Field evidence from exposed plutonic rocks indicates that mafic-felsic magma interaction is an important process during the construction and evolution of magma chambers. The exhumed, ~140 Ma, Halfmoon Pluton of Stewart Island, New Zealand is characterized by a sequence of mingled mafic sheets and enclaves preserved within an intermediate-felsic host, and provides a unique opportunity to directly investigate the physico-chemical processes that operate within an arc setting. Interpretation of mingling structures and textures in the field, in combination with extensive petrographic, geochemical and geochronological data, allow for conclusions to be reached regarding the nature of the mafic-felsic magma interactions, and the physical, chemical and thermal processes responsible for the generation and evolution of the calc-alkaline magmas. Detailed documentation and interpretation of mafic-felsic magma mingling structures and textures reveal that the Halfmoon Pluton formed incrementally as the result of episodic replenishments of mafic magma emplaced onto the floor of an aggrading intermediate-felsic magma chamber. Physico-chemical processes identified include fractional crystallization and accumulation of a plagioclase – hornblende – apatite – zircon mineral assemblage, episodic replenishment by hot, wet basaltic magmas, magmatic flow and compaction. Early amphibole and apatite crystallization played an important role in the compositional diversity within the Halfmoon Pluton. Variations in the style of magma mingling preserved within the magmatic “stratigraphy” indicate that processes operating within the chamber varied in space and time. Variations in mineral zoning and composition within hornblende indicate that the Halfmoon Pluton crystallized within a magma in which melt composition fluctuated in response to repeated mafic magma replenishments, fractionation, crystal settling and convection. Mineral assemblages, chemical characteristics, isotopic data and geochronological evidence indicate that the amphibole-rich calc-alkaline Halfmoon Pluton was emplaced into a juvenile arc setting, most probably an island-arc. Data are consistent with a model whereby ‘wet’ amphibole-rich basaltic magmas pond at the crust-mantle interface and episodically rise, inject and mingle with an overlying intermediate-felsic magma chamber that itself represents the fractionated product of the mantle melts.

No description available.

No description available.

Iceland straddles the Mid-Atlantic Ridge and overlies a mantle hotspot. This tectonic setting produces voluminous tholeiitic magmas. Volcanism in Iceland is focussed along three neovolcanic spreading ridges. During the Holocene, the Eastern Volcanic Zone (EVZ) in southeast Iceland has been the most volcanically active and has been the site of several large (>6 km3) eruptions, including the only floodbasalt type eruption in recorded history, the 1783-84 Laki eruption. Three eruptions of large volume have been sampled for this study: the 1783-84 Laki eruption (15.1 km3); the 3,000-4,000 yBP Thjórsárdalur eruption (probably >4 km3); and the ~8,600 yBP Thjórsá eruption (>21 km3). The products of these eruptions have been analysed using a range of analytical techniques, with the specific aim of investigating crystallisation, degassing and mixing processes in the magma reservoirs that feed large eruptions. The Laki eruption has been the particular focus of this study. Samples from different parts of the lava flow show fine-scale variations in trace element concentrations and ratios. This compositional variation is not fully explained by fractional crystallisation processes, but is strongly controlled by crystal accumulation as whole-rock incompatible trace element concentrations show a linear, negative correlation with the mass fraction of crystals in the sample. Simple crystal accumulation models, however, fail to explain the compositional variation, and one explanation is that the homogeneous Laki melt mixed with varying proportions of a crystal mush that contained its own liquid. The results of thermobarometry calculations indicate that the erupted Laki liquid was in equilibrium with olivine, plagioclase and augite at 1-3 kb. Most of the crystals carried by the flow are too primitive to have crystallised from the erupted liquid and barometry calculations indicate that clinopyroxene crystallised at 3-7 kb. The majority of the large crystals hosted in the Laki basalt samples are therefore antecrysts that grew within the same magma plumbing system as the Laki carrier melt but are not in direct chemical equilibrium with it. This finding is verified by the fact that olivine crystals that are too magnesian to be in chemical equilibrium with the Laki whole-rock composition contain melt inclusions with average La/Yb values that are the same within error as the whole-rock values. The wide range of La/Yb values in melt inclusions hosted in the most magnesian (Fo86) olivine crystals in comparison to the least magnesian (Fo<74) indicates the initial variability of the Laki magma prior to concurrent crystallisation and extensive mixing, which acted to homogenise the carrier melt composition. The preservation of a wide range of La/Yb within the melt inclusions in comparison to the whole-rock composition, and a range of La/Yb values in different inclusions from the same crystal, indicates short timescales between melt inclusion entrapment and quenching during eruption. Melt inclusion studies also reveal the dissolved volatile content of the Laki magma at the onset of olivine crystallisation, although the majority of H2O concentrations have almost certainly been reset by low pressure diffusive exchange with the host crystal or surrounding magma. Comparison of the behaviour of volatiles with that of incompatible elements in the melt inclusions indicates that CO2 was degassing during olivine crystallisation, but S, F and Cl were not. New estimates of total volatile loading to the atmosphere during the eruption based on melt inclusion volatile concentrations show SO2 and HCl loading comparable to previous estimates, but higher HF loading. Mass balance calculations show that the observed H2O and CO2 concentrations of melt inclusions hosted in olivines in chemical equilibrium with the Laki whole-rock composition are ~50% and ~93% lower respectively than would be expected if no pre-eruptive degassing of the magma reservoir had occurred, meaning that pre-eruptive degassing of H2O and CO2 from the magma must have been significant. Lava flows from Thjórsá are more compositionally variable than those from Laki, and have different average major and trace element compositions. Compositional variation within the Thjórsá whole-rock composition is explained by a combination of source variation and fractional crystallisation, and, unlike Laki, is not strongly controlled by crystal accumulation.

Die bruchhafte Fragmentation von höherviskosem Magma ist ein bedeutender Prozess im explosiven Vulkanismus. Deren Fragmentationsenergie ist linear an die Entstehung neugebildeter Bruchfläche gekoppelt. Aus diesem Grund ist es wichtig, die mechanische Energie der Schmelzefragmentation zu quantifizieren, um die physikalischen Vorgänge während dieses vulkanologischen Vorgangs besser verstehen zu können. Deshalb war es das Ziel der vorliegenden Arbeit eine Kenngröße der Fragmentationsleistung von vulkanischen Schmelzen der Phlegräischen Felder (Neapel/Italien) zu definieren und somit ihren vulkanischen Ablagerungen spezifische Fragmentationsenergien zuzuweisen. Das Vulkangebiet der Phlegräischen Felder ist durch langanhaltenden explosiven Vulkanismus gekennzeichnet. Die bruchhaft entstandenen Feinaschen-Ablagerungen intermediärer Zusammensetzung bedecken ein Gebiet von ca. 1000 km2. Dieses Gebiet wird heute von ca. 2 Millionen Menschen bevölkert. Diese Arbeit stellt eine Methodik vor, mit der die Fragmentationsenergie von solchen höher-viskosen Schmelzen intermediärer Zusammensetzung durch Laborexperimente und Granulometrie der erzeugten Partikel ermittelt werden kann. Die Materialparameter der kritischen Schubspannung und des kritischen Scherstress wurden mit einem sogenannten Gasdruck-Fragmentations-Versuch (GFV) experimentell bestimmt. Ferner konnten durch den GFV Erkenntnisse über das Fragmentationsverhalten dieser Schmelzen unter verschiedenen treibenden Gasdrücken gewonnen werden. Dieser spezielle Versuchsaufbau basiert auf Fragmentation von Schmelze durch Druckluft, die von unten in einen Tiegel geleitet wird. Ein individuell einstellbarer Gasdruckluftstoß führt in der Schmelze zu einer Zunahme der Schubspannung und einem Druckaufbau, der vergleichbar mit der Kraftrampe eines Biegeversuchs ist. Während dieser Zeit kommt es zur Mikrobruchbildung, die sich von der Schmelzeoberfläche nach unten fortsetzt. Nach dem Überschreiten der Bruchgrenze relaxiert das Schmelzematerial durch Ausbildung von Sprödbrüchen und wird nach oben ausgeworfen. Die Aufzeichnung der physikalischen Parameter und die optische Versuchsüberwachung erlaubten eine komplette Energiebilanzierung des Vorgangs. Die neugebildete Bruchfläche der entstandenen Partikel wird durch Granulometrie und Anwendung der Methode von Brunnauer-Emmet-Teller (BET) bestimmt. Somit kann die Fragmentationsenergie auf die Bruchfläche bezogen und als Materialparameter des kritischen Scherstresses ausgedrückt werden. Der GFV wurden durch normierte Laborexperimente an dem selben Schmelzematerial ergänzt. Dabei dienten statische Biegeversuche unter Raumtemperatur zur Überprüfung der über GFV bilanzierten Scherstresse. Die Rotationsviskosimetrie zeigte, dass der Materialparameter der Viskosität nicht geeignet ist, um Rückschlüsse über Materialverhalten im bruchhaften Regime zu ziehen. Anschließend wurde einer definierten Tephraschicht der Phlegräischen Felder eine spezifische Fragmentationsenergie zugeordnet, indem die experimentellen Ergebnisse auf Felddaten bezogen wurden. Diese spezifische Energie von ca. 8*1010 kJ entspricht der Sprengkraft von ca. 20.000 Tonnen Trinitrotoluol (TNT). Die Qualität eines hazard assessment gefährdeter Vulkangebiete wie z.B. der Phlegräischen Felder wird durch die Kenntnis der Energieaufteilung während des Eruptionsprozesses (Fragmentationsenergie, Auswurfenergie etc.) wesentlich verbessert. Die Kenntnis der Energien dient beispielsweise der Skalierung ballistischer Modelle, mit deren Hilfe dichtbevölkerte Zonen ausgewiesen werden können, die bei künftigen Eruptionen der Phlegräischen Felder durch den Niedergang von Pyroklastika bedroht sind. / The brittle fragmentation of highly viscous melt is a major part of explosive eruptions. It is important to quantify the mechanical energy needed for the melt-fragmentation in order to assess this volcanic physical process. The Phlegrean Volcanic Field (Naples/Italy) is characterized by long-term explosive volcanism. Fine-ash deposits of brittle origin and intermediate composition cover an area of about 1000 km2. Nowadays this area is inhabitated by about 2 Million people. This thesis presents a method to determine the fragmentation energy of such highly viscous melts of intermediate composition, using laboratory experiments and granulometry data of the produced particles. The rock parameters critical shear stress and fragmentation energy were determined, using the so called “Gas-Fragmentation-Test“ (GFV). Further, the GFV was useful to determine the fragmentation behaviour of these melts under varying driving pressures. This special fragmentation setup is based on a gas pressure blow (applying compressed air) which leads to a shear tension increase in the melt volume. The pressure built-up is comparable to a load force of a centre-loading flexural test. During this time microcrack propagation migrates from the upper melt surface downward. After exceeding the fragmentation limit the melt relaxes, resulting in brittle fractures. The monitoring of the partition of energies as well as the highspeed video recording of the process allows the calculation of the total fragmentation energy. The total fracture area of particles was quantified, using granulometry and the multipoint Brunnauer-Emmett-Teller method. Thus the fragmentation energy was related to the total fracture area and expressed as critical shear stress, which represents a material parameter. The GFV were complemented by standardised laboratory experiments. Static centre-loading flexural tests were carried out on the same material to check the ambient temperature shear stress values. Moreover measurements showed that determination of the melts viscosity this parameter is not useful to describe fragmentation in the brittle field. Afterwards the fragmentation energy needed to produce one tephra layer of the Phlegrean material was calculated, using the experimental results and field data. This energy value of approx. 8*1010 kJ corresponds to the explosive power of approx. 20.000 tons of trinitrotoluene (TNT). Finally, the knowledge of energy-partitioning is a useful tool to scale numerical models of the eruption and to improve the quality of hazard assessment in vulnerable volcanic regions like the Phlegrean Volcanic Field. In this way densely urbanized regions, which are threatened by the deposit of pyroclastics, were determined.

Magmas experiencing pressure changes can follow equilibrium or nonequilibrium degassing paths that determine the rate of gas exsolution and the composition of gases exsolved. Many variables influence timescales of equilibration between vapor and melt after a perturbation in pressure, temperature, or other factors, and the magnitude of this equilibration time determines whether the system experiences equilibrium degassing or not. In order to create a simplified framework for assessing degassing regime, we constructed a numerical diffusion model to test the sensitivity of equilibration time to variables such as bubble size, spacing, melt temperature, initial and final system pressures, and water content. We then determined the degassing regime for a range of bubble-spacing and decompression rates as an initial simplified framework to build on. We also attempted the first mixed-volatile continuous decompression experiments in order validate our model and further improve analyses and interpretations of volatile gradients in natural samples.

Abstract A progenitor developed by Robert T. Curtis is a type of infinite groups formed by the semi-direct product of a free group m∗n and a transitive permutation group of degree n. To produce finite homomorphic images we had to add relations to the progenitor of the form 2∗n : N. In this thesis we have investigated several permutations progenitors and monomials, 2∗12 : S4, 2∗12 : S4 × 2, 2∗13 : (13 : 4), 2∗30 : ((2• : 3) : 5), 2∗13 :13,2∗13 :(13:2),2∗13 :(13:S3),53∗2 :m (13:4),7∗8 :m (32 :8),and 53∗4 :m (13 : 4). We have discovered that the permutations progenitors produced the following finite homomorphic images, we have found P GL(2, 13), U3 (4) : 2, 2 × Sz (8), PSL(2,7), PGL(2,27), PSL(2,8), PSL(3,3), 4•S4(5), PSL2(53), and 13 : PGL2(53) as homomorphic images of this progenitors. We will construct double coset enumeration for the homomorphic images, 2 × Sz (8) over (13 : 4) Suzuki twisted group, P GL(2, 13) over S4,and PSL(2,7) over S4 and Maximal subgroups of 2×PGL(2,27) over 2•(13 : 2), P SL(2, 8) over (9 : 2), and P SL(3, 3) over (13 : 3). We will also give our techniques of finding finite homomorphic images and their isomorphism images.

No description available.

Studies of mid-ocean ridges have provided evidence of magma chambers beneath both fast and intermediate spreading ridges. However no such features have been observed to date beneath slow spreading ridges. These contradictory observations are in direct conflict with seismic studies which reveal that the resulting crustal structures are similar and hence crustal structure is independent of the spreading rate. These latter observations in turn lead to the implication that the accretionary processes operating at all ridge types must also be similar. The aim of this study is to attempt to resolve between this discrepancy in geophysical observations of magma chambers at fast, intermediate and slow spreading ridges and investigate the nature of accretionary processes operating such that the same crustal structure is achieved. Therefore an apparently currently magmatically active section of the slow spreading Mid-Atlantic Ridge at 57 45'N on the Reykjanes Ridge, was selected as the target of a multidisciplinary geophysical experiment to be conducted aboard the RRS Charles Darwin in 1993. Wide-angle seismic data recorded using 10 digital ocean bottom seismometers were used to generate models of the crustal structure along and across-axis. These models were confirmed and further constrained by modelling of normal incidence seismic and gravity data and by comparison with the results of modelling controlled source electromagnetic data. The resultant models indicate that a magma chamber exists beneath the axial volcanic ridge studied, providing the first geophysical observation of such a feature at any slow spreading ridge. This magma chamber is similar in dimensions to those observed beneath fast and intermediate spreading ridges and consists of a thin, narrow sill-like body which appears to be continuous along-axis, and which is underlain by a region of partial melt extending almost to the Moho. This latter feature also appears to be both longer-lived and more extensive than the magma chamber. The 2.5 km depth to the top of the magma chamber is only slightly greater than that observed at fast spreading ridges, which indicates that magma chamber depth does not vary significantly with spreading rate. However, there ore insufficient data available to fully constrain and develop this relationship to its fullest. Therefore the results of this study indicate that the processes of crustal accretion occurring at all spreading ridges are similar, with the lack of observations of magma chambers being due to the fact that the periods of magmatic activity at slow spreading ridges are considerably more widely separated in both space and time than for fast and intermediate spreading ridges. The main difference however, appears to occur in the process of emplacement of layer 2A, which is observed to thicken off-axis at fast spreading ridges due to the less viscous lavas produced at these ridges being able to flow further off-axis. The results of this study, and of two other studies at slow spreading ridges, show that layer 2A is completely formed on-axis and thins off-axis due to extensional faulting. The remainder of the crust is completely emplaced, and the Moho formed, on-axis at all spreading rates.

Universiẗat, Diss., 2001--München.