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Student Research 2004

Student Research   2004   2003   2002   2001   2000   1999   1998

Ian Anderson, Dan Hennessy, Jesse Bernhardt, and James Watkins

Faculty Advisor/Collaborator: Phillip Ihinger

Origin of Felsic Segregations in Mafic Magma Chambers: Comparing Tholeiitic and Calc-Alkaline Intrusive Complexes

Magmas formed from the melting of Earth’s mantle often crystallize within the crust before they reach the Earth’s surface. These magmas crystallize as large layered intrusions that reflect the evolving magma composition. The stratified intrusions illustrate how these magmas solidified from a mineral mush with interstitial residual liquid. In this way, geochemical measurements document progressive changes in both mineral and melt composition throughout the growing crystal cumulate pile. We note that silica-rich pipes and pods (felsic segregations) that cut across the grain of the layered cumulate pile are universal features observed in tholeiitic intrusions. The segregations formed after a skeletal crystal network was developed, and arise from the buoyancy difference between the partially crystallized mineral mush and the residual interstitial liquid. The importance of felsic segregations to the evolution of the magma bodies has only recently been appreciated. We present a comparison of the character of felsic segregation bodies from two widely varying magma types: the calc-alkaline, high-K intrusions from the Montana alkaline magmatic province and the tholeiitic intrusions from the Skaergaard, Stillwater, and Bushveld complexes. In particular, we show that syenite, which is common in the upper layers of the high-K intrusions, is analogous to granophyric segregations in the tholeiitic intrusions.

Mark Ciardelli
Faculty Advisor/Collaborator: Phillip Ihinger

Hydrous Impurities in Speleothems: Indicators of Growth Rates and Paleoenvironments?

Calcite crystals grown in the cave environment (speleothems) are abundant throughout the world and have been forming throughout much of earth history. Within the last ten years, research on caves has shown that speleothems are directly linked to the surficial conditions at the time of their formation. As such, they represent an exciting tool used for reconstructing climate change. Recently, our group has documented that micro infra-red (IR) spectroscopy can been used to map the distribution of H-bearing impurities contained in quartz crystals. In particular, the abundances of H-bearing species reflect the growth rates of the individual growth faces as the crystal grew from a hydrothermal solution; that is, they serve as speedometers that monitor the rate of growth. We believe this technique can be used to determine the growth rates of calcite formed in speleothems. We have analyzed a number of samples (speleothems, calcite rhombs, dolomite, and aragonite) using micro-IR, and show that H-bearing species are indeed present in single crystals of carbonate. Furthermore, the abundances vary greatly between samples. In future studies, we will apply our technique to well dated speleothems in order to calibrate the IR speedometer.

Ryan Dayton and Karilyn Niss
Faculty Advisor/Collaborator: Colin Shaw

Mesoproterozoic Deformation and Metamorphism at Cerro Colorado, NM: Insights into the Rheology of the Brittle-Plastic Transition

The Proterozoic lithosphere of the Southwestern U.S. was affected by a 1.4 Ga thermal event that is not widely understood. Mineral assemblages in the pelitic rocks of the Vadito Group in Cerro Colorado, New Mexico record the pressures and temperatures of amphibolite-grade metamorphism during this enigmatic event. Mineral assemblages were found using a petrographic microscope, plotted on an AFM diagram and placed on a petrogenetic grid to constrain the pressure and temperature at which these assemblages are in stable equilibrium. Typical assemblages include: garnet-biotite, garnet-biotite-staurolite, garnet-sillimanite, garnet-biotite-sillimanite, cordierite-sillimanite and staurolite-sillimanite. Preliminary estimates of metamorphic conditions range from temperatures of 550° C to 650° C with pressures ranging from 0.25 GPa to 0.5 GPa. Microprobe data from sample RD03CC01B can be used to estimate temperatures using the garnet-biotite (GARB) geothermometer and pressure using garnet-plagioclase (GASP) geobarometers. Style of deformation was interpreted from a foliation map of the area with insets of sketched thin sections placed in their field orientations. A NE-SW shortening direction prevails in the study area. Microstructures from porphyroblasts, matrix fabrics and structures from the map can be used to compile a kinematic history of deformation.

Chris Fell, Scott Formolo, and Kali Pace-Graczyk
Faculty Advisor/Collaborator: Colin Shaw

Ancestry and Reactivation of the Homestake Shear Zone

The Homestake Shear Zone (HSSZ) records ~1.7 Ga deformation associated with continental assembly and ~1.4 Ga reactivation during intracontinental orogeny. We have used geologic mapping, geochemistry, petrology, and 40Ar/39Ar thermochronology to constrain the tectonic history of the HSSZ. A package of mafic rocks near the HSSZ appears to be arc-related. XRF analysis of hornblendite pods, hornblende diorite and calc-silicate rocks show Nb deficiency, and Cr values running as high as 1370-2640ppm. Mg numbers for Hornblendite and amphibolite samples range from 43-66 suggesting genesis from a primitive mantle source. AFM and MnO-TiO2-P2O5 diagrams plot hornblendite, hornblende diorite, and amphibolite rocks in continental and oceanic arc fields. 40Ar/39Ar ages suggest the northern and southern sides of the HSSZ experienced different cooling histories; hornblende is older in the north while the micas are younger. The HSSZ contains both pseudotachylyte (pst), produced by frictional melting during earthquakes, and mylonite produced by slow creep. Cross-cutting relationships indicate synchronous brittle and plastic deformation indicating deformation near the base of the seismogenic layer. SEM and EDX analysis will be used to characterize pst, and how its genesis relates to ductile mylonites. Ancestry of the HSSZ is integral to understanding how the brittle-ductile transition occurs in the middle crust and the role of reactivation of pre-existing structures in intracontinental deformation.

Scott Formolo
Faculty Advisor/Collaborator: Phillip Ihinger

Micro-IR Investigation of Apatite Crystals

Apatite has recently become an important tool in the Earth sciences. It is found in igneous, metamorphic, and sedimentary environments, and is produced organically by many organisms. Small amounts of impurities contained within apatite crystals allow them to be used for determining when their host rock formed, as well as for determining when the rock was exposed on the Earth’s surface. These techniques treat individual apatite crystals as homogeneous domains. Recently, Infrared (IR) spectroscopic measurements on quartz crystals have delineated distinct sector zones that contain order-of-magnitude variations in impurity concentrations. The rate of a growing crystal face controls the abundance of impurities trapped in the crystal structure. The sector zones represent regions within the crystal that grew from different growth faces. Because apatite crystals grow in similar fashion to quartz crystals, we ask whether large variations in impurity concentrations exist within single apatite crystals. We present micro-IR measurements of apatite crystals from a variety of geologic environments and show that, indeed, they possess observable differences in H-bearing impurities. This indicates that there were significant variations in growth rates on individual growth faces. Our results must be accounted for by researchers utilizing apatite crystals in the dating applications described above.

Nick Freiburger
Faculty Advisor/Collaborator: Kristina Beuning

Widespread Drought in Tropical East Africa During the Grand Solar Maximum

New multi-proxy data from Lakes Edward, George and Kyoga, East Africa document significantly reduced lake levels between 1000 and 750 cal. years before present (BP). In the shallow lakes basins of George (central basin) and Kyoga, evidence for aridity is provided by a distinct desiccation surface of gray mottled clay with organic matter content below 20% and water content values of 60%, both well below typical values for the biogenic oozes that overlay these surfaces. Dates from these surfaces suggest that they may be linked to geochemical evidence for increased aridity around Lake Edward, which is hydrologically connected to Lake George. In Lake Edward sediments, a large positive spike in the % Mg in inorganic calcite occurs from 940 to 790 cal. years BP. The timing of all of these events, particularly the Edward record, corresponds closely with the Grand Solar Maximum and supports solar forcing of increased aridity in tropical East Africa at this time. Following this period of aridity, sedimentological evidence and biogenic silica profiles from George and Kyoga indicate that both basins supported a shallow marsh prior to a return to open water conditions dominated by non-silicic algal populations.

Lisa Grosvold
Faculty Advisor/Collaborator: Phillip Ihinger

Micro-IR Investigation of Doubly-Terminated Gemmy Quartz Crystals

Minerals provide important insights into the processes that shape the evolution of the Earth. Quartz is an abundant mineral in the Earth’s crust that is found commonly in igneous, metamorphic, and sedimentary environments. The familiar gemmy quartz crystals grow in fluid-filled fractures as a product of the precipitation from: 1) hydrothermal fluids released from pro-grade metamorphic environments, and 2) deuteric fluids released from degassing magmas in the igneous environment. Hydrothermal quartz crystals are composed of sector zones that are distinguished by different trace element concentrations. The sector zones are regions within the crystal that correspond to growth on different crystal faces such that faster growing faces trap higher concentrations of impurities. If the crystal is later subjected to higher temperatures, impurity concentrations will define a diffusion profile that reflects the timing of the thermal event. Deuteric quartz crystals have, as yet, not been analyzed for their impurities. In this study, we show that a gemmy deuteric crystal sampled from a Namibian lamprophyre has similar characteristics to previously studied hydrothermal quartz crystals. The doubly-terminated crystal shows distinctive sector zoning with diffusion profiles, which together delineate both the morphologic evolution and subsequent thermal history of the crystal. Our results provide new insights into how such doubly-terminated crystals form.

Emily Hauser
Faculty Advisor/Collaborator: J. Brian Mahoney and Sarah Gordee (University of British Columbia)

Magmatic Evolution of Central Coast Plutonic Complex

About 200 million years ago, coincident with the spreading of the north Atlantic, subduction began along the western edge of North America. The evolution of the magmatic arc is reflected in the petrographic and geochemical characteristics of a number of different plutons composing the Coast Plutonic Complex. Geologic mapping, geochronology, and geochemistry demonstrate that at least five episodes of magmatism occurred between 185-60 Ma. Petrographic and geochemical analyses are used to characterize five plutonic suites of intermediate to felsic composition, including Howe Lake (ca. 189-183 Ma), a diorite-tonalite characterized by metavolcanic screens and xenoliths; Firvale (ca. 133-149 Ma), a granodiorite-granite with a distinct pink and green color; Desire (ca. 123-110 Ma), a diorite-tonalite containing metavolcanic screens and several comagmatic intrusive dikes; Fougner (ca. 68 Ma) a sphene bearing diorite-granodiorite; and Four Mile (ca. 73-51 Ma), a garnet bearing two mica granite. The geochemistry of the five plutonic suites shows three distinct trends. The first four suites display very similar geochemical trends, with the strongly spiked trace element pattern characteristic of subduction-related magmatism. Conversely, the Four Mile suite, which overlaps geochronologically with the Fougner suite, displays a very erratic trace element pattern including a depletion in Ba, Sr, Ti, and Eu. This pattern suggests a high degree of crustal contamination during genesis of Four Mile plutonic suite.

Morgan Herrick
Faculty Advisor/Collaborator: Robert Hooper
Recipient of Third Place Award in Natural and Physical Sciences

Crystal Structure of a Pyroxene, CaFe3+[Fe3+SiO6], with Significant Tetrahedrally Coordinated Fe3+

Buchites resulting from partially fused calcareous shale/siltstone formed by the combustion of the Healy coal seam near Buffalo, Wyoming have generated Fe3+ and Al-rich, Si-poor pyroxenes, which have yet to be reported as occurring naturally. Of the pyroxenes found within these buchites, those of importance exhibit significant tetrahedrally coordinated Fe3+. However, the presence of tetrahedrally coordinated Fe3+ is an uncommon occurrence in almost all silicates. As a result of Fe3+ enrichment, the pyroxenes within this sample are restricted to the DI-FATS-FTS field, which requires the presence of tetrahedrally coordinated Fe3+ (Hooper and Foit, 1986). The preliminary samples do not represent an end-member composition, however, one sample (Pyx5a) has nearly entered the end-member composition field. Its formula is: (Ca0.864 Na0.13) (Ca0.084 Mg0.130 Fe0.748) [Fe0.314 Al0.406 Si1.28 O6], which correlates to ~ 17% tetrahedrally coordinated Fe3+. Structural analysis of this mineral sample has generated the following unit cell dimensions: a=9.8172(9), b=8.8506(5), c=5.3656(7) Å and ß=105.76(1)° Although preliminary sampling has not entered an end-member composition field, the samples have plotted in a new mineral composition field that requires tetrahedrally coordinated Fe3+. Essenite, the last reported pyroxene like this by definition, has no tertetrahedrally coordinated Fe3+.

Christopher Kohel and Kali Pace-Graczyk
Faculty Advisor/Collaborator: J. Brian Mahoney

Geochemical and Geochronologic Analysis of Volcanic Suites of the Bella Coola Region, West-Central British Columbia

Geological mapping in the Bella Coola area in west-central British Columbia, Canada has identified three distinct volcanic groups, including the Hazelton Group, Monarch volcanics, and unknown volcanics of Albian age. Geochemical and geochronologic analyses of these three units will provide constraints on arc magmatism from the Jurassic to Cretaceous time periods. The Hazelton Group is an early Jurassic (ca 180 Ma) package of basalt, rhyolite, and associated sediments that form a northwest trending belt on the east side of the map area. Geochemically, the Hazelton Group exhibits a strong linear trend on major element Harker diagrams and displays a strongly spiked trace element pattern characteristic of an uncontaminated island arc signature. The Monarch volcanics are Valanginian in age (ca 137-132 Ma) and are described as a thick succession of andesitic flows, fragmental rocks, volcanic sandstone, tuff, and argillite that occur as a northwest trending package that spans the entire map area. An unknown package of Ablian aged volcanic rocks is characterized by basal andesitic flows and overlying pyroclastic rocks that form isolated exposures between the Hazelton and Monarch outcrop belts. These and the Monarch volcanics are geochemically very similar, displaying a restricted SiO2 range of 48 to 55% and contain high Mg values relative to the Hazleton group.

Breck Johnson
Faculty Advisor/Collaborator: Phillip Ihinger and J. Brady Foust

Visual Display of the Cenozoic Evolution of the North American Cordillera

The North American (NA) Cordillera is represented by the mountainous region that extents more than 1000 km eastward from its western margin. The region has experienced extensive magmatic and tectonic activity over the last 80 million years, the origin of which is controversial. Any successful model for the evolution of the Cordillera must explain the wide variety of magma compositions and styles of structural deformation observed in it. The character of tectonic activity varied spatially and temporally across the region. No current model can account for all available observations. We have developed an interactive time progressive map that incorporates each of the tectonic attributes over the last 80 million years. We utilize a variety of colors in a time series that illustrates the changing magma compositions and deformational styles as a ‘movie’. The movie allows for clear visualization of the evolution of the region. Our approach is advantageous in that it allows us to image trends in tectonic activity. In a companion abstract, we present a new model for the evolution of the NA Cordillera, which can account for the unique tectonic events that shaped this region. Here, we utilize the movie to test the viability of this model.

Adam Lange
Faculty Advisor/Collaborator: Garry Running and Karen Havholm

Origin and Distribution of Fluvial Terraces along the Saskatchewan and South Saskatchewan Rivers: A GIS and GPS Approach

The project’s purpose is to provide archaeologists and geoscientists seeking sites suitable for their investigations with a model of terraces (number, location and stratigraphy) and landscape evolution within the South Saskatchewan River Valley through post-glacial time. Research conducted in 2002 revealed four terraces (T1-T4) and an active floodplain (T0) within the study reach. Terrace ages were determined, and their aerial extent was mapped. This year, more sites were sampled using a Geoprobe and mapped using dGPS. Elevation data, dGPS transects, was also collected. Terraces, including a newly identified terrace (T5) were remapped using aerial photographs constrained by dGPS elevation data. T3 through T5, previously mapped together, are cut into till or glaciolacustrine deposits and are graded to terminal late-Pleistocene levels of glacial lakes Saskatchewan and Agassiz. T1 and T2 (< 500->2000 BP, and < 4000 to ~9200 BP respectively) are composed of a silty vertical accretion facies with numerous thin, weakly expressed buried soil profiles over sand and gravel lateral accretion facies. Abandonment of T1 and T2 and subsequent incision resulted from adjustments to local base level changes controlled by glacial Lake Agassiz. Geoarchaeological investigations should focus on T1 and T2 where deeply buried, stratified archaeological sites are to be expected.

Kelly Plathe
Faculty Advisor/Collaborator: Robert Hooper

Using Transmission Electron Microscopy to Determine Arsenic Speciation in Mine Contaminated Sediment in the Lower Coeur d’Alene River Valley in Northern Idaho

Lakes in the lower CDA river system, Idaho, have been contaminated with high concentrations of metals from over 100 years of sulfide mining. To elucidate downstream variation in arsenic speciation, sediment cores from three different lakes (Killarney, Swan, and Thompson) were analyzed using HR-ICPMS on sequentially extracted (SE) samples and TEM. Unlike Swan Lake, whose only inlet is dammed, Killarney and Thompson are both connected to the main channel by distributaries, providing an inlet for sediment recharge during high water events. Arsenic SE patterns for Killarney and Thompson lakes are similar, showing increasing As in the exchangeable fraction with depth. Swan Lake, in contrast, shows negligible As in the exchangeable fraction, but indicates an increase in the detrital fraction with depth. TEM results show both Killarney and Thompson lakes having considerable As in both detrital (3.61wt %) and biogenic (1.91wt %) sulfidic phases. Near the pre-mining surface, detrital sulfides become rare and biogenic sulfidic phases are the primary form of As sequestration. Swan Lakes shows greater biogenic sequestration in the upper third of the contaminated column associated with ZnS and metal-rich sulfidic nano-particles up to 2.44 wt% As. The differences in As speciation are presumably due to periodic oxygenation of sediment in Killarney and Thompson during low water level periods.

Ryan Prechel
Faculty Advisor/Collaborator: Phillip Ihinger and Daniel Stevenson

Visual Simulation of Quartz Crystal Growth

Crystal growth in the natural environment is a poorly understood process. Analogous to snowflakes, every crystal has a unique size and shape. Although it has long been known that the angles between faces within a given mineral are invariant, what determines the final size and shape of a crystal is unclear. Recent measurements of chemical impurities within natural crystals offer important clues toward understanding crystal growth. In natural crystals, chemical heterogeneities are generated during growth because different crystal faces grow at different rates and therefore trap different amounts of impurities. Thus the relative growth rate of each face is recorded within the crystal, and the impurities act as a speedometer for the growth rate of the face. We present a visual simulation of a single quartz crystal growing in a hydrothermal solution. Our simulation mimics the morphologic and chemical characteristics observed in crystals grown in the natural environment. We show how growth on different crystal faces leads to chemically distinguishable sector zones inside the crystal. Our simulations generate the order-of-magnitude variations across sharp sector zone boundaries that are observed in natural crystals. Our visual simulation illustrates the morphologic evolution of a crystal as it grows from hydrothermal solution.

Sarah Prindiville
Faculty Advisor/Collaborator: J. Brian Mahoney

Geochemical Characteristics of Glaciogenic Sediments, Puget Lowland, Washington

Quaternary sediments in the Puget Lowland comprise a complex succession of intercalated glacial, glaciofluvial, and glaciomarine sediments deposited during repetitive glacial and interglacial periods. The primary source regions are the southern Canadian Cordillera (SCC), the Cascade Range and Olympic Mountains. Major glacial advances provided sediment from the SCC, and interglacial periods were dominated by locally derived Cascadian/Olympic sediments, but complex interfingering sedimentation over extended time periods and sediment reworking makes comprehensive basin analysis difficult. Major and trace element geochemistry from throughout the Puget Lowland suggest that it is possible to discriminate glacial and interglacial sedimentation events through bulk sediment composition. Major glacial advances produce sediment that is high in SiO2, Ba, Sr, Cr, Ni, and Cr/V, and low in TiO2, Nb, Ce, V, La, Zr and Th/Sc. These values suggest a source from continental arc plutons in southern British Columbia. Conversely, sediments deposited during interglacial periods were derived from Cascade volcanic rocks, and are correspondingly higher in TiO2, Nb, Ce, V, and La. However, sediment mixing, elemental division and element mobility can produce misleading geochemical signatures. Multiple geochemical indicators permit discrimination between glacial and non-glacial sediments, and permit stratigraphic correlations in complex Quaternary sediments of the Puget Lowland.

Laura Strumness and Jennifer Thornburg
Faculty Advisor/Collaborator: Robert Hooper and J. Brian Mahoney

Speciation and Mobility of Trace Metal Contamination in the Lower Coeur d’Alene River Valley, Idaho

Remediating fluvial systems impacted by sulfide mining requires characterization of contaminant mobility and trace metal speciation. Sequential extraction and electron microscopy (SEM and TEM) procedures provide unprecedented insight into metal speciation and behavior in different fluvial subenvironments. Three complete transects, including river channel, levee, wetland and lacustrine environments, along the lower Coeur d’Alene (CdA) River valley were studied. This investigation was designed to specifically test previously proposed speciation models (Hooper and Mahoney, 2000) along the entire length of the CdA River system. CdA River channel sediments contain abundant detrital and authigenic sulfide minerals (PbS, FeS2, ZnS), carbonates (PbCO3, FeCO3), and locally sulfide encrusted organic matter. During flooding, river bottom sediments are remobilized and supply trace metal contamination into adjacent fluvial subenvironments. River sediments are rapidly oxidized within the levee environment, resulting in extensive precipitation of oxy-hydroxides with subsequent dissolution of carbonate minerals. Both detrital and authigenic sulfides and oxy-hydroxides supply trace metals to adjacent wetland and lacustrine environments. The anaerobic conditions of the wetlands and lacustrine environments yield abundant biogenic sulfides. Analysis of three transects along the length of the lower CdA system indicates that there is minimal downstream variation within fluvial subenvironments but large differences between environments. Effective remediation in the lower CdA river valley requires limiting the disturbance of river bottom sediments during flood events.

Jennifer Thornburg
Faculty Advisor/Collaborator: Robert Barth and Phillip Ihinger

Geochemical Characterization of Red Lithic Artifacts in the Chippewa Valley, WI

Surveys of an archaeological site in Chippewa Falls produced four broken artifacts and approximately 1100 flakes of a distinctive red lithic material. Examination of the archaeological literature and site collections at the University of Wisconsin-Eau Claire revealed that artifacts and flakes of visually similar red material had been recovered at other sites in both the Chippewa River Valley and the Red Cedar River Valley. This lithic material had been classified by various archaeologists as pipestone, hematite, jasper, jasper taconite, and red flint. Geochemical analysis identified the material from Chippewa Falls as a unique silicified siltstone whose origin appears to be the Blue Hills of Barron and Rusk Counties. Analysis of the red lithic material from the other sites should easily verify if they are from the same geological source.

Jeremy Treague
Faculty Advisor/Collaborator: Steve Abbott (Southern Cross University Lismore, Australia)

Geological Context of Peat Deposition in Cumbebin Swamp, Northeast New South Wales, Australia: A Geological Framework for Carbon Sequestration Studies

Throughout the Twentieth Century, human activities have heavily impacted coastal wetland environments. Drainage and filling, clearing of vegetation, and pollution are a few of such activities that have contributed towards the loss of these habitats. The purpose of this project was to determine the geological origin and evolution of Cumbebin Swamp, a low-lying, estuarine/alluvial flood plain that is situated southwest of the township of Byron Bay along the northern New South Wales coastline, and to provide a geological framework for carbon sequestration studies. Vibracore equipment was used to collect a 53 cm-long, 12 cm-diameter wetland core. Swamp sediments were characterized in terms of thickness, location within the swamp stratigraphy, and environment of deposition. Detailed geology and soils maps of Cumbebin Swamp and surrounding coastal plain were compiled using NSW Department of Mineral Resources data and ArcView 8 software. Acidic, water-saturated, oxygen-poor conditions are responsible for peat deposition since the last high sea level (which peaked 1 m APSL and occurred 6.5 ka). This sediment was found up to 20 cm below the swamp surface. Fine-grained quartz sand was found 20-34 cm below the swamp surface and overlies a 6-cm thick bed of angular quartz gravel. This material may represent weathered Paleozoic rocks.

James Watkins
Faculty Advisor/Collaborator: Phillip Ihinger

Major Element Characterization of OIB Source Regions: Insights from the Hawaiian-Emperor Hotspot Chain

Magmatism on Earth’s surface provides clues to the origin, evolution, and present state of Earth’s interior. Mid-plate volcanoes on the ocean floor (OIB) directly reflect their mantle source characteristics because they are not influenced by significant contamination. Variations in trace element and isotope concentrations of OIB identify several different mantle reservoirs, each with a unique chemical composition. Complete characterization of these reservoirs is necessary in order to fully understand mantle dynamics; however, few attempts have been made to elicit the major element composition of OIB source regions. We note that geochemical measurements of Hawaiian lavas show correlations between isotope ratios and major element concentrations along the “Loa” segment of the Hawaiian-Emperor seamount chain. These systematic variations cannot be accounted for by common processes that affect magma composition, such as crystal fractionation, crustal contamination, alteration, or variable degrees of partial melting. Rather, these variations represent mixing between two or more distinct OIB mantle source regions. A multi-dimensional fit of Hawaiian geochemical data provides estimable major element abundances between OIB source regions represented by shield-stage lavas from Koolau and Loihi volcanoes. We demonstrate a method of calculating major element compositions for OIB source regions that can be applied to other centers of mid-plate volcanism.

James Watkins, Breck Johnson, and Jesse Bernhardt
Faculty Advisor/Collaborator: Phillip Ihinger

Origin of North American Cordillera: Lithospheric Response to Plume-Slab Interaction

Over the last 200 million years, the geologic evolution of western North America has been dominated by active subduction of the Farallon plate. During the early stages of its formation, the active plate boundary produced features similar to typical subduction margins. However, beginning ~ 80 million years ago, the style of magmatism and structural deformation changed abruptly, producing several enigmatic features: (1) the eruption of large, explosive volcanoes in New Mexico, (2) the formation of the Rocky Mountains in Montana, Wyoming, and Colorado, (3) the carving of the Grand Canyon in Arizona, (4) the formation of the unusual Basin and Range topography throughout Nevada and Utah, (5) the large outpouring of basalt lavas in Oregon, Washington, and Idaho, (6) the eastern extent of structural deformation, and (7) the coeval emplacement of two magma types derived from markedly different mantle source regions throughout the western US. Previous models for the geologic evolution of the western US cannot reconcile all of the observed features. We present a new model for the geologic evolution of the North American Cordillera that complies with existing observations of the structural and magmatic evolution of the region.

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