Melt generation beneath ocean islands

A U-Th-Ra isotope study from Lanzarote in the Canary Islands

L. E. Thomas*, C. J. Hawkesworth, P. Van Calsteren, S. P. Turner, N. W. Rogers

*Corresponding author for this work

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Abstract

New major and trace element data, and Sr, Nd, Pb, U, Th and Ra isotopes are presented for prehistoric and historic lavas from Lanzarote in the Canary Islands. These rocks are amongst the most primitive found on intraplate ocean islands ranging in composition from basanite to alkali basalt, with MgO contents >9.3% and Mg numbers >67. The youngest are from two of the three 1824 vents, the largest group of samples is from the best known eruption episode, the 1730-36 Timanfaya eruptions, a smaller group of samples are from the northeast Corona region (~50 ky) and the oldest samples are from the Famara complex and basement massif. The rocks have some of the characteristics of HIMU OIB, including high Ce/Pb, Nb/Ce and low Nb/U and restricted 87Sr/86Sr (0.70209-0.70332). There is significant 230Th/238U disequilibrium (230Th excesses range from 6-76%) with some of the intermediate silica composition Corona samples showing the greatest disequilibrium. The historic samples exhibit 226Ra excess. The major and trace element data have undergone fractionation corrections to Mg numbers of 70, requiring <5% olivine fractionation, and there inferred primary compositions are used to evaluate a number of melt generation and mixing models. The fractionation-corrected compositions for the 1824 and the 1730-36 have been modelled as 1-4% melts of a source similar to primitive mantle. However, Yb is incompatible, and so the amounts of residual garnet were < ~5%, suggesting that there was no significant contribution from garnet pyroxenite source rocks. Rather the REE and the FeO contents are both consistent with melting in the garnet-spinel transition, at depths of 60-70 km. (230Th/238U) increases slightly with increasing La/Yb in the younger rocks, and they require some form of dynamic melting model. In the preferred model the upwelling rate is kept constant, and the differences in the degrees of melting are attributed to different lengths of the melt column, with the smaller degree melts being extracted from greater depths. Strikingly, (226Ra/230Th) increases with increasing degrees of melting, and so it reflects the time since extraction from the melt column rather than variations in the melting processes. Intra-suite minor and trace element variations are due to magma mixing, and not to progressive changes in the degrees of partial melting, and it is envisaged that such magma mixing occurred during the dynamic melting processes. Dynamic melting at depths of 60-70 km suggests that the regional uplift around the Canary Islands is at least in part due to thermal erosion of the underlying lithosphere. Variations of average Ce/Yb, Tb/Yb, (230Th/238U), SiO2 and lithospheric age for different OIB highlight how the smaller degree melts tend to be generated at greater depths, and the mean pressure of melting increases with the thickness of the lithospheric lid. However, there is no general link between (230Th/238U) and the lithospheric age or thickness, and hence the integrated degrees of melting. High buoyancy fluxes result in higher degrees of melting and low (230Th/238U) (Chabaux and Allegre, 1994), but for OIB generated within low buoyancy plumes, (230Th/238U) and the degrees of melting primarily depend on the depths of melt extraction. Differences in the average composition of low buoyancy OIB depend on the thickness of the overlying lithosphere (Ellam, 1992; Haase, 1996), and the differences within an OIB suite, such as between the 1730-36 and 1824 lavas on Lanzarote, depend on the depth of extraction from the melt column.

Original languageEnglish
Pages (from-to)4081-4099
Number of pages19
JournalGeochimica et Cosmochimica Acta
Volume63
Issue number23-24
Publication statusPublished - Dec 1999
Externally publishedYes

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Thomas, L. E., Hawkesworth, C. J., Van Calsteren, P., Turner, S. P., & Rogers, N. W. (1999). Melt generation beneath ocean islands: A U-Th-Ra isotope study from Lanzarote in the Canary Islands. Geochimica et Cosmochimica Acta, 63(23-24), 4081-4099.