Other
Scientific paper
Dec 2006
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006agufmmr54a..01s&link_type=abstract
American Geophysical Union, Fall Meeting 2006, abstract #MR54A-01
Other
3672 Planetary Mineralogy And Petrology (5410), 3944 Shock Wave Experiments, 5112 Microstructure, 5420 Impact Phenomena, Cratering (6022, 8136), 5470 Surface Materials And Properties
Scientific paper
On Earth, the transformation of unconsolidated sediment (e.g., sand) to rock (sandstone) occurs via the process of lithification. Lithification typically occurs via burial within the upper crust at less than 150 degrees celsius, at depths of less than 5 km in the presence of liquid H2O. Liquid H2O is often important in the process of lithification because it is the transporting medium for dissolved and suspended ions and mineral species, which eventually precipitate as a cement that binds the unconsolidated grains. Lithification also applies to sedimentary deposits formed by precipitation of minerals from aqueous solutions at surface, or near- surface, conditions (e.g., to generate sulfate or carbonate-rich evaporites). However, for many planetary bodies in our solar system, there are no large sources of liquid H2O to facilitate this type of lithification process. Despite the absence of water on such bodies, the development of consolidated fragmental material is commonplace and it probably dominates the surface materials of Mercury, the Moon, Mars and many asteroids. This material, typically in the form of breccias, is a relatively coherent rock, yet the nature of the "glue" that binds the fragments is not well understood. Clearly, other processes are responsible for the lithification that we take for granted in many of the sedimentary rocks developed on our wet planet. This work explores these processes. For certain planetary bodies unconsolidated material may be bound by ices, such that it possesses rock-like properties in terms in strength and behaviour. In the absence of H2O, unconsolidated semi-molten material can be lithified by welding and compaction (e.g., certain pyroclastic discharges that fall and accumulate to form ignimbrites). This requires the production of hot volcanogenic or impact ejecta. In this work we explore the nature of the binding medium in different types of lunar breccia collected during the Apollo15, 16 and 17 missions, in meteorites of the howardite, eucrite and diogenite (HED) class and in laboratory shocked lunar regolith samples. Analytical scanning electron microscopy (SEM) and field emission SEM are used to explore the microstructures. The samples are grouped as: (1) being primarily derived from unconsolidated lunar regolith; (2) impact melt rocks, and (3) samples that were primarily derived from solid rock that were impact brecciated. There is significant overlap between these groups. All may exhibit relatively high degrees of porosity, especially regoliths and breccias. Features that affect the cohesiveness and coherence of the rock product are given particular attention. For this study, bridges are necks between grains, intergranular melts are melts that have developed between grains, fused grains are grains that are joined at a contact interfaces with no visible neck, and annealed fractures are fractures that have been partially or totally closed by either diffusion across the fracture or melting either side of the fracture. The various mechanisms of lithification are explored and discussed. Localized heating appears to be the dominant welding process. Heat generation can be attributed to (a) energy release due to frictional movement between grains during bulk (impact-generated) compression and (b) shock wave energy dissipation, especially between grains with high impedance contrasts and at grain (and other microstructural) boundaries. For unconsolidated materials, the resultant microtextures are akin to products of the industrial shock sintering of powders, and analogies are drawn between these two processes.
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