THE ROLE OF BASALTIC WATER CONTENT IN FACILITATING MAGMA MIXING BY REDUCING DENSITY AND VISCOSITY CONTRASTS

New mathematical calculations of densities and viscosities of hydrous mafic magmas and their high-silica counterparts suggest that the abilities of chemically distinct magmas to mix is facilitated when water is present in the mafic end member. Calculated mixing between granitic melts and mafic magmas containing 6-8 wt. % water can generate a range of rock types from granite to granodiorite to diorite; similar to the range observed in continental arcs.

The interplay of thermal, rheological and dynamical variables, such as density, melting temperature and viscosity, and their roles in the physical mixing and homogenization of hybrid magmas are difficult to constrain. However, in a very simplistic manner, a first step in understanding the ability of two magmas to mix begins with a better understanding of some of these variables. Although hybridization is widely recognized in volcanic systems, field, geochemical and isotopic evidence in granitoid plutons suggest that magma hybridization to generate chemically diverse rocks is also an important petrogenetic process at subvolcanic levels. However, calculations addressing the abilities of dry basaltic and granitic magmas to mix indicate that the type of rock generated is severely limited. New data from the Sierra Nevada batholith and other arcs suggest that mafic magmas involved in arc magmatism typically contain 6-8 wt. % water. Consequently, we present new estimates of the physical properties of hydrous mafic magmas that show a dramatic decrease in liquidus temperature and density. Furthermore, the lower liquidus temperature results in similar viscosities for basaltic and granitic magmas at much lower temperatures and may further facilitate hybridization.

These new calculations indicate that the tectonic setting may control the ways in which magmas interact. A range of rock types that are chemically and isotopically related by mixing are observed in arcs where basalts and diorites are generally water-rich. Extensional settings, on the other hand, are typified by bimodal magmatism and basalts there are typically dry. Calculations of physical properties are consistent with the field, geochemical and isotopic observations in diverse tectonic settings and may have dramatic implications for our understanding of continental crustal generation.