HR: 08:45h
AN: H41J-04    [Abstracts]
TI: The Impact of Different Acids on the Fractionation of Cu and Zn Isotopes During Leaching in Open Systems.
AU: * Fernandez, A
EM: afernandez2@miners.utep.edu
AF: Geological Sciences Department. University of Texas at El Paso., 500 W. University Ave., El Paso, TX 79968, United States
AU: Borrok, D
EM: dborrok@utep.edu
AF: Geological Sciences Department. University of Texas at El Paso., 500 W. University Ave., El Paso, TX 79968, United States
AB: Despite great promise, our ability to apply transition metal isotopic measurements to hydrologic systems remains limited by our knowledge of how metal isotopes are impacted by weathering under different chemical and mechanical pathways. With this in mind, we designed experiments to examine the impact of different anions (NO₃^-, Cl^-, and SO₄^2-) on the fractionation of Cu and Zn during leaching of a sulfide- rich rock by acidic (pH ~ 2.0) waters. Experiments consisted of five 6 hour cycles, where after each cycle the solid was separated from the solution and then allowed to react with a fresh batch of acidic solution. In this way, the experiments simulated an open system in which pulses of water leach the rock and are then removed. This is a significantly different approach than close-system leach experiments that have investigated metal isotopes. Zinc and Copper isotopic data shows that the magnitude and direction of isotope effects during cyclic leaching is partly dependent on the anion used during the leaching experiment. Moreover, the isotopic signatures of aqueous Cu and Zn in an open system fluctuate as leaching cycles progress. Leaching of zinc by HNO₃ results in aqueous zinc that shows a constant enrichment of δ⁶⁶Zn ~ 0.2‰ relative to the isotopic composition of Zn in the rock. In contrast, zinc leached by H₂SO₄ shows no resolvable fractionation relative to the zinc in the rock, and zinc leached by HCl is increasingly light with each progressive leaching cycle. Isotopic measurements of copper from the experiments in which HNO₃ and HCl were used demonstrate that Cu in leach fluids gets progressively lighter with each cycle. And, in the case of leaching by H₂SO₄, δ⁶⁵Cu values decrease in the first 3 cycles and then level to a value ~ 0.8 ‰ lighter than the isotopic composition of Cu in the rock. Our results suggest that the relationship between the isotopes of Cu and Zn source rocks and the isotopes of Cu and Zn in hydrologic systems is not straightforward, and isotopic fractionations of Cu and Zn during weathering are impacted both by the chemistry and degree of leaching.
DE: 1041 Stable isotope geochemistry (0454, 4870)
DE: 1806 Chemistry of fresh water
DE: 1886 Weathering (0790, 1625)
DE: 1894 Instruments and techniques: modeling
SC: Hydrology [H]
MN: 2007 Fall Meeting