Paul D. Lowman

The relation of tektites to lunar igneous activity

Abstract The theory that tektites are of lunar origin has frequently been criticized on the grounds that rocks of this chemical composition could not have been formed on the Moon. The purpose of this paper is to show that tektites may have been derived from silicic igneous rocks, specifically rhyolitic tuffs, forming the upper layers of the maria. The essentially igneous nature of tektites is indicated by their bulk composition, their restricted compositional range, and the high probability that they have not been derived from sedimentary rocks by random processes such as meteoritic impact. The differences between tektites and normal igneous rocks probably reflect their formation by extremely high temperature fusion of previously solid igneous rock. It is shown that if the Moon originally had a chondritic composition, the fusion curve of the lunar material should intersect the thermal gradients computed by G. J. F. MacDonald at the present time at 400 to 500 km depth, depending on the slope of the fusion curve, even if the Moon had originally been at 0°C. Additional heat sources such as the kinetic energy of accretion and capture-induced internal friction would probably have raised the temperature well above the “cold” Moon gradient. Magmas generated by partial fusion of the assumed chondritic material should be basaltic. It is proposed that the maria are the lunar equivalents of terrestrial lopoliths such as the Bushveld and Sudbury complexes, which are large basins filled mainly with basaltic rock overlain by tuffs, rhyolites, and granophyres. Tektites may have been derived from these silicic rocks by some process involving meteoritic impact.

Comparative planetology and the origin of continental crust

Abstract This paper summarizes crustal evolution in silicate planets (the Moon, Mercury, Mars, Venus and meteorite parent bodies), and outlines a general theory for the origin of continental crust based on comparative planetology and terrestrial evidence. Silicate planets and some meteorite parent bodies have undergone early global differentiation (nonplate-tectonic), followed by a late basin-forming bombardment and a long basaltic second differentiation. Evidence for analogous stages (except a late bombardment) can be found in terrestrial geology, modified by conditions of hydrous magma generation and persistent tectonism. Continental crust is inferred to have largely been formed in the early Archean by andesitic volcanism accompanying catastrophic early degassing. This global crust was disrupted by basin-forming impacts that initiated mantle upwelling and sea-floor spreading. Basaltic overplating and underplating, from mantle plumes, continued on the continents producing greenstone belts, mafic dike swarms, and, by partial melting of underplated ‘basalt’ and sialic crust the granitoids of Archean granite-greenstone terrane. This process produced most of the present continental crust by ∼2.5 Ga ago, when the global tectonic style evolved into the present one of tectonism marginal to and within continents. Subsequent crustal evolution has largely been deformation, metamorphism, anatexis and rearrangement of previously formed crustal segments, not separation of new sialic crust from the mantle. The crustal growth that has occurred since the Archean has primarily been crustal thickening resulting directly or indirectly from generation of basalt in the subcontinental tectosphere. Viewed in the light of evidence from comparative planetology, the continents are interpreted as the greatly altered remnants of an originally global primordial crust, the result of an evolutionary sequence shared by all the silicate planets to some degree depending on their mass, volatile content and location in the solar system. This theory can be tested by deep drilling in Archean terranes to locate remnants of the original crust, by determination of the composition of the highland crust of Mars, and by investigations of the crust of Venus.

Formation of the earliest continental crust: Inferences from the scourian complex of northwest Scotland and geophysical models of the lower continental crust

Abstract In this paper the structure, composition, and pre-metamorphic lithology of the Scourian Complex at its type locality are compared with models of the lower continental crust proposed by several authors on the basis of seismic reflection profiling and exposures in the Ivrea zone and the Vredefort dome. It is shown that the Scourian Complex resembles these models in major characteristics, consisting of gently-dipping granulite gneisses of intermediate bulk composition formed from partly or largely supracrustal precursors. In addition, seismic investigations of northwest Scotland indicate that granulites of the Scourian type make up most of the 26 km of continental crust in the area. It is therefore proposed that the Scourian Complex provides a representative exposure of the lower continental crust. If this is so, it implies that the lower crust is dominantly of supracrustal origin with normal superposition age relationships except for localized thrusting and intrusion. Such age relationships in turn imply that isotopic investigations (SmNd and RbSr) date only later additions to the early crust, either intrusions or supracrustal additions, not formation of entire crustal segments. It is therefore proposed that older continental crust may underlie even the oldest exposed rocks; that primordial crust may survive near the Moho; and that the earliest continental crust was formed largely by andesitic volcanism that produced a widespread, possibly global, layer now represented by granulites of the Scourian type.

Tektites as a Guide to the Structure of the Moon

Abstract Experience shows that morphological data by itself does not give decisive answers to the question of the mode of formation of the surface of the moon. Some chemical information is needed. If tektites are from the moon, they furnish that information. Tektites are the result of impact on some surface, because they contain nickel-iron spherules and coesite. The surface was probably not the surface of the earth because the same tektites that contain coesite consist largely of glass, and the glassmaking process is inconsistent with the survival of coesite. Hence the material was glass before the impact, but highly siliceous glasses with low water content of this kind are not found on the earth. Tektites do not come from interplanetary space because their distribution over the earth is explicable only if we suppose them to be fragments of orbiting bodies. They are, therefore, probably lunar. Accepting the Muong Nong tektites as essentially fragments of the lunar crust, we find a certain resemblance to terrestrial ash-flow tuffs. This resemblance is borne out by their dryness and by the following consideration: Tektites have been differentiated relatively late in the history of the solar system. If they are lunar, we would expect that they came from the maria. The maria are flat; if composed of acid rock we would expect that they were ash flows. The Ranger photographs support this view because they contain evidence of differential compaction. They also contain evidence of recent acid volcanism. Comparison of the Ranger photographs with the Valley of Ten Thousand Smokes supports the analysis which we have made here.

Candidate sites for lunar observatories, with a recommended example - The NE Orientale Basin

This paper discusses site selection criteria for a Moon‐based observatory, using an area on the NE flank of the Orientale Basin at 80° W on the lunar equator as a recommended example. Such a site would provide the following advantages: visibility of essentially the entire celestial sphere, continual line of sight to Earth, dynamic accessibility for minimum energy landings and takeoffs, closeness to the far side for installation of instruments requiring a radio silent environment, trafficable and workable terrain, closeness to geologically important features, and closeness to volcanic features with potential resources. Polar and far side sites have advantages as well, and no one site can meet all requirements. It is suggested that other candidate sites be evaluated by the criteria used for the Orientale Basin.