Computer Science – Sound
Scientific paper
Apr 2004
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004psrd.repte..82n&link_type=abstract
Planetary Science Research Discoveries
Computer Science
Sound
Lunar Crust, Lunar Magma Ocean, Anorthosite, Moon, Geochemistry, Petrology, Origin Of The Moon
Scientific paper
Anorthosites, rocks composed almost entirely of plagioclase feldspar, are the oldest rocks on the Moon. They appear to have formed when feldspar crystallized and floated to the top of a global magma ocean that surrounded the Moon soon after it formed. Not all ages determined for anorthosites, however, are as old as we expected--one appeared to be only 4.29 billion years old. While 4.29 billion years sounds very ancient, a magma ocean ought to have solidified well within 100 million years of lunar origin about 4.55 billion years ago. One possibility is that the young ages reflect impact events, not the original time of igneous crystallization. My colleagues Lars Borg (University of New Mexico) and Larry Nyquist and Don Bogard (Johnson Space Center) and I studied an anorthosite (rock 67215) relatively rich in pyroxene, allowing us to determine a precise crystallization age of 4.40 billion years. But even that age might have been affected by the subsequent shock heating event that reset the low-temperature components in this rock about 500 million years after it formed.
By examining data for all of the previously dated lunar anorthosites, we were able to show that plagioclase feldspar is more prone to shock damage than are the pyroxenes in these rocks, so we plotted only the pyroxene data for four different anorthosites on a samarium-neodymium isochron diagram. These data fall on a well-defined line indicating a crystallization age for the anorthosites of 4.46 billion years, consistent with very early, widespread melting of the Moon. Other data for 67215 show that it comes from a relatively shallow depth in the crust, giving us clues to the structure of the lunar crust. Studies like this one are filling in the picture of how the initial crust of the Moon formed, which in turn sheds light on the formation of the terrestrial planets.
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