Meteoritics & Planetary Science 53, Nr 1, 75–92 (2018) doi: 10.1111/maps.13011
Investigating the response of biotite to impact metamorphism: Examples from the Steen River impact structure, Canada E. L. WALTON
1,2,*
, T. G. SHARP3, J. HU
3
, and O. TSCHAUNER
4
1
2
Department of Physical Sciences, MacEwan University, Edmonton, Alberta T5J 4S2, Canada Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta T6G 2E3, Canada 3 Arizona State University, Tempe, Arizona 85287–1404, USA 4 Department of Geoscience, University of Nevada, Las Vegas, Nevada 89154–4002, USA * Corresponding author. E-mail:
[email protected];
[email protected] (Received 21 February 2017; revision accepted 19 October 2017)
Abstract–Impact metamorphic effects from quartz and feldspar and to a lesser extent olivine and pyroxene have been studied in detail. Comparatively, studies documenting shock effects in other minerals, such as double chain inosilicates, phyllosilicates, carbonates, and sulfates, are lacking. In this study, we investigate impact metamorphism recorded in crystalline basement rocks from the Steen River impact structure (SRIS), a 25 km diameter complex crater in NW Alberta, Canada. An array of advanced analytical techniques was used to characterize the breakdown of biotite in two distinct settings: along the margins of localized regions of shock melting and within granitic target rocks entrained as clasts in a breccia. In response to elevated temperature gradients along shock vein margins, biotite transformed at high pressure to an almandine-Ca/Fe majorite-rich garnet with a density of 4.2 g cm3. The shock-produced garnets are poikilitic, with oxide and silicate glass inclusions. Areas interstitial to garnets are vesiculated, in support of models for the formation of shock veins via oscillatory slip, with deformation continuing during pressure release. Biotite within granitic clasts entrained within the hot breccia matrix thermally decomposed at ambient pressure to produce a fine-grained mineral assemblage of orthopyroxene + sanidine + titanomagnetite. These minerals are aligned to the (001) cleavage plane of the original crystal. In this and previous work, the transformation of an inosilicate (pargasite) and a phyllosilicate (biotite) to form garnet, an easily identifiable, robust mineral, has been documented. We contend that in deeply eroded astroblemes, high-pressure minerals that form within or in the environs of shock veins may serve as one of the possibly few surviving indicators of impact metamorphism.
conditions experienced by a rock in response to hypervelocity impact. To a lesser extent, the response of olivine and pyroxene to shock has been investigated, driven largely by the presence of these two minerals in ordinary chondrites, in an effort to deconvolve parent body impact processing (e.g., Reimold and St€ offler 1978; Bauer 1979; St€ offler et al. 1991; Rubin et al. 1997). In contrast, little is known about how shock is manifested in other minerals such as double chain inosilicates, phyllosilicates, carbonates, and sulfates (Ferriere and Osinski 2013). Shock recovery experiments have been conducted to assess the response of mica minerals as a function of increasing shock
INTRODUCTION Shock metamorphic effects recorded in quartz and feldspar have been studied in great detail (e.g., French and Short 1968; Engelhardt and Bertsch 1969; St€ offler and Langenhorst 1994; Grieve et al. 1996; Langenhorst 2002; Ferriere and Osinski 2013; and many others), attributed largely to the abundance of these minerals in crustal rocks, their overall stability, and that petrographically observable deformation and transformation features occur over a range of shock pressures. These factors attest to the utility of these minerals for constraining the pressure–temperature
75
© The Meteoritical Society, 2017.
76
E. L. Walton et al.
Fig. 1. Simplified log of core ST003 showing the various breccia units, as logged by Walton et al. (2017), as well as the sampled locations for this study. Photographs on the right-hand side show the cm-sized clasts of granitic basement rocks entrained within breccia (sampled at depths of 285.6 and 324.5 m) and shock veins from the bottom 11 m of the core, which penetrated the side of the central uplift (378.5 m). Biotite in the cm-sized granitic clasts and along shock vein margins is the subject of this study. (Color figure can be viewed at wileyonlinelibrary.com.)
pressure (Lambert and Mackinnon 1984; Sazonova et al. 2005; Ogilvie et al. 2011), yet comparative investigation of naturally shocked biotite is lacking. In this study, we investigate shock metamorphic effects manifest in crystalline basement rocks from the Steen River impact structure (SRIS), a 25 km diameter buried complex crater in NW Alberta, Canada (Grieve 2006). Three continuous but shallow diamond drill holes sampled the crater fill deposits of the SRIS. One hole (ST003), with a total length of 381 m, encountered 164 m of polymict breccia containing clasts of cogenetic impact melt (Fig. 1). The bottom 11 m of the drill core intersected crystalline basement rock, which represents parautochthonous rocks of the flank of the central uplift. Crystalline basement rocks comprise plagioclase– orthoclase–quartz–pargasite–biotite gneiss crosscut by
an anastomosing network of black glassy veins, as described by Walton et al. (2016). Based on these lines of evidence—displacement of igneous minerals along vein margins, the direct correlation between vein margin and phase transformation (feldspar and quartz to diaplectic glasses; solid-state transformation of pargasite to majoritic [high-pressure] garnet), and crystallization of majoritic garnet within the vein interior—Walton et al. (2016) concluded that these veins of quenched melt formed and evolved during the shock compression regime. Therefore, in this manuscript, we will refer to them as “shock veins” to distinguish them from frictionformed pseudotachylite, and the material in the veins as “quench-crystallized shock melt” owing to the crystallization of 1–2 lm sized garnet grains with interstitial silicate glass.
Impact metamorphism of natural biotite
77
Fig. 2. Transmitted light image of a section of a shock vein cutting across the granitic host rock. Amphibole (strongly pleochroic in shades of green; ferro-pargasite) along shock vein margins has been transformed to an almandine–andradite–majorite garnet (bright orange), as documented by Walton et al. (2016). Biotite in the host rock is strongly pleochroic brown-yellow and has broken down to a fine-grained opaque assemblage along the shock vein margin (Btbd = biotite breakdown material). Abbreviations are as follows: Ab = albite; Bt = biotite; Gt = garnet; Prg = pargasite; SV = shock vein; Alt-Gl = altered glass. (Color figure can be viewed at wileyonlinelibrary.com.)
An array of advanced analytical techniques was used to characterize the response of biotite to shock in both cmsized clasts of granitic target rocks in the melt-bearing polymict breccia, and along the margins of sections of shock melt (now quench-crystallized). Thus, we are able to deconvolve deformation and transformation of biotite as a result of direct interaction with the shock wave (compression and rarefaction) from postshock thermal effects. This study documents the novel shock-induced transformation of biotite to an almandine–majorite– skiagite-rich garnet in response to high-pressure and hightemperature conditions along shock vein margins. We note here that the effects described should technically be attributed to impact metamorphism, a term encompassing melting, decomposition, and vaporization of target rocks, in addition to shock metamorphic effects (St€ offler and Grieve 2007), and hereafter we will use the term impact metamorphism, instead of the commonly used term “shock metamorphism.” Although volumetrically minor compared to quartz and feldspar, micas are an important component of crustal rocks. In this study, we make the first steps toward bridging the gap in knowledge between the diversity of shock effects in naturally shocked
tectosilicates and those of phyllosilicates, to shed light on shock conditions. SAMPLES AND METHODS Three polished thin sections of shock-veined crystalline basement rock, sampled at a depth of 378.5 m, were prepared for this study (Fig. 1). Centimeter-sized clasts of granitic target rock entrained within the polymict breccia were sampled at two depth intervals—324.5 m and 285.6 m, from which two thin sections spanning the clast-breccia contact were made (Fig. 1). The thin sections were initially investigated using standard optical microscopy with a petrographic microscope to identify areas of interest for follow-up advanced analytical characterization (Fig. 2). These areas of interest included biotite in nonveined regions of the host rock and where this mineral is in direct contact with now quench-crystallized shock melt in the 378.5 m sample, and opaque fine-grained pseudomorphs after biotite in granitic clasts (324.5 m and 285.6 m depth). Electron images were acquired using a ZEISS Sigma 300 field emission scanning electron microscope
78
E. L. Walton et al.
Fig. 3. Backscattered electron (BSE) image and X-ray elemental maps showing the distribution of Fe, Al, and K. The area imaged and mapped corresponds to biotite in contact with a shock vein (oriented NW–SE). Compared to biotite, the breakdown products are enriched in Al and Fe and depleted in K. Abbreviations are as follows: Ab = albite; Ap = apatite; Ilm = ilmenite; Bt = biotite; SV = shock vein matrix; Btbd = biotite breakdown material. The white box approximates the location of a higher magnification BSE image (Fig. 4), characterizing the microtexture of the biotite/shock vein margin. (Color figure can be viewed at wileyonlinelibrary.com.)
(FESEM) at the University of Alberta (UAb) in backscattered mode with a 20 kV accelerating voltage. This SEM is equipped with a Bruker energy-dispersive X-ray spectrometry (EDX) system with dual silicon drift detectors, each with an area of 60 mm2 and an energy resolution of 123 eV. The EDX system was used to aid in mineral identification and to obtain semiquantitative information on mineral composition. Commercial image analysis software, the freeware program ImageJ, was used to measure apparent dimensions of thin section features such as grain size and vein width, as well as to estimate modal abundances from acquired backscattered electron (BSE) images. Major and minor elemental abundances were quantified using a JEOL 8900R electron microprobe (EMP) at the UAb, equipped with
five wavelength dispersive spectrometers (WDS) using an accelerating potential of 15 kV and a 10 nA beam current. Analyses were acquired using a 1 lm diameter focused electron beam. The following natural and synthetic materials were used as standards: garnet (Si), rutile (Ti), spinel (Al), spessartine (Mn), fayalite (Fe), pyrope (Mg), diopside (Ca), albite (Na), sanidine (K), and scapolite (Cl). Detection limits at the 99% confidence limit for analyzed elements are: 0.01 wt% (Si, Al, K, Mg, Ca), 0.02 wt% (Mn, Fe, Ti, Na), and 0.03 wt% (Cl). For major elements, the precision was calculated to range from 0.2 to 0.8 wt% accuracy, up to 0.2 wt% for minor elements, with a 1r precision for major and minor elements ranging from 0.01 to 0.13 wt%. Vanadium, nickel, and zinc were also analyzed,
Impact metamorphism of natural biotite
standardized using V-metal, taenite, and gahnite, respectively, but were below the detection limit of the EMP (
