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Geology

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 istopes 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

Research at Pomona