J. Barry Dawson

Enrichment of the continental lithosphere by OIB melts: Isotopic evidence from the volcanic province of northern Tanzania

Alkali basalts and nephelinites from the southern end of the East African Rift (EAR) in northern Tanzania have incompatible trace element compositions that are similar to those of ocean island basalts (OIB). They define a considerable range of Sr, Nd and Pb isotopic compositions (87Sr/86Sr= 0.7035−0.7058,eNd = −5to+3, and206Pb/204Pb= 17.5−21.3), each of which partially overlaps the range found in OIB. However, they occupy a unique position in combined Nd, Sr and Pb isotopic compositional space. Nearly all of the lavas have radiogenic Pb, similar to HIMU with high time-integrated238U/204Pb coupled with unradiogenic Nd (+2 to −5) and radiogenic Sr (>0.704), similar to EMI. This combination has not been observed in OIB and provides evidence that these magmas predominantly acquired their Sr, Nd and Pb in the subcontinental lithospheric mantle rather than in the convecting asthenosphere. These data contrast with compositions for lavas from farther north in the EAR. The Pb isotopic compositions of basalts along the EAR are increasingly radiogenic from north to south, indicating a fundamental change to sources with higher time-integratedU/Pb, closer to the older cratons in the south. An ancient underplated OIB melt component, isolated for about 2 Ga as enriched lithospheric mantle and then remelted, could generate both the trace element and isotopic data measured in the Tanzanian samples. Whereas the radiogenic Pb in Tanzanian lavas requires a source with high time-integratedU/Pb, most continental basalts that are thought to have interacted with the continental lithospheric mantle have unradiogenic Pb, requiring a source with a history of lowU/Pb. Such lowU/Pb is readily accomplished with the addition of subduction-derived components, since the lower averageU/Pb of arc basalts (0.15) relative to OIB (0.36) probably reflects addition of Pb from subducted oceanic crust. If the subcontinental lithosphere is normally characterized by low time-integratedU/Pb it would appear that subduction magmatism is more important than OIB additions in supplying the Pb inventory of the lithospheric mantle. However,U/Pb ratios of xenoliths derived from the continental lithospheric mantle suggest that both processes may be important. This apparent discrepancy could be because xenoliths are not volumetrically representative of the subcontinental lithospheric mantle, or, more likely, that continental lithospheric mantle components in basalts are normally only identified as such when the isotopic ratios are dissimilar from MORB or OIB. Lithospheric enrichment from subaccreted OIB components appears to be more significant than generally recognized.

Hf isotopes of MARID (mica-amphibole-rutile-ilmenite-diopside) rutile trace metasomatic processes in the lithospheric mantle

The Hf isotope composition of rutile in mica-amphibole-rutile-ilmenite-diopside (MARID) xenoliths from the Kimberley area of South Africa has been analyzed in situ by laser ablation microprobe–multicollector–inductively coupled plasma mass spectrometry to investigate metasomatic processes in the lithospheric mantle. The 176Hf/177Hf ratio shows a wide range, from 0.2811 to 0.2858 (eHf = −55 to +110); much of this range is found within single samples and even within single grains. We suggest that the MARID rutiles initially resulted from the interaction of an asthenospheric melt with ancient depleted harzburgitic mantle with low 176Hf/177Hf ratios (<0.2812) and Lu/Hf ratios of ∼0.04, that dominated their Hf budget. The MARID rocks were later metasomatized by a fluid and/or melt that had caused the breakdown of eclogitic or lherzolitic garnet with Lu/Hf ratios of ∼0.6, providing a source of highly radiogenic Hf. The low 176Hf/177Hf ratios preserved in some MARID rutiles show that the Nd-Hf isotope systematics of kimberlites and lamproites can be explained by mixing between an asthenospheric melt and the ancient subcontinental lithospheric mantle.

Combeite

The third occurrence of combeite is in lapilli ejected from the active carbonatite volcano Oldoinyo Lengai, Tanzania. The combeite occurs as euhedra in equilibrium with nepheline, Na-melilite, Tiandradrite, and Ti-magnetite, and as coronas replacing corroded clinopyroxene and wollastonite crystals. Garnet and nepheline euhedra do not have combeite coronas. The combeite of the coronas is associated with minor Na-melilite, nepheline, and calcium silicates (rankinite? and larnite?). The coronal combeite is attributed to incomplete reaction between Ca-bearing silicates and sodium carbonatite magma. The mean of 11 analyses,

from Oldoinyo Lengai, Tanzania

Na_{2.33}Ca_{1.74}others_{o.12}Si_{3}O_{9}

Platinum-group element abundance patterns in different mantle environments

, indicates solid solution towards the

Carbonate–silicate immiscibility and extremely peralkaline silicate glasses from Nasira cone and recent eruptions at Oldoinyo Lengai Volcano, Tanzania

Na_{6}Si_{3}O_{9}