Monday, February 14, 2011

Silicic volcanism on the Moon


The heart of the Compton-Belkovich Thorium Anomaly (61.2°N, 99.4°) in a 90km x 90km segment of LROC Wide Angle Camera monochrome (643nm) observation M119212328M, LRO orbit 2702, January 27, 2010. At center is a high-reflectance feature known as the Compton-Belkovich High-Reflectance Feature (CBHRF). This area has been extensively surveyed by LRO, and has been determined to be a "unique occurrence" on the Moon's farside of "non-mare silicic volcanism" [NASA/GSFC/Arizona State University].

NONMARE, SILICIC VOLCANISM ON THE MOON’S FAR SIDE. B. L. Jolliff, Washington University; Tran, Lawrence & Robinson, Arizona State University; Scholten & Oberst, Institute of Planetary Research; B. R. Hawke, University of Hawaii; Hiesinger & van der Bogert, Universität Münster; B T. Greenhagen, JPL; S. A. Wiseman, Brown University; T. D. Glotch, Stony Brook University; D. A. Paige, UCLA

Introduction and Overview: At the center of the Compton-Belkovich Thorium Anomaly (CBTA) is a high-reflectance feature some 26×32 km in area (Fig. 1) [1], which we refer to as the Compton-Belkovich High-Reflectance Feature (CBHRF). Digital terrain models (DTM) derived from Wide-Angle (WAC) and Narrow-Angle (NAC) images show that this feature is elevated relative to its surroundings and contains positive relief features that we interpret to be of volcanic origin. The broad topographic feature also has irregular depressions that could be collapse features associated with volcanism.

Over the coming weeks, in anticipation of the 42nd Lunar & Planetary Science Conference, we continue highlighting some of the announced presentations related to lunar science:
LRO Diviner thermal emission measurements covering the CBHRF are consistent with an enrichment in silica or alkali feldspar, corresponding closely to the area of high reflectance. These observations, coupled with very high thorium contents inferred for this feature from the Lunar Prospector Gamma-Ray Spectrometer (LPGRS) data [2], are consistent with felsic (rhyolitic) material. These observations taken together provide strong evidence for a unique occurrence of non-mare, silicic volcanism on the Moon’s far side, in an area that is predominantly characterized as feldspathic, and located far from the Procellarum KREEP Terrane where silicic volcanism such as this might be anticipated.

Observations and Discussion: The CBTA is centered at 61.1°N, 99.5 °E and was first identified as a thorium ‘hotspot’ in data obtained by the LPGRS [2-4].

The site has a strong, focused concentration of thorium and is isolated in an area of Th-poor terrain on the Moon’s far side. It occurs between two large impact craters, Compton (162 km) and Belkovich (214 km).

Gillis and coworkers [4], using Clementine visible images, noted that the center of the CBTA corresponded to an area of unusually high reflectance, ~15×30 km in area; however, the Clementine image data were of insufficient resolution to reveal additional details about the feature. Lawrence et al. [2] considered the possibility that the highly reflective feature could be the cause of the thorium anomaly. Taking the broad spatial response function of the LPGRS into account, they calculated that the Th concentration could be as high as 40-55 ppm in the central feature. Few lunar igneous rock types have such high Th concentrations, among them granite, alkali-anorthosite, and monzogabbro [5]. The high-reflectance area is readily apparent in LRO WAC and NAC images (e.g., Fig. 1).

An image of the feature, draped over topography derived from a WAC DTM [6] shows that the high-reflectance area corresponds closely (but not precisely) to a topographically elevated terrain feature (Fig. 2). Notably, the bright material extends some 7-8 km to the eastsoutheast, beyond the topographically elevated terrain. This is significant because the Th anomaly is also “smeared out” to the east.

Within the CBHRF there are positive relief features ranging in size from small domes at an ~ 500 m scale, to an elongate (0.6×2.5 km) domical rock body, to an irregular massif about 6-7 km across at its base and up to a km high (Fig. 3) [see 7]. The two larger topographic features have steeper, inner dome areas (up to 20-26°of the feature), draped over topography derived from a WAC DTM [6] shows that the high-reflectance area corresponds closely (but not precisely) to a topographically elevated terrain feature (Fig. 2). Notably, the bright material extends some 7-8 km to the eastsoutheast, beyond the topographically elevated terrain.

LPSC 2011 #2224, Figure 1. LRO Wide-Angle Camera image of central region of Compton-Belkovich Thorium Anomaly.

This is significant because the Th anomaly is also “smeared out” to the east. Within the CBHRF there are positive relief features ranging in size from small domes at an ~ 500 m scale, to an elongate (0.6×2.5 km) domical rock body, to an irregular massif about 6-7 km across at its base and up to a km high (Fig. 3) [see 7]. The two larger topographic features have steeper, inner dome areas (up to 20-26° slopes) and broader, shallower aprons. The big dome has a summit plateau with a broad central depression. Boulders occur on the tops of these domes, especially “little dome” and “middle dome” (Fig. 3). Shapes and slopes suggest that these positive-relief features formed by eruption of a relatively high- viscosity lava compared to typical, low-viscosity mare lava. The east-west profile across the entire CBHRF (b-b’ in Fig. 2) shows a broad central depression composed of several irregular, scarp-bounded depressions that could be collapse features resulting from eruptive episodes. An overlay of Diviner data, using a measure of Christiansen Feature (CF) band position, an emissivity maximum between 7-9 μm for silicates, shows that the high-reflectance material corresponds to the area indicated to have a short-wave shift in the CF (Fig. 4) [8]. This short-wavelength CF position indicates the presence of a polymerized silicate such as quartz or alkali feldspar [9]. Close correspondence to the outline of the highly reflective area indicates that the mineralogical signature extends beyond the topographic feature by ~7-8 km to the east-southeast. The extension of the highly reflective area to the E-SE could be explained by deposition of pyroclastics beyond the extent of the topographic feature.

LPSC 2011 #2224, Figure 2. Topography of the Compton-Belkovich feature shown on a LROC WAC Digital Terrain Model. Close-dashed outline is the elevated topographic feature; the long-dashed outline is the high-reflectance feature from Fig. 1.

LPSC 2011 #2224, Figure 3. Positive relief features in the CBHRF inferred to be volcanic in origin.

LPSC 2011 #2224, Figure 4. Christiansen Feature position. Dashed white line shows the outline of the high-reflectivity area. The short-wavelength CF position, corresponding to blue, is consistent with polymerized silicate material.

We note a low density of small impact craters on flat surfaces within the CBHRF that indicates a relatively young age. This observation is consistent with the fact that the compositional signature has not been laterally mixed or overprinted significantly by impacts.

References: [1] Jolliff B. et al. (2010) LPSC, 41, #2412. [2] Lawrence D. et al. (2003) J. Geophys. Res 108, 6-1-6-25. [3] Lawrence D. et al. (1999) Geophys. Res. Lett. 26, 2681-2684. [4] Gillis J. et al. (2002) Lunar Planet. Sci. 33, #1934. [5] Jolliff B. (1998) Int. Geol. Rev. 40, 916-935. [6] Scholten F. et al. (2011) this Conf. [7] Tran T. et al. (2011) this Conf. [8] Greenhagen, B. et al. (2010) Science, 329, 1507-1509. [9] Glotch T. et al. (2010) Science, 329, 1510-1513.

Acknowledgements: We thank the LROC science operations team and the Diviner and LRO Ops teams, and NASA for support of the LRO project.


Mons Hansteen, unusually bright for a remnant volcanic feature, is a 30 km-wide, 300 meter high mountain that, together with the inundated 48 km-wide Billy crater below, are telescopic landmarks of south Oceanus Procellarum. Data from the Diviner instrument on LRO, analyzed at UCLA, identified the two southeast and southwest points of this feature among the few signatures of silicate detected on the Moon. The LROC Wide Angle Camera swept up this 50 km-wide scene over the course of LRO orbits 2496 & 2497, January 11, 2010 [NASA/GSFC/Arizona State University].

HANSTEEN ALPHA: A SILICIC VOLCANIC CONSTUCT ON THE MOON: Hawke & Lucey, University of Hawaii; T. A. Giguere, UH & Intergraph Corporation; Lawrence, Braden & Tran, Arizona State University; T. D. Glotch, Stony Brook University; B. T. Greenhagen, JPL; Hagerty & Gaddis, USGS; L. Jolliff, Washington University and the LROC Science Team

Introduction: Hansteen α is an arrowhead-shaped highlands feature located on the southern margin of Oceanus Procellarum just north of the crater Billy (Figures 1 and 3). The distinctive shape of Hansteen α has resulted in many workers referring to it simply as the “Arrowhead.”

The feature is a rough-textured triangular mound that is ~25 km on a side and exhibits a relatively high albedo. Hansteen α is a member of a class of lunar spectral anomalies known as red spots which are characterized by a relatively high albedo and a strong absorption in the UV [1,2]. In the immediate post-Apollo era, some workers presented evidence that at least some red spots were produced by highlands (i.e., nonmare) volcanism and suggested a connection with KREEP basalts or even more evolved highlands composition (e.g., dacite, rhyolite) [1,2,3].

Very recent research using Clementine, Lunar Prospector (LP), and Lunar Reconnaissance Orbiter (LRO) data have provided strong evidence that some red spots are dominated by Th- and silica-rich, highly evolved highlands lithologies [4,5,6,7,8]. The purpose of this study is to use images from the Lunar Reconnaissance Orbiter Cameras [9] and LRO Diviner Lunar Radiometer Experiment data [7,8] as well as Clementine UVVIS images to investigate the geology and composition of Hansteen α.

Geology and Morphology: McCauley [10] described Hansteen α as being made up of steep-sided, bulbous, very bright dome material that exhibits a hackly surface. He also described several small, linear, smooth-walled depressions at the crests of gentle individual highs and interpreted these depressions as probable volcanic vents. Wood and Head [1] noted that the Arrowhead appeared distinctive in its surface texture, color, and albedo from nearby highlands and is embayed by adjacent mare units. Wagner et al. [11] stated that Hansteen α had a flat summit region reminiscent of a mesa and noted that the summit surface as well as the flanks appear much more rugged than the Gruithuisen domes. Wagner et al. [11] determined a cratering model age of 3.67 Ga for two areas in the summit region.

LROC Wide Angle Camera (WAC) and Narrow Angle Camera (NAC) images were used to investigate Hansteen α. WAC images were used to produce a DEM for the region [12]. The outer margin of Hansteen α is marked by a steep slope that rises sharply from the surrounding mare surface (Figure 1).

LPSC 2011, #1652 Figure 1. Portion of LROC Wide Angle Camera frame M117826631.

The NE margin of the Arrowhead rises abruptly to a height of ~740 m above the mare surface and has an average slope of 18°. The NW margin stands ~690 m above the mare and has an average slope of 16°. The surface of the summit gently slopes up to the highest point (1036 m) which is located just SW of the center of Hansteen α. The summit surface is rough at a variety of scales. Numerous depressions and ridges can be seen in Figure 1. A high-resolution (0.5 meters/pixel) view of the surface of Hansteen α is provided by Figure 2. The area shown in Figure 2 is outlined in white in Figure 1. The arrows in Figure 2 indicate areas with high block abundances. Most blocks are associated with steep slopes or impact craters. However, some block fields occur on level surfaces that are not near impact craters.

Composition: Clementine UVVIS images were used to produce FeO, TiO2, and optical maturity maps of the Hansteen α region utilizing the algorithms of Lucey et al. [13,14]. Mare units in this region exhibit FeO abundances >16wt%, and TiO2 values range betteween 4wt% and 8wt%. In sharp contrast, much lower FeO and TiO2 values are exhibited by Hansteen α. FeO values range from 5wt% to 9wt% with slightly higher values occurring near the highland/mare contact. Most of Hansteen α exhibits less than 1wt% TiO2 The central core portion of the feature has an average FeO value of 6.9wt% and an average TiO2 value of 0.5wt%. Since this central region has suffered the least amount of contamination by surrounding mare units, its composition may most closely approximate that of the underlying lithology. The Imbrium-aged craters Billy and Hansteen emplaced relatively FeO and TiO2-rich ejecta.

The Arrowhead exhibits much lower FeO and TiO2 values. If the Arrowhead was present prior to the formation of Billy and Hansteen craters, it should have been covered with FeO and TiO2-rich ejecta since it is within one crater diameter of the rim crest of each crater. Since it is not, Hansteen α was emplaced on top of the FeO-rich ejecta deposits. Nonmare volcanism is the only viable process for the formation of Hansteen α.

LPSC 2011, #1652 Figure 2. Portion of LROC Narrow Angle Camera frame M133160305R.

The initial studies of the LP gamma ray data suggested that there was little or no Th enhancement associated with Hansteen α [4]. Later, Lawrence et al. [5] used forward modeling to show that the Th abundance at Hansteen α is not 6 ppm, as indicated by the LPGRS Th map, but is more likely closer to 25 ppm. A more recent forward modeling study by Hagerty et al. [6] reported a Th abundance of 20 ppm for Hansteen α. High Th values in the 20 to 25 ppm range are consistent with Th abundances measured in evolved lunar lithologies such as granites and felsites.

The Diviner Lunar Radiometer Experiment on the LRO is a multispectral radiometer that is well suited to detecting the mineral indicators of silicic volcanism [7]. Diviner has three narrow spectral bandpass filters centered at 7.8, 8.25, and 8.55 μm that were specifically designed to characterize the position of the Christiansen Feature [8,15], which is directly sensitive to silicate mineralogy and the bulk SiO2 content of a lithology [7]. Glotch et al. [7] used Diviner three-point spectra and two spectral parameters (C and I) that describe the spectra to investigate variations in silicate mineralogy and to search for the key indicators of silicic volcanism (quartz, silica-rich glass, and alkali feldspars). They determined that portions of four red spots (Hansteen α, Gruithuisen domes, Lassel massif, and Aristarchus south rim and ejecta) had spectral parameter or index values best explained by Si-rich glass, quartz, and/or alkali feldspars.

LPSC 2011, #1652 Figure 3. LRO Diviner concavity map overlaid on WAC frame.

The C parameter or concavity index provides an estimate of the abundance of silicic materials. Figure 3 shows a concavity index map of the Hansteen α region overlaid on a WAC image (M11782663ME). High values of the index (red) indicate increasingly silicic compositions. Lower concavity index values are green and blue. High values are more abundant near the center of Hansteen α and in the high terrain SW of the center. The lower values near the margins of the feature may be the result of contamination by mare debris transported to the lower slopes of the Arrowhead by impacts in the surrounding mare.

References: [1] Wood, C. and Head, J.W. (1975) Conf. on Origins of Mare Basalts, 189. [2] Head, J. and McCord, T. (1978) Science, 199, 1433. [3] Malin, M. (1974) EPSL, 21, 331. [4] Hawke, B. et al. (2003) JGR, 108(E7), 5069. [5] Lawrence, D. et al. (2005) GRL, 32, L0721. [6] Hagerty, J. et al. (2006) JGR, 111, E06002. [7] Glotch, T. et al. (2010) Science, 329, 1510. [8] Greenhagen, B. et al. (2010) Science, 329, 1507. [9] Robinson, M. et al. (2010) Space Sci. Rev., 150, 81. [10] McCauley, J. (1973) U.S.G.S. Map I-740. [11] Wagner, R. et al. (2010) JGR, 115, E06015. [12] Scholten, F. et al. (2010) LPS 41st, Abs. 2111. [13] Lucey, P. et al. (2000) JGR, 105(E8), 20,297. [14] Lucey, P., et al. (2000) JGR, 105(E8), 20,377. [15] Paige, D. et al. (2010) Space Sci. Rev., 150, 125.

42nd Lunar and Planetary Science Conference (2011)

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