Age and Sr-Nd-Hf isotopes of the sub-continental lithospheric mantle beneath the Cameroon Volcanic Line: Constraints from the Nyos mantle xenoliths

    Age and Sr-Nd-Hf isotopes of the sub-continental lithospheric mantle beneath the Cameroon Volcanic Line: Constraints from the Nyos mantle xenoliths Chuan-Zhou Liu, Liu-Yang Yang, Xian-Hua Li, Jean Pierre Tchouankoue PII: DOI: Reference:

S0009-2541(16)30364-3 doi: 10.1016/j.chemgeo.2016.07.022 CHEMGE 18011

To appear in:

Chemical Geology

Received date: Revised date: Accepted date:

31 March 2016 20 July 2016 26 July 2016

Please cite this article as: Liu, Chuan-Zhou, Yang, Liu-Yang, Li, Xian-Hua, Tchouankoue, Jean Pierre, Age and Sr-Nd-Hf isotopes of the sub-continental lithospheric mantle beneath the Cameroon Volcanic Line: Constraints from the Nyos mantle xenoliths, Chemical Geology (2016), doi: 10.1016/j.chemgeo.2016.07.022

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Age and Sr-Nd-Hf isotopes of the sub-continental lithospheric mantle beneath the

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Cameroon Volcanic Line: constraints from the Nyos mantle xenoliths

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Chuan-Zhou Liu1,2, Liu-Yang, Yang1, Xian-Hua Li1, Jean Pierre Tchouankoue3

1. State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics,

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Chinese Academy of Sciences, Beijing, 100029, China

2. CAS Center for Excellence in Tibetan Plateau Earth Sciences, Beijing 100101, China

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3. Department of Earth Sciences, University of Yaounde I, P.O. Box 812, Yaounde,

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Cameroon

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Corresponding author: Dr. Chuan-Zhou Liu

Tel: 0086-10-8299 8044

Fax: 0086-10-6201 0846

Email: [email protected]

(Submission for Chemical Geology)

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Abstract

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To better constrain the age and composition of the sub-continental lithospheric mantle (SCLM) beneath the Cameroon Volcanic Line (CVL), we present Sr-Nd-Hf-Os

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isotopes of the Nyos mantle xenoliths (nine lherzolites and four harzburgites). The Nyos lherzolites have fertile compositions with 37.69-41.12 wt% MgO and 2.49-4.38 wt%

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Al2O3, whereas the harzburgites are more refractory containing 43.2-45.42 wt% MgO and 1.43-2.1 wt% Al2O3. The MgO contents of the Nyos xenoliths overall show negative

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correlations with CaO, Al2O3, Na2O and TiO2 contents, indicating they represent mantle residues after melt depletion. Modal minerals like pargasitic amphibole have only been

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discovered in one Nyos lherzolite, suggesting modal metasomatism locally occurred. Clinopyroxenes in most Nyos xenoliths show enriched trace element compositions,

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indicating pervasive cryptic metasomatism within the SCLM. Clinopyroxenes in the Nyos lherzolites have less radiogenic Sr and more radiogenic Nd isotopes than the Nyos

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harzburgites. Clinopyroxene Hf isotopes of the harzburgites are comparable to those of the lherzolites. Although the CVL basalts have similar Sr-Nd isotopes with the Nyos harzburgites, they have different Hf isotopes. Therefore, the CVL basalts could not be sourced solely from the lithospheric mantle. The Nyos lherzolites show consistent HSE patterns, with slight depletion in both Re and Pt over Pd. The harzburgites are remarkably depleted in Pt, Pd and Re relative to Os, Ir and Ru. The Nyos lherzolites have variable 187

Os/188Os ratios ranging from 0.11700 to 0.13129, yielding Os model ages (TMA)

relative to the primitive upper mantle (PUM) of 0.27-2.18 Ga and Re depletion ages (TRD) of 0.12-1.76 Ga. In comparison, four harzburgites have more unradiogenic 187Os/188Os ratios of 0.11485-0.11705. Three of the four harzburgites with low Re/Os ratios yield similar TMA and TRD ages, i.e., 1.91-2.43 Ga vs 1.87-2.05 Ga, respectively. The 187

Os/188Os ratios of the Nyos xenoliths show a good correlation with bulk Al2O3 contents.

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An initial 187Os/188Os ratio of 0.11064 can be inferred from the 187Os/188Os-Al2O3

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correlation, giving an Archean TRD age of ~ 2.6 Ga. This suggests that the Nyos mantle

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xenoliths have been subjected to melt depletion events during the Paleoproterozoic convergence of the Congo and West Africa cratons, which formed the lithospheric mantle

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intruded by the CVL. The SCLM beneath the CVL is coeval with or slightly younger than the charnockite and tonalite-trondhjemite- granodiorite in the Ntem Complex of NW

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Congo Craton. We proposed that the old SCLM was a part of the Congo cratonic mantle. Therefore, our results suggest that old cratonic mantle beneath the CVL has not been

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completely removed during the Pan-African Orogeny, although the overlying crust might

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have been strongly deformed and reworked.

Keywords: Mantle xenoliths; Sr-Nd-Hf isotopes; Re-Os isotopes; Highly siderophile

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elements (HSE); Nyos; Cameroon Volcanic Line (CVL).

1. Introduction

The Cameroon Volcanic Line (CVL) comprise a genetically related series of Cenozoic volcanic and sub-volcanic complexes that straddle the continent-ocean boundary and extend for 1600 km from the Gulf of Guinea (i.e., Annobon or Pagalu Island) to the interior of the African continent (Marzoli et al., 2000; Rankenburg et al., 2005). Volcanic activity from 42 Ma to present ranges from basaltic to more evolved phonolitic to trachytic compositions (Njome and de Wit, 2014). The origin of the CVL basalts has been a subject of continuing controversy, and current debate concerns whether the CVL basalts were primarily derived from the asthenospheric mantle, and contaminated by lithospheric mantle, or if they originated from variably 'metasomatized' lithospheric mantle (Aka et al., 2004; Ballentine et al., 1997; Fitton and Dunlop, 1985; 3

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Gannoun et al., 2015; Halliday et al., 1990; Halliday et al., 1988; Marzoli et al., 2000;

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Rankenburg et al., 2004; Rankenburg et al., 2005). Fitton and Dunlop (1985) showed that

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basaltic rocks in the oceanic and continental sectors of the CVL are geochemically and isotopically (e.g., 87Sr/86Sr) similar, and concluded that the CVL basalts were derived

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from the asthenospheric mantle whereas the lithospheric mantle was not significantly involved. However, such a simple picture was challenged by later studies. Halliday et al.

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(1990) reported a distinctive HIMU Pb isotope signature for basalts from the continent-ocean boundary of the CVL and explained the signature by remelting of

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variably metasomatized lithospheric mantle rather than reflecting primary asthenospheric source heterogeneity. Since then, more and more studies have demonstrated the compositional difference in the CVL basalts from the oceanic and continental sectors, and

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invoked the involvement of enriched continental mantle in the formation of the CVL basalts (Aka et al., 2004; Ballentine et al., 1997; Marzoli et al., 2000; Rankenburg et al.,

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2004; Rankenburg et al., 2005). For example, it has been shown that basanites and alkali

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basalts from the Western Highlands (15-4 Ma) have anomalously high concentrations of Sr, Ba and P, and low contents of Zr, which has been explained as derivation from an incompatible element enriched, amphibole-bearing lithospheric mantle (Marzoli et al., 2000).

Existence of enriched lithospheric mantle beneath the CVL is supported by studies on mantle xenoliths, lamprophyres and nephelinites (Déruelle et al., 2007). The occurrence of pargasitic amphibole is widely reported in mantle xenoliths entrained in the CVL basalts, e.g., Mt. Cameroon (Déruelle et al., 1998), Mt. Bambouto (Marzoli et al., 2000), Mt. Oku (Lee et al., 1996) and Nyos (Pintér et al., 2015; Temdjim, 2012). Presence of fluid inclusions and interstitial carbonates in mantle xenoliths from São Tomé suggests metasomatism by carbonatitic melts (Caldeira and Munhá, 2002). Occurrence of carbonate pockets or interstitial carbonates in nephelinites from Mt. Etinde supports that

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they were derived from a metasomatic mantle source (Nkoumbou et al., 1995).

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Lamprophyres or camptonites discovered in Kokoumi and Tchircotché volcanoes were

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explained to stem from volatile-rich lithospheric mantle (Ngounouno et al., 2005; Ngounouno et al., 2001). Nevertheless, geochemical characteristics, in particular isotopic

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compositions of the enriched lithospheric mantle have not been well constrained by using mantle peridotites. This impedes the evaluation of the role of the metasomatized

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lithospheric mantle that played in the genesis of the CVL basalts. Furthermore, it remains unknown when the sub-continental lithospheric mantle beneath the continental sector of

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the CVL was formed, although studies on mantle xenoliths suggested the existence of Neoproterozoic sub-continental lithospheric mantle (SCLM) beneath the CVL (Lee et al.,

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1996).

Mantle xenoliths entrained in basalts or kimberlites can provide direct constraints on

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the compositional characteristics of the SCLM and important information on deep processes occurred in the mantle. In this study, we present comprehensive geochemistry,

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including major and trace elements, Sr-Nd-Hf isotopes, highly siderophile elements (HSE) and Re-Os isotopes, of mantle xenoliths entrained within the Cenozoic basalts from Lake Nyos, Mt. Oku. These data are used to decipher the partial melting and metasomatic histories experienced by the Nyos xenoliths, and evaluate the role of the enriched lithospheric mantle in the genesis of the CVL basalts. Both HSE and Re-Os isotopes are used to constrain the formation age of the lithospheric mantle beneath the CVL.

2. Geological setting and sample description 2.1. Geological setting A chain of Tertiary to Quaternary alkaline volcanoes, plutons and grabens extend over more than 1600 km in the western part of Cameroon (Njome and de Wit, 2014), 5

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which is usually referred to as the Cameroon Volcanic Line (CVL; Fig. 1a). The CVL can

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be divided into three zones: the oceanic sector including three islands of Annobon,

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Principe and São Tomé, the continent/ocean boundary including two volcanoes of Bioko and Mt. Cameroon, and the continental sector including Manengouba, Bamboutou, Oku,

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Ngaoundéré Plateau and Biu Plateau. The volcanic rocks are dominantly alkaline, ranging from transitional basalt to nephelinite and alkali rhyolite to phonolite (Fitton, 1987).

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Magmatic activity in the CVL extends over the whole Cenozoic but shows no systematic migration with time as might be expected for plume track migration (Déruelle et al.,

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2007), which was proposed to account for a mechanism of the CVL (Halliday et al., 1990).

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Mantle xenoliths selected in this study were collected from Lake Nyos, which belongs to the Oku volcanic field at the junction of the CVL with extension of the

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Adamawa volcanic massif. Lake Nyos occupies a crater of a phreatomagmatic origin with sub-vertical walls in a Precambrian basement that is made up mostly of highly deformed

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monzonitic granites (Fig. 1b). It consists of two well-preserved strombolian assemblages (Temdjim, 2012). The northeastern eruptive group close to Lake Njupi is formed by coalescence of three spatter-cones. The southern volcano is a cinder cone situated two kilometers south of the maar, which is capped by a horseshoe-shaped crater. The major volcanic period in the Nyos area is mainly the Quaternary and K-Ar dating of basalts yielded ages of 1.1-3.5 Ma (Freeth and Rex, 2000). 2.2. Sample description Thirteen spinel-facies peridotites, including nine lherzolites and four harzburgites, have been selected in this study. They are in a size of 5-10 cm and were collected from basaltic lavas and pyroclasts at the northern flank of Lake Nyos. All xenoliths are solely composed of spinel-facies peridotites. Most of the Nyos xenoliths display a protogranular

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texture, and two samples (NY03 and NY05) show a porphyroclastic texture. Both olivine

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and pyroxene crystals in the protogranular-textured samples show a curved and locally

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polygonal shape (Fig. 2a). Occasionally, they are completely recrystallized and display a triple junction approaching 120o (Fig. 2b). In the porphyroclastic samples, recrystallized

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olivine neoblasts at the border of both olivine and orthopyroxene porphyloclasts show undulose extinctions and triple junctions (Fig. 2c). In contrast, large olivine

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porphyroclasts do not show any apparent fabric. Clinopyroxenes are locally rimed by a reaction zone, in which secondary clinopyroxenes and orthopyroxenes are occurred (Fig.

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3a). More commonly, clinopyroxenes show a spongy texture (Fig. 3b). Exsolution lamellae are well developed in both orthopyroxenes (Fig. 2c & 3c) and clinopyroxenes (Fig. 2d), but commonly occur at their center. Melt inclusions are distributed along with

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the exsolution lamellae in clinopyroxenes (Fig. 2d), or randomly distributed at the rim of clinopyroxenes. In harzburgites, tiny clinopyroxenes are interstitial between olivine and

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orthopyroxenes (Fig. 2e, f). Brownish spinel is ubiquitous in Nyos xenoliths and

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commonly occurs as vermicular (Fig. 2a) or amoeboid (Fig. 2g) grains. In sample NY14, spinel is associated with brown amphibole (Fig. 2h). Alternatively, brown amphibole is also surrounded by a reaction zone that contains secondary clinopyroxenes (Fig. 3d).

3. Analytical methods Fresh Nyos xenoliths were cut from the host lavas and cleaned in ultra-pure (Milli-Q) water; they were then crushed to powders with sizes of ca 200 mesh. All analyses in this study were conducted at the Institute of Geology and Geophysics, Chinese Academy of Sciences (IGGCAS) at Beijing. 3.1. Major and trace elements Major element compositions of whole rocks were determined by XRF on fused glass 7

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disks, with an analytical uncertainty ranging from 1% to 3%. Mineral major elements

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were obtained on a JEOL JXA-8100 Electron Probe, using an accelerating voltage of 15

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keV and a beam current of 10 nA. Trace elements of both clinopyroxene and amphibole were analyzed using a laser ablation inductively coupled plasma mass spectrometer

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(LA-ICP-MS), which consists of a Analyte G2 193 nm ArF excimer laser coupled to an Agilent 7500a ICP-MS. Isotopes were measured in a peak-hopping mode. A spot size of

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80 µm and a repetition rate of 8 Hz were used. The NIST 612 glass standard was used as an external calibration standard and the isotope 43Ca was used as an internal standard.

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3.2. Clinopyroxene Sr-Nd-Hf isotopes

Prior to dissolution in HF-HNO3, clinopyroxene was handpicked and washed with

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ultra-pure (Milli-Q) water and subsequently leached by 6 M HCl at 100 oC overnight. Clinopyroxene Sr-Nd-Hf isotopes have been analyzed using the method previously

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described (Yang et al., 2010). About 150-300 mg of the leached clinopyroxene, together with the spike mixture of 87Rb-84Sr, 149Sm-150Nd and 176Lu-180Hf, were digested in a

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mixed acid (2 ml HF+1 ml HNO3+0.2 ml HClO4) on a hot plate at 150oC for more than one week. After dissolution and removal of all remaining flourides with HCl acid, sample solution was loaded onto the pre-conditioned 2 ml Ln Spec resin. Matrix elements including LREE and MREE were sequentially stripped with 3 M and 4 M HCl. Then the Lu fraction was eluted with 4 M HCl. Afterwards, the column was rinsed with 6 M HCl to effectively remove Lu and Yb residues, and with 4 M HCl +0.5% H2O2 mixture to strip Ti. Finally, Hf was extracted from the column with 5 ml 2 M HF. Sample solution consisting of matrix elements was loaded onto the pre-conditioned 2 ml AG50W-X12 resin. Rb was eluted with 1.5 ml 5 M HCl and Sr was eluted with 4 ml 5 M HCl. Separation of Sm and Nd was achieved using another Ln Spec resin column. Nd was stripped with 6 ml 0.25 HCl and Sm was stripped with 10 ml 0.4 M HCl.

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Both Rb-Sr and Sm-Nd isotopes were determined on the Finnigan MAT 262 thermal

Sr/86Sr and 143Nd/144Nd ratios were corrected for mass fractionation using 86Sr/88Sr of

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ionization mass spectrometer (TIMS) in static model using Faraday cups. Measured

0.1194 and 146Nd/144Nd of 0.7219, respectively. During data collection of this study, the

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measured values for the NBS-987 Sr standard and the JNdi-1 Nd standard were Sr/86Sr=0.710227±16 (2σ, n=9) and 143Nd/144Nd=0.512128±10 (2σ, n=12), respectively.

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Both Lu and Hf isotopes were measured on the Thermo Fisher Scientific Neptune multi-collector ICP-MS (MC-ICPMS). The mass bias behavior of Lu was assumed to

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follow that of Yb and calculated by 172Yb/173Yb of 1.35272 and 176Yb/172Yb of 0.5887. Hafnium isotopic ratios were normalized to 179Hf/177Hf of 0.7325, using the exponential law to correct instrumental mass bias. Eight analyses of Alfa Hf in-house standard gave

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an average 176Hf/177Hf value of 0.282174±12 (2σ, n=8), which is identical to the reference value within uncertainty (0.282189±19; (Wu et al., 2006)). The USGS reference

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materials, i.e., BHVO-2 and BIR-1, were analyzed together with samples to monitor the

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reproducibility of the method, yielding87Sr/86Sr, 143Nd/144Nd and 176Hf/177Hf ratios of 0.703493±11 (2σ) and 0.703103±12, 0.512995±10 and 0.513122±14, 0.283098±10 and 0.283244±24, respectively, which are consistent with the recommended values (GeoREM, http://georem.mpch-mainz.gwdg.de/). The total procedural blanks were about 40 pg for Rb, 300 pg for Sr, 20 pg for Sm, 60 pg for Nd, 20 pg for Lu and 60 pg for Hf. 3.3. Highly siderophile elements (HSE) and Re-Os isotopes Whole rock Re-Os isotopes were analyzed by isotope dilution method, following the procedure previously described (Chu et al., 2009). About 2g of powder, together with Re-Os (i.e., 187Re and 190Os) and HSE (99Ru, 105Pd, 191Ir and 194Pt) isotope tracers, was digested with reverse aqua regia (i.e., 3 ml 12N HCl and 6 ml 16N HNO3) in a Carius Tube at 240 oC for ca. 48-72 hours. Osmium was extracted from the aqua regia solution by solvent extraction into CCl4 and further purified by micro-distillation (Birck et al., 9

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1997). Afterwards, Ru, Pd, Re, Ir and Pt were sequentially separated from the solution by

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anion exchange resin (AG-1×8, 100-200 mesh).

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Osmium isotopes were measured by N-TIMS on a GV Isoprobe-T instrument in a static mode using Faraday cups. To increase the ionization efficiency, Ba(OH)2 solution

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was used as an ion emitter. The measured Os isotopes were corrected for mass fractionation using the 192Os/188Os ratio of 3.0827. The Nier oxygen isotope composition

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(17O/16O=0.0003708 and 18O/16O=0.002045) has been used for oxide correction. The in-run precisions for Os isotopic measurements were better than 0.2% (2σ) for all the

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samples. Johnson-Matthey standard of UMD was used as an external standard, yielding a Os/188Os ratio of 0.11378±2 (2σ; n=5). The concentrations of other HSEs were

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measured on a Neptune MC-ICPMS in peak-jumping mode or static mode, according to their measured signal intensities. In-run precisions for 185Re/187Re, 191Ir/193Ir, 99Ru/101Ru, Pt/196Pt and 105Pd/106Pd were 0.1-0.3% (2δ). The total procedural Os blank has a

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content of 3±1 pg (n=4) and a 187Os/188Os ratio of ~ 0.15. Total procedural blanks were

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about 3 pg for Re, 7 pg for Ir, 7 pg for Ru, 4 pg for Pt and 4 pg for Pd. The blank corrections were negligible (< 1%) for Ir, Ru, Pt and Pd, but up to 3% for Re.

4. Results

4.1. Whole rock and mineral major elements Whole rock and mineral major elements are listed in Table 1 and Table S1, respectively. Compositions of the porphyroclasts rather than the neoblasts were reported for olivines in the porphyroclastic-textured samples. Only cores of both clinopyroxene and orthopyroxene have been analyzed, when they show a compositional zonation. The Nyos lherzolites have relatively fertile compositions, containing 37.69-41.12 wt% MgO and 2.49-4.38 wt% Al2O3; in comparison, the harzburgites are more refractory, with 10

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43.2-45.42 wt% MgO and 1.43-2.1 wt% Al2O3. Their MgO contents overall show

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negative correlations with CaO, Al2O3, Na2O, TiO2 and V (Fig. 4a-e), but a positive

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correlation with Ni (Fig. 4f). Three of the four harzburgites have low Al2O3 and CaO contents, and plot within the field of mantle xenoliths from the Archean Tanzania craton

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(Fig. 4a, b).

The studied Nyos xenoliths show smaller variations of major elements in minerals

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compared to the literature data previously reported for mantle xenoliths from different CVL volcanoes (Caldeira and Munhá, 2002; Lee et al., 1996; Matsukage and Oya, 2010;

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Pintér et al., 2015; Temdjim, 2012). Overall, the constituent minerals in the Nyos harzburgites have more refractory compositions relative to the lherzolites, which is

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exemplified by their high forsterite contents of olivine (Fo; Fig. 5a), low Al2O3 contents of orthopyroxene (Fig. 5b), high Cr2O3 contents of clinopyroxene (Fig. 5c) and high Cr#

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values of spinel (Fig. 5d). Amphibole with a pargasitic composition has been only discovered in one lherzolite NY14; it contains 2.32 wt% TiO2, 1.08 wt% Cr2O3, 4.01 wt%

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Na2O and has a Mg# of 0.88.

4.2. Clinopyroxene trace elements Trace elements of both clinopyroxenes and amphibole are listed in Table 2. Clinopyroxenes in the Nyos lherzolites show flat patterns from HREE to MREE but variable patterns in LREE (Fig. 6a). Their Yb contents are of 7-21 times of CI chondrites (Anders and Grevesse, 1989). Clinopyroxene in sample NY01 has the most depleted LREE and the lowest HREE contents. Clinopyroxene in three lherzolites (i.e., NY04, NY05 and NY06) show similar REE patterns and are slightly depleted in LREE. In other three lherzolites (i.e., NY02, NY10 and NY14), clinopyroxene are slightly enriched in LREE. Clinopyroxene in sample NY09 displays a spoon-shaped REE pattern, which has been previously reported for Nyos xenoliths (Lee et al., 1996; Temdjim, 2012).

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Clinopyroxenes in all lherzolites are enriched in both Th and U, and show negative

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anomalies of Nb and Ti (Fig. 6b). Amphibole in sample NY14 shows a similar REE

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pattern to the co-existing clinopyroxene. It displays a LREE-enriched pattern and positive anomalies in Ba, Sr and Ti. Such characteristics have been previously described for

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amphiboles discovered in some CVL xenoliths (Lee et al., 1996).

Clinopyroxenes in the Nyos harzburgites show very similar patterns of both REE

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(Fig. 6c) and trace elements (Fig. 6d). They are strongly enriched in LREE and display flat patterns of MREE and HREE (Fig. 6c), with Yb contents of 4-9 times of CI

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chondrites. They show positive anomalies of Th and U (Fig. 6d), and negative anomalies of high field strength elements (HFSE; Zr, Hf, Nb, Ta and Ti).

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4.3. Clinopyroxene Sr-Nd-Hf isotopes

Clinopyroxenes from eight Nyos xenoliths (including six lherzolites and two

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harzburgites) have been analyzed for Sr-Nd-Hf isotopes and the data are shown in Table 3.

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Replicate analysis was conducted for sample NY02, and Sr-Nd-Hf isotopes identical within errors were reproduced. Overall, clinopyroxenes of the Nyos xenoliths have depleted Sr-Nd-Hf isotope compositions (Fig. 7a, b). They contain 0.05 -0.27 ppm Rb and 33-179 ppm Sr, and yield very low 87Rb/86Sr ratios of 0.004-0.01, resulting in a negligible correction of radioactive growth on the measured 87Sr/86Sr values (i.e., 0.701743-0.703736). They contain 0.45-1.84 ppm Sm and 1.17-11.52 ppm Nd, giving 147

Sm/144Nd ratios of 0.088-0.264. Their initial 143Nd/144Nd ratios vary from 0.512921 to

0.513666, and εNd(t) values from +5.6 to +19.1. In the Sr-Nd diagram (Fig. 7a), half of the analyzed samples with high 143Nd/144Nd values plot outside the field of the Atlantic MORB. Clinopyroxenes in all samples but one (NY08) have 143Nd/144Nd ratios more readiogenic than those of the CVL basalts. Clinopyroxenes contain 0.12-0.3 ppm Lu and 0.28-1.46 ppm Hf, yielding 176Lu/177Hf ratios of 0.02-0.087. Their initial 176Hf/177Hf

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ratios vary from 0.283224 to 0.285475, and εHf(t) values vary from +16.1 to +95.7. They

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have more depleted Hf isotopes than the CVL basalts, and most samples plot outside the

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field of oceanic basalts (Fig. 7b). Clinopyroxene of the lherzolite NY01 with high Nd/144Nd and 176Hf/177Hf ratios significantly deviates from the mantle array (Vervoort

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and Blichert-Toft, 1999). 4.4. HSE and Re-Os isotopes

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All studied Nyos xenoliths have been analyzed for HSE and Re-Os isotopes, and the

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data are listed in Table 4. All samples have consistent IPGE (i.e., Os, Ir and Ru) patterns (Fig. 8a), with (Os/Ir)N and (Ru/Ir)N (N: chondrite-normalized; Horan et al., 2003) ratios of 0.58-1.35 and 1.19-1.78, respectively. Most lherzolites also have consistent PPGE (i.e.,

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Pt, Pd and Re) patterns, with slight depletion in Pt and moderate depletion in Re. In contrast, the harzburgites and one lherzolite display strong depletion of PPGE relative to

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respectively.

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IPGE, with (Pt/Ir)N, (Pd/Ir)N and (Re/Ir)N ratios of 0.29-0.7, 0.24-0.54 and 0.03-0.34,

The Nyos xenoliths contain 0.8-4.97 ppb Os and 0.01-0.16 ppb Re, yielding 187

Re/188Os ratios of 0.01-0.3. All Nyos lherzolites have 187Os/188Os ratios varying from

0.11700 to 0.13129. In comparison, four harzburgites have relatively lower 187Os/188Os ratios of 0.11485-0.11705. Osmium model ages (TMA) of the lherzolites calculated relative to the primitive upper mantle (PUM; (Meisel et al., 2001)) vary from 0.27 Ga to 2.18 Ga, which are commonly older than their Re depletion ages (TRD; 0.12-1.76 Ga). Three of the four harzburgites have similar TMA and TRD ages, i.e., 1.91-2.43 Ga vs 1.87-2.05 Ga, respectively, as they have very low Re/Os ratios. However, the harzburgite NY07 has a TMA age of 3.24 Ga, which is much higher than its TRD age (i.e., 1.75 Ga). The 187Os/188Os ratios of the Nyos xenoliths show a rough positive correlation with their 187

Re/188Os ratios (Fig. 8b), and a good correlation with the bulk Al2O3 contents (Fig. 8c).

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5. Discussion

5.1. Partial melting and metasomatism of Nyos mantle xenoliths

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Mantle xenoliths are commonly thought to be derived from the sub-continental lithospheric mantle (SCLM), and thus their chemical compositions can provide valuable

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information on deep processes occurred within the SCLM. The Nyos xenoliths have variable mineral modes that range from fertile lherzolites to refractory harzburgites.

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Variation in mode contents is accompanied by changes in both whole rock and mineral compositions. Linear trends shown by MgO with both Al2O3 and CaO (Fig. 4a, b) can be explained by removal of basaltic magmas from the primitive mantle, which have been

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observed for xenoliths from worldwide occurrences (Canil, 2004). Melt depletion can also account for the positive MgO-Ni correlation (Fig. 4e) and the negative MgO-V

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correlation (Fig. 4f). Degree of partial melting experienced by mantle peridotites can be

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constrained using co-variation plots of modal abundances versus major elements (e.g., Al and Mg), in comparison with melting experiments (Herzberg, 2004). Comparison with the modeling results of mantle melting (Niu, 1997) indicates that the Nyos xenoliths might have experienced less than 10% degrees of polybaric batch melting from 25 kbar to 10 kbar, whereas the harzburgites have been subjected to more extensive (i.e., 10-25%) melting (Fig. 4a, b). Degree of partial melting subjected by mantle peridotites can be estimated by spinel Cr#, which is generally accepted as a sensitive indicator for the extent of melting (Dick and Bullen, 1984; Hellebrand et al., 2001). Using the function that was established between the extent of melting (F) and spinel C# (Hellebrand et al., 2001), i.e., F=10×ln(Cr#)+24, it can be estimated that the Nyos lherzolites have been subjected to 1-7% degrees of fractional melting of a depleted mantle (DM) starting source and the

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Nyos harzburgites have experienced 8-14% fractional melting. Besides spinel Cr#, it has

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also been suggested that moderately incompatible HREE contents of clinopyroxene can

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place quantitative constraints on degree of partial melting subjected by mantle peridotites. Commonly used models are for mantle melting within the spinel stability field (Norman,

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1998), in which both Y and Yb contents of clinpyroxenes are compared with the modeled fractional melting trend. Most Nyos lherzolites plot following the melting trend and have

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been subjected to < 7% degrees of fractional partial melting (Fig. 9), consistent with degrees of melting given by spinel Cr#. In comparison, the Nyos harzburgites have been

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subjected to relatively higher degrees (i.e., ~ 5-12%) of partial melting. It should be noted that sample NY02 deviates from the melting trend, probably due to secondary

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metasomatism.

Although the variations in major elements of the Nyos mantle xenoliths might be

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ascribed to melt extraction, the incompatible trace elements and mineralogical textures of the Nyos xenoliths indicate the occurrence of metasomatic enrichments after melt

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depletion. Clinopyroxene in some Nyos xenoliths contains silicate melt inclusions that are distributed in parallel with the exsolution lamellae (Fig. 2d). Fluid inclusions have also been observed at the rim of clinopyroxene (Fig. 2d). This indicates the infiltration of hydrous silicate melts within the lithospheric mantle. Reaction zone surrounding clinopyroxene supports the interaction between the infiltrated melts and the peridotites (Fig. 3a). Melt-peridotite interaction can account for the spongy texture of clinopyroxene in some Nyos xenoliths (Shaw et al., 2006). Finally, presence of pargasitic amphibole in one Nyos lherzolite (i.e., NY14) supports the local occurrence of modal metasomatism within the SCLM. Late-stage metasomatic enrichment is also supported by clinopyroxene trace elements. Among the predominant mantle minerals, clinopyroxnene is commonly considered as the major host of the incompatible trace elements (Bedini and Bodinier, 1999). 15

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Nevertheless, it has been suggested that orthopyroxene can host significant amount of

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trace elements, in particular HFSE (Witt-Eickschen and O'Neill, 2005). Clinopyroxenes

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in most Nyos xenoliths display LREE-enriched patterns (Fig. 6a, c), and show enrichment in LILE but depletion in HFSE (Fig. 6b, d). Depletion of HFSE relative to LILE is not

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uncommon in metasomatized mantle peridotites (Byerly and Lassiter, 2015). Depletion of HFSE in clinopyroxene of mantle peridotites might be due to equilibrium partitioning

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into orthopyroxene, as orthopyroxene has higher partition coefficients for HFSE relative to many LILE (Witt-Eickschen and O'Neill, 2005). Therefore, orthopyroxenes can be an

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important host for HFSE in harzburgites, which have high modal orthopyroxene/clinopyroxene ratios. Alternatively, HFSE can be concentrated in mineral grain boundaries of mantle peridotites (Bedini and Bodinier, 1999; Byerly and Lassiter,

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2015; Harvey et al., 2012), as their diffusion rates are at least an order of magnitude lower than REE diffusion rates (Van Orman et al., 2001). This might be the main reason

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for depletion of HFSE relative to LILE in clinopyroxenes of mantle peridotites that have

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been recently metasomatized. However, depletion of HFSE has been observed not only in Nyos harzburgites but also in Nyos lherzolites. Moreover, it has been suggested that the CVL mantle xenoliths have been subjected to Mesozoic or even older metasomatism. Therefore, we prefer to explain the enriched trace element characteristics of clinopyroxenes in the Nyos xenoliths as a result of melt metasomatism after melt depletion. Various metasomatic agents have been proposed to account for the observed incompatible element features and mineralogy in mantle xenoliths, including silicate melts (Zangana et al., 1999), hydrous (Downes, 2001) or CO2-rich fluids (O'Reilly and Griffin, 1988), and carbonatitic melts (Ionov et al., 1997; Rudnick et al., 1993; Yaxley et al., 1998). It has been suggested that mantle xenoliths metasomatized by carbonatite melts would be more enriched in LREE but depleted in HFSE (e.g., Ti and Zr) than those

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affected by silicate melts (Coltorti et al., 1999). In clinopyroxene La/Yb vs. Ti/Eu

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diagram (Fig. 10), clinopyroxenes in three harzburgites have high La/Yb ratios but low

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Ti/Eu ratios, plotting within the field of carbonatite metasomatism. This suggests that a portion of the SCLM beneath Nyos was metasomatized by carbonatitic melts, which

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could be the source of some CVL basalts enriched in Ba and Sr, and depleted in Zr (Marzoli et al., 2000). On the other hand, clinopyroxenes in all Nyos lherzolites and one

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harzburgite have high Ti/Eu but low La/Yb ratios, which are consistent with metasomatism by silicate melts (Fig. 10). Silicate melt metasomatism has been previously

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documented for Nyos mantle xenoliths (Pintér et al., 2015; Temdjim, 2012). Therefore, the SCLM beneath Nyos area has been affected by different kinds of metasomatic agents.

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5.2. Sr-Nd-Hf isotopes of the SCLM beneath the CVL Clinopyroxene Sr-Nd-Hf isotopes of mantle xenoliths provide a direct constraint on

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the isotope compositions of the SCLM, and play an important role in detecting whether the SCLM is involved in the genesis of continental basalts or not. Clinopyroxene in the

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studied Nyos xenoliths overall displays depleted Sr-Nd isotopes, with ranges similar to the previous data reported for the CVL mantle xenoliths (Lee et al., 1996). Lee et al. (1996) have shown that spinel lherzolites with LREE-depleted patterns are characterized by Sr-Nd isotopic compositions that are comparable with, but slightly more depleted than the Atlantic NMORB. This led to the conclusion that the unmetasomatized SCLM beneath the CVL might be isotopically similar to that of the lithsopheric mantle beneath the oceanic sector of the CVL. However, clinopyroxenes of our studied Nyos lherzolites have more depleted Nd and Hf isotopes than the CVL basalts (Ballentine et al., 1997), and the majority of them have more depleted Nd and Hf isotopes than the Atlantic MORB (Fig. 7a, b). This argues against the unmetasomatized SCLM beneath the CVL as the major mantle source of the CVL basalts.

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Clinopyroxenes in the analyzed harzburgites (i.e., NY07 and NY08) have more

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enriched Sr-Nd isotopes than those of the lherzolites. This might result from late-stage

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metasomatic processes, as clinopyroxene trace elements indicate that NY07 and NY08 have been subjected to silicate and carbonatite melt metasomatism (Fig. 10), respectively.

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Simple binary mixing modeling suggests that the Nd-Hf isotopes of the harzburgites can be explained by addition of ca 5% CVL basalts with the most enriched Nd-Hf isotopes to

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the lherzolite NY01 that has the most depleted Nd-Hf isotopes (Fig. 7b). Nevertheless, more than 30% of such a melt is required to explain the Sr-Nd isotopes of the

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harzburgites (Fig. 7a), which is unlikely from the perspective of major elements. Lee et al. (1996) reported that clinopyroxene in a harzburgite xenoliths from the Biu Plateau has Sr-Nd isotopes more enriched than the CVL basalts, indicating the existence of

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metasomatic agents more enriched in Sr-Nd isotopes than the CVL basalts. However, more than 20% of such an agent is still required to account for the enriched isotopes of

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the Nyos harzburgites. Simple binary mixing is unable to explain the enriched isotopic

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characteristics of the Nyos harzburgites. Other than simple addition of melts, melt-peridotite reaction, during which pyroxene dissolution is coupled with olivine crystallization (Dijkstra et al., 2003; Kelemen et al., 1995), should be more efficient to change the isotope compositions of peridotites (Ionov et al., 2002). Timing of metasomatism remains poorly constrained and a Mesozic enrichment was proposed according to studies on CVL mantle xenoliths and basalts (Coulon et al., 1996; Halliday et al., 1990; Lee et al., 1996; Lee et al., 1994). It has been suggested that the high 206Pb/204Pb anomaly focused at the CVL continent-ocean boundary region (e.g., Mt Cameroon) was inherited from relatively recent U/Pb fractionation at ca 125Ma during impregnation of the uppermost mantle by the St. Helena hotspot when the Equatorial Atlantic opened (Halliday et al., 1990). A better estimate of the timing of enrichment of the lithosphere underlying the Biu Plateau might be 147 Ma, based upon the earliest

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period of magmatic activity in the northern Benue Trough (Coulon et al., 1996).

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Moreover, the CVL mantle xenoliths show evidence of incompatible element enrichment,

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and yield U-Pb and Sm-Nd model ages consistent with Mesozoic metasomatism (Lee et al., 1996). On the other hand, it might also be possible that the enriched isotopic signature

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of the SCLM simply represents older lithosphere that was subjected to multiple metasomatic events rather than a single overprint in the Mesozoic (Rankenburg et al.,

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2005).

Most Nyos mantle xenoliths straddle along the mantle array, showing weak or no

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decoupling between Nd and Hf isotopes (Fig. 7a). Nevertheless, the lherzolite NY01 with decoupled Nd-Hf isotopes significantly deviates from the mantle array. The Nd-Hf

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decoupling has been widely demonstrated in peridotites from the SCLM (Doucet et al., 2015; Simon et al., 2007; Wittig et al., 2007) and oceanic mantle (Bizimis et al., 2003;

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Stracke et al., 2011). The common interpretation relates it to late-stage metasomatic overprinting in the lithospheric mantle following isolation from the convecting mantle

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(Bizimis et al., 2007; Pearson and Nowell, 2003), as melt metasomatism is more efficient in resetting the Sm-Nd isotope system than the Lu-Hf system. This is because Hf is more compatible than Nd in the melt-peridotite system, and is far more resistant to diffusional loss and metasomatism than Nd (Liu et al., 2012; Stracke et al., 2011). It should be noted that sample NY01 also has a higher 87Sr/86Sr ratio at a certain 144Nd/143Nd value, suggesting that its Sr isotopes were enriched during metasomatism. On the other hand, although two Nyos harzburgites have Sr-Nd isotopes similar to those of the CVL basalts (Fig. 7a), Hf isotopes of all Nyos xenoliths are clearly more depleted than the CVL basalts (Fig. 7b). This suggests that the continental mantle was not the major source of the CVL basalts, and other enriched components like continental crust should be invoked (Ballentine et al., 1997). 5.3. Age of the sub-continental lithospheric mantle (SCLM) beneath the CVL 19

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Both Sm-Nd and Lu-Hf isotope systems of the Nyos mantle xenoliths have been

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obviously obscured by late-stage metasomatic processes, and thus meaningful isochron or

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model ages cannot be obtained. Unlike the lithophile isotope systems (e.g., Rb-Sr and Sm-Nd), Os isotopes of mantle peridotites are more resistant to post-melting disturbance

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and able to provide robust age constraints, because of the compatibility of Os during mantle melting and high Os contents in mantle peridotites relative to metasomatic

187

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melts/fluids (Rudnick and Walker, 2009; Shirey and Walker, 1998). In the diagram of Re/188Os vs 187Os/188Os (Fig. 8b), the Nyos xenoliths are relatively scattered and an

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isochron cannot be constructed. The lherzolites show a rough positive correlation, whereas four harzburgites are horizontally distributed. This is not uncommon for mantle peridotites, as Re is relatively mobile during late magmatic processes (Becker, 2000).

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Timing of melt depletion experienced by mantle peridotites can be constrained by Re-Os model ages (TMA), in which the sample’s measured Re/Os ratio is used to calculate the

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time of its separation from the mantle reservoir (Shirey and Walker, 1998). All Nyos

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xenoliths except sample NY03 yield a big range of TMA ages varying from 0.27 to 3.24 Ga (Table 4), most of which are of Paleoproterozoic to Mesoproterozoic. The 187Os/188Os ratio of sample NY03 is slightly higher than the PUM, and thus gives a future age. The Re depletion age (TRD) is another way to constrain the melt depletion event of mantle peridotites, which is calculated with the assumption that the peridotite losts all Re at time of melt extraction. Therefore, the TRD age provides a minimum estimate of the true melt depletion age (Walker et al., 1989). Two Nyos harzburgite (i.e., NY08 and NY13) and one Nyos lherzolite (i.e., NY01) are strongly depleted in PPGE relative to IPGE (Fig. 8a), suggesting their HSE contents were not significantly affected by late-stage metasomatism, in particular by sulfide metasomatism (Alard et al., 2002; Harvey et al., 2010). The TMA ages (i.e., 1.56-2.09 Ga) of these three samples are slightly older than their TRD ages (i.e., 1.26-1.9 Ga), both of which support the occurrence of ancient melt depletion during the

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Paleoproterozoic.

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Age of a suite of peridotites can also be obtained through plotting their 187Os/188Os

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ratios against an immobile element that has a similar bulk partition coefficient to Re during peridotite melting, including aluminum, HREE and Y (Peslier et al., 2000;

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Reisberg and Lorand, 1995). By far, the most commonly employed element in this regard is aluminum, i.e., the so-called alumina-chron (Reisberg and Lorand, 1995). The Nyos

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xenoliths display a good linear correlation between the 187Os/188Os ratios and the whole-rock Al2O3 contents (Fig. 8c). It has been suggested that mantle residues still

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contain 0.7 wt% Al2O3 when Re is completely consumed during melting (Handler et al., 1997). Extrapolation of the alumina-chron of the Nyos xenoliths to 0.7 wt% Al2O3 yields

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an initial 187Os/188Os ratio of ~ 0.11064, corresponding to an Archean TRD age of ~ 2.6 Ga. This indicates that the Nyos xenoliths have been probably subjected to melt depletion

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events during the Neoarchean, which is slightly older than the TMA age of the most refractory harzburgite (NY12; ~ 2.4 Ga).

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However, the linear correlation between 187Os/188Os and Al2O3 might be attributed to physical mixing of the ancient mantle with the ambient juvenile mantle in the asthenosphere, which has been proposed as a mechanism to account for the wide spectrum of Os isotopes observed for abyssal peridotites (Liu et al., 2015). Previous Re-Os studies on abyssal peridotites suggested that ancient mantle domains are widely distributed in the asthenosphere (Brandon et al., 2000; Harvey et al., 2006; Lassiter et al., 2014; Liu et al., 2008; Parkinson et al., 1998), which have not been homogenized by convective stirring. Although Os isotopes of the Nyos xenoliths can seemingly be explained by a mix between the PUM-like ambient mantle (Al2O3=4.5 wt% and 187

Os/188Os=0.130) and an Archean mantle end-member (Al2O3=0.5 wt% and

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Os/188Os=0.115) with different Os contents within the asthenosphere (Fig. 11), physical

mixing cannot be applied to the Nyos mantle xenoliths. The Nyos mantle xenoliths have a 21

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similar range of 187Os/188Os ratios but remarkably higher Al2O3 contents than the abyssal

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peridotites. This is not expected if the Nyos xenoliths represent juvenile mantle that

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recently accreted from the asthenosphere, in which ancient depleted mantle domains are well mixed with the ambient convective fertile mantle. This is because recent melt

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extraction associated with the lithospheric mantle formation should decrease the Al2O3 contents but has a negligible effect on 187Os/188Os ratios. Therefore, the linear Os/188Os-Al2O3 correlation should be ruined or deviated to lower Al2O3 contents than

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the abyssal peridotites.

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Whole rock Re-Os isotopes suggest that the Nyos xenoliths were derived from the ancient SCLM with melt depletion during the Neoarchean to Paleoproterozoic. Previous

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studies have shown that the crust where the CVL is built on has a complex history of evolution. The crystalline basement of the CVL forms part of a mobile belt that was

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formed during the convergence of the Congo and West Africa cratons (Toteu et al., 2004; Van Schmus et al., 2008). The Pan-African granitic rocks intruded in this terrane have Nd

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model ages of Paleoproterozoic to Archean (Nkoumbou et al., 2014; Toteu et al., 2004). Recent field work and geochronology indicates a complex mosaic of Neoproterozoic crust with embedded fragments of Paleoproterozoic-Archean crust overlying the mantle lithosphere that the CVL intrudes (de Wit and Linol, 2015; Van Schmus et al., 2008). Therefore, there still exists continental lithospheric mantle with formation age of Archean to Paleoproterozoic beneath the CVL, which is temporally coupled with the overlying crust. Moreover, Os model ages of the Nyos mantle xenoliths are coeval with or slightly younger than the crust overlying the Archean Congo Craton, as numerous geochronological studies have revealed the existence of Neoarchean charnockite and TTG (tonalite-trondhjemite- granodiorite) in the Ntem Complex of NW Congo Craton (Li et al., 2016; Shang et al., 2004; Tchameni et al., 2010). There exist many evidences for a NW subduction of the Congo craton beneath the terrane that the CVL intrudes during the

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Pan-African Orogeny (Djouka-Fonkwe et al., 2008; Kwékam et al., 2013; Tadjou et al.,

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2009). Therefore, we suggest that the old SCLM beneath the CVL was a part of the

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Archean Congo cratonic mantle. The cratonic mantle beneath the Pan-Africa orogenic belt might have been delaminated (Black and Liégeois, 1993). Mantle delamination

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triggered the upwelling of the asthenosphere, which was responsible for the Bouguer gravity anomaly (Poudjom Djomani et al., 1997) and large volume of crustal-derived

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granotoids in this region (Black and Liégeois, 1993). Our results indicate that old cratonic mantle has not been completely delaminated or removed during the Pan-African Orogeny,

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although the overlying crust might have been strongly deformed and reworked.

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6. Conclusion

The Nyos mantle xenoliths represent residues of a depleted mantle source after

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variable degrees of fractional melting, i.e., < 7% for the lherzolites and 8-14% for the

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harzburgites. They show both chemical and textural evidence for late-stage metasomatic processes. Clinopyroxenes in most Nyos xenoliths are enriched in LILE and depleted in HFSE, suggesting a cryptic metasomatism by silicate or carbonatitic melts. Presence of pargasitic amphibole indicates the local occurrence of modal metasomatism. The Nyos xenoliths have more depleted Sr-Nd-Hf isotopes than the CVL basalts and the SCLM cannot be the major mantle source of the CVL basalts. Most Nyos lherzolites display consistent HSE patterns, whereas two harzburgites and one lherzolite show strong depletion in Pt, Pd and Re. The Nyos xenoliths display a good correlation between 187

Os/188Os and whole rock Al2O3 contents, i.e., "alumina-chron". This correlation is

unlikely caused by physical mixing between ancient mantle domains with the PUM-like ambient mantle within the asthenosphere, but might record an ancient melt depletion event. The Archean TRD age of ~ 2.6 Ga yielded by the "alumina-chron" is very close to

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the TMA age of the most refractory harzburgite (NY12; ~ 2.4 Ga). This supports that the

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Nyos xenoliths were derived from the SCLM, which was formed during the

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Paleoproterozoic convergence of the Congo and West Africa cratons. The old SCLM was a part of the Congo cratonic mantle, which has not been completely removed during the

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Pan-African Orogeny. Acknowledgements

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This study was financially supported by the National Natural Science Foundation of

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China (grants 41222017, 41273045, and 41521062) and a project from State Key Laboratory of Geological Processes and Mineral Resources (GPMR201506). We thank Chang Zhang, Yue-Heng Yang and Zhu-Yin Chu for help in geochemical analyses. This

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Figure Captions

Fig. 1 (a) The Cameroon Volcanic Line with localities of major volcanic centers,

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modified after (Reusch et al., 2010). (b) Geological map of Cameroon with major lithospheric units, modified after (Toteu et al., 2001). Fig. 2 Microtexture of the Nyos mantle xenoliths. (a) Olivine and pyroxene crystals with a curved and locally polygonal shape in sample NY01 with a protogranular texture; (b) recrystallized olivine and clinopyroxene showing a triple junction approaching 120o (NY10); (c) Orthopyroxene porphyroclast with inclusions of clinopyroxene and spinel (NY10); (d) Clinopyroxene with melt inclusions parallel with the orthopyroxene lamellae and fluid inclusions at the rim (NY09); (e) Tiny clinopyroxene interstitial among olivines (NY08); (f) Clinopyroxene interstitial between olivine and orthopyroxene (NY12); (g) Spinel in a amoeboid shape intergrown with orthopyroxene (NY04); (h) Spinel is associated with brown amphibole (NY14). Ol: olivine; Opx: orthopyroxene; Cpx:

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clinopyroxene; Sp: spinel; Amp: amphibole.

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Fig. 3 BSE image of Nyos xenoliths. (a) Clinopyroxene surrounded by secondary

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clinopyroxene and orthopyroxene (NY10); (b) Clinopyroxene with a spongy texture (NY13); (c) Exsolution lamellae in orthopyroxene (NY10); (d) Amphibole surrounded by

Cpx: clinopyroxene; Sp: spinel; Amp: amphibole.

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a reaction zone with secondary clinopyroxene (NY14). Ol: olivine; Opx: orthopyroxene;

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Fig. 4 Plot of whole rock major MgO against Al2O3 (a), CaO (b), Na2O (c), TiO2 (d), Ni

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(e) and V (f). The calculated curves for residual peridotites using the model of (Niu, 1997) are shown, which assume polybaric incremental melting from 2.5 GPa to 0.4 GPa. PM: Values of the primitive mantle (P.M.) are from (McDonough and Sun, 1995). The

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off-craton mantle xenoliths are compiled by (Canil, 2004) and data of Tanzanian cratonic mantle xenoliths are from (Lee and Rudnick, 1999).

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Fig. 5 Mineral compositions of Nyos xenoliths. Literature data are shown for comparison,

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Nyos (Pintér et al., 2015; Temdjim, 2012) and other CVL xenoliths (Caldeira and Munhá, 2002; Lee et al., 1996; Matsukage and Oya, 2010; Pintér et al., 2015). Fig. 6 Trace element compositions of clinopyroxene in Nyos xenoliths. REE (a) and trace element (b) patterns of clinopyroxene in Nyos lherzolites; REE (c) and trace element (d) patterns of clinopyroxene in Nyos harzburgites. Data are normalized to CI chondrite (Anders and Grevesse, 1989). Gray lines are literature data for Nyos xenoliths (Pintér et al., 2015; Temdjim, 2012). Fig. 7 Sr-Nd-Hf isotopes of Nyos xneoliths. Simple binary mixing for Sr-Nd isotopes is modeled between the clinopyroxene of sample NY06 (Sr=56.5 ppm, 87Sr/86Sr=0.701743; Nd=2.52 ppm, 143Nd/144Nd=0.513621) and a basaltic component (Sr=1128 ppm, 87

Sr/86Sr=0.704156; Nd=66 ppm, 143Nd/144Nd=0.512616). Simple binary mixing for

Hf-Nd isotopes is modeled between the clinopyroxene of sample NY01 (Hf=0.28 ppm, 37

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Hf/177Hf=0.285475; Nd=1.17 ppm, 143Nd/144Nd=0.513608) and a basaltic component

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(Hf=8.66 ppm, 176Hf/177Hf =0.282824; Nd=62.4 ppm, 143Nd/144Nd=0.512777; Ballentine

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et al., 1997; Rankenburg et al., 2005). Literature data of CVL basalts (Ballentine et al., 1997) and mantle xenoliths (Lee et al., 1996) are shown. Hf-Nd isotoeps of MORB and

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OIB are from (Stracke et al., 2011) and the mantle array is from (Vervoort and Blichert-Toft, 1999).

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Fig. 8 Highly siderophile elements (HSE) and Re-Os isotopes of Nyos xenoliths. HSE contents are normalized by chondritic values from (Horan et al., 2003). Data of primitive

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upper mantle (PUM) are from (Meisel et al., 2001).

Fig. 9 Comparison of the fractional melting trend with the Y and Yb contents of

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clinopyroxene in Nyos xenoliths. The fractional melting trend is from (Norman, 1998). Both Y and Yb are normalized to primitive mantle (McDonough and Sun, 1995).

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Literature data of Nyos xenoliths are shown for comparison (Pintér et al., 2015; Temdjim,

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2012).

Fig. 10 The Ti/Eu ratio vs. chondrite-normalized La/Yb ratio of clinopyroxene in Maguan peridotite xenoliths. Fields of carbonatite and silicate metasomatism are from (Coltorti et al., 1999). Literature data of Nyos xenoliths are shown for comparison (Pintér et al., 2015; Temdjim, 2012).

Fig. 11 Plots of 187Os/188Os ratios vs. whole-rock Al2O3 contents. The mixing trajectories are made, in an increment of 10%, between the PUM (Al2O3=4.5 wt%, Os=3.9 ppb and 187

Os/188Os=0.1296) and an Archean mantle (Al2O3=1 wt% and 187Os/188Os=0.11) with

different Os contents of 0.9 ppb, 3.9 ppb and 5.9 ppb. The data of global abyssal peridotite are shown for comparison (Brandon et al., 2000; Harvey et al., 2006; Lassiter et al., 2014; Liu et al., 2008; Parkinson et al., 1998; Snow and Reisberg, 1995). Table 1 Whole rock major elements of Nyos xenoliths. 38

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Table 3 Clinopyroxene Sr-Nd-Hf isotopes of Nyos xenoliths.

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Table 2 Clinopyroxene trace element compositions of Nyos xenoliths.

Table 4 Highly siderophile elements (HSE) and Re-Os isotopes of Nyos xenoliths.

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Table S1 Mineral major elements of Nyos xenoliths

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Table 1-Whole rock major and minor elements of Nyos mantle xenoliths

Lhz.

Lhz.

Lhz.

Lhz.

Lhz.

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NY01 NY02 NY03 NY04 NY05 NY06 NY07 NY08 NY09 NY10 NY12 NY13 NY14 Lhz. Harz. Harz. Lhz.

Lhz. Harz. Harz. Lhz.

SiO2

45.37 43.71 45.14 45.17 44.55 45.03 44.34 43.17 44.05 44.57 44.32 42.29 44.54

TiO2

0.03

0.09

0.13

0.09

0.13

Al2O3

2.49

3.25

4.38

2.83

4.17

3.94

Cr2O3

0.42

0.41

0.46

0.28

0.47

Fe2O3

8.06

8.28

8.02

8.66

MnO

0.12

0.12

0.12

0.13

MgO

40.56 40.57 37.69 41.12 38.69 38.46 43.20 44.18 40.60 39.48 44.11 45.42 39.52

CaO

2.43

2.85

Na2O

0.13

0.21

K2O

0.01

0.01

NiO

0.29

0.29

0.03

0.02

0.04

0.10

0.02

0.03

0.12

2.10

1.59

3.33

3.71

1.43

1.66

3.54

0.37

0.41

0.51

0.50

0.37

0.45

0.75

0.40

8.06

8.71

8.52

7.93

8.88

8.27

8.38

8.30

8.67

0.12

0.13

0.12

0.11

0.13

0.12

0.11

0.11

0.13

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0.12

1.69

3.14

2.92

1.01

2.07

2.18

2.48

0.85

1.08

2.60

0.26

0.12

0.26

0.25

0.06

0.11

0.17

0.21

0.05

0.04

0.24

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.02

0.01

0.01

0.01

0.27

0.28

0.25

0.26

0.30

0.33

0.29

0.28

0.32

0.34

0.26

LOI

-0.30 -0.28 -0.24

-0.34

-0.20 -0.34 -0.38 -0.38 -0.36 -0.14 -0.38 -0.38 -0.36

Total

99.61 99.52 99.68 100.04 99.65 99.87 99.72 99.65 99.81 99.47 99.68 99.64 99.67

Mg#

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3.44

0.91

Mg#=Mg/(Mg+Fe)

0.91

0.90

0.90

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Lerz.=Lherzolite; Harz.=Harzburgite.

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0.90

0.91

0.92

0.90

0.91

0.91

0.92

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NY02

NY03

NY04

NY05

NY06

NY07

NY08

NY09

Cpx

Cpx

Cpx

Cpx

Cpx

Cpx

Cpx

Cpx

Cpx

Sc

62

64

75

77

71

66

77

78

70

Ti

783

2303

2753

2996

3138

2747

1366

443

1032

V

217

267

300

307

277

260

267

205

262

Cr

6349

5109

5713

5409

5767

4934

6726

7041

Rb

0.01

0.13

0.36

0.22

0.22

0.14

0.14

Sr

10

50

105

58

56

52

Y

11

16

17

16

19

Zr

3

24

34

27

36

Nb

0.31

0.16

0.67

0.05

Ba

0.15

0.14

0.53

La

0.14

3.35

Ce

0.36

Pr

NY10

NY12

NY13

NY14

NY14

Cpx

Cpx

Cpx

Amp

71

79

74

68

38

2296

751

471

2624

13037

281

268

199

271

374

5610

5490

8537

8375

4977

7467

0.12

0.05

0.05

0.05

0.04

0.04

0.40

48

150

14

117

79

116

50

118

19

12

6

11

18

9

5

18

19

29

22

21

3

27

22

5

35

29

0.30

0.02

0.50

0.74

0.07

0.05

0.39

0.83

0.07

4.72

0.36

0.52

0.24

0.31

0.21

0.09

0.74

0.18

0.18

0.13

11.12

3.75

0.91

0.84

0.60

5.72

15.45

3.03

5.79

15.21

11.73

3.87

5.94

8.57

8.21

2.58

2.59

2.09

10.32

26.29

2.59

11.73

31.08

16.94

5.24

8.38

0.07

1.12

1.00

0.45

0.52

0.44

1.00

2.24

0.16

1.43

3.13

1.21

0.64

0.82

Nd

0.51

5.76

4.68

Sm

0.39

2.27

1.38

Eu

0.20

1.03

0.58

Gd

0.80

3.25

1.84

Tb

0.20

0.61

Dy

1.57

Ho

ED

MA

NU

SC

Cpx

PT

RI P

NY01

T

Table 2-Trace elements of clinopyroxene and amphibole in Nyos mantle xenoliths

3.58

3.13

3.77

6.46

0.66

6.78

10.80

3.31

3.90

4.30

1.22

1.60

1.51

1.05

0.88

0.43

1.94

1.82

0.56

1.66

1.74

0.50

0.64

0.67

0.44

0.37

0.22

0.78

0.58

0.22

0.70

0.79

1.78

2.31

2.34

1.39

0.80

0.97

2.50

1.47

0.62

2.34

2.54

0.32

0.32

0.43

0.48

0.30

0.16

0.23

0.48

0.25

0.12

0.46

0.50

5.01

2.54

2.45

3.10

3.32

2.12

1.02

1.83

3.32

1.56

0.85

3.17

3.48

0.38

0.94

0.50

0.50

0.66

0.74

0.52

0.26

0.44

0.74

0.32

0.20

0.70

0.77

Er

1.15

3.13

1.63

1.58

1.89

2.10

1.42

0.70

1.32

2.04

0.88

0.61

1.97

2.16

Tm

0.17

0.41

0.21

0.21

0.26

0.30

0.22

0.12

0.20

0.30

0.13

0.09

0.28

0.30

Yb

1.22

3.48

1.70

1.61

1.90

2.11

1.47

0.74

1.45

2.02

0.92

0.71

2.01

2.07

Lu

0.17

0.39

0.21

0.19

0.25

0.29

0.22

0.12

0.21

0.29

0.13

0.10

0.28

0.28

Hf

0.14

1.46

0.78

0.77

1.01

0.96

0.50

0.24

0.18

0.94

0.36

0.13

1.16

0.88

Ta

0.01

0.01

0.06

0.03

0.03

0.02

0.03

0.08

0.01

0.01

0.03

0.08

0.01

0.04

Pb

0.06

0.79

0.26

0.20

0.54

0.27

0.44

0.40

1.34

0.47

0.67

0.38

0.44

1.84

Th

0.03

1.07

0.33

0.13

0.17

0.16

1.23

0.65

4.86

0.78

2.49

0.84

1.05

1.02

U

0.03

0.39

0.11

0.07

0.16

0.08

0.39

0.09

1.21

0.25

0.66

0.19

0.38

0.39

AC CE

2.94

52

ACCEPTED MANUSCRIPT

T

Table 3-Sr-Nd-Hf isotopes of clinopyroxene in Nyos mantle xenoliths NY02

NY02-R NY03

NY04

NY05

NY06

t (Ma)

5

5

5

5

5

Rb (ppm)

0.05

0.11

0.09

0.12

Sr (ppm)

33.8

51.7

47.2

93.8

87Rb/86Sr

0.004

0.006

0.006

0.004

87Sr/86Sr

0.703361 0.702259 0.702269 0.702863 0.702531 0.702666 0.701743 0.703310 0.703736

2σm

0.000012 0.000013 0.000010 0.000012 0.000010 0.000010 0.000011 0.000014 0.000023

ISr

0.703360 0.702259 0.702269 0.702863 0.702531 0.702665 0.701743 0.703310 0.703736

Sm (ppm)

0.45

1.51

1.51

1.66

1.75

1.84

1.10

1.33

1.68

Nd (ppm)

1.17

4.07

4.09

4.79

5.66

4.56

2.52

5.91

11.52

147Sm/144Nd

0.230

0.224

0.223

0.209

0.187

0.244

0.264

0.136

0.088

143Nd/144Nd

0.513616 0.513620 0.513673 0.513233 0.513209 0.513455 0.513621 0.513091 0.512924

RI P

NY01

NY07

NY08

5

5

5

0.08

0.27

0.08

0.15

0.23

91.4

79.4

56.5

65.0

179.5

0.010

0.004

0.007

0.004

SC

5

ED

MA

NU

0.003

2σm

0.000012 0.000022 0.000011 0.000011 0.000008 0.000029 0.000008 0.000012 0.000016 0.513608 0.513613 0.513666 0.513226 0.513203 0.513447 0.513612 0.513087 0.512921

PT

(143Nd/144Nd)0

19.1

19.1

20.2

11.6

11.1

15.9

19.1

8.9

5.6

0.17

0.29

0.29

0.28

0.26

0.30

0.29

0.20

0.12

0.28

1.46

1.45

1.01

1.01

1.42

1.05

0.96

0.84

176Lu/177Hf

0.087

0.028

0.028

0.040

0.036

0.030

0.039

0.030

0.020

176Hf/177Hf

0.285483 0.283497 0.283502 0.283593 0.283645 0.283227 0.283452 0.283721 0.283583

2σm

0.000008 0.000026 0.000018 0.000017 0.000011 0.000017 0.000019 0.000015 0.000014

εNd(t)

Hf (ppm)

AC CE

Lu (ppm)

(176Hf/177Hf)0

0.285475 0.283495 0.283499 0.283589 0.283642 0.283224 0.283449 0.283718 0.283581

εHf(t)

95.2

25.2

25.4

R: replicate analysis; Decay constants: Sm-Nd system, λ =6.54E-12; Lu-Hf system, λ=1.93E-11; Depleted mantle (DM) values: 143Nd/144Nd=0.51315; 147Sm/144Nd=0.2137; 176Hf/177Hf=0.283251; 176Lu/177Hf=0.0384; Chondritic values (CHUR):

53

28.6

30.4

15.6

23.6

33.1

28.2

ACCEPTED MANUSCRIPT

143Nd/144Nd=0.512630; 147Sm/144Nd=0.1960;

T

176Hf/177Hf=0.282785;

AC CE

PT

ED

MA

NU

SC

RI P

176Lu/177Hf=0.0336;

54

ACCEPTED MANUSCRIPT

Ir

Ru

Pt

Pd

Re 187Re/188Os 187Os/188Os

(wt%) (ppb) (ppb) (ppb) (ppb) (ppb) (ppb) Lhz. 2.49 0.80 1.31 3.25 1.57 0.97 0.01

NY02

Lhz. 3.25 2.24 2.74 5.95 4.90 4.23 0.12

NY03

Lhz. 4.38 2.66 3.20 6.41 6.43 5.56 0.16

NY04

2SE

TMA TRD (Ga) (Ga)

0.08

0.12060

0.00001 1.56 1.26

0.26

0.12559

0.00031 1.45 0.57

0.28

0.13129

0.00044 -0.72 -0.24

Lhz. 2.83 3.05 4.08 6.83 5.27 2.75 0.05

0.08

0.11700

0.00004 2.18 1.76

NY05

Lhz. 4.17 1.60 2.19 4.14 4.11 3.31 0.08

0.23

0.12874

0.00018 0.27 0.12

NY06

Lhz. 3.94 1.83 2.45 4.80 4.75 3.66 0.11

0.30

0.12666

0.00035 1.43 0.41

NY07

Harz. 2.10 2.46 3.49 6.31 4.53 2.03 0.10

0.20

0.11705

0.00020 3.24 1.75

NY08

Harz. 1.59 4.97 3.48 7.47 1.86 0.89 0.01

0.01

0.11616

0.00000 1.91 1.87

NY09

Lhz. 3.33 1.97 2.82 5.62 5.07 3.88 0.11

0.27

0.12447

0.00030 1.98 0.72

NY10

Lhz. 3.71 2.19 2.79 5.48 4.82 3.49 0.10

0.21

0.12632

0.00020 0.93 0.46

NY12

Harz. 1.43 2.61 3.31 7.29 3.69 2.01 0.04

0.07

0.11485

0.00002 2.43 2.05

NY13

Harz. 1.66 2.13 1.86 4.70 1.32 0.51 0.02

0.04

0.11600

0.00001 2.09 1.90

NY14

Lhz. 3.54 1.65 2.09 4.32 3.55 2.53 0.05

0.15

0.12534

0.00008 0.94 0.60

relative to the primitive upper

NU

MA

ED

AC CE

mantle (PUM):

PT

TRD and TMA are caculated

SC

NY01

RI P

Litho. Al2O3 Os

T

Table 4-HSE and Re-Os isotopes of Nyos mantle xenoliths

187Re/188Os=0.4236;

187Os/188Os=0.1296; λ =1.6660E-11

55