Geoscience Reference
In-Depth Information
0.8
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0.0
400
450
500
550
600
650
700
Temperature (°C)
Figure 10.6
Metamorphic grid showing dehydration reaction curves that can be described by
(10.2)
and three solid-solid reactions for which the slopes are virtually constant.
matrix. As the tensile strength of the rock is generally low, this pressure contrast causes
hydro-fracturing and water escapes.
Metamorphic dehydration reactions entail a loss of water but also of dissolved solids,
and metamorphic rocks must be considered as open systems where very large quantities
of water may have circulated. During this process, the chemical composition of the initial
rock is transformed. As metamorphic reactions proceed, the distribution of trace elements
and chronometric systems are severely disrupted but, because the system is open to fluid
circulation, this rarely happens in a predictable way. Since, at the temperatures at which
metamorphism occurs,
18
O tends to preferentially fractionate into the fluid (contrary to
what happens at ordinary temperatures), the isotopic composition of the oxygen of rocks
drifts toward lower
18
O levels closer to those of the mantle. At temperatures in excess of
500
◦
C, the oxygen isotope fractionation coefficients tend to unity, there is little water left
to exchange oxygen with, and isotopic changes become less important.
Extreme metamorphic conditions cause rocks to melt when they have a high water con-
tent; the hydrated melting of common crustal metamorphic rocks is referred to as anatectic
melting. When water is absent or the dominant fluid is CO
2
, granulite facies conditions
pertain. Circulation of CO
2
promotes migration and the loss of elements that are normally
inert under hydrated conditions: it is known that granulite facies rocks, which are so com-
mon at the base of the continental crust, have lost much of their uranium and some of their
thorium. The production of heat in granulites of the deep part of the continental crust by the
radioactive elements U, Th, and K is therefore usually very low, with, as a consequence, a
distinctive isotopic composition of lead, which is in general unradiogenic.
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