Browsing by Subject "Oxygen isotopes"
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Item Oxygen isotope evidence for interaction of Franciscan high-grade blocks in the mantle wedge with sediment derived fluids, Ring Mountain (Tiburon) and Jenner Beach, California(2012-08) Errico, Jessica Cori; Barnes, Jaime D.; Cloos, Mark; Catlos, ElizabethOxygen isotopes and major and trace element geochemistry have been used to evaluate the geochemical and tectonic history of a Franciscan hornblende-amphibolite and a eclogite block from Ring Mountain, Tiburon and three eclogite/blueschist blocks from Jenner Beach, California, all blocks have experienced varying amounts of retrogression. Relative to the presumed basaltic protolith, enrichments in large ion lithophile elements (LILEs) indicate interaction with sediment derived fluids in the retrograde eclogite and retrograde blueschist samples and high Mg, Cr, and Ni in actinolite rind indicate interaction with ultramafic rock. The [delta]¹⁸O values of chlorite from the Ring Mountain hornblende-amphibolite and the eclogite block have a narrow range of [delta]¹⁸O values (+7.7-8.2%₀, n=8) and actinolite from actinolite rind on the eclogite block from Ring Mountain and the blueschist/eclogite blocks from Jenner Beach are (+7.8-8.5%₀, n=5). Chlorite-actinolite geothermometry yields temperatures of 200-280°C for actinolite rind formation. Additionally, the [delta]¹⁸O values of both chlorite and actinolite at these temperatures indicates equilibrium with the measured value of Tiburon serpentinites, (7.6 to 8.1%₀, n = 3 Wenner and Taylor, 1974). Oxygen isotope analyses of garnet mineral separates from the eclogite and hornblende-amphibolite from Ring Mountain have [delta]¹⁸O values of +6.8±0.3%₀ (n=7), and +8.2±0.2%₀ (n=7), respectively. Garnets from the three eclogite/blueschist blocks at Jenner Beach have a [delta]¹⁸O value of +9.8±0.7%₀, (n=23). The difference in [delta]¹⁸O values of garnets between the high-grade blocks is likely due to in situ hydrothermal alteration of the seafloor basalt prior to subduction. The geochemical trends can be explained by a model in which during the early stages of subduction pieces of altered oceanic crust are detached from the downgoing slab and incorporated into the mantle wedge soon after reaching peak eclogite or amphibolite facies conditions. As subduction continues, the hanging wall cools and fluids released from subducted sediments infiltrate the overlying mantle wedge. As the blocks cool they develop a retrograde blueschist facies overprint under relatively static conditions. With cooling of the hanging wall and infiltration of sedimentary fluids, serpentinization induces reaction between the blocks and surrounding mantle wedge and Mg-rich actinolite rind is formed. The blocks are then plucked from the mantle wedge and incorporated into the subduction channel where they flow back to the surface via corner flow.Item Reconstructing migration : using isotopic analysis to examine ancient Maya mobility in Northwestern Belize(2015-05) Locker, Angelina Jean; Valdez, Fred, Jr., 1953-; Breecker, Daniel OIsotopic analysis has proven to be beneficial to the field of archaeology, aiding in the understanding of changing climatic conditions, diet, and mobility. This report proposes the use of Oxygen and Strontium isotope ratios to understand migration patterns of the Ancient Maya within the Program for Belize Archaeological Project (PfBAP) research area in northwestern Belize. Research seeks to first identify immigrants and then try to understand sociopolitical factors that may have influenced population movement as well as the consequences of that movement upon a region. Currently, our understanding of mobility and migration within this region is severely lacking. This report presents a general background on migration in archaeology as well as a general background on oxygen and strontium isotopes, their application to the field of archaeology, and how isotopic ratios can shed light on possible reasons for population movement. Additionally, this report outlines a protocol for each isotopic system and proposes future research for the PfBAP region.Item Seasonality and Environmental Heterogeneity during a Late Carboniferous Highstand: Brachiopod Shell Geochemical Records from the Pangean Tropics(2014-12-10) Roark, AndrewThe relationship between Permo-Carboniferous glacial cycles and low-latitude climate remains a subject of vigorous debate. This study investigated seasonality and regional environmental variability in a portion of central equatorial Pangea during a late Pennsylvanian highstand using stable isotope and trace element analyses of brachiopod shells from the Virgilian Ames Member of the Conemaugh Group in the Appalachian Basin, U.S.A. Well-preserved, thick-shelled Neospirifer dunbari specimens were serially sampled across growth bands to elucidate a record of seasonal variability during the life of the organisms. Because Neospirifer only colonized this marginal basin during near-maximum highstands when stable marine salinities were established, these data are a direct proxy for intra-annual climate fluctuations during interglacial times. Additionally, well-preserved specimens of the thinner-shelled brachiopod Crurithyris planoconvexa, which has a wider spatial and stratigraphic distribution within the basin, were analyzed individually. Neospirifer specimens show remarkably little internal chemical variability, with ?^18O generally fluctuating by 0.4? or less and ?^13C by less than 1.5? within a single specimen. Moreover, total variability between all specimens is only ~1.5?. This lack of variation reflects a homogenous, nonseasonal to weakly seasonal climate during the Ames highstand. Both ?^18O and ?^13C are ~1.5 lower than those of other Virgilian specimens from regions with a more proximal connection to the open ocean, suggesting relatively high freshwater influence in the Appalachian Basin during this time, although salinities remained close to marine levels. Thus, brachiopod seasonal records indicate normal moist tropical conditions during this penultimate Carboniferous interglacial, with no evidence for strong monsoonal variations in temperature or rainfall. Crurithyris specimens show similarly homogenous isotopic values that are slightly depleted relative to more nearly marine specimens. There is a weak gradient towards increasing ?^18O and ?^13C in Crurithyris specimens to the west, consistent with decreasing influence of Appalachian runoff waters in that direction. Crurithyris from two sites show anomalously enriched isotopic values, potentially indicating rare, sporadic periods of net evaporation. However, because of Crurithyris? wider temporal distribution, these events may not have occurred during the highstand.