Janots et al., 2007
Spear and Pyle, 2010
Spear, 2010
Spear, 2010
Kelsey et al., 2008
“Coupled dissolution-reprecipitation is a well-established chemical reaction, driven by a minimization in the Gibbs free energy. In this process, a mineral phase, in the presence of a reactive fluid, is replaced either by an altered composition of the same phase or by an entirely new phase (Putnis, 2002)”.
ABSTRACT Electron-microprobe (EMP) U-Th-Pb dating on polyphase and discordant monazitesfrom polymetamorphic granulites of the Andriamena unit (north-central Madagascar)reveals inconsistent chemical ages. To explain these drastic variations, transmission electronmicroscopy (TEM) foils were prepared directly from thin sections by using the focusedion beam technique. The most important result of the TEM study is the demonstrationof the presence of small (;50 nm) Pb-rich domains where large variations inEMP ages occur. We suggest that radiogenic Pb was partially reincorporated in monaziteduring the recrystallization at 790 Ma. Because the excited volume of EMP is ;4 mm3,U-Th-Pb dating yielded various apparent older ages without geological significance. Inaddition, TEM analysis of the foils revealed the presence of an ;150-nm-wide amorphouszone along the grain boundary of monazite and its host quartz. This Fe-Si-Al–rich phasemay have formed as a result of fluid activity at 500 Ma, and the phase’s amorphous statemay be due to the irradiation from U and Th decay in the monazite. This demonstratesfor the first time the enormous potential of the TEM investigations on site-specific specimensprepared with the focused ion beam technique for the interpretation of geochronologicaldata.
“The most important result of the TEM study is the demonstration of the presence of small (50 nm) Pb-rich domains where large variations in EMP ages occur. We suggest that radiogenic Pb was partially reincorporated in monazite during the recrystallization…”
B. Budzyn (2009)
M-21
Assemblage: monazite, muscovite, albite, amorphous SiO2
Reagents: CaF2
Na2Si2O5
Experimental conditions 4.5 kbar, 450°C for a Duration: 16 days.
See: Budzyn (2009)
Experiment: hydrothermal apparatus
The monazite chosen for the experiment was taken from a heavy-mineral sand deposit at Cumuruxatiba, Bahia State, Brazil D.
Moderately rounded, semi-euhedral, relatively transparent, inclusion-free, 100 – 500 mm, amber-colored grains.
The monazite grains were hand-picked out of the heavy mineral sand, crushed to 50 – 150 mm size fragments and then washed in ethanol in an ultrasonic bath.
Starting monazite
ThO2: 7-8 wt %,
UO2: 0.5-0.75 wt%
Assemblage: monazite, muscovite, albite, amorphous SiO2
Reagents: CaF2
Na2Si2O5
Experimental conditions 4.5 kbar, 450°C for a Duration: 16 days.
See: Budzyn (2009)
M-21
5m
M-Y
M-X
ThO2= ~7wt%
ThO2= ~8wt%
5m
M-Z
5m
M-21
Bkg-1
Bkg-2
Bkg-3
5m
Brazil Monazite
Pristine Unaltered core Altered domains
Oxide MX MX
Weight Gr1 core Hi-Th-Rt LoThLo-Bk2
CaO 1.40 0.90 0.05
P2O5 29.64 29.24 28.48
ThO2 7.05 7.03 1.77
UO2 0.480 0.735 0.007
Y2O3 0.84 1.49 0.08
La2O3 12.490 12.686 15.576
Ce2O3 28.77 28.74 34.16
Nd2O3 12.28 12.43 13.71
Pr2O3 3.06 3.07 3.49
Sm2O3 1.694 1.436 1.148
Gd2O3 1.590 1.423 0.902
Dy2O3 0.349 0.432 0.044
PbO 0.129 0.144 -0.002
Total 100.8 101.2 100.5
Brazil Monazite
Pristine Unaltered core Altered domains
Oxide MX MX
Weight Gr1 core Hi-Th-Rt LoThLo-Bk2
CaO 1.40 0.90 0.05
P2O5 29.64 29.24 28.48
ThO2 7.05 7.63 1.77
UO2 0.480 0.735 0.007
Y2O3 0.84 1.49 0.08
La2O3 12.490 12.686 15.576
Ce2O3 28.77 28.74 34.16
Nd2O3 12.28 12.43 13.71
Pr2O3 3.06 3.07 3.49
Sm2O3 1.694 1.436 1.148
Gd2O3 1.590 1.423 0.902
Dy2O3 0.349 0.432 0.044
PbO 0.129 0.144 -0.002
Total 100.8 101.8 100.5
Brazil Monazite
Pristine Unaltered core Altered domains
Oxide MX MX
Weight Gr1 core Hi-Th-Rt LoThLo-Bk2
CaO 1.40 0.90 0.05
P2O5 29.64 29.24 28.48
ThO2 7.05 7.63 1.77
UO2 0.480 0.735 0.007
Y2O3 0.84 1.49 0.08
La2O3 12.490 12.686 15.576
Ce2O3 28.77 28.74 34.16
Nd2O3 12.28 12.43 13.71
Pr2O3 3.06 3.07 3.49
Sm2O3 1.694 1.436 1.148
Gd2O3 1.590 1.423 0.902
Dy2O3 0.349 0.432 0.044
PbO 0.129 0.144 -0.002
Total 100.8 101.8 100.5
Brazil Monazite
Pristine Unaltered core Altered domains
Oxide MX MX
Weight Gr1 core Hi-Th-Rt LoThLo-Bk2
CaO 1.40 0.90 0.05
P2O5 29.64 29.24 28.48
ThO2 7.05 7.63 1.77
UO2 0.480 0.735 0.007
Y2O3 0.84 1.49 0.08
La2O3 12.490 12.686 15.576
Ce2O3 28.77 28.74 34.16
Nd2O3 12.28 12.43 13.71
Pr2O3 3.06 3.07 3.49
Sm2O3 1.694 1.436 1.148
Gd2O3 1.590 1.423 0.902
Dy2O3 0.349 0.432 0.044
PbO 0.129 0.144 -0.002
Total 100.8 101.8 100.5
CaO
P2O5
ThO2 UO
2
Y2O3
La2O3
Ce2O3
Nd2O3
Pr2O3
Sm2O3
Gd2O3
Dy2O3
PbO
-1.20
-1.00
-0.80
-0.60
-0.40
-0.20
0.00
0.20
0.40
Th or U + Ca = 2 REE
Th + Si = REE + P
Compiled and modified by G. Dumond from Hoffman (1988), Tella et al. (2000), Ross (2002), Hajnal et al. (2005), van Breeman et al. (2005; 2007), Rainbird and Davis (2007), Berman et al. (2007), & http://www.lithoprobe.ca/transects