Data of good quality is necessary to further the study of rock lead isotopic interpretations. An internal standard (double-spike) was employed to correct for fractionation accompanying the single filament analyses, and lead isotope ratios with a standard deviation of 0.15% were obtained. Lead sulphide mounted on tantalum was employed for the lead analyses. Discrimination in the analyses using tantalum filaments was consistent with the theoretical fractionation laws. This was not found to be the case for rhenium. Lead and uranium analyses from the Vogt-Hobbs area near Lake Timagami, Ontario and from the Rice Lake-Beresford Lake Area, Manitoba revealed variations from a simple two stage model that could not be accounted for by experimental error. Interpretation of the data yields a three stage history for both of these regions, marked by major events at approximately 3400 and 2600 my ago. This earlier crustal history is supported by ore lead data from the vicinity. Both of the suites have a lower average value of (U238/Pb204) for the second stage, of their three stage history, than is characteristic of the source of single stage leads. Variations from closed system behaviour for the Ontario samples is attributed to lead remobilization at the time of the Grenville event. Remobilization of the lead tends to mask the 2600 my old event. The Vogt-Hobbs area is characterized by more acidic igneous rocks and more radiogenic lead than the Rice Lake-Beresford Lake area. These differences are thought to represent the environment 2600 my ago or earlier. The Manitouwadge lead analysed by Ostic resembles the lead - 2600 my ago in the Rice Lake-Beresford Lake rock samples. The Rice Lake-Beresford Lake rock samples are shown according to the interpretation presented, to have had a crustal history prior to 2600 my ago. This lends weight to arguments that exclude Manitouwadge from the class of single stage leads used in calculations of the age of the earth. / Science, Faculty of / Earth, Ocean and Atmospheric Sciences, Department of / Graduate

A systematic study of the Knight shift parameters has been carried out on single crystal specimens of tin and cadmium over a range from 1⁰ K to just below the melting points. The line widths observed in cadmium are approximately half the widths quoted by other workers, enabling more precise measurements to be made. These measurements show that the anisotropy of the Knight shift in cadmium changes sign between 4⁰ K and 77⁰ K. Pronounced changes in the Knight shift parameters are observed upon alloying cadmium with mercury. The temperature dependence of the isotropic Knight shift in tin can be explained by the volume effects and the phonon contributions when the temperature exceeds the Debye temperature. Comparing the temperature dependence data with pressure dependence results of other workers the explicit temperature dependence is extracted. It appears that the variation in the anisotropy of the Knight shift above the Debye temperature is largely caused by a change in lattice parameters, specifically a change in c/a ratio. Measurements of the line width at helium temperature in isotopically pure Sn¹¹⁹ and the same isotope in natural tin as a function of crystal orientation are reported. Second moments for the different crystal orientations have been computed. Analysis of the isotopically pure tin data leads to an evaluation of relative contributions, from the various shells, to the pseudo-dipolar interaction. For the isotopically pure tin, the measurements at the helium and room temperatures enable an evaluation of the spin-lattice relaxation time, T₁. T₁T is constant over the temperature range and closely isotropic. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

Pulsed magnetic resonance studies are reported for a gas of hydrogen atoms at densities of 3-10¹³ X 10 cm⁻³ and temperatures of 4.2-77 K. The gas was produced by dissociation in a room temperature R.F. discharge, and piped through glass tubing into the cryogenic apparatus for study. The magnetic resonance transition observed is between the two lowest hyper-fine levels of the 1s atom in a magnetic field of 6481 Gauss where this splitting has its minimum value of about 765.5 MHZ. At 77 K, spin-exchange broadening of the resonance is observed. By varying the number density of hydrogen atoms, the ratio of spin-exchange broadening to atomic hydrogen density is obtained, and from this ratio the spin-exchange cross section for this transition is calculated. The cross section obtained is 60% of the theoretical value. At liquid helium temperatures, the spin-exchange cross section is shown to be at least 15 times smaller than that at liquid nitrogen temperatures, as. predicted by theory. He⁴ and H₂ buffer gases . are used to limit the diffusion broadening of the resonance, allowing the observation of small frequency shifts of the free induction signal. A model is proposed in which interactions of the hydrogen atoms with the flow tube walls cause these shifts. From the diffusion broadening of the resonance, the diffusion cross sections for H in He⁴ at 4.2 K and H in H₂ at 5-9 K are inferred to be 500 Ų and 250 Ų respectively. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

Short-lived isotope systems can be utilized to track differentiation processes that had occurred during Earth’s early history. Both the 182Hf-182W and 146Sm-142Nd systems are sensitive to silicate fractionation events due to differing parent-daughter incompatibilities. The 182Hf-182W system is also affected by metal-silicate fractionation events due to the siderophile nature of W. An increasing number of mantle-derived rocks, mainly formed during the Archean (4.0-2.5 Ga), have presented variable anomalies in the daughter products of both systems, indicating their sources contain isotope signatures established shortly after the formation of the Earth. Some Archean Fe-rich primitive magmas known as ferropicrites, have been suggested to derive from mantle domains that differentiated after the crystallization of a Hadean (>4.0 Ga) magma ocean. In order to investigate the potential involvement of a Hadean source in the petrogenesis of Archean ferropicrites, we have studied the Nd and W isotopic compositions of rocks from the Pulpwood-Playter Harbour sequence in the Wawa subprovince, Ontario, Canada. This sequence is composed of ferropicritic intrusive rocks and lavas in association with tholeiitic mafic lavas. A 147Sm-143Nd isochron including all lithologies yields an age of 2681±51 Ma (MSWD =6.6) with an initial 143Nd of +2.5. This Nd initial isotopic composition indicates the rocks were derived from a long-term incompatible-element depleted mantle source. Both the ferropicritic and the tholeiitic rocks plot on the same isochron, suggesting they derived from the same mantle source, despite their different geochemical compositions. Negative 142Nd anomalies compared to the Nd terrestrial standard were found in the majority of the rocks studied here, yielding an average μ142Nd value of -2.0±3.9. Although not resolvable from the terrestrial standard given iii the current analytical precision, the fact that almost all analysed samples exhibit negative μ142Nd values could suggest the influence of a Hadean source in their formation. If this is the case, a single early silicate fractionation event occurring between 4.56 and 4.47 Ga could explain both the ε143Nd and μ142Nd values obtained for the studied rocks. Alternatively, the involvement of eclogitic material, with a Hadean basaltic protolith, interacting with Archean peridotitic mantle could explain the Nd isotopic composition of the ferropicrites, but this would not account for the identical isotopic composition of the tholeiites – thus rendering a garnet pyroxenite source improbable. One intrusive ferropicritic sample yielded a resolvable 182W excess of +14.1 ±6.7 ppm. If this excess 182W is characteristic of the Pulpwood-Playter Harbour sequence, it indicates the decoupling of 182Hf-182W and 146Sm-142Nd systems. This decoupling could be explained by early metal-silicate fractionation recorded in the Hf-W systematics of these rocks or the contribution of Fe-rich meteoritic material into the source of ferropicrites.

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