Geochemistry of East African Rift basalts: An overview

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Journal of African Earth Sciences 48 (2007) 147–160

Geochemistry of East African Rift basalts: An overview

Tanya Furman *

Department of Geosciences, Pennsylvania State University, University Park, PA 16802, USA

Received 17 May 2005; received in revised form 21 October 2005; accepted 1 June 2006Available online 24 February 2007

Abstract

Mafic lavas erupted along the East African Rift System from the Afar triangle in northern Ethiopia to the Rungwe province in south-ern Tanzania display a wide range of geochemical and isotopic compositions that reflect heterogeneity in both source and process. Inareas with the lowest degree of crustal extension (the Western and Southern Kenya Rifts) primitive lavas record the greatest extentof lithospheric melting, manifest in elevated abundances of incompatible elements and highly radiogenic Sr–Nd–Pb isotopic composi-tions. Where prolonged extension has removed most or all of the mantle lithosphere (the Turkana and Northern Kenya Rifts), a largerrole for sub-lithospheric processes is indicated. At intermediate degrees of extension (the Main Ethiopian Rift) both lithospheric and sub-lithospheric contributions are observed, and crustal assimilation occurs in some cases. Despite the wide compositional range of AfricanRift basalts, a restricted number of source domains contribute to magmatism throughout the area. These individual domains are: (1) thesubcontinental mantle lithosphere; (2) a plume source with high-l Sr–Nd–Pb–He isotopic affinities, present in all areas within and southof the Turkana Depression; and (3) a plume source with isotopic signatures analogous to those observed in some ocean islands, includinghigh 3He/4He values, present throughout the Ethiopian Rift and the Afar region. The two plume sources may both be derived from theSouth African Superplume, which is likely to be a compositionally heterogeneous feature of the lower mantle.� 2007 Elsevier Ltd. All rights reserved.

Keywords: East Africa; Geochemistry; Basalt; Petrogenesis; Mantle domains

1. Introduction

The East African Rift system is the classic modernexample of continental rifting above one or more mantleplumes, and provides an excellent framework in which toinvestigate extensional mafic magmatism. Large succes-sions of basalts occur along much of the rift system,emplaced over a broad range both spatially and tempo-rally. The East African Rift volcanics outcrop from theAfar depression to southern Tanzania, and span pre-rift,syn-rift and post-rift periods. The rift itself traverses twoareas of continental uplift, the Ethiopian and Kenyadomes, separated by the low-lying Turkana Depression innorthern Kenya (Fig. 1). Throughout Ethiopia, the riftdefines a single zone of extension and volcanic activitytermed the Main Ethiopian Rift. South of Turkana, the rift

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is manifest in two branches that encircle the mechanicallyrobust Tanzania craton, referred to herein as the WesternRift and Kenya Rift. The eruptive style, chemistry and vol-ume of volcanic products exposed along the three rifts varyin an overall coherent fashion, allowing each rift to be dis-cussed as a tectono-magmatic entity. As a result, the geo-chemistry of East African Rift basalts is important indetermining the along-axis thermochemical structure ofthe underlying mantle. By documenting spatial variationsin magmatic processes as well as the source regions contrib-uting to this mafic volcanism we gain important insightinto the evolution of plume-driven rifting.

The primary objective of this paper is to place con-straints on the thermal, mineralogical and chemical fea-tures of the mantle source regions that contribute tomafic magmatism throughout the African Rift. In recentyears, publication of several detailed geochemical and pet-rological studies of individual volcanic systems along theentire Rift system have made it feasible to put forward

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Fig. 1. Map of the East African Rift System showing topography andstudy areas discussed in the text (base map from R. Keller). Volcanismoccurs throughout the Ethiopian and Kenya Rifts but in the Western Riftit is restricted to the four provinces shown; Muhavura and Karisimbivolcanoes are both within the Virunga province.

148 T. Furman / Journal of African Earth Sciences 48 (2007) 147–160

an integrated interpretation of this magmatic province(e.g., Rogers et al., 1992, 1998; Class et al., 1994; Stewartand Rogers, 1996; Black et al., 1998; Furman and Graham,1999; Macdonald et al., 2001; le Roex et al., 2001; Spathet al., 2001; George and Rogers, 2002; Furman et al.,2004, 2006, in press). Major and trace element data as wellas Sr–Nd–Pb isotopic values measured on Pliocene throughRecent lavas are used to probe the nature of the mantlesource regions underlying the African Rift as well as thethermal structure of the associated lithosphere. Of relatedinterest is determining the scale of physical and geochemi-cal heterogeneities within the lower and middle mantle asmanifest in the South African Superplume (e.g., Nybladeand Robinson, 1994; Gurnis et al., 2000; Zhao, 2001; Niet al., 2002), which is broadly viewed as the source regionfor heat and/or magmas erupted on the African continent(e.g., Ebinger and Sleep, 1998; Janney et al., 2002; Furmanet al., 2004; Kieffer et al., 2004).

2. Overview of rifting and rift volcanism

2.1. Tectonic and magmatic evolution of the rift

The earliest recorded volcanic activity in the AfricanRift took place 40–45 Ma in the northern Turkanadepression (i.e., southernmost Ethiopia; Ochieng, 1988;Ebinger et al., 1993; George et al., 1998; Knight et al.,2003). The Turkana region has remained volcanicallyactive, albeit on an episodic basis, since that time. Despitethis observation, the modern East African Rift magmaticprovince is generally described with respect to the onset of

flood basalt activity in modern Ethiopia, Eritrea andYemen that occurred in the late Oligocene and is attrib-uted to impact of a mantle plume head at the base ofthe lithosphere. Within Ethiopia and Eritrea, the Oligo-cene flood basalt province covers an area of about600,000 km2, with an estimated total volume of about350,000 km3 (Mohr, 1983; Mohr and Zanettin, 1988).The lava pile thickness varies, reaching up to 2000 m innorthwestern Ethiopia and thinning to �500 m towardsboth north and south. Most of the basalts and associatedfelsic rocks were emplaced over a period of �1 millionyears at �30 Ma (Zumbo et al., 1995; Baker et al.,1996; Hofmann et al., 1997; Pik et al., 1998). This Oligo-cene flood basalt event is associated with the breakup ofthe Afro-Arabian shield, forming the Red Sea �28 Ma(e.g., Ukstins et al., 2002; Wolfenden et al., 2004) andthe Gulf of Aden �35 Ma (e.g., Watchorn et al., 1998).The African Rift represents the third arm of this triplejunction, although recent regional plate kinematic models(e.g., Acton et al., 2000; Eagles et al., 2002; Wolfendenet al., 2004) indicate that the three rift zones were notjoined until about 11 Ma.

The earliest extension documented in the East AfricanRift System occurred in the Turkana area �25 Ma (Morleyet al., 1992; Hendrie et al., 1994). Volcanism and faultingpropagated from this region to both the north and south,forming the familiar structures of the modern rift branches(Fig. 1). In southwestern Ethiopia a marked episode of vol-canism and extension occurred 18–19 Ma (Ebinger et al.,2000; Stewart and Rogers, 1996; George et al., 1998), andreached the central and northern portions of the MainEthiopian Rift as the Afar triple junction developed�11 Ma (e.g., Chernet et al., 1998; WoldeGabriel et al.,1990; Wolfenden et al., 2004). At the same time, magma-tism and faulting moved progressively southward in boththe Kenya and Western Rifts, reaching central Tanzaniaby roughly 1 Ma (e.g., Baker et al., 1971; Ebinger et al.,1989; Kampunzu et al., 1998).

2.2. Magmatic provinces

Structural and geochemical considerations allow eachrift to be divided into a small number of reasonably coher-ent provinces that provide a framework for discussion.First-order geochemical distinctions between provincesare apparent in Fig. 2, where the level of enrichment inNa2O, K2O and P2O5 are shown for representative volcaniccenters within each province.

The Ethiopian and Kenya branches of the rift are thesite of substantially greater volcanism than is observed inthe Western Rift, where eruptive activity is restricted tofour spatially distinct provinces along the rift axis(Fig. 1). From north to south these are: the Toro-Ankoleregion in western Uganda, the Virunga and Kivu provincesalong the border of the Democratic Republic of the Congowith Uganda, Rwanda and Burundi, and the Rungwevolcanic field in southwestern Tanzania. These areas are

a

b

Fig. 2. Major element variations in African Rift mafic lavas. (a) Within-suite variations in total alkalis against silica define consistent regionalpatterns. The dotted line is the alkaline-tholeiitic division of Macdonaldand Katsura (1964). Trend 1: Silica-saturated lavas from Naivasha inthe southern Kenya Rift (SKR) overlap the field of quartz- andhypersthene-normative 30 Ma Ethiopian flood basalts (shaded). Trend 2:Lavas from the northern Kenya Rift (NKR; Turkana, Emuruangogolak,Silali, Huri Hills) as well as basalts from the Main Ethiopian Rift(MER) define a transitional-tholeiitic sequence. Trend 3: The mosthighly silica-undersaturated samples are found in the Western Rift (WR;e.g., Rungwe, Muhavura) and southern Kenya (Chyulu Hills). (b)Variations in P2O5 against MgO show similar regional trends. Tholeiiticbasalts from the southern Kenya Rift (SKR: Ol Tepesi, Singaraini,Kirikiti) have the lowest overall incompatible element contents. Tran-sitional tholeiites from Turkana, Huri Hills and the northern Kenya Rift(NKR) and Main Ethiopian Rift (MER) have higher incompatibleelement abundances, and silica-undersaturated lavas from Chyulu Hillsand the Western Rift (WR; here represented by the Rungwe province)have the highest incompatible element contents. (Sources of data: Daviesand Macdonald, 1987; Rogers et al., 1992, 1998; Class et al., 1994;Furman, 1995; Black et al., 1998; Pik et al., 1999; le Roex et al., 2001;Spath et al., 2001; Furman et al., 2004, 2006, in press; Kieffer et al.,2004).

T. Furman / Journal of African Earth Sciences 48 (2007) 147–160 149

characterized generally by silica-undersaturated mafic vol-canism (Fig. 2), the products of which include ultrapotas-sic, hypersodic and carbonatitic compositions. Themagmatic provinces of the Western Rift are confined toaccommodation zones formed by major basin-boundingfaults.

Within Ethiopia, the Main Ethiopian Rift basalts aretransitional-tholeiitic overall (Fig. 2), which evolved prod-ucts encompassing primarily rhyolites and trachytes. Well-developed polygenetic calderas are situated in the center ofthe rift axis, and recent basalts are erupted from within-riftfissures adjacent to and distal from these central volcanoes.The southwest Ethiopian magmatic province is character-ized by episodic volcanism that becomes increasingly sil-ica-undersaturated through time, ranging from essentiallytholeiitic compositions 40–35 Ma to nepheline-normativebasalts since �19 Ma. These features are similar to thosedisplayed by contemporaneous lavas from the Turkanaregion which is located immediately to the south.

The northern Kenya Rift comprises the Turkana, HuriHills and Marsabit regions as well as the Quaternary cen-tral volcanic complexes of Emuruangogolak, Namurunu,Silali, Paka and Korosi. Mafic lavas from these volcanoesare transitional-tholeiitic basalts (Fig. 2); the correspond-ing felsic lavas are dominantly trachytic in composition.

Volcanic complexes in central and southern Kenyainclude the within-rift centers Menengai, Eburru, Naiva-sha, Longonot, Suswa and Lenderut as well as the off-riftvolcanic field at Chyulu Hills. The rift axis volcanoes aredominantly trachytic to pantelleritic with little to no expo-sure of associated basalts. Where present, the basalts aredominantly transitional tholeiitic, although the ChyuluHills lavas are markedly silica-undersaturated and havehigh concentrations of P2O5.

3. Petrogenesis of rift lavas

In this section, the geochemical and isotopic characteris-tics of mafic lavas from the individual magmatic provincesare considered in the context of processes and parametersthat may affect melt composition. It is clear that the pro-cesses of melt generation and aggregation can also play sig-nificant roles in determining eruptive composition, but forthe most part the data needed to assess these phenomenaare not available for the East African Rift basalts. Of par-ticular importance in this regard are recent studies of meltinclusions in primitive Turkana lavas that require small-scale isotopic heterogeneities in the melting region (Waterset al., 2004). Similarly, recent work on U-series disequilib-rium shows great promise for understanding the time scalesof melt generation and emplacement (e.g., Black et al.,1998; Rogers pers. comm.).

3.1. Evaluating crustal assimilation

The East African Rift System crosses cratonal regions ofthick continental lithosphere, making it important to assessthe contribution of crustal assimilation in the genesis oferupted lavas. Crustal assimilation is likely greatest inregions where magma supply rates are low and crustalthicknesses are large, resulting in greater residence timesfor individual magma batches within the shallow crust.Incorporating melted crustal material should result in


marked enrichments in the mobile large ion lithophile ele-ments, such as Pb and Rb, but not in the more refractoryrare earth and high field strength elements.

Values of Ce/Pb measured in mafic lavas are thus sensi-tive to contamination, and this ratio is well-defined for pri-mary mantle derived liquids (25 ± 5, Hofmann et al.,1986), making it a useful indicator of crustal assimilation.Fig. 3 reveals that the majority of mafic lavas from theMain Ethiopian and Turkana Rifts – areas where post-Miocene extension has markedly thinned the crust andlithosphere – have Ce/Pb values within the range of man-tle-derived basalts. In contrast, basalts from Naivasha inthe southern Kenya Rift have consistently low Ce/Pb val-

a

b

c

ues that indicate substantial interaction with the thick crus-tal section in this area. Western Rift lavas from theRungwe and Kivu provinces and Virunga volcanoesMuhavura and Karisimbi plot primarily within the fieldof mantle values, but also extend to both higher (i.e., litho-spheric?) and lower (i.e., crustal) values. This observation isconsistent with the low rate of magma supply in this area,where extension of only 10–15% is estimated (Ebingeret al., 1989).

A consistent picture emerges from the relationship of143Nd/144Nd against Rb, where abundances of a mobileelement are compared to variations in the Sm/Nd isotopicsystem which is far more robust with respect to assimila-tion of crustal melts, particularly given the high Sr andNd contents of many Kenya Rift basalts (Fig. 3). Rbbehaves incompatibly during fractionation, but the Nd iso-topic ratio should not change during closed-system differ-entiation. Low Nd isotope values are found in Africancrustal materials (e.g., Davidson and Wilson, 1989)although no single value is appropriate for the entire riftsystem (indeed, the mantle lithosphere is likely quite heter-ogeneous in its Nd isotopic composition), so the fraction-ation–assimilation curves in Fig. 3 are only schematic.The northernmost Kenya Rift systems – Turkana and HuriHills – have basalt compositions that are dominated byfractionation in the absence of crustal assimilation. Maficlavas from Emuruangogolak and Silali (NKR) as well asfrom Chyulu Hills, Kirikiti, Ol Esayeiti and Ol Tepesi(SKR) provide evidence for minor assimilation during theearly stages of crystallization (i.e., among the most MgO-rich lavas), followed by prolonged fractionation towardsevolved compositions with little or no crustal melting.SKR central volcanoes dominated by felsic eruptives (Nai-vasha and Lenderut), like the Western Rift Rungwe prov-ince, appear to show more extensive assimilation overall,again particularly among the more mafic lavas. The Viru-nga volcanoes Muhavura and Karisimbi have extremely

Fig. 3. (a) Values of Ce/Pb are sensitive to crustal contamination, asinferred for lavas from Naivasha (shaded field). Most MgO-rich lavasfrom the Rungwe province, the northern and southern Kenya Rifts andthe Main Ethiopian Rift have Ce/Pb values within the range of mantle-derived melts (Hofmann et al., 1986). Samples from Chyulu Hills and HuriHills do not have published Pb data. (b) Plot of 143Nd/144Nd against Rbshows the effects of combined assimilation and fractional crystallizationon mafic lavas. This panel shows the range of values observed in theWestern Rift, Kenya Rift and southern Ethiopia (Rungwe data shown asshaded circles). Detail: Values of r (the ratio of assimilation to fraction-ation) are shown schematically for assimilation of a crustal contaminantfrom central Sudan (Davidson and Wilson, 1989). Starting compositionsare identified on the basis of isotopic relationships discussed in the text; nogenetic link among samples from different areas is implied. The starlabeled ‘‘1’’ represents the least radiogenic composition reported in thesouthern Kenya Rift; star ‘‘2’’ is the end-member composition identified inTurkana and Kivu. (Sources of data: Davies and Macdonald, 1987;Rogers et al., 1992, 1998, 2000; Class et al., 1994; Furman, 1995; Blacket al., 1998; le Roex et al., 2001; Macdonald et al., 2001; Spath et al., 2001;Furman et al., 2004, 2006, in press).

b


high Rb contents and low Nd isotope values, indicatingextensive crustal or lithospheric melting.

3.2. The role of hydrous phases in the source region

The mineralogical characteristics of the meltingdomains beneath the East African Rift must be con-strained as part of an overall assessment of the source

a

b

Fig. 4. Incompatible trace element variations in African Rift lavas. (a) IncomSun and McDonough, 1989) of representative lavas with >8 wt.% MgO are shoare highest in the silica-undersaturated samples from Chyulu Hills (south Kenyfrom the Main Ethiopian Rift (MER), Huri Hills and Turkana (north Kenya) h(K2O*10,000/Th) against Th for African Rift basalts shows the effects of progrean essential structural constituent of these phases, so it will be retained in the seach suite, samples with highest Th and lowest K/Th* record the lowest degreesof Main Ethiopian Rift basalts is extremely small, as many of these lavas lack K1987; Rogers et al., 1992, 1998, 2000; Class et al., 1994; Furman, 1995; Black eFurman et al., 2004, 2006, in press).

regions. Of particular importance are the roles of amphi-bole and phlogopite, which may indicate a history ofmetasomatism by silicate or carbonatitic fluids. K abun-dances do not co-vary consistently with similarly incom-patible elements (Nb, Th; Fig. 4) and in places reflectsthe presence of residual phases in the source region. Thisobservation has been made for ultrapotassic through sil-ica-saturated volcanics of the Western Rift (Lloyd et al.,

patible trace element abundances (normalized to primitive mantle values;wn for individual portions of the East African Rift. Elemental abundancesa) and Muhavura (Virunga province, Western Rift). Transitional tholeiitesave lower overall incompatible element contents. (b) Covariation of K/Th*

ssive melting of an amphibole- or phlogopite-bearing source. Potassium isource region until they are consumed through progressive melting. Withinof melting. The inferred abundance of hydrous phases in the source regionanomalies (Furman et al., 2006). (Sources of data: Davies and Macdonald,t al., 1998; le Roex et al., 2001; Macdonald et al., 2001; Spath et al., 2001;

a

b

Fig. 5. Radiogenic isotopic variations in African Rift mafic lavas.(a) Covariations in 87Sr/86Sr and 143Nd/144Nd define a broad negativetrend from values in northern Kenya (NKR = Turkana, Huri Hills,Emuruangogolak, Silali and Kenya Rift Mobile Belt) and the Kivuprovince (Western Rift) that are less radiogenic than the modern Afarplume (heavy circle enclosing MgO-rich lavas from Djibouti and Erta‘Ale) towards highly radiogenic compositions observed in the Western Rift(Rungwe [Run], Toro Ankole, Muhavura, Karisimbi). Basalts with non-radiogenic Sr-Nd isotopic compositions trend towards the fields of HIMUlavas and mid-ocean ridge basalts from the Western Sheba Ridge (WSR).Shaded field labeled S. Kenya Rift encloses basalts from the within-riftvolcanoes Ol Tepesi, Singaraini and Kirikiti. Hawaiites and benmoreitesfrom Lenderut, southern Kenya, are displaced from the main trendtowards extremely low 143Nd/144Nd values observed in granulite xenolithsfrom Tanzania (not shown). Isotopic end-member compositions areindicated for depleted MORB mantle (DMM), HIMU, and bulk silicateEarth (BSE). (b) Variations in 206Pb/204Pb and 208Pb/204Pb show stronggeographic control, as MgO-rich samples from all portions of the KenyaRift system have higher values than lavas from the Main Ethiopian Rift.Both groups overlap the range of modern Afar plume lavas. Western Riftlavas with radiogenic Sr–Nd trend to extremely high 208Pb/204Pb values.Evolved lavas from Lenderut trend towards low Pb isotope valuescharacteristic of crustal xenoliths. (Sources of data: Davies and Macdon-ald, 1987; Rogers et al., 1992, 1998, 2000; Class et al., 1994; Deniel et al.,1994; Furman, 1995; Barrat et al., 1998; Black et al., 1998; Schilling et al.,1998; Furman and Graham, 1999; Pik et al., 1999; le Roex et al., 2001;Macdonald et al., 2001; Spath et al., 2001; Furman et al., 2004, 2006, inpress; Kieffer et al., 2004).


1991; Rogers et al., 1992, 1998; Furman, 1995; Furmanand Graham, 1999) as well as among basalt suites fromthe Ethiopian (Hart et al., 1989; Chernet and Hart,1999; Furman et al., 2006), Turkana (Furman et al.,2004, in press) and Kenya Rifts (Davies and Macdonald,1987; Macdonald et al., 2001; le Roex et al., 2001; Spathet al., 2001). Bulk partition coefficients for K and Th (andNb, though not discussed here) are similar enough in spi-nel and garnet lherzolite that processes taking place in theabsence of a K-bearing phase cannot produce the relativedepletion in K observed in the mafic lavas. Fractionationof K/Th could result from crystallization of amphibole orphlogopite, but there is no petrographic evidence to sup-port this interpretation. Rather, we concur with previousauthors (Class and Goldstein, 1997; le Roex et al., 2001;Spath et al., 2001) that in many cases the K/Th variationsare most simply explained by melting in the presence ofeither amphibole or phlogopite. Elevated Rb/Sr valuesof ultrapotassic mafic Western Rift lavas require phlogo-pite in the source area, although the overall geochemicalsignatures of these lavas also reflect melting of sourceregions with ancient enrichment histories (Davies andLloyd, 1989; Lloyd et al., 1991; Rogers et al., 1992,1998). Basalts from the remaining regions are interpretedas being derived from source regions with amphibole pres-ent through at least part of the melting process.

Taking the African Rift lavas together within this con-text, Fig. 4 suggests that the silica-undersaturated lavasof the Chyulu Hills province are derived by the lowestdegrees of melting (i.e., highest Th) of amphibole-bearingsource material (i.e., lowest K/Th); Muhavura and Kar-isimbi lavas derived by melting of phlogopite–lherzolitehave similar Th contents but are offset to higher K/Thby virtue of the higher K content in phlogopite. Thisanalysis indicates that, to first-order, mafic lavas fromthe remaining provinces of the Western Rift, as well aslavas from throughout Kenya and the Turkana areas,derive by higher degrees of melting of amphibole lherzo-lite, whereas selected Turkana basalts and Main Ethio-pian Rift lavas record the highest degrees of meltingand, correspondingly, the lowest proportion of amphi-bole in the source rock. Although the isotopic differencesdescribed above preclude defining a realistic along-axiscontinuum in source domain, this exercise is useful incategorizing patterns in source mineralogy and the extentof melting, thereby helping constrain regional differencesin both metasomatic history and modern thermalenvironment.

3.3. Isotopic characteristics of regional source domains

The Sr–Nd isotopic compositions of East African Riftrocks have been shown to reflect mixing of melts fromlithospheric and sub-lithospheric sources, with or withoutdirect contributions from one or more mantle plumes. Ingeneral terms the mafic lavas define a negative correlationbetween 87Sr/86Sr and 143Nd/144Nd that ranges from rather

depleted values (�0.703, �0.5130) to highly enriched com-positions (�0.707, �0.5123); individual volcanic systems orprovinces have weaker negative trends with more restrictedSr–Nd isotopic ranges (Fig. 5).


The Sr–Nd isotopic compositions of samples from theMain Ethiopian, Turkana and Eastern Rifts plot generallywithin a triangular region defined by three mantle end-members (Fig. 5): DMM (the shallow convective astheno-sphere that is the source region for depleted mid-oceanridge basalts), HIMU (a reservoir with a high time-inte-grated U/Pb ratio) and BSE (the bulk silicate earth). Themodern Afar plume plots within this triangle (Fig. 5).The physical location of these three end-member sourcedomains is not fully constrained, but the low Sr-isotopevalues and the occurrence of these compositions in oceanisland suites worldwide suggest they are found at sublitho-spheric levels.

Mafic lavas from the Western Rift trend towards highlyradiogenic Sr–Nd compositions that require direct meltingof the thick continental lithosphere, particularly in theRungwe, Virunga and Toro Ankole provinces (Daviesand Lloyd, 1989; Lloyd et al., 1991; Furman, 1995; Rogerset al., 1992, 1998) and to a lesser extent in the Kivu area(Furman and Graham, 1999). The only Eastern Rift local-ity to share these radiogenic compositions is Naivasha,where the preponderance of highly evolved lavas suggestsextensive crustal melting.

The Pb isotope systematics of African Rift mafic lavasare less well-characterized, but source domains defined onthe basis of available data are consistent with those inferredfrom the Sr–Nd isotopic evidence. Compelling regional dif-ferences in Pb isotope data distinguish the Ethiopian lavasfrom those erupted in the Turkana and Eastern Rift zones.Ethiopian Rift basalts overlap the isotopic composition ofthe Afar plume and trend towards less radiogenic values,while the northern and southern Kenya Rift lavas havecompositions that extend from the Afar plume towardsmore highly radiogenic values that approach the HIMUlavas of St. Helena (e.g., Hanan et al., 1986; Chaffeyet al., 1989; Fig. 5). Mafic lavas from southern Ethiopiawith Ce/Pb values within the range of mantle basalts havePb isotope values that overlap those of neighboring Tur-kana basalts; south Ethiopian samples with lower Ce/Pbvalues have non-radiogenic Pb isotope values that fallwithin the range of Main Ethiopian Rift samples. ToroAnkole, Muhavura and Karisimbi lavas from the WesternRift trend towards unusually high 208Pb/204Pb (Fig. 5) and207Pb/204Pb values (not shown) suggesting melts of a crus-tal or lithospheric source.

3.4. Depth and degree of melting

The depth at which mantle melting occurs can be con-strained using two different approaches that make use ofmajor and trace element data on African Rift mafic lavas.At the most general level, the melting depth can be evalu-ated by documenting the presence of garnet or phlogopitein the source region. Experimental stability estimates forphlogopite-bearing assemblages suggest this phase is stableat pressures near 30–35 kbar, or depths of at least 90–100 km (Olafsson and Eggler, 1983; Lloyd et al., 1985;

Wallace and Green, 1988; Sato et al., 1997). As indicatedabove, only the ultrapotassic volcanics of the Western Riftshow evidence for melting of phlogopite-bearing lherzolite;this interpretation is fully consistent with the highly radio-genic isotopic signatures of these lavas that indicate melt-ing of thick, metasomatized continental lithosphericmaterial.

Evaluating the presence of garnet is less clear-cut; thegarnet-spinel transition occurs at �60–70 km but the com-patibility of many key elements is pressure-sensitive (e.g.,Van Westrenen et al., 2001), making it difficult to distin-guish between melting of spinel- and garnet-lherzolite. Val-ues of Tb/Yb, however, will be substantially higher in meltsof a garnet-bearing residuum given the much greater com-patibility of Yb relative to Tb in this mineral (e.g., Shimizu,1975; Irving and Frey, 1984; Hauri et al., 1994). Among theEast African Rift suites, moderate values of Tb/Ybn (2.2–2.8) are observed at Muhavura in the Western Rift, andin selected samples from Turkana and Huri Hills in north-ern Kenya. Melting of garnet-bearing source material can-not be inferred categorically in these regions on thisevidence alone but moderate to high Tb/Ybn values are agood indicator of melting at depths appropriate to themantle lithosphere. Mafic lavas from Chyulu Hills insouthern Kenya have chondrite-normalized Tb/Yb valuesranging from 2.7 to 5.6, significantly and consistentlyhigher than those reported in other provinces; these highvalues are attributed to melting of an enriched sourceregion rather than requiring a garnet-bearing source (Spathet al., 2001).

A second route to constrain the depth of mantle meltingcompares the major element contents of individual primi-tive lavas, back-corrected for olivine fractionation toMg# = 72 (Fig. 6), to liquid compositions determinedexperimentally on melts of peridotite at various pressures.The actual FeO* and SiO2 contents of natural and experi-mental liquids are highly dependent on the starting sourcecomposition (e.g., fertile vs. depleted peridotite) but Fig. 6provides a framework in which to evaluate relative pres-sures of melt generation. The depths of melting inferredon the basis of this calculation scheme are consistent withthe trace element evidence presented above: lavas fromChyulu Hills and Ol Esayeiti in southern Kenya indicatethe greatest pressures of melting (>25–30 kbar). Plio-Pleis-tocene samples from Huri Hills and Turkana in northernKenya record similar to slightly lower melting pressuresthat are suggestive of, but do not require, garnet in thesource region. Mafic lavas from the Western Rift, the MainEthiopian Rift, northern Kenya (Quaternary lavas) and theRed Sea indicate progressively lower melting pressures thatare likely consistent with along-axis variations in litho-spheric thickness as discussed below.

Fig. 6 is also qualitatively useful in estimating the degreeof mantle melting, as melts formed after clinopyroxene iscompletely exhausted trend towards higher FeO*–SiO2

contents (Kushiro, 1996). To first order, the majority ofEast African Rift lavas reflect melting in the presence of

Fig. 6. Calculated FeO* and SiO2 contents of primary mafic East AfricanRift lavas and experimental melts in equilibrium with mantle peridotite(Baker and Stolper, 1994; Kushiro, 1996). Heavy-lined fields encircleexperimental runs for melts coexisting with cpx + oliv + opx, and labeledtic-marks indicate corresponding experimental melts coexisting witholiv + opx after higher degrees of melt removal. Major element compo-sitions were renormalized with total iron as FeO, then corrected for olivinefractionation to correspond to experimentally derived melts of fertileperidotite (Mg# � 72); only samples with Mg# > 62 were corrected usingthis procedure and are plotted here. Experimental data are plotted asreported. Pressures estimated on the basis of this diagram are approxi-mate, as isobaric experiments are not directly analogous to naturalsystems generated by adiabatic decompression melting; however, therelative pressures inferred for basalt suites are useful for comparativepurposes. The greatest pressures of melting are indicated for the SouthernKenya Rift (SKR; individual samples indicate within-rift lavas from OlEsayeiti and Kirikiti, and the field encompasses Chyulu Hills and within-rift and rift margin suites) and Plio-Pleistocene basalts from the NorthernKenya Rift (NKR-P, including Turkana and Huri Hills). Calculatedprimary melts for the Western Rift (WR, all provinces) and the MainEthiopian Rift and Erta ‘Ale (MER/E) overlap at slightly lower pressures.The lowest calculated pressures of melt generation are found amongQuaternary basalts from the Northern Kenya Rift (NKR-Q; Turkana)and the Red Sea. See text for discussion. Sources of data (Rogers et al.,1992, 1998; Class et al., 1994; Furman, 1995; Barrat et al., 1998; Blacket al., 1998; Furman and Graham, 1999; le Roex et al., 2001; Macdonaldet al., 2001; Spath et al., 2001; Furman et al., 2004, 2006, in press). See textfor discussion.


clinopyroxene indicating either a highly fertile sourceregion, or low to moderate degrees of melting (<�7%;Wasylenki et al., 2003), or a combination of these two fac-tors. Early Miocene picrites from Turkana record greaterdegrees of melting consistent with the higher temperaturesassociated with this type of lava (Furman et al., in press)and suggesting that this approach, although qualitative,makes geological sense.

4. Defining regional source characteristics

By integrating the observations presented above with Heisotopic results (Marty et al., 1993, 1996; Darling et al.,1995; Scarsi and Craig, 1996; Furman et al., in press; Piket al., 2006) it is possible to define the mineralogy and iso-topic composition of source regions that contribute tomafic volcanism throughout the African Rift System.Regional differences in source composition may reflect ver-tical layering or region-scale lateral heterogeneity of the

lithosphere. Geochemical and geophysical evidence alsopoint to the influence of one or more mantle plumesbeneath the African Rift (e.g., Ebinger and Sleep, 1998;George et al., 1998; Rogers et al., 2000; Nyblade et al.,2000; Weeraratne et al., 2003; Montelli et al., 2004). Thereview presented here can help evaluate the relative contri-butions of these various source domains.

4.1. Western Rift and Kenya Rift magmatism

Mafic lavas erupted in the Western Rift and southernKenya (south of �2�N) record the highest degrees of crus-tal assimilation and lithospheric melting, consistent withthe considerable lithospheric thicknesses (KRISP, 1987,1991; Mechie et al., 1997; Henry et al., 1990) and lowmagma supply rates in these portions of the rift system.Sr–Nd–Pb isotopic values in these basalts vary over a widerange, but define three groups in Fig. 5: (1) MagnesianChyulu Hills alkali basalts with the least radiogenic Sr–Nd isotopic compositions (87Sr/86Sr �0.7033, 143Nd/144Nd�0.51284) have some of the most radiogenic Pb isotopevalues (206Pb/204Pb �20.0; 207Pb/204Pb �15.62; 208Pb/204Pb�39.8). (2) Ultrapotassic lavas from the Western Rift andselected Naivasha basalts display particularly high87Sr/86Sr, 207Pb/204Pb and 208Pb/204Pb. (3) RemainingSouth Kenya Rift samples have Sr–Nd isotope values inter-mediate to the first two groups that, along with their traceelement contents, suggest crustal or lithospheric assimila-tion prior to eruption. However, these same samples trendtowards higher 206Pb/204Pb–208Pb/204Pb values which areinconsistent with assimilation of crustal compositions asmeasured in xenoliths from the Sudan (Davidson and Wil-son, 1989), Tanzania (Cohen et al., 1984) and northernKenya (P. Curtis, unpublished data).

The geochemistry of basalts from northern Kenya canbe explained primarily in terms of melting sub-lithosphericsource regions. These lavas include some of the most MgO-rich compositions found throughout the East African Rift,and overall exhibit the lowest degree of crustal interactionduring fractionation (Fig. 3). These observations are con-sistent with the regional crustal section, which thins dra-matically towards Turkana; the seismically determinedcrustal thickness is �20 km and lithospheric removal isinterpreted as complete (KRISP, 1987, 1991; Mechieet al., 1997; Henry et al., 1990). The inferred depth of melt-ing in this region decreases through time, apparentlyrecording the thinning and removal of the lithosphere.Interestingly, amphibole–lherzolite melting is observedamong Tertiary through Pleistocene basalts and basanites(Class et al., 1994; Furman et al., in press) whereas theQuaternary lavas reflect melting of anhydrous spinel lherz-olite, suggesting that the degree of interaction with litho-spheric (metasomatized) components diminishes throughtime as the lithosphere itself is removed.

Northern Kenya samples define a restricted range of Sr–Nd isotope values (Fig. 5) that overlap Chyulu Hills basaltsand extend towards less radiogenic compositions,


approaching values characteristic of high-l systems such asSt. Helena (Chaffey et al., 1989). The Pb isotopic composi-tions of these samples cluster at 206Pb/204Pb 19.0–19.6,207Pb/204Pb �15.6 and 208Pb/204Pb 38.8–39.5; lavas fromTurkana and southern Ethiopia erupted between 19 and40 Ma include more highly radiogenic Pb isotope values(206Pb/204Pb �20.4, 207Pb/204Pb �15.72; 208Pb/204Pb�40.0) also approaching those of high-l ocean islands.This distinctive isotopic composition found in the North-ern Kenya Rift samples overlaps the low-Sr, high-Ndend-member observed at Kivu (Furman and Graham,1999), although Pb isotopic data for the Kivu lavas arelacking.

To summarize, the most primitive mafic lavas from Chy-ulu Hills (SKR) and Turkana (NKR) have overlappingend-member Pb isotope compositions that approachhigh-l ocean islands, but the Sr–Nd isotope values of thesetwo systems differ beyond analytical error. In manyvolcanic systems interaction of mafic melts with the thicklithosphere may interfere with direct sampling of thisend-member and may also obscure isotopic heterogeneitywithin the source region. One possibility is that the ChyuluHills and Turkana lavas are derived from geochemicallydistinct sub-lithospheric source domains (the low Sr-iso-tope values are unlikely to be found in the lithosphere).Alternatively, all lavas erupted in the Western, NorthKenya and South Kenya Rifts may reflect mixturesbetween lithospheric source material and a single sub-litho-spheric composition with high-l isotopic characteristics.The occurrence of this non-radiogenic Sr–Nd compositionin selected Kivu basalts (Furman and Graham, 1999) pro-vides support for this interpretation.

The regional dataset of 3He/4He measurements on maficphenocrysts and geothermal fluids is not extensive, andprovides limited additional insight into the question ofsource domains and their heterogeneity. A recent compila-tion by Pik et al. (2006) demonstrates that all samples ana-lyzed from throughout the Kenya and Western Rifts haveHe isotopic values between 6 and 9 RA (after the effects ofcrustal influence are eliminated). This range of values issimilar to that observed world wide for mid-ocean ridgebasalts (7–8 RA; Graham, 2002) but could also reflect mix-tures between a deep mantle reservoir with high 3He/4Heand mantle, crustal or lithospheric source regions withmuch lower 3He/4He signatures. It is noteworthy that Tur-kana samples with Sr–Nd–Pb isotopic values that most clo-sely approach those of the HIMU mantle reservoir have3He/4He �6 RA, i.e., within the characteristic range ofHIMU as well (Furman et al., in press). The prevalenceof this component within the broader context of the KenyaDome lavas cannot be assessed convincingly on the basis ofthe current data set but awaits further analysis.

4.2. Ethiopian Rift magmatism

Basalts erupted in the Main Ethiopian Rift display geo-chemical features consistent with melting of fertile spinel

peridotite at pressures that correspond to the lithosphericthickness inferred from seismic reflection and gravity anal-yses (e.g., Dugda et al., 2005). Mafic lavas derived at higherpressures and/or through lower degrees of melting (i.e.,nepheline-normative compositions) have more depleted Srand Nd isotopic signatures than hypersthene-normativelavas that are derived more shallowly and/or throughgreater extents of melting (Furman et al., 2006). The degreeof crustal interaction is inferred to be low overall amongthe MgO-rich lavas, though more highly evolved sampleshave clearly assimilated a crustal component (e.g., Truaet al., 1999). Minor amphibole may be present in the sourceregion for some or all of the Ethiopian basalts, but itsabundance is low and its impact on bulk lava geochemistrysubdued relative to the other systems investigated.

The Ethiopian lavas, like those from Kivu, show a widerange in Sr-Nd isotopic composition that suggests adynamic rifting environment characterized by sharp topog-raphy along the lithosphere-asthenosphere boundary. TheSr–Nd isotopic values observed in central Ethiopia clusternear the Bulk Silicate Earth composition; they do not over-lap the non-radiogenic compositions observed at ChyuluHills, Kivu or Turkana. Similarly, the Pb isotopic valuesof recent Ethiopian Rift basalts are consistently lower thanthose measured in all localities south of the Ethiopian pla-teau. Their most radiogenic compositions overlap the mod-ern Afar plume as observed in Djibouti and Erta ‘Ale (i.e.,206Pb/204Pb �19, 207Pb/204Pb �15.6; 208Pb/204Pb �38.8;Deniel et al., 1994; Barrat et al., 1998). The Pb isotopicdata require that modern Ethiopian lavas are derived froma source region that is not sampled by basalts from por-tions of the East African Rift that surround the Kenyadome. This result is consistent with He isotopic evidence:phenocrysts in young mafic lavas from the Main EthiopianRift have values between 9 and 19RA that require a contri-bution from an undegassed lower mantle source region,i.e., an upwelling plume (Marty et al., 1993, 1996; Piket al., 2006).

5. Geophysical considerations

While the regional patterns of Pb and He isotopic com-position indicate that the source regions for the Ethiopianand Kenyan portions of the East African Rift are chemi-cally distinct, they do not constrain the structure of theshallow or deep mantle that underlies the rift itself. Thatis, the geochemical data alone cannot distinguish betweenmodels that call for a single homogeneous plume beneathEast Africa (e.g., Ebinger and Sleep, 1998), a complex het-erogeneous plume (e.g., Furman et al., 2004), or two geo-chemically distinct mantle plumes (e.g., George et al.,1998; Rogers et al., 2000). Further, there is a need toresolve the relationship between mantle melting processesoccurring beneath the Ethiopian and Kenya Rifts and themajor thermochemical anomaly in the deepest mantleknown as the South African Superplume. The Superplumewas identified on the basis of tomographic modeling (e.g.,


Ritsema et al., 1999; Gurnis et al., 2000; Zhao, 2001; Niet al., 2002), but poor resolution in the uppermost mantlehas not made it possible to determine whether this featurepersists across the transition zone beneath north-centralAfrica and thereby supports mafic volcanism in the AfricanRift.

Recent geophysical studies provide evidence for possibleplume stems in the upper mantle beneath both northern

Fig. 7. Representative cross-sections through the crust and upper mantleindicate the approximate depth of melting and lithospheric structureinferred beneath each portion of the East African Rift. In each case thetopography and sub-surface geometries are schematic, and question marksindicate areas of substantial structural uncertainty. (a) Main EthiopianRift (MER) at 8�N. In this region melting is initiated at �100 km depthwithin a plume source and, locally, metasomatized mantle lithosphere(mml). The transition zone is anomalously thin as the 410 km disconti-nuity is depressed and the 660 km discontinuity elevated, indicating thepresence of warmer-than-normal material interpreted as part of anascending plume (Benoit et al., 2004). (b) Turkana and the NorthernKenya Rift (NKR) at 4�N. This region is underlain by unusually thin crustand geochemical evidence suggests that melting takes place at �40–60 kmin sub-lithospheric source material. Individual eruptions do samplemetasomatized mantle lithosphere, suggesting it is still present locally inthe region. (c) The Western (WR) and Southern Kenya Rifts (SKR) at 4�S(adapted from Nyblade et al., 2000). Geophysical evidence indicates adepression of the 410 km discontinuity, but does not provide constraintson the location of the 660 km horizon. Geochemical evidence requiresmelting of metasomatized continental lithosphere beneath both riftsectors, likely accompanied by sub-lithospheric contributions in the SKR.

Ethiopia and the Tanzania craton (Debayle et al., 2001;George et al., 1998; Nyblade et al., 2000; Weeraratneet al., 2003; Benoit et al., 2004; Montelli et al., 2004; Bas-tow et al., 2005). In the Afar region of northern Ethiopia,S-wave tomographic imaging shows a thermal anomalythat appears to narrow with depth down to �500 km belowthe surface (Debayle et al., 2001). Montelli et al. (2004)report evidence for the continuation of this feature todepths approaching 1200 km, well below the transitionzone and hence requiring a deep mantle origin as inferredfrom the He isotopic evidence presented above.

To the south, seismic velocity models (Nyblade et al.,2000) show a �20-km depression of the 440-km discontinu-ity beneath the eastern margin of the Tanzania craton thatrequires the presence of a thick lens of anomalously hotmaterial; the thickness and spatial distribution of thismaterial makes it unlikely to have accumulated throughchanneled flow from a single plume located to the north(e.g., Ebinger and Sleep, 1998). Similarly, Weeraratneet al. (2003) document a 2-pronged low velocity zone inter-preted as an ascending plume head beneath the Kenya andWestern Rifts and the Tanzania craton itself. The continu-ation of this structure as a plume tail at depth is unclear,however, as it may be located to either the north or thesouth.

The geophysical and geochemical data thus suggest thatEast African Rift volcanism is supported by two distinctregions of upwelling at upper mantle levels, both withlikely connections to deep mantle reservoirs (Fig. 7). Forreasons of physical simplicity, the data thus favor a modi-fied 1-plume model that allows for multiple plume stemsarising from a common large plume at depth (Ishidaet al., 1999; Davaille et al., 2003). The two plume stemsproposed here are isotopically distinct, and also interactwith regionally unique crustal and sublithospheric sourcesduring ascent. Tomographic investigation of other plumepairs (Azores and Canaries, Ascension and St. Helena)indicates that two areas of upwelling can merge at mantledepths over 1000 km, thus tapping a common source atdepth (Montelli et al., 2004).

6. The South African Superplume and regional volcanism

This integrated model implicitly attributes East AfricanRift magmatic activity to the South African Superplume,and thus enables discussion of the apparent geochemicalheterogeneity contained within this feature. The tworegions of mantle upwelling documented in East Africareflect two source domains recognized world-wide in oce-anic islands: (1) a high-3He, high eNd source analogous tothose sampled at Hawaii and Iceland (e.g., Kurz et al.,1996, 2004; Dixon, 2003) and (2) a lower-3He, lower eNd

component that approaches high-l compositions, similarto that sampled in the Cameroon Line, the Canary Islandsand St. Helena (e.g., Barfod et al., 1999; Hilton et al., 2000;Graham, 2002). The spatial proximity of lavas with thesetwo characteristics is not unique to East Africa, as it is also


observed in the southern Pacific, where basalts from Samoawith high-3He and those from Raivavae with high-l affin-ities are associated with the South Pacific Superplume (e.g.,Courtillot et al., 2003; Lassiter et al., 2003).

Geophysical and numerical studies suggest that geo-chemically heterogeneous plumes are indeed likely. Numer-ical simulations of plume geochemical structuresemploying tracer techniques (Samuel and Farnetani,2003) demonstrate that large contrasts in helium isotopicsignatures, caused by the plume sampling both an unde-gassed mantle component and recycled oceanic crust, canexist over time and length scales that are consistent withthose observed in East Africa. These results show that dis-tinct components present in the lower mantle can ascendwithout complete mixing and thus preserve their isotopicfeatures in erupted basalts. The large physical dimensionof the South African and South Pacific superplumes – thelargest sources of heat and mass transfer from the lower-most portion of the mantle – makes it likely that a greatvariety of source materials coexist within the ascendingregion, leading to the possibility that chemically distinctplumes (plumelets or mini-plumes) could derive from thesame deep mantle thermal and chemical anomaly (Bregerand Romanowicz, 1998; Romanowicz and Gung, 2002;Courtillot et al., 2003).

A full discussion of the origin of high-l source regions isbeyond the scope of this paper; most authors attribute thedistinctive geochemistry of high-l lavas to melting of recy-cled (subducted) oceanic crust (e.g., Hofmann and White,1982; Graham et al., 1992; Hilton et al., 2000; Bell and Til-ton, 2001; Hoernle et al., 2002; Lassiter et al., 2003) or con-tributions from the mantle lithosphere (e.g., Bertrand et al.,2003). In the African Rift system this component is mostprevalent in areas underlain by locally thin lithosphere(i.e., Turkana, Kivu) and is either absent or masked inareas where melting of hydrated lithosphere is documented.These results are consistent with previous interpretations ofhigh-l contributions in alkaline rocks of South Africa (Jan-ney et al., 2002) and basalts from the continental and oce-anic segments of the Cameroon Line (Barfod et al., 1999)that this component originates through recycling of oceancrust at mid- to lower-mantle depths. This material maypreferentially melt and metasomatize the lithosphere, thuseffectively get ‘‘stored’’ in the lithosphere. Alternatively, itmay be brought to the shallow mantle as a component ofthe South African superplume.

7. Summary

The geochemistry and isotopic composition of maficlavas from throughout the East African Rift System arepresented here within the context of defining the primarysource regions for basaltic volcanism. The erupted compo-sitions reflect mixing of melts from lithospheric and sub-lithospheric sources, with or without direct contributionsfrom one or more mantle plumes. Although each volcanicsystem has some distinctive geochemical features, there are

regional commonalities that reflect the depth and composi-tion of the melting environment as well as the degree of tec-tonic extension and the rate of magma supply.

At the broadest scale, three distinct source componentscontrol magmatism throughout the area. First, the conti-nental lithospheric mantle is the ultimate source of ultra-potassic and other silica-undersaturated lavas erupted inthe Western and Southern Kenya Rifts. Lithosphericinvolvement can also be documented to a limited extentin other portions of the African Rift, but in this area ittypically overprints any geochemical signature from sub-lithospheric sources. The lithosphere in this regioncontains amphibole and, in many places, phlogopiteindicating a metasomatic event potentially associated withfluids derived from an ascending mantle plume. The plumesource itself can be identified locally in rare tholeiitic lavas(i.e., from Kivu in the Western Rift). Second, all volcanicsystems within and south of the Turkana depression(including those within the Western and Southern KenyaRifts) record a source component with an isotopic compo-sition that approaches the HIMU mantle end-member.This source likely comprises a portion of an upwellingmantle plume identified on geophysical evidence beneaththe Kenya dome. Third, all Quaternary and younger vol-canic systems located along the Main Ethiopian Rift andnorthward to the Afar triangle record a source contribu-tion with non-radiogenic Sr–Nd–Pb isotope values buthighly radiogenic He isotopic signatures. This sourcedomain is also located within a mantle plume, assupported by geophysical evidence of anomalously hotmaterial extending through the entire transition zone(410–660 km). It is important to note that the two mantleplume sources identified on the basis of isotopic data areobserved exclusively in distinct portions of the AfricanRift.

The geochemical observations presented here are consis-tent with a model in which magmatism throughout theAfrican Rift is driven by a single, yet chemically heteroge-neous, plume: the South African Superplume. The signifi-cant physical constraint imposed by the geochemical datais that the Superplume must sample and transport a heter-ogeneous source at deep mantle levels (i.e., below the tran-sition zone and perhaps to the core-mantle boundary). Thisrequirement is likely to be met easily within a thermo-chemical anomaly as large as the Superplume, andnumerical investigations reveal that such heterogeneitiesare preserved during ascent from the lowermost mantle(Samuel and Farnetani, 2003).

Acknowledgments

This work was supported in part by NSF Grants EAR-9508112 and EAR-0207764 and a George H. Deike, Jr.grant from the College of Earth and Mineral Sciences atthe Pennsylvania State University. I am grateful to N. Rog-ers and an anonymous reviewer for thoughtful and con-structive comments that improved both the manuscript


and my own thinking and to G. Yirgu for careful editorialhandling.

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Geochemistry of East African Rift basalts: An overview

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