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Journal of Petrology | Volume 45 | Number 5 | Pages 1069-1088 | 2004
Journal of Petrology 45(5) © Oxford University Press 2004; all rights reserved.
East African Rift System (EARS) Plume Structure: Insights from Quaternary Mafic Lavas of Turkana, Kenya
1 DEPARTMENT OF GEOSCIENCES, PENNSYLVANIA STATE UNIVERSITY, UNIVERSITY PARK, PA 16802, USA
2 DEPARTMENT OF EARTH SCIENCES, UNIVERSITY OF NEW HAMPSHIRE, DURHAM, NH 03824, USA
3 DIVISION OF EARTH AND OCEAN SCIENCES, DUKE UNIVERSITY, DURHAM, NC 27708, USA
4 VIA GRAZIANO 15, 00165 ROME, ITALY
RECEIVED JANUARY 20, 2003; ACCEPTED OCTOBER 31, 2003
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONRESULTSDISCUSSIONSUPPLEMENTARY DATAREFERENCES |
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Quaternary mafic lavas from Lake Turkana (northern Kenya) provide information on processes operating beneath the East African Rift in an area of anomalous lithospheric and crustal thinning. Inferred depths of melting beneath Turkana (15–20 km) are shallower than those recorded elsewhere along the rift, consistent with the anomalously thin crustal section. The mafic lavas have elevated incompatible trace element contents when compared with mid-ocean ridge basalts, requiring an enrichment event in the source region. Basalts with low Sr isotopic ratios (
0·7030) have high 143Nd/144Nd (>0·5129) and 206Pb/204Pb values (19·4) and incompatible trace element abundances that indicate derivation from a sub-lithospheric mantle source region. Quaternary mafic rocks with 10–15 wt % MgO record contributions from a mantle plume that is isotopically similar to the deep mantle source region for global hotspots. These Turkana basalts have isotope and incompatible trace element ratios that overlap with those of Quaternary mafic lavas from the Red Sea, the western Gulf of Aden, and northern Kenya, interpreted as being derived from mixtures of plume and ambient mantle sources. The Turkana data imply a common and long-lived mantle plume composition beneath both the Ethiopia and Kenya domes. This scenario is supported by tomographic results indicating a discontinuous thermal and chemical anomaly that originates in the deep mantle beneath southern Africa, and is also consistent with the seismically determined shallow mantle structure beneath East Africa. KEY WORDS: basalts; geochemistry; plume; rifting; East Africa
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONRESULTSDISCUSSIONSUPPLEMENTARY DATAREFERENCES |
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The East African rift system (EARS) provides an excellent opportunity to investigate plume-driven continental rifting. The eastern branch of the EARS extends over 2000 km from the Red Sea southward to Mozambique (Fig. 1). It crosses two regions of topographic uplift, the Ethiopian and Kenyan domes, both regarded as the surface manifestation of mantle plumes (e.g. Thiessen et al., 1979
). Between the domes lie the Turkana Depression and the older Anza graben, a zone of NW–SE extension that developed in the Early Cretaceous. The Turkana Depression is anomalous in that it is a site of Quaternary volcanism yet displays no uplift attributed to a mantle plume. For this reason the origin of the Turkana basalts is significant to understanding the thermal and chemical structure of the underlying mantle, and to determining the history of plume-related volcanism in the EARS.
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Volcanism in the EARS began in southern Ethiopia near 45 Ma (George et al., 1998
), followed by flood basalt activity in northern Ethiopia and Yemen at 30 Ma (Schilling et al., 1992; Pik et al., 1999). The temporal and spatial gap between the initial phase of volcanism and plume-driven flood basalt activity represents a fundamental geodynamic controversy surrounding the evolution of the EARS. Ebinger & Sleep (1998) suggested that Cenozoic magmatism throughout the EARS and west–central Africa results from the impact of a single, large mantle plume beneath the Ethiopian lithosphere at 45 Ma. However, the geochemical and geochronological results of George et al. (1998) and Rogers et al. (2000) suggest that magmatism in the Ethiopian (Afar) and Kenyan sectors of the EARS involved more than one mantle plume. Significantly, the geochemical and isotopic compositions of the earliest (45 Ma) southern Ethiopian basalts (George & Rogers, 2002) are distinct from those attributed to the Afar mantle plume (Hart et al., 1989; Vidal et al., 1991; Schilling et al., 1992; Deniel et al., 1994; Stewart & Rogers, 1996; Pik et al., 1999). This interpretation is complicated by the strong effects of lithospheric melting on basalt geochemistry observed in these and other areas of the EARS (Macdonald, 1994; Stewart & Rogers, 1996; Furman & Graham, 1999; Rogers et al., 2000; Macdonald et al., 2001; Späth et al., 2001).
We focus on the geochemistry of the Turkana rift basalts to document the thermal, mineralogical and chemical characteristics of the source region beneath this unique intra-domal region of the EARS. As part of determining the source characteristics for primitive Turkana basalts, we explore their relationship to compositions attributed previously to plumes underlying both Afar and central Kenya (Hart et al., 1989; Vidal et al., 1991; Schilling et al., 1992; Deniel et al., 1994; Pik et al., 1999; Rogers et al., 2000), and interpret these results in light of numerical models of plume formation and ascent (Farnetani et al., 2002). We also explore how spatial and temporal variations in the mafic lavas reflect heterogeneities in the source composition and melting conditions along the EARS over the past 40 Myr.
Recent tomographic studies indicate that Turkana at present overlies a region of anomalously low-velocity upper mantle that appears to be linked at depth to a plume that originates along the core–mantle boundary beneath southernmost Africa (Zhao, 2001; Ni et al., 2002). The geophysical results are ambiguous as to the connection between the anomalous velocity zones in the upper and lower mantle, leaving open the interpretation that Turkana may be part of one of the largest plumes on Earth. Alternatively, Turkana magmatism may derive from a shallow thermal and compositional anomaly related to asthenospheric upwelling during passive extension (e.g. Buck, 1986; King & Ritsema, 2000). Our results indicate that mafic Turkana lavas, as well as basalts from other portions of the Ethiopian and Kenya Rift, are derived from a source composition consistent with that observed in plume-derived lavas worldwide (FOZO, Hart et al., 1992; or C, Hanan & Graham, 1996). These results suggest that the depth of plume initiation beneath the EARS could be at the core–mantle boundary.
Geodynamic setting of the Turkana Rift volcanics
The broad Turkana Depression lies between the geophysical and topographic expressions of the Kenyan and Ethiopian domes (Fig. 1). The area comprises a wide (>150 km) zone of faulting that links the relatively narrow (<30 km), well-defined rift segments located to the north and south, but its relationship to the remainder of the EARS is poorly understood. Baker et al. (1977) and Cerling & Powers (1977) suggested initially that Lake Turkana is located west of the active rift zone, which has migrated gradually eastward to the Kinu Sogo fracture zone, the southern terminus of the Main Ethiopian Rift. Seismic surveys carried out by Project PROBE of Duke University, however, found a significant rift structure underlying the lake (Dunkleman et al., 1988) that suggests the area is a northern continuation of the Kenya Rift.
The greatest crustal extension within the EARS occurs in the Turkana region, where thermal doming is absent. The modern Turkana rift is about 150 km wide, roughly three times the normal width of most other portions of the EARS (Morley, 1994). Maximum ß stretching factors calculated for episodic crustal extension in Turkana increase from 1·25 for the Paleogene to 1·4 at the end of the Miocene, and reach a maximum value of 1·6 in the late Pliocene (Hendrie et al., 1994). A ß factor of 1·6 is unusually high for the EARS, but it is lower than the theoretical limit for decompression melting (ß 
2) during adiabatic upwelling of asthenosphere of ‘normal’ potential temperature (1280°C; McKenzie & Bickle, 1988). These observations suggest that a region of elevated mantle temperatures is needed to generate basalt lavas. We demonstrate below that the geochemistry of the mafic Turkana lavas indicates the influence of a mantle plume rather than the presence of anomalous (i.e. hydrated) mantle that is readily melted at normal ambient mantle temperatures.
Five composite volcanic centers are located within the Turkana Rift (Fig. 1): the Korath Range, North, Central and South Islands, and the Barrier. Two small volcanic centers, Bird Nest and Bug Island, are parasitic vents of Central Island. The Quaternary volcanoes are located near the centers of individual half-graben basins that are likely sites of maximum extension (Dunkleman et al., 1988). The rift basins have an axial spacing of 51 ± 6 km, somewhat greater than observed elsewhere in the eastern rift (43 ± 5 km; Mohr & Wood, 1976). The Quaternary geology and petrology of the Turkana region have been described previously (Brown & Carmichael, 1969, 1971; Williams et al., 1984; Curtis, 1987, 1991; Bloomer et al., 1989; Dunkley et al., 1993; Karson & Curtis, 1994) and are only summarized here.
Geology and petrology of the Turkana Rift volcanoes
The Turkana volcanoes show along-axis variations in both geochemistry and eruptive style. Basanites and tephrites from the Korath Range are the most silica-undersaturated lavas, with several percent normative nepheline (Brown & Carmichael, 1969). Korath tephrites contain amphibole phenocrysts and groundmass phlogopite whereas all other Turkana lavas lack hydrous phases. North, Central and South Islands have erupted mildly alkalic suites of lavas and pyroclastic rocks, with compositions ranging from olivine basalt to trachyte and phonolite (Bloomer et al., 1989; Karson & Curtis, 1994). North Island has only a few major lava flows and subaerial exposures primarily consist of pyroclastic units. Central Island is also dominantly pyroclastic, but has several basalt and hawaiite lava flows, and the eastern half of the island is covered by mafic lavas. South Island is covered primarily by basaltic lava flows and spatter cones, with smaller volumes of evolved lavas and pyroclastics. The Barrier has a wide compositional range of lavas, with subordinate pyroclastic units, and includes phonolites and basanites as well as a mildly alkalic basalt– trachyte sequence (Williams et al., 1984).
We focus on mafic lavas from South and Central Islands (including Bird Nest) as well as the Barrier to explore variations in lava composition and source chemistry. Samples were collected by P. Curtis and J. Karson, and petrographic descriptions have been reported by Curtis (1991). Most samples contain phenocrysts of olivine, clinopyroxene and plagioclase feldspar within a holocrystalline, intergranular groundmass of these minerals plus Fe–Ti oxides. There is no petrographic evidence for olivine accumulation among the highly magnesian basalts. Quaternary lavas reported by Bloomer et al. (1989) were reanalyzed by W. Meurer at Duke University for consistency with the present dataset (Table 1), which includes 34 additional mafic lavas. Sr and Pb isotopic data obtained at Woods Hole Oceanographic Institution (Curtis, 1991) have been augmented by additional Sr–Nd–Pb isotope analyses performed at the University of California, Santa Barbara (UCSB; Table 2).
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RESULTS |
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TOPABSTRACTINTRODUCTIONRESULTSDISCUSSIONSUPPLEMENTARY DATAREFERENCES |
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Major and compatible trace elements
Quaternary lavas from Turkana are geochemically similar to alkali basalts from the Huri Hills, Samburu and Marsabit volcanic fields on the eastern flank of the rift in northern Kenya (Key et al., 1987
; Tatsumi & Kimura, 1991; Class et al., 1994). Each Turkana volcano has erupted lavas over a wide compositional spectrum, but we focus here on the most mafic compositions (additional data are available at http://www.petrology.oupjournals.org). Basalts with 9–14 wt % MgO were sampled at every volcanic center except North Island. Turkana mafic lavas are generally mildly alkaline (
7·5% normative nepheline) and are classified as basalts and hawaiites on the basis of their total alkali and silica contents (Fig. 2). Three samples from Central Island and Bird Nest (CI-32, BN-4, BN-2C) are classified as picrites on this basis, although BN-2C contains only 7·8 wt % MgO.
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Curtis (1991)
demonstrated that major element variations among Central Island lavas were consistent with progressive fractionation of olivine, with subordinate clinopyroxene and minor plagioclase feldspar, from parental basalts but that some geochemical features of the suite required source heterogeneity. This heterogeneity is apparent in, for example, a plot of TiO2 against MgO (Fig. 3a); the relationship between P2O5 and MgO is remarkably similar. It should be noted that the compositional range observed in Quaternary Turkana basalts is comparable with that of southern Ethiopian lavas that date from 45 Ma to Recent (Stewart & Rogers, 1996; George & Rogers, 2002), or of young basalts from axial portions of the entire Kenya Rift (Macdonald et al., 2001).
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Primitive Turkana lavas are among the most highly magnesian basalts sampled from the Kenyan and Ethiopia rifts (Fig. 3). Sparsely phyric Turkana lavas with 10–15 wt % MgO (Mg number 69–75) have compatible trace element abundances that indicate they have undergone little fractionation since separating from their mantle source (i.e. 200–350 ppm Ni, 350–600 ppm Cr, 25–35 ppm Sc).
Incompatible trace elements
In general, Turkana mafic lavas show restricted variations in incompatible trace element abundances and ratios. Abundances of high field strength elements (HFSE) and rare earth elements (REE) do not vary smoothly when plotted against MgO, but correlate strongly with one another (Fig. 4). La/Nb and Ba/Nb values measured at Turkana are slightly lower than those of global ocean island basalts (Fig. 5a). Similar ranges in La/Nb–Ba/Nb are observed among Miocene and younger lavas from southern Ethiopia, Huri Hills, the Red Sea, Djibouti and the South Chyulu Hills, as well as high-titanium Oligocene flood basalts from northern Ethiopia that Pik et al. (1999) interpreted as plume-derived melts. In contrast, older basalts from southern Ethiopia and low-titanium Oligocene flood basalts (derived from a mixture of plume material and depleted mantle; Pik et al., 1999) extend to higher values of both Ba/Nb and La/Nb.
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) and Central Island. The majority of mafic lavas from South Island do not appear to have been formed from an amphibole-bearing source region.Bloomer et al. (1989)
noted that patterns of relative trace element enrichment vary along the Turkana rift axis. Primitive mantle-normalized incompatible trace element diagrams show generally coherent behavior throughout the Turkana suite, but some elements show substantial heterogeneity that requires variations in the source material, melting conditions, and/or crystal fractionation histories. The greatest variation is observed in K, which has prominent negative anomalies in samples from Bird Nest and Central Island but has small negative to slight positive anomalies among South Island basalts (Fig. 5b). Lavas from Central Island and the Barrier show variable Ba enrichment and U depletion that are not observed at South Island. Mafic lavas from South Island typically have the lowest values of Ba/Th and Ba/Rb and the highest values of (K/Nb)n and (K/La)n measured at Turkana (Fig. 5c). A few samples from Central Island and Bird Nest are geochemically anomalous with, for example, unusually high Ba/Rb and low (K/Nb)n and (K/La)n; these samples will be discussed separately.
Rare earth elements
Chondrite-normalized REE diagrams for all Turkana basalts show light REE enrichment relative to heavy REE. Values of La/Smn and La/Ybn correlate well with La, and with one another, in the Turkana suite as a whole (Fig. 6a); lavas from the Barrier have slight positive Eu anomalies. Chondrite-normalized Tb/Yb values are roughly constant among the Turkana suite, and overlap values for the 45 Ma to Recent basalts from southern Ethiopia (Stewart & Rogers, 1996; George & Rogers, 2002). In contrast, Oligocene high-Ti mafic lavas from the Ethiopian plateau (Pik et al., 1999) and young basalts from much of the Kenya Rift (Macdonald et al., 2001; Späth et al., 2001) have higher Tb/Ybn values (Fig. 6b). Tb/Ybn values are difficult to fractionate in the absence of residual garnet, and most of the Turkana data thus suggest melting in the absence of garnet. The highest Tb/Ybn value measured at Turkana is that of the anomalously incompatible-element-rich sample BN-2C. Taken together, these REE data suggest that mafic Turkana lavas are derived by variable degrees of partial melting over a depth range that is located dominantly above the garnet–spinel transition.
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Radiogenic isotopes
Lavas from South Island have among the lowest 87Sr/86Sr values reported to date from the EARS (0·7030). Turkana mafic lavas plot within the field defined by Kenya Rift lavas from mobile belt regions (Class et al., 1994
; Macdonald et al., 2001) and overlap mafic samples from the Red Sea (Volker et al., 1997), but are displaced towards consistently lower 87Sr/86Sr values than mafic lavas from the remainder of the Ethiopian and Kenya Rifts (Fig. 7a). The Turkana samples have Nd-isotope ratios comparable with those of mafic lavas from Djibouti (Deniel et al., 1994) and portions of the Main Ethiopian Rift and Afar (Hart et al., 1989), as well as Oligocene Ethiopian flood basalts (Pik et al., 1999); however, the Sr-isotope ratios measured at Turkana are consistently lower than those determined in these other suites.
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Sr isotope variations within the Turkana suite correlate with the incompatible trace element characteristics of the mafic lavas. For example, the incompatible element-rich samples from Central Island and Bird Nest have anomalously high 87Sr/86Sr values (Fig. 7a). It seems unlikely that the high 87Sr/86Sr values result from open-system processes in crustal magma chambers, as these lavas have MgO > 7·5 wt % and Sr concentrations >800 ppm. We suggest instead that these samples represent melts of enriched mantle lithosphere; their relatively high 143Nd/144Nd values indicate that the enrichment is recent.
Although the Quaternary Turkana basalts have low 87Sr/86Sr values they have relatively radiogenic Pb isotope compositions (Fig. 7b). This relationship is observed also among 11–19 Ma basalts from neighboring southern Ethiopia (George & Rogers, 2002), and Miocene and younger mafic lavas from the central Kenya Rift (Davies & Macdonald, 1987; Class et al., 1994; Macdonald et al., 2001; Späth et al., 2001), although these latter groups extend to even higher Pb isotope values (Fig. 7b). Lavas from the Barrier, South Island, and Bird Nest have more highly radiogenic Pb isotope signatures than those from Central and North Islands (Table 2), with little overlap among mafic samples from individual centers. Values measured at Turkana overlap the field of Red Sea basalts (Volker et al., 1997) and extend to more radiogenic compositions than samples from Djibouti (Deniel et al., 1994) and the mobile belt portions of the Kenya Rift (Macdonald et al., 2001). Excluding the 11–19 Ma suite described above, Ethiopian basalts ranging in age from Eocene to Recent (Hart et al., 1989; Stewart & Rogers, 1996; Pik et al., 1999; George & Rogers, 2002) are restricted to less radiogenic Pb isotope compositions than the Turkana lavas.
206Pb/204Pb values show a weak negative covariation with Ce/Pb in mafic Turkana samples and selected Kenya Rift basalts (Fig. 8). Samples with the highest 206Pb/204Pb have typical mantle ratios of Ce/Pb (25 ± 5; Hofmann et al., 1986), whereas lavas with lower 206Pb/204Pb have Ce/Pb ratios up to 35. This trend cannot be explained by involvement of continental crust, in which Ce/Pb ratios are below 15. In contrast, samples of all ages from southern Ethiopia—as well as the high-TiO2 Oligocene lavas—have low Ce/Pb values that are suggestive of crustal contamination (Fig. 8).
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, samples from the Barrier reported by Macdonald et al. (2001) |
DISCUSSION |
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TOPABSTRACTINTRODUCTIONRESULTSDISCUSSIONSUPPLEMENTARY DATAREFERENCES |
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Source regions for Quaternary EARS mafic lavas
Geodynamic considerations make the Turkana region ideal for addressing the linkages between continental rift evolution and extensional magmatism above a mantle plume. Beneath most of the EARS, trace element and isotope signatures indicate consistently that mafic volcanism is dominated by lithospheric melts (e.g. Macdonald, 1994
; Rogers et al., 2000; Macdonald et al., 2001). In contrast, lavas from Turkana, Huri Hills (Class et al., 1994) and southernmost Ethiopia (Stewart & Rogers, 1996; George & Rogers, 2002) provide insight into processes occurring beneath the highly extended region between the hotspot domes. This volcanic area is unusual because the seismically determined depth to Moho is locally only about 20 km (Simiyu & Keller, 1997, and references therein), indicating significant thinning of the lithosphere (Hendrie et al., 1994; Mechie et al., 1994) and/or underplating of mafic magmas (Karson & Curtis, 1989). The relatively high extension rates and thin lithosphere favor eruption of near-primary basalts. We address below the question of whether the Turkana rift basalts are predominantly derived from sublithospheric sources, or whether crustal and lithospheric mantle components play a critical role in their petrogenesis. Finally, we discuss possible spatial and temporal variations in source composition as indicated by the geochemistry of a broad suite of EARS mafic lavas.
Depth and degree of melting
The SiO2 and FeO* contents of the most primitive Turkana lavas (back-corrected for olivine fractionation; Fig. 9) plot near the range of experimentally derived melts of fertile peridotite at pressures of 15–20 kbar (Hirose & Kushiro, 1993; Baker & Stolper, 1994; Kogiso et al., 1998). These compositions also correspond to experimental melts of depleted peridotite at 20–30 kbar (Hirose & Kushiro, 1993; Baker & Stolper, 1994), but as discussed below we find no compelling evidence for garnet in the Turkana source region.
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The approximate degree of melting can be constrained by analogy to experimental melts of peridotite at shallow mantle conditions (10 kbar; Wasylenki et al., 2003
). The low FeO* and SiO2 contents of the Turkana mafic lavas suggest melting in the presence of residual clinopyroxene, corresponding to a melt fraction of at most 7%. For the purpose of this discussion, the persistence of clinopyroxene throughout the melting interval is more important than the exact melt percentage itself.
Geochemical characteristics of the Quaternary source regions
Crustal contamination
For most primitive Turkana mafic lavas, there is no compelling evidence for assimilation of crustal materials. There is no correlation between either MgO or SiO2 and 
Nd or 87Sr/86Sr, and trace element ratios sensitive to contamination (e.g. Ce/Pb, Ba/U) generally fall within the range of mantle values. There are a few exceptions, such as samples BR-05 and KCH-05, which have anomalously low Ce/Pb (<15) and high Ba/U (>500), and their parent magmas may well have interacted with the continental crust. We cannot rule out any incorporation of crustal materials into the magma supply, but we can identify these outliers where they occur, and concur with previous workers (e.g. Baker et al., 1977; Class et al., 1994; Furman & Graham, 1999; Rogers et al., 2000; Macdonald et al., 2001) that geochemical variations among the primitive mafic lavas reflect primarily heterogeneities in the mantle source region rather than the effects of crustal contamination.
Evidence for lithospheric melting
It has long been apparent that lithospheric mantle melting plays an important role in magmatism throughout the EARS, where Quaternary and older lavas erupt through regions of moderate crustal thickness along the main rift axes (e.g. Macdonald, 1994; Stewart & Rogers, 1996; Furman & Graham, 1999; Rogers et al., 2000; Macdonald et al., 2001; Späth et al., 2001). Indeed, mafic lavas from southern Ethiopia, Afar, and the Main Ethiopian and Kenya Rifts trend convincingly towards enriched mantle isotopic compositions (Fig. 7). Among the Quaternary Turkana suite, a subset of samples from Central Island and the parasitic Bird Nest vent is apparently dominated by contributions from the lithospheric mantle. These lavas (CI-31, CI-32, CI-37B, BN-2C, BN-4) do not define a single coherent geochemical composition, but share common features including generally high incompatible trace element contents, anomalously high Ba/Rb and 87Sr/86Sr values, and low K/Nb and K/La. The trace element signatures are consistent with melting of an amphibole-bearing source region, and the radiogenic Sr isotope values clearly require prolonged isolation of this source from the convecting asthenosphere. It is interesting to note that the Pb isotope signatures of these unusual samples include both the most radiogenic (BN-2C, BN-4) and least radiogenic (CI-31, CI-32) values recorded in the Turkana suite. This heterogeneity is not easily interpreted and is the subject of continuing study.
The spatial scale of isotopic variation at Turkana is very small: mafic lavas from Bird Nest and Central Island span the entire range of data, and include both non-radiogenic and highly radiogenic (i.e. mantle lithospheric) compositions. This juxtaposition requires that individual volcanic centers can tap small-scale heterogeneities within the source region or that there is a steep compositional gradient across the zone of melting, presumably reflecting either local penetration of upwelling sub-lithospheric material or mechanical removal of thinned lithosphere.
Evidence for a mantle plume source beneath Turkana
Sr–Nd–Pb isotopic compositions indicate that sub-lithospheric melting is important beneath the Turkana rift. This result supports predictions made on the basis of the thin crustal section observed seismically (Mechie et al., 1994) and inferred from surface structures. The range of isotope compositions observed among primitive Quaternary Turkana lavas (87Sr/86Sr 0·7030–0·7035, 143Nd/144 Nd0·5129–0·5130 and 206Pb/204Pb 19·0–19·4; Fig. 7) is similar to proposed common source values for hotspot magmas worldwide, termed FOZO (Hart et al., 1992) or C (Hanan & Graham, 1996). The lithophile isotopic compositions are far less radiogenic than products from metasomatized Tanzanian lithosphere (see Paslick et al., 1995; Rogers et al., 2000; Macdonald et al., 2001). The source composition that we recognize in Turkana has slightly lower 87Sr/86Sr than that suggested for the modern Afar mantle plume (87Sr/86Sr 0·7033–0·7034, 143Nd/144Nd 0·5129, 206Pb/204Pb 19·4; Schilling et al., 1992), and is found in several areas that lie within the proposed radial extent of this plume (i.e. Turkana, Huri Hills, Red Sea, West Sheba Ridge). The regional geochemical similarities we identify are important because they show the distribution of common source materials. These results suggest that a single mantle plume—or multiple plumes with a common composition—could contribute to Quaternary volcanic activity from the central Red Sea to northern-most Kenya.
Several lines of geochemical evidence support the interpretation that a plume contributes materially as well as thermally to magmatism at Turkana, particularly at South Island. First, most South Island mafic lavas show no evidence for melting in the presence of hydrous mantle phases (amphibole or phlogopite) as indicated by their high K/Nb and K/La (Fig. 5b), and low Ba/Rb and Rb/Sr values. These same primitive lavas—considering only samples that show no petrographic evidence of olivine accumulation—have higher Mg numbers than those reported from other portions of the EARS, and thus require high degrees of melting and low extents of fractionation relative to conditions inferred, for example, along the Main Ethiopian Rift (Hart et al., 1989). The major element compositions and low Tb/Ybn values of the Turkana samples (Figs 6 and 9) indicate that these lavas are derived at the lowest pressures of melting of all sample suites of the EARS, comparable only with lavas from the spreading ridge axis in the central Red Sea. At the same time, the overall enriched incompatible trace element abundances in those Turkana mafic lavas with 87Sr/86Sr values 0·7030 are difficult to reconcile strictly with a depleted mid-ocean ridge basalt (MORB) mantle source, but are instead more consistent with contributions from a mantle plume.
The inferred source mineralogy beneath Turkana is that of fertile peridotite, again suggesting that a mantle plume component is involved in melt generation. Figure 10 is a plot of Ce/Y against Zr/Nb, showing the range of melt compositions expected to form through non-modal fractional melting of fertile spinel and garnet peridotite (Hardarson & Fitton, 1991). The Turkana and Huri Hills mafic lavas cluster tightly along the spinel peridotite melting curve, whereas Kenya rift lavas from the mobile belt and cratonic regions range over inferred pressures that span the garnet–spinel transition (Macdonald et al., 2001). Ethiopian Oligocene high-Ti flood basalts plot along the garnet peridotite melting curve, consistent with major element evidence from Fig. 9.
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Values of La/Smn and Zr/Y are positively correlated among mafic Turkana lavas and Huri Hills alkali basalts (Fig. 10 inset). This covariation is expected during progressive melting of a clinopyroxene-rich source region because the relative compatibilities of these elements in clinopyroxene are Y > Zr > Nb and Sm > La (Johnson & Kinzler, 1989
; Kelemen & Dunn, 1992; Hart & Dunn, 1993; Hauri et al., 1994; Johnson, 1994; Blundy et al., 1998). As noted above, we infer these samples represent moderate degrees (5–7%) of melting; it is important in the present context to recognize the presence of this clinopyroxene-rich source material at shallow levels beneath Turkana. This interpretation has important implications for the thermal structure of the underlying mantle. The shallow depths of anhydrous melting require elevated mantle potential temperatures beneath Turkana, and thus support the presence of a mantle plume as suggested on the basis of lithophile radiogenic isotopic geochemical data.
Source mixing beneath the EARS
The distinctive geochemical characteristics of the Turkana basalts are found in three other Quaternary volcanic regions: Huri Hills, central Red Sea and the Gulf of Aden. Mafic lavas from these areas have Ba/Nb–La/Nb, Zr/Nb–Ce/Y and La/Smn values and Sr–Nd–Pb isotope ratios that overlap those of the Turkana samples. The spatial proximity of Huri Hills to Turkana makes it reasonable to suggest that volcanism in these two areas could reflect a common source material. The distance to the Red Sea and Gulf of Aden, however, makes this direct analogy inappropriate, particularly as the geochemistry of the mafic lavas erupted along the Ethiopian dome is markedly different from those observed in Turkana.
At the same time, there are geochemical differences between the Quaternary mafic lavas of the Main Ethiopian Rift (including Erta Ale, Afar and Djibouti) and those described herein from Turkana, Huri Hills, the Red Sea and the Gulf of Aden that have led previous workers (e.g. Rogers et al., 2000; Macdonald et al., 2001) to suggest the presence of two geochemically distinct plumes beneath the Ethiopian and Kenya domes. First, the Sr-isotope compositions of the Ethiopian and Djibouti basalts are consistently higher at a given Nd-isotope value than those observed at Turkana (Fig. 7). Second, mafic lavas with high 3He/4He (15–19 R/RA; Marty et al., 1996) have been sampled in the Ethiopian Rift (the average 3He/4He value measured is 8·24; Marty et al., 1996). In Kenya, the highest 3He/4He value measured in groundwater is 8 R/RA (Darling et al., 1995; Marty et al., 1996; Scarsi & Craig, 1996), whereas in the Red Sea, Moreira et al. (1996) reported 3He/4He values up to 14·5 R/RA (one sample >10 R/RA). Unfortunately, there is no comparable dataset of He-isotope analyses of mafic lavas from Erta Ale, the Kenya Rift or the Turkana region. It is significant in this context that the geochemical signatures observed at Turkana are also present to the north of the Ethiopian dome, complicating the geodynamic picture somewhat with respect to one- and two-plume models.
We interpret the Turkana, Huri Hills, Red Sea and Gulf of Aden suites as representing mixtures of plume material with ambient upper mantle (and, in a few cases as described above, lithosphere) in two highly extended regions some 1000 km apart on either side of the Ethiopian dome. This interpretation is consistent with earlier investigations of the Red Sea (e.g. Volker et al., 1997). In this scenario, the lithophile radiogenic isotope values of the Turkana mafic lavas reflect mixing of sources with similar Nd-isotope values but different Sr-, Pb- and He-isotope signatures. Specifically, we interpret the Turkana data to indicate mixing between a composition equivalent to that of the modern Afar plume (Schilling et al., 1992) and one derived from a distinct upper-mantle source region, perhaps analogous to that of the East Sheba Ridge (Schilling et al., 1992).
Mixing among different source regions is seen in both the Sr–Nd–Pb isotope and trace element characteristics of the Quaternary suites. At Turkana, increasing Sr-isotope ratios at near-constant Zr/Nb indicate lithospheric involvement, whereas variations in Zr/Nb at a given Sr-isotope value probably reflect variable degrees of source melting (Fig. 11a). It is clear that mixing between lithosphere and an Afar plume composition cannot explain the low 87Sr/86Sr–Zr/Nb values observed at Turkana, Huri Hills and the Red Sea. We infer involvement of ambient asthenosphere for these samples, which define a field distinct from Djibouti and Erta Ale (the apparent locus of modern Afar plume magmatism; Schilling et al., 1992; Deniel et al., 1994; Barrat et al., 1998). Variations in La/Smn against 87Sr/86Sr also support this mixing model (Fig. 11b). In this case, MORB samples from the Eastern and Western Sheba Ridges (Schilling et al., 1992) clearly provide an appropriate contribution to the observed geochemical signatures. The covariations in 87Sr/86Sr–Zr/Nb–La/Smn observed in these suites are not readily explained through variable degrees of melting of a homogeneous source, but are consistent with the mixing scenario proposed here. We suggest that the plume incorporates ambient mantle during ascent in areas of high extension, or alternatively that the rising hot plume induces melting of fertile regions within the overlying mantle column.
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This scenario is consistent with predictions based on numerical modeling of mixing within plume heads and tails (Farnetani et al., 2002
). These numerical models suggest that upwelling plume heads do not entrain significant volumes of upper-mantle material, but rather incorporate geochemical heterogeneities from their source region near the core–mantle boundary. That is, basalts with deep-mantle isotopic characteristics erupted in the Red Sea, the Gulf of Aden, and the Turkana Depression may represent deep mantle material that ascends as heterogeneities within the plume itself (Farnetani et al., 2002). Alternatively, this relatively undepleted mantle material may form the majority of the plume tail but may be preferentially erupted in these selected zones of lithospheric thinning. In contrast, in portions of the EARS where extension is less well developed there is a greater role for melting of the mantle lithosphere.
Variations in 206Pb/204Pb–La/Smn support the mixing scenario proposed above (Fig. 11c), although Pb isotopic signatures are remarkably variable in EARS lavas with identical Sr and Nd isotopic compositions. One possibility for the variations in Pb isotopic composition may come from melt generation within a region that is isotopically rather heterogeneous with respect to Pb on short length- and time-scales. Our results are in concert with several recent Pb isotopic studies of oceanic basaltic rocks. High-precision Pb isotopic measurements in Hawaiian lavas have been used to identify several distinct Pb isotopic reservoirs that are tapped by individual Hawaiian volcanoes (see Abouchami et al., 2000; Eisele et al., 2003), which are erupted over time-scales significantly shorter than those represented in the products of the EARS. Recent studies of Pb isotopic compositions in phenocrysts (Bryce & DePaolo, 2004) and melt inclusions (Saal et al., 1998) in oceanic and continental basaltic rocks provide evidence for highly variable Pb isotopic compositions that are delivered to the same magma chamber over even shorter time-scales: those of phenocryst crystallization (1–1000 years). Another possibility, which clearly applies to some suites of EARS lavas in Fig. 7, is that this Pb heterogeneity results from lithospheric contamination (Paslick et al., 1995; Furman & Graham, 1999; Macdonald et al., 2001).
The documented presence of a deep mantle component beneath the Turkana Depression as well as the Red Sea and Gulf of Aden regions does not enable us to distinguish between models that require one plume and those that favor two plumes with a common isotopic component. To address this question, we turn first to a temporal comparison of Oligocene and modern magmatism, and then to inferences based on seismic tomography.
Temporal history of plume-related magmatism along the EARS
The presence of plume-derived mafic lavas in the modern Turkana Depression raises significant geodynamic issues related to the evolution of the EARS. It is important to reconcile this geochemical evidence with the topographic and geophysical indications of asthenospheric upwelling both north and south of Turkana, with the temporal evolution of basalt geochemistry observed along the Rift, and also with recent tomographic data on the thermal structure of the shallow and deep mantle beneath eastern Africa.
There is a general consensus that the modern Afar plume underlies the Ethiopian plateau and the triple junction region that encompasses the southern Red Sea and Gulf of Aden. It is also widely accepted that the onset of volcanism in northern Ethiopia corresponds to the impact of a mantle plume head beneath the modern Afar region, as reflected in the Oligocene Ethiopian continental flood basalt province (e.g. Hart et al., 1989; Vidal et al., 1991; Schilling et al., 1992; Marty et al., 1996; Pik et al., 1999). Two significant questions remain: (1) Are both Oligocene Ethiopian flood basalts and Recent Afar basalts related to a single long-lived plume? (2) What is the relationship between the Afar plume and eruptive activity in the Kenya Rift? Ebinger & Sleep (1998) have suggested that magmatism throughout most of Africa could be related to a single deep mantle plume that initiated basalt volcanism in southern Ethiopia at 45 Ma. In this model, upwelling plume material and associated basalt melts are channeled along pre-existing ‘thin’ zones in the lower lithosphere until they reach regions of thin crust, such as that developed at Turkana, where eruptive activity takes place. In contrast, George et al. (1998), Rogers et al. (2000) and George & Rogers (2002) have suggested that two distinct plumes are required—one beneath modern Kenya and the other beneath the modern Afar. In this model, the southward migration of basalt activity from southern Ethiopia towards Tanzania reflects the northeastward movement of Africa over the Kenya plume for about 50 Myr, whereas magmatism in northern Ethiopia reflects 30 Myr of sustained activity of the Afar plume. The present results on mafic lavas from the Turkana Depression are clearly central to this debate.
Temporal variations in the isotopic and trace element signatures of basalts from southern Ethiopia led George et al. (1998) and George & Rogers (2002) to suggest that progressive rifting is associated with melting of more than one distinctive source region. This feature is seen clearly in Fig. 7, where 11–19 Ma lavas from southern Ethiopia are isotopically distinct from older (30–45 Ma) basalts from the same region. The low Ce/Pb and generally high 87Sr/86Sr values of these mafic lavas (Figs 7 and 8), however, suggest that enriched mantle source regions—rather than a mantle plume—control their eruptive chemistry. Our analysis indicates that the geochemistry of all but a few individual basalts from southern Ethiopia reflects melting of mantle lithosphere, and thus does not help to constrain the composition of one or more mantle plumes. Interestingly, the inferred lithosphere composition for southern Ethiopia basalts erupted from 30 to 45 Ma is clearly distinct from that involved in magmatism at 11–19 Ma (Fig. 11). Identification of the latter composition at Turkana suggests that the local lithosphere is vertically heterogeneous, and that progressive rifting has exposed different compositional units to melting through time. This observation is supported by isotopic and compositional studies of continental mantle xenoliths (e.g. Carignan et al., 1996; Lee & Rudnick, 1999).
Turkana basalts from 2 to 40 Ma have geochemical signatures that generally overlap those of the Quaternary suite (Figs 10 and 12), although older samples tend to have higher Sr-isotope values (>0·70311; Knight & Furman, 2003). This long-term dataset will be explored more fully elsewhere (Knight & Furman, in preparation) but is useful here for comparative purposes. The Sr-isotope values of both Tertiary and Quaternary suites at Turkana are lower than values inferred for either of the two plumes proposed by Rogers et al. (2000), whereas the Nd-isotope values of the Turkana lavas overlap those of the modern Afar plume rather than the proposed Kenya plume (Fig. 12). These observations require that the combined contributions to Turkana magmatism have not changed significantly since before the onset of plume-related magmatism in central Ethiopia and hence are inconsistent with a model that requires two compositionally distinct plumes.
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There is some indication that the portion of the plume undergoing melting may have changed over the past 30 Myr. Oligocene high-Ti mafic flood basalts from central Ethiopia are interpreted to represent the original plume head composition of the Afar plume (Pik et al., 1999
). These lavas do not have geochemical signatures suggesting that lithosphere was involved in their genesis (unlike the low-Ti suite, for which lithospheric involvement can be demonstrated convincingly). Modern Afar plume lavas from Djibouti and Erta Ale have lower Sr-isotope ratios than the Oligocene samples, although the Nd-isotope compositions of these two groups overlap. It is possible that the portion of the plume undergoing melting is now compositionally distinct from the portion that melted in the Oligocene. Alternatively, the Djibouti and Erta Ale samples—the center of the inferred modern Afar plume region—may represent mixtures of Oligocene-like plume material with more depleted mantle as well. These two models cannot be distinguished from the available data, and this distinction does not affect our interpretation of the Turkana mafic lavas.
Geophysical evidence
Geophysical evidence for both shallow and deep mantle structure beneath the EARS supports the implications drawn from our geochemical data. Nyblade et al. (2000b) presented seismic evidence for a depression of the 410 km discontinuity beneath central Tanzania, noting an absence of corresponding topography along the 660 km discontinuity. These features suggest recent ascent of a mantle plume head along the eastern side of the Tanzania craton, followed by lateral flow of plume material along the craton margin beneath the eastern and western rift zones. The tail of the inferred plume deepens beneath Kenya, and indeed may be centered underneath the Kenya dome, yet there is no evidence to suggest that it extends northward to the Afar region. Farther north, the Afar region shows a strong thermal anomaly that is located above the 410 km discontinuity. Unlike the situation in central Tanzania, this thermal anomaly does not perturb either the 410 km or the 660 km mantle structure, suggesting that it is confined to the shallowest region of the upper mantle.
These shallow mantle structures do not support the presence of a single, continuous plume as proposed beneath Ethiopia (Ebinger & Sleep, 1998) or the Afar (Schilling et al., 1992), which cannot reasonably affect the 410 km discontinuity beneath Tanzania while remaining at very shallow levels beneath Afar itself. Rather, the data support a scenario in which thermally anomalous material was derived from a discontinuous (or ‘blobby’) plume head at some earlier time—perhaps during the onset of flood basalt volcanism at 30 Ma.
Further insight comes from tomographic studies, which indicate a strong low-velocity anomaly over 1000 km wide in the deep mantle beneath East Africa (Ritsema et al., 1999; Zhao, 2001; Ni et al., 2002). What remains unclear, however, is the relationship between this deep ‘superswell’ that is probably both thermal and chemical in origin (Ritsema et al., 1999; Gurnis et al., 2000; Ni et al., 2002) and the pronounced thermal anomaly beneath northern Ethiopia observed at depths less than about 400 km (Nyblade et al., 2000a; Benoit et al., 2003; Nolet et al., 2003). Recent modeling by Zhao (2001) is consistent with a single discontinuous plume model for EARS volcanism, indicating a thermal anomaly located beneath the EARS in the region between Afar and Lake Victoria—perhaps coincidentally in the vicinity of Lake Turkana. The resulting structure suggests that a single deep mantle thermal anomaly beneath south–central Africa is deflected by mantle flow to form a sloping conduit as detailed by Ni et al. (2002). These tomographic results are compatible with the geochemical data presented here, as the recognition of a deep mantle component at Turkana as well as in the Red Sea area strongly favors a deep mantle origin for the source materials. Further resolution of this issue awaits detailed seismic investigation across the central and northern EARS.
Summary
Quaternary mafic lavas from the Lake Turkana region, northern Kenya, provide insight into processes operating beneath the East African Rift in an area of anomalously well-developed lithospheric thinning. Turkana eruptive products display a wide range in incompatible trace element abundances and Sr–Nd–Pb isotope ratios that are probably the result of variations in both source composition and melting conditions. We attribute the distinctive chemistry at Turkana to interaction between a mantle plume and the overlying lithosphere. Extreme compositional diversity exists on a spatial scale of <1 km, requiring steep topography across the asthenosphere– lithosphere boundary or lateral dike injection from distinct source regions. The apparent depths of melting beneath Turkana (15–20 kbar) are shallower than those recorded elsewhere along the EARS, suggesting that basaltic liquids are derived from decompression melting of ascending asthenospheric material, which we infer to be a mantle plume.
The Sr–Nd–Pb isotope ratios of mafic rocks with 10–15 wt % MgO define a tight cluster within the range of the deep mantle component common to plumes worldwide. Basalts from Turkana and the neighboring Huri Hills province (Class et al., 1994) are the only EARS suites to record this composition, although it is found also in young basalts from the Red Sea (Volker et al., 1997) and the western Gulf of Aden (Schilling et al., 1992). These sample suites also have overlapping incompatible trace element ratios (e.g. Ba/La, Ba/Nb, Zr/Nb) that indicate derivation from a common source region. The combined geochemical data for these suites, however, do not overlap the modern Afar plume composition, but rather are consistent with mixing between a mantle plume and ambient asthenospheric mantle. Our data thus suggest that a common source composition exists at present beneath both the Ethiopian and Kenyan domes. By integrating geochemical and geophysical evidence, we suggest that EARS volcanism derives in large measure from a discontinuous plume that originates in the deepest mantle beneath southern Africa (Zhao et al., 2001; Ni et al., 2002). Future coupled geochronologic, isotopic and geochemical studies of Tertiary and Quaternary centers in the EARS will provide a means to distinguish whether the plume has undergone compositional evolution over time, or whether the EARS plume has a complex spatial zonation apparent in other plumes elsewhere.
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SUPPLEMENTARY DATA |
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TOPABSTRACTINTRODUCTIONRESULTSDISCUSSIONSUPPLEMENTARY DATAREFERENCES |
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Supplementary data for this paper are available on Journal of Petrology online.
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ACKNOWLEDGEMENTS |
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Samples and extensive field, analytical and petrographic notes were kindly made available by P. Curtis. We are grateful to E. Klein and W. Meurer for major and trace element analyses. J.G.B. thanks G. Tilton for access to the UCSB isotope facilities. This work was supported in part by NSF CAREER EAR-9508112, EAR-0207764 and a George H. Deike Jr. grant from the Pennsylvania State University to T.F. J.G.B. received support from NSF EAR-0207939 and EAR-0338385 during the final stages of writing. The manuscript benefited greatly from thoughtful reviews by W. K. Hart, R. Macdonald and S. Gibson, as well as from comments on an earlier version by N. Rogers.
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FOOTNOTES |
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* Corresponding author. Telephone: 814-865-5782. Fax: 814-863-7823. E-mail: furman{at}geosc.psu.edu
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