Chronostratigraphy, composition, and origin of Ni-rich spinel from the Late Eocene Fuente Caldera section in Spain: One impact or more?

Abstract

Abstract-Here we report on the stratigraphic distribution and chemical composition of Ni-rich spinel, a specific mineral tracer of meteorite impacts, in the Fuente Caldera section in Spain. A major peak in spinel abundance is observed in a biostratigraphic interval defined by the last occurrence of the planktic foraminifera Porticulasphaera semiinvoluta and the first occurrence of the planktic foraminifera Turborotalia cunialensis. Two other peaks of lower abundances are observed higher up in the same biostratigraphic interval, but geochemical considerations suggest that they likely originate from redeposition by turbiditic currents. Biostratigraphic correlations with the global stratotype section and point for the Eocene/Oligocene boundary of Massignano in Italy give an age of 35.4 ± 0.2 Ma (1σ) for the major peak. This age is indistinguishable from the age of the impact horizon at Massignano (35.5 ± 0.2 Ma) and within the age uncertainties for the Popigai (35.7 ± 0.2 Ma) and Chesapeake Bay (35.5 ± 0.5 Ma) craters. The Fuente Caldera spinel, as the Massignano spinel, is assumed to be a relic mineral of microkrystites, which are believed to derive from a unique source related to the Popigai impact crater. The morphologies and Cr compositions of the Fuente Caldera and Massignano spinel crystals are markedly different, however: the Fuente Caldera spinel occurs mostly as octahedral and skeletal crystals with 85% of the grains belonging to the Cr-rich magnetite series and 15% to the Fe-rich chromite series, whereas the Massignano spinel occurs mostly as dendritic crystals with 90% of the grains belonging to the Cr-poor magnetite series. It is unlikely that these differences are the result of post-depositional alteration processes because the compositions of the crystals, as well as their morphologies, are in general very similar to those reported for primary spinel crystals, i.e., spinel crystals present in meteorite fusion crust or synthetized from meteoritic material. In addition, spinel crystals have quite homogeneous compositions except for a few grains (<10%) showing Cr zonations, but these are assigned to primary crystallization processes. One possible explanation that is consistent with a single impact event producing spatial variations in spinel compositions and morphologies is that microkrystites are locally generated by the ablation in the atmosphere of impact debris. An alternative explanation is that Fuente Caldera and Massignano microkrystites derive from two closely spaced impact events, which however requires another, so-far unknown source crater for microkrystites.

Incipient devitrification of impact melt particles at Bosumtwi crater, Ghana: Implications for suevite cooling history and melt dispersion

“…The K/Pg boundary spinel has been described to have high Fe 2 O 3 /FeO ratios, high Ni concentrations, and low content of Ti and Cr (Kyte and Bostwick ) that is different from virtually all of the known terrestrial metamorphic or igneous spinels. Its high nickel content may reflect abundant Ni in the parent material, and a high ferric/ferrous ratio results from crystallization in an O 2 ‐rich environment (Robin and Molina ). Ni‐rich spinel is also commonly found in meteorite fusion crusts and cosmic spherules (Robin et al.…”

Magnetic particle characterization in the Seine river system: Implications for the determination of natural versus anthropogenic input

“…For SEM analyses, magnetic extracts were prepared using the method described by Robin and Molina [2006]. Such concentrates are biased toward strongly ferrimagnetic mineral phases [e.g., Franke et al , 2007a] and very often, weaker magnetic minerals, such as antiferromagnetic goethite and hematite, are not fully extracted.…”

“…The particle classification technique can be adjusted as defined by the user for the analysis of many other mineral phases, such as shown for barite containing samples [ Robin et al , 2003] or as applied in this study on iron oxide and sulphide particles. The ACC technique allows for absolute quantifications of the different identified minerals with respect to a grain size of ≥0.5 μ m. For further instrumental details see also Robin and Molina [2006].…”

Osmium, tungsten, and chromium isotopes in sediments and in Ni‐rich spinel at the K‐T boundary: Signature of a chondritic impactor

“…For some authors, the spinel crystals constitute a mineralogical marker specific to the meteoritic material melted and oxidized at high temperature in the terrestrial atmosphere (Robin and Rocchia 1998), while for others they correspond to the material that first accreted in the impact cloud at high temperature from a part of the cloud strongly enriched in material coming from the projectile (Kyte et al 1996). The K-T spinel is therefore expected to carry the true isotopic signature of the impactor, especially given that it is not affected by diagenesis (Robin and Molina 2006). Thus the Ni-rich spinels are the only relict crystals available to constrain the composition of the impactor at the K-T boundary (Kyte and Smit 1986).…”

“…Spinel crystals were separated from the Bidart sediments with a magnet following the procedure described in Robin and Molina (2006). Scanning electron microscopy coupled with energy dispersive spectroscopy (SEM-EDS) analysis of a fraction leached for 30 min in 27N HF revealed the proportions of minerals to be 20% silicates and 80% spinel.…”