Effect of Carbon Dioxide on Dehydration Melting Reactions and Melt Compositions in the Lower Crust and the Origin of Alkaline Rocks

Abstract

Dehydration melting experiments of alkali basalt associated with (e.g. Thompson, 1982). Much recent effort examines the origin of granite by simulating dehydration melting in a the Kenya Rift were performed at 0·7 and 1·0 GPa, 850–1100°C, 3–5 wt % H2O, and fO2 near nickel–nickel oxide. Carbon dioxide variety of protoliths, including graywackes (Vielzeuf & Montel, 1994; Patino Douce & Beard, 1996; Skjerlie & [XCO2 = molar CO2/(H2O + CO2) = 0·2–0·9] was added to experiments at 1025 and 1050°C. Dehydration melting in the Johnston, 1996), pelitic rocks (Patino Douce & Johnston, 1991; Carrington & Harley, 1995; Patino Douce, 1996), system alkali basalt–H2O produces quartzand corundum-normative trachyandesite (6–7·5 wt % total alkalis) at 1000 and felsic igneous rocks (Rutter & Wyllie, 1988; Skjerlie & Johnston, 1992, 1993), mixed, interlayered lithologies 1025°C by the incongruent melting of amphibole (pargasite– magnesiohastingsite). Dehydration melting in the system alkali (Skjerlie et al., 1993; Skjerlie & Patino Douce, 1995), and mafic gneisses and amphibolites (Ellis & Thompson, 1986; basalt–H2O–CO2 produces nepheline-normative tephriphonolite, trachyandesite, and trachyte (10·5–12 wt % total alkalis). In the Patino Douce & Beard, 1995). Other studies explore the origin of tonalite and trondhjemite by dehydration latter case, the solidus is raised relative to the hydrous system, less melt is produced, and the incongruent melting reaction involves melting of felsic gneisses ( Johnston & Wyllie, 1988) and basalts and amphibolites (Hacker, 1990; Beard & Lofgren, kaersutite. The role of carbon dioxide in alkaline magma genesis is well documented for mantle systems. This study shows that carbon 1991; Rapp et al., 1991; Rushmer, 1991; Winther & Newton, 1991; Wolf & Wyllie, 1991, 1994; Rapp & dioxide is also important to the petrogenesis of alkaline magmas at the lower pressures of crustal systems. Select suites of continental Watson, 1995; Winther, 1996; Springer & Seck, 1997). Water in hydrous phases is a critical constituent in alkaline rocks, including those containing phonolite, may be derived these dehydration melting studies. After H2O, carbon by low-pressure dehydration melting of an alkali basalt–carbon dioxide is probably the second most important volatile dioxide crustal system. species at volcanic centers (e.g. Blank & Brooker, 1994; Jambon, 1994). Within certain systems, such as continental rifts, CO2 even exceeds H2O in importance