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3.6 a. The effect of normative anorthite on the viscosity of haplogranitic melts (D.B. Dingwell and K.-U. Hess)

The chemical compositions of intermediate members of the calcalkaline intrusive series represent deviations from the haplogranitic system that may in large part be expressed as normative anorthite. The physical properties of such intermediate magmas are poorly known. Existing models for the calculation of melt density and viscosity are not capable of predicting those properties over the entire P-T-X range of interest.

As a first step towards defining better models the influence of the anorthite component on the viscosity of haplogranitic melts has been investigated. Viscosity measurements have been performed using the concentric cylinder method at low viscosities and micropenetration at high viscosities. Measurements are currently underway using centrifuge falling sphere viscometry to fill in the intermediate viscosity range. Viscosity data for melts representing the additions of 10, 20, 50 and 75 wt% anorthite to a HPG8 (haplogranite) base composition are compared with the endmember viscosities of HPG8 and anorthite as a function of reciprocal temperature in Fig. 3.6-1. The effect of anorthite on the viscosity of HPG8 is very sensitive to temperature. At the highest temperatures investigated the viscosity decrease associated with anorthite addition is a maximum falling over 3 orders of magnitude in a nearly linear fashion. At low temperatures the viscosity of the HPG8 melt is far less sensitive to the addition of anorthite. At the extreme of experimentally observed high viscosities the viscosity decreases by less than one order of magnitude with anorthite addition and increases again beyond 75 wt%. The nonlinearity of the effect of the addition of anorthite on the HPG8 melt viscosity at low temperatures means however that the effect of small additions of anorthite (10-20 wt%) to the granitic melt viscosity is still relatively small and constant over the entire investigated range of viscosity. Thus we may be able to parameterize the anorthite effect relatively simply in these melts. The question of the effect of water on the viscosity of these intermediate liquids is the subject of current investigation.

Fig 3.6-1: Log viscosity as a function of reciprocal temperature. Viscosities shown for compositions along the metaluminous alkali-feldspar - anorthite join (numbers in sample name denote anorthite component expressed in wt%).

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