Chemical interdiffusion between Na-series tephritic and phonolitic melts with different H<sub>2</sub>O content, temperature, and oxygen fugacity values
oleh: D. González-García, D. González-García, F. Pohl, F. Marxer, S. Krasheninnikov, S. Krasheninnikov, R. Almeev, F. Holtz
| Format: | Article |
|---|---|
| Diterbitkan: | Copernicus Publications 2024-08-01 |
Deskripsi
<p>The diffusive exchange of major elements in Na-series tephrite–phonolite diffusion couples with compositions relevant to the Canary Islands magmatism was determined at 300 MPa and variable H<span class="inline-formula"><sub>2</sub></span>O concentrations (0.3 wt % to 3.3 wt %), temperatures (1150 to 1300 °C), and <span class="inline-formula"><i>f</i></span>O<span class="inline-formula"><sub>2</sub></span> levels (NNO<span class="inline-formula">−1.5</span> to NNO<span class="inline-formula">+1.7</span>). Composition-dependent effective binary diffusion coefficients were determined from concentration–distance profiles. Results show a wide range of diffusivities for different cations, consistently following the sequence Na <span class="inline-formula">≫</span> Al <span class="inline-formula">≫</span> K <span class="inline-formula">≥</span> Mg <span class="inline-formula">=</span> Fe <span class="inline-formula">=</span> Ca <span class="inline-formula">></span> Si <span class="inline-formula">></span> Ti, with a mild diffusivity contrast (0.2–0.8 log units) between tephritic and phonolitic melts. Na is the fastest component, with diffusivities falling <span class="inline-formula">∼1.0</span> log units above those of Si for any given condition. An anomalously fast Al diffusion is observed, with <span class="inline-formula"><i>D</i><sub>Al</sub></span> falling <span class="inline-formula">∼0.4</span> log units above Si and <span class="inline-formula">∼0.6</span> log units below Na, suggesting a prevalence of Al–alkali coupling across our range of run conditions. The relationships between log <span class="inline-formula"><i>D</i></span> and H<span class="inline-formula"><sub>2</sub></span>O content in melt for all cations in an intermediate composition are strongly nonlinear and can be fitted using an exponential function with a convergence in diffusion coefficients for different temperatures with increasing H<span class="inline-formula"><sub>2</sub></span>O content. Thus, Arrhenius analyses result in a decrease in activation energies from 222–293 kJ mol<span class="inline-formula"><sup>−1</sup></span> at 1.7 wt % H<span class="inline-formula"><sub>2</sub></span>O to 48–112 kJ mol<span class="inline-formula"><sup>−1</sup></span> at 3.0 wt % H<span class="inline-formula"><sub>2</sub></span>O. These results provide new data on chemical interdiffusion in highly alkaline Na-rich melts and suggest that H<span class="inline-formula"><sub>2</sub></span>O content plays a key role in increasing the chemical efficiency of magma mixing at low temperatures. The obtained dataset is used to test chemical controls of magma mixing in the El Abrigo ignimbrite, Tenerife, where banded pumices involving basanitic–tephritic to phonolitic magmas are common in several compositionally bimodal ignimbrite units.</p>