Magmas and volatile components

Abstract

The trivial quantities of CO_2 and H_2O (or reduced combination of C-H-O-S) in the upper mantle have little effect on the abundant basalts, but H_2O influences magmas generated in peridotite overlying subducted, hydrated oceanic crust, and CO_2 causes the generation of alkalic subsilicic magmas from peridotite beneath continental shields. PT sections through the (partly schematic) phase diagram for peridotite-CO_2-H_2O with low (CO_2 + H_2O) and selected CO_2/H_2O, 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 H_2O/CO_2: only in regions of upwelling can CO_2-rich vapor exist. The solidus surface for peridotite-CO_2-H_2O (P, T, X^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-H_2O from magma sources at depth to the surface. Data from scattered sources have been synthesized in a PTX(SiO_2)X(H_2O) framework to 35 kbar, and illustrated in sections and projections, including liquidus surfaces for the rock series to 35 kbar, dry, with 5 percent H_2O, and with excess H_2O (saturated). The volatile components become prominent in residual magmas: pegmatites from granitic magmas are enriched in H_2O, and carbonatites from alkalic magmas are extraordinarily enriched in CO_2. The behavior of another volatile component is illustrated by experimental data in NaAlSi_3O_8-H_2O-HF at 2.75 kbar. The vapor-saturated liquidus field boundary extends from 808°C-8.5 percent H_2O to 777°C-(9.2 percent H_2O + 1.0 percent HF), and three-phase boundaries demonstrat1i 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.

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