Alan B. Binder

Post-imbrian global lunar tectonism: Evidence for an initially totally molten Moon

Evaluation of all reasonable sources of stress in the lunar crust indicates that compressional thermoelastic stresses are the only ones which have been tectonically significant on the global scale during the last 3.5×109 yr of lunar history — i.e., the post-Imbrian. However, the thermoelastic stresses calculated for lunar models which have accretional heating profiles at the beginning of lunar history; i.e., a molten zone only a few hundred kilometers deep and a cool deep interior, are less than 1 kbar in the crust. Such stresses are lower than the more than 1 to 7 kbar needed to initiate thrust faulting in the outer crust according to Andersons theory of thrust faulting. Thus such accretional models predict that no significant global thrust faulting has occurred during the post-Imbrian and that the crust should currently be seismically quiet on the global scale.In contrast, the compressional thermoelastic stresses generated in a Moon which was initially totally molten, as is the case if the Moon formed by fission, are up to 3.5 kbar in the outer few km of the crust at present. These stresses are well within the range needed to cause thrust faulting in the outer 4 km of the crust. According to this model there should be modest scale (10 km), young (≤ 0.5 to 1×109 yr old) thrust fault scarps in the highlands.Photoselenological investigations confirm that scarps with the expected age and geometric characteristics are found in the highlands. Thus the currently available photoselenological data support the stress model derived for an initially totally molten Moon, but not one which was molten only in the outer few hundreds of km.

On the origin of the Moon by rotational fission

Based on simple CIPW norms for the proposed terrestrial upper mantle material, it is shown that if the Moon fissioned from the Earth and gravitationally differentiated, it could have a 72 km thick anorthosite (An97) crust, a calcium poor (3.8% by weight) pyroxenite upper mantle 100 Mg/Mg + Fe = 75 to 80) ending at a depth of 313 km and a dunite (Fo93_95) lower mantle below a depth of 313 km. Refinements of these simple norm models, based on the cooling history, crystallization sequence and the variations of the 100 Mg/Mg + Fe ratio of the liquid and crystals during the crystallization sequence, indicate that the final form of such a Moon could have the following properties: (1) a primitive, cumulate anorthosite - minor troctolite crust with intrusive and extrusive feldspathic basalts and KREEP rich norites; the thickness of this crust would be 75 km; (2) a zone in the bottom of the crust and the top of the upper mantle which is rich in KREEP, the incompatible elements, silica, and possibly voltiles; this zone would be the source area for the upland feldspathic basalts, KREEP rich norites and KREEP and silica rich fluids; (3) an upper mantle between the depths of 75 km and 350 to 400 km which consists of peridotite containing 80–85% pyroxene (Wo10En68_72Fs18_22) and 15–20% olivine (Fo75_80); the Al2O3 content of the upper mantle is ∼ 3%; the peridotite layer would be the source area for mare basalts and; (4) a lower mantle below a depth of 350–400 km which consists of dunite (Fo93_97).The cooling history of such a moon indicates that the primitive anorthosite crust would have been completely formed within 108 yr after fission. The extrusion and intrusion of upland basalts and KREEP rich norites and the metamorphism of the crustal rocks via KREEP and silica rich fluids would have ended about 4 × 109 yr ago when cooling well below the solidus reached a depth of 150 km. As cooling continied, the only source of magmas after 4 × 109 yr ago would have been the peridotite upper mantle, i.e. the source area of the mare basalts. Extrusion of mare basalts ended when cooling below the solidus reached the top of the refractory dunite lower mantle 3-3.3 × 109 yr ago.Thus, it is shown that the chemistry, primary lithology, structure and developmental history of a fissioned Moon readily match those known for the real Moon. As such, the models presented in this paper strongly support the fission origin of the Moon.

On the thermal history of a moon of fission origin

Model calculations show that the thermal history of a Moon which originated by fission from the proto-Earth is the same as that for the Moon as it is currently understood. In particular, a fissioned Moon currently has a small percent of partial melt or at least near solidus temperatures below depths of 800 km in accord with the seismic data which show that the deep interior of the Moon has a very lowQ. The models have moderate (20–50%) degrees of partial melting in the upper mantle (depths < 300 or 200 km) in the period between 3 to 4 × 109 years ago and, therefore, can account for the mare filling epoch. Finally the heat flow of the models is 18 ergs cm−2 s−1 which is close to the average of 19 ergs cm−2 s−1 derived from the Apollo heat flow experiments. These findings add further support for the fission origin of the Moon.

On the compositions and characteristics of the mare basalt magmas and their source regions

Based on a synthesis of available mare basalt data, it is shown that the samples which were returned to Earth via the various Apollo and Luna missions were derived from at least 16 separate eruptive events. The currently published data are sufficient to allow reasonably good estimates of the compositions of the parental magmas of 12 of these units to be made. At the present, only first order estimates of the compositions of the magmas of the remaining four units are possible.It is further shown that, when these 16 magmas are plotted on the pseudo-ternary phase diagram for the system anorthite-olivine-quartz and the quaternary phase diagrams for the systems which include augite or ilmenite, the magmas all lie along a common, equilibrium melting path. This path is defined by the high aluminum basalt magmas and the majority of the high TiO2 basalt magmas which plot near the 5kb olivine-pyroxene cotectic and by the high olivine magmas which plot along or near a single olivine control line. The fact that all the high olivine magmas plot near a single olivine control line is a direct consequence of the equilibrium partial melting of an olivine dominated mantle, but is statistically very unlikely (1 chance in 106) if the mantle is dominated by pyroxene as is widely accepted. Based on the reasonable assumption that the degree of partial melting which produced the magmas was no greater than 50%, and noting that the composition of the mantle is constrained to lie on the olivine control line around which the high olivine magmas plot in the ternary and quaternary phase diagrams, then the normative composition of the lunar upper mantle must be about 64% olivine (Fo70), 23% pyroxene, 9% anorthite, and 4% ilmenite - though olivine richer models are possible. This composition is essentially the same as that for pyrolite, the proposed composition of the Earths mantle. This observation is taken to add further support for the fission origin of the Moon.

On the implications of an olivine dominated upper mantle on the development of a moon of fission origin

In a series of previous papers, a petrological model for the Moon has been developed based on the assumption that the Moon is a globe of differentiated terrestrial mantle material which fissioned from the Earth. One of the major constraints which this model matches is the hypothesis that the lunar upper mantle is dominated by pyroxene. However, it has been recently shown that olivine is most probably the major constituent of the lunar upper mantle and that, at least that part of the Moon has a composition which is very similar to that of pyrolite - the proposed composition of the Earths mantle. As a result of this new model constraint, the previously proposed differentiation scheme for a Moon of fission origin is reviewed and found to be inadequate, despite modification, for explaining the near pyrolite composition of the lunar upper mantle. As a result, a solidification sequence, which has been proposed to explain the rhythmic banding in terrestrial ultra mafic complexes, is investigated and found to be able to account for the high olivine content of the upper mantle, assuming a pyrolite composition for the Moon.

On the heat flow of a gravitationally-differentiated moon of fission origin

It is shown that the mean value for the heat flow of a gravitationally-differentiated Moon of fission origin is about 13 erg cm−2 s−1 and that the heat flow varies regionally from about 3 erg cm−2s−1 to more than 45 erg cm−2s−1. These regional variations in the heat flow are caused by a non-uniform distribution of K, U and Th in the KREEP zone at the crust-upper mantle boundary and the redistribution of crustal materials and K, U and Th rich KREEP materials by basin-forming impacts. The scale of these regional variations is hundreds of km. The models presented are in accord with the Apollo 15 and 17 heat flow measurements.

On the petrology and structure of a gravitionally differentiated moon of fission origin

In a previous paper, it was shown that the basic properties and the developmental history of a gravitationally differentiated Moon of fission origin match those known for the Moon. In the first part of this report, the models of a differentiated Moon are critically reviewed based on second order considerations of some of the chemical systems used to develope the earlier models and based on new lunar data. As a result, slightly updated models are developed and the results indicate that a Moon of fission origin has a feldspar rich crust (≈70% Or0.8Ab5.3An93.9 with ≈30% pyroxene and olivine) reaching an average depth of ≈65 km. A KREEP rich layer is located at the interface of the crust and the upper mantle. The upper mantle consists of peridotite (≈80% Wo10En70Fs20 and ≈20% Fo75–80 with ≈3% Al2O3 and ≈ 2% TiO2) and reaches a depth of 300–400 km. Below 300–400 km lies a dunite (≈Fo95) lower mantle.A simple model for the distribution of K, U and Th (and by inference, KREEP) in the differentiated Moon model is developed using a distribution coefficient of 0.1 for the three elements. This coefficient is derived from published data on the distribution of U in Apollo 11 basalts. The simple model successfully accounts for the observed K, U and Th contents of the various mare basalts and upland rocks and yields a heat flow of 21 erg cm−2s−1 for the Moon.A model for the fine structure of the peridotite upper mantle of the model Moon is developed based on the TiO2 and trace element variations observed in the various mare basalts. It is proposed that the upper mantle is rhythmically banded on the scale of 10s of km and that this banding leads to local variations of a factor of ±3 in the K, U and Th content,-10+5 in the TiO2 content and-∞+2 in the olivine content of the peridotite. It is also proposed that this banding leads to large scale horizontal inhomogenuities in the composition of the upper mantle.It is also shown that the formation of the primitive suite of upland rocks is easily explained by the cumulation of plagioclase, which carried varying amounts of pyroxene, olivine and melt with it, during the peritectic crystallization of the last 20% of the differentiating Moon. It is found that the 100 Mg/(Mg+Fe) ratios of the mafics and the An contents of the plagioclases of the rocks are controlled by several factors, the most important of which is the ratio of melt to crystals which together formed the various upland rocks. The inverse relationship between the An contents and the Mg contents of the upland rocks is a direct consequence of the differentiation sequence proposed.The results and models presented in this paper further support the hypothesis that the Moon formed as a result of fission from the proto-Earth.

Linear crater chains: Indication of a volcanic organ

A mathematical investigation of the alignment of the craters of the four best preserved lunar linear crater chains, which lack the characteristics expected if they were of secondary impact origin, shows that, first, the craters lie along the distinct lines with very small deviations. This suggests that the craters were formed along deep crustal fissures. Second, the strikes of the lines or fissures indicate that they are reactivated lunar grid system structures. Third, the morphology of the craters is similar to that of volcanic diatremes. These results, especially the excellent geometric alignment of the craters along the lines, all indicate that these linear crater chains are of volcanic origin.