Spinel-Bearing Lherzolite Xenoliths from Hosséré Garba (Likok, Adamawa-Cameroon): Mineral Compositions and Geothermobarometric Implications

Spinel-bearing lherzolite xenoliths from the Hossere Garba (1272 m.a.s.l) volcano on the Adamawa Plateau, is located in Likok village, at about 35 km to WSW of Ngaoundere. These xenoliths (~11 cm size) have been sampled into the host basaltic lava flows from the NE flank of the Hossere Garba volcano. These xenoliths characterized by porphyroclastic texture consisted of olivine (~55 vol.%), orthopyroxene (~19 vol.%) and clinopyroxene (~21 vol.%) crystals. Spinel crystals (~5 vol.%) are red brown and interstitial between the crystals of olivine and pyroxenes. CaO contents are low (<0.08 wt%) in olivine and similar to those estimated (CaO: 0.05 - 0.1 wt%) for the mantle origin. The values of AlVI/AlIV ratio range between 1.1 and 1.3 for the Cr-diopside crystals from Hossere Garba xenoliths. The constant value of the volumes V(Cell) and V(M1) for clinopyroxene compositions, indicates the similar pressures. Hossere Garba represents a residual sequence issued from partial melting of a mantle source. Similar compositions have been recorded in minerals of ultramafic xenoliths from other ultramafic xenoliths domains of the Cameroon Line and the Adamawa Plateau.


Introduction
Ultramafic xenoliths generally are considered as small parts broken or separated off the lithospheric mantle [1]; or the residues of partial melting of the upper-mantle [2] hosted in basaltic lava during volcanic eruption. These ultramafic xenoliths ejected from volcanoes at the surface, as solid fragmentary material, are important to the knowledge of the textural features, chemical evolutions, temperature and pressure conditions of the upper mantle. Upper-mantle xenoliths were found in basaltic lavas from several localities along the Cameroon Volcanic Line [3]- [5]. However, beneath the volcanoes of the Adamawa Plateau, there are few available data aside from ultramafic xenoliths described from the Dibi volcano [6] [7], Youkou maar [8] and Ngao Voglar volcano [4]. Lherzolite xenoliths described in this paper are discovered in the Hosséré Garba (1272 m.a.s.l) volcano on the Adamawa Plateau, which consist of basaltic lava flows, more or less fragmented into rounded sparse blocks. Hosséré Garba is located in Likok village, at about 35 km to WSW of Ngaoundéré (Figure 1).
In this paper, we present the first petrography and mineral chemistry of spinel-bearing lherzolite xenoliths from Hosséré Garba (Likok) in the Adamawa Plateau. Chiefly, the objective is to enhance the knowledge of upper mantle beneath the Adamawa Plateau volcanic domain.
Spinel-bearing lherzolite xenoliths have been sampled into the basaltic lava flows from the NE flank of the Hosséré Garba volcano (Likok sector, Adamawa Plateau). These xenoliths (~11 cm size) characterized by porphyroclastic texture [21] with a cataclastic appearance are greenish yellow and slightly dark. Olivine (~55 vol.%), orthopyroxene (~19 vol.%) and clinopyroxene (~21 vol.%) crystals identified in these xenoliths are more or less equigranular (up to 0.4 -1 cm). Spinel crystals (~5 vol.%) are red brown and interstitial between N. Dagwai et al. 1437 the crystals of olivine and pyroxenes. The "kink-bands" structure characterizes some olivine crystals. Many triple-points involving olivine, pyroxene and spinel show evidence of a particular equilibrium condition between these minerals [22]. The tiny host basalt veins are present in rim sections of the lherzolite xenoliths from Hosséré Garba.

Analytical Methods
Chemical analyzes ("Université de Bretagne Occidentale-Brest, France") of major elements were performed using an electron microprobe with beams of: 10 and 40 μm, 15 kV and 15 nA. Errors considered for these analyzes are between 5% and 10% of measured values < 1% and between 1% and 5% of the measured values > 1%. Measurements of trace elements of the different phases have been produced using a technique laser ablation of a mass spectrometer coupled plasma source (LA-ICP-MS). The beam size varies between 40 and 70 μm. Counts where repeated several time for each spot analysis and averaged. The number of repeated analyses and the duration of the counting times depend on the concentration level of the elements studied. To examine homogeneity of constituent mineral, more than seventeen crystals each of clinopyroxène were selected at random from each thin section. Calcium is used as an internal standard, while the NIST 612 is used as an external standard. The accuracy is better than 10%. The whole rock analyzes were performed previously in solution using ICP-AES for major elements and ICP-MS for trace elements.

Olivine
The forsterite component of olivine range from 89 to 90 (Table 1) These values are similar to those calculated for the lherzolite xenoliths from Ngao Voglar. NiO contents reaching 0.41 wt% in the olivine from Hosséré Garba xenoliths. CaO contents are low (<0.08 wt%) and similar to those estimatited (CaO: 0.05 wt% -0.1 wt%) by [23] for the mantle origin, which characterize equilibrium environment of high pressure (>0.2 GPa; [24]).
This is due to the Al IV and R 3+ (Al VI , Fe 3+ , Cr and Ti 4+ ) relationship. The constant value of the volumes V(Cell) and V(M1) (see Table 2) for clinopyroxene compositions, indicates the similar pressures [26] in the spinelbearing lherzolite xenoliths from Hosséré Garba. V(Cell) and V(M1) relationships are more sensitive than the only chemical composition, to estimate the equilibration pressure of clinopyroxene [27].

Spinel
Spinel crystals display homogenous compositions, with Mg# and Cr# ranging from 81.1 to 81.9 and from 10.6 to 10.8 respectively (Figure 4).

Geothermobarometry
Using the orthopyroxene-clinopyroxene geothermometry of [28], equilibrium temperature of ~905˚C has been estimated for the Hosséré Garba spinel-bearing lherzolite xenoliths. Applying the geothermometer of [29], based on the solubility of Al 2 O 3 in orthopyroxene and spinel, the temperature expression calibrated at the pressure of 1.5 GPa in a natural spinel lherzolite system, gives the equilibrium temperatures of Hosséré Garba spinel-bearing lherzolite xenoliths of 1215˚C -1245˚C. The lack of plagioclase or gamet in Iherzolites and the fact that the lherzolite xenoliths have reached a high degree of internal chemical equilibrium, indicate that equilibration took place at pressures ranging between 0.7 and 1.1 GPa (25 -36 km). Hosséré Garba xenoliths have probably been sampled by the host-basalt in the upper mantle. At depths of 25 -36 km, the calculated temperatures are about 200˚C above those of the oceanic geotherm of [30].
These equilibrium temperatures of spinel-bearing lherzolite xenoliths from Hosséré Garba fall more or less in the similar range as equilibrium temperatures of other lherzolite xenoliths along the Cameroon Volcanic Line and the Adamawa Plateau [7]- [23].
A maximum pressure can be calculated from the composition of spinel [31] or established with the clinopyroxene crystal chemistry data to determine the variability of the pressure during the partial melting. Hosséré Garba mantle xenoliths as those from Ngao Voglar [4] are spinel peridotites, containing neither plagioclase nor garnet, and were derived from the spinel stability field in the mantle, which lies in a pressure range of ~0.8 to ~2 GPa [32]- [34]. These estimated pressures can be associated to the depths ranging between 30 and 60 km as reported for the other spinel lherzolite xenoliths [35]- [40].

Spinel-Bearing lherzolite Xenoliths Origin
The refractory elements as Cr are enriched in the residue, as demonstrated for the Cr-rich clinopyroxene from Hosséré Garbé xenoliths. Thus, the studied spinel-bearing lherzolite xenoliths, characterized by the presence of Cr-diopside, represent probably portions of the upper mantle [41].
However, the compositions of olivine and spinel are strongly dependent on the degree of melting, a feature characterized by a nearly constant values of (Mg#) Ol , (Mg#) Sp , and (Cr#) Sp . Based on these data, the xenoliths from Hosséré Garba represent a residual sequence issued from partial melting of a mantle source. Experiments have shown that the composition of solid solutions, such as olivine, is very sensitive to temperature changes, and olivine could therefore evolve towards refractory compositions during the partial melting. So, an extraction of the melt could leave a residue depleted in highly incompatible elements (e.g. Na, Al, K, and Ti) and enriched in Cr and Ni relative to the most primitive mantle [21]. The range of Ni contents of olivine of lherzolites from Hosséré Garba confirms that they are residues of melting. It appears that high contents of Ni (2500 -4100 ppm) in olivine suggest that xenoliths from Hosséré Garba could derive from a low degree of melting (~30%) of a relatively homogeneous mantle source. Thus, the model of [41] for portions of the earth's upper mantle appears appropriate for the origin of the Hosséré Garba spinel-bearing lherzolite xenoliths.

Conclusion
The spinel-bearing lherzolite xenoliths from Hosséré Garba represent a residual sequence issued from partial melting of a mantle source. The compositions of their silicate minerals are typical of equilibrated Mg-rich olivine, Al-rich enstatite and Cr-diopside in the upper mantle. Similar compositions have been recorded in minerals of ultramafic xenoliths from other ultramafic xenoliths domains of the Cameroon Volcanic Line and the Adamawa Plateau. Thus, ultramafic xenoliths from Hosséré Garba represent peridotites sampled from lithospheric mantle and hosted in the basaltic lava flows.