Mineral reactions - Geochemistry: An Introduction - page 209

ly

Open Open

Closure temperature interval

Closed Closed

Time

Figure 10.7 The common thermal pathways of rock samples from the mantle (top) and crust (bottom). The

shaded zone represents the blocking temperature of a mineral reaction used to provide
temperature and pressure estimates or the closure temperature of a chronometer such as garnet
176 Lu- 176 Hf ages. Temperatures of mantle rocks typically exceed 900 ◦ C: their minerals therefore
remain continuously open to the exchange of elements and isotopes. In contrast, after original
cooling to rather low temperatures, the PT -age estimates derived from crustal rocks may be
episodically reset by geological events.

Metamorphic assemblages and, at depth, the metasomatic assemblages derived by
reaction between mantle rocks and percolating fluids and magmas, are commonly used
to derive equilibration temperatures and depth (pressure)-temperature-time (through
geochronology) pathways in the mantle and the crust. Mineral assemblages can only be
considered as equilibrium assemblages (they are then referred to as parageneses) and can
therefore be used as thermometers or barometers if they cool rapidly enough to freeze min-
eral compositions under the conditions prevailing at high temperatures. In Chapter 5 , we
saw that, in the most general case, diffusion may maintain chemical exchange among min-
erals and between minerals and surrounding fluids. It is important that such exchanges do
not affect a large fraction of mineral elements, in other words that cooling is fast. In the
case of moderate cooling rates, metamorphic assemblages reflect not equilibrium PT -age
conditions but the blocking (closure) temperature of chemical exchanges. This is a com-
mon situation of high-temperature assemblages, such as granulites and eclogites, in slowly
cooling metamorphic terranes: the ubiquitous temperature range of 700-850 ◦ C observed
for these rocks corresponds to the closure temperatures of many minerals, notably pyroxene
and garnet.
Two additional complexities arise with pressure-temperature estimates in metamorphic
and metasomatic assemblages ( Fig. 10.7 ) . First, it must be checked that the values observed
reflect the temperature and the age of the original cooling and not those of a resetting dur-
ing a subsequent reheating event above the closure temperature of the different mineral