Melt generation processes have been investigated by
U-Th isotope studies of selected OIB-like basalts from the Azores, the Canaries, and the Auckland field, New Zealand. The first two are above hotspots with low inferred bouyancy fluxes, whereas the Auckland rocks appear to have been generated in response to regional tectonic movements rather than a mantle plume. All are characterised by significant (230Th/238U) disequilibria. Within the Azores the islands of Pico, Sao Jorge, and Terceira are ~250 km closer to the MAR than Sao Miguel. For most samples the 230Th excess is greater in the rocks from Pico, Sao Jorge and Terceira, and so the degree of disequilibrium is greater closer to the MAR, rather than adjacent to, for example, the Terceira Rift. On the MAR near the Azores 230Th excesses range from 20 to 35% (Bourdon et al., 1995), similar to those reported here for the islands of Pico, Sao Jorge, and Terceira. However, the larger 230Th excesses are found in the samples from the shallower water depths, and the overall negative correlation between (230Th/238U) and water depth suggests that there is a positive correlation between (230Th/238U) and 87Sr/86Sr. That is the opposite to what is observed on the Azores islands within which (230Th/238U) decreases with increasing 87Sr/86Sr. Excess 230Th is widely attributed to residual garnet during partial melting, as are high Nb/Y ratios. However, within the Azores (230Th/238U) decreases with increasing Nb/Y, and since the latter are correlated positively with 87Sr/86Sr, and negatively with Th/Nb and Th/U, they would appear to have been influenced by the changing source compositions. The low Th/Nb and Th/U ratios of the high 87Sr/86Sr rocks from Sao Miguel are consistent with a significant contribution from recycled sediment and that material is different from the plume component sampled at the islands of Pico, Sao Jorge and Terceira, and by implication on the adjacent MAR.
Rocks from the 1730-36 and the 1824 eruptions on Lanzarote are among the most primitive known from ocean islands. SiO2 = 44-51%, MgO = 9.5-12.5%, 87Sr/86Sr = 0.703062-0.703315, (230Th/232Th) = 0.95-1.3 and most samples have significantly lower (230Th/232Th) than other oceanic basalts with similar Sr isotope ratios. (230Th/238U) ranges from 1.05-1.85, but the higher values are observed in the rocks which are silica undersaturated and have high Nb/Y and Th abundances. (230Th/238U) in the silica saturated rocks are consistent with melt generation rates of ~ 10-4 kg m-3 yr-1, but the negative trend between (230Th/238U) and Th contents in the silica undersaturated rocks requires more complex melting models. One interpretation is that there was an inverse relation between the degree of melting and the time spent in the melting regime, with the smaller degree melts having been extracted in a shorter time than the higher degree melts. The Auckland volcanic field is unusual in that small volumes of silica under saturated magmas with the trace element characteristics of HIMU OIB were erupted in a non-plume intraplate setting: SiO2 = 41-48%, MgO = 10-13%, 87Sr/86Sr = 0.70287-0.70327, 206Pb/204Pb = 19.06-19.2, (230Th/232Th) = 1.12-1.28 and (230Th/238U) = 1.24-1.51. Variations in fractionation-corrected trace element abundances are used to evaluate the relative KD values during partial melting in all three examples. Batch melting, and rapid melt extraction, may explain the inferred fractionation in Th/U and Sm/Nd for the Auckland rocks. However, those for Lanzarote and the Azores would appear to require more complex dynamic melt generation models with melt generation rates in the range 10-3 to 10-5 kgm-3 yr-1 (e.g. Chabaux and Allègre, 1994).
Bourdon, B. et al., Abs., Terra Nova 7, 144 (1995).
Chabaux, F. & Allègre, C.J., Earth Planet. Sci. Lett. 126, 61 (1994).