Casey, John F.2018-11-212018-11-21August 2012018-08August 201http://hdl.handle.net/10657/3374A bulk rock Li isotope, immobile and mobile trace element traverses through the crustal section of the Bay of Islands Ophiolite Complex (BIOC) in Western Newfoundland, Canada was analyzed to understand the extent of hydrothermal alteration at different pseudo-stratigraphic levels in the oceanic crust. Lithium systematics together with previously obtained Strontium isotope and Oxygen isotope data (Casey et al., in prep) were incorporated to give a better understanding of alteration extent. Of key importance are mylonite zones discovered that penetrate to the lower gabbroic crust which may provide strain localization zones and permeable fluid pathways. Trace element analysis was conducted by QQQ-ICP-MS and Li isotopic composition was analyzed by MC-ICP-MS at the University of Houston. The bulk rocks from BOIC have a wide range of 7Li values from -5.80 ‰ to +19.00 ‰, reflecting water/rock interaction at various temperature and water/rock (w/r) ratios. Generally, alteration temperatures increase with depth close to the ridge axis. The York Harbor Mine Basalts were collected in 630 to 680 m depths in the basaltic section. They have the most negative 7Li values between -5.80 ‰ to -1.43 ‰ as well as very high Li contents with 13.9 g/g to 51 g/g, suggest an upwelling zone of high temperature hydrothermal fluids. Mylonitic zones from 2500 m to 2710 m show slightly enriched 7Li value with +3.31 ‰ to +7.42 ‰ compared to fresh MORB (+3.4 ‰±1.4 ‰, (Tomascak et al., 2008)) reflecting the reaction temperature for late stage of alteration between 200 °C to 400 °C based on modeling results. Most mylonitic plutonic rocks, however have higher temperature (480 °C-800 °C) amphibolite facies, indicating retrograde metamorphic alteration effects likely occurred off axis at lower temperatures. The layered gabbroic rock (2915 m to 4307 m) have both depleted and enriched 18O values from +4.08 ‰ to +7 ‰ (Casey et al., in prep) relative to unaltered MORB (+5.7 ‰±0.3 ‰) (Harmon and Hoefs, 1995), reflecting a relatively low temperatures (200 °C-250 °C) hydrothermal alteration. All the observations indicated that the lower oceanic crust at this location may have been constructed by hybrid model and cooled by hydrothermal fluids/seawater circulation at both on-axis and off-axis locations.application/pdfengThe author of this work is the copyright owner. UH Libraries and the Texas Digital Library have their permission to store and provide access to this work. Further transmission, reproduction, or presentation of this work is prohibited except with permission of the author(s).Lithium IsotopeOceanic crustHydrothermal alterationBay of Island Complex2018-11-21Thesisborn digital