The Azores hotspot: A lower mantle origin for Terceira magmas as shown by Ne isotopic data

Abstract

The collection of helium isotopic data in the last twenty years has shown different signatures for MORB and OIB basalts and this has been used as the basis for the two-layer mantle model. However, for some OIB that appear in the vicinity of mid-ocean ridges, He isotopic ratios are similar to those typical of MORB, leading some authors to argue against the deep mantle plume hypothesis particularly in areas where geophysical evidence is lacking (Anderson, 2000). For Terceira island (Azores), located close to the Mid- Atlantic Ridge, we analysed neon from olivine phenocrysts in order to better constrain the nature of the Azorian plume. Some of the analysed samples show higher 20Ne/22Ne and 21Ne/22Ne ratios than air suggesting the presence of a mantle component in the olivine melt inclusions. Moreover, in a three-Ne isotope diagram, the samples define a trend with a higher slope than the MORB line (Sarda et al., 1988) arguing for a lower mantle origin. One basaltic sample coming from the 1998 submarine Serreta eruption is plotted along this line, suggesting that the influence of a deep plume is still active. In a plot of 21Ne/22Necorr vs. 4He/3He, Terceira data is plotted in the same hyperbolic mixing line that encompasses Iceland, Shona and Discovery data (Moreira et al., 1995, 2001; Sarda et al., 2000). The data suggests the existence of plume-ridge interaction for these hotspots resulting on mixing between degassed MORB-type and deep plume-type magmas. In such an environment the data shows that neon is a better tracer of primordial material than helium.