Seminars Archive

Magmatic differentiation and bimodality in oceanic island settings - Implications for the petrogenesis of magma in Tenerife

Abstract
The most recent succession of lavas on Tenerife (<200ka) show a marked bimodality in the composition of erupted lavas. To constrain the petrogenetic causes for that, this succession has been studied for their isotopic compositions of groundmass (Sr, Nd, Pb and O) and feldspar phenocrysts (Sr). The results have been combined with pre-existing stratigraphical and age constraints and a whole-rock major and trace element dataset. Intermediate lavas on Tenerife are, apart from fractional crystallisation and re-melting of country rock, likely formed by mixing of a mafic and a felsic magma. The Montaña Reventada composite eruption consists of a basanite and a phonolite member. The phonolite contains abundant mafic inclusions. Major and trace element and isotope data (Sr, Nd, Pb and O) allow to determine that these have been affected by mixing and diffusion to form intermediate composition magmas. Unequal crystal exchange between phonolite and inclusions may account for irregularities in mass balance. Individual zones of feldspars from these lavas were analysed for their major and trace elements and 87Sr/86Sr composition. A homogeneous range of 87Sr/86Sr ratios is found in plagioclases, with similar ratios in the majority of the lavas, apart from the most evolved phonolites discussed above. Empirical equations allow to model the MgO (wt.%) and Sr (ppm) composition and the temperature of the primitive melts these feldspars crystallised from. The melts with the highest crystallisation temperature of feldspar are the ones that show a lower Zr and Sr concentration than the remaining mafic suite. EC-RAχFC modelling, where cumulate rock that previously crystallised from the same principal magma served as the main assimilant, allowed reproducing the composition of high-Sr/high-Zr primitive lavas, using the low-Sr/low-Zr (hotter) lava composition as primary magma. Various amounts of assimilation and fractional crystallisation in permanent magma chambers at upper mantle/lower crustal depth appear to affect the primitive lavas on Tenerife, most strongly in the high-Sr/high-Zr group than in the low-Sr/low-Zr mafic suite. Pb and Sr isotopes in the succession under study indicate that an open system is at work in the recent Tenerife plumbing system, while at the same time excluding an influence of sediment as contaminant. Elevated 87Sr/86Sr ratios in phonolites require an end-member of comparable isotope characteristics, however, existing data is insufficient to explain the variations observed in phonolites. In turn, a hypothetical contaminant, well within the limits of existing data, shows that the two phonolites with the highest 87Sr/86Sr ratio may be derived almost exclusively from crustal melting. The low Sr/Nd of this contaminant requires a highly differentiated rock to be assimilated. Major and trace and d18O data of highly differentiated Tenerife ignimbrite are consistent with this and shows that this type of rock may be the real end-member. AFC modelling, in turn, emphasises the importance of fractional crystallisation for the bulk of the succession. This study presents evidence for a distinct combination of differentiation processes (fractional crystallisation, crustal melting and magma mixing) characteristic to each compositional group of lava and, hence, a discontinuous differentiation of magma on Tenerife.