The density distribution of the lithosphere is non-linear and
discontinuous due to complex mineralogy and, most importantly, phase
transitions. We evaluate the influence of changes in mantle composition
on lithospheric density and its evolution during horizontal stretching,
using thermodynamic calculations of the density as a function of
pressure, temperature and composition. We also develop a simple
parameterization based oil end-member mineral reactions and geometric
relationships between the geotherm and the phase boundary for
comparison. The garnet-spinel peridotite transition leads to a moderate
decrease in density of the mantle part of the lithospheric column at the
initial stages of stretching. When the crust is sufficiently thinned and
temperature is relatively high, plagioclase peridotite becomes stable in
the upper part of the mantle. The density reduction due to the
plagioclase-in reaction is controlled by bulk Al(2)O(3) in the mantle
and by the depth of the plagioclase-in reaction. which is mainly
governed by the Na(2)O/Al(2)O(3) ratio. Since Na(2)O and Al(2)O(3)
increase with the fertility of the mantle the phase transition effect is
most pronounced for relatively fertile mantle (and strong extension) and
can lead to 2.3% density reduction. This is equivalent to heating the
entire lithosphere by 700 degrees C if only the effect of thermal
expansion on density is taken into account. The formation of plagioclase
peridotite can explain syn-rift uplift in sedimentary basins that
experienced large mantle stretching without invoking an unrealistically
strong increase in temperature. it might also be responsible for the
break-up unconformity observed at continental margins. (C) 2008 Elsevier
B.V. All rights reserved.