Mantle Xenoliths from Ibal-Oku (Oku massif, North-west Region, Cameroon): Imprints of Superimposed Carbonatitic and Silicic Metasomatisms

Mantle xenoliths have been discovered in Ibal-Oku basalts from Oku Massif, Cameroon Volcanic Line. These xenoliths analyzed in term of major elements by scanning electron microscope, atomic emission spectrometry, traces and rare earth elements by mass spectrometry are peridotites and pyroxenites. Peridotites comprise Fe-rich lherzolites, harzburgites and wehrlites. Pyroxenites comprise websterites, olivine-websterites, clinopyroxenites and olivine-clinopyroxenites. Mineralogically, olivine Fo% values and NiO content vary from 85 to 91 and 0.26 to 0.43 wt.%, respectively. Orthopyroxene is enstatite, Mg# values and Al content ranging from 0.83 to 0.92 and 0.12 to 0.27 atom per formula unit (apfu), respectively. Clinopyroxene is augite and diopside, Mg# values and Al content ranging from 0.83 to 0.93 and 0.23 to 0.37 apfu, respectively. Spinel is aluminous, Cr# and Mg# values ranging from 0.07 to 0.23 and 0.67 to 0.82, respectively. Micas are biotites (Fe#: 0.52-0.76). Feldspars, which are secondary are sanidine, andesine and labradorite. Geochemically, peridotite Mg# values vary from 82.7 to 89.9 and pyroxenites from 80.1 to 83.6. The major element variations and some compatible elements are described in terms of partial melting (14-15 vol.% in lherzolites and 17-18 vol.% in harzburgites), whereas the heterogeneities in trace elements are related to carbonatitic/silicic metasomatism. www.scholink.org/ojs/index.php/ees Energy and Earth Science Vol. 3, No. 2, 2020 81 Published by SCHOLINK INC.

xenomorphic to sub-automorphic. Similar to lherzolite olivine porphyroblasts, porphyroblastic olivine often exhibit kink-bands ( Figure 3G). Orthopyroxene crystals vary from 0.1 to 4 mm in size. A few of them are poikilitic and enclose small clinopyroxene grains. They are sub-automorphic. Clinopyroxene crystals vary from 0.01 to 1 mm in size. They are xenomorphic to sub-automorphic. A few of them are also poikilitic and enclose small orthopyroxene and spinel grains. Spinel crystals vary from 0.1 to 3 mm in size. They are xenomorphic to sub-automorphic. In contact with glass, they usually exhibit spongious borders ( Figure 3H).

Pyroxenites
Olivine-websterites and websterites exhibit cumulative texture. They differ mainly in their mineral  Figure 3K). Feldspar, olivine and oxide crystals are interstitial between pyroxene grains or present in the mineral pocket ( Figure 3L). They are porphyroclastic. However, few rocks exhibit cumulative texture. www.scholink.org/ojs/index.php/ees Energy and Earth Science Vol. 3, No. 2, 2020 Published by SCHOLINK INC.

Host Lavas
Olivine phenocrysts  ) are always more forsteritic than microcrysts

Major Elements
Major elements ranges as shown by Table 6 portray the broad ultramafic rocks suite described here.
MgO varies thus widely between 15.85 and 40.5 wt. %. The variation is however reasonable in each of the petrographic type: 29 to 40.5 wt. % in peridotites, 15.85 to 26.6 wt. % in pyroxenites. Although peridotites display the highest deviation, it should be noted that one sample show an uncommon (29 wt. %) content for peridotites. The mg# varies between 82.7 and 89.9 for peridotites and 80.1 to 83.6 for pyroxenites. These peridotite mg# are rather low when compared to values so far recorded along the CVL. They however reveal their iron-rich nature, similarly to Fe-rich lherzolites and wehrlites from Tok, SE Siberia (Ionov et al., 2005) or from Horní Bory, Bohemian Massif (Ackerman et al., 2009).
MgO correlates variably with the other major elements. Figure 5 shows that, Refractory Lithophiles

Figure 6. Variation Diagram of Selected Traces Elements vs MgO
Chondrite-normalized spidergram yields mirroring spectra for the two petrographic facies (Figure 7a), though sections of overlapping or detachment may exist (Figure 7b). Rb, K and P depict severe negative anomalies of a factor of 45, 50 and 300 respectively, meanwhile Ba exhibits a rough positive anomaly of a factor of 4.5. Chondrite-normalized diagram (Figure 7c and 7d) show an overall LREE enrichment relative to HREE. The (La/Lu)n ratios vary between 4.2 and 10 for peridotites and between 2.9 and 9.52 for pyroxenites. It is obvious to note that the pyroxenites LREE enrichment over HREE is more spread. On the other hand, all the Oku rocks are REE enriched with respect to chondrite but its order of magnitude is far beyond two units (35). The pyroxenites spectra have a more regular pattern than that of peridotites and moreover, they all have higher contents. Some of the peridotites display rough Ce and Tm positive anomalies of a factor of 1.7 and 2 respectively.

Discussions
The Ibal-Oku ultramafic rocks described here are included in dismantled basaltic flows. A few of these xenoliths are veined by the host basalt meanwhile the outer contact is sometimes reactive, leading to the development of augitic clinopyroxene. Contrarily to the other ultramafic xenolith reservoirs along the CVL, pyroxenites are more represented than peridotites and although their textures are similar to the main ones recorded elsewhere in Cameroon, their petrology and iron-rich signature are specific features that request thorough considerations.

Origin of Peridotites
Ultramafic xenolith sizes are very important. In fact, small size are problematic as far as their magmatic or mantellic origin is concerned. Magmatic xenoliths have either to do with a precocious crystallization from the host lava or with the phenomenon at the origin of the host magma itself meanwhile mantellic origin is fragment detached either from the source region or from the wall rocks along the magma's route to the surface. Because the latter are accidentally sampled, not all depth intervals are necessarily represented in a xenolith population; nevertheless, their pressures and temperatures estimates provide some spatial context amongst samples within a xenolith population, which may perhaps be divisible according to their textures and/or compositions (Pearson et al., 2014). One of the discriminative features of the two origins is their texture, cumulative in the first case, metamorphic in the broad sense since they result from the combination of thermo-barometric and differential stress conditions for the second. Magmatic ultramafic xenoliths have been described along CVL (Caldeira & Munha, 2002;Ngounouno et al., 2006;Wandji et al., 2009;Matsukage & Oya, 2010) and they all fit in this distinctive cumulative signature. The Oku samples however display metamorphic textures and low whole rocks and silicate mg#. In the latter view, Ionov et al. (2005)

Textural Evolution
The Ibal-Oku wehrlites are protogranular, with as lherzolites few olivine crystals displaying straight boundaries that sometimes form triple junctions. Lherzolites and harzburgites are either secondary protogranular or porphyroclastic. Kink-bands are often observed on some olivine crystals. The typology of mantle rocks textures (Mercier et Nicolas, 1975;Coisy et Nicolas, 1978) reveal that xenoliths displaying protogranular textures derive from tectonically inactive mantle areas meanwhile porphyroclastic textures are typical of mantle active zones. As a consequence of the high deformation, a rock can experience a complete process of textural evolution leading to a secondary protogranular texture, characterized by mechanically dispersed spinel which may be included in the rock silicate mineral or form string-grains and/or atolls. Such a secondary protogranular texture thus portrays active mantle areas. The Oku SCLM is likely active, a phenomenon which in the exception of the Kumba SCLM (Teitchou et al., 2007) is almost general along the CVL. Deformation textures are known to indicate the presence of shear zones in the mantle, likely resulting from (i) the replay of large horizontal shear zones during oceanic opening, (ii) asthenospheric diapirism within the lithosphere (Coisy et Nicolas, 1978;Witt et Seck, 1987) or (iii) the mechanic response to lithosphere/asthenosphere fluxes coupling (Kennedy et al., 2002;Tikoff et al., 2004). Along the CVL where the crust in thin (36 km after Tokam et al., 2010) however, the basal lithospheric erosion by the asthenosphere seems to explain the origin of the mantle shearing (Elsheikh et al., 2014). In addition to protogranular and porphyroclastic textures in Oku, poikilitic spongy microtextures are sometimes superimposed. Such microtextures originate either from local melting through metasomatism prior to the sampling by the host magma or from melt/xenolith interaction during host magma ascent (Ionov et al., 1994;Qi et al., 1995;Carpenter et al., 2002). Poikilitic and spongy Cpx are often associated to festooned border spinel and vugs, but unlike the Dibi (Dautria & Girod, 1986), Kapsiki (Tamen et al., 2015), Wum (Aziwo,

Chemical and Geochemical Evolution
Using the degree of partial melting (F) from the relation of Hellebrand et al. (2001;F=10×ln(Cr#) Ibal-Oku is strikingly veined by diversified pyroxenites. As we pointed out in a previous section, the origin of pyroxenites is highly debated. We have however, partially discarded the magmatic origin for our samples. The presence of numerous fluid inclusions in Opx and Cpx crystals together with the occurrence of calcite and apatite are strong arguments for mantle metasomatism. In fact, the presence of minerals such as biotite, apatite and calcite is characteristic of modal metasomatism (Pearson et al., 2003 and references there in). Along the CVL, phlogopite and pargasite on contrary to that last three mineral phases are usually discuted for modal metasomatism (Nana, 2001;Temdjim et al., 2004;Matsukage et Oya, 2010;Temdjim, 2012). The nature of the agents involved in the metasomatism is of chief importance and the contribution of mineral compositions (trace, REE and isotope) is paramount in the modelling of this phenomenon. In the mantle, melt and fluids circulating are of various signatures, ranging from silicic to carbonate-rich with associated CO 2 and brines (Stagno & Frost, 2010;Stagno et al., 2013;Frezzotti & Touret, 2014). These fluids/melts enrich the mantle and modify chemical compositions, Cpx will react with carbonate to produce calcite (Ackerman et al., 2012). In some cases, minerals of the same nature as those present in the primary rock may be added through  Figure 7) likely significant of cryptic metasomatism as it is the case for Nunivak (Brown et al., 1980;Pearson et al., 2003), Kumba (Teitchou et al., 2007), Ataq (Al-Malabeh, 2009), Nyos (Temdjim, 2012) and Ngao Bilta (Temdjim et al., in press lherzolite-wehrlite xenoliths from Tok (Ionov et al., 2005) and likely result from Fe-enrichment.
Various elements enrichments have been described in mantle rocks worldwide. Fe-improvement in particular can be achieved through either solid-state diffusion by Mg-Fe equilibration between mantle rocks and Fe-rich cumulus veins (Kempton, 1987;Abe et al. 2003) or exchange with percolating melts (Navon & Stolper, 1987;Kelemen et al., 1990Kelemen et al., , 1992Takazawa et al., 1992;Nielson & Wilshire, 1993;Ionov et al., 2005;Ackerman et al., 2009Ackerman et al., , 2013. Mg-Fe solid-state equilibration of the Oku peridotites with their veining pyroxenites as prospective origin of Fe-enrichment is uncertain for the host rocks have FeO contents slightly higher than that of the pyroxenites, unless they originate from different depths, the equilibrating rocks actually missing or yet to be sampled. Melt-rocks interaction potentially fits well with Fe-enrichment as witnessed by comparable amounts of FeO in both the peridotites and their host basaltic lavas (12.96-14.99 wt. %, Asaah et al., 2015(12.96-14.99 wt. %, Asaah et al., , 2019. In this case, post-entrainment modification as well as in situ long-term impregnation and equilibration can be evoked. Post-entrainment enrichments are often associated with textural modification such as spongy and sieve textures, reaction borders or compositional zoning on crystals (Shaw & Edgar, 1997;Carpenter et al., 2002;Wang et al., 2012), features almost absent in the Ibal-Oku samples. Thus, Fe-enhancement through melt percolation is more likely at the origin of high FeO contents. If so doing, it appears preoccupying that such a process is restricted to Ibal-Oku, giving the ubiquitous melt/mantle rock time and the physical contact intimacy beneath the CVL. All the xenoliths plot in silicate metasomatism field, which is also an indicator of interaction between xenoliths and silicate melt (Figure 12-13).

Conclusion
The Ibal-Oku SCLM is heterogenous and veined by pyroxenites. Major elements in general and Al 2 O 3 /CaO decoupling in particular are significant of mantle enrichment/depletion. Alike several sections of the mantle underlining the CVL, this sector is tectonically active and metasomatized. The metasomatism here, probably carbonatitic and silicate has acted cryptically, modally and likely stealthily. Hydrous (biotite) and anhydrous (apatite and calcite) metasomatic minerals are present, feature which together with the prevalence of pyroxenites on peridotites are specific of signatures of Oku SCLM that deserve further consideration.