Below are recent summer research projects completed by students studying Geology at Pomona College.


Titanite Geochronology of the Sierras

Grace Hruska ’18; Advisor: Jade Lackey

Sierra Nevada batholiths and neighboring plutons have been previously dated using Zircon-geochronology methods but have yet to be studied utilizing titanite until now. Titanite (CaTiSiO5), is an accessory mineral found in calc-alkaline plutons that can be used to calculate the ages of plutonic batholiths. Titanite has an unusual crystal structure that takes up uranium (U), zircon (Zr) and rare earth elements (REE), and has low diffusivity rates of U. Unlike zircon, titanite has a high closure temperature of 650-700 C which indicates that it cooled before zircons, this information can then be used to calculate cooling rates of granitic rocks. Methodology included selection of magmatic and metamorphic titanite grains, mounting grains onto an epoxy puck, SEM imaging and measurement of isotope concentrations with ICP-MS. Results from age calculation have yet to be fully analyzed for calculation of cooling rates and relationship of previously zircon-dated rocks. We need to reduce a few of the data sets and interpret them but preliminary findings suggests that the young Eastern Sierra titanites have good dating systematics, a sufficient amount of common lead, and can be used to determine cooling rates.
Funding Provided By: Department Funding

Trace elements in the San Gabriel streams

Olivia Mendoza ’20; Advisor: Jade Lackey

Trace elements are depicted at different concentration levels depending on the discharge levels at each stream. The goal of this project was to investigate trace metal levels in the San Gabriel mountains following a high rainfall year. We sampled water from 24 different sites in 6 different streams and ponds, using the US Fish and Wildlife Service water sampling protocol. Old USGS data showed us that elements such as arsenic had higher concentrations during lower discharge years, we wanted to find similar patterns with silver, aluminum, beryllium, zinc, and other elements that indicate geological processes. We sampled at places that had interesting settings such as landslide zones, fault areas, and springs. Samples were run through an inductively coupled plasma mass spectrometer (ICP-MS) to determine trace element levels. Some elements, including strontium and tin, had high levels in all sites sampled. In contrast, zinc levels were high in two of the six streams and zero in all others. Manganese and aluminum levels also varied by stream, but these differences could be due to variation in sampling methodology. We then compared our own data do historical data on trace element levels and water flow rates. We are still in the process of comparing high rainfall years to lighter rainfall years in our specific zones but our data has already shown us that sampling techniques, heavy debris zones, and springs indicate massively different trace level element concentrations.
Funding Provided By: Department Funding


Defining Development of Channelized Lava Flows of Krafla, Iceland

Esme Faneuff ’16; Mentors: Eric Grsofils, Rick Hazlett, and Jeff Karson (Syracuse University)

Through the Keck Geology Consortium, I studied the lava flows of the Krafla Caldera in Northern Iceland, and later created experiments to model lava formations by pouring molten lava at Syracuse University. My study compares three different channelized flows, each of which has a different slope and source, that formed during the 1984 eruption at Krafla. I measured physical dimensions and took samples along each flow. This study is significant because it will help predict eruption conditions for future volcanic events. Also, this study builds upon a current query in volcanology: what is the relationship between morphology and petrology? The Syracuse University Lava Project aims to create a model of basaltic lava flows. The parent material, 1.1 billion year old Keweenawan basalt, is heated in a 1,000-pound gas-powered furnace to different temperatures depending on the needs of the experiment. This molten lava is then poured down a trough and onto a bed of sand or a steel plate, or into a different type of vessel depending on the experiment. Data are collected using video cameras, a FLIR camera, thermocouples, and still photos. These experiments allow for the manipulation of variables, like temperature and slope, to test the relationship between the variable and lava morphology. The goal of my study is to define lava channel development, and from this I hope to create a classification scheme in order to explain the physical characteristics of lava channels specific to Krafla.
Funding Provided By: Keck Geology Consortium

Magma Chamber Zonation and Eruptive Inversion in the Peach Springs Tuff: Geochemical and Paleomagnetic Evidence

Erin Barry ’17: Mentors: Jade Star Lackey, Robert Varga, and Calvin Miller ’69 (Vanderbilt University)

The 18.8 Ma Peach Springs Tuff (PST) is a >640 km3 welded ash flow tuff in SE CA, NW AZ, and S NV (Glazner et al 1986, Ferguson et al 2013). Exposures of outflow PST near Kingman, AZ, reveal four zones based on morphology and pumice, lithic, and phenocryst abundance (Ferguson & Cook 2015). Samples of these zones were analyzed to test for indicators of erupting chamber zonation. From base to top of this section we observe: 1) increase of phenocryst content from ~5 to ~20%; 2) increase of mafic magmatic enclaves, <<1%, max. 2 cm to prominent, max. 20 cm at top; 3) increase in size and abundance of pumice. Major phenocrysts include sanidine>plagioclase and biotite. Biotite, feldspar, and bulk rock compositions are similar throughout, supporting evidence for minimal chemical zonation within this portion of the magma chamber. Zircon saturation thermometry (Boehnke et al 2013) indicates a temperature of 793±22˚C, further suggesting a homogeneous magma. A sample from a fifth uppermost layer is markedly different (35% phenocrysts; no mafic enclaves; high pumice content), implying an increase in phenocryst content deeper in the chamber and a major increase in the deepest portion erupted. Elemental composition is uniform but shows a change at the crystal-rich base, consistent with a single eruptive event that ejected the contents of the magma chamber, tapping down to a crystal mush. Preliminary results of paleomagnetic remanence and AMS of PST zones suggest a single eruptive event.
Funding Provided By: National Science Foundation (Vanderbilt University)

Mecca Hills Topographical Evolution Analysis

Supasiri Rittiron PZ ’18; Mentor: Linda Reinen; Collaborators: Rodrigo Mejias (CMC '18), Patcharaporn Maneerat (PZ '17), Kai Fukutaki '17;

The Mecca Hills active fault region formed along the San Andreas Fault in Southern California as a result of transpressional uplift, sediment transport, and erosion. We quantitatively analyzed the region to investigate the potential topographic evolution models using several geomorphic parameters. Gray et al. 2014 investigated two models for the region: the radial growth model and the ‘structural knot’ model. Through their data, Gray et al. concluded that the radial growth model best fits the Mecca Hills region. The radial growth model suggests a focal point of uplift that spreads outwardly while the ‘structural knot’ model suggests a region of uplift that started in the southeast corner and travelled to the northwest corner. We find the hypsometry integrals, stream profiles, stream-length gradients and basin elongation ratios suggest that the Northern region is more tectonically active than the Southern region, supporting the structural knot model. The asymmetry factor analysis suggests a strong correlation of northwestwardly tilt near the San Andreas Fault, also supporting the structural knot model. In contrast, the radial growth model is supported by an analysis of drainage divides which suggests that the Mecca Hills grows radially, not just in the Northwest direction. The geomorphic parameters investigated support a ‘structural knot’ topographic growth model in the Mecca Hills region; these results may be useful for future seismic hazard analysis in the region.
Funding Provided By: Pomona College Research Fund (Fukutaki), Pomona College Geology Department (Fukutaki), Pitzer Internship Fund (Rittiron), Pomona College Research Fund (Maneerat), Claremont McKenna Interdisciplinary Science Scholarship (Mejias)

Recognizing Your Faults: San Andreas and the Mecca Hills

Kai Fukutaki ’17; Mentor: Linda Reinen; Collaborators: Supasiri Rittiron (PZ '18), Patcharaporn Maneerat (PZ '17), Rodrigo Mejias (CMC '18);

Seeking to better understand the subsurface structure of the San Andreas Fault (SAF), we analyzed the southern section in the Mecca Hills in order to determine which out of three published models for the SAF (Fattaruso et al., 2014) best fit observations and predictions for surface deformation. Using a combination of ArcGIS for mapping analysis, Stereonet9 for orientation, and Poly3D for model generation, the observations were compared with the corresponding strain, orientation, and topographic predictions for each model. All of these agreed that the best model of the three for the region is the dipping SAF, as opposed to vertical SAF or vertical SAF with a hidden subsurface fault.
Funding Provided By: Pomona College Research Fund (Fukutaki), Pomona College Geology Department (Fukutaki), Pitzer Internship Fund (Rittiron), Pomona College Research Fund (Maneerat), Claremont McKenna Interdisciplinary Science Scholarship (Mejias)

Structural Effects of Trace La3+ and Ce3+ on Solution-Precipitated HAP

Michael Wucher ’17; Collaborator: Jade Star Lackey

Hydroxylapatite (HAP) [Ca5(PO4)3OH] is one of the most abundant phosphate minerals  and has applications in geology, biology, and engineering. Apatite group minerals, IXM12VIIM23(IVPO4)3X, can experience a wide range of cation substitutions in M1 and M2 sites and can therefore contain a variety of metals. Other research has investigated the effect of La and Ce on HAP growth kinetics, but the substitution effects of La and Ce on HAP growth morphology and structure have not yet been addressed. Therefore, this study evaluates the structural effects of trace concentrations of La and Ce at 1% and 2% of total M site occupancy on space group P63/m micrometer scale solution-precipitated HAP. In addition, geologically formed HAP samples of similar composition from a San Gabriel nelsonite are analyzed for comparison. X-ray diffraction (XRD) and scanning electron microscopy (SEM) are used to analyze lattice parameters and surface growth morphology respectively to evaluate the structural effects of La and Ce substitutions. Preliminary results indicate a-axis widening to accommodate La3+ and Ce3+, resulting in morphologically more developed {10Ī0} faces and euhedral prismatic crystals at a lanthanide concentration of 2% compared to standard HAP. Conversely, concentrations of 1% La and Ce formed more anhedral equant crystals which may indicate that La and Ce provide periodic space filling in non-stoichiometric solution-precipitated HAP.  
Funding Provided By: Fletcher Jones

Using GIS to Investigate Erosionof Mecca Hills, Southern San Andreas Fault, CA.

Patcharaporn Maneerat (PZ ’17); Mentor: Linda Reinen; Collaborators: Kai Fukutaki '17, Supasiri Rittiron (PZ '18), Rodrigo Mejias (CMC '18)

The Mecca Hills (MH) have formed due to transpression along southern San Andreas Fault. This geomorphic feature results from the interplay between uplift and erosion. The MH is mostly covered by uniform sedimentary rocks and >70% is from Palm Spring Formation suggesting same denudation rate over the region. However, Gray et al. (2014) found a wide range of erosion rates (20 to 150 m/My) by using 10Be concentrations in active-channel alluvial sediment. We investigate potential causes of erosion and examine maturity of watersheds in the MH. We use ArcGIS to find the best geomorphic proxy for the published erosion rates by considering elevation, lithology, mean slope and active faults. We apply the best geomorphic proxy to the overall MH to predict spatial variation over the region. We use hypsometric integral and basin elongation ratio (BER) to study the maturity of the MH watersheds. We found that active faults are a main factor of erosion in the MH. The watersheds located closer to the active faults have higher erosion rates than the others. Most watersheds show mature stage of erosion cycle. The watersheds in the central MH are in more youthful stage of erosion cycle than the ones in north and south while BER values suggest the central MH formed earlier and have higher degree of integration within the basins. Although the watershed in the central MH formed earlier than the others, they experience more youthful stage of erosion cycle due to the active faulting in the region.
Funding Provided By: Pomona College Research Fund (Fukutaki), Pomona College Geology Department (Fukutaki), Pitzer Internship Fund (Rittiron), Pomona College Research Fund (Maneerat), Claremont McKenna Interdisciplinary Science Scholarship (Mejias)


Ring Fault and Caldera Formation: A 3D Modeling Approach

John Albright (2016); Student Collaborator(s): Robert Goldman (2015); Mentor(s): Eric Grosfils

Abstract: Major calderas form when large sections of the crust collapse into an underlying magma reservoir, resulting in some of the most destructive and least well-understood eruptions on both Earth and the other terrestrial planets. While previous studies have used computer modelling to explore the conditions and factors that contribute to caldera formation, they do not address the possible effects of regional tectonic stresses. In order to advance our understanding of how these features form, we developed 3D finite element model that can incorporate external extension and compression. After calibrating against the simpler, established models, we simulated inflating magma reservoirs of different shapes, and depths, subjected varying amounts of tectonic stress. In each case, we increased the pressure inside the reservoir and then observed how the stresses in the overlying rock reacted. We found that while tectonic stresses increase the likelihood of reservoir failure, the orientations of the resulting faults decrease the possibility of collapse and caldera formation. With further refinement, such results can help assess the risks and chances of eruption in volcanically active regions, like California’s Long Valley or Italy’s Campi Flegrei.
Funding Provided by: NASA Planetary Geology and Geophysics Program # NNX12AQ01G: PI Grosfilis

Using Map-Derived Hoop Strain and Elastic Models of Reservoir Inflation to Quantify the Degree of Dike Emplacement at Giant Radial Lineament Systems on Venus

Erin Barry (2017); Student Collaborator(s): Annika Deurlington (2016 CMC), Amanda Yin (2017), Michael Wucher (2017); Mentor(s): Eric Grosfils

Abstract: Over 150 giant radial lineament systems characterized by extensional structures were first identified on Venus via Magellan C1-MIDR resolution (225 m/pix) reconnaissance survey (Grosfils and Head, 1994). To investigate the roles of shallow dike emplacement and domical uplift in constructing these giant radial lineament systems, ArcGIS and COMSOL Multiphysics are used compare mapping-derived hoop strain to modeled hoop strain at 8 radially fractured systems (15S, 215E; 28.5S, 232E; 14N, 39E; 6S, 262E; 8S, 243.5E; 13.5N, 291.5E; 39.5S, 296.5E; 34N, 21.5E). By varying strain-per-lineament values and exploring different edifice construction/uplift ratios when accounting for the observed topography, we quantify the minimum degree of dike involvement needed to create these features. By optimizing the case against dike injection and then examining three additional construction/uplift scenarios, our strain quantification method shows that lateral dike injection is needed to produce observed degrees of hoop strain even if uplift plays a role in the formation of a volcanic edifice. Greater initial construction/uplift ratios yield proportionately smaller strains due to uplift, and hence more dike involvement is necessary to explain map-derived strain. Even in a case where only 25% of the existing topography is explained by construction, dike emplacement is necessary to account for ~50% of the flank deformation for all features analyzed.
Funding Provided by: Sherman Fairchild Foundation (EB); National Science Foundation Grant #OCE- 1338842: PI Lackey (MW); National Aeronautics Space Administration # NNX12AO49G: PI Grosfilis (AY)

Evidence of Active Faulting in the Mecca Hills and Recreating its Geomorphology

Sarah Granke (2017); Student Collaborator(s): Patcharaporn "Nam" Maneerat (2017 PZ); Mentor(s): Linda Reinen

Funding Provided by: Fletcher Jones Foundation (SG)

Using GIS To Investigate The Growth History Of Mecca Hills, Southern San Andreas Fault, CA.

Patcharaporn Maneerat (2017 PZ); Student Collaborator(s): Sarah Granke (2017); Mentor(s): Linda Reinen

Lava Ponds: Variation in Outflow as a Product of Morphology

Nicholas Browne (2015); Additional Collaborator(s): Jeffrey Karson (Syracuse University), Robert Wysocki (Syracuse University); Mentor(s): Richard Hazlett; Eric Grosfils

Abstract: Lava ponds are portions of lava flows that experience stagnation such that the surface of the lava cools while portions remain molten underneath. The Krafla Lava Flows in Northern Iceland from 1984 include multiple areas where topography has caused the lava to slow and stagnate. The parameters of this process are not well understood, and in order to document their process of formation the dimensions, depth, and bearings of outflow in different ponds at Krafla were measured in the field, with the texture of the ponds' edges being studied as well. It was found that the formation of ponds is dependent on there being significant negative relief, and it was also likely that their formation is dependent on the presence of pre-existing lava that causes incoming lava to stagnate before exiting a depression. At Syracuse University, simulated pours employing remelted Keweenawan Basalt were undertaken to study the formation processes of ponds when different-size depressions were constructed. A larger-sized depression produced immediate stagnation and only limited outflow via breakouts. A group of smaller depressions, however, caused only a small front of lava to stagnate, around which the lava continued to flow via a channel. This suggests that the method of outflow may in fact be dependent on how much of the lava stagnates and solidifies, which in turn is determined by the size of the pre-existing depression.
Funding Provided by: Sherman Fairchild Foundation; Keck Geology Consortium

Investigating the geochemical weathering of continents as a potential trigger for the Cambrian Explosion

Bryan Gee (2016); Mentor(s): Robert Gaines

Abstract: The Cambrian explosion (542 Ma) was a significant biological and geological event, characterized by the rapid origination and diversification of the majority of modern animal phyla, the cause(s) of which remains under debate. By studying elemental compositions of paleosols (ancient soils) at various depth profiles from globally distributed samples of Cambrian-aged continental material, we propose that chemical weathering and alteration of the exposed Cambrian continents acted as a geological trigger for this event. Our preliminary results show a trend of increasing losses of several major elements from shallow-depth paleosols, which would have been more susceptible to surficial weathering, particularly iron (Fe), phosphorus (P), and calcium (Ca). Iron and phosphorus are limiting nutrients for primary production at the base of ecological food chains, suggesting that the preferential weathering of these elements from continental rocks into the Cambrian oceans could have triggered the rapid trophic and evolutionary expansion documented in the fossil record. Additionally, the influx of calcium, a critical component of invertebrate exoskeletons, to the oceans, could have produced the widespread biomineralization that originated in the Cambrian fauna via the external sequestration of excess calcium to avoid internal accumulation to toxic levels.
Funding Provided by: Sherman Fairchild Foundation


Experimental constraints on microbial liberation of structural iron from common clay minerals in marine sediments

Kyle Metcalfe (2014); Mentor(s): Robert Gaines

Abstract: Iron is a limiting nutrient in many marine settings, and thus plays a large role in the marine carbon cycle, influencing both primary productivity and the oxidation of sedimentary organic matter. The primary constituents of marine sediments are clay minerals, which can contain lattice-bound Fe. In marine settings, the pool of Fe bound within silicate mineral lattices has long been considered reactive only over long timescales, and thus non-bioavailable. In vitro experimental evidence has shown that lab cultures of Fe-reducing bacteria are able to utilize structurally-bound Fe (III) from the crystal lattice of nontronite, a particularly Fe-rich smectite. Importantly, this process is capable of liberating Fe (II) to solution, where it is available to biotic processes as an electron donor. In order to constrain the capacity of naturally-occurring marine bacteria to liberate structurally-coordinated Fe from the lattices of common clay minerals, we exposed a suite of 16 different clay minerals to both lab cultures and to a natural consortia of Fe-reducing microbes over timescales ranging from 7-120 days. Clay minerals were treated with Na-dithionite to extract surface-bound Fe prior to exposure. Crystallographic data and direct measurements of Fe in solution demonstrate the release of structural Fe from all clay minerals analyzed. The array of clay minerals and microbes used in this experiment complement past findings, suggesting that common clay minerals may represent a large and previously unrecognized pool of bioavailable Fe in the world ocean.
Funding Provided by: Paul K. Richter and Evelyn E. Cook Richter Memorial Fund

Metamorphism and Alteration of Mafic and Ultramafic Rocks at Black Mountain, Southern Mojave Desert, CA

Rachel Havranek (2014); Mentor(s): Jade Star Lackey

Abstract: Black Mountain (BM), near El Mirage, CA exposes massive serpentinite, amphibolite that grades into massive garnet(Grt)-diopside(Di) rock, and actinolite bodies that are cross cut by diorite, granite and veins of dolomite + quartz. In this study, field mapping, whole rock geochemistry, U-Pb dating, and O and C isotope analyses have been used to evaluate the relationship among the different units. Zircon 206Pb/238U ages measured by LA-ICP-MS establish an approximate age of metamorphism at BM and are 245.8 ± 5.3 and 239.5 ± 5.2 Ma (2SD) for the diorite and granite, respectively. These ages expand westward the previously recognized limit of Triassic magmatism in the Mojave. In the field, transitions between the different rock types are gradual, suggesting that they were metamorphosed as a single package. Because Grt-Di rocks are transitional from amphibolite we ascribe an origin at BM similar to that producing the rodingites by metasomatic alteration of gabbro during serpentinization. High Ni and Cr in the Grt-Di rocks is consistent with this model, and values δ18O(Grt) of 5.8 to 7.2‰ overlap with those reported for rodingites (Wenner, 1979, GCA 43:603–614). Metamorphic mineral assemblages occurring in the Grt+Di rocks are consistent with amphibolite facies metamorphism. Dolomite+quartz veining records carbonation of serpentites at BM, that was limited by permeability. Dolomite δ18O (PBD) values cluster tightly around -5‰, while δ13C (PBD) values fall between -9‰ and -13‰. These values suggest late-stage low T and low P formation of carbonates at BM.
Funding Provided by: Sherman Fairchild Foundation

Revisiting Emplacement Depths of the Fine Gold Intrusive Suite, West-Central Sierra Nevada

Dulcie Head (2014); Mentor(s): Jade Star Lackey

Abstract: The Fine Gold Intrusive Suite (FGIS) is a large intrusive complex in the Sierra Nevada Batholith. FGIS Bass Lake Tonalite (BLT) samples were petrographically characterized to identify which contained mineral assemblage and crystallization textures appropriate for application of the Aluminum-in-Hornblende barometer of Hammarstrom and Zen (1986) re-calibrated by Anderson and Smith (1995). FGIS amphiboles are typical magnesio-hornblende on average: K0.2Na0.1Ca1.8 [Mg2.4(Al,Fe3+)(0.2-0.6)]Si6.7Ti0.1Al1.3O22(OH)2. Plagioclase compositional ranges are Ab(54-69)An(30¬45)Or(0-1). BLT data of Ague and Brimhall (1988) were re-calculated for typical plagioclase composition in the BLT (Ab62An37Or1), yielding slightly higher crystallization pressures (3.3 to 5.8 kbar) than the original range (2.4 to 4.5 kbar). New FGIS crystallization pressures of 2.6 to 3.5 kbar match the recalculated data well, thus providing larger coverage for estimates of emplacement depth. FGIS crystallization pressures generally decrease from about 3 to 3.5 kbar in the northeastern regions to closer to 2.5 to 3.0 kbar in the southwestern regions. The slight trend toward lower pressures in the southwest, where the FGIS abuts the Foothills belt metamorphic rocks, is consistent with the general westward tilting of the batholith exposing deeper levels eastward in the FGIS. We find no evidence for major changes in emplacement levels of the FGIS associated with structural breaks or with major differences of intrusive age.
Funding Provided by: Sherman Fairchild Foundation; Pomona College Department of Geology

Mud and microbes in the Salton Sea geothermal field

Nicholas Sbardellati (2014); Mentor(s): Jade Star Lackey

Abstract: Microbial communities within geothermal systems have given insight into Earth’s earliest simple organisms. Thermophiles thrive under high temperatures like those present over geologic hot spots and rift zones (Lynch). One such setting in California is active rift zone that defines the Salton Trough in Imperial County, CA. In this study, surficial geothermal features of Salton Sea geothermal field were studied to evaluate their geology and traces of biological communities. Specific analyses included characterization of clay and sulfide minerals and compositions of bulk mud at two sites: Mullet Island and Schrimp-Davis Rds. A range of temperatures (35– ¬92°C) were measured from mud volcanoes, gryphons, and near-boiling mud-pots. Sulfide minerals from mud samples were separated gravimetric methods, and clays were isolated by standard techniques including glycolation. Morphology of samples varied between framboidal, cubic and orbicular and all three pyrite morphologies were found at both study sites indicating uniform processes in pyrite formation, despite fluctuations in temperature between the two sites. Percent sulfur concentration was distinctly higher in the Mullet Isl. site by approximately 50% despite lower amounts of pyrite present in samples. Clay analysis indicated samples were mostly comprised of quartz and two samples showed characteristic peaks of chlorite and kaolinite. Sulfur analysis investigated microbial diversity within sulfide minerals. Sulfur isotope δ34S analyses by ion microprobe show isotope variations of –25 to 18‰ in individual samples, showing a range of microbial and abiotic processes contributing to precipitation of pyrite.
Funding Provided by: Research Corporation for Science Multi-Investigator Cottrell College Science Award

Oxygen isotopes of California skarn garnets: monitors of fluid infiltration and decarbonization in mesozoic arcs

Anne Fulton (2015); Additional Collaborator(s): Callie Sendek (2012 SCR); Jamie Barnes (University of Texas at Austin); Mentor(s): Jade Star Lackey

Abstract: Study of skarn mineralization furthers understanding of decarbonation reactions and volatile budgets in continental magmatic arcs (Lee et al. Geosphere 2013). Many skarn localities in Cordilleran arcs represent ideal targets to study how the interplay of magma and wallrock type and crustal depth control mixtures of volatiles that accompany skarn formation and control the extent of decarbonation progress. Oxygen isotopes are powerful tracers of fluid and wallrock reservoirs. Skarn garnet is ideal to study because it retains 18O of primary skarn-forming fluid and preserves compositional zoning that reflects changing fluid flow conditions (D’Errico et al. Geology 2012). Our 18O analyses of skarn garnets from localities in the Peninsular Ranges, Mojave Desert and Owens Valley span key periods of skarn formation in Mesozoic arcs. When compared with 18O skarn garnet values elsewhere in the Cordillera, skarn volume and reaction progress from this study show distinct patterns: Skarns showing limited reaction progress as cm-thick veneers between plutons and carbonate wallrock are often buffered by metamorphic fluids. We hypothesize that emplacement of early increments of magma stifle continued reaction in skarns as plutons are constructed in the middle crust. Massive, >10m thick skarns formed in the presence of fluids dominated by magmatic to meteoric water. Inflow of meteoric water provides a potent driver of decarbonation reactions in shallow levels of continental magmatic arcs.
Funding Provided by: Sherman Fairchild Foundation


Stratigraphy and preliminary geochemical analysis of Lake Bonneville Marl, Tule Valley, UT

Laura Haynes (2013); Mentor(s): Robert Gaines

Abstract: During the Last Glacial maximum, increased precipitation in the Great Basin created large networks of pluvial lakes, such as Lake Bonneville, in which finegrained carbonate sediments were deposited out of the water column. These deposits of lacustrine marl have enormous geochemical potential for the reconstruction of regional paleoclimate in this critical period of earth's climatic history by using carbonate mineralogy, stable isotope ratios, and sedimentary analysis. Fieldwork was conducted in the Tule Valley of Southwestern Utah, where two >1m marl outcrops were continuously sampled at the opposite ends of the valley in order to investigate intra-basinal climate effects. The Constitution marl outcrop, which likely retains its pelagic sequence, is directly overlain by Provo-aged beach gravels, indicating the quick decline in lake level following the Bonneville flood at 14.5 kya. The aragonite to calcite ratio of the Constitution marl, which may be used as a proxy for lake level and water conditions (Oviatt 1997), increases continuously until ~60cm above base, where it falls and stays relatively constant. X-radiographs of the sequence show alternating periods of finely laminated and massive marl, which will be analyzed in thin section for variations in Ca/Mg ratios. Radiocarbon dates of gastropods and δ18O, δ13C, and clumped oxygen isotope measurements will further illuminate the late Pleistocene paleoclimate record retained by this detailed sequence.
Funding Provided by: Pomona College SURP; Pomona College Geology Department

Evidence for microbial liberation of structurally bound iron in clay minerals as a source of nutrient in the world ocean

Kyle Metcalfe (2014); Mentor(s): Robert Gaines; E.J. Crane

Abstract: Clay minerals are the most abundant materials found at the surface of earth and are the primary constituents of marine sediments. Iron, a limiting nutrient in many marine settings, is a common constituent of clay minerals. Recent experimental evidence has shown that lab cultures of Fe-reducing bacteria are able to utilize structurally-bound Fe from the crystal lattice of nontronite, an uncommon and Fe-rich smectite. Reduction of structurally-coordinated Fe results in liberation of Fe(II) to solution, where it is available for other biotic processes. However, it has remained unclear: 1. if Fe-reducers are able to access structurally coordinated Fe found in low wt.% in common clay minerals; 2. if naturally occurring populations of Fe-reducers are able to reduce structurally coordinated Fe as some lab strains are. In order to address these questions, we conducted experiments using a suite of 16 clay minerals with iron contents ranging from low to high weight percents. Clays were treated with Na-dithionite solution to remove surface Fe. Experimental evidence indicates that Fe(III) bound in common clay minerals is available for reduction by the lab strain S. oneidensis MR-1 as well as by a naturally occurring consortia of Fe-reducers cultured from the San Pedro and Santa Monica Basins. Our findings suggest that common clay minerals may represent a large, previously unrecognized pool of bioavailable Fe in the world ocean that contributes to biogeochemical cycling of Fe and C.
Funding Provided by: Sherman Fairchild Foundation

The Sedimentary Rocks of Long Valley Caldera: A Record of the Post-Eruption History of a Restless Caldera

Benjamin Murphy (2013); Mentor(s): Robert Gaines; Jade Star Lackey

Abstract: 760,000 years ago, caldera collapse after a huge volcanic eruption on the eastern flank of the Sierra Nevada created the large (17 x 32 km) valley north of Bishop, California, that is now known as Long Valley. Almost immediately after formation of this valley, a lake filled the topographic depression. Sedimentary rocks that record the post-eruption history of this volcanic system were subsequently deposited within this lake; the continuing goal of this project is to document and interpret these sedimentary rocks in order to better understand the post-eruption history of what continues to be one of the most restless volcanic systems in the United States. This summer, more than twenty measured stratigraphic sections, over one hundred rock samples, and countless pages of detailed field observations were collected. Preliminary analysis of this information indicates several depositional environments within the caldera lake. Spatially pervasive coarse-grained sandstones and conglomerates indicate that a large delta system existed within the lake. In specific areas, siltstones that include fossilized plant remains record a calm marsh environment that would have existed along shore. Very fine sediments that indicate deep-water deposition contain diatom plankton and ostracods. Cemented and non-cemented beach and terrace gravels record lake level over time. Continuing analysis throughout the upcoming school year will provide a more detailed insight into the history of Long Valley Lake.
Funding Provided by: Sherman Fairchild Foundation; Pomona College Geology Department; American Chemical Society Petroleum Research Fund - 50152- UN18

Probing the Origins of the Kaweah Peaks Volcanics, South-Central Sierra Nevada

James Gordon (2015); Mentor(s): Jade Star Lackey

Abstract removed upon request.

Surface Wave Dispersion from Recent Australian Earthquakes

Dulcie Head (2014); Additional Collaborator(s): Kazunori Yoshizawa†; Mentor(s): Hrvoje Tkalcic*
*The Australian National University; †Hokkaido University

Abstract: Various imaging techniques have been deployed recently to make maps of the subsurface of Australia. These maps contribute to the understanding of structure and evolution of the Australian lithosphere, and can also improve understanding of the associated geohazards. Surface wave group velocity dispersion is one of the tools used to image the shallow subsurface, especially the crust and the uppermost mantle. Most such velocity maps of Australia’s subsurface were made using data from earthquakes that occurred at teleseismic distances. The longer travel paths used in previous studies mean that the average velocity information inevitably included structures from offshore and away from the landmass. Data from recent earthquakes large enough to have excited surface waves that occurred on the continental crust in Australia are thus, arguably, a unique tool to scrutinize the current maps and provide additional information. This study incorporates data from 6 earthquakes and about 200 new paths within the Australian continent. The newly obtained dispersion curves are compared with dispersion curves and tomographic maps obtained using long source-receiver paths from the updated model of Yoshizawa and Kennett (2004) and with the PREM predictions (Dziewonski and Anderson, 1981). This allows for insight into how the offshore distance affects the results and presents the potential to create a better, higher resolution map of crustal and lithospheric properties of Australia.
Funding Provided by: National Science Foundation; Incorporated Research Institutions for Seismology