不纯碳酸盐样品年龄的铀系法测定

ON THE U-SERIES DATING OF DETRITUS-LADEN CARBONATE DEPOSITS

The uranium-series disequilibrium methods fulfil the need for investigating late Quaternary events in the time interval (ca. 5,000 to 400,000 yr B. P.) which bridges the applicable ranges of the K-At and ^14C methods. Since the methods were developed some twenty-five years ago, they have been applied to pure carbonate with notable success. However,many carbonate deposits, especially those of continental origin, such as travertine, tufa and soil caliche, are impure and contain detrital material composed largely of silicate minerals.In order to date these carbonates with U-series isotopes, the practical approach is to chemically separate the carbonate matrix from the embedded detritus using dilute .acid leaching.This process inevitably also solubilizes some of the U and Th isotopes from the detrital component. The problem is then to correct for this detrital contamination. This paper reviews the detrital correction schemes that have been used, discusses the relative merit and mathematical aspects of the Roshoh-Szabo mixing line plots used for making- the corrections,and recommends analytical procedures pertinent to the use of such plots.

The uranium-series disequilibrium methods fulfil the need for investigating late Quaternary events in the time interval (ca. 5,000 to 400,000 yr B. P.) which bridges the applicable ranges of the K-Ar and 14C methods. Since the methods were developed some twenty-five years ago, they have been applied to pure carbonate with notable success. However, manv carbonate deposits, especially those of continental origin, such as travertine, tufa and soil caliche, are impure and contain detrital material composed largely of silicate minerals. In order to date these carbonates with U-series isotooes, the practical approach is to chemically separate the carbonate matrix from the embedded detritus using dilute acid leaching. This process inevitably also solubilizes some of the U and Th isotopes from the detrital component. The problem is then to correct for this detrital contamination. This paper reviews the detrital correction schemes that have been used, discusses the relative merit and mathematical aspects of the Rosholt-Szabo mixing line plots used for making the corrections, and recommends analytical procedures pertinent to the use of such plots.