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<p>Although volumetrically minor, ultramafic magmatism in arc settings is globally widespread. The genesis of arc-related ultramafic magmas is hypothesized to require efficient slab dehydration that results in high water concentrations in the hottest portion of the sub-arc mantle wedge where mantle-equilibrated magmas attain picritic compositions. Using the highly variable concentrations of Ca in olivine (Fo<sub>89.6−93.5</sub>), measured by EPMA, we show that ultramafic volcanic rocks of the Late Triassic Nicola Group in the North American Cordillera record the ascent of crystal-rich picritic slurries (~ 40–60 wt% accumulated olivine + Cr-spinel) driven by limited escape of exsolved H<sub>2</sub>O ± other volatiles. The picrites record initial crystallization of low-Ca olivine (~ 400 ppm) from an oxidized and water-rich (≥ 8wt.%) near-primary magma containing ~ 19.3 wt% MgO, followed by rapid degassing-driven ascent, during which olivine with high Ca contents (&gt; 2000 ppm) crystallized. The concomitant changes in Mn content and Fe/Mn ratios of olivine and the increasing Fe<sup>3+</sup> contents in coexisting Cr-spinel suggest that degassing also resulted in significant oxidation of the melt, possibly through SO<sub>2</sub> degassing.</p>

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Degassing-driven ascent and explosive eruption of picritic slurries in arc settings

  • 期刊论文

  • 出版模式：

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  • 发表日期：

    2026年1月23日

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  • Dejan Milidragovic,
  • Dylan W. Spence

摘要

Although volumetrically minor, ultramafic magmatism in arc settings is globally widespread. The genesis of arc-related ultramafic magmas is hypothesized to require efficient slab dehydration that results in high water concentrations in the hottest portion of the sub-arc mantle wedge where mantle-equilibrated magmas attain picritic compositions. Using the highly variable concentrations of Ca in olivine (Fo89.6−93.5), measured by EPMA, we show that ultramafic volcanic rocks of the Late Triassic Nicola Group in the North American Cordillera record the ascent of crystal-rich picritic slurries (~ 40–60 wt% accumulated olivine + Cr-spinel) driven by limited escape of exsolved H2O ± other volatiles. The picrites record initial crystallization of low-Ca olivine (~ 400 ppm) from an oxidized and water-rich (≥ 8wt.%) near-primary magma containing ~ 19.3 wt% MgO, followed by rapid degassing-driven ascent, during which olivine with high Ca contents (> 2000 ppm) crystallized. The concomitant changes in Mn content and Fe/Mn ratios of olivine and the increasing Fe3+ contents in coexisting Cr-spinel suggest that degassing also resulted in significant oxidation of the melt, possibly through SO2 degassing.