High spatial resolution geochemistry and textural characteristics of ‘microtektite’ glass spherules in proximal Cretaceous–Paleogene sections: Insights into glass alteration patterns and precursor melt lithologies

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Abstract

Using laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS), we have conducted spatially resolved trace element analysis on fresh, unaltered microtektite glasses linked to the Cretaceous–Paleogene (K–Pg) boundary Chicxulub crater and on their surrounding alteration phases. This unique approach offers the opportunity to study in situ and at high spatial resolution both the mixing of different target lithologies and the variation of the major and trace element budget during the alteration process. In addition, two-dimensional element distribution maps reveal important geochemical information beyond the capabilities of single spot laser drilling. Glasses from two localities in opposite quadrants from the source crater were studied. At the Beloc locality (Haiti), the glass population is dominated by the presence of yellow high-Ca glass and black andesitic glass formed by admixture of carbonate/dolomite/anhydrite platform lithologies with crystalline basement. These glasses alter according to the well-established hydration–palagonitization model postulated for mafic volcanic glasses. REEs become progressively leached from the glass to below the detection limit for the applied spot size, while immobile Zr, Hf, Nb, and Ta passively accumulate in the process exhibiting both inter-element ratios and absolute concentrations similar to those for the original glass. In contrast, The Arroyo El Mimbral locality (NE Mexico) is characterized by abundant green glass fragments high in Si, Al and alkalis, and low in Mg, Ca, Fe. Low Si black glass is less abundant though similar in composition to the black glass variety at Beloc. The alteration pattern of high-Si, Al green glass at the Mimbral locality is more complex, including numerous competing reaction processes (ion-exchange, hydration, dissolution, and secondary mineral precipitation) generally controlled by the pH and composition of the surrounding fluid. All green, high-Si, Al glasses are hydrated and variably enriched in Sr, Ba, and Cs, indicating preferred adsorption from seawater during hydration. Despite the onset of ion-exchange reactions, which only seem to have affected the alkalis, the trace element composition of the green high-Si, Al glass is still largely representative of the original melt composition. Refining the geochemical signature of (altered) melt lithologies may advance our current understanding of glass stability in the natural environment and provide insight into the origin and emplacement of ejecta material during crater formation.

Introduction

A large number of studies recognize that impact melt holds key information on important parameters to understand the formation of an impact crater (Hörz, 1982, Shaw and Wasserburg, 1982, Glass et al., 1984, Koeberl, 1986, Schuraytz et al., 1994, Warren et al., 1996, Dressler and Reimold, 2001, Claeys et al., 2003, Kettrup and Deutsch, 2003). Because impact melt essentially represents the molten, quenched state of the precursor target, its chemical and isotopic signatures provide a unique fingerprint that is characteristic of the different target rock compositions (Premo and Izett, 1992). Comparative geochemical studies between melt and precursor target materials provide information on depth of excavation, zones of melting and the relative contributions of different target lithologies and (possible traces of) the meteorite impactor variably admixed within the melt. Such parameters are necessary to gain insight into the basic processes of complex crater formation, refine mathematical models, estimate the amount of energy involved in the impact process, etc. (Melosh, 1989, Kring, 1995, Pierazzo et al., 1997, Pierazzo et al., 1998, Pierazzo and Melosh, 1999, Kettrup et al., 2000, Wünnemann et al., 2008).

In the proximal ejecta deposits located all around the Gulf of Mexico and in the Caribbean area, impact melt from the Chicxulub crater occurs predominantly in the form of mm- sized ‘glassy spherules’, termed ‘microtektites’ (Bohor and Betterton, 1993). Microtektites are thought to represent droplets of melt, which travelled ballistically from the point of impact and quenched rapidly during flight (Smit, 1999). This interpretation relies on (1) their aerodynamic shape, (2) the lack of crystals or primary microlites, (3) the presence of vesicles documenting an exsolving gas phase that became trapped in a rapidly solidifying melt, (4) an almost zero gas pressure within these bubbles (Smit, 1999), and (5) their stratigraphic position (see Smit et al., 1992a, Smit et al., 1992b, for detailed overview). Chicxulub spherules commonly occur at the base of reworked impact-induced tsunami-wave and deep water oscillatory-wave deposits triggered by the impact event (Smit et al., 1992a, Smit et al., 1992b, Smit, 1999). They are physically separated from the uppermost platinum group element (PGE)-enriched clay layer that marks the late-stage settlement from a mixed target-projectile-rich impact vapor plume (Fig. 1).

Section snippets

Geological background

Relict glass from the 66 Ma Chicxulub crater (Yucatán, Mexico) was first described at the Beloc locality (Southern Peninsula of Haiti) (Maurasse et al., 1979, Izett et al., 1990, Sigurdsson et al., 1991a) and at Arroyo El Mimbral (Northeastern Mexico, Smit et al., 1992a, Smit et al., 1992b) (Fig. 1). The discovery of the microtektites allowed the first isotopic dating of the K–Pg impact event (Izett et al., 1991, Swisher et al., 1992) and the acquisition of geochemical data (Izett et al., 1991,

Samples and analytical methods

Novel, present-day techniques offer extraction of in situ geochemical information at high spatial resolution (μm-scale). Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) is a fast, quasi non-destructive analytical technique that provides multi-element determination of major and trace elements at low limits of detection and high spatial resolution. Based on LA-ICP-MS results, combined with whole rock wet chemistry for the determination of major and trace elements,

Beloc

At Beloc, glass spherules are pseudomorphically transformed into a brown Fe–Mg alteration product qualified as ‘smectite’ by several authors (Izett, 1991a, Izett, 1991b, Koeberl and Sigurdsson, 1992, Glass and Simonson, 2012). However, Bohor and Glass (1995) emphasize the presence of both poorly crystalline smectite and ‘palagonite’. Palagonite has been used as a descriptive term to denote yellow and brown alteration products of basaltic glass (Berkgaut et al., 1994). Palagonite forms as a

Glass alteration processes

Numerous authors have assessed the geochemical instability of natural and synthetic glasses in aqueous solutions and their subsequent alteration products (see Bunker, 1994, for an overview). It is a complex process that depends on both the intrinsic properties of the glass and the specific environmental conditions in which the alteration takes place (Utton et al., 2013). One of the primary factors is the composition of the glass. The alteration of low SiO2 basaltic glass (sideromelane) is known

Conclusion – Summary

This study provides a detailed investigation of different types of microtektite-like glasses linked to the formation of the Chicxulub crater, and their associated alteration products. Both glass composition and fluid properties exert a major control on the type of alteration that has taken place. However, particular textural and geochemical constraints can still be used to deduce the original composition of the glass. This can be particularly useful in areas where pristine glass phases are

Acknowledgements

This project was supported by FWO Research Foundation––Flanders (Projects G009113 & GoB8513). We thank Research Foundation-Flanders (FWO) for its support in funding a Ph.D. fellowship to J. Belza and a Postdoctoral Fellowship to S. Goderis. This research has been funded by the Interuniversity Attraction Poles Program Planet Topers initiated by the Belgian Science Policy Office. Oscar Steenhaut and Marc Raes are gratefully acknowledged for technical support with SEM-EDX and FEG-SEM. We would

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