Klaudia Hradil 1, Ulrich Schüssler 1, Kord Ernstson 2

The Rubielos de la Cérida structure is located within the Iberian Chain of northeastern Spain, south of Zaragoza. Neighbouring the well established Azuara impact structure and being of the same stratigraphic age, it is interpreted as one part of a doublet impact structure, caused by a paired projectile during the Lower to Mid-Tertiary [1, 2, 3]. Between the central uplift and the northern rim of the structure, various kinds of melted rocks form blocks of variabe size in a decimetre to metre range, intermixed in a polymict megabreccia.

One kind of porous and soft silicate melt rocks is composed of more than 90% glass. This is typically shown by an amorphous "glass hump" occurring in x-ray powder diffractograms. The glass consists of two different phases. The dominant one is a whitish glass which forms tiny spheroids and lens-shaped bodies in a size of around 0.5 mm. The second phase is a translucent greyish glass interstitial within the white phase. From microprobe analyses, both phases are rather similar and match the whole-rock RFA data of SiO₂ around 58 wt.%, variable Al₂O₃ up to 21%, MgO 5-6%, CaO 1.5%, variable Na₂O+K₂O around 2%, LOI around 10%. This roughy fits the average composition of shales [4] which are assumed to be the pre-impact educt rocks.

A very special kind of former melt was found near the village of Barrachina within the Rubielos de la Cérida crater and consists of amoeba-like carbonate particles, embedded within a glassy matrix (Fig.1). The carbonate bodies are coarse-grained in their centres, with decreasing grain size and perpendicular orientation towards the rims. The isotropic matrix is pervaded by tiny microcrystals. From microprobe investigations, the carbonate is pure calcite. The glassy matrix consists of nearly pure Ca-phosphate which locally may contain some additional Si. In part, this glass is recrystallized to form apatite, as veryfied by x-ray diffraction analysis. The diffraction peaks of this apatite, however, are much wider compared to those of a well crystallized one, indicating its very unperfect crystal structure (Fig.2). A similar melt rock has been reported for the suevite of the Ries crater [5]. In the suevite, the calcite particles have identical structure and composition compared with the melt rocks of Barrachina and are interpreted as quench products of a carbonate melt [5]. Different from the Barrachina melt rocks, the matrix in the Ries samples is silicate glass as a result of immiscibility of carbonate melt and silicate melt [5]. In our case, the melt rock displays a small-scaled immiscibility of coexisting former carbonate melt and phosphate melt. Beside these mixed melt rocks, fragments of pure former Ca-phosphate melt with glassy parts and unperfectly recrystallized apatite, but without carbonate were also found.

The composition of the various melt rocks displays the composition of the pre-impact stratigraphy of the target region which is dominated by carbonates with interlayers of shales (see above). Phosphate-rich material can be deduced from coprolite layers well known within sediments of the regional Lower Tertiary. Furtheron, recrystallized baryte which was locally observed in the melt rocks may reflect strongly Ba-enriched rocks also known to occur in the target region.

References: - [1] Ernstson, K. et al., 1985, Earth Planet. Sci. Lett., 74, 361-370. [2] Ernstson, K. & Fiebag, J., 1992, Geol. Rundschau, 81, 403-427. [3] Ernstson, K. et al., this volume. [4] Wedepohl, K.H. (ed.), 1969, Handbook of Geochemistry, Springer. [5] Graup, G., 1999, Meteoritics Planet. Sci, 34, 425-438.