NEAR SURFACE MAGMATIC LENSES IN THE CENTRAL KENYA PERALKALINE PROVINCE, EAST AFRICA RIFT: EVIDENCE FROM URANIUM SERIES DISEQUILIBRIA AND THERMODYNAMIC MODELING OF GLASS MINERAL DATA
The most recent 230Th/232Th activity ratios for calderas and peripheral fissure fed eruptions plot as horizontal lines on an equiline diagram and decrease towards the equline as the units become older. Therefore, the mafic and felsic eruptions originate from the same shallow magma lenses. Furthermore, the data confirm that the source region for both mafic and felsic magmas was the garnet peridotite mantle and the magmas, including the most evolved pantellerites, experienced minimal crustal interactions. The sole exceptions are recent comendites from the Greater Olkaria Volcanic Complex, which plot on the equiline and probably represent melting of a syenite.
Mineral glass data have been used for QUILF and MELTS modeling for Eburru (Ren and others, 2006) and Suswa (White and others, 2012). We are currently revisiting Suswa using models from Putirka and others (2003) and Masotta and others (2013). Early activity saw construction of a trachytic shield volcano, followed by mafic-felsic magma mingling, explosive volcanism, and caldera collapse at 41 ka. Continued activity produced a second, inner caldera and eruption of phonolites. Our preliminary thermobarometric results suggest the pre- and syn-caldera eruptions originate from a magma chamber at 400 MPa, which represents the Conrad discontinuity in this region and thus would serve as a density filter. The earliest phonolite eruptions originate at 250 to 350 MPa and 927 C. The recent eruptions (10 ka to recent) originate at 135 MPa to 80 MPa. These shallow depths agree with geothermal drilling at Menengai, which encountered magma mush at 2-5 km.