Preliminary data on volatiles (H₂O, S, Cl, F) in primitive magmas of Azores islands, Northern Atlantic

N.L. Mironov*, K.Hoernle**, M.V. Portnyagin**^,*

* Vernadsky Institute of Geochemistry and analytical chemistry, Moscow, Russia;

** IFM-Geomar, Kiel, Germany

The Azores islands in the Northern Atlantic (nine islands at ~ N 36.9° - 39.7°, W 25° - 31.3°) represent onland parts of a large Azores submarine plateau, likely related to plume magmatism near the Mid-Atlantic Ridge at the junction of the Eurasian, African and American plates (e.g. Krause, Watkins, 1970). Most of the islands are volcanically active. Azores volcanic rocks, ranging from alkaline Ol basalts to trachytes (GEOROC database), are typical of oceanic island basalts (OIBs). The most primitive volcanic rocks are geochemically very similar to the average OIB (Sun, McDonough, 1989).

Volatiles play a fundamental role in the evolution and magmatic processes of the Earth (e.g. Volatiles and Volatile-Bearing Melts in the Earth's Interior, 2009). They influence mantle melting, magma composition, magmatic crystallization and volcanic eruption. They can also have a serious impact on the environment, influencing the Earth’s climate. In respect to OIB genesis, knowledge about volatile contents in primitive OIB magmas can tell us about 1) volatile contents and their distribution in deep mantle sources, 2) assimilation processes (lithosphere/crust) that occur as magmas ascend to the surface, 3) volatile behavior during recycling processes, 4) the origin of isotopic heterogeneities in these volcanic rocks, and 5) the potential environmental risks connected with volcanic hazards.

It has long been questioned if the Azores are a “wet or hot spot” (e.g. Bonatti, 1990). Additional support that the Azores source is wet comes from elevated water contents in glasses from the Mid-Atlantic Ridge segment adjacent to Azores islands (Kingsley, Schilling, 1995; Asimow et al., 2004). However data from direct measurements of water and other volatiles in Azores island volcanic rocks have not yet been published.

Here we present preliminary data on volatiles (H₂O, S, Cl, F), trace and major elements in primitive Azores magmas based on study of naturally quenched melt inclusions (MI) in olivines Fo_90-78(Fig. 1, 2) from 4 tephra samples from two Azores islands: Pico (P) and Sao Miguel (SM). Isotopically the samples represent a component common to all Azores islands, similar to the FOZO-type plume component (e.g. Beier at al., 2007; Millet et al., 2009). 25 melt inclusion glasses were analyzed for major elements, S and Cl using an electron micro-probe and 14 of them for trace elements, F and H₂O with ion-probe (SIMS). Melt inclusions have basaltic and trachybasaltic composition (SiO₂=42-49, on average 47 wt.%) with TiO₂ variation from 2.8 to 5.6 wt.% (on average 3.5 wt.%). Glasses mostly belong to high-potassium series (K₂O average = 1.5 wt.%). They correspond very well in composition to Azores mafic (MgO>5 wt.%) volcanic rocks and show similar trends. Trace element content and their patterns in melt inclusions are similar to average OIB and Azores rocks. Geochemically SM inclusions are more enriched then P inclusions (e.g. La/Sm=4.2 and 3.3), which possibly reflects smaller degrees of mantle source melting beneath SM island (e.g. Beier et al., 2008).

Melt inclusions contain 0.1-1 wt.% of water (0.7 wt.% on average), 0.04-0.28 wt.% of sulphur (0.16 wt.%), 0.013-0.117 wt.% of chlorine (0.066 wt.%) and more than 0.1 wt.% of fluorine (0.107-0.165, 0.135 wt.% on average). The volatiles show no correlation with olivine-host composition (Fig. 2).

H2O concentration is quite typical for OIB magmas (Fig. 2A), yet quite high (up to 1 wt. % for MI in primitive olivine Fo>85) but is lower then could be predicted from general OIB systematics based on H₂O/Ce ratio (~200-250 - e.g. Dixon et al., 2002). H₂O/Ce is ~50-90 for SM and slightly higher (130-140) for P inclusions. Lower values for SM can be explained by larger contribution from highly dehydrated (recycled) material (e.g. Simons et al., 2002) as supported by elevated proportion of pyroxenite in the source of SM (Sobolev et al., 2007). It should be also taken into consideration that water could be partially lost from inclusions after their entrapment and so represents minimum values (e.g. Portnyagin et al., 2008). Present data on water content combined with relatively high mantle potential temperatures (~1450-1500 ºC versus 1300-1400 ºC for ambient MORB mantle) calculated for primitive P and SM rocks and for studied melt inclusions (Herzberg, Asimow, 2008) suggest wet and hot spot character of Azores plume magmatism.

Most primitive inclusions (Fo_89-90) have 0.13-0.17 wt.% of sulphur, which correspond to the most sulphur rich OIB composition (S>0.14 wt.% - Fig. 2B). This concentration can be regarded as a minimum value for parental Azores magmas due to occurrence of small sulfide globules inside the inclusions. S/Dy ratio for all Azores inclusions (~260 on average) is close to OIB (290) and MORB (250) values (e.g. Koleszar et al., 2009).

Chlorine concentration shows systematic variation depending on studied localities. While P island inclusions (0.037 wt.% Cl on average, Cl/K=0.02-0.05) have quite typical values for OIB, SM inclusions show ~2 times higher concentration (0.07 wt.% Cl, Cl/K=0.06-0.1) (Fig. 2C). This correlates with an increase of lithosphere thickness and possibly reflects smaller degrees of melting beneath SM, but also can be related to interaction of primary magmas with brine-impregnated oceanic crust or lithosphere (e.g. Kent el al, 1999; Lassiter et al., 2002). Alternatively higher Cl content could be due to a larger contribution from recycled altered and less dechlorinated oceanic lithosphere (e.g. Stroncik, Haase, 2004).

Studied melt inclusions have relatively high fluorine content, which is more then 2 times higher then average value for OIB (Fig 2D). In contrast to chlorine there is no difference in fluorine content between Sao Miguel and Pico islands. The high values seem to correlate with enriched character of Azores mantle source. Higher F values in evolved trachytic and much more explosive magmas can be expected due to enrichment of F during fractionation. High F contents could have a dramatic effect on agriculture, cattle-breeding and the civilian population on the Azores.

Fig. 1 Naturally quenched melt inclusions in primitive olivine Fo₈₉ from Sao-Miguel island.

Fig. 2 Volatiles (H₂O, S, Cl, F) in studied olivine-hosted (Fo) melt inclusions. Histograms show a distribution of volatiles in studied Azores samples and published data on melt inclusions from oceanic islands settings (GEOROC data base – melt inclusions, plus Galapagos data – Koleszar et al., 2009; Portnyagin et al., 2008 and unpublished)

The research was supported by RFBR grant 09-05-01234 (Russia) and SFB 574 (Germany).

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