Isotopic and trace element evidence from lavas, bearing on mantle heterogeneity beneath Kenya

Abstract

Nd, Sr and Pb isotope data, together with new major and trace element data are presented for lavas from northern Kenya. A general trend towards silica saturation and decreasing incompatible element contents is observed from the Miocene to the present day. Significantly, the abundances of different incompatible elements decrease The Nd, Sr and Pb isotope compositions of the basic lavas are similar to those observed on the Atlantic ocean islands. Comparison of the Sm/Nd ratios required to produce the Nd isotope ratios with those observed in the rocks indicates that light rare earth elements (r.e.e.) have probably been added to the source region of the lavas comparatively recently. A model involving recent metasomatism of the subcontinental mantle beneath Kenya, which could account for the correlated silica undersaturation and incompatible element content of the lavas, is proposed.

Magmatism of the Kenya Rift Valley: a review

“…The Sr and Nd isotope systematics of lavas from the rift valley provide evidence that the compositional heterogeneity is at least partly related to one or more chemical fractionations (depletions) in the Proterozoic, and a series of metasomatic events, one of which undoubtedly occurred shortly before and during the Cenozoic activity, others of which may be the result of subduction, accretion and magmatic events related to Archaean, Proterozoic and Panafrican orogenies (Norry et al 1980;Cohen et al 1984;Davies & Macdonald 1987;RehkaÈ mper et al 1997;Dawson & Smith 1988;Rudnick et al 1993;Chesley et al 1999;Burton et al 2000;SpaÈ th et al 2000).…”

“…Generally, the degree of silica-saturation of the basaltic parents is re¯ected in that of the daughter magmas; mildly alkaline basalts are associated with trachytes which are mildly nepheline-or quartz-normative; nephelinitic and basanitic magmas generated phonolitic residua (Sceal & Weaver 1971;Baker et al 1977;Weaver 1977;Brotzu et al 1983;Price et al 1985;Baker 1987;Macdonald et al 1995;Kabeto et al 2001a,b). Isotopic data, where available, indicate that fractional crystallisation was commonly accompanied by usually minor assimilation of crustal rocks (Norry et al 1980;Davies & Macdonald 1987;Macdonald et al 1995;Kabeto et al 2001a,b). Dolerite blocks, containing residual glasses varying from mugearitic to peralkaline phonolitic compositions and thought to have been disrupted from a high-level, sill-like intrusion under the Silali basalt±trachyte caldera volcano (see Fig.…”

Island-arc magma sources: A geochemical assessment of the roles of slab-derived components and crustal contamination.

“…Some con tinental-flood basalts and other less voluminous continental rock types such as kimberlite host magmas (BASU and TATSUMOTO, 1979) are ap parently derived from CHUR, whereas other continental rock types are derived from sources either (a) depleted in Nd relative to Sm (NORRY et al, 1980), or (b) enriched in Nd relative to Sm (e.g. DEPAOLO and WASSERBURG, 1976b) There is a large range of 143Nd/144Nd and $' Sr/86 Sr in IAB (Fig .…”

“…17) indicating that a simple two component mix cannot account for the data array. Independent, complex magma sources NORRY et al (1980) reported Nd, Sr, and Pb isotopic data for a range of lavas from the Eastern Rift, including those samples described by WEAVER (1976-77). The isotopic data for the Emuruangogolak suite deviate markedly from the mantle array (Fig.…”

“…Such a contaminant is not thought to exist beneath the Rift Valley, so NoRRY et al (1980) appealed to contamination by upper-continental crust to explain the eNd-versus-esr characteristics.…”