Physics
Scientific paper
Mar 1996
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996lpi....27..975o&link_type=abstract
Lunar and Planetary Science, volume 27, page 975
Physics
Crust: Lunar, Granites, Quartz Monzodiorites
Scientific paper
Compositionally evolved lunar rocks (quartz monzodiorites and granites) are represented by rare small clasts in Apollo 15 impact melt breccias and Apollo 14 soils. The great majority of these clasts have incompatible trace element and rare earth element abundances remarkably similar to those of KREEP. Most lines of evidence point to these rock types as having been derived from a KREEP basalt magma by extreme fractional crystallization accompanied by late-stage liquid immiscibility. However, some quartz monzodiorite clasts have textures, compositions and mineral assemblages reminiscent of cumulates. In an attempt to elucidate some of the physical conditions under which QMDs formed, we have examined the fine structure of their pyroxenes. Pyroxenes in QMDs are ferropigeonites and/or ferroaugites and almost all have well-developed, albeit fine-scale, exsolution features formed during cooling. Using a method similar to that previously described, we have attempted to extract cooling rates from a simulation of lamellar growth in these pyroxenes and from these cooling rates to compute a depth of burial at the time of crystallization. The fine-scale of the lamellae-host intergrowths necessitated the development of a modified analytical technique since conventional microprobe step-scanning with step intervals of 1 micron or less proved totally inadequate to determine the details of the compositional variation across these features since, in many cases, the lamellae were 1 micron or less in width. In such cases, the diameter of the x-ray excitation volume was 2 to 3 times larger than the lamellae being analyzed requiring the use of a complex method of deconvolution.
McCallum Stewart I.
O'Brien Hugh E.
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