Thesis
Geochemical variations within monogenic scoria cones, southern Cascades, northeast California
Washington State University
Master of Science (MS), Washington State University
2001
Handle:
https://hdl.handle.net/2376/67
Abstract
Basaltic to andesitic scoria cones from the southern Cascades exhibit large internal chemical variations within the products of single eruptions. We have identified two types of chemical variation that occur within monogenic cones. First, there are several instances where the chemistry of the late-stage lava flows is significantly different from that of the scoria. In each case, the two magma types cannot be related to each other by fractional crystallization and instead seem to be derived from different sources. Second, we report several examples of chemical variations that occur within the scoria, most easily recognized in the large-ion lithophile element / high field-strength element (LILE/HFSE) ratios. In some cases up to a twofold variation in Ba/Nb is observed in the scoria. We consistently see evidence for an oceanisland basalt (OIB)-like component that erupts contemporaneously with the calc-alkaline basalts. The generation of the array of compositions observed requires two distinct mantle sources (one OIB-source-like and one normal mid-ocean ridge basalt (NMORB)-source-like) as well as the variable addition of slab-derived fluids. Such variations suggest that either magma chambers do not exist and the melt migrates through dike-like structures without homogenization, or magma chambers are inefficient in their ability to mix. Small volumes of basalt, which may be thermally challenged to breach the surface, might ascend through the crust through pathways recently traveled by previous melt batches, causing pairs of magmas to erupt at each vent.
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Details
- Title
- Geochemical variations within monogenic scoria cones, southern Cascades, northeast California
- Creators
- Mel Strong
- Contributors
- John A. Wolff (Degree Supervisor)
- Awarding Institution
- Washington State University
- Academic Unit
- Environment, School of the (CAHNRS)
- Theses and Dissertations
- Master of Science (MS), Washington State University
- Publisher
- Washington State University; Pullman, Wash. :
- Identifiers
- 99900525108901842
- Language
- English
- Resource Type
- Thesis