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Item Oceanic and continental lithospheric mantle in the 1.95 Ga Jormua Ophiolite Complex, Finland: implications for mantle and crustal evolution(Oxford, 2023-10-31) Finlayson, Valerie; Haller, Mitchell; Day, James M.D.; Ginley, Stephen; O'Driskoll, Brian; Kontinen, Asko; Hanski, Eero; Walker, Richard J.The ca. 1.95 Ga Jormua Ophiolite Complex (JOC), Finland, is a rare Paleoproterozoic ophiolite that preserves a record of diverse upper mantle materials and melting processes. Meter-scale grid sampling of four JOC outcrops, as well as non-grid samples, permits evaluation of meter- to kilometer-scale mantle heterogeneity within the JOC. Significant heterogeneity is observed between the four grids, and also among a number of the non-grid samples examined. Variations in the concentrations of fluid-mobile elements are particularly large among different samples and locations. New whole-rock major, lithophile trace, and highly siderophile element data (HSE: Os, Ir, Ru, Pt, Pd, Re), including 187Re-187Os isotopic data, for serpentinized harzburgites indicate the presence of two distinct compositional types and probable modes of origin within the JOC. This is consistent with prior findings. Type 1 is similar to modern refractory abyssal-type mantle. Type 2 is more highly refractory than Type 1, and most likely represents samples from sub-continental lithospheric mantle (SCLM). Type 1 mantle is moderately heterogeneous with respect to chemical and Os isotopic compositions at both the meter and kilometer scales. By contrast, Type 2 mantle is considerably more homogeneous than Type 1 grids at the meter scale, but is more heterogeneous at the kilometer scale. The median initial γOs value for Type 1 mantle, calculated for 1.95 Ga, is ~-2.0 (where γOs is the % deviation in 187Os/188Os relative to a chondritic reference calculated for a specified time). This isotopic composition is consistent with a moderate, long-term decrease in Re/Os relative to the estimate for Primitive Mantle, prior to JOC formation. The similarity in this γOs value to the value for the modern abyssal mantle, as well as the initial values for several Phanerozoic ophiolites suggests that the upper mantle achieved a Re/Os ratio similar to the chondritic reference by ~2 Ga, then evolved along a subparallel trajectory to the chondritic reference since then. For this to occur, only limited Re could have been permanently removed from the upper mantle since at least the time the JOC formed. A localized secondary metasomatic event at ~2 Ga, concurrent with the estimated obduction age for the JOC and subsequent Svecofennian Orogeny, affected the HSE systematics of some Type 1 samples. By contrast, late Archean Os TRD model ages for Type 2 rocks indicate a depletion event superimposed upon the long-term Re depletion of the abyssal mantle. This event was established no later than ~2.6 Ga and may have occurred during a period of significant, well-documented crustal production in the Karelia craton at ~2.7 Ga.Item RE-OS AND OXYGEN SYSTEMATICS OF VARIABLY ALTERED ULTRAMAFIC ROCKS, NORTH CAROLINA(2020) Centorbi, Tracey; Walker, Richard J.; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This study focuses on the origin and modification of six ultramafic bodies located in the Blue Ridge Province of North Carolina. The bodies consist mainly of harzburgites and dunites with associated chromites. Some of the bodies are associated spatially and genetically with mafic lithologies while others are fault bounded. All the bodies in the study are characterized by variations in their initial Os isotopic compositions, assuming a formation age of 490 Ma (187Os/188Osinitial 0.1114 to 0.1360). Most of the initial 187Os/188Os ratios are chondritic to subchondritic and can be explained by Re depletion during a partial melting event prior to ophiolite formation. By contrast, some initial 187Os/188Os ratios, particularly for those bodies in the Tallulah Falls formation, are suprachondritic suggesting the addition of radiogenic Os during a melt percolation or melt/rock reaction event, most likely during the event that led to the formation of the bodies. Oxygen isotopic δ18O values of the bodies range from +4.85 to +7.60 which overlap with and extend above mantle estimates. The cause of the higher values remains unresolved, but serpentinization and contamination by large amounts of crustal material can be excluded. It is concluded that the six bodies in this study have a common history as the residues of mantle partial melting, with chemical compositions and isotopic systematics similar to Phanerozoic ophiolite peridotites associated with the same collisional event, as well as modern abyssal peridotites. Nevertheless, Os isotopic characteristics indicate different processes acted within the bodies despite their relatively close spatial association.Item Patterns and Mechanisms of Melt Distribution in Partially Molten Harzburgite under Hydrostatic Pressure(2017) Hou, Jiangyi; Zhu, Wenlu; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Geophysical and geochemical properties of partially molten regions of the Earth’s upper mantle are strongly affected by the distribution of melt. I investigated the mechanisms affecting the 3-dimensional (3D) melt distribution of partially molten harzburgite samples. The 3D melt distributions of experimental charges of various melt fractions were recorded using synchrotron-based X-ray microtomography. In most samples the melt fraction increases along the axial direction of the sample, but the mineral phases exhibit no systematic heterogeneities. Analysis of time-series samples confirmed that the melt fraction heterogeneity was generated during sintering and took more than 84 hours to develop. To elucidate the mechanisms of melt focusing, I evaluated the importance of gravity, surface tension, lithologic partitioning and thermal migration as potential driving forces. The scale of the melt fraction variations and the speed at which they develop appear compatible with the thermal migration model.