Geoscience Reference
In-Depth Information
Fig. 2.1
Cell assembly for double
capsule experiments at 12-16 GPa.
capsules Au-Pd alloy is considered to be least
permeable for hydrogen (Nishihara
et al
., 2006).
To reduce hydrogen diffusion into the sample
capsule a hematite (Fe
2
O
3
) sleeve can be used in
piston-cylinder or multianvil experiments (Liu
et al
., 2006).
Buffering techniques control
fO
2
in the sample
during experiment typically using metal-metal
oxide pairs and can be considered as most suitable
for studies of volatile-bearing systems. In studies
of systems with reduced C-O-H fluid we used
modified double capsule methods (Sokol
et al
.,
2009, Sokol
et al
., 2010, Litasov
et al
., 2013b),
where the thick-walled outer capsule was made of
buffer material (Molybdenum or Iron). In this case
the sample was placed into an Au-Pd or Pt capsule
and separated from the outer capsule talc, which
transforms to enstatite and H
2
O-fluid or H
2
O-
bearing silicate melt upon heating (Figure 2.1).
temperature profiles in the asthenosphere corre-
spond to nearly adiabatic conditions of convecting
mantle. The depth of the transition from mainly
conductive to convective thermal regime varies
from 10-20 km beneath Mid-Ocean ridges to
>
250 km beneath the ancient cratonic areas (e.g.,
Priestley & McKenzie, 2006). Figure 2.2 shows
a series of geotherms constrained by mantle
xenoliths from continental basalts (South-East
Australia) and kimberlites of the Udachnaya pipe
(Siberia) and Jericho pipe (Canada).
The temperature in subducting oceanic plates
is significantly lower than in the surrounding
mantle. The coldest part may correspond to
the slab Moho (5-7 km into the slab or slightly
deeper). The PT-profiles of hottest modern
slabs may be similar to cratonic geotherms
(Figure 2.2). The PT-profiles of ancient subduc-
tion might be significantly hotter than those
of modern subduction. The PT-estimations
for Precambrian metamorphic rocks related to
subduction environments indicate that they
correspond to modern continental rifts, whereas
most estimations for Phanerozoic rocks are con-
sistent with modern hot subduction (Figure 2.2).
Model PT-profiles used for comparison with
volatile-bearing solidi of mantle rocks are
summarized in Figure 2.3. The average mantle
adiabat corresponds to a potential temperature
of 1315
◦
C
2.3 Temperature Profiles and Oxidation
State in the Mantle
The temperature profiles in the mantle and redox
conditions largely control melting in the presence
of the volatiles. The temperature profiles have
large uncertainties even for average mantle due to
uncertainty in the estimated mantle composition
and parameters for equations of state for mantle
phases. The thermal regimes of lithosphere and
asthenosphere are different. In the lithosphere
heat is transported by conduction and temper-
ature increases rapidly with depth, whereas the
100 - according to thermobarom-
etry and thermoelastic properties of mantle
phases (McKenzie & Bickle, 1988; Katsura
et al
., 2004; Putirka, 2005). The PT-profiles of
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