5:15 PM - 7:15 PM
[SMP28-P15] Visualization of features and spatial patterns of retrograde metamorphism in metamorphic rocks using XRD: example of the East Iratsu body of the Sanbagawa metamorphic belt.
Keywords:Sanbagawa metamorphic belt, Hydration reactions, UMAP, PCA, XRD reatbelt analysis
Fluid activity at subduction zone plate boundaries is crucial for understanding large-scale elemental transport and dynamic processes. Direct evidence of fluid activity includes hydration reactions due to retrograde metamorphism, which occurs as deeply subducted rocks ascend. Okamoto et al. (2005) examined the chemical zoning of amphibole in mafic schists from the Sanbagawa metamorphic belt and discussed the spatial heterogeneity of retrograde metamorphism using the growth ratio of actinolite as an indicator. However, high-resolution EPMA analysis has a limited area and it is costly for large datasets. This study investigates the retrograde metamorphism of metagabbros that reached eclogite-facies conditions in the Besshi region of the Sanbagawa metamorphic belt. Samples were collected from the East Iratsu body and surrounding mafic schists. High-precision XRD measurements were conducted, and dimensionality reduction techniques were applied to XRD spectra to elucidate spatial patterns.
The East Iratsu body is a garnet-bearing metagabbro that subducted to ~70 km depth and underwent multiple stages of retrograde metamorphism during exhumation (Ota et al., 2004). Mafic and pelitic schists are distributed around it. A total of 82 samples were collected along two transects from the outer to the inner body. Thin sections were prepared, and XRD measurements were conducted at beamline BL19B2 of the SPring-8 synchrotron radiation facility. Rietveld analysis quantified mineral mode, and dimensionality reduction was applied using principal component analysis (PCA) and uniform manifold approximation and projection (UMAP). PCA is a technique that reduces dimensionality by identifying linear axes that maximize data variance, while UMAP is a method that projects high-dimensional data into a lower-dimensional space, forming clusters of similar samples. We examined the relationships between the dimensionality-reduced result and mineral mode obtained from Rietveld analysis, mineral compositions, thin-section observations, and their spatial distribution on maps.
The explained variance ratios of PC1, PC2, and PC3 were 0.28, 0.17, and 0.10, respectively. PC1 primarily indicated an increase in amphibole, epidote, and plagioclase, while quartz decreased. Furthermore, dimensionality reduction using UMAP revealed that the samples were divided into six clusters, as shown in the figure. Samples in Clusters 2, 3, and 4 were distributed within the metagabbro, whereas those in Clusters 1, 5, and 6 were dispersed across both the metagabbro and mafic schist areas. Notably, the PC1 score increased from Cluster 1 to Cluster 2. The mineral assemblage of Cluster 1 consisted of garnet, chlorite, epidote, amphibole, phengite, paragonite, quartz, plagioclase, and Ti-bearing minerals. Among these, hydrous minerals (chlorite, epidote, amphibole, phengite, and paragonite) were dominant, whereas garnet content was relatively low. This cluster was distributed within the geological unit of mafic schists affected by retrograde metamorphism. Cluster 2 exhibited a similar mineral assemblage. However, in terms of mineral proportions, the amount of hydrous minerals decreased, while garnet content increased. This cluster was distributed along the boundary of the metagabbro body, where the degree of retrograde metamorphism was relatively weak. Moreover, both clusters exhibited well-developed foliation structures. Therefore, PC1 is interpreted as an indicator of hydration reactions associated with deformation, particularly the replacement of garnet by hydrous minerals.
This study demonstrates that dimensionality reduction of XRD spectral data can quantitatively analyze and visualize retrograde metamorphism without prior knowledge. Future work will refine the relationships between dimensionality reduction results, mineral compositions, and microstructural features while discussing hydration reactions across the study area.
Reference
・A. Okamoto and M. Toriumi (2005). J. metamorphic Geol., 23, 335–356
・Ota et al. (2004). Lithos, 73, 95-126, doi: 10.1016/j.lithos.2004.01.001
The East Iratsu body is a garnet-bearing metagabbro that subducted to ~70 km depth and underwent multiple stages of retrograde metamorphism during exhumation (Ota et al., 2004). Mafic and pelitic schists are distributed around it. A total of 82 samples were collected along two transects from the outer to the inner body. Thin sections were prepared, and XRD measurements were conducted at beamline BL19B2 of the SPring-8 synchrotron radiation facility. Rietveld analysis quantified mineral mode, and dimensionality reduction was applied using principal component analysis (PCA) and uniform manifold approximation and projection (UMAP). PCA is a technique that reduces dimensionality by identifying linear axes that maximize data variance, while UMAP is a method that projects high-dimensional data into a lower-dimensional space, forming clusters of similar samples. We examined the relationships between the dimensionality-reduced result and mineral mode obtained from Rietveld analysis, mineral compositions, thin-section observations, and their spatial distribution on maps.
The explained variance ratios of PC1, PC2, and PC3 were 0.28, 0.17, and 0.10, respectively. PC1 primarily indicated an increase in amphibole, epidote, and plagioclase, while quartz decreased. Furthermore, dimensionality reduction using UMAP revealed that the samples were divided into six clusters, as shown in the figure. Samples in Clusters 2, 3, and 4 were distributed within the metagabbro, whereas those in Clusters 1, 5, and 6 were dispersed across both the metagabbro and mafic schist areas. Notably, the PC1 score increased from Cluster 1 to Cluster 2. The mineral assemblage of Cluster 1 consisted of garnet, chlorite, epidote, amphibole, phengite, paragonite, quartz, plagioclase, and Ti-bearing minerals. Among these, hydrous minerals (chlorite, epidote, amphibole, phengite, and paragonite) were dominant, whereas garnet content was relatively low. This cluster was distributed within the geological unit of mafic schists affected by retrograde metamorphism. Cluster 2 exhibited a similar mineral assemblage. However, in terms of mineral proportions, the amount of hydrous minerals decreased, while garnet content increased. This cluster was distributed along the boundary of the metagabbro body, where the degree of retrograde metamorphism was relatively weak. Moreover, both clusters exhibited well-developed foliation structures. Therefore, PC1 is interpreted as an indicator of hydration reactions associated with deformation, particularly the replacement of garnet by hydrous minerals.
This study demonstrates that dimensionality reduction of XRD spectral data can quantitatively analyze and visualize retrograde metamorphism without prior knowledge. Future work will refine the relationships between dimensionality reduction results, mineral compositions, and microstructural features while discussing hydration reactions across the study area.
Reference
・A. Okamoto and M. Toriumi (2005). J. metamorphic Geol., 23, 335–356
・Ota et al. (2004). Lithos, 73, 95-126, doi: 10.1016/j.lithos.2004.01.001