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

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

Genesis of ^(230)Th excess in basalts from mid-ocean ridges and ocean islands:Constraints from the global U-series isotope database and major and rare earth element geochemistry

Based on 230Th-238U disequilibrium and major element data from mid-ocean ridge basalts(MORBs) and ocean island basalts(OIBs),this study calculates mantle melting parameters,and thereby investigates the origin of 230Th excess.(230Th/238U) in global MORBs shows a positive correlation with Fe8,Po,Na8,and Fmelt(Fe8 and Na8 are FeO and Na2O contents respectively after correction for crustal fractionation relative to MgO = 8 wt%,Po=pressure of initial melting and Fmelt=degree of melt),while 230Th excess in OIBs has no obvious correlation with either initial mantle melting depth or the average degree of mantle melting.Furthermore,compared with the MORBs,higher(230Th/238U) in OIBs actually corresponds to a lower melting degree.This suggests that the 230Th excess in MORBs is controlled by mantle melting conditions,while the 230Th excess in OIBs is more likely related to the deep garnet control.The vast majority of calculated initial melting pressures of MORBs with excess 230Th are between 1.0 and 2.5 GPa,which is consistent with the conclusion from experiments in recent years that DU>DTh for Al-clinopyroxene at pressures of >1.0 GPa.The initial melting pressure of OIBs is 2.2-3.5 GPa(around the spinel-garnet transition zone),with their low excess 226Ra compared to MORBs also suggesting a deeper mantle source.Accordingly,excess 230Th in MORBs and OIBs may be formed respectively in the spinel and garnet stability field.In addition,there is no obvious correlation of K2O/TiO2 with(230Th/238U) and initial melting pressure(Po) of MORBs,so it is proposed that the melting depth producing excess 230Th does not tap the spinel-garnet transition zone.OIBs and MORBs in both(230Th/238U) vs.K2O/TiO2 and(230Th/238U) vs.Po plots fall in two distinct areas,indicating that the mineral phases which dominate their excess 230Th are different.Ce/Yb-Ce curves of fast and slow ridge MORBs are similar,while,in comparison,the Ce/Yb-Ce curve for OIBs shows more influence from garnet.The mechanisms generating excess 230Th in MORBs and OIBs are significantly different,with formation of excess 230Th in the garnet zone only being suitable for OIBs.