Major element concentrations of minerals vary in response to both melt composition and magma chamber conditions including pressure, temperature, volatile content and oxygen fugacity (fO 2 )12. However, trace element concentrations are almost entirely a function of melt composition and are largely independent of changes in intensive parameters11. With the exception of rare patchy crystals, concentric zonation is evident (Fig. 2), implying little alteration by post-crystallization diffusion12. Within crystal mantles, compositions oscillate (Fig. 3), but zonation generally shows increasing An# (Ca/(Ca+Na) × 100) in plagioclase and either flat or slightly increasing Mg# (Mg/(Mg+Fe) × 100) in clinopyroxene towards the outer rim (i.e. reverse zoning). This is also reflected in trace element transects by increasing FeO in plagioclase (Fig. 3). However, the most striking feature of the cumulus zoning is a sharp decrease in plagioclase An# and clinopyroxene Mg# at the rims of these crystals, with a corresponding drop in Fe content and Al/Ti over the same distance.

Figure 3 Compositional profiles across representative plagioclase and clinopyroxene cumulus phases. (a) plagioclase and (b) clinopyroxene crystals from the La Caleta Formation, (c) plagioclase crystal from the Poris Formation, (d) clinopyroxene crystal from the Fasnia Formation. Photomicrograph images above the compositional profiles show optical zoning and position of compositional transects, shown below, which were collected from core to rim. The boundary between core- and mantle-regions is shown with a dashed line, where applicable. The grey shading in compositional profiles highlights the evolved zone at the crystal rims. Full size image

In plagioclase, An# correlates positively with melt temperature and H 2 O content, with changes in chamber pressure exerting only a minor control13. Additionally, fluctuations in clinopyroxene Mg# can occur in response to changes in melt fO 2 (Ref. 14). Therefore, the oscillatory major element zoning may result from closed system processes, including crystal movement along thermal or compositional gradients, or open system processes, such as magmatic recharge11,13. However, the large magnitude changes in An# and Mg# approaching the rim of the cumulate minerals are unlikely to result from variations in intensive parameters alone12 and are more consistent with a sudden switch to a more evolved (felsic) melt composition.

As a trace element in plagioclase, Sr correlates negatively with An# through the crystal mantles (Fig. 3). Although bulk melt composition may influence plagioclase-melt Sr partitioning, particularly in more evolved systems10,13, the dominant control on Sr is its increasing compatibility in plagioclase with decreasing An#15. Thus, the observed Sr zonation pattern is predicted by its changing partition coefficient, in response to this crystal-chemical control. In contrast, melt composition has the greatest effect on plagioclase Fe content13. This is known to increase with melt fO 2 (ref. 16) and correlates negatively with temperature and An#13,17. However, within the oscillatory zoned plagioclase mantles, An# correlates positively with FeO (Fig. 3). As such, An-content and temperature may not have had a large influence on the Fe content of plagioclase. fO 2 -induced variations in plagioclase-liquid Fe partitioning are also unlikely to have significantly influenced Fe zoning, as this can not simply result in the positive correlation between An# and FeO. Variations in FeO are more readily explained by changes in melt composition resulting from repeated recharge of the fractionating magma chamber.

Variation in Al/Ti is a useful indicator of melt evolution in Cr-deficient clinopyroxene crystals18. While Al and Ti concentrations may be affected by temperature, pressure and rate of crystal growth19, the Al/Ti ratio more strongly reflects changes in melt composition14. A minor increase in clinopyroxene Al/Ti with temperature may occur due to the stronger partitioning of both AlIV and AlVI19. Although temperature fluctuations could cause the positive correlation between Al/Ti and Mg# observed within oscillatory zoned clinopyroxene mantles (Fig. 3), Ti concentration also shows a well-defined anticorrelation with Mg#, which cannot result from variations in temperature alone19. Variable pressure is also an unlikely explanation for oscillatory Al/Ti zoning, as closed system convection would only cause small pressure changes (<1 kb)20. Increased crystal growth rates relate to the degree of undercooling and correlate positively with Al and Ti21. This could explain the anti-correlation between Ti concentration and Mg#, so increased growth rates cannot be fully discounted as the cause of chemical Al/Ti zonation. However, all the clinopyroxenes exhibit concentric, rather than hourglass zoning, which would be expected if growth rate strongly influenced chemical zonation21. As such, clinopyroxene trace element zoning more likely records changes in melt composition.

Concordance of the plagioclase and clinopyroxene zoning patterns through the mantles of the cumulate phases is found in each of the ignimbrite units. This is consistent with a fractionating magma chamber, periodically refilled by more primitive melt, rather than fluctuations in parameters such as pressure and temperature (e.g. refs. 22, 23). Petrological evidence, such as sieve textured plagioclase phenocrysts, compositionally distinct phenocryst cores and overgrowth mantles also suggest that open system mixing occurred during chamber development24. However, elemental oscillations cannot be correlated between crystals, indicating these events did not affect the whole chamber equally.

Changes in trace element concentrations accompany major element variations observed at crystal rims. In one plagioclase (La Caleta), the drop in An# at the rim correlates with a substantial (≤ 11%) decrease in FeO concentration. This suggests that the rim zone reflects a significant change in melt composition, with lower FeO and An# indicative of crystallisation from a more evolved liquid. In contrast, rim zones in other plagioclase crystals from the three ignimbrites show an increase in FeO concentration, mirroring the drop in An#. This can be explained by cooling and rapid crystal growth, potentially accompanied by an increase in melt fO 2 , associated with a change to more evolved melt compositions. During rapid crystal growth, a chemical boundary layer, enriched in plagioclase incompatible elements such as Fe, may form at the crystal-melt interface13. Although such kinetic effects could contribute to the high Fe concentrations at crystal rims, they cannot account for the concurrent drop in An#. Contrasting Fe enrichment and depletion trends observed within rims of different plagioclase crystals are likely to result from varying degrees of undercooling. Both require a significant change in melt composition, regardless of the concentration shift direction. A large drop in Al/Ti accompanies the decrease in Mg# at the rim of most clinopyroxene phenocrysts analysed in this study and is taken as a further indication of a large-scale change in melt chemistry.

To test if the interstitial melt is in equilibrium with Tenerifian basaltic liquids or more evolved phonolitic compositions, we recovered and analysed interstitial material from nodules in each unit. Figure 4 shows that these interstices are displaced towards more phonolitic compositions relative to the liquids in equilibrium with the cumulus crystal-forming melts (excluding rims). As such, the “frozen” final liquids within these cumulates confirm that mixing occurred between phonolitic and basaltic magmas before each eruption.