V.M. Oversby

Lead isotopic systematics and ages of Archaean acid intrusives in the Kalgoorlie-Norseman area, Western Australia

Abstract Lead-lead mineral isochrons consisting of K-feldspar + plagioclase + total rock have been determined for eight intrusive bodies in the Kalgoorlie-Norseman area of Western Australia. The ages found range from 2630 to 2760 m.y. Variation of initial lead isotopic composition within plutons has been identified in two cases. For the remaining plutons, whole rock lead-lead isochrons give the same age as the mineral isochrons. Attempts to determine uranium-lead ages reveal large-scale loss of uranium from surface rocks in recent times. The initial lead isotopic composition of the intrusions, as estimated from age corrected K-feldspar data, shows that the source regions for the plutonic magmas must have experienced multistage histories prior to magma generation. The time integrated μ value for the source region in all cases exceeds the mean μ value for the Earth. Multistage model calculations show that continental type rocks must have been present in the Norseman area as early as 3300 m.y. ago. Original formation of continental crust near Kalgoorlie either occurred much later than at Norseman, or consisted of more basic rock types.

Isotopic ages and geochemistry of Archaean acid igneous rocks from the Pilbara, Western Australia

Abstract Lead isotopic ages were determined for seven localities of gneissic granite and granodiorite from the Pilbara Region of Western Australia. For four of the localities Rb-Sr ages were also measured. In the lead isotopic system all localities showed some evidence of post-emplacement disturbance; lead redistribution varied from very slight effects to complete equilibration of K-feldspars during metamorphism. In one case, lead and Rb-Sr ages agreed within experimental error; in two cases, Rb-Sr mineral ages were younger than Pb-Pb ages, and in one case, the Rb-Sr age was intermediate between the primary and metamorphic ages recorded by the Pb isotopic system. Four localities show evidence of metamorphism at 2950 my. It is suggested that this represents the time of formation of the granite dome structures in the southeastern Pilbara. At least two of these localities were also affected by post-tectonic metamorphism (2600–2770 my). The other three localities show a more extended history of events starting at about 2900 my and ending between 2000 and 2250 my. The younger metamorphic ages are interpreted to record a thermal event at the time of outpouring of the Proterozoic Fortescue Group which formerly overlay the Archaean rocks.

The behaviour of lead isotopes during granulite facies metamorphism

Abstract The U-Pb decay scheme can give the age of granulite facies metamorphism accompanied by the depletion of U, because of its sensitivity to changes in U/Pb ratio. Model Pb ages, which are generally reliable to within 100 Myr in the period considered, allow the dating of these U-deficient systems. Fractionation-corrected Pb isotopic compositions for central Australian granulites and their separated potassium feldspars were determined. Initial Pb for both rocks and feldspars has a model age which approximates one of two ages given by the Rb-Sr total rock method; the age is that of the granulite facies metamorphism. Isotopic data and bulk analyses demonstrate the depletion of U in all rocks, but in contrast show that Th is erratically depleted. The essentially single-stage nature of the Pb means that premetamorphic fractionation of U from Pb is not detected.

Genetic relations among the volcanic rocks of Réunion: chemical and lead isotopic evidence

Abstract Lead isotopic compositions and major element chemical data are presented for volcanic rocks from the younger Oceanite Series and Differentiated Series of volcanic rocks from the island of Reunion, Indian Ocean. Several lead isotopic domains are identified, in contrast to the uniform strontium isotopic composition previously found for the island. The lead domains represent predominantly horizontal heterogeneity in the U Pb ratio of the mantle under Reunion. The Differentiated Series lavas have chemical characteristics which are transitional from hypersthene normative in the older units to nepheline normative in the younger lavas. Although the differentiated lavas underwent extensive low pressure crystal fractionation, it is inferred that the parental magmas were also of transitional nature, and that parental magmas of the older members of the Differentiated Series were derived by a slightly greater degree of partial melting than parental magmas for the younger members.

Potassium distribution between metal and silicate and its bearing on the occurrence of potassium in the earth's core

Abstract The hypothesis that a large fraction of the earths potassium is present in an Fe-FeS core is discussed. Data on the distribution of potassium in meteorites, and experimental work on potassium distribution between silicate and metal-sulfide phases limit the amount of potassium in the core to a very small percentage of the total potassium in the earth.

Lead isotopic study of aplites from the Precambrian basement rocks near Ibadan, Southwestern Nigeria

Abstract Pb isotopic compositions for three total-rock samples of aplite and their constituent K-feldspars from the Nigerian basement assemblage near Ibadan show lead homogenization during the Pan-African thermo-tectonic event. A secondary isochron formed by the K-feldspars data points is used to calculate a primary age of about 2750 m.y. for the aplites. The aplites do not register any Pb isotopic effect from the intrusion of granite gneiss in the area at 2330 m.y.

Lead isotopes in Archaean Plutonic rocks

Abstract Archaean intrusive rocks have initial Pb isotopic compositions which show a varied and complex history for the source regions of the rocks. Even the oldest rocks from Greenland indicate heterogeneous U and Pb distribution prior to 3800 m.y. ago. Source regions with μ values less than 7 must have played a significant role in the early history of the earth. By late Archaean time U/Pb ratios of source regions had increased substantially. Data from Australia and North America show distinct regional differences, both within and between continents.

Lead in oceanic islands : Faial. Azores and Trindade.

Abstract Lead isotopic analyses of three basaltic rocks from the 1958 Capelinhos eruption of Faial, Azores, show a uniform isotopic composition. The 207 Pb/ 204 Pb ratio of 15.65 is high relative to the 206 Pb/ 204 Pb of 19.24, placing the composition on the upper edge of the band of compositions observed from other oceanic islands. Chemical and isotopic analyses are given for three phonolites and two basic rocks from the island of Trindade. The rocks are all much more undersaturated than volcanics from other islands in the Atlantic Ocean, probably indicating that they were derived from hydrous mantle. The internal isotopic pattern has a slope in the 206 Pb/ 204 Pb vs 207 Pb/ 204 Pb plot of 0.086, which for a two stage model gives a fractionation time of 1.4 × 10 9 yr. This slope is the same as the composite slope of the lead isotope composition band from all oceanic islands.

Redetermination of lead isotopic composition in Canyon Diablo troilite

Abstract Redetermination of the lead isotopic composition in Canyon Diablo troilite shows that the original analysis was overcorrected for analytical fractionation. The new analysis indicates that the lead in nodule CDN-1 is a mixture of primordial lead plus a small amount of common lead.

ISOTOPIC ANALYSIS OF MICROGRAM QUANTITIES OF LEAD IODIDE USING AN ELECTRON- BOMBARDMENT SOURCE MASS SPECTROMETER.

A method for routine isotopic analysis of small amounts of lead using PbI2+ ions in a modified commercial gas-source mass spectrometer has been developed. Samples as small as 2 μg of lead have been analyzed successfully. Mass fractionation averages + 0.06% per mass unit with maximum observed fractionation of + 0.13% per mass unit. Chemical separation of lead by a standard anion exchange column produces samples of sufficient purity for mass spectrometry by PbI2.