講演情報
[SMP31-P12]Aragonite under stress: Deformation and fluid-driven calcite replacement in lawsonite blueschist facies Ward Creek marbles, California
*帆足 碧夏1、辻森 樹1、横山 裕晃1、武藤 潤1 (1.東北大学)
キーワード：
アラゴナイト大理石、変形双晶、冷たい沈み込み帯、ローソン石青色片岩相、高圧変成作用
Carbonate rheology plays a key role in deep carbon cycling and subduction dynamics. CaCO3 remains stable as aragonite under lawsonite blueschist facies conditions, widespread calcite replacement during exhumation commonly overprints aragonite microtextures, obscuring the link between deformation and the aragonite–calcite transformation. This study investigates aragonite marble from the Ward Creek (Franciscan Complex, California: Coleman and Lee, 1962; Hoashi, Tsujimori et al., this conference) to clarify how aragonite deforms and transforms to calcite under lawsonite blueschist facies conditions.
The marble is predominantly aragonitic and partially calcitized, and both phases occurs in various textural forms (e.g., coarse-grained, fine-grained, and flake-like aragonite; fine-grained replacive and veinlet calcite). FE-SEM (BSE) imaging and EBSD mapping reveal pervasive deformation twinning in aragonite regardless of grain size, together with locally bent cleavage traces and dynamic recrystallization expressed by grain-size reduction and neoblast development. Coarse aragonite grains (typically >2 cm across) exhibit gradual intragranular misorientation gradients, recording progressive internal strain accumulation and crystal-plastic deformation. Twin boundaries are consistent with the dominant aragonite twin law ({110}, 63–64°about <001>). Notably, calcite replacement preferentially occurs along these twin boundaries, indicating that twin-related strain localization created mechanically weakened and reactive domains that promoted selective calcitization. In contrast, late-stage veinlet calcite occurs along some grain boundaries and fractures, interpreted as pathways for retrograde fluid infiltration.
Overall, the microstructural relationships suggest a multi-stage evolution of the sample: (1) crystal-plastic deformation of aragonite within its stability field, recorded by deformation twins; (2) enhanced dislocation activity and dynamic recrystallization accompanied by fracturing during the phase transformation; and (3) localized, fluid-driven calcite replacement along mechanically weakened domains. These results suggest that strain localization not only records deformation processes but also creates fluid pathways that facilitate phase transformation and fluid–rock interaction.
References
Hoashi, A., Tsujimori, T., Takayanagi, H., Iryu, Y., 2026, Carbon retention and oxygen-isotope overprint during aragonite–calcite transformation in lawsonite-bearing aragonite marble from Ward Creek, Franciscan Complex. JpGU 2026 abst.
Coleman, R.G., Lee, D.E., 1962, Metamorphic aragonite in the glaucophane schists of Cazadero, California. Amer. J. Sci. 260, 577–595, https://doi.org/10.2475/001c.58941
The marble is predominantly aragonitic and partially calcitized, and both phases occurs in various textural forms (e.g., coarse-grained, fine-grained, and flake-like aragonite; fine-grained replacive and veinlet calcite). FE-SEM (BSE) imaging and EBSD mapping reveal pervasive deformation twinning in aragonite regardless of grain size, together with locally bent cleavage traces and dynamic recrystallization expressed by grain-size reduction and neoblast development. Coarse aragonite grains (typically >2 cm across) exhibit gradual intragranular misorientation gradients, recording progressive internal strain accumulation and crystal-plastic deformation. Twin boundaries are consistent with the dominant aragonite twin law ({110}, 63–64°about <001>). Notably, calcite replacement preferentially occurs along these twin boundaries, indicating that twin-related strain localization created mechanically weakened and reactive domains that promoted selective calcitization. In contrast, late-stage veinlet calcite occurs along some grain boundaries and fractures, interpreted as pathways for retrograde fluid infiltration.
Overall, the microstructural relationships suggest a multi-stage evolution of the sample: (1) crystal-plastic deformation of aragonite within its stability field, recorded by deformation twins; (2) enhanced dislocation activity and dynamic recrystallization accompanied by fracturing during the phase transformation; and (3) localized, fluid-driven calcite replacement along mechanically weakened domains. These results suggest that strain localization not only records deformation processes but also creates fluid pathways that facilitate phase transformation and fluid–rock interaction.
References
Hoashi, A., Tsujimori, T., Takayanagi, H., Iryu, Y., 2026, Carbon retention and oxygen-isotope overprint during aragonite–calcite transformation in lawsonite-bearing aragonite marble from Ward Creek, Franciscan Complex. JpGU 2026 abst.
Coleman, R.G., Lee, D.E., 1962, Metamorphic aragonite in the glaucophane schists of Cazadero, California. Amer. J. Sci. 260, 577–595, https://doi.org/10.2475/001c.58941