Water content of a granite magma deduced from the sequence of crystallization determined experimentally with water-undersaturated conditions

Abstract

The sequence of crystallization in a biotite-granite from the Bohus batholith of Norway and Sweden, deduced from its texture, was magnetite, plagioclase, microcline, quartz, and finally biotite. Several sequences of crystallization were determined experimentally at 2 kb in the presence of varying only for H_2O contents below 1.2% by weight. The rock was fused to a homogeneous glass, and each experiment included samples of finely crushed rock and glass. The samples were reacted in Ag-Pd capsules with measured H_2O content in coldseal pressure vessels with NNO buffer. With excess H_2O (more than 6.5%) the crystallization interval extends from 865° C to 705° C. In the H_2O-deficient region, the solidus temperature remains unchanged as long as a trace of vapor is present, but the liquidus temperature increases as H_2O content decreases; with 0.8 % H_2O the liquidus temperature is 1125° C, the crystallization interval is 420° C, and a separate aqueous vapor phase is evolved only a few degrees above the solidus at 705° C. The biotite phase boundary increases slightly from 845° C with excess H_2O to 875° C with 1% H_2O, and it intersects the steep phase boundaries for quartz and feldspars; the sequence of crystallization changes at each intersection point. Similar diagrams at various pressures for related rock compositions involving muscovite, biotite and amphibole will provide grids useful in defining limits for the water content of granitic and dioritic magmas. Applications are considered for the Bohus batholith, other granitic rocks, and rhyolites. The Bohus magma could have been formed by crustal anatexis as a mobile assemblage of H_2O-undersaturated liquid and residual crystals with initial total H_2O content less than 1.2%, or it could have been derived by fractionation of a more basic parent with low H_2O content from mantle or subduction zone, but it could not have been derived from a primary andesite generated from mantle peridotite. We consider it unlikely that the H_2O content of large granitic magma bodies exceeds about 1.5% H_2O; these magmas are H_2O-undersaturated through most of their histories. Uprise and progressive crystallization of magma bodies produces H_2O-saturation around margins and in the upper regions of magma chambers. H_2O-saturated rhyolitic and dacitic magmas with phenocrysts can be tapped from the upper parts of the magma chambers.

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