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<p>The structure and evolution of the magma plumbing system that produced the dacitic Yukiura Pumice (YP) at Iwate volcano, northern Honshu, are constrained by petrography and rhyolite–MELTS simulations. The basaltic Yukiura Scoria (YS) eruption immediately preceded YP, marking a bimodal shift in erupted compositions. For YS, whole-rock MELTS runs that reproduce the observed phenocryst assemblage support hydrous basalt (3–4.25 wt% H<sub>2</sub>O). Plagioclase (An<sub>86–92</sub>) crystallized during 200–70&#xa0;MPa decompression/cooling, followed by olivine (Fo<sub>73–75</sub>) at 90–40&#xa0;MPa. Plagioclase in YP dacite shows clear, normally zoned, patchy, and oscillatory textures. Correlating domains yields a thermal history: An fell from ~ 90 to ~ 60 with cooling, rose to 65–80 during a resorption event, then declined again; later, local fluctuations persisted within An<sub>45–60</sub> (mostly 50–55). Whole-rock MELTS calculations for YP match modal mineralogy and crystallinity but underpredict plagioclase An, demonstrating disequilibrium between crystals and bulk magma. Accordingly, most plagioclase—and cotectic mafic phases—are interpreted as antecrysts derived from less evolved magma. To reconcile petrography and chemistry, we model MECE (melt extraction with crystal entrainment): residual liquid is extracted from a high-crystallinity mafic mush while entraining crystals as antecrysts. Using groundmass as melt and observed phenocrysts as crystal phases, MELTS runs on a 90% crystalline, high-Fe/Mg basaltic mush (H<sub>2</sub>O &gt; 3.5 wt%) at 350&#xa0;MPa reproduce the observed phase relations, mineral chemistry, groundmass composition, and REE patterns. The preferred thermal path involves cooling from liquidus to 815&#xa0;°C, episodic reheating and dehydration near the solidus, and final warming to 880&#xa0;°C immediately before eruption. We infer that before the YS–YP sequence, basalt ascending beneath East Iwate intruded laterally at shallow depth and built a crystal mush that became the YP source. Injection of YS-type magma reheated this mush and triggered the extraction and eruption of the YP dacite.</p> Graphical Abstract <p></p>

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Melt extraction with crystal entrainment from basaltic crystal mush, causing a dacitic pumice eruption from the mafic-dominant Iwate volcano, northern Honshu

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  • 发表日期：

    2025年10月21日

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  • Tsukasa Ohba,
  • Masamichi Ikeda,
  • Akira Chiba,
  • Arata Murayama

摘要

The structure and evolution of the magma plumbing system that produced the dacitic Yukiura Pumice (YP) at Iwate volcano, northern Honshu, are constrained by petrography and rhyolite–MELTS simulations. The basaltic Yukiura Scoria (YS) eruption immediately preceded YP, marking a bimodal shift in erupted compositions. For YS, whole-rock MELTS runs that reproduce the observed phenocryst assemblage support hydrous basalt (3–4.25 wt% H2O). Plagioclase (An86–92) crystallized during 200–70 MPa decompression/cooling, followed by olivine (Fo73–75) at 90–40 MPa. Plagioclase in YP dacite shows clear, normally zoned, patchy, and oscillatory textures. Correlating domains yields a thermal history: An fell from ~ 90 to ~ 60 with cooling, rose to 65–80 during a resorption event, then declined again; later, local fluctuations persisted within An45–60 (mostly 50–55). Whole-rock MELTS calculations for YP match modal mineralogy and crystallinity but underpredict plagioclase An, demonstrating disequilibrium between crystals and bulk magma. Accordingly, most plagioclase—and cotectic mafic phases—are interpreted as antecrysts derived from less evolved magma. To reconcile petrography and chemistry, we model MECE (melt extraction with crystal entrainment): residual liquid is extracted from a high-crystallinity mafic mush while entraining crystals as antecrysts. Using groundmass as melt and observed phenocrysts as crystal phases, MELTS runs on a 90% crystalline, high-Fe/Mg basaltic mush (H2O > 3.5 wt%) at 350 MPa reproduce the observed phase relations, mineral chemistry, groundmass composition, and REE patterns. The preferred thermal path involves cooling from liquidus to 815 °C, episodic reheating and dehydration near the solidus, and final warming to 880 °C immediately before eruption. We infer that before the YS–YP sequence, basalt ascending beneath East Iwate intruded laterally at shallow depth and built a crystal mush that became the YP source. Injection of YS-type magma reheated this mush and triggered the extraction and eruption of the YP dacite.

Graphical Abstract