Nanoscale deformation twinning in xenotime, a new shocked mineral, from the Santa Fe impact structure (New Mexico, USA) Aaron J. Cavosie1,2,3, Pedro E. Montalvo3, Nicholas E. Timms1, and Steven M. Reddy1 1TIGeR (The Institute for Geoscience Research), Department of Applied Geology, Curtin University, Perth, WA 6102, Australia 2NASA Astrobiology Institute, Department of Geoscience, University of Wisconsin–Madison, Madison, Wisconsin 53706, USA 3Department of Geology, University of Puerto Rico–Mayagüez, Mayagüez, Puerto Rico 00681, USA ABSTRACT 105º 53’W 105º 50’W Shock microstructures in refractory accessory minerals such as zircon and monazite A provide crucial evidence for deciphering impact-related deformation in a wide variety of Santa planetary materials. Here we describe the first occurrence of shock deformation in xeno- Fe 14NM10 time, YPO4, from a shocked quartz–bearing shatter cone in granite at the Santa Fe impact 35º 44’N structure (New Mexico, USA). Backscattered electron imaging shows that shocked xenotime H grains near the surface of a shatter cone contain multiple orientations of closely spaced pla- 2 km nar fractures. High-resolution electron backscatter diffraction mapping reveals that some 475 of the planar microstructures in {112} contain deformation twin lamellae that range from 50 nm to 200 nm in width on the polished surface and occur in up to three crystallographic B orientations. Other features attributed to impact, such as planar low-angle boundaries and planar deformation bands, record crystal-plastic deformation. Shatter cone formation and co-existing shocked quartz constrain minimum shock pressure experienced by the xenotime grains to 5–10 GPa. An upper limit of 20 GPa is tentatively assigned based on the absence of YPO4 polymorphs and shock twins in co-existing zircon. We propose that {112} deforma- tion twins in xenotime constitute a diagnostic record of shock metamorphism, similar to {112} twins in zircon; they have not previously been reported in nature and occur in a rock with conspicuous evidence of shock deformation. Documentation of deformation twins in xenotime, a widely applied U-Pb geochronometer, can be used to identify hypervelocity deformation in shocked rocks, detrital grains, and other materials, and may be particularly ideal for recording low-pressure (<20 GPa) impact conditions that do not produce diagnostic 10 cm shock microstructures in zircon. C INTRODUCTION and supracrustal rocks of the southern Sangre Shocked zircon grains have long been rec- de Cristo Mountains, ~8 km northeast of Santa ognized in impact environments (Krogh et al., Fe, New Mexico, USA (Fig. 1A). Confirma- 1984), and discovery of diagnostic impact-gen- tion of an impact origin for the structure was erated microstructures such as {112} twinning based on discovery of well-developed shatter (Moser et al., 2011; Timms et al., 2012a; Erick- cones that contain shocked quartz; the quartz son et al., 2013a, 2013b; Thomson et al., 2014; grains were described as being confined to a Cavosie et al., 2015a) and reidite (e.g., Cavosie narrow zone within 1 mm of shatter cone sur- et al., 2015b; Reddy et al., 2015) has resulted in faces (Fackelman et al., 2008; French and Koe- 50 µm 50 µm its wide application for studying impact processes. berl, 2010). The structure is highly deformed Shock-deformed monazite grains have also been and may be deeply eroded, as shocked bedrock Figure 1. Location and sample photos. A: Loca- described at several terrestrial impact struc- has only been found along the southern mar- tion of shatter cone sample 14NM10 on State tures (Schärer and Deutsch, 1990; Moser, 1997; gin of a 2 × 3 km fault-bound block that con- Highway 475, 8 km northeast of Santa Fe, New Cavosie et al., 2010; Erickson et al., 2013a; Toh- tains shatter cones exposed along New Mexico Mexico, USA (see inset); horizontal rule defines ver et al., 2012; Erickson et al., 2016). Like zir- State Highway 475 (Fig. 1A). Fackelman et al. fault block where shatter cones (filled circles) are reported (after Fackelman et al., 2008). H— con, shocked monazite grains can contain a range (2008) interpreted the shatter cones as a rem- Hyde Memorial State Park. B: Granite shatter of microstructures, including deformation twins, nant of the central uplift of the impact structure, cone from outcrop near where sample 14NM10 that provide diagnostic evidence of impact (Erick- however no other supporting morphological was collected. C: Shocked quartz grains in son et al., 2016). Here we describe twinning and evidence for crater features has been reported. sample 14NM10. Arrows indicate orientation of other deformation microstructures in xenotime, an Estimates of impact age (ca. 1200–300 Ma) decorated planar deformation features. accessory rare-earth phosphate compositionally and crater diameter (6–13 km) thus remain similar to monazite yet isostructural with zircon. poorly constrained (Fackelman et al., 2008). SAMPLE AND ELECTRON The motivation of this study was to investi- BACKSCATTER DIFFRACTION THE SANTA FE IMPACT STRUCTURE gate accessory phases for shock deformation METHODS The Santa Fe structure is located in meta- in order to elucidate additional constraints on A shatter cone in shocked granite, sample morphosed 1.7–1.4 Ga Proterozoic granitoid impact conditions. 14NM10, was collected in 2014 from the same GEOLOGY, October 2016; v. 44; no. 10; p. 803–806 | Data Repository item 2016260 | doi:10.1130/G38179.1 | Published online 23 August 2016 GEOLOGY© 2016 Geological | Volume Society 44 | ofNumber America. 10 For | www.gsapubs.orgpermission to copy, contact [email protected]. 803 outcrop exposure along State Highway 475 described by Fackelman et al. (2008) (Figs. 1A and 1B). The hand sample contains conspicu- AB C ous striations defining shatter cone surfaces (Items DR1–DR3 in the GSA Data Repository1). A thin section cut normal to a shatter cone sur- face is dominated by alkali feldspar, with lesser amounts of quartz, plagioclase, muscovite, bio- tite, chlorite, and opaque oxides. The thin sec- tion contains shocked quartz grains with deco- rated planar deformation features (PDFs) (Fig. 1C) and muscovite with kink bands (Item DR1). Accessory minerals located by backscattered 5 µm 10 µm 10 µm electron (BSE) imaging and identified using c-axis t1 energy dispersive spectroscopy (EDS) include t3 zircon, xenotime, and other phases. No obvious c-axis impact-related microstructures were observed in t2 zircon (Item DR2). In contrast, five xenotime t1 t1 c grains with planar microstructures were found -a x is as inclusions in alkali feldspar and muscovite. twin The xenotime grains are small, 28–50 µm across, 200 nm located <2 mm from the shatter cone surface twin LAB LAB SCS LAB SCS SCS (Item DR2), and are the focus of this study. PDB 0˚ 5˚ 0˚ 6˚ Three representative xenotime grains (labeled 0˚ 5˚ 3, 4, and 5) were mapped by electron backscatter Figure 2. Backscattered electron (BSE) images (top) and orientation maps (bottom) of the three diffraction (EBSD) with a 50 nm step size, using shocked xenotime grains. A: Grain 3, with three orientations of planar fractures, indicated by conditions similar to those for zircon (Item DR3; arrows, and low-angle boundaries (LABs). B: Grain 4, with one orientation of planar fractures, Table DR1; Reddy et al., 2007). Xenotime and LABs, and single twin lamella (t1, inset). Faint oscillatory zoning is visible in BSE. C: Grain 5, zircon are isostructural [tetragonal, 4/m 2/m 2/m, with three orientations of planar fractures. Three sets of twin lamellae (t1–t3) are present, as are LABs and planar deformation bands (PDBs). Maps are colored for misorientation relative space group I41/amd and therefore yield simi- to reference orientation (red cross). SCS—shatter cone surface. lar EBSD patterns that are difficult for EBSD mapping systems to distinguish. For this reason, EDS maps were collected simultaneously dur- ing EBSD analysis to identify xenotime (Item DR4). EBSD patterns from the xenotime were of high quality and were indexed using a xeno- A B step size = 50 nm time match unit based on crystallographic data B for YPO4 from Milligan et al. (1982), yielding average mean angular deviation values from twin 1 0.32° to 0.34°. twin 3 200 nm RESULTS Xenotime Microstructures Several types of microstructures were observed in EBSD data for the three analyzed (001) (010) xenotime grains, including planar fractures with- twin boundary twin 2 2 µm out associated lattice misorientation, discrete (110) 10 µm low-angle (<5°) boundaries, planar deformation C (001) {110} {112} t2 bands (PDBs), and broad areas of lattice mis- t2 t1 orientation (Fig. 2C); each grain displays 5°–6° of cumulative misorientation accommodated by t3 t1 t3 these features (Fig. 2). Twin lamellae, found in t2 grains 4 and 5 (Figs. 2 and 3), are discussed t1 separately below. host t3 1 GSA Data Repository item 2016260, Item DR1 n=438,761 (field and sample images), Item DR2 (SEM images of 65°<110> aligned {112} zircon and xenotime), Item DR3 (sample location and analytical details), Item DR4 (additional EBSD maps Figure 3. Shock-twinned xenotime (grain 5). A: Orientation map using inverse pole figure and figures), and Table DR1 (analytical conditions), is color scheme (IPF-Y, where Y is vertical on the map). B: Closeup of inset in A showing three available online at www.geosociety.org/pubs/ft2016 orientations of twin lamellae. C: Pole figures showing host-twin relations. Stereonets are .htm, or on request from [email protected]. equal area, lower hemisphere projections. t1, t2, t3—twin set 1, 2, 3. 804 www.gsapubs.org | Volume 44 | Number 10 | GEOLOGY Planar fractures, visible in BSE images and DISCUSSION located further away (~1.5 mm), tentatively EBSD maps of all three grains, form closely supporting the suggestion by Fackelman et al. spaced (~0.25–1 µm) sets of parallel features Xenotime Deformation (2008) that higher pressure shock deformation that extend nearly across the grains and occur occurs near shatter cone surfaces.