Geochemical processes in the roots of the Azores magmatic systems

• Published in: Contributions to mineralogy and petrology 2024-06, Vol.179 (6), p.64, Article 64
• Main Authors: Zanon, Vittorio, Métrich, Nicole, D’Oriano, Claudia
• Format: Article
• Language: English
• Subjects: Aluminum oxide; Archipelagoes; Basalt; Chemical evolution; Earth and Environmental Science; Earth Sciences; Fault lines; Fractional crystallization; Geochemistry; Geology; Halogens; Inclusions; Islands; Lava; Magma; Mid-ocean ridges; Mineral Resources; Mineralogy; Oceanic crust; Olivine; Original Paper; Petrogenesis; Petrology; Ratios; Roots; Tephra; Volcanic activity; Volcanic islands; Volcanoes; Zirconium
• ISSN: 0010-7999; 1432-0967
• DOI: 10.1007/s00410-024-02142-z

The Azores archipelago, situated east of the Mid-Atlantic Ridge, comprises volcanic islands arranged along sub-parallel spreading systems and rests on a thick oceanic crust. Magma is supplied directly from the roots of the volcanic systems. Located at or nearby the boundary between the crust and the mantle, they consist of mafic cumulates and mafic mush layers. This work focuses on tephra samples and a submarine lava younger than 40.000 years, collected from both central volcanoes and fissure zones. Our report details a new dataset of major, trace, and volatile elements analysed in glassy melt inclusions trapped in olivine (Fo 75.8–85.6 ) which are extracted from cumulative bodies at the vicinity of the crust-mantle boundary. Their compositions cover a range from subalkaline to mildly alkaline basalt, and trachybasalt, which match those of Azores lavas. They registered a chemical evolution through fractional crystallisation of olivine alone, as well as olivine and clinopyroxene, as both the FeO t /MgO (1.4–3.1) and CaO/Al 2 O 3 (0.4–1.0) ratios of the melt decrease. Incompatible element ratios of Zr (40–352 ppm), Ba (135–612 ppm), and Rb (5–77 ppm), as compared to Nb (5–82 ppm), exhibit variability within a limited but significant range of values. The ranges in the Nb/Zr, Ba/Nb and Rb/Nb ratios recorded by melt inclusions possibly reveal distinct geochemical sources (at least two), and mixing between partial melts as they move upward. The halogen signature is characteristic of the shallow mantle. The majority of melt inclusions show Cl/K ratio (0.06) similar to E-MORB, although some of them are comparable to N-MORB (Cl/K = 0.03). Their F/Nd ratio may achieve a rather high value (27.8).