Department of Civil & Environmental Engineering &Earth Sciences
Environmental Engineering & Earth Sciences Lecture Series
FRIDAY, April25th, 2014
4:00 – 5:00 pm, 356 FITZPATRICK HALL
PROF. CHARLES SHEARER
Dept. Earth & Planetary Sciences
The University of New Mexico
“Formation and Evolution of Early Planetary Crusts”
An integrated petrology, geochemistry, chronology, and remote sensing perspective for deciphering the origin of the lunar highlands
ABSTRACT:
The lunar highlands crust is dominated by numerous “pristine” magmatic lithologies. These “pristine” igneous rocks include the ferroananorthosites (FANs), alkali-rich rocks, and Mg-rich rocks. The FANs have been interpreted as representing flotation cumulates produced during primordial differentiation (Lunar Magma Ocean, LMO) of the Moon. The ancient, “post-LMO” Mg-suite rocks are plutonic to hypabyssal in origin with a range of rock types including ultramafics, troctolites, spinel troctolites, anorthositictroctolites, norites, and gabbronorites. The origin and chronology of these major crustal lithologies have long been a point of controversy. Mg-suite rocks in the Apollo collection have the paradoxical chemical characteristics of very Mg-rich mafic silicates, which indicate a primitive parental magma and highly elevated abundances of incompatible trace elements, indicating an evolved parental magma. It is generally assumed that the Mg-suite was emplaced into the primordial FAN crust and that they formed layered intrusions. However, incontrovertible evidence, such as observed layering or clear genetic intrusive relationships among samples of different petrologic types, for this scenario does not exist. Based on the sample suite from the Apollo landing sites, the distribution of FANs and Mg-suite rocks implies a lateral and vertical crustal association rather than an intrusive relationship. The nearly contemporaneous crystallization ages of FANs and Mg-suite rocks also obscures the relationship between these crustal suites. Interpreting the origin for the Mg-suite using an integrated approach (petrology, geochemistry, chronology, and remote sensing) has produced a number of profound implications for its petrogenesis, relationships to other suites of crustal lithologies, links to the lunar differentiation and thermal history, and the origin of the Moon.