K, Rb, Cs, Sr and Ba contents and Sr isotope ratios of ocean floor basalts

Abstract

Reliable analyses of K, Rb and Cs in dredge basalts require samples which show no petrographic evidence of alteration minerals and which have H 2 O + contents less than 0.7 %. Very fresh glass samples usually have H 2 O + levels of 0.1 to 0.2 %, and this probably represents the primary level of H 2 O in most submarine basalt magmas. A dredge haul containing both basalt and andesite was studied for major element and trace element variations. The major elements were consistent with a differentiation model involving crystallization of plagioclase, olivine, clinopyroxene and titanomagnetite. This differentiation had little effect on Sr concentration and on K/Rb and K/Cs ratios; these parameters are thus especially useful in studying mantle chemistry and partial melting processes. Twenty-eight unaltered dredge basalts were analysed, with K/Rb ratios varying from 360 to 1350. K contents of most samples, after correction for high level (shallow) differentiation processes, fall in the range 500 to 1200 parts/10 6 . A comparison of unaltered basalts from ‘fast-spreading ridges’ and ‘slowspreading ridges’ shows that K, Rb, Cs and Sr contents and K/Rb, K/Cs and Rb/Sr ratios are identical for both environments, while Ba contents and 87 Sr/ 86 Sr ratios may be significantly different. Thus mantle chemistry appears to be largely decoupled from the dynamic processes of plate movement. It is shown that submarine ridge basalts have lower 87 Sr/ 86 Sr ratios than the basalts of the oceanic islands, suggesting an early depletion of parts of the mantle in the dispersed elements. Average values for 15 samples from 11 different ridge localities: K = 1160 parts/10 6 ; Rb = 1.11 parts/ 10 6 ; Cs = 0.016 parts/10 6 ; Sr = 135 parts/10 6 ; Ba = 10 parts/10 6 ; K/Rb = 1060; K/Cs = 70000; Rb/Sr = 0.0082; 87 Sr/ 86 Sr = 0.70265.