Typescript; Thesis (M.S.)--University of Oregon, 1998; Includes vita and abstract; Includes bibliographical references (leaves 183-186); CD-ROM contains SEM backscatter electron images for appendix B
98 collapse. The oxide shells are clearly preserved around the non-collapsed areas of the bubbles, but also encircle recently deflated portions. While it is likely that bubble collapse occurs in lava flows during emplacement, the same line of evidence can be used to argue vesicle collapse in the tephra populations as well, even though it may occur by a different mechanism. Lava flow samples, therefore, contain a small range in vesicularity, but a wide range of feldspar microlite distributions, which typically correlate with localized vesicular zones. This may result from rapidly changing equilibrium conditions immediately surrounding vesicles (Manley, 1996) which allows for the preferential formation of either feldspar or oxides. Additionally, the flow samples illustrate the concentration of oxides through bubble collapse. Summary The purpose of this chapter was to describe and document fully the range in textural variations seen in BSE images. The juvenile sub-populations each have distinctive textural parameters, which can also vary within that population. Differences in vesicularity of pumices (bubble size, shape and wall thickness) correlate best with measured density variations. Yet all imaged pumice clasts, even low density white pumice, show varying degrees of vesicle deformation and collapse, as well as varying concentrations of microlites. The microlite population is heterogeneous on such a large scale that it is difficult to identify any relationship to density or other parameter. The microlites are, however, useful in demonstrating bubble collapse, especially where oxide strings outline the remnants of flattened vesicles. The range of vesicle collapse and deformation may be the primary cause of density variations exhibited by the Cleetwood tephra, suggesting that the erupting magma experienced variable conditions in transit to the surface. However, the geochemistry of the juvenile sub-populations is discussed in the following chapter to eliminate compositional differences as the cause of the documented textural variations. 99 115 Summary In summary, the geochemical data present conclusive evidence that the juvenile sub-populations are not formed by variation of chemical compositions. Instead, the textural distinctions must originate after the homogenous magma leaves the chamber, and reflect variable conditions experienced by the rising magma. Chapter VI discusses possible models for formation of the five sub-populations, and their relation to overall eruption dynamics.
98 collapse. The oxide shells are clearly preserved around the non-collapsed areas of the bubbles, but also encircle recently deflated portions. While it is likely that bubble collapse occurs in lava flows during emplacement, the same line of evidence can be used to argue vesicle collapse in the tephra populations as well, even though it may occur by a different mechanism. Lava flow samples, therefore, contain a small range in vesicularity, but a wide range of feldspar microlite distributions, which typically correlate with localized vesicular zones. This may result from rapidly changing equilibrium conditions immediately surrounding vesicles (Manley, 1996) which allows for the preferential formation of either feldspar or oxides. Additionally, the flow samples illustrate the concentration of oxides through bubble collapse. Summary The purpose of this chapter was to describe and document fully the range in textural variations seen in BSE images. The juvenile sub-populations each have distinctive textural parameters, which can also vary within that population. Differences in vesicularity of pumices (bubble size, shape and wall thickness) correlate best with measured density variations. Yet all imaged pumice clasts, even low density white pumice, show varying degrees of vesicle deformation and collapse, as well as varying concentrations of microlites. The microlite population is heterogeneous on such a large scale that it is difficult to identify any relationship to density or other parameter. The microlites are, however, useful in demonstrating bubble collapse, especially where oxide strings outline the remnants of flattened vesicles. The range of vesicle collapse and deformation may be the primary cause of density variations exhibited by the Cleetwood tephra, suggesting that the erupting magma experienced variable conditions in transit to the surface. However, the geochemistry of the juvenile sub-populations is discussed in the following chapter to eliminate compositional differences as the cause of the documented textural variations. 99 115 Summary In summary, the geochemical data present conclusive evidence that the juvenile sub-populations are not formed by variation of chemical compositions. Instead, the textural distinctions must originate after the homogenous magma leaves the chamber, and reflect variable conditions experienced by the rising magma. Chapter VI discusses possible models for formation of the five sub-populations, and their relation to overall eruption dynamics.