Geoscience Reference
In-Depth Information
reactions and chronometric systems. Migration of magmas or hot fluids on top of rift or
subduction zones are prime potential culprits for such resetting events. Second, most rocks
from the mantle and the deep crust never cooled below these blocking temperatures: they
remained essentially open systems since they formed. If such an open system behavior is
nearly perfect, the thermobarometric estimates can be used to derive modern geotherms,
e.g. the
P
-
T
relationship in the local mantle. Such a behavior is best assessed by check-
ing that the chronometric ages correspond to the time these rocks rose to the surface (e.g.
the eruption age of ultramafic inclusions in volcanic rocks). If, however, the chronomet-
ric ages of these rocks are substantially older than the time of their final ascent, the
P
-
T
observations should be treated with utmost care. In the absence of zircons in the mantle,
garnet
176
Lu-
176
Hf ages have proved to be particularly helpful in this context. It is there-
fore worth stressing that most rocks from the mantle should be a priori treated as open
systems and that attempts to interpret the major and trace element concentrations and the
isotopic properties of their minerals as reflecting those prevailing at the time they formed
are probably bound to be erroneous. The lack of equilibration among minerals of mantle
rocks is therefore not a sufficient indication of a metasomatic event. In contrast, most of
the crustal rocks have cooled through the closure temperature and some of them have been
reworked over different tectonic and magmatic events.
10.4 Water/rock ratios
The motivations for finding out how much water a given rock sample has “seen” during
diagenetic, hydrothermal, or metamorphic processes are varied: we may want to assess
the reserves of a particular element of economic value remaining in the source rock of a
particular ore deposit, we may need to know how far water circulation affected the ther-
mal regime of a given area, or we may worry that too much circulating water disturbed
the initial geochemical properties of a rock, a mineral, or fossil remains. In order to pro-
vide a quick estimate of the extent of water-rock interaction, geochemists often refer to
the concept of a water/rock ratio. Given a set of geochemical observations on an altered
rock or a diagenetic/hydrothermal solution, geology may often give some hints at what the
untransformed rock may have looked like (a basalt, a granite), while other constraints may
be good enough to let us infer the geochemistry of the reacting fluid (meteoritic water
at a given latitude, for instance). A number of geochemical properties can be used as
well, concentrations and isotopes, but probably the most popular are
18
O and
87
Sr/
86
Sr.
δ
18
O
HR
of a hydrothermally altered
Let us suppose, for instance, that we measured the
δ
basalt sample to be
, while examination of thin sections suggests that the rock was
hydrothermally altered at 350
◦
C in the greenschist facies. Paleogeography also suggests
that when this particular basalt was erupted, the latitude was such that meteoric water had
a
−
2
18
O
MW
of
18
O
FR
of the fresh basalt was in the range
δ
−
10
. We will assume that the
δ
of mantle-derived magmas, say
. Can we find out how much water interacted with
this particular basaltic sample before it turned into a metabasalt?
+
5.5
