Volatile components (H2O, CO2, Cl, F, and S) in basic magmas of various geodynamic settings: Data on melt inclusions and quenched glasses

Using data on melt inclusions and quenched glasses, we estimated the averag e concentrations of H2O, CO2, Cl, F, S and their variations in natural basi c magmas in different geodynamic settings: spreading zones of oceanic ridge s and backarc basins, oceanic islands, island arcs, active continental marg ins, continental rifts, and hot spots (intracontinental areas). Variations of volatile components are correlated with mg# of the magmas and their pota ssium and phosphorus concentrations. The water contents in the less magnesi an and potassium-rich magmas of spreading zones come to 2.5 wt % (on averag e, the quenched glasses contain 0.31 wt % H2O in oceanic ridges and 0.88 wt % H2O in the backarc basins), while the magmas of oceanic islands contain up to 1.4 wt % H2O, The H2O contents range from 0 to 6 wt % in magmas of is land arcs and active continental margins and from 0 to 2.5 wt % in magmas o f intracontinental areas. The chlorine contents in magmas vary from 0 to 0. 2 wt % in spreading zones and from 0-0.1 to 0.4 wt % in oceanic islands. Re guardless of the silica concentrations, most magmas of island arcs and acti ve continental margins have 0-0.5 wt % Cl, while the magmas of intracontine ntal areas bear up to 1.6 wt % Cl. With decreasing mg# and increasing K2O c oncentrations the F concentrations increase from 0 to 0.08 wt % in magmas o f spreading zones and from 0.0n to about 0.31 wt % in magmas of oceanic isl ands. The F contents vary from 0 to 0.7 wt % in magmas of island arcs and a ctive continental margins and from 0 to 1.4 wt % in magmas of intracontinen tal areas. The S contents in magmas of all specified geodynamic settings de crease from 0.3 to 0.0n wt % with increasing SiO2, and they are relatively high (up to 0.5-0.8 wt %) in intracontinental magmas containing 0.5-9 wt % K2O. Sulfur shows a distinct positive correlation with iron in magmas of sp reading zones. The CO2 contents vary from 0 to 0.2 wt % in most magmas. The lowest CO2 concentrations are found in magmas of intracontinental areas (0 .1 wt %), while CO2 is most abundant in the magmas of active continental ma rgins (up to 1.6 wt %). Variations in the volatile concentrations in magmas of various geodynamic settings are controlled by differentiation and depen d on the modal mineralogy of their mantle sources. Judging from the ratios of volatile components to K2O and P2O5, they could be concentrated in mantl e micas, amphiboles, and apatites variously abundant in magma sources in di fferent geodynamic settings, as well as in a hypothetical aqueous-carbonate fluid. Most magmas in the oceanic spreading zones are derived from sources bearing amphibole and H2O-CO2 fluid, as well as small amounts of apatite a nd mica. The sources of oceanic-island magmas are richer in micas, while th e magma sources in intracontinental areas may contain a water-free potassiu m phase, additionally to mica and apatite. Amphibole or fluid and apatite d ominate the sources of magmas in the island arcs and active continental mar gins. Experimental petrology demonstrates that mica and amphibole definitel y occur in sources of basic magmas in island arcs and intracontinental area s, while ordinary amphibole is not stable in magmas generated in the spread ing zones. However, we can not exclude the existence of K-, F-, and Ti-rich amphiboles, or mantle fluid enriched in components of amphibole and mica i n the sources of these magmas.