In-situ Laser Microprobe 40Ar 39Ar Geochronology of Apollo 16 Impact Melts-Student: Cameron Mercer

Project: Research project

Description

Establishing an absolute chronology of lunar impact events is one of the highest priorities for NASA in the robotic and human exploration of the Moon, and has significant implications for understanding the bombardment history and early evolution of the Earth and the other planets of the inner solar system. This requires the precise and accurate dating of lunar rocks that have experienced one or more impact events. The samples that are most useful for 40Ar/39Ar geochronology are impact melt breccias, which are typically complex admixtures of (solidified) glassy or crystalline melt, and relict mineral and lithic fragments, i.e., clasts, that did not melt. Some lunar melt breccias also can contain multiple distinct generations of melt. Samples of the size used in conventional step-heating 40Ar/39Ar experiments (a few to tens of milligrams) are large enough to include multiple melt phases and incompletely reset clasts. The resultant age spectra are often complex, and can make geologically meaningful age interpretations difficult. For this project, we are employing high spatial-resolution laser microprobe techniques to measure multiple in-situ 40Ar/39Ar dates for each of several impact melt breccias from the Apollo 16 and 17 archives. This approach allows us to target individual generations of impact melt while avoiding clasts as much as possible, and allows direct interpretations of the age relationships preserved in complex, multi-phase samples. Samples returned from the Descartes Mountains and Cayley Plains formations at the Apollo 16 landing site contain abundant impact melts from four major geochemical groups, and exhibit conventional 40Ar/39Ar ages that span nearly 400 million years and likely reflect multiple impact events. Previous researchers have attempted to constrain the formation ages for the Nectaris and Imbrium basins by analyzing Apollo 16 impactites, but the genetic relationships between dated samples and a particular basin are uncertain at best. This is largely due to the unknown and undoubtedly mixed provenance of impact breccia materials in the Descartes Mountains and Cayley Plains formations, which were presumably emplaced as ejecta from the Nectaris and Imbrium basins, respectively. Impact melt breccias collected from the Taurus Littrow Valley during the Apollo 17 mission have traditionally been considered as products of the Serenitatis impact basin. However, researchers using Lunar Reconnaissance Orbiter data have suggested that most of the impact melts sampled at Apollo 17 may be derived from Imbrium rather than Serenitatis ejecta. It is therefore important to quantify the ages of all the generations of impact melts that were sampled at the Apollo 16 and 17 sites. This may improve our understanding of the ages of the Nectaris and Imbrium impact basins, and could help elucidate the provenance of materials collected at the Taurus Littrow Valley. In the second year of this project, we have received six 200 m-thick thick sections and corresponding 32 m-thick thin sections of Apollo 16 impact melt breccias. I have obtained backscattered electron mosaics of the thick sections and am currently preparing them for irradiation at the Oregon State University TRIGA research reactor. I will conduct detailed characterization of the thin sections to inform the design of our laser microprobe analysis campaigns, which will be completed during the fall of this year. Additionally, we have completed laser microprobe 40Ar/39Ar work on two Apollo 17 samples, and have submitted a manuscript for consideration to the journal Science. We found one sample to have a simple, single-event impact history while the other records a complex, multipleevent history spanning at least several hundred million years. Finally, I have continued the development of my analytical software, and I anticipate submitting one of my programs for publication to Computers and Geosciences in the next year.

Description

One of the highest priorities in lunar science for NASA is to establish an absolute chronology of lunar impact events, with significant implications for the bombardment history of the Earth and other planets of the inner Solar System. Achieving this goal requires the precise and accurate dating of lunar rocks that have been affected by impacts (impactites). The 40Ar/39Ar method, based on the K-Ar isotopic system, has been widely used to date impact events since its systematics may be reset during impact, and because it allows the effective analysis of small quantities of rare samples. Conventional 40Ar/39Ar techniques typically involve step-heating small whole-rock fragments or mineral separates in a furnace or by infrared lasers to release gasses for analysis in a mass spectrometer. In most cases, the petrographic context of the dated material is lost in the process of isolating it for analysis, and the presence of unmelted minerals may engender age interpretations that are ambiguous or that have little geological significance for the timing of impact. Additionally, the thermal histories of lunar impactites are complex, and may be reflected by multiple generations of impact melts and shock effects within individual samples. The application of cutting-edge ultraviolet (UV) laser ablation techniques in 40Ar/39Ar experiments provides the ability to date material in petrographic context. UV laser microprobes have already revolutionized 40Ar/39Ar studies of complex, partially-melted materials (pseudotachylites) along terrestrial fault zones, revealing multiple stages of fault movement. We propose to conduct detailed petrology and UV laser microprobe 40Ar/39Ar geochronology of Apollo 16 impactites in order to better understand their complex thermal histories, and hopefully to expand our knowledge of the impact record in the Descartes- Cayley Plains. In this way, we hope to clarify or expand upon previous impact chronologies developed with conventional 40Ar/39Ar techniques, and hence, to address some of NASAs top priorities in lunar science.
StatusFinished
Effective start/end date9/1/128/31/15

Funding

  • NASA: Goddard Space Flight Center: $89,385.00

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geochronology
student
laser
melt
history
clast
chronology
basin
in situ
ejecta
thin section
solar system
provenance
mineral
planet
rock
heating
lithic fragment
valley
mountain