Impact glass spherules in the Chicxulub K‐Pg event bed at Beloc, Haiti: Alteration patterns

Abstract

International audienceWe have investigated six impact glass spherules from the K-Pg event bed at Beloc, Haiti, using optical and electron microscopy, electron microprobe and in situ laser ablation-mass spectrometry (LA-ICP-MS; 37 trace elements, spot size 90-35m), in order to understand geochemical changes during alteration. The mm-sized glass spherules are partly or totally altered to smectite, but original textural features are preserved. The average trace-element composition of glass matches that one of the upper continental crust. Hints for a meteoritic component are lacking (Ni/Cr<1.3; Pt below detection limit). Compared to this fresh glass, smectites are strongly depleted in trace elements, except for Li, Sc, V, Ni, Ga, Ge, and Ba. The chondrite-normalized REE distribution patterns are flat with subchondritic abundances, related to their very low degree of crystallinity. We observe a positive Eu and a strong negative Ce anomaly; the latter is explained by formation of an organic Ce4+-complex, soluble under reducing conditions. Zr/Hf of glasses and smectites is chondritic to superchondritic (35-40), whereas Nb/Ta in smectite is subchondritic (5-12) compared to Nb/Ta in the glass (similar to 14-18). The low Nb/Ta is due to the low Nb concentrations in the smectite. Using in situ techniques with high spatial resolution, we have documented for the first time the significant changes in diagnostic elemental ratios during alteration of glass spherules. This has to be taken into account in the interpretation of geochemical data of not only impact materials but also volcanic glass, especially if bulk rock methods are used

The Cretaceous/Paleogene (K‐Pg) boundary at the J Anomaly Ridge, Newfoundland (IODP Expedition 342, Hole U1403B)

“…Comparing the results of this study with those for the K‐Pg event bed from Beloc, Haiti, we noticed that all altered material (“spherule layer” of the U1403B K‐Pg sample, “smectite”—Ritter et al. ; and the palagonite‐like material “Beloc Layer L1” of the Beloc profile—Belza et al. ) show low Nb/Ta and variable Zr/Hf (Fig.…”

“…; Ritter et al. ; this work). While working with large samples can circumvent problems due to irregularly distributed nuggets, yielding “average” values for PGE concentrations (bulk rock), the method with extremely high spatial resolution allows insight into the μm‐scale and particular deposition modes.…”

“…As postulated by Ritter et al. (), the newly formed clay minerals were unable to retain the REE released from the glass.…”

“…() and Ritter et al. (). The latter authors explained the near quantitative loss of REE during alteration of the glass by the very low crystallinity and small grain size of the smectites, hindering adsorption of the ions as well as their fixture in the lattice.…”

“…Ritter et al. () tentatively explained the loss of Nb to be related to different solubility of Nb in its two oxidation stages (Nb 3+ and Nb 5+ ). According to Wood (), Nb transport at low temperatures seems possible in sulfate complexes at acidic pH over short distances.…”