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- micrographs 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

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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 Phanerozoic 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 microanalysis (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 two ichnogenera and fi ve lar and granular textures and in some tubular Analysis of the 100–300-nm-wide rim of a tun- ichnotaxa based on a selection of samples from forms (e.g., Furnes et al., 1996; Giovannoni nel in glass shows that the glass lost Fe (Alt and the in situ oceanic crust and Phanerozoic ophio- et al., 1996; Torsvik et al., 1998; Banerjee Mata, 2000) and this loss of Fe is consistent lites. This atlas expands on these fi ve ichnotaxa and Muehlenbachs, 2003). It has been shown with a gain in Fe in the alteration material near or morphotypes, and outlines seven morphologi- theoretically that basaltic glass can yield suf- “biotic” alteration (Storrie-Lombardi and Fisk, cal criteria and provides names for features that fi cient energy to support chemolithoautotro- 2004). Transmission electron microscopy and can help unify discussion of the morphotypes. phic growth (Bach and Edwards, 2003), and synchrotron-based X-ray microprobe analy- culture-independent sequencing studies have sis show the presence of partially oxidized Fe Abundance and Distribution of shown that the microbial population inhabiting (Benzerara et al., 2007; Knowles et al., 2011; the Alteration Textures the sub-seafl oor is distinct from that found in Fliegel et al., 2012) and organic carbon in both overlying seawater and seafl oor sediments tunnel-fi lling smectite (Benzerara et al., 2007). The percentage of glass alteration in sub- and is up to 4 times larger (Mason et al., 2009; The X-ray microprobe analysis also shows that seafl oor basalts can be estimated visually, and Santelli et al., 2008). Controlled laboratory tunnels are produced by the dissolution of the the percent of that total alteration that is attrib- experiments have found that enhanced, local- glass. Nano-Secondary Ion Mass Spectrometry uted to biotic versus abiotic processes has been ized dissolution occurs in volcanic glass inocu- analyses of carbon, nitrogen, and manganese derived by point counting of thin sections from lated with microorganisms, relative to abiotic associated with micropores in glass suggest the Mid-Atlantic Ridge, the Costa Rica Rift, and controls (Thorseth et al., 1995; Staudigel et al., that these are remnants of manganese-oxidizing Lau Basin (Furnes et al., 2001a). From 2% to 1995). Comparative analysis of pillow-basalt bacteria (McLoughlin et al., 2011). Raman 60% of the glass was altered, with about half of rims and interiors suggests that biological spectroscopy indicates that tunnels contain this alteration being granular and tubular and the activity has lowered the δ13C of the rim rela- complex organic compounds such as amides remainder being “abiotic” secondary minerals. tive to the basalt interior (e.g., Furnes et al., and esters, which could be left by microbial The amount of granular and tubular alteration 2001c). Partially fossilized, mineral-encrusted inhabitants (Preston et al., 2011). These studies has also been visually estimated in basalt glass microbial cells have been observed in or near show that microbes and microbial processes are at the marine/land transition (Cousins et al., etch pits on altered glass surfaces, and these associated with some of the alteration features 2009; Montague et al., 2010). One of these pits have forms and sizes resembling the asso- described in this paper. studies (Cousins et al., 2009) from the glacial/ ciated microbes suggesting that the microbes The biogeochemical controls on the abun- marine transition of James Ross Island, Antarc- are involved in pit formation (Thorseth et al., dance, distribution, and diversity of alteration tica, found that the samples exposed to sea water 1992, 2001, 2003). Although these studies sup- textures in volcanic glass, however, are yet to tended to have more granular and tubular altera- port the hypothesis of biologically mediated be identifi ed, and we hope that the framework tion than samples exposed to fresh water. Alter- tunnels, it has not yet been possible to culti- presented herein will aid future investigations of ation of a Hawaiian subsurface hyalo clastite vate microorganisms that create tunnel shapes, the these controls on (bio)alteration. In addition, was indexed from 1 to 6, with 1 being no glass and abiotic mechanisms of tunnel production this collection of alteration textures may be an alteration to 6 being complete alteration (Mon- have been proposed such as for the Archean informative companion for those studying the tague et al., 2010). Indices were mostly 2–3. mafi c tuffs, which experienced conditions very alteration of (meta)volcanic glass in ophiolites From these three studies, it appears that granular different from basalts in our collection (e.g., and/or Precambrian greenstone belts and pro- and tubular alteration is ubiquitous and more Lepot et al., 2011). So although the weight of vide a context for interpreting proposed trace abundant in marine water than fresh water. evidence is in favor of the biological formation fossils that are hypothesized to represent some Although most photographic documenta- of complex tunnels, the question has not been of the earliest evidence for life on Earth (Furnes tion of tunnels has come from basalts, there are answered. et al., 2004). Likewise, for astrobiologists who

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may one day study alteration in igneous rocks 4×, 10×, 20×, and 40× magnification. Thin Tubular Alteration from other planetary bodies (cf. Fisk et al., sections are nominally 30 μm thick, which is 2006), this will provide a terrestrial reference 3–30 times the diameter of most tunnels and The tunnel forms are more complex and frame for the range of currently known textures. thus permits viewing tunnels in three dimen- varied than granular textures and are therefore sions by changing the elevation of the micro- further characterized by their shape, density, SAMPLES scope stage. The observation strategy was contents, and their relationship to other fea- to survey the whole thin section with the 4× tures in the thin sections such as fractures and Samples for this study are from existing col- objective lens to locate areas of glass. These vesicles. Seven major characteristics are used lections and are primarily cored sub-seafl oor glasses were then viewed at 10× and/or 20× to to describe the variety of tunnel morpholo- igneous rocks that were collected and archived locate regions that contained altered glass and gies. These are: (1) size (length and width); by the Deep Sea Drilling Project (DSDP) and glass-alteration textures. Glass commonly was (2) spacing between similar tunnels; (3) cur- the Ocean Drilling Program (ODP). The loca- limited to less than 25% of the thin section vature; (4) roughness; (5) changing width tion of volcanic glass within the archived area and most regions of altered glass were along the length of the tunnel; (6) branching; cores was determined by fi rst reviewing Ini- examined at one of these two higher magni- and (7) tunnel contents. In addition we recog- tial Reports volumes of the Deep Sea Drilling fi cations. The alteration was photographed at nize differences in how tunnels are distributed Project and the Initial Results volumes of the 40× and sometimes at 10× for larger features relative to fractures, minerals, and vesicles, Ocean Drilling Program. Then, during visits or to show the context of the feature being and we note that some thin sections have a to the sample repositories at Scripps Institution illustrated. All of the morphotypes of altera- single type of alteration but others exhibit of Oceanography, Texas A&M University, and tion found in our selection of thin sections are multiple forms. Lamont-Doherty Earth Observatory, the pres- illustrated in the fi gures. The term thin is applied to tunnel widths ence of glass was verifi ed visually and samples The optical images shown here were obtained (diameters) less than 3 µm from edge to edge were collected from the working halves of the using a Nikon LV100Pol polarizing microscope (Fig. 8A) and long tunnels are more than 50 cores. This collection of DSDP and ODP sam- at the Centre for Geobiology in Bergen, Nor- times longer than they are wide (Fig. 6A). ples was supplemented with a small number way, and photographed using an DS-Fi1 color The length of short tunnels is less than 10 of samples from an Integrated Ocean Drilling camera with 5.24-megapixel resolution coupled times their width (Fig. 6B). The density term Program (IODP) expedition as well as from to NIS-Elements BR 2.30 software. The images (Fig. 2) is based on how closely packed the seafl oor outcrops that were collected by sub- were saved in Joint Photographic Experts Group tunnels are along the fracture or other surface mersibles. In total, the samples come from 21 (.jpg) format (2560 × 1920 pixels). from which they originate: close tunnels have DSDP expeditions, 15 ODP and IODP expedi- a center-to-center distance that is less than 10 tions, and 5 manned and unmanned submersi- RESULTS times the tunnel width (Fig. 7A), whereas the ble expeditions. Samples are from the Pacifi c, center-to-center distance of separated tunnels Indian, and Atlantic Oceans, the Mediterranean Guide to Illustrations are apart by more than 10 times their diam- Sea, and some adjacent seas. Basalts from eters (Fig. 7B). Tunnels are usually curved and ocean rifts, seamounts, and back-arc spread- The alteration features are summarized in directed, such as away from a fracture (e.g., ing ridges are included in the study. The cored line drawings in Figure 2, and each line draw- Figs. 5A, 6A). These directed tunnels can be samples come from a range of depths into the ing references a photograph that illustrates the nearly linear (curvilinear), kinked with sharp volcanic basement, ranging from the sediment/ feature (Figs. 3–31). The diversity of alteration changes in direction (Figs. 5A, 9B), or appear basalt contact (<0.5 m into basement, mib) to features can be illustrated with a subset of 26 have a tangled knotted appearance along their 320 mib. Samples collected by submersible are thin sections (identifi ed by bold italics font in length (Fig. 8A). Convoluted tunnels turn back from outcrops on the seafl oor. The samples are Table 1). Each photograph is labeled with the toward their point of origin (e.g., Fig. 24A) primarily the rims of pillow basalts, sheet-fl ow sample identifi cation, a scale bar, and descrip- and do not appear to be directed away from margins, and interfl ow breccias. The youngest tive terms. Also the glass and major features their point of origin. Some tunnels have nearly cored basalt examined was <0.4 Ma and the old- such as fractures, vesicles, and minerals are constant width ±20% over their entire lengths est was 167 Ma. At some cored sites, samples annotated. (Figs. 5A, 7A), but others are variable (e.g., from multiple depths were examined. Standard Features are fi rst separated into major cate- Figs. 14, 15, 20). Some taper from their origin 26 mm × 46 mm polished petrographic thin gories of granular and tubular forms (Fig. 2, at a free surface to a point in the glass (Fig. sections were examined. These are listed in upper left panel) as previously described (Furnes 13B). Others have repeated variations in their Table 1, and Figure 1 shows the global distribu- et al., 2001a). In addition to the granular form, width resulting in a rhythmic annulated tunnel tion of samples. All samples, except 482D 11R2 a bud-shaped form is recognized here that is an (Fig. 13A), or have a single bump between the 32, which appears to have been at ≤150 °C at intermediate form between granular and tubu- tunnel origin and end termed engorged tunnels the time of collection (Duennebier and Blackin- lar. The photographs are grouped by the primary (Figs. 11B, 22B), or multiple irregular bumps ton, 1980), were from the seafl oor or shallow feature that is being illustrated such as granular (Fig. 12A). Rarely, a tunnel will broaden into subsurface where ambient temperatures were forms, simple tubes, branching, distribution, and a mushroom shape (Fig. 14A) or central disk compatible with life (<100 °C). overprinting. In many photographs, more than (Fig. 12B). In addition to tunnels of variable one textural feature is present but typically only width, there are round bud and bubble textures METHODS the feature being demonstrated is described. A that are present at the margins of fractures glossary of terms is provided in Table 2 to aid (Figs. 16, 17A). The petrographic thin sections listed in in the description of the alteration features. In The surfaces of some tunnels are smooth, Table 1 were examined with a petrographic the text below, terms that are from the glossary having irregularities that are less than 0.5 μm microscope fi tted with objective lenses with are italicized. (Fig. 10A). Rough tunnel walls (Figs. 7B, 10B,

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TABLE 1. SAMPLES EXAMINED FOR THIS STUDY Leg Thin section Basin Feature Lithology 11 105 41R3 46 Atlantic West flank, Mid-Atlantic Ridge Basal t 22 215 18R1 67 Indian Ninety East Ridge, hotspot track Basalt 25 240 7R1 30 Indian West flank Carlsberg Ridge, Somali Basin Basal t 28 266 23R1 64 Indian South flank Southeast Indian Ridge Basal t 37 332B 20R2 106 Atlantic West flank Mid-Atlantic Ridge Ol-plag basalt 37 335 5R3 7 Atlantic West flank Mid-Atlantic Ridge Plag-ol basalt 37 335 9R5 46 Atlantic West flank Mid-Atlantic Ridge Plag-ol basalt breccia 38 337 15R2 77 Atlantic East of Aegir Rift, Norway Basin Basalt breccia 42 373A 3R3 128 Mediterranean Central Tyrrhenian Abyssal Plain Basalt breccia 45 395A 17R1 83 Atlantic North Pond, west flank Mid-Atlantic Ridge Plag-ol-cpx basal t 45 395A 17R1 91 Atlantic North Pond, west flank Mid-Atlantic Ridge Plag-ol-cpx basal t 45 395A 17R1 131 Atlantic North Pond, west flank Mid-Atlantic Ridge Plag-ol-cpx basal t 45 395A 56R3 8 Atlantic North Pond, west flank Mid-Atlantic Ridge Aphyric basal t 45 395A 58R2 112 Atlantic North Pond, west flank Mid-Atlantic Ridge Hyaloclastit e 45 395A 65R1 102 Atlantic North Pond, west flank Mid-Atlantic Ridge Aphyric basal t 45 395A 67R1 146 Atlantic North Pond, west flank Mid-Atlantic Ridge Aphyric basal t 45 396 14R6 108 Atlantic East flank Mid-Atlantic Ridge Plag-ol basalt 45 396B 20R3 108 Atlantic East flank Mid-Atlantic Ridge Plag-ol basalt 52 417D 54R1 129 Atlantic West flank Mid-Atlantic Ridge Plag-cpx-ol pillow basalt 52 417D 66R6 35 Atlantic West flank Mid-Atlantic Ridge Plag-cpx-ol pillow basalt 53 418A 68R3 41 Atlantic West flank Mid-Atlantic Ridge Plag-cpx-ol pillow basalt 53 418A 71R3 87 Atlantic West flank Mid-Atlantic Ridge Plag-cpx-ol pillow basalt 53 418A 86R2 81 Atlantic West flank Mid-Atlantic Ridge Plag-cpx-ol pillow basalt 58 442B 19R1 17 Pacific Shikoku Basin, Philippine Sea Aphyric basalt 58 443 54R8 33 Pacific Shikoku Basin, Philippine Sea Aphyric basalt 58 443 62R3 105 Pacific Shikoku Basin, Philippine Sea Aphyric basalt 59 447A 18R2 34 Pacifi c East flank Palau-Kyushu Ridge, Philippine Sea Plag-ol basalt 59 447A 21R1 124 Pacifi c East fl ank Palau-Kyushu Ridge, Philippine Sea Plag-ol-cpx basalt 59 447A 31R3 118 Pacifi c East flank Palau-Kyushu Ridge, Philippine Sea Plag-ol-sp pillow basalt 59 447A 32R2 48 Pacifi c East fl ank Palau-Kyushu Ridge, Philippine Sea Plag-ol-sp pillow basalt 59 448A 10R2 142 Pacific Palau Ridge, Philippine Sea Aphyric basalt 59 448A 15R2 18 Pacific Palau Ridge, Philippine Sea Plag-ol basalt 59 448A 39R2 32 Pacific Palau Ridge, Philippine Sea Volcanic breccia 59 449 16R1 23 Pacific West of Palou-Kyushu Ridge, Philippine Sea Plag-ol-sp pillow basalt 63 469 50R2 17 Pacifi c Base of Patton Escarpment, California Borderland Aphyric basalt 63 472 14R1 26 Pacific Baja Seamount Province Aphyric basalt 63 472 15R1 102 Pacific Baja Seamount Province Aphyric pillow basalt 64 474A 48R1 92 Pacific Mouth, Gulf of California Plag pillow basalt 65 482D 11R2 32 Pacifi c Flank of East Pacifi c Rise south of Tamayo Fracture Zone Aphyric basalt 65 483 21R2 34 Pacifi c Flank of East Pacifi c Rise south of Tamayo Fracture Zone Plag-cpx-ol basalt 69 504B 4R2 80 Pacifi c South flank Costa Rica Rift Plag-ol pillow basalt 73 520 31R1 18 Atlantic East flank Mid-Atlantic Ridge Aphyric basalt 73 522B 5R1 105 Atlantic Angola Abyssal Plain, east fl ank Mid-Atlantic Ridge Aphyric pillow basalt 78 543A 16R3 11 Atlantic East of Barbados Ridge, west fl ank Mid-Atlantic Ridge Plag-ol pillow basalt 78 543A 16R7 60 Atlantic East of Barbados Ridge, west fl ank Mid-Atlantic Ridge Plag-ol basalt 81 555 71R1 21 Atlantic East flank Mid-Atlantic Ridge Phyric basalt 82 556 4R4 33 Atlantic West flank Mid-Atlantic Ridge Aphyric basalt 82 556 4R4 108 Atlantic West flank Mid-Atlantic Ridge Plag basalt 82 558 31R1 37 Atlantic West flank Mid-Atlantic Ridge Aphyric pillow basalt 82 559 2R2 37 Atlantic West flank Mid-Atlantic Ridge Aphyric pillow basalt 82 562 9R1 39 Atlantic West flank Mid-Atlantic Ridge Plag pillow basalt 107 655B 1R1 107 Mediterranean Vavilov Basin, Tyrrhenian Sea Basalt 107 655B 12R1 34 Mediterranean Vavilov Basin, Tyrrhenian Sea Basalt 115 706C 2R2 76 Indian Mascarene Plateau, Reunion Hotspot track Plag basalt 115 713A 20R5 89 Indian Chagos Ridge, Reunion Hotspot track Plag basalt hawaiite 115 713A 20R5 98 Indian Chagos Ridge, Reunion Hotspot track Plag basalt hawaiite 115 713A 20R6 24 Indian Chagos Ridge, Reunion Hotspot track Plag basalt 121 758A 71R2 133 Indian Ninety East Ridge, hotspot track Aphyric pillow basalt 123 765D 5R1 12 Indian Argo Abyssal Plain, eastern Indian Ocean Basalt 123 765D 5R1 53 Indian Argo Abyssal Plain, eastern Indian Ocean Basalt 123 765D 5R8 81 Indian Argo Abyssal Plain, northeastern Indian Ocean Basalt 123 765D 13R1 96 Indian Argo Abyssal Plain, eastern Indian Ocean Breccia 123 765D 24R3 142 Indian Argo Abyssal Plain, eastern Indian Ocean Pillow breccia 124 770B 16R3 142 Pacific Celebes Sea Plag-ol basalt 124 770C 4R1 14 Pacific Celebes Sea Plag-ol basalt breccia 126 791B 47R1 106 Pacific Izu-Bonin back arc Basalt breccia 126 793B 105R1 134 Pacific Isu-Bonin forearc Basalt breccia 129 801C 12R1 85 Pacifi c West fl ank East Pacific Rise Ol-plag basal t 185 801C 17R3 85 Pacifi c West fl ank East Pacific Rise Aphryic basal t 185 801C 48R2 121 Pacifi c West fl ank East Pacific Rise Aphryic basal t 185 801C 42R2 126 Pacifi c West fl ank East Pacific Rise Aphryic basal t 129 802A 58R2 97 Pacifi c West fl ank East Pacific Rise Ol-plag basal t 130 803D 69R1 50 Pacific Ontong Java Plateau Aphryic basalt 130 803D 70R2 117 Pacific Ontong Java Plateau Aphryic basalt 130 807C 75R2 8 Pacific Ontong Java Plateau Aphryic basalt 130 807C 82R5 14 Pacific Ontong Java Plateau Aphryic basalt 130 807C 92R2 110 Pacific Ontong Java Plateau Aphryic basalt 134 833B 81R2 73 Pacific Interarc basin, New Hebredes Plag basalt 135 836A 3H7 Pacific Western Lau Basin backarc Aphryic basalt (continued)

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TABLE 1. SAMPLES EXAMINED FOR THIS STUDY (continued) Leg Thin section Basin Feature Lithology 135 839B 22R1 20 Pacific Western Lau Basin backarc Cpx-ol basalt 135 839B 42R1 1 Pacific Western Lau Basin backarc Cpx-pl basaltic andesite 135 841B 49R1 11 Pacific Forearc, Tonga Ridge Rhyolite breccia 141 862C 5R1 24 Pacific Taitao Ridge, Chile Triple Junction Plag-hbl dacite 168 1026B 3R1 Pacifi c East flank, Juan de Fuca Ridge Basalt breccia 183 1140A 26R1 81 Indian Kerguelen Plateau, hotspot platform Plag basalt 183 1140A 35R1 31 Indian Kerguelen Plateau, hotspot platform Plag-cpx basalt 197 1203A 19R2 24 Pacifi c Emperor Seamounts, Hawaii-Emperor hotspot track Plag-ol basalt 197 1203A 31R1 118 Pacifi c Emperor Seamounts, Hawaii-Emperor hotspot track Plag-ol basalt 329 1365E 11R1 83 Pacifi c Ridge flank Aphyric basalt 329 1367F 2R3 53 Pacifi c West fl ank East Pacific Rise Aphyric basal t 329 1367F 5R1 27 Pacifi c West fl ank East Pacific Rise Aphyric basal t 329 1367F 6R1 0 Pacifi c West fl ank East Pacific Rise Aphyric basal t 329 1368F 9R1 78 Pacifi c West fl ank East Pacific Rise Aphyric basal t 329 1368F 11R1 108 Pacifi c West fl ank East Pacific Rise Aphyric basal t 329 1368F 13R2 102 Pacifi c West fl ank East Pacific Rise Aphyric basal t 329 1368F 13R2 120 Pacifi c West fl ank East Pacific Rise Aphyric basal t 329 1368F 13R3 20 Pacifi c West fl ank East Pacific Rise Aphyric basal t — DSV Alvin 3807 A1 Pacifi c Warwick Seamount, Cobb-Eichelberger hotspot track Basalt — DSV Alvin 3816 F Pacific CoAxial segment, Juan de Fuca Ridge Basalt — DSV Alvin 3821 A1 Pacifi c Warwick Seamount, Cobb-Eichelberg hotspot track Hyaloclastite — DSV Alvin 3823G Pacifi c Cobb Seamount, Cobb-Eichelberg hotspot track Basalt — DSV Alvin 3825 C Pacifi c Brown Bear Seamount, Cobb-Eichelberg hotspot track Basalt — DSV Alvin 3853 R Pacifi c Crest, East Pacific Rise Basal t — DSV Alvin 4026 5C Pacific Denson Seamount Basalt — DSV Alvin 4027 2, 3, 8, 15 Pacific Denson Seamount Basalt — DSV Alvin 4028 9 Pacific Dickins Seamount Basalt — DSV Alvin 4029 2, 3, 8, 11 Pacific Dickins Seamount Basalt — DSV Alvin 4030 2, 4, 8 Pacific Dickins Seamount Basalt — DSV Alvin 4031 18 Pacific Dickins Seamount Basalt — DSV Alvin 4032 15 Pacific Welker Seamount Basalt — DSV Alvin 4033 3, 5, 9, 10 Pacific Welker Seamount Basalt — DSV Alvin 4038 19 Pacific Pratt Seamount Basalt — DSV Alvin 4039 25, 30 Pacific Pratt Seamount Basalt — DSV Alvin 4040 5 Pacific Giacomini Seamount Basalt — DSV Alvin 4041 15 Pacific Giacomini Seamount Basalt — DSV Alvin 4042 3 Pacific Giacomini Seamount Basalt — DSV Sea Cliff MR3 - 3A Pacific Mendocino Ridge, transform fault Basalt — ROV Tiburon MRF1-10R Pacific Mendocino Ridge, transform fault Basalt — ROV Tiburon MRF1-8R Pacific Mendocino Ridge, transform fault Basalt Note: For drilled samples, the Leg indicates the Deep Sea Drilling Project (DSDP), Ocean Drilling Program (ODP), or Integrated Ocean Drilling Program (IODP) expedition. Thin section number includes the core site number, the hole at that site, the core barrel number, the type of core (R for rotary rock core, H for hydraulic piston core, and CC for core catcher), section of core (the cored material was archived in 1.5 m sections), and distance in centimeters from the top of the section to the sample. (For example, 332B 20R2 106 indicates: Site 332, Hole B, Core 20, a core type of rotary, Section 2, and 106 cm from the top of Section 2). Numbers for the samples collected with submersibles are the vehicle designation, the dive number, and the sample number assigned when the rocks were archived. Entries in bold italic font are illustrated in Figures 3–31. In addition, the ocean basin, geological feature, and lithology are indicated. Data are from the DSDP Initial Reports volumes and the ODP Preliminary Reports and Scientifi c Results volumes. DSDP data are available online from the National Geophysical Data Center (http://www.ngdc.noaa.gov/mgg/geology/dsdp/start.htm); ODP data are available online from the Ocean Drilling Program (Publications—Leg-Related Publications) (http://www-odp.tamu.edu/publications/pubs_ir.htm). The table is modified from a previously compiled table (Josef, 2006).

11) are embellished with ﬁ ne (>1 μm) extensions into the glass uniformly or periodically distributed along the tunnel. The widths of individual extensions from the walls of tunnels are much thinner than the width of the tunnel. In contrast to this, branches (Fig. 2) are commonly the same width as the main tunnel. Branching may be simple with the nodes widely spaced along a tunnel, (Fig. 17B), or mossy (Fig. 4A), or networks (Figs. 18A, 19), where branches are crowded together and branch repeatedly. In the case of simple, mossy, or network branching, daughter branches are the same diameter as the parent branch. In other cases branches are narrower than their parent tunnels (Figs. 20, Figure 1. Locations of all samples examined as part of this study. Larger (white) circles 21A). Some tunnels have crowns, composed indicate the locations of samples that are documented with photographs in this article; other of multiple radiating tunnels that have a dif- samples examined are indicated by smaller (blue) circles. The map was generated with Geo- ferent form than the tunnel from which they MapApp, http://www.geomapapp.org (Ryan et al., 2009). originate (Figs. 21A, 23A, 25B).

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Figure 2. Schematic diagram summarizing the morphological characteristics used to describe alteration textures. The box (top left) shows the main alteration morphotypes: tubular, granular, and the less common sub-types of buds and bubbles. In the rest of the fi gure, seven characteristics are illustrated with line diagrams: (1) length and width; (2) density; (3) cur- vature; (4) roughness; (5) variations in width; (6) branching; and (7) tunnel contents. Each of the morphological forms is given a simple descriptive term, and a typical example from our photographic atlas is given. Descrip- tive terms are defi ned in Table 2. This diagram expands on the ichnotaxonomic classification scheme shown in McLoughlin et al. (2009, their fi gure 1).

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A phenocryst light granular border glass

palagonite

Figure 3. (A) Tan glass (upper ﬁlled fracture and lower areas of photo) with a fracture ﬁ lled with banded, yellow to tan palagonite. The border micro of the glass with the palagonite phenocrysts (arrows) has a granular texture. darker granular border The lower border is darker than 1140A 35R1 31 the upper border. (B) Light-tan 20 μm glass MRF 2012 glass in the lower part of the B photo, palagonite in the upper palagonite part of the photo with a granular micro boundary at the palagonite/glass phenocryst interface. A crystalline quench feature (variole) is on the right side; a microphenocryst is below the focal plane in the center of the palagonite. variole

granular

glass

474A 48R1 92 MRF 2012 20 μm

mossy

granular

Figure 4. (A) Circular altera- palagonite tion features along a fracture. The center of the fracture is palagonite, which is surrounded glass by a band of dark granu- 803D 69R1 50 lar texture at the palagonite/ 20 μm MRF 2012 glass border. A mossy texture B extends from the granular band into the glass. (B) Short, dark, glass rough tunnels <5 μm wide and variole <20 μm long at the glass/granu- granular lar-alteration boundary. Mossy palagonite tunnels appear to be random as opposed to directed tunnels.

variole dark zone mossy

20 μm 765D 5R8 81 MRF 2012

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granular glass

mossy

random

Figure 5. (A) Five styles of alteration are illustrated: (i) granular, (ii) mossy, (iii) simple directed tunnel with constant width, 447A 21R1 124 (iv) kinked directed tunnel, and 20 μm MRF 2012 (v) random tunnels. (B) Simple B 50-μm-long, 3–5-μm-wide tunnels radiate from a central point on the edge of a fracture at high angles.

radiang tunnels

glass 20 μm

Sea Cliﬀ MR3 3RA MRF 2012

glass palagonite

long granular mossy

Figure 6. (A) Long, directed tunnels extend more than 500 μm from the granular and mossy 500 μm texture at the right edge of the glass. Tunnels are ~3 μm wide. 765D 5R1 53 MRF 2012 (B) Simple, short, close tunnels extending from clay along the B margin of an olivine grain and from an area of bubble texture. variole glass These tunnels are 1–2 μm wide and 10 μm long and 1 or 2 μm bubble texture apart. Nearby bubble texture has no tunnels emerging from it.

short tunnels

olivine

395A 17R1 83 20 μm MRF 2012

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A 713A 20R2 89

Figure 7. (A) Close, smooth tunnels are 5–7 μm wide and 30–40 μm apart along the fracture. (B) Separated tunnels (the smooth tunnels separated by 30 μm spacing along the fracture is at their intersecons with the more than 10 times the tunnel MRF 2012 fracture 20 μm width). In this case, the tunnel B 765D 5R1 53 width is 5–7 μm and the distance between them is 100 μm. These channels are ornamented with 3–5 μm extensions and separated contain septa.

20 μm MRF 2012

A 395A 65R1 102

Figure 8. (A) Thin, <3-μm-wide, directed, hair tunnels originat- ing from the edge of the fracture. The fracture contains a 10-μm-wide layer of brown material adjacent to the glass. The image also contains tunnels that randomly change glass tangles direction within a segment of the tunnel. A tunnel may have 20 μm MRF 2012 more than one of these tangled regions. (B) Alternating dark B 396 14R6 108 glass and light concentric rings in yellow palagonite are centered short wide tunnels along the fracture. Granular or granular incursions texture extends from the fracture or from the concentric rings to a dark granular zone. Short, wide, granular incur- granular palagonite sions emerge from the dark, granular texture. The fracture is ﬁ lled with an undetermined white transparent mineral.

20 μm concentric rings fracture MRF 2012

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A 765D 5R8 81

Figure 9. (A) Directed, simple, empty tunnel empty tunnels with a constant width of 5 μm emerge from granular material. The granular alteration is separated from an original fracture by a crack

that was produced possibly dur- glass ing the manufacture of the thin section. Yellow granular mate- MRF 2012 20 μm rial has replaced glass along the B original fracture. (B) Kinked, palagonite directed, opaque tunnels ~3 μm wide emerge from the dark granular material. A fracture just visible in the upper right of the photo is surrounded by kinked tunnels palagonite that grades from yellow to brown to black. glass

447A 32R2 48 20 μm MRF 2012

A 543A 16R3 11

smooth

Figure 10. (A) Smooth, simple, 5–10-μm-wide tunnels emerge from yellow granular altera-

20 μm glass MRF 2012 tion. Two tunnels are indicated with arrows. (B) Simple tunnels B 105 41R3 46 with rough surfaces emerge from brown alteration at the quench crystals edge of a fracture. Tunnels are smooth glass 5–10 μm wide.

rough alteraon

MRF 2012 20 μm

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A microphenocryst

intermediate enlargement μ Figure 11. (A) 15- m-wide tunnel glass with rough exterior expands at terminal the end to ~35 μm wide. The enlargement tunnel also has an enlargement in the midsection. Short tun- 807C 75R2 8 nels, 5–10 μm long, also emerge from the fracture in lower part microphenocryst of the photo. (B) Fracture in the 20 μm short MRF 2012 lower left contains brown ﬁ ll B and is surrounded by altered 807C 75R2 8 glass. From this comes wide (10–15 μm), rough tunnels that μ taper to 10 m or less at their intermediate ends and have an intermediate enlargements enlargement (15–20 μm wide) (engorged) with a single ovoid body with dark edges. glass

20 μm MRF 2012

A 10 μm glass

Figure 12. (A) Fractures with several styles of alteration. Yel- low granular alteration occurs granular at the margins of the fractures. Mossy alteration and 3-μm- mossy alteraon terminal wide tunnels with variable enlargement width emerge from the fracture 1140A 26R1 81 annulated on the left. Annulated tunnels tunnels and tunnels with club-shaped B ends emerge from the fracture with yellow ﬁ ll. (B) Fracture surrounded by several styles of alteration, one of these (arrow) being a smooth tunnel, 5 μm wide, that has a 25-μm-wide, disk dark disk 100 μm from the fracture and 60 μm from the tunnel terminus.

395A 67R1 146 glass 20 μm MRF 2012

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Figure 13. (A) Smooth, 10-μm- wide tunnel with spiral fi lament has repeated rings (annulations) μ every 3–5 m near its end. The annulaons glass photo also contains smooth, thin, simple tunnels on both sides of a fracture that runs vertically through the image, 20 μm 713A 20R5 8 MRF 2012 and thin, kinked, dark tunnels B on the left side of the image. 765D 5R8 81 (B) Fracture with little alteration has three smooth fi ngers or tunnels of variable width that variole taper from 10 μm at their bases to points ~30 μm from the frac- glass ture. Brown patches are areas of quench crystals and varioles. tapered fingers

Quench crystals MRF 2012 20 μm

A 807C 92R2 110

glass Figure 14. (A) There is a fracture in the lower left from which a tunnel that is 20 μm wide expands to 40 μm just below a mushroom-shaped cap that is 70 μm wide. (B) Fracture runs diagonally from the upper left corner and has smooth tun- 20 μm MRF 2012 nels, three of which are broad B and flattened, petal shapes. petal 337 15R2 77 One (i) has a honeycomb con- (ii) glass structed of 5–10-μm-wide cellular pattern. Another (ii) has dark lineaments spaced 5 μm apart. Boxed areas are shown in Figure 15. Spotted texture is an artifact of thin section pro- honeycomb petal with honeycomb duction. paern

(i)

50 μm NM 2012

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A 337 15R2 77

petals petals with honeycomb paern

Figure 15. (A) Flattened petal- shaped tunnels have a 10-μm- wide cell-like pattern of dark glass 20 μm partitions. (B) Flattened petal- MRF 2012 shaped tunnel with ribs sepa- B glass 337 15R2 77 rated by 5 μm. Smooth tunnel with spiral ﬁ lament is on the left side of the photograph.

petal ribs

spiral

20 μm

MRF 2012

A 765D 5R8 81

buds

Figure 16. (A) Two short, wide upper truncaon (15–20 μm), smooth, round, lower of tunnel truncaon yellow-brown buds extend from of tunnel the granular area next to the fracture. Also present is a row glass MRF 2012 20 μm of tunnels that are truncated at the upper and lower surfaces of B granular 472 14R1 26 the thin section. (B) Coalesced 5–8-μm-diameter spheres produce a bubble texture. The bubbles grade from transpar- bubble ent yellow spheres on the left to texture brown spheres with dark rims.

glass

20 μm MRF 2012

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A 482D 11R2 32

glass

buds Figure 17. (A) A 10-μm-wide fracture has 5–10-μm-diameter spherical buds along its edge. The buds lack any internal

structure and are not granular. 20 μm MRF 2012 (B) Simple branching; a single B tunnel can have more than one node. Branches have the same diameter as the trunk. Tunnel width varies in a sawtooth or thorny pattern. nodes

glass

20 μm 447 32R2 48 MRF 2012

A 335 9R5 43

Figure 18. (A) Dark, 1-μm- wide, separated, branched tunnels form random networks that extend 50–60 μm from networks the fracture. (B) Semicircular palagonite area is centered on a fracture (top of photo) and surrounded by a black mass glass 20 μm MRF 2012 of kinked tunnels. Branched, B straight, dark tunnels are granular palagonite 3–5 μm wide. 30-μm-long tun- dark, kinked nels with undulating, smooth tunnels outlines emerge from the black mass of dark, mess. The branches fork at kinked tunnels ~60° angles from each other. Most appear to have only one glass branch per tunnel. Boxed area enlarged in Figure 19A.

branched tunnel with single node

472 14R1 26 NM 2012 50 μm

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annulated Figure 19. (A) Annulated tun- branched nels branch at 60° angle. Mossy, tunnels kinked tunnels also branch. (B) Fracture at the bottom of glass the photo has granular alteration along one side that grades 20 μm 472 14R1 26 mossy branched tunnels into a dark zone. Emerging MRF 2012 from the dark zone are kinked, B dark, kinked, branched network of tunnels dark, branched, random tun- 396B 20R3 108 nels that are ~3 μm wide and glass 30–50 μm long. This is the type specimen of Tubulohyalichnus stipes isp., ﬁ rst described in McLoughlin et al. (2009).

dark zone alteraon fracture

20 μm glass NM 2012

A NM 2012 annulated Figure 20. (A) Fracture at the palmate bottom of the photo has tunnels branching up to 300 μm long emerging bulbs nodes from it. One tunnel branches at nodes into a palmate arrange- ment of tunnels that terminate glass in 2 μm bulbs. Some tunnels petal shape are annulated; one has a petal

50 μm shape. Some branches emerge from an expanded node; some tunnels are partitioned with 337 15R22 77 a filigree of dark material. fracture (B) Fracture (out of view below B the bottom of the photo) is sur- 20 μm glass kinked, dark, rounded by reddish palago- branched nite (granular texture), which crowns grades into a dark zone. Emerg- bulb ing from the dark zone are dark, simple tunnels that are 5–10 μm wide. Some have blunt terminations as close as 10 μm to the dark zone. Others penetrate 100–120 μm into the glass blunt and expand to 20 μm wide or produce crowns of 3–5-μm- palagonite wide tunnels that end in bulbs. 472 15R1 102 NM 2012

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A Alvin 3807 A1 glass

tunnel crowns

Figure 21. (A) Separated, smooth dark, helical tunnels, 3–4 μm wide and 20 μm tunnels MRF 2012 20 μm long, with crowns composed of multiple, dark helixes that are B 1 μm wide. (B) Three forms of 447 32R2 48 tunnel ﬁ lling of simple, directed, smooth tunnels: granular, inter- mittent dark material, and con- constant tinuous dark material. width

connuous intermient dark contents dark contents

granular contents

20 μm MRF 2012

A glass glass 765D 5R1 53

septae

Figure 22. (A) Smooth, simple, 2-μm-wide tunnels contain 2 μm ovoid bodies ovoid bodies spaced 10–20 μm 20 μm MRF 2012 apart and septae that divide B the tunnel at 2 μm intervals. glass (B) Tapering tunnels with intermediate enlargements that are divided by dark partitions. intermediate enlargements

enlargement with divisions

MRF 2012 807C 75R2 8 20 μm

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A glass 807C 75R2 8 crown septae

variable Figure 23. (A) Dark alteration width enlarged segment at the right of the ﬁ gure has a microphenocryst single, rough, variable-width tunnel that is 100 μm long and expands from 12 μm at its base

to 20 μm just below the 30-μm- 20 μm MRF 2012 wide, rough crown. The tunnel B has prominent septae and an 543A 16R3 11

enlarged segment below the glass crown. (B) Smooth, simple, 5-μm-wide tunnels contain spi- spiral filaments ral fi laments. One tunnel may also have a fi lament and septae.

septae MRF 2012 20 μm

A 447A 21R1 124 glass

variole

convoluted

Figure 24. (A) Fracture at the left has separated patches of dark, granular material. From MRF 2012 20 μm one of these patches emerges a 1–2-μm-wide, simple, directed, B convoluted tunnel with dark glass contents. (B) Annulated tun- dark object in nel contains a 10-μm-wide and annulated tunnel 20-μm-long, brown oval.

20 μm 713A 20R2 89 MRF 2012

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A glass

ovals Figure 25. (A) A group of tunnels radiate from a center on a fracture at the right of the photo. One smooth, 8-μm-wide tunnel in the group contains numerous 8 μm × 5 μm ovals that are 5–10 μm apart. A sec- 713A 20R2 89 20 μm ond tunnel also contains ovals. MRF 2012 (B) A 3–5-μm-wide fracture B with granular alteration along 483 21R2 34 glass its edges and rough, simple tunnels (5–10 μm wide and up to 40 μm long) with rough crowns, 10–20 μm wide. Tunnels fracture vary in width and several have bumpy

three bulges plus the crown. crown

tunnel 20 μm MRF 2012

A 765D 5R1 53 glass tunnels

vesicle Figure 26. (A) A fracture cuts across the center of the photo. vesicle Two spherical vesicles, one 70 μm in diameter and the fracture olivine other 30 μm in diameter, are above the fracture. Empty, rough tunnels with broadened crowns extend from the rims of the vesicles. (B) Smooth, simple,

MRF 2012 20 μm 1–3-μm-wide tunnels emerge from the rim of a vesicle. The B 765D 5R1 53 vesicle contains tan clay and 1 to 2 μm an opaque sulﬁ de. The tunnels tunnel emerging upward bodies and turning to the right extend from the vesicle in oppo- olivine site directions. Some tunnels micro- contain ovoid bodies. The photo phenocryst also shows two smooth tunnels with ovoid bodies that are not connected to the vesicle and which wrap around an olivine microphenocryst. tunnels glass

tunnels wrapping around olivine

MRF 2012 20 μm

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glass granular incursions Figure 27. (A) A vesicle lined rough crowns with light-tan clay. The glass rim of the vesicle has been altered to granular palagonite. Granu- altered glass lar incursions into the glass are clay precipitate 5–20 μm wide at their contact with the granular/glass bound- empty vesicle ary and have rough crowns that are 20 μm wide. (B) A dark brown, 150-μm-long and μ Tiburon MRF1 8R 70- m-wide variole has numer- 20 μm MRF 2012 ous empty, smooth tunnels that fracture are 1–3 μm wide and up to B 80 μm long that emerge from altered glass the contact of the spherule with glass. The tunnels only emerge from the side of the variole that faces the fracture. At the top of the photo, there is a fracture surrounded with light-colored alteration with a dark border tunnels and associated, dark, kinked tunnels. variole glass

543A 16R3 11 20 μm MRF 2012

A glass 543A 16R3 11

fracture

variole Figure 28. (A) A fracture on the left and a dark quench feature (variole) on the right are connected by clear, smooth tunnels that are 1–3 μm wide and μ MRF 2012 150–200 m long. (B) A thin, 20 μm directed network of branched B tunnels between the fracture glass on the left and olivine micro- pheno cryst on the right. In some locations, the network appears to initiate from the network olivine and in others, it emerges from the fracture.

olivine microphenocryst 335 9R5 43 20 μm MRF 2012

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A glass

preserved Figure 29. (A) Glass that once vesicle preserved, surrounded a vesicle has been smooth, replaced with palagonite. The tapered, short

outlines of smooth, tapered microphenocrysts tunnels tunnels with dark tips that sur- round the vesicle are preserved. 765D 5R1 53 (B) Fracture in the lower part of 20 μm MRF 2012

the photo has dark networks of B tunnels that were overprinted second generaon with secondary alteration that glass 100 μm left parallel dark laminations of tunnels outlining the groups of tunnels. A second episode of similar tunnels formed along the dark upper margin of the alteration.

fracture preserved network parallel dark 443 62R3 105 laminaons MRF 2012

A 469 50R2 17 glass

(4)

Figure 30. (A) A fracture across (3) the lower part of the photo is bordered by an early stage of 100 μm dark, granular alteration (1). This is followed by hemispherical alteration centered on what (2) is assumed to be the edge of an preserved alteraon advancing alteration front (2). textures A second dark zone (3) is asso- fracture (1) MRF 2012 ciated with dark, mossy alteration. The ﬁ nal glass/alteration B boundary is dark and rough (4). The white crack occurred overprinted during or after sample col- tunnels lection. (B) Granular texture μ granular texture surrounded by 5–8- m-wide, tapered channels. Similar but tunnels slightly larger tunnels are present to the right of the granular texture and are overprinted with palagonite. glass

20 μm 765D 5R1 53 MRF 2012

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mented with transmitted-light photomicrographs 765D 5R8 81 (Giovannoni et al., 1996). Also at this time and from the same Costa Rica Rift site, granular and tubular features were illustrated in back- Figure 31. Simple, smooth, scattered electron images (Furnes et al., 1996). empty tunnels start at the gran- More detailed close-up photomicrographs (Fisk ular boundary of the fracture et al., 1998a) illustrated that the tubular phe- and extend parallel to the frac- glass nomenon was more varied than the tunnels seen ture. Tunnels are 5 μm wide and in Costa Rica Rift basalts. In these new photo- 40 μm long or longer. Granular graphs, mossy and branching tunnels, granular texture extends 5–20 μm from alteration, as well as tunnels with pronounced the edge of the fracture. parallel to fracture septae and cell-sized inclusions were illustrated from separate seamounts in the Pacifi c Ocean (one dredged and one cored basalt), from the MRF 2012 20 μm Mid-Atlantic Ridge (two cored basalts), and from the Indian Ocean (one cored basalt) (Fisk et al., 1998a). Content of Alteration Textures Timing of Alteration In the 2000s the growing literature of oceanic glass alteration reported additional morpholo- The contents of tunnels can vary as well. The photographs also illustrate the temporal gies including: budding along a tunnel (Furnes Some tunnels appear to be empty (Fig. 9A), but relationship of alteration in some samples. For et al., 2000a), similar to what we have called others are partially or completely fi lled (Fig. example, Figure 29A shows tapered tunnels tangled texture (Fig. 8A), and bifurcating tun- 21B) with opaque material. Tunnel contents radiating from a vesicle. Originally the tunnels nels (Furnes et al., 2001a, 2002). Annulated and can be segregated into oval or ovoid bodies extended into glass, but the vesicle, tunnels, convoluted tunnels were described by Banerjee typically 1–2 μm in diameter that are com- and surrounding glass have now been replaced and Muehlenbachs (2003) in basalts from the monly evenly distributed along the interior of with phyllosilicate. Overprinting of a network Ontong Java Plateau, which we have also illus- the tunnel (e.g., Figs. 22A, 25A). However, a is obvious in Figure 29B. Here a granular bor- trated with samples from Chagos Ridge (Fig. single, large, 20 μm ovoid body is present in der evolves into a dark network in glass in the 13A) and from the Philippine Sea (Fig. 24A), some tunnels and the tunnel walls swell around upper half of Figure 29B. In the lower half of respectively. Josef (2006) identifi ed a number of the body (Fig. 11). In one example the large the fi gure, glass containing a previous dark net- textures not previously described, such as mush- ovoid body is divided by dark septae into fi ve work has been transformed into a yellow phyllo- room (Fig. 14A) and engorged (Fig. 11B). Net- separate bodies (Fig. 22B). Septae can also silicate. In Figure 30A a semicircular alteration works of branched tunnels were fi rst described divide a tunnel into multiple chambers that are pattern, which is similar to that in Figure 8B, has by McLoughlin et al. (2009), and the type 5–10 μm long (e.g., Figs. 22A, 23). Broad fl at been replaced by a subsequent phase of altera- example is illustrated in Figure 19B. Here we tunnels are divided by a honeycomb or pat- tion, which suggests conditions changed during also report networks made of thinner branched terned with a fi ligree of dark material (Figs the formation of this alteration boundary. tunnels (Figs. 18A, 28B). 14B, 15). Tunnels may also have spiral fi laments (e.g., Figs. 13A, 23B). STATE OF KNOWLEDGE ON Comparison to an Ichnotaxonomic ALTERATION TEXTURES Classifi cation Distribution and Directionality IN VOLCANIC GLASS of Alteration An ichnotaxonomic framework was advanced This Atlas Compared to Earlier Studies for glass alteration by McLoughlin et al. (2009) The distribution of alteration textures within that considers the textures as trace fossils, and glass has also been documented in this study. Morgenstein (1969) illustrated granular and two new ichnogenera were proposed, corre- Tunnels are usually found distributed along tubular textures in three dredged basalts from sponding to the two broad granular and tubu- fractures (e.g., Figs. 5A, 6A) but sometimes the Mid-Atlantic Ridge and one from a frac- lar morphotypes discussed here and in earlier they are distributed around vesicles (Figs. ture zone along the Pacifi c-Antarctic Ridge. His reports. Five ichnospecies were also defi ned on 26, 27A, 29A), varioles (Figs. 27B, 28A), or transmitted-light images showed granular altera- the basis of morphological characteristics and phenocrysts and microphenocrysts (Figs. 6B, tion forming a semicircle around a fracture (sim- these are compared to the range of textures illus- 28B). Tunnels are commonly directed away ilar to Fig. 4A) and 20-µm-wide zones between trated here, which includes new morphological from fractures, but sometimes they are paral- glass and palagonite along fractures (similar to variants: lel to fractures (Fig. 31). Tunnels may also Fig. 4B). His photographs also include what he 1. Granulohyalichnus vulgaris isp. has a radiate from a point at the edge of a fracture called “hair tunnels” (see Table 2) extending granular form (McLoughlin et al., 2009, their (Fig. 5B). Some tunnels turn sharply from their 50 μm from a granular texture into glass (similar fi gure 2), and is very common and comparable to initial direction perpendicular to the surface to Fig. 6A) and from fractures into glass (simi- the examples illustrated here (e.g., Figs. 3 and 4). where they originate to a direction parallel to lar to Fig. 9A). He also described a “solid solu- 2. Tubulohyalichnus simplus isp. has an the alignment of other tunnels in the glass (Fig. tion” border that is similar to the dark zone un orna mented tubular form (McLoughlin et al., 26B). In one example, the alignment of tunnels between glass and palagonite (Fig. 4). 2009, their fi gure 3). This alteration morphol- is parallel to the major axis of elongation of It was not until 1996 that tubular and granu- ogy is also illustrated here (Figs. 5–31) and with vesicles in the glass (not shown). lar features were again emphasized and docu- further descriptors including short or long, thick

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TABLE 2. GLOSSARY OF TERMS, DESCRIPTIONS, AND ILLUSTRATIVE FIGURES SPECIFIC TO THIN SECTIONS IN THIS STUDY Term R Description Figure* Annulated f Periodic increases and decreases in tunnel width 13A Branch f Bifurcation of a tunnel 17B Bulb f Round enlarged end of a tunnel 20A Close o Distance between tunnels is <10 times the width of the tunnels, as measured on the surface of the 7A thin section along the feature from which tunnels emerge Concentric rings ts Laminations that form semicircles around a point on a fracture 8B Constant width f Tunnel diameter changes less than 20% 21B Convoluted f Tunnel direction turns back toward starting point 24A Crack ts Break in the material of the thin section that is interpreted to have been produced during or after the 9A collection of the sample Crown f Enlarged end of a tunnel giving rise to multiple tunnels or other alteration features 20B Dark border f Thin opaque granular/glass boundary 3A Dark zone f Broad opaque granular region between a fracture and glass 4B Directed o Multiple tunnels are parallel or nearly parallel to each other and trend in the same direction 5A Empty c No visible contents 9A Engorged c Tunnel with enlargement between its origin and end 11B Fracture ts A break in the material of the thin section that is interpreted to have been produced in situ 16A Glass ts Quenched silicate magma (sideromelane) All Granular f Micron- and submicron-size spherical and subspherical pits in glass or replacing glass 3B Granular incursion f Granular alteration extending from fracture into glass without tunnels 8B Hair tunnel f Thin (typically <1 µm), linear , colored or dark feature without separate margins 8A Helical (form of tunnel) f Coiled tunnel 21A High angle o Tunnels emerge nearly perpendicular to a glass/alteration border or fracture 22A Honeycomb c Dark or opaque hexagonal pattern 15A Intermediate enlargement f Single enlargement (annular bulge) between the origin and tip of a tunnel 11 Kinked f Multiple sharp angular bends in a tunnel 5A Laminations ts Alternating thin, dark lines and broader, lighter areas in an area of alteration 29B Light border f A thin granular/glass boundary with some darkening but not opaque 3A Long f A tunnel whose length is >50 times its width 6A Microphenocryst ts Mineral <300 microns long in glass or fine-grained matrix 28 B Mossy f Mass of dark, branched, short tunnels along the glass/alteration border 4B Mushroom f Terminal enlargement that tapers 14A Network f Multiple, branching, directed tunnels 19B Node f Point of branching (bifurcation) 17B Ovoid bodies c Cell-like features 22A Palagonite ts Aqueous alteration of silicate glass 3A Palmate f Branches that radiate from a point 20A Parallel o Tunnels originate at high angle to a fracture and then turn parallel to the fracture 31 Pattern c Intricate filigree of dark or opaque material 15A Petal f Flat tunnel form 14B Preserved texture ts Alteration texture that remains visible after a second phase of alteration has occurred 29A Quench crystal ts Crystal that formed as the magma was rapidly cooling 10B Radiating o Tunnels originate and diverge from a point, microphenocryst, or other feature 25A Random o Tunnels have many changes in direction and are not directed, and the length of the tunnel between 5A changes of direction is not uniform Rough f Walls of the tunnel are irregular on a scale of 1 micron or less 10B Separated o The distance between tunnels is >10 times the width of the tunnel, as measured on the surface of the 7B thin section along the feature from which tunnels emerge Septa c Evenly spaced, dark divisions in a tunnel 23A Short f Length of tunnel is <10 times its width 6B Simple f Tunnel without branches, enlargements, or sharp changes in direction 9A Smooth f The walls of the tunnel have no irregularities >0.5 microns 10A Spiral c Linear filament wrapped inside a tunnel 13A Tangle f Region of knotted appearance between origin and tip of a tunnel 8A Tapered f Tunnel tapers to a point 13B Terminal enlargement f Broadening of the tunnel at its terminus 11A Thin f Alteration feature <3 microns from edge to edge 8A Tunnel f Linear cavity with identifiable margins 22A Variable width f Tunnel diameter changes more than 20% over its length 23A Variole ts Cluster of quench crystals often nucleated on and radiating from a microphenocryst 24A Wide f Alteration feature >10 microns from edge to edge 11A Note: The second column (R) indicates if the term refers to an alteration feature (f), tunnel contents (c), organization of alteration features (o), or characteristics of thin sections (ts). *Many terms are illustrated in more than one fi gure, but usually only one fi gure number is listed.

or thin, closely spaced or separated, curvilinear, (Fig. 13A), and those with irregularly sized and sinistrally or dextrally coiled. This ichno species smooth, or constant width, possibly with tunnel spaced annulations termed engorged and bumpy may show a linear or curved growth axis with contents (Fig. 2). (Figs. 11B and 12A, respectively). The latter the spacing and diameter of the whorls chang- 3. Tubulohyalichnus annularis isp. has an can be compared to the “string-of-beads” form ing along its length. This morphotype was annulated tubular form (McLoughlin et al., previously described by Fisk et al. (1998a). not identifi ed in the selection of sub-seafl oor 2009, their fi gure 4). This morphotype is also 4. Tubulohyalichnus spiralis isp. has a heli- samples described here, although we did fi nd illustrated here (Fig. 13A); however, we have coidal tubular form with a coiled or helical axis spiral-shaped fi laments within some of the tun- subdivided the annulated type further into those (McLoughlin et al., 2009, their fi gure 5), with nels (Fig. 23B) and extending from the crown in with regularly sized and spaced annulations up to 12 rotations reported, and can be either another example (Fig. 21A).

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5. Tubulohyalichnus stipes isp. has a branched lar alteration, which was determined by point probably refl ect changing conditions in the sea- tubular form (McLoughlin et al., 2009, their fi g- counting, with measured and inferred subsur- fl oor; however, the time between these episodes ure 6) with dichotomously branching tubes, in face conditions in the ocean crust. At these sites, of texture formation is not known. which the diameters of the daughter branches are measured temperatures increase with depth, and equal to that of the parent branches, and occur in porosity is highest at ~75 m below the sediment/ SUMMARY dense intergrowths that may be hemispherical- basalt interface and decreases below this depth. shaped clusters or more irregular bands. The Permeability also decreases with depth, and it Alteration textures in oceanic basalt glass have type material is illustrated in this atlas (Fig. is inferred that the oxygen content of formation been documented in a comprehensive sample set 19B); however, many additional variations of fl uids decreases with depth. They found that from the ocean basins (Fig. 1). The alteration branching are also illustrated here, for example, granular alteration was present from the basalt/ textures originate at the glass/water interface with more widely spaced branches (Fig. 17B) or sediment interface to a depth where the cur- both on the seafl oor and along the fractures and palmate crowns (Fig. 20A). rent temperature was ~115 °C (500 m). Tubular surfaces of glass that is buried in the crust. The alteration had a more restricted range, being rare alteration transforms the glass to palagonite and Tunnel Contents near the basalt/sediment interface and at depths creates cavities in the glass that exhibit a wide where the temperature was greater than 90 °C. range of simple, intricate, or ornate forms, which The contents of tunnels have been noted in The tubular style had a peak abundance of ~10% are summarized in graphical form (Fig. 2) and previous photomicrographs (Fisk et al., 1998a; at ~120–130 m depth. This depth corresponds to documented in photomicrographs (Figs. 3–31). Josef, 2006), and here tunnel contents are used a temperature of ~70 °C where formation per- The photographs illustrate prevalent as well as as a distinguishing characteristic of the altera- meability is high and the presence of celadonite rare forms. The granular form is common and it tion. Tunnels are often transparent and appar- indicates that alteration occurred in a relatively is often abundant along most of the altered frac- ently empty; however, others have dark contents oxidizing environment (Furnes and Staudigel, tures in some samples. Also quite common is a (Fig. 21B) that we interpret to be either foreign 1999; Furnes et al., 2001a). Their work shows mossy texture. Tubular or tunnel forms are less material introduced during drilling, sampling, or that granular alteration is limited to tempera- common. Often cohorts of tunnels have a com- thin-section preparation, or alternatively as pri- tures less than 115 °C (500 m) and that tubular mon orientation away from their region of origin mary organic residue (cf. Preston et al., 2011). alteration is limited to shallower regions of the and although within a few microns of each other The single or multiple ovoid bodies spaced crust where temperatures are lower and fl uid they do not intersect. Some tunnels have random along the tunnel (Fig. 22A) are intriguing, and fl ow is likely to be higher than in deeper parts tracks and do not appear to be directed away further investigation, for example with fl uo- of the crust. from their point of origin. Tunnel diameters are rescent microscopy techniques, could test the Boreholes on the fl anks of Juan de Fuca and often constant, not varying more than 20% over hypothesis that they are cellular bodies. The Mid-Atlantic Ridges that are fi tted with devices a 100 µm length of tunnel. In some basalts only origin of septae (Fig. 23), spiral fi laments, orna- (CORKs) that monitor physical and chemi- a single type of alteration, such as the granular ments inside tunnels (Fig. 23B), and petaloid cal conditions in the holes (Fisher et al., 2005, form, is present. However, some rocks contain or fl attened tubes with ribbing and/or honey- 2011; Expedition 336 Scientists, 2012) can also multiple forms of alteration within a few tens of comb ornament (Fig. 14B, 15) may be revealed provide basalt samples in which alteration fea- micrometers of each other. The superposition with microanalytical techniques (Alt and Mata, tures can be documented. Although alteration of alteration types in single thin sections prob- 2000; Knowles et al., 2011a, 2011b). textures may form at conditions different from ably refl ects changing conditions in the sub- the current conditions at the site, CORKed holes seafl oor. The wide range of alteration features Environmental Controls on Alteration may enable studies where sub-seafl oor environ- in some individual basalts suggests variations mental conditions are compared to the distri- in chemical or physical conditions over time We expect that certain styles of alteration bution, abundance, and variability in alteration or on micrometer scales at a given time. The of basalt glass will be correlated with environ- features like those described in this atlas. images presented here are a starting point for mental variables at the time of alteration. Fac- correlating alteration features with physical and tors such as temperature, aqueous chemistry, History of Alteration chemical conditions in the seafl oor. We consider and oxygen abundance are known to affect the photo graphic atlas to be the current state of the secondary mineral chemistry and therefore The period over which the alteration textures knowledge, which will likely be expanded as new assumedly the textural styles of alteration. In in basalt glass forms is not known. The tex- features are discovered. Biogenicity has been one attempt to relate alteration styles to environ- tures may form any time between eruption into proposed for alteration features that are similar mental conditions, Josef (2006) studied 140 thin the cold, oxygenated seawater to when the basalt to some of those illustrated here; however, if sections from 63 DSDP and IODP boreholes is deeply buried and surrounded by warm anoxic future work with microbial cultures and envi- and found no correlation. This may be because fl uids. In the shallow ocean crust at both the ronmental samples documents the biogenic pro- in the sub-seafl oor the temperature and com- 5.9 Ma Costa Rica Rift and the 110 Ma Western duction of some features, we would not extend position of the formation water are usually not Atlantic oceanic crust, there are similar maxima this interpretation to the broad range of features measured and must be inferred, and the condi- in the amount of granular and tubular alteration illustrated here. tions at the time of sample collection may not as a percentage of the total alteration (see Furnes ACKNOWLEDGMENTS represent the conditions when the alteration et al., 2001a, their fi gure 11). This suggests that occurred. a substantial portion of this alteration is estab- Thanks to the U.S. Fulbright Program, the U.S.– In another study at DSDP Site 504B and ODP lished early in the crustal history, i.e., within Norway Fulbright Foundation, and the Centre for Geo- biology at the University of Bergen for their fi nancial Site 896A of the 5.9 Ma Costa Rica Rift, Furnes ~6 m.y. Images presented here indicate that there support. The Centre for Geobiology was a gracious and Staudigel (1999) and Staudigel et al. (2006) can be at least two episodes of texture formation host during this study. Jeff Karson provided a scanned compared the abundance of granular and tubu- at some locations (Figs. 29, 30). These episodes version of Maury Morgenstein’s Master’s thesis

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