Mineralogy and phase-chemistry of an Ir-enriched pre-K/T layer from the Lattengebirge, Bavarian Alps, and significance for the KTB problem

Abstract

Three distinct iridium anomalies are exposed in the Cretaceous-Tertiary boundary (KTB) section of the Lattengebirge, Bavarian Alps, comprising a pelagic calcareous sequence from below a continental slope. Multiple Ir anomalies are in contrast to most reported K/T sections which are characterized by a single Ir peak. Two of these Ir spikes are contained in the 20 cm thick actual KTB termed “clayey interval” with the main peak of 4.4 ppb precisely at the boundary, and the second peak of 2.8 ppb 10 cm above. A third peak of 3.6 ppb was detected at 16 cm below the KTB in a thin, turbiditic and coal-bearing sandstone layer of Upper Maastrichtian age. The distribution of Ir anomalies provides evidence of episodic Ir-delivering events over an extended period of time. Whereas the worldwide distributed single-spiked KTB layers are composed entirely of diagenetically altered materials, various phases are discernible in the well preserved Ir-bearing pre-K/T layer: (1) sandstone clastic minerals in a carbonate matrix, (2) coal which is partly burned, (3) sulfides (chalcopyrite) included in coal, and (4) silicate melt glasses. The glasses represent the first ones reported from a K/T section, and were formed by combustion metamorphism from sedimentary parent rocks of the coal as indicated by their major element chemistry and REE patterns. Phase-chemistry of the layer components reveals Ir concentrations of up to 110 ppb indicating that Ir and chalcophile trace elements (TRE) are primarily contained in the sulfides. This carrier-phase points to Ir enrichment by processes unrelated to impact, and its stratigraphic position demonstrates availability of Ir for incorporation into sediments before the KTB. The uptake of Ir and chalcophile TRE into the coal and subsequent combustion-metamorphism occurred on the continent during a time interval of 14,000–9000 yr prior to the KTB as constrained by radiometric age determinations, stratigraphy, and sedimentation rates. In the Lattengebirge K/T sediments no indications of shock are found. The results suggest a volcanic origin for the multiple Ir-bearing horizons. Like the worldwide KTB layers, they are concomitantly enriched in Ir and chalcophile TRE and, hence, have a common source. The Lattengebirge K/T profile is largely continuous, and was recently reexamined by micropaleontological methods (contribution by D. Herm and K.F. Weidich). The preservation of multiple Ir anomalies is attributed to high sedimentation rates of 1.75 cm/kyr and the absence of carbonate dissolution and condensation of sediments usually encountered in the worldwide KTB layers.