Geology and the origin of trachytes and pantellerites from the Eburru volcanic field, Kenya Rift

The Eburru volcanic field is located in the Kenya Rift, where it is part of the very young axial volcanic activity. The Eburru field belongs to the complex of volcanoes -- Menegai, Eburru, Olkaria, Longonot, and Suswa -- that are centered on the Kenya Dome. All of these volcanoes are prime targets for geothermal energy, with Kenya's one geothermal plant at Olkaria.. Correlation with dated volcanism implies that the activity at Eburru is at most approximately 500,000 years. The surfaces preserved on the youngest flows suggest that they erupted within the last 1,000 years. Mapping indicates that the volcanic field is divided into an older western section, composed of pantellerites (Er1) and overlying, faulted trachytes (Et1), and a younger eastern section. The eastern section has a mapable ring structure, and is composed of trachytes (Et2) and pantellerites (Er2). Some of these flows may be contemporaneous, but the final phase of eruption is exclusively pantellerite. We have chemical data for all units except the older pantellerites. The data indicate that the trachytes and rhyolites are both pantelleritic in terms of their alumina and iron contents. This is in contradistinction to the rhyolites immediately adjacent at Olkaria, which are comenditic. Concentrations for all elements are highly elevated, except for Ba, Sr, K, P, and Ti that show deep negative anomalies. The relationship between the trachytes (Et2) and pantellerites (Er2) is one in which the pantellerites consistently have the highest concentrations in all elements, including those with negative anomalies. Correlation coefficients for pairs such as Zr and Rb support the field evidence for the western Et1 trachytes being a separate magmatic event from the Et2 and Er2 units of the eastern field. Sanidine is the principal phenocrystic phase in these rocks, and thus the elevated Sr and Ba in the pantellerites preclude simple crystal fractionation to derive pantellerite from trachyte. Bailey and Macdonald (1975, Min. Mag. 40, 405-414) reached the same conclusion and noted high correlation coefficients among F, Zr, and Rb on one hand and Cl, Nb, and Y as a second group. They argued that a halogen-bearing vapor is important to the genesis of the pantellerites. We have verified the same elemental correlations for our data set. Lowenstern (1994, Amer. Min. 79, 353-369) documented immiscible halide fluids in fluid inclusions from the type locality Pantelleria, Italy. This direct observation of halide lends credence to the importance of fluids in peralkaline rhyolites. We are currently examining fluid inclusions from the Eburru samples for similar evidence of a complex volatile phase.