Basalt samples dredged from the ocean floor were experi­mentally fractured in tension while being observed microscopi­cally. Five basaltic textural types were chosen: (1) cotectic crystallization of olivine, plagioclase, and pyroxene Juan de Fuca, (2) dark glass matrix with phenocrysts of plagioclase and clinopyroxene Gorda Ridge, (3) vesicular black glass matrix with trachytic micro-phenocrysts Mid Atlantic Ridge Rift Valley, (4) black glass pillow with an hollow-centered phenocrysts Mid Atlantic Ridge Rift Valley, (5) black glass matrix with micro-­phenocrysts of olivine, pyroxene, plagioclase Mid Atlantic Ridge Rift Valley. Fracturing was done in the ¼ inch direction of precisely machined chips 1 x ¾ x ¾ inches. Test conditions were dry at 20°C, wet at 20°c, and 300°c. Modal integration of the crack path length and general modal analyses indicate that regardless of texture or test parameter: (1) the fracture path seeks out voids, glass, and grain boundaries to the exclusion of mineral grain cleavage, (2) a propagating fracture is offset by pre-existing frac­tures perpendicular to the propagation direction, (3) within the locus of the propagation direction, a pro­pagating fracture coalesces with a pre-existing parallel or sub-parallel fracture. The off-setting behavior of pre-existing fractures for newer fractures bears a striking geometric resemblance to the Mid Atlantic Ridge and its transform faults. / Earth and Environmental Science

The Rock Creek Flow, a Miocene tholeiitic basalt of the Columbia River Plateau Province, is a geographically well defined flow more than 400 feet thick and belonging to the "Basaltic flood" type. Having received little prior geologic attention, it is now the object of an eight-university study with the ultimate goal of comparing its petrology to that of the lunar maria basalts. This thesis is an experimental study of the melting relationships (the solidification temperatures) of samples collected in vertical sequence at one outcrop of the Rock Creek Flow. These natural basalts were investigated experimentally at a series of temperatures. The remelt solidus of the Rock Creek Flow is 1025°C ± 6°C. The remelt liquidus of the flow is 1210°C ± 10°C , and estimated eruption temperature is 1175° C ± 15°C. The melting behavior of the Rock Creek Flow Basalt indicates that this flow had nearly isothermal crystallization except for chilled zones. Either a single out­welling of lava or many converging flow units formed this single major (more than 400 feet thick) flow unit prior to the solidifica­tion of lava. The petrology of the Rock Creek Flow was studied by Drs. Hooper and Rosenberg (1971.) and they made chemical analyses of samples collected in vertical sequence at five different out­crops of the Rock Creek Flow including the outcrop sampled for the determination of melting relationship. Their chemical analyses show that the Rock Creek Flow has a nearly isochemical nature having very little vertical chemical variation. Very probably this isochemical nature of the flow is responsible for the nearly isothermal crystallization determined by experimental study. The results of the melting relationship study of the Rock Creek Flow do not support any major crystal settling for this 440-foot-thick flow. / Earth and Environmental Science

published_or_final_version / abstract / Earth Sciences / Doctoral / Doctor of Philosophy

The Sabie River Basalt Formation is a group of tholeiitic basaltic rocks erupted ca 190 Ma ago in the eastern zone of the Karoo Igneous Province of southern Africa. It is traceable over a distance of 700 km from Zululand, northwards along the Lebombo monocline into the Transvaal and south-east Zimbabwe. An abrupt compositional change in this formation occurs about halfway down its length in the vicinity of the Sabie and Komati Rivers: basalts to the north are known to be enriched in certain incompatible elements relative to basalts in the south, which are comparable in geochemistry to most basaltic rocks in the southern part of the Karoo Igneous Province. New data obtained in this work include 134 major and trace element whole-rock analyses, some 400 analyses of constituent minerals, 38 ⁸⁷Sr/⁸⁶Sr ratio determinations, 19 ¹⁴³Nd/¹⁴⁴Nd ratio determinations, 16 common Pb determinations and 12 oxygen isotope analyses. The "normal" (N) and "enriched" basaltic rocks are distinguished by differences in the concentrations of Ti, P, Zr, Nb, Y, La, Ce and Nd (high field strength elements). Broadly these differences are substantiated by K, Rb, Ba and Sr, but with much more overlap. The "enriched" group of basaltic rocks has been further subdivided into a low-Fe "enriched" (LFE) group and a high-Fe "enriched" group (HFE). The LFE-group basalts, which predominate at the base of the stratigraphic sections, are considered to be equivalent to basalts occurring in the N. Lebombo. In the central Lebombo N-group basalts predominate in the mid- and upper portions of the sections and HFE-group basalt occurs near the top of each section. Interbedding of all basalt groups occurs in the Sabie River section at the northern end of the study area, while the N- and HFE-group basalts are interbedded in the Crocodile and Komati River sections further to the south. The decrease in LFE-group basalt abundance southwards is accompanied by an increase in N-group basalt abundance. HFE-group basalts appear to be unique to the central Lebombo area of the Karoo Igneous Province and are volumetrically less significant than N- or LFE-group basalts. Petrogenetic models involving closed-system fractional crystallization; coupled assimilation (of granitic crust) fractional crystallization; replenished, tapped and fractionated magma chambers and partial melting are examined. Granitic crustal contamination appears to have been significant only in some samples of the N group where assimilation of granitic material has proceeded in a bulk fashion described by an AFC model. RTF models are dynamically more realistic than closed-system fractional crystallization models and explain increases in incompatible elements with decreasing MgO in the LFE and HFE groups. Variations in the N group, however, require varying degrees of partial melting of a N-type source to be explained fully. RTF models may explain the absence of any stratigraphic correlations of element abundances in the three groups. The HFE group may be related to an uncontaminated N-type parent composition by a combination of continued fractional crystallization from an N-group parent composition and varying degrees of partial melting of an N-type source. The only petrogenetic process by which the N and LFE groups may be related is different degrees of partial melting. However, this demands a source composition which has no resemblance on trace element and isotopic grounds, to observed mantle xenolith compositions. The preferred model is one in which the LFE group is derived from old sub-cratonic mantle similar to garnet-bearing "cold" peridotite xenoliths and the N group from a source similar in composition to estimates of primitive mantle. The existence of two types of mantle derived continental flood basalt magmas occurs in other Mesozoic basalt provinces in "southern" Gondwanaland (e.g. Kirwanveggan of Antarctica, Etendeka of Namibia and the Parana Basin of South America). It is suggested that there is a geographical association of LFE-type basalts with Archaean crust (or Archaean crust re-worked in low temperature - high pressure events) and N-type basalts with post-Archaean crust (or Archaean crust re-worked in high temperature - low pressure events). This model suggests the derivation of the LFE group, from old sub-cratonic lithospheric mantle relatively enriched in incompatible elements and the N group being derived from more recently accreted and less enriched lithospheric mantle underlying younger crustal terraines.

The impact of subduction initiation on regional to global tectonics and the compositions of major Earth reservoirs are topics of vigorous ongoing research. Here, pristine glasses extracted from ~51.9 Myr old basalts and younger boninites that erupted in the Izu-Bonin Mariana forearc immediately after subduction initiation were analyzed by microbeam techniques, with goals of characterizing the mantle sources and the conditions under which melting occurred to produce nascent arc crust. Forearc basalts (FAB) have relatively differentiated major element compositions. Thus, to determine melting conditions and source compositions, primitive melt compositions were restored through an inferred crystallization history based on melt liquidus associations. Subsequent modeling indicates that they were generated at high temperatures and low pressures relative to a mid ocean ridge basalt (MORB). Incompatible trace element compositions of FAB show that they are similar to MORB in that they were generated largely by decompression melting. Differences in several trace element ratios between MORB and FAB indicate that the mantle sources for FAB were unusually depleted. Differences between FAB sub-units indicate a range of petrogenetic histories. Upper FAB sub-units are weakly enriched in fluid-mobile elements which may indicate that fluids from the subducting Pacific plate contributed to melting. Boninites are separated into high and low silica types based on preexisting whole rock analyses. Glasses separated from these boninites are highly differentiated and thus classify as high-Mg andesites rather than boninites on MgO-SiO2-TiO2 diagrams. These glasses are also enriched in a suite of fluid mobile elements indicating that they are products of flux melting of the mantle involving fluids and melts from the subducting plate. Olivine calcium concentrations are consistent with hydrous parental boninite melts. Aluminum partitioning between olivine and hosted spinel inclusions constrains the temperatures of initial crystallization between 1170 and 1330 degrees Celsius. The change from decompression melting which generated forearc basalts to flux melting which generated high silica boninites illustrates an evolution of the subduction system over the course of the initiation process. Based on trace element ratio plots, mixing relationships between upper forearc basalts and highly enriched fluids probably released by the nascent subducting slab suggest that both decompression melting and fluid fluxing operated to produce low silica boninite during subduction initiation. This melt composition progressively becomes dominated by fluid flux melts with additional components derived from the slab to make high silica boninite. These late volcanic rocks record melting of a highly depleted mantle source. The fact that heavy rare earth element concentrations become increasingly depleted from FAB to low silica boninite to high silica boninite indicates that the mantle source changed in composition over time. The progressive decrease suggests that the initial mantle source for FAB remained the mantle source for the duration of subduction initiation related magmatism.

Seismic imaging of sub-basalt sedimentary layers is difficult due to high impedance of the basalt layer, the roughness of the top and bottom of the basalt layer and sometimes the heterogeneities within the basalt layer. In this thesis we identify specific problems within the modern imaging technology which limit sub-basalt imaging. The basic framework for the identification of this limitation is that we are able to group most basalt layers into the following four categories: A basalt layer having smooth top and bottom surfaces. A basalt layer having rough top and bottom surfaces. Small-scale heterogeneities within the basalt layer. Intra-basalt velocity variation due to different basalt flows. All the above models of basalt layers obviously have high impedance with respect to the surrounding sedimentary layers. These four models encapsulate all the possible heterogeneities of basalt layers seen in areas like the Voring and More basins off mid- Norway, basins in the Faroes, W. Greenland, Angola and Brazil margins, and the Deccan Traps of India. In this work, problems in seismic processing and imaging specific to these models have been presented. For instance, we have found that the application of the multiple attenuation technique, which first predicts the multiples and then subtracts them from the data, using least-squares criteria, can be effective for all the models except for the model, which has intra-bedded layers within the basalt. The failure in the second case is due to the destructive interference of multiple scattering from the intra-bedded layers within the basalt and the multiples located below the primary associated with the top of the basalt layer. This interference degrades the signal-to-noise (S/N) ratio of the multiples contained in the data, whereas the predicted multiples, which are constructed from the reflectors above the basalt, have a much higher signal-to-noise ratio. Our recommendation is to subtract the predicted multiples from the data using either leastabsolute- value criteria or any other higher-order-statistics-based criteria.

No description available.

No description available.

Thesis (M.Ag.Sci.)--University of Adelaide, Faculty Agricultural Science,1955. / Typewritten copy. Includes bibliographical references.

Thesis (Ph. D.)--University of Wisconsin--Madison, 1971. / Typescript. Vita. eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references.