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Geochemistry of the Tatara-San Pedro continental arc volcanic complex and implications for magmatism in the Chilean Southern Volcanic ZoneJweda, Jason January 2014 (has links)
Reconnaissance work and high-density sampling of volcanic rocks at the Quaternary Tatara-San Pedro complex (TSPC) in the Southern Volcanic Zone (SVZ) of Chile has yielded one of the most complete eruptive chrono-stratigraphies and comprehensive geochemical datasets of any arc volcano on Earth. The TSPC is a large frontal arc stratovolcano within the SVZ that exhibits a wide compositional diversity of lavas from basalt to rhyolite, covering most of the ranges in major and trace element contents across the SVZ. The TSPC occupies a pivotal position within the SVZ, where it is "intermediate" in terms of geophysical and geochemical characteristics between northern and southern SVZ volcanoes. The large TSPC dataset and stratigraphic control provides a unique opportunity to elucidate magma source heterogeneity and distinguish between contributions from upper mantle, subducted slab, and crust in a volcanic complex overlying relatively thick continental crust. Furthermore, the results of this investigation provide important constraints about the role of various recycled materials in generating the SVZ mantle and implications for along-arc magmatism and geochemical variability. TSPC magmas least impacted by crustal contamination (evolved lavas are filtered out on the basis of > 56 wt.% SiO2 and Rb/Y >1.75) have compositions bounded by three chemically and isotopically distinct mantle-derived end-members. The `prevalent TSPC mantle' end-member, which includes the largest number of analyzed lavas, is interpreted to represent melts of the upper mantle below TSPC that has been modified by long-term subduction. A second end-member shows extreme depletions in incompatible high field strength elements (HFSE) and the lowest concentrations of fluid-immobile incompatible elements, but has the highest aqueous fluid-mobile/immobile element ratios at the volcano (e.g., Sr/Nd and Pb/Ce). The source of these `low HFSE' magmas is `prevalent TSPC mantle' that experienced previous melt extraction, followed by more recent melting due to infiltration of solute-rich fluid from the subducting basaltic Nazca oceanic crust. A third end-member is enriched in incompatible elements and has the lowest Nd-Hf and highest Sr isotope ratios. This `TE enriched' end-member has common chemical characteristics with behind-the-arc basalts, indicating derivation from trace element-enriched behind-the-arc South American mantle that has been advected trenchward into the convecting mantle wedge. Determining the composition and relative input of slab-derived components to the SVZ mantle wedge has remained elusive for the last 2+ decades because of inadequate datasets and the controversial role of crustal contamination within the thick Andean continental crust. The `prevalent TSPC mantle' magmas, which best represent melts of the subduction-modified mantle wedge composition beneath the TSPC, provide important constraints on both the composition of the "pre-subduction" mantle and geochemical modifications by way of subduction. Mass-balance modeling suggests that the source of `prevalent TSPC mantle' magmas has been generated by a two-stage, three component mixing process. Isotopic and trace element evidence indicate that ~7-11% bulk subducted Chilean trench sediment has been added to an `E-MORB-like' pre-subduction mantle composition. This mantle mixture is further infiltrated by ~4% solute-rich fluid derived from the subducted Nazca basaltic oceanic crust. Trace element patterns of end-member `prevalent TSPC mantle' magmas are best fit by a two-stage partial melting model whereby the residual mantle, after a small degree melt extraction (F = 0.1%), undergoes F = 22% partial melting. The high melt fraction appears to potentially correspond with large volumes of solute-rich fluid released from the subducted Mocha Fracture Zone (MFZ). Although geochemical attributes of mafic TSPC magmas suggest that they are all derived from the same general mantle framework operating below the complex, one lava sequence appears to deviate. The mantle origins of the Upper Placeta San Pedro Sequence (UPSPS) have remained elusive since first being studied. It is a well-characterized basaltic lava series that erupted over a short interval at ~235-240 ka with highly variable incompatible element abundances and a large xenocrystic cargo. The new comprehensive chemical and radiogenic isotope (Sr-Nd-Pb-Hf) dataset, along with stratigraphic control and understanding of the larger-scale geochemical variability at the TSPC, provides fresh perspectives about the mantle sources and evolution of UPSPS magmas. While the UPSPS magmas are derived from the same sources as other TSPC magmas, they have undergone a unique petrogenetic evolution. This is evident from decoupled trace element-isotopic trends that are difficult to reconcile with other mafic TSPC magmas. Based on the Nd-Hf isotope ratios and trace element ratios, the two UPSPS unit magmas are derived from depleted-`TE enriched' and `prevalent TSPC mantle' sources. High ratios of aqueous fluid-mobile/immobile elements, such as high Pb/Ce and Sr/Nd, as well distinctive Sr and Pb isotope ratios, indicate that the UPSPS magmas were generated through fluxing of the mantle wedge, already depleted by melt removal by an solute-rich fluid derived from the subducted Pacific oceanic crust as well as the overlying trench sediment, which caused it to melt.
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Volcanic Glass as a Paleoenvironmental Proxy: Comparing Preparation Methods on Ashes from the Lee of the Cascade Range in Oregon, USACarlson, Tessa Boe 06 July 2018 (has links)
Deuterium ratios (δD) of hydrated volcanic glass have been used to reconstruct paleoenvironments, although the reliability and proper sample preparation protocol have been debated. In this study, hydrated volcanic ash samples from the lee of the Cascades were prepared using two separate methods. Method 1 involves sonicating and rinsing samples with hydrochloric acid (HCl) followed by hand-selection of glass shards (125-212µm). Method 2 requires hydrochloric acid (HCl) and hydrofluoric acid (HF) abrasion as well as heavy liquid separation of shards (70-150µm). Method 2 produced more consistent results with decreased intra-replicate variability in both water content (-0.92 wt. %) and deuterium values (-2.5‰ δD). Method 2 δD values of ≥99% isotropic glass were also 2.5-10 % more negative relative to Method 1 values, with an increasing discrepancy with age (3.68-32.66 Ma). Method 2 results suggest volcanic glass did not re-equilibrate with modern water, based on 1) < 2‰ discrepancies between samples of the same ash flow taken from unique sample localities and 2) a ~20‰ difference between samples of different ages (~8 Ma apart) from the same locality. These results support the specified use of HF abrasion and heavy liquid separation on 70-150 µm glass shards to minimize the impact of contaminants on reconstructed paleowater δD values.
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Age, chemistry, and tectonic significance of Easter and Sala y Gomez IslandsClark, James Gregory, 1948- 11 April 1975 (has links)
Easter Island and Sala y Gomez are part of the Sala y Gomez
Ridge, a broad band of high topography and scattered seamounts
extending ESE from the East Pacific Rise. It has been proposed that
the Sala y Gomez Ridge results from the movement of the Nazca Plate
over a fixed melting spot in the mantle. To test this hypothesis volcanic
rocks from Easter Island and Sala y Gomez were analyzed for
their K-Ar ages and major element abundances.
Subaerial Easter Island was constructed in three distinct episodes,
occurring at 2.5 m.y., 0.9 m.y., and 0.4 m.y. ago. The
youngest rocks on the island are the Roiho olivine basalts, and are
probably less than 50,000 years old. Eruptive activity on Sala y
Gomez was essentially contemporaneous with the early volcanism on
Easter Island. No migration of volcanism with time is apparent along
the Sala y Gomez Ridge, thus a major criterion of the melting spot hypothesis is not fulfilled.
Volcanic rocks from Easter Island constitute a tholeiitic differentiation
series; they are chemically similar to those from other
islands situated near mid-ocean rise crests. The wide compositional
spectrum is most likely the result of fractional crystallization from a
basaltic parent liquid, though the data is ambiguous for the highly
silicic differentiates. The youngest basalts possess more alkaline
affinities which are probably not related to fractional crystallization
from the earlier basalts. The alkaline nature of these rocks may be
the result of a downward migration of the fusion zone with time, as
the island moved eastward over a progressively thickening lithosphere.
Volcanic rocks from Sala y Gomez belong to an alkali olivine basalt
series. The fundamental chemical differences between the Easter
Island and Sala y Gomez suites suggest that the two islands were not
derived from a common source, as predicted by the melting spot
hypothesis.
The evidence does not support a melting spot origin for Easter
Island, Sala y Gomez, and the Sala y Gomez Ridge. An alternative
model involving diapiric intrusion and decompression melting of
asthenosphere material along a major fracture in the Nazca Plate
provides a better explanation for the data. Synchronous volcanism
along the eastern extension of the Easter Island transform fault has
given rise to the islands and seamounts on the Sala y Gomez Ridge. / Graduation date: 1975
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Petrology of the reversely zoned Mickey Pass Tuff, west-central NevadaTempleton, Jeffrey H. 03 September 1998 (has links)
Graduation date: 1999
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Fracture and permeability analysis of the Santana Tuff, Trans-Pecos TexasFuller, Carla Matherne, January 1990 (has links) (PDF)
Thesis (M.A.)--University of Texas at Austin, 1990. / Vita. Includes bibliographical references (leaves 96-101).
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The effects of molecular diffusion on groundwater solute transport through fractured tuffWalter, Gary R. January 1985 (has links) (PDF)
Thesis (Ph. D. - Hydrology and Water Resources)--University of Arizona, 1985. / Includes bibliographical references (leaves 182-187).
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Geochemical and isotopic investigation of the rate and pathway of fluid flow in partially-welded fractured unsaturated tuffDavidson, Gregg Randall, January 1995 (has links) (PDF)
Thesis (Ph.D. - Hydrology and Water Resources)--University of Arizona. / Includes bibliographical references (leaves 200-207).
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Perched water in fractured, welded tuff mechanisms of formation and characteristics of recharge /Woodhouse, Elizabeth Gail. January 1997 (has links) (PDF)
Thesis (Ph.D. - Hydrology and Water Resources)--University of Arizona. / Includes bibliographical references (leaves 248-255).
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Geology and petrologic evolution of the silicic to intermediate volcanic rocks underneath Steens Mountain basalt, SE Oregon /Langer, Vera W. January 1992 (has links)
Thesis (M.S.)--Oregon State University, 1992. / Includes map in pocket. Typescript (photocopy). Includes bibliographical references (leaves. Also available on the World Wide Web.
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Surface-subsurface geology of the middle to upper Eocene sedimentary and volcanic rock units, western Columbia County, northwest Oregon /Berkman, Thomas Anthony. January 1990 (has links)
Thesis (M.S.)--Oregon State University, 1990. / Typescript (photocopy). Includes mounted photographs. Includes folded plates in pocket. Includes bibliographical references (leaves 377-396). Also available via the World Wide Web.
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