Petrogenesis of Magnesian-Suite Troctolites and Norites

Details Petrogenesis of Magnesian-Suite Troctolites and Norites Petrogenesis of Magnesian-Suite Troctolites and Norites

Jan 1999

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999nvm..conf...26h&link_type=abstract

Workshop on New Views of the Moon 2: Understanding the Moon Through the Integration of Diverse Datasets, p. 26

Computer Science

Geochemistry, Kreep, Lunar Crust, Magma, Mineralogy, Petrogenesis, Pyroxenes, Lunar Geology, Petrography, Lunar Maria, Radiogenic Materials, Ejecta, Anorthosite, Chondrites

Scientific paper

Models for the petrogenesis of magnesian-suite troctolites and norites are severely constrained by a number of important geochemical features. I. Most troctolites have olivines with Mg* values that range from 84 to 92. Rare spinel troctolites have extremely primitive olivines with Mg* 95-96. Hess showed that a peridotite source with Mg*>91 was required to produce the most magnesian troctolite parent liquids known at that time. But these parent liquids were strongly undersaturated with respect to plagioclase at the low pressures expected of the lunar anorthosite crust. Such liquids must crystallize significant quantities of olivine in order to bring plagioclase onto the liquidus; the fractionation of olivine must reduce the Mg* value of the plagioclase-saturated liquids. It follows that troctolites with Mg* of 90-92 cannot be produced in this way. The problem is far worse for the spinel troctolite with Mg*96. II. The crystallization sequence of magnesian suite magmas is non-tholeiitic; i.e., the crystallization of low-CaO pyroxene precedes high-CaO pyroxene. Indeed, troctolites and norites are nearly free of discrete CaO-rich pyroxene, a feature implying a melt and a source with low normative diopside, certainly much lower than in typical terrestrial lherzolite. III. The parent magmas for magnesian norites and troctolite have calculated REE contents typically between 1 and 2x high-K KREEP or about 300-600x chondrite! IV. The parent magmas to the most primitive troctolite are hot and have temperatures that exceed 12700C at 1 bar. V. Radiogenic isotope ages for magnesian norites imply that magma generation was long lived, contemporary with the formation of ferroan anorthosite and extending to (and possibly beyond) 4.2 AE. Three models of petrogenesis are tested against the petrochemical features listed above. These models include (1) impact melting, (2) cumulate remelting, and (3) assimilation of cumulates by late-stage liquids of the magma ocean. A. In this model, the parent magmas to the magnesian-suite troctolite are impact melts formed by melting the ferroan anorthosite crust and the early-most cumulates of the magma ocean. The mafic cumulates exist below the crust through the agency of cumulate overturn as the original cumulate pile establishes gravitational equilibrium. A magma ocean with Mg* 84-88 is required to produce olivines with Mg* 92-95. Such high Mg* values require that the magma ocean experienced fractional rather than equilibrium crystallization even at the earliest periods of crystallization. The high and variable REE content is explained by melting of variable amounts of the KREEP layer that survived the overturn. The mafic cumulate cannot be solely of olivine, however, since the magnesian norites require an orthopyroxene-bearing source. An orthopyroxene-bearing dunite or even an olivine-rich harzburgite is required. This would imply that the magma ocean was less olivine normative than typical terrestrial mantle. The impact model nicely reconciles constraints I to IV, whereas V simply requires multiple impacts. Indeed, since magnesian-suite rocks were collected from Apollo 11, 12, 14, 15, 16, and 17, impact melting in this model was a common and widespread event. If this model is correct, why are the ferroan anorthosite and magnesian suite plutonic trends so distinct? Why isn't there a continuum of compositions? It is also noteworthy that upper mantle samples are rare to nonexistent in the lunar collection. In fact, neither ferroan anorthosites or cumulate mantle samples are typically found within the ejecta deposits that include magnesian-suite samples. B. In the cumulate remelting model, the high Mg* values of the magnesian troctolite are contributed also by a source dominated by early olivine cumulates of the magma ocean. The normative plagioclase and orthopyroxene components are supplied by sinking parcels of quenched crust of the convecting magma ocean or perhaps by interaction with partial melts generated by melting the primitive interior of the Moon, if indeed such an interior exists. In either case, the mafic cumulates of olivine, perhaps with small amounts of orthopyroxene (see earlier discussion), must dominate the source. Pressure-release melting associated with the cumulate overturn creates the parent magmas for the magnesian suite. Sufficient melting is required to leave only orthopyroxene and olivine in the source to maintain low normative diopside contents. This model satisfies constraints I, II, and IV but has difficulty with III, the high KREEP contents of the Mg-suite parent liquids. Possibly the high KREEP contents are the result of assimilation of high level KREEP deposits. However, if KREEP is akin to ferroan basalt then the assimilation of this mass would lower Mg* values. If KREEP is quartz normative and granitic, assimilation would compromise the olivine-normative character of the parent liquids; KREEP-rich troctolite parent liquids would evolve to norite parent liquids. This latter viewpoint is probably the more acceptable one. If this model is correct, then convective overturn and pressure release melting must not only begin promptly to create old Magnesian-suite rocks but must also continue for 200-300 more m.y. to account for the youngest members of the Magnesian-suite. The rapid formation of the cumulate pile is consistent with the 182W-isotopic anomalies that require that heterogeneous and discrete source regions must be established in the first 50 m.y. of the origin of the Moon. Perhaps thermal rather then chemically driven convection is needed to explain the origin of young Magnesian norites. Additional information is contained in original document.

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