Mathematics – Logic
Scientific paper
Jul 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27r.217e&link_type=abstract
Meteoritics, vol. 27, no. 3, volume 27, page 217
Mathematics
Logic
Scientific paper
In 1980, Alvarez et al. reported high Ir concentrations for the Cretaceous-Tertiary (hereafter, K/T) boundary layer, suggesting an impact of extraterrestrial material as a possible cause of the sudden mass extinction at the end of the Cretaceous period. Since then, high Ir abundances have been reported for K/T layers all over the world. Iridium enrichments were alternatively explained in terms of volcanic eruptions (Officer and Drake, 1982) or sedimentation (Zoller et al, 1982). Thus, abundances of Ir only cannot be critical in explaining the cause of the mass extinctions at the K/T boundary. In contrast to the fairly large number of Ir data for K/T boundary geological materials, only limited data are available for other siderophile elements. Relative abundances of siderophiles must be more informative in considering the causes of extinction, and provide further data on the type of extraterrestrial material of the projectile if siderophile abundances are in favor of an impact as the cause of the mass extinction at the K/T boundary. Thus, we analyzed additional K/T boundary materials for trace elements, including some of the siderophiles. A total of 7 samples collected from the K/T boundary near Gubbio, Italy (three from Bottaccione, four from Contessa) were analyzed. For comparison, we analyzed three additional samples, one from a Cretaceous sediment layer and the remaining two from a Tertiary layer. Four siderophile elements (Ir, Pt, Au, and Pd) were measured by RNAA and more than 25 elements, including 9 lanthanoids, were measured by INAA. The siderophiles listed above and Ni were found to be present in all of the boundary clay samples. They have C1-normalized abundances of 0.02 for Ni, Ir, and Pt, 0.04 for Pd, and Au was exceptionally depleted at 0.005. Both Ni and Ir show fairly small variations in abundances among the clay samples, whereas the other three elements show quite large variations, exceeding error limits. We believe that similar enrichments for these siderophiles in the K/T boundary clays were caused by an impact of extraterrestrial material having siderophiles that have not been largely fractionated. Similar abundance patterns of REE were confirmed not only for clay samples but also for the Cretaceous and Tertiary sediments. This suggests that sedimentation continued in similar circumstances without a large disturbance at the K/T boundary. We confirmed excellent correlations among Ir, As, and Sb abundances in the K/T samples, suggesting that they had a similar solution chemistry when sedimentation occurred. Both As and Sb show similar abundances, even for the Cretaceous as well as the Tertiary sediments, while Ir does not. Neither Pd nor Pt shows any correlation with these elements or with each other. This suggests that Ir was trapped into the clay together with As and Sb, but not with Pd or Pt. It is highly unlikely that these siderophiles were supplied only from sea water, and were eventually greatly enriched in clay materials, with the relative elemental abundances coinciding with those in chondrites. Thus, our data strongly suggest that a large impact of extraterrestrial material (chondritic?) caused the enrichment of siderophiles at K/T boundary. Acknowledgment. We are indebted to M. Ozima and S. Amari for samples analyzed in this work. References Alvarez, L.W., Alvarez, W., Asaro, F., and Michel, H.V. (1980) Science 208, 1095-1108. Officer, C.B. and Drake, C.L. (1982) Science 219, 1383-1390. Zoller, W.H., Parrington, J.R., and Kotra, J.M.P. (1983) Science 222, 1118-1120.
Ebihara Mitsuru
Miura Takashi
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