Computer Science
Scientific paper
Jul 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993metic..28r.396m&link_type=abstract
Meteoritics, vol. 28, no. 3, volume 28, page 396
Computer Science
Cretaceous-Tertiary Boundary, Osmium Isotopes
Scientific paper
Osmium isotope measurements have been performed on the boundary clay at different Cretaceous-Tertiary boundary (KTB) sites [1-5] since [6] suggested that Os isotopes are an indicator of an extraterrestrial component. The debate over "impact vs. volcanic" could not be resolved, but an isotope ratio close to chondritic could be demonstrated. The study of the distribution of iridium in the stratigraphy of the KTB cannot distinguish the contribution of chondritic and/or terrestrial Ir respectively, whereas the Os isotopes allow us to better constrain a mixing model. The ^187Os/^186Os ratio of the continental crust and chondritic reservoirs differ by at least 10-30 times. Assuming certain parameters, we should be able to calculate the proportion of the reservoirs making up the sediments of the KTB section. We studied a complete section of the KTB of Sumbar, Turkmenistan [7], for its Os isotopic composition. In the section 0-30 cm above the boundary clay, the ^187Os/^186Os ratio increases from 1.15 to 1.47, whereas the Ir concentration decreases from 66 to 1.4 ng/g or 66 to 4.7 ng/g on a carbonate-free basis respectively. Calculations show that the chondritic component makes up 9% at the boundary layer and decreases down to 0.6% at +30 cm. The data cannot be simply explained by varying admixtures of a chondritic component to a sediment of constant Os concentration and isotopic signature. To explain the Os ratios completely it is necessary to consider a mixture of four components (extraterrestrial, ejecta material, local terrigeneous, and carbonacous sediments) with certain assumptions: (1) The extraterrestrial source is chondritic in its Os and Re content and has an initial Os isotope ratio of 1.12 at 65 Ma (time of impact), which is above the average for normal chondrites but is within the range measured so far (e.g., Murray). (2) The ejecta material has a higher Os concentration (0.2 ng/g) than the sediments and is only present in the first 5 cm of the sequence above the basal layer. (3) The local sediments have an Os concentration of 80 pg/g and the ^187Re/^186Os increases with time. Rhenium concentration has not been determined. The Re abundance is a not very sensitive parameter, since the long decay time of ^187Re (42.3 Ga) cannot account for the higher ratios within 65 Ma. It has been observed that Re is highly enriched above the basal layer [9] of the Caravaca section. This might also be true for the Sumbar section, and thus point 3 is plausible. Further analysis of Maastrichtian samples will give additional constraints on the concentration and isotopic composition of the terrigenous source material. References: [1] Luck J. M. and Turekian K. K. (1983) Science, 222, 613-615. [2] Lichte F. E. et al. (1986) Nature, 322, 816-817. [3] Esser B. K. and Turekian K. K. (1989) EOS, 70, 717. [4] Kraehenbuehl U. et al. (1988) Meteoritics, 23, 282. [5] Smitt R. A. (1990) LPS XXI, 1085-1086. [6] Turekian K. K. (1982) Geol. Bull. Am. Spec. Pap., 190, 243-249. [7] Alekseyev A. S. et al. (1988) Int. Geol. Rev., 30, 121-135. [9] Kyte F. T. et al. (1985) EPSL, 73, 183-195. Fig. 1, which appears here in the hard copy, shows the distribution of Ir concentration and Os-isotope ratio over the Sumbar K/T boundary section. Iridium data are from [7].
Krähenbühl Urs
Meisel Th.
Nazarov Michael A.
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