[S-MP24] Deformed rocks, Metamorphic rocks and Tectonics

[SMP24-04] Re-evaluation of the thermobaric structure of the Besshi eclogite-facies unit in the Sanbagawa metamorphic belt based on synchrotron micro-XAFS spectroscopy

*Taisuke Ito¹, Simon Richard Wallis¹, Yoshio Takahashi¹, Shunsuke Endo² (1.Department of Earth and Planetary Science, The University of Tokyo, 2.Department of Earth Science, Interdisciplinary Faculty of Science and Engineering, Shimane University)

Keywords:subduction zone, eclogite, omphacite, garnet-clinopyroxene geothermometry, XAFS spectroscopy

Thermal structure of convergent plate boundaries is an important piece of information essential for understanding seismogenic and magma-forming regions. However, the estimation of thermal structure is accompanied by large errors, and numerical models based on geophysical observations such as heat flow and seismic waves show temperature discrepancies of several hundred degrees or more (Penniston-Dorland et al., 2015). In particular, convection patterns in the overlying mantle can cause significant differences in estimated thermal structures, thus requiring validation using geological methods.
Petrological analyses of low-T/high-P metamorphic rocks of the Late Cretaceous Sanbagawa belt, central Shikoku, suggests that there is a remarkable increase in temperature gradient at P > ~2.0 GPa (> ~65 km depth) (Aoya et al., 2009). This thermobaric feature has been considered to represent high temperature conditions caused by mantle convection induced by a mechanical coupling between the subducting slab and the overlying mantle and may support the numerical models of subduction zone by van Keken et al. (2002). On the other hand, numerical models that account for convection within subduction channels, such as the one by Gerya et al. (2002), do not estimate such an increase in temperature gradient, and thus may lack validity.
However, metamorphic temperature estimates for eclogite, which is widely stable at these P–T condition, have been subject to notoriously large errors (>±100°C; Carswell & Zhang, 1999) due to uncertainties associated with indirect estimates of Fe³⁺/Fe²⁺ ratios of omphacite, and therefore large estimation errors are expected in the petrologically derived thermobaric structure. To address this issue, it is necessary to analyze iron speciation of omphacite in micro-areas comparable to EMP analyses so that mineral inclusions and compositional heterogeneities reflecting metamorphic history are sufficiently resolved.
In this study, we re-evaluated the thermobaric structure of the eclogite-facies unit in the Besshi area, central Shikoku, SW Japan to validate the subduction zone numerical models. Non-destructive measurements of micro-area Fe³⁺/Fe²⁺ ratios in omphacite using synchrotron X-ray absorption fine structure (XAFS) spectroscopy. XAFS spectroscopy was performed at Photon Factory, KEK, and Fe K-edge XANES spectra were collected by the fluorescence yield method. The Fe³⁺/Fe²⁺ ratios were determined based on the analysis of the pre-edge peak attributed to the 1s→3d/4p transition (~7112 eV). To improve the accuracy of the regression analysis, several clinopyroxene standards were prepared and their Fe³⁺/Fe²⁺ ratios were determined by Mössbauer spectroscopy.
Fe³⁺/Fe²⁺ ratios of omphacite showed a wider range of values than those calculated by conventional indirect estimation methods (charge balance, Fe³⁺=Na–^VIAl–Cr). For omphacite with a true Fe³⁺/Fe²⁺ ratios of about 0.5 or higher, such indirect estimation methods always underestimated Fe³⁺/Fe²⁺ ratios. At the same time, such underestimates were positively correlated with cation deficiencies in the pyroxene formula, and end-member analysis showed the presence of Ca-Eskola molecule (Ca_0.5[vacancy]_0.5AlSi₂O₆). Ca-Eskola molecule is particularly prominent in omphacite in the quartz-rich kyanite eclogite of the Gongen body, suggesting that non-stoichiometry due to Ca-Eskola substitution is responsible for the underestimates of indirect Fe³⁺/Fe²⁺ ratios. The discovery of Ca-Eskola component in natural high-pressure metamorphic rocks with quartz stable as SiO₂ phase contradicts the conventional idea that Ca-Eskola component is one of the indicators of ultrahigh-pressure metamorphism (e.g., Smyth, 1980), suggesting the lower pressure limit of Ca-Eskola component extends from coesite to quartz stable conditions with dependence on temperature and/or bulk-rock composition.
Re-evaluated thermobaric structure of the Late Cretaceous (89-85 Ma) Sanbagawa subduction zone showed an increase in temperature gradient at P > ~2.0 GPa, suggesting that hot mantle flow toward the subduction zone induced by the coupling between the subducting slab and overlying mantle at greater depths than ~65 km. This result supports the subduction zone numerical models by van Keken et al. (2002) from natural rock record. In the future, determination of eclogite-facies metamorphic ages for coarse-grained eclogitic rocks is needed, and quantification of orientation effect of X-ray absorption in anisotropic materials on speciation is needed to refine the accuracy.