Bingham fluid behavior of plagioclase-bearing basaltic magma: Reanalyses of laboratory viscosity measurements for Fuji 1707 basalt

抄録

Datasets obtained by viscometry of Fuji 1707 basalt at a pressure of one atmosphere (Ishibashi, 2009) were analyzed by using the Bingham fluid model, and both the yield stress (τ_y) and Bingham viscosity (η_B) were determined. The relation between η_B and the crystallinity (Φ) was compared with the Krieger-Dougherty equation, and both the maximum packing fraction of crystals (Φ_m) and intrinsic viscosity (ν) for Bingham viscosity were determined as Φ_m = ∼ 0.45 and ν = ∼ 5.25, respectively; thus, it was found that Φ_m decreased and ν increased concomitantly with an increase in the shape anisotropy of crystals. However, the obtained value of νΦ_m (∼ 2.36) was similar to that in the case of uniform, isotropic particles (2.5). This indicates that the effect of crystal shape anisotropy on η_B might be predicted only on the basis of a change in Φ_m. For Φ > 0.133, τ_y was found to be finite; it increased with Φ, which suggests that the critical crystallinity for the onset of yield stress, Φ_c, is at least lower than 0.133. The upper limit of Φ_c is close to the value calculated numerically for randomly oriented uniform particles by Saar et al. (2001) (their value is 0.10-0.15 for a width/length (W/L) ratio of 0.1 to 0.2, which is similar to the ratios in the case of basalt).