Origin of KREEP
Most KREEPy samples are polymict breccias. Most model of KREEP petrogenesis base on partial melting of a variety of sources fail to explain the veritable absence of variations in incompatible element patterns over the sampled potion of the moon. e have defined a KREEP component based on the average composition of Apollo 14 breccias having extremely high concentrations of incompatible elements. Normalization of accurate incompatible data for KREEPy samples from Apollo sites to this component virtually always shows no resolvable fractionation (e.g., <10% variation in La/Lu ratio), whereas partial melting models typically produce larger fractionations ( +- 20-25% in La/Lu) from a factor of 2 difference in degree of partial melting. The anorthositic crust of the moon is commonly attributed to the flotation of plagioclase on a deep, moon-wide magma ocean. Fractional crystallization of this magma ocean would have produced large enrichments of incompatibles in a residual liquid. No other plausible major source of incompatibles has been proposed. We designate this residual liquid 'urKREEP. We propose that all KREEPy rocks originated by dilution of urKREEP with crustal or mantle materials during assimilation, or zone-refining (pristine samples), or impact-induced brecciation (breccias and melt rocks). The formation of urKREEP cannot be dated precisely. Correction of breccia Rb-Sr model ages for Rb loss or gain during the early intense bombardments yields ages that cluster in the range 4.4--4.5 Gy. This implies that crystallization of the magma ocean was essentially complete at this time. Assuming that the moon had the composition of an H-group chondrite depleted in Fe-Ni and FeS and that half the incompatibles fractionated into materials other than urKREEP, the thickness of a moon-wide urKREEP layer was <2 km. Th concentrations determined by gamma ray spectroscopy indicate that about 4% of the incompatibles in an H chondritic moon are now in the outermost kilometer.
- Research Organization:
- Institute of Geophysics and Planetary Physics and the Department of Earth and Space Sciences, University of California, Los Angeles, California 90024
- OSTI ID:
- 6463755
- Journal Information:
- Rev. Geophys. Space Phys.; (United States), Vol. 17:1
- Country of Publication:
- United States
- Language:
- English
Similar Records
Chronology of alkali anorthosite 14304 clast “b” records basin forming impact at ~3.95 Ga
Ion and electron microprobe study of troctolites, norite, and anorthosites from Apollo 14: Evidence for urKREEP assimilation during petrogenesis of Apollo 14 Mg-suite rocks