Asteroids - Extreme Events: A Physical Reconstruction and Risk Assessment - page 241

effects produced on it from the impact and nature of the bolide. Together with
Ir anomalies, spinels with high Ni content are often the only preserved evidence
of distal impact deposits.
Robin et al. (1992)haveclassified spinel occurrences into three groups accord-
ing to their chemical compositions.

(1) Amicrometeorite spinel formed by oxidation of cosmic dust particles

in the upper part of the atmosphere, at relatively low oxygen fugacities.
It is mostly magnetite characterised by an Fe 3 + /Fe total ratio up to 75--80
atom%.

(2) Ameteorite spinel formed by ablation of meteorites in the lower part

of the atmosphere, at higher oxygen fugacities. It is a solid solution
of magnetite and magnesioferrite characterised by an Fe 3 + /Fe total ratio
between 75 and 90 atom%.

(3) Ameteorite spinel formed by interaction of impact debris with the

highly compressed atmosphere. It is essentially magnesioferrite char-
acterised by an iron oxidation state (Fe + 3 /Fe total

> 90 atom%).

The Barberton Greenstone Belt of Southern Africa contains four impact layers
(which is disputed by Koeberl et al. , 1993)and two of these layers contain Ni spinel
and Ir anomalies (Byerly and Lowe, 1994). Of the other layers, one exhibits only
Ir anomalies and the other none. However, all layers are characterised by the
presence of spherules. Despite the debate concerning the extraterrestrial origin
of these beds in South Africa, the presence of Ni-rich oxidised spinel elsewhere
around the globe as a signature of bolide impacts is not disputed.
One of the most widely observed spinel-bearing geological units (often mul-
tiple microtektite--microkrystite layers) occurs at the boundary between the
Eocene and Oligocene time epochs around 35 Ma. Pierrard et al. (1998)exam-
ined the global type section for the Eocene--Oligocene boundary in Massignano,
Italy where they aimed to determine whether Ni-rich spinel is present. Pier-
rard et al. (1998), using the Robin et al. (1992)chemical classification, established
that the spinel from the Massignano site belongs to the first described class
as it consists mainly of magnetite and has an Fe 3 + /Fe total ratio between 70 and
80 atom%. It is possible, therefore, that the spinel layer found in the upper
Eocene could have resulted from a comet shower, although Pierrard et al. (1998)
suggest that this is not likely as the magnitude and high accretion rate of spinel
in this horizon indicate a sudden event. A comet shower is likely to span over
aperiod of a few million years. It is more likely that the spinel layer origi-
nated from a large cometary impact. Because dust particles are imperative in
the formation of the micrometeorite spinels in group 1, Pierrard et al. (1998)
lean toward a comet impact rather than an asteroid as comets consist mainly