An experimental study of high-alumina basalt differentiation and the effect of H2O and pressure on plagioclase – melt equilibria

Water-rich high-alumina basalts are widely implicated in models of subduction zone magma genesis and porphyry copper mineralisation, yet their phase relationships at high pressures have been very little studied since the pioneering work of Yoder and Tilley (1963). To fill this gap an experimental study has been carried out on water-saturated phase relationships of a high-alumina basalt (HAB) from St. Kitts, Lesser Antilles volcanic arc (Eastern Caribbean), in the pressure range 1.5 to 20 kbar. Experimentally produced glasses, mineral compositions and mineral assemblages match whole rock data of Lesser Antilles high-alumina basalts and high-alumina basaltic andesites, phenocryst assemblages and mineral chemistry. I show that the liquidus silicate mineral phase changes from olivine and plagioclase at low pressures, through clinopyroxene and amphibole at intermediate pressures, to garnet above 14 kbar. Experimentally produced mineral assemblages correspond well to plutonic xenolith found in Lesser Antilles and change from dunnite, and olivine and hornblende gabbro dominated lithology at pressure ≤ 10 kbar to hornblende pyroxenite, hornblendite and eclogite at pressures ≥ 10kbar.

Melt compositions describe liquid lines of descent that resemble those of many magmatic arc sequences; only alumina shows any strong correlation with pressure. Water saturation (as determined by the difference method) is lower than predicted by most solubility models. The evolved melts generated at low crustal conditions from these experimental series do not resemble silicic melts associated with porphyry copper deposits.

I also address the long-debated origin of high-An plagioclase (An > 90), which occurs in many volcanic and plutonic rocks associated with arc magmatism. Additional experiments on HAB at water-undersaturated and fluid-saturated experiments with 2M CaCl₂ solution and CaCO₃ have been carried out at 7 kbar to evaluate fluid composition and aH₂O on An content of plagioclase. These experiments, combined with existing experiments on broadly basaltic compositions, demonstrate that high-An plagioclase (An ≥ 90) crystallises readily from fluid-saturated high-alumina basaltic melts at pressures of 1 to 10 kbars and temperatures 850 – 1100 ºC. However, the limit of An content produced by experiments is An96. Addition of CaCl₂ to the fluid and CaCO₃ to the system does not have a significant effect on An content. To produce plagioclase with An content more than 96 mol% a secondary process such as re-melting/re-crystallisation must be involved (Melekhova et al 2022).

Melekhova E., et al. (2022) Journal of Petrology 63.5 (2022): egac033.