Geology topics

Charles K. Shearer

Publications and source records attributed to Charles K. Shearer.

4 recordsLinked to original sources

USGS2016

Geologic history of Martian regolith breccia Northwest Africa 7034: Evidence for hydrothermal activity and lithologic diversity in the Martian crust

The timing and mode of deposition for Martian regolith breccia Northwest Africa (NWA) 7034 were determined by combining petrography, shape analysis, and thermochronology. NWA 7034 is composed of igneous, impact, and brecciated clasts within a thermally annealed submicron matrix of pulverized crustal rocks and devitrified impact/volcanic glass. The brecciated clasts are likely lithified portions of Martian regolith with some evidence of past hydrothermal activity. Represented lithologies are primarily ancient crustal materials with crystallization ages as old as 4.4 Ga. One ancient zircon was hosted by an alkali-rich basalt clast, confirming that alkalic volcanism occurred on Mars very early. NWA 7034 is composed of fragmented particles that do not exhibit evidence of having undergone bed load transport by wind or water. The clast size distribution is similar to terrestrial pyroclastic deposits. We infer that the clasts were deposited by atmospheric rainout subsequent to a pyroclastic eruption(s) and/or impact event(s), although the ancient ages of igneous components favor mobilization by impact(s). Despite ancient components, the breccia has undergone a single pervasive thermal event at 500–800°C, evident by groundmass texture and concordance of ~1.5 Ga dates for bulk rock K-Ar, U-Pb in apatite, and U-Pb in metamict zircons. The 1.5 Ga age is likely a thermal event that coincides with rainout/breccia lithification. We infer that the episodic process of regolith lithification dominated sedimentary processes during the Amazonian Epoch. The absence of pre-Amazonian high-temperature metamorphic events recorded in ancient zircons indicates source domains of static southern highland crust punctuated by episodic impact modification.

Journal of Geophysical Research E: Planets↗

USGS2006

Heat-producing elements in the lunar mantle: Insights from ion microprobe analyses of lunar pyroclastic glasses

We provide new estimates for the abundance of heat-producing elements in the lunar mantle by using SIMS techniques to measure the concentrations of thorium and samarium in lunar pyroclastic glasses. Lunar pyroclastic glasses are utilized in this study because they represent quenched products of near-primary melts from the lunar mantle and as such, they provide compositional information about the mantle itself. Thorium and samarium were measured because: (1) Th is not significantly fractionated from Sm during partial melting of the pyroclastic glass source regions, which are dominated by olivine and pyroxene. Therefore, the Th/Sm ratios that we measure in the pyroclastic glasses reflect the Th/Sm ratio of the pyroclastic glass source regions. (2) Strong correlations between Th, U, and K on the Moon allow us to use measured Th concentrations to estimate the concentrations of U and K in the pyroclastic glasses. (3) Th, Sm, U, and K are radioactive elements and as such, their concentrations can be used to investigate heat production in the lunar mantle. The results from this study show that the lunar mantle is heterogeneous with respect to heat-producing elements and that there is evidence for mixing of a KREEP component into the source regions of some of the pyroclastic glasses. Because the source regions for many of the glasses are deep (⩾400 km), we propose that a KREEP component was transported to the deep lunar mantle. KREEP enriched sources produce 138% more heat than sources that do not contain KREEP and therefore, could have provided a source of heat for extended periods of nearside basaltic magmatism. Data from this study, in conjunction with models for the fractional crystallization of a lunar magma ocean, are used to show that the average lunar mantle contains 0.15 ppm Th, 0.54 ppm Sm, 0.039 ppm U, and 212 ppm K. This is a greater enrichment in radiogenic elements than some earlier estimates, suggesting a more prolonged impact of radiogenic heat on nearside basaltic volcanism.

Geochimica et Cosmochimica Acta↗

USGS2006

Identifying the effects of petrologic processes in a closed basaltic system using trace element concentrations in olivines and glasses: Implications for comparative planetology

We use trace-element concentrations in olivines and glasses from a closed basaltic system to identify the effects of petrologic processes on the trace-element record of that system. The closed basaltic system in question is the Makaopuhi Lava Lake (MLL), which is closed with respect to magma mixing. Detailed studies of this lava lake have provided important information about system variables and petrologic processes that have been measured and observed at the lake. These previous studies show that olivine crystallized from the lava lake at all stages of the lake’s evolution, which means that olivine and residual glasses contain a record of the lake’s petrologic history. We use this information, in conjunction with variations in trace-element concentrations in olivines and glasses, to show that mineral crystallization, gravitational settling, convective flow, filter pressing, and mineral-melt interface kinetics have characteristic effects on the trace element record of a closed basaltic system. These results are pertinent to the field of comparative planetology because they can be used to evaluate petrologic information in small samples from other planetary bodies, where information about system variables and/or petrologic processes is limited.

Hawaii↗

USGS2005

Petrogenesis of the Apollo 14 high-alumina basalts: Implications from ion microprobe analyses

In this study, ion microprobe analyses of individual minerals are used to investigate the petrogenesis of the Apollo 14 high-Al basalts. We use trace element concentrations from individual minerals in the Apollo 14 high-Al basalts to evaluate both endogenic and exogenic models. The data show that if the Apollo 14 high-Al basalts were produced by melting within the lunar mantle, these basalts cannot be related to one another by closed-system fractional crystallization of a single basaltic melt. Rather, the trace element data show that variable amounts of a KREEP component were added to the basalts by either assimilation, mixing into mantle sources, or impact melting. Single-stage assimilation–fractional crystallization models can only explain the data from this study if an excessively large mass of urKREEP is assimilated into the parent magma before olivine crystallization. Alternatively, the trace element data can be explained if the Apollo 14 high-Al basalts were produced by melting multiple Al-rich mantle sources that contain different amounts of urKREEP. Finally, for impact melting to be a relevant process, the data require that multiple large impact melts be formed from mixed KREEP-rich target lithologies. The resulting impact melts must then crystallize to produce basalts with igneous textures, high Al 2 O 3 concentrations, uniform major element compositions, and a wide range of incompatible trace element concentrations.

Geochimica et Cosmochimica Acta↗