Figure 4

Empirical calibration of the reflectance signal for water concentration. Composition of synthesized glasses and their measurement conditions are listed in Table 1. (a) Basaltic composition. Two regression lines correspond to the difference of aperture size; larger (100 × 100 μm) (dotted line): water (wt.%) = (51.92 ± 1.55) × ΔR/R_baseline − 0.06 (r² = 0.990), and smaller (20 × 20 μm and 30 × 30 μm) (solid line): water (wt.%) = (49.76 ± 1.10) × ΔR/R_baseline − 0.08 (r² = 0.992). (b) Andesitic composition: water (wt.%) = (46.74 ± 2.52) × ΔR/R_baseline + 0.10 (r² = 0.975). (c) Rhyolitic composition: water (wt.%) = (43.51 ± 1.02) × ΔR/R_baseline − 0.004 (r² = 0.998).