http://hdl.handle.net/1957/1718 2013-05-21T21:41:27Z http://hdl.handle.net/1957/38577 Re and Os isotopes of the central Oregon Cascades and along the arc indicate variable homogenization and mafic growth in the deep crust Schmidt, Mariek E.; Grunder, Anita L.; Rowe, Michael C.; Chesley, John T. The rhenium-osmium isotopic system has the potential to track the maturation of arc crust at lower crustal levels because the ¹⁸⁷Os/¹⁸⁸Os in magmas is sensitive to the age and composition of crust with which they interact. We here present Re and Os isotopic data for a suite of samples from the extensional central Oregon Cascade arc that includes diverse, primitive (high Mg) basalts and low-and medium-K basaltic andesites from North Sister and Little Brother volcanoes. The basaltic andesites contain higher ¹⁸⁷Os/¹⁸⁸Os than the two most common basalt types, calc-alkaline basalt and low-K tholeiites (0.17-0.19 vs. 0.14-0.15), indicating interaction with mafic crust. The evolution of ¹⁸⁷Os/¹⁸⁸Os in the deep crust depends on Re partitioning that in turn depends on sulfide and magnetite stability and oxygen fugacity. We estimate bulk mineral-melt partition coefficients for Re to ~4 at ƒO₂ ≈ QFM and ~10 at ƒO₂ ≈ + 1 to + 3 ΔQFM in keeping with near liquidus phase equilibria at deep crustal conditions (Mercer and Johnston, 2008) and on ƒO₂ of central Oregon primitive magmas (Rowe et al., 2009). Modeling fractional crystallization of the central Oregon primitive basalts indicates that the deep arc crust is likely to have ¹⁸⁷Re/¹⁸⁸Os similar to the range measured in North Sister basaltic andesites (~55-260). The age of the mafic crustal contaminant in the central Oregon Cascades is then estimated to be 9-29 Ma and is younger than the start of Cascade arc volcanism at ~45 Ma in central Oregon. The range in Os¹⁸⁷/Os¹⁸⁸ at the Cascade volcanic centers Mt. Lassen and Mt. Adams is greater than is observed for central Oregon arc magmas. Weestimate a range in mafic crustal contaminant ages of 8-340 Mafor Mt. Lassen and 0-70 Mafor Mt. Adams and are older than suggested by previous studies (Hart et al., 2002, 2003). Mafic contaminant age ranges reflect the degree to which the crust is homogenized, or the extent to which the composition of the crust is a product of recent arc magmatism vs. pre-existing protoliths. Intra-arc extension in central Oregon facilitates homogenization and leads to assimilation of relatively young arc crust. Where extension is less narrowly focused, the range in Re and Os isotopes is greater and reflects isotopic heterogeneities, including variably aged crust at Mt. Lassen or variable enrichment of Re and Os in the lithosphere beneath Mt. Adams. (C) 2013 Elsevier Ltd. All rights reserved. This is the publisher’s final pdf. The published article is copyrighted by Elsevier and can be found at: http://www.elsevier.com/. 2013-02-14T00:00:00Z http://hdl.handle.net/1957/38575 Post-fire vegetation response to snow in the western United States Blauvelt, Katie The western United States is experiencing significant changes in wildfire and snow regimes as a result of warming temperatures. An amplification of wildfire activity and reduction in snow water equivalent, snow covered area, and earlier spring snowmelt are documented trends that are projected to continue into the future. With an increase in wildfire activity, it is important to understand how a reduction in snow will impact regenerating vegetation in the western United States. The first objective of this study was to assess summer vegetation biomass response to antecedent winter snow on a local scale by determining the physiographic characteristics that influence the relationship between snow and vegetation in the case of the 2002 Biscuit Fire. The second objective was to assess the broad scale regional patterns of regenerating vegetation response to snow, by comparing the correlation between summer vegetation biomass and antecedent winter snow before and after large wildfires across the western United States. Remote sensing data and spatial-temporal statistics were used to analyze the relationship between snow and vegetation. In the local scale analysis, the 2002 Biscuit Fire was analyzed, which burned over 2,000 km² in southwest Oregon and northern California. Nonparametric Multiplicative Regression (NPMR) was used to explore the complex relationships between multiple predictor variables (winter snow frequency, elevation, slope, aspect, and burn severity) and the summer vegetation response variable (enhanced vegetation index, or EVI), before and after the Biscuit Fire burned. The burned area was subset by soil type to determine how soil texture influenced the snow and vegetation relationship. In the regional scale analysis, the Pearson's Correlation Coefficient was calculated to analyze the relationship between winter snow frequency and summer EVI before and after 23 wildfires across the western United States. In the case of the Biscuit Fire, summer EVI responded negatively to snow before the fire, and responded positively to snow after the fire. EVI in coarse-textured skeletal soils exhibited the clearest shift to a positive response to snow after the fire burned, while EVI in fine-textured clay soils did not exhibit this type of shift. The regional analysis proved that wildfire disturbances affect the relationship between snow and vegetation differently across the western United States. Seven fires clustered near the Biscuit Fire in northern California and southwestern Oregon behaved similar to the Biscuit Fire, shifting from a negative pre-fire snow and EVI correlation to a less negative or positive post-fire snow and EVI correlation. The majority of these fires had relatively low average elevations (430 to 1708 m) with greater than 80% forest land cover. Ten fire areas exhibited a significant positive pre and post-fire snow and EVI correlation. The majority of these fires had relatively high average elevations (1612 to 2291 m) and consisted of greater than 50% shrub, scrub, and grass land cover. The local scale analysis suggests that the condition of the vegetation (undisturbed vs. regenerating) and the soil texture in which it grows affects its response to winter snow. The low water holding capacity of coarse-textured soils and the short root-lengths of regenerating vegetation may result in greater dependence on snow as a water resource. Regionally, vegetation type and elevation may affect the vegetation's response to snow; short-rooted shrubs at higher elevations above the transient snow zone may be more dependent on snow as a water resource. These results suggest that the relationship between snow and vegetation is not constant, depending on the condition of the vegetation. Increases in wildfire activity and a reduction of snow in the future may impact successional trajectories in certain regions where vegetation may have historically relied on snowmelt to regenerate. Graduation date: 2013 2013-05-02T00:00:00Z http://hdl.handle.net/1957/38254 Geology, geochemistry, and mineral potential of cretaceous and tertiary plutons in the eastern part of the Soldier Mountains, Idaho Lewis, Reed S. (Reed Stone) Graduation date: 1991 1990-05-21T00:00:00Z http://hdl.handle.net/1957/38243 Stratigraphy, structure, and metamorphism near Saidu Sharif, Lower Swat, Pakistan DiPietro, Joseph A. Graduation date: 1990 1990-02-16T00:00:00Z