Mathematics – Logic
Scientific paper
Oct 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994e%26psl.127...25l&link_type=abstract
Earth and Planetary Science Letters (ISSN 0012-821X), vol. 127, no. 1-4, p. 25-38
Mathematics
Logic
16
Arid Lands, Earth Crust, Earth Mantle, Geochemistry, Geochronology, Isotopes, Mineral Deposits, Structural Basins, Chemical Composition, China, Fractionation, Mathematical Models, Sediments, Spectral Signatures
Scientific paper
Quaternary deposits of various types (lacustrine, desert sand, river, loess and loess-like deposits) from the southwestern Tarim Basin, North Tianshan, northwest China and the central Loess Plateau exhibit an epsilonNd(0) range from -5 to -12.8 and a Sm-147/Nd-144 range from 0.110 to 0.140. Their Nd-depleted mantle model ages are between 1.50 and 1.92 Ga, with a mean value of 1.7 Ga. Th/Sc ratios vary over a wide range from 0.1 to 2. Comparison of these geochemical signatures with those of the old Australian shales and modern Pacific Ocean dust suggests a continuous input of mantle-derived material into the crust over geological time. The deposits from the North Tianshan show clearly different chemical and isotopic features, with significantly higher Sm/Nd, high Nd-143/Nd-144, low Sr-87/Sr-86, and low La/Lu and Th/Sc ratios relative to those in the deposits from the other areas. This indicates a geological control on the local source, which has an overall young age and an erichment of basic components. The epsilonCe(0) values of two loess samples from the central Loess Plateau are +1.8 and 3.9, corresponding to depleted mantle model ages of 2.2 and 3.4 Ga. Like the Rb-Sr system, the evolution of the La-Ce isotopic system in the crust may be to some extent decoupled from the Sm-Nd system, due to La/Ce fractionation by sedimentary processes. We conclude that the loess can have originated from desert processes, since the difference in the chemistry of the desert sand and loess may result from mineral sorting during desert processes and aeolian transportation. The loess deposits on the central Loess Plateau chemically and isotopically resemble the desert loess deposits in the southwestern Tarim Basin but do not resemble those of the North Tianshan.
Fan Zi-Li
Liu Cong-Qiang
Masuda Akimasa
Okada Akihiko
Yabuki Sadayo
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