Geochemical Investigation of Fluid-Rock Interactions During Chemical Weathering and Subduction

• Main Author: Hanna, Heather Dawn
• Other Authors: College of Arts and Sciences, Department of Geological Sciences, Liu, Xiao-Ming, Coleman, Drew, Barzyk, Julia, Benninger, Larry, Teng, Feng-Zhen
• Format: Doctoral or Postdoctoral Thesis
• Language: English
• Published: University of North Carolina at Chapel Hill Graduate School 2020
• Subjects: Pedogenic Processes; Galapagos Islands; San Cristobal Island; Geochemistry; Lithium Isotopes; Basalt Weathering; Aleutian Arc; Galapagos; Aleutian Island
• Online Access: https://doi.org/10.17615/5w3w-k022; https://cdr.lib.unc.edu/downloads/cz30q2514?file=thumbnail; https://cdr.lib.unc.edu/downloads/cz30q2514

This research examines fluid-rock interactions in two significantly different environments. First, Li isotopic values are evaluated as a potential slab component tracer in Aleutian island arc rocks. The δ7Li values of Aleutian lavas do not exhibit the spatial trends observed in other slab component tracers, nor do they or correlate with these tracers. However, mixing models using 7Li values in conjunction with 143Nd/144Nd ratios suggest most Aleutian samples can be explained by addition of <1-2% sediment-derived aqueous fluid and ≤3% sediment melt to depleted mantle. The study demonstrates that slab signatures may be deciphered via modeling even in arcs where 7Li values do not correlate with slab component indicators, and where sediment and mantle 7Li values overlap.Next the impact of weathering intensity and climate on basalt weathering and pedogenic processes are examined for soil and saprolite samples from San Cristobal island, Galapagos. This research suggests different mineralogical controls dominate element retention in different climate zones. When age is held constant, congruent weathering of olivine appears to dominate element retention in dry climates, with amorphous phases becoming important as humidity and weathering intensity begin to increase. Crystalline secondary phases, in particular gibbsite, become the dominant influence under high humidity and weathering intensity, while goethite and hematite strongly influence elemental retention in samples from older, drier climate. Additionally, Rare Earth Element (REE) distribution provides insight into variations in pedogenic processes under differing climate and weathering intensities. In dry to seasonally humid climates, the flux of REE-rich aqueous fluids into the profile from more weathered areas primarily controls REE distribution, with plagioclase leaching promoting REE precipitation and thus, REE-enrichment. Heavy REE (HREE) are more enriched than Light REE (LREE) in these samples. As humidity and weathering intensity increase, vertical ...