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Popigai impact ejecta layer and extraterrestrial spinels recovered in a new Italian location—The Monte Vaccaro section (Marche Apennines, Italy)

By
Samuele Boschi
Samuele Boschi
Astrogeobiology Laboratory, Division of Nuclear Physics, Department of Physics, Lund University, 223 63 Lund, Sweden
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;
Birger Schmitz
Birger Schmitz
Astrogeobiology Laboratory, Division of Nuclear Physics, Department of Physics, Lund University, 223 63 Lund, Sweden
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;
Fredrik Terfelt
Fredrik Terfelt
Astrogeobiology Laboratory, Division of Nuclear Physics, Department of Physics, Lund University, 223 63 Lund, Sweden
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;
Linus Ros
Linus Ros
Division of Nuclear Physics, Department of Physics, Lund University, 223 63 Lund, Sweden
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;
Mikael Elfman
Mikael Elfman
Division of Nuclear Physics, Department of Physics, Lund University, 223 63 Lund, Sweden
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;
Per Kristiansson
Per Kristiansson
Division of Nuclear Physics, Department of Physics, Lund University, 223 63 Lund, Sweden
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;
Camilla Sulas
Camilla Sulas
Dipartimento di Scienze della Terra, Università degli Studi Firenze, 50121 Firenze, Italy
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;
Simonetta Monechi
Simonetta Monechi
Dipartimento di Scienze della Terra, Università degli Studi Firenze, 50121 Firenze, Italy
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;
Alessandro Montanari
Alessandro Montanari
Osservatorio Geologico di Coldigioco, 62021 Apiro, Italy
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Publication history
15 January 201910 May 2019

ABSTRACT

The Popigai (100 km in diameter) and the Chesapeake Bay (40–85 km diameter) impact structures formed within ~10–20 k.y. in the late Eocene during a 2 m.y. period with enhanced flux of 3He-rich interplanetary dust to Earth. Ejecta from the Siberian Popigai impact structure have been found in late Eocene marine sediments at numerous deep-sea drilling sites around the globe and also in a few marine sections outcropped on land, like the Massignano section near Ancona in Italy. In the Massignano section, the Popigai layer is associated with an iridium anomaly, shocked quartz, and abundant clinopyroxene-bearing (cpx) spherules, altered to smectite and flattened to “pancake spherules.” The ejecta are also associated with a significant enrichment of H-chondritic chromite grains (>63 µm), likely representing unmelted fragments of the impactor. The Massignano section also contains abundant terrestrial chrome-spinel grains, making reconstructions of the micrometeorite flux very difficult. We therefore searched for an alternative section that would be more useful for these types of studies. Here, we report the discovery of such a section, and also the first discovery of the Popigai ejecta in another locality in Italy, the Monte Vaccaro section, 90 km west of Ancona. The Monte Vaccaro section biostratigraphy was established based on calcareous nannoplankton, which allowed the identification of a sequence of distinct bioevents showing a good correlation with the Massignano section. In both the Monte Vaccaro and Massignano sections, the Popigai ejecta layer occurs in calcareous nannofossil zone CNE 19. The ejecta layer in the Monte Vaccaro section contains shocked quartz, abundant pancake spherules, and an iridium anomaly of 700 ppt, which is three times higher than the peak Ir measured in the ejecta layer at Massignano. In a 105-kg-size sample from just above the ejecta layer at Monte Vaccaro, we also found an enrichment of H-chondritic chromite grains. Because of its condensed nature and low content of terrestrial spinel grains, the Monte Vaccaro section holds great potential for reconstructions of the micrometeorite flux to Earth during the late Eocene using spinels.

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GSA Special Papers

250 Million Years of Earth History in Central Italy: Celebrating 25 Years of the Geological Observatory of Coldigioco

Geological Society of America
ISBN electronic:
9780813795423

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