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%).