Asteroids - Extreme Events: A Physical Reconstruction and Risk Assessment - page 240

Precambrian spherule deposits are found in thicker accumulations and are more
crystallised. These spherules appear to have formed from basaltic melts rather
than from continental target rocks as is the case with spherule layers from the
Phanerozoic. No glass or original minerals have been found in early Precam-
brian layers. Simonson and Harnik (2000)suggest that while these differences
could indicate that there have been secular changes to variations in the nature
of impact ejecta, changes in the nature of Earth's hydrosphere and lithosphere
might be an equally important reason. As most of the mass in distal melt and
condensate particles from impacts comes from the target crust, it is likely that
changes through time in the average composition of spherules probably reflect
changes in the average composition of crustal rocks. It is likely that the volume
of continental crust was significantly smaller in Archean times and the ocean
crust proportionately larger. This would have resulted in a greater percentage of
mafic rocks in the target material and thus likely to be recorded in the distal
impact ejecta. More open-ocean impacts could also skew the nature of distal
ejecta in stratigraphic layers towards thicker accumulations. Another important
fact to consider is that only the largest impacts, where the diameter of the
projectile is approximately the same as the water depth, excavate and disperse
target rocks from the sea floor; the water mass shields the target area in the
case of smaller impacts. A predominance of mainly mafic target rocks could also
explain the lack of shocked quartz in early Precambrian spherule layers.

Spinel

Spinel is a mineral frequently found in igneous rocks. However, when
it is high in nickel (Ni) and is oxidised, it is more likely to have formed during
hypervelocity meteoritic interactions with the Earth's atmosphere. Meteorites
have relatively high Ni content along with a high ferric/ferrous ratio result-
ing from crystallisation in the O 2 -rich environment of the Earth's atmosphere.
Pierrard et al. (1998)notethat spinel is found in meteorite fusion crusts, cos-
mic spherules from deep sea sediments, polar ice and a variety of meteoritic
debris in the sedimentary record. Oxidised Ni-rich spinel does not form by vol-
canic processes as terrestrial magmas are depleted in nickel and evolve under
extremely low oxygen fugacity environments (Robin et al. , 1992). Spinel is found
at a number of distinct time periods in the geological record such as the K--T
boundary, the lower to middle Jurassic in the southern Alps of Italy, late Eocene
sediments in Italy, late Oligocene sediments ( ∼ 30 Ma) from the central north
Pacific and in late Pliocene ( 2 Ma) sediments in Antarctica. The presence of

spinel and the size of spherules can also potentially provide constraints on the
size of the impact, the nature of the early Archean atmosphere and transient

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