicate the evacuation of an exceptionally complex and heterogeneous magma system. The onset of widespread ignimbrite deposition, inferred to relate to caldera collapse, occurred after ~ 50 km3 of magma had been discharged. Although external controls were important as an eruption trigger, depressurisation of the system led to caldera collapse with the eruption of numerous discrete melt-dominant domains.  相似文献   

4.
The nature of compositional heterogeneity within large silicic magma bodies has important implications for how silicic reservoirs are assembled and evolve through time. We examine compositional heterogeneity in the youngest (~170 to 70 ka) post-caldera volcanism at Yellowstone caldera, the Central Plateau Member (CPM) rhyolites, as a case study. We compare 238U–230Th age, trace-element, and Hf isotopic data from zircons, and major-element, Ba, and Pb isotopic data from sanidines hosted in two CPM rhyolites (Hayden Valley and Solfatara Plateau flows) and one extracaldera rhyolite (Gibbon River flow), all of which erupted near the caldera margin ca. 100 ka. The Hayden Valley flow hosts two zircon populations and one sanidine population that are consistent with residence in the CPM reservoir. The Gibbon River flow hosts one zircon population that is compositionally distinct from Hayden Valley flow zircons. The Solfatara Plateau flow contains multiple sanidine populations and all three zircon populations found in the Hayden Valley and Gibbon River flows, demonstrating that the Solfatara Plateau flow formed by mixing extracaldera magma with the margin of the CPM reservoir. This process highlights the dynamic nature of magmatic interactions at the margins of large silicic reservoirs. More generally, Hf isotopic data from the CPM zircons provide the first direct evidence for isotopically juvenile magmas contributing mass to the youngest post-caldera magmatic system and demonstrate that the sources contributing magma to the CPM reservoir were heterogeneous in 176Hf/177Hf at ca. 100 ka. Thus, the limited compositional variability of CPM glasses reflects homogenization occurring within the CPM reservoir, not a homogeneous source.  相似文献   

5.
Rock 14318 is a complex microbreccia consisting of lithic fragments, chondrules, glass spherules, and glass and mineral fragments that are embedded into a fine-grained, partly glassy matrix. Rock fragmenta, chondrules, and glasses are tightly welded to the matrix and partly recrystallized, indicating a relatively high-temperature agglomeration history. Few lithic fragments have igneous textures; most are miorobreccias that have suffered various degrees of recrystallization before they were embedded into rock 14318. Compositions of lithic fragments, glasses and chondrules, in terms of compositional rock and rock suite equivalents, represent members of the ANT (anorthositic-noritic-troctolite) suite; the alkalic high-alumina basalt (KREEP) group; high-alkali quartz basalt; basalt; and dunite. The polymict nature of many lithic fragments suggests that rook 14318 require at least two, and probably more, impact episodes for its formation. Final agglomeration took place while part of the material was hot, as is indicated by the welded texture, suggesting that the final impact event was a large one, producing a fiery cloud similar to a nuée ardente. The close similarity in texture of lunar rock 14318 to certain polymict-brecciated meteorites such as Siena suggests that meteorites of this type were also formed by complex and successive impact events on the surface of the meteorite parent body, rather than during agglomeration of the parent body.  相似文献   

6.
An analytical artefact is reported here related to differences in instrumental mass fractionation between NIST SRM glasses and natural geological glasses during SIMS boron isotope determinations. The data presented demonstrated an average 3.4‰ difference between the NIST glasses and natural basaltic to rhyolitic glasses mainly in terms of their sputtering-induced fractionation of boron isotopes. As no matrix effect was found among basaltic to rhyolitic glasses, instrumental mass fractionation of most natural glass samples can be corrected by using appropriate glass reference materials. In order to confirm the existence of the compositionally induced variations in boron SIMS instrumental mass bias, the observed offset in SIMS instrumental mass bias has been independently reproduced in two laboratories and the phenomenon has been found to be stable over a period of more than one year. This study highlights the need for a close match between the chemical composition of the reference material and the samples being investigated.  相似文献   

7.
Lunar meteorite Northwest Africa 773 (herein referred to as NWA773) is a breccia composed predominantly of mafic volcanic components, including a prominent igneous clast lithology. The clast lithology is an olivine-gabbro cumulate, which, on the basis of mineral and bulk compositions, is a hypabyssal igneous rock related compositionally to volcanic components in the meteorite. The olivine-gabbro lithology exhibits cumulus textures and, in our largest section of it, includes some 48% olivine (Fo64 to Fo70, average Fo67), 27% pigeonite (En60Fs24Wo16 to En67Fs27Wo6), 11% augite (En50Fs17Wo33 to En47Fs13Wo40), 2% orthopyroxene (En70Fs26Wo4), 11% plagioclase (An80 to An94), and trace barian K-feldspar, ilmenite, Cr-spinel, RE-merrillite, troilite, and Fe-Ni metal. The Mg/Fe ratios of the mafic silicates indicate equilibration of Fe and Mg; however, the silicates retain compositional variations in minor and trace elements that are consistent with intercumulus crystallization. Accessory mineralogy reflects crystallization of late-stage residual melt. Both lithologies (breccia and olivine cumulate) of the meteorite have very-low-Ti (VLT) major-element compositions, but with an unusual trace-element signature compared to most lunar VLT volcanic compositions, i.e., relative enrichment in light REE and large-ion-lithophile elements, and greater depletion in Eu than almost all other known lunar volcanic rocks. The calculated composition of the melt that was in equilibrium with pyroxene and plagioclase of the cumulate lithology exhibits a KREEP-like REE pattern, but at lower concentrations. Melt of a composition calculated to have been in equilibrium with the cumulate assemblage, plus excess olivine, yields a major-element composition that is similar to known green volcanic glasses. One volcanic glass type from Apollo 14 in particular, green glass B, type 1, has a very low Ti concentration and REE characteristics, including extremely low Eu concentration, that make it a candidate parent melt for the olivine-gabbro cumulate. We infer an origin for the parent melt of NWA773 volcanic components by assimilation of a trace-element-rich partial or residual melt by a magnesian, VLT magma deep in the lunar crust or in the mantle prior to transportation to the near-surface, accumulation of olivine and pyroxene in a shallow chamber, eruption onto a volcanic surface, and incorporation of components into local, predominantly volcanic regolith, prior to impact mixing of the volcanic terrain and related hypabyssal setting, and ejection from the surface of the Moon. Volcanic components such as these probably occur in the Oceanus Procellarum region near the site of origin of the green volcanic glasses found in the Apollo 14 regolith.  相似文献   

8.
Melt glasses for seven types of rock ranging from acid to basic were prepared under 1.0 GPa on a multi-anvil pressure apparatus, YJ-3000 ton press. Densities and elastic properties of the melt glasses were compared with those described in previous studies. It was found that the glasses melted under 1.0 GPa were consistent in density with both naturally-occurring glassy rocks and artificially prepared glasses melted at ambient pressure. The densities of glasses are negatively correlated with the SiO2 contents and positively correlated with the （MgO＋FeO） contents. The compressive velocity （Vp） of glass tends to increase with decreasing SiO2 contents and increasing （MgO＋FeO） contents. The shear velocity （Vs） of glass tends to increase slightly with increasing SiO2 contents, which has little connection with the （MgO＋FeO） contents. It was calculated from densities and velocities that the elastic moduli of glasses are negatively correlated with the SiO2 contents and positively correlated with the （MgO＋FeO） contents.  相似文献   

9.
The exposed crust consists mainly of plagioclase (35%), quartz (20%), K-feldspar (11%), volcanic glass (12%), biotite (8%), and muscovite (5%). Quartz is a resistate, thus feldspars and glass represent approximately 75 percent of the labile minerals. The weathering characteristics of these constituents are summarized in the context of thermodynamic, mass balance and kinetic considerations.Experimentally determined release rate constants were used to predict the proportions of Ca, Na and K released by feldspars of plutonic rocks (granites to gabbros) to weathering solutions. The chemical weathering trends of the weathered residues, calculated from the kinetic data, conform closely to the initial trends observed in some recent weathering profiles, demonstrating the accuracy of the predictions. Since the weathering of feldspars is controlled by processes that should not change through geological time, the relative release rates of Ca, Na, and K from the feldspars of granitic rocks can be calculated for future and past episodes of continental weathering.Experimentally determined release rate constants are not available for a wide range of volcanic glass compositions, but the limited data indicate that compositional trends are predictable in weathering profiles developed on volcanic rocks. The kinetic data available for rhyolitic glasses accurately predict the initial weathering trends observed in a recent rhyolite weathering profile.  相似文献   

10.
 Interstitial glasses and glasses in small inclusions in Mg-rich phenocrysts of 14 boninites from New Caledonia, the Mariana Trench, Cape Vogel and Chichi-jima were analysed by electron microprobe and the water contents measured in situ by ion microprobe. The glasses are remarkably fresh and abundant (∼30–90 vol.%), and the phenocrysts are often skeletal with glass inclusions. Broad-beam analyses (∼1030) of interstitial glasses and ∼180 point analyses of glass inclusions were carried out, as well as ∼100 hydrogen analyses. Most glasses have low water-free totals, high water contents, very low MgO, and low total iron; they are almost entirely quartzofeldspathic and with few exceptions (Q+or+ab+ an+C) lies in the range 83–96. The interstitial glasses from New Caledonia, the Marianas and most of the glasses from Chichi-jima are dacitic, those from Cape Vogel straddle dacitic and andesitic compositions, whereas the glasses in a highly glassy sample from Chichi-jima are high-Mg andesitic or boninitic with up ∼9 wt% MgO, and are, with the exception of a few high-Ca boninites from Tonga, the most Mg-rich interstitial glasses so far described in boninites. Glasses included in orthopyroxene, olivine or clinoenstatite are boninitic or high-Mg andesitic in the highly glassy rock and dacitic to high-silica dacitic in the others. They are in general slightly more differentiated than the interstitial glasses, because of more-extensive crystallization on the host crystal in small inclusions. The interstitial glass compositions show a direct relationship between silica and Al2O3 and, for most glasses, alkalis, and inverse relationships between silica and CaO, FeO and MgO; alkalis and TiO2 show, however, a broad spread in values in glasses from the Marianas and New Caledonia. Included glasses show similar variations. Water contents in interstitial glasses are ∼2 wt% for the highly glassy high-Mg andesitic glasses from Chichi-jima, ∼5.4 wt% for the more differentiated andesitic to dacitic glasses from Cape Vogel, and ∼6.7–7.0 wt% in the most differentiated dacitic ones from the Marianas and New Caledonia. Water contents in glass inclusions in olivine, orthopyroxene and clinoenstatite are in the range ∼1.9–3.3 wt%. The interstitial glasses are black and not vesicular, showing that the liquids did not reach supersaturation after eruption on or intrusion near the sea floor, or were insufficiently so to allow nucleation of water vapour bubbles. The water is inferred to be primary and to increase strongly with crystallization in the residual liquid down to the glass-transition T. Received: 19 December 1994 / Accepted: 5 October 1995  相似文献   

11.
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   

12.
松散物质是流域侵蚀演化的产物,对于泥石流流域而言,物质供给能力影响着泥石流的易发程度和活动频率。以岷江上游都江堰—汶川部分区域147个泥石流小流域为例,运用面积?高程法和面积?坡度积分对流域地貌演化阶段和侵蚀强度定量划分,并结合地貌演化阶段和侵蚀强度开展泥石流物质供给能力研究。研究结果表明:采用单一的地貌演化阶段或侵蚀强度解释泥石流的易发程度具有一定局限性,泥石流暴发主要集中于壮年期、壮年偏幼年期及侵蚀强度Ⅲ～Ⅴ级;随着物质供给能力的提升,泥石流的暴发率上升,而面积对泥石流的暴发有一定限制作用,初步确定研究区内供给能力处中、强、极强三种水平泥石流流域,其优势发育面积范围分别为:20～35 km2、10～50 km2、10～40 km2;对于物质供给能力水平较高、面积处于优势发育范围内、且长期未有明显泥石流活动迹象的流域,应进一步排查流域松散物质储量和分布特征,确定泥石流活动稳定性,采取合理的防灾减灾措施。  相似文献   

13.
The distribution of H2O, F, Cl and S in the Campanian Ignimbrite (CI) magma chamber was investigated through study of primary glass inclusions and matrix glasses from pumices of the Plinian fall deposit. The eruption, fed by trachytic to phono-trachytic magmas, mainly produced a trachytic non-welded to partially welded tuff, underlain by a minor cogenetic fallout deposit. The entire chemical variability of the eruptive products is well represented in the pumices of the Plinian fall deposit, which we divide into a basal Lower Fall Unit (LFU) and an overlying Upper Fall Unit (UFU). Primary glass inclusions were only found in clinopyroxenes associated with the LFU pumice and contain a mean of 1.60ǂ.32 wt% H2O (analysed by FTIR), 0.11ǂ.08 wt% F, 0.37ǂ.03 wt% Cl and 0.08ǂ.04 wt% SO3 (EMP analysis); CO2 concentrations were below the FTIR detection limit (10-20 ppm). The coexisting matrix glasses contain similar amounts of halogens and sulfur but less water (~0.60 wt%). Partially degassed matrix glasses from UFU pumices contain a mean of 0.30ǂ.02 H2O, 0.28ǂ.10 F, 0.04ǂ.02 SO3 and 0.80ǂ.04 wt% Cl. To reconstruct the total amount of volatiles dissolved in the most evolved trachytes we have used experimental solubility data and mass balance calculations concerning the amount of crystal fractionation required to produce the most evolved trachyte from the least evolved trachyte; these yield an estimated pre-eruptive magma volatile content (H2O + Cl + F) of ~5.5 wt% for the most evolved magmas. On the basis of new determinations of Cl solubility limits in hydrous trachytic melts coexisting with an aqueous fluid phase + hydrosaline melt (brine), we suggest that the upper part of the magma chamber which fed the CI eruption was fluid(s) saturated and at a minimum depth of ~2 km. Variations in eruptive style (Plinian fallout, pyroclastic flows) do not appear to be related to significant variations in pre-eruptive volatile contents.  相似文献   

14.
Melt droplets formed by the impact of a large meteorite impact event(s) have been found in upper Eocene marine sediments from the Atlantic, Pacific, and Indian oceans. The number of discreet impacts that occurred to form these melt droplets can be gleaned by compositionally analysing the droplets and establishing the number of distinct groups. Previous studies have inferred two, three, or more source impact events, although we believe the statistical techniques used to distinguish the groupings are open to criticism. Multivariate and univariate analysis (including discriminant analysis) of the major-element composition of an increased data set of impact melt ejecta droplets have been performed. The results demonstrate that the uppermost ejecta layer is geochemically distinct from other late Eocene melt ejecta. Our statistical analysis suggests two underlying, purportedly stratigraphically separate ejecta layers, possess minimal differences that cannot be distinguished clearly from one another by discriminant analysis, which adds to the plausibility that they have a common source. Finally, our results reveal apparent disparities between the new major-element data from this study and data compiled from existing sources.  相似文献   

15.
Glasses have been analysed from six mantle-derived xenoliths (5 orthopyroxene and/or olivine-rich, 1 clinopyroxene-rich) from the Quaternary volcanics S.E. of Gees, West Eifel, Germany. The glasses in these xenoliths occur as pools surrounding and embaying spinels, as inclusions in spinels, as veins and stringers within phlogopiterich veins, and as jackets partially surrounding some of the xenoliths. Glasses analysed are felsic and characterised by low to intermediate SiO2 (40–62 wt.%), variable CaO (1–11 wt.%) and MgO (1–4 wt.%), high Al2O3 (14–21 wt.%), and up to 11 wt.% Na2O + K2O. The jacket glasses have the lowest SiO2, highest CaO and MgO. Variations in all of the glass compositions are similar and imply a unifying factor or process in their formation. Glass as pools and stringers within veins of phlogopite forms part of the same trends shown by all the glasses when plotted on bivariate (oxide vs SiO2) diagrams but can be distinguished from glass surrounding and enclosed by spinels. Glasses occurring as jackets are similar in composition to those in pools and veinlets associated with phlogopite but are of quite different composition to the glasses found within the xenoliths that they partially enclose. The occurrence and chemistry of the glasses do not support such glasses as representing original or differentiated magma trapped during formation of the xenolithic assemblages. The chemistry of the glasses also makes it unlikely that they were produced by dissociation of phlogopite during ascent of the xenoliths. The most likely origin for the glasses is that they represent volatile-rich melts which migrated through upper mantle material. These melts are likely to be responsible for the heterogeneous nature of the mantle underlying this part of the West Eifel region.  相似文献   

16.
Six siliceous glass spheres, five siliceous glass-bonded agglutinates and one breccia fragment from Luna 20 LRL sample number 22003 were analyzed by optical microscope, scanning electron microscope, scanning electron microprobe and energy-dispersive techniques. The data suggest that most of the glass spheres were probably derived locally by meteoritic impact processes and that most craters on their surfaces may have occurred from impacts of relatively high velocity particles in the impact-produced debris cloud while the glass sphere was at elevated temperatures. This is suggested by the nature of the craters, the partially buried fragments of plagioclase surrounded by radiating fractures and by the apparent absence of craters on the glass surfaces of the glass-bonded agglutinates. One glass sphere has a surface suggestive of a complex multiple impact origin involving liquid siliceous material and numerous siliceous spherules from 0.1 μm to 1 μm in diameter that may have formed from vaporization and condensation processes possibly in a relatively large scale meteoritic impact event.The surfaces of the siliceous glass spheres have several different types of materials. Concentration of metallic iron spherules on the surfaces of the glass spheres is generally lower than for similar Apollo 11 and 12 glass spheres. This is consistent with reduction processes being of primary importance in the formation of this metallic iron. Surface material composed only of Ca, C and O2, possibly CaCO3, is probably derived from carbonaceous chondrites. Splashes of material rich in Ca, Al, Fe, K and Cl occur. The origin of the relatively low temperature chlorine-bearing melt is unknown but it may be related to vaporization and condensation processes, possibly volcanic in nature, or possibly to partial fusion of components of carbonaceous chondrites. Siliceous surface material rich in potassium may represent either fused splash material of granitic composition or material enriched by vaporization and condensation processes.  相似文献   

17.
About 12.3 km3 of basaltic magma were erupted from the Lakagigar fissure in Iceland in 1783, which may have been derived from the high-level reservoir of Grimsvotn central volcano, by lateral flow within the rifted crust. We have studied the petrology of quenched, glassy tephra from sections through pyroclastic cones along the fissure. The chemical composition of matrix glass of the 1783 tephra is heterogeneous and ranges from olivine tholeiite to Fe–Ti rich basalt, but the most common magma erupted is quartz tholeiite (Mg#43.6 to 37.2). The tephra are characterized by low crystal content (5 to 9 vol%). Glass inclusions trapped in plagioclase and Fo86 to Fo75 olivine phenocrysts show a large range of compositions, from primitive olivine tholeiite (Mg#64.3), quartz tholeiite (Mg#43–37), to Fe–Ti basalts (Mg#33.5) which represent the most differentiated liquids and are trapped as rare melt inclusions in clinopyroxene. Both matrix glass and melt inclusion data indicate a chemically heterogeneous magma reservoir, with quartz tholeiite dominant. LREE-depleted olivine-tholeiite melt-inclusions in Mg-rich olivine and anorthitic-plagioclase phenocrysts may represent primitive magma batches ascending into the reservoir at the time of the eruption. Vesicularity of matrix glasses correlates with differentiation, ranging from 10 to 60 vol.% in evolved quartz-tholeiite glasses, whereas olivine-tholeiite glasses contain less than 10 vol.% vesicles. FTIR analyses of olivine-tholeiite melt-inclusions indicate concentrations of 0.47 wt% H2O and 430 to 510 ppm for CO2. Chlorine in glass inclusions and matrix glasses increases from 50 ppm in primitive tholeiite to 230 ppm in Fe–Ti basalts, without clear evidence of degassing. Melt inclusion analyses show that sulfur varies from 915 ppm to 1970 ppm, as total FeO* increases from 9 to 13.5 wt%. Sulfur degassing correlates both with vesicularity and magma composition. Thus sulfur in matrix glasses decreases from 1490 ppm to 500 ppm, as Mg # decreases from 47 to 37 and vesicularity of the magma strongly increases. These results indicate loss of at least 75% of sulfur during the eruption. The correlation of low sulfur content in matrix glasses with high vesicularity is regarded as evidence of the control of a major exsolving volatile phase on the degassing efficiency of the magma. Our model is consistent a quasi-permanent CO2 flux through the shallow-level magmatic reservoir of Grimsvotn. Following magma withdrawal from the reservoir and during eruption from the Lakagigar fissure, sulfur degassing was controlled by inherent CO2-induced vesicularity of the magma.  相似文献   

18.
Extensive compositional heterogeneity is shown to affect at least twenty four of the doped trace elements in the NIST SRM 610-617 glasses.
Compositional profiling and mapping using laser ablation ICP-MS reveals that all NIST SRM 610-617 wafers examined here contain domains that are significantly depleted in Ag, As, Au, B, Bi, Cd, Cr, Cs, Mo, Pb, Re, (Rh), Sb, Se, Te, Tl and W, and antithetically enriched in Cu (and Pt), with large enrichments in Cd, Fe and Mn also being encountered in some cases. These domains are visible in doubly polished wafers by unaided visual inspection and by transmitted light and schlieren microscopy. They occur in close proximity to the wafer perimeters and also as stretched and complexly folded forms within wafer interiors. The chemical and optical properties of these heterogeneous domains are consistent with those of compositional cords, a phenomenon of glass manufacture where glass bulk composition and physical properties are modified by loss of volatile components from the molten glass surface. The NIST SRM 610-617 glasses may be considered reliable reference materials for microanalysis of only between one half and two thirds of the trace elements with which they were doped, including Be, Mg, Sr, Ba, Sc, Y, REE, V, Zr, Hf, Nb, Ta, Th, U, Ga, In, Sn, Co, Ni and Zn. These elements show no evidence of significant heterogeneity, indicating that the original glass constituents and possible residues remaining in the furnace from preceding glass batch fusions were well homogenised during manufacture.  相似文献   

19.
In Russia, highly radioactive liquid wastes from recycling of spent fuel of nuclear reactors are solidified into Na–Al–P glass for underground storage. The properties of the matrix including the radionuclide fixation will change with time due to crystallization. This is supported by the results of study of the interaction between glassy matrices, products of their crystallization, and water. The concentration of Cs in a solution at the contact of a recrystallized sample increased by three orders of magnitude in comparison with an experiment with glass. This difference is nearly one order of magnitude for Sr, Ce, and Nd (simulators of actinides) and U due to their incorporation into phases with low solubility in water. Based on data on the compositional change of solutions after passing through filters of various diameters, it is concluded that Cs occurs in the dissolved state in runs with a glass and recrystallized matrix. At the same time, Sr, lanthanides, and U occur in the dissolved state and in the composition of colloids in runs with glass, and mostly in colloid particles after contact with the recrystallized sample. These results should be regarded for substantiation of safety for geological waste storage.  相似文献   

20.
In the suevite breccia of the Ries impact crater, Germany, glasses occur as bombs, and small particles in the groundmass. These glasses were formed from melt produced by shock fusion of crystalline basement rocks. Ejection from the crater resulted in the formation of aerodynamically shaped bombs, a few homogeneous spherules and a large mass of small glass particles which were deposited in the suevite breccia. Bombs and small particles included within chilled bottom and top layers of suevite deposits have been preserved in vitreous state, whereas glasses within the interior of the suevite devitrified, due to slower cooling rates.This paper summarizes the results of petrographical and chemical investigations of suevite glasses and their devitrification products. Conclusions are derived on origin and history of bombs and glass particles.Vitreous bombs and glass particles consist of schlieren-rich glass, mineral fragments (mainly quartz), rock fragments and vesicles. Wet chemical, trace element and microprobe analyses reveal that a primary melt was formed by shock fusion of a basement complex, consisting of about 80% biotite granite and 20% amphibolite. The, originally, more than 1800° C hot melt then incorporated shocked and desintegrated rocks of outer zones of the impact. Partial fusion of the rock debris resulted in a polyphase mixture consisting of melts, different in composition, accumulations of refractory mineral fragments and vesicles.Devitrified bombs and glass particles which are found in the interior of suevite deposits show alterations of texture and composition, due to microcrystallite growth and action of hydrothermal and weathering solutions. Incipient devitrification is indicated by brown staining of the glasses, originating, probably, by exsolution of minute magnetite particles. By optical microscopy and X-ray analysis, plagioclase and pyroxenes have been identified as main devitrification products. Shapes and textures of microcrystallites indicate fast crystal growth in a viscous and supercooled medium. Hot fluids permeating the suevite deposited microcrystalline quartz in vesicles and cracks. Later, montmorillonite was precipitated by solutions corroding the glass. Action of solutions on glasses which were weakened in coherence by devitrification resulted in oxidation of iron, leaching of iron and magnesium, and enrichment in alkalis.  相似文献   

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