High pressure phase relations of primitive high-alumina basalts from Medicine Lake volcano,northern California

Anhydrous phase relations were determined at 1 atm and 10 to 15 kbar for primitive high-alumina basalts (79–35g and 82–72f) from Giant Crater at Medicine Lake volcano. These compositions are multiply saturated with olivine+augite+plagioclase+spinel+/-orthopyroxene near the liquidus at about 11 kbar. Experiments on mixtures of sample 79–35g with orthopyroxene and olivine determined the location of the multiple saturation boundaries where liquid coexists with the assemblage olivine+augite+orthopyroxene+plagioclase at 10 kbar and olivine+augite+orthopyroxene+spinel at 15 kbar. The mix experiments showed that primitive Medicine Lake high alumina basalts (HABs) are close in composition to liquids in equilibrium with a mantle lherzolite source containing olivine+augite+ orthopyroxene+spinel+plagioclase at 11 kbar. Orthopyroxene observed as a near liquidus phase in an 11 kbar experiment on sample 82–72f supports this conclusion. The most primitive HABs from Medicine Lake are low in K₂O (0.07 wt.%), high in MgO (>10 wt.%) and Ni (231 ppm), and have light-rare earth element depletions and large ion lithophile element enrichments. A model for the origin of these near-primary high-alumina basalts is that they are partial melts of a MORB-like mantle lherzolite source that has been enriched by a fluid component derived from the subducted slab. The HAB magma segregated from its mantle residue just below the base of the crust near the crust-mantle boundary.     

Laboratory observations of gravity wave critical-layer flows

A new laboratory facility for studying gravity wave critical-layer interactions is described, and the results from one experiment are presented. In the experiment, a forced, monochromatic gravity wave is allowed to propagate into a stratified shear flow containing a critical level for the gravity wave. The early evoluation of the flow is characterized by turbulent wavebreaking and mean flow modifications which are in qualitative agreement with previous numerical simulations. The late-time critical layer flow is characterized by internal mixing regions which are phase-locked to the incoming gravity wave.     

“Calcic amphibole equilibria and a new amphibole-plagioclase geothermometer”: Reply to the comments of Hammarstrom and Zen,and Rutherford and Johnson

Conclusions We reaffirm the barometric potential of our amphibole-plagioclase thermometer for rocks where (and only where) the final equilibration temperature is very well constrained. In the advantageous case of granitoids with simple thermal histories this temperature may be the wet granitoid solidus at the pressure of interest. We emphasise the need for careful petrographic assessment of equilibrium between plagioclase and amphibole before attempting total-Al barometry and recommend that a relatively large number of pairs are analysed for each sample. In rocks with complex post-consolidation or fluid-flow histories such barometry requires an independent means of determining amphibole equilibration temperature.     

Using Dummy Variables to Estimate Economic Base Multipliers

Economic base analysis is a cost-effective and accurate means of predicting employment impacts in relatively small and uncomplicated regional (community) economies. Extending previous research using the Arizona Community Data Set, this paper estimates economic base multipliers in different types of communities by introducing dummy variables into regression equations. Total employment, transfer payments, and population potential are used as explanatory variables. Manufacturing centers are shown to have larger multipliers than diversified places, mining settlements, service and trade communities, and utility towns.     

Use of Total Organic Carbon as an Indicator of Contamination from an Oil Refinery, South-Central Kansas

Water samples collected from 26 sites at an abandoned oil refinery in south-central Kansas were analyzed for total organic carbon (TOC) and specific volatile and semivolatile organic compounds by gas-chromatography/mass-spectrometric methods. Results from a Spearman-rho correlation analysis between TOC concentration and the number of compounds (correlation coefficient = 0.71) and TOC concentration and total concentration of compounds identified (correlation coefficient = 0.83) indicate correlations significant at the 0.01 level.
Although TOC data alone would not be sufficient to evaluate hazards posed by oil-refinery wastes, results of the correlation analysis performed using data collected from the site in Kansas indicate that TOC data can be used effectively to delineate petroleum-related ground water contamination and to help identify sources of ground water contaminants. TOC data collected from a large number of temporary sampling points during the initial phases of an investigation will provide an estimate of the extent of hydrocarbon contamination and allow placement of monitoring wells and more detailed sampling in appropriate areas.     

Compositional evolution of the zoned calcalkaline magma chamber of Mount Mazama,Crater Lake,Oregon

The climactic eruption of Mount Mazama has long been recognized as a classic example of rapid eruption of a substantial fraction of a zoned magma body. Increased knowledge of eruptive history and new chemical analyses of 350 wholerock and glass samples of the climactic ejecta, preclimactic rhyodacite flows and their inclusions, postcaldera lavas, and lavas of nearby monogenetic vents are used here to infer processes of chemical evolution of this late Pleistocene — Holocene magmatic system. The 6845±50 BP climactic eruption vented 50 km³ of magma to form: (1) rhyodacite fall deposit; (2) welded rhyodacite ignimbrite; and (3) lithic breccia and zoned ignimbrite, these during collapse of Crater Lake caldera. Climactic ejecta were dominantly homogeneous rhyodacite (70.4±0.3% SiO₂), followed by subordinate andesite and cumulate scoriae (48–61% SiO₂). The gap in wholerock composition reflects mainly a step in crystal content because glass compositions are virtually continuous. Two types of scoriae are distinguished by different LREE, Rb, Th, and Zr, but principally by a twofold contrast in Sr content: High-Sr (HSr) and low-Sr (LSr) scoriae. HSr scoriae were erupted first. Trace element abundances indicate that HSr and LSr scoriae had different calcalkaline andesite parents; basalt was parental to some mafic cumulate scoriae. Parental magma compositions reconstructed from scoria wholerock and glass data are similar to those of inclusions in preclimactic rhyodacites and of aphyric lavas of nearby monogenetic vents.Preclimactic rhyodacite flows and their magmatic inclusions give insight into evolution of the climactic chamber. Evolved rhyodacite flows containing LSr andesite inclusions were emplaced between 30000 and 25000 BP. At 7015±45 BP, the Llao Rock vent produced a zoned rhyodacite pumice fall, then rhyodacite lava with HSr andesite inclusions. The Cleetwood rhyodacite flow, emplaced immediately before the climactic eruption and compositionally identical to climactic rhyodacite (volatile-free), contains different HSr inclusions from Llao Rock. The change from LSr to HSr inclusions indicates replenishment of the chamber with andesite magma, perhaps several times, in the latest Pleistocene to early Holocene.Modeling calculations and wholerock-glass relations suggest than: (1) magmas were derived mainly by crystallization differentiation of andesite liquid; (2) evolved preclimactic rhyodacite probably was derived from LSr andesite; (3) rhyodacites contain a minor component of partial melt from wall rocks, and (4) climactic and compositionally similar rhyodacites probably formed by mixing of evolved rhyodacite with HSr derivative liquid(s) after replenishment of the chamber with HSr andesite magma. Density considerations permit a model for growth and evolution of the chamber in which andesite recharge magma ponded repeatedly between cumulates and rhyodacite magma. Convective cooling of this andesite resulted in rapid crystallization and upward escape of buoyant derivative liquid which mixed with overlying, convecting rhyodacite. The evolved rhyodacites were erupted early in the chamber's history and(or) near its margins. Postcaldera andesite lavas may be hybrids composed of LSr cumulates mixed with remnant climactic rhyodacite. Younger postcaldera rhyodacite probably formed by fractionation of similar andesite and assimilation of partial melts of wallrocks.Uniformity of climactic rhyodacite suggests homogeneous silicic ejecta from other volcanoes resulted from similar replenishment-driven convective mixing. Calcalkaline pluton compositions and their internal zonation can be interpreted in terms of the Mazama system frozen at various times in its history.     

Damming Knowledge Flows: POWER as a Constraint on Knowledge Pluralism in River Flow Decision-making in the Saskatchewan River Delta

Rivers and dams are increasingly contested venues where knowledge pluralism is critical for effective governance. To navigate change, decision-makers can adopt culturally-sensitive interventions to address the needs of diverse stakeholders and rights holders. Calls for Indigenous flows have become important as dam operators seek to renew their legal and social licenses to operate. Knowledge pluralism is needed to enhance decision-making about flows that better address complexity and change to Indigenous livelihoods and cultural practices. However, leveraging diverse knowledge types to inform these interventions is not simply a matter of changing management practice. Power relations can constrain knowledge pluralism. We contribute an empirical example that links power in a decision-making process about a dam in Saskatchewan, Canada, to the losses experienced by downstream Indigenous communities. This paper operationalizes power to illustrate its methodological utility and documents the interconnected losses, experienced by resources users, resultant from the marginalization of Indigenous knowledge.     

Developing the Next Generation of Community-Based Scholars

As we train the next generation of scholars, we should be cognizant that the academy is increasingly being asked to justify its value to broader society. We discuss ways in which students can prepare for the changing directions of the academy as they participate in a community geography research training program. Our results reveal the impacts of our program, contextualize lessons learned from creating community-based research experiences, and provide data to support a framework for other scholars to develop community-based research training programs.