Experimental deformation of partially melted granite revisited: implications
for the continental crust
Journal of Metamorphic Geology, 23 (1):
19--28 (January 2005)DOI: 10.1111/j.1525-1314.2005.00555.x
Abstract
A review and reinterpretation of previous experimental data on the
deformation of partially melted crustal rocks reveals that the relationship
of aggregate strength to melt fraction is non-linear, even if plotted
on a linear ordinate and abscissa. At melt fractions, Phi < 0.07,
the dependence of aggregate strength on Phi is significantly greater
than at Phi > 0.07. This melt fraction (Phi = 0.07) marks the transition
from a significant increase in the proportion of melt-bearing grain
boundaries up to this point to a minor increase thereafter. Therefore,
we suggest that it is the increase of melt-interconnectivity that
causes the dramatic strength drop between the solidus and a melt
fraction of 0.07. We term this drop the 'melt connectivity transition'
(MCT). A second, less-pronounced strength drop occurs at higher melt
fractions and corresponds to the breakdown of the solid (crystal)
framework. This is the 'solid-to-liquid transition' (SLT), corresponding
to the well known 'rheologically critical melt percentage'. Although
the strength drop at the SLT is about four orders of magnitude, the
absolute value of this drop is small compared with the absolute strength
of the unmelted aggregate, rendering the SLT invisible in a linear
aggregate strength v. melt-fraction diagram. On the other hand, the
more important MCT has been overlooked in previous work because experimental
data usually are plotted in logarithmic strength v. melt-fraction
diagrams, obscuring large strength drops at high absolute strength
values. We propose that crustal-scale localization of deformation
effectively coincides with the onset of melting, pre-empting attainment
of the SLT in most geological settings. The SLT may be restricted
to controlling flow localization within magmatic bodies, especially
where melt accumulates.