To estimate the Continental growth rate on the Earth; (1) Age distribution of detrital zircons from Mississippi River sand, North America1) Shuji Rino 1) Tsuyoshi Iizuka 1) Ikuo Katayama 2) Akihisa Motoki 1) Takefumi Hirata 1) Shigenori Maruyama
1)Tokyo Institute of Technology, 2-12-1, O-okayama, Meguro-ku 152-8551. Japan.
EOS, Transactions, American Geophysical Union, 82-47. Nov. 20, F1318, V22B-1040, San Francisco, California.
In order to understand the continental history of the Earth, we must evaluate the recycled, we must evaluate the recycled granitic materials to their protolith ages, because the Phanerozoic orogenies occupy the continents more than 50% on the Earth and are composed of dominantly sedimentary rocks over 90%. To overcome this difficulty, the age population of detrital zircon from river sand is critical, because of (1) resistance of zircon against recrystallization, (2) common occurrence of granitic rocks, and (3) high blocking temperature (ca. 900° C) of U.Pb isotope system. We initiated systematically to analyze those zircons from major 25 rivers over the world. The remained unknown parameters are statistic analysis to obtain how many analysis satisfactorily represent the space ratio of provenance with different ages. As a first step. we investigated U-Pb ages of zircons (1000 grains) from Mississippi River sand, North America. Th/U ratio was also analyzed to distinguish the origin of zircon, either metamorphic or igneous simultaneously. The Mississippi River and its tributaries cover North American continent about 40%, and transport the sediment, into the river mouth. Age diatribution of zircon is compared with spatial ratio of basement with different age (Hoffman, 1991). The results indicated that zircon age ranges from 36 Ga to 0.2 Ga with a strong peak at 15-19 Ga occupying 45% among total analysis of 125 grains. This overall patter is very similar to that of space ratio of basement rock with different ages (Hoffman, 1988), Slight difference appears on the peak at 2.7 Ga and 32 Ga calculated from space ratio on basement, i.e.. no strong peaks on zircon ages. Metamorphic source zircons (Th/U ratio is above 4) are less than 1% of all. It may be tentatively concluded that the number of analysis of zircon may be 100 enough to conclude the general tendency as a first approximation.