Untersuchungen zur Geochemie des Bleis

Abstract

After discussing literature on the geochemical distribution of lead, a quantitative spectrochemical method is described (0.01% Bi standard). The sensitivity of 0.0001% Pb is attained by relative concentration of light volatile elements with a double-arc instrument. In this investigation more than 500 analyses for lead of representative samples are discussed (more than 250 own determinations). During the last 3.3 . 10 9 years the total amount of lead of the earth's crust has increased only by 20% from radiogenic sources. The average lead content of micas and potassium feldspars (~0.0025%) surpasses that of the lithosphere (0.0015%). Remarkable amounts of 0.010% Pb are gathered in the feldspars of pegmatites. Olivine and quartz (excepting rose-quartz) have the lowest contents (~0.0001% Pb); those are expected after their structural properties. Lead replaces potassium in its structural positions owing to similar ionic dimensions. In the course of magmatic differentiation the proportions K/Pb and Ba/Pb are increasing, only excepting some late minerals. Adulars, feldspathoids and zeolites are favoured in the replacement of K, Na and Ca. The feldspars contain the main lead content of eruptive rocks. The following figures are average Pb abundances of a great number of samples: granite 0.0019%, granodiorite 0.0015%, diorite 0.0010%, basalt 0.0006%, ultrabasic rocks 0.0003%. Galena seems to be a very rare compound in normal rocks. The behaviour of lead in rock alteration (weathering, mobilization, concentration, etc.) is discussed. Factors for concentration are low solubility (PbS, Bleiphosphat adsorption (clay minerals, Mn hydroxides, ferric hydroxides) and action of organisms (foraminiferae, plants, etc.). The clay minerals contain the bulk of lead in sediments. The relative high contents of Pb, Zn and Cu of pelagic sediments cannot have been derived entirely from continental weathering. It must be assumed that a part is due to volcanic exhalations. The following averages are results of sediment analyses: nonpelagic argillaceous rocks 0.0020%, deep-sea clays (Atlantic Ocean) 0.0043%, deep-sea clays (Pacific Ocean) 0.014%, sandstones 0.0007%, limestones and dolomites 0.0009% Pb. Part of the lead content of metamorphic rocks can be derived from their parent material. It increases from greenschists, amphibolites, granulites (0.0010%), to mica-schists and gneisses (0.0023% Pb). A number of lead analyses from specific minerals of metamorphic rocks are reviewed. Natural environments very rarely permit the formation of Pb ⃜ . The Goldschmidt rules on the relationships between distribution of an element and its ionic dimensions and ionic charge give a sufficient explanation of most geochemical characteristics of lead.