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<p>Melt inclusions in autocrysts may represent a composition of a magma at different conditions as it rises prior to eruption. Mineral hosts for melt inclusions, however, may be from previous intrusions within a magma chamber or conduit that were picked up by a rising magma. The melt inclusions are in this case not representative of the magma that incorporated them but may give insights on reservoir conditions. Melt inclusions found in olivines that are xenocrysts describe a different magmatic temporal and/or spatial environment. Thus, part of the interpretations for selected deposits from Tongariro Volcanic Complex is an evaluation for possible xenocryst and autocryst hosts for melt inclusions. The 1500AD lava flow from Te Maari Crater contains xenocrystic olivines, and melt inclusions hosted by these crystals describe reservoir conditions wherein the crystals formed in terms of H<sub>2</sub>O (0.6–2.3 wt.%) and CO<sub>2</sub> (286–470&#xa0;ppm). Melt inclusions from autocryst olivines in pyroclasts of Wharepu Tephra (ca. 10&#xa0;ka eruptions) may show a degassing trend for an evolving/rising magma in terms of H<sub>2</sub>O (0.9–6.7 wt.%). Unlike the Te Maari xenocrysts that were stored in a reservoir, the host crystals for Wharepu melt inclusions were forming at different depths as the magma moved to the surface. These melt inclusions recorded changing volatile contents interpreted here as regassing and degassing of H<sub>2</sub>O, early release of CO<sub>2</sub>, and retention of most of the sulfur. However, additional melt inclusions within Wharepu Tephra autocrysts are needed to see if these trends are consistent. Presented here are interpretations of melt inclusion major element compositions including volatiles (CO<sub>2</sub>, H<sub>2</sub>O, and S) to describe crystallization environments for magma that is rising and in storage.</p>

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Melt inclusions from autocrysts and xenocrysts: considering crystal origin in interpreting volatile content and magmatic processes

  • 期刊论文

  • 发表日期：

    2025年8月6日

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  • Maria Carmencita Arpa,
  • Bruce Christenson,
  • Georg F. Zellmer

摘要

Melt inclusions in autocrysts may represent a composition of a magma at different conditions as it rises prior to eruption. Mineral hosts for melt inclusions, however, may be from previous intrusions within a magma chamber or conduit that were picked up by a rising magma. The melt inclusions are in this case not representative of the magma that incorporated them but may give insights on reservoir conditions. Melt inclusions found in olivines that are xenocrysts describe a different magmatic temporal and/or spatial environment. Thus, part of the interpretations for selected deposits from Tongariro Volcanic Complex is an evaluation for possible xenocryst and autocryst hosts for melt inclusions. The 1500AD lava flow from Te Maari Crater contains xenocrystic olivines, and melt inclusions hosted by these crystals describe reservoir conditions wherein the crystals formed in terms of H2O (0.6–2.3 wt.%) and CO2 (286–470 ppm). Melt inclusions from autocryst olivines in pyroclasts of Wharepu Tephra (ca. 10 ka eruptions) may show a degassing trend for an evolving/rising magma in terms of H2O (0.9–6.7 wt.%). Unlike the Te Maari xenocrysts that were stored in a reservoir, the host crystals for Wharepu melt inclusions were forming at different depths as the magma moved to the surface. These melt inclusions recorded changing volatile contents interpreted here as regassing and degassing of H2O, early release of CO2, and retention of most of the sulfur. However, additional melt inclusions within Wharepu Tephra autocrysts are needed to see if these trends are consistent. Presented here are interpretations of melt inclusion major element compositions including volatiles (CO2, H2O, and S) to describe crystallization environments for magma that is rising and in storage.