A U-Series Disequilibrium Study of the Longonot Trachyte Magma Chamber, Kenya

Evans, Peter James (1999). A U-Series Disequilibrium Study of the Longonot Trachyte Magma Chamber, Kenya. PhD thesis The Open University.

DOI: https://doi.org/10.21954/ou.ro.0000ff52

Abstract

Longonot volcano is situated in central Kenya in the vicinity of Lake Naivasha. It is one of a series of Quaternary volcanoes aligned along the inner trough of the Gregory Rift Valley, part of the East African Rift. The 10 ka history of eruptions at Longonot is subdivided into an early explosive caldera pyroclastic stage (9150-5650 yrs BP), followed by a dominantly effusive series of emptions, the lava pile stage (5650 -1000 yrs BP). The Longonot volcanic rocks are extremely restricted in composition, being almost exclusively peralkaline trachyte.

Individual beds of the caldera pyroclastic stage are characterised by major and trace element compositional zonation, with no systematic relationship between beds. Successive flows of the lava pile stage are increasingly enriched in FeO(T) (7-9 wt %) and incompatible trace elements such as Th (12.5 - 23.6 ppm) but depleted in trace elements such as Ba (198 - 4.7 ppm) that are compatible in sanidine, which accounts for >80% of all phenocrysts. All trachytes have generally flat chondrite normalised rare earth element profiles but with negative Eu anomalies and a restricted range in Pb and Nd isotope ratios (207/Pb/204Pb: 15.66-15.73 n = 36, 143Nd/144Nd: 0.51257 - 0.51265 n= 34).

Trace element modelling shows that the trachytes are derived by partial melting of alkali basalts, possibly related to mafic underplating associated with rift extension. The caldera pyroclastics comprise a series of separate magma batches, whereas the lava pile stage comprises only two coexisting magmas from which successive eruptions sampled the top of the chamber as it became compositionally zoned. Trachyte differentiation took place in a near-surface magma chamber ( < 1 kb) with a complex structure as indicated by the co-existence of different magma batches, slight Pb-Nd isotopic heterogeneities and compositional zonation. Differentiation of the main lava pile trachytes is modelled by 50% closed-system fractional crystallisation.

High precision thermal ionisation mass spectrometry (TIMS) has been used to analyse Useries isotopes in the Longonot volcanics. The lava pile samples define a single nearhorizontal linear array of constant (230Th/232Th) (0.796 +/- 0.01 n = 22) but variable (238U/232Th) (0.601 - 0.715). The caldera pyroclastics have variably lower (230Th/232Th) than the lava pile (0.745 - 0.792 n = 14), but it is constant for each zoned pumice bed whereas (238U/232Th) is variable. Early lava pile trachytes have 226Ra excess but Ra partitioning into sanidine during differentiation results in (226Ra/230Th) < 1 (1.132 - 0.749 n =16) for most samples.

238U-230Th fractionation has been modelled as a two-stage process. The first stage involved partial melts ponding in a deep reservoir in almost closed system for 60 -220 ka, possibly with limited differentiation. Magma ascent was sporadic, and may have related to fault movements along the rift. Secondary 238U - 230Th fractionation during differentiation was due to variable crystallisation of accessory phases in a compositionally zoned near surface magma chamber.

Combined (230Th/238U) and (226Ra/230Th) disequilibria indicate that differentiation in the near surface magma chamber was rapid, possibly much less than 10 ka for the 32 km3 of magma that formed the lava pile and that compositional zonation may have developed in under 1 ka. This rapid differentiation is most readily achieved through complex sidewall crystallisation.

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