Navajo Volcanic Field xenoliths of the Colorado Plateau : a window into subduction processes from the Proterozoic to the present




Marshall, Edward Wayne, IV

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Mantle xenoliths from the central Colorado Plateau record geochemical evidence of both ancient and modern subduction. These xenoliths are sampled from unusual serpentinized ultramafic microbreccia diatremes, and are both modally hydrated and metasomatized. The hydration and metasomatism of the xenoliths is related to Farallon flat-slab subduction beneath the Colorado Plateau. In Chapter 1, Sm-Nd and Re-Os isotopes systematics are used to see through Farallon hydration and metasomatism and learn more about pre-Farallon magmatic events. From Re-Os systematics, the lithospheric mantle beneath the Colorado Plateau has been retained since it formed between 2.0 to 1.6 Ga. Unmetasomatized xenoliths lie on a Sm-Nd isochron that is 1.45 Ga in age, suggesting a major isotopic resetting event at this time. Combining these two observations with those of previous studies suggests that subduction triggered the ~1.4 Ga granite magmatism event in Laurentia. In Chapter 2, oxygen isotope compositions of olivine and hydrogen isotope compositions of hydrous minerals were collected to investigate the sources and effects of Farallon flat-slab fluids on the Colorado Plateau lithospheric mantle. Hydrogen isotope compositions of hydrous minerals are consistent with equilibration with slab-derived fluids. Oxygen isotope compositions of olivines correlate with indices of metasomatism from the same xenoliths. From this correlation, metasomatism in the lithospheric mantle is related to fluids derived from the serpentinized lithosphere of the Farallon slab. In Chapter 3, hydrogen concentrations in nominally anhydrous minerals (NAMs; e.g. pyroxene) are measured and evaluated for connections between metasomatism, melt extraction, and hydrous mineral growth. There is no convincing clear connection between metasomatism, melt extraction, or hydrous mineral growth and NAM water content either in the Colorado Plateau xenoliths or in studies of NAM water contents from other localities. A key observation is that NAM water content has significantly less variability than similarly incompatible species, such as Ce. This can be explained if water diffuses rapidly through the mantle, smoothing out variability in concentration and decoupling it from indices of melt extraction, metasomatism, or hydrous mineral growth.


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