Recognizing isotopic reservoirs

Rock type Sr/ Sr Nd/ + Nd lOTpb/IOtpb Pb/ Pb Dominant mantle component Ref.  

type Nd/ Nd jDSpb/JWp, lB7pb/ pb Dnminant Pb/ n b mantle component Ret  

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Calcite cement is the dominant cement type in the central basin. Cemented zones can be visually recognized in cores and are from 10 cm to, in a few cases, more than 1 m thick (Boles Ramseyer, 1987). Cement zones cannot be easily traced between wells spaced as close as 100 m, suggesting that the intensely cemented zones are relatively isolated and discontinuous, certainly on a basin scale and in most cases on a reservoir scale. Most cement zones have not been studied in sufficient detail to establish growth patterns. A few detailed analyses of individual zones show that some have a composite history (i.e. variable isotopic compositions) on a scale of less than 0.5 m (e.g. cement zone at North Coles Levee, well NCL 488-29, 2621 m depth), whereas others show little variation (Schultz et al., 1989). Systematic growth patterns, such as are typical for concretions in shales (e.g. Raiswell, 1971 Boles et al., 1985) or in concretions that coalesce to form continuous cemented beds (Bjor-kum Walderhaug, 1990), have not been recognized in the zones studied to date. Apart from extensively cemented zones, calcite occurs as scattered crystals in many samples. 

The solar system formed from a molecular cloud fragment—traditionally called the solar nebula—that was rather well mixed. Therefore, isotopic abundances in almost all available solar system materials are very similar to each other, and elemental abundances in primitive meteorites are also similar to the values in the Sun. The major exceptions to this rule are the noble gases. Because they are chemically inert and volatile, they are very strongly depleted in solid matter. As a consequence, numerous noble gas components can be recognized throughout the solar system which are not necessarily related to the composition of the bulk nebula. Still, one major question in cosmochemistry is to what extent planetary bodies contain reservoirs that reflect the noble gas composition in the nebula or the presolar cloud. 

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