Volatile components (H2O, CO2, Cl, F, and S) in basic magmas of various geodynamic settings: Data on melt inclusions and quenched glasses
Kovalenko, VI; Naumov, VB; Yarmolyuk, VV; Dorofeeva, VA
Using data on melt inclusions and quenched glasses, we estimated the average concentrations of H2O, CO2, Cl, F, S and their variations in natural basic magmas in different geodynamic settings: spreading zones of oceanic ridges and backarc basins, oceanic islands, island arcs, active continental margins, continental rifts, and hot spots (intracontinental areas). Variations of volatile components are correlated with mg# of the magmas and their potassium and phosphorus concentrations. The water contents in the less magnesian and potassium-rich magmas of spreading zones come to 2.5 wt % (on average, 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 island arcs and active continental margins and from 0 to 2.5 wt % in magmas of 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. Reguardless of the silica concentrations, most magmas of island arcs and active continental margins have 0-0.5 wt % Cl, while the magmas of intracontinental areas bear up to 1.6 wt % Cl. With decreasing mg# and increasing K2O concentrations the F concentrations increase from 0 to 0.08 wt % in magmas of spreading zones and from 0.0n to about 0.31 wt % in magmas of oceanic islands. The F contents vary from 0 to 0.7 wt % in magmas of island arcs and active continental margins and from 0 to 1.4 wt % in magmas of intracontinental areas. The S contents in magmas of all specified geodynamic settings decrease 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 spreading 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 margins (up to 1.6 wt %). Variations in the volatile concentrations in magmas of various geodynamic settings are controlled by differentiation and depend on the modal mineralogy of their mantle sources. Judging from the ratios of volatile components to K2O and P2O5, they could be concentrated in mantle micas, amphiboles, and apatites variously abundant in magma sources in different 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 and mica. The sources of oceanic-island magmas are richer in micas, while the magma sources in intracontinental areas may contain a water-free potassium phase, additionally to mica and apatite. Amphibole or fluid and apatite dominate the sources of magmas in the island arcs and active continental margins. Experimental petrology demonstrates that mica and amphibole definitely occur in sources of basic magmas in island arcs and intracontinental areas, while ordinary amphibole is not stable in magmas generated in the spreading 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 in the sources of these magmas.
Physical Sciences - Other Topics; subduction components, submarine lavas, kilauea volcano, silicate-melt,; upper-mantle, basalts, chemistry, vesuvius, hawaii, metasomatism