Noble Gas Constraints on the Origin of the Azores Hotspot

Abstract

Noble gas geochemistry is a valuable tool for assessing the nature of contributors to magma mantle sources. In this chapter, we analyse previously published data regarding helium and neon in the Azores to discuss the origin of the Azores archipelago. After the pioneering works of Kurz et al. (1982a, 1990) examining helium isotopic variations along the Mid-Atlantic Ridge (MAR) between 28°N and 53°N and on São Miguel Island, a systematic study was conducted by Moreira et al. (1999) on several Azores islands. These authors analysed He isotopic ratios from phenocrysts collected in five islands of the archipelago. Helium isotopic data from Terceira (minimum 4He/3He ratio of 63,700; R/Ra 11.3) were interpreted as the presence of a relatively primitive component in the mantle source, whereas the radiogenic 4He/3He ratios found at São Miguel (from 121,600 to 276,800; R/Ra from 5.9 to 2.6) have been interpreted as resulting from the melting of an enriched and ancient recycled material. Moreira and All ègre (2002) analysed noble gas isotopic data for MORB glasses dredged along a MAR segment (from 21.25°N to 39.9°N), which also includes the Azores triple junction area. The obtained 4He/3He ratios decrease from 90,000 at 37°N to 75,000 at 38.5°N and later increase to 100,000 at 40.5° N. The low 4He/3He ratio measured on the ridge at 38.5°N (76,000; R/Ra = 9.5) was interpreted as the result of present-day interaction between the ridge source and the Azores plume (see also Madureira et al. 2014), as sampled by lava erupted at Terceira and São Jorge islands the latter with 4He/3He ratios down to 40,000; R/Ra 18). Madureira et al. (2005) focused their study on Terceira Island, for which He and Ne isotopic ratios were determined from olivine phenocrysts. Ne isotopic data corroborated the presence of a relatively primitive component in the Terceira mantle source (21Ne/22Necorr = 0.052), which is, however, dominated by a 53 MORB-type component. Jean-Baptiste et al. (2009) presented helium isotope data for thermal waters and gas emissions sampled at Terceira, Graciosa and São Miguel islands, as well as Faial, Pico and Flores islands. The results were interpreted as the presence of relatively primitive He isotopic ratios at Terceira (4 He/3He 53,500; R/Ra 13.5), which also extend to Graciosa Island (4He/3He 64,500; R/Ra 11.2). These values contrast with those obtained by these authors for São Miguel Island, which are significantly more 65 radiogenic than those typical of MORB ( 4He/3He from 120,400 to 138,900; R/Ra from 5.2 to 6.0) and are in agreement with the helium measurements in lavas from the eastern part of São Miguel (Moreira et al. 2012). Noble gas geochemistry suggests, despite the strong dilution of its primitive signature by MORB-like material, that a lower mantle-derived mantle plume is located under the central group of islands, particularly under São Jorge, in agreement with seismic tomographic images. The peculiar helium isotopic ratios observed in 77 São Miguel lavas can be explained by different scenarios invoked for the Sr–Pb isotopic and trace element systematics, which suggest that the São Miguel source contains recycled mafic material *3 Ga in age (e.g., Beier et al. 2007; Elliott et al. 2007).