The origin of Na-alkaline lavas revisited: new constraints from experimental melting of amphibole-rich metasomes + lherzolite at uppermost mantle pressure

Grützner, T; Prelevic, D; Berndt, J; Klemme, S

Zusammenfassung

We present a new experimental dataset for reaction experiments between natural amphibole-clinopyroxene metasomes (hornblendite) and synthetic lherzolite that produced Na-rich alkaline melts. Experiments were conducted at 1, 3 and 4 GPa and 1000 to 1300 °C. The generated melts range from foidite over basanite to phonotephrite. At 1 GPa between 1000 and 1100 °C amphibole decompression-breakdown products generate phonotephritic melt. Among the breakdown components rhönite was found to be stable up to 1100 °C and 1 GPa. At 3 and 4 GPa the melt compositions are additionally influenced by phlogopite melting and shift to more foiditic composition. We find that the melting of metasome produces wehrlitic residue. The reaction of the melt with the lherzolite produces again a wehrlitic residue with different olivine : clinopyroxene ratios. Therefore, wehrlite formation does not always require separate metasomatic processes, but can be a direct by-product of alkaline volcanism. We applied a metasome melting model to the magmas of the Kula volcanic province, Turkey, and show that at 1 GPa basanite melts and phonotephrite melts cover the whole range of known Kula lava compositions. The Kula lava compositional trend can be therefore generated not by differentiation of basanite but by basanite-phonotephrite melt mixing. A comparison of high-pressure (3-4 GPa) melts with natural nephelinite data shows overlap with many major, minor, and trace elements but differences in SiO2, FeO, and TiO2 argue that the natural nephelinite data do not represent primary metasome melts.