Uranium series, major and trace element geochemistry of lavas from Tenerife and Lanzarote, Canary Islands.
Ocean Islands Basalts provide important windows into the compositional variations of
the Earth's mantle, which in tum constrains models for mantle convection and evolution.
The Canary Islands show contrasting styles of eruption and evolution of magmas in an ocean
island setting. U-Th-Ra disequilibria have been used to constrain rates and timescales of melt
generation and differentiation beneath ocean islands, and to estimate the buoyancy flux, potential
mantle temperature and the depth and degree of melting. The Canary Islands provide a rare
opportunity to observe U-Th-Ra disequilibrium, because they are underlain by a region of low
buoyancy flux, and were expected to show significant disequilibrium.
Tenerife is underlain by numerous magma chambers, in which magmas have time to
differentiate from basanites to phonolites, erupting to form large strato-volcano complexes. The
fissure and small vent eruptions of unusually primitive basanites and alkali basalts from
Lanzarote show little evidence of magma chambers, unless of substantial size and longevity at
depth. The U-Th results indicate that lavas underwent rapid transport from the melt region.
The historic and recent pre-historic eruptions (1824, 1730-36, Corona) from Lanzarote
have some of the most primitive compositions found on oceanic islands with low Si02 contents
« 51 %), Mg numbers of 67-74 and high Cr and Ni contents. The rocks are restricted in Sr,
Nd and Pb isotopes, being displaced from MORB towards the HIMU om field. The major and
trace elements have been modelled by mixing a deep smaller degree (1 %) melt and a shallower
larger degree (4%) melt. Negative K anomalies were observed in the small degree melts
indicating that melt generation may have continued at a shallow level, perhaps to within the
lithospheric mantle with melting in the presence of residual phlogopite. The Lanzarote source
was modelled as a mixture of HIMU and EMIl asthenospheric mantle, with a small contribution
from a shallow, lithospheric source. Thermal erosion of the lithospheric mantle is required for
melting at depths (58 and 73 km) modelled from the ma~or and trace elements. The Lanzarote
lavas exhibit significant e~h/238U) disequilibrium with 3!7h excesses of 6 - 81 %. This was
modelled by dynamic melting giving a calculated melt rate of 0.125 x 10-3 kg.m-'.yr- I
timescale of melt generation (matrix transfer time) of 270 ka for the 1 % melt and 1,100 ka for
the 4 % melt. A consistent upwelling rate of I cm.yr-I and an assumption that the melting
process has remained consistent over tens of km at depth.
The Teide-Pico Viejo complex lavas have undergone fractionation and mixing to form
compositions from basanite to phonolite. Crystallising phases differ in the Pico Viejo series,
where amphibole is dominant in the more evolved lavas, and Pico Teide series, where olivine in
the major control. The more evolved lavas require assimilation and fractional crystallisation to
explain the range in 87Sr/86Sr. e3!7h/238U) ranges from 1.004-1.39 and gives information
regarding the timescales of differentiation within the magma chambers, not least because the
youngest mafic rocks have the highest CZ3!7h/238U) and the most evolved phonolites have the
lowest. The timescale of differentiation from basanite to phonolite is of the order of 150,000
years, which links to the periodicity of the eruption cycles on the island. A Ra-Th 'pseudo'
whole rock isochron gave an age of fractionation for the Montafia Blanca eruption of 2.3 ka ±
80, which is a maximum of 300 years prior to eruption, indicating that fractionation of
plagioclase as a possible trigger of an eruption.