Experimentelle und Analytische Planetologie
Der Einfluss von Mikrostrukturen auf die geologische Signifikanz radiometrischer Alter von
Datierungsmineralen: Beispiel Monazit
Dating with the U-(Th)-Pb, Rb-Sr, and K-Ar decay systems is one of the most fundamental techniques in geoscience.
Yet frequently disturbances in the isotope systems of the dated minerals hamper an assessment of the geologic
relevance of the measured data ("ages"). Such disturbances include partial loss of radiogenic isotopes, and
parent-daughter element fractionation. These effects are explained by diffusion along lattice defects,
re-crystallization, dissolution - precipitation processes, and most popular, by volume diffusion. The latter process
stood in the center of most experimental and theoretical studies in the field of "opening and resetting of isotope
systems", whereas, the essential role of the mineral's micro-structures is persistently underestimated. - We have
studied the micro-structure of monazite using scanning electron microscopy (SEM), electron microprobe analysis
(EMP), X-ray diffraction patterns (XRD) and transmission electron microscopy (TEM). Four well-characterized
monazites were investigated, with concordant U-Pb ages ranging from 24 to 1928 Ma, and up to ~15 wt.%
ThO2, and ~0.94 wt.% UO2. The single crystal fragments lack significant chemical
zoning. The monazites are not metamict, despite their old ages, very high abundances of radionuclides, and hence,
high time-integrated radiation doses. Except for the youngest one, the monazite crystals are composed of a mosaic of
crystalline but slightly distorted domains. This structure is responsible for the presence of (1) mottled diffraction
contrasts on the TEM, and (2) a second structural phase (B), with very broad XRD reflection patterns. For the 1928
Ma monazite, XRD reveals only the broad reflections of phase B, implying that the whole monazite was affected by
radiation damage that resulted in total distortion of the lattice. It is concluded that radiation damage in the form of
amorphous domains does not accumulate in monazite because self-annealing heals the defects as they are produced
by a-decay damage. The only memory of irradiation-induced defects is the presence of
distorted domains. As the diffusion rate of Pb in an undisturbed monazite lattice is extremely low, Pb loss due to
volume diffusion out of the monazite lattice is virtually impossible. This is the basic reason why almost all
monazites have concordant U-Th-Pb ages. Moreover, as long-term self-irradiation effects are limited in monazite, we
consider this phase as good candidate for the storage of high-level nuclear waste under the aspect of its high
resistance to irradiation.
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