International Geology Review

Ruzicka, Alex; Snyder, Gregory A.; Taylor, Lawrence A.

doi: 10.1080/00206819809465242pmid: N/A

Geochemical data for Ni, Co, Cr, V, and Mn have played an important role in theories for the Moon's origin. It has been argued that the data for these elements strongly support formation of the Moon as ejecta from the Earth, either as a result of one giant or numerous smaller impacts on the proto-Earth. These theories have come to be known as the “Giant Impact” and “Impact-triggered Fission” hypotheses, respectively, and the first of these has been the leading explanation for the origin of the Moon over the past decade. Data for these same “diagnostic” elements also have been used to argue for significant distinctions between the bulk compositions of the Moon and a eucrite (HED) parent body, which otherwise appear to be remarkably similar in their compositions. We review geochemical evidence pertaining to the origin of the Moon, focusing on the diagnostic elements, and find that there is no strong geochemical support for either the Giant Impact or Impact-triggered Fission hypotheses. We show that basalts produced in the Moon and a HED parent body (mare basalts and eucrites, respectively) were derived by the melting of source regions with similar compositions. Mare and eucrite basalts differ in Ni and Co abundances but lie on the same igneous fractionation trend. Chromium, Mg# (Mg/[Fe+Mg]), and V abundance systematics suggest a close similarity between mare and eucrite basalts, and a significant difference from terrestrial volcanic rocks, which are depleted in Cr. Mare and eucrite basalts differ in their Mn abundances and Fe/Mn ratios, but the same can be said for mare and terrestrial basalts. On the whole, the Moon appears to be more chemically similar to the HED parent body than to the Earth. This suggests that either: (1) the HED parent body (probably the asteroid Vesta) formed by an impact mechanism and is an escaped satellite, or (2) the Moon is a captured body that formed independently of Earth. Similar conclusions were reached long ago by Anders and colleagues (e.g., Anders, 1977), long before the Giant Impact Hypothesis attained popularity.

Roonwal, Ganpat S.; Wilson, Graham C.

doi: 10.1080/00206819809465243pmid: N/A

This paper outlines the potential of India's mineral resources, which comprise a wide range of deposit types scattered across the subcontinent, within an area of 3,287,590 km2. The geological environment is extremely diverse, ranging from extensive Archean cratonic nuclei, with large areas of high-grade metamorphic rocks of lower-crustal provenance, to Proterozoic sedimentary basins and mobile belts, to Tertiary mountain ranges and Quaternary surficial deposits. The mineral endowment of these disparate regions includes world-class deposits of iron, manganese, and chromium, plus base metals (copper, lead, zinc), precious metals, and a spectrum of nonmetallic resources from bauxite to gemstones. India is a major exporter of Fe ore, Mn ore, chromite, mica, and granite. The Exclusive Economic Zone (EEZ) around the shores of India totals 2,014,900 km2, 61% the size of the landmass, holding additional potential for commodities such as heavy mineral sands (monazite, rutile, and other materials) and Mn and Co-rich ferromanganese nodules and crusts. A selective overview of India's mineral potential is provided with reference to 22 metals and groups of metals and 6 non-metallic commodity groups, excluding fuels. These are Cu, Zn, Pb, and Ag; Fe, Ti, V, and Mn; Cr, Ni, Co, and PGE; Au; Sn, W, and Mo; Nb, Ta, and Li; bauxite; U, Th, and the REE; barite, garnet, and graphite; and diamonds and colored gemstones. The effectiveness of the domestic mineral industry in locating and exploiting these resources varies greatly among commodities. Some sectors where the country was once a noted producer, such as diamonds and gold, have been eclipsed by major discoveries in other lands and by stagnant domestic output. In some cases, such as gold and nickel, known reserves and output are small compared to both the area of prospective terrain and high domestic demand (Table 1). Some indication is provided of likely future trends, in view of the Government of India's current initiatives to entice foreign investment in many sectors. This recently developed open policy extends to exploration and mining ventures, construction of power generation units, and modernization of existing industrial plant, transport, and communications. With an eye on this mineral-economic context, we present a synthesis of the issues faced by the mineral sector as it enters the 21st century.

Kraemer, Pablo E.

doi: 10.1080/00206819809465244pmid: N/A

A 100 km long balanced structural transect is presented for the Patagonian Andes at 50° S Latitude. The area studied is characterized by a fold belt in the eastern Andean foothills and basement-involved thrusts in a western-basement thrust zone. The basement thrust zone exposes pre-Jurassic, polydeformed sedimentary and layered metamorphic rocks emplaced over Lower Cretaceous rocks above an E-vergent thrust located at the western end of the fold belt. The fold belt is developed in a 3 km thick deformed Cretaceous–Paleogene sedimentary cover with few basement outcrops and scarce calc-alkaline magmatism. Cover structures related to shallow décollements have a N-S to NW-SE strike, with fold wavelengths from 1100 to 370 m in the east to 20 to 40 m in the west. However, long-wavelength basement-involved structures related to deeper décollements have a dominant N-S to NE-SW trend along the eastern and western parts of the fold belt. Field evidence showing different degrees of inversion of N-S–trending normal faults suggests that the orientation of the Cenozoic compressive basement structures was inherited partially from the original geometry of Mesozoic normal faults. The deformation propagated toward the foreland in at least two events of deformation. The effects of Paleogene (Eocene?) compressive episode are observed in the western fold belt and a Neogene (Late Miocene) compressive episode is present in the eastern fold belt. Basement-involved structures typically refold older cover structures, producing a mixed thick and thin-skinned structural style. By retrodeforming a regional balanced cross section in the fold belt, a minimum late Miocene shortening of 35 km (26%) was calculated.

Jolliff, Bradley L.

doi: 10.1080/00206819809465245pmid: N/A

Mafic impact-melt breccias (IMB) from the Apollo landing sites—particularly Apollo 14, Apollo 15, Apollo 16, and Apollo 17—are abundant and form compositionally distinct groups. These groups exhibit a range of major-element compositions and incompatible-element enrichments. Although concentrations of incompatible elements span a significant range, inter-element ratios vary little and have been used in the past to infer a common KREEP component (KREEP = rich in potassium, rare-earth elements, phosphorus, and other alkali and high-field-strength elements). On the basis of an extensive, high-precision data set for melt-breccia groups from different Apollo landing sites, variations in trace-element signatures of the mafic impact-melt breccias reflect significant differences in KREEP components of source regions. These differences are consistent with variable enrichment or depletion of source regions in those trace elements that fractionated during the latest stages of residual-melt evolution and are more or less related to “lunar granite.” Compared to other sites, the source region of Apollo 14 impact melts had an excess of the elements that are concentrated in lunar granite, suggesting either than this source region was enriched in such a component (K-frac) or that it lost a corresponding mafic component (REEP-frac). Because these are impact-melt breccias formed in large (probably basin) impacts, the indicated geochemical separations must have occurred on a broad scale. Variations in the incompatible-element concentrations of the IMB groups reported in this paper are used to calculate a revised KREEP incompatible-element composition. On the basis of several extremely enriched lunar samples that retain the incompatible elements in KREEP-like ratios, the KREEP composition is extended to a level of 300 ppm La, or about three times the concentration of high-potassium KREEP as estimated by Warren (1989).

Colson, Russell O.

doi: 10.1080/00206819809465246pmid: N/A

If temperature and composition dependencies of partitioning are taken into account in modeling equilibrium processes, unexpected chemical trends are derived. Failure to consider variations in partition coefficients with temperature and composition in modeling equilibrium melting or crystallization processes leads to wildly erroneous results. Because of changes in partition coefficients with temperature and composition, incompatible elements can exhibit apparently compatible behavior, complex parallel or perpendicular trends can develop from a single process and bulk composition, and trends that appear to require several controlling phases can be explained with only one phase of variable composition. The exercises illustrated here demonstrate that complex trends can be misinterpreted if the variations in partition coefficients are not taken into account, and the “average” phase proportions and compositions controlling an evolutionary trend can be erroneously estimated. Complex trends, such as those in Apollo 15 green glass beads, can possibly be understood in terms of simple partial equilibrium melting of one or two phases.