Other
Scientific paper
Jul 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27r.295t&link_type=abstract
Meteoritics, vol. 27, no. 3, volume 27, page 295
Other
Scientific paper
Following the general acceptance of the magma ocean hypothesis, models for the evolution of the highland crust of the Moon have become increasingly complicated, just as religious and philosophical systems have always diverged from the teachings of their founder. Three components make up the highland crust: the ferroan anorthosite, which crystallizes early from the magma ocean, depletes the deep interior in Eu, and adds a large Eu enrichment to the crust. KREEP, choked with incompatible trace elements from the residual 2% melt resulting from the crystallization of the magma ocean is pervasively mixed into the crust by cratering. KREEP adds a deep Eu depletion, with high abundances of the other REE parallel to those of the ferroan anorthosites. The third well-recognized component is the Mg Suite, commonly about 100-200 Ma younger, with intermediate REE patterns parallel to the ferroan anorthosites and KREEP (Fig. 1). If the highland crust were formed from many igneous events, in which the Mg suite comes from several separate plutons, crystallization and separation of mineral phases would surely result in REE patterns with diverse slopes, as is observed on Earth. This does not seem to have occurred. For example, the deep-seated troctolite 76535 has a well-established age of 4236 +- 15 Ma (Premo and Tatsumoto, 1992), much younger than the 4440 +- 20 Ma crystallization age of the lunar crust (Carlson and Lugmair, 1988), and the 4400-Ma closure ages for the source regions of the lunar mare basalts. If 76535 formed as a separate intrusion by partial melting during "serial magmatism" 200 Ma after the ferroan anorthosites crystallized, why is its REE pattern parallel to those of all the other highland rocks (Fig. 1)? Two explanations seem viable. The first possibility is that a diverse crust may have been homogenized by cratering. Alternatively, only one major igneous event produced the lunar highland crust. All subsequent complexity in ages and production of "igneous" rocks is due to mixing and melting during massive impact events. Formation of the Mg Suite, "pseudo-igneous" rocks (e.g., 14310), and perhaps "pseudo-pristine" rocks may be expected. However, these events did not change the slope of the REE patterns. For example, the patterns of the impact-melts (Fig. 1) are strictly parallel to those of the other highland rocks. On Earth, the Sudbury impact event produced 10,000 km^3 of melt from the continental crust (Grieve et al., 1992). Lunar basin-forming events were orders of magnitude larger, so that much remelting of the lunar highland crust must have occurred. This later remelting may have produced the observed petrographic and textural complexities, but did not change the REE patterns. Carlson, R. W. and Lugmair, G. W. (1988) Earth Planet. Sci. Lett., 90, 119. Grieve, R.A.F., Stoffler, D. and Deutsch, A. (1992) J. Geophys. Res. 96, 22753. Koeberl, C., Kurat, G. and Brandstatter, F. (1991) Geochim. Cosmochim. Acta 55, 3073. Premo, W. R. and Tatsumoto, M. (1992) Proc. Lunar Planet. Sci. 22, 381 Taylor, S.R. (1982) Planetary Science: A Lunar Perspective; LPI Houston, Table 5.3. Figure 1, which in the hard copy appears here, shows that lunar highland crustal rocks of diverse ages, petrography, and depths show parallel REE patterns (except for enrichment and depletion of Eu; data from Taylor (1982) and Koeberl et al. (1991)) implying a common petrogenesis or very efficient mixing.
Koeberl Christian
Taylor Stephen R.
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