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• 1. [Article] Temporal and Crustal Effects on Differentiation of Tholeiite to Calcalkaline and Ferro-trachytic Suites, High Lava Plains, Oregon, USA

  Strongly bimodal, basalt-rhyolite volcanism of the High Lava Plains Province of Oregon followed the Middle Miocene flood basalts of the Pacific Northwest and extends to recent time. During the 8 m.y. of ...

  Citation × Citation Citation Abstract Copy Title: Temporal and Crustal Effects on Differentiation of Tholeiite to Calcalkaline and Ferro-trachytic Suites, High Lava Plains, Oregon, USA Author: Streck, Martin J., Grunder, Anita L. Year: 2012 Strongly bimodal, basalt-rhyolite volcanism of the High Lava Plains Province of Oregon followed the Middle Miocene flood basalts of the Pacific Northwest and extends to recent time. During the 8 m.y. of volcanism recorded in the central High Lava Plains, in western Harney Basin, three distinct mafic magmatic trends originate from primitive high-alumina olivine tholeiites (HAOT); they are tholeiitic, calcalkaline and ferro-trachytic. Tholeiitic basalts occur throughout the history and their compositions are derived by crystal fractionation while traversing the crust and mixing with evolved mafic magmas. Scavenging of apatite from crustal rocks and minor contamination with felsic melts accounts for P, incompatible element enrichments and increasing tilts of incompatible element patterns with differentiation. The calcalkaline mafic suite occurs in temporal association with abundant silicic volcanism and is the only suite with Fe decreasing with Mg. Calcalkaline compositions are derived from evolved tholeiitic basalt by crystal fractionation coupled with assimilation of felsic crust or crustal melts. The ferro-trachytic suite occurs mainly late, is highly enriched in incompatible element with patterns parallel to tholeiites from which it is derived by protracted fractionation and recharge. The three suites primarily reflect changes in magma flux and crustal interactions in time. High magma flux promotes crustal melting and contamination of tholeiite to make the calcalkaline suite. On the other hand, ferrotrachytic magmas erupted mainly late in the sequence, during magmatic waning and after significant basaltification of the crust. Title: Temporal and Crustal Effects on Differentiation of Tholeiite to Calcalkaline and Ferro-trachytic Suites, High Lava Plains, Oregon, USA Author: Streck, Martin J., Grunder, Anita L. Year: 2012 Strongly bimodal, basalt-rhyolite volcanism of the High Lava Plains Province of Oregon followed the Middle Miocene flood basalts of the Pacific Northwest and extends to recent time. During the 8 m.y. of volcanism recorded in the central High Lava Plains, in western Harney Basin, three distinct mafic magmatic trends originate from primitive high-alumina olivine tholeiites (HAOT); they are tholeiitic, calcalkaline and ferro-trachytic. Tholeiitic basalts occur throughout the history and their compositions are derived by crystal fractionation while traversing the crust and mixing with evolved mafic magmas. Scavenging of apatite from crustal rocks and minor contamination with felsic melts accounts for P, incompatible element enrichments and increasing tilts of incompatible element patterns with differentiation. The calcalkaline mafic suite occurs in temporal association with abundant silicic volcanism and is the only suite with Fe decreasing with Mg. Calcalkaline compositions are derived from evolved tholeiitic basalt by crystal fractionation coupled with assimilation of felsic crust or crustal melts. The ferro-trachytic suite occurs mainly late, is highly enriched in incompatible element with patterns parallel to tholeiites from which it is derived by protracted fractionation and recharge. The three suites primarily reflect changes in magma flux and crustal interactions in time. High magma flux promotes crustal melting and contamination of tholeiite to make the calcalkaline suite. On the other hand, ferrotrachytic magmas erupted mainly late in the sequence, during magmatic waning and after significant basaltification of the crust. Close

• 2. [Article] Plagioclase Populations and Zoning in Dacite of the 2004–2005 Mount St. Helens Eruption: Constraints for Magma Origin and Dynamics

  We investigated plagioclase phenocrysts in dacite of the 2004–5 eruption of Mount St. Helens to gain insights into the magmatic processes of the current eruption, which is char­acterized by prolonged, ...

  Citation × Citation Citation Abstract Copy Title: Plagioclase Populations and Zoning in Dacite of the 2004–2005 Mount St. Helens Eruption: Constraints for Magma Origin and Dynamics Author: Streck, Martin J., Broderick, Cindy A., Thornber, Carl R., Clynne, Michael A., Pallister, John S. Year: 2008 We investigated plagioclase phenocrysts in dacite of the 2004–5 eruption of Mount St. Helens to gain insights into the magmatic processes of the current eruption, which is char­acterized by prolonged, nearly solid-state extrusion, low gas emission, and shallow seismicity. In addition, we investigated plagioclase of 1980–86 dacite. Light and Nomarski microscopy were used to texturally characterize plagioclase crystals. Electron microprobe analy­ses measured their compositions. We systematically mapped and categorized all plagioclase phenocrysts in a preselected area according to the following criteria: (1) occurrence of zones of acicular orthopyroxene inclusions, (2) presence of dissolution surface(s), and (3) spatial association of 1 and 2. Phenocrysts fall into three main categories; one category con­tains four subcategories. The range of anorthite (An) content in 2004–5 plagio­clase is about An57–35 during the last 30–40 percent crystal­lization of plagioclase phenocrysts. Select microphenocrysts (10–50 μm) range from An30 to An42. Anorthite content is lowest near outermost rims of phenocrysts, but zonation pat­terns between interior and rim indicate variable trends that correlate with textural features. Crystals without dissolution surfaces (about 14 percent of total) show steadily decreas­ing An content outward to the crystal rim (outer ~80 μm). All other crystals are banded as a consequence of dissolu­tion; dissolution surfaces are band boundaries. Such crystals display normal outward An zoning within a single band that, following dissolution, is then overgrown abruptly by high-An material of the next band. Swarms of acicular orthopyroxene inclusions in plagioclase are characteristic of 2004–5 dacite. They occur mostly inward of dissolution surfaces, where band composition reaches lowest An content. The relative propor­tions of the three crystal types are distinctly different between 2004–5 dacite and 1980s dome dacite. We propose that crystals with no dissolution surfaces are those that were supplied last to the shallow reservoir, whereas plagioclase with increasingly more complex zoning patterns (that is, the number of zoned bands bounded by dissolution surfaces) result from prolonged residency and evolution in the reservoir. We propose that banding and An zoning across multiple bands are primarily a response to thermally induced fluctuations in crystallinity of the magma in combination with recharge; a lesser role is ascribed to cycling crystals through pressure gradients. Crystals without dissolution surfaces, in contrast, could have grown only in response to steady(?) decompression. Some heating-cooling cycles probably postdate the final eruption in 1986. They resulted from small recharge events that supplied new crystals that then experi­enced resorption-growth cycles. We suggest that magmatic events shortly prior to the current eruption, recorded in the outermost zones of plagioclase phenocrysts, began with the incorporation of acicular orthopyroxene, followed by last resorption, and concluded with crystallization of euhedral rims. Finally, we propose that 2004–5 dacite is composed mostly of dacite magma that remained after 1986 and under­went subsequent magmatic evolution but, more importantly, contains a component of new dacite from deeper in the mag­matic system, which may have triggered the new eruption. Title: Plagioclase Populations and Zoning in Dacite of the 2004–2005 Mount St. Helens Eruption: Constraints for Magma Origin and Dynamics Author: Streck, Martin J., Broderick, Cindy A., Thornber, Carl R., Clynne, Michael A., Pallister, John S. Year: 2008 We investigated plagioclase phenocrysts in dacite of the 2004–5 eruption of Mount St. Helens to gain insights into the magmatic processes of the current eruption, which is char­acterized by prolonged, nearly solid-state extrusion, low gas emission, and shallow seismicity. In addition, we investigated plagioclase of 1980–86 dacite. Light and Nomarski microscopy were used to texturally characterize plagioclase crystals. Electron microprobe analy­ses measured their compositions. We systematically mapped and categorized all plagioclase phenocrysts in a preselected area according to the following criteria: (1) occurrence of zones of acicular orthopyroxene inclusions, (2) presence of dissolution surface(s), and (3) spatial association of 1 and 2. Phenocrysts fall into three main categories; one category con­tains four subcategories. The range of anorthite (An) content in 2004–5 plagio­clase is about An57–35 during the last 30–40 percent crystal­lization of plagioclase phenocrysts. Select microphenocrysts (10–50 μm) range from An30 to An42. Anorthite content is lowest near outermost rims of phenocrysts, but zonation pat­terns between interior and rim indicate variable trends that correlate with textural features. Crystals without dissolution surfaces (about 14 percent of total) show steadily decreas­ing An content outward to the crystal rim (outer ~80 μm). All other crystals are banded as a consequence of dissolu­tion; dissolution surfaces are band boundaries. Such crystals display normal outward An zoning within a single band that, following dissolution, is then overgrown abruptly by high-An material of the next band. Swarms of acicular orthopyroxene inclusions in plagioclase are characteristic of 2004–5 dacite. They occur mostly inward of dissolution surfaces, where band composition reaches lowest An content. The relative propor­tions of the three crystal types are distinctly different between 2004–5 dacite and 1980s dome dacite. We propose that crystals with no dissolution surfaces are those that were supplied last to the shallow reservoir, whereas plagioclase with increasingly more complex zoning patterns (that is, the number of zoned bands bounded by dissolution surfaces) result from prolonged residency and evolution in the reservoir. We propose that banding and An zoning across multiple bands are primarily a response to thermally induced fluctuations in crystallinity of the magma in combination with recharge; a lesser role is ascribed to cycling crystals through pressure gradients. Crystals without dissolution surfaces, in contrast, could have grown only in response to steady(?) decompression. Some heating-cooling cycles probably postdate the final eruption in 1986. They resulted from small recharge events that supplied new crystals that then experi­enced resorption-growth cycles. We suggest that magmatic events shortly prior to the current eruption, recorded in the outermost zones of plagioclase phenocrysts, began with the incorporation of acicular orthopyroxene, followed by last resorption, and concluded with crystallization of euhedral rims. Finally, we propose that 2004–5 dacite is composed mostly of dacite magma that remained after 1986 and under­went subsequent magmatic evolution but, more importantly, contains a component of new dacite from deeper in the mag­matic system, which may have triggered the new eruption. Close