Accepting that Gorely's gas plume composition carries a strictly magmatic volatile signature, as argued above, our results provide new constraints on both the CO2 abundance and the origins of volatile components in the Kamchatka mantle source. The H2O, S, Cl and F contents of K parental magmas have recently been quantified by Portnyagin et al. . By combining their parental melt S content (0.07–0.27%) with our inferred CO2/SO2 ratio (1.2) in Gorely's volcanic gas, a CO2 content of 0.13–0.45% in these parental melts is inferred. This is in the range estimated for most arc magmas [Fischer and Marty, 2005; Wallace, 2005]. Following the procedure of Duggen et al. , the CO2-completed volatile content of Kamchatka parental melts (Figure 4) can then be used to calculate the volatile composition of the K mantle source. We did just this by solving a batch melting equation for each volatile, assuming a 10% mantle melting fraction [Duggen et al., 2007] and using vapour-melt distribution coefficients fromSaal et al. . The so-calculated volatile ratios in the K mantle (KM) are clearly distinct from DMM (data fromSaal et al. ), pointing to pervasive mantle fertilization by slab-derived fluids (Figure 4). Furthermore, on the basis that KM involves 0.3–1 wt% addition of a slab component (SC) to DMM [Duggen et al., 2007] and, using simple mass balance equations, we compute a SC bulk fluid molar composition of ∼87% H2O, ∼5.8% CO2, ∼3.2% S, ∼2.9% Cl and ∼0.4% F (Figure 4). Since major silicate components are not included in the calculations, our inferred SC volatile contents are maxima, whilst the chemical ratios should be close to the actual values. According to mass balance calculations, the SC would contribute 94–98% of total H2O and Cl, ∼70% of the CO2, and 38–41% of total S and F in generated magmas. Our estimated SC composition diverges from the average volatile composition of subducted materials entering the slab - the K arc input fluxes (IF) - hypothesised byTaran  (Figure 4). We stress, however, that the latter estimate is based upon compositions of sediments and altered ocean crust for Pacific arc segments other than Kamchatka (for which such data, unfortunately, are unavailable): this may explain, at least partly, the mismatch between SC and IF found here. In particular, the carbon-poorer signature of our SC relative toTaran's  IF, in accord with the low CO2/SO2 signature of the KK gases, could be explained by the relative absence of subducted carbonate sediments in the NW Pacific, in comparison to other arc segments [Plank and Langmuir, 1998]. Alternatively, less effective carbon extraction from subducted materials (with respect to the more easily released Cl), and/or carbon-rich sediment accretion/off-scraping [Hilton et al., 2002] might contribute to this discrepancy. Further studies will be needed to resolve this issue.