Bennett, Kristen -- Joint M3 and Diviner analysis of the mineralogy, glass composition, and country rock content of pyroclastic deposits in Oppenheimer Crater


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Kristen Bennett

Arizona State University

1709 S Parkside Dr




Here we present initial results from a global study of the near- and mid-infrared spectral properties of lunar pyroclastic deposits in order to constrain the range of eruption processes on the Moon. We have employed a new method of spectral analysis of the 1 μm iron absorption band in Chandrayaan-1 Moon Mineralogy Mapper (M3) spectra. By analyzing both the position and shape of the 1 μm band we can detect and map the distribution of minerals, glasses, and mixtures of these phases in pyroclastic deposits. This method has successfully been used to detect glass in volcanic and impact deposits on Mars (Horgan and Bell, 2012; Horgan et al., 2012) and the Moon (Horgan et al., 2013).

We are also using mid-infrared spectra from the Lunar Reconnaissance Orbiter Diviner Lunar Radiometer Experiment to develop Christiansen Feature (CF) maps at a resolution of ~200 m/pixel. The CF value is the emissivity maximum that occurs near 8 μm, which correlates with silica abundance. Lower CF values are representative of plagioclase while higher CF values are representative of pyroxene or olivine (Salisbury and Walter, 1989). One of the benefits of using CF maps for analysis of pyroclastic deposits is that they can be used to detect silicic country rock that may have been emplaced by Vulcanian-style eruptions, and are sensitive to iron content in glasses (Allen et al., 2013), which the near-infrared is not.

Here we present results from pyroclastic deposits within Oppenheimer Crater that are hypothesized to be Vulcanian in origin (Gaddis et al., 2013). M3 analysis reveals that the primary spectral endmembers are low-calcium pyroxene and iron-bearing glass, with only minor high-calcium pyroxene, and no detectable olivine. The pyroclastic deposits clearly exhibit significant glass concentrations, most likely 80 wt.% or more of the mafic component; however, glass concentrations do appear to vary between deposits. The large (~40km) southern deposit exhibits spectra consistent with a glass content on the order of tens of wt.% lower than other deposits in the crater, perhaps suggesting a higher country rock content, and therefore, a larger and/or deeper eruption.

Analysis of publicly available Diviner CF maps of the Oppenheimer crater floor indicates an average CF value of 8.23 ± .05, consistent with a pyroxene-rich substrate as interpreted from the M3 analysis. The anomalous southern deposit has a median CF value of 8.33, and a range between 8.6 and 8.1. While this variation may indicate a heterogeneous deposit, the median indicates a more mafic composition than the crater floor. M3 analysis indicates a pyroxene-rich substrate and a moderately glass-rich deposit. While we cannot determine the iron content of the glass from M3 spectra alone, Diviner CF values suggest that the glass is relatively iron-rich. Indeed, Diviner CF values for this deposit are considerably higher than for sampled soils from the Apollo landing sites (Allen et al., 2013), also supporting our hypothesis that the glass in this deposit is relatively iron-rich.



Kristen, great video for you poster!