Mathematics – Logic
Scientific paper
Jan 1999
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999nvm..conf...68w&link_type=abstract
Workshop on New Views of the Moon 2: Understanding the Moon Through the Integration of Diverse Datasets, p. 68
Mathematics
Logic
Lunar Mantle, Lunar Rocks, Lunar Core, Lunar Evolution, Lunar Composition, Selenology, Core-Mantle Boundary, Earth Core, Earth Mantle, Lunar Prospector, Geochemistry
Scientific paper
The most highly siderophile elements (HSEs), i.e., the most sensitive indicators of planetary metal-silicate differentiation, are the heavier platinum-group elements: Os, Ir, and Pt, and their periodic-table neighbors Re and Au. The HSE that has been determined most often and most successfully in lunar rocks is Ir. Even so, previous estimates of the concentration of Ir in the lunar mantle indicate great uncertainty, as they range from 0.01 to 4 ng/g, or 0.00002 to 0.009 x CI chondrites. This uncertainty stems from several factors, including the "nugget effect" and other analytical difficulties. The lunar HSE database is scattered amidst many papers, and in most cases analyses for Ir (and other HSE) did not simultaneously determine compatible-lithophile elements. Previous interpretations of these data in terms of mantle concentrations have involved little more than comparison between mean concentrations in lunar vs. terrestrial basalts. Such interpretations are inadequate, because basalts; are diverse. Even the most extremely siderophile elements exhibit compatible behavior under metal-absent conditions. I have compiled a database that is a comprehensive blend of siderophile data with compatible-lithophile data for identical basaltic samples. Over 50% of the HSE data are from the compilation of Wolf et al.; many of the other data are from past publications from my UCLA lab. This database manifests correlations between siderophile elements and several compatible elements that imply relatively tight constraints on the lunar mantle siderophile composition. The notion that FeNi metal may have been a residual mantle phase that determined the HSE compositions of lunar basalts is precluded by a correlation between W (which would be markedly siderophile in the presence of metal) and U. Conceivably residual sulfides controlled the HSE. But textural-mineralogical observations preclude S saturation in the low-Ti basalts, and experimental data indicate that even the most S-rich high-Ti types erupted about 50% undersaturated in S. Nickel, although not quite a HSE like Ir, is a strongly siderophile element with a very extensive database. A plot of MgO vs. Ni manifests clearly significant, albeit nonlinear, correlation trends. The lunar trend parallels that for Earth basalts and komatiites (and a similar trend for martian rocks), but is displaced to lower Ni. The Mg0 contents of the two mantles differ slightly, but mostly this offset reflects an about 5x lower (Ni) in the lunar mantle vs. its terrestrial counterpart. The terrestrial trend passes through the MgO content of Earth's mantle at Ni approximately equals 2200 micro g/g. The implied Ni content of the lunar mantle is about 400 micro g/g. The hypothesis that (Ni) is the same in the lunar and terrestrial mantles is clearly untenable. Ni shows similarly strong correlations vs. Cr and V (except among terrestrial rocks, where V is generally incompatible with olivine). A plot of MgO vs. Ir shows greater scatter, but the same basic relationships. Again, the lunar data form a trend that parallels the Earth trend, but at a displacement indicating about 6x lower Ir) in the lunar mantle vs its terrestrial counterpart. This same inference extends at least to Os, which shows an excellent correlation vs. Ir, at chondritic Os/Ir, among lunar basalts. The lunar mantle ReAr (and Re/Os) is also at least nearly chondritic; Au/Ir, however, gives some indication of being as much as 3x greater than chondritic. The inferred composition implies that Ni/Ir is enhanced in the lunar mantle by nearly the same factor, about 30x Cl chondrites, as in Earth's mantle. In a sense, it is remarkable that the lunar mantle siderophile depletions are not much greater. In the case of the Earth and Mars, the hypothesis that siderophiles were added as a veneer after differentiation of the core remains controversial. In the case of Earth, high-pressure dampening of metal/silicate partitioning behavior may have played a more important role. Recent Lunar Prospector results indicate that the Moon possesses a core amounting to 1-2 wt% of its bulk mass. The depletion factor delta implied by an equilibrium between the silicate portion of the planet and its core can be calculated from the simple mass balance Delta = 1/(Dc+[1-c]) where c is the weight fraction of the core and D is the core/silicate (metal/silicate?) distribution coefficient. With a low-pressure metal/ silicate D of the order 5 x 105 and perhaps much higher, Ir should have been depleted in the lunar mantle to about 0.0002x the bulk-Moon concentration (which in any event is presumably not chondritic for siderophile elements, given the Moon's gross depletion in Fe metal). For Ni, the metal/silicate D is roughly 25x lower, so core-mantle equilibration for Ni should have resulted in Delta= 0.005 and Ni/Ir = 25x the bulk-Moon ratio (and the implication regarding Ni/Ir is approximately the same for all finite values of c). By many models, the Moon formed largely from, and thus inherited its bulk composition largely from, Earth's mantle. If a core subsequently formed within the Moon, the Moon-mantle Ir/Ni should have been depleted by an additional factor of 25 vs. the Earth-mantle value. Thus, it is remarkable that Ir/Ni is so nearly identical in the two mantles. A gross disequilibrium between the mantle and core, i.e., accretion of a "late veneer," is most unambiguously implied for the Moon, even though the proportion of the veneer (0.1 wt% of Cl-like material, based on Ir) is much less for the Moon's mantle than often suggested (about 0.7 wt%) for the Earth's mantle. Additional information contained in original.
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