Mathematics – Logic
Scientific paper
Sep 1973
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1973gecoa..37.2099r&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 37, Issue 9, pp.2099-2124
Mathematics
Logic
2
Scientific paper
A regional survey of lead isotope abundances from galena found in the gold mines of the Rhodesian and Tanzanian cratons has been made as an aid to understanding the early geochemical history of the Earth's crust and mantle. In all, isotopic analyses of eightly galenas from Rhodesia and twenty-five galenas from Tanzania were made, most of them at least in duplicate. The Tanzanian results include a group from the older part of the craton lying slightly above the 2700 m.y. point on a conformable growth curve, and a group from the Lupa Goldfield whose ratios indicate a three-stage history. The Rhodesian results can be divided isotopically and geologically into two groups-Que Que-type and Bulawayo-type. Geologically, most of the Que Que-type leads are associated with the oldest volcanics of the Sebakwian Series or nearby ancient (3400 m.y.) banded gneisses and migmatites, whereas the Bulawayo-type leads are almost always found in shear zones and granite stocks in the younger overlying Bulawayan volcanic rocks. Isotopically, they differ in that Que Que-type leads lie on a straight line well above and not touching or intersecting the conformable growth curve. The Bulawayan-type leads fall in a cluster closer to the conformable growth curve. Often the ratios from individual suites of samples containing Bulawayan-type lead gathered from a specific area fall along short, straight lines almost tangent to the growth curve at 2900-2700 m.y. One interpretation of the pattern is: 1. 1. The Que Que-type leads developed initially in a homogeneous environment with a high ( = 9.8) U 238 / Pb 204 ratio, while the Bulawayo-type leads developed in a homogeneous but lower ( = 8.9-9.0) U / Pb environment. The high ratio of 9.8 reflects an early stage of differentiation within the Earth which occurred before an event about 3500 m.y. ago. 2. 2. An event about 3500 m.y. ago transported the Que Que-type leads into a more heterogeneous U 238 / Pb 204 environment. 3. 3. An event about 2700 m.y. ago mineralized the Que Que-type leads. The Bulawayo-type leads were removed at this time from their initial environment and mineralized as well. In some cases, they had experienced a short period of anomalous lead development before being mineralized. This pattern is significantly more complicated than that obtained on an earlier work on the Slave Province, N.W.T., Canada. It neither unequivocally supports nor rules out the 4000 m.y. event discussed in that work which is thought to correspond to the formation of a protocrust. One may conclude from the data that the source of terrestrial (vein) leads before 2700 m.y. ago is not compatible with the source required by the conformable terrestrial growth curve.
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