Interpreting an alkaline lava series from Gran Canaria
In a 400 m roadcut through the Tejeda caldera on Gran Canaria, you sample five lavas in stratigraphic order. From base to top they are: basanite (SiO2 44 wt%, MgO 12 wt%, Ni 280 ppm, Zr 140 ppm); hawaiite (SiO2 50, MgO 6, Ni 90, Zr 280); mugearite (SiO2 54, MgO 3, Ni 35, Zr 430); trachyte (SiO2 61, MgO 1.2, Ni 10, Zr 620); and phonolite (SiO2 57, MgO 0.8, Ni 8, Zr 780). A phonolitic dyke cuts the sequence. In the basanite you find centimetre-sized xenoliths of spinel lherzolite with olivine Fo89 and Cr-spinel. The roadcut is part of a proposed geosite, and the local council asks you to write a short interpretative panel for visitors. You have no thin sections and no isotopic data, only the field relations and the numbers above. The panel must be scientifically defensible but readable by non-specialists, and it must explain why the phonolite has lower SiO2 than the trachyte even though it is the most evolved rock in the sequence.
1. Identify the process that best explains the chemical variation from basanite to phonolite. Use at least three elements and the stratigraphic order to support your answer.
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The best explanation is fractional crystallisation of a parental alkaline magma, with the lavas representing progressively more evolved liquids. MgO falls from 12 to 0.8 wt% and Ni from 280 to 8 ppm; both are compatible elements removed by olivine and clinopyroxene crystallisation. Zr rises from 140 to 780 ppm because it is incompatible and concentrates in the residual melt. SiO2 rises from 44 to 61 wt% and then falls to 57 wt% in the phonolite, which is not a reversal of evolution: the series is silica-undersaturated, and phonolite contains alkali feldspar and feldspathoids such as nepheline rather than quartz. The stratigraphic order, from basanite upward to phonolite, is consistent with progressive differentiation of a single magma batch or a repeatedly replenished reservoir. Without isotopic data I cannot exclude crustal assimilation or magma mixing, but the smooth compatible–incompatible trends and the field succession make fractional crystallisation the most economical interpretation. The phonolitic dyke shows that evolved magma was still available after the trachytes erupted.
2. Explain the significance of the spinel lherzolite xenoliths for the depth and speed of magma ascent, and why they matter for the geosite panel.
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Spinel lherzolite is a mantle rock stable in the upper mantle, roughly 30–60 km deep, so the xenoliths are direct samples of the Canary mantle rather than crustal material. Their presence in basanite means the magma originated at mantle depth and rose fast enough to carry centimetre-sized, dense xenoliths to the surface without stalling or settling. Olivine Fo89 and Cr-spinel support a primitive, little-evolved mantle source. For the geosite panel, this is the key message: the basanite is not just a lava, it is a transport vehicle that accidentally sampled the mantle. The xenoliths also imply a rapid ascent path, probably along a dyke or fracture, which is relevant to how volcanic hazard is assessed. I would avoid overclaiming a specific depth without thermobarometry, but I would state that the xenoliths prove a mantle origin and fast ascent. This makes the outcrop scientifically valuable and gives visitors a tangible link between the lava and the deep Earth.