Skip to Main Content
Skip Nav Destination

Information pertinent to meteoritic contamination at the Cretaceous Tertiary (C/T) boundary, and other Ir-enriched layers, can be obtained from the study of contaminated formations in impact craters. Target-mixing considerations and volumetric estimates of Rochechouart breccias are combined with geochemistry of major and siderophile trace elements to evaluate how the chemistry of the preserved mixture of target rock-projectile evolved since it was deposited. Target and impact formations are characterized by a large scatter of their major element compositions. Si and Al are the least affected by the impact and/or post-impact events. Their overall fluctuation is less than 10%. Impact melts (E) are strongly enriched in K (150% to 200%) and Fe3+ and depleted in Ca (80% to 85%), Na (95% to 90%), Fe2+ (85% to 90%), and Mg (35% to 70%). Similar anomalies are found in basement clasts in E. These anomalies also affect suevite-like breccia (D), glass-free mixed breccia (C) and autochthonous breccia (B). The amplitude of these anomalies follows the trend E>D>C>B. However, the amplitude of the anomalies is variable from one sample to another. Similar but unrelated variations are observed for the meteoritic contamination. The degree of contamination varies also from one siderophile element to another. Over 99% of the mass of extraterrestrial Ir and Os in preserved formations at Rochechouart is located in suevite-like breccias and impact melts. As indicated by Pd and Ni, a substantial part of the meteoritic contamination (10% and 30%, respectively) resides in glass-free breccias. Most of the Ni contamination (60%) is actually contained and/or mixed with the nonmelted portion of the preserved formations. These data are interpreted as evidence of differential mobility of the various siderophile elements and redistribution of the Ni from the melt toward the unmelted fractions. Ni commonly occurs in alteration minerals, principally chlorites. Redistribution of siderophiles puts severe constraints on projectile identification and mass balance considerations. These difficulties are reduced when the number of samples is increased and when a larger fraction of material (not only the most contaminated) is considered. Even when the entire preserved allochthonous formations (C + D + E +) are considered, the projectile identification at Rochechouart is still uncertain because of the limited number of analyses for siderophiles other than Ni. The total mass of extraterrestrial Ni in the preserved formations can be estimated with reasonable accuracy (27 × 103 to 32 × 103 T). The projectile represents between 0.04 and 0.06% of the preserved formations in case of an IIA iron meteorite (3 to 14 times more in case of a chondrite). Hydrothermal alteration and/or weathering are the most likely processes to explain both major and trace element redistribution in Rochechouart formations. A detailed study of phase chemistry is required to better understand the relative redistribution of siderophiles at Rochechouart and by analogy to better understand the geochemical record at the C/T boundary.

You do not currently have access to this chapter.

Figures & Tables




Citing Books via

Related Book Content
Close Modal

or Create an Account

Close Modal
Close Modal