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[SCG45-P14] Silica-sealing zone at the base of seismogenic zone revealed by geochemical machine-learning of pelitic schists
Keywords:Silica sealing, Machine learning, Subduction zone, Pelitic rocks
Silica transport and precipitation have significant implications for fault mechanics, as they control fault sealing, regulate fluid pressure variations, and influence the recurrence interval of earthquakes. Such silica-sealing is also considered important to control the seismicity in subduction zones [1]. Geochemical analyses of metapelite evidence transport of silica during subduction-related metamorphism [2]. However, the location of silica dissolution, transport, and precipitation in the subduction plate interface is largely unknown, because protolith (i.e., pre-metamorphic mudstone) information needed for mass transfer analyses is usually not accessible.
Recent advances in machine learning on geochemistry have facilitated the estimation of protolith basalt compositions from compositions of immobile elements in metamorphosed basalt [3]. Here, we developed protolith reconstruction models (PRMs) to estimate the silica concentration of the protolith of pelitic schists, which enables unbiased sample-to-sample mass transfer analyses. By integrating field surveys of high-pressure metamorphic rocks, geochemical analyses, and machine learning mass transfer analyses, we evaluated the extent of silica transport in subduction zones.
PRMs were designed to estimate the SiO2, TiO2, Nb, Zr, Y, Th, Al2O3 concentrations in sediments from only the concentration ratios of 6 immobile elements (TiO2/Zr, Nb/Zr, Y/Zr, Th/Zr, Al2O3/Zr). Using the 13740 global sediment database as training data, the model can estimate SiO2 concentration with the accuracy of 5-6 wt%.
This study focused on the Asemigawa region of the Sanbagawa metamorphic belt, a well-exposed subduction complex that preserves a wide range of metamorphic grades from chlorite (Chl) zone (~360°C) to oligoclase-biotite (Oc-bt) zone(~600°C). 13 outcrops were investigated, covering a temperature range of 288-536°C [4], and 95 pelitic schists were collected. Whole-rock chemical compositions were analyzed using X-ray fluorescence (XRF) spectrometer, while mineral distributions were mapped using elemental imaging techniques to assess mineral phases and textures.
At the outcrop scale, one specific Chl zone outcrop exhibited a high density of 0.1-90 mm thick quartz veins, whereas higher-grade Albite-biotite (Ab-bt) and Oc-bt zones outcrops showed a lower density of thick veins (~15 cm). Elemental mapping confirmed a larger modal amount of quartz within the Chl-zone outcrop samples than higher-grade rocks.
Whole-rock SiO2 and Zr concentrations of pelitic schists scatter 52-83 wt% and 56-197 ppm, with some Chl zone samples scatter to SiO2-rich, Zr-poor side, while some higher-grade rocks tend to have lower SiO2 and higher Zr. The protolith compositions estimated by PRMs converged to SiO2 63-76 wt% and Zr 96-176 ppm, indicating SiO2 addition in some Chl zone outcrop, and SiO2 removal in some high grade rocks. Furthermore, a clear increase in silica content was observed in one Chl-zone outcrop with peak metamorphic temperature of 302°C. The estimated amount of SiO2 transfer ranges a addition of +18 wt% in the Chl zone outcrop and a maximum silica loss of -15 wt% in Oc-bt zone. These results suggest that silica has been transported from deeper (i.e., ~536 °C) to shallower (i.e., ~300°C) depths within the subduction plate interface, with substantial precipitation occurring at approximately 300°C.
The ~300°C of silica sealing corresponds to the transition between the source region of slow-slip events and the seismogenic zone, suggesting that silica precipitation at ~300°C may play a significant role in modulating fault behavior. The formation of silica-rich seals at ~300°C may contribute to localized fluid pressure increases, potentially influencing the boundary between slow and fast earthquakes.
1: Audet and Burgman, 2014 Nature, 510, 389-392.
2: Breeding and Ague, 2002 Geology, 30, 499-502
3: Matsuno et al., 2022 Science Reports, 12, 1385
4: Kouketsu et al., 2021 J Metamorph Geol., 39, 727-749
Recent advances in machine learning on geochemistry have facilitated the estimation of protolith basalt compositions from compositions of immobile elements in metamorphosed basalt [3]. Here, we developed protolith reconstruction models (PRMs) to estimate the silica concentration of the protolith of pelitic schists, which enables unbiased sample-to-sample mass transfer analyses. By integrating field surveys of high-pressure metamorphic rocks, geochemical analyses, and machine learning mass transfer analyses, we evaluated the extent of silica transport in subduction zones.
PRMs were designed to estimate the SiO2, TiO2, Nb, Zr, Y, Th, Al2O3 concentrations in sediments from only the concentration ratios of 6 immobile elements (TiO2/Zr, Nb/Zr, Y/Zr, Th/Zr, Al2O3/Zr). Using the 13740 global sediment database as training data, the model can estimate SiO2 concentration with the accuracy of 5-6 wt%.
This study focused on the Asemigawa region of the Sanbagawa metamorphic belt, a well-exposed subduction complex that preserves a wide range of metamorphic grades from chlorite (Chl) zone (~360°C) to oligoclase-biotite (Oc-bt) zone(~600°C). 13 outcrops were investigated, covering a temperature range of 288-536°C [4], and 95 pelitic schists were collected. Whole-rock chemical compositions were analyzed using X-ray fluorescence (XRF) spectrometer, while mineral distributions were mapped using elemental imaging techniques to assess mineral phases and textures.
At the outcrop scale, one specific Chl zone outcrop exhibited a high density of 0.1-90 mm thick quartz veins, whereas higher-grade Albite-biotite (Ab-bt) and Oc-bt zones outcrops showed a lower density of thick veins (~15 cm). Elemental mapping confirmed a larger modal amount of quartz within the Chl-zone outcrop samples than higher-grade rocks.
Whole-rock SiO2 and Zr concentrations of pelitic schists scatter 52-83 wt% and 56-197 ppm, with some Chl zone samples scatter to SiO2-rich, Zr-poor side, while some higher-grade rocks tend to have lower SiO2 and higher Zr. The protolith compositions estimated by PRMs converged to SiO2 63-76 wt% and Zr 96-176 ppm, indicating SiO2 addition in some Chl zone outcrop, and SiO2 removal in some high grade rocks. Furthermore, a clear increase in silica content was observed in one Chl-zone outcrop with peak metamorphic temperature of 302°C. The estimated amount of SiO2 transfer ranges a addition of +18 wt% in the Chl zone outcrop and a maximum silica loss of -15 wt% in Oc-bt zone. These results suggest that silica has been transported from deeper (i.e., ~536 °C) to shallower (i.e., ~300°C) depths within the subduction plate interface, with substantial precipitation occurring at approximately 300°C.
The ~300°C of silica sealing corresponds to the transition between the source region of slow-slip events and the seismogenic zone, suggesting that silica precipitation at ~300°C may play a significant role in modulating fault behavior. The formation of silica-rich seals at ~300°C may contribute to localized fluid pressure increases, potentially influencing the boundary between slow and fast earthquakes.
1: Audet and Burgman, 2014 Nature, 510, 389-392.
2: Breeding and Ague, 2002 Geology, 30, 499-502
3: Matsuno et al., 2022 Science Reports, 12, 1385
4: Kouketsu et al., 2021 J Metamorph Geol., 39, 727-749