A reconnaissance study of oxygen, hydrogen and strontium isotopes in geochemically diverse lakes, Western Nebraska, USA

Reconnaissance ¹⁸O,, D, and ⁸⁷Sr data for fifteen lakes in the Western Lakes Region of the Sand Hills of Nebraska indicate dynamic hydrologic systems. The rather narrow range of ⁸⁷Sr from lake water (1.1 to 2.1) and groundwater (0.9 to 1.7) indicates that the groundwater is generally unradiogenic. Groundwater residence times and relatively unradiogenic volcanic ash within the dune sediments control the ⁸⁷Sr values. Based on the mutual variations of ¹⁸O and D, the lakes can be divided into three groups. In Group 1, both ¹⁸O and D values increase from spring to fall. The ¹⁸O and D values in Group 2 decreased from spring to fall. Group 3 are ephemeral lakes that went dry some time during 1992. The data and isotopic modeling show that variations in the ratio of evaporation relative to groundwater inflow, local humidity conditions, and the _a has substantial influence on the isotopic composition. In addition, isotopic behavior in ephemeral lakes can be rather unusual because of the changing activities of water and mineral precipitation and redissolution. The annual and interannual isotopic variability of these lakes which is reflected in the paleonvironmental indicators may be the rule rather than the exception in these types of systems.     

High-temperature deformation during continental-margin subduction & exhumation: The ultrahigh-pressure Western Gneiss Region of Norway

A new dataset for the high-pressure to ultrahigh-pressure Western Gneiss Region allows the definition of distinct structural and petrological domains. Much of the study area is an E-dipping homocline with E-plunging lineations that exposes progressively deeper, more strongly deformed, more eclogite-rich structural levels westward. Although eclogites crop out across the WGR, Scandian deformation is weak and earlier structures are well preserved in the southeastern half of the study area. The Scandian reworking increases westward, culminating in strong Scandian fabrics with only isolated pockets of older structures; the dominant Scandian deformation was coaxial E–W stretching. The sinistrally sheared Møre–Trøndelag Fault Complex and Nordfjord Mylonitic Shear Zone bound these rocks to the north and south. There was moderate top-E, amphibolite-facies deformation associated with translation of the allochthons over the basement along its eastern edge, and the Nordfjord–Sogn Detachment Zone underwent strong lower amphibolite-facies to greenschist-facies top-W shearing. A northwestward increase in exhumation-related melting is indicated by leucosomes with hornblende, plagioclase, and Scandian sphene. In the western 2/3 of the study area, exhumation-related, amphibolite-facies symplectite formation in quartzofeldspathic gneiss postdated most Scandian deformation; further deformation was restricted to slip along biotite-rich foliation planes and minor local folding. That the Western Gneiss Region quartzofeldspathic gneiss exhibits a strong gradient in degree of deformation, implies that continental crust in general need not undergo pervasive deformation during subduction.     

A Rubidium-Strontium study of the Twilight Gneiss,West Needle Mountains,Colorado

The Precambrian trondhjemitic Twilight Gneiss (Twilight Granite of Cross and Howe, 1905b) of the West Needle Mountains, southwestern Colorado, and its interlayered amphibolite and metarhyodacite yield a Rb-Sr isochron of 1,805±35 m.y. A low initial Sr⁸⁷/Sr⁸⁶ ratio of 0.7015 implies that metamorphism of these rocks to amphibolite facies took place soon after their emplacement. The mild metamorphism of Uncompahgran age, prior to 1,460 m.y. ago, and Laramide volcanism did not affect the Rb-Sr system in the Twilight. Rb contents of 26.5 to 108 ppm, Sr contents of 114 to 251 ppm, and K₂O percentages of 1.23 to 3.64 in the Twilight Gneiss, in conjunction with high K/Rb ratios and the low initial ratio of Sr⁸⁷/Sr⁸⁶, lend support to geologic data that suggest the Twilight originated as volcanic or hypabyssal igneous rocks in a basaltic volcanic pile.Publication authorized by the Director, U.S. Geological Survey.     

Bimodal tholeiitic—dacitic magmatism and the Early Precambrian crust

Interlayered plagioclase-quartz gneisses and amphibolites from 2.7 to more than 3.6 b.y. old form much of the basement underlying Precambrian greenstone belts of the world; they are especially well-developed and preserved in the Transvaal and Rhodesian cratons. We postulate that these basement rocks are largely a metamorphosed, volcanic, bimodal suite of tholeiite and high-silica low-potash dacite—compositionally similar to the 1.8-b.y.-old Twilight Gneiss — and partly intrusive equivalents injected into the lower parts of such volcanic piles.We speculate that magmatism in the Early Precambrian involved higher heat flow and more hydrous conditions than in the Phanerozoic. Specifically, we suggest that the early degassing of the Earth produced a basaltic crust and pyrolitic upper mantle that contained much amphibole, serpentine, and other hydrous minerals. Dehydration of the lower parts of a downgoing slab of such hydrous crust and upper mantle would release sufficient water to prohibit formation of andesitic liquid in the upper part of the slab. Instead, a dacitic liquid and a residuum of amphibole and other silica-poor phases would form, according to Green and Ringwood's experimental results. Higher temperatures farther down the slab would cause total melting of basalt and generation of the tholeiitic member of the suite. This type of magma generation and volcanism persisted until the early hydrous lithosphere was consumed.An implication of this hypothesis is that about half the present volume of the oceans formed before about 2.6 b.y. ago.     

Geochemical and C, O, Sr, and U-series isotopic evidence for the meteoric origin of calcrete at Solitario Wash, Crater Flat, Nevada, USA

Calcite-rich soils (calcrete) in alluvium and colluvium at Solitario Wash, Crater Flat, Nevada, USA, contain pedogenic calcite and opaline silica similar to soils present elsewhere in the semi-arid southwestern United States. Nevertheless, a ground-water discharge origin for the Solitario Wash soil deposits was proposed in a series of publications proposing elevation-dependent variations of carbon and oxygen isotopes in calcrete samples. Discharge of ground water in the past would raise the possibility of future flooding in the unsaturated zone at Yucca Mountain, Nevada, site of a proposed high-level nuclear waste repository. New geochemical and carbon, oxygen, strontium, and uranium-series isotopic data disprove the presence of systematic elevation-isotopic composition relations, which are the main justification given for a proposed ground-water discharge origin of the calcrete deposits at Solitario Wash. Values of δ¹³C (−4.1 to −7.8 per mil [‰]), δ¹⁸O (23.8–17.2‰), ⁸⁷Sr/⁸⁶Sr (0.71270–0.71146), and initial ²³⁴U/²³⁸U activity ratios of about 1.6 in the new calcrete samples are within ranges previously observed in pedogenic carbonate deposits at Yucca Mountain and are incompatible with a ground-water origin for the calcrete. Variations in carbon and oxygen isotopes in Solitario Wash calcrete likely are caused by pedogenic deposition from meteoric water under varying Quaternary climatic conditions over hundreds of thousands of years.     

Strontium isotope systematics of mixing groundwater and oil-field brine at Goose Lake in northeastern Montana,USA

Groundwater, surface water, and soil in the Goose Lake oil field in northeastern Montana have been affected by Cl⁻-rich oil-field brines during long-term petroleum production. Ongoing multidisciplinary geochemical and geophysical studies have identified the degree and local extent of interaction between brine and groundwater. Fourteen samples representing groundwater, surface water, and brine were collected for Sr isotope analyses to evaluate the usefulness of ⁸⁷Sr/⁸⁶Sr in detecting small amounts of brine. Differences in Sr concentrations and ⁸⁷Sr/⁸⁶Sr are optimal at this site for the experiment. Strontium concentrations range from 0.13 to 36.9 mg/L, and corresponding ⁸⁷Sr/⁸⁶Sr values range from 0.71097 to 0.70828. The local brine has 168 mg/L Sr and a ⁸⁷Sr/⁸⁶Sr value of 0.70802. Mixing relationships are evident in the data set and illustrate the sensitivity of Sr in detecting small amounts of brine in groundwater. The location of data points on a Sr isotope-concentration plot is readily explained by an evaporation-mixing model. The model is supported by the variation in concentrations of most of the other solutes.