Natural and experimental fluorine substitution in biotite: Implications for fluid-rock thermochronometry and application to the Serido Belt, northeastern Brazil

作者：Sallet Ricardo; Price Jonathan D; Moritz Robert

来源：Chemical Geology, 2018, 482: 32-45.

DOI：10.1016/j.chemgeo.2018.01.019

• Fluorine diffusion
• Biotite
• Cooling rates
• Orogeny

We modeled non-steady-state, fluid-assisted diffusion in a sheet plane, infinite cylinder and finite thin cylinder using analytical solutions of the diffusion equation and the experimental diffusivity of F-in-biotite. Diffusion experiments on natural biotite in the presence of hydrofluoric acid at 0.4 GPa and 650, 700, and 750 degrees C produced diffusive influx perpendicular to the c-axis. EPMA mapping and profiling allow us to define the following Arrhenius expression of F diffusion in biotite:
D =7.04 m(2)/s x 10(-4) exp[-221 kJ/mol (RT)(-1)]
The results yield the discrimination of a high-T homogenization domain where cooling paths continuously reequilibrate F-in-biotite, an intermediate-T zonation domain where cooling paths record F-in-biotite diffusion profiles and a low-T closure domain where F-in-biotite remains unchanged. The zonation domain significantly expands with increasing cooling rates and decreasing diffusion time.
We systematically analyzed F-in-biotite with electron microprobe in samples collected along representative cross-sections within the Serido Belt, northeastern Brazil. Its metasedimentary units reached upper amphibolite grade metamorphic conditions, with constant F-in-biotite (F approximate to 0.33 wt.%) in grains from mica schist and paragneiss. The Ti-in-biotite geothermometer yielded nearly constant temperatures around 623 degrees C. The F-in-biotite regional background values remain unchanged at the contact with the major Ediacaran F-rich Acari pluton. It indicates the absence of magmatic fluid influx into the host rocks and is evidence for a fluid-absent nature of melts.
By contrast, the F-in-biotite of the metric mica schist enclaves within dykes and sills of Cambrian pegmatitic granites was reequilibrated by interaction with exsolved fluids. The Ti-in-biotite geothermometer indicates temperatures around 642 degrees C, and our model suggests 100 kyr as a minimal duration for fluid-rock interaction. The preservation of intra-grain F-in-biotite homogeneity requires fluid flux cessation before cooling or fluid- present cooling rates below 100 degrees C Myr(-1). Higher cooling rates would generate F-in-biotite intra-grain zonation. At the contact with the pegmatitic granites, the metasedimentary rocks show meter-scale F-in-biotite gradients due to thermal profiles setting the fluid (f(H2O)/f(HF)) gradient. The preservation of such gradients is related to a relatively fast fluid flux episode compared to the overall duration of the elevated thermal profile. The absence of intra-grain zonation indicates that F-in-biotite re-equilibration was fast relative to that of the fluid flux duration.
We found intra-grain F-in-biotite zonation within an orthogneiss sample from the Paleoproterozoic Caico complex basement nearby the metasedimentary belt and intruded plutons. The fit of the natural profile by modeled profiles for cooling rates below 10 degrees C Myr(-1), obtained from available Serido Belt U-Pb and Ar-40/Ar-39 ages, indicates that the orthogneiss cooled from 475 degrees C. It suggests the existence of a thermal gradient as the inner part of the belt cooled from 623 degrees C.

• 出版日期2018-4-5