Geoscience Reference
In-Depth Information
1.10
27.5
1.05
25.0
26
(18
−
35)
18
(13
−
24)
22.5
21
(17
−
27)
13)
12
(8
−
18)
1.00
10
(7
−
26
(21
−
32)
20.0
16
(12
−
21)
0.95
17.5
16
(12
−
22)
0.90
15.0
8
(6
−
11)
0.85
12.5
16
(12
−
21)
0.80
10.0
0.75
0.70
7.5
800
850
900
950
1000
1050
1100
T (
°
C)
Fig. 1.8
Solubility of rutile (TiO
2
) in water, as determined by direct observation in an externally heated diamond
anvil cell. For each experiment, the solubility in ppm TiO
2
by weight is given together with the confidence interval
of the measurement. Isochors for fluid densities between 0.7 g
/
cm
3
and 1.10 g
/
cm
3
are also given. After Audetat
and Keppler (2005). Reproduced with permission of Elsevier.
remains sufficiently low that aqueous metasoma-
tism of the mantle usually does not lead to a
significant redistribution of these elements over
long distances. The depletion of HFSE relative to
fluid-mobile LIL (large ion lithophile) elements
such as Rb
+
or Ba
2
+
in metasomatically altered
rocks and in silicate melts is therefore usually
a good indicator for the involvement of aqueous
fluids in chemical transport processes.
the serpentinized peridotites of the subducting
slab releases hydrous fluids (Manning, 2004) that
infiltrate into the mantle wedge. The melting
point depression due to water is the ultimate
cause of melt formation in subduction zones
(Tatsumi, 1989); only for unusually young and
hot slabs, direct melting of the subducted slab
may occur. Part of the trace element budget
in arc magmas produced in this environment
was transported by the hydrous fluid from
the subducted slab into the zone of melting.
Since HFSE elements are hardly transported by
aqueous fluids, they are typically depleted in arc
magmas relative to fluid-mobile elements. The
enrichment pattern observed for elements such
as U, Th, Ba, Pb, Rb and others likely requires
that subduction zone fluids are relatively oxi-
dizing and contain significant concentrations of
chloride (Keppler, 1996; Bali
et al
., 2011).
1.3.5 Water and melting in the mantle
Melting in the shallow part of the mantle
occurs primarily in subduction zones and below
mid-ocean ridges, with minor melt production
at hot spots, such as oceanic islands. Figure 1.9
shows schematically the processes occuring
in subduction zones. A series of dehydration
reactions in the sediments, altered basalts and in
