Geoscience Reference
In-Depth Information
Figure 10.6.
(a) A schematic representation of equilibrium metamorphic facies and
melting of basaltic oceanic crust. Note that all the boundaries are gradational, not
sharp, and that many of the reactions involved have not been particularly well
defined. (b) A schematic representation of the breakdown of serpentinite and
melting of peridotite. The two heavy lines are the solidus in the presence of excess
water (wet melting) and the solidus for the dry rock (dry melting). The large arrow
indicates the probable range of the temperature-depth profile of the subducting
oceanic crust. (After Wyllie (1981) and Turner (1981).)
Prior to subduction, much of the oceanic crust is altered to low-grade brown-
stone, or at greater depth is in the greenschist metamorphic facies (Fig. 10.6(a)).
This alteration was produced by the hydration and metamorphism of basalt in
the near-ridge hydrothermal processes. During the initial stage of subduction,
at shallow depths and low temperatures, the oceanic basalt passes through the
pressure-temperature fields of the prehnite-pumpellyite and blueschist facies.
At this stage, extensive dehydration and decarbonation take place as the basalt is
metamorphosed beginning with expulsion of unbound water and followed by sig-
nificant metamorphic dehydration that commences at a depth of about 10-15 km.
During the descent of the slab, the pressure increases and the slab slowly heats up.
The heat which warms the slab is transferred from the overlying wedge of mantle
and is also produced by friction. The basalt then undergoes further dehydration
reactions, transforming from blueschist to eclogite. During this compression, the
water released, being light, moves upwards. Any entrained ocean-floor sediment
also undergoes progressive dehydration and decarbonation. These processes are
illustrated in Fig. 10.6(a).
The temperatures and pressures at which oceanic basalt, in the presence
of excess water, produces significant amounts of melt are markedly different
from the temperatures and pressures at which dry basalt produces copious melts
(Fig. 10.6(a)). At 10 km depth, wet basalt begins to produce significant melt at
about 850
◦
C and dry basalt at 1200
◦
C. For increasing amounts of water, the
point at which copious melting begins lies at positions intermediate between the
