Mathematics – Logic
Scientific paper
Jan 1999
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999nvm..conf...16g&link_type=abstract
Workshop on New Views of the Moon 2: Understanding the Moon Through the Integration of Diverse Datasets, p. 16
Mathematics
Logic
Crystallization, Glass, Igneous Rocks, Lunar Maria, Lunar Rocks, Volcanoes, Volcanology, Moon, Lunar Geology, Selenography, Absorption Spectra, Clementine Spacecraft, Remote Sensing, Spectral Reflectance, Ultraviolet Spectra
Scientific paper
In recent analyses, the 5-band Clementine UV-VIS data have been used to examine the compositions of lunar pyroclastic deposits. A primary goal of these analyses has been the characterization of the primary volcanic or juvenile components of these deposits. The compositions, physical and morphological characteristics, and spatial distributions of juvenile volcanic materials provide information on the distribution of primary mafic materials on the Moon, conditions required for their eruption at the surface, and the behavior of lunar volcanic processes over time. Using current analytical techniques with the new Clementine UV-VIS global mosaic, and data from the GLGM2 geophysical models, to supplement ongoing work with Earth-based spectral reflectance analyses and laboratory investigations, we have adopted a three-pronged approach to these issues involving: (1) compositional analyses of lunar pyroclastic deposits; (2) characterization of the relations between effusive and explosive lunar volcanism; and (3) examination of the global occurrence and distribution of lunar pyroclastic deposits. This report and related work describe progress toward remote characterization of the compositions of juvenile materials in the pyroclastic deposits located at Taurus-Littrow and J. Herschel. These studies have implications for characterization of the relations between the products of effusive and explosive volcanism on the Moon. Analyses of lunar pyroclastic materials, primarily the juvenile picritic glasses, provide unique information on the composition of the mantle and on the nature and origin of associated volatile elements in an otherwise volatile-depleted environment. Possible fundamental differences between picritic glasses and mare basalts, (e.g., lesser fractional crystallization and greater depth of origin for glasses) support their identification as the best examples of primitive materials on the Moon, and attest to their importance in characterizing the lunar interior and as a starting place for understanding the origin and evolution of basaltic magmatism on the Moon. Remote-sensing analyses of these deposits have helped us to identify the characteristic components of some of these deposits, to begin to constrain the distribution of lunar volcanic deposits, and to understand the styles of eruption and emplacement of basalts on the Moon. To fully appreciate the role of pyroclastic volcanism on the Moon, we must understand the ranges of composition, spatial and temporal distribution, relationship to effusive volcanic deposits, and modes of occurrence and formation of pyroclastic deposits. Until additional samples are available, remote analyses of lunar pyroclastic deposits are a primary means of fully characterizing these deposits. More than 100 pyroclastic deposits are identified on the Moon. Their characteristics, summarized here briefly, are described in detail elsewhere. Lunar pyroclastic deposits are dark and smooth surfaced, and they are observed in association with sinuous rilles, irregular depressions, or endogenic craters within highlands and/or on the floors of old impact craters situated along the margins of many mare-filled basins on the lunar nearside and farside. Lunar pyroclastic deposits are divided into two classes on the basis of size, morphology, and occurrence. The about 12 large deposits are of regional extent (up to several tens of thousands sq. km), while small deposits (about 90 in number) are more localized, typically only several hundred sq.km. Large deposits were probably emplaced via Strombolian-style or continuous fire-fountain eruptions, with wide dispersion of wellsorted pyroclasts. A significant component of Fe+2-bearing volcanic glass beads was identified in many of the regional pyroclastic deposits. At Taurus-Littrow, black beads (the crystallized equivalent of Fe+2-bearing orange glasses) are the characteristic ingredient of the regional pyroclastic deposit. Both orange and black beads were recognized as pyroclastic, with variations in cooling time in a fire fountain producing quenched, crystallized, and/or composite droplets. Small pyroclastic deposits were formed via Vulcanian-style or intermittent eruptions, with explosive decompression removing a plug of lava or caprock within a conduit and forming a vent depression. Small pyroclastic deposits were divided into three classes on the basis of their "1-mm" or mafic absorption bands. Most Group 1 deposits are mixtures of highlands-rich country rock and glassy juvenile material with small amounts of basaltic caprock material. Group 2 deposits consist largely of fragmented basaltic material, with insignificant amounts of highland and glassy materials. Group 3 deposits are dominated by olivine, and orthopyroxene; the olivine is almost certainly associated with juvenile material, and the orthopyroxene is likely to have been emplaced as a result of erosion and entrainment of the wall rock. Although differing origins have been proposed for the large and small pyroclastic deposits, comparison of Clementine UV-VIS spectral reflectance data for these deposits shows a linear trend, suggesting some compositional relationship between the two types of deposits. Weitz et al in studying (Taurus-Littrow, Sulpicius Gallus, Mare Vaponim, Rima Bode, Sinus Aestuum, Aristarchus Plateau, and Orientale), proposed that this linear trend represents a "mixing line" between those large deposits with about 100% crystallized beads and those with about 100% glass beads. A Sinus Aestuum dark spot is the crystallized end member, and Aristarchus is the glass end member. However, the overlap between the small and large deposits and between the pyroclastic deposits and nearby mare and highlands units suggests that (1) the juvenile pyroclastic materials are not glasses in all cases and (2) substantial spectral contribution from highland and mare units maybe observed for large and small deposits.(Additional information is contained in the original)
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