14:15 〜 14:30
[SMP24-03] Contrasting chemical reactions and fluid transport by melt and aqueous fluids during middle crustal fracturing (Sør Rondane Mountains, East Antarctica)
キーワード：岩石-流体反応、反応輸送モデル、角閃岩相，グラニュライト相、東南極セール・ロンダーネ山地、塩素
Fluids in the deep crust promote heat and mass transport, control rock rheology and fracturing, and play essential roles on the dynamics of plate boundaries. Although fluid activities are geophysically recognized as migrations of hypocenters and seismic velocity anomalies (e.g., Hasegawa et al., 2005; Yoshida et al., 2018), roles of melt and aqueous fluids in deep crustal fracturing are not always clear, due to limitations in distinguishing melt or aqueous fluids in geophysical observations. Exceptionally well-exposed crust-fluid reaction zones in the Sør Rondane Mountains (SRM), East Antarctica provides an ideal opportunity to evaluate the fluid flow in the deep crust from mm to km-scale (e.g., Adachi et al., 2010; Osanai, et al., 2013; Higashino et al., 2013, 2019; Kawakami et al., 2017; Uno et al., 2017; Mindaleva et al., 2020). Here we examine the amphibolite-facies melt–rock and aqueous fluid–rock reaction zones of hosted in felsic gneiss (so called “bleached” zones) that are widely distributed in the SRM, and discuss the contrasting modes of chemical transport between melt and aqueous fluids during crustal fracturing.
We have analyzed amphibolite-facies melt/aqueous fluid–rock reaction zones, Mefjell area, SRM. Orthopyroxene-bearing felsic gneiss are crosscut by numerous granitic veins and biotite veins (m to 100s m in length) and are associated with cm– to decimeter–scale whitish reaction zones composed of hornblende-biotite-bearing felsic gneiss, showing “bleached” zones. The bleached zones are characterized by complete hydration of orthopyroxene and clinopyroxene to grunerite, hornblende and biotite, representing fluid infiltration at 0.40–0.55 GPa, 600–670°C (Uno et al., 2020). Mineralogy, mineral chemistry and temperature conditions of bleached zones around granitic veins and biotite veins are essentially identical, except for characteristic minor element profiles described below.
Bleached zones associated with granitic vein (“granitic vein-bleached zones”) are characterized by relatively high MnO contents at the vein wall (hornblende 2.0 wt%; biotite 0.6 wt%; ilmenite 6.0 wt%) and they gradually decrease towards the host rock within ~15 mm length (≅ bleached zone width). Similar gradual profiles are observed for Be, Ga, Zn, Sn, Rb, Cs and Pb in biotite and/or hornblende. TiO2 in biotite are within the range of host rock (3.9–4.5 wt%). The Cl contents in biotite and hornblende are low (<0.08wt%). Contrarily, in bleached zones associated with biotite vein (“biotite vein-bleached zones”), the Cl contents in hornblende and biotite are relatively high and constant within the middle of the bleached zones (0.40–0.52 wt%; ~7 mm length from the vein wall) and sharply decrease towards the host rock (<0.04 wt%; ~15 mm length). MnO in biotite and hornblende are low (<0.4 wt%), and TiO2 contents in biotite are depleted in bleached zone (~2 wt%) compared to those in the host rock (~4 wt%).
Reactive-transport analysis of MnO and Cl in the bleached zone show that transports from the vein wall to the host rock are diffusion-dominant (Pe = 0) in granitic vein-bleached zone, whereas those in biotite vein-bleached zone are advection dominant (Pe ~300). The MnO, TiO2 enriched, Cl-depleted nature of the granitic vein-bleached zone is likely to be explained by infiltration of granitic melt, whereas Cl-enriched, MnO, TiO2 depleted nature of the biotite vein-bleached zone is affected by infiltration of Cl-bearing aqueous fluids. Assuming near lithostatic fluid pressure of the veins, the differences of the modes of chemical transport (diffusional vs. advectional) of the two bleached zones suggest contrasting viscosity of the granitic melt and aqueous fluids during infiltration from the veins to the host rock.
The results of this study indicate that both viscous granitic melt and Cl-bearing aqueous fluids contribute to fracturing at middle crustal conditions (0.40–0.55 GPa, 600–670°C). Further reactive-transport analyses of these reaction zones would reveal timescales of fluids infiltration and associated hydraulic parameters operated during middle crustal fracturing.
References
Adachi, et al., 2010. Polar Sci. 3, 222–234.
Hasegawa et al., 2005. Tectonophysics 403, 59–75.
Higashino et al., 2013. Precambrian Res. 234, 229–246.
Higashino et al., 2019. J. Petrol. 60, 329–358.
Kawakami et al., 2017. Lithos 274–275, 73–92.
Mindaleva et al., 2020. Lithos 372–373, 105521.
Osanai et al., 2013. Precambrian Res. 234, 8–29.
Yoshida et al., 2018. Tectonophysics 733, 132–147.
Uno et al., 2017. Lithos 284–285, 625–641.
Uno et al., 2020. Abstract for the 11th Symposium on Polar Science, OGo02.
We have analyzed amphibolite-facies melt/aqueous fluid–rock reaction zones, Mefjell area, SRM. Orthopyroxene-bearing felsic gneiss are crosscut by numerous granitic veins and biotite veins (m to 100s m in length) and are associated with cm– to decimeter–scale whitish reaction zones composed of hornblende-biotite-bearing felsic gneiss, showing “bleached” zones. The bleached zones are characterized by complete hydration of orthopyroxene and clinopyroxene to grunerite, hornblende and biotite, representing fluid infiltration at 0.40–0.55 GPa, 600–670°C (Uno et al., 2020). Mineralogy, mineral chemistry and temperature conditions of bleached zones around granitic veins and biotite veins are essentially identical, except for characteristic minor element profiles described below.
Bleached zones associated with granitic vein (“granitic vein-bleached zones”) are characterized by relatively high MnO contents at the vein wall (hornblende 2.0 wt%; biotite 0.6 wt%; ilmenite 6.0 wt%) and they gradually decrease towards the host rock within ~15 mm length (≅ bleached zone width). Similar gradual profiles are observed for Be, Ga, Zn, Sn, Rb, Cs and Pb in biotite and/or hornblende. TiO2 in biotite are within the range of host rock (3.9–4.5 wt%). The Cl contents in biotite and hornblende are low (<0.08wt%). Contrarily, in bleached zones associated with biotite vein (“biotite vein-bleached zones”), the Cl contents in hornblende and biotite are relatively high and constant within the middle of the bleached zones (0.40–0.52 wt%; ~7 mm length from the vein wall) and sharply decrease towards the host rock (<0.04 wt%; ~15 mm length). MnO in biotite and hornblende are low (<0.4 wt%), and TiO2 contents in biotite are depleted in bleached zone (~2 wt%) compared to those in the host rock (~4 wt%).
Reactive-transport analysis of MnO and Cl in the bleached zone show that transports from the vein wall to the host rock are diffusion-dominant (Pe = 0) in granitic vein-bleached zone, whereas those in biotite vein-bleached zone are advection dominant (Pe ~300). The MnO, TiO2 enriched, Cl-depleted nature of the granitic vein-bleached zone is likely to be explained by infiltration of granitic melt, whereas Cl-enriched, MnO, TiO2 depleted nature of the biotite vein-bleached zone is affected by infiltration of Cl-bearing aqueous fluids. Assuming near lithostatic fluid pressure of the veins, the differences of the modes of chemical transport (diffusional vs. advectional) of the two bleached zones suggest contrasting viscosity of the granitic melt and aqueous fluids during infiltration from the veins to the host rock.
The results of this study indicate that both viscous granitic melt and Cl-bearing aqueous fluids contribute to fracturing at middle crustal conditions (0.40–0.55 GPa, 600–670°C). Further reactive-transport analyses of these reaction zones would reveal timescales of fluids infiltration and associated hydraulic parameters operated during middle crustal fracturing.
References
Adachi, et al., 2010. Polar Sci. 3, 222–234.
Hasegawa et al., 2005. Tectonophysics 403, 59–75.
Higashino et al., 2013. Precambrian Res. 234, 229–246.
Higashino et al., 2019. J. Petrol. 60, 329–358.
Kawakami et al., 2017. Lithos 274–275, 73–92.
Mindaleva et al., 2020. Lithos 372–373, 105521.
Osanai et al., 2013. Precambrian Res. 234, 8–29.
Yoshida et al., 2018. Tectonophysics 733, 132–147.
Uno et al., 2017. Lithos 284–285, 625–641.
Uno et al., 2020. Abstract for the 11th Symposium on Polar Science, OGo02.