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

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 (τy) and Bingham viscosity (ηB) were determined. The relation between ηB and the crystallinity (Φ) was compared with the Krieger-Dougherty equation, and both the maximum packing fraction of crystals (Φm) and intrinsic viscosity (ν) for Bingham viscosity were determined as Φm = ~ 0.45 and ν = ~ 5.25, respectively; thus, it was found that Φm decreased and ν increased concomitantly with an increase in the shape anisotropy of crystals. However, the obtained value of νΦm (~ 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 ηB might be predicted only on the basis of a change in Φm. For Φ > 0.133, τy was found to be finite; it increased with Φ, which suggests that the critical crystallinity for the onset of yield stress, Φc, is at least lower than 0.133. The upper limit of Φc 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).