U-Pb Systematics

The isotopic composition of lead in the alkali-rich lavas on Ross Island, on Mt. Morning and Mt. Discovery, and from Taylor Valley loosely define a straight line in Fig. 16.8 (Sun and Hanson 1975). The slope of this line is the ratio of radiogenic Pb to 

Sun and Hanson (1975, p. 83) concluded that the date indicated by the isotope compositions of lead is about 1500 Ma and offered two alternative interpretations of its geological significance  

The basalts have been contaminated with basement material and the linear array is a result of mixing of two components. 

The ° Pb/ ° Pb ratios represent primary variations in the mantle source which have existed for 1500 m.y. 

We prefer the second alternative because the Sr/ Sr ratios of most Cenozoic lavas analyzed by Stuckless and Ericksen (1976) and by Jones and Walker (1972) provide no evidence that the magma was contaminated by strontium derived from the granitic basement rocks. The only exception is the sample of trachyte (CC-82, Fig. 16.7) which has a high Sr/ Sr ratio of 0.70501 (Stuckless and Ericksen 1976). 

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Gopel, C., Manhes, G. and Allegre, C.-J. (1994) U-Pb systematics of phosphates from unequilibrated ordinary chondrites. Earth and Planetary Science Letters, 121, 153-171. 

Hawkins D. P. and Bowring S. A. (1997) U-Pb systematics of monazite and xenotime case studies from the Paleoprotero-zoic of the Grand Canyon, Arizona. Contrib. Mineral. Petrol. 127, 87-103. 

U-Pb systematics could be enhanced by fluids even at moderate temperatures (<600 °C Rubatto et aL, 1999 Liati et al., 2000). Consequently, zircon may yield no clear petrological record of the P-T path, and it may be difficult to link SHRIMP U-Pb ages with a specific stage of metamorphic evolution. 

Eikenberg J., Signer P., and Wieler R. (1993) U-Xe, U-Kr, and U-Pb systematics for dating uranium minerals and investigations of the production of nucleogenic neon and argon. Geochim. Cosmochim. Acta S7, 1053-1069. 

Th-U-Pb systematics and Sm-Nd systematics address in part different aspects of Earth differentiation. First, Th/Pb and U/Pb ratios are affected by core formation, whereas the Sm/Nd ratio is not. Second, in the silicate Earth Th/Pb... 

Deliens M, Delhal J, Tarte P (1977) Metamictization and U-Pb systematics-a study by infrared absorption spectrometry of Precambrian zircons. Earth Planet Sci Lett 33 331-344... 

Gdpel C, Manhes G, Allegre CJ (1991) Constraints on the time-scale of accretion and thermal evolution of chondrite parent bodies by precise U-Pb dating of phosphates. Meteoritics 26 338 Gdpel C, Manhes G, Allegre CJ (1994) U-Pb systematics of phosphates from equilibrated ordinary chondrites. EarthPlanet Sci Lett 121 153-171 Hartmaim WK (1975) Lunar "cataclysm" A misconception. Icaras 24 181-187... 

There are several mechanisms by which metamorphic phosphates can grow. In the case of monazite this is especially important because recrystallization will reset the Th-U-Pb systematics, and the age measured will be the age of the recrystallization event. Recrystallization can occur by heterogeneous reactions in response to changes in pressure and temperature (prograde or retrograde metamorphism). Recrystallization is also... 

Kingsbury JA, Miller CF, Wooden JL Harrison TM (1993) Monazite paragenesis and U-Pb systematics in rocks of the eastern Mojave Desert, California, USA Implications for thermochronometiy. Chem Geol 110 147-167... 

Nakamura N, Unruh DM, Tatsumoto M, Hutchison R (1982) Origin and evolution of the Nakhla meteorite inferred from the Sm-Nd and U-Pb systematics and REE, Ba, Sr, Rb, and K abundances. Geochim Cosmochim Acta 46 1555-1573 Nakamura T, Imae N, Noguchi T, and 12 additional persons (1999) Antarctic micrometeoiites collected at the Dome Fuji Station. Antarctic Meteorite Research, vol. 12—. Tokyo, Japan, pp 183-198... 

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