# Wehrlite xenoliths and petrogenetic implications, Hosséré Do Guessa volcano, Adamawa plateau, Cameroon

Oumarou Faarouk Nkouandou, Jacques-Marie Bardintzeff, Zénab Nouraan Njankouo Ndassa, Aminatou Fagny Mefire, Adama Haman

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## To cite this version:

Oumarou Faarouk Nkouandou, Jacques-Marie Bardintzeff, Zénab Nouraan Njankouo Ndassa, Aminatou Fagny Mefire, Adama Haman. Wehrlite xenoliths and petrogenetic implications, Hosséré Do Guessa volcano, Adamawa plateau, Cameroon. Open Geosciences, 2022, 14, 10.1515/geo-2022-0408. insu-03846933

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Research Article

Oumarou Faarouk Nkouandou, Jacques-Marie Bardintzeff*,
Zénab Nouraan Njankouo Ndassa, Aminatou Fagny Mefire, and Adama Haman

# Wehrlite xenoliths and petrogenetic implications, Hosséré Do Guessa volcano, Adamawa plateau, Cameroon

https://doi.org/10.1515/geo-2022-0408

received January 27, 2022; accepted September 15, 2022

**Abstract:** Peridotite xenoliths of wehrlite composition, scarcely known in Adamawa plateau, Cameroon, were sampled by Mio-Pliocene basanites from Hosséré Do Guessa volcano. Their origins are discussed and elucidated trough petrography and mineral chemistry. Studied wehrlites exhibit poikilitic or protogranular textures and are composed of four main mantle phases (high Mg-olivine, augite, enstatite and Al-spinel). Petrographic and microprobe (Camebax SX100) chemical data (Fo90.8-91.4 olivine, Wo39.4-42.0 augite, En90.5-91.1 enstatite and Al-spinel) suggest a mantle origin for the Hosséré Do Guessa wehrlites. Hence, these rocks could not be considered cumulate. They have been equilibrated between 1,140 and 1,220°C, at pressures of 1.5–2.0 GPa, at 50–66 km deep, below the crust-mantle boundary. Wehrlites might result in reactions with carbonate/carbonatite melt, accompanying CO₂ degassing and metasomatism by fluid phases. They suffered transpressional tectonics, during movement at Tertiary times of Pan-African strike-slip-faults, after solid-state tectonic relaxation.

**Keywords:** wehrlite, metasomatism, Hosséré Do Guessa, Adamawa plateau, Cameroon

<figure><img src="image_4.png" /><figcaption>Graphical abstract</figcaption></figure>

# 1 Introduction

The Cameroon Volcanic Line (CVL) is a Y-shaped oceanic and continental tectono-magmatic megastructure. Numerous volcanoes and plutonic-volcanic ring complexes, dated from Cenozoic to Quaternary compose the N30°E-trending CVL s.s. and the N70°E-trending Adamawa plateau, intersecting at Tchabal Mbabo volcano [1]. The internal structure of the Adamawa sub-continental mantle is still debated according to different studies.

Seismic and gravity studies [2–12] evidence (1) a crustal uplift resulting of the upward migration of the lithosphere-asthenosphere boundary, (2) an abnormally hot upwelling upper mantle located at the depth of 70–90 km [13] and (3) two broad negative gravity anomalies (–80 to –100 and –120 mGal/cm), attributable to lithospheric and crustal (crust is locally only 20 km thick) thinning.

Petrographical and mineral chemistry studies of numerous peridotite xenoliths found in the basaltic lavas evidence that they should be considered as fragments of the sub-continental lithosphere [14]. Ultra-mafic mantle xenoliths were sampled by Mio-Pliocene basanite lavas and pyroclastite projections on their

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<footnote>* Corresponding author: Jacques-Marie Bardintzeff, Univ Paris-Saclay, Department of Sciences de la Terre, Volcanologie-Planétologie, UMR CNRS 8148 GEOPS, bât. 504, F-91405 Orsay, France, e-mail: jacques-marie.bardintzeff@universite-paris-saclay.fr</footnote>

<footnote>Oumarou Faarouk Nkouandou: Department of Earth Sciences, Faculty of Sciences, University of Ngaoundéré, P.O. Box. 454, Ngaoundéré, Cameroon; Department of Mineral Engineering, Chemical Engineering and Minerals Industry School, University of Ngaoundéré, P.O. Box. 454, Ngaoundéré, Cameroon</footnote>

<footnote>Zénab Nouraan Njankouo Ndassa, Aminatou Fagny Mefire, Adama</footnote>

<footnote>Haman: Department of Earth Sciences, Faculty of Sciences, University of Ngaoundéré, P.O. Box. 454, Ngaoundéré, Cameroon</footnote>
way to the surface [3,15–18]. In the whole Adamawa plateau, ultramafic xenoliths display a compositionally diverse suite (spinel lherzolite, garnet lherzolite, olivine websterite, harzburgite). Mineral compositions, textures and thermobarometric conditions (820–1,160°C, 0.8–2.5 GPa, corresponding to different sampling depths of those rocks) evidence compositional heterogeneity of ~57-km-thick lithospheric mantle (from 26 to 83 km in depth) beneath the Adamawa plateau with limited asthenosphere uprise [17,18].

This article describes newly discovered wehrlite xenoliths. They occur in Hosséré Do Guessa volcano, close to Mazélé, in the northernmost region of Ngaoundéré. The study includes mineral chemistry of xenoliths compared to those of host rock and geothermobarometry.

## 2 Geological setting

Hosséré Do Guessa volcano is located within Adamawa plateau (AP), a south-trending asymmetrical horst limited by a cliff in its northern side. The asymmetry could result from the difference between the north, where the plateau is elevated on a rather thin 23 km crust and the south with a classical ~30–33 km crust [19]. The granulitic Pan-African (500–600 Ma) basement [20,21] is bounded to the north and to the south (Figure 1) by Pan-African N70° strike-slip faults [22,23]. These faults extend from Cameroon to Sudan along about 2,000 km [24] and crosscut the entire Adamawa lithosphere (crustal basement and upper mantle) down to depths greater than 190 km [6,13,19]. They have favored the ascent of alkaline magmas [22,25], which sampled fragments of sub-continental mantle xenoliths on their way to the surface. Geophysical studies carried out on the Adamawa plateau [6–12] show that a deflected and thin lithosphere with an effective elastic thickness of about 20 km [6]. Three major density contrasts were evidenced [6,19]: (1) the shallowest (4–15 km) boundary may be correlated to thrust structures and/or bottoms of sedimentary basins; (2) the 33–37 km boundary corresponds to the Moho; and (3) the 70–90 km boundary represents the lithosphere-asthenosphere boundary. From these different studies, a crust of 20–30 km thick and a lithosphere of 80–90 km thick seem to be evidenced in this region.

Previous petrologic studies on Adamawa mantle xenoliths [4,15–18,26] have shown that ultramafic xenoliths differ in nature and composition from each volcanic center to others. The present work is focused on the newly discovered ultramafic wehrlite xenoliths entrained by Mio-

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**Figure 1:** (a) Location of region of study in Africa continent, (b) main volcanic structures in Cameroon after [29]; red star shows the studied Hosséré Do Guessa volcano area in northern Ngaoundéré and (c) geological sketch map of Adamawa plateau (after [29]). (1) lateritic breastplate; (2) basalt of upper lava series; (3) trachyte and rhyolite; (4) basalt; (5) sedimentary rocks of southern Adamawa ditch, (a) metamorphic conglomerate and (b) conglomerate and arkose; (6) late panafricain granites; (7) schiste from Lom; (8) panafricain granites; (9) migmatite and gneiss; and (10) faults (a) with uprisde morphology and (b) with mylonite. (d) Location of Hosséré Do Guessa volcano near the village of Mazélé (yellow star).
<figure><img src="image_8.png" /><figcaption>**Figure 2:** (a) View of the NE flank of Hosséré Do Guessa volcano. Peridotite xenoliths in host basanitic lava of Hosséré Do Guessa volcano. (b and c) rounded and (d and e) sub-angular shapes. Blue and red pens are 10 cm long.</figcaption></figure>

Pliocene basaltic lavas along the Pan-African cracks at the northernmost edge of Adamawa plateau in Hosséré Do

Guessa volcano. Wehrlites were only scarcely described in Adamawa plateau [26].
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**Figure 3:** Photomicrographs of host basaltic lava (a) and xenoliths (b–e): wehrlite poikilitic texture (b and c), BSE images of representative peridotite samples showing: intimate contact between olivine and orthopyroxene (d), symplectite reaction with vermicular spinel between orthopyroxene crystals and exsolution (exsol) lamellae of orthopyroxene in clinopyroxene (e). cpx = clinopyroxene, ol = olivine, opx = orthopyroxene, sp = spinel.

**3 Materials and methods**

Petrographic studies have been carried out on about ten polished thin sections prepared from representative samples of ultramafic xenoliths at GEOPS laboratory, Université Paris-Saclay, France. Modal proportions of the four major mineral phases (olivine, orthopyroxene, clinopyroxene and spinel) have been estimated under

scanning electron microscope (SEM), partly in Laboratory of Physics, University of Alexandru Ioan Cuza, Iasi, Romania, and partly in the GEOPS laboratory, Université Paris-Saclay, France.

Mineral compositions of host lavas and ultramafic xenoliths were analyzed on Camebax SX100 microprobe at Camparis, Université Paris-Sorbonne, France. The operating conditions were: olivine and clinopyroxene:
15 kV accelerating voltage and 40 nA beam current, 20 s counting time, except Si for olivine (10 s) and Ti for pyroxene (30 s); plagioclase: 15 kV, 10 nA, 10 s; titano-magnetite: 15 kV and 40 nA; Si, Ca, Ni: 10 s; Mn: 25 s; Cr: 15 s; Al: 30 s; and Ti, Fe, Mg: 40 s. Standards used were a combination of natural and synthetic minerals: diopside for Si, Ca and Mg; Fe₂O₃ for Fe; MnTiO₃ for Ti and Mn; Cr₂O₃ for Cr; albite for Na; and orthoclase for K and Al. Data corrections were made using the PAP correction [27].

# 4 Results

## 4.1 Field work and petrography

### 4.1.1 Hosséré Do Guessa volcano

The basement of Adamawa Plateau is cut by a Panafrican fault network. Small volcanic centers were emplaced at fault crossings. These are also fissural flows without noticeable crater.

Hosséré Do Guessa volcano is located at N07°37′32″ and E13°41′06″. It culminates at 1,436 m, 17 m above the surrounding floor and presents a low elevation of rather rectangular form of 350 m × 260 m, elongated along the SW-NE direction (Figure 2a). Volume of volcanites ranges between 1.0 and 1.5 × 10⁶ m³. Lava occur as dark grey loose blocks, which have a diameter of 1–1.5 m on the top of the hill and 20–50 cm in diameter on the flanks. They show a 1–3-mm-thick brown patina, and angular or rounded centimeter-size cavities (Figure 2b–e).

Basaltic host lava presents microlitic porphyritic texture (Figure 3a) that displays euhedral and subhedral

olivine (15–18 vol%), clinopyroxene crystals (5–10 vol. %) sometimes with skeletal shape and oxides phenocrysts (<5 vol%). Plagioclase (20–25 vol%) and clinopyroxene (5 vol%) microliths are oriented along the flow foliation.

### 4.1.2 Peridotite xenoliths

Studied peridotite mantle xenoliths are collected in Mio-Pliocene basaltic lava flows of Hosséré Do Guessa volcano, 35 km at the northern edge of Ngaoundéré, near the so-called “Ngaoundéré cliff” (Figure 1b). The basement of this locality is delineated by numerous NW-SE shortening Pan-African faults [28,29], which may be related to the effect of the tectonic compression on the Adamawa plateau [29].

Yellowish, greenish and dark peridotite xenoliths have sizes of 8–15 cm and exhibit various shapes (angular, elongated or rounded shape, Figure 2). Contacts between xenoliths and host basaltic lavas are sharp (Figure 2c–e).

Modal compositions of xenoliths, estimated by microscopic observations and SEM images, are listed in Table 1. Their mineralogy is largely dominated by olivine (75–85%) and clinopyroxene (7–18%). The rest (5–8%) is composed of orthopyroxene and spinel.

According to the IUGS classification [30] (Figure 4), they are wehrlites, or lherzolites close to wehrlite-lherzolite boundary (samples TZ21 and TZ110). They present some similarities with wehrlites from Mt. Cameroon [31], West Eifel, Germany [32] and central Spain [33].

Textures are poikilitic (Figure 3b) or protogranular (Figure 3c–f) following [34], with some recrystallized olivine crystals.

Large (up to 7 mm) olivine crystals are weakly altered with thin micrometre-scale iddingsitized rims (Figure 3c).

<table><caption>**Table 1:** Sample location and modal mineral compositions (in volume%) of Hosséré Do Guessa wehrlites</caption><thead><tr><th>Locality</th><th>Sample number</th><th>GPS coordinates</th><th>Olivine</th><th>Clinopyroxene</th><th>Orthopyroxene</th><th>Spinel</th></tr></thead><tbody><tr><td>Mazélé</td><td>TZ13</td><td>N: 07° 37'31.4"<br>E: 13° 41'11.1"</td><td>76</td><td>16</td><td>4</td><td>4</td></tr><tr><td>Mazélé</td><td>NZ-B1</td><td>N: 07° 37'31.9"<br>E: 13° 41'07.2"</td><td>77</td><td>18</td><td>4</td><td>1</td></tr><tr><td>Mazélé</td><td>TZ21</td><td>N: 07° 38'23.9"<br>E: 13° 40'51.0"</td><td>79</td><td>13</td><td>5</td><td>3</td></tr><tr><td>Mazélé</td><td>BM1</td><td>N: 07° 38'22.7"<br>E: 13° 40'51.8"</td><td>79</td><td>13</td><td>4</td><td>4</td></tr><tr><td>Mazélé</td><td>TZ110</td><td>N: 07° 38'25.6"<br>E: 13° 40'47.8"</td><td>75</td><td>17</td><td>5</td><td>3</td></tr><tr><td>Mazélé</td><td>MZ11</td><td>N: 07° 38'22.53"<br>E: 13° 41'08.4"</td><td>85</td><td>7</td><td>4</td><td>4</td></tr></tbody></table>
Smaller (200 µm) light-colored olivine crystals are included within large clinopyroxene crystals (Figure 3b) and exhibit no indications of deformation, or alteration.

Clinopyroxene crystals are large (500 µm to 5 mm) and constitute the poikilitic phase enclosing small olivine crystals (Figure 3b). Orthopyroxene crystals are interlocked with olivine as shown trough Back-Scattered Electron

(BSE) images (Figure 3d). Spinel occurs as intercrystalline dark crystals (50 µm to 3 mm) (Figure 3b) or surrounds clinopyroxene (Figure 3d). Elongated (10–100 µm, rarely up to 0.5 mm long) exsolution lamellae of orthopyroxene within clinopyroxene occur seldom (Figure 3f). Dark brown spinel crystals exhibit vermicular shape. Symplectite textures of clustered pyroxene and spinel are frequent (Figure 3f).

<figure><img src="image_10.png" /><figcaption>**Figure 4:** Ternary plot of olivine (Ol), orthopyroxene (Opx) and clinopyroxene (Cpx) modal compositions of the studied peridotite xenoliths after the IUGS nomenclature [30]. Data from Cameroon, Germany and Spain are added for comparison.</figcaption></figure>

<table><caption>**Table 2:** Host lava (LTZ13) chemical composition and CIPW-normative composition calculated with $Fe_2O_3$/FeO = 0.15 [35,36]</caption><thead><tr><th colspan="14">LTZ13</th></tr><tr><th colspan="14">Major elements (wt%)</th></tr><tr><th>SiO<sub>2</sub></th><th>TiO<sub>2</sub></th><th>Al<sub>2</sub>O<sub>3</sub></th><th>Fe<sub>2</sub>O<sub>3</sub></th><th>MnO</th><th>MgO</th><th>CaO</th><th>Na<sub>2</sub>O</th><th>K<sub>2</sub>O</th><th>P<sub>2</sub>O<sub>5</sub></th><th>LOI</th><th>Sum</th></tr></thead><tbody><tr><td>43.26</td><td>2.68</td><td>12.26</td><td>11.49</td><td>0.17</td><td>14.02</td><td>9.95</td><td>3.20</td><td>1.48</td><td>0.64</td><td>0.20</td><td>99.35</td></tr><tr><th colspan="14">CIPW normative analysis (wt%)</th></tr><tr><td>Or</td><td>Ab</td><td>An</td><td>Ne</td><td>Dio</td><td>OI</td><td>Mt</td><td>Ilm</td><td>Apt</td><td></td><td></td><td></td></tr><tr><td>8.75</td><td>4.58</td><td>14.72</td><td>12.18</td><td>24.41</td><td>24.95</td><td>1.98</td><td>5.09</td><td>1.48</td><td></td><td></td><td></td></tr><tr><th colspan="14">Trace elements (ppm)</th></tr><tr><td>Be</td><td>Rb</td><td>Sr</td><td>Cs</td><td>Ba</td><td>V</td><td>Cr</td><td>Co</td><td>Ni</td><td>Cu</td><td>Y</td><td>Zr</td></tr><tr><td>2.0</td><td>38</td><td>759</td><td>0.40</td><td>538</td><td>220</td><td>65.0</td><td>59.7</td><td>382.8</td><td>38</td><td>22.5</td><td>237</td></tr><tr><td>Hf</td><td>Ta</td><td>Th</td><td>Nb</td><td>Zn</td><td>La</td><td>Ce</td><td>Pr</td><td>Nd</td><td>Sm</td><td>Eu</td><td>Gd</td></tr><tr><td>5.5</td><td>3.70</td><td>7.40</td><td>73</td><td>73</td><td>54.5</td><td>97</td><td>11.1</td><td>42</td><td>8.0</td><td>2.5</td><td>7.20</td></tr><tr><td>Tb</td><td>Dy</td><td>Ho</td><td>Tm</td><td>Yb</td><td>Lu</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>0.980</td><td>5.04</td><td>0.91</td><td>0.28</td><td>1.80</td><td>0.24</td><td></td><td></td><td></td><td></td><td></td><td></td></tr></tbody></table>
## 4.2 Whole rock and mineral chemistry

### 4.2.1 Host lava

The lava (sample LTZ13) host of xenolith sample TZ13 was studied in detail [35,36]. Its whole-rock chemical composition is listed in Table 2. It is a typical basanite, rather silica-poor (43.26 wt% SiO₂), but alkali rich (Na₂O + K₂O = 4.68 wt%, Ba = 538 ppm), with CIPW-normative nepheneline (12.18 wt%). Its rather primitive character is shown by high Ni content (383 ppm). Data of mineral chemistry

were previously published [35,36]. Additional data are listed in Table 3.

Olivine phenocrysts have Fo71-79, decreasing from cores to rims and from phenocrysts to microcrysts, while olivine xenocrysts have high Fo contents (Fo88-90). CaO contents of olivine phenocrysts of host lava are high (0.25-0.55 wt% CaO) compared to low contents (0.10-0.15 wt%) in olivine xenocrysts. NiO contents of olivine phenocrysts are low (<0.2 wt%) whereas those of xenocrysts are higher (0.37-0.40 wt%).

<table><caption>Table 3: Representative microprobe compositions of major rock-forming minerals of Hosséré Do Guessa wehrlites host lava</caption><thead><tr><th>Sample</th><th colspan="10">LTZ13</th></tr><tr><th>Mineral Type</th><th colspan="3">Olivine</th><th colspan="3">Xenocryst</th><th colspan="2">cpx</th><th>pl</th><th colspan="2">mt</th></tr><tr><th></th><th></th><th>Phenocryst</th><th></th><th></th><th></th><th></th><th>Phenocryst</th><th></th><th></th><th></th><th></th></tr></thead><tbody><tr><td>SiO<sub>2</sub> (wt%)</td><td>38.37</td><td>36.48</td><td>36.51</td><td>37.08</td><td>36.33</td><td>36.65</td><td>39.89</td><td>40.49</td><td>39.89</td><td>41.94</td><td>52.93</td><td>51.97</td><td>0.08</td><td>0.69</td></tr><tr><td>TiO<sub>2</sub></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td>5.12</td><td></td><td></td><td>21.89</td><td>21.79</td></tr><tr><td>Al<sub>2</sub>O<sub>3</sub></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td>9.32</td><td>29.04</td><td>29.42</td><td>1.75</td><td>1.58</td></tr><tr><td>Cr<sub>2</sub>O<sub>3</sub></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td>0.04</td><td></td><td></td><td>0.12</td><td>0.17</td></tr><tr><td>FeO</td><td>19.42</td><td>24.68</td><td>23.65</td><td>24.04</td><td>24.36</td><td>22.84</td><td>10.13</td><td>9.55</td><td>11.94</td><td>4.90</td><td>0.86</td><td>0.89</td><td>67.79</td><td>65.01</td></tr><tr><td>Fe<sub>2</sub>O<sub>3</sub></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td>4.54</td><td></td><td></td><td></td><td></td></tr><tr><td>MnO</td><td>0.40</td><td>0.48</td><td>0.54</td><td>0.52</td><td>0.60</td><td>0.42</td><td>0.15</td><td>0.12</td><td>0.21</td><td>0.11</td><td></td><td></td><td>0.90</td><td>1.03</td></tr><tr><td>MgO</td><td>40.04</td><td>37.10</td><td>37.44</td><td>37.62</td><td>36.66</td><td>38.71</td><td>49.66</td><td>49.41</td><td>47.31</td><td>10.54</td><td></td><td></td><td>3.54</td><td>3.41</td></tr><tr><td>NiO</td><td>0.23</td><td>0.14</td><td>0.09</td><td>0.24</td><td>0.13</td><td>0.19</td><td>0.40</td><td>0.37</td><td>0.40</td><td></td><td></td><td></td><td>0.00</td><td>0.00</td></tr><tr><td>CaO</td><td>0.25</td><td>0.49</td><td>0.46</td><td>0.49</td><td>0.45</td><td>0.30</td><td>0.10</td><td>0.12</td><td>0.15</td><td>22.14</td><td>11.27</td><td>12.03</td><td>0.07</td><td>0.70</td></tr><tr><td>Na<sub>2</sub>O</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td>0.56</td><td>4.55</td><td>4.47</td><td></td><td></td></tr><tr><td>K<sub>2</sub>O</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td>0.41</td><td>0.19</td><td></td><td></td></tr><tr><td>Sum</td><td>98.71</td><td>99.37</td><td>98.69</td><td>99.99</td><td>98.53</td><td>99.11</td><td>100.33</td><td>100.06</td><td>99.90</td><td>99.21</td><td>99.06</td><td>98.97</td><td>96.14</td><td>94.38</td></tr><tr><td>Si (apfu)</td><td>0.999</td><td>0.965</td><td>0.968</td><td>0.972</td><td>0.969</td><td>0.962</td><td>0.973</td><td>0.990</td><td>0.989</td><td>6.395</td><td>2.418</td><td>2.384</td><td>0.024</td><td>0.206</td></tr><tr><td>Ti</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td>0.587</td><td></td><td></td><td>4.813</td><td>4.872</td></tr><tr><td>Al<sup>IV</sup></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td>1.605</td><td></td><td></td><td></td><td></td></tr><tr><td>Al<sup>VI</sup></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td>0.070</td><td></td><td></td><td></td><td></td></tr><tr><td>Cr</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td>0.005</td><td></td><td></td><td>0.029</td><td>0.040</td></tr><tr><td>Fe<sup>3+</sup></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td>0.521</td><td></td><td></td><td>5.527</td><td>5.081</td></tr><tr><td>Fe<sup>2+</sup></td><td>0.423</td><td>0.546</td><td>0.524</td><td>0.527</td><td>0.544</td><td>0.502</td><td>0.207</td><td>0.195</td><td>0.248</td><td>0.624</td><td>0.033</td><td>0.034</td><td>11.047</td><td>11.082</td></tr><tr><td>Mn</td><td>0.005</td><td>0.011</td><td>0.012</td><td>0.011</td><td>0.014</td><td>0.009</td><td>0.003</td><td>0.002</td><td>0.004</td><td>0.014</td><td></td><td></td><td>0.224</td><td>0.259</td></tr><tr><td>Mg</td><td>1.554</td><td>1.462</td><td>1.480</td><td>1.470</td><td>1.458</td><td>1.515</td><td>1.806</td><td>1.801</td><td>1.747</td><td>2.395</td><td></td><td></td><td>1.545</td><td>1.513</td></tr><tr><td>Ni</td><td>0.009</td><td>0.003</td><td>0.002</td><td>0.005</td><td>0.003</td><td>0.004</td><td>0.008</td><td>0.007</td><td>0.008</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Ca</td><td>0.007</td><td>0.014</td><td>0.013</td><td>0.014</td><td>0.013</td><td>0.009</td><td>0.003</td><td>0.003</td><td>0.004</td><td>3.617</td><td>0.552</td><td>0.591</td><td>0.022</td><td>0.224</td></tr><tr><td>Na</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td>0.166</td><td>0.403</td><td>0.397</td><td></td><td></td></tr><tr><td>K</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td>0.024</td><td>0.011</td><td></td><td></td></tr><tr><td>Mg#</td><td>78.61</td><td>72.81</td><td>73.85</td><td>73.61</td><td>72.83</td><td>75.11</td><td>89.72</td><td>90.23</td><td>87.57</td><td>67.65</td><td></td><td></td><td></td><td></td></tr><tr><td>% Usp</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td>61.49</td><td>64.51</td></tr><tr><td>Wo</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td>50.44</td><td></td><td></td><td></td><td></td></tr><tr><td>En</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td>33.40</td><td></td><td></td><td></td><td></td></tr><tr><td>Fs</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td>16.17</td><td></td><td></td><td></td><td></td></tr><tr><td>An</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td>56.30</td><td>59.12</td><td></td><td></td></tr><tr><td>Ab</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td>41.26</td><td>39.77</td><td></td><td></td></tr><tr><td>Or</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td>2.44</td><td>1.11</td><td></td><td></td></tr></tbody></table>

Structural formulas: olivine on the basis of 4 oxygen anions, clinopyroxene on the basis of 16 cations, plagioclase on the basis of 8 oxygen anions, Ti-magnetite on the basis of 32 oxygen anions.
Clinopyroxene phenocrysts are augite and diopside (Wo 43.2–48.4), some with fassaitic composition (Wo 50.4) as those described in Noun Plain, Cameroon [37]. TiO₂ contents (2.7–5.1 wt%) and Al₂O₃ contents (from 6.6 to 9.3, until 12.4 wt%) are high. Mg# ratios (68–76) are in the range as olivine.

Oxide phenocrysts and microcrysts are Ti-magnetite (TiO₂: 21.8–21.9 wt% and FeOt: 65.0–67.8 wt%). Cr₂O₃ contents are low (0.12–0.17 wt%), while MgO contents are fairly constant (3.4–3.5 wt%).

### 4.2.2 Wehrlite xenoliths

Representative microprobe analyses of olivine, orthopyroxene, clinopyroxene and spinel are listed in Tables 4–6.

Olivine varies in Fo between 90.8 and 91.4 (Table 4) and CaO contents are very low (<0.1 wt%). High NiO contents range between 0.24 and 0.48 wt%.

Clinopyroxene (Table 5) is augite (Figure 5) with Wo39.4–42.0 En48.5–51.2 Fs9.0–9.6. TiO₂ (0.76–1.27 wt%) contents are fairly low and Al₂O₃ (5.00–5.88 wt%) contents are high. Cr₂O₃ contents range between 0.89 and 0.99 wt%. Na₂O contents between 0.68 and 1.18 wt% are somewhat high for mantle pyroxene. Mg# (=Mg/Mg + Fe) are fairly constant, between 84.2 and 85.8. In TiO₂ (wt%) in pyroxene versus Mg# in coexisting olivine diagram (Figure 6, according to [38]), wehrlite minerals plot higher than the lherzolite field. In the MgO versus

Al₂O₃ diagram (Figure 7, after [39]), clinopyroxene compositions plot within the spinel peridotite field.

Orthopyroxene (Table 5) is enstatite (after [40]) with Wo0.8–1.0 En90.5–91.1 Fs7.9–8.6. Al₂O₃ contents (0.72–1.28 wt%) are low and fairly constant whereas Cr₂O₃ contents (0.02–0.05 wt%) are very low. Mg# ratios (91.7–92.2) are higher than in clinopyroxene and slightly higher than in olivine.

Spinel (Table 6) is Al-spinel after [41]. It shows some contrasted compositions, with varying Cr# (11.9–27.3) and Fe³⁺# (5.3–10.7). In Fo versus Cr# OSMA diagram (after [42], Figure 8), wehrlites plot in the mantle spinel array.

## 4.3 Thermobarometry

Equilibrium temperatures, pressures and depths of wehrlite from Hosséré Do Guessa volcano have been estimated through several currently available and appropriate thermobarometers based on different exchange mechanisms (Table 7, Figure 9). Crystal core compositions have been preferentially used for calculations.

### 4.3.1 Host lava

Tentative calculations were based on olivine – liquid/glass [43] and clinopyroxene – liquid/glass [44] geothermometers.

<table><caption>**Table 4:** Representative microprobe compositions of olivine of Hosséré Do Guessa wehrlite and structural formulas on the basis of 4 oxygen anions</caption><thead><tr><th>Sample</th><th colspan="14">TZ13</th></tr><tr><th>n0</th><th>1</th><th>2</th><th>3</th><th>4</th><th>5</th><th>6</th><th>7</th><th>8</th><th>9</th><th>10</th><th>11</th><th>12</th><th>13</th><th>14</th></tr></thead><tbody><tr><td>SiO2 (wt%)</td><td>41.16</td><td>41.12</td><td>40.77</td><td>40.36</td><td>40.77</td><td>40.81</td><td>40.63</td><td>40.92</td><td>40.78</td><td>40.87</td><td>40.66</td><td>41.12</td><td>40.61</td><td>40.78</td></tr><tr><td>FeO</td><td>8.52</td><td>8.39</td><td>8.52</td><td>8.86</td><td>8.80</td><td>8.70</td><td>8.76</td><td>8.73</td><td>8.35</td><td>8.68</td><td>8.75</td><td>8.51</td><td>8.64</td><td>8.86</td></tr><tr><td>MnO</td><td>0.20</td><td>0.11</td><td>0.14</td><td>0.14</td><td>0.06</td><td>0.16</td><td>0.19</td><td>0.18</td><td>0.17</td><td>0.20</td><td>0.18</td><td>0.17</td><td>0.12</td><td>0.17</td></tr><tr><td>MgO</td><td>50.51</td><td>50.32</td><td>50.75</td><td>50.28</td><td>50.46</td><td>50.15</td><td>50.15</td><td>50.11</td><td>50.13</td><td>50.32</td><td>50.61</td><td>50.36</td><td>50.44</td><td>50.30</td></tr><tr><td>CaO</td><td>0.03</td><td>0.03</td><td>0.04</td><td>0.06</td><td>0.05</td><td>0.02</td><td>0.05</td><td>0.08</td><td>0.03</td><td>0.04</td><td>0.06</td><td>0.06</td><td>0.06</td><td>0.03</td></tr><tr><td>NiO</td><td>0.36</td><td>0.48</td><td>0.43</td><td>0.44</td><td>0.37</td><td>0.24</td><td>0.42</td><td>0.46</td><td>0.45</td><td>0.34</td><td>0.36</td><td>0.36</td><td>0.37</td><td>0.41</td></tr><tr><td>Sum</td><td>100.78</td><td>100.45</td><td>100.65</td><td>100.14</td><td>100.51</td><td>100.08</td><td>100.20</td><td>100.48</td><td>99.91</td><td>100.45</td><td>100.62</td><td>100.58</td><td>100.24</td><td>100.55</td></tr><tr><td>Si (apfu)</td><td>0.996</td><td>0.998</td><td>0.989</td><td>0.987</td><td>0.991</td><td>0.995</td><td>0.991</td><td>0.995</td><td>0.996</td><td>0.994</td><td>0.988</td><td>0.997</td><td>0.990</td><td>0.992</td></tr><tr><td>Fe²⁺</td><td>0.172</td><td>0.170</td><td>0.173</td><td>0.181</td><td>0.179</td><td>0.177</td><td>0.179</td><td>0.177</td><td>0.171</td><td>0.176</td><td>0.178</td><td>0.173</td><td>0.176</td><td>0.180</td></tr><tr><td>Mn</td><td>0.004</td><td>0.002</td><td>0.003</td><td>0.003</td><td>0.001</td><td>0.003</td><td>0.004</td><td>0.004</td><td>0.003</td><td>0.004</td><td>0.004</td><td>0.003</td><td>0.002</td><td>0.003</td></tr><tr><td>Mg</td><td>1.823</td><td>1.821</td><td>1.836</td><td>1.832</td><td>1.829</td><td>1.823</td><td>1.824</td><td>1.817</td><td>1.825</td><td>1.824</td><td>1.834</td><td>1.821</td><td>1.833</td><td>1.824</td></tr><tr><td>Ca</td><td>0.001</td><td>0.001</td><td>0.001</td><td>0.001</td><td>0.001</td><td>0.000</td><td>0.001</td><td>0.002</td><td>0.001</td><td>0.001</td><td>0.002</td><td>0.001</td><td>0.002</td><td>0.001</td></tr><tr><td>Ni</td><td>0.007</td><td>0.009</td><td>0.008</td><td>0.009</td><td>0.007</td><td>0.005</td><td>0.008</td><td>0.009</td><td>0.009</td><td>0.007</td><td>0.007</td><td>0.007</td><td>0.007</td><td>0.008</td></tr><tr><td>Fo%</td><td>91.17</td><td>91.35</td><td>91.26</td><td>90.88</td><td>91.03</td><td>90.98</td><td>90.90</td><td>90.93</td><td>91.29</td><td>91.00</td><td>91.00</td><td>91.18</td><td>91.12</td><td>90.85</td></tr></tbody></table>
<table><caption>**Table 5:** Representative microprobe compositions of clinopyroxene and orthopyroxene of Hosséré Do Guessa wehrlite and structural formulas on the basis of 16 cations</caption><thead><tr><th>Sample<br>Mineral<br>n0</th><th colspan="12">TZ13<br>Clinopyroxene</th></tr><tr><th></th><th>1</th><th>2</th><th>3</th><th>4</th><th>5</th><th>6</th><th>7</th><th>8</th><th>9</th><th>10</th><th>11</th><th>12</th></tr></thead><tbody><tr><td>SiO<sub>2</sub> (wt%)</td><td>50.79</td><td>51.19</td><td>51.16</td><td>50.82</td><td>51.06</td><td>51.89</td><td>51.44</td><td>51.62</td><td>50.77</td><td>51.77</td><td>52.15</td><td>50.70</td></tr><tr><td>TiO<sub>2</sub></td><td>0.96</td><td>0.86</td><td>0.87</td><td>0.84</td><td>0.77</td><td>0.76</td><td>0.85</td><td>0.83</td><td>1.27</td><td>0.81</td><td>0.82</td><td>0.75</td></tr><tr><td>Al<sub>2</sub>O<sub>3</sub></td><td>5.88</td><td>5.66</td><td>5.45</td><td>5.41</td><td>5.37</td><td>5.51</td><td>5.44</td><td>5.43</td><td>5.00</td><td>5.33</td><td>5.40</td><td>5.51</td></tr><tr><td>Cr<sub>2</sub>O<sub>3</sub></td><td>0.95</td><td>0.98</td><td>0.90</td><td>0.98</td><td>0.95</td><td>0.95</td><td>0.90</td><td>0.99</td><td>0.89</td><td>0.94</td><td>0.94</td><td>0.94</td></tr><tr><td>FeO*</td><td>4.04</td><td>4.71</td><td>4.34</td><td>3.52</td><td>2.93</td><td>4.09</td><td>4.31</td><td>3.88</td><td>4.33</td><td>3.92</td><td>4.62</td><td>0.02</td></tr><tr><td>Fe<sub>2</sub>O<sub>3</sub>*</td><td>1.57</td><td>0.84</td><td>0.90</td><td>2.00</td><td>2.77</td><td>1.22</td><td>1.09</td><td>1.60</td><td>1.24</td><td>1.02</td><td>0.55</td><td>5.41</td></tr><tr><td>MnO</td><td>0.04</td><td>0.16</td><td>0.10</td><td>0.16</td><td>0.09</td><td>0.11</td><td>0.08</td><td>0.13</td><td>0.23</td><td>0.10</td><td>0.12</td><td>0.08</td></tr><tr><td>MgO</td><td>16.29</td><td>16.47</td><td>16.34</td><td>16.73</td><td>16.62</td><td>16.87</td><td>16.24</td><td>16.50</td><td>16.27</td><td>16.39</td><td>16.55</td><td>16.80</td></tr><tr><td>NiO</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td>0.16</td></tr><tr><td>CaO</td><td>18.47</td><td>18.34</td><td>18.29</td><td>17.93</td><td>18.45</td><td>18.42</td><td>18.79</td><td>18.73</td><td>19.62</td><td>19.26</td><td>18.80</td><td>18.89</td></tr><tr><td>Na<sub>2</sub>O</td><td>1.04</td><td>0.92</td><td>1.07</td><td>1.09</td><td>1.18</td><td>1.05</td><td>1.05</td><td>1.09</td><td>0.68</td><td>1.02</td><td>1.03</td><td>1.06</td></tr><tr><td>Sum</td><td>100.03</td><td>100.13</td><td>99.42</td><td>99.48</td><td>100.19</td><td>100.87</td><td>100.19</td><td>100.80</td><td>100.30</td><td>100.56</td><td>100.98</td><td>100.32</td></tr><tr><td>Si (apfu)</td><td>7.396</td><td>7.448</td><td>7.485</td><td>7.429</td><td>7.416</td><td>7.477</td><td>7.478</td><td>7.457</td><td>7.407</td><td>7.491</td><td>7.513</td><td>7.311</td></tr><tr><td>Ti</td><td>0.105</td><td>0.094</td><td>0.096</td><td>0.092</td><td>0.084</td><td>0.082</td><td>0.093</td><td>0.090</td><td>0.139</td><td>0.088</td><td>0.089</td><td>0.082</td></tr><tr><td>Al<sup>liV</sup></td><td>0.604</td><td>0.552</td><td>0.515</td><td>0.571</td><td>0.584</td><td>0.523</td><td>0.522</td><td>0.543</td><td>0.593</td><td>0.509</td><td>0.487</td><td>0.689</td></tr><tr><td>Al<sup>vi</sup></td><td>0.406</td><td>0.419</td><td>0.424</td><td>0.362</td><td>0.336</td><td>0.412</td><td>0.410</td><td>0.382</td><td>0.267</td><td>0.401</td><td>0.430</td><td>0.247</td></tr><tr><td>Cr</td><td>0.109</td><td>0.113</td><td>0.104</td><td>0.113</td><td>0.109</td><td>0.108</td><td>0.103</td><td>0.113</td><td>0.103</td><td>0.107</td><td>0.107</td><td>0.108</td></tr><tr><td>Fe<sup>3+</sup></td><td>0.172</td><td>0.092</td><td>0.099</td><td>0.220</td><td>0.303</td><td>0.132</td><td>0.119</td><td>0.174</td><td>0.136</td><td>0.111</td><td>0.059</td><td>0.587</td></tr><tr><td>Fe<sup>2+</sup></td><td>0.492</td><td>0.573</td><td>0.531</td><td>0.430</td><td>0.356</td><td>0.492</td><td>0.524</td><td>0.469</td><td>0.529</td><td>0.474</td><td>0.556</td><td>0.002</td></tr><tr><td>Mn</td><td>0.005</td><td>0.020</td><td>0.012</td><td>0.020</td><td>0.011</td><td>0.013</td><td>0.010</td><td>0.016</td><td>0.028</td><td>0.012</td><td>0.015</td><td>0.010</td></tr><tr><td>Mg</td><td>3.535</td><td>3.571</td><td>3.563</td><td>3.645</td><td>3.598</td><td>3.623</td><td>3.518</td><td>3.552</td><td>3.538</td><td>3.535</td><td>3.554</td><td>3.611</td></tr><tr><td>Ni</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td>0.019</td></tr><tr><td>Ca</td><td>2.882</td><td>2.859</td><td>2.867</td><td>2.809</td><td>2.871</td><td>2.844</td><td>2.927</td><td>2.899</td><td>3.067</td><td>2.986</td><td>2.902</td><td>2.919</td></tr><tr><td>Na</td><td>0.294</td><td>0.260</td><td>0.304</td><td>0.309</td><td>0.332</td><td>0.293</td><td>0.296</td><td>0.305</td><td>0.192</td><td>0.286</td><td>0.288</td><td>0.297</td></tr><tr><td>Mg#</td><td>84.19</td><td>84.31</td><td>84.97</td><td>84.86</td><td>84.53</td><td>85.30</td><td>84.55</td><td>84.68</td><td>84.18</td><td>85.81</td><td>85.23</td><td>85.96</td></tr><tr><td>Wo</td><td>40.67</td><td>40.19</td><td>40.54</td><td>39.43</td><td>40.22</td><td>40.03</td><td>41.23</td><td>40.78</td><td>42.03</td><td>41.96</td><td>40.96</td><td>40.94</td></tr><tr><td>En</td><td>49.89</td><td>50.20</td><td>50.37</td><td>51.17</td><td>50.40</td><td>50.99</td><td>49.57</td><td>49.96</td><td>48.47</td><td>49.66</td><td>50.15</td><td>50.65</td></tr><tr><td>Fs</td><td>9.44</td><td>9.62</td><td>9.08</td><td>9.41</td><td>9.38</td><td>8.98</td><td>9.20</td><td>9.26</td><td>9.50</td><td>8.38</td><td>8.90</td><td>8.41</td></tr></tbody></table>

<table><thead><tr><th>Sample<br>Mineral<br>n0</th><th colspan="12">TZ13<br>Orthopyroxene</th></tr><tr><th></th><th>13</th><th>14</th><th>15</th><th>16</th><th>17</th><th>18</th><th>19</th><th>20</th></tr></thead><tbody><tr><td>SiO<sub>2</sub> (wt%)</td><td>57.74</td><td>58.00</td><td>57.73</td><td>57.89</td><td>58.17</td><td>57.92</td><td>53.41</td><td>54.41</td></tr><tr><td>TiO<sub>2</sub></td><td>0.00</td><td>0.00</td><td>0.00</td><td>0.00</td><td>0.00</td><td>0.00</td><td>0.30</td><td>0.32</td></tr><tr><td>Al<sub>2</sub>O<sub>3</sub></td><td>1.28</td><td>0.76</td><td>0.84</td><td>0.84</td><td>0.72</td><td>0.74</td><td>4.58</td><td>4.37</td></tr><tr><td>Cr<sub>2</sub>O<sub>3</sub></td><td>0.04</td><td>0.02</td><td>0.04</td><td>0.03</td><td>0.04</td><td>0.05</td><td>0.73</td><td>0.75</td></tr><tr><td>FeO*</td><td>5.27</td><td>5.22</td><td>5.08</td><td>4.09</td><td>5.12</td><td>5.06</td><td>4.84</td><td>6.49</td></tr><tr><td>Fe<sub>2</sub>O<sub>3</sub>*</td><td>0.45</td><td>0.14</td><td>0.32</td><td>1.45</td><td>0.53</td><td>0.57</td><td>4.01</td><td>1.98</td></tr><tr><td>MnO</td><td>0.29</td><td>0.19</td><td>0.19</td><td>0.18</td><td>0.12</td><td>0.11</td><td>0.13</td><td>0.09</td></tr><tr><td>MgO</td><td>35.11</td><td>35.39</td><td>35.31</td><td>35.90</td><td>35.67</td><td>35.47</td><td>30.70</td><td>30.59</td></tr><tr><td>NiO</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td>0.10</td></tr><tr><td>CaO</td><td>0.46</td><td>0.50</td><td>0.47</td><td>0.54</td><td>0.51</td><td>0.47</td><td>1.45</td><td>1.38</td></tr><tr><td>Na<sub>2</sub>O</td><td>0.07</td><td>0.05</td><td>0.05</td><td>0.06</td><td>0.02</td><td>0.06</td><td>0.11</td><td>0.11</td></tr><tr><td>Sum</td><td>100.71</td><td>100.27</td><td>100.03</td><td>100.98</td><td>100.90</td><td>100.45</td><td>100.36</td><td>100.49</td></tr><tr><td>Si (apfu)</td><td>7.881</td><td>7.937</td><td>7.920</td><td>7.865</td><td>7.916</td><td>7.916</td><td>7.417</td><td>7.538</td></tr><tr><td>Ti</td><td>0.000</td><td>0.000</td><td>0.000</td><td>0.000</td><td>0.000</td><td>0.000</td><td>0.031</td><td>0.033</td></tr><tr><td>Al<sup>liV</sup></td><td>0.119</td><td>0.063</td><td>0.080</td><td>0.135</td><td>0.084</td><td>0.084</td><td>0.583</td><td>0.462</td></tr><tr><td>Al<sup>vi</sup></td><td>0.087</td><td>0.059</td><td>0.056</td><td>0.000</td><td>0.031</td><td>0.035</td><td>0.166</td><td>0.251</td></tr><tr><td>Cr</td><td>0.004</td><td>0.002</td><td>0.004</td><td>0.003</td><td>0.004</td><td>0.005</td><td>0.080</td><td>0.082</td></tr><tr><td>Fe<sup>3+</sup></td><td>0.046</td><td>0.015</td><td>0.033</td><td>0.148</td><td>0.054</td><td>0.059</td><td>0.419</td><td>0.206</td></tr></tbody></table>

(Continued)
<table><caption>**Table 5: Continued**</caption><thead><tr><th rowspan="2">Sample<br>Mineral</th><th colspan="8">TZ13<br>Orthopyroxene</th></tr><tr><th>13</th><th>14</th><th>15</th><th>16</th><th>17</th><th>18</th><th>19</th><th>20</th></tr></thead><tbody><tr><td>n0</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Fe<sup>2+</sup></td><td>0.601</td><td>0.598</td><td>0.583</td><td>0.465</td><td>0.583</td><td>0.578</td><td>0.563</td><td>0.752</td></tr><tr><td>Mn</td><td>0.034</td><td>0.022</td><td>0.022</td><td>0.021</td><td>0.014</td><td>0.013</td><td>0.015</td><td>0.011</td></tr><tr><td>Mg</td><td>7.142</td><td>7.217</td><td>7.219</td><td>7.269</td><td>7.234</td><td>7.225</td><td>6.354</td><td>6.316</td></tr><tr><td>Ni</td><td></td><td></td><td></td><td></td><td></td><td></td><td>0.012</td><td>0.000</td></tr><tr><td>Ca</td><td>0.067</td><td>0.073</td><td>0.069</td><td>0.079</td><td>0.074</td><td>0.069</td><td>0.216</td><td>0.205</td></tr><tr><td>Na</td><td>0.019</td><td>0.013</td><td>0.013</td><td>0.016</td><td>0.005</td><td>0.016</td><td>0.030</td><td>0.029</td></tr><tr><td>Mg#</td><td>91.69</td><td>92.18</td><td>92.14</td><td>92.22</td><td>91.90</td><td>91.90</td><td>86.62</td><td>86.82</td></tr><tr><td>Wo</td><td>0.85</td><td>0.93</td><td>0.87</td><td>0.98</td><td>0.93</td><td>0.87</td><td>2.86</td><td>2.74</td></tr><tr><td>En</td><td>90.52</td><td>91.07</td><td>91.08</td><td>91.07</td><td>90.88</td><td>90.96</td><td>83.97</td><td>84.31</td></tr><tr><td>Fs</td><td>8.63</td><td>8.00</td><td>8.05</td><td>7.95</td><td>8.18</td><td>8.18</td><td>13.17</td><td>12.94</td></tr></tbody></table>

FeO* and Fe₂O₃* recalculated according to a total of 16 cations.

However, calculated K_D are respectively 0.658 for olivine Fo 78.6, 0.902 for olivine Fo 72.8 and 0.960 for olivine Fo 71.1, that differ from the experimental equilibrium K_D of 0.300. Similarly, K_D are 0.715 for clinopyroxene Wo 50.4, 0.661 for Wo 48.4 and 0.850 for Wo 43.2, that differ from the experimental equilibrium K_D of 0.275. Actually, the basanite sample LTZ13 corresponds to a partly cumulative lava ("ankaramite" facies), containing also olivine xenocrysts (Table 3). Considering an assumed liquid (MgO = 8–9 wt%, FeO = 14–15 wt%) in equilibrium with olivine and pyroxene, temperature of 1,300°C and pressure of 11 kbar (1.1 GPa) are estimated.

### 4.3.2 Xenoliths

#### 4.3.2.1 Thermometry

Graphical estimations in quadrilateral pyroxene diagram according to [45] suggest a temperature range from 920 to 1,080°C for a pressure of 5 kbar (0.5 GPa) (Figure 5) but note that these thermometric limits are nearly the same between 1 atm and 10 kbar.

More precise equilibrium temperatures of xenolith were determined by applying (1) the empirical single pyroxene thermobarometer of [46] based on Cr–Al solubility in (orthor- or clino-) pyroxene, (2) the two-pyroxenes thermometer of [47], based on Ca-exchange between clino- and ortho- pyroxenes, (3) the partitioning of Na between ortho- and clino- pyroxenes and Ca (apfu) content of orthopyroxene alone proposed by [48], (4) olivine-spinel exchange of [49] and (5) clino-ortho-pyroxenes equilibrium of ref. [50]. For these calculations, only data on well crystallized and identified phases have been used to avoid any unknown mantle

process which may occur during recrystallization process. Calculated temperatures are listed in Table 7.

Calculated temperatures using single pyroxene thermometer of [46] yield values between 840 and 900°C with orthopyroxene data, except two values at 1,260°C

<table><caption>**Table 6: Representative microprobe compositions of spinel and structural formula on the basis of 24 total cations**</caption><thead><tr><th rowspan="2">Sample<br>Mineral<br>n0</th><th colspan="3">TZ13<br>Spinel</th></tr><tr><th>1</th><th>11</th><th>13</th></tr></thead><tbody><tr><td>SiO<sub>2</sub> (wt%)</td><td>0.92</td><td>0.25</td><td>0.21</td></tr><tr><td>TiO<sub>2</sub></td><td>0.91</td><td>1.77</td><td>1.28</td></tr><tr><td>Al<sub>2</sub>O<sub>3</sub></td><td>50.51</td><td>37.89</td><td>44.07</td></tr><tr><td>Cr<sub>2</sub>O<sub>3</sub></td><td>10.15</td><td>21.17</td><td>15.49</td></tr><tr><td>FeOt</td><td>18.94</td><td>18.08</td><td>22.15</td></tr><tr><td>MgO</td><td>17.70</td><td>19.65</td><td>17.01</td></tr><tr><td>NiO</td><td>0.35</td><td>0.46</td><td>0.22</td></tr><tr><td>CaO</td><td>0.02</td><td>0.01</td><td>0.08</td></tr><tr><td>Na<sub>2</sub>O</td><td>0.03</td><td>0.01</td><td>0.01</td></tr><tr><td>Sum</td><td>99.53</td><td>99.29</td><td>100.52</td></tr><tr><td>Si (apfu)</td><td>0.197</td><td>0.055</td><td>0.046</td></tr><tr><td>Ti</td><td>0.147</td><td>0.296</td><td>0.211</td></tr><tr><td>Al<sup>t</sup></td><td>12.786</td><td>9.947</td><td>11.397</td></tr><tr><td>Cr</td><td>1.724</td><td>3.728</td><td>2.687</td></tr><tr><td>Fe<sup>3+</sup></td><td>0.812</td><td>1.629</td><td>1.406</td></tr><tr><td>Fe<sup>2+</sup></td><td>2.590</td><td>1.738</td><td>2.658</td></tr><tr><td>Mg</td><td>5.667</td><td>6.525</td><td>5.565</td></tr><tr><td>Ni</td><td>0.061</td><td>0.082</td><td>0.039</td></tr><tr><td>Ca</td><td>0.005</td><td>0.003</td><td>0.018</td></tr><tr><td>Na</td><td>0.010</td><td>0.005</td><td>0.003</td></tr><tr><td>Mg#</td><td>68.63</td><td>78.96</td><td>67.67</td></tr><tr><td>Cr#</td><td>11.88</td><td>27.26</td><td>19.08</td></tr><tr><td>Fe<sup>3+</sup>#</td><td>5.30</td><td>10.65</td><td>9.08</td></tr></tbody></table>
<figure><img src="image_11.png" /><figcaption>**Figure 5:** Pyroxene composition of Hosséré Do Guessa wehrlites and host lava in quadrilateral diagram after [40]. Thermometry between 500°C and 1,300°C according to [45] for P = 5 kbar (0.5 GPa).</figcaption></figure>

(for orthopyroxene analyses 19 and 20 which are Cr-richer that witness deeper equilibrium) while values obtained when using clinopyroxene data are between 1,090 and 1,220°C (Table 7, Figure 9). Empirical two pyroxene method of [47] give the temperature values of 1,070–1,130°C while the range of 750–900°C (except 1,190°C for 19 and 20) have been obtained through formulae of [48] when using Na and Ca of pyroxene porphyrocrysts. Olivine-spinel exchange method of [49] give the temperature values

of 1,005–1,220°C. Two-pyroxenes geothermometer of [50] was used. We obtain 1,183.4°C for cpx Ca-poorest versus opx Ca-poorest and 1,124.5°C for cpx Ca-richest versus opx Ca-poorest. Values with Cr-orthopyroxene 19 and 20 are up to 1,135°C.

Results are scattered. Values obtained with three thermometers (two single orthopyroxene and one cpx–opx of [46] and [48] are rather low (750–900°C). They do not likely correspond to wehlite crystallization but reflect probably late re-equilibration. Values obtained with the single clinopyroxene [46], olivine–spinel [49] and cpx–opx [50] give higher temperatures (1,005–1,220°C), reflecting more probably crystallization temperatures of mantle xenoliths (see discussions in [48,51,52]). The same value of 1,180°C is

<figure><img src="image_12.png" /><figcaption>**Figure 6:** Mg# in Ol versus TiO₂ in Cpx in Hosséré Do Guessa wehlite following [38]. Dashed line with arrow indicates the mantle refertilization.</figcaption></figure>

<figure><img src="image_13.png" /><figcaption>**Figure 7:** MgO versus Al₂O₃ for clinopyroxene of Hosséré Do Guessa wehrlites compared with other peridotites of Cameroon, plotted in the fields defined by ref. [39].</figcaption></figure>
obtained with cpx Ca-poorest versus opx Ca-poorest thermometry of [50] and corresponds to the highest result with olivine-spinel thermometry. A bracket of 1,180 ± 40 = 1,140–1,220°C (maximum calculated) might match the effective temperature.

### 4.3.2.2 Barometry

Unfortunately, there is no direct method for determining equilibrium pressure in spinel peridotite xenoliths. Despite use of the Ca-in-olivine barometer [53] in some investigations, it has been demonstrated that it is incorrect, and its application may not provide reliable results [52,54,55].

Using formulas of [46], values from clinopyroxene mainly range from 0.58 to 0.94 GPa (and some values from 3.0 to 3.5 GPa corresponding to very deep pressures) whereas, from orthopyroxene, two groups of values are identified, one around 0.16 GPa and the second from 2.24 to 2.58 GPa (Table 7, Figure 9).

The two-pyroxenes geobarometer [50] give values between 1.4 and 2.1 GPa. In addition, assuming a temperature of 1,150°C, clinopyroxene yields pressures of 1.08–1.68 GPa (barometer of [56]). Several values, between 1.5 and 2.0 GPa, seem possible pressures of xenoliths formation. These values are close of those experimental equilibria (1.61–1.87 GPa) between spinel lherzolite and garnet lherzolite determined by [57]. Later, xenoliths have been reequilibrated from 0.9 to 0.6 GPa and possibly down to 0.16 GPa.

The corresponding depths using the conversion of 33 km/GPa are between 49.5 km (for 1.5 GPa) and 66 km (for 2.0 GPa), that is to say 33–37 km of crust plus 17–29 km of mantle.

When plotted in P-T diagram (Figure 9, adapted from [58]), calculated values form four clusters. The group with temperatures close to 1,100–1,200°C and pressures around 1.5–2.0 GPa obtained following [50] follow the same geotherm as the Youkou spinel lherzolite, in Adamawa, proposed by [58]. It corresponds to a fairly hot geotherm of 60 mW/m², between the “continental geotherm” and the “static rift geotherm.” It is consistent with geophysical data obtained in Adamawa. Such high geotherm might result from the regional active tectonic and/or the occurrence of a rising mantle plume, as discussed by [59] for Ethiopian plateau.

## 5 Discussion

### 5.1 Mantle origin of Hosséré Do Guessa wehrlite xenoliths

Mio-Pliocene basanite lava flows and volcanic deposits in the Adamawa plateau [36,60,61] are characterized by the occurrence of a large number of ultramafic xenoliths [17,18,58]. Wehrlite xenoliths, discovered in Hosséré Do Guessa volcano, were unknown so far in the Adamawa Plateau.

These wehrlites exhibit poikilitic textures, which are sometimes ascribed to cumulate rocks crystalized from

<figure><img src="image_14.png" /><figcaption>**Figure 8:** Fo (olivine) versus Cr# (spinel) diagram (after [42]). OSMA = olivine spinel mantle array. Data for peridotite xenoliths from Cameroon [18,31,75], São Tomé [76], Madagascar [77] and elsewhere in the world [42] are added for comparison.</figcaption></figure>
<figure><img src="image_15.png" /></figure>

**Figure 9:** Equilibrium conditions of Hosséré Do Guessa wehrlites plotted in a temperature versus pressure (depth) diagram (adapted from [58]). Four values calculated between 3.04 and 3.48 GPa (Table 7) are not plotted in this diagram. Curves 1 and 2 are, respectively, the hydrous (0.4 wt% H₂O) and anhydrous solidi [78].

magma liquid [62,63]. However, many other features suggested by texture and mineral compositions show that Hosséré Do Guessa wehrlites are xenoliths from mantle origin. They are not consistent with the hypothesis of large volume of cumulates, since the latter are commonly associated with considerable compositional variation [64]. The poikilitic texture seems to be acquired during shallow mantle deformation which have led to strong recrystallisation of olivine [65]. As the northern part of Ngaoundéré basement have been transected in its large part by numerous Pan-African transpressive stick-slip-faults down to the mantle [13,22,28,29], reworking of these faults might have affected the shallow upper lithospheric mantle, as suggested by [42] for peridotite xenoliths of Iraya volcano in Japan.

Additionally, mineral composition of studied wehrlites strongly supports the mantle origin hypothesis of these rocks. Olivine is highly magnesian with Fo between 90.8 and 91.4 and has low CaO (<0.1 wt%, see [66]) and high NiO (0.24–0.48 wt%) contents, compatible with of

mantle origin [67]. Cr# (Cr/Cr + Al) of spinel is low (12) and its composition plots in the mantle stability field of [41,68] with TiO₂ and Fe²⁺/Fe³⁺ values close to mantle spinel composition. Clinopyroxene is Al-rich (5.0–5.9 wt% Al₂O₃) and orthopyroxene is Al-poor (0.7–1.3 wt% Al₂O₃, leading to inheritance from original garnet phase [69] after uplift [70].

Vertical movements under the Adamawa plateau have been suggested by geophysical studies [7]. The crust was uplifted by upward migration of the lithosphere-asthenosphere boundary from 180 to 80 km [2–6]. Poikilitic textures of Hosséré Do Guessa wehrlites may thus result from the solid-state tectonic relaxation as have been suggested by [34,71], consistent with the tectonic scenario. Hosséré Do Guessa wehrlites would not be comparable to cumulative wehrlites and xenoliths of magmatic origin from crystal mush, such as wehrlites described in Batoke, Mt. Cameroon volcano [31].

As the crustal thickness is estimated around 33–37 km at the north of Ngaoundéré, the studied xenoliths should have been sampled below, in the upper mantle, 50–66 km deep. The referring depths locate the wehrlite source close to the mantle-crust transition zone as that have been suggested by ref. [72]. It is thus possible that the wehrlite represents the uprising part of the high-temperature mantle peridotites, which have been metasomatized by magma fluids during upwelling (see below).

## 5.2 Melt-rock-fluid interactions: the wehrlite and carbonatite paradigm

Petrographic studies point out the presence of discrete orthopyroxene crystals and symplectite microtexture that are characteristic features of peridotites of mantle origin [73].

It is pointed out [74] that orthopyroxene-poor peridotites result, in general, from reactions with small volumes of carbonate/carbonatite melt, accompanying CO₂ degassing, according to the reaction: enstatite + dolomite (melt) → forsterite + diopside + CO₂ (vapour) (“wehrlitisation”).

Fluids-rock interaction might have affected the lithospheric mantle beneath the Adamawa plateau as have been suggested [58]. Fluids might have circulated during or after tectonic recrystallization along Pan-African stick-slip-faults at high temperatures. The nature of the circulating fluids might be carbonatitic, as proposed by ref. [58] for Youkou sub-continental mantle. Pockets of
carbonate were identified in basalts, north and east of Ngaoundéré [61].

Later, re-equilibrations occurred at low depths (ca. 7 km) in the upper crust, following volcanic eruptions, with cooling from 900°C down to 750°C.

Numerous lukewarm (<22°C) springs are described north of Ngaoundéré [12]. Carbonated hot springs and geysers occur at 200–300 km north of Ngaoundéré, close to Tchabal Mbabo volcano.

## 6 Conclusion

Wehrlites from Hosséré Do Guessa volcano in Adamawa plateau have a mantle origin. They suffered movements of Pan-African strict-slip-faults during Adamawa upwelling at Tertiary times, after the solid state tectonic relaxation. Hosséré Do Guessa wehrlites result from reactions with carbonate/carbonatite melt, accompanying CO₂ degassing and then a metasomatic event after fluids phase circulation at depth. They have crystallized at 1.5–2.0 GPa, which corresponds to a depth of 50–66 km, at a temperature of 1,140–1,220°C, and then been equilibrated until 900–750°C, between 0.9 and 0.6 (perhaps until 0.2) GPa at shallow depth.

**Acknowledgments:** Authors greatly thank the “Agence Universitaire de la Francophonie (AUF)” through the BAGL (Bureau Afrique Centrale et des Grands Lacs), for financial support of “Le Projet de soutien aux équipes de recherche 2012/2013_No 51110SU201” in all aspect (from field works to laboratory analyses). The contribution of “Department of Earth Sciences, UMR CNRS 8148 GEOPS” of the University Paris-Saclay, France, is greatly appreciated. B. Bonin and A. Pouclet are warmly thanked for useful remarks. Fruitful remarks by F. Casetta and an anonymous reviewer have greatly helped us to improve the manuscript.

**Author contributions:** OFN, ZNNN, AFM and AH made field studies, JMB made electron microprobe analyses and OFN and JMB prepared the article.

**Conflict of interest:** Authors state no conflict of interest.

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