Rates and Timescales of Fractional Crystallization from 238U-230Th-226Ra Disequilibria in Trachyte Lavas from Longonot Volcano, Kenya

Abstract

A suite of peralkaline trachytes from Longonot volcano, Kenya, which erupted during the last 6000 years, has been analysed for major and trace elements, Pb and Nd isotopes, and U–Th–Ra disequilibria. The lavas are divided into three stratigraphic groups of trachytes (Lt2a, Lt2b and Lt3), and hybrid lavas, designated LMx1 and LMx2, which, respectively, pre-date and post-date the Lt2 lavas. Major and trace elements are consistent, with up to 37% within-group fractional crystallization of predominantly alkali feldspar. The parental magma for the different trachyte groups had a more mafic composition—probably hawaiitic. Nd and Pb isotopes show minimal variation, both within and between magma groups, and indicate that up to 10% comendite magma from the neighbouring Olkaria volcanic field may have intermixed with the Longonot magma. (230Th/238U) disequilibria indicate that limited U/Th fractionation occurred during the past 10 kyr, whereas (226Ra/230Th) disequilibria reflect the effect of alkali feldspar fractionation >8 kyr ago in the Lt2a lavas, between 3 and 7 kyr ago in the Lt2b lavas and in the past 3 kyr for the Lt3 lavas. (226Ra/230Th) disequilibria in the Lt2b lavas are interpreted using a model that combines the equations of radioactive decay and in-growth with Rayleigh crystallization to give fractionation rates of about 0·2 x 10–4/year for the evolution of hawaiite to trachyte, but more rapid rates of up to 3 x 10–4/year for fractionation within the trachyte sequence. (226Ra/230Th) from two whole-rock–alkali feldspar pairs are interpreted to show the crystals formed at 5800 years BP (Lt2b) and 2800 years BP (Lt3), implying that phenocryst formation continued almost up to the time of eruption. The results strongly indicate that fractionated magmas can be stored for periods on the order of 1000–2500 years prior to eruption, whereas other magmas were erupted as fractionation was proceeding.