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The ionization of the most abundant elements in planetary nebulae has been determined for a number of models of nebulae at different epochs in their expansion. The values used for the temperatures and radii of the central stars and the sizes and densities of the shells have come from Seaton's evolutionary sequence. The ionizing radiation field has been taken from model atmosphere calculations of the central stars by Gebbie and Seaton, and Biihm and Deinzer. Emission -line fluxes have been calculated for the models and compared with observations of planetary nebulae by O'Dell, Osterbrock's group, and Aller and his collaborators. Results indicate that the central stars have strong He+ Lyman continuum excesses, similar to those predicted by Gebbie and Seaton. The mean abundance determinations for the nebulae made by Aller are confirmed, with the exception of nitrogen, which appears to be 3 or 4 times more abundant than his value. It is also seen that the electron temperatures of the nebulae are higher than previous theoretical determinations, providing better agreement with empirically derived values.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric and Planetary Sciences, 1987. / Vita. / Bibliography: leaves 131-133. / by Yung-An Lee. / Ph.D.

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Earth, Atmosphere and Planetary Sciences, 1987. / Bibliography: v. 2, leaves 260-271. / by Alan Edward Leinbach. / M.S.

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Earth, Atmosphere and Planetary Sciences, 1987. / Vita. / Bibliography: leaves 95-100. / by John William Nielsen. / M.S.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 2003. / Includes bibliographical references. / This thesis examines the petrology and geochronology of high-pressure granulites and eclogites within the Snowbird tectonic zone of the western Canadian Shield. The focus of this study is the East Athabasca mylonite triangle (EAmt), a well-exposed terrane of granulite facies mylonitic rocks along the trace of the Snowbird tectonic zone in northern Saskatchewan. This study focuses on the 400 km² Southern Domain of the EAmt, which contains a spectacular suite of high- pressure granulites and eclogites that have been metamorphosed at conditions exceeding 1.5 GPa and 1000⁰C. Each chapter of this thesis focuses on a different lithology within the Southern Domain - mafic granulite, eclogite, sapphirine granulite, and felsic granulite. The approach that is taken in understanding each of these rock types is an integrated study of the petrological and geochronological constraints that yield important information about the metamorphic evolution of these unique rocks. The main discovery through these integrated studies is that the Snowbird tectonic zone records a significant Paleoproterozoic high-pressure metamorphism that was previously unrecognized. Petrological and geochronological studies of each of these rock types constrains the pressure-temperature-time path of these rocks from their initial formation to their ultimate exhumation. The granulites and eclogites of the Southern Domain are derived from Archean igneous and sedimentary protoliths. / (cont.) The dominant rock type of the Southern Domain, the felsic gneiss, is interpreted to be derived from a pelitic protolith that underwent an early metamorphism at 2.62- 2.60 Ga. Protoliths of the mafic lithologies intruded at mid-crustal levels by 2.55-2.52 Ga. The eclogite protolith was derived from a plagioclase-bearing cumulate source at pressures <1.0 GPa. The eclogite and mafic granulite, and, to a lesser extent, the felsic gneiss record high-pressure metamorphism at 1.9 Ga. Near-isothermal decompression P-T paths in the mafic granulite and eclogite record rapid exhumation to medium-pressure granulite facies conditions of [approx.]1.0 GPa, 800⁰C. Sapphirine-bearing veins within the eclogite record further decompression, cooling, and re-equilibration in the middle crust at [approx.]0.6 GPa, 600-700⁰C. Final exhumation of these rocks occurred by more protracted erosional processes from 1.9 to 1.8 Ga. / by Julia A. Baldwin. / Ph.D.

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 2006. / Includes bibliographical references (p. 19-22). / Rising over 5 km along the border of Uganda and the Democratic Republic of the Congo, the Rwenzori Mountains represent an extreme example of basement rift-flank uplift in the western branch of the East African Rift, a phenomenon common throughout the East African Rift System and characteristic of continental rift systems in general. A thermochronologic study combining (U-Th)/He and U-Pb analysis of apatite, titanite, and zircon separated from crystalline basement rocks was conducted across the Rwenzori block to characterize the timing and rate of rift-flank exhumation related to continental extension in east-central Africa. The thermochronologic data coupled with field and remote sensing observations make the case for recent and non-steady state uplift of the massif. Uranium-lead thermochronology indicate that, prior to Upper Neogene rifting, the rocks of the Rwenzori experienced a protracted history of slow cooling without major tectonothermal perturbation since at least the Paleoproterozoic (ca. 1900 Ma). / (cont.) Stream channel steepness profiles and thermochronometry along the western slope of the range show it to be the main active scarp that accommodates uplift. Relatively old (U-Th)/He zircon and apatite dates (>400 Ma, >70 Ma respectively) along the high peaks and eastern slope of the range reflect a transient lag period resulting from yet-insufficient exhumation to remove the inherited pre-rift cratonic thermal structure. This non-steady state condition of rapid uplift outpacing erosion has resulted in preservation of relict landsurfaces, truncated spurs, hanging valleys, uplifted river terraces, and vast stranded bogs at high elevation. Given the low cooling rate and geothermal gradient prior to rifting implied by U-Pb thermochronometry we determine that no more than 1.7 km of erosion could have accompanied uplift on the order of at least 5 km in the Rwenzori region. Biostratigraphic evidence suggests the range rose from beneath local baselevel within the last 2.5 Ma. This requires a minimum average uplift rate of 1.6 km/Myr. Regardless of the active rock uplift rate of the Rwenzori, net exhumation cannot yet have exceeded the depth of the (U-Th)/He closure isotherm in apatite (<1.7 km). These results highlight the danger of modeling young orogenic systems using the simplifying assumption of topographic steady state. / by Daniel MacPhee. / S.M.

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric and Planetary Sciences, 1984. / Microfiche copy available in Archives and Science. / Vita. / Bibliography: leaves 233-238. / by Jane Hsiung Wojcik. / Ph.D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, June 2002. / This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections. / Pages 460-462 blank. / Includes bibliographical references. / Three extensional fault systems intersect in the central Nepal Himalaya: the South Tibetan fault system (STFS); the Thakkhola graben; and structures bounding the Upper Mustang Massif (UMM). Interactions between these systems are investigated using integrated field studies, remote sensing analyses, geodynamic modeling, and a wide variety of geochronologic tools. The goals are to characterize the relationships between the three extensional systems, document their neotectonic histories in relation to their older manifestations, and develop a model for how the geodynamic phenomena driving them have produced the observed patterns of tectonism and deformation. 14C geochronology, remote sensing analyses, and field observations in the southern Thakkhola graben show that the STFS has been active in the Quaternary and that the kinematically-linked Thakkhola graben acts as a tear structure in its hanging wall. In the northern Thakkhola graben, neotectonic, structural, and 40Ar/39Ar and U-Pb geochronologic data from the UMM record its Late Miocene to Quaternary exhumational history and reveal a kinematic and geometric relationship between core-complex extension in the UMM and rift extension in the Thakkhola graben. / This work shows that core-complex development and east-west extension in southern Tibet are closely related to the STFS. In the adjacent Dhaulagiri Himalaya, remote sensing and field analyses of several generations of STFS structures - some of which appear to be active - show that the most recently active fault traces do not necessarily follow pre-existing lithologic contacts defined by older strands, but instead follow less contorted traces, cutting across allochtons and even reactivating segments of the Main Central Thrust. The distribution of high-grade rocks and the interactions between the STFS, east-west extension, and core-complex development are hypothesized to reflect the response of the orogen to gravitational potential energy (GPE) anomalies. GPE anomalies in the Himalaya are calculated from geoid data and interpreted in terms of pressure gradients driving flow in an intracrustal channel. From the distribution of GPE anomalies, I suggest that orogen parallel flow may be as significant as orogen-perpendicular flow, indicating that both removal and redistribution of GPE is important in the development of extensional structures in the Himalayan orogen and in southern Tibet. / by José Miguel Hurtado, Jr. / Contents of CDROM: Standard Adobe PDF files of the disserataion -- PDF compatible (Adobe Illustrator) files of the official defense notices -- Microsoft Powerpoint presentation used during the defense talk -- Apple Quicktime animation of the Powerpoint slides -- Standard Adobe PDF of the author's CV. / Ph.D.

Thesis: S.B., Massachusetts Institute of Technology, Department of Earth, Atmospheric, and Planetary Sciences, 2007. / Cataloged from PDF version of thesis. / Includes bibliographical references (pages 40-41). / Scientists at NASA's Goddard Space Flight Center have developed a Combined X-ray Diffraction/Fluorescence (CXRDF) instrument. CXRDF performs simultaneous chemical and structural analysis of an unprepared sample, making it ideal for planetary mineral identification. In an effort to analyze the effectiveness of CXRDF, samples were chosen from a list of minerals that are important in the debate about the origin of the outcrops at Meridiani Planum on Mars. These samples were run on both CXRDF and a laboratory X-ray diffractometer. The datasets were compared, looking at peak identification, d-spacing resolution, and whether the instruments could definitively identify each sample. CXRDF successfully measured the d-spacings for each mineral, and the chemical analysis data were very valuable. However, for CXRDF to be able to definitively identify minerals, its d-spacing range and resolution will need to be improved, in addition to its data analysis software. / by Caitlin A. Murphy. / S.B.