The scope of this project is to collect and interpret geochemical and textural data of iron-nickel (Fe-Ni) spherules in impact melt from the Monturaqui Impact Crater, Chile to investigate the processes accounting for the compositional variations. Major element data are collected on in situ spherules in one thin section and three epoxy rounds by electron microprobe and major element, trace element, and platinum group element data are collected by inductively coupled plasma mass spectrometry on spherules separated from impact glass by electric pulse disintegration (Figure 1C-1D). Major element, trace element, and platinum group element data are collected on a piece of iron shale from Monturaqui by using both instruments. Though the variation of the major elements within Fe-Ni spherules has been well documented, the variations of trace elements have not been rigorously investigated. This study aims to document the variations seen in major elements, trace elements, and platinum group elements in the Fe-Ni spherules relative to the iron impactor. The impactor was originally classified as a group I coarse octahedrite based on the Ni content of Fe-Ni spherules in the impact melt as well as the morphology of the iron shale (Bunch and Cassidy, 1972; Buchwald, 1975). For this study, major element, trace element, and platinum group element data were collected on a piece of iron shale from Monturaqui to provide a more robust classification.

Impact spherules are sand-sized spherical particles that have been interpreted to have formed by the cooling, crystallization, and quenching of melt droplets condensed from vapor plumes that are created during large meteor impacts. Spherules may be deposited globally as unique marker beds, such as at the K-Pg boundary. A minimum of 11 spherule beds have been identified in the Archean and Paleoproterozoic, and provide a record of impact events that predate any known craters. This study of 3.24 Ga impact spherules from the S3 spherule layer in the Barberton Greenstone Belt (BGB) in the Kaapvaal Craton of South Africa focuses on the heterogeneity of textures and geochemistry produced during the cooling and crystallization of spherules within a vapor plume. Type 4b spherules are layered phyllosilicate spherules with discrete differences in texture and composition between the inner and outer layer, even after alteration. Compositionally layered phyllosilicate spherules were analyzed using Energy Dispersive X-ray Spectroscopy (EDS) and Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) to measure major, trace, and rare earth element (REE) concentrations. Backscatter Electron (BSE) images and elemental X-ray maps indicate a range of compositional differences between the inner and outer layers of type 4b spherules. The majority of REE plots have nearly flat patterns, with little to no light to heavy REE fractionation; however, the outer layers consistently have higher concentrations, averaging about 10x chondritic, whereas the interiors are at or below chondritic levels with a mid-REE enrichment. The trace and REE patterns of the type 4b spherules are consistent with a more mafic inner layer and a more intermediate outer layer. Mechanisms to produce this layered texture may include: (1) accretion of less mafic material from the plume onto existing melt droplets as the plume continues to fractionate, (2) collision of melt droplets of different viscosities, (3) by differentiation within the melt droplet prior to crystallization, or (4) by diagenetic effects. Based on textures, such as distinct boundaries between layers, and compositional patterns, such as an enrichment of Ti and REE in the outer layer, the data best fits the particle collision formation mechanism hypothesis, which has important implications for impact plume studies, such as plume density, turbulence, temperature, and opacity. / Geology

Carbonaceous materials found in Archean rocks have been the source of study and controversy for the last two decades due to questions of the biogenecity of these particles. One of the key locations for these studies is the Barberton Greenstone Belt (BGB), in South Africa which contains some of the oldest known rocks on Earth, ranging in age from 3.5 to 3.2 billion years old. Preserved within the Onverwacht and Fig Tree Groups of the BGB are spherules that formed by the condensation of an impact-produced global vapor plume. The spherules are distal deposits that would have been deposited globally, but are only preserved at this location and in western parts of Australia. Like several other sediments in the BGB, there is evidence of minor amounts of carbonaceous particles contained within the spherule beds. Four individual impact events are preserved in distinct beds designated as S1, S2, S3, and S4. Due to the wide distribution of this bed in a variety of depositional settings, including both protected shallow and deep water depositional settings where there is little evidence of reworking, the S3 bed is an ideal choice for mineralogical, geochemical, and petrographic studies of impact spherules. This research examines samples from four different locations of the S3 spherule bed layer, the Barite Syncline, Maid-of-the-Mist, Sheba Mine, and Loop Road locations, in order to determine the origin of carbonaceous particles contained within the bed. Several geologic processes could account for the presence of the carbonaceous materials within the S3 spherule bed layer. These processes include: (1) Diagenetic processes, (2) Fisher-Tropsch Synthesis, (3) Microbial activity, and (4) Primary condensates from the impact plume. In order to distinguish between these processes, the spatial distribution of the carbonaceous matter was mapped, noting the mineral associations with these grains. Petrographic and electron microanalytical studies of the S3 samples reveal the presence of carbonaceous material in the sections with highly concentrated spherules, Barite Syncline, Loop Road, and Sheba Mine locations, but not in the samples from the Maid-of-the-Mist location, where there is a low concentration of spherules and abundant admixed volcanic detritus. Only Fischer-Tropsch Synthesis can be excluded as a process responsible for the origin of carbonaceous materials in the S3 beds. Though there is no direct evidence of the biogenecity of the observed carbonaceous materials, other textual observations within the S3 spherule bed are consistent with microbial activity, including Ambient Inclusion Trails and an unusual feature with a cyanobacteria-like morphology. While microbial activity cannot be ruled out as a process responsible for the origins of the carbonaceous materials, the findings of this study indicate that the carbon was mobilized from within the spherules during diagenesis. The location of carbon along spherule rims and microfractures within the spherules can also be attributed to diagenetic processes, such as fracture flow, dissolution, and replacement. A plausible explanation is that the carbon was a primary condensate from the impact plume, but has been diagenetically remobilized locally into microfractures and along the rims of the spherules. / Geology

The ultimate goal of this dissertation was to better understand what the Apollo sample collection tells us about the impact history of the Moon. My main research tool is a computer code called Cratered Terrain Evolution Model (CTEM). CTEM is a Monte Carlo landscape evolution code developed to model a planetary surface subjected to impacts. While the main effect of impact cratering that CTEM simulates is elevation changes of the landscape through the excavation process of craters and the deposition of ejecta, I worked to extend the capabilities of the code to study problems in material transport. As impact cratering is a dominant process on the surface of Moon, the stratigraphy of lunar geology is thought to be composed of stacks of impact-generated ejecta layers. Each individual impact generates ejecta that is sourced from varying depths of the subsurface. This ejecta contains a rich abundance of material containing information, including composition and datable impact products, such as impact glasses. The extensions to the CTEM code that I developed allows me to track all ejecta generated during a simulation and model the complex history of the lunar regolith.

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The Gatun Structure (N 09º 05’ 58.1”, W 79º 47’ 21.8”, situated in the triple-canopy rainforest 10 km to the WSW of the Gamboa and about 2 km south of the Isle of Barbacoas, Republic de Panama), is a partially inundated, quasi-concentric surface feature ~3km in diameter, which appears in aerial photographs and in radar imagery as an arcuate chain of islands with a raised center. Although the structure has been heavily weathered and altered, it has retained morphology consistent with complex craters: an elevated circular central uplift 500-600 m in diameter and approximately 70 m high, and arcuate boundary ridges (a rim structure?) ranging from ~50-110 meters high. Within the central peak, highly altered and fractured siltstone of the Gatuncillo Formation (?) (Eocene) ± older rocks are uplifted and exposed through surrounding calcareous units of the Caimito Formation (Oligocene) and the Las Cascadas Formation (Miocene), the major target rocks in the region. Lithologies in the structure include highly fractured siliciclastic rocks (siltstone, sandstones and greywackes), limestones with anomalous spherical glass inclusions, both black and white hypocrystalline glasses (possible melt rocks), lithic fragmental breccias, and melt-bearing breccias (possible impact melt breccias and suevites) containing flow banding and evidence for selective melting of minerals. Three types of spherules (glass, fluid-drop and lithic), a pyroxenequartz “necklace” disequilibrium structure (coronas), plagioclase feldspars exhibiting mosaicism and partially amorphization and zeolitization, possible liquid immiscibility between melts of calcite and felpspathic glass, as well as decomposition of titano-magnetite, are all petrographic criteria that suggest a hypervelocity impact event. The structure is crosscut by numerous dikes of unshocked basalt and basaltic andesite related to volcanism along the Panamanian segment of the Central American arc to the south. However, the lithologies of the Gatun Structure are chemically inconsistent with the regional volcanic rocks and the unshocked volcanic rocks that crosscut the structure. An impact origin is our preferred interpretation for the Gatun structure due to the lack of an igneous relationship between the Gatun structure and the explosive volcanism of Panamanian arc, the presence of classical impactite lithologies within the site, the occurrence of spherules, maskelynite (as suggested by Raman Spectroscopy) and other disequilibrium shock features in the Gatun suite of rocks.

Title from PDF of title page. / Document formatted into pages; contains 558 pages. / Original thesis submitted in HTML and can be accessed at http://www.lib.usf.edu/ETD-db/theses/available/etd-10122001-142859/unrestricted/frame.html / td.pdf / Thesis (M.S.)--University of South Florida, 2002. / Includes bibliographical references. / Text (Electronic thesis) in PDF format. / ABSTRACT: The Gatun Structure, (Latitude N 09° 05&softsign; 58.1", Longitude W 79° 47&softsign; 21.8", situated in the triple-canopy rainforest 10 km to the WSW of the Gamboa and about 2 km south of the Isle of Barbacoas, Republic de Panama), is a partially inundated, quasi-concentric surface feature 2.2 - 3km in diameter, which appears in aerial photographs and in radar imagery as an arcuate chain of islands with a raised center. Although the structure has been heavily weathered and altered, it has retained morphology consistent with complex craters: an elevated circular central uplift 500-600 m in diameter and 50m high, and arcuate boundary ridges (a rim structure?) ranging from 50-100 meters high. Within the central peak, highly altered and fractured siltstone of the Gatuncillo (?) formation (Eocene) (+-) older rocks are uplifted and exposed through surrounding calcareous units of the Caimito formation (Oligocene) and the Las Cascadas formation (Miocene), the major target rocks in the region. / ABSTRACT: Lithologies in the structure include highly fractured siliciclastic rocks (siltstone, sandstones and greywackes), limestones with anomalous spherical glass inclusions, both black and white hypocrystalline glasses (possible melt rocks), lithic fragmental breccias, and melt-bearing breccias (possible impact melt breccias and suevites), some of which contain flow banding and evidence for selective melting of minerals. Three types of spherules (glass, fluid-drop and lithic), a pyroxene-quartz "necklace" disequilibrium structure (coronas), plagioclase feldspars exhibiting mosaicism and partially amorphization, possible liquid immiscibility between melts of calcite and felpspathic glass, as well as decomposition of titanomagnite or ulvospinel, are all petrographic indicators of a hypervelocity impact event. / ABSTRACT: The structure is crosscut by numerous dikes of unshocked basalt and basaltic andesite related to volcanism along the Panamanian segment of the Central American arc to the south. However, the lithologies of the Gatun Structure are chemically inconsistent with the regional volcanic rocks and the unshocked volcanic rocks that crosscut the structure. The lack of an igneous relationship between the Gatun structure and the explosive volcanism of Panamanian arc the presence of classical shock lithologies within the site, and the occurrence of spherules, maskelynite and other disequilibrium shock features in the rocks, an impact origin is our preferred interpretation for the Gatun structure. / System requirements: World Wide Web browser and PDF reader. / Mode of access: World Wide Web.

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