DO THE IMPACT MELT ROCKS WITHIN THE CHICXULUB IMPACT STRUCTURE PEAK RING (YUCATÁN PENINSULA, MEXICO) PRESERVE AN IMPACTOR SIGNATURE?

The IODP-ICDP Expedition 364 drilling recovered a ~829 m continuous core of impactites and basement rocks within the peak ring of the ca. 200-km-diameter Chicxulub impact structure. Until now, no unambiguous meteoritic contribution has been detected in impact melt rocks (IMR) from the peak ring.

In order to identify a possible meteoritic component within the IMRs, selected highly siderophile element (HSE) abundance measurements and Re–Os isotopic analyses were performed. Suevites, as well as pre-impact lithologies (dolerite, dacite, granite), were also investigated.

The IMR chemical compositions reflect primarily a mixture of mafic and felsic components. The HSE contents in the IMRs are generally low (<39 ppt Ir, <96 ppt Os, <149 ppt Pt; similar to those of upper continental crust, UCC). Only three IMRs (from a total of 12 IMRs investigated) exhibit higher Os (125–410 ppt) and/or Ir (250–324 ppt) contents relative to the other IMRs studied. Dolerite is characterized by high HSE contents (245 ppt Os, 156 ppt Ir, and 346 ppt Pt). On the other hand, granites, dacite, and suevites have very low Ir contents (1–10 ppt). The ¹⁸⁷Os/¹⁸⁸Os ratios of the IMRs differ widely (0.18–2.09). The dolerite, thought to be the main mafic component of the IMR, displays a ¹⁸⁷Os/¹⁸⁸Os ratio of 0.17, close to the chondritic value of ~0.13, which makes the unambiguous identification of an extraterrestrial component challenging. Moreover, granites show less radiogenic ¹⁸⁷Os/¹⁸⁸Os ratios (~0.16) compared to typical crustal rocks, while IMRs and suevites broadly follow a mixing trend between UCC and chondritic/mantle material. Based also on Cr and Ir contents, we hypothesize the presence of a possible, highly diluted meteoritic component, contributing up to 0.05%, in only one of the investigated IMRs. The IMRs and pre-impact lithologies were affected by post-impact hydrothermal alteration processes, probably remobilizing Re and Os. Consequently, the interpretation of relatively HSE-enriched IMRs as having a meteoritic component remains ambiguous, as is the case at other impact structures in which mafic target rocks are present and/or post-impact hydrothermal alteration occurred. Our study suggests that a heterogeneously distributed, but extremely low amount of meteoritic material (if any) occurs in impactites within the Chicxulub peak ring, in contrast to the distal K–Pg boundary ejecta, in which a significantly higher amount (up to ~5%) of meteoritic component is present.