HR: 09:15h
AN: V11E-06    [Abstracts]
TI: Mechanisms of Oxygen Isotopic Exchange and Isotopic Evolution of 18O/16O-Depleted Periclase Zone Marbles in the Alta Aureole, Utah, USA—Insights From ion Microprobe Analysis of Calcite
AU: * Bowman, J R
EM: bowman@earth.utah.edu
AF: Dept. of Geology and Geophysics, University of Utah, Salt Lake City, UT 84112, United States
AU: Valley, J W
EM: valley@geology.wisc.edu
AF: Dept. of Geology and Geophysics, University of Wisconsin, Madison, WI 53706, United States
AU: Kita, N
EM: noriko@geology.wisc.edu
AF: Dept. of Geology and Geophysics, University of Wisconsin, Madison, WI 53706, United States
AB: Infiltration of water-rich fluids during prograde metamorphism has produced significant but variable ¹⁸O/¹⁶O depletion in dolomitic marbles within the periclase (Per) zone of the Alta Stock aureole, Utah. At one location, a marble layer containing calcite (Cal) and forsterite (Fo) and an adjacent layer containing Cal + Per (replaced by brucite) + humite are depleted to δ¹⁸O values of 17.2 and 11.8 permil, respectively, from original protolith values (>25 permil). Detailed ion microprobe and millimeter-scale dental drill sampling traverses across the boundary between the two layers define a steep, coherent gradient in δ¹⁸O that is displaced a short distance (4 cm) into the higher δ¹⁸O Cal + Fo layer. Textural studies (optical, SEM) and ion microprobe analyses show that there are two isotopically and texturally distinct types of calcite at the grain scale in this traverse. Clear (well polished) calcite grains are isotopically homogeneous (within analytical uncertainty; ±0.2 to 0.45 permil, two SD) and their δ¹⁸O values confirm the basic geometry and location of the gradient defined by the dental drill samples. More poorly polished (pitted), texturally retrograde ‘turbid'-looking calcite has lower and more variable δ¹⁸O values, and replaces clear calcite along fractures, cleavage traces or grain boundaries. Within the interiors of both layers there is no systematic spatial variation of δ¹⁸O in either ion microprobe or dental drill results. However there are systematic differences (up to 0.8 permil) between the ion microprobe analyses and the dental drill samples in these layer interiors, which suggest that minor amounts of retrograde calcite are incorporated at the sub-mm scale into these dental drill samples. Despite significant and pervasive depletion of ¹⁸O/¹⁶O in calcite throughout both layers during prograde metamorphism, ion microprobe analyses indicate that clear calcite grains are now isotopically homogeneous within analytical uncertainty (±0.2 to 0.45 permil, two SD) across the entire gradient in δ¹⁸O, not just within the relatively homogeneous interiors of the two layers. Diffusion calculations indicate that conservative time scales required for isotopic homogenization of calcite grains by volume diffusion, 2000 to 62,000 years at 575-600^oC, exceed significantly the timescale (approx. 1250 yrs) estimated for the prograde development of the δ¹⁸O gradient at the boundary between these two marble layers. The ion microprobe data and these diffusion calculations suggest instead that surface reaction mechanisms (e.g., dissolution-reprecipitation) accompanying recrystallization are responsible for the observed oxygen isotope homogeneity of these calcite grains. The ion microprobe data are consistent with, but do not require that metamorphic calcite forms in oxygen isotope exchange equilibrium with infiltrating fluid during prograde reaction and recrystallization. However the presence of retrograde calcite-dolomite temperatures (450- 550^oC) recorded in calcite from the periclase zone suggests that both Mg exchange and oxygen isotope homogenization accompanied recrystallization during the early stages of retrograde cooling.
DE: 1034 Hydrothermal systems (0450, 3017, 3616, 4832, 8135, 8424)
DE: 1041 Stable isotope geochemistry (0454, 4870)
DE: 3653 Fluid flow
DE: 3660 Metamorphic petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting