THE RELATIONSHIP BETWEEN SEISMIC VELOCITY, MINERAL-COMPOSITION AND TEMPERATURE AND PRESSURE IN THE UPPER-MANTLE - WITH AN APPLICATION TO THE KENYA RIFT AND ITS EASTERN FLANK

Using the elastic properties of individual minerals the velocity of a rock at a known temperature and pressure can be calculated. Alternativ ely, from P- and/or S-wave seismic velocities at a particular depth (p ressure and temperature) the mineralogical composition of a rock may b e estimated. In this study, this procedure is applied to the uppermost mantle beneath the Kenya Rift and its eastern shoulder. It will be sh own that, for estimated in-situ temperatures and pressures, the minera logical compositions derived from mantle xenoliths found on the easter n shoulder are compatible with the uppermost-mantle P-n velocities der ived from the KRISP 90 seismic experiment beneath the eastern shoulder . Here the P-n velocities are compatible with an average undepleted sp inel peridotite composition at temperatures of 300-400 degrees C, corr esponding to surface heat flows of 35-50 mW/m(2), at 30-35 km depth an d in certain areas with a composition containing a 10-15% increase in olivine at temperatures of 600-670 degrees C, corresponding to a surfa ce heat flow of about 75 mW/m(2), at 30-35 km depth. Beneath the Rift itself, where no mantle xenoliths have been found, the mineralogical c omposition derived from mantle xenoliths found on the eastern shoulder is not compatible with the low observed uppermost-mantle P-n velociti es of 7.5-7.7 km/s. Under both the northern Rift and the southern Rift where Moho temperatures of respectively 630 degrees C at 21 km depth (5.6 kbar) and 1000 degrees C at 35 km depth (9.6 kbar) have been esti mated, corresponding in both cases to a surface heat now of 105-120 mW /m(2), the observed P-n velocities are 0.1-0.3 km/s lower than the pre dicted velocities. In order to explain these low observed velocities, the presence of 3-5% basaltic melt rising from greater depths and bein g trapped just below the Moho is invoked. At depths greater than those penetrated by the P-n-wave, for the northern Rift a temperature-depth profile of 830 degrees C at 35 km depth (10.0 kbar), 940 degrees C at 45 km depth (12.9 kbar), 1030 degrees C at 55 km depth (16.0 kbar) an d 1100 degrees C at 63 km depth (18.4 kbar) has been used, while for t he southern Rift values of 1200 degrees C at 45 km depth (12.9 kbar), 1300 degrees C at 55 km depth (16.0 kbar) and 1350 degrees C at 63 km depth (18.4 kbar) have been utilized. Beneath the southern Rift the ob served low velocities which are less than 7.8 km/s, can again be expla ined by the presence of basaltic melt which probably exists as in-situ partial melt below 45 km depth. The high-velocity layers beneath the northern Rift with velocities of 8.05-8.15 km/s at a high estimated te mperature of about 940 degrees C at 45 km depth and about 8.3 km/s at a high estimated temperature of about 1100 degrees C at 60-65 km depth require some kind of preferred mineral orientation. Possible models i nclude either a transverse isotropic structure in which the slow b-axi s of olivine is oriented vertically and the faster a- and c-axes are r andomly oriented in the horizontal plane or an orthorhombic structure in which some 40-55% of the olivine has the fast a-axis oriented horiz ontally along the rift axis. In the case of the layer at 60-65 km dept h, if a transverse isotropic structure exists, then depletion of basal t may additionally be required in order to reconcile the observed and predicted seismic velocities.