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Shallow Magmatic Intrusion in the Lunar Nearside Highlands?

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Shallow Magmatic Intrusion in the Lunar Nearside Highlands? 46th Lunar and Planetary Science Conference (2015) 1583.pdf SHALLOW MAGMATIC INTRUSIONS IN THE LUNAR NEARSIDE HIGHLANDS? L. M. Jozwiak1 and J. W. Head1, 1 Dept. Earth, Environmental and Planetary Sciences, Brown University, Providence, RI 02906 ([email protected]) Introduction: The moon displays a dichotomy in strengthening the excavated magmatic intrusion mare exposure with the majority of deposits located on formation hypothesis. the lunar nearside. The nearside is also comprised of the Regional Investigation. We used the standard southern nearside highlands, which lack exposed mare Clementine RGB color composite image [12] and deposits. Morphologic and spectroscopic studies do not analyzed the nearside highlands for dark-halo craters show evidence of cryptomare exposures in this region with similar multispectral properties to Buch B, and also [1]. Additionally, maps of floor-fractured crater for exposures of similar multispectral properties within distributions [2] show a paucity of floor-fractured craters crater floors and walls. Figure 2a shows the crater Buch in this region, suggesting a lack of shallow magmatic B in a Clementine RGB composite image where the dark intrusions below crater floors (Figure 1). halo is readily identifiable by its green color. We located Data presented in Jozwiak et al. (2015) [3] builds 21 areas of interest using the Clementine dataset upon a paradigm proposed by Head and Wilson (1992) including several small dark halo craters similar to Buch [4] using crustal thickness to explain the distribution of B, and also several craters floors with similar surficial lava deposits and shallow intrusive deposits multispectral properties to Buch B, for example craters (floor-fractured craters). In this context, crust of superposed on the floor-fractured crater Janssen (Figure intermediate thickness (~20-30 km) is predicted to be 3a). favorable to shallow intrusions. Despite this, the nearside The large crater superposed on the floor of the floor- highlands contain only 3 floor-fractured craters (Fig. 1). fractured crater Janssen (41.5°E, 44.9°S) clearly displays Large craters in this region show significant infill by the same spectral characteristics (green) in Clementine smooth deposits interpreted to be emplaced by secondary RGB color ratio imagery as seen in the dark halo of crater ejecta from craters and basins [5] which could crater Buch B. Floor-fractured craters are postulated to obscure mare deposits or morphologies associated with form in response to magmatic intrusion and sill floor-fractured craters. To search for evidence of formation beneath the existing crater floor [2, 13]. The potentially buried or obscured magmatic bodies, we use sills are predicted to form at a depth of a few kilometers Clementine multispectral image data [6] to search for beneath the crater floor. The superposed crater in mafic signatures of material excavated by impact craters, Janssen (Figure 3b) is approximately 10 km in diameter M3 (Moon Mineralogy Mapper [7]) data to refine our and excavates to a depth of approximately 3 km [14]. compositional analysis, and GRAIL (Gravity Recovery Thus, if this crater formed after sill formation in Janssen, and Interior Laboratory) [8] data, to assess Bouguer it is possible that the crater formation could excavate and gravity anomalies potentially associated with dense incorporate mafic material from the intrusion, resulting mafic intrusions. in the signal observed in the Clementine composite. The Dark-Halo Craters: Dark-halo craters have been large crater Fabricius is superposed on the northern rim used to infer the presence of a low-albedo mafic unit of Janssen and is itself a floor-fractured crater. There are underlying a higher albedo surface unit [9, 10]. Whitten small exposures of possible mafic material on the floor and Head (2014) used the identification of excavated of Fabricius, visible in the Clementine RGB composite mafic deposits to map the spatial extent of lunar image (Figure 2a). These signatures, if indeed basaltic in cryptomare deposits, focusing primarily on previously nature, could either be excavated from a pre-existing proposed regions of cryptomare. This study did not intrusion including a sill or dike, or they could represent identify any cryptomare deposits in the nearside transported mafic material from the proposed intrusion highland region [1]. beneath Fabricius itself. Crater Buch B. Hawke et al. (2002) [11] used Although the geologic setting and deposit Clementine data to identify a dark-halo crater, Buch B morphology suggest a mafic, magmatic intrusion source (17.0°E, 39.9°S) (Figure 2a), located within the nearside for both Buch B and the superposed crater in Janssen, highlands. They postulated that the dark rays are the the mineralogical identification using Clementine result of excavated mafic deposits from a magmatic multispectral imagery is not conclusive. We are intrusion as opposed to cryptomare because of the expanding our investigation to include the use of M3 isolated nature of the deposit and the surrounding spectral data to better identify the mineralogy of our morphology of the region [11]. Additionally, the FeO previously identified areas of interest. values for the low-albedo halo of Buch B are only Investigation of Gravity Anomalies: Data solutions slightly lower than lunar mare basalt values [11], from the recent GRAIL mission now resolve 46th Lunar and Planetary Science Conference (2015) 1583.pdf gravitational anomalies at a spatial scale of <10 km [15, Meteor. and Planet .Science 34, 25-41. [7] Pieters, C.M., et al. (2009) Curr. 16]. Lunar basaltic magmas are denser than lunar crustal Sci. 96, 500-505 [8] Zuber, M.T., et al. (2013) Science 339, 668-671. [9] material [17], and as such, magmatic intrusions are Schultz, P.H. and Spudis, P. (1979) Proc. Lunar Planet. Sci. Conf. 10th, predicted to produce a positive Bouguer anomaly. Sori et 2899-2918. [10] Hawke, B.R. and Bell, J.F. (1981) Proc. Lunar Planet. Sci. al. (2013) [18] used GRAIL data to investigate Conf. 12th, 665-678. [11]Hawke, B.R., et al. (2002) JGR 107, E12. [12] cryptomare locations, and identified possible cryptomare Pieters, C. M., et al. (1994) Science 266, 1844-1848. [13] Schultz, P.H. locations including the Shiller-Schikard region and the (1976) The Moon 15, 241-273. [14] Pike, R.J. (1980) USGS Prof. Paper Maurolycus region. The Shiller-Schikard region was also 1046-C, C1-C77. [15] Lemoine, F.G., et al. (2014) GRL 41, 3382-3389. identified by Whitten and Head (2014) as a location of [16] Konopliv, A.S., et al. (2014) GRL 41, 1452-1458. [17] Kiefer, W.S., et cryptomare, although no cryptomare were identified in al. (2012) GRL 39, L07201. [18] Sori, M.M., et al. (2013) LPSC XLIV, the Maurolycus region. Abstract #2755. [19] Wieczorek, M. A. et al. (2013) Science 339, 671-675. Using the GRGM900c spherical harmonic Bouguer solution [15] band-filtered to orders 100-600 to strongly attenuate anomaly contributions from regions deeper than 34 km, we examine the Bouguer anomaly for both Buch B and for the floor-fractured crater Janssen. The Bouguer anomaly for crater Buch B is shown in Figure 2b. There is a clear positive anomaly beneath the crater, although there are anomalies of similar magnitude throughout the crust in this region. There are several interesting features in the Bouguer solution for the crater Janssen seen in Figure 3b. A broad positive anomaly Figure 1: Locations of floor-fractured craters [2] plotted on a basemap of underlies the center of the crater Janssen; additionally crustal thickness [19]. Floor-fractured craters are preferentially located in another distinct broad positive anomaly underlies the regions of intermediate (20-30 km) crustal thickness. Mare deposits are located primarily on the lunar nearside and in regions of thinner crust. floor of the crater Fabricius. These anomalies are most readily explained as arising from subcrater magmatic intrusions. This conclusion is strengthened by the surface observation of spectral signatures associated with mafic compositions, although the heterogeneous nature of the gravity solutions precludes identification of magmatic sources by gravity alone. Conclusions: The lunar crust is predicted to host numerous intrusive magmatic features. Despite favorable crustal thickness conditions, there is a paucity of identified intrusive features in the lunar nearside highlands. We use Clementine multispectral image data Figure 2: Crater Buch B (17.0°E, 39.9°S) A) Band-filtered Bouguer gravity and GRAIL data to search for previously unidentified anomaly data showing a high-density anomaly beneath the crater Buch B. B) Clementine RGB composite color ratio image of Buch B showing the magmatic intrusion in the lunar nearside highlands. dark halo and rays surrounding the impact crater Buch B, the halo and rays Beneath our two test regions, the small crater Buch B having characteristics corresponding to a mafic mineralogy. and the large crater Janssen, we see both spectral signatures suggestive of mafic mineralogy and gravity anomalies consistent with a high density crustal component. We interpret this as support for the existence of a magmatic intrusion beneath the floor-fractured craters Janssen and Fabricius and a possible smaller intrusion, such as a dike, beneath the crater Buch B, as suggested by Hawke et al. (2002). We are extending this dual investigation to all 21 regions of interest, and expanding the mineralogical analysis to include M3 data. Figure 3: Crater Janssen (41.5°E, 44.9°S), crater rim marked by dashed line. References: [1] Whitten, J. and Head, J. W. (2014) Icarus 247, A) Band-filtered Bouguer gravity anomaly data showing a broad positive 150-171. [2] Jozwiak, L.M., et al. (2012) JGR 117, E11. [3] Jozwiak, L.M., anomaly beneath the center of the floor-fractured crater Janssen, and also beneath the superposed floor-fractured crater Fabricius. B) Clementine et al. (2015) Icarus 248, 424-447. [4] Head, J.W. and Wilson, L.
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