Xenoliths Evidence of Alkaline Magmatic Infiltrations Beneath Lake Nyos (Cameroon Volcanic Line, West Africa) ()
1. Introduction
The Cameroon Volcanic Line (CVL; Figure 1(a)) is a chain of Tertiary to Recent, less tholeiitic, transitional to strongly alkaline intraplate volcanoes extending from the south Atlantic island of Pagalu to the continental interior of West Africa (Biu Plateau), trending averagely N30˚E and extending over a distance of 1600 Km [1] [2] [3]. The Nyos volcanic unit (6˚26'N and 10˚18'E) is part of the Oku massif, one of the continental CVL massifs. In this unit, volcanism has been concentrated in distinct eruptive centers, the largest one being to take in the Lake Nyos area. The Nyos volcanic rocks, mainly basalts have been grouped into two successive episodes. The first manifestation has been termed the first episode [4] and is made up of large basaltic lava flows. The second episode (recent series) activity contrasts with the relatively quiet volcanism of the earliest volcanic activity in that it was highly explosive, giving rise to steep pyroclastic cones and maars. A recent series is a group of craters, tuff cones, and flows which, on the basis of their youthful morphology, are regarded as the products of some of the most recent activity in Oku massif. The volcanism of the Lake Nyos area is essentially basic and different from that of the CVL continental swells which can be bimodal (basic and/or acid [4]. Lava flows, maar pyroclastics, and cinder cones from the Nyos volcano enclose abundant ultramafic xenoliths of spinel lherzolites and harzburgites and a diverse suite of crustal rock. Websterites have also been described by Temdjim et al. [5]. Ultramafic xenoliths have not been found in the first episode. Ultramafic xenoliths comprise varying combinations of olivine, orthopyroxene, clinopyroxene, spinel, and amphibole. New petrogeochemical investigations allow us to portrait special petrographic (wehrlites and lherzolites transitional porphyroclastic to equigranular) and mineralogic (pargasites and phlogopites) features and to precise their origin and highlight the alkaline affinity of the magma that brought out these peridotites.
2. Geological Setting and Xenolith Petrography
The geological setting of the Lake Nyos area (Figure 1(b)) has been revisited by Teitchou [4]. In addition, it is important to note that Lake Nyos constitutes the lower down part of the four massifs (Babanki, Oku, Nkambe, and Nyos) which form mounts Oku (CVL). Volcanic formations of Lake Nyos, principally lava flows and maar deposits, rest uncomfortably over Pan-African metamorphic formations (mostly biotite gneiss) intruded by granitoids. Petrochemical data [6] [7] [8] point out that lavas of the Nyos volcano area are essentially basaltic and tholeitic and outlet by a low degree of partial melting as most lavas of the continental volcanic plains of the CVL formed in lower asthenospheric mantle
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Figure 1. (a) Localization of the Cameroon volcanic line and (b) geological map of the Lake Nyos volcanic unit area.
sources [9]. Lavas are sodic and share some OIB characteristics. Basaltic lavas of the lake Nyos volcanic unit display petrological and geochemical similarities with other continental plains of the CVL, especially those of Kumba, Tombel and Noun [4].
Peridotite xenoliths recovered from Nyos area are found in recent lava flows and maar pyroclastics. These xenoliths can be enclosed in stratified deposits or in base surges ejecta, sometimes, occupying the heart of basaltic bombs. They also form real bombs of variety lengths up to 10 cm. The contact with the host lavas generally is sharp. Unbreaking bombs can be spherical, angular or ovoid.
Modal compositions of peridotites (Table 1) were determined following the method described by Teitchou et al. [10] [11]. Modal compositions, using Streckeisen diagram [12] allow recognizing lherzolites, harburgites and a free wehrlite (Figure 2).
Spinel lherzolites (samples NK01, NK05, NK07, NK09, NK11, NK13 and NK14) are the most abundant xenoliths in Nyos volcanic unit (more than 60% of collected samples) and are composed of olivine (55% - 70%), orthopyroxene (7% - 30%), clinopyroxene (6% - 20%) and spinel (1% - 6%). Harzburgites (samples NK02, NK04, NK10a, NK10b and NK15) are less abundant (30%) and are composed of olivine (71% - 83%), orthopyroxene (10% - 20%) and clinopyroxene (2% - 4%). Studied lherzolites and harzburgites have predominantely
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Figure 2. Modal composition of the studied Nyos xenoliths. Classification is after [12]. Ol—olivine; Opx—orthopyroxene; Cpx—clinopyroxene.
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Table 1. Modal compositions of the investigated Nyos mantle peridotites (in vol%).
porphyroclastic and protogranular textures respectively, except wehrlite NK08 (modal composition: Ol: 63, Cpx: 36 and glass patches or veinlets < 1%) which is the only sample that displays a cumulative texture mostly consisting of sub-euhedral olivines associated to interstitial anhedral clinopyroxenes commonly poekilitic. Wehrlite also contains localy a strip of clinopyroxene which cristallyse in disavantage of orthopyroxene (Figure 3). Similar processes transforming
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Figure 3. Microphotographs of (a) wehrlite NK08 show a strip of Cpx which crystallize in favor of Opx; (b) lherzolite NK14 showing transitional porphyroclastic to equigranular texture.
Opx into Cpx have been reported in other peridotite xenolith occurrences in the world, e.g. by Boivin [13] in composite xenoliths of Tallante in Spain), Neuman and Wulf-Pederson [14], lherzolites of Canaries Islands, Grégoire et al. [15] in harzburgites of Kerguelen Islands and Remaïdi [16] in lherzolitic massif of Ronda. Sample NK14 have transitional porphyroclastic to equigranular texture and displays pyroxene-Cr spinel symplectites with CO2 inclusions.
3. Analytical Methods
Major and trace elements of minerals were analysed at the UMR 5563 (LMTG, Observatoire Midi-Pyrénées) of University Paul Sabatier (Toulouse III, France). Major-element compositions of minerals were determined with the CAMECA SX50 electron microprobe and a standard program: beam current of 20 nA and acceleration voltage of 15 kV, 10 to 30 s/peak, 5 - 10 s/background counting times, and natural and synthetic minerals as standards. Nominal concentrations were subsequently corrected using the PAP data reduction method [17]. The theoretical lower detection limits are about 100 ppm (0.01%). The concentrations of Rare Earth Elements and other trace elements (La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb, Lu, Rb, Ba, Th, Sr, Zr, Ti, Y, Ni, V and Sc) in clinopyroxenes and amphiboles were analysed in situ on 100 - 120 mm thick polished sections with a Perkin-Elmer Elan 6000 ICP-MS instrument coupled to a CETAC laser ablation module that uses a 266 nm frequency-quadrupled Nd-YAG laser. The NIST 610 and 612 glass standards were used to calibrate relative element sensitivities for the analyses. The analyses were normalized using CaO values determined by electron microprobe. The analyses were performed on inter-cleavage area from the cores of the freshest cpx and amphibole grains in order to get homogeneous results unaffected by alteration or exsolution processes. A beam diameter of 50 - 100 µm and a scanning rate of 20 µm/s were used. The typical relative precision and accuracy for a laser analysis range from 1% to 10%. Typical theoretical detection limits range from 10 to 20 ppb for REE, Ba, Rb, Th, Sr, Zr and Y; 100 ppb for Sc and V; and 2 ppm for Ti and Ni (see in Dantas et al. [18] for more details).
Major element compositions of the same sample were carried out with XRF techniques (Philips 2400) and measured on fused disks at Saint-Etienne school of Mines, France.
Trace element composition in Nyos peridotites were obtained using ground rock powders (~100 mg) dissolved in HF-HNO3 mixtures. Dried samples were taken up in HNO3 and diluted to 1:1000 in 2% HNO3. Reference sample BEN was used for calibration. Chemical blanks and 2 reference materials (UB-N, JP-1) were run with each sample batch.
4. Mineral Chemistry
Nyos peridotites display four typical mantle minerals: olivine, orthopyroxene, clinopyroxene and spinel. Disseminated dark pargasitic amphibole and brown phlogopite plates are observed in harzburgites NK02 and NK04 respectively.
4.1. Major Element Distribution
Selected electron microprobe analysis of minerals phases are given in Tables 2(a)-(e).
Olivine compositions in lherzolites and harzburgites range between Fo87 and Fo91 (mole %) and CaO contents are 0.07% - 0.09% (wt%). Magnesium rich olivine appears in harzburgites with Fo88-91. Harzburgites have high NiO contents (0.30 - 0.45 wt%) like ophiolitic and oceanic ridge peridotites [19] [20]. High NiO and Fo contents of harzburgites are mineralogical fingerprints of their refractory characters [21]. Wehrlite NK08 has low Fo contents (Fo79.17-78.75) and is rich in CaO (mean composition: 0.45 wt%) in comparison to lherzolites and harzburgites. Wehrlite NK08 and lherzolite transitional porphyroclastic to equigranular NK14 have low NiO content compare to harzburgites and porphyroclastic lherzolites.
Orthopyroxenes plot in the field of enstatite (En89.97-86.63) with narrow variation of Mg# (=100 × Mg/Mg + Fetotal, between 88.26 - 90.88). Orthopyroxene of Nyos peridotites are similar to those described by Caldeira and Munha [22] in São Tomé islands and by Tamen [23] in Mandara massifs, respectively in the oceanic and continental sectors of the CVL.
Diopsidic clinopyroxenes of Nyos (En48.44-41.02 - Fs11.50-4.83 - Wo47.48-46.16) are rich in Cr2O3 (until ~1.3%) and Al2O3: 3.22% à 8.10%. Mg# Cpx increase from wehrlite
(Mg#: 78.10) to lherzolites and harzurgites (Mg#: 87.50 - 90.86). Al2O3 content of Cpx is high in wehrlite (Al2O3: 8.10%) compare to lherzolites and harzburgites (3.22% - 5.71%) and therefore express a disequilibrium in the initial gathering or paragenesis of minerals. Cpx of Nyos xenoliths have low Ti contents (~0.01) except wehrlite NK08 (Ti = 0.05). Cr contents increase from wehrlite (0.01) to harzburgites (0.04). Cpx of Nyos peridotites are similar to those of Lake Nji peridotites in the volcanic area of Mount Oku [24] which belong to the continental sector of CVL.
Spinels Mg# (=100 × Mg/Mg + Fe2+) and Cr# (=100 × Cr/Cr + Al) are in the range 55 - 78 and 12 - 39 respectively. Al2O3 (31.79% - 55.48%) and Cr2O3 (11.20% - 29.85%) contents vary widely. TiO2 contents are very low (<0.4%) except for lherzolite NK14 with mean composition is 0.53%. Porphyroclastic lherzolites display a fertile character (Cr# < 13) while lherzolite NK14 and harzburgites display refractory character (Cr# > 31). Consequently, Porphyroclastic lherzolites contain Al spinel interpreted as residues of relative low degree of partial melting of the mantle component [25] while harzburgites and lherzolite NK14 defines Cr spinel, witnesses of an important degree of partial melting compare to Al spinel. Low values of Cr# and medium to high values of Mg# excluded a magmatic origin of these minerals. Thus, most peridotites of Nyos seem to have experienced a low degree of partial melting, attested by the low proportion of harburgites and the absence of dunites. Spinels of Nyos volcanic unit are similar to those of the Loihi Seamount [26] and comparable to those of Adamawa mantle xenoliths [27].
Sample NK02 contain calcic amphibole (Ca/Na > 1; Ca + Na > 1.34 and Na < 0.67; AlIV > Fe3+) and therefore correspond to pargasite [28]. Nyos pargasitic amphibole have slightly high MgO (15.08% - 17.77%), TiO2 (1.12% - 1.62%), Na2O (2.97% - 3.17%), Cr2O3 (2.03% - 2.21%) and low K2O (1.06% - 1.26%) contents. Al2O3 (13.43% - 13.66%) and Mg# (98) contents are less variable. Nyos pargasites are similar to those of equigranular lherzolites of Orania [29] and different of brown metasomatised amphibole of the Caussou in French Pyrénées [30].
Phlogopites occur as sub-euhedral grains along grain boundaries in lherzolite xenoliths or along orthopyroxene lamellae exsolved from intercumulus clinopyroxene in the wehrlite xenoliths. They do not show any reaction relationships with other phases in any of the xenoliths studied indicating that the phlogopites have equilibrated with these coexisting phases. Phlogopites (Fe/Fe + Mg: 0.10 to 0.11) in sample NK04 have high TiO2 (~3%) and low Cr2O3 (<1%) contents. They have Mg/Mg + Fe2+ similar to that of coexisting olivine, clinopyroxene, and orthopyroxene and are similar to phlogopites of type I wehrlite of Oranie describe by Zerka et al. [31].
4.2. Trace Element Distribution
LA-ICP-MS analyses of mineral phases are given in Table 3(a) and Table 3(b). In general, olivines, orthopyroxenes and spinels have REE contents below the detection limit except for transitional elements whereas clinopyroxenes and amphiboles are rich in trace elements and micas remain poor in REE.
4.2.1. Olivines
Transition element concentrations are homogeneous or less variable (Sc: 4.5 - 6 ppm; Ti: 42 - 49 ppm; Ni: 2740 - 3090 ppm). Olivine wehrlite is slightly rich in Ti (125 ppm) but poor in Ni (830 ppm). Its vanadium content (17 ppm) is higher compare to olivine bearing lherzolites and harzburgites which are below the detection limit. Incompatible trace elements have generally low concentrations or stand below the detection limit. Wehrlite NK08 and lherzolite NK14 are slightly rich in REE than other peridotites.
4.2.2. Orthopyroxenes
Sc (19 - 23 ppm) and V (105 - 125 ppm) contents of orthopyroxenes are closer to Cl chondrites while their Ni (709 - 759 ppm) content are higher. They also display a low composition of Sr (0.3 - 0.5 ppm), Zr (0.3 - 0.7 ppm), Ti (435 - 590 ppm) and Y (0.8 - 1.2 ppm). HREE contents are generally less variable ([Yb]N: 0.5 - 0.9).
4.2.3. Clinopyroxenes
Transitional element concentration of clinopyroxene (Sc: 55 - 74 ppm; V: 135 - 255 ppm; Ni: 228 - 276 ppm) are relatively less variable in lherzolites and harzburgites while wehrlite clinopyroxenes have high V (295 ppm), low Ni (102 ppm) and equivalent Sc (70 ppm) contents. Ti contents yield broad variation ranges, varying from 460 ppm in harzburgite bearing phlogopite NK02 to 10465 ppm in wehrlite NK08.
Clinopyroxenes from selected Nyos xenoliths, normalised to the estimated primitive mantle values [32] display three distinct REE patterns (Figure 4(a)): 1) “spoon” shape patterns or group 1 samples characterized by HREE enriched
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Table 3. (a) Representative clinopyroxene trace-element analyses (LA-ICP-MS) of Nyos peridotites (values in ppm); (b) Representative amphibole and phlogopite trace-element analyses of Nyos peridotites (values in ppm).
compare to MREE and LREE; [La/Sm]N: 0.08 - 1.48; [Sm/Yb]N: 0.47 - 0.57 and [La/Yb]N: 0.03 - 0.69. Corresponding rocks are porphyroclastic lherzolites and harzburgites NK10. 2) “convex” shape patterns enriched in LREE and MREE compare to HREEs; [La/Sm]N: 0.72 - 1.36; [Sm/Yb]N: 2.85 - 6.23 et [La/Yb]N: 3.27 - 4.52. Corresponding peridotites are lherzolite transitional to porphyroclastic NK14, wehrlite NK08 and phlogopite bearing harzburgites, sample NK04. 3) convex shape patterns enriched in LREE and display flat MREE and HREE patterns, corresponding rock is harzburgite bearing amphibole NK02 ([La/Sm]N: 5.50; Sm/Yb]N: 1.17 and [La/Yb]N: 6.45).
Globaly, clinopyroxenes trace element patterns of Nyos can be divided in two groups, those enriched in LREE (type ii and iii) and those depleted in LREE (type i).
Nearly, all Nyos clinopyroxenes have deep negative anomalies of Ti and Nb and positive anomalies in U and Th (Figure 4(b)).
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Figure 4. Clinopyroxene REE (a) and multi-element abundance (b) patterns normalised to the estimated primitive mantle values of Nyos peridotites. Normalization values after [32].
4.2.4. Amphiboles and Phlogopites
Transitional element contents in pargasitic amphibole are medium (Sc: 44 ppm; V: 208 ppm; Ni: 296 ppm) while Ti (2132.40 ppm) content is high like other incompatible elements. Analysed amphiboles point out REE enriches compare to the estimated primitive mantle values after McDonough and Sun [32]. They display enriched LREE compare to MREE and HREE; [La/Sm]N: 5.08; [Sm/Yb]N: 1.16 and [La/Yb]N: 5.89. Nyos amphiboles are similar to those of Orania in Algeria [29].
In general, phlogopites have high contents of Ti (17,413 ppm), Ni (1334 ppm), V (213 ppm), Ba (2974 ppm) and Rb (361 ppm) while their Pr, Dy, Ho and Lu contents are below the detection limits. Phlogopites remain poor in REE (less than 1% of the estimated primitive Cl chondrites).
5. Bulk-Rock Geochemistry
5.1. Major Element Distribution
Analysed xenoliths by XRF technics and ICP-MS solution have generally homogeneous compositions (Table 4). Their Mg# = (100 × Mg/Mg + Fetotal) vary from
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Table 4. Bulk Rock major-element data (X-Ray fluorescence) and trace-element data (ICP-MS solution) of Nyos peridotites.
86.20 to 88.53. Harzburgites NK04 and NK10 with Mg# < 88, respectively 86.20 and 87.87, are classified as ferric mantle peridotites while lherzolites with Mg# > 88 are considered as normal mantle peridotites [33].
K2O and P2O5 contents are closed to the detection limit while Na2O contents are null. K2O contents are also null except for harzburgite bearing phlopopite NK04 (K2O: 0.03) also characterized by low Al2O3 (<2%), CaO (<1%) and TiO2 (<0.2%) contents which reflect lacking in Cpx and Sp.
Harzburgites have Al2O3/CaO ratio inferior to 1.1; estimated value for fertile mantle by Jagoutz et al. [34] or 1.2 proposed by McDonough and Frey [35]. These particularities are indication of their depleted or refractory characters. Nyos harzburgites are similar to harzburgitic xenoliths of Kerguelen [15] and Canaries islands [36].
Lherzolites show a relative homogeineity of their Mg# (88.02 - 88.62) and Al2O3/CaO (1.0 - 1.3) ratios and display a composition similar to the estimated fertile mantle. They have Cr# Sp < 13 and high Al2O3 and CaO contents. Nyos lherzolites are similar to that of the Barombi Mbo in the Kumba plain [10].
5.2. Trace Element Distribution
Analysed lherzolites and harzburgites are depleted in REE compare to the estimated primitive mantle values [32] except harzburgite NK04 which concentrations vary up to 5% of chondrites. The 2 harzburgites analysed are enriched in LREE compare to lherzolites. In general, 2 types of REE patterns (Figure 5) are identified:
1) Lherzolites and harzburgites NK10 have “spoon shape” REE patterns (Figure 5(a)) and display LREE depletion compare to MREE and less depleted HREE; ([La/Sm]N: 1.37 - 2.53; [Sm/Yb]N: 0.33 - 0.47 et [La/Yb]N: 0.46 - 1.28). A slight La-Ce “inflexions” is observed. Their multi-element REE patterns (Figure 5(b)) are similar and characterize by positive anomalies in Nb, Ti and Sr and negative anomalies in U, La and Nd.
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Figure 5. Bulk rock REE (a) and multi-element (b) patterns of Nyos péridotites.
2) Phlogopite REE patterns of harzburgite NK04 are characterised by a “convex” shape enriched in LREE compare to MREE and HREE; [La/Sm]N: 0.43 - 1.34; [Sm/Yb]N: 0.33 - 0.55 et [La/Yb]N: 0.46 - 1.28). Multi-element REE patterns of harzburgite NK04 point out a deep negative anomalies of Sr and Rb and positive anomalies in Nb and Ba.
6. Discussion: Evidence of Lake Nyos Mantle Percolated by Alkaline Magmas and Characterization of Induced Enrichment Phenomena
6.1. Magmatic Trace Infiltrations in Nyos Upper Mantle
Major element compositions of minerals are consistent with normal equilibration of typical minerals formed in the upper mantle except those of wehrlite. Indeed, olivine of wehrlite is slightly poor in Fe (Fo79) compare to those (Fo90) of depleted residual lherzolites [34] or olivine primitive mantle [37]. The low Fo values of olivines can be impute to the re-equilibration between olivine and a liquid rich in Fe [38] or to mantle metasomatism generated by basaltic magma infiltrations (“Fe-Ti metasomatism” [39] [40]. These low values also suggest that after their formation, wehrlite were percolated by, and re-equilibrated within basaltic magmas outled by low degrees of partial melting. Similar processes of re-equibration of wehrlite with magma of moderate Mg# have been proposed by Berger and Vannier [41] in order to explain the presence of dunites enclosed in oceanic island alkaline basalts. The same processes were proposed by Bodinier et al. [42] for Lanzo dunites.
Altogether, Nyos wehrlite represents mantle residues forms by “magmatic impregnation” [38], i.e. solid residus were percolated by a basaltic magma, enabling Cpx to crystallize in interstices between olivine crystals. Traped magmas, out of percolator filters, contribute to the formation of wehrlite. The percolating basaltic liquids were probably formed in deep seated zone by partial melting of a lherzolitic peridotite. The percolation of peridotites by basaltic magmas may be responsible of olivine CaO abnormal enrichments in wehrlite NK08 (mean composition CaO = 0.45), conferring them a magmatic origin whereas they are primary of mantle origin. Evidence of liquid percolation and associated dissolution/recrystallisation processes are attested by the presence of glass veins in wehrlite NK08 [43], coupled with the diminution of Mg# ratio of minerals (consequently the magnesium content of the magma plummets is poor) and the LREE enrichment of Cpx (≥10% chondrites, (Figure 6) which is generally related to the “chromatographic effects” of melt percolation, in mixture with magmatic liquid of alkaline affinity rich in LREE [44] [45] [46]. The major element features of olivine and clinopyroxene of the wehrlite appear to be significantly different than those of their equivalents in the other peridotites. They are indeed less magnesian (Fo: 78.75 - 79.17) vs Fo (86.44 - 87.95); Mg#Cpx: (77.08 - 78.93 vs 86.93 - 90.49), the olivine is higher in CaO (0.16 - 0.33 vs 0.05 - 0.12) and the clinopyroxene displays much higher Al2O3 (7.63 - 8.48 vs 3.15 - 4.7) and TiO2 (1.6 - 1.9 vs 0.17 - 0.76) contents and a lower Cr2O3 content (0.09 - 0.19 vs 0.61 - 1.16).
High NiO contents of harzburgites in some cases have been attributed to the re-equilibration between olivine and a liquid rich in Fe [38] [47].
The genesis of mantle amphibole is generally considered as a result of interactions between magma or a fluid and it surroundings. Interactions may be as a result of mantle metasomatism initiated by the percolation between a magma or a fluid rich in H2O, Ti, Fe, Na, K and others incompatible elements through peridotites [15] [38] [48] [49] [50]. A similar process may justify the presence of pargasites in harzburgite NK02.
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Figure 6. Clinopyroxenes REE patterns of Nyos wehrlite NK08.
The occurrence of phlogopites in fluid inclusions containing Na, K, Cl, P, and S, coupled with the fact that phlogopite and amphibole have similar trace element patterns, (their enriched in LREE and MREE compared to HREE which are nearly constant) suggest that incompatible element-enriched hydrous fluids/melts fluxed this part of the upper mantle beneath the CVL.
6.2. Metasomatic Events
The systematic investigations concerning the mineralogical and geochemical fingerprints of mantle metasomatism beneath Lake Nyos have been thorouhgfully debated (see Teitchou et al. [11] and associated figure captions). We therefore resume the main results in relation with the magmatic infiltration signature.
Trace element variations in mineral phases of Nyos peridotites seem more important in clinopyroxenes with two main types of REE patterns indicating different degrees of partial melting and enrichment processes by fluids (metasomatism). Indeed, Nyos clinopyroxene trace element compositions of type 1) are consistent with partial melting residues while trace element compositions of type 2) are not expressive with a simple partial melting process characterize by a systematic depletion in LREE and MREE compare to HREE [15] [35] [46] [51] [52]. Enrichments in LREE are geochemical fingerprints of metasomatism generate by the circulation and reaction with magmas (or fluids) in more or less residual mantle peridotites [15] [38] [46] [47]. This type of pattern usually appoints metasomatic enrichments later on partial melting [15] [40]. Metasomatism can be modal (by hydrous melts) and generate new hydrous mineral phases like phlogopites and pargasites [15] [38] [53] [54] or cryptic (anhydrous) characterize simply by chemical modifications (e.g, enrichments in LREE). This suggests that pargasites of sample NK02 and phlogopites of sample NK04 are witnesses of modal metasomatism. The metasomatic liquid which percolated the lithospheric column was a densely silicate rich in volatile and able to transport a high quantity of Ti, confirm by positive anomalies of Ti in harzburgites. The presence of amphibole is therefore reliable to the magmatic episode which brought out these xenoliths.
The following results were evidenced [4]:
Clinopyroxenes are the main hosts of trace elements during partial melting of Nyos peridotites.
The HFSE depletion trends appear to reflect low HFSE abundances in the metasomatic hydrous melts compared to the LILE, consequently, the flux of HFSE through the mantle wedge was low, corroborate by high LREE/HFSE and LILE/HFSE ratios.
Anhydrous metasomatised peridotites experience enrichments only in HFSE melts while hydrous metasomatised peridotites experienced both enrichment in HFSE and alkali melts.
Clinopyroxenes, amphiboles and phlogopites are good reservoirs of HFSE of the studied peridotites while REE budget is shared by amphiboles and clinopyroxenes, confirm by their parallel REE patterns enriched in LREE compared to constant MREE and HREE, and also by their strong positive anomalies in Sc, Th and La and negative anomalies in Ti and Ta.
Major element distribution in minerals reveals also the fundamental action of partial melting and metasomatic events due to percolation through peridotites of a liquid rich in H2O, Ti, Fe, Na and K (Fe-Ti metasomatism), mineralogically recorded by pargasitic amphiboles. Partial melting and metasomatism events are also documented by bulk rock REE patterns and overall trace element in minerals, particularly clinopyroxenes which can be consider as major hosts of trace elements in Nyos peridotites. The presence of pargasite in Nyos peridotites indicates an important metasomatic event beneath the Nyos volcano. Its Ti contents, relatively high (Ti > 0.4), implies that the metasomatic agent was a silicate liquid rich in volatile than low density, H2O-CO2 rich fluid, unable to transport an important quantity of Ti [55]. The significant variations of K, Cr and Ti contents combine with the high Mg# ratio of pargasite are the direct consequences of the crystallization of this mineral in interstices between the mineralogical assemblage made up of olivine + clinopyroxene + spinelle. Pargasites K, Cr, Ti, Mg and Fe contents thus depend both on the composition of metasomatic liquid and previous silicate phases in the upper mantle. Therefore, the presence of amphibole, even in small amount in Nyos peridotites (modal compositions < 2%) may be reliable to alkali magmatism which brought out these peridotites. REE data indicates that peridotites without amphibole are also enriched in LREE. In addition, their (Ce/Sm)N ratio range between 1.5 - 2.5 and (Ce/Yb)N ratio range between 1 - 2 are similar to those resulting from the percolation by basaltic magmas, during or after the partial melting [56].
Harzburgites and some lherzolites display low Fe content (<0.10) but high in samples NK04 and NK14 (0.12 and 0.20 respectively). The enrichment in incompatible elements of sample NK04 is due to the presence of phlogopite which reflects modal metasomatism while sample NK14 is a “Fe-Ti” metasomatic fingerprint, recorded by primary clinopyroxenes. Those characteristics are in agreement with the fact that sample NK14 (lherzolites transitional porphyroclastic to equigranular) and the phlogopite-bearing harzburgite NK04 are metasomatised mantle peridotites whereas the wehrlite NK08 is a magmatic cumulate.
6.3. Nature of the Percoling/Metasomatizing Liquid
Petrogeochemical data on CVL Lavas indicate three mains series: 1) series of tholeitic affinity [7] [8] [57], 2) series of transitional affinity [58] [59] [60] [61] and 3) series of alkaline affinity [4] [62] [63]. The transitional series, seem older (51 - 40 Ma; [61] than the alkaline series (40 Ma to present; Nana [6]). The succession of magmas evolving in time from tholeitic, transitional to more typical alkaline compositions is still debated, allowing us to open the debate on the magmatic affinity of the metasomatizing agent that percolate the Nyos mantle column.
Data obtained from earlier work evidence the alkaline affinity of liquids reacted with peridotites of Nyos upper mantle. These findings are consistent with: 1) high contents of Al and Ti of the clinopyroxenes of wehrlite NK08; 2) crystallysation of amphibole in harzburgite NK02; 3) formation of wehrlite rich in CaO; 4) LREE enrichments (in some sample) patterns typical of upper mantle infiltrated by alkaline magmas and 5) enrichment or depletion transitional trace element in most minerals phases, particulary Ti (olivine, amphibole, phlogopite) and V (clinopyroxene) enrichments and 6) depletion in Ni both in wehrlite and lherzolite-harzburgite series. The metasomatic melts that has affected the samples NK04 and NK14 is also probably the parental melt of the wehrlite cumulate. These findings are in agreement with the fact that sample NK14 (lherzolites transitional porphyroclastic to equigranular) and the phlogopite-bearing harzburgite NK04 are metasomatised mantle peridotites whereas the wehrlite NK08 is a magmatic cumulate.
In order to test the alkaline affinity of the magma or fluid react with Nyos peridotites showing clear evidence of metasomatism (Group 2 samples): harzburgite bearing phlogopite NK04; lherzolite transitional porphyroclastic to equigranular NK14; wehrlite NK08 and precisely to test if the metasomatising magma/fluid displays similar characteristics with the alkaline lavas of the Kumba plain whose volcanism is comparable to Nyos volcanism [4]. REE contents of the theorical liquid in equilibrium with clinopyroxenes of metasomatised xenoliths were calculated. We used partition coefficients Clinopyroxene/mafic silicate liquid compilated by Chazot et al. [64]. Calculation were made exclusively on REE because they concentrate preferentially on clinopyroxene of anhydrous peridotites and their REE contents can be considered as reflecting the composition of the metasomatizing agent (magma/fluid). Others incompatible trace elements like LILE (Rb, Ba, Th, U, Pb) or HFSE (Nb, Ta, Ti , Zr, Hf) may preferentially concentrate in other metasomatic phases like amphibole, phlogopite or microphases (e.g. oxides) which may triggered sub-solidus equilibration processes between these metasomatic phases, erasing the primary metasomatic history previously affected these peridotites. Calculations clearly indicate similarities between REE contents of two theorical liquids in equilibrium of metasomatised clinopyroxenes of Nyos peridotites type (ii) and the one in equilibrium with clinopyroxene of the alkaline lavas of the Kumba plain (Figure 7) because no in situ analyses were available for Nyos lavas. Altogether, the model evidence firmly establishes a link between the alkaline affinity of the metasomatizing agent of Nyos peridotites type (ii) and the composition of magma/liquid nearly the magmas at the origin of the volcanism of the studied area.
The degrees of partial melting previously affected type (i) peridotites were estimated at ~1% - ~5% [11]. We suggest that Nyos peridotite types (i) have experienced two types of events: 1) partial melting processes and 2) mantle metasomatism related to the percolating of depleted mantle by an alkaline silicate liquid probably rich in CO2, regardless of his high anomalies in HFSE (Nb and Ti). High LREE/HFSE ratio in basalts and peridotites suggest that melt percolation in
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Figure 7. REE diagrams normalised [32] to primitive mantle (PM) values of theorical liquids in equilibrium with Nyos peridotites type (ii) [harzburgite bearing phlogopite NK04; lherzolite transitional porphyroclastic to equigranular NK14; wehrlite NK08], compared to those in equilibrium with clinopyroxene megacrysts of the Kumba lavas (samples K3, O2, NKB3, MC4 and T14 [4]).
xenoliths have chemical characteristic of Enriched Mantle (EM1) which is not corresponding to tectonic environments prone to tholeitic and transitional basalts characterize respectively by moderate rate of partial melting, little or no olivine in basalts and rich magnesium content of the magma plummets (not recorded in our samples).
The depth of reactive melt migration in which melt is penetrating into the protolith and reacting with it is still to be studied. Nevertheless, from the chemical evolution described above coupled with the fact that sample NK14 displays pyroxene-Cr spinel symplectites interpreted as old garnet [65]. It is suggested that in the deeper parts of the Nyos upper mantle, garnet may be a possible potassium-bearing mineral. At shallower levels of the Nyos upper mantle, (e.g. less than 40 km) amphibole may be a potassium-bearing mineral instead of or in addition to phlogopite, while in the transitional context should be probably the formation of olivine poor-host basalts (not yet described in Nyos area) fluxed by EM1 components and the absence of wehrlites respectively at deeper and shallower levels of the mantle beneath the Nyos volcano.
7. Conclusion
To sum up, sample NK 14 corresponding to lherzolite transitional porphyroclastic to equigranular, wehrlite NK08 and hydrous harzburgites (harzburgites bearing phlogopites or pargasites) display particular mineralogical characters compared to lherzolites and anhydrous harzburgites and therefore documented uplifting of the mantle and interactions between initial peridotitic upper mantle (lherzolites and harzburgites) and infiltrated basaltic magmas of alkaline affinity. These interaction processes triggered the formation of wehrlite. Thus, wehrlite NK08 corresponds to products of interactions between percolating basaltic magmas and encasing mantle peridotites and then represents percolated peridotitic vents in which Opx, a member of the primary lherzolitic or harzburgitic mineral association, were quasi consumed during interactions with percolated magmas [66]. The study of these peridotites reveals a complex evolution of Nyos upper mantle which involved at least two stages. The first one was due to partial melting which produce depleted lherzolites and more refractory harzburgites from a fertile lherzolitic mantle. Later on, hydrous phases, Ti-rich Cpx, CaO rich Ol, Ti and V rich Ol wehrlite precipited from alkali enrichments due to the percolation of the mantle by basaltic magmas. Metasomatism of the upper mantle beneath Nyos probably occurred prior to recent alkaline magmatism and after the initial volcanism which formed the wehrlite cumulates. Metasomatism caused overall oxidation of the upper mantle beneath this area but probably was not responsible for the anomalously Fe-rich nature of some xenoliths attributable to the infiltration of alkaline magmatic fluid/melts beneath the lake eruptive centre.
Acknowledgements
This work was supported by a French government visiting researcher grant and the Institute of Geological and Mining Research, IRGM Cameroon. C. Dantas, Kamani Toko, and Dr. Diouta G of ISTIC Bangangte are thanked for their collaboration.