http://hdl.handle.net/10090/39 Search the Channel search http://dspace.nitle.org/simple-search http://hdl.handle.net/10090/6434 Title: Petrology and geochemistry of a metamorphosed anorthosite-gabbro intrusive suite near Dresden, NY <br/> <br/>Authors: Lake, Ethan T. <br/> <br/>Abstract: Massive type anorthosite suites are complex large scale igneous bodies consisting primarily of plagioclase rich anorthosites and some residual mafic Fe-Ti bearing gabbros. These bodies formed during the early to mid Proterozoic, some 2500 to 1000 million years ago and are relatively common throughout the Adirondacks. This study involves the mapping and sampling of a small anorthosite suite near Dresden, New York. The site was mapped during a week of field days and 30 samples were collected for petrographic analysis, whole rock geochemical analysis using an ICP-AES, and rare earth element analysis using an ICP-MS. Much of the field area contains significant high grade metamorphic overprinting from the late stages of the Grenville Orogeny (around 1080 Ma). Field mapping revealed the presence of a gradient of plagioclase bearing rocks ranging from mafic ferro-gabbro to more plagioclase rich gabbroic anorthosite. Petrographic analyses confirmed the subdivision of the field area into three lithologies based on percent plagioclase composition as well as identified younger mafic intrusions into the anorthosite suite. Rare earth element analysis indicated that the suite likely differentiated from a single plagioclase rich magma source. Both the rare earth element analysis and whole rock geochemistry linked the anorthosite suite at Dresden to the broader Adirondack Proterozoic anorthosite event and indicated that the Dresden suite likely comprised a small, mafic off-shoot of that regional event. <br/> <br/>Description: 63 leaves : ill. Includes bibliographical references. Thesis (B.A.)--Middlebury College, 2008. Tue, 18 Nov 2008 18:28:01 GMT http://hdl.handle.net/10090/6428 Title: Analysis of biogenic silica from marine sediment in Maxwell Bay, Antartica: a look at holocene climate history. <br/> <br/>Authors: Klein, Allison W. <br/> <br/>Abstract: The Shallow Water Drilling project (SHALDRIL) retrieved an 108-meter core in 2005 from Maxwell Bay, a fjord in the South Shetland Islands of the Antarctic Peninsula. Electric Resistivity (ER), Magnetic Susceptibility (MS), grain size, weight percent biogenic content using QXRD analysis and other physical properties of the core were determined (Michalchuk, 2006). The goal of this study is to perform a Holocene climate reconstruction using biogenic silica (BSi) and analyze it with respect to other climate proxies. BSi content was determined at 50-cm intervals downcore using a NaOH leaching analysis procedure (DeMaster, 1981). Duplicates were done at random intervals and reproducibility is predominantly within 1.1% with a few having a slightly higher difference than the original values. Salient results are: 1) Based on BSi bulk accumulation as well as the presence of certain diatoms and grain size analysis, seven distinct climate stages are identified within the core including deglaciation of the region between 11,000 and 10,000 cal yr BP as well as the mid-Holocene climatic optimum between 6,500 and 7,300 cal yr BP. 2) There is an extreme difference between the biogenic content determined from QXRD and BSi from the NaOH leach in not only magnitude, but also overall trend. The most likely reason for this is that the volcanic glass in the sediment from volcanism on Deception Island is not picked up by the QXRD as one of the key minerals due to its lack of crystalline structure and is therefore attributed to biogenic content. Using QXRD as a proxy to determine biogenic content, although possible in other environments, is not appropriate in an environment with volcanic and volcaniclastic rocks. <br/> <br/>Description: 59 leaves : ill. Includes bibliographical references. Thesis (B.A.)--Middlebury College, 2008. Mon, 17 Nov 2008 18:47:20 GMT http://hdl.handle.net/10090/795 Title: Evaluation Of A Potential Ultramafic Source Of Arsenic Contamination In Bedrock Water Wells In Central Vermont <br/> <br/>Authors: Sullivan, Colleen Marie <br/> <br/>Abstract: Analysis of 40 bedrock groundwater well, spring and surface water samples in the vicinity of Stowe, Vermont reveals three wells with arsenic concentrations that exceed the EPA MCL of 10 ppb, with two of the wells producing water containing 86 and 275 ppb As. Eight additional wells contained between 1.5 and 7.0 ppb As. The wells are located in the Stowe, Ottaquechee, Moretown, and Hazens Notch Formations, which consist of mainly schists and greenstones along with small fault-bounded serpentinite bodies. The geochemistry of water samples shows that only 1 out of the 40 samples received a potentially significant contribution from ultramafic rocks (based on Mg:Ca ratios), a stream sample with 2.7 ppb As from a site near serpentine tailings of the Belvedere mine in Lowell, Vermont. Geochemical analysis of the Stowe, Ottaquechee, Moretown and Hazens Notch Formations from the present research and previous work (Bright, 2006; Anderson, 2006; Morris, 2006) are combined to form a suite of 99 samples. Complete geochemical analyses have been performed on 76 of the samples, and document relatively low concentrations of arsenic in schists and quartzites (mean = 8.6 ppm; SD = 15.3; N = 20) and greenstones (mean = 4.1 ppm; SD = 13.9; N = 33) compared to ultramafic rocks which contain high arsenic concentrations between 9.5 and 449 ppm (mean = 63.7 ppm; SD = 94.1; N = 23). Base metals (e.g., Cu, Zn, Pb) in ultramafic rocks are not elevated above average crustal abundances to the same extent as As. Metasedimentary rocks show low to moderate correlations between arsenic and elements expected of a sulfide source (e.g., R2 for As and Ni = 0.42) which suggests a potential mixture of sulfide and non-sulfide sources of arsenic. Scanning electron microscopyenergy dispersive spectrometry (SEM-EDS) analysis of ultramafic thin sections have not identified a particular trace mineral that contains arsenic; instead, it appears that the arsenic is disseminated throughout the serpentine, possibly substituted into tetrahedral layers. Such speculation is supported by recent work by Hattori et al. (2005), who indicate that arsenic becomes incorporated into serpentine during hydration and metamorphism of ultramafic rocks associated with orogenic events. The elevated concentrations of arsenic within the rocks from the Barnes Hill and other ultramafic bodies along a transect from Waterbury to Lowell, and the relative lack of elevated arsenic in regional metasedimentary rocks and greenstones, suggest that the ultramafic rocks are the source of the high arsenic concentrations in the ground water. However, the presence of As in wells that do not exhibit ultramafic signatures indicate that sulfidic schists are also a likely As source. <br/> <br/>Description: Submitted in partial fulfillment of the requirement for the degree of Bachelor of Arts Department of Geology Middlebury College Middlebury, VT Sat, 28 Apr 2007 22:58:59 GMT http://hdl.handle.net/10090/793 Title: Uranium-Enriched Ground Water, Knox Mountain Pluton, Vermont: Occurrence And Geologic Controls <br/> <br/>Authors: Gleason, Michael Jeffrey <br/> <br/>Abstract: Since 2003, seven ground water wells in Marshfield and Peacham, VT have demonstrated uranium concentrations exceeding U.S. EPA and VT Department of Health (VDH) maximum contaminant levels (MCLs). The contaminated wells tap deep fractured-bedrock aquifers of the Knox Mountain Pluton, implicating lithologies within this Devonian intrusion as likely sources. This study uses a multidisciplinary approach to assess: (1) the distribution of uranium-enriched ground water across the northern Knox Mountain Pluton; and (2) the lithologic, geochemical, and hydrologic factors controlling uranium concentrations in the ground water. Of 19 private wells sampled in the study area, only three contain uranium in concentrations above the VDH MCL of 20 ppb (i.e. 21, 24, 37 ppb U), and only two contain gross alpha (GA) in levels exceeding the EPA MCL of 15 pCi/L (i.e. 18.3, 28.9 pCi/L). Nevertheless, uranium levels are generally above the average of 1.8 ppb U found in ground water throughout the U.S (Longtin, 1990). While the geochemistry of ground water in the study area seems to have minimal effects on uranium concentrations, distinct spatial patterns of high and low uranium wells suggest the importance of lithologic variation. Nevertheless, bulk-rock concentrations of uranium for most lithologies within the pluton demonstrate a narrow range (1.20 to 5.70 ppm) near the typical concentration of 4 ppm found in granitic rocks (Legrand, 1987), and do not correlate with the observed distribution of uranium levels in wells. Analysis of the solubility of uranium in the different rock types reveals that pegmatites and muscovite granite may have a greater propensity to contribute uranium to ground water during leaching than others. This observation is supported by bulk-rock geochemistry and optical analyses of thin sections, which indicate the common siting of uranium in highly soluble secondary minerals within these lithologies. One sample of the Waits River Formation taken from the area near the tested wells is characterized by particularly low uranium solubility; thus, water originating in the country rock likely may dilute uranium concentrations in areas of ground water convergence. When combined with hydrologic setting, the high solubility of uranium in pegmatites and muscovite granite and low solubility of uranium in the Waits River Formation can account for the observed spatial patterns of uranium-enriched and uranium-depleted wells. In light of this, hydrologic setting and the lithologic variability in uranium solubility seem to be the two dominant factors controlling uranium concentrations in the ground water of this region. <br/> <br/>Description: Submitted in partial fulfillment of the requirements for the degree of Bachelor of Arts Department of Geology Middlebury College Middlebury, Vermont Thu, 03 May 2007 22:58:59 GMT