Rare Gas Signature of the Azores Archipelago

Manuel Moreira Laboratoire de Géochimie et Cosmochimie et Université Paris 7, 4 Place Jussieu,

T14 3ème étage, 75252 Paris Cedex 05, France


Claude-Jean Allègre Laboratoire de Géochimie et Cosmochimie et Université Paris 7, 4 Place Jussieu,

T14 3ème étage, 75252 Paris Cedex 05, France


We present here data of Helium isotopes obtained by crushing on phenocrysts from five islands from the Azores archipelago: Santa Maria, Sao Miguel, Pico, Terceira and Faial. The volcanism in the archipelago began more than
6 m.y. ago at the Santa Maria Island and is still active at present time (last eruption in Faial island: 1957), thus we can follow the evolution of isotopic ratios of the Azores hotspot with time.

Analytical procedure for the Helium

Because of the very low concentration of Helium in phenocrysts (olivines, pyroxenes) from oceanic basalts, we had to develop a new procedure for the crushing, to improve the blanks of Helium. This new procedure is nonly applicable for He (and Ne). The 4He blank was 5 10-10 CCSTP (with the normal procedure: 1.5 10-8). We installed a charcoal trap close to the crusher and trapped all the gases, except He and Ne, during 30 minutes with liquid nitrogen. We did not use Ti purifications but inlet only the He and Ne in the mass spectrometer just after this 30 minutes. The cold trap was sufficient to retain all active gases, thus we never observed any CO2 or other gases in the mass spectrometer. We analysed the 3He on a electron multiplier and the 4He in a faraday collector, or sometimes in a electron multiplier. The H2 is also measured for the correction of the HD on the 3He. Thus, it was possible to measure samples with very low concentration of He. With this procedure, we need between 1g and 2g of phenocrysts to have a precise result.

Results and discussion

We present here our preliminary results on the Helium from five islands from the Azores. The concentration of 4He vary between 2.10-9 and 5.10-8 CCSTP/g and the 4He/3He isotopic ratio vary between 63,000 (R/Ra=11.5) to 400,000 (R/Ra=1.8). The highest value of the 4He/3He ratio was found for Santa Maria and Sao Miguel islands, and the lowest ratios were found for the recent volcanism of Faial, Terceira and Pico. Thus, it seems that in the same archipelago, two types of component can be observed for the Helium. One is a radiogenic component observed at Santa Maria, Sao Miguel and along the Mid Atlantic Ridge between 36°N and 41°N where the 4He/3He ratio is upper than for the rest of the north Atlantic (Kurz et al. 1983). The second is a slightly "high 3He" component observed by us at Pico, Faial and Terceira. Recently, Hilton et al. (1995) observed two types of He isotopic ratios (radiogenic and primordial) in Heard island (Indian Ocean) and attributed
the high 4He/3He ratios to a contamination at shallow depth radiogenic rather than to a deep component like a subducted slab.

The high 4He/3He component observed along the
Mid Atlantic Ridge close to the Azores (Kurz et al., 1983) indicates that the radiogenic component observed at Santa Maria and Sao Miguel is a deep component rather than a shallow one. The evolution with time of the 4He/3He ratio in basalts from Azores to values lower than MORB ratio reflects probably the evolution of the source itself rather than interactions with the crust or magmatic chambers outgassing. We should note that the lowest 4He/3He ratios measured in the Hawaii chain are for the most recent manifestations of volcanism, which is the Loihi seamount.