Geochemistry and geochronology of mafic rocks from Bamenda Mountains (Cameroon): Source composition and crustal contamination along the Cameroon Volcanic Line

Mafic rocks from the Bamenda volcanic province along the Cameroon Volcanic Line have been dated from 17 to 0 Ma. Associated with some trachytes and rhyolites, this volcanism covers a period of more than 25 Ma. The studied rocks are basalts to mugearites. Most of them have been contaminated by continental crust during their transit to the surface. The oldest rocks are the most contaminated. One group of samples shows high Eu, Sr and Ba contents. This characteristic is not due to crustal contamination process, but has a mantle source origin. We argue that these characteristics have been acquired by mixing of melts formed by partial melting of mantle pyroxenites with melts formed in mantle peridotites. Such pyroxenites have been observed as mantle xenoliths in the Adamaoua province, and their chemical and isotopic compositions are consistent with such a model.     

Extreme source heterogeneity and complex contamination patterns along the Cameroon Volcanic Line: New geochemical data from the Bamoun plateau

We investigated mafic and felsic volcanic rocks from the Bamoun plateau, a magmatic province located north of Mount Cameroon, in the continental part of the Cameroon Volcanic Line (CVL). Basalts and dacites were probably emplaced more than 40 Ma ago, while basanites represent very young volcanic eruptions. Among the basalts, some of them have suffered crustal contamination during their uprise through the continental crust, and their primary trace element and isotopic compositions have been slightly modified. The formation of the dacites was also accompanied by some crustal contamination. Non-contaminated rocks show that the oldest magmas are transitional basalts formed by relatively high degrees of partial melting of a moderately enriched mantle source, probably containing pyroxenites. Recent basanites were produced by very low partial melting degrees of an enriched mantle source with HIMU composition, but different from the source of the nearby Mount Cameroon lavas. The mantle beneath the CVL is thus very heterogeneous, and the tendency towards more alkaline mafic-ultramafic compositions in the youngest volcanic manifestations along the CVL seems to be a general feature of all CVL.     

Tchabal Gangdaba massif in the Cameroon Volcanic Line: a bimodal association

Tchabal Gangdaba (TG) volcanic massif, which is a part of the continental sector of the Cameroon Volcanic Line (CVL), is dated between 34.4 and 25.1 Ma. It displays mafic lavas (picrobasalt and basanite, 41–43 wt % SiO₂) and felsic lavas (rhyolite, 68–73 wt % SiO₂). The lack of intermediate rocks evidences a pronounced Daly gap between 43 and 68 wt % SiO₂, which corresponds to an important time span of 3.4 Ma. It is interpreted as due to extensive fractional crystallization under peculiar thermodynamical conditions. Felsic lavas yield strong negative anomalies in Ba, Sr and Eu (0.1?206Pb/²⁰⁴Pb?207Pb/²⁰⁴Pb?208Pb/²⁰⁴Pb?相似文献   

Mantle Dynamics: A Nd, Sr, Pb and O Isotopic Study of the Cameroon Line Volcanic Chain

The Cameroon line comprises a 1600-km long Y-shaped chain of< 30 m.y. old volcanoes and <70 m.y. old plutons extendinginto mainland Africa from the Atlantic island of Pagalu. Thedistribution of basaltic volcanic centres is ideal for comparingsub-continental and sub-oceanic sources for basalts and forstudying the influence of the lithosphere on magma generation.We report Nd, Sr, Pb and O isotopic data for more than thirty(principally basaltic) samples from all the main volcanic centrestogether with data for two granulite facies xenoliths. Thosebasalts which display no obvious evidence of crustal contaminationyield initial ⁸⁷Sr/⁸⁶Sr ratios ranging from 0.7029 to 0.7035,Nd between +2 and +7 and ²⁰⁶Pb/²⁰⁴Pb between 19?0 and 20?6.The Nd and Sr isotopic compositions define a field on the lefthand side of the ‘mantle array’ (that is with relativelyunradiogenic Sr) and include some data which show overlap withcompositions observed for St. Helena. ²⁰⁸Pb/²⁰⁴Pb ratios extendto 40?4—amongst the more radiogenic observed for alkalibasalts. The Nd and Sr isotopic data are similar in oceanicand continental sectors indicating that the magmas are derivedfrom generally similar mantle sources. Despite this overallsimple picture, the source of the Cameroon line volcanics hasin fact been variable in both time and space. Pb is less radiogenicand Sr is more radiogenic in transitional to hypersthene-normativecompositions. There is a progression to more radiogenic leadisotopic compositions with time for the Cameroon line as a wholethat is most strikingly displayed in the 30 m.y. eruptive historyof Principe. These space-time data are difficult to reconcilewith conventional plume models or with some dispersed ‘plumpudding’ models. The heterogeneities require isolationtimes considerably longer than the age of the south Atlanticsea floor (120 Ma). The eruptive lavas with the most radiogenicPb observed (accompanied by unradiogenic Nd) precisely straddlethe continental edge (i.e. occur in both oceanic and continentalsectors) with no dependency on Nd and Pb concentrations. A modelis proposed which links these observations with the destructionof lithosphere, and the impregnation of the uppermost mantleby the St. Helena hot spot during the formation of the SouthAtlantic ocean. This mantle was subsequently melted to formthe Cameroon line which appears to be derived from a risinghot zone initiated by the early plume activity. The magmaticproducts reflect the mantle mixing that took place during continentalbreakup, the consequent cooling and thickening of the lithosphereand the continued interaction between rising plume componentsand this lithosphere. The depth from which magmas are currentlybeing tapped at the continent/ocean boundary is estimated atless that 150 km.     

Geochemical and isotopic characteristics of fluids in the Niutuozhen geothermal field,North China

Niutuozhen geothermal field is located in the Jizhong graben, belonging to the northern part of Bohai Bay Basin in North China. Chemical and isotopic analyses were carried out on 14 samples of the geothermal fluids discharged from Neogene Minghuazhen (Nm), Guantao (Ng), and Jixianian Wumishan (Jxw) formations. The δ²H and δ¹⁸O in water, δ¹³C in CH₄, δ¹³C in CO₂, and ³He/⁴He ratio in the gases were analyzed in combination with chemical analyses on the fluids in the Niutuozhen geothermal field. The chemical and isotopic compositions indicate a meteoric origin of the thermal waters. The reservoir temperatures estimated by chemical geothermometry are in the range between 60 and 108 °C. The results show that the gases are made up mainly by N₂ (18.20–97.42 vol%), CH₄ (0.02–60.95 vol%), and CO₂ (0.17–25.14 vol%), with relatively high He composition (up to 0.52 vol%). The chemical and isotopic compositions of the gas samples suggest the meteoric origin of N₂, predominant crustal origins of CH₄, CO₂, and He. The mantle-derived He contributions are calculated to be from 5 to 8% based on a crust–mantle binary mixing model. The deep temperatures in the Jxw reservoir were evaluated based on gas isotope geothermometry to be in the range from 141 to 165 °C. The mantle-derived heat fraction in the surface heat flow is estimated to be in the range of 48–51% based on ³He/⁴He ratios.     

Eruptive history of the Barombi Mbo Maar, Cameroon Volcanic Line, Central Africa: Constraints from volcanic facies analysis

his study presents the first and detail field investigations of exposed deposits at proximal sections of the Barombi Mbo Maar (BMM), NE Mt Cameroon, with the aim of documenting its past activity, providing insight on the stratigraphic distribution, depositional process, and evolution of the eruptive sequences during its formation. Field evidence reveals that the BMM deposit is about 126m thick, of which about 20m is buried lowermost under the lake level and covered by vegetation. Based on variation in pyroclastic facies within the deposit, it can be divided into three main stratigraphic units: U₁, U₂ and U₃. Interpretation of these features indicates that U₁ consists of alternating lapilli-ash-lapilli beds series, in which fallout derived individual lapilli-rich beds are demarcated by surges deposits made up of thin, fine-grained and consolidated ash-beds that are well-defined, well-sorted and laterally continuous in outcrop scale. U₂, a pyroclastic fall-derived unit, shows crudely lenticular stratified scoriaceous layers, in which many fluidal and spindle bombs-rich lapilli-beds are separated by very thin, coarse-vesiculatedash-beds, overlain by a mantle xenolith- and accidental lithic-rich explosive breccia, and massive lapilli tuff and lapillistone. U3 displays a series of surges and pyroclastic fall layers. Emplacement processes were largely controlled by fallout deposition and turbulent diluted pyroclastic density currents under “dry” and “wet” conditions. The eruptive activity evolved in a series of initial phreatic eruptions, which gradually became phreatomagmatic, followed by a phreato-Strombolian and a violent phreatomagmatic fragmentation. A relatively long-time break, demonstrated by a paleosol between U₂ and U₃, would have permitted the feeding of the root zone or the prominent crater by the water that sustained the next eruptive episode, dominated by subsequent phreatomagmatic eruptions. These preliminary results require complementary studies, such as geochemistry, for a better understanding of the changes in the eruptive styles, and to develop more constraints on the maar’s polygenetic origin.     

Chemical and Isotopic Characteristics of Gabbroic Xenoliths from Hannuoba,China

Nine pieces of gabbroic xenoliths from Hannuoba were examined for their major and trace elements and Nd,Sr and Pb isotopes.The results show that the gab-broic xenoliths are of more mafic basaltic composition .Their abundances show narrow variations in major elements.The trace element contents are highly variable in contrast with those of host basalts and lherzolite xenoliths.The gabbroic xenoliths are rich in Nd(0.51159-0.51249),Sr(0.70491-0.70768) and low in radiogenic Pb(16.283-17.046, 15.191-15.381 and 36.999-37.476),significantly different from basalts and lherzolites in isotopic space.The calculated Nd and Pb model ages are about 3.0-3.5 Ga.The rocks have relatively low equilibrium T(-850℃) and P(0.8-0.9 Gpa).They could be inter-preted to be the product of upper mantle melting at the boundary between the lower crust and the upper mantle.Their chemical and isotopic variations can be ascribed to different degrees of melting,segregation and long-term evolution.     

Hydrochemical and isotopic characteristics of groundwater in the Ndop plain,northwest Cameroon: resilience to seasonal climatic changes

Shallow groundwater (>30 mbgl) is an essential source of drinking water to rural communities in the Ndop plain, northwest Cameroon. As a contribution to water management, the effect of seasonal variation on the groundwater chemistry, hydrochemical controls, drinking quality and recharge were investigated during the peaks of the dry (January) and rainy (September) seasons. Field measurements of physical parameters were preceded by sampling 58 groundwater samples during both seasons for major ions and stable isotope analyses. The groundwater, which was barely acidic (mean pH of 6) and less mineralised (TDS < 272 mg/l), showed no significant seasonal variation in temperature, pH and TDS during the two seasons. The order of cation abundance (meq/l) was Na⁺ > Ca²⁺ > Mg²⁺ > K⁺ and Na⁺ > Mg²⁺ > Ca²⁺ > K⁺ in the dry and rainy seasons, respectively, but that of anions ( \( {\text{HCO}}_{3}^{ - } \)  >  \( {\text{NO}}_{3}^{ - } \)  > Cl^? >  \( {\text{SO}}_{4}^{2 - } \)  > F^?) was similar in both seasons. This suggests a negligible effect of seasonal variations on groundwater chemistry. The groundwater, which was CaMgHCO₃ and NaHCO₃, is chemically evolved rainfall (CaMgSO₄Cl) in the area. Silicate mineral dissolution and cation-exchange were the main controls on groundwater chemistry while there was little anthropogenic influence. The major ions and TDS concentrations classified the water as suitable for human consumption as per WHO guidelines. The narrow cluster of δ¹⁸O and δD of same groundwater from both seasons between the δ¹⁸O and δD values of May–June precipitation along the Ndop Meteoric Water Line indicates meteoric origin, rapid recharge (after precipitation) and timing of recharge between May and June rainfall. Diffuse groundwater recharge mainly occurs at low altitudes (<1,400 m asl) within the plain. Besides major ions and TDS, the similar δ¹⁸O and δD of groundwater from both seasons indicate a consistent groundwater recharge and flow pattern throughout the year and resilience to present day short-term seasonal climatic variations. However, controlled groundwater abstraction is recommended given the increasing demand.     

北京市通州区地热流体水化学和同位素特征及其地热学意义

通过对北京市通州区蓟县系岩溶热储中地热流体的水化学组分、2H、18O、3H、14C、锶同位素组成的研究,论述了区域内地热系统中水的补给径流循环特征和地温分布特征。研究区内蓟县系热储中地热水的出水温度分布在35~91 ℃范围内,补给来源为北京市西北部或北部山区大气降水,平均补给高程为1510m。热储中地热水年龄和热储温度均呈现出明显的构造控制特点。通州区西部,大兴迭隆起构造单元内,热水年龄从西北（18ka）向东南（27ka）增加,运移速度约1.5m/a,热储温度从57.4 ℃增至86.5 ℃。东南部的夏垫断裂是一个导水导热断裂,其上地热水年龄减小至8.4ka,同时热储温度增至107.8 ℃。地热水中锶含量和锶同位素值均沿着地下水的径流方向增加,揭示了两个过程的叠加影响：蓟县系碳酸盐岩中锶的溶解与87Rb的衰减,后者呈现出明显的时间累积效应,在研究区东南部体现的更明显。     

北京市通州区地热流体水化学和同位素特征及其地热学意义

通过对北京市通州区蓟县系岩溶热储中地热流体的水化学组分、2H、18O、3H、14C、锶同位素组成的研究,论述了区域内地热系统中水的补给径流循环特征和地温分布特征.研究区内蓟县系热储中地热水的出水温度分布在35～91℃范围内,补给来源为北京市西北部或北部山区大气降水,平均补给高程为1510 m.热储中地热水年龄和热储温度...     

The Cameroon Volcanic Line Revisited: Petrogenesis of Continental Basaltic Magmas from Lithospheric and Asthenospheric Mantle Sources

The volcanic activity of Mts Bambouto and Oku (Western Highlands)and of the Ngaoundere Plateau, in the continental sector ofthe Cameroon Volcanic Line, Equatorial West Africa, ranges inage from Oligocene to Recent. It is characterized by basanitic,alkali basaltic and transitional basaltic series. Mineral chemistry,major and trace element bulk-rock compositions, and geochemicalmodelling suggest that the magmatic series evolved mainly atlow pressure (2–4 kbar) through fractional crystallizationof clinopyroxene and olivine ± magnetite, at moderatelyhydrated (H₂O = 0·5–1 wt %) and QFM (quartz–fayalite–magnetite)to QFM + 1 fO₂ conditions. Basalts from Ngaoundere (Mioceneto Quaternary) and from the early activity (31–14 Ma)of the Western Highlands have incompatible trace element andSr–Nd isotopic compositions similar to those of oceanicCameroon Line basalts, pointing to a similar asthenosphericmantle source. By contrast, the late (15–4 Ma) WesternHighlands basanites and alkali basalts have anomalously highconcentrations of Sr, Ba and P, and low concentrations of Zr,which are exclusive features of continental Cameroon basalts.The genesis of these latter magmas is consistent with derivationfrom an incompatible element enriched, amphibole-bearing lithosphericmantle source. Western Highlands basalts show a continuous spectrumfrom high to low Sr–Ba–P compositions, and may resultfrom variable amounts of mixing between melts derived from ananhydrous lherzolite source (asthenospheric component) and meltsfrom an amphibole-bearing peridotite source (lithospheric HSrcomponent). New ⁴⁰Ar/³⁹Ar ages for Mts Oku and Bambouto basalts,combined with previous ⁴⁰Ar/³⁹Ar and K/Ar ages of basaltic andsilicic volcanics, and with volcanic stratigraphy, suggest aNE–SW younging of the peak magmatic activity in the WesternHighlands. This SW younging trend, extending from the Oligocenevolcanism in northern Cameroon (e.g. Mt Oku) to the still activeMt Cameroon, suggests that the African plate is moving abovea deep-seated mantle thermal anomaly. However, the age and locationof the Ngaoundere volcanism does not conform to the NE–SWyounging trend, implying that the continental sector of theCameroon Volcanic Line cannot be easily interpreted as the surfaceexpression of a single hotspot system. KEY WORDS: Cameroon Line basalts;⁴⁰Ar/³⁹Ar geochronology; lithospheric and asthenospheric mantle source; hotspot     

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

Acta Geochimica - Pozzolans from the Tombel Plain, Bamileke Plateau, and Noun Plain, 3 monogenetic volcanic fields in the central part of the Cameroon Volcanic Line (the Tombel Plain, Bamileke...     

承德北部茅荆坝地热田地热流体的水化学和同位素特征及其成因

承德北部茅荆坝地区地表出露的地热水温度高达98.7 °C,赋存于裂隙较发育的侏罗世中粗粒二长花岗岩热储,${\mathrm{SO}}_4^{2-} $含量较高,但关于该区地热流体的补给来源、循环演化过程与成因机制研究尚少。为了认识该基岩山区地热系统的成因以合理开发利用地热资源,在区域地热地质调查的基础上,测试分析了不同水体的水化学组分、地下水年龄（3H和14C）、硫酸盐硫、氧同位素（δ34S-${\mathrm{SO}}_4^{2-} $和δ18O-${\mathrm{SO}}_4^{2-} $）、碳酸盐碳同位素（δ13C-$ {\mathrm{HCO}}_3^-$）、锶同位素（87Sr、86Sr）等特征值。结果表明：（1）茅荆坝地区地热水化学类型以${\mathrm{SO}}_4 $—$ {\mathrm{Na}}$型为主,硅酸盐矿物的溶解及阳离子交换作用促进了地热水中${\mathrm{Na}}^+ $、${\mathrm{K}}^+ $和SiO2的富集,水中${\mathrm{SO}}_4^{2-} $并非来源于硫酸盐岩矿物溶解,推测为H2S气体从深部还原环境上升到浅部氧化后生成${\mathrm{SO}}_4^{2-} $,也可能来源于高温地热水与硫反应形成的硫酸盐;（2）地热水n(87Sr)/n(86Sr)均值为0.7092,与海相碳酸盐岩比值接近,揭示热储深部可能存在海相碳酸盐岩储层;（3）地热水属于古地下水,14C校正年龄为11.9～14.9 ka,循环更新能力差,由周边山区的大气降水补给,补给高程在1532～1632 m;（4）地热系统深部热储温度为142～144 °C,高温中心位于热田北部。研究结果对冀北山地地热资源的可持续开发利用具有重要意义。     

Mineralogy, geochemistry and petrogenesis of the recent magmatic formations from Mbengwi, a continental sector of the Cameroon Volcanic Line (CVL), Central Africa

The Mbengwi recent magmatic formations consist of volcanics and syenites belonging to the same magmatic episode. Lavas form a bimodal basanite-rhyolite alkaline series with a gap between 50 and 62?wt.% SiO₂. Mafic lavas (basanite-hawaiite) are sodic while felsic rocks (trachyte-rhyolite-syenites) are sodi-potassic, slightly metaluminous to peralkaline. The geochemical and isotopic characteristics (0.7031?<?(⁸⁷Sr/⁸⁶Sr)_initial?<?0.7043; 1.03?<?ε_Ndi?<?5.17) of these rocks are similar to those of other rocks from the CVL. The main differentiation process is fractional crystallization with two trends of fractionation. Their Rb/Sr isochron age of 28.2?Ma, almost similar to 27.40?±?0.6?Ma?K/Ar age obtained in a trachyte from neighboring Bamenda Mountains system, precludes any local age migration of an hypothetic hotspot. Mafic lavas have OIB features displaying an isotopic signature similar to that of HIMU mantle source different from FOZO known as source of most parental magmas along the CVL.     

Geological and Geochemical Characterization of Iron-titanium Mineral Indices in the Tertiary Anorogenic Complex of Bana (Cameroon Volcanic Line)

Please?refer?to?the?attachment(s)?for?more?details.     

Petrology of spinel lherzolite xenoliths from Youkou volcano,Adamawa Massif,Cameroon Volcanic Line: mineralogical and geochemical fingerprints of sub-rift mantle processes

The basaltic maar of Youkou, situated in the Adamawa Volcanic Massif in the eastern branch of the continental segment of the Cameroon Volcanic Line, contains mantle-derived xenoliths of various types in pyroclastites. Spinel-bearing lherzolite xenoliths from the Youkou volcano generally exhibit protogranular textures with olivine (Fo_89.4?90.5), enstatite (En_89???91Fs_8.7?9.8Wo_0.82?1.13), clinopyroxene, spinel (Cr#_Sp?=?9.4–13.8), and in some cases amphibole (Mg_#?=?88.5–89.1). Mineral equilibration temperatures in the lherzolite xenoliths have been estimated from three–two pyroxene thermometers and range between 835 and 937 °C at pressures of 10–18 kbar, consistent with shallow mantle depths of around 32–58 km. Trends displayed by bulk-rock MgO correlate with Al₂O₃, indicating that the xenoliths are refractory mantle residues after partial melting. The degree of partial melting estimated from spinel compositions is less than 10%: evidences for much higher degrees of depletion are preserved in one sample, but overprinted by refertilization in others. Trace element compositions of the xenoliths are enriched in highly incompatible elements (LREE, Sr, Ba, and U), indicating that the spinel lherzolites underwent later cryptic metasomatic enrichment induced by plume-related hydrous silicate melts. The extreme fertility (Al₂O₃?=?6.07–6.56 wt% in clinopyroxene) and the low CaO/Al₂O₃ ratios in the spinel lherzolites suggest that they could not be a simple residue of partial melting of primitive mantle and must have experienced refertilization processes driven by the infiltration of carbonatite or carbonated silicate melts.     

Clinopyroxene of spinel-peridotite mantle xenoliths from Lake Nji (Cameroon Volcanic Line, W Africa): crystal chemistry and petrological implications

The crystal chemistry of six clinopyroxenes enclosed in protogranular spinel-peridotite mantle xenoliths from Lake Nji (Cameroon, W Africa) was studied by means of single-crystal X-ray diffraction and electron microprobe. These spinel-peridotite xenoliths are characterised by clinopyroxene contents distinctly higher than those reported by Lee et al. (1996) for spinel-peridotite xenoliths of the same region (19–11% and 15–8% respectively) and by high clinopyroxene/orthopyroxene ratios: 0.7–2.7 (present study) and about 0.4 (Lee et al. 1996). The clinopyroxene crystal chemistry indicates that the xenoliths show the compositional features of a mantle source contaminated by small-volume melts, responsible for the high clinopyroxene content. The protogranular texture of the Cameroon xenoliths (upper portion of lithospheric mantle) testifies that the spinel peridotite and the associated small-volume melts completely re-equilibrated at the spinel-peridotite facies. This is also supported by the petrological and geochemical data of Lee et al. (1996) on the Etinde–Biu Plateau spinel-peridotite xenoliths, which underwent Late Proterozoic–Early Paleozoic enrichment in incompatible trace elements. Received: 6 October 1999 / Accepted: 27 March 2000     

Geochemical and isotopic characteristics of lower crustal xenoliths, San Francisco Volcanic Field, Arizona, U.S.A

A wide variety of xenoliths has been entrained in Miocene-to-Recent alkali olivine and hypersthene-normative basalts in the San Francisco Volcanic Field (SFVF), northern Arizona, U.S.A. Based on petrography, mineralogy, bulk rock chemistry and Sr-Nd isotopic characteristics, SFVF xenoliths can be divided into two major groups: cumulates and granulites. The cumulates are genetically related to the Cenozoic volcanic rocks and represent under- and/or intraplated additions to the crust of the Colorado Plateau. Assemblages are mafic to ultramafic and are dominated by clinopyroxene-orthopyroxene-plagioclase-spinel-amphibole-olivine. The granulites are probably Proterozoic in age, mafic-to-intermediate/felsic in bulk composition, either two pyroxene-plagioclase-spinel or plagioclase-alkali feldspar-quartz-magnetite-amphibole-biotite assemblages. Many of the granulites show evidence of partial melting. Some high SiO₂, very high Rb/Sr glasses are close in composition to erupted rhyolites, and probably represent end-member melts that have interacted with basalt to produce a variety of hybrid intermediate lavas. The major element, trace element and Sr-Nd isotope geochemistry is highly variable in the SFVF xenoliths. Extremely high Ba contents and Ba/Nb of a number of the granulites are equivalent to values characteristics of modern supra-subduction zone magmas. The considerable variation of chemical and isotopic composition depends upon mineral proportions, assemblages and chemistry. Isotopically, three end-members can be identified within the granulites: (i) lowest ⁸⁷Sr/⁸⁶Sr (0.702870) with low ¹⁴³Nd/¹⁴⁴Nd (0.511541, εNd-21.4); (ii) high ⁸⁷Sr/⁸⁶Sr (0.711069) with the lowest ¹⁴³Nd/¹⁴⁴Nd (0.511434, εNd-23.5); (iii) highest ⁸⁷Sr/{86}Sr (0.715306) with low ¹⁴³Nd/¹⁴⁴Nd (0.511793, εNd-16.5). Two important age ranges deduced from the isotopic data probably relate to episodes of crustal-growth beneath the SFVF (1.88 ± 0.33 Ga and Cenozoic). Thermobarometric calculations assuming equilibrium show that the xenoliths are derived from the lower crust (0.6–1.3 GPa, 850–1050 °C). The average SFVF lower crust is mafic in composition. In the absence of partial lithospheric delamination, the lower crust may become mafic with time due to under- and intraplating of continental crust by mafic magmas derived from the mantle.     

Petrology of ultramafic nodules from São Tomé Island, Cameroon Volcanic Line (oceanic sector)

Ultramafic xenoliths were found in recent alkali basalts from São Tomé Island. These include spinel peridotites (lherzolites, harzburgites and dunites) and pyroxenites (orthopyroxenites and clinopyroxenites). Textures and mineral compositions indicate that pyroxenites originated from crystal/liquid separation processes operating on magmas similar to those giving rise to their present host rocks whereas spinel peridotite xenoliths had an accidental origin; Fo (>89) and Ni (>0.36 wt.%) contents in olivines, Mg# (91–95) of orthopyroxenes and low Ti in clinopyroxene (primary crystals: TiO₂<0.06 wt.%) and in spinel (TiO₂<0.1 wt.%) are within the range reported for abyssal peridotites, indicating São Tomé spinel peridotites represent refractory residues of melting. Nevertheless, the lack of correlation between mineral chemistry and modal composition suggests that spinel peridotite xenoliths are not simple residues and were affected by infiltration of fluid/melts within the mantle. The wide temperature range obtained for spinel peridotites (700 to >1150 °C) is compatible with a long period of pre-entrainment cooling supporting Fitton's [Tectonophysics 94 (1983) 473] hypothesis that proposes oceanic lithosphere uprising in the Cameroon Volcanic Line prior to the initiation of the current thermal regime, related to São Tomé magmatism. The association of upper mantle (peridotite) xenoliths with igneous cumulates (pyroxenites) suggests that the spinel peridotite suite originated in the uppermost mantle above the São Tomé magma storage zone(s), probably in a region of high strain rate, near the boundary between the mantle and the overlying oceanic crust.     

Chemical and isotopic characteristics of hydrocarbon gases from Mendeleev and Golovnin volcanoes,Kunashir Island

This paper reports the analysis of the distribution of hydrocarbon and other gases in Mendeleev and Golovnin volcanoes and other occurrences of Kunashir Island. It was shown that, in addition to methane, the solfatara gases and gases dissolved in thermal waters contain ethane, propane, and butane. The volume concentration of methane in gas samples was no higher than 0.4 vol %. Carbon isotopes were analyzed in methane, ethane, and carbon dioxide in free and dissolved states. The hydrocarbon gases of Kunashir I. are of the thermogenic and metamorphogenic types, and their distribution depends on the position of springs relative to the eruptive centers of the volcanoes. Thermogenic gases are probably products of the high-temperature transformation of sedimentary organic matter buried during the formation of volcanic edifices. General gas geochemical characteristics are presented for Kuril volcanoes, and it was found that hydrocarbon generation in the sediments of the Kuril Basin increases toward volcanic edifices.