Raymond L. Walker

Age and origin of the Stone Mountain Granite, Lithonia district, Georgia

Petrographic, geochemical, and strontium isotopic studies have been conducted on samples of the Stone Mountain Granite (leucocratic quartz monzonite) from the Lithonia district, within the Inner Piedmont province of Georgia. A Rb-Sr isochron obtained from 10 whole-rock and 3 mineral samples yielded an age of 291 ± 7 m.y. and an initial Sr 87 Sr 86 ratio of 0.7250 ± 0.0005. Petrologic and geochemical data, combined with the high initial strontium isotopic ratio, suggest that the origin of this peraluminous quartz monzonite can best be explained by the anatexis of an older peraluminous, granitic crustal material during Late Pennsylvanian time. The depth of intrusion was probably around 12 km, with initial magma temperatures of 700°C or less. Magma generation could have occurred at depths of 22 to 28 km, depending on the regional geothermal gradient at the time. The most likely source material is the Lithonia Gneiss, which has a peraluminous, granitic composition and underlies the area. The Stone Mountain pluton extends the 300-m.y.-old intrusive belt in the southern Appalachians across the Piedmont province to just south of the Brevard lineament. Its implied depth of intrusion fits well with certain models for the development of the Inner Piedmont and Blue Ridge provinces in the Carolinas because it requires rapid removal of considerable overburden. The origin and magmatic history of this intrusive unit may find use as a model for other peraluminous granitic plutons having similar petrologic, geochemical, and isotopic characteristics.

Uranium series dating of stalagmites from Blanchard Springs Caverns, U.S.A.

Abstract Because stalagmites are enriched in 234U beyond the equilibrium value at the time of deposition, they are potentially datable by measuring their progress toward equilibrium. Procedures for uranium-thorium and uranium disequilibrium dating of speleothems were developed, making use of alpha spectrometry for thorium determinations and mass spectrometry for uranium determinations. The stalagmites tested were generally beyond the age range of the U-Th age dating method and, as a result, 234U/238U ratios provided the most useful dating technique. The estimated age range for a stalagmite from Blanchard Springs is 135,000 to 785,000 years BP with an interval of slow or suspended growth from about 320,000 to 570,000 years BP.

Early Cretaceous basement rocks in Hispaniola

A medium-grained, foliated amphibolite found in the Cordillera Central of the Dominican Republic near the Haitian border yields concordant K-Ar ages of 123 ± 2 m.y. for hornblende and plagioclase. The rock exhibits a low Rb/Sr ratio (0.0217) and present-day 87Sr/86Sr ratio (0.7024) but differs in chemical composition from metamorphosed mafic rocks of the Duarte Formation. A previously reported K-Ar age of 127 ± 6 m.y., obtained on a hornblendite body from the Duarte Formation, agrees closely with the amphibolite age. The hornblendite also differs in composition from the metamorphosed mafic rocks of the Duarte Formation and from the amphibolite. The 123- to 127- m.y. ages are interpreted to represent a minimum date for a metamorphic event that affected the basement complex of Hispaniola. An unpublished K-Ar date for the Bermeja complex in Puerto Rico falls within this range, but older ages have been obtained from rocks in Cuba. A date of about 120 m.y. indicated by one interpretation of a previously reported conformable lead isotopic composition for galena from the Pueblo Viejo gold deposit in the upper part of the Los Ranches Formation volcanic rocks in the Dominican Republic also agrees closely with this 123- to 127-m.y. range; together with other data, it suggests that the extrusion of Los Ranchos volcanic rocks and metamorphism of the Duarte Formation could be genetically related.

A Bulk Resin Bead Procedure To Obtain Uranium And Plutonium From Radioactive Solutions For Mass Spectrometric Analysis

A method is described for extracting representative uranium and plutonium samples from highly radioactive solutions for isotopic mass spectrometric analysis. Anion resin beads in the nitrate form are used to effect separation from fission products and other actinides. Conditions required to achieve separation are proper adjustment of the uranium and nitric acid concentrations. Once uranium and plutonium are adsorbed, each bead serves as a sample for mass spectrometric analysis, with plutonium and uranium being run sequentially from the same bead. Quantitative determination of the two elements is effected through the technique of isotopic dilution.

Rb-Sr Whole-Rock Age of the Siloam Granite, Georgia: A Permian Intrusive in the Southern Appalachians

The Siloam granite crops out in the southern half of Greene County, Georgia, within the Outer Piedmont. The granite is similar to the coarsely porphyritic postmetamorphic granites of the Charlotte Belt of North Carolina and South Carolina. The Rb-Sr whole-rock date of the Siloam granite is 269 ± 3 m.y., making this intrusive the youngest of the southern Appalachian region. Its low initial 87Sr/86Sr ratio of 0.7052 ± 0.0001 suggests an upper mantle affinity with no appreciable crustal contamination during emplacement.

Early island-arc intrusive activity, Cordillera Central, Dominican Republic

Reconnaissance studies of early island-arc intrusions in the Cordillera Central of the Dominican Republic demonstrate that these rocks are mainly hornblende tonalite with lesser amounts of hornblende diorite, quartz diorite, granodiorite and quartz monzonite. Two plutons (El Bao, Medina) are petrographically and chemically homogeneous, whereas two others (El Rio and Loma de Cabrera) are compositionally heterogeneous. Samples from these intrusions range in SiO2 from 49 to 70% with most rocks in the 59 to 62% range. K2O ranges from 0.24 to 3% and averages 1.2%. Cu, Zn, Co, Ni, V and possibly Cr decrease with increasing SiO2. Rb/Sr values for the intrusions are low but variable. Present-day 87Sr/86Sr values range from 0.7031 to 0.7045 for the El Bao and Loma de Cabrera batholiths and 0.7033 to 0.7091 for the Medina stock. These data do not generate isochrons. The Cordillera Central tonalite intrusions are the most abundant plutonic rock type in the Greater Antilles, although small, younger granodiorite and quartz monzonite stocks are present. The Cordillera Central intrusions are lower in SiO2, K2O, Rb, and Sr than the average composition of the Sierra Nevada batholith, but they are similar to the tonalites and trondjhemites from the western margin of the Sierra Nevada batholith. The low Rb/Sr ratios and low initital Sr87/Sr86 ratios for the Cordillera Central intrusions combined with the high liquidus temperatures required for the generation of tonalite magmas strongly favor a subcrustal source for these magmas in an island-arc setting.

Radiohalos in Coalified Wood: New Evidence Relating to the Time of Uranium Introduction and Coalification

The discovery of embryonic halos around uranium-rich sites that exhibit very high 238U/206Pb ratios suggests that uranium introduction may have occurred far more recently than previously supposed. The discovery of 210Po halos derived from uranium daughters, some elliptical in shape, further suggests that uranium-daughter infiltration occurred prior to coalification when the radionuclide transport rate was relatively high and the matrix still plastically deformable.

Analysis of Resin-Bead Loaded U And PU Samples with a Single-Stage Mass Spectrometer

Abstract The resin bead sample loading technique has been applied to a conventional single-stage mass spectrometer equipped with pulse counting. Isotopic data obtained for U and Pu are comparable to those obtainable from similar instruments using conventional techniques.

Determination of the abundance of krypton in the earth's atmosphere by isotope dilution mass spectrometry☆

The natural abundance of elemental krypton in earths atmosphere has been redetermined by mass spectrometry using isotope dilution with highly enriched 36Kr. Surface air was collected in evacuated tanks of calibrated volume, spiked with a known amount of krypton isotope, and a small volume of the sample processed to recover enough krypton for determination of its isotopic composition in a static mass spectrometer. Ten determinations made over a period of seven months give a result of 1.1074±0.0083 ppm by volume. An error analysis is included.

Isotope Dilution Resin Bead Mass Spectrometry—An Ultra Trace Technique for Measuring Nuclides in Three-Mile Island Water

Abstract From the onset of the Three-Mile Island reactor incident (March 28, 1979), the distinct possibility existed that actinide and fission product pollutants could escape into the surrounding environment. Experienced analytical chemists in command of techniques which achieve the highest sensitivity were called upon for assistance in performing analyses. Cooling water was analyzed for uranium and plutonium, certain fission products, additives, and possible corrosion products by isotope dilution mass spectrometry. In particular, the thermal emission isotope dilution technique, in which an anion exchange resin bead was used to concentrate the uranium and plutonium, provided technical information on samples orders of magnitude smaller than those necessary for conventional counting techniques. The resin-bead-loaded sample acts as a point source in a pulse counting two-stage high abundance sensitivity mass spectrometer, enhancing the sensitivity so that levels as low as 10−13 plutonium and 10−11M uranium co...