Lunar and Planetary Science XLVIII (2017) 1059.pdf

SILICON XANES ASSESSMENT OF THE SILICONE OIL CONTENT OF GEMS IN IDPS . G. J. Flynn 1, S. Wirick 2, A. L. Butterworth 3, Z. Gainsforth3, A. J. Westphal 3, B. Kaulich 4, T. Araki 4, M. Abyaneh 4, and T. Ty- liszczak 5. 1Dept. of Physics, SUNY-Plattsburgh, Plattsburgh, NY 12901 USA ( [email protected] ), 2Focused Beam Enterprises, Westhampton, NY 11977, 3Space Sciences Laboratory, University of California, Berke- ley, CA 94720, 4Diamond Light Source, Didcot, Oxfordshire OX11 0DE, UK, 5Advanced Light Source, 6 Cyclotron Rd., Berkeley, CA 94720

Introduction: Glass with Embedded Metal and copy, and Si x-ray absorption near-edge structure Sulfides (GEMS) is a major component in many chon- (XANES) spectroscopy provides a technique to assess dritic porous interplanetary dust particles (CP IDPs). the silicone oil content of GEMS. Barranco et al. [8] Based on the similarity of GEMS to the properties of performed Si-XANES on SiO xCyHz polymeric thin interstellar silicates, inferred from astronomical obser- films having compositions with different C/O atomic vations, Bradley [1] proposed that GEMS are surviv- ratios. They also measured Si-XANES spectra of Si- ing interstellar silicates. The GEMS in CP IDPs have carbide and SiO 2 and found that the energy of the onset been extensively studied over the past two decades. of the Si K absorption edge varied with the composi- Synchrotron-based infrared spectroscopy showed a tion from 1840 eV in SiC to 1844 eV in SiO 2. Because good match between the 10 micron Si-O absorption the Si-C and Si-O bonds absorb at significantly differ- feature of GEMS in CP IDPs and the silicates in two ent energies, Si-XANES is a particularly effective molecular clouds as well as in the regions surrounding technique to assess the ratio of Si-C to Si-O bonding at a T-Tauri star and a post-main sequence M star [2]. the nanoscale. Oxygen isotopic measurements by Messenger et al. [3] We performed Si-XANES on silicone oil, which is established that 4 of 177 GEMS grains they analyzed dominated by Si-C bonds, and amorphous silicate (sili- exhibited isotopic anomalies inconsistent with Solar ca aerogel), which has only Si-O bonding (Figure 1) to Nebula material and consistent with origins from AGB determine if Si-XANES could be used to determine the stars, supernova, and low metallicity stars. However, silicone oil content of GEMS. We found about a 5 eV later work by Keller and Messenger [4] indicated that shift between the strongest Si K-edge absorption peak most GEMS are condensates from our own Solar Ne- of silicone oil and the absorption feature of Si-O in the bula, with element/Si ratios too low to match the inters- aerogel, consistent with the shift reported by Barranco tellar grain composition. et al. [8], with absorption peaks at 1842 eV for silicone The incomplete removal of silicone oil, the collec- oil and 1847 eV for amorphous silicate. tion medium generally used for the Johnson Space The Scanning X-ray Microscope (SXM) operating Center cosmic dust collection effort, by the hexane on Beamline I08 of the Diamond synchrotron has an incident x-ray beam tunable from 250 to 4,400 eV, wash used to clean IDPs has long been recognized as a covering the K- or L-lines of most major and minor problem, and results in most IDP elemental analyses elements in extraterrestrial material. We used this SXM being reported as element/Fe ratios rather than ele- to perform x-ray fluorescence (XRF) and XANES ment/Si ratios. Bradley [5] suggested that about 30% of absorption spectroscopy, with a spatial resolution of Si in GEMS might be from silicone oil and, and once ~200 nm, smaller than the GEMS clusters in CP IDPs. the silicone oil contamination is accounted for, GEMS would match the interstellar silicate abundance pattern. Figure 1: Comparison of the Si-XANES spectra of Keller et al. [6] used infrared spectroscopy to assess an amorphous silicate, with the onset of the K-edge the silicone oil contamination of GEMS, based on the 8 at 1844.5 eV, like the GEMS, to that of silicone oil, and 12.5 micron silicone oil absorption features being with an onset of the K-edge at 1440 eV, consistent below the detection limit. However, this effort is ham- with the results of Barranco et al. [8] for Si-C vs. Si- pered by the relatively large footprint of the infrared O bonding. analysis beamspot, which is limited by diffraction to ~6 Solid -- silicone oil microns for the 12.5 micron silicone oil feature, signif- Dashed -- amorphous icantly larger than the typical 500 nm GEMS grain. silicate (silica aerogel) More recently, Messenger et al. [7] examined GEMS in CP IDPs collected without the use of silicone oil, and found similarly high element to Si ratios in two GEMS grains, consistent with their earlier results. Method: X-ray microscopy is better matched to the size of the GEMS grains than is infrared spectros- Lunar and Planetary Science XLVIII (2017) 1059.pdf

Area 1 Si-C Si-O

GEMS

Area 2

Area 3

Area 4

Area 4a

Area 5 Area 6

Area 7

Area 8 Area 9

GEMS GEMS Area

Other Side of GEMS Area

Figure 3: Si-XANES absorption spectra of various Si-rich regions of a CP IDP. The bottom two spec- tra are the left and the right areas of the GEMS grain identified by TEM.

Figure 2: (Top) TEM image of L2071 Clu 17,3 Grid energy than the peaks in the crystalline silicate grains A2 S2 with the arrow marking the GEMS grain and (Figure 3), but neither of the two GEMS spectra show the box indicating the area included in the SXM any detectable silicone oil absorption feature near 1842 image. (Bottom) SXM absorption image of the same eV, demonstrating that the silicone oil to silicate ratio section, with 200 nm resolution, showing the areas is substantially less than 30% in this GEMS grain. from which the Si-XANES spectra were extracted. Conclusions : Si K-edge XANES provides the ca- pability to assess silicone oil contamination at the size Results: We performed Si-XANES stack imaging scale of GEMS, and the Si-XANES measurements of an ultramicrotome section of a CP IDP from the could be done on the same TEM sections on which L2071 collector. This section was previously studied element/Si ratios were determined. The silicone oil by TEM, and an ~400x500 nm GEMS grain, along contamination of the GEMS grain in L2071 Clu 17,3 is with other silicate phases, were identified (Figure 2). well below the amount required to significantly alter We extracted Si-XANES absorption spectra from the element/Si ratios in this grain. ten individual silicate areas identified in this ultrami- References: [1] Bradley, J. P. (1994) Science , 265 , crotome section as well as the GEMS area (see Figure 925-929. [2] Bradley, J. P. et al., (1999) Science, 285 , 2). The spectrum of each of the ten non-GEMS grains 1716-1718. [3] Messenger, S. M. et al., (2010) MAPS shows a strong absorption near 1846 eV, consistent Supplement, 45 , id.5384. [4] Keller, L. P. and Mes- with the presence of Si-O bonding, either crystalline or senger, S.M. (2011) Geochim. Cosmochim. Acta , 75 , amorphous silicate. The GEMS region showed two 5336-5365. [5] Bradley, J. P. (2013) Geochim. Cosmo- slightly different spectra, consistent with earlier obser- chim. Acta , 107 , 336-340. [6] Keller, L. P. and Mes- vations by Keller and Messenger [6] that an ~500 nm senger, S.M. (2012) Geochim. Cosmochim. Acta , 107 , GEMS grain is actually an aggregate of several small- 341-344. [7] Messenger, S. M. et al. (2015) Meteorit. er, chemically distinct, GEMS grains. The absorption Planet. Sci., 50 , 1468-1485. [8] Barranco, A. et al. peak in the GEMS spectra are shifted to slightly higher (2006) J. App. Phys., 100 , 033706.