Nd, Sr and Pb isotope data, together with new major and trace element data are presented for lavas from northern Kenya. A general trend towards silica saturation and decreasing incompatible element contents is observed from the Miocene to the present day. Significantly, the abundances of different incompatible elements decrease The Nd, Sr and Pb isotope compositions of the basic lavas are similar to those observed on the Atlantic ocean islands. Comparison of the Sm/Nd ratios required to produce the Nd isotope ratios with those observed in the rocks indicates that light rare earth elements (r.e.e.) have probably been added to the source region of the lavas comparatively recently. A model involving recent metasomatism of the subcontinental mantle beneath Kenya, which could account for the correlated silica undersaturation and incompatible element content of the lavas, is proposed.
Summary The island of Sao Miguel in the Azores exhibits bimodal volcanism: eruptives from Agua de Pau volcano and the ‘Waist’ consist almost entirely of basalt or trachyte with rare intermediate products. Although some intermediate lavas are fractionates from basalt, the majority are mixed-magma hybrids anomalously enriched in europium, barium and potassium. These features clearly indicate that they have also accumulated alkali feldspar. The source of the alkali feldspar contaminant must be the least evolved trachyte in the Agua de Pau system, because barium, europium and strontium are rapidly depleted in liquids and crystals with differentiation to more evolved trachytes. Many less evolved trachytes are also accumulative in alkali feldspar, suggesting a genetic association with the hybrid lavas. Geochemical modelling indicates that the hybrid lavas are the result of contamination of basalt by accumulative trachyte containing up to 75% alkali feldspar. Field and petrological evidence points to the episodic injection and ponding of basalt beneath trachytic magma at Agua de Pau volcano. One trachytic lava, with alkali feldspar-contaminated basaltic inclusions, represents a sample of this interface. Density calculations indicate that highly accumulative trachyte can have a neutral or negative buoyancy with respect to basalt and could descend through the interface. Resorption of alkali feldspar will lower the bulk density of the contaminated basalt, with the effect of favouring even greater contamination by overlying accumulative trachyte. The most attractive mechanism for achieving the accumulative trachyte compositions is the slumping of side-wall cumulates and the formation of magmatic density currents which flowed along the compositional interface between trachyte and basalt. Flowage differentiation and crystal settling would result in local ‘parcels’ of denser highly accumulative trachytic magma. The process is illustrated with results from a simple laboratory experiment.
Integrated Ocean Drilling Program Expedition 313 recovered siliciclastic sediments from three holes cored through a series of Miocene clinoforms offshore New Jersey.The clinoform sequence that lies between seismic reflectors m5.2 and m4.1 has been dated as mid-Miocene within an interval of major climatic change and displays interesting sedimentological and petrophysical features.However, the depth ties between surfaces in the recovered succession and seismic reflectors, correlation across sites, and depositional environments are not all well defined in this interval.Additionally, features observed in the two more proximal boreholes are absent from the most distal borehole.This report presents X-ray fluorescence (XRF) measurements from scanning the split surface of archive sediment cores and from individual core samples from Holes M0027A, M0028A, and M0029A.Major trends are identified and described, aided by statistical analyses (correlation coefficient matrix plots and principal component analyses).Si/Al and Zr/Rb ratios and, to a lesser extent, Th correspond with changes in the ratio of clay minerals to quartz.In the most proximal Hole M0027A, an alternating sequence of dark and light bands in an extended sequence of clays is characterized by distinctive high and variable magnetic susceptibility and equivalently variable Fe/S ratios.A similar sequence is observed in Hole M0028A, with both sequences located above the inferred m4.1 seismic sequence boundary.Redox elements in this interval are highly variable and suggest the influence of postdepositional processes.In Hole M0029A, despite an expanded sequence, similar characteristics are absent from the clay sequences, which are also geochemically more homogeneous.The analyzed geochemical compositions are compared with sedimentological observations and petrophysical analyses before discussing in their wider context.
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