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Experimental determination of liquidus H2O contents of haplogranite at deep-crustal conditions
Ist Teil von
Contributions to mineralogy and petrology, 2017-09, Vol.172 (9), p.1, Article 77
Ort / Verlag
Berlin/Heidelberg: Springer Berlin Heidelberg
Erscheinungsjahr
2017
Link zum Volltext
Quelle
Alma/SFX Local Collection
Beschreibungen/Notizen
The liquidus water content of a haplogranite melt at high pressure (
P
) and temperature (
T
) is important, because it is a key parameter for constraining the volume of granite that could be produced by melting of the deep crust. Previous estimates based on melting experiments at low
P
(≤0.5 GPa) show substantial scatter when extrapolated to deep crustal
P
and
T
(700–1000 °C, 0.6–1.5 GPa). To improve the high-P constraints on H
2
O concentration at the granite liquidus, we performed experiments in a piston–cylinder apparatus at 1.0 GPa using a range of haplogranite compositions in the albite (Ab: NaAlSi
3
O
8
)—orthoclase (Or: KAlSi
3
O
8
)—quartz (Qz: SiO
2
)—H
2
O system. We used equal weight fractions of the feldspar components and varied the Qz between 20 and 30 wt%. In each experiment, synthetic granitic composition glass + H
2
O was homogenized well above the liquidus
T
, and
T
was lowered by increments until quartz and alkali feldspar crystalized from the liquid. To establish reversed equilibrium, we crystallized the homogenized melt at the lower
T
and then raised
T
until we found that the crystalline phases were completely resorbed into the liquid. The reversed liquidus minimum temperatures at 3.0, 4.1, 5.8, 8.0, and 12.0 wt% H
2
O are 935–985, 875–900, 775–800, 725–775, and 650–675 °C, respectively. Quenched charges were analyzed by petrographic microscope, scanning electron microscope (SEM), X-ray diffraction (XRD), and electron microprobe analysis (EMPA). The equation for the reversed haplogranite liquidus minimum curve for Ab
36.25
Or
36.25
Qz
27.5
(wt% basis) at 1.0 GPa is
T
=
-
0.0995
w
H
2
O
3
+
5.0242
w
H
2
O
2
-
88.183
w
H
2
O
+
1171.0
for
0
≤
w
H
2
O
≤
17
wt% and
T
is in °C. We present a revised
P
-
T
diagram of liquidus minimum H
2
O isopleths which integrates data from previous determinations of vapor-saturated melting and the lower pressure vapor-undersaturated melting studies conducted by other workers on the haplogranite system. For lower H
2
O (<5.8 wt%) and higher temperature, our results plot on the high end of the extrapolated water contents at liquidus minima when compared to the previous estimates. As a consequence, amounts of metaluminous granites that can be produced from lower crustal biotite–amphibole gneisses by dehydration melting are more restricted than previously thought.