Magmas and volatile components

The trivial quantities of CO₂ and H₂O (or reduced combination of C–H–O–S) in the upper mantle have little effect on the abundant basalts, but H₂O influences magmas generated in peridotite overlying subducted, hydrated oceanic crust, and CO₂ causes the generation of alkalic subsilicic magmas from peridotite beneath continental shields. PT sections through the (partly schematic) phase diagram for peridotite-CO₂–H₂O with low (CO₂ + H₂O) and selected CO₂/H₂O, compared with isotherms, illustrate the petrological structure of the upper mantle for different tectonic environments. If vapor is present, the cooler the geotherm, the higher is H₂O/CO₂: only in regions of upwelling can CO₂-rich vapor exist. The solidus surface for peridotite-CO₂–H₂O (P, T, A^v) has been mapped with tentative boundaries marking changes in normative compositions of near-solidus magmas. The restricted area for quartz-normative magmas suggests that if these are to be generated from mantle in subduction zones, there must be active asthenospheric convection carrying hot mantle to shallow levels above dehydration fronts in subducted oceanic crust. Changes in the relative positions of dehydration fronts and solidus boundaries for warm and cool subduction models are illustrated in new diagrams, unencumbered by the need for precise temperatures and depths. Tests for processes require knowledge of the phase relationships in the system basalt-andesite-dacite-rhyolite-H₂O from magma sources at depth to the surface. Data from scattered sources have been synthesized in a PTX(SiO₂)X(H₂O) framework to 35 kbar, and illustrated in sections and projections, including liquidus surfaces for the rock series to 35 kbar, dry, with 5 percent H₂O, and with excess H₂O (saturated). The volatile components become prominent in residual magmas: pegmatites from granitic magmas are enriched in H₂O, and carbonatites from alkalic magmas are extraordinarily enriched in CO₂. The behavior of another volatile component is illustrated by experimental data in NaAlSi₃O₈–H₂O–HF at 2.75 kbar. The vapor-saturated liquidus field boundary extends from 808°C-8.5 percent H₂O to 777°C-(9.2 percent H₂O + 1.0 percent HF), and three-phase boundaries demonstrate strong partition of HF into liquid compared with vapor. Details remain uncertain for the sources of volatile components, in mantle reservoirs or recycled through subduction, and of oxygen fugacity and temperature variation with depth in different tectonic environments.