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前言铷是一个碱金属,它的离子半径(1.48(?))与钾的离子半径(1.33(?))十分相近,因此它可以取代所有含钾矿物中的钾离子。Shaw(1968)曾经建立了一个有关各类火成岩钾和铷变化的“主要趋势”(MT)(图1),其中K含量的变化范围是0.01—10%,K/Rb比值的相应变化范围是433—195。在一组同源岩浆的花岗质岩石中,铷在残余岩浆中的富集程度要高于钾,因此随岩浆分异作用的进行,K/Rb比值将随钾的增加而减小。但是,在许多前寒武纪的麻粒岩中,随钾的减少K/Rb比值则有急剧的增长(Tarney & Windley,1977)。这说明,在麻粒岩的形成过程中,随着钾的丢失,铷有更大程度的丢失,因而在麻粒岩中往往有低的Rb含量和高的K/Rb比值(Heier, 1973; Green et al., 1972; Sheraton et al. , 1973; Field&Clough, 1976; Tarney & Windley, 1977),K/Rb比值高者往往可以超过1000。在其它类型岩石中,仅洋脊玄武岩有这么高的K/Rb比值(Shaw,1968)。但是麻粒岩中的高K/Rb
Introduction Rubidium is an alkali metal whose ionic radius (1.48 (?)) Is very similar to the ionic radius of potassium (1.33 (?)), So it can replace potassium in all potassium-containing minerals. Shaw (1968) has established a “major trend” (MT) of changes in potassium and rubidium in various igneous rocks (Figure 1), where the K content ranges from 0.01 to 10% and the K / Rb ratio varies from 433-195. In a group of granitic rocks with the same magma, the enrichment of rubidium in the residual magma is higher than that of potassium. Therefore, with the differentiation of magma, the K / Rb ratio will decrease with the increase of K. However, in many Precambrian granulites, the K / Rb ratio decreases sharply with decreasing K (Tarney & Windley, 1977). This indicates that during the formation of granulites, there is a greater loss of rubidium with the loss of potassium, and therefore there is often a low Rb content and a high K / Rb ratio in granulites (Heier, 1973; Green et al., 1972; Sheraton et al., 1973; Field & Clough, 1976; Tarney & Windley, 1977). K / Rb ratios can often exceed 1000. Of the other types of rock, only the ridge basalts have such a high K / Rb ratio (Shaw, 1968). But the high K / Rb in granulite