Petrology of Spinel-Lherzolite Xenoliths from Mazélé and Others Northen Xenoliths Localities of Cameroon Volcanic Line: Exchange Reactions and Equilibrium State ()
1. Introduction
The “Cameroon Volcanic Line” (LVC) is an active tectono-magmatic alignment oriented N30˚E, from the island of Pagalù to Lake Chad, [1] Figure 1. This line is a major geological structure in Central Africa whose importance can be compared to the “Rift-Valley system” of East Africa [2] . This chain of intraplate volcanoes covers more than 1600 km long [3] and 100 km wide.
Several scientific researches have been the subject of study both on the geophysical and geological level in the continental part of the Volcanic Line of Cameroon. We recall that according to the authors, geophysical studies have made it possible to better understand the composition, the constitution of its subsoil and its internal structure. In addition, we note that the study of ultramafic xenoliths in general and spinel lherzolites in particular makes it possible to determine the state of stress, pressure and temperature which reign above the upper mantle of this part. The tectonic phenomena which reigned in this part will undoubtedly bring about a slight modification in the lithological composition. Gravity data characterized by a negative anomaly of maximum amplitude 120 mgals [4] - [9] have made it possible to characterize the Adamaoua horst in the Ngaoundéré region. The seismological study [10] reveals a propagation of relatively slow wave speeds (waves P = 7.8 km/s) at the level of Adamaoua indicating the presence of an abnormal upper mantle. It is generally accepted that the “uplift” zones come from the isostatic response linked to a thinning of the lithosphere, itself due to a crustal extension [11] . The crust is thin (about 23 km) and the P-wave velocity in the upper mantle is slow (about 7.8 km/s) compared to the southern part of the plateau where normal values are observed (about 33 km and 8 km/s respectively). The tectonic model according to which the uplift of the Adamaoua would be a thermal bulge resulting from the thinning of the lithosphere and the crust does not on its own explain the results obtained which highlight a significant asymmetry [12] . In recent years, mantle metasomatism has been invoked to account for the geochemical and isotopic inhomogeneities within the upper mantle documented in peridotite xenoliths entrained in kimberlite and alkali basalts. Although metasomatism in the xenoliths is partly attributed to the transporting magma, most metasomatism takes place before entrainment and is of interest, particularly if its effect can be correlated
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Figure 1. Location map of mantle xenoliths along the “Cameroon Volcanic Line” modified after [1] .
with coeval and/or later magmatism and tectonic events such as plateau uplift and rifting [13] . New ultramafic deposits that are the subject of this study are harvested in the northern part of the LVC of Cameroon. Most are affected by cryptic metasomatism in which the chemical composition of the rock is changed by the addition or removal of elements. The theme discussed in this article concerns the mechanisms that are transmitted between the different mineral phases. More particularly, the study focuses on the comparison between the xenoliths of the mantle, mentioned in several places of the Cameroon volcanic line and on the Adamaoua plateau. These are precisely the locality of Liri (south of the Kapsiki plateau) and four localities of Adamaoua (Ganguiré, Mazele, Likok, Tello). The data relating to the sampling sites are reported in the following Table 1.
The example serving as support for this work is the continental domain of the Cameroon Volcanic Line, where certain volcanic edifices have emitted lava which contains peridotite xenoliths.
Xenoliths are fragments of mantle rock brought to the surface by magma during volcanic eruption. The upper mantle of the Earth is made up of peridotites at least in its superficial part. Peridotite xenoliths brought to the surface by alkaline basalts or kimberlites thus provide us with direct information on the processes and composition of the upper mantle.
These xenoliths constitute the major source of information concerning the state of stress, pressure and temperature prevailing in the deep mantle. They are thus a source of petrological and geochemical information which is not generally available on the Earth’s surface
2. Geological Setting
2.1. Case of the Kapsiki Plateau
The Kapsiki Plateau (altitude ≈ 1000 m) is a volcanic area located at the far north of the “Cameroon Volcanic Line” [13] . The Precambrian basement of this zone is recovered by numerous trachyte needles and dykes, phonolite and rhyolite, and partially overlain by basalt flows. The age of the basalts is between 33.2 ± 1.3 Ma [14] and 27.0 ± 0.5 Ma [12] . Trachytes have been dated at 29.6 ± 0.6 Ma, [15] and 35.3 ± 2.4 Ma [15] . The age of the rhyolites is between 32 ± 0.5 Ma and 29.0 Ma [14] . The needles are more than fifty and rise above the Kapsiki plateau. Many of them are emplaced for the benefit of feeder dykes (Teki dyke). Needles from Roumsiki, Gouldé, Mogodé, Roumzu and Re Dilili, Omtémalé and Mchirgui are the most typical [16] .
The presence of xenoliths in the Liri sector (south of the Kapsiki Plateau) was initially reported by [17] . According to the work of [16] , it is proved that the structural and Geophysical (gravimetric and seismological) features of the granite-gneissic base complex are still poorly defined. The Liri sector is located along the Cameroon Line with a basement constituted of granite and gneiss. This basement is crossed by faults and locally covered by basaltic lava flows. The lava flows are broken up into the small centimetric blocks. The spinel-lherzolite xenoliths (size: 7 - 30 cm) with sub-rounded shapes were discovered in these dark basaltic lavas.
2.2. Case of the Adamaoua Plateau
Satellite documents [14] have made it possible to highlight alignments and lineamentary structures on the Adamaoua plateau. A main direction N70˚E emerges, called the “Adamaoua beam”. This direction is a megaline several hundred kilometers long, which reflects the surface conditions and more or less materializes the deformations of the lithosphere under the influence of the stresses. It’s formed during the Tertiary and then was uplifted up to 1 km relatively to the surrounding areas [18] . The plateau is limited by the N70˚E Adamaoua and Mbéré-Djerem faults [19] . The establishment of volcanism in this region is probably linked to these pan-African fractures [20] . Xenoliths crop out in basalt flows in the shape of balls or metric-sized blocks. A few rare clinopyroxenite xenolites are encountered at the summits of the massifs. It is also necessary to note the presence of dolerites which form a planeze in certain sectors (case of Likok).
3. Results
3.1. Petrography
3.1.1. Petrography of Host Basanite
Host-lavas exhibit a massive structure. Phenocryst phases consist of euhedral to subhedral large olivine crystals (1.5 - 3 mm, 15 to 20 volume %), euhedral clinopyroxene (1 - 2 mm, 15%), Fe-Ti oxides (0.5 - 1 mm, 5%), set in a groundmass (60%) containing the same minerals and needle plagioclase microlites. The weathered surfaces are greenish gray in color, irregular with numerous multicentimeter to multimillimeter grains in relief giving the rock an irregular appearance due to meteoric alteration. The silicate minerals (olivine and pyroxene) remained in relief.
3.1.2. Petrography of Mantle Xenoliths
Basanitic lavas have entrained mantle peridotite xenoliths during their ascent to the Earth’s surface. Ultramafic xenoliths sampled in these areas are spinel- and plagioclase-bearing [21] lherzolites. In our study only the following four minerals are observed.
Olivine
Olivine is the dominant mineral in these xenoliths. Some small crystals (≤0.5 mm or neoblasts) are sometimes nested between the orthopyroxene grains. They form by recrystallization at the periphery of granoclasts. Others of large size (>2 mm or porphyroclasts) contain small crystals of other minerals (cpx) (Figure 2) and having automorphic forms (curved limits). The olivine crystals present in places a fractured aspect along crack planes and locally show polygonal limits.
Orthopyroxene
Large orthopyroxene crystals are 3 to 6 mm in diameter and have curved outlines (Figure 2). Small polygonal crystals (0.5 - 1 mm) or undeformed neoblasts are present among the porphyroclasts of olivine crystals.
Clinopyroxene
Clinopyroxene crystals surround small olivine crystals with rounded edges. The clinopyroxene presents lamellae of exsolutions or inclusions of orthopyroxene. Two families are observed: one almost fibrous showing signs of fracturing and the other showing forms of annealing (Figure 2). Clinopyroxene (11-12) and orthopyroxene (15) crystals nest the olivine crystal (13). The large crystals of clinopyroxene present a distorted appearance and give the blade a destabilized structure. The mineralogical assemblage shows subautomorphic minerals. The clinopyroxene crystals appear here much more destabilized compared to the crystals of the other localities. Subautomorphic clinopyroxene and olivine crystals rarely form sharp joints.
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Figure 2. Microphotographs showing spongy clinopyroxenes and spinels scattered in the rocks and included in the silicate phases: the numbers for each mineral correspond to its analysis number (Photos 1 to 7 relate successively to the microphographies of Mazele1 (Maz1); Mazele 1 (Maz 2); Ganguire (GA); Liri, Tello 1 (TO1); Likok (LO) and Tello 2 (TO2)).
Spinel
Spinel grains arranged in a chain mold automorphic to subautomorphic orthopyroxene and clinopyroxene minerals. These grains are at times rounded or elongated (Figure 2) and give the blade a granoblastic texture. The contact between the spinel and the magmatic liquid leaves traces of oxidation, black spots. On the other hand, no sign of reaction between the different minerals in the xenoliths is observed. Overall, the bottom of the blade shows inclusions or infiltrations of the basaltic liquid. The automorphic spinels, arranged in clusters with brown grain boundaries, have diameters that vary between 1 and 2 mm. They are arranged side by side with non-oriented olivine neoblasts with almost polygonal contours and half-millimeter dimensions.
3.2. Analytical Procedures
Mineral phases of the xenoliths and host basalt were analyzed using the CAMECA SX50 electronic microanalyzer at the Pierre and Marie Curie University in Paris (France) and in Canada. The samples analyzed are thin sections of rocks of different natures (basalts and xenoliths) and the analyzes are made on polished sections. Before each analysis, the samples are metallized with carbon. Radiation is then analyzed by spectrometers, each of which explores a specific range of wavelengths. Finally, the intensity of the radiation measured at the detector is compared with that produced under the same conditions by standards containing the desired elements at a known concentration. The final quantification of the elements (atoms, % by weight) is carried out by the PAP program of [22] which allows the correction of the analyses. The sensitivity threshold depends on the atomic number of the elements measured, the counting time (s), the voltage (Kv) and the current (nA) used. The analytical conditions are as follows: olivine: 15 kV, 40 nA, 20 s per element, except Si: 10 s; clinopyroxene: 15 kV, 40 nA, 20 s per element, except Ti: 30 s (Fe3+ recalculated according to [23] ; orthopyroxene: 15 kV, 40 nA, 20 s per element, except Ti: 30 s (Fe3+ recalculated according to [23] ; Fe-Ti oxides: 20 kV, 40 nA, 40 s per cell except Al and Cr: 30 s (Fe3+ recalculated from [24] ; plagioclase: 15 kV, 10 nA, 10 sec; spinel: 10 kV, 80 nA, 40 s per element except Al and Cr: 30 s, and Zn and Ni: 15 s.
In the same way, the mineral phases of the xenoliths were also produced under the same conditions on the Cameca SX 100 microprobe of the Microsonde Ouest service in Brest (France).
3.3. Mineralogy
1) The mineralogy of the xenolites of the Cameroon Volcanic Line and the others localities based on the following four mineral phases.
Olivine
Olivine crystals from lherzolites (Table 1) are not zoned. Olivine crystals display a wide range of Fo (88.58 - 92.15). They are as rich in MgO (Fo~89.5) as that of the peridotite nodules of Sao Tomé (Fo90-92) [25] ; Bioko [26] and of the continental sector of “the Cameroon Ligne” [27] [28] [29] . The chemical compositions corroborate with recent data from Ngao Voglar in Adamaoua [30] which show that olivine crystals are relatively homogeneous and varies in composition from Fo88.58to Fo92.15 with Mg# of 89.1 - 89.8 [Mg# = 100 × Mg/(Mg + Fe2+)] and their NiO content varies from 0.03% - 0.5% and a low MnO value (0.11% - 0.2%). The CaO contents vary between 0.00% and 0.14%.
Clinopyroxene
The lherzolites containing chromium diopside (Table 2, Figure 3), nomenclature according to [31] , low in TiO2 (0.50% - 0.75%) with medium content in Cr2O3 (0.63% - 0.77%) and the content en MgO ~ 49%. Diopside compositions vary between (Wo40.9-42.2En50.4-51.6Fs7.1-7.6).
Orthopyroxene
The lherzolites contain enstatite Wo2.6En86.2Fs11.2 (Table 3, Figure 3), nomenclature according to [31] low in TiO2 (<0.27%), Al2O3 (2.27% - 5.98%) and Cr2O3 (0.28% - 0.59%). Similarly, the chemical compositions of the orthopyroxenes from Mazélé are similar to the Ngao Voglar data with an enstatite rich in MgO (Mg# = 90.1; En89-91Wo1Fs8-10), low in TiO2 (0.02% - 0.12%) and Cr2O3 (0.28% - 0.43%), and a high content of Al2O3 (5.71% - 6.01%) and CaO (0.48% - 1.14%) (Table 3)
Spinel
The spinels (Table 5, Figure 3) of the different samples have contents which vary slightly in magnesium and chromium from one sample to another. The Cr2O3 contents vary between 9.0% and 9.6% and those of MgO are close to 20%. Their TiO2 content is ~0.34%. The edges of the spinels in the sample show in places an enrichment in FeOT and a decrease in Cr2O3. The TiO2 contents are low but these spinels are rich in Al2O3 (55.28 - 56.07).
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Table 2. Chemical analyzes of olivines (% by weight and a.p.f.u. based on 4 oxygens).
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Table 3. Chemical analyzes of orthopyroxenes (% by weight and a.p.f.u. based on 6 oxygens).
2) Mineralogy of Mazele spinel lherzolites
The mineralogical analyzes are almost identical to those observed in other localities which constitute the north and the Cameroon Volcanic Line.
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Figure 3. Composition of the pyroxenes of xenoliths according to [31] .
Olivine
The Fo contents (91%) are relatively high, characteristic of mantle olivine (88 < Fo < 92) (Table 2). Nevertheless, their CaO contents are also high (0.0% and 0.13%) and are rather typical of phases having crystallized under low pressure and symptomatic of a volcanic origin [32] . The temperatures calculated using the thermometer of [33] are relatively high (1250˚C ± 50˚C) but consistent for the distribution of Mg, Fe and Mn.
Pyroxenes
The clinopyroxenes of xenoliths are diopsides (Wo41.09-49.14En47.25-51.82Fs2.74.-7.82: [31] (Table 3), with Mg# ranging from 93.76 - 100. Their Cr2O3 contents (0.64% - 1.56%) qualify them as chromiferous diopsides (Cr a.f.u: >0.01: The fact that Mg#Cpx > Mg#Opx > FoOl, leaves to believe that these xenoliths are typical of residual peridotites [25] [26] ). The TiO2 and Al2O3 concentrations in the clinopyroxenes of these peridotites are high by 0.5 and, respectively with AlVI < 0.2 and AlIV < 0.2.
Orthopyroxenes (Table 4) are enstatites [31] with a composition varying from Wo2.5-2.6En86.3-86.5Fs10.9-11.2. The Mg# of orthopyroxenes correlates with the Fo of olivines and indicates that Mg#Opx > Mg#ol [27] [34] . The orthopyroxenes of the peridotites have a high Al2O3 content (≥5%) compared to the contents obtained in the peridotites of TOb, GA and Liri.
Spinel
spinel (Mg, Fe2+) (Cr, Al, Fe3+)O4 (Table 5) is a mineral sensitive to chemical variations and plays, in particular, an important geochemical role in the partitioning of chromium. In the spinels analyzed, the Cr# content varies between 9.04% and 10.07%; the TiO2 contents vary between 0.30% and 0.45%. Spinels occur in various habitus: in atoll, in automorphic grain, in complex association with other mineral phases.
3.4. Global Chemistry of Some Host Basalts
Note that the alkaline nature of these basalts has been confirmed by the study of certain trace elements and rare earths. The diagram in Figure 3 shows the spectra of trace elements and rare earths normalized to chondrites from [35] of the host lavas of Ganguiré, [30] , Mount Cameroon, [36] and the Kapsiki Plateau.
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Table 4. Chemical analyzes of clinopyroxenes (% by weight and a.p.f.u. based on 6 oxygens).
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Table 5. Chemical analyzes of spinels (% by weight and a.p.f.u. based on 32 oxygens).
It is observed that the trace element and rare earth spectra of these different lava units show similar trends. But we note the presence of a negative anomaly in Th and positive anomalies in U and Sr in the host lavas of Ganguiré. They all show a slight anomaly positive in Eu and negative in Dy. The spectra of these lavas are more enriched in light rare earths (La, Ce, Pr and Nd) compared to heavy rare earths (Dy, Yb and Lu) which is a characteristic of alkaline lava (Figure 4).
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Figure 4. Incompatible element patterns for the host basalts of Liri (BL), Ganguiré (Mazélé; MA), Mount Cameroon (C10J), Kapsiki plateau (2X), normalized to the mean of the chondrites of [35] .
4. Discussion
Exchange reactions and equilibrium state
Mineralogically, xenoliths contain four mineral phases: olivine (ol), orthopyroxene (opx), clinopyroxene (cpx) and an aluminous phase, spinel (sp). The elements are not randomly distributed among the various mineral phases. Indeed, they are governed by a set of exchange and transfer reactions governed by strict thermodynamic laws. Here, we will review a number of these reactions, which should later make it possible to characterize the chemical composition of the upper mantle of each region and test the state of equilibrium of the phases between them.
4.1. Exchange Reactions
Fe-Mg exchange between olivine and orthopyroxene
In the case of these peridotites, visual observation under a microscope reveals that olivine and orthopyroxene constitute more than 70% - 90% of the modal composition, and are chemically very close to the FeO-MgO-SiO2 (FMS) system. We can therefore characterize the chemical variation of the Fe and Mg systems with the olivine/orthopyroxene couple.
According to the work of [37] , the Fe-Mg exchange between olivine and orthopyroxene can be described by assuming a monosite ideal solution behavior for these two phases at high temperatures (>1000˚C), but such a model also remains valid for lower temperatures (800˚C - 900˚C) especially when studying the field of interest around Fo = 90, except for a few cases of the values of TOb and TO1 which slightly exceed (Fo = 92) and that of MAZ whose values are lower (Fo = 88). We can therefore use the following reaction to express this exchange
Mg2SiO4 + Fe2Si2O6 <==> Mg2Si2O6 + Fe2SiO4
Fo(ol) Fs(opx) En(opx) Fa(ol)
The partition coefficient is expressed as follows for the exchange of one atom for another:
KD = ((XFe)ol * (XMg)opx)/((XMg)ol * (XFe)opx
For application to the natural system, considering that the exchange between Mg and Fe is limited to the octahedral sites not occupied by Ca, Na, Cr, Al (the other elements can be neglected), the concentrations corresponding to the FMS system can be estimated as being equal to:
(XMg)ol = (Mg/Mg + Fe2+)ol (XFe)ol = 1 – (XMg)ol
(XMg)opx = (Mg/Mg + Fe2+)opx (XFe)opx = 1 – (XMg)opx
Knowing the ratios (Mg/Mg + Fe) of olivine and orthopyroxene of peridotites we can construct the Roozeboom diagram (Mg/Mg + Fe) olversus (Mg/Mg + Fe) opx and determine the partition coefficient at compare with the theoretical model. Figure 5 shows a correlation between olivine and orthopyroxene, indicating partition coefficients greater than 1 with the exception of peridotites from Ganguirét having a mean value ≤0.94). The forsterite content of olivine ranges from 88 - 92. This range of values is typical of mantle rocks.
Al-Cr exchange between pyroxene and spinel
Since Al Indeed, [38] experimentally found by measuring the interatomic distance that, in pyroxene, Al occupies both tetrahedral and octahedral sites, while Cr only occupies octahedral sites. The presence of Na in pyroxene also requires an equivalent quantity of trivalent ions (Al, Cr and Fe3+) to maintain the electroneutrality of the phase. The proportion of these three species is relatively constant in the presence of a given aluminous phase for the peridotites of the mantle [39] . In our case, for the spinel facies, the concentration of trivalent Fe in the pyroxenes is very low and no significant value can be calculated from the results of the microprobe analyses.
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Figure 5. Roozeboom type diagram for the exchange of Fe―Mg between olivine and orthopyroxene which are two phases of the reaction, the value 0.94 corresponds to the constant kd [35] .
Reaction in order of importance is:
(Mg, Fe)3Al(AlSi3O12) + 1/2(Mg, Fe)Cr2O4 <==> (Mg, Fe)3Cr(AlSi3O12) + 1/2(Mg, Fe)Al2O4
(opx) (sp) (opx) (sp)
The experiment shows that by measuring the interatomic distance in pyroxene, Al occupies both tetrahedral and octahedral sites, while Cr only occupies octahedral sites. The presence of Na in pyroxene also requires an equivalent quantity of trivalent ions (Al, Cr and Fe3+) to maintain the electroneutrality of the phase. The proportion of these three species is relatively constant in the presence of a aluminous phase for the peridotites of [40] . In our case, for the spinel facies, the concentration of trivalent Fe in the pyroxenes is very low and no significant value can be calculated from the results of the microprobe analyses.
In summary, to express the Al-Cr exchange between orthopyroxene and spinel, we should calculate the amount of Al in the octahedral site not associated with Na, i.e. VIAl* = (Al − Cr − Na)/2. Tetrahedral aluminum is equal to IVAl = (Al + Cr − Na)/2. A Roozeboom diagram (Cr/Cr + Al)Sp – (Cr/AlIV) Opx is used to describe this exchange appropriately. The trend in Figure 4 implies spinel non-ideality at the Y site for a Cr-Al substitution. This observation is in agreement with the experimental and theoretical data which show that the simple model of regular solution is well suited to describe this Al-Cr partition between spinel and orthopyroxene.
X2 = X1/(X1 + K(1 – X1) exp(WG(2X1 – 1)/RT for fixed T with X1 = (Cr/Cr + Al)spX2 = (2Cr/Al + Cr − Na)opx; X1 and X2 being respective concentrations of Cr in spinel and orthopyroxene. WG(2X1 – 1)RT being the excess energy with a value of 11.3 kJ for the Margules parameter WG, from [41] . This expression allows us to calculate the theoretical Al-Cr partition between spinel and orthopyroxene which is represented in the inset in Figure 6 by asymmetrical curves contrasting with those of Figure 6. The majority of the points lie on a curve corresponding to an equilibrium constant practically equal to 2.5.
Al-Cr exchange between the two pyroxenes
The Al-Cr exchange between the two pyroxenes is well described by an ideal solution model [42] , using the couple (Cr/AlIV)cpx and (Cr/AlIV)opx, and an equilibrium constant written as follows:
KD = (XAl)opx(XCr)cpx/(XCr)opx(XAl)cpx
avec (XCr)opx = 2Cr/Al + Cr − Na
(XAl)opx = 1 – (2Cr/Cr + Al − Na)
Figure 7 indicates that the equilibrium constant is significantly different from unity and varies from 1.5 to 3.5 except for the samples from Ganguiré and Tello 2 whose distribution coefficients Kd are greater than 3.5. The Cr and Al contents of clinopyroxene are about twice as high as those of orthopyroxene. The correlation observed here confirms what we have seen in the previous diagrams: all the samples of peridotite show a good state of equilibrium, and are therefore completely characteristic of the normal mantle.
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Figure 6. Roozeboom diagram for Cr―Al exchange between spinel and orthopyroxene. The regular solution model was calculated at 900˚C (inset) with an excess energy of 11.3 KJ [40] ) for the Cr―Al interaction at Y sites of spinel.
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Figure 7. Roozeboom diagram for the exchange of Cr―Al between the two pyroxenes. Only Cr and Al in non-bonded octahedral site are taken into account in the exchange.
Diagram (Mg/Mg + Fe2+)sp versus (Cr/Cr+Al)sp
It is well known that spinel cannot be considered as a single-site ideal solution with respect to Fe―Mg substitution due to interactions between X and Y sites, called reciprocal effects. The interactions of the cations (Al, Cr) in the Y sites of spinel with the cations (Fe, Mg) in the X sites can be described by the following reaction:
FeAl2O4 + MgCr2O4 == FeCr2O4 + MgAl2O4
Since spinel is a Cr-rich mineral compared to other phases in which Cr is almost trace, the ratio (Cr/Cr + Al)sp reflects that of the system and controls (Mg/Mg + Fe2+)sp. An autocorrelation is observed in Figure 8. This figure which shows the reciprocal nature of the spinel solution, also illustrates a significant variation in (Cr/Cr + Al) of the system.
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Figure 8. Mg#-Cr# autocorrelation diagram for the spinel phase. The ophiolite reference is reported from [34] . (a) Domain of ultrabasic enclaves (Lherzolites and Harburgites) in basic lavas [42] ; (b) Domain of ultrabasic rocks in ophiolites [41] .
We can clearly see a well-defined domain for the xenoliths studied which is placed above that corresponding to the ophiolitic massifs (field of ophiolites according to [41] . This is the normal position of spinel peridotites typical of a lithospheric mantle sampled by alkaline magma quenched within hours. This characteristic leads us to believe that these peridotites are not fragments of an ultrabasic body (tectonic scale) in the continental crust, but that they have a deep origin in the mantle. This observation is completely analogous with that of other xenoliths in the alkaline basalts of Cameroon [27] . The difference between the position of xenoliths and ophiolites was explained by the kinetics of this exchange reaction [43] .
4.2. Transfer Reactions
Balance state
Exchange reactions are well suited for verifying the global equilibrium state of paragenesis, whereas transfer reactions are more suitable for estimating equilibrium conditions. There are two types of transfer reactions classically applied to pyroxenes for spinel facies peridotites: one concerns the mutual solubility of orthopyroxene and clinopyroxene, and the other concerns the solubility of aluminum in buffered orthopyroxene by the spinel:
En(opx) <=> En(cpx) et Di(opx) <=> Di(cpx)
and Py(opx) + Fo(ol) <==> En(opx) + Sp(sp)
The theoretical equilibrium curves were calculated under different pressures according to the different thermometers (Bertrand et al., 1987; Bertrand and Mercier, 1985; Benoît, 1987).
The combination of these reactions is also a particularly suitable tool for testing the state of equilibrium of the pyroxenes of ultrabasic parageneses [44] .
Equilibrium conditions
Previous studies have shown us that the xenoliths studied are completely in equilibrium. This therefore allows us to estimate the physical conditions to which they were subjected just before the eruptions, using the transfer reactions described by the thermodynamic laws.
Temperature
Several geothermometers have been calibrated from experimental data and/or data from the natural system [40] , but the choice among these thermometers is not particularly easy. The fundamental problem for thermometry is the extrapolation of P-T from the experimental region when determining the pressures and temperatures of fairly simple synthetic systems to the complex natural compositions of xenoliths. Here we will arbitrarily consider three thermometers (one of which is for an exchange reaction) to determine the equilibrium conditions and check the consistency of the results.
4.3. Fe―Mg Exchange between Olivine and Spinel
[45] [46] calibrated a thermometer for the Fe―Mg exchange reaction between olivine and spinel:
T (˚K) = (4250(YCr)sp + 1343)/(LnKD + 1.825(YCr)sp + 0.571)
où (YCr)sp= (Cr/Cr + Al)sp; KD is an equilibrium constant
KD = (XMg)ol * (XFe)sp/(XMg)sp * (XFe)ol
Given that the forsterite content of the olivine of the peridotites in the samples is between 88% and 92% and that the Fe3+ concentration of the spinel is on the whole very low, it seems reasonable to use this thermometer. The calculation gives a temperature of 845˚C - 1050˚C.
It is well known that as with all thermometers based on exchange reactions, this thermometer is easily disturbed. The temperatures calculated from this thermometer therefore reflect the equilibrium conditions of the last thermal state, that is to say when the peridotites were sampled by the magma which brought them up.
Pression
Pressure estimation remains problematic. Although some calibrations using the solubility of aluminum in orthopyroxene [39] or of calcium in orthopyroxene [40] have been proposed, there is no reliable and precise geobarometer for spinel facies mantle rocks.
As we did not observe garnet in the samples, it is plausible to consider a maximum pressure of 2.5 GPa and a minimum pressure of 0.9 GPa can also be estimated [46] due to the absence of plagioclase.
Other parameters are also sensitive to pressure, in particular the constant Na (Opx)/Na(Cpx) for the sodium exchange reaction between pyroxenes, and Ca in olivine buffered by the couple of reactions between the two pyroxenes. No calibration exists for Na but the Ca (Ol) geothermobarometer has been studied experimentally for CMS and CFMS systems for Fe = 0.16. The problem is the quality of analyzes of this trace element. As the silicon of spinel is a good thermometer, it is practical to study these two traces simultaneously with spinel, despite its empiricism. Calculation indicates that pressures from diopside are outside the allowable range (0.9 - 2.5 GPa) we mentioned, while pressures from enstatite are more reasonable (1.4 - 2 GPa). To calculate the pressure, we used the following [47] formula:
P(kbar) = 771.48 + 4.956.AlT – 28.756.Fe2+M1 – 5.345.Fe3+ + 56.904.AlM1 + 1.848.Ti + 14.825.Cr – 773.74.Ca – 736.57.Na – 754.81.MgM2 – 763.20.Fe2+M2 + – 759.66Mn – 1.185 (MgM2)2 – 1.876.(FeM2)2.
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Figure 9. Diagram of variation of SiO2 vs TiO2, SiO2 vs Al2O3, V(M1) vs Mg1, AlIV vs Mg1, AlIV vs Mg1 and Mg2 vs AlIV in the cpx of the different localities.
4.4. Behavior of Some Major and Trace Elements in Clinopyroxens
Here we observe a characteristic negative correlation of melt residue between the TiO2 and SiO2 content for the clinopyroxenes (Figure 8). All samples follow the evolution of the trend. The TiO2 contents decrease from the high cpx values of MAZ2 and MAZ1 ≥ 0.7, to the average values of LO, TO2 and Liri varying between 0.2 and 0.5. The lowest TiO2 values occupying the bottom of the slope are those of TiO2 present in the Cpx of Tob and To1 whose values are almost zero < 0.1. The same trend is observed with the evolution of the Al2O3 content as a function of SiO2 which shows a negative correlation leaving higher values (7 - 8) of MAZ1 and MAZ2. On the other hand, the TiO2 and Al2O3 contents are roughly anti-correlated with SiO2 (behavior of a fusion residue). Their anticorrelations reflect their low levels. The proportion of Cpx can vary significantly within the same series or between the xenoliths chosen. The M1 and M2 octahedral sites of the Cpx have a very marked difference in terms of morphology. The variation of V(M1) as a function of Mg1 makes a positive correlation (Figure 9c). This testifies to a state of equilibrium between the minerals and their appropriate sites [47] [48] . Similarly for the other chemical elements Mg1, Aliv which fill the same sites, we observe tendencies of a positive correlation contrary to the elements which do not share the same site (Aliv, Mg2). A high Mg1 content in the M1 site automatically leads to an increase in the Aliv content (Figure 9).
5. Conclusions
The study of the mineral phases of spinel lherzolites from Mazélé in particular and the Adamaou plateau in general present results which corroborate with other spinel lherzolites from the continental sector of the Cameroon Volcanic Line. Of the above, the homogeneous character of the texture, mineralogy and chemistry is manifested in the mantle xenoliths of lherzolite of the volcanic massif of Mazélé and others northen xenoliths, reflecting the stability of the lithospheric mantle in the northern continental domain of the Cameroon volcanic line. The mineralogical assembly of the four phases (olivine, clinopyrones, orthopyroxe and spinel) reflects the balance [49] [50] . The homogeneity of the physical conditions (P, T) as well as the homogeneity of the texture suggest that the peridotites in enclaves in the alkaline basalts of the regions located on the Adamaoua and Kapsiki plateau undoubtedly prove to be from an upper mantle level mostly located between 0.6 and 1.4 GPa (22 - 45 km) for a temperature of 1000˚C at the time of the eruption.
Such mechanisms in mantle rocks can be related to low degrees of partial melting of liquids from a fertile lherzolite to a melt residue [51] [52] . In some spinel-rich porphyroclastic lherzolites (CaO/Al2O3 < 1) rich in Al2O3 and CaO, the spectra of MORB-type rare earths have observed that they have undergone the effects of one or more partial melting processes of low degree and remained rather representative of a weakly depressed mantle. The protogranular textures of lherzolites can also be interpreted more as the result of equilibration during reactions with a metasomatic agent during crystallization as well as the light rare earth enriched spectra of these rocks [51] [53] . This type of spectra is in fact compatible with a chromatographic effect due to the percolation of small fractions of basaltic liquids through peridotites initially depleted in light rare earths [52] [54] [55] . Fine-grained equigranular textures are formed in the most superficial levels of the lithospheric mantle, thanks to shear likely to connect the upper mantle and the upper crust [56] . The absence of dunites and garnet demonstrates that these xenoliths are compatible with a context of thinned and diapiric lithosphere in the superficial levels. Lithospheric upwellings may have started as early as the Triassic in the region, as evidenced by the presence of spinel lherzolites [57] .