[Eoas-seminar] Reminder: Geology Dissertation Defense - Rupsa Roy - July 20, 10:30am, A321 MagLab

eoas-seminar at lists.fsu.edu eoas-seminar at lists.fsu.edu
Wed Jul 19 14:45:23 EDT 2017

Title: Geochemical Study of Mantle Processes and Paleoclimate Reconstruction: Perspective from Mantle Xenoliths and Modern Freshwater Snail Shells

Major Professors: Yang Wang and Vincent Salters

The major elements, trace elements, and isotopic composition of clinopyroxene and garnet fractions from mantle xenoliths from Colorado Plateau area were analyzed in this study to examine the sources of the xenoliths and their role in fractionation of Nb from Ta during the subduction process. Unlike most of the other elemental ratios, Nb/Ta ratio in the earth's crust and depleted mantle do not add up to the chondritic ratio (~18). This discrepancy has led to the assumption of existence of a "hidden" reservoir enriched in Nb content in the Earth's interior (core or mantle). Studies show that Nb/Ta ratio is controlled predominantly by Ta mobility in hydrous fluid. Subduction related hydration, therefore has the potential to fractionate Ta from Nb, resulting in a reservoir high in Nb in the subducted crust. In order to test this hypothesis and to find out the sources of the xenoliths, clinopyroxene and garnet fractions of eclogites and garnet pyroxenites from Moses Rock and Garnet Ridge, and clinopyroxene fractions of high MgO and low MgO clinopyroxenites from Moses Rock have been analyzed. These xenoliths are emplaced in the serpentinized ultramafic microbreccia and minnette diatreme hosts, that erupted during Laramide Orogeny (80-40 Ma) when Farallon plate subducted beneath the Navajo Volcanic Field (NVF) at a low angle. Elemental and isotope signature of the eclogites and garnet pyroxenites suggest that they are derived from the primitive mantle that underwent subsequent metasomatism followed by multiple stages of melting before and Laramide Orogeny. In contrast, high MgO pyroxenites have residual mantle origin, later altered by hydration and low MgO pyroxenites are part of the lower crust metasomatized by a carbon rich silicate magma. Presence of fragments of lower crust and residual mantle in the minnette diatremes is a result of scraping off part of the overlying mantle wedge and lower crust that was dragged along by the flat-subducting Farallon plate. Bulk rock Nb/Ta ratio of the eclogites and garnet pyroxenites suggest that they are not a potential reservoir for high Nb. The lowest Nb/Ta ratio in the high MgO pyroxenites indicates that hydration of mantle wedge has fractionated Nb from Ta due to mobility of Ta in fluid resulting in a low Nb/Ta ratio in the rocks. The near chondritic Nb/Ta ratio in the low MgO pyroxenites emphasizes their melt metasomatized lower crustal origin.
Fossil mollusk shells are widely distributed in the geologic records and their oxygen isotopic compositions have been used to reconstruct the seasonal changes in temperature, local climatic and elevation conditions. However, interpretation of isotope data from fossil shells in terms of past climate and elevation is often ambiguous. This study examines the stable isotopic systematics in the carbonate shells of modern freshwater gastropods Bellamya and Radix along an elevation/climate transect in the Asian monsoon region in China in order to improve the accuracy of paleoclimate and paleoelevation reconstruction based on stable isotopes in fossil shells.  The results from sclerochronological stable isotopic analyses of the shells show that the intra-shell oxygen isotopic pattern in a shell is determined by seasonal variations in lake water temperature and water isotopic composition as well as the life history (time of birth and life span) of the organism.  The data suggest that both Bellamya and Radix are able to reproduce at any time from early spring to the fall and grow all year in the low to mid altitude/latitude lakes. Bellamya is not found in high altitude Tibetan lakes. Radix in the high elevation cold habitat, although prefers to grow in the summer months, can survive through the freezing temperature. The δ18O patterns in the gastropod shells are similar across the elevation/climate transect, showing high δ18O values in the winter months and low δ18O values in the summer months, which is consistent with the expected pattern in the Asian summer monsoon region. The average growth rates of the snails analyzed in this study were highest at the lowest elevation site characterized by warm and humid climatic conditions but were similar at the mid to high elevation sites. The growth rate of Radix appears to decrease towards the late ontogeny.  Bellamya and Radix from the same lake have similar δ18O values, suggesting that there is no species-dependent isotope fractionation in the shells of these freshwater snails. The serial isotope data suggest that both Bellamya and Radix shells, if they are full-sized or nearly full-sized, are excellent archives of lake environmental conditions and most suitable for paleo-environmental studies. Smaller sized shells (<1.5 cm for Radix and <2.1cm for Bellamya), on the other hand, are not ideal for paleo-environmental reconstruction as they represent short periods (<1 year) of growth and would yield biased information about paleo-environment towards the times of their shell formation.
The oxygen isotopic compositions of bulk shells from these lakes vary widely and are in part controlled by life histories of the individual snails, reflecting the average environmental conditions during the shell formation.  Although the δ18O values of the shells do not show a clear relationship with elevation, the temperatures calculated from the measured δ18O values of bulk shells and lake water δ18O decrease with increasing elevation. The average calculated temperature for each lake is similar to the mean annual water temperature (MAWT) within the analytical uncertainty except Liuxihe Reservoir where only limited and small-sized samples were analyzed.  Based on the isotopic variability observed in these modern shells, it was estimated that a minimum of 12 individual shells would be needed in order to provide a reliable estimate of the MAWT for a given location.
The carbon isotope variations in the shells appear to be mainly controlled by variations in the carbon isotopic composition of dissolved inorganic carbon (DIC), with higher values indicating either increased primary productivity in the lake or increased fraction of DIC derived from dissolution of limestone (relative to that originated from decomposition of organic matter).  Shells from the low altitude lake (in a warm climate) are more depleted in heavy carbon isotope 13C compared to those from higher elevation lakes, due to higher rates of organic matter decomposition contributing a greater amount of 13C-depleted carbon to the DIC and resulting in lower δ13C values of the shells.  Radiocarbon analyses of modern shells from the studied lakes confirm the previous observation that lake radiocarbon reservoir effect varies greatly among lakes depending on local hydrology and bedrock lithology. The reservoir effect also varies in space and time within a lake.

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