We present an updated and expanded version of the Fast
Grain Boundary (FGB) program, originally developed by [1].
In its current form, FGB forward models the oxygen-isotope
compositional evolution of a rock resulting from diffusive
oxygen isotope exchange. FGB is a tool for 1) constraining
thermal histories (cooling rates and durations) from measured
intragrain oxygen isotope zoning profiles, and 2) predicting
oxygen isotope zoning that results from coupled volume and
grain boundary diffusion. The FGB model is mass-balance
constrained through exchange with a finite, grain-boundary
reservoir and does not require a Dodson-like infinite reservoir
assumption. The new FGB program code is written in Python
and includes a graphical user interface.
The inverse modeling capabilities for FGB are currently
under development. We present preliminary results for
thermal history inversion from a test case using oxygenisotope
diffusion zoning data from titanite. Both the gradient
descent and the Levenberg-Marquardt (LM) algorithms are
applied to the FGB model in search of cooling histories that
maximize agreement between the model output and recorded
data. Various schemes of regularization are employed to
ensure meaningful realizations of cooling histories.
Additionally, to prevent local extrema entrapment, the results
of these algorithms are compared to long-run brute force
methods that have been implemented through Amazon’s
cloud computing services.
The observed zoning profiles in the example titanite
dataset can be forward modeled with several arbitrary (and
potentially biased) cooling histories. The preliminary results
of inverse modeling reduce initial bias and suggest an
episodic cooling history that may include an isothermal
period or even a reheating event. These results demonstrate
both the potential for oxygen-isotope zoning to preserve
prevailing thermal events and the potential of the FGB model
for recovering these events.
[1] Eiler, Baumgartner & Valley (1994), Computers &
Geoscience 20, 1415-1434.
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Time-temperature paths from intragrain oxygen isotope zoning
Oxygen isotopes are a well-known geochemical tool with applications to equilibrium thermometry, fluid tracing, and magma and ore petrogenesis. High-precision, high-spatial resolution oxygen isotope analysis by SIMS has also enabled the development of oxygen isotopes as a tool for geospeedometry. Here, we detail Fast Grain Boundary (FGB), an updated computational approach and software tool for determining time-temperature (T-t) histories through modeling of oxygen isotope diffusion. FBG models a rock system, rather than a single phase (cf. thermochronometry based on He, Ar, Pb), and has the potential to constrain continuous thermal histories over a wide range of temperatures, including at high temperature (500-800°C). The new FGB also allows for inversion of the FGB model to extract thermal histories from intragrain oxygen isotope zoning data using the Levenberg-Marquardt (LM) algorithm. Tests with synthetic datasets show that the LM algorithm is able to distinguish between simple linear cooling and more complex thermal histories containing, for instance, reheating events. Inversion of an actual oxygen isotope data set from titanite are consistent with the previously determined T-t path for the sample region, showing a brief period of >700 °C conditions, followed by cooling below 500 °C in <5 m.y.. However, the inversion suffers from a flat-bottomed minimum and does not produce a well-converged T-t path. These results point to analytical precision as a continuing challenge in recovering tightly constrained thermal histories for the real data set and emphasize the need for further development of high-precision microanalytical oxygen isotope standards. In the meantime, we use FGB modeling to explore sampling and analytical approaches that improve the resolution of inversion solutions for current analytical capabilities. For instance, inversion most successfully recovers a well constrained T-t path solution when SIMS analysis targets oxygen isotope gradients developed near grain rims, as opposed to oxygen isotope values in grain centers. Additional tests that probe the sensitivity of the inversion results to modal mineralogy and relative grain sizes suggest that careful targeting of samples in the field can enhance the recovery of unique T-t paths.
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- Award ID(s):
- 1650355
- NSF-PAR ID:
- 10144064
- Date Published:
- Journal Name:
- AGU 2019 Fall Meeting
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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