SILICATE, PHOSPHATE AND CARBONATE MELTS AS INDICATORS FOR AN IMPACT-RELATED HIGH-TEMPERATURE INFLUENCE ON SEDIMENTARY ROCKS OF THE RUBIELOS DE LA CÉRIDA STRUCTURE, SPAIN
Klaudia Hradil1, Ulrich Schüssler1, Kord Ernstson2
1Institut für Mineralogie, Universität Würzburg, 2Fakultät für Geowissenschaften, Universität Würzburg
The Rubielos de la Cérida structure is located within the Iberian Chain of northeastern Spain, south of Zaragoza (Fig. 1). 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]. Intermixed in a polymict impact megabreccia, various kinds of melted rocks occur as blocks of variabe size in a decimetre to metre range.
Fig. 1: Sketch-map showing the location of the Azuara (A) and the Rubielos de la Cérida (R) structures
Fig. 2: Rafts of former melt within the polymict impact megabreccia
2a: Silicate glass (length about 1 m)
2b: Phosphate melt rock (ca. 10 cm)
2c: Carbonate-phosphate melt rock (length about 20 cm)
Fig 3: X-ray powder diffraction pattern of a silicate melt rock, showing the typical glass hump and peaks of additional minerals
Fig 4: Silicate melt rock, consisting of spheroids and lenses of whitish glass and interstitial greyish glass (field 8 mm wide)
Phosphate and carbonate melts
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 (Figs. 2c and 5a). The carbonate bodies are coarse-grained in their centres, with decreasing grain size and perpendicular orientation towards the rims. From microprobe investigations, the carbonate is pure calcite. The isotropic glassy matrix consists of nearly pure Ca-phosphate which locally may contain some additional Si. This matrix is pervaded by lots of tiny microcrystals which are identified by x-ray diffraction analysis to be apatite and which are interpreted as a result of recrystallization from the glass. The diffraction peaks of this apatite are much wider compared to those of a well crystallized one, indicating its very unperfect crystal structure (Fig. 6).
Fig. 6: X-ray powder diffraction pattern of apatite, unperfectly recrystallized from phosphate melt of the Rubielos de a Cérida crater (upper pattern), compared to well crystallized apatite from a pegmatite (middle) and to theoretical pattern of apatite and calcite (PDF data base, lower pattern)
References: -  Ernstson, K. et al., 1985, Earth Planet. Sci. Lett., 74, 361-370.  Ernstson, K. & Fiebag, J., 1992, Geol. Rundschau, 81, 403-427.  Ernstson, K. et al., this volume.  Wedepohl, K.H. (ed.), 1969, Handbook of Geochemistry, Springer.  Graup, G., 1999, Meteoritics Planet. Sci, 34, 425-438.
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