Sr-Nd-Pb isotopic studies of lavas of Mt . Oku volcano , North West Cameroon : A case involving HIMU , depleted and enriched mantle sources

Sr-Nd-Pb isotopic studies have been carried out on the lavas of Mt. Oku, the central part of the continental sector of the Cameroon volcanic line (CVL). The lavas define a considerable range of Sr-NdPb isotopic compositions characterized by 143 Nd/ 144 Nd = 0.512626 – 0.512937, 87 Sr/ 86 Sr = 0.699411 0.704279 and more radiogenic isotopic ratios 206 Pb/ 204 Pb = 18.3800 – 19.4480. 207 Pb/ 204 Pb = 15.4216 – 15.6448, 208 Pb/ 204 Pb = 37.1114 – 39.7507, with radiogenic Pb similar to the HIMU of the continent ocean boundary of the Cameroon volcanic line. These isotopic data display a linear array on the 143 Nd/ 144 Nd vs 87 Sr/ 86 Sr diagrams trending from MORB towards bulk earth. 143 Nd/ 144 Nd and 87 Sr/ 86 Sr vs. 206 Pb/ 204 Pb show a continuum of compositions that suggest differing contributions to the parental magmas from three end-members. One end-member corresponds to a mixture of high uranium high lead (HIMU) and depleted mantle (DM) source components while the other end-member corresponds to an enriched mantle source thought to be the MORB source or sub-lithospheric source. The enriched geochemical signatures (EM) of the Oku magmas are unlikely to be the result of crustal assimilation during magma ascent. Rather, they are thought to be derived from a sub-continental lithospheric mantle enriched in incompatible trace elements by ancient metasomatic processes. The HIMU component is thought to have been inherited from the fossil plume that underlay the Equatorial Atlantic during the period 130 to 100 Ma before migrating to its present site. The radiogenic initial 206 Pb/ 204 Pb ratios of the samples (~18.3800 – 19.44804) furthermore require the involvement of an ancient HIMU mantle plume in the magmatism and this may have been formed by metasomatism of fluids derived from St. Helena hot plume ca 120 Ma.

Highlands basanites and alkali basalts have anomalously high concentrations of Sr, Ba and P and low concentrations of Zr, which are exclusive features of continental Cameroon basalts.The genesis of these latter magmas is consistent with derivation from an incompatible element enriched, amphibole-bearing lithospheric mantle source (Marzoli et al., 2000).
Mt. Oku volcanic field lies in the continental sector of the CVL.It is a complex stratovolcanic edifice 90 km across rising to a height of 3011 m (Mt.Oku).About 2000 m of lava pile would have been erupted ranging from basalt through hawaitte, mugearite to trachyterhyolites, high level intrusions and intercalated pyroclastics.Njilah (1991) and Njilah et al. (2007) give K-Ar ages, major and trace element including Rare Earth Elements (REE) geochemistry as well as petrographic studies of the spectrum of lavas of this massif.
An interesting aspect of lavas of the massif is the occurrence of patchy and complex optical and chemical zoning of the pyroxenes described in Njilah et al. (2008).The silicic volcanism here occurred between 25 Ma to recent and is represented by voluminous quartznormative trachytes and minor rhyolitic ignimbrites.These silicic volcanics are associated with slightly to moderately alkaline basalts and minor basanites.In general, onset of the silicic volcanism migrated from NE (Oku: 25 Ma) to SW (Sabga: 23 Ma; Bambouto: 18 Ma; and Mt.Manengouba: 12 Ma).The silicic volcanism of the Ngaoundere plateau (eastern branch of the CVL) is instead dominated by nepheline-normative trachytes which are associated with strongly alkaline basalts and basanitic rocks (Marzoli et al., 1999).We provide new Sr-Nd-Pb isotope compositions for the Mt.Oku volcanics in order to identify the geochemical character of the magma source region.Our data show that the parental basaltic magmas of Mt.Oku underwent a complex evolutionary history that is reflected in the mineralogy, major, trace and isotopic chemistry (Njilah, 1991, Marzoli et al., 2000).The new Sr-Nd-Pb isotope chemistry presented in this work suggests the involvement of three mantle source components, enriched, depleted and HIMU in the petrogenesis of the lavas of Mt.Oku.

Geological setting
1600 km long Y-shaped chain of tertiary to recent, generally alkaline, volcanoes called the CVL (Figure 1) represents Cenozoic volcanism in Cameroon.The CVL follows a trend of crustal weakness that stretches from the Atlantic Island of PagaIu, through the Gulf of Guinea, to the interior of the African continent (Fitton, 1987;Halliday et al., 1988;Déruelle et al., 1991).It is unique amongst intraplate volcanic provinces in that it straddles the continental margin and includes both oceanic and continental intraplate volcanism.
The primitive alkali basaltic magmas of the oceancontinent boundary (Mts.Cameroon, nephilinites of Mt.Etinde and basalts of the Island of Bioko) display a distinctive Pb-isotope anomaly which diminishes away from this zone over a radius of 400 km on either side (Njilah et al., 1999).The elevated 206 Pb/ 204 Pb ratios are similar to those associated with the mantle source of the oceanic Island of St. Helena (Chaffey et al., 1989;Richard et al, 1997), commonly referred to as high uranium high lead (HIMU), whose hotspot trace can be projected back to the ocean-continent boundary at ca 120 Ma (O'Connor and Duncan, 1990).
Mt. Oku (Figure 1) occupies a central position inland of an almost continuous chain of volcanism stretching north east from Mt. Cameroon and which cuts through and sits on a basement of Pan-African granite-gneisses (approx.600 Ma) migmatites, and biotite diorites.These rocks were uplifted during the general doming that accompanied the eruption of the CVL.Erupted rocks include basalts, trachytes, ryholites, ashflow tuff scories and volcanic ash (Njilah, 1991).The massif is a complex volcanic edifice approximately 90 E-W and 60 km N-S and rising at height of 3011 m (Mt.Oku).It is composed of four major adjacent stratovolcanoes: Mt.Oku, Mt.Babanki, Nyos and Nkambe.A 2000 m thick volcanic sequence could have existed here prior to erosion.Trachyte, rhyolite and ignimbrite constitute 85% of the volcanics We obtained a spectrum of mafic to felsic rocks from the Mt.Oku of which the mineralogy, major and trace element geochemistry and K-Ar age determinations have been discussed elsewhere (Njilah et al., 2007).
Here we discuss the Sr-Nd-Pb isotopic evidence for mantle heterogeneity under Mt.Oku volcano and show that three mantle sources are responsible for the generation of the spectrum of lavas of the Mt.Oku: HIMU, MORB (depleted) and Lithospheric (enriched) mantle sources.

METHODS
Whole rock analyses for major and trace elements were done by X-Ray Fluorescence (XRF) techniques method as described in Njilah (1991) in the geochemistry Laboratory of the University of Leeds UK.For Sr and Nd isotope analyses, (thanks to G. R. Davies at the University of Michigan, USA who did the analyses).50 to 150 mg of sample powder was weighed accurately into Teflon bombs, (Njilah) and spiked with 85 Rb-84 Sr and 150 Nd-14 Sm spikes.These powders were digested in HF-HNO3 in an oven overnight, evaporated to dryness, and converted with 15 M HNO3 and 6 M HCI completely into solution.A11 reagents used were quartz distilled.Sr and REE were separated on preconditioned cation-exchange columns with 1 N HCl and 6 N HCI as eluants.Separation of Nd from the REE fraction was achieved using 0.25 N HCI as eluant on HDEHP (Hydrogen Diethylxylphosphate) coated Bio-Rad bead resin.For Pb analyses, aliquots of 80 mg were dissolved in Savillex beakers at 15°C using a HF/HNO3 solution.After redissolving in HCI and converting to a 0.5 N HBr solution, purification of Pb essentially followed standard HBr-HCl chemistry on columns filled with 100 µ AG-1X8, 200-400 mesh resin prior to elution with 6 N. Isotopes were measured in static mode on a Fisons-VG-Micromass-54-30 Sector multicollector mass spectrometer.Pb was loaded on single Re filaments using a H3PO4-silica gel mixture as reported in detail in Cameron et al. (1967).Pb was analysed at 1350°C and the data were corrected for instrumental mass fractionation of 0.7% per atomic mass unit.This value was established by two loads of NBS-981 during the course of this study calculated from 202 Pb/ 205 Pb double spiked measurements.The total Pb procedure blank during the course of this study is about 120 pg per analysis.
Sr was loaded in 1 M NHNO3 on a Ta single filament and Nd was ionized on triple filament assemblies with a center Re ionization filament.For Sr and Nd isotope ratios, an exponential fraction correction was applied using 86 Sr/ 88 Sr = 0.1194 and 146 Nd/ 144 Nd = 0.7279.The accuracy was monitored by replicate measurements of NBS-987 and the Ames ® Nd metal standard giving 87 Sr/ 86 Sr = 0.710257 (n=8) and 143 Nd/ 144 Nd = 0.511978 ± 12 (2s) respectively.

RESULTS
Chemical analyses for major, trace and REEs of representative samples of the spectrum of lavas of the Mt.Oku are presented in major Table 1.These data indicate that they are under-saturated or saturated alkali basalts (Ne N > 15), and are characterized by high tenors in alkalis (Na 2 O + K 2 O >), Ni, Co, Nb and the relations LREE/HREE (La N /Yb N > 15).Furthermore, the chemical analyses of morphological and chemically zoned clinopyroxene have clearly proved the existence of more than one magma chamber beneath Mt.Oku that gave rise to these lavas (Njilah et al., 2008).

Isotope chemistry
On a plot of 143 Nd/ 144 Nd, 87 Sr/ 86 Sr against MgO wt.% (Figure 2), the basic samples from Oku show trends which suggest combined fractional crystallization and crustal contamination by assimilation as the main differentiation processes that affected the magma.Nd-Sr and Pb isotope geochemistry of the CVL has been presented elsewhere (Halliday et al., 1988(Halliday et al., , 1990(Halliday et al., , 1994;;Marzoli et al., 1999;Marzoli et al., 2000) (Njilah, 1991), the lavas plot on a linear array trending away from the field of present day MORB towards bulk earth.Njilah et al. 127 Figure 3 shows 208   4) show a continuum of compositions that suggests differing contributions to the parental magmas from two end-members, A and B. End-member A appears to be a mixture of HIMU and depleted mantle (DM) source component while endmember B comprises of an enriched mantle source thought to be a MORB source or sub-lithospheric source.

DISCUSSION
Crustal rocks, evolving with higher Rb/Sr ratios, develop much higher present day 87 Sr/ 86 Sr ratios than the upper mantle, which has evolved with very much lower Rb/Sr ratio.MORB and OIB reach the surface having suffered little or no contamination due to the thin crust which they traverse, and the fact that they are traversing a crust of basaltic composition.In intra-continental plate domains like Mt. Oku, the magmas will have a tendency to interact with high Nd > 0.51278 to the associated alkaline basalts suggesting differentiation processes without appreciable interaction with crustal materials in Mt.Oku and neighboring Bambouto.Since the mobility of Rb and Sr in the melt (in situ silicate) (H 2 O + CO 2 and chlorite medium) interaction is considerably higher than Sm and Nd Light Rare Earth Elements (LREE), the fluid induced melt in the upper mantle becomes isotopically more homogeneous with respect to Rb-Sr compared to Sm-Nd causing variable ٤ Nd (~Nd +5.3 to -3.24).Such heterogeneity can be interpreted by the participation of two mantle reservoirs: a depleted MORB and a HIMU OIB-type source.The isotopic systematics of the Oku cannot be accounted for by simple two-component mixing between an unmetasomatized mantle and a HIMU component.Rather, they are most reasonably explained by contamination by some enriched subcontinental lithosphere magmatic plume juice component characterized by high 206 Pb/ 204 Pb.Nd-Sr isotopic data for primitive basalts from the CVL suggest that the mantle source is heterogeneous and that this heterogeneity could have evolved in the last 125 to 100 Ma since the initiation and the opening of the Gulf of Guinea  (Halliday et al., 1990).Homogeneity of trace element characteristics of the mantle source, on the scale of 1000 km or more is implied by the geochemical similarity between the alkali basalts erupted in the oceanic and continental sectors of the CVL, though small-scale heterogeneity may persist (Fitton and Dunlop, 1985).The enriched geochemical signatures are thought to be derived from a subcontinental lithospheric mantle enriched in incompatible trace elements by ancient metasomatic processes.The radiogenic initial Pb ratios of the samples (~17.3165-20.2503) furthermore require the involvement of an ancient high µ mantle reservoir in the magmatism and this may have been formed by metasomatism involving fluids derived from St. Helena hot plume ca 120 Ma.

Origin of the three mantle sources
Isotopic variations cannot be related to a simple temporal evolution of a single component but rather imply a mixture in variable proportions between the two end-members (Dieter et al., 1991;Zendler et al., 1982;Karmalkar and Rege 2002;Escrige et al., 2003) or more end members (Njilah et al. 1999).Temdjim et al. (2004) have, from the study of ultramafic xenoliths, shown upper-mantle heterogeneity in Nyos (northern flanks of the Oku massif).Moundi et al. (2007) have put in evidence the data which indicate a source in an enriched lithospheric mantle (EM) that produced the transitional basalts of the Bamoun plateau 75 km to the south, in contrast to a source closer to HIMU, which could have produced alkali basalts.Our studies show that the melts produced in Mt.Oku and the rest of the CVL have signatures from three sources: plume components (HIMU), asthenospheric components (Depleted) and lithospheric components (Enriched).On the basis of Sr-Nd-Pb isotopic studies, it is clear that the parental magmas were derived from a source region which is largely insensitive to the gross differences that one might expect in the lithosphere between oceanic and continental regions.The old lithospheric mantle beneath Africa should be chemically and isotopically very different from the young Atlantic Ocean lithosphere and, therefore, if it had been involved extensively in the petrogenesis of the continental sector magmas, they should bear a distinctive geochemical fingerprint, which they do not.Sr-Nd-Pb isotopic studies have revealed that the basic magmas from Mt. Oku, and the CVL, as a whole, could be derived from mixing of partial melts derived from a three source components: HIMU, depleted and enriched mantle.The question now is, what is the origin of these three mantle sources?
The sub continental lithospheres in most areas studied have been found to be enriched in incompatible elements at various times throughout geological time.To explain the origin of these three mantle sources of the Oku volcano, we think that, at ca 120 Ma the low density St. Helena plume up welled, undergoing adiabatic decompression resulting in partial melting.Normally, a deep mantle plume originates from a depth within the lower mantle and the core mantle boundary (Roy et al., 2004).The melts segregated from their source and ascended towards the surface, but because they were low degree partial melts, they became trapped in the mechanical boundary where they caused metasomatic enrichment of the upper mantle (Mckenzie, 1989).Sea floor spreading, starting at 120 Ma  (Fitton, 1987;Halliday et al., 1988).This, in effect, caused migration of the presently active St. Helena hotspot to its present-day position as described by O'Connor and Duncan (1990), leaving the metasomatised enriched mantle coupled to the lithosphere.
At 65 Ma, magmatic activity was initiated along the CVL with the emplacement of sub-volcanic intrusions (ring complexes).Melts were probably generated from the metasomatised domains (Vollmer, 1983;Karmalkar and Rege, 2002) or enriched mantle.
From 35 Ma to present, the partial melts of the metasomatised mantle (composed of HIMU, depleted and enriched components) have erupted through channel ways to the surface forming the extrusive series of the CVL.The HIMU characteristics of the lava of Mt.Oku are, therefore, fingerprints of the involvement of the Cretaceous St. Helena hotspot in the petrogenesis of the magmas of the CVL as a whole.More detailed geochemical and geochronological studies are required to fully understand the part played by the St. Helena hot plume in time and space in the generation of magmas of the Mt.Oku and the CVL as a whole.

Conclusion
Radiogenic isotope compositions (Sr, Nd, Pb) of mafic rocks of Mt.Oku show continuous variations connecting three distinct mantle reservoirs: HIMU, enriched and depleted mantle reservoirs.The trends of radiogenic isotopes have been interpreted as metasomatism induced in the mechanical boundary (lithospheric upper mantle) layer by fluids from the St. Helena mantle uprising plume ca 120 Ma before migration to its present site.
The melts became trapped in the mechanical boundary where metasomatic enrichment of the upper mantle occurred.The strong regionality of major and trace elements together with structural geology and geophysical evidence, support shallow present day mantle processes in the generation of lavas of the Mt.Oku and the CVL as a whole.Present day magmatism is therefore suggested to be derived partial melting of this mechanical boundary layer by adiabatic decompression.

Figure 1 .
Figure 1.The Cameroon Volcanic Line (CVL) showing (a) location of Mt.Oku volcano and (b) Geological map of Mt.Oku volcano.
granite/gneiss crust to generate hybrid magmas.Marzoli et al. (1999) however, showed that the least differentiated silicic volcanics are isotopically similar 87 Sr/

Figure 2 .
Figure 2. Variation of 143 Nd/ 144 Nd and 87 Sr/ 86 Sr plotted against MgO.Vectors show the effects of fractional crystallization (FC) and assimilation (A).

Figure 3 .
Figure 3. Variation of 208 Pb/ 204 Pb and 207 Pb/ 204 Pb plotted against 206 Pb/ 204 Pb for mafic rocks from Oku compared with data from other CVL lavas.NHRL is the northern hemisphere reference line.

Figure 4 .
Figure 4. Variation of 143 Nd/ 144 Nd and 87 Sr/ 86 Sr, plotted against 206 Pb/ 204 Pb for the lavas of Oku compared with oceanic, ocean-continent boundary and continental sector volcanics (from literature cited in the text).The data may be described in terms of three mantle components described in the text i.e. depleted mantle (DM), enriched mantle or continental lithospheric mantle and HIMU sources.

Table 2 )
. On a 143 Nd/ 144 Nd vs. 87 Sr/ 86 Sr variation diagram Pb/ 204Pb and 207 Pb/204Pb plotted against206Pb/ 204 Pb in relation to other volcanic centres.Some of the lavas have radiogenic Pb similar to the HIMU of the continent-ocean boundary sector of the CVL.This HIMU signature contrasts with lavas from other volcanic centres around Mt. Oku.The lavas overlap those of the continent-ocean boundary and trend towards MORB, paralleling the NHRL.The trends of Pb isotope variation for Mt.Oku are paralleled by Sr-Nd variation, that is, samples with the most radiogenic Pb have the least radiogenic Nd.

Table 1 .
Major, trace and REEs of representative samples from the Mt.Oku.

Table 2 .
Radiogenic Sr-Nd-Pb²isotopic data for selected basic volcanic rocks from the Oku-Ndu area.Also included are data for MgO, Ni and Cr.87Sr/86Sr, indicates age corrected data.