Other
Scientific paper
Jan 1997
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997lpico.922...62w&link_type=abstract
Large Meteorite Impacts and Planetary Evolution, p. 62
Other
Hypervelocity Impact, Impact Melts, Meteorite Collisions, Lunar Craters, Selenology, Igneous Rocks, Moon
Scientific paper
The Moon is a body where meteoritic impacts have played an even more important evolutionary role than on Earth. A key issue is the role of impact as a cause of melting on a large enough scale to engender igneous differentiation. What fraction of Apollo rock samples generally viewed as "pristine" products of early endogenous magmatism are actually products of impact melt? Cold clastic debris has a strong chilling effect on impact melt. Consequently, the melting displacement ratio m/d is probably the paramount influence on the potential for an impact-generated magma to undergo internal igneous differentiation (fractional crystallization). For calibration of the relationship between m/d and the potential for impact magma igneous differentiation (MIID), we must look to large terrestrial structures with well-exposed melt sheets. IMID was evidently vastly greater at Sudbury (m/d - 0.46) than at the two next largest such structures, Popigai and Manicouagan (both m/d - 0.24). The Sudbury IMID may have been enhanced by endogenous melting potential (deep-crustal heat) in the Penokean Sudbury region. Considering that at Manicouagan and Popigai the melt sheets did not differentiate even slightly, it seems likely that a robust IMID requires m/d > 0.3. On the Moon m/d is much smaller for a given crater size than on Earth. Of the known (young enough to be seen) large basins, only a few formed with m/d > 0.3, m/d calculated a la Melosh. Any differentiated lunar melt sheet is probably capped by a clastic debris-rich, heterogeneous but nondifferentiated Onaping analog. Igneous differentiates (cumulates) making their way into an Apollo rock collection would require a later large crater to excavate from several km in depth, with sufficient lateral transport (anticorrelated with depth of excavation in any single event) to bring the rock into the small Apollo sampling region. Except by starting from Procellarum (or some other basin not currently visible), this series of events appears unlikely. A strong objection to the hypothesis that a single basin (Procellarum) is the source of the apparent pristine rocks is the correlation among these samples between trace-element compositions and Apollo sampling locations. The distinction between impact melt and endogenous, mantlederived melt begins to blur when extremely large events are considered. In big lunar events, impact melt that avoids immediate dissemination (ejection) is of mainly mantle derivation. For Sudbury, the same approach predicts crust/ (mantle + crust) is not <100% and could well be 1. On the early Moon, big impacts probably influenced the times and places where magmas intruded the crust, but except for basins too young to have yielded samples, the heat for the magmatism was probably mainly endogenous. (Additional information is contained in the original)
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