Li Isotope Analysis And Experiments To Understand Fluid Exchange During Bay of Islands Ophiolitic Crust Formation And Dynamic Metamorphism Of Its Sole



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Three Li isotopic studies were conducted to understand: 1) hydrothermal alteration of crustal levels of the Bay of Islands Ophiolite (BOIC), 2) Li alteration during dynamic metamorphism of protoliths in the welded metamorphic sole, and 3) Li partitioning and isotope fractionation factors at 150 °C and 300 °C in basalt and gabbro during hydrothermal reaction. Compared to unaltered MORB (Li = 6.1 ug/g, d7Li = 3.4 ‰), the upper pillow lava and dike section (Li = 1.24 - 40.66 ug/g, d7Li = -2.97 – 20.69 ‰) and lower plutonic section (Li = 0.087 - 8.16 mg/g, d7Li = 0.79 - 18.83 ‰) show extensive hydrothermal alteration. Hydrothermal fluid penetration appears limited to ~2km depth in the region above axial magma chamber but circulates as brine fluids off-axis to deep levels that create a hybrid system of magma chamber cooling. The HT-LP metamorphic sole subcreted to the base of BOIC ophiolite during subduction consists of 3 primary protoliths: sedimentary, altered basaltic, and altered gabbroic rocks. Amphibolite to granulite metabasites exhibit a high Li (10.55 -57.33 ug/g) and light d7Li (-1.39 - +2.80 ‰) compared with altered oceanic crust (AOC, Li = 14 ug/g, d7Li = 7.5 ‰). Metasediments have enriched Li (53.86 - 66.24 ug/g) but low d7Li (0.81 - 1.52 ‰), compared with Global Subducting Sediments (GLOSS- II, Li = 45 ug/g, d7Li = 2.42 ‰). Two-stage process during subduction and formation of BOIC sole includes: (1) dehydration of protoliths during subduction resulting in initial depletion of both Li and d7Li followed by (2) metasomatism by later retrograde Li-rich fluids derived from metasediments subducted and dehydrated at deeper levels of subduction interface resulting in Li enrichment and slight increase of d7Li compared to dehydrated values. Laboratory hydrothermal experiments yielded new Li partitioning coefficients (Kd) and Li isotopic fractionation factors (a) to aid models presented. Gabbroic samples show higher Kd (12.41) and lower a (0.9816) compared to basaltic glass (Kd = 6.08, a = 0.9863) at 150 °C but show lower values of Kd = 1.53 and a = 0.9964 compared to basaltic glass which has Kd = 2.79 and a = 0.9908 at 300 °C.



Lithium isotope, Trace element geochemistry