Oxygen isotope geospeedometry by SIMS

Geospeedometry, a discipline closely related and complimentary to thermochronology, exploits the phenomenon of diffusion in order to extract rate and duration information for segments of a rock's thermal history. Geospeedometry data, when anchored in absolute time by geochronologic data, allow for the construction of detailed temperature-time paths for specific terranes and geologic processes. We highlight the developing field of SIMS-based oxygen isotope geospeedometry with an application from granulites of the Adirondack Mountains (New York) and discuss potential future applications based on a recently updated and expanded modeling tool, the Fast Grain Boundary diffusion program (FGB; Eiler et al. 1994). Equilibrium oxygen isotope ratios in minerals are a function of temperature and bulk rock composition. In dynamic systems, intragrain oxygen isotope zoning can develop in response to geologic events that affect the thermal state of a rock and/or induce recrystallization, especially tectonic deformation and fluid infiltration. As an example, titanite grains from late-Grenville shear zones in the northwestern Adirondack Mountains exhibit a range of δ¹⁸O zoning patterns that record post-peak metamorphic cooling, episodic fluid infiltration, and deformation-facilitated recrystallization. Many titanite grains preserve smooth, core-to-rim decreasing, diffusional δ¹⁸O profiles, which are amenable to diffusion modeling. FGB models that best fit the measured δ¹⁸O profiles indicate cooling from ~700-500°C in just 2-5 m.y., a rapid thermal change signaling the final gravitational collapse of the late-Grenville orogen. Titanite can also be utilized as a U-Pb chronometer, and comparison of δ¹⁸O and U-Pb age zoning patterns within the Adirondack titanites pins the episode of rapid cooling inferred from the δ¹⁸O record to some time between 1054 and 1047 Ma. The expanded capabilities of FGB also allow for evaluation of a range of heating-cooling histories for the Adirondack granulites. Diffusional δ¹⁸O zoning profiles in titanite are best fit by complete re-equilibration at temperatures above 675 °C followed by rapid, monotonic cooling; FGB models that include only partial re-equilibration and/or episodes of reheating along the retrograde path do not fit the observed δ¹⁸O profiles. Beyond the Adirondack titanite example, FGB can be used as a predictive tool to target either specific minerals within a rock or specific rock types within a terrane for oxygen isotope geospeedometry and zoning studies. FGB generates predictions of δ¹⁸O zoning for all minerals in a rock of a given mineralogy and heating-cooling history. Different minerals within the same rock will record different segments of the thermal and fluid history based on their individual diffusivities, phase stabilities, and propensities for deformation-induced/facilitated recrystallization. It should therefore be possible to extract long thermal histories from a single sample by measuring oxygen isotope zoning profiles across several minerals with different partial retention zones for oxygen.