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Atlas of Alteration Textures in Volcanic Glass from the Ocean Basins

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Atlas of Alteration Textures in Volcanic Glass from the Ocean Basins Atlas of alteration textures in volcanic glass from the ocean basins Martin Fisk1 and Nicola McLoughlin2 1College of Earth, Ocean and Atmospheric Sciences, Oregon State University, Corvallis, Oregon 97330, USA 2Department of Earth Sciences, Centre for Geobiology, University of Bergen, 5020 Bergen, Norway ABSTRACT sized etch pits and tunnels that are located at and fractures in the glass. These minerals are the interface of fresh glass and its alteration indicative of conditions in the seafl oor and may We provide a comprehensive photo- products. This petrographic atlas aims to bring be useful for correlating conditions of alteration graphic atlas of the intricate alteration fea- together and illustrate the full spectrum of alter- with glass alteration features; however, this is tures found in glass in igneous rocks from the ation textures in marine lavas to show the variety the subject of an ongoing study. If the alteration ocean basins. The samples come from sur- of alteration textures found in vol canic glass and textures are biotic and if specifi c textures can be face and subsurface rocks from oceanic rises hyaloclastites collected from the ocean crust. In correlated with subsurface conditions, then they and seamounts of the ocean basins and some particular we focus on the size, morphology, could help researchers understand the evolution marginal seas. These textures have previ- distribution, and infi lling of granular cavities of the marine subsurface environment from the ously been termed “bioalteration textures” and tubular tunnels. Archean to the present. by those who consider them as potentially A selection of annotated petrographic images biogenic in origin, or as “etch pits” by those from a collection of 119 samples spanning the Previous Work who prefer a non-biogenic interpretation. world’s ocean basins is provided to systemati- Here, transmitted-light color photomicro- cally illustrate the key textural characteristics Granular and tubular alteration textures of graphs are provided to illustrate the range of of glass alteration. A guide and glossary to the oceanic volcanic glass have been illustrated in granular and tubular textures as well as their principal features is provided and an accompa- transmitted-light photomicrographs since the relation to fractures, minerals, vesicles, and nying classifi cation scheme is given to iden- 1960s (Morgenstein, 1969). In that fi rst study, multiple episodes of alteration in the same tify the key morphotypes of glass alteration. glass/palagonite alteration boundaries and linear sample. The tubular forms are described This expands on earlier classifi cation schemes features in black-and-white photographs were using seven morphological characteristics: (Furnes and Staudigel, 1999; Josef, 2006; described as “micro-channels” and “hair chan- (1) length and width; (2) density; (3) curva- Staudigel et al., 2006, 2008; McLoughlin et al., nels.” More recently, transmitted-light photo- ture; (4) roughness; (5) variations in width; 2009) and identifi es several previously unrec- micro graphs of alteration features in seafl oor (6) branching; and (7) tunnel contents. The ognized morphotypes. and sub-seafl oor basalt glass have been pub- photomicrographs are a starting point for The atlas is intended as an illustrated guide lished by a number of authors (e.g., Giovannoni understanding the factors that control the for geologists, microbiologists, and astrobiolo- et al., 1996; Fisk et al., 1998a, 2006; Furnes and formation of the alteration textures, for eval- gists studying glass alteration. We realize that as Staudigel, 1999; Fisk and Giovannoni, 1999; uating the biogenicity of the various forms, researchers further explore their collections and Christie et al., 2001; Furnes et al., 2001a, 2002; for inferring subsurface conditions during as more deep-sea environments are sampled, Banerjee and Muehlenbachs, 2003; Storrie- alteration, and for making comparisons to new forms of glass alteration will be found Lombardi and Fisk, 2004; Ivarsson et al., 2008; similar textures in ancient ophiolites, some of and documented; thus, this guide represents the Staudigel et al., 2008; Cockell and Herrera, which have been attributed to the earliest life current state of knowledge. Some alteration 2008; McLoughlin et al., 2009, 2010; Heber- on Earth. textures have previously been argued to repre- ling et al., 2010). These studies have, in general, sent biological alteration products and trace included a limited number of images to illustrate INTRODUCTION AND fossils (e.g., Fisk et al., 1998a; Torsvik et al., the granular or tubular structures, and they have PREVIOUS WORK 1998; Furnes et al., 2001a, 2001b, 2002, 2008; not documented the full range of alteration tex- Furnes and Muehlenbachs, 2003; Banerjee and tures now known from oceanic igneous glass. Aims and Scope of This Atlas Muehlenbachs, 2003; Thorseth et al., 2003; An extensive unpublished collection of photo- McLoughlin et al., 2009; Staudigel et al., 2008); micrographs also exists (Josef, 2006). The interaction of sub-seafl oor volcanic glass however, this study is not designed to support or Over this more recent period (1996 to the pres- with circulating fl uids produces secondary min- refute claims of biogenicity of the alteration of ent), alteration features in oceanic basalt glass erals as well as alteration textures that penetrate basaltic glass. Also, this work does not investi- have also been illustrated in backscattered elec- into the glass (e.g., Thorseth et al., 1995; Fisk gate the secondary mineralogy of altered glass, tron images, transmission electron images, and et al., 1998a; Alt and Mata, 2000; Furnes et al., referred to as palagonite, a mixture of iron oxy- energy-dispersive X-ray spectroscopy (EDS) 2001a; Josef, 2006). These alteration textures hydroxides and phyllosilicates (Stronick and maps (e.g., Furnes et al., 1996, 1999; Torsvik are found in basalts from the fl anks of ocean Schmincke, 2002), and we have not character- et al., 1998; Alt and Mata, 2000; Thorseth et al., rifts, seamounts, back-arc basins, and marginal ized the secondary minerals, such as carbonates, 2003; Kruber et al., 2008; Cockell et al., 2009). seas. The alteration textures include micron- zeolites, and phyllosilicates, that occur in voids Also, similar features have been documented Geosphere; April 2013; v. 9; no. 2; p. 317–341; doi:10.1130/GES00827.1; 31 fi gures; 2 tables. Received 26 May 2012 ♦ Revision received 17 November 2012 ♦ Accepted 20 November 2012 ♦ Published online 5 February 2013 For permission to copy, contact [email protected] 317 © 2013 Geological Society of America Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/9/2/317/3345052/317.pdf by guest on 02 October 2021 Fisk and McLoughlin with transmitted-light photo graphs of meta- examples of tunnels from other silicates. There In attempts to understand the origin of the morphosed pillow-lava rims from Archean to are two examples from felsic rocks—one of alteration textures, several geochemical tools Phanero zoic ophiolites (Furnes et al., 2001b, these is from a rhyolite tuff from central Ore- have been used to examine the contents of tun- 2004, 2008; Furnes and Muehlenbachs, 2003; gon (United States) (Fisk et al., 1998b) and the nels and the chemistry of the surrounding glass Staudigel et al., 2006, 2008) and an Archean other is from a submarine clastic tuff from the and alteration products. These studies have mafi c tuff (Lepot et al., 2011). Photographs of western Pacifi c (Banerjee and Muehlenbachs, included: electron probe micro analysis (Furnes granular and tubular alteration in basalts from 2003). Also, tunnels have been documented in et al., 1996; Torsvik, et al., 1998; Storrie- the marine/land transition have also been pub- olivine from an olivine basalt collected from the Lombardi and Fisk, 2004); scanning and trans- lished (Fisk et al., 2003; Walton and Schiff- marine/land transition in Hawaii and in dunites mission electron microscopy (Alt and Mata, man, 2003; Walton, 2008; Cousins et al., 2009; from central Oregon and northern California 2000; Thorseth et al., 2003; Benzerara et al., Montague et al., 2010). Interestingly, similar (Fisk et al., 2006). 2007; McLoughlin et al., 2011; Knowles et al., transmitted-light photomicrographs of altera- 2012); Raman and/or infrared spectroscopy tion features in pillow lavas erupted into fresh Origin of Alteration Textures in (Preston et al., 2011); and synchrotron-based water are not evident in the literature. Volcanic Glass X-ray microprobe techniques (Benzerara et al., Common alteration textures, such as tunnels 2007; Staudigel et al., 2008; Knowles et al., in volcanic glass, were until recently informally It has been hypothesized based on several 2011, 2012; Fliegel et al., 2012). It has been classifi ed by several authors, so synonyms for lines of evidence that some of the tunnel and hypothesized that Fe(II) is an energy source for these textures exist in the literature. A more granular alteration features are produced bioti- microbial metabolism, and electron microprobe formal ichnotaxonomic classifi cation was sug- cally. In support of this, biological staining has analyses of palagonite near “biotic” alteration gested by McLoughlin et al. (2009), which con- revealed that nucleic acids can be found at the has higher Fe than palagonite near “abiotic” sidered potential bioalteration textures as trace interface of fresh and altered glass near tubu- alteration (Storrie-Lombardi and Fisk, 2004). fossils and recognized
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