Large Meteorite Impacts VI 2019 (LPI Contrib. No. 2136) 5035.pdf

IMPACT COESITE: FORMATION AND SURVIVAL. L. Folco1, F. Campanale1,2, B. P. Glass3, E. Mugnaio- li2, M. Masotta1, M. Lee4 and M. Gemmi2, 1Dipartimento di Scienze della Terra, Università di Pisa, V. S. Maria 53, I-56126, Pisa, Italy ([email protected]; [email protected], [email protected]), 2Center for Nanotechnology Innovation@NEST, Istituto Italiano di Tecnologia (IIT), Piazza San Silvestro 12, 56127 Pisa, Italy ([email protected]; [email protected]), 3Department of Geosciences, University of Delaware, Newark, Delaware 19716, USA ([email protected]), 4Department of Geographical and Earth Sciences, University of Glas- gow, Glasgow G12 8QQ, UK ([email protected]).

Introduction: This is an overview of the results of Methods: Samples were studied using a combina- our ongoing research (in part published, [1], in part tion of FE-SEM-EDS microscopy, EDX mapping, under review, [2] and [3]) aiming at better understand- µRaman spectroscopy, EPMA, FIB, TEM and Electron ing of the formation and survival of impact coesite - a Diffraction Tomography (EDT) and crystal orientation debated issue in impact cratering and shock metamor- mapping (PACOM) analyses. phism studies. Impact coesite occurs in the form of Results: Coesite in Kamil sandstone. The studied nanometer-sized grains with polysynthetic twinning on sandstone ejecta is a fragment of a medium-grained (010) grains, typically embedded in silica glass. Its quartzarenite dominated by heavily shocked, equigran- presence in rocks that experienced shock conditions ular quartz grains with an average grain size of 1 mm beyond the stability field is an intriguing and contro- (~78 vol%) and including accessory tourmaline and versial issue. Models, widely accepted since its discov- zircon. Intergranular veins and pockets (up to 1 mm ery in 1960 [4], predict that coesite forms during crys- across) of silica glass contain microcrystalline coesite. tallization from highly densified silica melts [5], [6], These domains are known in the literature as symplec- [7] or from diaplectic glass [8] during shock unloading, tic regions, first described in the Coconino Sandstones when the decompression path intersects the coesite from the Barringer Crater, USA [12]. stability field (pressure 3–10 GPa, temperature <3000 Orientations and frequency of PDF in shocked K). In contrast to these mechanisms, we show miner- quartz are {10-13}, 23%, and {10-12}, 14% [6]; the alogical and petrographic evidence of subsolidus direct amount of silica glass is ~22 vol%. Intergranular quartz-to-coesite transformation in quartzose impac- symplectic regions show microstructural zoning. From tites from different geological contexts, including a the core of the quartz crystals to the core of the plausible mechanism for this polymorphic transfor- symplectic regions, we can distinguish a "quartz zone", mation. These results have implications on the recon- a "coesite zone" and a "silica glass" zone. The quartz struction of the P-T-t paths experienced by target rocks zone consists of PDF-bearing shocked quartz. The and on the definition of impact scenarios. coesite zone, up to several tens of µm in thickness, Samples: Our investigation focused on a shocked typically consists of polycrystalline aggregates of mi- sandstone from Kamil Crater (Egypt), microscopic cro-to-nanocrystals (<5 µm) coesite set in pure silica ejecta particles of silica composition from the Austral- glass, i.e. lechatelierite. Coesite shows fine polysyn- asian microtektite layer and a Muong Nong-type Aus- thetic (010) twinning. Polycrystalline aggregates, ar- tralasian tektite. All samples lack evidence for post- ranged along planes that are parallel (or nearly so) to shock thermal overprint and alteration due to hydro- PDFs of the quartz crystals in the adjacent quartz zone, thermal activity. consists of fine coesite plus quartz intergrowths, caus- The shocked sandstone sample from Kamil Crater ing tartan-like microtextures. Flame-like corrosion is a fist-sized ejecta. It was collected ~350 m from the textures characterize the margins of the coesite aggre- crater rim in the main downrange ejecta ray during the gates. The silica glass zone consists of homogeneous 2010 Italian/Egyptian geophysical expedition as re- lechatelierite with usually one central bubble up to ported by [9]. several tens of µm across. The microscopic ejecta particles (n = 4; 200-700 Coesite in ejecta particles from the Australasian µm size) from Australasian microtektite layers are microtektite layer. The particles are dominated by from two deep-sea sediment cores in the South China coesite regions locally intergrown with relic PDF- Sea: core 37 from ODP Hole 1144A and core SO95- bearing quartz microstructures. Up to 4 sets of PDF 17957-2, less than 2000 km away from a proposed were observed with the most frequent sets belonging to impact location in Indochina [10]. the {10-13} and {10-12} families. Coesite regions The Muong Nong tektite was found in the general consist of polycrystalline aggregates of nm-sized, area of Muong Phin, Laos (16°32’N, 106° 01’ E) as (010) polysynthetically twinned grains with rounded or reported by [11]. elongated habit. Coesite and quartz are in direct con- Large Meteorite Impacts VI 2019 (LPI Contrib. No. 2136) 5035.pdf

tact with no detectable amorphous or ‘glassy’ volume (010) planes of coesite, corresponding to the direction in between. Quartz boundaries are always lobate or of disorder and twinning in impact coesite. saw-tooth-like, with euhedral coesite crystals penetrat- Muong Nong tektite [3]. The lack of detailed na- ing through the quartz boundaries, and partially erasing noscale investigation of the silica phases in the coesite the PDF microstructures. 3D ED and crystal orienta- bearing inclusion cores precludes a definite conclusion tion mapping (PACOM) show a recurrent pseudo iso- of the origin of coesite. However the observed petro- orientation between the (1-11) vector in quartz and the graphic similarities with the symplectic regions in (010) vector of neighboring coesite crystals. Kamil sandstones hints for a similar origin in a quartz- Coesite in Muong Nong tektite. Coesite occurs in rich domain of the tektite precursor rock. We propose opaque, vesicular inclusions up to several 100s of µm that the survival of coesite was possible due to the across embedded in and elongated along the composi- froth layer that acted as a heat sink during bubble ex- tional banding typical of Muong Nong-type tektites. pansion and then as a thermal insulator. Finally, the The inclusions usually consist of a core, surrounded by distribution and textural relationships between the froth layer, and quartz neoblast layer. The cores are coesite-bearing inclusions and the tektite matrix point composed of a mixture of silica glass, coesite, and to an in situ formation of the coesite due to an impact, quartz in varying proportions. Overall, many of the rather than to an infall into tektite melt during the aeri- inclusion cores have petrographic features similar to al burst of a bolide [14]. the symplectic regions observed in the shocked sand- Conclusions: i) Impact coesite forms locally, stones from Kamil. through a direct subsolidus transformation from quartz Discussion: Kamil [1]. Coesite forms locally, in as a result of shock reverberation at medium disconti- symplectic regions. These are interpreted as inter- nuities (pore spaces, grain boundaries). ii) The mech- granular pore space in origin. The quartz-coesite inter- anism of the direct quartz-coesite transformation is growths with tartan-like textures in the "coesite zone" likely a solid-state martensitic-like process involving a suggest that coesite grew at the expenses of PDF bear- relative structural shift of {-1011} quartz planes, which ing quartz. PDF orientation in the quartz zone indicate turns into coesite (010) twin planes. iii) We emphasize shock pressures of 20–25 GPa according to [13]. Thus the role of shock reverberations at medium discontinui- coesite growths during the decompression-pressure ties (e.g., pores, grain boundaries, etc.) in producing amplification path associated with the collapse of localized PTt gradients, eventually resulting in the co- pores, as predicted by numerical models in the litera- occurrence of shock metamorphic features indicating ture [14]. The corrosion texture of coesite observed in variable shock levels at the microscopic scale. iv) The the coesite zone indicate subsequent melting of the widespread occurrence of coesite in impact ejecta of pre-existing crystalline silica phases, as a result of the the Australasian tektite-mcicrotektite strewnfield, and temperature increase associated with the friction in particular its in situ formation in tektites lends sup- caused by the collapse of the pores. port to the impact-cratering origin of Australasian tek- Ejecta particles [2]. The features of the contact be- tites, and not to the airburst scenario invoked to ex- tween coesite and PDF-bearing quartz indicate direct plain the missing crater [15]. transformation of quartz into coesite, after PDF for- References: [1] Folco L. et al. (2018) Geology, 46, mation. The frequency and orientation of PDF indicate 739–742. [2] Campanale F. et al. (2019) Geochim. pressure of 20–25 GPa according to [13]. Similarly to Cosmochim. Acta (under review). [3] Glass B. P. et al. what observed and proposed for Kamil sandstones, we (2019), Meteoritics & Planet. Sci. (under review). [4] suggest that coesite formed as result of shock wave Chao E. et al. (1960) Science, 132, 220–222. [5] reverberation at medium discontinuities (grain bounda- Langenhorts F. 2003, [6] Chen M. et al. (2010) Earth ries, fractures, dislocations, inclusions?), followed by a & Planet. Sci. Lett., 297, 306–314,. [7] Fazio et al. rapid pressure and temperature drop of P and T, with- (2017) Meteoritics & Planet. Sci., 52, 1437–1448. [8] out entering the liquid stability field of the silica sys- Stähle V. et al. (2018) Contrib. Mineral. & Petrol., tem. 3D EDT analysis of neighboring crystals of quartz 155, 457–472. [9] Fazio et al. (2014). [10] Glass B. and coesite shows that the [010]* reciprocal vector of and Fries M. (2008) Meteoritics & Planet. Sci., 43, coesite is often sub-parallel to the [-101]* reciprocal 1487–1496. [11] Schnetzler C. C. and McHone J. F. vector of quartz. We thus propose that quartz structure (1996) Meteoritics & Planetary Science, 31, 73–76. can transform into coesite by a martensitic-like trans- [12] Kieffer S. W. et al. (1976) Contrib. Mineral. & formation. This involves consecutive shift along (-101) Petrol., 59, 41–93,. [13] Stöffler D. and Langenhorst F. planes, which triggers a rearrangement of Si-tetrahedra (1994) Meteoritics, 29, 155–181. [14] Güldemeister N. chains. The (-101) planes of quartz turn then into the et al. (2013) Meteoritics & Planet. Sci., 48, 115–133. [15] Wasson J. T. (2003) Astrobiology 3, 163–179.