Browsing by Subject "Mantle"
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Item Acquiring Bulk Compositions of Spinel Peridotite Xenoliths Using GIS and Digital Image Processing Techniques(2022-12) Roberts, AmberThis study investigates the viability of using GIS techniques to estimate modal abundances of minerals in thin sections of peridotite and uses this data to calculate bulk chemical compositions of peridotitic mantle xenoliths from Oahu, Hawaii to determine their provenance. To answer these questions, I performed classifications using ArcGIS on nine xenolith samples to determine their modal abundances, which were then used to calculate their bulk chemical compositions. My results show that GIS has the potential to be a useful tool for non-destructive analysis of modal abundances. They also support a role for melt-rock reaction occurring between migrating melts and peridotite in the oceanic lithospheric mantle, resulting in the production of dunites. This study is a first step in utilizing GIS to assist with thin section analyses and fills a gap in existing chemical data for Hawaiian mantle xenoliths.Item The Development Of Olivine Textures In Complex Deformation Geometries(2023) Wagner, NicoleThe analysis of crystallographic preferred orientations (CPOs) in olivine is a crucial tool in our understanding of the Earth’s upper mantle. The development of CPOs is controlled by the activation of slip systems, which are sensitive to various thermochemomechanical conditions such as stress, temperature, and water content. Laboratory experiments have offered insight on the conditions in which different olivine CPOs develop. However, the relationship between these thermochemomechanical conditions and CPOs is complicated. Recent studies have challenged some of our current understanding on olivine CPO development and have brought attention to the importance of kinematics. Here, we aim to experimentally investigate the role of kinematics in the development of olivine CPOs, particularly pertaining to complex deformation geometries involving simultaneous pure and simple shear. To explore this topic, cylindrical samples of dry, polycrystalline San Carlos olivine were deformed in either simultaneous extension and torsion or simultaneous shortening and torsion at a temperature of 1523 K and confining pressure of 300 MPa in a Paterson gas-medium apparatus at the University of Minnesota. Following deformation, the CPO was measured at different sections along the sample radius using electron backscatter diffraction analysis. For each section, the kinematic vorticity number, the equivalent strain, the J-index, and the M-index were determined. The fabric index angle was also calculated to classify the resulting CPO for each section. In total, two samples were deformed in extension and torsion and three samples in shortening and torsion. Of the two extension and torsion experiments, one sample localized thus allowing for multiple sections along the length to be analyzed. Moreover, the samples subjected to extension and torsion went out to axial strains of 0.15, 0.26, and 0.36 and to shear strains between 1.5 and 2.2. These samples generally produced stronger textures and displayed an evolution from C-type to E-type to D-type CPO with increasing kinematic vorticity. The presence of an E-type CPO aligns with numerical simulations of CPO development in simultaneous extension and simple shear geometries and suggests that olivine does not necessarily develop as a function of water content. Conversely, the samples deformed in shortening and torsion went to axial strains of -0.12, -0.28, and -0.35 and shear strains between 1.0 and 1.7. These samples had weaker textures and typically evolved from an AG-type to an A-type CPO with increasing kinematic vorticity, which also reasonably aligns with numerical simulations of this given geometry. Although different textural evolutions were observed between these two deformation geometries, the fabric index angles began to converge as kinematic vorticity increased for both sets of experiments, ultimately resulting in textures expected in simple shear. The geometries achieved in this study were then simulated using a modified director textural model. The simulations produced from this model reasonably reproduced the CPO characteristics observed in the experimental samples, but the CPO classification, as determined by the fabric index angle, did not align as well. All in all, the findings presented in this study highlight the role of kinematics in CPO development, emphasize the caution needed when interpreting the thermochemomechanical conditions of the mantle from seismic anisotropy and exhumed peridotites, and contribute valuable information on how we can use olivine CPOs as a field tool.Item Experimentally generated partition coefficients for the first-row transition elements during eclogite partial melting at three gigapascals(2020-10) Regula, AndrewThe earth’s mantle is primarily composed of a rock called peridotite. Subduction results in the reinjection of basaltic crust into the mantle, which is metamorphosed into a rock called eclogite. The extent of lithological heterogeneity has implications for the rate of convective stirring and chemical homogenization of the mantle but is not well-constrained. Basaltic melts extracted from the mantle carry geochemical information about the composition of the mantle, most clearly through their trace element and isotopic signatures. This experimental study provides measurements of the partitioning of the first-row transition elements (FRTE) between eclogite and melt, which could be used to fingerprint the source lithology of ocean island basalts (OIB) and infer participation of recycled lithologies in their generation. My simple forward melting modeling shows that partial melts of peridotite match OIB FRTE signatures just as well as melts from mixed peridotite plus eclogite sources, but there are some signatures that are not well matched by either.Item Ferric Iron Partitioning between Pyroxene and Melt: Experiments, Microbeam analysis, and Consequences for Mantle Redox(2021-12) Rudra, AvishekPyroxene is the chief reservoir of Fe3+ in upper mantle peridotite, but experiments exploring pyroxene/melt Fe3+ partitioning have been restricted to 100 kPa and pyroxene with low alumina. Here we present Fe3+ partitioning experiments between clinopyroxenes (cpx) and mafic melt at elevated pressures (1–2.5 GPa). Experiments were conducted with fO2 buffered and modulated by Ru+RuO2 and Fe-Pt alloy capsules, respectively, between ∆QFM -2.68 and +5.13. Fe3+/FeT of both cpx and melt were determined by Fe K-edge X-ray absorption near edge structure spectroscopy. The experimentally synthesized cpx compositions (Al2O3 = 2.36–6.01 wt.%, CaO = 19.33–22.21 wt.%) approximate those expected in basalt source regions. We find that Fe3+ is moderately incompatible in cpx and correlates with cpx Al2O3 content, increasing from 0.05±0.09 to 0.81±0.04. Comparison between experimentally synthesized cpx with those from natural peridotites indicates influences of both temperature and composition on Fe3+/FeT for cpx in spinel and garnet peridotites. The combined effects of decreased pyroxene Al2O3 concentration and pyroxene mode with progressive partial melting of peridotite diminishes the bulk partition coefficients of Fe3+, leading to greater Fe2O3 contents in high degree partial melts, and this accounts for an inverse relationship between Na2O and Fe2O3 observed in mid-ocean ridge basalts (MORB). Comparison to numerical experiments with pMELTS and the model of Jennings and Holland (2015) show that these models overpredict for partial melting of the mantle, and so they do not accurately determine the relationship between the fO2 and Fe2O3 of peridotite in basalt source regions. To estimate the Fe3+/FeT ratio of the mantle source of MORB, we modeled liquid Fe2O3 during isentropic batch melting of peridotite at three potential temperatures (1320 °C, 1400 °C, and 1440 °C) for peridotitic sources with Fe3+/FeT ratios between 0.02–0.06. A source with an Fe3+/FeT ratio of 0.038±0.007 matches most of the span of natural MORB. This ratio is similar to that typical of continental lithospheric mantle sampled by xenoliths, but lower than that surmised by several recent experimental and thermodynamic studies. Considering this source Fe3+/FeT but extending the partial melting calculations to higher pressures (2.5–4 GPa) reveals that bulk significantly decreases for garnet peridotite relative to spinel peridotite because the cpx become significantly less aluminous with increasing pressure. This results in high pressure partial melts with greater liquid Fe3+/FeT ratios. Therefore, elevated Fe3+/FeT ratios observed from some oceanic island basalts (OIB), such as those from Hawaii and Iceland, reflect in part the differences in conditions of melting and may not require mantle source regions more oxidized than those that produce MORB.Item Iron-Nickel-Sulfur-Carbon System Under High Pressure, With Implications To Earth’S Mantle(2016-10) Zhang, ZhouFe-Ni-S-C phases are accessory phases in the Earth’s mantle, but carry important geochemical and geophysical implications. According to their chemical behavior, Fe-Ni-S-C phases preferentially store siderophile and chalcophile elements (and potentially noble gases). Physically, Fe-Ni-S-C phases have distinctly higher densities, surface tensions, and electrical conductivities, and lower melting points than mantle silicates. Understanding the geochemical and geophysical impacts caused by Fe-Ni-S-C phases requires accurate quantification of the basic properties of Fe-Ni-S-C phases under mantle conditions. This PhD thesis uses both high-pressure experiments and thermodynamic calculations to constrain the melting temperatures and compositions of Fe-Ni-S-C phases in the Earth’s upper mantle mantle, and their potential for deep carbon storage. This study suggests that monosulfides in the upper mantle are mostly molten, even in significant portions of cratonic roots under continental geotherms. Incorporation of carbon depresses the monosulfide solidus by 50-100˚C. Experiments and calculations of reactions between Fe-Ni-S melts and silicates at mantle conditions suggest that Fe-Ni-S melts are Ni-rich (Ni/(Ni+Fe)~0.6) monosulfides ((Fe+Ni)/S~1 or Xs~0.5) under oxidized (FMQ -2 to FMQ) conditions at 2 GPa. With increasing depth in the mantle (thus decreasing fO2), Fe-Ni-S melts become increasingly Ni- and S-poor, characterized by Ni/(Ni+Fe)~0.4, (Fe+Ni)/S~3, and Xs~0。4 at 8 GPa, and Ni/(Ni+Fe)~0.2, Xs~0.05 and (Fe+Ni)/S~10 at 12 GPa. Carbon solubility in Fe-Ni-S melts determined by high-pressure experiments suggests that carbon solubility decreases exponentially with increasing Xs. Based on mantle Fe-Ni-S melt compositions, C-S relations in carbon-saturated melts, and the typical mantle P-T-fO2 profile and sulfur abundance (200 ppm), it is suggested that significant amounts (40-100%) of deep carbon could potentially be stored in Fe-Ni-S melts in the Earth’s reduced deep upper mantle.Item Seismic structure of the mantle beneath the Pacific Hemisphere(2011-06) Bagley, Brian C.Aside from xenoliths, the Earth's mantle is a region that is inaccessible directly, leaving us with limited tools to investigate its characteristics indirectly. Seismology is a tool well-suited for this purpose, and has provided valuable insight regarding many fundamental processes occurring within the mantle. It is fortuitous that the mantle is layered, and that these layers are often punctuated by distinct changes in density and/or velocity that are seismically detectable. By investigating the seismic structure of the mantle we are able to infer properties such as composition, temperature, anisotropy, and water content. Seismic tomography has informed our understanding of subduction and the fate of slabs, and we are beginning to realize that the lower mantle might also be rich with heterogeneity. Our picture of the Earth's mantle is becoming clearer, however, there is much that we do not understand. Receiver function studies of the oceans are fewer and suffer the common malady of looking beneath oceanic islands, not generic oceanic crust. Most of the detailed information regarding the seismic discontinuity structure of open ocean mantle comes from bottom-side reflections that are precursors to SS phases (a shear wave that has traveled from source to receiver with one bottom-side surface bounce in between). SS and PP (a compressional wave with a path analogous to SS) precursors offer extensive geographic coverage and good sensitivity to small velocity contrasts and reasonable localization. They do not perform well for shallow reflectors, or reflectors near the larger transition zone discontinuities. In our studies we use multiple ScS reverberations to gain better resolution of these features. The primary goals of this research are to study mantle discontinuities, and fill in some of the missing detail regarding mantle heterogeneity. We do this by examining the Pacific ocean, beginning with the open ocean mantle, then moving to the subduction zones in the west Pacific. This region, containing the Boso Triple Junction, is one of the most complex subduction zones on the planet. Finally we continue west beneath the Sea of Japan, the Sea of Okhotsk, and the northeast Chinese craton. The changes in mantle structure across the Pacific reveal many interesting differences between the open ocean mantle and the mantle in regions of subduction.Item Stability of reduced carbon in the mantle(2013-01) Hastings, Patrick Timothy JrReduced carbon in the mantle is commonly thought to be chiefly in diamond, but experiments suggest that at >250 km the mantle contains small amounts (0.1-1 %) of FeNi alloy. [1, 2, 3]. Thus, alloy may be a significant host of reduced C [4], but little is known about C solubility of FeNi alloy under mantle conditions. To determine the carbon solubility in FeNi alloy and melt, we conducted experiments in the system Fe-Ni-C with bulk compositions having 5 wt. % C and variable Fe/(Fe+Ni) at 3 to 7 GPa and 1000 - 1400°C. Experiments at 3 GPa and 1000-1250 °C were performed in an end loaded piston cylinder apparatus; those at 5 and 7 GPa and 1200-1400°C were performed in a 1000 ton Walker-style octahedral multianvil. At 3 GPa, Fe-rich melts contain up to 4.5 wt. % C, but Ni-rich (>25 mole% Ni) compositions remain subsolidus at 1250°C. The solubility of carbon in pure Fe and Ni metal are 2 wt. % and 1 wt. % respectively, but in the alloy passes through a minimum of 0.4 wt. % for Fe0.2Ni0.8. Assuming that these concentrations apply at higher temperatures and pressures (as will be tested by future experiments) allows a first estimate of the potential storage of C in FeNi alloy in the mantle. If the mantle at 250 km contains 0.1-0.2 wt.% Ni-rich (Fe0.4Ni0.6) alloy, increasing with depth to 1 wt.% Fe-rich (Fe0.88Ni0.12) alloy at 700 km [1,2], then maximum storage of C in alloy rises from 5 ppm in the deep upper mantle to 180 ppm in the shallow lower mantle. For mantle similar to the MORB source, with ~10-30 ppm C [5], alloy cannot store all C in the deep upper mantle but can in the lower mantle. For OIB sources with 33-500 ppm C [5], complete storage in alloy is less likely. Additional phases will be diamond in the upper mantle, as our experiments and previous work [6] indicate that carbide is not stable in equilibrium with Ni-rich alloy, and carbide melt in the lower mantle. [1] Frost et al (2004) Nature 428 409-412 [2] Frost and McCammon (2008) EPSL 36 389-420 [3] Rohrbach et al. (2011) J.Petrol 52 #717-731 [4] Dasgupta et al. (2009) GCA 73 6678-6691 [5] Dasgupta and Hirschmann (2010) EPSL 298 1-13 [6] Romig and Goldstein (1978) Metal Trans. Met. AIME 9a 1599-1609.