THE LITHIUM ISOTOPIC COMPOSITION OF ALTERED MAFIC AND ULTRAMAFIC ROCKS ASSOCIATED WITH MID-ATLANTIC RIDGE OCEANIC CORE COMPLEXES FROM IODP LEG 209
Benavidez, Raul 1983-
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Oceanic core complexes (OCC) provide the opportunity to collect and analyze a unique set of rocks from the lower crust and upper mantle, which have been exposed along the ocean floor and are associated with slow spreading ridges. Fluid circulation at these settings is believed to be directly associated with the low-angle detachment system and ongoing exhumation of deep crustal and mantle rock. The lithium isotopic analysis of IODP Drill Core provides a unique opportunity to better understand alteration profiles at OCCs and complex hydrothermal systems. Forty-eight bulk rock samples were analyzed for lithium concentration and isotopic ratios (δ7Li) from cores collected along the Mid-Atlantic Ridge (MAR), near the 15°20’N Transform. Lithologies analyzed include diabase, gabbro, olivine gabbro, troctolite, dunite, harzburgite, and multiple gabbroic veins, which are interpreted to be intruding into harzburgite, troctolite and gabbro. Four of the samples included veins intruding within host troctolitic rock, two veins are intruding into gabbroic host rock and one vein is intruding into harzburgite. For all host rock/vein pairs, lithium concentration was lower in the host rock than in the intruding vein. Lithium concentrations in mafic plutonic samples ranged from 0.30 ppm to 74.7 ppm while δ7Li ranged between +0.29‰ to +16.3‰. For ultramafic samples, lithium ranged between 1.01 ppm to 18.8 ppm and δ7Li ranged between -4.15‰ and +12.97‰. Thirty two samples were analyzed from the deepest hole, 1275D, with sample depths between 9.76 and 204.55 meters below seafloor (mbsf). Closed-system mixing models were used to evaluate the water/rock ratios and temperatures at which the rocks were altered. The model assumes mixing of a rock and a fluid of different starting compositions and demonstrates that several mixing solutions can be derived. Modeling demonstrates that elevated water/rock ratios persist from the seafloor to depths of up to ~210 mbsf and alteration temperatures ranged between 50°C to 400°C. We infer that large volumes of fluid have migrated through and along the detachment fault based on modeling, lithological Li concentrations, δ7Li compositions, and features observed within the cored samples.