M. Rabinowicz et al., Compaction in a mantle mush with high melt concentrations and the generation of magma chambers, EARTH PLAN, 188(3-4), 2001, pp. 313-328
Most compaction models in the partially molten mantle have addressed the ca
se of low intergranular melt concentrations (up to a few percent). Here we
develop a mathematical and numerical formalism adapted to the two-dimension
al modeling of compaction in mushes with higher melt concentrations (up to
a few tens of percents). Experimental data on mantle-like mushes (olivine c
rystals+basaltic melts) suggest that the mush viscosity depends in a comple
x way on the melt concentration: three rheological thresholds occur at melt
concentrations of about 5%, 20%, and 40%, respectively. The first threshol
d corresponds to the establishment of full interconnectivity of the intergr
anular melt, the second to the formation of a very dense suspension of crys
tals and the last to the development of crystal clusters in the suspension.
The present models take into account the stiff and drastic viscosity drops
associated with these rheological thresholds. Intergranular melt migration
associated with an initial melt pulse generates a horizon of high melt/cry
stal ratio. If the melt concentration in the initial pulse presents a local
excess, the horizon becomes slightly tilted. As a consequence, melt percol
ates upslope inside the tilted horizon, pools at its summit and generates a
'pocket-like' zone. Due to the higher melt concentration. the upward Darcy
velocity in the pocket markedly exceeds that in the horizon. The result is
that the pocket-like impregnation is rapidly disconnected from the horizon
and a new pocket develops at the summit of the partially fragmented horizo
n. Eventually, the intergranular melt contained in the horizon is completel
y redistributed into pockets. Increasing the background melt concentration
in the mush from 5%, 20%, and 40% leads to an increase of the maximum melt
concentration of 10%, 40%, and 100%, in the transient horizons and of 25%,
60%, and 100% in the pockets. These models suggest that magma chambers with
a kilometer extent naturally result from the compaction of a mantle mush w
ith an initial melt concentration exceeding 5%. (C) 2001 Elsevier Science B
.V. All rights reserved.