The proposed high-level radioactive waste repository at Yucca Mountain, Nevada, would be constructed in the high-silica rhyolite (Tptp) member of the Miocene-age Topopah Spring Tuff, a mostly welded ash-flow tuff in the {approx}500-m-thick unsaturated zone. Strontium isotope compositions have been measured in pore water centrifuged from preserved core samples and in leachates of pore-water salts from dried core samples, both from boreholes in the Tptp. Strontium isotope ratios ({sup 87}Sr/{sup 86}Sr) vary systematically with depth in the surface-based boreholes. Ratios in pore water near the surface (0.7114 to 0.7124) reflect the range of ratios in soil carbonate (0.7112 to 0.7125) collected near the boreholes, but ratios in the Tptp (0.7122 to 0.7127) at depths of 150 to 370 m have a narrower range and are more radiogenic due to interaction with the volcanic rocks (primarily non-welded tuffs) above the Tptp. An advection-reaction model relates the rate of strontium dissolution from the rocks with flow velocity. The model results agree with the low transport velocity ({approx}2 cm per year) calculated from carbon-14 data by I.C. Yang (2002, App. Geochem., v. 17, no. 6, p. 807-817). Strontium isotope ratios in pore water from Tptp samples from horizontal boreholes collared in tunnels at the proposed repository horizon have a similar range (0.7121 to 0.7127), also indicating a low transport velocity. Strontium isotope compositions of pore water below the proposed repository in core samples from boreholes drilled vertically downward from tunnel floors are more varied, ranging from 0.7112 to 0.7127. The lower ratios (<0.7121) indicate that some of the pore water in these boreholes was replaced by tunnel construction water, which had an {sup 87}Sr/{sup 86}Sr of 0.7115. Ratios lower than 0.7115 likely reflect interaction of construction water with concrete in the tunnel inverts, which had an {sup 87}Sr/{sup 86}Sr < 0.709. These low Sr ratios indicate penetration of construction water to depths of {approx}20 m below the tunnels within three years after construction, a transport velocity of {approx}7 m per year. These studies show that construction activities locally may alter the characteristics of the ambient hydrologic system at Yucca Mountain.
Research Article| October 01, 1981 Proterozoic zircon from augen gneiss, Yukon-Tanana Upland, east-central Alaska John N. Aleinikoff; John N. Aleinikoff 1U.S. Geological Survey, Federal Center, Denver, Colorado 80225 Search for other works by this author on: GSW Google Scholar Cynthia Dusel-Bacon; Cynthia Dusel-Bacon 2U.S. Geological Survey, Menlo Park, California 94025 Search for other works by this author on: GSW Google Scholar Helen L. Foster; Helen L. Foster 2U.S. Geological Survey, Menlo Park, California 94025 Search for other works by this author on: GSW Google Scholar Kiyoto Futa Kiyoto Futa 3U.S. Geological Survey, Federal Center, Denver, Colorado 80225 Search for other works by this author on: GSW Google Scholar Author and Article Information John N. Aleinikoff 1U.S. Geological Survey, Federal Center, Denver, Colorado 80225 Cynthia Dusel-Bacon 2U.S. Geological Survey, Menlo Park, California 94025 Helen L. Foster 2U.S. Geological Survey, Menlo Park, California 94025 Kiyoto Futa 3U.S. Geological Survey, Federal Center, Denver, Colorado 80225 Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1981) 9 (10): 469–473. https://doi.org/10.1130/0091-7613(1981)9<469:PZFAGY>2.0.CO;2 Article history First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation John N. Aleinikoff, Cynthia Dusel-Bacon, Helen L. Foster, Kiyoto Futa; Proterozoic zircon from augen gneiss, Yukon-Tanana Upland, east-central Alaska. Geology 1981;; 9 (10): 469–473. doi: https://doi.org/10.1130/0091-7613(1981)9<469:PZFAGY>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract U-Th-Pb analyses of zircons from an ortho-augen gneiss body in the Yukon-Tanana Upland of east-central Alaska yield strong evidence for the presence of early Proterozoic material in this area. U-Pb data define a chord that intersects concordia at about 2,300 and 345 m.y. We consider two interpretations: (1) the protolith was intruded during the Proterozoic and was subsequently metamorphosed in the Paleozoic or, more likely, (2) the protolith was intruded in the Paleozoic and incorporated material of Proterozoic age. An Sm-Nd model age of about 1,900 m.y. on a whole-rock sample of augen gneiss is additional evidence for the presence of Proterozoic material in the gneiss. K-Ar and U-Th-Pb dating of mica and sphene, respectively, reveal that younger thermal events occurred at least as recently as 110 m.y. ago. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
Type-I Cr-diopside ultramafic xenoliths found in the Pleistocene Pali-Aike alkali basalts, the southernmost units of the Patagonian plateau lavas of southern South America, include both garnet-free and garnet-bearing peridotites, the latter uncommon in alkali basalts (Skewes and Stern, 1979).Spinels occur in both the garnet-free and the garnet-bearing xenoliths so that a division between spinel-and garnet-peridotite is not suitable for the Pali-Aike xenoliths.The xenoliths are lithologically diverse, consisting dominantly of Iherzolites and harzburgites, but also including dunites and orthopyroxenites.All the Pali-Aike peridotites have coarse granular textures.
Abstract Geochemical evaluation of the sources and movement of saline groundwater in coastal aquifers can aid in the initial mapping of the subsurface when geological information is unavailable. Chloride concentrations of groundwater in a coastal aquifer near San Diego, California, range from about 57 to 39,400 mg/L. On the basis of relative proportions of major‐ions, the chemical composition is classified as Na‐Ca‐Cl‐ SO 4 , Na‐Cl, or Na‐Ca‐Cl type water. δ 2 H and δ 18 O values range from −47.7‰ to −12.8‰ and from −7.0‰ to −1.2‰, respectively. The isotopically depleted groundwater occurs in the deeper part of the coastal aquifer, and the isotopically enriched groundwater occurs in zones of sea water intrusion. 87 Sr / 86 Sr ratios range from about 0.7050 to 0.7090, and differ between shallower and deeper flow paths in the coastal aquifer. 3 H and 14 C analyses indicate that most of the groundwater was recharged many thousands of years ago. The analysis of multiple chemical and isotopic tracers indicates that the sources and movement of saline groundwater in the San Diego coastal aquifer are dominated by: (1) recharge of local precipitation in relatively shallow parts of the flow system; (2) regional flow of recharge of higher‐elevation precipitation along deep flow paths that freshen a previously saline aquifer; and (3) intrusion of sea water that entered the aquifer primarily during premodern times. Two northwest‐to‐southeast trending sections show the spatial distribution of the different geochemical groups and suggest the subsurface in the coastal aquifer can be separated into two predominant hydrostratigraphic layers.
Two distinct granite plutons occur above the Main Central Thrust north of Uttarkashi in the upper valley of the Bhagirathi River and its source, the Gangotri Glacier, in Garhwal, India. The structurally lower pluton is a biotite granite with mineralogic and major and trace element characteristics similar to late Precambrian to early Paleozoic plutons of the Northern and Lesser Himalayan belts of Indian shield granites. The structurally higher pluton, intruded into the Martoli Formation and Vaikrita Group of the Tethyan sedimentary rocks, is an aluminous S-type muscovite-tourmaline leucogranite similar to other Cenozoic High Himalayan leucogranites with respect to its mineralogy and major element chemistry, as well as its high concentrations of Rb, Cs, and U, combined with low concentrations of Sr, Zr, Th, and rare-earth elements. Whole-rock Rb-Sr data for five samples from the main leucogranite define two clusters on an isochron of 64 ± 11 Ma. This isochron may reflect (1) a significant age, (2) variations of initial 87Sr/86Sr in the magma at the time of crystallization, or (3) multiple pulses of magma with different isotopic compositions. An Rb-Sr mineral isochron for one of the leucogranite samples yields an age of 21.1 ± 0.9 Ma, whereas a K-Ar age on a muscovite separate from the same rock yields an age of 18.9 ± 1.3 Ma.
It is commonly accepted that beneath the continental crust lies a keel of lithospheric mantle, which extends 50–200 kilometres downward to a transition zone into the asthenosphere. The chemical and physical properties of this reservoir are best known through studies of the basalts and xenoliths that provide samples of the subcrustal mantle. Although sharing many characteristics with oceanic island basalts, some continental basalts become increasingly distinct isotopically as crustal age increases, strongly supporting a permanent association between crust and mantle. Consequently, the distinctive trace element and isotope composition of the lithospheric mantle is able to give important clues to its origin and evolution. The mantle under newly‐created crust is typified by a radiogenic isotope variability that emphasizes the materials from which the continental lithosphere is assembled. Old lithospheric mantle, on the other hand, exhibits more evolved isotopic patterns that attest to the existence of long‐lived, chemically complex systems. A comparison of the Pb, Sr and Nd isotopes in alkalic to sub‐alkalic basalt derived from Phanerozoic (Patagonia) and Middle Archaean to Early Proterozoic (eastern China) subcrustal mantle is useful for identifying 'end‐member' components of the lithosphere. One component, having an isotopic composition close to PREMA, either continues to evolve virtually unchanged after incorporation into the lithosphere or is, itself, a relatively new addition even to old lithosphere. Another component, beginning with the isotopic composition of BSE, undergoes significant reduction in U/Pb and Sm/Nd (but not Rb/Sr) upon incorporation into the lithosphere and, with time, shows an increasingly retarded evolution of 206Pb/204Pb and negative εNd‐values approaching the isotopic composition of EMI. Five models are discussed that relate the isotopic composition of the continental lithospheric mantle to that of other parts of the terrestrial system, which may be involved in its origin and evolution. The potential locations of the contributing components and the mechanisms and timing of their assembly into lithosphere are considered. Current knowledge, however, does not allow us to distinguish unequivocally among the various scenarios for the creation and evolution of this reservoir. Key words: basaltscontinental lithosphereeastern ChinaPatagoniaSrNd and Pb isotopes
In this article, we compare chemical ( 87 Sr/ 86 Sr and elemental) analyses of archaeological maize from dated contexts within Pueblo Bonito, Chaco Canyon, New Mexico, to potential agricultural sites on the periphery of the San Juan Basin. The oldest maize analyzed from Pueblo Bonito probably was grown in an area located 80 km to the west at the base of the Chuska Mountains. The youngest maize came from the San Juan or Animas river floodplains 90 km to the north. This article demonstrates that maize, a dietary staple of southwestern Native Americans, was transported over considerable distances in pre-Columbian times, a finding fundamental to understanding the organization of pre-Columbian southwestern societies. In addition, this article provides support for the hypothesis that major construction events in Chaco Canyon were made possible because maize was brought in to support extra-local labor forces.
As part of the paleohydrology study of the Yucca Mountain Project, strontium-isotope analyses of carbonate deposits, ground water, and major rock reservoirs of strontium are in progress. This paper presents a summary of the strontium-isotope data obtained through 1989. Calcium carbonate is ubiquitous in the vicinity of Yucca Mountain, where it occurs as pedogenic horizons and rhizoliths, small veins and fracture fillings in Tertiary volcanic rocks, large veins and masses along faults, and freshwater and marine limestones. With the exception of marine limestones, which are Paleozoic, the calcium carbonate has been precipitated directly from water during the past 10 7 years. This paper reports strontium-isotope compositions of the following carbonate groups: 1) limestones of the Paleozoic basement, 2) calcite-silica veins, 3) small calcite veins, 4) pedogenic carbonate deposits, and 5) spring deposits (i.e., tufa). The authors have also analyzed the strontium from samples of Tertiary volcanic rocks and from ground water.
'/%3e%3c/defs%3e%3cpath fill='%23012169' d='M0 0h10080v5040H0z'/%3e%3cpath stroke='%23fff' stroke-width='.6' d='m0 0 6 3m0-3L0 3' clip-path='url(%23b)' transform='scale(840)'/%3e%3cpath stroke='%23e4002b' stroke-width='.4' d='m0 0 6 3m0-3L0 3' clip-path='url(%23c)' transform='scale(840)'/%3e%3cpath stroke='%23fff' stroke-width='840' d='M2520 0v2520M0 1260h5040'/%3e%3cpath stroke='%23e4002b' stroke-width='504' d='M2520 0v2520M0 1260h5040'/%3e%3cg fill='%23fff'%3e%3cuse xlink:href='%23d' x='2520' y='3780'/%3e%3cuse xlink:href='%23a' x='7560' y='4200'/%3e%3cuse xlink:href='%23a' x='6300' y='2205'/%3e%3cuse xlink:href='%23a' x='7560' y='840'/%3e%3cuse xlink:href='%23a' x='8680' y='1869'/%3e%3cuse xlink:href='%23e' x='8064' y='2730'/%3e%3c/g%3e%3c/svg%3e)