Improving U-Pb concordance of zircon through sequential dissolution in a microwave oven

Conference · Wed Jan 01 04:00:00 EST 1992 · Geological Society of America, Abstracts with Programs; (United States)

OSTI ID:5809464

Palais, D G; Mukasa, S B ^[1]

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1. Univ. of Michigan, Ann Arbor, MI (United States). Dept. of Geological Sciences

Seven zircon fractions, ranging in age from early Proterozoic to late Mesozoic, have been studied following short-term exposure to microwave in the presence of HF acid using a conventional microwave oven. The HF acid leaches, zircon residues, and untreated fractions were examined by thermal ionization mass spectrometry to compare their U and Pb concentrations and apparent ages. Six untreated fractions from a Proterozoic paragneiss from northwestern Arizona are discordant and define an array with an upper concordia intercept age of 1,699 [+-] 10 Ma. Following the microwave treatment, residues representing the least (4%) and most (8%) discordant fractions were less than 0.6% discordant and had Pb-207/Pb-206 ages of 1,711 [+-] 2 and 1,707 [+-] 3 Ma, respectively. Elongate, prismatic zircons from the northwest Arizona paragneiss were extremely fragile following the microwave cycle and broke apart upon application of the slightest pressure. SEM imaging suggests that these grains broke along preexisting fractures that were preferentially attacked by hot HF acid during the microwave cycle. Zircons from three Mesozoic plutonic rocks collected from West Antarctica show similar improvement of concordancy following the microwave leaching cycle. In one case, an untreated fraction yielded concordant U-Pb ages of 101.8 [+-] 1 Ma. The residue following the microwave procedure contained 83% of the total Pb budget for the sample and was concordant at 105.7 [+-] 1 Ma. The leach from this sample yielded slightly discordant U-Pb ages at 82 [+-] .4 and 83 [+-] .4 Ma. When recombined, the leach and residue yield the same age as the untreated sample. This suggests that the untreated fraction may in reality be discordant, a factor which is typically difficult to resolve in relatively young zircons.

• OSTI ID: 5809464
• Report Number(s): CONF-921058--
• Conference Information: Journal Name: Geological Society of America, Abstracts with Programs; (United States) Journal Volume: 24:7
• Country of Publication: United States
• Language: English

Related Subjects

58 GEOSCIENCES
580000* -- Geosciences
ACCURACY
ACID HYDROLYSIS
ACTINIDES
AGE ESTIMATION
ANTARCTIC REGIONS
ANTARCTICA
ARIZONA
CHEMICAL REACTIONS
CRYOSPHERE
DECOMPOSITION
DEVELOPED COUNTRIES
ELECTROMAGNETIC RADIATION
ELEMENTS
FLUORINE COMPOUNDS
FRACTURE PROPERTIES
GEOLOGIC FORMATIONS
HALOGEN COMPOUNDS
HYDROFLUORIC ACID
HYDROGEN COMPOUNDS
HYDROLYSIS
INORGANIC ACIDS
ISOTOPE RATIO
LEAD
LYSIS
MECHANICAL PROPERTIES
METALS
MICROWAVE RADIATION
MINERALS
NORTH AMERICA
POLAR REGIONS
RADIATIONS
SAMPLE PREPARATION
SILICATE MINERALS
SOLVOLYSIS
URANIUM
USA
ZIRCON