Ti-bearing aenigmatite from Djinga Tadorgal (Adamawa plateau) and Sao Tomé (Cameroon Line) phonolites: geochemical implications and application of the QUILF thermobarometer for the crystallization conditions

The chemical and structural compositions of aenigmatite from Djinga Tadorgal and Sao Tomé phonolites were compared to those previously described in the felsic lavas and nephelinite from the Adamawa Plateau and Cameroon Line, in order to determine their crystallization temperatures and pressures. Chemical analysis results and the application of QUILF thermobarometer, indicated the equilibrium temperatures between 655 °C and 791 °C, oxygen fugacity (fO2) below the FMQ buffer, pressures near 0.1 GPa and silica activities less than 1.0 for Djinga Tadorgal and Sao Tomé phonolites. The Ti-bearing aenigmatite from Djinga Tadorgal and Sao Tomé phonolites are a late and accessory mineral, resulting from the reaction between Tirich magnetite and Na2O-rich magmatic liquid.Keywords: Peralkaline lavas, mineral chemistry, silica activity, thermodynamic condition

A Typical Sublithospheric Mantle beneath the Tello Region, SE-Ngaoundéré (Cameroon Line)

Peridotite xenoliths, raised to the surface by alkaline basalts or kimberlites,provide us direct information on the processes and composition of the upper mantle. They are the major source of information on the state of stress,pressure and temperature in the deep mantle. They are thus a source of petrological and geochemical information that is generally not available on the Earth’s surface. Fresh spinel-lherzolite xenoliths exhibit a protogranular components of the Tello volcano. The Tello is the continental sector of the Cameroon Line, located in the South East of the town of Ngaoundéré at 75 km approximately between (N7° 13’, N7° 14’) and (E13° 40’ and E13° 60’). Minerals’ composition of the xenoliths is ~64% olivine, ~24% orthopyroxene, ~11% clinopyroxene and ~1% spinel. Significant variation in (Cr/Cr + Al) of the system shows the reciprocal nature of the spinel solution.The Tello spinel lherzolites show internal chemical homogeneity and represent a normal upper mantle. Their mineral chemistries suggest equilibrium condition of 830° - 925° and 1.4 GPa-2.3 GPa. These data suggest that there is good correlation bracket between increasing activity of Al2O3 and decreasing of practionning of TiO2 into spinel. The AlIV and AlVI contents vary by 0.05-0.2 and 0.03-0.2 respectively. The majority of samples caracterise the lithospheric mantle

Peralkaline Felsic Magmatism of the Atlantic Islands

The oceanic-island magmatic systems of the Atlantic Ocean exhibit significant diversity in their respective sizes, ages, and the compositional ranges of their eruptive products. Nevertheless, almost all of the Atlantic islands and island groups have produced peralkaline felsic magmas, implying that similar petrogenetic regimes may be operating throughout the Atlantic Ocean, and arguably elsewhere. The origins of peralkaline magmas are frequently linked to low-degree partial melting of enriched mantle, followed by protracted differentiation in the shallow crust. However, additional petrogenetic processes such as magma mixing, crustal melting, and contamination have been identified at numerous peralkaline centers. The onset of peralkalinity leads to magma viscosities lower than those typical for metaluminous felsic magmas, which has profound implications for processes such as crystal settling. This study represents a compilation of published and original data which demonstrates trends that suggest that the peralkaline magmas of the Atlantic Ocean islands are generated primarily via extended (up to ∼ 95%), open system fractional crystallization of mantle-derived mafic magmas. Crustal assimilation is likely to become more significant as the system matures and fusible material accumulates in the crust. Magma mixing may occur between various compositional end-members and may be recognized via hybridized intermediate magmas. The peralkaline magmas are hydrous, and frequently zoned in composition, temperature, and/or water content. They are typically stored in shallow crustal magma reservoirs (∼ 2–5 km), maintained by mafic replenishment. Low melt viscosities (1 × 101.77 to 1 × 104.77 Pa s) facilitate two-phase flow, promoting the formation of alkali-feldspar crystal mush. This mush may then contribute melt to an overlying melt lens via filter pressing or partial melting. We utilize a three-stage model to account for the establishment, development, and termination of peralkaline magmatism in the ocean island magmatic systems of the Atlantic. We suggest that the overall control on peralkaline magmatism in the Atlantic is magma flux rate, which controls the stability of upper crustal magma reservoirs. The abundance of peralkaline magmas in the Atlantic suggests that their development must be a common, but not inevitable, stage in the evolution of ocean islands

Pétrologie du massif de Djinga Tadorgal (Adamaoua, Cameroun) (comparaison avec le volcanisme des îles de São Tomé et Principe et du lac Tchad ("Ligne Chaude du Cameroun"))

Djinga Tadorgal est un volcan de l Adamaoua et les volcans de São Tomé et Príncipe et du lac Tchad appartiennent à la "Ligne Chaude du Cameroun" (LChC). Les laves sont alcalines. Leur structure est microlitique porphyrique.Les teneurs en TiO2, Fe2O3*, MgO, CaO diminuent avec SiO2 croissante, celles en Al2O3, Na2O, K2O et P2O5 augmentent puis diminuent. Les teneurs en Rb, Sr, Ba croissent puis décroissent, celles en éléments de transition décroissent, celles en Zr, Nb, Th, Y croissent. Les spectres normalisés de terres rares des laves felsiques ont une forme en cuillère. Les rapports isotopiques du Sr, Nd et Pb sont proches de ceux des laves de la LChC. La série a évolué par cristallisation fractionnée. Les benmoréites résultent d'un mélange entre magmas basaltique et trachytique. Les magmas proviennent de la fusion partielle d'un manteau HIMU. La contamination crustale a affecté des laves felsiques. Les valeurs des rapports isotopiques du Pb sont de type FOZO comme défini pour la LChC.PARIS-BIUSJ-Physique recherche (751052113) / SudocSudocFranceF

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

International audienceSpinel-bearing lherzolite xenoliths from the Hosséré Garba (1272 m.a.s.l) volcano on the Adama-wa Plateau, is located in Likok village, at about 35 km to WSW of Ngaoundéré. These xenoliths (~11 cm size) have been sampled into the host basaltic lava flows from the NE flank of the Hosséré 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 Al VI /Al IV ratio range between 1.1 and 1.3 for the Cr-diopside crystals from Hosséré Garba xenoliths. The constant value of the volumes V(Cell) and V(M1) for clinopyroxene compositions, indicates the similar pressures. Hosséré Garba represents a residual sequence is-sued from partial melting of a mantle source. Similar compositions have been recorded in miner-als of ultramafic xenoliths from other ultramafic xenoliths domains of the Cameroon Line and the Adamawa Plateau

Petrology of the Late Cretaceous peralkaline rhyolites (pantellerite and comendite) from Lake Chad, Central Africa

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Geochemistry of the volcanic rocks from Bioko Island (“Cameroon Hot Line”): Evidence for plume-lithosphere interaction

Bioko Island (3008 m a.s.l) is located in the presently more active volcanic zone of the Cameroon Line and composed essentially of alkaline basalts and hawaiites, and lesser mugearites. The rocks show microlitic porphyritic texture with phenocrysts of olivine (83% < Fo < 87%) and clinopyroxene in a matrix of plagioclase, clinopyroxene and oxides. Hawaiites and mugearites also include phenocrysts of plagioclase (An62-67Ab35-32Or3-1). Major element variation diagrams show an increase in SiO2, Al2O3, Na2O and K2O with increasing MgO for the studied rock groups. The rocks are characterized by low (86Sr/87Sr)i ratios (0.70320–0.70406), high ɛNd(t) values (2.56–4.33) and high (206Pb/204Pb)i ratios (20.032–20.035) values. Basalts are enriched in LILE and LREE, and have (Hf/Sm)N = 0.57–1.16. These geochemical signatures are similar to those of the Mount Cameroon rocks, and might be attributed to low degrees of partial melting from a garnet-amphibole-bearing mantle source. The trace elements and isotopic compositions suggest that the parental magma source might have involved HIMU- and EM1-components.SCOPUS: ar.jinfo:eu-repo/semantics/publishe

Taking advantage of iron contained in natural volcanic ash for catalytic degradation of Rhodamine 6G

Using Fe-containing silica-alumina materials as heterogeneous catalyst for Fenton-like degradation of organic pollutants has attracted a lot of attention during the last decades. In this work the catalytic activity of an iron-rich natural silica-alumina material (namely volcanic ash) for the abatement of Rhodamine 6G (Rh6G) has been evaluated. The X-ray diffraction (XRD) and Scanning electron microscopy coupled with energy dispersive spectroscopy (SEM-EDS) results showed that such material contains about 13 wt% of iron mainly present in augite and ferroan forsterite crystalline phases. Such iron content was sufficient to activate hydrogen peroxide via Fenton process. Indeed, 78% of Rh6G was degraded after 120 min of reaction in the presence of volcanic ash ([VA] = 5 g/L; [Rh6G] = 25 mg/L and initial pH = 3). The dye abatement efficiency increased with decrease of initial dye concentration, and with increase of catalyst dosage. However, the process was efficient only in acid medium (pH = 2.5 – 3) in which the leaching of the catalyst could not be avoided. As a consequence, a drastic decrease in abatement efficiency was observed during the recyclability test. This suggests that the studied volcanic ash can be used as low cost material for wastewater treatment. However it needs to be pre-treated for efficient use as heterogeneous catalyst

Mineralogy and geochemistry of pozzolans from the Tombel Plain, Bamileke Plateau, and Noun Plain monogenetic volcanoes in the central part of the Cameroon Volcanic Line

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