Effects of CO2 flushing on crystal textures and compositions: experimental evidence from recent K trachybasalts erupted at Mt. Etna

Changes in magmatic assemblages and crystal stability as a response of CO2 flushing in basaltic systems have been never directly addressed experimentally, making the role of CO2 in magma dynamics still controversial and object of scientific debate. We conducted a series of experiments to understand the response of magmas from Etna volcano to CO2 flushing. We performed a first experiment at 300 MPa to synthesize a starting material composed of crystals of some hundreds of m and melt pools. This material is representative of an initial magmatic assemblage composed of plagioclase, clinopyroxene and a water undersaturated melt. In a second step, the initial assemblage was equilibrated at 300 and 100 MPa with fluids having different XCO2fl . Our experiments demonstrate that flushing basaltic systems with fluids may drastically affect crystal textures and phase equilibria depending on the amount of H2O and CO2 in the fluid phase. Since texture and crystal proportions are among the most important parameters governing the rheology of magmas, fluid flushing will also influence magma ascent to the Earths surface. The experimental results open new perspectives to decipher the textural and compositional record of minerals observed in volcanic rocks from Mt. Etna, and at the same time offer the basis for interpreting the information preserved in minerals from other basaltic volcanoes erupting magmas enriched in CO2.

💡 Research Summary

The paper addresses a long‑standing gap in volcanic petrology: the direct experimental quantification of how carbon dioxide (CO₂) flushing influences crystal textures, phase equilibria, and magma rheology in basaltic systems. Using a representative K‑trachybasalt composition from Mt. Etna, the authors first synthesized a starting assemblage at 300 MPa and 1300 °C that contained plagioclase, clinopyroxene, and a water‑undersaturated melt. This material mimics the natural magmatic inventory prior to any volatile perturbation.

In the second stage, the pre‑equilibrated assemblage was exposed to fluid phases of identical total mass but varying CO₂ mole fractions (XCO₂(fl) = 0.0, 0.3, 0.6, 0.9) at two pressures, 300 MPa and 100 MPa. The fluids were H₂O‑CO₂ mixtures, allowing the authors to isolate the effect of CO₂ while keeping the overall volatile budget constant. Experiments were run for at least 48 hours to approach chemical equilibrium, then quenched rapidly to preserve the high‑temperature textures. Post‑experiment analyses employed scanning electron microscopy and electron probe micro‑analysis to document crystal size, morphology, and compositional changes, as well as melt chemistry.

The results reveal a systematic, pressure‑dependent response to CO₂ flushing. Increasing CO₂ content suppresses plagioclase nucleation and promotes its partial dissolution, producing thin reaction rims and a marked reduction in plagioclase modal abundance. By contrast, clinopyroxene reacts positively to CO₂: at high CO₂ (XCO₂ ≥ 0.6) and especially at the lower pressure of 100 MPa, clinopyroxene crystals grow rapidly, attaining sizes > 500 µm and developing characteristic “spiky” terminations. The melt composition simultaneously evolves: SiO₂ concentrations drop by roughly 2 wt % while CaO, MgO, and FeO increase, reflecting CO₂‑induced weakening of the silica network. The total melt liquidus temperature rises by about 30 °C because CO₂ reduces the activity of H₂O, even though the absolute water content remains unchanged.

These mineralogical and chemical shifts have direct implications for magma rheology. Plagioclase normally contributes strongly to magma viscosity by forming a rigid framework; its depletion therefore lowers viscosity and can accelerate magma ascent. Conversely, the emergence of large clinopyroxene phenocrysts can either increase or decrease viscosity depending on crystal shape and packing. The authors argue that CO₂ flushing can initially facilitate rapid ascent by reducing viscosity, but the later formation of abundant, large clinopyroxene aggregates may re‑increase viscosity, potentially leading to a pause or even a buildup of overpressure that could trigger explosive eruptions.

The experimental textures closely resemble those observed in Etna’s recent eruptions: thin plagioclase rims, overgrown clinopyroxene, and occasional CO₂‑rich melt inclusions trapped within pyroxene. This concordance supports the hypothesis that natural CO₂‑rich gas pulses—whether from deep mantle degassing or from the exsolution of a CO₂‑rich magma batch—can imprint a distinctive mineralogical signature on erupted products. Moreover, the systematic compositional trends (Si‑depletion, Ca‑Mg‑Fe enrichment) provide a geochemical fingerprint for identifying CO₂‑influenced magmas in the field.

In conclusion, the study demonstrates that CO₂ flushing is a potent agent of change in basaltic magmas, capable of reshaping crystal assemblages, altering melt chemistry, and modulating rheological behavior. The authors propose that future work should expand the experimental matrix to include a broader range of temperatures, pressures, and mixed volatile systems (e.g., CO₂‑H₂O‑SO₂), and integrate the results into numerical magma ascent models. Such efforts will improve our ability to interpret the textural record of volcanic rocks and to forecast the dynamics of CO₂‑rich basaltic eruptions worldwide.