HR: 1330h
AN: V52A-0417    [PDF]
TI: Lithium Isotopic Fractionation in Subduction Zones: Clues From Clays
AU: * Williams, L B
EM: lynda.williams@asu.edu
AF: Arizona State University, CSSS PO 871704, Tempe, AZ 85287 United States
AU: Hervig, R L
EM: hervig@asu.edu
AF: Arizona State University, CSSS PO 871704, Tempe, AZ 85287 United States
AB: Lithium isotope ratios show such large variations in nature ($>$30 per mil), that many areas of geosciences are exploring the usefulness of this system in explaining the evolution of particular rocks. Here we show how the lithium isotope ratios change during the transformation of smectite clay minerals to illite during burial metamorphism. Such a transition may be a common feature in the shallow regions of subduction zones and may ultimately affect the Li isotope compositions of fluids contributing to arc magmatism. Lithium is a ubiquitous trace element in natural formation waters that, like B, shows large isotopic fractionation especially during interactions with clay minerals. Lithium is adsorbed in the interlayer region of expandable clay minerals but is easily exchanged. Lithium is also incorporated into the octahedral sites. The substitutions of Li in two crystallographic sites of clay minerals may complicate interpretations of bulk Li-isotope ratios. We suggest that the magnitude of the isotopic fractionation of Li between fluid and clay is different in the interlayer sites of clay minerals than in the octahedral sites of clay minerals. Examination of Li contents and isotope variations in experimental reactions of smectite to illite (300C, 100MPa) shows changes with structural re-arrangement of the clay layers. The Li-isotope trend declines (from ~+6 to -13 per mil, expressed as ratios of 7/6) throughout R1-ordering of the mixed-layered illite smectite (I/S). However, the equilibrium end products of the reaction have R3-ordering and show a heavier isotope ratio (~0 per mil). This observation is very similar to the trends we observed for B-isotopes, where the interlayer B initially overprinted the tetrahedral-layer B isotope composition, but as the interlayer sites were collapsed during illitization, the equilibrium isotope composition was approached. The significant Li and B isotopic changes that occur during ordering of I/S coincides with the temperatures experienced during the early stages of subduction and undoubtedly influences the isotopic makeup of those higher pressure phases that contribute Li and B to the region of arc magma generation.
DE: 1000 GEOCHEMISTRY (New field, replaces Rock Chemistry)
DE: 1040 Isotopic composition/chemistry
DE: 3620 Crystal chemistry
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting