U-Pb systematics and alteration trends of Pennsylvanian-aged aragonite and calcite
- Univ. of Toronto, Ontario (Canada)
- Brock Univ., St. Catharines, Ontario (Canada)
Carbonate allochems from the mid-Pennsylvanian Buckhorn fauna of the Boggy Formation, Oklahoma, USA, consist of primary aragonite preserved in nautiloids and of secondary low-Mg calcite present in crinoids and corals. Unaltered nautiloids contain 11-38 ppb U and 23-422 ppb Pb, and have low [sup 238]U/[sup 204]Pb ([mu]) values from 4.1-14.8. They give a [sup 238]U-[sup 206]Pb isochron age of 311 [+-] 48 Ma (2[sigma]), which is in agreement with the assigned stratigraphic age (mid-Desmoinsian; [approximately]305 Ma) of the Boggy Formation sediments. Their initial lead isotope ratios are [sup 206]Pb/[sup 204]Pb = 19.11 [+-] 0.08, [sup 207]Pb/[sup 204]Pb = 15.70 [+-] 0.04, and [sup 208]Pb/[sup 204]Pb [approximately] 38.7, and reflect an upper crustal source for Pb in mid-Pennsylvanian seawater. These values are similar to those of coals of this age. The contents of both U and Pb in the Buckhorn nautiloids are significantly higher than those found in the modern nautilus Nautilus pompilius. These enrichments are attributed to bitumen which was added to the carbonates during diagenesis. The diagenetic transformation of aragonite to low-Mg calcite in the Buckhorn nautiloids was apparently associated with a net loss of Pb, thereby increasing their [mu] values. The secondary carbonates of crinoids contain 265-352 ppb U, 136-262 ppb Pb, and their [mu] values range from 68-111; consequently, their present-day uranogenic lead isotopic ratios are higher than those of the Buckhorn nautiloids. The U-Pb data for some diagenetic low-Mg components indicate an age of [approximately] 280 Ma suggesting an early timing for the post-depositional alteration process. The apparent initial lead isotope ratios of crinoids are variable, and some are significantly higher than the coexisting aragonite ([sup 206]Pb/[sup 204]Pb = 20.50, [sup 207]Pb/[sup 204]Pb = 15.73).
- OSTI ID:
- 5239388
- Journal Information:
- Geochimica et Cosmochimica Acta; (United States), Vol. 58:1; ISSN 0016-7037
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
CARBONATES
DIAGENESIS
ISOTOPE DATING
LEAD ISOTOPES
ISOTOPE RATIO
URANIUM 238
LEAD 204
LEAD 206
LEAD 207
ACTINIDE ISOTOPES
ACTINIDE NUCLEI
AGE ESTIMATION
ALPHA DECAY RADIOISOTOPES
CARBON COMPOUNDS
EVEN-EVEN NUCLEI
EVEN-ODD NUCLEI
HEAVY NUCLEI
HOURS LIVING RADIOISOTOPES
ISOMERIC TRANSITION ISOTOPES
ISOTOPES
MILLISEC LIVING RADIOISOTOPES
NUCLEI
OXYGEN COMPOUNDS
RADIOISOTOPES
SECONDS LIVING RADIOISOTOPES
SPONTANEOUS FISSION RADIOISOTOPES
STABLE ISOTOPES
URANIUM ISOTOPES
YEARS LIVING RADIOISOTOPES
580000* - Geosciences