Journal of Petrology

Journal of Petrology Advance Access originally published online on March 21, 2006
Journal of Petrology 2006 47(6):1221-1244; doi:10.1093/petrology/egl009

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Tertiary Mafic Lavas of Turkana, Kenya: Constraints on East African Plume Structure and the Occurrence of High-µ Volcanism in Africa

TANYA FURMAN^1,*, KELLY M. KALETA², JULIA G. BRYCE³ and BARRY B. HANAN⁴

¹ DEPARTMENT OF GEOSCIENCES, PENNSYLVANIA STATE UNIVERSITY, UNIVERSITY PARK, PA 16802, USA
² SHELL EXPLORATION AND PRODUCTION COMPANY, 200 N. DAIRY ASHFORD, HOUSTON, TX 77079, USA
³ DEPARTMENT OF EARTH SCIENCES, UNIVERSITY OF NEW HAMPSHIRE, DURHAM, NH 03824, USA
⁴ DEPARTMENT OF GEOLOGICAL SCIENCES, SAN DIEGO STATE UNIVERSITY, SAN DIEGO, CA 92182, USA

RECEIVED SEPTEMBER 16, 2004; ACCEPTED FEBRUARY 13, 2006

	ABSTRACT

TOP ABSTRACT INTRODUCTION TECTONIC SETTING GEOLOGY AND PETROLOGY OF... SAMPLING AND ANALYTICAL... RESULTS DISCUSSION SUMMARY APPENDIX: ANALYTICAL METHODS REFERENCES

 
The East African Rift System is important to understanding plume-initiated rifting as manifest in the geochemistry of mafic lavas erupted along the rift throughout its evolution. We present new data from high-MgO Tertiary lavas from Turkana, northern Kenya, to investigate regional melt source components, to identify the depths and degrees of melting, and to characterize spatially and temporally the chemical structure of the underlying mantle. The Turkana area is a region of high lithospheric extension that sits between two topographic uplifts thought to be surface expressions of one or more upwelling mantle plumes. Thinning of local crust is believed to be accompanied by widespread removal of the mantle lithosphere, causing the asthenosphere to be in close contact with the overlying crust. New geochemical data on basanites, picrites and basalts (MgO >7 wt %) tightly constrain the primary melt source regions of Tertiary volcanism. Initial isotopic signatures (¹⁴³Nd/¹⁴⁴Nd = 0·51267–0·51283, ⁸⁷Sr/⁸⁶Sr = 0·7031–0·7036) and trace element abundances (Ce/Pb 30, La/Nb = 0·6–0·8 and Ba/Nb = 3–10) in these lavas are consistent with derivation from sub-lithospheric sources. Basalts and picrites erupted between 23 and 20 Ma have Sr–Nd–Pb–He isotopic characteristics indicative of high-µ influence, record high depths and degrees of partial melting, and are associated with rift propagation to the north and south. Accordingly, these lavas sample a source region that is geochemically distinct from that reflected both in Oligocene Ethiopian flood basalts and in the modern Afar region. The geochemical data support numerical and theoretical models as well as tomographic results providing for a complex thermal structure in the mantle beneath East Africa and are interpreted to reflect isotopically distinct plume heads beneath Tanzania and Afar that are derived from the chemically heterogeneous South African superplume.

KEY WORDS: East African Rift System; mantle plumes; HIMU; geochemistry; Afar

	INTRODUCTION

TOP ABSTRACT INTRODUCTION TECTONIC SETTING GEOLOGY AND PETROLOGY OF... SAMPLING AND ANALYTICAL... RESULTS DISCUSSION SUMMARY APPENDIX: ANALYTICAL METHODS REFERENCES

 
The East African Rift system is a classic recent example of continental rifting, and provides an excellent framework in which to investigate extensional magmatism. Voluminous and/or thick successions of basalts crop out from northern Ethiopia to southern Tanzania, and span pre-rift, syn-rift and post-rift periods. The Turkana Depression in northern Kenya is a region of anomalously high extension, forming a broad (150 km wide), diffuse fault zone that is nearly three times the width of the rift observed in other areas. This highly extended region sits between two topographic uplifts (Fig. 1), thought to be surface expressions of an underlying mantle plume (e.g. Thiessen et al., 1979). The Turkana Depression is part of a failed Mesozoic rift system, allowing for the possibility that the plateaux are actually part of one large zone of uplift extending from southern Africa to the Red Sea (e.g. Nyblade & Robinson, 1994; Gurnis et al., 2000). By documenting spatial and temporal variations in the magmatic source regions contributing to Turkana mafic volcanism we gain important insight into the temporal evolution of plume-driven rifting in East Africa.

View larger version (35K): [in this window] [in a new window]   Fig. 1. (a) Sketch map of the East African Rift System. Bold dotted lines enclose areas of the Ethiopian and Kenyan plateaux with elevation >1 km (after Ebinger et al., 2000). Major structural features of the Rift are indicated with dashed lines. (b) Geological features of the Turkana rift. , Eocene to Pliocene sample localities; Quaternary volcanic centers within Lake Turkana are labeled (NI, North Island; CI, Central Island; SI, South Island; B, the Barrier). KS, Kino–Sogu fault zone.

 
Volcanism in the Turkana Depression occurred in three distinct episodes. The first took place in the Early Oligocene (between 40 and 30 Ma) prior to initiation of widespread rifting in the Late Oligocene to Early Miocene (25–20 Ma; Morley et al., 1992), and is manifest locally in northern Kenya and southern Ethiopia (Morley et al., 1992; George et al., 1998). A second, more voluminous, phase took place between roughly 26 and 16 Ma. This magmatic and tectonic episode apparently propagated northward from Kenya into southern Ethiopia, where it is reflected in a volcanic pulse between 20 and 11 Ma (Stewart & Rogers, 1996; George et al., 1998), as well as southward into the Kenya and Western Rifts. The third phase of volcanism began in the Plio-Pleistocene (3·5 Ma) and continues at present; recent magmatism occurs in several discrete areas (Fig. 1). These three magmatic events are spatially overlapping, but are characterized by lavas with distinct geochemical features.

Our primary objective is to place constraints on the long-term thermal, mineralogical and chemical evolution of the melt source region(s) beneath this highly extended portion of the African Rift. Despite the tectonic significance of the Turkana Depression, and the extensive history of both rifting and volcanism, geochemical studies of mafic lavas from this area are limited (Bloomer et al., 1989; Karson & Curtis, 1994; Furman et al., 2004). We employ new major and trace element data as well as Sr–Nd–Pb–He isotopic ratios to explore the nature of the mantle source region, document its heterogeneities, and infer its temporal evolution. In particular, we investigate the HIMU-like isotopic signature of Miocene picrites (²⁰⁶Pb/²⁰⁴Pb 20·4, ²⁰⁸Pb/²⁰⁴Pb 40 and ⁸⁷Sr/⁸⁶Sr 0·7032) and determine the origin of widespread African volcanism with this isotopic character (e.g. Janney et al., 2002). Morley (1994) interpreted structural aspects of early Turkana volcanism to suggest a link to the mantle plume that impacted beneath northern Ethiopia at 30 Ma. This interpretation is consistent with numerical modeling (Ebinger & Sleep, 1998) that suggests that Cenozoic to Recent magmatism in eastern and west–central Africa is associated with a single large mantle plume. In contrast, George & Rogers (2002) interpreted the geochemistry of the oldest (45 to 30 Ma) southern Ethiopian mafic lavas to require two distinct mantle plumes, one at present beneath Afar and a second plume below the Turkana Depression. Recent tomographic studies also suggest that there are two distinct thermal anomalies that extend to mid-mantle depths—one beneath the Afar and the other located beneath central Kenya (Debayle et al., 2001; Nyblade et al., 2000; Montelli et al., 2004). The results from Turkana provide critical insight into this debate because of the long history of volcanism in this area and its unique geographical position in the inter-domal region of the East African Rift.

	TECTONIC SETTING

TOP ABSTRACT INTRODUCTION TECTONIC SETTING GEOLOGY AND PETROLOGY OF... SAMPLING AND ANALYTICAL... RESULTS DISCUSSION SUMMARY APPENDIX: ANALYTICAL METHODS REFERENCES

 
The Turkana Rift lies at the northern terminus of the eastern arm of the East African Rift system (Fig. 1). This site is tectonically unusual within this region in that it has had a prolonged rifting history. Earliest documented rifting began to the east of the modern rift axis when the Anza graben developed during the Cretaceous. Reactivation of existing Anza faults and formation of new grabens have both occurred in Neogene time. Miocene–Recent fault systems initiated in the Lotikipi basin around 25 Ma have generally propagated southeastward through time to the Kinu Sogo fracture zone, the southern terminus of the main Ethiopian Rift (Fig. 1). Seismic surveys carried out by Project PROBE, however, found a significant rift structure underlying Lake Turkana (Dunkleman et al., 1988) that suggests the area is a northern continuation of the Kenya Rift (Morley et al., 1992; Hendrie et al., 1994). This modern rift is characterized by a series of half-grabens that alternate polarity along the length of Lake Turkana. Individual rift basins are approximately 20 km long and 10 km wide, and Quaternary volcanism occurs at basin midpoints, forming discrete volcanic centers that are axially aligned with a spacing of 51 ± 6 km (Dunkleman et al., 1988).

The modern Turkana rift is about 150 km wide. The superposition of three distinct rifting episodes in North Kenya (Late Jurassic–Cretaceous, Paleogene and Miocene–Recent) has produced cumulative stretching factors approaching two (Ebinger & Ibrahim, 1994). Maximum stretching factors calculated for episodic 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 for the late Pliocene (Hendrie et al., 1994). The cumulative stretching factor approaches the theoretical limit for decompression melting during adiabatic upwelling of asthenosphere of ‘normal’ potential temperature (1280°C; McKenzie & Bickle, 1988), but the episodic values are substantially lower. These observations suggest that a region of elevated mantle temperatures has been needed to generate basalt lavas throughout the history of Turkana magmatism. We demonstrate below that the geochemistry of Tertiary Turkana lavas supports 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.

	GEOLOGY AND PETROLOGY OF TERTIARY VOLCANIC CENTERS

TOP ABSTRACT INTRODUCTION TECTONIC SETTING GEOLOGY AND PETROLOGY OF... SAMPLING AND ANALYTICAL... RESULTS DISCUSSION SUMMARY APPENDIX: ANALYTICAL METHODS REFERENCES

 
Magmatism in Turkana was widespread in the Tertiary, but occurred in a restricted series of eruptive centers during the Quaternary (Fig. 1). Earliest volcanism in western Turkana preceded half-graben rift formation by some 10–15 Myr (Brotzu et al., 1984; Morley et al., 1992). Volcanism in Turkana comprises fissure-derived flood basalts as well as products from discrete composite volcanic centers. A wide range of eruptive products exist in the Tertiary Turkana suite, with abundant rhyolitic lavas and pyroclastics present at most locations.

In this study we focus on 50 mafic lavas from seven eruptive areas located primarily on the eastern side of Lake Turkana, spanning three discrete volcanic episodes over a time period of almost 40 Myr (Table 1). The oldest lavas investigated (32–40 Ma) are from the Kajong area and Mt. Porr in the south–central portion of the Turkana Rift, and comprise part of the Balsea Kormoto basalts (Table 1; Walsh & Dodson, 1969; Ochieng' et al., 1988). A second volcanic episode is characterized by primitive mafic lavas from Lodwar, west of Lake Turkana (23–26 Ma), and from Jarigole, east of the lake (17–23 Ma; Ochieng' et al., 1988) (Fig. 1). The Jarigole Plateau comprises some 400 m of flood basalts interspersed with alkaline dikes and plugs as well as nepheline syenite intrusives (Ochieng' et al., 1988; Wilkinson, 1988). Lavas from nearby Allia Bay and Loiyangalani are also probably early to mid-Miocene in age (P. Curtis, personal communication, 1996). Finally, at the southern end of Lake Turkana, the early Pliocene Longipi basalts erupted from cinder cones to form the Longipi shield (Table 1; Fig. 1); volcanic sequences here thicken to the north, forming beds in excess of 300 m (Ochieng' et al., 1988). Adjacent to Longipi is the elongate Mt. Kulal shield, where several cones have produced mid-Pliocene to Quaternary basalts, tuffs and pumice lapilli (beginning at 2·9 Ma; Ochieng' et al., 1988).

View this table: [in this window] [in a new window]   Table 1: Major and trace element geochemistry

 
The majority of samples used in this study are non-vesicular and nearly aphanitic, with <5 vol. % phenocrysts (0·5–2 mm) of olivine, plagioclase feldspar and, less commonly, clinopyroxene in a fine-grained matrix of olivine, plagioclase feldspar, clinopyroxene and opaque oxides. MgO-rich samples from Jarigole and Lodwar contain up to 20 vol. % euhedral olivine crystals ranging up to 8 mm in length. Accumulated olivine phenocrysts in the two highest-MgO picrites have an average composition of Fo_85·4±2·3 (based on 12 grains analyzed in triplicate); moderately high CaO contents of these olivines (0·30–0·47 wt %) indicate that they are accumulative from melts rather than entrained mantle xenocrysts (Thompson & Gibson, 2000). Kajong lavas comprise two groups: Group A samples (KJ 1, KJ 13 and KJ 16) have devitrified matrix glass that is locally altered with veins of trace calcite. The remaining Kajong samples are holocrystalline and fresh; alteration is not observed in samples from other areas.

	SAMPLING AND ANALYTICAL TECHNIQUES

TOP ABSTRACT INTRODUCTION TECTONIC SETTING GEOLOGY AND PETROLOGY OF... SAMPLING AND ANALYTICAL... RESULTS DISCUSSION SUMMARY APPENDIX: ANALYTICAL METHODS REFERENCES

 
All samples were collected by P. Curtis and J. Karson as part of a structural investigation of the Turkana area. Samples used in our study were selected from over 350 available specimens on the basis of freshness and spatio-temporal control, and are biased towards mafic lavas with visible modal olivine.

Sample preparation involved crushing centimetre-thick slabs that had been polished to remove saw marks, and then powdering the chips in a WC disc mill. Bulk-rock analyses for major and minor elements were obtained by direct current plasma (DCP) and inductively coupled plasma mass spectrometry (ICP-MS) at Duke University (Table 1; see Appendix for details). Sr–Nd–Pb isotopic analyses (Table 2) were performed at the Keck Center for Geochemistry in the Department of Earth and Space Sciences at the University of California, Los Angeles using conventional thermal ionization mass spectrometry (TIMS) techniques on VG Instruments Sector 54-E multicollector mass spectrometers. The Pb isotopic compositions of three Jarigole picrites were reanalyzed at San Diego State University by multi-collector (MC)-ICP-MS using a Nu Plasma system. Sample powders were leached prior to analysis with dilute HBr. Isotopic values of standards were corrected using the Tl-doping technique (White et al., 2000; Thirlwall 2002; Albarède et al., 2004). Sample isotope ratios were corrected for within-run fractionation using standard–sample–standard bracketing techniques. The higher precision MC-ICP-MS data (Table 2) plotted in the figures confirm the HIMU-like nature of the Jarigole picrites (see Appendix for details).

View this table: [in this window] [in a new window]   Table 2: Isotopic data

 
Hand-picked olivine separates from three Jarigole picrites were analyzed for ³He/⁴He isotope compositions by D. Graham at Oregon State University (Table 2). Precise ⁴⁰Ar/³⁹Ar age determinations on three bulk-rock samples from Longipi and Kajong were obtained by R. Duncan at Oregon State University; weighted-mean plateau ages are reported in Table 3.

View this table: [in this window] [in a new window]   Table 3: 40Ar/39A age data

 

	RESULTS

TOP ABSTRACT INTRODUCTION TECTONIC SETTING GEOLOGY AND PETROLOGY OF... SAMPLING AND ANALYTICAL... RESULTS DISCUSSION SUMMARY APPENDIX: ANALYTICAL METHODS REFERENCES

 
Major elements
Samples analyzed in this study include some of the most primitive lavas reported from eastern Africa. Mafic lavas with >8 wt % MgO are found at all study locations, although these same centers also produced more evolved lavas, including phonolites and rhyolites. Tertiary Turkana lavas reported here are primarily basaltic and basanitic, based on the silica–alkali classification scheme (Fig. 2). The 39–35 Ma lavas from Mt. Porr and Kajong and 3·5–1·5 Ma lavas from Longipi extend to basanitic compositions; picrites are found at Jarigole (23–17 Ma).

View larger version (30K): [in this window] [in a new window]   Fig. 2. Classification and nomenclature of Turkana lavas. (a) Total alkalis (Na2O + K2O wt %) vs SiO2 variations in Turkana lavas [igneous rock classification from Le Maitre (1976)]. Shaded field of Quaternary Turkana data is from Furman et al. (2004). (b) Normative compositions of Turkana mafic lavas. Symbols as in (a). Field labeled KJ(A) encloses Kajong Group A lavas; field labeled JRG, LOD picrites encloses mafic lavas from Jarigole and Lodwar with 14·3 wt % MgO.

 
Both the oldest and youngest suites are dominated by nepheline-normative compositions, whereas the intermediate volcanic period is represented by nepheline-, hypersthene- and quartz-normative lavas (Fig. 2). Accumulative olivine in select Jarigole and Lodwar samples is manifest in high normative olivine contents. Group A Kajong lavas (39 Ma) have high normative nepheline contents that correlate with high loss-on-ignition values (LOI; Table 1); no correlation is found among the nepheline-normative Mt. Kulal and Longipi lavas (3·5–1·5 Ma).

The highly evolved nature of many Tertiary Turkana samples indicates an important role for fractionation. However, it is clear that fractional crystallization alone cannot explain many geochemical features of the mafic Tertiary samples. Major element oxides (Al₂O₃, TiO₂, P₂O₅ and CaO/Al₂O₃) show essentially no correlation with MgO among the non-accumulative (MgO 15 wt %; Fig. 3) Jarigole and Lodwar series (Fig. 3). Basanite lavas from Mt. Porr, Kajong and Longipi have anomalously high contents of K₂O, resulting in high normative nepheline values (17–24 %), and are also enriched in TiO₂ and P₂O₅. These geochemical features suggest a genesis distinct from that of the majority of the less alkaline samples investigated (Fig. 3). Most of the youngest Tertiary lavas have major element compositions that overlap Quaternary Turkana lavas (Furman et al., 2004).

View larger version (39K): [in this window] [in a new window]   Fig. 3. Whole-rock geochemical characteristics of Turkana lavas as a function of wt % MgO. The effects of olivine addition are indicated by the labeled vector. The shaded fields encompass Quaternary Turkana mafic lavas (Furman et al., 2004). Geochemical variations among basalts from portions of the Kenya Rift underlain by rifted margins and ancient cratons are shown by the open field (Macdonald et al., 2001); this group includes samples for which continental crustal contamination can be demonstrated.

 
Trace elements
Compatible trace elements
Several Tertiary Turkana samples with 8–12 wt % MgO have undergone little fractionation since initially forming as partial melts from the mantle. These primitive samples contain up to 350 ppm Ni, 450 ppm Cr and 35 ppm Sc, indicating only minor removal of olivine and/or clinopyroxene from primary mantle melts, and are thus important for interpreting source components in Turkana magmatism. Jarigole and Lodwar picrites with 12·9–28·8 wt % MgO have up to 900 ppm Ni, indicating accumulation of olivine phenocrysts (Fig. 3).

Incompatible trace elements
Tertiary Turkana lavas are broadly enriched in highly incompatible trace elements with respect to the primitive mantle (Fig. 4). All of the samples are characterized by positive Nb and negative K anomalies; selected samples display marked positive Ba anomalies and/or rare earth element (REE) enrichment relative to the high field strength elements Zr and Ti. Individual trace element abundances generally correlate negatively with MgO among the Tertiary Turkana lavas, although variations between and within suites preclude their derivation from a common and homogeneous source.

View larger version (53K): [in this window] [in a new window]   Fig. 4. (a–d) Primitive mantle-normalized incompatible trace element (ITE) variation diagrams for samples from different locations and time periods. (a) Kajong Group A and (b) Mt. Porr mafic lavas have the highest degree of ITE enrichment; negative K anomalies indicate melting in the presence of residual amphibole. (c) Jarigole and Lodwar basalts and picrites have the lowest degree of ITE enrichment; their patterns overlap HIMU basalts from St. Helena (Chaffey et al., 1989). (d) Longipi and Kulal basalts have ITE contents intermediate to those of the older suites. (e) Tertiary Turkana La/Nb–Ba/Nb values cluster near those of HIMU basalts (represented by the star) and Quaternary Turkana mafic lavas (field labeled TQ), and are lower than those observed among <2 Ma basalts from Djibouti (the modern Afar plume; Deniel et al., 1994). Kajong Group A, Kulal and Longipi lavas have high La/Nb–Ba/Nb values similar to 35–45 Ma basalts from southern Ethiopia (Stewart & Rogers, 1996; George & Rogers, 2002) that are characteristic of enriched mantle or mantle lithosphere sources (EM). (f) Ba/Rb–Rb/Sr variations in Turkana lavas indicate a limited role for amphibole (elevated Ba/Rb) and either phlogopite or carbonatite metasomatism (elevated Rb/Sr). Jarigole and Lodwar lavas fall within the range of HIMU basalts from St. Helena as indicated by the star (Chaffey et al., 1989).

 
Successive generations of Turkana lavas have distinctive trace element abundance patterns. The oldest basanites from Kajong and Mt. Porr have the highest degree of incompatible trace element enrichment observed, whereas Jarigole and Lodwar basalts and picrites show the least enrichment over primitive mantle. Latest Tertiary to Quaternary mafic lavas tend to have trace element abundances intermediate to those measured in the oldest two series of Turkana basalts.

Basalt and picrites from Jarigole and Lodwar have a restricted range in incompatible trace element abundances that indicate derivation from a sub-lithospheric mantle source region. Their Ba/Nb–La/Nb values overlap those of HIMU ocean island basalts (Chaffey et al., 1989; Sun & McDonough, 1989), and their Ce/Pb–Nb/U and Ba/Rb–Rb/Sr values are characteristic of global basalts derived from anhydrous mantle source regions (Fig. 4). Most Tertiary and Quaternary Turkana samples also have incompatible trace element ratios within the ranges of mid-ocean ridge and HIMU basalts (Fig. 4), although a small number of lavas (KJ 2, KJ 19, LGP 12, KL 18) plot amongst the suite of 35–45 Ma mafic lavas from southern Ethiopia (Stewart & Rogers, 1996; George & Rogers, 2002) and define a trend towards an EM1-like component or enriched mantle lithosphere.

Variations in incompatible trace element abundances provide useful constraints on the accessory phase mineralogy of potential mantle source regions for primitive Turkana mafic lavas. For example, high Ba/Rb and low values of K/Nb and K/La suggest melting of amphibole-bearing material in the source region of Group A Kajong lavas (KJ 1, KJ 13 and KJ 16; Fig. 4); a lack of correlation between these parameters and LOI values suggests that they are a feature of the source region rather than of post-eruption hydrous alteration. A minor role for carbonatite metasomatism is indicated by the extreme Ba enrichment and relative negative Zr–Hf–Ti anomalies in the entire Kajong suite and in selected Mt. Porr lavas (e.g. Yaxley et al., 1991). In contrast, there is no evidence to suggest that phlogopite or oxide phases (ilmenite, rutile) control the minor and trace element chemistry of the Tertiary Turkana mafic rocks.

Rare earth elements
Chondrite-normalized REE patterns appear smooth with negative, shallow slopes; Eu anomalies are generally absent. Samples from individual localities—excluding Kajong—have parallel patterns that typically do not cross. Group A Kajong samples, interpreted as melts of an amphibole-bearing source, have weakly concave-upwards patterns (Fig. 5), reflecting the greater compatibility of the middle REE (MREE) in this phase (e.g. Zack et al., 1997). The REE patterns of these samples mimic those of lithosphere-derived samples from Bird Nest Island (Fig. 5; Furman et al., 2004). Anomalously high light REE (LREE) abundances in Group A Kajong lavas (Fig. 5) may be associated with carbonatite metasomatism (e.g. Yaxley et al., 1991; Rudnick et al., 1993).

View larger version (28K): [in this window] [in a new window]   Fig. 5. (a) Chondrite-normalized REE patterns for Turkana mafic lavas. Kajong Group A samples (39 Ma) have patterns that match those of Quaternary basalts interpreted as melts of subcontinental lithospheric mantle (e.g. BN-2C; Furman et al., 2004). (b) La/Smn–Tb/Ybn variations in mafic lavas with >6 wt % MgO. The high values of Tb/Ybn observed in Tertiary Turkana lavas and other portions of the Kenya Rift with moderately thick crust (Macdonald et al., 2001; Späth et al., 2001) suggest melting of enriched lithosphere in the presence of residual garnet (or clinopyroxene rich in Ca-Tschermak component; Blundy et al., 1998).

 
Tertiary Turkana samples have consistently higher Tb/Yb_n values [the subscript n indicates values normalized to the chondritic abundances of Boynton (1984)] than the Quaternary suite (Fig. 5). The Tertiary values are within the range measured on basalts from throughout the Kenya Rift (Macdonald et al., 2001) although they do not extend to values as high as those observed in the Chyulu Hills, southern Kenya, where the crustal thickness exceeds 100 km (Späth et al., 2001).

Isotope data
Precise ⁴⁰Ar/³⁹Ar age determinations were made on three whole-rock samples from Kajong and Longipi (Table 3). Kajong lava KJ 16 (39·2 ± 0·4 Ma) is the oldest volcanic sample dated from the Turkana region (Walsh & Dodson, 1969; Zanettin et al., 1983; Ochieng' et al., 1988; Morley et al., 1992; Hendrie et al., 1994). Longipi basanite samples LGP 607 and LGP 207 are the first from this area to be dated, and indicate activity at this volcanic center between 3·5 and 1·5 Ma.

Radiogenic Sr–Nd–Pb isotope data for the Tertiary Turkana lavas (Fig. 6; Table 2) indicate substantial source heterogeneity, both within and between eruptive episodes. For example, the oldest basanites from Kajong and Mt. Porr have a narrow range of (⁸⁷Sr/⁸⁶Sr)_i values (0·70308–0·70344) but widely variable (²⁰⁶Pb/²⁰⁴Pb)_i values (18·75–20·33). In contrast, Plio-Pleistocene lavas from Mt. Kulal and Longipi display the greatest variability in initial Sr isotope ratios (0·70318–0·70472) but define a much tighter range of initial ²⁰⁶Pb/²⁰⁴Pb values (19·26–19·81). Jarigole picrites and transitional basalts are more compositionally restricted, with Sr–Pb isotope values that are markedly similar to HIMU-like lavas from St. Helena and South Africa (Chaffey et al., 1989; Janney et al., 2002).

View larger version (44K): [in this window] [in a new window]   Fig. 6. Regional isotope variations along the EARS. (a) Sr–Nd isotope values in Tertiary Turkana samples are intermediate between those of Quaternary Turkana lavas (Furman et al., 2004) and basalts of all ages from all the Ethiopian Rift. Turkana samples with 87Sr/86Sr >0·7035 are interpreted as melts of lithospheric mantle. MER, Main Ethiopian Rift (Trua et al., 1999; Furman et al., 2006); SEth, southern Ethiopia (Hart et al., 1989; Stewart & Rogers, 1996; George & Rogers, 2002); StH, HIMU St. Helena basalts (Chaffey et al., 1989); A, modern Afar plume (Deniel et al., 1994; Schilling et al., 1998); WSR, West Sheba Ridge (Schilling et al., 1998). (b) Values of 87Sr/86Sr–206Pb/204Pb in Jarigole picrites define a regional high-µ end-member. Most Turkana basalts record mixing between this high-µ end-member or the Afar plume with a more depleted mantle component. Southern Ethiopian basalts from 35 to 45 Ma and modern Main Ethiopian Rift lavas record involvement of ‘Ethiopian Lithosphere’, whereas southern Ethiopian basalts from 11 to 19 Ma and <3 Ma Turkana basalts sample ‘Turkana Lithosphere’. (c) A few Turkana lavas have high 207Pb/204Pb values at a given 206Pb/204Pb, suggesting involvement of lithospheric mantle. This feature is also seen in selected Ethiopian lavas. (d) Values of 206Pb/204Pb–208Pb/204Pb in primitive Turkana Tertiary and Quaternary basalts show significant overlap, although Tertiary lavas extend towards HIMU-dominated St. Helena basalts. (e) Regional 143Nd/144Nd–206Pb/204Pb variations indicate mixing among high-µ, lithospheric and Afar plume source components. (f) Jarigole basalts and picrites have Pb–Sr isotopic compositions that plot within the field of HIMU lavas (Stracke et al., 2005). Tie-lines connect samples that were analyzed by both TIMS and higher precision MC-ICP-MS methods following leaching of 2–5 mm chips. It should be noted that the more precise data consistently displace the analyses towards the field of HIMU.

 
Initial ⁸⁷Sr/⁸⁶Sr ratios of mafic Tertiary Turkana rocks are among the lowest measured in the East African Rift; only two samples (LGP 4 and KL 18) have values >0·7035. Many of the Tertiary samples have Sr–Nd isotope compositions that overlap those of Quaternary Turkana basalts (Fig. 6). Two Pliocene samples (KL 18 and LGP 4) have substantially higher ⁸⁷Sr/⁸⁶Sr and lower ¹⁴³Nd/¹⁴⁴Nd, similar to values observed in selected Quaternary samples interpreted as lithospheric melts (Furman et al., 2004) and falling within the ranges of both Recent and Miocene Ethiopian basalts (Stewart & Rogers, 1996; Furman et al., 2006). When considered as a group, the Sr–Nd isotopic signatures of the Tertiary lavas broadly suggest mixing between a HIMU-like component and an enriched source component.

Tertiary Turkana lavas have highly variable radiogenic Pb isotope signatures with ²⁰⁶Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb extending to nearly the highest values of silicates documented in the African Rift (Fig. 6). The oldest lavas from Kajong and Mt. Porr span a wide range of ²⁰⁶Pb/²⁰⁴Pb values comparable with that observed in contemporaneous southern Ethiopian basalts (Stewart & Rogers, 1996). In contrast, early Miocene Jarigole picrites have consistently high ²⁰⁶Pb/²⁰⁴Pb and low ⁸⁷Sr/⁸⁶Sr values that fall on the edge of the field defined by HIMU basalts from St. Helena (Chaffey et al., 1989) and in other African volcanic provinces (e.g. the Cameroon Line, Halliday et al., 1988; the Western Cape melilitite province, Janney et al., 2002). We refer to the Pb isotopic signatures of the Turkana lavas as high-µ (high U/Pb) and restrict use of the term HIMU to indicate the mantle end-member (e.g. Stracke et al., 2005). The Pb isotope signatures of Pliocene to Quaternary Turkana lavas range from moderately radiogenic values (at Mt. Kulal) to much lower values (at Longipi) that overlap those for modern mafic lavas of the Afar plume from Djibouti (Deniel et al., 1994; Fig. 6). Tertiary Turkana mafic lavas show greater scatter in ²⁰⁷Pb/²⁰⁴Pb values than is observed in the Quaternary suite or basalts from MORB suites such as the West Sheba Ridge (Schilling et al., 1998), although the Jarigole picrites again define a consistent narrow compositional range.

Helium isotopic data obtained on three Jarigole picrites fall within analytical error (6·2–6·6 R_A, where R_A indicates the ratio of ³He/⁴He measured in the sample to that of the atmosphere); these values are significantly lower than data obtained on Oligocene to Recent basalts from central Ethiopia (up to 19 R_A; Marty et al., 1993, 1996; Scarsi & Craig, 1996) and are consistent with sites of HIMU volcanism (e.g. Graham et al., 1992; Barfod et al., 1999; Hilton et al., 2000).

	DISCUSSION

TOP ABSTRACT INTRODUCTION TECTONIC SETTING GEOLOGY AND PETROLOGY OF... SAMPLING AND ANALYTICAL... RESULTS DISCUSSION SUMMARY APPENDIX: ANALYTICAL METHODS REFERENCES

 
Source regions and melting history in the Turkana Rift
Lithospheric assimilation
Most samples within the Turkana suite have geochemical signatures that are inconsistent with significant contamination by crustal or mantle lithospheric melts. One Pliocene lava (KL 18) has the most enriched ⁸⁷Sr/⁸⁶Sr and _Nd values (0·7047 and 0·7, respectively) observed at Turkana, as well as high Ba/Nb (18·3) and Ba/Th (217), suggesting a contribution from either the crust or the lithospheric mantle. It is noteworthy for the discussion that follows that this sample does not have the most radiogenic ²⁰⁶Pb/²⁰⁴Pb of the suite, but does have the most radiogenic ²⁰⁷Pb/²⁰⁴Pb at the edge of the resolution by the TIMS technique. A second Pliocene sample (LGP 4) with similar Ba enrichment has a radiogenic Sr isotopic signature (0·7042) that is accompanied by more depleted values of _Nd (5·0), ²⁰⁶Pb/²⁰⁴Pb and ²⁰⁷Pb/²⁰⁴Pb, suggesting either a sublithospheric source or a different lithospheric component. These features are also observed in two Quaternary Turkana lavas, interpreted as lithospheric mantle melts (Furman et al., 2004), but are absent among samples older than 3·5 Ma. The isotopic signatures of these samples are similar to the radiogenic end of the Red Sea array (Schilling et al. 1998), but their trace element compositions suggest a lithospheric origin.

Minor variations in chemical parameters sensitive to crustal contamination also exist among mafic lavas with ⁸⁷Sr/⁸⁶Sr values <0·7035 (e.g. K₂O, Ba/Th, Ce/Pb, particularly among Group A Kajong lavas), but the magnitude of these variations is small and could, in some cases, result from pressure-dependent partitioning behavior of these elements during progressive melting of a pyroxene-bearing source (Blundy et al., 1998). We emphasize that the majority of Turkana samples are MgO-rich with incompatible trace element abundances within the range of mantle-derived melts, making it unlikely that they have experienced substantial crustal assimilation. In assessing the mineralogy and thermal structure of the Turkana source regions we focus on mafic lavas that display the least evidence for interaction with crustal and mantle lithospheric melts.

Depth and extent of melting
Isotopic and trace element considerations indicate that all Turkana mafic lavas are derived from a source region that is enriched relative to the depleted mantle, and for this reason we compare them with experimental melts of fertile peridotite. The SiO₂ and FeO^* contents of primitive Turkana lavas (back-corrected for olivine fractionation; Fig. 7) plot near the range of experimentally derived melts of fertile peridotite at pressures of 20–30 kbar (Baker & Stolper, 1994; Kushiro, 1996). This result corresponds well with inferences based on the Tb/Yb_n values (Fig. 5) of Tertiary Turkana lavas that indicate a depth of melting at or above the garnet–spinel transition (80 km depth, or 25 kbar). There is a strong temporal pattern to the inferred depths of melting. The oldest samples (Kajong and Mt. Porr, 35–39 Ma) record the lowest apparent pressures of melting, whereas Plio-Pleistocene lavas (Longipi) and 20–23 Ma basalts and picrites (Lodwar and Jarigole) plot at substantially higher pressures.

View larger version (25K): [in this window] [in a new window]   Fig. 7. FeO* vs SiO2 for selected EARS mafic suites and experimental melts in equilibrium with mantle peridotite (Baker & Stolper, 1994; Kushiro, 1996). Dotted lines encircle fields for experimental melts coexisting with cpx + oliv + opx; continuous lines enclose fields for liquids coexisting with oliv + opx after higher degrees of melting. Major element compositions were renormalized with total iron as FeO, then corrected for olivine fractionation to correspond to experimentally derived melts of fertile peridotite (Mg-number 72); only samples with Mg-number >62 were corrected using this procedure. Smaller symbols indicate the original data for comparison. Tertiary Turkana basalts define three groups: (1) Kajong and Mt. Porr lavas indicate derivation at pressures 20 kbar, similar to depths inferred for melting along the Main Ethiopian Rift and in Erta' Ale (Barrat et al., 1998; Hart et al., 1989; Furman et al., 2006). (2) Jarigole and Lodwar picrites and basalts record pressures of 25–30 kbar. These samples have unusually high FeO* contents consistent with high extents of melting. (3) Basalts from Mt. Kulal and Longipi also indicate melting pressures of 25–30 kbar, but at substantially lower FeO* values. Quaternary basalts from Turkana (Furman et al., 2004) and the Red Sea (Volker et al., 1997) indicate the lowest melting pressures. Pressures and degrees of melting estimated on the basis of this diagram are approximate.

 
The low FeO^* and SiO₂ contents of most Turkana mafic lavas (relative to the experimental fields; Fig. 7) suggest melting in the presence of residual clinopyroxene. The Jarigole and Lodwar picrites have higher FeO^* and SiO₂ contents, suggesting higher degrees of melting (15% for fertile peridotite, see Kushiro, 1996). It should be noted, however, that there is no evidence to suggest that the source region for the Turkana mafic lavas is either clinopyroxenitic or olivine-free (e.g. Sobolev et al., 2005). Lavas derived from pyroxenite are expected to have unusually high contents of Na₂O and SiO₂ (Hirschmann, 2000; Pickering-Witter & Johnston, 2000; le Roux et al., 2002) that are not observed in this sample suite.

Sub-lithospheric mantle contributions
Sr–Nd–Pb isotopic compositions of the Tertiary Turkana mafic lavas require material inputs from one or more sub-lithospheric sources. A similar conclusion was reached by Furman et al. (2004) with regard to the Quaternary Turkana basalts, and this study extends that interpretation back in time to the onset of regional extension and volcanism. Tertiary lavas define a wider range in radiogenic lithophile isotopic compositions than the Quaternary suite, although the two groups overlap substantially; all Plio-Pleistocene lavas plot within fields defined by the Quaternary basalts (Fig. 6).

The voluminous second phase of Turkana volcanism, represented here by 20 Ma basalts and picrites from Jarigole and Lodwar, has a restricted range of Sr–Nd–Pb–He isotopic values that require a high-µ mantle source. This composition apparently represents an end-member that contributes to Turkana magmatism—as well as that of southern Ethiopia—throughout the regional 40 Myr volcanic history. Turkana samples that are clearly not affected by crustal or lithospheric processes define mixing trends between the proposed common source composition for global magmatism—FOZO (Hart et al., 1992) or C (Hanan & Graham, 1996)—and this high-µ end-member (Fig. 6).

Geodynamic evolution of the Turkana region
Rifting and volcanism
The Turkana Rift occupies a significant position within the overall architecture of the East African Rift System. It preserves evidence for the earliest phases of both magmatism and extension along the entire rift, and marks the transitional junction between the north–south-trending Kenya Rift and the NE–SW-trending Main Ethiopian Rift. Understanding the evolution of mantle source regions during progressive lithospheric thinning and/or removal therefore provides insight into the overall mechanisms of continental extension. We interpret the Turkana data in terms of plume–lithosphere interaction during rifting as shown schematically in Fig. 8 and described below.

View larger version (20K): [in this window] [in a new window]   Fig. 8. Schematic illustration of the linked evolution of Turkana rifting and magmatism (see text for discussion). The different shades of ascending material indicate chemical heterogeneity within the inferred plume; dark blobs represent source material of high-µ isotopic character that are present throughout the history of Turkana magmatism but are dominant for a brief episode at 20 Ma.

 
Relationships between FeO^* and SiO₂ indicate that the oldest (39–35 Ma) basanite lavas from Kajong and Mt. Porr were derived at pressures of 15–20 kbar (Fig. 7), corresponding to depths of 50–65 km. A few of these early lavas record interaction with continental crust or mantle lithosphere, presumably during ascent through a thick, cool section (Fig. 8). The Sr–Pb isotopic composition of the primitive basanites includes a high-µ component that is, however, clearly unrelated to crustal and/or mantle lithospheric contamination; this component is not observed in contemporaneous lavas from southern Ethiopia (Stewart & Rogers, 1996), which are derived largely from the mantle lithosphere. The 39–35 Ma episode of Turkana mafic volcanism precedes faulting and regional uplift, suggesting that it does not reflect adiabatic decompression melting as a passive response to lithospheric stretching, but rather requires substantial heat input, probably from a mantle plume (e.g. Ebinger et al., 2000).

Primitive Jarigole and Lodwar basalts and picrites provide strong evidence for a thermal and chemical anomaly (i.e. plume) beneath the Turkana area at 20 Ma (Fig. 8). Jarigole and Lodwar mafic lavas appear to be derived by the greatest degree of melting recorded in the Turkana suite, and their inferred depth of origin is 85–100 km (Fig. 7); these observations suggest a significant thermal event that is constrained in both space and time, such as a pulse in plume activity. The 20 Ma phase of activity is associated with both northward and southward propagation of rifting and volcanism, as indicated by progressively younger ages of fault-bounded rift basins and basalts in southern Ethiopia away from Turkana, as well as to the south along the Kenya Rift (George et al., 1998; Ebinger et al., 2000). Rift basins in southern Ethiopian that developed at 18–10 Ma (e.g. Getra-Kele, Chew Bahir, Chamo; Fig. 1) are oriented north–south, like those in Turkana and Kenya, rather than NE–SW as in the Main Ethiopian Rift, where they are probably related to Red Sea rifting (e.g. Ebinger et al., 2000). Miocene faults, basins and volcanic rocks in the Kenya and Western Rifts also young consistently away from Turkana, supporting our interpretation that a major pulse of magmatic activity triggered rift propagation.

The distinctive Sr–Nd–Pb–He isotopic signatures of the Jarigole picrites approach those of the HIMU mantle end-member (e.g. Chaffey et al., 1989; Graham et al., 1992; Stracke et al., 2005). The incompatible trace element ratios of these lavas fall consistently within the range of HIMU basalts and other mantle-derived melts, and show no evidence of contamination by crustal materials. The lack of crustal overprint probably results from the greater volume of lava erupted during this period relative to that of the 39–35 Ma episode, as well as the progressive maturation of transport pathways through the lithosphere. The isotopic signatures of 19–11 Ma southern Ethiopian basalts appear to show mixing between the high-µ component and a source area with more enriched Sr–Nd isotopic characteristics (Fig. 6), which could be the Afar mantle plume, the Ethiopian lithosphere, or a combination of the two. It is worth noting that the southern Ethiopian lavas do not include primitive lavas with >8·7 wt % MgO. Although many of them have ²⁰⁶Pb/²⁰⁴Pb values characteristic of high-µ volcanism, a subset also show the high ²⁰⁷Pb/²⁰⁴Pb values indicative of crustal involvement (Fig. 6).

Plio-Pleistocene to Recent Turkana mafic lavas show the effects of a shift from localized volcanism to a more distributed and laterally extensive array of rift structures. During this period the locus of extension moved eastward, with development of the Ririba Volcanic Zone and related north–south-trending features in southern Ethiopia (e.g. Hendrie et al., 1994; Ebinger et al., 2000) although the rift axis beneath Lake Turkana remained a zone of active faulting and magmatism (Dunkleman et al., 1988). Latest Tertiary Turkana lavas from Longipi (east of Lake Turkana; Fig. 1) record melting at pressures of 20–30 kbar (Fig. 7), whereas Quaternary lavas from neighboring South Island formed at 15–20 kbar, consistent with the extremely thin crustal section observed seismically (20 km; Simiyu & Keller, 1997, and references therein). Some of the 3·5 Ma Turkana lavas record melting of amphibole-bearing peridotite, presumably within the lithospheric mantle. The youngest southern Ethiopian lavas also show both lithospheric and sub-lithospheric contributions as indicated by extremely 2heterogeneous Sr–Nd isotopic signatures (Fig. 6). The short spatial scale over which melting and eruptive processes sample both lithospheric and sub-lithospheric source materials (<1 km at Bird Nest Island; Furman et al., 2004) suggests that magmas ascend along the locally rough topography of the lithosphere–asthenosphere boundary (Fig. 8).

Occurrence of high-µ volcanism in East Africa
Mafic lavas erupted in many parts of Africa and the Near East since 100 Ma display radiogenic isotope signatures that indicate contributions from high-µ component(s) (Fig. 9 and references therein). Some of these suites are alkalic, and most have isotopic compositions that do not overlap those of the Turkana picrites (²⁰⁶Pb/²⁰⁴Pb 20·5 and ⁸⁷Sr/⁸⁶Sr 0·703). Among both recent and pre-Miocene volcanic rocks in the African Rift there is a broad increase in ²⁰⁶Pb/²⁰⁴Pb values with increasing distance from the center of the inferred modern Afar plume as one moves southward towards Turkana (Fig. 9). Oligocene flood basalts associated with plume head impact beneath Ethiopia and Yemen apparently lack this compositional component, as they have ²⁰⁶Pb/²⁰⁴Pb 19·0 (Pik et al., 1999; Kieffer et al., 2004). In contrast, Eocene to Oligocene mafic lavas from Turkana (this study) and southern Ethiopia (Stewart & Rogers, 1996) as well as carbonatites from several localities in eastern Uganda (Bell & Tilton, 2001) extend to higher ²⁰⁶Pb/²⁰⁴Pb values that are more readily associated with high-µ source contributions (Fig. 9). Farther south, olivine melilitites and alkali basalts erupted at 58–76 Ma in the Western Cape region also include high-µ compositions (Janney et al., 2002).The He isotopic database for this region is more limited, but where data are available they indicate high ³He/⁴He values associated with the Afar plume (up to 19 R_A; Marty et al., 1996; Scarsi & Craig, 1996) and lower values more typical of high-µ basalts along the Cameroon Line (Barfod et al., 1999; Aka et al., 2004) and at Turkana.

View larger version (23K): [in this window] [in a new window]   Fig. 9. Variation in 206Pb/204Pb as a function of distance from the Afar region. The map indicates areas from which data were used to construct the graphs; only samples with >6 wt % MgO and 87Sr/86Sr isotope values <0·7040 are plotted. Grey squares indicate mafic silicate lavas; , carbonatites. Pb isotope values (206Pb/204Pb) for Jebel Marra, the Cameroon Line, Canary Islands and South Africa are shown for comparison and are not plotted. Distance to Afar is calculated by projecting each locality onto the black lines. Pb isotope compositions observed along the EARS south of Afar fall within a triangular envelope defined by Bulk Silicate Earth (BSE), the modern Afar plume (open star), and East African carbonatites (white rectangles). This relationship is not observed among samples from the Arabian Shield and Red Sea. (See text for a full discussion.) Sources of data: Davies & Macdonald, 1987; Halliday et al., 1988, 1990; Davidson & Wilson, 1989; Hart et al., 1989; Chazot & Bertrand, 1993; Deniel et al., 1994; Simonetti & Bell, 1995; Bell & Simonetti, 1996; Stewart & Rogers, 1996; Kalt et al., 1997; Volker et al., 1997; Barrat et al., 1998; George et al., 1998; Pik et al., 1999; Hilton et al., 2000; Rogers et al., 2000; Bell & Tilton, 2001; Macdonald et al., 2001; Späth et al., 2001; Janney et al., 2002; Bertrand et al., 2003; Shaw et al., 2003; Furman et al., 2004, 2006; Kieffer et al., 2004.

 
The origin of the high-µ signature in African volcanic rocks and the broader geochemical systematics of high-µ ocean island rocks remain topics of debate. Recently, Stracke et al. (2005) suggested that this component represents recycled ocean crust or carbonate that is sampled within a mantle plume (e.g. Graham et al., 1992; Bell & Tilton, 2001; Hoernle et al. 2002). Janney et al. (2002) proposed that the high-µ signature associated with post-Cretaceous alkaline rocks throughout Africa arises from sublithospheric melting of pods of recycled oceanic material that are mobilized by a large regional mantle upwelling, perhaps the South African superplume. On the other hand, widespread volcanic rocks with high-µ tendencies (²⁰⁶Pb/²⁰⁴Pb = 18·6–19·5) in the Arabian Plate and the Red Sea region are interpreted as melts of the lithospheric mantle (Bertrand et al., 2003; Shaw et al., 2003).

The isotopic signatures of the Jarigole picrites are a testament to the influence of a high-µ source in Turkana in the Tertiary: the ²⁰⁶Pb/²⁰⁴Pb values (averaging 20·3 in the picrites) fall on the border of the new HIMU classification proposed by Stracke et al. (2005). These radiogenic Pb isotopic signatures are associated with depleted ⁸⁷Sr/⁸⁶Sr values (averaging <0·7032) and HIMU-like ³He/⁴He signatures (6 R_A). The four Turkana samples with high-µ isotopic characteristics have narrow ranges in diagnostic incompatible trace element ratios (e.g. Nb/U, Ce/Pb, Ba/Nb, Ba/Th, Ba/Rb) that overlap those of St. Helena HIMU lavas (Chaffey et al., 1989), whereas the remaining Turkana samples and proposed high-µ lavas from the Arabian Shield (Bertrand et al., 2003) have much more variable isotopic and incompatible trace element ratios that are consistent with contributions from the mantle lithosphere.

Several lines of evidence indicate that the high-µ source component does not reside within the lithospheric mantle beneath Turkana. First, the inferred depth of melting (85–100 km) is below the base of the continental lithospheric mantle at 20 Ma, as deduced from seismic information and structural basin analysis (Morley et al., 1992; Hendrie et al., 1994). Second, although melting of eclogite detached from the subcontinental lithospheric mantle in the Turkana region during rifting could produce the observed picritic melts, the time scales typically inferred for this process (30–75 Myr; Anderson, 2005) are substantively longer than the offset between the initiation of extension and the occurrence of high-µ magmatism in Turkana. Finally, the high-µ influence in Turkana is observed most clearly in basalts and picrites from a discrete pulse of volcanic activity (20–23 Ma) associated with the onset of northward and southward propagation of the Main Ethiopian and Kenya Rifts. This short-lived voluminous magmatic pulse and the moderate to large extents of melting inferred on the basis of major element compositions (Fig. 7) require an anomalously warm environment, in keeping with the expected melting regime within a mantle plume. In this regard, the Turkana high-µ lavas differ significantly from the small-volume high-µ melilitites from the Western Cape, South Africa (Janney et al., 2002).

Collectively, the high-µ lavas record the highest degrees of melting at the greatest pressures (25–30 kbar) observed over a 40 Myr period of volcanism in Turkana. The existence of the high-µ signature in Turkana, in products with trace and major element signatures indicative of high degrees of polybaric melting originating at high pressures, is best reconciled with melting of exotic source materials in the asthenosphere. The spatial and temporal distribution of lavas with high-µ isotopic characteristics indicates that this geochemical component is not associated with the Afar plume head (Fig. 9). As outlined below, the occurrence of the high-µ influence across and throughout Africa is consistent with an association with the upwelling South African superplume (see Janney et al., 2002).

Sublithospheric structure beneath the East African Rift System
Early two-plume models (George et al. 1998; Rogers et al. 2000) were based on the Sr–Nd isotopic compositions of lavas from southern Ethiopia and the Kenya Rift. The current study, which incorporates Sr–Nd–Pb–He isotopic observations over a much larger region and time frame, requires long-lived, isolated and geochemically distinct source domains to produce the diverse range of magmas originating from beneath the African lithosphere. Specifically, we need to explain the contemporaneous occurrence of high-µ (relatively high ²⁰⁶Pb/²⁰⁴Pb, low ⁸⁷Sr/⁸⁶Sr and low-³He) volcanism at Turkana and rather lower ²⁰⁶Pb/²⁰⁴Pb, high-³He magmatism in Afar. The question remains whether two geochemically distinct plumes are required to explain the regional geochemical and geophysical observations.

The geochemical variations alone do not require two distinct plumes, but could reflect heterogeneity within one plume. This suggestion follows from a single-plume model proposed by Ebinger & Sleep (1998) attributing Cenozoic African Rift volcanism to plume head impact beneath southern Ethiopia at 45 Ma, with lateral transport of plume material along zones of thinned lithosphere. The spatial proximity of lavas with this degree of Pb and He isotopic variability is not unique to East Africa, as it is also observed in the southern Pacific, where basalts from Samoa, Pitcairn and some of the Society Islands (high-³He), and those from the Cook–Austral islands (HIMU-affinity), are all associated with the South Pacific Superplume (e.g. Farley et al., 1992; Hanyu et al., 1999; Courtillot et al., 2003; Lassiter et al., 2003).

The geophysical evidence, however, may require two separate upper-mantle structures. There is a growing body of seismic information indicating plume stems beneath Afar and Tanzania that are linked to lower mantle features. Beneath Afar, S-wave tomographic imaging shows a thermal anomaly that reaches 500 km below the surface (Debayle et al., 2001; Benoit et al., 2003). Montelli et al. (2004) suggested that this feature extends to depths approaching 1200 km, well below the Transition Zone, hence requiring a deep mantle origin. Beneath the Tanzania craton, Nyblade et al. (2000) and Weeraratne et al. (2003) documented an upper mantle thermal anomaly that is too large to be sustained by flow of plume material along the rift axis, and is interpreted as an ascending plume head. The continuation of this structure as a plume tail at depth is unclear, as is its possible connection to the South African superplume. The geophysical evidence thus supports two distinct upper- to mid-mantle thermal anomalies, or plumes. The depth of origin of these two features is unclear, as they may merge below 1000 km as seen with other plume pairs (Azores and Canaries, Ascension and St. Helena; Montelli et al., 2004). In this case, both thermal anomalies may be derived from the South African superplume, a well-documented (Ritsema et al., 1999; Gurnis et al., 2000; Zhao, 2001; Ni et al., 2002) feature of the deep mantle. Current tomographic models are unable to resolve the fine structure of the mid-mantle, leaving open the question of whether this feature reaches the shallow mantle beneath the African Rift.

The observations presented here can be reconciled through a modified one-plume model that allows for multiple plume stems arising from a common large plume at depth (Ishida et al. 1999; Davaille et al. 2003; Farnetani & Samuel, 2003). The plume stems supporting volcanism in Turkana and in the Afar are proposed to contain lenses of isotopically distinct materials, and hence record large-scale heterogeneity within the South African superplume. This result is consistent with numerical simulations of plume geochemical structures (Samuel & Farnetani, 2003), which demonstrate that large geochemical and isotopic contrasts can exist over time and length scales consistent with those observed in eastern Africa. These simulations show that chemically distinct components present in the lower mantle can ascend through the mantle without full-scale mixing, and suggest that chemical heterogeneities in the lower mantle may in fact lead to a complex range in plume shapes and sizes.

	SUMMARY

TOP ABSTRACT INTRODUCTION TECTONIC SETTING GEOLOGY AND PETROLOGY OF... SAMPLING AND ANALYTICAL... RESULTS DISCUSSION SUMMARY APPENDIX: ANALYTICAL METHODS REFERENCES

 
Primitive mafic lavas of Tertiary age from Turkana, Kenya, provide insight into the magmatic processes associated with the earliest phase of continental extension in the central East African Rift. Alkalic and tholeiitic basalt lavas from Turkana 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 as a function of time. The oldest basanite lavas (up to 40 Ma) show evidence of crustal contamination resulting from interaction along immature magmatic pathways in the early evolution of the rift. A significant volume of primitive basalts and picrites with high-µ Sr–Nd–Pb–He isotopic signatures was erupted for a restricted period of time from 23 to 20 Ma. This eruptive episode is associated with rift propagation both to the north and south, suggesting that it is tectonically significant to the overall evolution of the African Rift. Mafic lavas from this period were derived by the greatest extents of melting recorded at Turkana and at the greatest pressures (>25 kbar). Late Tertiary Turkana lavas, like the Quaternary Turkana suite (Furman et al., 2004), show local involvement of continental mantle lithosphere that suggests the rapid development of steep topography across the lithosphere–asthenosphere boundary associated with the interaction of the Main Ethiopian and Kenya rifts.

The high-µ isotopic signature observed in the Jarigole mafic lavas is not derived from the in situ mantle lithosphere, but rather requires a sub-lithospheric origin. These isotopic features are not observed in lavas of any age erupted in the Afar region, and are interpreted as evidence for a long-lived mantle plume that is spatially, temporally and geochemically heterogeneous. We suggest that the high-µ component—presumably a pod of recycled ancient oceanic crust—was entrained by upwelling plume material, rises passively at the same time as the plume, or forms an integral part of the upwelling plume itself. We attribute this heterogeneity to the South African superplume, where large-<1?h0,14?>scale internal variations in source composition are manifest in a wide range of eruptive isotopic signatures across Africa.

	APPENDIX: ANALYTICAL METHODS

TOP ABSTRACT INTRODUCTION TECTONIC SETTING GEOLOGY AND PETROLOGY OF... SAMPLING AND ANALYTICAL... RESULTS DISCUSSION SUMMARY APPENDIX: ANALYTICAL METHODS REFERENCES

 
Bulk-rock analyses for major and minor elements (including Ba and Sr) were obtained by DCP on an ARL-Fisons Spectraspan 7; P₂O₅ and remaining trace elements were analyzed by ICP-MS using a VG PlasmaQuad-3 at Duke University. Precision based on replicate analyses of samples and natural basalt standards is generally <1% for SiO₂, Sr, Y, Zr, Nb, La and Ce; <3% for other major elements, Ba, Sr, Rb, Cs, Cr, Sc, V, Co, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Hf and Ta; <5% Ni, Yb, Lu, Pb and Th; and <8% for U.

Strontium isotopic compositions are corrected with an internal normalization of ⁸⁶Sr/⁸⁸Sr = 0·1194. Nd isotopic compositions were normalized to ¹⁴⁶Nd/¹⁴⁴Nd = 0·7219 to correct for within-run fractionation; _Nd values are reported in this study relative to the chondritic uniform reservoir (CHUR) value of 0·512638. Sr and Nd isotopic standards were routinely run to monitor analytical performance. Average NIST SRM 987 was ⁸⁷Sr/⁸⁶Sr = 0·710240 (0·000012, 2, n = 2); average La Jolla was ¹⁴³Nd/¹⁴⁴Nd = 0·511850 (0·000020, 2, n = 4). NIST SRM 981 was analyzed at UCLA to monitor machine performance and to provide a means for fractionation correction. Average NIST SRM 981 values were ²⁰⁶Pb/²⁰⁴Pb = 16·896 (0·007, 2, n = 3), ²⁰⁷Pb/²⁰⁴Pb = 15·435 (0·004, 2, n = 3), and ²⁰⁸Pb/²⁰⁴Pb = 36·5333 (0·010, 2, n = 3).

The Pb isotope compositions analyzed at SDSU were determined by MC-ICP-MS on the Nu Plasma HR using the T1 doping technique using NIST SRM 997 Tl (White et al., 2000; Albarède et al., 2004) and ²⁰⁵Tl/²⁰³Tl = 2·3889 (Thirlwall, 2002) to monitor fractionation. The Tl-corrected NIST SRM 981 Pb standard averaged ²⁰⁶Pb/²⁰⁴Pb = 16·9267 ± 0·0002, ²⁰⁷Pb/²⁰⁴Pb = 15·4781 ± 0·0002, and ²⁰⁸Pb/²⁰⁴Pb = 36·6553 ± 0·0008 during the analysis period. The NIST 981 standard was run every third analysis. The sample Pb isotope ratios were corrected for instrumental mass fractionation and machine bias using the bracketing standard runs and a delta correction to the Todt et al. (1996) values for NIST 981. Uncertainties reported on the measured isotope ratios represent error propagation of the in-run 2/n analytical errors plus the 2/n reproducibility of the standard.

Argon–argon dates shown are weighted mean plateau ages determined on fine-grained whole-rock samples. Samples were leached to remove minor alteration products by washing in 5% HF for 30 s, followed by a water rinse, ultrasound in dilute nitric acid for 20 min, another water rinse, and then drying. Cleaned samples were examined under a binocular microscope and altered crystals removed by hand. All samples were irradiated in quartz vials at the Oregon State University TRIGA research reactor; neutron fluence was monitored with the Fish Canyon Tuff biotite standard (28·03 ± 0·16 Ma, Renne et al., 1998). Typical irradiation conditions were 6 h in a dummy fuel rod in the reactor's central ring, at 1 MW power. Following irradiation and initial decay of short-lived radionuclides, samples were loaded in an ultrahigh-vacuum gas extraction line. After gas clean-up with Zr–Al getters the isotopic composition of Ar released at each heating step was measured with a MAP 215/50 mass spectrometer. Samples were heated in 50–100°C increments, from 400°C to fusion in 6–13 steps, depending on K content. Ar data were acquired in a peak-hopping mode (for m/z = 35, 36, 37, 38, 39, 40) by computer. Peak decay was linear and typically <10% over 12 sets of peaks and backgrounds. Mass discrimination on the MAP system was measured with zero age samples run in the same way as samples, and was constant at 1·005 (for 2 a.m.u.). The sensitivity of the mass spectrometer is 4 x 10^–14 mol/V and measured backgrounds were 1·5 x 10^–18 mol at m/z = 36, 2 x 10^–18 mol at m/z = 39 and 1·5 x 10^–16 mol at m/z = 40. Procedure blanks for the resistance furnace ranged from 3·0 x 10^–18 mol ³⁶Ar and 9·0 x 10^–16 mol ⁴⁰Ar at 600°C to 6·4 x 10^–18 mol ³⁶Ar and 1·9 x 10^–15 mol ⁴⁰Ar at 1400°C, whereas laser chamber blanks measured before each sample and after every third step during sample analysis were (2–3) x 10^–18 mol ³⁶Ar and (6–9) x 10^–16 mol ⁴⁰Ar. ArArCALC v2.2 software from Koppers (2002) was used to reduce data and make plots. Step ages were calculated using the corrected Steiger & Jager (1977) decay constant of 5·530 ± 0·097 x 10^–10 yr^–1 (2) as reported by Min et al. (2000), assuming that initial sample Ar compositions were atmospheric (initial ⁴⁰Ar/³⁶Ar = 295·5), and plotted against cumulative proportion of gas released (per cent ³⁹Ar) as age spectrum, or ‘plateau’ diagrams.

For He isotope analysis, olivine phenocrysts were obtained from lightly crushed rocks by sieving and hand-picking under a binocular microscope. Grains were cleaned ultrasonically in deionized water and then in acetone, then air-dried and re-examined under the microscope. Crystals showing evidence of alteration or adhering rock matrix were rejected. Olivine separates were analyzed by in vacuo crushing to liberate gases trapped within melt and/or fluid inclusions as described by Graham et al. (1998).

	ACKNOWLEDGEMENTS

 
This paper grew out of the M.S. thesis of K.M.K., who gratefully acknowledges fellowship support from the Department of Geosciences at Penn State. We are very grateful for He isotope analyses performed by D. Graham, and for Ar dates obtained by R. Duncan. Major and trace element analyses at Duke University were performed by M. Rudnicki and G. Dwyer, and we thank E. Klein for making her facility available to us. We thank Haibo Zou for conducting the TIMS analyses. T.F. supported this work through NSF EAR-0207764 and a George H. Deike, Jr grant from the College of Earth and Mineral Sciences at Penn State. J.G.B. is appreciative of support during the preparation of this manuscript from NSF EAR-0338385, as well as funds from the UNH College of Engineering and Physical Sciences. Thoughtful comments from S. Gibson, J. Mahoney and D. Geist have served to clarify our thinking and improve the manuscript.

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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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